Note: Descriptions are shown in the official language in which they were submitted.
5 ~
The inventio~ h~rein de~cribed w~ made in the
course of or und~r a contract or subcontract with the
National Science FoundatisnO
This invention relate~ to an improved electrlcAlly
conductiYe current collactor ~u~t~ble for use in high te~p
~rature application~ in tho pre30nce of ~orra~ive environ-
m~nt~.
More particularly thi~ inven~on ralate~ to a
m~thod f~r manufactuxing ~ high str~ngth el~ctronic~lly
condùetlve polycxy~talline titanium dioxide c~ramlo m~mbBr
u~eful in ~n electronlcally condu~t~ve curront coll~otor
or current collector/çont~inor for u~e in energy aonvorslo~
d~vla~s ~uch ~9 the ~odlum-~ul~ur batt~y.
Suoh curr~nt collec~ors ar~ crib~d ~nd cl~l~ad~
ln.Appllcation No. 272017 from whlch th~ p~aoa~t a~pllc~t~on
: has b~n dividsd.
Thor~ aro ~ numb~r of e~lectr~oal applicatlon~ ln-
volvlng v~lou~ ~n~rgy conv~xsloll tovl~ ln whlch th-
, cnvl~onma~t to ~hl~h tho curr~nt coll~ctor of th~ ~o~lc~ 1
- 20 axpoo~d i~ ~xtrQm~ly co~roolv~D For oxamplo i~ rgy con-
var~lon ~lc-a of th~ typo ~omp~s1~g a molton ~athodl~
x~actant ~u~h a~ sodlum.poly~ulf~d~ th~ ~12i~tlon of a
~u~t~ çu~r~n~ ~olloctor ~B wall as ~ ~u~tablo ~ontain~r
h~ n a ~ou~oo o~ aon~lder~bl~ conc~rn.
On~ of ~ho prlmo o~nd~dato~ to date for u3e a3 a
. , .
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current collector or current collector/container for such
devices have been certain metals. However, metal ~ystems,
both pure and alloyed, often exhibit the phenomenon of
severe plastic deformation under stresses (external and
from their own weight). For this rèason and because of
severe corrosion problems, many metals are not practical
for u~e in ~uch high temperature or corrosive (oxidative)
environments.
Since the thermodynamic stability of ceramic
materials such as oxides and sulfide~ in the pre~ence of
corrosive environme~t~ is well established ànd since it is
also known that the thermodynamic stability of such materials
is maintained to temperatures much higher than is com-
patible for metal systems, it ha5 been sugge~ted to employ
a ceramic coating on a metal which is used as the load
bearing element of the current collector or container.
Where a metal system operates as the load bearing element
and includes a protective covering separating the metal
from the corrosive substance, the l3elect$0n of a suitable
covering must be made from materials which ~1) are non-
corrosive and impermeable to the corrosive sub~tance, (2)
adhere well under c~nditions of thermal cycling and (3)
have suf~icient electronic conductivi~y. Oftentims~ a
thermal expansion mismatch between the attached metal and
ceramic covering re~ults in fractures, microcrack~ and
eventual spalling of the coating from the metal surface.
In addi~ion to mechanic~l incompatibility, conventional
me~hod of applying the ceramic coating such as by
anodizing often result in an insulative rather than a con-
ductive coating.
~. .
5.~
In summary, the concurrent development of the requisite non-
corrosive character, good adherence, and adequate con-
ductivity in a coating which will be mechanically compatible :.
under recurring cycles of thermal expansion has long ~::
presented a difficult challenge in this field of art.
In view of the above discussed inherent limitations : .
of current collecting systems comprising a metal load
bearing element with a corrosion resistant ceramic coating, ~ -
the u~a of corrosion resistant ceramic per se has been
~uggested. However, the vast majority of useful ceramics
are electrical insulators, thus making them unsuitable for ~-
current collection purposes. Of course, some ceramics are
known to be conductive in the metallic sense, but are not
economically attractive,
A large class of ceramics can be made moderately
cond~ctive, but with conductivities whi~h are much less
than metals. Consequently, a curr~nt collector constructed
of an electronically conductive ceramic will exhibit a much ; .
higher resistance than that of a si.milarly shaped metal
current collector.
According to the present invention there i5
providea a method for manufacturing a high strength, non-
corrosive, electronically conductiv~ polycrystalline .
titanium dioxide ceramic mambex exhibiting high resistance
to thermal shock and uniform grain size with an average
size of less than about 25 micrometers and consistin~ of
titanium dioxide in the rutlle crystallographic form doped
with a homogeneously distributed ionic metal species
selected from tantalum and niobium, comprising:
(A) forming a slurry comprising titanium dioxide and
-4-
. . ~ , . .
:, ....... . : . , :,
.. - . ' ' : :. ` ` ' ; - . : ~
said metal specie~, said metal specie~ being pre~ent in an
amount to provide from 0.01 to 8 atomic percent of sai~
metal species in said ceramic member, said slurry being
formed either
(i) by dissolving i.n a solvent ~elected from water and
aliphatic alcohols the pentafluoride salt of said metal
species in an amount adapted to provide from about .01 to
about 8 atomic percent of caid metal ~pecies in said cerami~
member and then adding titanium dioxide powder in either the
rutile or anatase form to the 801ution of (1~ to produce a
mixed slurry; or
(ii) di~persing a mixture of a pent~chlorido-dle~hyl-
etherat~ complex of ~aid metal 3pecie~ and tltanium dioxld~
powder in either the rutile or anatas~ ~orm to form a
slurry, said slurry components bolng ~nalud~d in amount#
adap~ed to provide from about .O:L to about 8 atomic p~rc~nt
of said metal species in 9aid cernmic memb~rt
(~) Drylng said ~lurry to powdor ~orm at a t~mpQrature
adapted to avold ~v~poration of l3aid metal apecl~
~0 (C) ~reen form~ng ~ld powld~r to th~ d~lr~A ~hapet
(D~ Heating the ~haped gre2n body to about 350~C. ~t
a rate of at le~t about 10C. por minute; or ln th~ ca5a
where slurry of s~ep A is formed u~ing the pentachloride
di~th~r~t~ compl~x heating to about 500C. at a rate at
le st 40C. par mlnute
(E) Sin~aring sald ~haped gr~en body at a temperatur~
of at least about 1330C.; and
~ (F) ~nne~ling said sintered shaped body ~n a reducing
atmo~phere having an oxygeh partlal pres~ure of about 10 5
to about 10 25 atmo~pheres at a temperature of from about
.~ ,
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:
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P~l
850C. to about 1400C. to increase the conductivity of said
ceramic member.
The invention will be more fully understood from
the following detailed description of the invention taken in
conjunction with the drawings in which the Figure shows a
graph depicting the reduction in resistivity achieved in
rutile titanium dioxide by annealing.
Preparation of highly corrosive oxide ceramics is
accomplished in the art by four commonly accepted methods:
tl) intrinsic high conductivity, (2) reduction of the oxide
ceramic causing a deficiency in oxygen ions and subsequent
electrical compensation by the addition of conducting
elect~ons, t3) controlled addition of an ionic species
difering from the solute cationic species in both con-
stitution and electric charge, the added speci~s occupying
an interstitial crystal site, c~arge neutrality con- ;
siderations creating conducting electrons and higher con-
ductivity, and (4) controlled addition of an ionic species
difering from the solute cationic species in both con-
stitution and electric charge, the added species occupying
by substitution the sites of the parent cationic species
with charge neutrality considerations producing conducting
electrons.
Intrinsic high conductivity is exhibited by
ruthenium oxide, a compound normally considered uneconomical
because of the rare occurrence of ruthenium in nature.
Methods (2) and t3) normally result in the creation of
charged, mobile atomic entities which can move easily under
the force due to an electric field. Method (4) offers the
3~ greatest promise for applicability in the current collector
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described in that the addition of impurity ions in a sub-
stitutional manner usually produces a nearly immobile
impurity except at very high temperatures.
Three economically viable metal oxides which may
be made conducting and which are economically viable because
of natural abundance are calcium titanate (CaTiO3),
strontium titanate (SrTiO3) and one of the derivatives of
both titanates, titanium dioxide (TiO2) in the rutile
crystallographic form. Common substitutional additive ions
for all of these oxides include iron in the +3 oxidation
state¢and aluminum in the ~3 oxidation state. Greater
electronic conductivity increase may bP accomplished by the
addition of an ionic metallic species having a stable
valence in said ceramic of at least ~5. Tantalum in the +5
oxidation state or niobium in the -~5 oxidation state are
preferred because of the solubility of these elements and
because the charge carriers created from the niobium or
tantalum impurity additions remain nearly free for electronic
current flow.
By far the most common prlor art method of adding
niobium or tantalum to these metal oxideq, when the resulting
ceramic is to be polycrystalline, i~ the simple mixture of
fine powders of niobium pentoxide (Nb205~ or tantalum
pentoxide (Ta205) with fine powders of the solute subqtance
CaTiO3, SrTiO3 or TiO2. Subsequent processing by commonly
known arts of pressing the mixed powders into green ceramic
form and sintaring at a suitable temperature yield a black,
dense ceramic w~th conductivity drastically enhanced over
the pure ceramic.
Grain size is generally very nonuniform coincident
_7_
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... .
with the inhomogeneous distribution of the impurity in the
solute. That is, the tantalum or niobium is not distributed
in a homogeneous fashion, the large ceramic grains contain
smaller amounts, the smaller grains existing as such because
tantalum and niobium tend to serve as grain growth inhibi-
tors. Excessive heating at very high temperatures may be
employed to further homogenization but at the expense of
further grain growth and excessive processing costs.
Barium titanate, homologous to calcium titanate
and strontium titanate, may be formed as a powder and in-
timatèiy mixed with tantalum oxide or niobium oxide by a gel
process deRcribed in U.S. patent 3,330,697. Even if this
method is extendable to CaTiO3, SrTiO3 and TiO2, a need
exi~ts to simplify the powder preparation proce~s ~or
economlc reasons when large quantities of conducting ceramic
are required.
Relative abundance of CaTiO3, SrTiO3 and TiO2 makes
these materials attractive for use in corrosion resistance,
electronically conducting current collectors provided these
materials may b~ processed at low CQSt and exhibit electronic
conductivities adequate for the planned use of the current
collector. Of special interest i~ the use of electronically
conducting Ca~iO3, SrTiO3 and ~iO2 as materials for a
current coll~ctor or current collector/container for the
~odium-~ul~ur battery, such as is disclosed in ~he U.S.
patents 3,404,035 an~ 3,468,70g.
The above ceramic oxideR not only may be made
electronically conducting in the sense that resistivities
at room temperatur~ are less than 50 ohm-centimeters but
0 also are known to be resistant to corrosive attack by
-8-
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commercial and/or electrolytic grade sodium polysulfides at
400C or below.
Disc shaped samples of about one and one~fourth
inch diameter and one-eight inch thickness have been formed
and sintered for the following chemical compositions and
te~ted for corrosion resistance.
1) TiO2 (rutile) containing 1% tantalum
2) SrTiO3 reduced in water vapor atmospheres
3) CaTiO3 containing 3.0% iron
4) LaO 84Sr0 16CrO3 with no other additives.
Also, a single crystal sample of Tio2 (rutile) con-
taining 0.05 percent tantalum has been subjected to the
corrosion tests above, which are performed by the method of
recording th~ initial ~ample weight, subjecting the sample
to the sodium polysulfides (in either commercial grade or
electrolytic grade quality) at 400C by immersion for 14
days and subsequently weighing the cleaned samples after
immarsion for detection of weight 1089 or weight gain due
to corrosive reactions with the ~odium polysulfides. All
o the above named samples exhibited either no weight change
or a very small change after ~he above tests (see ~able
belo~) lndicating g~od corro ion resistance to these liquids.
Weight Change in .
Sodium Sulfide
at 400C. (PCT)
Material Form after 14 davs
~ . .
CaTiO3 + 3,0% Fe Sintered -1.45
SrTiO3 - xed~ced Sintered -.141
SrTiO3 Sintered -S.0
TiO2 + 0.5% Ta Single Crystal 0
~i2 ~ 1.0% Ta Sintered 0
;~ 9- ' "
.
The ideal electronically conductive ceramics for
use in this invention are the tantalum and niobium doped
titanium dioxides prepared in accordance with the method of
this invention which was briefly described above and will be
more fully discussed hereinafter.
As mentioned previously the ideal high strength,
noncorrosive, electronically conductive polycrystalline
ceramic for use in preparing the current collectors are
prepared in accordance with either one of two variants of
a basic method. The ceramics are titanium dioxide in the
rutile crystallographic form which is doped with a
homogeneouqly distributed ionic metal species selected from
tantalum and niobium. This homogeneous distribution of
dopant i8 a result of the processes employed and results in
ceramics having excellent properties including high
resistance to thermal shock, uniform grain size and an
average grain size of less than about 25 micrometers,
The first of the two proce~ses comprises:
A) The formation of a slurry o~ titanium dioxide
in either the rutile or anatase crystallographic forms in
water solution (or aliphatic alcohol solution) of tantalum
or nio~ium pentafluoride. The weights of titanium and th~
pentafluoride salt constituent will be determined by the
desired final concentration of dopant in the titanium
dioxide. In general, it is desirable to provide from abou~
.01 to about 8 atomic percent of the ionic metal species in
the sintered ceramic. It will be appreciated that the amount
of dopant which may be added with continuous ~lectrical
property enhancement of the ~iO2 due to homogeneous tantalum
or niobium distribution i5 ultimately limited by the
1 0-- ,
~L$~
solubility of tantalum or niobium in TiO2, but certainly
quantities of up to 3 percent Ta or Nb on a titanium ion
basis is possible. For no tantalum or niobium percentage
should the sum of all percentages of cationic impurities of
normal ionic charge less than +4 or greater than +5 be
greater than about 0.10 percent and concurrent with this
restriction, the sum of all percentages of cationic
impurities of ionic charge less than +4 or greater than ~5
should not exceed 10 percent of the added tantalum con-
centration. Impurity levels greater than these mentioneds_rve to drasticalIy limit the attainable conductivity
resulting in a highly inefficient use of the tantalum panta-
fluoride addition~.
B) The slurry is next dried to powder at a tem-
lS perature adapted to avoid evaporat:ion of the metal species.The drying of the slurry is generally accomplished by slowly
heating to a temperature o not more than 110C and should
be accompli~hed in a time of not rnore than 10 hours.
Stirring of the slurry enhances the rate o~ drying. During
thiq process addition of a suitable binder useful in green
forming of the ceramic body may be accomplished. Penta-
fluoride saltq of the metal ion which remains after drying,
melt~ at temperatures less than 100C and the ~apor pressure
of the substance rises to an unreasonably high value before
the drying slurry reache~ 130C. Long term heating at
temperatures above 120C results in the evaporation loss of
the pentafluorlde when experimental conditions are equivalent
to open container heating.
G) The powder with water removed is pressed into
a ~uitable or desired form and sintered at elevated
temperatures in air or oxygen with critical heating rate of
at least 10C per minute maintained between ambient and
350C to prevent loss of tantalum or niobium via a
vaporization process. Above 350C oxidation of tantalum or
niobium and fluorine to succeedingly more stable oxides
occurs, each succeeding form being less susceptible to
vapori2ation loss until the final form of tantalum or
niobium pentoxide is reached and which is stable to
vaporization loss throughout common sintering temperature
ranges. A wide range of final sintering temperatures and
holding times at the sintering temp2rature may be useful to
those skilled in the arts of ceramic processing. A temp-
erat~re of at least about 1330C and more preferably a
range of about 1380C to about 1440C may be used. ~owever,
the preferred conditions for optimal densification,
homogenization of the tantalum or niobium ion by diffusion
into the rutile powders and minimization of uniform grain
size i~ about 1400C ~or approximately 3 hours. A minimum
temperatUrQ of 1330C is required ~or reasonable rates of
homogenization. These above stated conditions apply to the
processing regardles~ of the quantity of tant~lum or niobium
fluoride added to the rutile. The resulting dense material
may be cooled to room temperature from 1400C in as little
~s 10 minutes for samples containing 1 percent tantal~m or
niobium. Cooling rate~ must decrease for samples with
decreasing pe~centages of tantalum additive.
The above stated m~thod is conductive to the fab-
rication of highly conductive tantalum or niobium doped
rutile ~n a batch sintering mode and may be subjected to
0 many minor modifications to suit available processing
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apparatus. Samples of dried TiO2 powders in both the rutileand anatase forms and under several forming conditions may
be processed with the method described including about 1
atom percent of Ta and produce very nearly identical values
for properties of electrical conduckivity (i.e. about 1
mho/cm), uniform grain size less than 25 micrometers,
fracture strength of 18,000 psi or greater and density
greater than 9B percent of the theoretical density of Tio2
in the rutile ~orm. Some variations of processing methods,
starting TiO2 powders and green forming methods are
discussed in ~pecific examples will make this point clear.
The second method of processing TiO2 and small
percentages of tantalum or niobium additive into a highly
conducting, thermally shock resistant ceramic with uniform
grain size o~ less than 5 micrometerC and fracture strength
of 18,000 psi or greater comprises:
A) The addition of tantalum or niobium penta-
chloride to diethyl ether to form the molecular complex
tantalum or niobium pentachloride di.ethyletherate, with
any excess ether to serve as a liquid into which Tio2 powder
in eith~r the rutile or anatase form may ~e stirred to form
a 31urry. Con3tant stirring causes the ether to evaporate
rapidly a~ room ~emperature leaving essentially dry, mixed
powders to which a binder may be added. This simple method
is ideally suited for rapid, ~ontlnuous raw material prep-
arat~on. At this stage of processing, the concen~ration of
tantalum or niobium is adjusted by varying the amount of
tantalum or n~obium pentachoride used to react with the
diethyl ether. As in the first method, it will be
appreciated that the amount of tantalum or niobium which may
-13-
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be added with continuous electrical conductivity enhancement
of the rutile due to homogeneously distributed tantalum or
niobium i5 ultimately limited by the solubility of the metal
species in TiO2, but certainly quantities of up to 3 atomic
percent are possible. For no tantalum or niobium percentages
should the sum of all percentages of cationic impurities of
normal ionic charge less than +4 or greater than ~5 exceed
10 perc~nt of the added tantalum or niobium concentration.
Impurity levels greater than these mentioned serve to ~-
drastically limit the attainable conductivity resulting in
a highly inefficient use of the tantalum or niobium penta-
chloride additions. The resultin~ mixed powders may not
be dried at temperatures exceeding 65C for times exceeding
a few minutes due to the loss of tantalum or niobium by
vaporization o ~aOC13, a product of the tantalum chloride
diethyletherate decomposition at 65C or the comparable
niobium compound.
B) The powder with a suitable binder is pressed
into a suitable form and sintered a~t elevated temperatures
in air or oxygen with a critical heating rate of at least
40C per minute maintained between ambient and 500C to
prevent loss of tantalum or niobium via vaporization
processes. ~bove 500C, more stable oxides of tantalum or
niobium and chlorine form and are less suscQptible to
vaporization, the final form being tantalum or niobium
pentoxide which i3 stable to vaporization loss throughout
the common sintering temp~rature ranges. The methods,
rAnges of sinterlng conditions, cooling rates and properties
are very similar to the first method o~ preparation except
0 that the grain size is uniform at 5 micrometers or less and
-14-
has fracture strength of greater than 18,000 p8i.
For elther o the above method~, in order to
produc~ rutile doped with tantalum or niobium or repe~table
elec~rical characteri~tics an anneal of tha caramic in a
relativ21y low oxygen pressure environment is useful.
Such a reducing treatment serves to increa~e the conductivity
ef the ~ample to valuec as high aq 5 (ohm-cm1 1 ~t room
temperature for material containing about 0.5~ tantalum and
9 (ohm-cm) at room temperature for m~t~rial contalnlng 1
atom percent tantalum. The approxlmate rang~ o~ useful
oxygen partial presiure iq from 10 5 to 10 25 atmooph~r~
(optimum being 10 10 to 10-2 ~tmcsph0r~s) with the ann~llng
temperature ranging from a~out 850C ~o ~bout 1400~C. ~t
the lower temperature of B50C, th~ ann~ ho~ld b~
out for appxoxlm~tely 3 hour~ whll~ at 1400-C ~pproxl~
1/2 hour ~hould b~ adequatQ~ ~hoge ~klll~d ln tha art ca~
ea~ily determine optimum time~ fo~ ~n~Als carria~ out ln
the middle of tha above temperatur0 rang~. A~t~r ~uch ~n
anneal, tha conductivity of the cex~mic con~lnlng from O. 1
to 3.0 atomlc p~rcent of tantalum or nioblum ln th~ t~mp~
ar~tuE~ r~nge of ambion~ to 350~ 1~ ral~tlv~ly ~n~p-n~nt
o~ temp~r~turo.
The Figure i~ a graph sho~in~ the relationship
between the resisti~i~y of the ceramic and the anne~}ing ~tep.
EXAMP~E 1
~n ord~r to produ~e 10~.28 g~am~ of Tio2 powd2r-
cont~inlng 1 p~rcont Ts catlons, 3.49 ~ram~ o~ tantalum
panta~luorid~ o~! 99~ purity on a me~ b~ d~ssolved
ln 20 ml o~ wat-r at about 25C in a poly~thylen~ b~3aker.
Th~ aolutlon i~ a~d to an ~th~l alcohol ~lurry cont~ir,ing
- --15--
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100.0 grams of reagent grade TiO2 powder in the anatase form
while constantly stirring until the slurry becomes very
viscous and nearly dryO The powder, still slightly wet, is
placed in an oven at 100C for overnight drying.
To the dried powders in added 1 ml. of a binder
composed of a solution of polyvinyl alcohol (PVA) in water.
The PVA-powder combination is ground in a mortar and pestle
until apparently well mixed. The combination is then
3creened into nearl~ round pellet form by forcing the powder-
binder co~bination through a screen of 20 wires per lineal
inch. Subsequently, the powders are poured into molds and
pressed at about 20,000 psi after which they are prefired
~o a temperature of 950C. During prefiring the elapsed
time between ambient and 350C is less than about 35 minute~.
The heating rate in this temperature range is approximately
constant; the average hèating rate i~ about 10C per minute.
The green ceramic bodies are held at 950C for approximately
3 hours and then cooled in about 4 hours to room temperature.
The prefired ceramic bodies are placed on a small
brick slab which has been coated with unpressed powders of
~he ceramic body composition and placed in a sintering
furnace at 1000C. The sintering furnace is composed of a
tubular, nonporous aluminum oxide t~be and aluminum oxide
flat shelf and electrically heated by silicon carbide rods.
The sample is introduced into the highest temperature zone
of the furnace ovex a period of 5 to 10 minutes, after which
the furnace is he~ted to a sintering temperature of 140QC
in about 2 t~ 3 hGur~ in air. The ceramic body is held at
the sintering te~perature for 3 hours and immediately
removed by pu~hing the brick sample slab to the end of the
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furnace tube and placing in ambient air to cool.
The ceramic bodies are placed upon powders of Ta
doped rutile within a tubular furnace, the ends of which are
sealed with water-cooled rubber rings. The furnace is
evacuated with a mechanical pump after which nitrogen gas
with less than 8 ppm oxygen is passed over metallic titanium
powder and through the furnace. The furnace temperature is
raised to 1200C at the point of placement of the ceramic
body and at 1050C at the point of placement of the metallic
titanium powder. The temperatures are maintained ~or
approximately 3 ho~rs before slowly cooling over 8 hours to
ambient. 1~ -
The ceramic body is sectioned for density mea~ure- -
ment by hydrostatic weighing and a section is removed for
the addition of metal electrodes required for a determination
of electrical resistivity and degradation of the metal-
ceramic interface upon exposure to oxygen at various (higher
than ambient) temperatures.
The conductivity o~ a sample treated by thi~
method will b~ approximately 9 mho per centimeter at room
temperature, increasing to 16 mho per centimeter or greater
above 300~C. The fracture strength of ceramics prepared in
this fashion, as measured by the 4 point bending test is
greater than 18,000 psi.
2S It is speculated that uniform grain size and high
~lectrical conductivity is largely due to the homogeneous
distribution of tant~lum in the rutile phase of ~iO2.
~pparently the most critical step is the addition of a
volatile ~luoride and rapid prefiring to prevent its loss
through vapori2ation.
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EXAMPLE II
In ord~r to produce 102.29 gram~ of Tio2 powder
containing, 4.5284 grams of tantalum pentachloride (TaC15) ~ -
of 99.5+~ purity on a metal~ basl~ is dissolved in 20 ml of
diethul e~ is a Pyrex beaker`'Pyrex" is a ~rade mark. ~ solution
is added to a diethyl ether slwny of 100.0 ~x~ of reagent ~b TiO2
powder in the anatase form. The combination ~lurry i9 then
stirred for several minute~ to assure mixing, Sub~equ~ntly
the slurry is poured into 5 inch diamater watch gl~s~s
from which the excess dlethyl ether i8 allowed to evaporate.
A~ no time i9 the temperature of th~ chemical sy~tom rai~ed
abov~ ambient. . ,
To the dried powder3 i8 add~d about 2 ml of ~lne
carbon powder which i8 mixed with a moxtar and p~tlo.
F~ollowing thls procedure ~bout 2 ml of ~thyl alcohol i~ i
added and mixed with a plastlc 9poon. Tho powders with the
carbon and ethyl alcohol binder axe scraen~d in~o ne~rly I .
round pellet form by forcing the powder blndar aomblna~on
through a ~creen of 20 wire~psr lineal inch. The powder3
are then poured into mold~ and praEsad at pressure~ of about
20,000 psi after whlch thoy ~r~ prefired to a temperatur~
of 950C. Dur~ng p~eflring th~ elap~d tim~ b~twe~n ~mbl~nt
and 950C is le~ than about 25 minuto~. Tha av~rage
heating rate in thi~ temperature range ~ 8 about 38C p~r
minute and great~r th~n 40C per minute between ambi~nt and
500C. The green ceramlc bodies are held at 950C for
approximately 3 hour~ and then cooled in about 4 hour~ to
room temperatu~e.
The p~sflred cera~ic bodies are placed in ~ small
br~ck slab which h~s b~en coa~ed with unpre~sed powder~ of
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the ceramic body composition and p:Laced in a sintering
furnace at 1000C. The sintering furnace is composed of a
tubular, nonporous aluminum oxide tube and aluminum oxide
flat shelf and electrically heated by silicon carbide rods.
The sample is introduced into the highest temperature zone
of the furnace over a period of S to 10 minutes, after
which the furnace is heated to the sintering temperature of
1400C in about 2 to 3 houxs in air. The ceramic body is
held at the sintering temperature for 3 hours and immediately
removed by pushing the brick sample slab to the end of the
furnace tube and placing in ambient air to cool.
The ceramic bodies are placed upon powders of Ta L:
doped rutile within a tubular furnace, the ends of which are
sealed with water-cooled rubber rings. The furnace is
evacuated with a mechanical pump after which nitrogen gas
with less than 8 ppm oxygen is passed over metallic titanium
powder and through the furnace. The furnac~ temperature is
raised to 1200C at the point o placement of the ceramic
body and at 1050C at the point of placement of the metallic
titanium powder. The temperatures are maintained for
approximately 3 hours before slowly cooling over 8 hours to
ambient.
The ceramic body is sectioned for density measure-
ment by hydrostatic weighing and microscopic examination and ~-~
a section is removed for the addition of metal electrodes
required for a determination of electrical conductivity.
The density of the ceramic body prepared in this
way is greater than 4.10 grams per cubic centimeter, the
electronic conductivity is greater than or about 9 mho per
centimeter at 25C. The very uniform grain size in a
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ceramic prepared in this fashion will be less than or about
5 micrometers and the corresponding fracture strength as
measured by the 4 point bending method is greater than
18,000 psi.
It i5 speculated that small uniform grain size and
relatively high electrical conductivity is largely due to
the homogeneous distribution of tantalum in the rutile phase
of TiO2. Apparently the most critical processing step in
the formation of TaOC13 upon decompositon of tantalum penta-
chloride diethyl operate 65C and the very rapid prefiring
to prevent its loss through vaporization.
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