Note: Descriptions are shown in the official language in which they were submitted.
~117Z~9
~ .
Description of the Invelltion
The literature describes a variety of methods for preparinq hard
refractory metal borides such as titanium diboride. For example, element~l
titanium and boron can be fused together at about 3G30 F. This method
(synthesis by fusion) produces products that are relatively impure and requires
isolation of the boride product by chemical treatme~t. Other sintering pro-
cesses involve the reaction of elemental titanium with boron ca~-bi~e (U.S.
Patent 2,613,154), the reaction of titanium hydride with elemental boron
~U.S. Patent 2,735,155), and the reaction of ferro~itanium and ferroboron
alloys in a molten metal matrix, e.q., iron (U.S. Patent 3,096,149). A fused
salt bath containing an alXali metal or alkaline earth metal reducing agent
and titanium- and boron-containing reactants has been used to produce titanium
diboride (U.S. Patent 3,S20,656). U.S. Patent 3,775,271 describes the electro-
lytic preparation of titanium and zirconium diborides by using a molten sodium
salt electrolyte and rutile or zircon concentrates as the source of titanium
and zirconium, respectively.
The preparation of the borides of titanium, zirconium, and hafnium
by the vapor phase reaction of the correspondin~ metal halide, e.g., titaniwn
tetrachloride, and a boron halide, e.g., boron trichloride or boron tribromide,
~20 in the presence of hydrosen at temperatures of from 1000-1330 C., 1700-2500 C.,
and 1900-2700 C., respectively, has been reported in Refractory Hard Metals,
by Schwarækopf and Kieffer, the MacMillan Company, N.Y., 1953, pages 277, 281
and 285. Typically, these vapor phase reactions have been conducted by heating
the reactants in the presence of an incandescent tungsten filament. Such pro-
cedures, however, produce a coating of the metal boride on a heated substrate
rather than a powdery product. The aforementioned vapor phase reaction for
preparinq titanium diboride has been conducted at temperatures less than 1200 C.
using sodium vapor in lieu of hydrogen (U.S. Patent 3,244,482).
-- 2 --
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7Z~9
A ~:idely reported commercial process used for prepariny refractory
metal borides, e.g., titanium diboride, is the carbothermlc process. In this
process, refractory metal oxide, e.g., titanium dioxide, an oxide of boron,
e.g., B203, and carbon are heated in an electric arc or high frequency carbon
furnace. ~s an alternative to the electric arc furnace, it has been proposed
to prepare titanium diboride by inject~ng powdered activated charcoal impre~-
nated with boron oxide and titania (anatase) into an argon plasma (British
Patent Specification 11273,523). This process produces about one gram of
product in ten minutes and is not, therefore, considcred commercially attractive.
The product obtained from the aforementioned carbothermic process is ground in,
for example, jaw-crushers and mills, and screened. To obtain a finely-divicled
product, extensive milling is required. For example, U.S. Patent 3,052,538
describes the necessity for milling intermetallic compounds such as titanillm
diboride and titanium carbide to obtain a fine particle size useful for dis-
persion strengthening of titanium. A milling time of 300 hours (12-l/2 days)
in a porcelain mill using hardened steel balls as the grinding medium is
recited as being required.
The reported average size of the p~oduct produced from such lengthy
milling ranges from about 2 to about lO microns. Moreover, the product is
contaminated with metallic impurities abraded from the materials of construction
of the mill and grinding surface. Thus, it is common to find impurities in
the product such as tungsten, iron, chromium, cobalt, and nickel. Moreover,
extensive milling produces a significant amount of ultrafine/ i.e. less than
0.05 micron, fragments. These fragments are produced during milling and com-
prise irregular pieces of the principal particles that have been chipped orground away from the edge or face of the particle. Thus, extensive milling
produces particles having fractured irregular surfaces and a relatively large
amount of fines.
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It llas no~l been discovered that suhmicron refLactcry me~al boride
powder, such as titanium diboride, zirconium diboride and hafnium aiboride
powders, that contain minor amounts Oe a carbon-containing additive can be
produced by reacting in the vapor phase, the corresponding metal halide, e.g.,
S titanium halide, boron source, e.g., boron hydride or boron halide, and car~on
source, e.g., readily volatile hydrocarbons or haloyenated hydrocarbons,
reactants in the presence of hydrogen, e.g., a hot hydrogen gas stream produced
by a hydrogen plasma heater, and in the substantial absence of oxygen, either
combined or elemental. Preferably, hydrogen is heated in a plasma l;eater to
form a highly heated hydrogen gas stream, which is introduced into the reactor
and into the reaction zone. The metal halide, boron source, and carbon
source reactants are introduced into the reactor and pxeferably into the hot
hydrogen stream and the resulting reactant gas mixture permitted to react in
a zone maintained at metal boride forming temperatures. The solid metal
diboride formed is removed from the reactor, ~uenched, usually by indirect
heat exchange means, and recovered in conventional fine particle collection
equipment, e.g., cyclones, electrostatic precipitators, dust collectors, etc.
When the metal halide is titanium halide, solid, submicron carbon-containing
titanium diboride powder is produced, the titanium diboride particles of
which are characterized by well developed individual crystals that have well
developed faces. Substantially all, i.e., at least 90 percent, of the
particles have a nominal sectional diame-ter of less than one micron. The
preponderant number, i.e., greater than 50 percent, of the particles less than
one micron are in the particle si~e range of between 0.05 and 0.7 microns.
The powder product can be produced containing less than 0.25 weight percent
oxygen and less than 0.20 weight percent halogen, e.g., chlorine.
By the aforementioned process, an intimate mixture of refractory
metal boride powder containing carbon, either as free carbon and/or c~emically
~ 4 -
72~g
combined carbon, probably as submicron refractoxy metal carbidc, i5 produced.
The resulting powder composition, i.e., submicron refîactory metal boride
powder having dispersed therein a carbon-containing additive, either as free
carbon, refractory metal carbide o~ both, can be cold pressed and sintered,
or hot pressed to dense articles, e.q., articles havinq a density of at least
90 percent, more usually at least 95 percent, of the theoretical density for
titanium diboride. Thus, coproduced powders of, for example, titanium diboride
and titanium carbide and/or carbon in intimate admixture and in most any pro-
portion can be prepared by the above-described process. For use in aluminum
reduction or refining electrolytic cells, consolidated articles prepared from
such refractory metal boride powder, e.g., titanium diboride, preferably
contain between above 0.1 and about S weight percent of total carbon, which
is the sum of the carbon present in the powder as free carbon and chemically
combined carbon. For other uses, a boride powder product containing higher
amounts of total carbon can be produced.
Alternatively, it has been found that submicron carbon-containing
refractory metal boride powder compositions can be prepared by preparing the
submicron ~itanium diboride powder as described, but in the absence of the
carbon source reactant, and blending with the resulting powdered product the
desired added amount of submicron carbon and/or refractory metal carbide. For
example, submicron carbon or titanium carbide can be blended with suhmicron
titanium diboride to produce powder cornpositions that l-ave a carbon content
of from above 0.1 to about 5 weight percent total carbon. Such compositions
also can be cold pressed and sintered and hot pressed to articles having a
density of at least 90 percent, more usually at least 95 percent, of the
theoretical density for titanium diboride. Powdery compositions in which the
carbon-containing additive is coproduced, i.e., formed simultaneously in the
reactor with the metal boride, provide compositions in which the carbon-
,~ - S - :,
.
~~ ~11721~9
c~ntaining additive is more homogeneousiy dispersed throuqnout the metal boride
powder. Consequently, less carbon-contailling addi~ive is requir~d to producc
the same results as when physical blends are usecl.
Brief Description cf the Drawinc~s
The process described herein for preparing submicron refractory
metal boride pohder, submicron, carbon-containing refractory metal boride
powder and articles preparod from such powder can be better understood by
reference to the accompanying drawings and photomicrograpns wherein:
FIGVRE 1 is a diagram of an assemblage, partially broken away in
section, comprising arc plasma gas heatincJ means, two slot reactant mixer
means for introducing reactants to the hot gas stream emanating from the
plasma heater, reactor means, and auxiliary product recovery equipment means
(cyclones and bag filter) for recovering the metal boride powder product
suspended in the reactor gaseous effluent;
FIGUXE 2 is a diagrammatic sectional view of the lower portion of
the arc plasma gas heating means and upper portion of the reactor of FIGURE 1
combined with three slot reactant mixer means in place oE the two slot
reactant mixer means illustrated in FIGURE 1.
FIGVRE 3 is a scanning electron micrograph having a magnification
factor of 25,000 of a sample of titanium diboride powder having a B.E.T.
surface area of ll.S square meters per gram that was prepared in a manner
similar to that described in Example II.
FIGURE 4 is a transmission electron micrograph having a magnification
factor of 25,000 of a sample of the titanium diboride described in connection
with FIGURE 3.
FIGURE 5 is a photomicrograph, having a magnification factor of
2100, of a polished etched sectio~ of the ho-t pressed plate prepared in
Example X;
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~1~72~9
. ~ ' ' ,.
FIGURE 6 is a photomicrogral~-,, havirg a magnirication factor of 2100,
of a polished etched section of the hot pressed plate o~ Example IX;
FIGURE 7 is a photomicro~raph, having a magnification factor of 2100,
of a polished etched section of the isostatically pressed and sintered rod
prepared in Example XII from 7.0 square me-ters per gram titanium diboride; and
FIGURE ~ is a photomicroyraph, having a magnification factor of 2100,
-of a polished etched section of the isostatically pressed and sintered rod of
Example XIII.
Detailed Description
The present invention relates to submicron refractory metal boride
powders containing minor amounts of submicron carbon-containing additive and
particularly relates to submicron titanium diboride powder compositions that
contain minor amounts of added carbon and consolidated dense articles prepared
from such compositions. It has been found that submicron carbon, notably in
the elemental form or in the form of metal carbides, aids the densification of
submicron refractory metal boride powder compositions (promotes sintering)
to the extent that the powder composition can be consolidated to highly dense
articles by cold pressing and sintering. Thus, submicron titanium diboride
containing as little as 1 weight percent submicron carbon can be cold pressed
and sintered to densities of at least 90, e.g. 95 percent of the theoretical
density for titanium diboride. As used herein with respect to metal boride
powders or compositions, the terms '`carbon" or "total carbon", unless otherwise
defined, are intended to mean the carbon present therein both as elemental
carbon and chemically combined carbon, e.g., as a metal carbide.
Consolidated articles prepared from submicron refractory titanium
boride powder compositions which contain from above 0.1 to about 5 weight
percent total carbon, preferably from above 0.1 to about 2 weight percent,
e.g., 0.15 to 1 height percent, more preferably, about 1 weight percent, total
~ .
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72
~ .
car~on based on titanium diboride are cspecially useful in aluminum rcduction
or aluminum refininy cells. For other uses, refractory metal boride powders
containing higl-er amounts of tota carbon, e.g., up to lO weight percent or
more, are contemplated. Thus, powder compositions ~and articles prepared
therefrom) containing from O.l to lO weight percent total carbon are contempiated
herein.
The carbon-containing additive can be introduced into the boride
powder in any convenie~it manner, however, it is prefcrred that the carbon be
introduced into the powder in the reactor when the metal boride powder i.s being
formed. Various advantages accrue when the carbon is introduced into the
boride powder at that time. First, a more homogel-eous distribution of carbon
in the boride powder product results than can be achieved by physically
blending. ~ homogeneous distribution of carbon throughout the boride powder
hinders grain growth during sintering and helps provide a fine grain structure.
A fine grain structure generally has greater strength than a coarse grained
structure. Second, elimination of possible oxygen and metal contamination as
a consequence of such blending is achieved. Third, the presence of ultrafine
carbon particles in the reaction zone provides also a source of nuclei which
often results in a boride powder product of higher surface area than a powder
~20 prepared in a reaction system that does not have such nuclei. Finally, it has
been observed that less reactor added carbon is required to obtain the same
d~gree of densification than is required with physically blended carbon.
,` Results obtained with reactor added carbon compare favorably with those obtained
using twice as much carbon that has been physically blended with preformed
~25 refractory metal boride. It is postulated that the essentially homogeneous
dispersion of reactor added carbon throughout the refractory metal boride
powder is a major reason for this result. Further, titanium diboride containing
reactor added carbon provides a sintered article having an essentially equia~ed
8 -
72~9
.
~rain structure ~hile titanium diboride containing physically blended carbon
provides a sintered a~ticle having l~ss pronounced cquiaxed grains and more
elongated grains.
Metal boride, e.g., titanium diboride, powder compositions containing
from, for example, above 0.1 to 5 weight percent total carbon can be prepared
also by blending physically submicron ~letal carbide powder, e.g., titanium
carbide powder, and/or finely-divided carbon with submicron metal boride,
e.q., titanium diboride po~der in amounts sufficient to provide a total
carbon level within the aforesaid range. Submicron titanium carbide and other
metal carbides can be prepared by the processes exemplified by U.S. Patents
3,485,586, 3,661,523, 3,761,576, and 3,340,020. Briefly, such processes
comprise subjec-ting a halide of an element of the metals of the 3rd to the 4th
group of the Periodic Table or the metalloids of the 3rd and 4th group and a
hydrocarbon to the action of a hydro~en plasma. In Example I of U.S. 3,340,020,
a mixture of tantalum pentachloride and methane was introduced into the flame
of a hydrogen plasma and tantalum carbide haviny an average particle size of
0.01 micron and a B.E.T. surface area of 40-60 m /gram was reported produced.
Generally, the submicron metal carbide, e.g., titanium carbide, used will
have a number median particle size of bet~een about 0.1 and 0.9 microns,
although submicron metal carbides having a number average particle size of
between 0.01 to 0.9 microns can be used. Usually, the added submicron
refractory metal carhide will have substantially the same surface area, i.e.,
the same number average or number median particle size, as the refractory metal
boride. Submicron carbon is commercially available and such materials can be
used directly; a co~mercial carbon product having a particle size larger than
desired can be used, preferably by first being reduced in size by grinding the
carbon in conventional milling equipment, e.g., fluid energy mills. For
example, commercially available NllO carbon black having a surface area of
_ g _
1~ 89
~1-19 m /gram can bc used.
The carbon-containing additive can be prcsent as elemental submicron
~finely-divided? carbon; ho~ever, it is preferred that it be present as sub-
micron refractory metal carbide powder, e.g., hafnium carbide, titanium carbide,
tantalum carbide, zirconium carbide, boron carbide, silicon carbide, etc. The
carbides of refractory metals of Groups 9b, 5b and 6b of the Periodic Table
of the Elements (identified hereinafter) boron and silicon are contemplated.
Mixtures of metal carbides can be used; but, usually the refractory metal of
the carbide will be the same as the refractory metal of the boride. Identity
of refractory metal between the metal boride and carbon~containing additive
is not required. Thus, powder compositions such as titanium diboride powder
containing carbon as hafnium carbide, tantalum carbide, zirconium carbide,
boron carbide, silicon carbide, or mixtures thereof are contemplated. Other
similar combinations of refractory metal boride powders and refractory metal
carbide powders are also contemplated.
Refractory metal borides of Group 4b of the Periodic Table of the
Elements (Handbook of Chemistry and Physics, 45th edition, published by The
Chemical Rubber Co., 1964) prepared by the process described hereinafter,
namely, titanium diboride, zirconium diboride and hafnium diboride, are grey
to black powders composed predominantly of well developed crystals having
well defined faces. FIGURES 3 and 4 which are electron micrographs (25,000
magnification) of submicron titanium diboride prepared in accordance with
the present invention, show examples of the typical crystalline particles
produced. The product contains varying proportions of equidimensional and
tabular single crystals, which are freely dispersible by virtue of extremely
limited crystal intergrowth. The equidimensional crystals are bounded either by
planar crystal faces or smooth rounded surfaces. The tabular crystal forms
consist dominantly of hexagonal prisms terminated by the basal pinacord. The
10 -
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tabular crystals are flattened perpendicular to the c - crystallos~aDhic axis
as a result of sreater development of the pinacoidal faces relative to the
prism faces. Consequently, the crystal habit of the product can be described
as tabular to equidimensional hexagonal. Based on visual observations of the
S powdery product through an electron microscope, the tabular hexagonal crys1_als
exhibit a nominal sectional diameter to thickness ratio within the range of
l.S:l to 10~
Submicron metal boride powders, e.g., titanium diboride, that can be
prepared utili~iny the proess described in more detail hereinafter are sub-
io stantially free of untlesirable metal contaminants, i.e., the powders areessentially pure, as estahlished by emission spectrographic analysis. Since
carbon is added to or confom~ed with the metal boride powder, carbon i5 not
considered an impurity.
~;etal impurities ~as el~mental metal) normally represent less than
4,000 parts per million parts of the boride po~der (ppm), i.e., less than
0.4 weight percent, and often represent less than 3,000 ppm (0.3 weight percent).
Among the metals that can comprise the aforementioned impurities are the
following: alurninum, barium, calcium, chromium, copper, iron, potassiurn,
lithium, magnesium, manganese, sodium, nickel, silicon, vanadium and tungsten.
T~e source of such metal impurities, if present, in the boride po~der product
is normally the reactants or equipment used to prepare the product.
Oxygen and halogen, e.g., chlorine, normally make up the largest
individual non-metallic impurities that are introduced into the product from
- the reactants. By virtue of the described process, it is readily feasible to
obtain boride powders with less than 0.20 weight percent halogen, e.g., chlorine,
and less than 0.25 weight percent oxygen. By careful recovery, e.g., degasifi-
cation, and handling techniques to avoid exposure of the boride po~der product
to the atmosphere (oxygen) or moisture, boride powders with less than 0.15,
- 1 1 -
:.. ~- . ... .
za~
often ]ess than 0.10, weight percent halogen, asld less than 0.20, e.g., less
than 0.15 weight percent oxygen can be obtained. Ths~ aforementioned values for
halogen and oxygen are based upon analysis for such impuritis~s obtained by use
- of X-ray spectrographic analys.~s and by the use of a Leco oxygen analyzer
(model 534-300) respectively. The aforementioned X-ray spectrographic
technique analyzes principally for unrPacted metal halides and subhalides
present in the boride powder. Adsorbed hydrogen halide, e.g., hydrogen
chloride, on the boride powder may not be detected by that technique.
Thus, despite the use of substantially pure reactants and careful
handling and recovery techniques, a small amount of metal impurities, llalogen
and oxygen can be present in the boride product. When not added lntentionally,
carbon can also be found in the boride powder product; however, the carbon
level is typically less than 0.1 weiqht percent. The total amount of the
aforesaid impurities in the boride powder product (other than added amounts
of carbon) is usually less than 1.0 weight percent, and typically is less
than 0.75 weight percent. Stated another way, refractory metal boride powders
of the present process that are produced in the absence of carbon source
reactant are usually at least 99 percent pure and typically are at least 99.25
percent pure.
The metal boride powders produced by the present process, e.g.,
titanium boride, are, as indicated, predominantly submicron in si e. The
surface area of the boride powder product can vary between about 3 and about
35 square meters per gram, tm /gram), more typically between about 4 and
about 15 m /gram, e.g., between 5 and 10 m /gram, as measured by the method
of Brunauer, Emmett, and Teller, J. ~m. Chem. Soc., ~0, 309 ~193~). This
method, which is often referred to as the B.E.T. method, measures the absolute
surface area of a material by measuring the amount of gas adsorbed under
special conditions of low temperature and pressure. The B.E.T. surface areas
- 12 -
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reported hereirl were o~)tained using nitrogell as the gas adsorbed and li~uid
nitrogen temperatures ~-19G C) and a pressure of 150 mm of mercury (0.2
relative pressure).
The surface area of the boride powder is, of course, a function of
the particle size of the boride particles produced, i.e., the smaller the
particle si~e, the higner the surface ~rea. The average spherical particle
si~e diameter, in microns, of the refractory metal boride, e.g., titanium
diboride, powder particles-produced can be estimated roughly by the
expression:
~verage Spherical Particle Size Diameter ~ 1.33/Surface ~rea tm ~gram)
` which assumes that each particle is a sphere (regular shaped polygon).
; Substantially all, i.e., at least 90 percent (by number) of the metal -~
; boride particles comprising the boride powder composition are submicron, i.e.,
;~ have a nominal sectional diameter of less than one micron. The nominal
sect~onal diameter is the nominal diameter of a particle viewed under high
magnification, e.g., 25,000 magnification, such as_seen by an electron micro-
scope and depicted in electron micrographs. The nominal diameter is based on
the two dimensional surface viewed under high rnagnification. The preponderant
number, i.e., greater than 50 percent, of the particles less than one micron
~20 are in the particle size range of between 0~05 and 0.7 microns. Particles as
small as 0.03 microns and as large as 2 mlcrons can be present in the powdery
product; but, particles greater than 2 microns rarely represent more than one
percent by nurnber of the product. The aforesaid crystalline particles less
than 0.05 microns are distinguishable from the ultrafine fragments less than
0.05 microns found in metal diboride powder that has been milled extensively.
The metal diboride powders described herein are substantially free of fragments
less than 0.1 micron, e.g., the ultrafine fragments less than 0.05 microns.
- 13 -
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~ 728~
.
t is estimated from a study of the refractory metal borldc powders of the
present invention with a Zeiss* TGZ-3 Particle Si~c Analyzer that at least
60 percent on a number basis, more usually at least 70 percent, e.g., 98 per-
cent, of the boride particlcs comprising the po~der are 0.7 microns or less.
It is not uncommon to find that the aforesaid percentages represent also the
particles within the particle size range of between 0.05 and 0.7 microns.
It is estimated further that less than 10 percent on a number basis of the
boride particles are greater than 1 micron. The aforementioned values
respecting the percentage of boride particles 0.7 microns or less depends
on the partlcle size distribution of the powder: Generally, the particle
size distribution is relatively narrow. The num`oer median particle si~e of
the boride particles comprising the boride powder composition is usually
; between about 0.08 and about 0.6 micron, more usually between 0.1 and 0.5
microns, and varies directly with the surface area of the powder. Because
of its high surface area, the metal boride powder tends to adsorb readily
r~ oxygen or moisture.
: The refractory metal boride powders of the present invention are
useful when consolidated into dense articles as current conducting elements,
e.g,, as high temperature electrical conductors, as electrodes in metal
manufacturing and refining such as aluminum manufacture. The relatively low
electrical resistivities of consolidated shapes prepared from these boride
powders make them especially desirable as electrical conductors and electrodes.
Moreover, it has been found that the electrical resistivity of hot pressed or
cold pressed and sintered forms p~epared from the boride powder products, e.g.,
titanium diboride, produced in accordance with the process described herein
are lower than values reported in the literature. For example, electrical
resistivity values for titanium diboride have been reported as being greater
than 10 microohm centimeters, e.g., from 10 to ?sO microohm centimeters and
* Trade Mark - 14 -
7Z~
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~ypically from 15 to 2S microohm centimeters. In contra~t, hot presscd or col~
pressed and sintered titanium diboride forms prepared from titanium dibori~le
; powder compositions produced in accordance with the present invention are
typically less than lO microohm centimeters, e.g., usualLy from S to 9 microohm
centimeters. The electrical resistivity of zirconium diboride and hafnl~l
diboride is also typically less than lO microohm centimeters at room tempera-
ture, e.g~, 25 C.
Electrical resistivity can be measured in the conventional Manner.
Briefly, such measurement is obtained by applying direct current from two
elcctrodes across the specimen to be measured, e.y., a square or rectangular
plate, and the potential (voltage) difference between two points on the speci-
men equidistance from the electrodes recorded by an electrometer. For example,
a 2 inch x 2 inch x 1/2 refractory metal boride plate is clamped at the l/2
inch sida between two copper electrodes and a direct current applied acrcss
the plate. A distance of 4 centim~ters along the line of current flow (2
; centimeters on either side of the center line) is measured and the end points
marked. The probes from the electrometer are placed on the end points of the
measured 4 centlmeter length and the potential difference measured. Generally,
electrical resistivity is taken at 25 C. and the values reported in the
examples herein ~ere measured at that temperature. The electrical resistivity
value is calculated from the following expression:
Resistivity (ohm cm.) = (Potential Difference~ vo]ts)(Cross Sectional Area, cm .)
(Applied ~mperage, Amps)(Distance between voltage probes, cm.)
Refractory metal boride powders prepared in accordance with the
process described herein can be consolidated into shapes or forms of high
density by conventional hot pressing, or cold pressing and sintering techniques.
The refractory metal boride powders, e.g., titanium diboride, of the present
invention can be consolidated by hot pressing by subjecting a mold containing
,:
- 15 -
' .
r~ z~
~e po~ders to a con~irluously applied pressure o~ ~rom a~out 0.5 to S0 tons
- per square inch, e.g., 1 to 3 tons per square inch, while raising slowly its
temperature to between 1600 C. and 2700 C., e.g., 1800 C.-2500 C. Tne com-
pacting, heating and subsequent cooling operations are typically carried out
in an inert atmosphere, e.g., arc3On or in a vacuum. The operation is often
oarried out in a graphite die having a cavity of the appropriate desired
- cross-sectional shape. The pressure is preferably applied to the powder by
plungers acting on opposite ends of the powder, e.g., a column of powder.
' The nature of the hot pressing process is such AS to render it difficult to `
- 10 form shapes other than flat plates and other relatively simple shapes. ~ore-
over, hot pressing is a relatively expensive process and is hard to adapt to
large scale production by continuous processing.
The refractory metal boride powders of the present invention can
be consolidated by cold pressing and sintering by pressing the powder into
the desired shape followed by sintering the resulting form at temperatures
betwèen 1800 C. and 2500 C. either in a vacuum or in a neutral ~incrt)
atmosphere. For simple shapes such as cylinders, platcs, or the like, the
- powders can be dry pressed in matched metal dies. For complicated shapes,
slip casting, tape casting, pressure casting, compression castin~, extrusion
- 20 or injection molding can be used to cold form the article. Further, a wax
binder can be incorporated into the powder by spray drying techniques and the
resulting powder blend molded into the desired shape in rubber molds. Typical-
ly, the powder composition is mixed with a small portion of binder, e.g., 1
weight percent of paraEfin wax dissolved in l,l,l-trichloroethane solvent. ~ ;
The solvent is evaporated prior to consolidating the powder. The resulting
powder composition-binder mixture can be consolidated by applying pressure
to the mixture, e.g., isostatically or between matched metal dies, either at
ambient temperature or at slightly elevated temperatures, but, significantly
- 16 -
. .
7;2 ~
li 5S tl;an sintering ternpeLat~lres. The pressure applied is in the rangc of
0.5 to 50 tons per square inch., e.g., 2-10 tons per square inch. ~lternati~Jely,
the powder composition-binder mixture can be extruded into the desir2d shape.
Sinterin~ is accomplished by heating the consolidated shape in vacuum or inert
atmosphere at temperatures of from 1800 C. to 2500 C. Prior to sintering,
it may be necessary to first heat the green compacts at temperatures sufficient
to remove any organic binder material (if used). Heating at about 200 to 400 C.
for about one hour in a vacuum or inert atmosphere is usually sufficient to
remove such binder materials. ~he term "cold formed" as used herein means that
tile metal boride powder composition is compacted and shapecl, as by pressing ormoiding, prior to the sintering operations, as distinguished from hot formed
or hot pressed bodies ~hich are shaped and pressed by the application of
pressure during sintering.
- 17 -
' ~ ..
:
~ 7 v~
.. It has been found th.-t carbon-contai~ g refractory
metal boric1e, e.~., titanium diboride, po~der colnpositio1ls
of the prcscnt invention can be cold prcssed ancl sin-t:ercd
to high.densities, i.e., at least 90 percent of the theor-
: etical density of the refractory metal boride. Depending
upon the particular powder composition, densi.ties in
e~cess of 93 percellt of theoretical, e.g., in excess of
95 percent and often ln excess of 98 percent of theore-
. tical, can be achieved. Stated another way, cold pressed
- and sintered elements fabricat~d from titanium diboride
. . powder compositions having a porosity level of not more
than l0 percent now can be obtained. The aforesaid re-
fractory metal boride powders and powder composi.tions also
can be hot pressed to densities at least equa]. to that
obtalned by cold pressing and si.ntering and more usually
to densities approaching the theoretical density. S:ince
the technique of hot pressing l.imits to a great extent
the shape and size of fabricated shapes, the availability
20 of cold pressing and sintering as a conso]idation tech- :~
nique provides engineering design opportunities which
. were not possibl.e earlier. .
~he apparent g*ain size, i.e., average diameter, of
- . the refractory metal boride grain as measured on an e-tched -
. metallographically polished surface of a sintered refrac-
tory metal boride specimen is predominantly fine. As
~easuredion photomicrographs of the polished surface,
the grain size of the borilde grains is generally less than 20
microns, and predominantly in the range of about l to l0
microns. The grains are relatively uniform size and
occur in a microstructure characterized by contiguous
- grain boundaries and low porosity resulting in high density
.. and strength of the sintered bodies.
~ 18 - ~ 2~
..
.
2i~
~ llot pressed or cold pressed and sintered articles
havin~ dcnsities of greater than 90 percen-t of theoretical
`~ of the refractory me~al boride density, e.g., at leas-t 92
or 93 percent of -theoretical, are generally considered
impermeable. Thus, when such articles are used in, for
example, aluminum reduction or refining electrolytic cells,
they are substantially impermeable to mol-ten material to
. which they are exposed in such cells. The refractory
: metal boride powder compositions O r the present invention
, 10
`~ can be fabricated into articles having such densities and,
accordingly, such articles are userul as current conclucting
elements in the aformentioned typ~ electrolytic cells.
Refractory metal boride compositions comprislng mi.x
tures of more than one metal boride powder and carbon-con-
taining additive(s) are also contemplated herein. Thus,
blends of titanium diboride powder with zirconium dibor-
ide powder and/or hafnium diboride powder in most any pro
portion can be cold pressed and sintered, or hot pressed
in the same manner as heretofore described. Such mixtures
of boride powders can be prepared by blend:ing the pre
formed boride powders in the relatlve amounts desired;
or, the boride powders can be co-produced by introducing
into -the reactor, ususally simultaneously, the refractory
metal halides of the metal borides desired and in the
proportion desired in the end product. Further, mixtures
of the carbides of the aforementioned refractory metals
with such boride powder mixtures can be blended physically
with the powder or simultaneously prepared with the
aforementioned refractory metal borides in the amount
described previously by introducing a carbon source into
the reaction zone of the reactor.
.
.
Generally, any volatile inorganic t:itclllium ZirC~lliUIll ~72~3
- -or hafniurn halide, e.g., ~ compound of on:ly the aforc)llen-
tioned rneta]. and ha~ogen (chlorine, bromine, :fluorine ancl
iodine), can be used as the source of the aforemen-tioned
metal ln the refractory metal bor~de powder product pre-
pared by the process described herein. ~s used herein
the terms "metal halide" and "metal boride" or "metal
diboride" are intendcd to means and include the halides
and borides respectively of titanium, zirconium and hafn-
ium, i.e., the elements of Group 4b of the aforesaid
Periodic Table of the Elements. However, for the sake of
convenience and brevity, reference w.i]l be made sometimes
to only one of the aformen-tioned metal halides or borides.
Exemplary of the refractory metal halides -that can
be employed in the present process include: titaniwn
tetrachloride, titanium tetrabrornide, titanium tetraio-
dide, titanium tetrafluoride, zirconium tetrabromide,
zirconium tetrachloride, zirconium tetrafluoride, zir-
conium tetraiodide, hafnium tetrabromide, hafnium tet-
rachloride, hafnium tetrafluoride, hafnium tetraiodide,
as well as subhalides of titanium and zirconium such as
: titanium dichloride, titanium trichloride, titanium tri-
fluoride, zirconium dibromide, zirconium tribromide, zir-
conium dichloride and zirconium trichloride, Of course,
SUbhalideS other than -the subchlorides and subfluorides
; can be used in the same manner. Fu~ther, inorganic metal
~ halide corresponding to the refractory metal carbide(s)
:; :
desired coproduced with the metal boride, if different
than the metal boride powder bein$ produced, can be used.
For example, the halides of hafnium, tantalum, silicon
and other refractory metals, the carbides of which are
desired can be used. Mixtures of metal halides of the
same metal such as
_ 20 -
... .
2~
- ;the cI-loricIes and the biomides, e.g., ti~aIlium tetrac~lo-
ride and titanium ~etrabromide can be emplGycd as the
metal halide reactaJlt. Fur-ther, mixtures of halides of
different metals can be used when it is desired to co-
produce more than olle rnetal boride powder, e.g., ti-tan-
ium dlboride and zirconium diboride, or more than one
metal carbide. Prefer~bly, the halogen portion of the
metal halide reactant (s) is the same to avoid separation
and recovery of different hydrogen halides from the pro-
10 . '
duct stream. The metal halide reactant(s) can be intro-
duced into the reactant inlet asscmbly (mixer means) used
to introduce the reactants into the reac-tor as a liquid
or vapor; but, should be introduced in such a manner that
the reactant(s) is a vapor in reactant mixing zone and
subsequent reaction zone. Economically preferred as the
metal halide reac-tant are the tetrachlorides, e.g., titan-
ium tetrachloride. The metal halide reactant (s) should
be substantially pure, i.e. substantially free of metal
contaminants and free or chemically combined oxygen so as
to produce a metal boride powder having the purity des~
; cribed earlier.
- The boron source reactant like the metal halide reac-
tant should be also oxygen-free and substantially pure to
avoid the introduction of oxygen and metal contaminants
into the metal diboride product. 8y oxygen-free is meant
that the boron source is substantially free of chemically
combined oxygen, e.g., the oxides of boron, as well as un-
combined oxygen. Despite the precau-tions of reac-tant
purity, a small amount of oxygen contamination occurs in
-the boride powder, as earlier described. As a suitable
source of boron for the metal borides, there can be
, ~
- 21 -
. ~ ' .
2~9
,
mentioned inorganic boron compounds such as boron
tribromide, boron tri:iodide, boron trichloride,
boron trifluoride and
' ' , ' ' ~ ~
.
~.
10,
'',' ' ~,:
'' ' ' , , ' ~:
'
.
- ~:
-' , . . . :
.. , : .
~, ", " ' ' ~ '':
:
- 21a -
- 30
~1~7~Zi~l~
" ~ . .
the hydrol~orides (boranes), e.g., B21~6, B511~, Blol~
and B6i~2 boron tric}-loride is preferred. ~s in the
case of the metal halide reactan-t, the boron source reac--
tant is introducecl illtO the reac-tor in sucl- a manner that
is is present in the reactant mixing zone and reaction
zone as a vapor. ~`he metal halide source and boron source
should be chosen from those compounds which, in combin-
atlon provide a thermodynamically favorable reaction at
the desired reaction temperature.
10 ' , ' , '
the reaction of titanium tetrachloride with boron triflu-
oride ls thermodynarr.ically less favorable at 2000 K. than
at 2500 lC. Thus, such thermodynamlcally less favorable
reactions will requlre higher reactlon ternperatures.
rrhe amount of` boron source reactant introduced into
the reaction zone in the reactor will preferably in at
leas-t stoichiometric quantities, i.e., in amounts suffic~-
ient to provide at leas-t two atoms of boron for each atom
of metal, e.g., titanium introduced into the reaction
- zone ln the reactor as metal halide, e.g., titaniurn halide,
reactant. The ratio of the boron source reactant to the
metal halide reactant can, of course, vary from stoichio-
metric quantities. Thus, the boron source reactant can
be introduced in amounts sufficient to provide in the
reaction zone between about 1.8 and about 3 a-toms of` boron
per atom of metal, e.g., titanium. Preferably, greater
than the stoichiometric ratio is used. For example, the
mole ratio of reactants boron trihalide to titanium tetra-
halide (BX3/TiX~), wherein X is halogen, can vary from
about 1.8:1 to 3:1 and preferably is about 2. When a
stoichiometric e~cess of the boron source is used, less
- - 22 -
7Z~
: residual unreac~ed metal l~lide reactant is l`ound
in the product. I~hen a stoichiome-tric excess o~
rnetal halide is used,
'
,' ''
.
;,
; .
- 22a ~ ::
7~
sub-llLIlides of tl-e metal are found in the product. ;~hile
it is pre-f`erred t:hat t:he boron SOlll'Ce reactant be used in
stoichiome-tric excess either of the metal halide or boron
source reactants can be used in stoich:iometIic excess in
amounts of from 5 to 30 percen-t by weight.
The carbon source reactant should also be of the
type -that is readily volatile in the reaction zone and is
capable of reacting in a thermodynamically favorable man-
ner at the temperatures at which the reaction is ~conduc-tecl.
In the
aforesaid embodiment, volatile hydrocarbons, halogenated
hydrocarbons or mixtures thereof that are substantially
pure and oxygen-free, as defined above, can be used as
the carbon source. As used herein, the term "halogena-ted
i~ydrocarbons", e.g., "chlorinated hydrocarbon", is in-
tended to means and include both compounds of carbon, hal-
ogen and hydrogen and compounds only o~ carbon and hal-
ogen, e.g., carbon tetrachloride.
Typical hydrocarbons that can be used as the carbon
source include the normally gaseous or liquid but rela
- tively volalile hydrocarbons including saturated and un-
saturated Cl - C12 hydrocarbons, such as methane, ethane
propane, the butanes, the pentanes, decanes, dodecanes,
ethylene, propylene, the butylenes and amylenes, symmetr-
ical dimethylethylene and like alkenes, cycloaliphatic
and aromatic hydrocarbons, such as cyclopentane, cyclohex-
ane toluene, benzene, etc. t and acetylenic compounds of
which may be noted acetylene, methyl ace-tylene, ethyl
acetylene, and dimethyl acetylene. Methane or propane
are economically preferred for this purpose. Rarely are
hydrocarons of more than twelve carbons used.
- 23 -
Exclmples Or hc~loge~ ted hydrocarbons tl-at can be ~Ised
as tlle source of carbon in the process clescl7ibed hereill 1 ~ 1 7 Z~
include satul-a~ed ancl unsaturated compourlds containing
from one to twelve, more usual:ly one -to eigllt, carbon
atoms, such as methyl chloride, ethyl chloride, chloro-
form, methylene chloride, carbon te-tr-achloride, dichloro-
difluorometl-ane, amyi chloride, chloroethane, viny] chlor-
ide, l,l-dichloroethylene, 1,2-dichlorocthylene, l,l-dich-
loroethane, 1,2-dichloroethane~ ethylene dibromide,
trichloroethylene, perchloroethylene, propylene dichloride,
1,1,2-trichloroethane, 1,1,1-trichloroethane, 1,1,1,2-
and 1,1,2,2-tetrachloroethane, hexachloroethane, and like
aliphatle chlorides, fluorides, bromides or :iodides con-
taining up to about twelve carbon atoms, most preferably
up to about six carbon atoms. Aromatic halocarbon compounds
e.g., chlorocarbon compounds, also can be used. Such
compoullds include C6 - Cg halogenated aromatic compounds
such as monochlorobenzene, orthodichlorobenzene, paradich-
lorobenzene and the like. Cycloaliphatic halides, such
as the C5 - C6 aliphatic halides, e.g., chlorinated cyc-
lopentane and cyclohexane, etc., can also be used.
Typically, the above-described hydrocarbons and halo-
genated hydrocarbons should be readily vaporizable (vola-
-tile) without tar formation since otherwise unnecessary
difficulties which are unrelated to the process itself
can arise, such as the plugging of transfer lines by de-
compositio;l of polymerization products produced in the
course of vaporizing the carbon source reactant. The
Cl - C3 hydrocarbons and halogenated hydrocarbons have b
been found very useful.
The amount of carbon source reactant, e.g., hydrocar-
bon or halogenated hydrocarbon, used will of course de-
pend on the amount of
-24-
~L~17;Z ~9
carbon desired in the final boride po~vder produc-t. 'I`he
amourlt of total carbon in the meta]. diboride powder, e.g.,
titanium diboride pot~der, or diboride powder composition
can range from abo~e 0.1 to about 5 weight percent, pre-
ferably from above 0.1 e.g., 0.15, to about 1 or 2 weigllt,
percent. The use of about 1 weight percent total carbon
has been observed to be very usefu]. Whcn a carbon source
reactant is introduced into -the reactor, it-is expected
that carbide(s) of metal-(;s) present in the reactor, e.g.
titanium carbide are co-produced in situ with metal di-
boride. At low levels of carbon, i.e., less than 1
weight percel~t total carbon, the X-ray patte~ characteri-
stic of metal carbides, such as -titani-un carbide, :in the
diboride powder is not fairly evident. By "total carboll"
is meant the total amount of both free carbon and chemi-
cally combined carbonJ e.g., metal carbide, in the metal
diboride powder product. If, for example, all of the co-
formed carbon in titanium boride powder is present as
titanium carbide a total carbon content of between above
0.1 and about 5 weight percent corresponds to a titanium
carbide content of between above 0.5 and about 25 wei~ht
percent. On the same basis, a to-tal carbon con-tent o:E`
0.15 to 2 e,~, 1 weight percent
corresponds ~o a titanium carbide content of between about
0.75 and 10, e.g., 5 weight percent. From the evidence
at hand, it -'s believed that when a carbon source is
added to the reaction zone, the carbon in the metal boride
powder product is present pricipally as the metal carbide.
The vapor phase reaction of metal halide and boron
source reactants with or without a volatile carbon source
is conducted in -the presence of hydrogen. The amount of
hydrogenu-tlized in the above-described process is at le~t
that amount which is required stoichiometrically to sat-
isfy the theoretical demand of the reac-tion. Preferably,
- 25 -
7~
~~~ ~lle- alnount of hydrogen used is in exccss Or tlle theo-
re~ical amo-lnt. \~hell, for example, the metal hal:ide reac-
tant used is titallium tetrachloride ancl the boron source
reactant used is boron trich]oride, the theore-tical amolnt
or demand of hydrogen required can be e~pressed by the
equation:
I TiC1~ + 2BC13 t 5~l2 ~ 2 t
Often the amount of hydrogen utilized will be in excess
of ten times and as high as lOD times the amount of hydro-
gen shown to be required by the above equation or required
to equal the chemical eq-livalents of halogen oI` the metal
halide arld/or boron halide, and halogenated hydrocarbon
tif used) reactants. ~hen the boride source is a hydro-
boride, the hydrogen available from the hydroboride can
be used to satisfy all or a part of the hydrogen demand.
Typically, the mole ratio of hydrogen to metal halide
reactant ranges between about 20 and ~0, e.g.~ 25 moles
of hydrogen per mole of metal halide.
The temperature at which the vapor phase reaction of
metal halide and boron source reactants is conducted will
depend on the reactants selected and will be those temp-
eratures at vhich submicron metal boride powder is pro-
duced with the selected reactants under thermodynamically
favorable conditions, i.e., metal boride powcler forming
temperatures. The average reaction zone temperature for
the aforementioned vapor phase production of metal boride
powder such as titanium diboride powder typically is above
1000 C~ and usually ranges upwardly of 1000C. to about
3500 C. The process
- 26 - ;
'
c.arl l~e cond-lctc-d at; subatmospheric, a1~rnos;~ er~ic, ancl snpe~
. - .
-- atmospheric press-lres. Typically, thc proccss is con~-lc-t.ed
at between abdut 1 ancl about ~ atmos~ eres, nornlally L,e
tween 1 and 1.5 atmosphercs pressure.
The process and handling eqllipment utlizecl in the
aforementioned process for producing met~al diboride powder
(as more specif'ically described thereinaf-ter) are constru- -
cted from materials resistant to the temperatures and
corrosive environment to which they are exposed during
the ~arious steps of the procedure, as outlined herein-
after. The present invention will be more fully understood
by reference to tl-e accompanying drawings. Referring now
to ~IGURE 1, there is shown apparatus comprising plaslna
generator heating means 1 mounted atop reactant inlet
assembly (mixer) means 30 which, in turn, is mounted atop
reactor 34. Al-though the aforesaid apparatus is shown in
vertical alignment, other alignments away from the verti-
cal including a horizontal aligment are contemplated.
While the plasma generator heating means shown is an arc
heater, other plasma heater types, e.g., an induction
(high frequency) hea-ter, can also be used. Further, o-ther
heating means sucl- as electrical resistance heaters, can
be used to heat hydrogen to the temperatures required by
the process described herein. The hydrogen is heated
typically to temperatures which is sufficient to establish
and maintain metal boride forming temperatures in the
r`eaction zone bearing in mind that it is mixed with the
metal halide and boron source reactants which are intro-
duced into the reaction zone at below the reaction temper-
ature, usually significantly below reaction temperatures.
- 27 --
'I`hus, thc principal source of heat f`or ~lle reaction is
~enerally the highly hydrogen gas stream. I']asma heat :L
consists essential]y of an an~ Lar anode ll whicl) is
aligned coaxially with cathode rod 3. Both anocle and
cathode are mounted in a cylindrical sleeve 9 which is
electrically norl-conduc-tive In the embodiment illus-
tratecl, the cathode rod tapers conical]y at its end essen-
tially to a point. The anode and catho~'e are construc-ted
out of conventional electrode type materials, such as
copper, tungsten, etc. The cathod often has a thoria-ted
tungsten tip or inserts which assist in cooling of the
cathode.
As is convent:ional with plasma heaters, the anode is
surrounded by an annular cooling chamber 13 through which'
coolant e.g., water, or other cooling medium is circulated
by means (not shown) in order to hold the anode at a suit-
ably low temperature and prevent undue erosion thereo~.
In a similar manner, the interior of the cathode is pro-
vided with cooling chamber 7 and with means (no-t shown)
to circulate water or other suitable cooling fluid there- ;~
in in order to hold the cathode at a suitable operating
temperature. Tube 2 serves -to help support and align
cathode rod 3 and provide a conduit for coolant flow.
Cathode 3 can be provided with means for moving it in a
vertical direction so that the distance between cathode 3
and anode ll can be varied.
The anode and cathode are axially aligned but spaced
longitudinally to provide annular space 21 which tapers
conically to a coaxial outlet conduct 23. The assemblage
i5 also provided with plasma or work gas inlet means 15 ~'
having conduit 17 which communicates through annular coni-
cal conduit l9 with the annular
- 28 -
space 2~ . 'Il-e catllode and anode are co~ ect~-~ by cleetlL~
eal connecting meal-ls (not showrl) -to a power supp].y (llOt
silown). Typically, the power source is a clirect curren-t
power souree.
~ eaetant mixer means 30 is adjaeent to the anode e.nd
of eylindrieal sleeve 9, and as shown, comprises two co-
axial, longitudinally spaced annulclr eonduits 42 ancl 47
that are provided with inle-t nozzle means 4G and 45, res-
peetively. ~s shown, e~it port 48 of annular conduit 47
is retracted from exit port 43-of annular conduit 42 to
form a conieal reaetant introduction zone 24. Reaetants
from reactant supp:Ly means tnot shown) are introduced into
eonduits 42 and ~7 through nozzle means 40 and 45 respee-
tively. Ihe flow pa-th of the reaetants diseharged thro-lgh
exit ports 43 and 48 ean be perpenclicular to the exiting
gas from conduit 23, as shown. If desired, exit ports
43 and 48 also can be positioned away from the perpendicu- ;
lar, i.e., downwardly or upwardly, at an angle of from
l to 45 frorn the horizontal position shown so that the
20~
reactant gas flow is direeted at sueh angle into or in
eontaet with the stream of hot gas emanating from the
plasma heater. The reaetant gas ean be projeeted radially,
tangentially or at any suitable angle therebetween into
the downwardly flowing stream of heated plasma gas eman-
ating from outlet eonduit 23. The -top of reaetant mixer
means 30 eontains opening 31 whieh is eoaxially aligned
with outlet condui-t 23 of anode ll to provide an overall
direet straight-line path for the heated plasma gas from
plasma generator 1 through reaetant mixer means 30 into ~ :
reaetor 34. Preferably, the heated plasma gas is intro-
dueed into the eenter of reaetor 34 and spaeed from the
- 29 -
2~
~alls t~creof` ~o thereby assist in pos-i~ion:itlg the reac~
-~~tion zone away from ~he ~alls Or the retlctor.
- I`ypically, hydrogen ia used as the gas wl~ich is
heated by the aforementioned hea-ting means, e.g., ?lasma
heater l; however, other gases, e.g., the noble gases can
be used. Argon and helium are suitable plasma gases.
The use of hydrogen as the plasma gas is advantageous
s~nce it insures the establishment of a reducirlg atmos~
phere and provides a llalogen, e.g., chlorine, acceptor
thereby removing halogen released from the metal halide,
bolon halide and/or halocarbon compound reactants as hydro-
gen halide. Mixtur;es of hydrogen with other gases, such
as the noble gases, e.g., argon or helium, can also be
employed as the plasma gas. When a noble gas is used as
the plasma gas, the hydrogen required for the vapor phase
reaction is introduced into the reactor by mixing it with
the reactants, as a part of the boron source reactant
e.g., the boranes, and/or as a separate stream through
mixer means 3Q.
As the heated plasma gas stream moves past the zone
of reactant in-troduction 24, it mixes with the reactants
introduced through reactant mixer menas 30. The reactants
are introduced usually at below reaction temperatures. ~:
Because of the high heat content of the hot hydrogen
stream no special efforts to heat the reactants to temper- ;i
atures above which they are gaseous are required. The
resulting gaseous mixture is forwarded into the interior
of reactor 34 and reacted therein. Reactor 34 is typic~
ally externally water cooled (not shown). Typically, the
reactants and reaction mixture are in turbulent flow al~
though
- 30 -
,
larninar flow can be used. Ihe relaction mi~t~lle flowill~sr
LntO reactor 3~-} ~vl~ich :is a recirculclting-tyl~e reac-tor ns
opposed to a plug flow-type reac-tor, typically has an
apparent residence time therein of between about O.05
and about O.5 seconds, more usually between about O.l alld
0.2 secollds. The apparen-t residence time can be calu-
lated by ~iv~ding the reactor volume by the gas flow
through the reactor.
As shown in FIGURE l, finely-divided metal diboride
powder product, which is suspended in reaction product
gases as well as excess reactant gas, hereinafter collec-
tively referred to as product gases or other equivalent
terms, is removed from reactor 34 through conduit 36 and
introduced into cyclones 33 and 39, in order to separate
tl-e solide metal diboride powder from the product gases.
The submicron particles of metal diboride are formed
comple-tely in the reactor and since the reactor effluent
is cooled to below metal boride forming tempera-tures sub-
stantially immediately, substantially no metal borideformation or individual particle growth (other than by
physical aggregation) occurs ou-tside the reactor. Cyc-
lones 38 and 39 are normally colled, e.g., externally
water cooled to cool the powder product. For example -the
cyclones can be traced with tubing through the coolant,
e.g., water, is passed. As shown, the discharge from
conduit 36 is introduced tangentially in-to cyclone 38 and
from there into cyclone 39 by means of conduit 51. Titan-
ium diboride powder drops out into receivers 25 and 26respectively, while gaseous èffluent leaves cyclone 39
through conduit 52 and into solids separation chamber 28
in which there is disposed a bag filter 29, electrostatic
- 31 -
~9
,_
prccipitator or other convellir,-n-t means f~'r separating
suspended solids from a gas. Cyclones 38 and 39, and
receivers 25 and 26 are closed to the atmosphere to
prevent contamination of the product with oxygerl. 'I'hus,
the metal diboride powder that'is fol-med in t,he reacf;or
at metal diboride forming tempera-tures is removed immed-
iately from the reactor and forwarded to product collec--,
tors that are substant~ally below temperatures found in
the reactor. The powder product is typically cooled or
allowed to cool to room temperature. ~-lowever, if the
cooling capacity o~ the cyclones and receivers is not
suff:icicnt to provide a powder product at room tempera-
ture, the product in the receivers may be above room `~
temperature, i.e., from about 20 C. to 100 C., because
of the residual heat content of -the powder. Higher
temperatures in the receiver may be used intentionally,
as described hereinafter, to promote degasifica-tion of ;1!
the powder product. Separation chamber 28 as shown also
has an exit or exhaust 50 on the opposi-te side Or the bag
filter. As shown, the bag filter has engaged therewith ~ ;
a suitable shaking rneans 59 to clear the filter of metal
diboride powder. While only two cyclones and receivers
are shown; more than two can be used. Alternatively a ~ ,
singlè receiver and cyclone can be used.
Solid,-separation chamber 28 can also be a caustic
water scrubber, often containing packing of some sort e.g.
ba;;s. sadd;es. etcé fpr greater cpntact. The scrubber
separates the fine solids from the gas stream and neutra-
lizes acidic species therein before -the gas is discharged
to -the atmospheré or to a flue. To recover unreacted
reactants, hydrogen, hydrogen chloride,
etc. from the product ~ases substa~tial]y ~evoicl of' its
iolid burden, conven-tional separa-tion and recovery Means
for such materials can be ins-talled between exit conduit
52 and the flue. Further, if the heat removal from the
product recovery apparatus, i.e., the cyclones and re-
ceivers, is insufficient, the product transfer line 36
can be externally cooled. Moreover, a cold or cooler
compa-tible gas can be mixed with tlle exiting product
effluent to thereby cool it.
Referring now to FIGURE 2, there is shown a partial
assembly, in cross-section, similar to that of FIGURE 1,
except that three-slot reactant mixer means 32 instead of
two-slot reactant mixer means 30 is shown. In addition
to ann-llar conduits 42 and 47, there is shown a coaxial,
annular conduit 44 which is spaced longitudinally from
annular conduits 42 and~47. The exit port 49 of conduit
44 is retracted from that of conduit 47 to further extend
conical reactant introduction zone 24. Annular conduit
20 44 is connected to nozzle means 41 for introducing reac- ~
tant gas into said conduit. Nozzle means 41 is~ in turn, ~ ;
connected with reactant gas supply means tnot shown).
Reactant mixer means 30 and 32 can be constructed of any
suitable material, such as graphite, molybdenum, refrac-
tory or any other,material which will withstand the heat
and corrosive environment present in the reactant in-tro-
duction zone 24. The mixer means can be n~nternally cooled
thereby permitting the use of conventional metal fabri-
cation~
~ hen a source of carbon is introduced into the reactor
to prepare acarbon-containing metal diboride powder (pre-
sumably as simultaneously produced metal carbide), the
carbon source reactant
~v~
~an be introduced by ar-y convenient means. ]`hus, the car-
bon source reactant can be introduced into the reactor
mixed with one or both of the metal halide and boron
source reactants. Alterna-tively, the carbon source can
be introduccd as a separate reactant stream. Thus, appar~-
atus such as described and shown in FIGU~ 2, provides
individual conduits for each of the reactants wl-en the
aforesaid embodiment is used. The reactants can be intro-
duced into the reactor in any sequence; however, the metal
halide, e.g., titaniwn halide, reactant is introduced
preferably upstream of the boron source reactant. Prefer-
ably the carbon source reactant is introduced prior to
the meta]. halide and boron source reactants. Further,
one or more of the reactant gases can be introduced
through the same conduit in the reactant mixer means
(provided the reactants are at a temperature at which in-
ter-reaction does not occur) thereby leaving a conduit
for the use of a sheath gas. Still further, mixer means
with four, five or more slots are contemplated so that
each reactant and gas stream introduced through said
mixermeans can be introduced separately.
When it is desired to produce metal boride, e.g.,
titanium diboride, powder in the absence of co-formed
metal carbide, me-tal halide, e.g., titanium tetrachloride,
reactant can be introduced through the top slot of the
three-slot mixer means depicted in FIGURE 2, hydrogen, is
introduced through the middle slot thereby ac-ting as a
sheath gas between the meal halide reactant and the boron
source reactant, e.g., boron trichloride, which is intro-
duced through the bottom slot of the mixer. Alternatively
metal halide can be introduced through the top slot,
boron source reactant
- 3
z~
throllgh the middle slot and she~th g~s, e.g., hydrogell,
through tlle bottom 510t . I`he sheath gas serves to prevent
contact of tlle reactant gases wil:h exposed surfaces of the
mixer means 32, such as lip 75, and the reactor, e.g.,
the upper lip ~6 of reac-tor 34. ~hen rnetal boride~ e.g.,
titanium cliboride, is to be produced wi-th co-formed metal
carbicle, e.g., titanium carbide, the carbon source reac-
tant can be introduced through the top slot, the metal
halide reactant introduced through the middle slot and the
boron source reactant introduced through the bo-ttom slo-t.
Other reactant introduction sequence can, of course, be
used if desired.
The metal halide, carbon source and boron source
reactants are mixed commonly with a carrier gas to facili-
tate their introduction into reactant introduction zone
24. The carrier gas can be hydrogen, recycle hydrogen,
recycle solids-free product gas, or a chemically inert,
(i.e., inert with respect to the reactant with which it
is admixed) gas such as the noble gases, e.g., argon and
helium. Hydrogen is not used commonly wi-th the boron
source reactant, e.g., boron trichloride for the reason
. ., - ~.
that hydrogen has been observed to react with the boron
halide reactant within the reactant inlet condui-ts there-
by causing blockage thereof. The amount of carrier gas
used to ~acilitate the introdiction of the reactants can
vary; but, generally will range between 250 and 1200 mo]e
per~ent based on the reactant with which the carrier gas
is admixed. The carrier gas assists in cooling the mixer
means, in keeping reactant conduits free of condensibles
and has some effect in controlling the mixing of the
reactants in zone 24 with a consequent effect on the sur_ ¦
face area of the metal boride powder product.
- l
, ~
.~ ..
~7~
I`he mean particle si~e ( ancl thus sur:r`ace al ea) Or
the refractory metal boride par-ticles comprising the
powdery product prepared by the process:described herein
is a function of many variables within the process sys-
tem some Or which can be interrelated. From -thc evide!lce
at hand some gencral observations can be macle. Partic]e
size tends to increase with an increcase in the rate of
production. Particle size does not appear to change sig-
nificantly with changes in the hydrogen plasma gas flow.
Particle size tends to decrease with an increase in the
intensity of mixing resulting from the use of larger
amounts of carrier gas (or inert gas) introduced into the
reactor other -than by means of the plasma gas. Finally,
increasing the arnount of nuclei :~rom additives, such as
hydroaarbons tends to decrease the particle size.
In carrying out the pre~ration of refractory metal
diboride powder by the process and with the apparatus
described herein, and particularly with reference to
; 20 FIGURE 1 adapted with reactant mixer means 32 of FIGURE
2, a hydrogen-containing gas or noble gas, e.g., argon
is in-troduced into plasma generator rneans 1, -through con-
duit 17 from whence it is directed by means of annular
conduit 19, into space 21, between cathode 3 and anode 11.
The plasma gas can be introduced in a manner such that
- - the gas flows in a spiral or helical fashion through out
let conduit 23. Alternatively, the plasma gas can be
introduced radially into the space 21 between the cathode
and anode so that there is no helical flow pattern estab-
lished by the plasma gas and the heated plasma gas exits
the plasma heater in substantially linear low path.
An electric arc is established between the anode and
cathode and as the arc passes through the plasma gas,
the gas is heated to high
- 36 -
.~ 7~ . . . ~ ~15 ?r~ ~rr~ ~?r ?~?.
i:elnl~erat~lreS, ~lsually tell~peratlLres abov e reac-l:ioll zolle ~ 9
`~ temperat-lrcs. ~ hydrogcll--cont.~ g l~:L~stna g.~s C~lll h~ve
an entIlal~)y ol` bet~een 20,000 ancl 60,000 BrlJ per p~un~
of gas, more commonly between 30,000 and 40,000 BTU/
pound. The heated plasma gas is projected direc;~ly into
reactor 34, passed reactant introduction zone 24 formed
by the lower lip of anode 11 and the exit ports of reac-
tant inlet conduits 42,~7 and 4~.
Reactant gases, metal halide and boron source reac-
10 tant are introduced, in one embodiment, into nozzles 40
.
and 41, respectively, and thence into reactant introduc-
tion zone 24 and into the env:ironmerIt of the downwardly
flowing streaM of hot plasma gas. The reactant gases
can be introduced at a mass velocity sucl- that they are
aspirated by the movement of thc projc-cted plasma stream,
or they can be introduced into the plasma s-tream at a
mass velocity such -th~t the plasma stream is momentarily
constricted. }-Iydrogen can be introduced into nozzle 45
of reactant mixer 32 and thence into the reactant intro-
duction zone 24 thereby acting as a gas sheath between
the metal halide and boron source reactants.
The formation of refractory metal diboride powder by
the gas phase reaction of the corresponding metal halide
and boron source reactants in the presence of hydrogen
and in the substantial absence of oxygen (combined or
elemental) commences essentially immediately with the
mixing of the reactants in the reaction zone at metal bor-
.~ .
ide forming temperatures. Optimally, the gas phase reac-
tion is confined to a zone within reactor 34 away from
the hot surfaces of the reactant mixer means and the reac-
tor. This minimizes deposition of the metal boride
powder product on the wall surfaces, which, if not other-
wise removed, will continue to build-up until causing in-
terruption of the process. The powder that builds-up on
the walls of the reactor tends
- 37 -
~o be coarser thall the po-vdery product l~emovecl flom t~le ~ g
reactor soon arter it is formed. Co-minglil-lg the bui:!.d-
up powder on'the wall with the principal dibc)ride powder
product'contributes to the production of a norl-~lniform
- product. When the principal powder product becomes nol~-
unlform because of coarse powder from the reactor wall ;
the powder product should be classified to remove over-
si~ed particles before being used.
Finely-divided ret`ractory metal diboride powder sus-
pended in reactor effluent product gas is removed immedi-
10 ately from reactor 34 and introduced into cyclone 38.
A portion of the powder product is removed in cyclone 38
and recovered in receiver G5. Powder product retained in
the gas effluent from cyclone 38 is forwarded via conduit
51 to cyclone 39 wherein furtiler amounts of metal di.boride
powder products are removed and recovered in receiver 26.
Addi'tional'cyclones and receivers can be used if needed.
In most cases, the products from receivers 25 and 26 are
blended into a single product.
The reactor effluent product gas, now subs-tantially ~ '
:free of its solid metal diboride powder content, is for-
' .warded to gas separation chamber 28 where it is treated
to free it from any remaining suspended metal diboride
powder. As shown, the product gas passes through a bag
filter 29 and is removed from chamber 28 by means of
conduit 50. The product gas no~v removed of its metal di-
boride and/or other soli~ burderl can be treated further
to recover valuable by-products and remove noxious
components therefrom before being burned or discharged to
the atmosphere. If desired, the product gas can be
treated to recover hydrogen and/or hydrogen halide, e.g.,
hydrogen chloride, for
- 38 -
us~- in the presellt process or in sorne other pl'OCeSS or
the cooled product effluel-t stream can be recycled -to ~7
the reactor as a source or cooling or diluent gas.
'rhe meta] diboride powder product prep~lred in acco.r-
dance witl- the aforementioned process is a finely-divided
powder that can adsorb gases such unreacted reactants
that may be present in the receiver in which the procluct
is collected. To av~id contamination by adsorption~ re-
ceivers 25 and 2G are heated generally to temperatures
above about 200 F. (93 C.), e.g., from abou-t 200 ~-600 F.
(93 C -316 C.) to assist in degassing of the product
during collection of the product. Simultaneously, it is
advantageous to maintain a stream of hydrogen or an inert
noble gas, e.g., argon, percolating through the product
to fur-ther assist in the degasification step while the
product cools. If the product is not substantially free
of unreacted reactants such as the metal halide, e.g.,
titanium halide, and boron source, e.g., boron halide,
such compounds can react with moinsture or oxygen in the
`~
atmosphere to form oxides or hydroxides of the metal, e.g.,
titanium or boron, thereby introducing oxygen contamina-
tion into tlle product. Advan-tageously, -the produc-t is
handled without exposure to the atmosphere; however, in
some cases, some exposure to the atmosphere cannot be pre-
vented. In the event the metal diboride powder produc-t
contains adsorbed chlorine-containing species, e.g., the
subhalides of the metal halide reactant such as tibaniwn
trichloride and titanium dichloride, such species can be
removed by heating the product to between abut 400 and
lO00 C., e.g., 500-700 C. and preferably about 600 C. for
between about l and 4 hours. In performing such heating
step, the metal diboride powder is charged to a calciner
or similar furnace, preferably a rotating calciner, and
heated to the indicated temperatures~for
- 39 _
the indicatc~ tillle. A strearn of hyclrosen or inert ~as,
, ~ .
s~lch as argon, is mailltained over the heatecl procluct to ~1~7~9
he]p remove unclesirable adsorbed gases I`rom t:he procluc~
- and preven-t exposure of o~ygerl. After clegassing, the
boride prod-lct can be coated with 2 paraffin wax or other
similar binder material to minimize the rate of oxygen
piclc-up during storage and handling.
The boride po\vder eompositions having carbon-eont:ain-
ing additive described herein, particularly titanium and
b zirconium diboride compositions, when hot pressed or cold
pressed and sintered into solid shapes are especially
useful as current conducting elements, e.g., electrodes,
in electrolytic cells for the production of me-tals, e.g.,
al~iminum. The term "electrolytic cell" as used herein
with respeet -to al~minum produetion is in-tended to in-
elude both reduction cells and three-layer cells for tile
refining oi purification of aluminum. When used as a
eurrent eondueting element, titanium and zirconium dibor-
ide can eomprise at least par-t of the cathode of the
electrolytie eell or of the elements used for eonducting
electrolyzing eurrent to and/or from the eleetrolytie cell,
and ean be exposed to the molten metal either in the re-
duetion cell or in the purifieation eell.
Current condueting elements prepared with metal
diboride powders of the present invention ean be disposed
in a vertical or inelined position in the eleetrolytie
eell for the reason that mo]ten aluminum wets the surfaee
of such elements. l`hus, a ea-thode prepared for the titan-
30 ium diboride powder of the present invention can be arr-
anged in the electrolytie eell so that the operative faee
or faees of the eathode are disposed at a relatively large
angle, i.e., 60 or 90 degrees, to the horizontal, thereby -
allowing the deposited aluminum to eontinuously drain from
the faee or faees of the eathode and preferably to collect
_ ~0 --
.. . ~ ,
~~~~ in a pool in contac-t wi-tll a lower part ot the
;atllode from ~vhicll F)ool it rnay be withdrawill from
- time to time in the usual manner. Due to the inclined
.
or substantially ~ertical
, " ,
.
.
, .
~-
. ' ' ' " ' ' ' ' .
:~
- 40a -
m~ m~ ~~m~7~ e-~
.. . ~ . : .
- arrallgenlenl: of the cathode, the floor Sp;l.c occ:~lpiecl l~y
,he electrolytic cell is very considera~ly reducecl in
relation to that which is conventionally req-lired. Pcr- -
haps the largest advantage to the use of inclined or
subs-tantially ver-tically arranged electrodes ot the in-
stant metal diborides is -tha-t surg:ing of the molten al.~lm-
inum is less likely to occur so -that the spacing o.[ the
anode and cathode can be substantially reduced compared
with the adopted in aluminum reduction cells heretofore
knwon and the dissipation of e;ectrical energy in the
electrolyte correspondingly reduced. Moreover, current
conducting elements prepared from titanium diboride comp-
ositions have relatively high electrical conductivity,
i.e., a low electrical resistiv~ty, and therefore the
voltage drop due to the passage of the operating current
is less than that experienced in cells of orthodox con-
struction. The effect of sludge formation at the bottom
of the cell which causes an undesirable addi.tional vol-
tage drop at the cathode in existing horizontal cells
can also be avoided. Thus, the use of current-conducting
.elements prepared from metal diboride powder of the pres-
ent invention in aluminum reduction cells improves the
passage of electrolyzing current through the cell because
of the low electrical resistivity of the compositions,
and further, when such elements are us.ed in a substant-
ially vertical or inclined position, the voltage drop
across the electrolytic cell is significantly reduced
thereby p~ovidlng significant savings in power. Such
power savings have become increasingly more important due
to the continuing rising cost o~ power.
The use of titanium diboride current-conducting
elements in electrolytic cells for the production or rc-
fining of aluminum is described in the following U.S.
Patents, 2,915,442, 3,028,324, 3,215,615, 3,314,876,
- 41 -
:
_~ 3,330,756, 3,1S6,G39, 3,27~l093 and 3,~00,061. Despite
e rather e~tensive effort these paten-ts indicate was
mounted and tl-e potential advantages f`or using -titanium
diboride and titanium diboride compositions as curren-t-
conducting elements in electrolytic cells for the pro-
duction of alumin-lrn as described in -the aforementlonecl
patents, such compositions do not appear to have been
commercially aclopted on any significan scale by the alum-
inum industry. The leasons for such laclc of acceptance
are believed to be related to ~he lack of stability of
the current-conducting elements prepared from the titanium
diboride powders of the prior art durin$ service in
electrolytic reduction cel]s. It has been reported -that
such current-conducting elements prepared with composi-
tlons of the prior art fail after relatively shor-t periods
in service. Such failure has been a sociated in the past
with penetration of the current-conducting element struc-
ture by the electrolyte, e.g., c~yolite, thereby causing
critical weakening of the self-bonded structure with con-
sequent cracking and failure. Other reasons proposed
have been the solubility of the compositions in molten
aluminum, molten flux or electrolyte, or the lack of mech-
anical streng-th and resistance to thermal shock.
Ideally, a current-conducting element should have the
following characteristics:
1. Good electrical conductivity
2. It must not react w1th nor be soluble in either
molten aluminum or, under cathodic conditions, in
molten fluY or electrolyte, at least to any apprec-
iable extent at the operating temperature of the cell.
The
- ~2 -
7~
~~solubility of -~he material in mo:Lten al~nlin-lm is an
importan-t consideration as it determines both ~hc
usefu] life o~ tile currerlt_conduc-ting elemellt ancl
the degree of contamination of the aluminum procluced
through the agency oI` such current-conductirlg elemen~:.
3. Iletability by molten aluminum.
4. Capable of being produced and fabricated lnt:o required
shapes economically.
5. High stability and under the conditions existing at
10the cathode of` the cell, i.e., it should possess
good resistance to penetration by the molten elec-tro-
lyte (cryolite) and to craclcing.
6. LQW thermal conductivity.
7. Good mechanical streng-th and resistance to thermal
shock.
In order to have high stability under service condi- `,
tioning and resistance to penetration by the electrolyte,
the current-conducting element prepared of titanium dibor-
ide powder compositions must have a relatively high den-
sity. In the past, high densities have been achieved wi-th
meta] boride powder compositions of the prior ar-t by hot
pressing only. The metal boride powders of the present
invention can be cold-formed and sintered to high densi
ties. These metal boride powders provide the opportunity
for preparing current-conducting elements of simple and
complex shapes at a reasonable cost. Such current-conduc-
ting elements are resistant to the environment existing
in electrolytic cells
- ~3 -
~1'7~
~or the reductioll or purification of a]~tnil-lum and have
improved stability coml)red to prior ar-t boride composit-
ions in such electroly-tic cells.
The present invention is more par-ticular]y described
in the followlng examples which are in-tended as illustra~
tive only since numerous modifications and varia-tions
therein will be apparent to those slcilled in -the art. In
the followir~g examples, some volumes of gas are expres:sed
in cubic feet per hour a-t standard conditions ~4.7 pounds
10 per square inch (101.3 lcPa) pressure and 70 F. (21 C.~
or SCFH. Reactant and o-ther gas stream rates were meas-
ured at nominal laboratory conditions, i.e., 1 atmosphere
and 70 E. (21C), and are reported as measured if other
than SCFH. Unless otherwise spccified all percentages
are by weight.
The following examples illustrate the preparation
of refractory metal borides with and without added carbon
by vapor phase reaction of` the corresponding metal halide
20 and a boron source in the presence of a hot hydrogen ; ~;
stream and in the substantial absence of o~ygen, combinecl
or elemental.
EXAMPLE 1
Apparatus analogous to FIGURE 1 modified with the
reactant inlet assembly means of FIGURE 2 was used to pre-
pare finely-divided titanium diboride. The power to the
plasma heater was 22.5 kilowatts. Hydrogen in the amount
of 300 SCFH was used as the plasma gas. 0.71 grams per
minute of 1,1,2-trichloroethane together wi-th 45 SCFH
hydrogen as a carrier gas was introduced through the top
slot of the three-slot reactant inlet assembly means,
which was fabricated from graphite. Titanium te-trachlor-
ide in the amoun-t of 18.8 grams per
- 4~ -
z~
minute toget~ler witll 20 SCFIl hydrogc-n allCI 5 S~] 1I hydr'Og~'ell
~hloride was introduced through t~e midclle slot of thc
reactant inlet assembly means. Boron trichloride in the
amount of 21.7 grams per minu-te, together with ?2 SCFH
argon was introduced -through the bottom slot of the reac-
tant inlet assembly means. This run was continued fcr
989 minutes and produced titanium diboride ]-aving a B.E.T.
surface area of 24.0 square meters per gram. ;rhe product
was analyzed for carbon and found to have 0.5 percent
total carbon.
EXAMPEE II
The apparatus and general procedure of Example I
was. used except that titanium tetrachloride in the amount
of 72.2 grams per minute and 15 SCFH of hydrogen were
introduced into the reactor throu~h the top slot of the
reactan-t mixer assembly means. 1.26 grams per minute of
1,1,2-trichloroethane, 45 SCFH of hydrogen and 20 SCFH
of hydrogen chloride were introduced through the middle
slot and boron trichloride in an amount calculated to
represent a lO percent stoichiometric excess (basis the
titanium tetrachloride) and 8 SCFH of argon were intro-
duced through the bottom slot of the reactant mixer
assembly. The tibanium diboride powder product recovered
had a B.E.T. surface area of 11.5 square meters per gram
- and was found to contain about 31.6 percent boron, 0.08
percent chlorine, 0.19 percent oxygen and l percent total
carbon.
EXAM LE III
Run _
Apparatus similar to FIGU~E l was used to prepare
titanium diboride. The arc heater was a medium voltage,
medium amperage
- 45 -
', ~
ZB9
heater havirlg a power input of 28 kilowatts. 'I'he arc
heater was operated at between 24-28 kilowa'~ts. I-lydl~o-
gen in tlle amount of 300 SCFH ~vas in-troduced into the
arc heater as the plasma gas. Gaseous titanium tetra-
chloride in t:he amount of' 18.7 grams per minute together
with hydrogen as the carrier ~as in the amount of` 20 SCFH
was introduced through tlie top slot of the reactant in
let assembly means. Gaseous boron trichloride in the
amount of 26.9 grams per minute with an argon carrier
gas in the amount of 22 SCFH was introduced through the
bottom slot of the assembly means. The run continued
for 95-1/2 minutes and titanium diboride having a B.E.T.
surface area of about 14 square meters per gram was
obtained. Titanium diboride deposits on the bottom lip
of the reactant inlet assembly were observed at the
end of the run.
Run B
l'he procedure of Run A was repeated except that
boron trichloride was introduced through the top slot and
titanium tetrachloride through the bottom slot of the
reactant inlet assembly means. 25.6 grams per mirlute of
gaseous boron trichloride with 22 SCFII argon and lB.7
grams per minute of titanium tetrachloride together with
12SCFH of hydrogen chloride were utilized as ~he reactants.
The run was continued for 120 minutes to produce titanium
diboride, having a B.E.T. surface area of about 9.1
square meters per gram. A this skin of titanium diboride
powder deposits on the inlet assembly were observed a-t
the end of the run. Most of the deposit was found to be
attached to the bottom exposed portion of the inlet assebly
e.g., lip 46 of mixer means 30 in FIGURE 1, and the ex-
posed top lip of reactor 34.
- 46 -
-- Rur~ z~ ~
The procedllre of Run ~ was repeated, except that 12
SCFH of hyclrogen chloride was utilized as the carrier gas
for the titanium tetrachloride instead of the 20 SCFH of
hydrogen and 27.~ grams per m:inute of boron trichloride
was fed to the reactor. This run continued fr 150 min-
utes and the titanium diboride product was -found to
ahve a B.E.T. surfac~ area of about 5.8 square meters per
grams. No growth of titanium d~iboride deposits on -the
inlet assembly means was observed.
Run D '
The procedure of Run C was repeated, except thc~t the
titanium tetrachloride feed rate averaged about 21 grams
per minute and the boron trichloride feed rate averaged
about 29.8 grams per minute. This run continued for 975
minutes and the titanium diboride product had a B.E.T.
surface area of about 6.3 square me-ters per gram. No
growth of titanium diboride deposits on the reactant in-
let assembly means was observed at the end of the run.
In all of the above runs, the powder product obtainedwas calcined in the presence of hydrogen at 1000C. to
degasify the product. Some of the calcined produc-t
remained pyrophoric.
' Run E
,, , Run D was repeated except that 23 SCF~I of hydrogen
was added to the titanium tetrachloride reactant
introduced through -the top slot of the reactant inlet
assembly means. The titanium tetrachloride and hydrogen
chloride reactant addition rates averaged
- ~7 -
72~
~. 19 2 gran~s ~er millutc- and 2.5 SCII-I, respectively. Boro
trichloride in the amount of 27.0 grarlls per minu-te to-
gether with ?2 SCII~ argon was.introduced through the
bottom slot of the reactant inlet assembly means. This
run conti.nued foIl 1,072 minutes and produced titanium
diboride having a B.E.T. surface area of about 14.1 square
meters per gram.
. Example I-III show that submi.cron titaniwn diboride
.. can be produced by the vapor phase reaction of titanium
halide an~ a boron source compound with or without a car-
bon source reactant. The submicron titanium diboride
powder produced is composed of well formed, individual
crystals of titanium dibori.de. Typical scannillg and
transmission electron micrographs of such titanium dib0r-
. . ide is shown in FIGURE 3 and 4 which are described in more
detail hereinbefore.
EXAMPLE IV
Apparatus similar to FIGURE 1, which is described in
Example III, Run A, was used to prepare zirconiwm dibor-
ide. Hydrogen in the amount of 300 SCFH was introduced
into and heated by the medium voltage, medium amperage arc
. heater. Gaseous zirconiwn tetrachloride, at a rate of
20.5 grams/minute, and 100 SCFH argon were in-troduced
through the bottorn slot of the reactant. inlet assembly
into the hot hydrogen stream emanating from the arc heater.
Gaseous boron trichloride, at a rate of 4.93 liters/min-
. ute (a 25 percent stoichiometric excess based on zirconiwm
tetrachloride), and 22 SC~IF of argon were introduced
through the top slot of the reactant inlet assembly. The
process was continued for 42 minutes. The zirconium
diboride product recovered had a B.E.T. surface area of
7.7 square meters per gram.
48 -
. . " '
~72E~
~ .
-~ .
~i ` .
rx~ x.~ v
The procedure and -~ppara~us of Example IV is used to
prepare hafnium diboride and a finely-divided, submicroQ product
similar in si~e and surface area to the zirconium diboride of
Example IV is recovered.
.
E~A~PLE VI
Apparatus analogous to that used in Example I was used
to prepare titanium diboride. 300 SCFII of hydrogen was used as
the plasma ga~. Propane (89 standard cc/minute), and 4S SCFII
hyclrogen as a carrier gas were introduced into the reactor through
the top slot of the three-slot reactant inlet assembly rneans.
Titaniilm tetrachloride (52 grams/minute) together ~7ith 9 SCFH of
hydrogen and 24 SCFH of hydrogen chloride were introduced through
the middle slot, and boron trichloride (13,000 standard cc./minute)
and 22`SCFH of argon wer~ introduced through the bottom slot of the
inlet assembly. Titzniunl diboride powder product was recovered and
degassed under a hydrogen flow of 11 SCFII at 600C. for 4-3/4 hours.
The tltanium diboride powder product had an elemental analysis of
31.9 percent boron, 0.09 percent oxygen, 0.78 percent carbon and
0.088 percent chlorineJ and had a B.E.T. surface area of about
6,4 m2/gram
E _ LE VII
Apparatus analogous to that used in E~ample VI was used
to.prepare titanium diboride. 300 SCFII of hydrogen was used as the
plasma gas. Titanium tetrachloride in the amount of about 41.5
grams/minute, 9 SCFII of hydrogen and 24 SCFII of hydrogen chloride
~L~L172
r ~
weL-e introduce~ into the reactor throu~II the top slot of the
three-slot reactant i.nlet as~embly means. About 22 SCIH o
hydrogen was introduced through the middle slot; and, boron tri-
chloride in the amount of about 10,700 standard cc./minute ~about
a 10 percent stoichiometric excess) and about 22 SCFII of argon
were introduced through the bottom slot of the inlet assen~bly.
TItanium diboride powder was recoverecl and degassed under hydrogen
at 600C. for 3 hours. The titanium diboride powder product had
an elemental analysis of 32.3 percent boron, 0.44 percent oxygen
and 0.03 percènt chlorine, and had a B.E.T. surface area of 3.3 m /
gram.
E~'IPLE VIII
The procedure of Example VII was repeated and titanium
diboride powder having an elemental analysis of 32.3 percent boron,
0.60 percent oxygen and 0.10 percent chlorine was recovered. The
product had a B.E.T. surface area of 4.5 m /gram.
The following examples illustrate the utility of the
refractory metal borides.
EX~`IPLE IX
A portion of the titanium diboride powder of ~xample VI
was hot pressed at about 2100C. and 3500 pounds per square inch into
a plate 2 inchPs x 2 inches x 1/2 inch. The plate had a density of
97 percent of the theoretical density of TiB2 and a resistivity of
about 7 microohm centimeters. The plate was analyzed for oxygen,
which was found to be about 0.05 percent. Ihe plate was operated as
a catllode in an aluminum reduccion cell for 100 hours at 960C. a~ an
3~ anode current density of 6.5 amperes/inch2. At the end of the test
5~
period, the plate was removed, ~ractllred, ancl inspected. No
deterloration of the plate alld no penetratioll of electroly~e into
the plate was observcd. Fracture of the plate was observed to be
primarily transgranular.
A piece of the test plate was cut out aEter the test was
completed and polislled and etchcd. FIGURE 6 is a photomicrograph,
having a magnification factor of 2100, of a polished and etched
- section of the plate. The microstructure of FIGURE 6 shows a mosaic
of equidimerlsional TiB2 grains witll contiguous. grain boundaries and
a limited grain size range. The TiB2 grains range from about one to
fifteen microns in diameter; but, are predominantly in the four to
twelve micron range in size. Titanium carbide occurs as occlusions
les6 than one micron in si~e within the tltanium diboride grains.
EXA~IPLE X
A blend of the titanium diboride powders of Examples VII
and VIII in a weight ratio of about 58.5/41.5 was mixed witll about
5 weight percent of titanium carbide powder havino a B.E.T. surface
area of about 4.5 m /gram. The titanium carbide powder was prepared
in accordance with the procedures described in U. S. Patent 3,485,586.
The titanium diboride and titanium carbide powders were mixed with
1 percent paraffin wax in l,l,l-trichloroethane with a high speed
Cowles mixer. The blended mixture was vacuum dried and hot pressed
at about 2000~C. and 3500 pounds per square inch into a 2 inch x 2
inch x 1/2 inch plate. The plate wac allowed to cool overnight in
the mold under vacuum. The plate had a density of abou~ 93 percent
of the theoretical density of TiB2 and was found to have an oxygen
~i L
,. .
content o~ about 0.33 percent. The electrical resistivity o~ the
plate was 6 microoht!~ centimeters The plate was operatec' as a
cathode in an aluminum reduction cell under the same conditions
as recitecl in Example IX. At the end of the test period, the place
was removed, fractured and inspected. Some minor spalling and
erosion of the plate had taken place; but, no penetration of the
electrolyte into the plate was observed. Eracture of the plate was
observed to be primarily transgranular.
A piece of the test plate was cut out after the test was
completed and polishecl ancl etchecl. FIGU~E S is a photomicrograph,
having a magnification factor o 2100 of a polished and etched
section of the plate. The microstructure of FIGURE 5 is fine and
shows interlock.ing grains of ~hite, lath-shaped TiB2 with grey TiC
grains dispersed in the structure. The TiB2 grains range in size
from less than one micron to five ,nicrons. TiC grains are up to
three microns in diameter.
EX~IPI,F XI
,
A blend of 95 parts of titanium diboricde powder prepared
in a manner similar to Example III Rune E ancl 5 parts of titanium
carbide powder was mixed with about l percent paraffin wax in l,l,l-
trichloroethane and ball milled for about one hour. The titanium
diboride powder hacl a B.E.T. surface area of 4.9 m /gram and the
titanium carbide powder had a .B.E.T. surface area of about 5.0 m /gram.
The blended mixture was vacuum dried and isostatically pressed at
about 20,000 pounds per square inch into a cylindrical rod l-1/2 inch
in diameter x 16-3/8 inches long. A well 3/8 inch in diameter and
about 15 inches deep was drilled out of the rod and the resulting rod
~2
~. . . ; :
~.~17;2.~
.
was VZCUUID sintered at about 1900C. for abo~lL 1 hour. The rod
had a density of 95 percent of t.he theoretical density OL TiB2.
The sinterecl rod was tested as a thermocouple well in an aluminulll
reduction cell. The rod showed excellellt thermal shock resistallce
- and resistance to the bath.
E~YA~rLE XII
Rods similar to that of ~xample XI were preparecl using
til:anium dibori.cle powder having B.E.T. sur~ace areas of 6.6 m /gram
and 7.0 m /gram. The sintered rods had densities of 96 percent and
greater than 99 percent of the theoretical density of TiB2 respec-
tively. A piece of the rod prepared with the 7.0 m2~gram titanium ~ '~
diboride was polished and etched. FIGURE 7 is a photomicrograph,
having a magnification factor of 2100, of a polished and etched
section of the rod. The microstructllre of FIGURE 7 shows a mosaic
of relatively equidimensional TiB2 grains wi.th the light-grey TiC
predominately localized ln interstices between TiB2 grains or
occurring as occlusions within the TiB2 grains. Electron microprobe
analysis has indlcated that a gold color induced in the TiC signifies
scavenging of oxygen and nitrogen to produce a solid solution phase
represented by Ti (C,O,N).
EXAMPLE XIII
Titanium diboride powder prepared in a manner similar to
Example VI and having a B.E.T. surface area of 24 m /gram and 0.46
percent carbon was isostatically pressed ~t 20,000 pounds per square
inch into a cylindrical rod. The rod was vacuum sintered at about
2000C. for about 30 minutes. The slntered rod, wllich had dimensions
~3
:
of about 1 inch x 5 inclles, had a clensity of about 98 percent of
the theoretical density of TiB2 and a resistivity of about 9 microollm
centimeters.
A piece of the rod was polished and etched. FIGURE 8
is a photomicrograph, having a magnification factor of 2100, of
,
a polished and etched section of the rod. The microstructure of
FIGU~E 8 shows a mosaic of equidimensional TiB2 grains with
contiguous grain boundaries and a limited grain si~e range. The
TiB2 grains are predon~inantly three to ten nnicrons in diameter. The
Ti (C,O,N~ phase occurs as occlusions iess than one micron in si~e
within the TiB2 grains.
Although the present process has been described with
reference to specific details of certain embodiments thereof, it is
not intended that such details should be regarded as limitations upon
the scope of the invention except as and to the extent that they are
included in the accompanying claims.
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