Note: Descriptions are shown in the official language in which they were submitted.
Description _f t~le Invention
The literature descri~es a variety of methods fox preparing hard
refractory metal borides such ~s titanium diboride. For example, elemental
titanium and boron can be fused together at about 3630F. This method
(synthesis by fusion) produces products that are relatively impure and requires
isolation of the boride product by chemical treatment. Other sintering pro-
cesses involve the reaction of elemental titanium with boron carbide (U.S.
Patent 2,613,154), the xeaction of titanium hydride with elemental boron
(U.S. Patent 2,735~155), and the reaction of ferrotitanium and ferroboron
alloys in a molten metal matrix, e.g., iron ~U.S. Patent 3,096,149). ~ fused
salt bath containing an alkali metal or alkaline earth metal reducing agent
and t.itanium- and boron-containing reactants has been used to produce titanium
diboride (U.S. Patent 3,520,656). U.S. Patent 3,715,271 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, ~irconium, and hafnium
by the vapor phase reaction of the corresponding metal halide, e.g., titanium
tetrachloride, and a boron halide, e.g., boron trichloride or boron tribromide,
in the presence of hydrogen at temperatures of from 1000-1330 C., 1700-2500 C.,
and 1900-2700 C., respectively, has been reported in Refractory Hard Metals,
by Schwarzkop 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
preparing titanium diboride has been conducted at temperatures less than 1200 C.
~; using sodium vapor in lieu of hydrogen (U.S. Patent 3,244,482).
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A widely reported con~rcial process used for preparing refractory
metal borides, e.g., titani~l diboride, is the carbothermic process. In this
process, refractory metal oxide, e.g., titanium dioxide, an oxide of boron,
e.g., B2O3, and carbon are heated in an electric arc or high frequency carbon
furnace. As an alternative to the electric arc furnace, it has been proposed
to prepare titanium diboride by injecting powdered activated charcoal impreg-
nated with boron oxide and titania (anatase) into an argon plasma (~ritish
Patent Specification 1,273,523). This process produces a~out one gram of
product in ten minutes and is not, thereore, considered 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-divided
product, extensive milling is required. For example, U.S. Pa~ent 3,052,538
describes the necessity for milling intermetallic compounds such as titanium
diboride and titanium carbide to obtain a fine particle size useful for dis-
persion strengthening of titanium. A milling time of 300 hours (12-1/2 days)
in a porcelain mill using hardened steel balls as the grinding medium is
recited as being required.
The reported average size of the product produced from such lengthy
milling ranges from about 2 to about 10 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 oP 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 mi}ling
produces particles having fractured irregular surfaces and a relatively large
amount of fines.
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8~57
It has now boen discovered t:hat submicron refractory metal boride
powder, such as titanium diboride/ zirconium diboride and hafnium diboride
powders, that contain minor amounts of a carbon-containing additive can be
produced by reacting in the vapor phase, the corresponding metal halide, e.g.,
titanium halid~, boron source, e.g., boron hydride or boron halide, and carbon
source, e.g., readily volatile hydrocarbons or halogenated 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. Praferably, hydrogen is heated in a plasma heater to
form a highly heated hydrogen gas stream, which is in~roduced into the reactor
and into the reaction zone. The metal halide, boron source, and carbon
source reactants are introduced into the reactor and preerably 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, quenched, usually by indirect
heat exchange means, and recovered in conventional fine particle collection
equipment, e.g., cyclones, electrostatic precipitators, dust co1lectors, etc.
When ~he 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 havs a nominal sectional diameter of less than one micron. The
preponderant number, i.e., greater than 50 percent, of the par~icles less than ;
one micron are in the particle siza range of be~ween 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.
~ y the aforementioned process, an intimate mixture of refractory
metal boride powder containing carbon, either as free carbon and/or chemically
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combined carbon, probably ag submicror- refractory metal carbide, is produced.
The resulting powder composition, i.e., submicron refractory metal boride
powder having dispersed therein a carbon-containing additive, either as free
carbon, refractory metal carbide or both, can be cold pressed and sintered,
or hot pressed to dense articles, e.g., articles having 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 5 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 titanium diboride powder as described, but in the absence of the
carbon source reactant, and blending with the resul~ing powdered product the
desired added amount of submicron carbon andJor refractory metaI carbide. For
example, submicron carbon or titanium carbide can be blended with submicron
titanium diboride to produce powder compositions that have 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-
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c~ntainin~ additive ic: mor~ homogeneollsly disE~ersed throughout the m~tal boride
po~der. Consequently, less carborl-cont~ining aclditiva is requlred to produce
the same results as when physical blends are used.
Brief Description of the Drawings
The process described herein for preparing submicron refractory
metal boride powder, sukmicron, carbon-containing refractory metal boride
powder and articles prepared from such powder can be better understood by
reference to the accompanying drawings and photomicrographs wherein:
FIGVRE 1 is a diagram of an assemblage, partially broken away in
section, comprising are plasma gas heating means, two slot reactant mixer
means for introducing reactants to the hot gas stream emanating from the
plasma heater, reactor means, and auxiliary produet reeovery equipment means
(cyelones and bag $ilter) for recovering the metal boride powder product
suspended in the reactor gaseous effluent;
FIGURE 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 of the two slot
reactant mixer means illustrated in FIGURE 1. ~
FIGURE 3 is a scanning electron mierograph having a magnification `
faetor of 25,000 of a sample of titanium diboride powder having a B.E.~.
surfaee area of 11.5 square meters per gram that was prepared in a manner
similar to that deseribed 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 etehed see~ion of the hot pressed plate prepared in
Example X;
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FIGURE 6 is a photomicroyra~ , ha~ing ~ magnification factor of 2100,
of a polished etched sectioll o~ the hot pressed plate of ~x~mple lX;
FI~URE 7 is a photomicrograph, 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 meters per gram titanium diboride; and
FIGURE 8 is a photomicrograph, having a magnification factor of 2100,
of a polished etched section of the isostatically pressed and sintered rod of
Example XIII.
The present invention rala~es to submicron refractory metal boxide
powders containing minor amounts o submicron carbon-containing additi~e and
particularly relates to submicron titanium diboride powder compositions that
contain minor amounts of added carbon and consolidated dense artlcles 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 therPin both as elemental
carbon and chemically combined carbon, e.g., as a metal carbide.
2; 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 weight percent, more preferably, about 1 weight percent, total
_ 7 _
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~8~57
carbon based on titanium diboride are especially useful in aluminum reduction
or aluminum refining cell~. For other uses, refractory metal boride powders
containing higher amounts of total carbon, e.g., up to 10 weight perce~t or
more, are contemplated. Thus, powder compositions (and articles prepared
therefrom) containing from 0.1 to 10 weight percent total carbon are contemplated
herein.
The carbon-containing additive can be introduced into the boride
powder in any convenient manner, however, it is preferred that the carbon be
introduced into the ~owder in the reactor when the metal boride powder is being
formed. Various advantages accrue when the carbon is introduced into the
boride powder at that time. First, a more homogeneous distribution of carbon
in the boride powder product results than can be achieved by phy~icà}ly
blending. A homogeneous distribution of carbon throughout the boride powder
hinders grain growth during sintering and helps provide a fine grain structure.
A fine ~rain structure generally has greater strength than a coarse g~ained
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
prepared in a reaction system that does not have such nuclei. Finally, it has
been observed that less reactor adaed carbon is required to obtain the same
degree 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
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 h~ving an essentially equiaxed
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grain structure while titanium diboride con~aining physically blended carbon
provides a sintered article having l~s9 pronounced equiaxed grains and more
elongated grains.
~etal 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 metal carbide powder, e.g., titanium
carbide powder, and/or finely-divided carbon with submicron metal boride,
e.g., titanium diboride powder 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 subjectlng 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 hydrogen plasma. In Example I of U.S. 3,340,020,
a mixture of tantalum pentachloxide and methane was introduced into the flame
of a hydrogen plasma and tantalum carbide having 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 wil}
have a number median particle size of between about 0.1 snd 0.9 microns,
although submicron metal earbides having a number average particle size of
between 0.01 to 0.9 microns can~be used. Usually, the added submicron
refractory metal carbide w111 have substantially the same surface area, i.e.,
the same number average or numbex median particle size, as the refractory metal
boride. Submicron carbon is commercially available and such materials can be
used directlyi a commercial 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
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57
ll-l9 m /gram can be used.
The carbon-containing additive can be present as elemental submicron
(fin~ly-divided) carbon; however, it i9 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 4b, 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 m~tal 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 composPd 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
producad. 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 eLther by
planar crystal faces or smooth rounded surfaces. The tabular crystal forms
consist dominantly of hexagonal prisms terminated by the ba~al pinacord. The
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tabular crystals are flattened perpendicular to the c - crystallo~raphic axis
as a result of greater development oE the pinacoidal faces relative to the
prism faces. Consequently, the crystal habit of the product can be described
as tabular to equidimensional hexagonal. ~ased on visual observations of the
powdery product through an electron microscope, the tabular hexagonal crystals
exhibit a nominal sectional diameter to thickness ratio within the range of
1.5:1 to 10:1.
Submicron metal boride powders, e.g., titanium diboride, that can be
prepared utili~ing the process described in more detail hereinafter are sub-
stantially free of undesirable metal contaminants, iOe., the powders areessentially pure, as established by emission spectrographic analysis. Since
carbon is added to or conformed with the metal boride powder, carbon is not
considered an impurity.
Metal impurities (as elemental metal) normally represent less than
4,000 parts per million parts of the boride powder (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: aluminum, baxium, calcium, chromium, copper, iron, potassium,
lithium, magnesium, manganese, sodium, nickel, silicon, vanadiu~ and tungsten.
The source of such metal impurities, if present, in the boride powder 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. sy virtue of the described proc~ss, 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. sy careful recovery, e.g., degasifi-
cation, and handling techniques to avoid exposure of the boride powder product
to the atmosphere ~oxygen) or moisture, boride powders with less than 0.15,
-- 1 1 --
often 1~es~ than 0.10, weight percent halogen~ and less than 0.~0, e.g., less
than 0.15 weight percent oxygen can be obtained. The aforem~ntioned values for
halogen and oxygen are based upon-analysis for such impurities obtained by use
of X-ray spectrographic analysis and by the use of a Leco oxygen ~nalyzer
(model 534-300) respectively. The aforementioned X-ray spectrographic
technique analyzes principally for unrPacted metal halides and suhhalides
present in the boride powder. Adsorbed hydrogen halide, e.g., hydrogen
chloride, on the boride powder may not be de~ected by that technique.
Thus, despite the use of substantially pure reactants and careful
handling and recovery techniques, a small amount of metal impurities, halogen
and oxygen can be present in the boride product. When not added intentionally,
carbon can also be found in the boride powder product; however, the carbon
level is typically less than 0.1 weight 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 siæe. The
surface area of the boride powder product can vary between about 3 and about
35 square meters per gram, (m /gram), more typically between about 4 and
about 15 m /gram, e.g., between 5 and 10 m jgram, as measured by the method
of Brunauer, Emmett, and Teller, J. ~m. Chem. Soc., 60, 309 (1938). This
method, which ~s 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 s.E.T. surface areas
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reported h~rein were obtained using nitrogen as the yas adsorbed and liquid
nitrogen temperatures (-196 C) and a pressure of 150 n~ of mercury (0.2
relative pressure).
The surface area of ~he boride powder is, of course, a function of
the particle size of the boride particles produced, i.e., the smaller the
particle size, the higher the surface area. The average spherical particle
size diameter, in microns, of the refractory metal boride, e.g., titanium
diboride, powder particles produced can be estimated roughly by the
expression:
Average Spherical Particle Si~e Diameter = 1.33/Surface Area ~m /gram)
which assumes that each particle is a sphere ~regu~ar shaped polygon).
Substantially all, i.e., at least 90 percent (by numberl 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
sectional 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 magnification. The preponderant
number, i.e., greater than 50 percent, of the particles less than one micron
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 microns can be present in the powdery
product; but, particles greater than 2 microns rarely represent more than one
percent by number 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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It is es~imated from a study of the refractory metal boride powders of the
present invention with a Zeiss* TGZ-3 Particle Size Analyzer that at least
60 percent on a number basis, more usually at least 70 percent, e.g., 98 per-
cent, of the boride particles comprising the powder are 0.7 microns or less.
It is not uncommon to find that the aforesaid percentages represent also the
particles within the particle si~e 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 particle size distribution of the powder. Generally, the particle
size distribution is relatively narrow. ~he nu~ber median par~iGle size 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
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 prepared 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 30 microohm centimeters and
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typically from 15 to 25 microohm centimeters. In contrast, hot pressed or coldpressed and sintered titanium diboride forms prepared from titanium diboride
powder compositions produced in accordance with the present invention are
typically less than 10 microohm centimeters, e.g~, usually from 5 to~9 microohm
centimeters. The electrical resistivity of zirconium diboride and hafnium
diboride is also typically less than 10 microohm cen~imcters at room tempera-
ture, e.g., 25C.
Electrical resistivity can be measured in the conventional manner.
~riefly, such measurement is obtained by applying direct current from two
electxode~ across the specimen to be measured, e.g., 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 1/2
inch side between two copper electrodes and a direct current applied across
the plate. A distance of 4 centimeters along the line of cuxrent 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 centimeter length and the potential difference measured. Generally,
electrical resistivity is taken at 25 C. and the values reported in the
examples herein were measured at that temperature. The electrical resistivity
value is calculated from the following e~pression:
Resistivity (ohm cm.) = (Potential Difference, volts)(Cross Sectional Area, cm .)
(Applied Amperage, 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
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8;~57
the powdPrs to a continuously applied E~ressure of from about 0.5 to S0 tQns
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. The com-
pacting, heating and subsequent cooling operations are typically carried out
in an inert atmosphere, e.g., argon or in a vacuum. The operation i9 often
carried out in a graphite die having a cavity of the appropriate desired
cross-sectional shape. The pressure is pre~erably 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
form shapes other than ~lat plates and other relatively simple shapes. More-
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
between 1800 C. and 2500 C. either in a vacuum or in a neutral ~inert)
atmosphere. For simple shapes such as cylinders, plates, or the like, the
powders can be dry prsssed in matched metal dies. For complicated shapes,
slip casting, tape casting, pressure casting, compression casting, extrusion
or injection molding can be used to cold fonm the article. Further, a wax
binder can be incorporated into the powder by spray drying techni~ues 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 paraffin 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
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l~sss than sintering t~mperatures. The pressure applied is in the range of
0.5 to 50 tons per square inch., e.g., 2-10 tons per square inch. Alternatively,the powder compcsition-bin~er mixture can be extruded into the desired shape.
Sintering is accomplished by heating the consolidated shape in vacuum or inert
atmosphere at temperatures of from 1800C. to 2500C. 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 m~terials. The term "cold formad" as used herein means that
the metal boride powder composition is compacted and shaped, as by pressing or ~ -
moldinq, prior to the sintering operations, as distinguished from hot formed
or hot pressed bodies which are shaped and pressed by the application of
pressure during sintering.
- 17 -
ii7
tt has been f'o-~nd that c~:lrbon-c:ontainlng refr~ct~ry
metRl boridc, e.g., -t:itanLu~ diboride, powder compositions
of` the presellt invention can be cold pressed and sintered
to high densi-ties, i.e., at least 90 percent of the theor-
etical densi-ty of the refractory me-tal boride. Depending
upon the particular powder composition, densities in
excess of 93 percent of theore-tical, e.g., in excess of
95 percent and often in excess of 98 percent of theore-
tical, can be achieved. Stated another way, cold pressed
and sintered elemen-ts fabricated from titanium diboride
powder compositions having a;porosity level of not more
than 10 percent now can be obtained. The aforesaid re-
fractoryi!metal boride powders and powder compositions also
can be hot pressed to densities at least equal to that
obtained by cold pressing and sintering and more usually
to densities approaching the theoretical density. Since
the -techni~ue of hot pressing limits to a great extent
the shape and size of fabricated shapes, the availability
of cold pressing and sintering as a consolidation tech-
nique provides engineering design opportunities which
were no-t possible earlier.
The apparent g*ain size, i.e., average diameter, of
the refractory metal bor1de grain as measured on an etched
metallographically polished surface of a sintered refrac-
tory metal boride specimen is predominantly fine. As
measuredlon photomicrographs of the ~po~ished surface,
the grain size of the boride grains is~generally~less~than 20
microns, and predominantly in the range of about 1 to 10
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 -
`~ :
8257
I-lot pressed or- cold pressed and s:intered article~
having densi-ties of greater -th~n ~0 percent o~ theoretical
of the re~ractory me-tal boride density, e.g., at least 92
or 93 percen-t of theoretical, are generally con~idered
impermeable. Thus, when such articles are used in, for
example, aluminum reduction or refining electrolytic cells,
they are substantially impermeable to molten material to
which they are exposed in such cells. The refractory -~
metal boride powder compositions of the present invention
can be fabricated into articles having such densities and,
accordingly, such articles are useful as current conduc-ting
elem~nts in the aformentioned t~p~ electrolytic cells.
Refractory metal boride compositions comprising ~ix
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 mlxtures
of boride powders can be prepared by blending the pre-
formed boride powders in the relative 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, mix-tures
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.
~'
- 19 ~
:~:
1~482~ii7
Cener~lly, c~rly volatile inorgarlic -tltanium zirconiwn
or haf~ m hcllicle, e.~., a compoullcl of only -tl~e a~oremen-
tioned metal and halogen Ichlorlne, brom:ine, fluorine and
iodine), can be used as the source o~ the aforementioned
metal in the refrac-tory metal boride powder product pre-
pared by the process described herein. As used herein
-the terms "metal halide" and "metal boride" or "metal
diboride" are intended to means and include the halides
and borides respectively of titanium, zirconium and hafn-
ium9 i.e., -the elements of Group ~b of the aforesaid
Periodic Table of the Elements. However, for the sake of
convenience and brevity, reference will be made sometimes
to only one of the aformentioned metal halides or borides
Exemplary of the refractory metal halides that can
be em~`oyffd in the present process include: titanium
tetrachloride, titanium tetrabromide, 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 subhalldes 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,
~bhalides other than the subchlor-ides and subfluorides
can be used in the same manner. Fu~ther, inorganic metal
halide corresponding -to the refractory metal carbide~s)
desired coproduced wi-th the metal boride, if different
than the metal boride powder being produced, can be used.
For example, the halides of hafnium, tantalum, si~icon
and other refractory m~-tals, the carbides of which are
desired can be used. Mixtures of metal halides of the
same metal such as
- 20 -
4~3Z57
-the chlorides and the bromides, e.g., -titanium tetrachlo-
ride and titanium te-trabromide can be employed as the
metal halide reactant. Further, mixtures of halides of
different metals can be used when it is desired to co-
produce more than one metal boride powder, e.g., titan-
ium diboride and zirconium diboride, or more -than one
metal carbide. Preferably, the hallogen portion of the
metal halide reactant (s) is the same to avoid separation
and recovery of different hydrogen halides from the pro-
duct stream. The metal halide reactant(s) can be intro-
duced into the reactant inlet assembly (mixer means) used
to introduce the reactants into -the reactor 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 reactant are the tetrachlorides, e.g., titan-
ium tetrachloride. Ihe 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. By oxygen--free is ~leant
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 precautions of reactant
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 -
~41!3Z57 ~:
mentioned inorganie boron compounds such ~s boron
tribromide, boron -triiodide, boron trichloride,
boron trifluoride and ~ .
'
,
' ~;
..~,
:
.
- 21a -
:: :
'~
~0~8ZS~ ;
the hydroboricles (bor~ e~), e.g., B2~-l6, B5~19, Blo~
ancl B6~i2 bororl trichloride :i9 preferred. As in the
case of the metal halide reactant, -the boron source reac--
-tant is introduced in-to -the reactor in such a manner that
is is present in -the reac-tant mixing zone and reac-tion
zone as a vapor. Ihe metal halide source and boron source
should be chosen -~rom -those compounds which, in combin-
a-tion provide a thermodynamically favorable reaction at
-the desired reaction temperature.
the reaction of titanium tetrachloride with boron triflu-
oride is thermodynamically less favorable at 2000 K. than
at 2500K. Thus, such thermodynamically less favorable
reactions will require higher reaction temperatures.
The amount of boron source reactant introduced into
the reaction zone in the reactor will preferably in at ~;
least stoichiometric quantit~es, i.e., in amounts suffic-~
ient to provide at least two atoms of boron for each atom ~-
of metal, e.g., titanium introduced into the reaction
zone in the reactor as metal halide, e.g., titanium 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
reac-tion zone between abo~t 1.8 and about 3 atoms of boron
per atom of metalj e.g., titanium. Preferably, greater
than the stoichiometric ratio is used. For example, the
mole ratio o~ reactants boron trihalide to titanium tetra-
halide (BX3/TiX4), wherein X is halogen, can vary from
about 1.8:1 to 3:1 and preferably is about 2. When a
stoichiometric excess of the boron source is used, less
~ 22 -
~4~S7
residu~l unreacted metal hlide reactant is found
in the product. When a stoichiometric excess o~
metal halide is used,
- 22a -
~4~57
sub-llalides o~` the m~tcll are E`ouncl in the product. While
it is pre~erred th~lt the bc>ron source reclctan-t be used in
stoichiornetric ex~ess ei-ther of` the me-tal halide or boron
source reacta~ts can be ~sed in stoichiome-tric excess in
amounts of from 5 -to 30 percen-t by weight.
The earbon source reactan-t should also be of the
type tha-t is readily volatile in the reaction zone and is
eapable of ~eaeting in a thermodynamieally -favorable man-
ner at the temperatures at which;~he r~ction is conducted.In the
aforesaid embodiment, volatile hydroearbons, halogenated
hydroearbons or mixtures thereof that ~re subs-tantially `'~`;
pure and oxygen-free, as defined above, can be used as
the carbon source. As used herein, the term "halogena-ted
~ydrocarbons", e.g., "ehlorina-ted hydrocarbon", is in-
tended to means and include both eompounds of earbon, hal-
ogen and hydrogen and eompounds only of earbon and hal-
ogen, e.g., carbon tetrachloride.
Typieal hydroearbons that ean be used as the earbon
souree include the normally gaseous or liquid but rela~
tively volatile hydroearbons ineluding saturated and un-
saturated Cl - C12 hydroearbons, sueh as methane, ethane
propane, the butanes, the pentanes, deeanes, dodecanes,
ethylene, propylene, the butylenes and amylenes, symmetr~
ieal dimethylethylene and like alkenes, eycloaliphatic
and aromatie hydroearbons, sueh as cyelopentane, eyelohex-
,
ane toluene, benzene, ete., and aeetylenie eompounds o:fwhieh may be noted aeetylene, methyl aeetylene, ethyl
aeetylene, and dimethyl aeetylene. Methane or propane
are eeonomieally preferred for this purpose. Rarely are
hydroearons of more than twelve earbons used~.l
- 23 - ;~
'~ .
~ ~ :
1~48Z5~ :
E~clmples of halogenclted hydrocarbons that can be used
as t:he source of carbon in -the process described herein
include saturated and unsaturated compounds containing
from one to twelve, more usually one to eight, carbon
atoms, such as methyl chloride, e-thyl chloride, chloro-
form, methylene chloride, carbon -tetrachloride, dichloro-
difluoromethane, amyl chloride, chloroethane, vinyl chlor~
ide, l,l-dichloroethylene, 1,2-dichloroethylene, 1,1-dich-
loroethane, 1,2-dichloroethane, ethylene dibromide,
trichloroethylene, perchloroethylene, propylene dichloride,
1,1,2-trichloroethane, l,l,l-trichloroethane,ll t 1,1,2
and 1,1,2,2-tetrachloroethane, hexachloroethane, and like
aliphatic chlorides, fluorides, bromides or iodides con-
taining up to about twel~e carbon atoms, mos-t preferably
up to about six carbon ~oms. Aroma-tic halocarbon compounds
e.g., chlorocarbon compounds, also can be used. Such
compounds 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 cyclohexane9 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-
composition 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 hydrocarbon9 used will of course de- `~
pend on the amount of
-2
~ :
~ ---
~14~3~S7
carbol~ clesired in the firlal boricle powder prod-lc-t. 'ihe
amount oE` total carbon in -the me-tal diboride powder, e.g.,
titanium diboride powder, or diboride powder composition
can range ~rom above 0.1 to abou-t 5 weight percen-t, pre-
ferably from above 0.1 e.g., 0.15, to abou-t 1 or 2 weigh-t
percent. Ihe use of about 1 weigh-t percent total carbon
has been observed to be very useful. When a carbon source
reactant is introduced into the reactor, i-t is ex~ected
that carbide(s) Qf 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 perce~t total carbon, -the X-ray patte~ characteri-
stic of metal carbides, such as titanium carbide, in the
diboride powder is not fairly evident. By "total carbon"
is meant the total amount of both free carbon and chemi~
cally combined carbon, 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.~ and about 5 weight percent corresponds t-o a titanium
carbide content of between above~ 0.5 and about 25 weight
percent. On the same~basis,i a total carbon conten~-o~
0.15 to 2 e,g, 1 weight percent ~ -
corresponds to a titanium carbide content of between abou-t
0.75 and 10, e.g., 5 weight percent. From the evidence
at hand, it -s believed that when a carbbn 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 me-tal halide and boron
source reactants with or without a volatile carbon source
is conducted in the presence of hydrogen. The amount of
hydrog~nutlized in the above-desc*ibed process is at least
that amount which is required stoichiometrically to sat-
isfy the theoretical demand of the reaction. Preferably, ~-
- 25 -
~0~8;~57
the amo~tnt o~ hydrogen used is :in excess of the theo-
retical amount. l~hen, for exclmple, the metal ha:Lide reac-
tant ~Ised :is titanium te-trachloride and the b~ron source
reactan-t used is boron trichloride, tlle theore-tlcal amoult
or demand of hydrogen required can be expressed by the
equation:
I ~iC14 + 2~C13 + 5H2 ~ >liB2
Often the amount of hydrogen utilized will be in excess
of ten times and as high as 100 times the amount of hydro-
gen shown to be required by the above equation or required
to equal the chemical equivalents of halogen of the metal~
halide and/or boron halide, and halogenated hydrocarbQn
~if used) reactants. When the boride source is a hydro-
boride, the hydrogen available ~rom the hydroboride can
be used to satisfy all or a part of the hydrogen demand.
Typlcally, the mole ratio of hydrogen to metal halide
reactant ranges between about 20 and 40, 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 wh1ch subm1cron metal boride powder is pro-
duced with the selected reactants under thermodynamically
favorable conditions, i.e., metal boride powder forming
temperatures. The average reaction zone temperature for
the aforemen-tioned vapor phase production of metal boride
powder such as titanium diboride powder typically is above
1000C. and usually ranges upwardly of 1000C. to about
3500 C. The process
- 26 -
1048;~57
call he conduc-ted at sut-atmospheric, atmospher1c, and super~ ~-
a-tmospheric press~lres. Iypically, the process is conductcd
at between abdu-t 1 and a~ou-t 3 atmospheres, normal]y be
tween 1 and 1.5 atmospheres pressure.
~ `he process and handling equipment utlizecl in the
aforementioned process for producing metal diboride powder
(as more speci~ically described thereinaf-ter) are constru-
cted from materials resistant to the temperatures and
corrosive environment to which they are exposed during ~-
the various steps of the procedure, as outlined herein-
a~ter. The present invention will be more fully unders~od
by reference to the accompanying drawings. Referring now
to FIGURE 1, there is shown apparatus comprising plasma
generator heating means 1 mounted atop reactant inlet
assembly (mixer) means 30 which, in turn, is mounted atop
reactor 34. Although the a~oresaid apparatus is shown in
vertical alagnment, 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) heater, can also be used. Further, other
heating means such 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 su~icient to establish
and maintain metal boride forming temperatures in the
reaction 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 -
-, . : . , .; . - . : , . . . .
~(~4~ZS~
I`hus, -the princip~l source o~ heat f`or the reaction is
generally the hlghly hydrogen gas stream. Plasma heat 1
consists essen-tially of an annu:lar anode 11 which is
aligned coaxially with cathode rod 3. Both anode and
cathode are mounted in a cylindrical sleeve 9 which is
electrically non-conductive. In the embodiment illus-
trated, the cathode rod tapers conically at its end essen-
tially to a point. The anode and catho~ are constructed
out of conventional electrode type materials, such as
copper, tungsten, etc. The cathod often has a thoriated
tungsten tip or inserts which assist in cooling of the
cathode.
As is conventional with plasma heaters~ the anode is
s-urrounded by an annular cooling chamber 13 through which- ~
coolant e.g., water, or obher cooling medium is circulated ;
by means (not shown) in order to hold the anode at a suit-
ably low temperature and prevent undue erosion -thereof.
In a similar manner, the interior of the cathode is pro-
vided with cooling chamber 7 and with means (not shown)
to circulate water or other suitable cooling fluid there-
in in order to hold the cathode at a sultable operatlng
temperature. Tube 2 serves to help support and align
cathode~ rod 3 and provide a conduit for soolant flow. ~
Cathode 3 can be provided with means for moving it in a ~ ~;
vertical direction so that the distance between cathode 3
and anode 11 can be varied.
The ano~e and cathode are axially aligned but spaced ;
longitudinally to provide annular space 21 which tapers
conically to a coaxial outlet conduct 23. The assemblage
is also provided with plasma or work gas inlet means 15
having conduit 17 which communicates through annular coni~
cal conduit 19 with the annular
- 28 -
space 2l. Ihe cathode and anode are conrlected by elec-tri-
cal connectin~ me.-ns (rlo-t shown) -to a powcr supply (not
shown). rl`ypic~:lly, the power source i9 a direc-t current ;~
power source.
Reactant mixer means 30 is adjacent to the anode ~nd
of cylindrical sleeve 9, and as shown, comprises two co-
axial, longitudinally spaced annular conduits 42 and 47
that are provided with inlet nozzle means 40 and 45, res-
pectively. As shown, exit port 48 o:~ annular conduit 47
is retracted from exi-t port 43 of annular conduit 42 to
form a conical reactant introduc-tion zone 24. Reactants
from reactant supply means (not shown) are introduced into
condui-ts 42 and 47 through nozzle means 40 and 45 respec-
tively. The flow path of the reactants discharged through
exit ports 43 and 48 can be perpendicular 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
1 to 45 from the horizontal position shown so that the
reactant gas flow is directed at such angle into or in ~"
contact with the stream of` hot gas emanating from the
plasma heater. The reactant gas can be projected radially,
tangentially or at any suitable angle therebetween into -
the downwardly flowing stream of heated plasma gas eman-
ating from outlet conduit 23. The top of reactant mixer
means 30 contains opening 31 which is coa~ially aligned
with outlet conduit 23 of anode 11 to provide an overall
direct straight-line path for the hea-ted plasma gas from
plasma generator l through reactant mixer means 30 into
reactor 34. Preferably, the heated plasma gas is intro~
.. . ..
duced into the center of reactor 34 and spaced from the
- 29 -
11:i 4~ 7
w-llls ~hereoL` tc~-~hereby assist in positioning -the reac-
tion ~one awil~ L`rom the walls of` the reac-tor.
lypically, hydrogen is used as the gas which is
hea-ted by the aforementioned heating rneans, e.g., plasma
heater l; however, o-ther 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
since i-t insures -the es-tablishment of a reducing atmos-
phere and provides a halogen, e.g., chlorine, acceptor
thereby removing halogen released E`rom the metal halide,
boron halide and/or halocarbon compound reactants as hydro-
gen halide. Mixtures 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 usecl as
the plasma gasJ 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.J the boranes, and/or as a separate stream through
mixer means 30.
As the heated plasma gas stream moves past the zone
of reactant introduction 24, it mixes with the reactants
introduced through reactant mixer menas 30. The reactan-ts
are introduced usually at below reaction temperatures.
Because of the high heat content o~ the hot hydrogen
stream no special efforts to hea-t -the reactants to temper-
a~ures 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). TypicallyJ the
reactants and reaction mixture are in turbulent flow al-
though
- 30 -
~048~57
lamin~r L`low can be used. I`he reaction mix-ture flowl~g
in-to reac-tor 34 which :i S a recircl~lating-type reactor as
opposed -to a plug flow-type reactor, -typically has an
apparent residence time therein of between about O.OS
and about 0.5 seconds, more usually between about 0.1 and ~ ?
0.2 seconds. The apparent residence -time can be calcu-
lated by dividing the reactor volurne by the gas flow
through the reactor.
As shown in FIGURE l, finely-divided metal diboride
powder product, which is suspended in reac-tion product `
gases as well as excess reactant gas, hereina~-ter collec-
tively referred to as product gases or other equivalen-t
terms, is removed from reactor 34 through conduit 36 and
introduced into cyclones 38 and 39, in order to separate
the solide metal diboride powder from the product gaqes.
The submicron particles of metal diboride are formed
completely in the reactor and since the reactor effluent
is cooled to below metal boride forming temperatures sub-
stantially immediately, substantially no metal boride ;
formation or individual particle growth (other than by
physical aggregation) occurs outside 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 into cyclone 38 and
from there into cyclone 39 by means of conduit 51. Titan-
ium diboride powder drops out lnto receivers 25 and 26
respectively, while gaseous ~ffluent leaves cyclone 39 ~
through conduit 52 and into solids separation chamber 28 ~;
in which there is disposed a bag filter 29, electrostatic ~
"'~ ,'.,.
- 31 - ~ `~
''' ':
5~
precipi~ator or other convellient means f~r s~parating
s~spended sol:ids from a gas. Cyclones 38 and 39, and
receivers 25 and 26 are closed to the atmosphere to
prevent contamination of the product with oxygen. lhus,
-the metal diboride powder that is formed in the reactor
at me~al diboride forming -temperatures is removed i~med-
iately from the reactor and forwarded to product collec~
tors that are substantially below temperatures found in
the reactor. The powder product is typically cooled or
allowed to cool to room temperature. However, if th~
cooling capacity of the cyclones and receivers is not
sufficient to provide a powder product at room tempera-
ture, the produc-t 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
the powder product. Separation chamber 28 as shown also ;
has an exit or exhaust 50 on the opposite side of the bag
filter. As shown, the bag filter has engaged therewith
a suitabl~- shaking means 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
sing~e receiver and cyclone can be used.
Solic~s-separation chamber 2a can also be a caustic
wa-ter scrubber, often containing pack~ng of some sort e.g.
ba;;s. sadd;es. etcé fpr greater contac~. The scrubber
separates the fine solids from the gas stream and neut~a-
.
lizes acidic species therein before the gas is discharged
to the atmosphere or to a flue. To recover unreacted
reactants, hydrogen, hydrogen chloride,
- 32 -
7 ~ .
etc. f`rom the procluct gases substallticll:Ly devo:id of its
solid burden, conventional sepRration and recovery means
for such materi~:ls can be ins-talled between exi~ conduit
52 and the flue. ~urther, if the hea-t removal from the
product recovery apparatus, i.e., the cyclones and re-
ceivers, is insufficient, the product -transfer line 36
can be externally cooled. Mor-eover, a cold or cooler-
compatible gas can be mixed wi-th the exiting product
effluent to thereby cool it.
Referring now to FIGURE 2, there is shown a partial ~ -
assembly, in cross-sec-tion, similar to that of FIGURE l,
except that three-slot reactan-t mixer means 32 instead of
two-slot reactant mixer means 30 is shown. In addition
to annUlar conduits 42 and 47, there is shown a coaxial,
annular conduit 44 which is spaced longitudinally from `~
annular conduits 42 and;ll47. The exit port 49 of conduit
44 is retracted from that of conduit 47 to fur-ther extend
conical reactant introduction zone 24. Annular conduit
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 (not shown).
.: , .
Reactant mixer means 30 and 32 can be constructed of any
suitable material, such as graphite, molybdenum, refrac-
tory or any other~,lmaterial which will withstand the heat
and corrosive environment present in the reactant intro-
duction zone 24. The mixer means can be n~nternally cooled
thereby permitting the use of ~onventional metal fabri~
cation. ~
When a source of carbon is introduced into the reac~r ;;~ -
to prepare acarbon-containing metal diboride powder (pre~
sumably as simultaneously produced metal carbide), the `~
carbon source reactant
~'
- 33 -
l32S7
can ~e introcl-lce~l by arly conven:ient mQarls. rrhus, the car-
bon source reactFIllt ctln be introdllced into the rleactor
mi.Yed wi-th one or both of -the metal halide and ~oron
source reactants. ~lternatively, the carbon source can
be introduced as a separate reac-tant strea~. Thus, appar-
atus such as described and shown in FIGURE 2, provides
individual conduits for each of the reactants when the
aforesaid embodiment is used. The reactants can be intro-
duced into the reactor in any sequence; however, the metal
halide, e.g., titanium halide, reactant is introduced
preferably upstream of the boron source reactant. Prefer- --
ably the carbon source reactant is introduced prior to
the metal halide and boron source reactants. Further,
one or more of the reactant gases can be in-troduced
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 borlde, e.g.,
titanium diboride, powder in the absence of co-formed
metal carbide, metal 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 acting 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
'
- 34 -
~04~3257
througfh the midcl:le s:Lot ~lnd shea-th gas, e.g., hydrogen,
through the bottom slot. ~I~he ~hea-th gas serves to preven-t
contact of the re~ctant gases with exposed surfaces of the
mixer means 32, such as lip 75, and -the reactor, e.g.,
the upper lip 76 of reactor 34. When metal boride~ e.g.,
titanium diboride, is to be produced with co-formed meta1
carbide, e.g., titanium carbide, the carbon source reac-
tant can be introduced through the -top slot, the metal
halide reactant introduced through the middle slo-t and the
boron source reactant introduced -through -the bo-ttom slot.
Other reactant introduction seqùence can, of course, be
used if desired.
The metal halide, carbon source and boron source
reactants are mlxed 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 respec-t to the reactant with which it
is admixed) gas such as the noble gases, e.g., argon and `
helium. Hydrogen is not used commonly with 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 conduits there-
by caus~ng blockage thereof. The amount of carrier gas
used -to facilitate the introdiction of` the reactants can
vary; but, generally will range between 250 and 1200 mole
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.
- 35 -
, . .
,, , '' ~ - . :
-
5~
I`he rnecln part:icle si~e ~ and -th~s surf`ace area) of
t}-e re~ractory metal boride partic:Les comprising the
powdery product prepared by the process described herein
is a function of many variables within the process sys~
tem some of which can be interrelated. From the evidence
at hand some general observations can be made. Particle
size tends -to increase with an increas~ 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 ~`rom 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 amount o~ nuclei from additives, such as
hydro~arbons 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
FIGURE 1 adapted with reactant mixer means 32 of FIGU~E
2, a hydrogen containing gas or noble gas, e.g., argon
is introduced into plasma generator means 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 ou~
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 lo~ 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 -
-
Z57 ~ ~
temperat~lres, usllally -temper~-tures above reac-tion ~one
-temperatures. ~ hydro~en-containing plasma gas can have
an enthalpy of` b~-tween 20,000 and 60,000 BTU per pound
o~ gas, more commonly between 30,000 and 40,000 BTU/
pound. The hea-ted plasma gas is projec-ted directly 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 42,47 and 44.
Reactan-t gases, metal halide and boron source reac- -
tant are introduced, in one embodiment, into nozzles 40
and 41, respectively, and thence into reactan-t introduc-
-tion zone 24 and into the environment of the downwardly /-
flowing stream of hot plasma ga.s. The reactant gases
can be introduced at a mass velocity such tha-t they are
aspirated by the movement of the projected plasma stream,
or -they can be introduced into the plasma stream at a
mass velocity such th~t the plasma stream ls momentarily
constricted. Hydrogen can be introduced into nozzle 45
of reactant mixer 3Z and thence into the reactant in-tro-
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 reac-tion 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 deposltion of the metal borid~
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 -
82S7
to be coars~r than -the powclery product removed ~rom -the
reactor soon af-ter it is t`ormed. Co-mingling -the build
up powder on -the wall with the principal diboride powder
produc-t contribu-tes -to the produc-tion of a non-uni~orm
product. When the principal powder product becomes non-
uniform because of coarse powder from the reactor wall
the powder product should be classified to remove over-
sized particles before being used.
Finely~div-ded refractory metal diboride powder sus-
pended in reactor effluent product gas is removed immedi-
ately from reactor 34 and introduced into cyclone 38.
A portion of the powder product is removed in cyclone 38
and recovered in receiver 25. Powder product retained in
the gas effluent from cyclone 38 is forwarded via conduit `
51 to cyclone 39 wherein further amounts of me-tal diboride
powder products are removed and recovered in recelver 26.
Additional cyclones and recelvers can be used if needed. ~
In most cases, the products from receivers 25 ànd 26 are ;
blended into a single product.
The reactor effluent product gas, now substantially
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 ls removed from chamber 28 by means of
condult 50. The product gas now removed of its metal dl-
boride and/or other soli~ burden 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 chlorlde, for
_ 38 - ~
~;:
.
3Z~7
u~e in the pr~serlt process or in some o-ther process or
the c~ooled procl-lct ef`fLuerlt streEIrn carl be recyc:led t~
the reactor as a source of cooling or diluerlt gas.
I`he metal dibo~ide powder product prepared in accor-
dance with the aforemen-tioned process is a finely-divided
powder -tha-t can adsorb gases 5uch unreacted reactants ' i
that may be present in the receiver in which -the product
is collected. To avoid contamination by adsorption, re-
ceivers 25 and 26 are heated generally to temper~tures
above about 200 F. (93 C.), e.g., ~rom about 200 F 600 F.
(93C -316C.) to assist in degas~ing 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 further assist in the degasif`ication step while the
product cools. I~ 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 con-tamina-
tion into the product. Advantageously, the product is `~
handled without exposure to the a-tmosphere; however, in
some cases, some exposure to the atmosphere cannot be pre-
vented. In the event the metal diboride powder product
contains adsorbed chlorine-containing species, e.g., the
subhalides of the metal halide reactant such as titanium
trichloride and titanium dichloride, such species can be
removed by heating the product to between abut 400 and
1000C., e.g., 500-700C. and preferably about 600C. for
between abo~t 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 temperaturesl`for
- 39 -
~4(?~257
the indic~ted t:ime. A stream of hydrogen or :iner-t ~as,
such as argon, i~ mu:in-tc~inecl over the hea-ted produc-t to
help remove undesirable adsorbed ~a~es from -the product
and preven-t exposure of oxygen. After degassing, the
boride product can be coated with ~ paraffin wax or other
similar binder material to minimize the rate of oxygen
pick-up during storage and handling.
The boride powder compositions having carbon-con-~ain-
ing additive described herein, particularly titanium and
zirconium diboride compositions, when hot pressed or cold
pressed and sintered into solid shapes are especially
useful as current conducting elemen-ts, e.g., electrodes,
in electrolytic cells for the production of metals, e.g., ~
aluminum. The term "electrolytic cell" as used herein '
with respect -to aluminum production is in-tended to in-
clude both reduc-tion cells and three-layer cells for the
refining or: purification of aluminum. When used as a
current conducti'ng elemen-t, ti-tanium and zirconium dibor-
ide can comprise at least part of the cathode of the
electrolytic cell or of the elements used for conducting
electrolyzing current to and/or from the electrolytic cell,
and can be exposed to the molten metal either in the re~
duction cell or in the purification cell. "
Current conducting elements prepared with metal
diboride powders of the present invention can be disposed
in a vertical or inclined position in the electrolytic
cell for the reason that molten aluminum wets the surface
of such elements. Thus, a cathode prepared for the titan-
ium diboride powder of the present invention can be arr* ~'
anged in the electrolytic cell so that the operative face
or faces of the cathode are disposed at a relatively large
angle, i.e., 60 or 90 degrees, to the horizontal, thereby
allowing the deposited aluminum to continuously drain from
the face or faces of the cathode and preferably to collect
~ - 40 -~
1~4~3Z57 ~
in a pool in contact wl-th a :Lower part o:~ -the
cathode from which pool i-t may be wi-thdrawin from
time to time in the usual manner. Due to the inclined
or substan-tially vertical
- 40a - ~
: ,:
.
~8~S7
arrangemerlt o~ the cathode, the floor space occupied by
the electrolytic cell :is very considerably reduced in
relation to that which is conventionally required. Per-
haps the larges-t advantage to the use of inclined or
substantially vertic~lly arranged electrodes oF the in-
stan-t metal diborides is that surging of the molten alum-
inum is less likely to occur so that the spacing of the
anode and cathode can be substantially reduced compared
with -the adopted in aluminum reduction cells heretofore
knwon and the dissipation of electrical 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 a-t the bottom
of the cell which causes an undesirable additional 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 used in a substant-
ially vertical or inclined position, the vol-tage drop
across the electrolytic cell is significantly reduced
thereby p~oviding significant savings in power. Such
power savings have become 1ncreasingly more important due
to the continuing rising cost of power.
The use of titanium diboride current-conducting
elements in electrolytic cells for the production or re-
fining of aluminum is described in the following U.S.
Patents, 2,915~442, 3,028,324, 3,215,615, 3,314,876,
- 41 -
~04~5~
3,33Q,756, 3,15~,63~, 3,274,0~3 ~ncl 3,~00,061. Despite
t~e rather extellsive eL`fort these patents indicate w~s
mounted ~Incl the potential advan-tages ~or using -titanium
diboride and titanium diboride compositions as curren-t-
conduc-ting elements in elec-trolytic cells for the pro-
duction of aluminum as described in the aforementioned
patents, such composi-tions do not appear to have been
commercially adopted on any significan scale by -the alum-
inum indus-try. The reasons for such laclc of acceptance
are believed to be related to the lack of stability of
the current-conducting elements prepared from -the titani~
diboride powders of the prior art during service in
electrolytic reduction cells. It has been reported that
such current_conduc-ting elements prepared with composi~
tions of -the prior art fail after relatively short periods
in service. Such failure has been associated in the past
with penetration of the current-conducting element struc-
ture by the electrolyte3 e.g., c~yoli-te, 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 strength and resistance to thermal shock.
Ideally, a current-conducting element should have the
following characteristics:
1. Good electrical conductivity
2. It must not react with nor be soluble in either
molten aluminum,or, under cathodic conditions, in
molten flux or electrolytey at least to any apprec-
iable extent at the operating temperature of the cell.
The
- 42 -
:~0~32S~ ~
sol~lbility of the materi.ll in moltcn aluminum is an
impo~tant cons:ider~-tion as i-t de~ermines both the
usef~ll lit`e o~ the curren-~ conducting element and
the degree of` con-tamination of the aluminum produced
through the agency of such curren-t~conduc:ting element.
3. Wetability by molten aluminum.
4. Capable of bcing produced and fabricated into required
shapes economically.
5. High stability and under the conditions existing at
the cathode of the cell, i.e., it shollld possess
good resis-tance to penetration by the molten electro-
lyte (cryolite) and to cracking.
6. Low thermal conductivity.
7. Good mechanical s-trength and resistance to thermal
shock.
In order to have high stabi~lity 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 with
metal boride powder compos1tions of the prior art by hot
pressing only. The metal boride powders of the pr-esent ~;
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
- 43 -
~' ~
~4~Z57
~or -the recl-lction or pur:if'ica-tio~ of al-lminum and have
improved stability compred -to prior art boride composit-
ions in such electrolytic cells.
The presen-t invention is more particu]arly described
in the following examples which are intended as illustra~
tive only since numerous modifications and varia-tions
therein will be apparent -to -those slcilled in the art. In
the followi~g examples J some volumes of gas are expressed
in cubic feet per hour at standard conditions ~4.7 pounds
per square inch~ (].01.3 kPa) pressure and 70 F. (21 C.~ -
or SCFH. Reactant and o-ther gas s-tream rates were meas-
ured at nominal laboratory conditions, i.e., l atmosphere
and 70 F. (21 C), and are reported as rneasured if other
than SCFH. Unless otherwise specified all percentages
are by weight.
The following examples illustra-te the prepara-tion
of refractory metal borides with and,without added carbon
by vapor phase reaction of the corresponding metal halide
and a boron source in the presence of a hot hydrogen~
stream and in the substantial absence of oxygen, combined
or elemental.
EXAMPLE l
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 kllowatts. Hydrogen in the amount
of 300 SCFH was used as the plasma gas. 0.71 grams per
minute of 1,1,2-trichloroethane together with 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 tetrachlor-
ide in the amount of 18.8 grams per
- 44 _
., ~ - : : :.
- ..
. .
: L048257
minute together with 20 SCF`H hydrogen and 5 SCT`~I hydrogen
chloride was introduced -through -the middle slot of the
reactan-t in:let assembly means. Boron trichloride in the
amount of 21.7 grams per minute, -together with 22 SCFH
argon was introduced through the bottom slot of the reac- -
tant inlet assembly means. This run was continued for
989 minutes and produced -titanium diboride having a B. E.T.
surface area of 24.0 square meters per gram. The product
was analyzed for carbon and found to have 0.5 percent
total carbon.
EXAMPLE 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~ the top slot ~`f the
reactant mixer assembly means. 1.26 grams per minute of
l,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 hottom slot of the reactant mixer
assembly. The titanium 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.
EXAMPLE III
:
Run _ ~
. ;. ,
Apparatus slmilar to FIGURE 1 was used to prepare -
titanium diboride. The arc heater was a medium voltage,
medium amperage
- 45 -
104BZ57
heater having a power :inpu-t of 28 ki:lowat-ts. Ihe arc
heater was operat~ct at be~tweerl 2~-28 kilowatts. Ilydro-
gen in the amount of 300 SC~H was introduced into -the
arc heater as the plasma gas. Gaseous titanium -tetra-
chloride in the amount of 18.7 grams per minu-te, together
with hydrogen as the carrier gas in the amount of 20 SCFH
was introduced through the -top slot of the reactant in
let assembly means. Gaseous boron trichloride, in -the
amount of 26 . 9 grams per minute wi-th an argon carrier
gas in the amount of 22 SCFI-I was introduced through the
bottom slot of the assembly means. The run continued
for 95-l/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 bo-ttom lip
of the reactant inlet assembly were observed at the
end of the run.
Run B
The 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 minute of
gaseous boron trichloride with 22 SCFH argon and 18.~7
grams per min~te o~ titanium tetrachloride together with
12SCFH of hydrogen chloride were utilized'-asthe 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 inle-t assembly were observed at
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 l , and the ex-
posed top lip of reactor 34.
- ~6 -
~ n ~ ~ ~4B~7
~ rhe proceclure of Run ~ was repea-ted, excep-t tha-t 12
~CFH of hydrogen chloride was utilized as the carrier gas
for the titanium tetrachloride instead of the 20 SCF'H of
hydrogen and 27.8 grams per minute of boron trichloride
was fed to the reactor. This run continued fr 150 min~
utes and the titanium diboride product was f`ound to
ahve a B.E.T. surface area of abou-t 5.8 square meters per
grams. No growth of titanium diboride deposi-ts on the
inlet assembly means was observed. ;
Run D ~;
The procedure of Run C was repeated9 except th~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 meters 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 obtained ~`
was calcined in the presence of hydrogen at 1000 C. to
degasify the product. Some of the calcined product
remained pyrophoric.
. ,:
Run E -
Run D was repeated except tha-t 23 SCFH of hydrogen
was added to the titanium tetrachloride reactant
introduced through the top slot of-the~reàctant inlet
assembiy means. The titanium tetrachloride and hydrogen
chloride reactant addition rates averaged
.
- 47 - ~
~0482S~ ~
19.2 grams per minute and 2.5 SCF~I, respec-tively. Boron
trichloride in -the amount of 27.0 grams per minute to-
gether with 22 SC~I~ argon was introduced through the
bottom slot of the reactant in:Let assembly means. This .
run continued for 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 submicron titanium diboride
can be produced by the vapor phase reaction of titanium
halide and a boron source compound with or without a car-
bon source reactant. The submicron ti-tanium diboride
powder produced is composed of well formed, individual
crystals of titanium diboride. Typical scanning and
transmission electron micrographs o~ such titanium dibor-
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 III9 Run A, was used to prepare zirconium dibor-
ide. Hydrogen in the amount of 300 SCFH was introduced
into and heated by the medium voltage, medium amperage arc
heater. Gaseous zirconium -tetrachloride, at a rate o~
20.5 grams/minute, and 100 SCFH argon were introduced
through the bottom slot of the reactant inlet assembly ;~
into the hot hydrogen stream emana-ting from the arc heater.
Gaseous boron trichloride, at a rate of 4.93 liters/min-
ute (a 25 percent stoichiometric excess based on zirconium
tetrachloride), and 22 SCHF 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 - ;
~f~ 57
EX~MPL~ V
The procedure and apparatus of Example IV is used to
prepare hafniutn diboride and a finely-divided, submicron product
~imilar in size and surface area to the zirconium diboride of
Example IV is recovered.
'
FuY~MPLE VI
Apparatus anaIogous to that used in Lxample I was u&ed
to prepare titanium diboride. 300 SCFH of hydrogen was used as
the plasma gas. Propane (89 standard cc/minute), and 45 SCFH
hydrogen as a carrier ga& were introduced into the reactor through
the top slot of the three-slot reactant inlet assembly means.
Titanium tetrachloride (52 grams/minute) together with 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 were introduced through the bottom &lot of the
inlet assembly. Titanium diboride powder product was recovered and
degassed under~a hydrogen flow of 11 SCFH at 600C. for 4-3/4 hours.
The titanium diboride powder product had an elemental analysis of
31.9 percent boron, O.O9 percent oxygen, 0.78 percent carbon and
0.088 percent chlorine, and had a B.E.T. surface area of about
6.4 m /gram.
EXA~LE VII
Apparatus analogous to that used in Example VI was used
to prepare titanium diboride. 300 SCFH of hydrogen was used as the
plasma ga&. Titanium tetrachlorlde in the amount of about 41.5 ~ ~
grams/minute, 9 SCFH of hydrogen and 24 SCFH of hydrogen chloride
,,
., .~ .
were introcluced into the reactor through the top 810t of the
three-slo~ reactant inlet assembly means. About 22 SCFH of
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 SCFH of argon
were introduced through the bottom slot of the inlet assembly.
Titanium diboride powder was recovered and degassecl under hydrogen
at 600~C. for 3 hours. The titanium diboride powder product had
an elemental analysis of 32.3 percent boron, 0.44 percen~ oxygen
and 0.03 percent chlorine, and had a B.E.T. surface area of 3.3 m2/
gram.
EX~LE VIII
The procedure oP Example VII was repeated and titanium
diboride powdeF 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.
E ~MPLE IX
A pDrtion of the titanium diboride powder of Example VI _
was hot pressed at about 2100C. and 3500 pounds per square inch into
a plate 2 inches x 2 inches x 1/2 inch. The plate had a density of
97 percent of the theoretical density of TiB2 and a resistivlty of
about 7 microohm centimeters. The plate was analyzed for oxygen,
which was found to be about 0.05 percent. The plate was operated as
a cathode in an aluminum reduction cell for 100 hours at 960C. at an
anode current density of 6.5 amperes/inch2. At the end of the test
.:,
57
period, the plate was removed, ~ractured, and inspectecl. No
deterioration of the plate and no penetration of electrolyte into
the plate was observed. Fracture of the plate was observed to be
primarily transgranular.
A piece of the test plate was cut ou~ a~ter the test was
colllple~ed and polished and etched. FIW RE 6 is a photomlcrograph,
having a magnification factor of 2100, of a polished and etched
section of the plate. The microstructure of FIGURE 6 shows a mosaic
of equidimensional TiB2 grains with contiguous grain boundaries and
a limited grain size raLIge. The TiB2 grains range from about one to
fifteen microns in diameLer; but, are predominantly in the four to
twelve micron range in size. Titanium carbide occurs as occlusions
less than one micron in size within the tltanium diboFide grains.
EXAMPLE X
~ blend of the titanium diboride powders of Examples VII
and VIII in a weight ratio of about 58.5/41.5 was mixed with~about
5 welght percent of titanium carbide powder having 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 2000C. and 3500 pounds per square inch into a 2 inch x 2
inch x 1/2 inch plate. The plate was allowed to cool overnight in
the mold under vacuum. The plate had a density of about 93 percent
of the theoretical density of TiB2 and was found to have an oxygen
5 1 ' ::
- . ~
~ ~ .
,
., .
1~413257
content of abo~lt 0.33 percent. The electrical resistivlty of the
plate was 6 microohm centimeters. The plate was operated as a
cathode in an aluminum reduction cell under the same conditions
as recited in Example IX. At the end oE the test period, the plate
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. Fracture of the plate was
observed to be primarily transgranular.
A piece of the test plate was cut out after the test was
completed and polished and etched. FIGURE 5 is a photomicrograph,
having a magnification factor o~ 2100 of a polished and e~ched
section of the plate. The microstructure of FIGURE 5 is fine and
shows interlocking grains of white, lath-shaped TiB2 with grey TiC
grains dispersed in the structure. The TiB2 grains range in si~e
from less than one micron to five microns. TiC grains are up to
three microns in diameter.
EXAMPLE XI
A blend of 95 parts of titanium diboride powder prepared
in a manner similar to Example III Rune E and 5 parts of titanium
carbide powder was mixed with about l percent paraffin wax in 191
trichloroethane and ball milled for about one hour. The titanium
diboride powder had 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 m2/gram.
The blended mixture was vacuum dried and isostatically pressed at ~ ~
~ ~',..
about 20,000 pounds per :quare inch into a cylindrical rod 1-1/2 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
~ - ~-- .
- . . . ~ , . . ~ .
4~3257
was vacuum sintered at about 1900C. or about 1 hour. The rod
had a density of 95 percent of the theoretical densi~y o TiB2.
The sintered rod was tested as a thermocouple well in an aluminum
reduction cell. The rod showed excellent thermal shock resistance
and resistance to the bath.
EX~MPLE XII
Rods similar to that of Example XI were prepared using
titanium diboride powder having B.E.T. surface areas of 6.6 m /gram
and 7.0 m2/gram. The slntered rods had densities of 96 percent and
greater than 99 percent of the theoretical de~sity of TiB2 respec- -
~ively. A piece of ~he 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 ~he rod. The microstructure of FIGURE 7 shows a mosaic
of rela~ively equidimensional TiB2 grains with the light-grey TiC
predominately localized in interstices between TiB2 grains or
occurring as occlusions within the TiB2 grains. Electron microprobe
analysis has indicated 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).
EXAM2L~ 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 ~at 20,000 pounds per square
inch into a cylindrical rod. The rod was vacuum sintered at about
2000C. for about 30 minutes. The sintered rod, which had dimensions
s~
of about 1 inch x 5 inches, had a density of about 98 percent of
the theoretical density of TiB2 and a resistlvity of about 9 microohm
centimeters.
A piece of the rod was polished and etched. FIGUR~ 8
is a photomicrograph, having a magnification factor of 2100, of
a polished and etched section of the rod. The microstructure of
FIGUR~ 8 shows a mosaic of equidimensional TiB2 grains with
contiguous grain boundaries and a limited grain size range. The
TiB2 grains axe predominantly three to ten microns in diameter. The
Ti (C,O,N) phase occurs as occlusions less than one micron in size
within the TiB2 grains.
Although the present process has been described with
reference to speciflc 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.
