Note: Descriptions are shown in the official language in which they were submitted.
11 ~87374
--1--
- m is application relates to a stabilized slurry
of Beta-type alumina powder as well as to a method of
stabilizing said slurry and to a method of spray drying
said stabilized slurry to produce a highly flowable and
fillable powder suitable for use in automatically-filled
die cavity pressing machinery, the pressed articles from
said machinery to be sinterable to dense beta-type alumina
ceramic articles of suitably low electrical resistivity.
More particularly, this application relates to a method
for preparing flowable powder which, after pressing, may
be fired to form high density ~" or ~-alumina containing
~ ceramic bodie~ suitable for use in a variety of electrical
; convers~on devices. Still more parti~ularly, this appli-
cation relates to a method for preparing flowable powders,
which, after pressing, may be fired by conventional sin-
tering techniques to form shaped, polycrystalline ~" or
~-alumina containing bodies which are ideally 3uited for
use as reaction zone separators or solid electrolytes in
certain electrica} conversion devices.
Among the polycrystalline bi- or multi-metal
oxides which are most suitable for use in electrical con-
version devices, particularly those employing molten metal
and/or molten metal 8alt~ a~ reactants, are those in the
family of Beta-aluminas, all of which exhibit a generic
crystalline structure which is readily identifiable by
X-ray diffraction. Thus, ~eta-type-alumina ~sometimes
referred to as sodium-Beta-type-alumina) is a material
which may be thought of as a series of layers o aluminum
oxide ~A1203) held apart by columns of linear Al-0-bond
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D873~
--2--
chains with sodium ions occupying sites between the afore-
mentioned layers and columns. Numerous Beta-type-alumina
polycrystalline materials exhibiting this generic
crystalline structure are disclosed in the following U.S.
Patents, 3~404,035, 3,404,036; 3,413,150; 3,446,677;
3,458,356; 3,468,709; 3,468,719; 3,475,220; 3,475,223;
3,475,225; 3,535,163, 3,719,531: and 3,811,943.
Among the numerous polycrystalline ~eta-type-
alumina materials disclosed in those patents and which
may be processed in accordance with the method of this
invention are the following:
(1) Standard Beta-type-aluminas which exhibit the
above-discus~ed crystalline structure comprising a series
of layers of aluminum oxide held apart ~y columns of
linear Al-O bond chains with sodium ions occupying sites
between the aforementioned layers and aolu~J As dis-
cussed in the aforementioned patents, Beta-type-alumina
i8 formed from compositions comprising at least about
80 percent by weight, preferably at least about 85 per-
cent by weight of aluminum oxide and between about 5 andabout 15 percent by weight, preferably between about 8
and about 11 percent by weight sodium oxide. ~here are
two well known crystalline forms of Beta-type-alumina,
both of which demonstrate the generic Beta-type-alumina
crystalline structure discussed hereinbefore and both of
which can easily be identified by their own characteristic
X-ray diffraction pattern. ~-Alumina is one crystalline
form which may be repressented by the formula Na20,11A1203.
~he second crystalline form is ~"-alumina which may be
represented by the formula Na20.6A1203. It will be noted
the the ~" crystalline form of Beta-type-alumina contains
approximately twice as much soda ~sodium oxide) per unit
weiyht of material as does the B-alumina. It i8 the
; ~"-alumina crystalline structure whi¢h is preferred for
the formation of solid electrolytes or reaction zone
separators for electrical conversion devices becauQe of
its inherent lower electrical resistivity.
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;L0873~4
3--
(2) Boron oxide B2O3 modified Beta-type-alumina
wherein about ~.1 to about 1 weight percent of boron oxide
is added to the composition. This modification of the
Beta-type-alumina is more thoroughly discussed in afore-
mentioned U S. Patent 3,404,036.
(3) Substituted Beta-type-alumina wherein the sodium
ions of the composition are replaced in part or in whole
with other positive ions which are preferably alkaline
metal ions.
(4) Beta-type-alumina which is modified by the
addition of a minor proportion by weight of a metal ion
having a valence not greater than 2 such that the modified
Beta-type-alumina composition comprises a major proportion
by weight of ions of aluminum and oxygen and a minor pro-
portion by weight of mètal ions in crystal lattice com-
bination along with cations which migrate in relation to
the crystal lattice as a result of an electric field, the
preferred em~odiment being wherein the metal ion having
a valence not greater than 2 is either lithium or mag-
nesium or a combination of lithium and magnesium. ~hesemetals may be included in the composition in the form of
lithium oxide or magnesium oxide or mixtures thcreof in
amounts ranging from about 0.1 to about 5 weight percent,
preferably from about 0.1 to about 1.5 weight percent.
This type of modified Beta-type-alumina is more thoroughly
discussed in V.S. Patent 3,475,225 and 3,535,163 mentioned
above. Such lithia and magne3ia stabilized ~-aluminas
are preferred compositions for the preparation of Beta-
type-alumina bodies demonstrating the ~" crystal structure.
me energy conversion devices for which the
bodies prepared from compacts made in accordance with this
invention are particularly useful as solid electrolytes
are disclosed in some detail in the aforementioned patents.
In the operation of such energy conversion devices, the
catiDns, such as sodium in the ~"-alumina, or some other
cation which has been substituted for sodium in part or
~ in whole, migrate in relation to the crystal lattice as
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1087374
a result of ef~ects caused by an electric field. Thus,
the solid ceramic electrolytes which may be prepared from
the dense, green ceramic compacts made by the method of
this invention are particularly suited to provide selective
cationic communication between the anodic and cathodic
reaction zonès of the energy conversion device6 and are
essentially impermeable to the fluid reactant~ employed
in the device when the reactants are in the elemental,
compoundor anionic states. Among the energy conversion
devices in which ~"-alumina containing ceramics are use-
ful are: (l) primary 4atterie~ employing electrochemically
reactive oxidants and reductants in contact and on opposite
sides of the solid electrolyte or reaction zone separators;
t2) secondary batteries employ~ng electrochemically re-
versably reactive oxidants and reauctants in contact with
and on opposite sides of the solid electrolyte or reaction
zone separator; (3) thermo-electric ge~erators wherein a
temperature and pressure differential is maintained between
; anodic and cathodic reaction zones and/or between anode and
cathode and a molten alkaline metal which is converted to
ionic form, passed through the polycrystalline ~"-alumina-
containing ceramic wall or inorganic membrane and re-
converted to elemental form; and (4~ thermally regenerated
fuel cells.
~rior art techniques for the preparation of
shaped, dense green ceramic bodies suitable for forming
~"-containing ceramic bodies have typically involved
pouring a fine powder with poor flow characteristics, this
~ powder having a composition comprising at least about 80
30 weight percent of aluminum oxide and between about 5 and
about 15 weight percent of sodium oxide, into a suitably
shaped mold and then is~statically pressing at high pres-
sure ~typically around 60,000 psi~.
In order to obtain uniform filling of the iso-
static pressing mold by the prior art powder it has been
found necessary to cause vibration of the mold during
powder addition or after powder addition is complete, or,
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873~4
preferrably, both. For acceptably dense and uniform
filling with the prior art powder a total time of mold
filling exceeding five minutes has been typically used.
The uniformity of filling is necessary becaUse thlckness
variations typically occur in the isostatically pressed
solid electrolyte or reaction zone separator green ceramic
body if the mold has been filled non-uniformly prior to
isostatic pressing. These thickness variations give rise
to corresponding regions of minimum thickness in the sin-
tered solid electrolyte or reaction zone separator part.Such regions of minimum thickness offer low resistance
paths for sodium ion conduction. Such low electrical
resistance paths may lead to premature failure of the
solid electrolyte or reaction zone separator part under
conditions of service in energy conversion devices.
The prior art process, including mold vibration,
while entirely suitable for forming high quality compacts,
is time consuming and not readily adaptable to large
scale commercial production of compacts. In order to
; 20 make the aforementio~ed electrical energy conversion
devices cost compet~tive with other electrical conversion
devices, it is, among other things, necessary to devise a
rapid and efficient method for the production of suchc~cts.
The wall thickness of solid electrolyte or re-
action zone separator parts should be large enough to give
reasonable mechanical strength to the part but small
enough to present an acceptably low resistance to sodium
ion conduction. A practical desirable wall thickness has
been found to be about 1 to 3 millimeters. This wall
thickness after sintering dictates a width for the corres-
ponding dimension of the opening of the isostatic pressing
mold of about 2.5 to about 7.5 millimeters. That this
mold opening width is typical is seen by inspection of,
for example, U.S. Patent 3,903,225. The mold opening
width dimension corresponding to wall thickness after sin-
tering given in the example in U.S. Patent 3,903,225 is
3.15 millimeters.
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` ~87374
--6--
The above considerations show the need for a
flowable powder. A flowable powder is defined for the
purposes of this patent to be one which will ~low con-
tinuously and unaided under the influence of gravity
through a 2.5 millimeter or smaller orifice in a stem-
less funnel without the application of vibration to the
funnel and without other application of energy such as
might be supplied by a current of gas This definition
of flowability is consistent in principle with that
given in Metal Powder Industries Federation Standard
No. 03, as revised 1972. It is, more particularly, con-
sistent with that given in Degussa Technical Bulletin
No. 31, "Aerosil~ for Improving the Flow ~ehavior of
Powder Substances". A series of stemless glaæs funnels
of different exit orifice diameters is described in the
latter publication. Funnels of this design, rQferred to
hereafter as sand timers, have been used to assess the
flowability of powders produced by the prior art and of
powders produced by this invention.
The flowability test using sand timers is an
i indirect measure of the frictional and cohesive forces
; acting between particles of a powder. Such forces not
only impede the flowa~ility of a powder but also limit
the mass of powder per unit volume of container which
may be achieved by adding a standard mass of powder to a
~ standard container and vibrating said container in a
-~ standard manner until the powder ceases to settle. Therate at which the powder achieves its final mass per
volume value and the final mass per volume value itself
are both useful indirect measures of the frictional and
cohesive forces between powder particles a~ to how these
forces affect the ability of a powder to effectively fill
a container rapidly. A powder which will rapidly achieve
a relatively high mass per volume of container under such
; 35 conditions is defined for the purposes of this patent as
being a fillable powder.
The prior art of spray drying as applied to
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~lS 7374
alumina ceramic powders has involved the employment of
deflocculating agents to produce deflocculated slurries.
Such prior art slurry stabilizations have typically
employed adjustment of the pH of aqueous slurries to
between 3 and 9. Deflocculants of ~he polyelectrolyte
type such as gum arabic and Darvan(R~No. 7 (R.T.
Vanderbilt Company, Inc., Norwalk, Conn.) have also
been used.
Such techniques are ineffective in the case
of Beta-type-alumina slurries due to the relatively
high pH, greater than 13, which can be produced by the
leaching of basic components from the powder into the
aqueous phase during the course of slurry preparation
and stabilization. Such very basic (caustic) conditions
make neutralization of the excess basicity impractical
due to the high percentage of acid anion which i9 required,
which would, in turn, l~wer the density of the individual
spray dried agglomerate particles by introducing excessive
voidage into them. Furthermore, the neutralization of
this high basicity speeds the leaching of further quan-
tities of ba ic components from the powder, leading to
rapid re-establishment of the original basic conditions.
Those skillea in the art to which this invention
pertain~ will readily recognize that such slurry sta-
bilization techniques which are useful for alpha-alumina
slurries would not be expected to be useful for Beta-type
alumina slurries ina6much as Bçta-type alumina slurries
are chemically distinct from alpha-alumina slurries.
Structurally, the distinction between ~età-type
alumina and other species of the generic class of aluminas
is the presence of substantial sodium ion with the A1203
primary constituent. Unfortunately, the genealogy of
nomenclature for the A1203 materials developed outside
the framework of any unified nomenclature system.
One compound of the A1203 class was first
identified as Alpha-alU~, being pure A1203 with an
hexagonal crystalline struc~ure. The later discovery
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of additional materials containing what was thought to
be a mere variance of structure of the same A1203 re-
sulted in its denomination as Beta-al~mina. Had the
discoverer known of the presence of Na in the crystalline
~- 5 matrix, the term would have more properly been a sodium
aluminate.
Sodium aluminates are formed by reacting Na20
and A1203 to form new compounds which vary in sodium
; content from Na20 - A1203 down to Na20- 11 A1203. As
would be expected with the change in molecular com-
position, the Beta-type-alumina is easily distinguished
from common alumina materials by dramatic and unpredict-
able chemical and physical variations.
Among the more pronounced differences are the
changes which occur in the liquid phas~ of the slurry
comprising water and Beta-type-alumina. A reaction
appears to result between the water and calcined Beta-
type-alumina causing a Qtrong increase in pH a~d vis-
cosity. These changes are demonstrated in the following
summary, showing the effects of dispereing agents as well.
Visco9ity is measured as stated in the subject patent
application.
TABLE A
Beta-type Alumina/Water Slurry
25 Slurry Mixture Viscosity (minutes from time of mixing
slurrY)
5 min 30 min. 60 min.
(1) H O and 40% 14.4 17.9 19,6
Beta-alumina* sec
pH = 13.1 (Act~al)
pH - 7.0 ~Calculated)
(2) Item (1) plus.
.0888 M Ta~tari~
Acid 10.5 sec 10.4 10.5
pH = 12.6 (Actual)
pH = 2.0 (Calculated)
~ ~.85% Na2O, 0.75% Li2O, 90.48% A12O3 calcined for 2 hours
at 1260C. and milled for 3 hours.
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~087374
TABLE A (CONTINUED)
.
- (3) Item ~1) plus
` .178 M HCl 12.0 sec 13.3 14.0
pH = 13.1 ~ctual)
p~= 0.75 (Calculated)
When Alpha-alumina i8 mixed with .103 M NaO~
under the conditions and measures of the corre~ponding
item number of Table~, the vi8c08ity increase during
the 60 minute period i5:
(1) 10.9 sec to 11.6 sec
(2) 11.0 sec to 11.3 sec
~3) 11.0 sec to 10.4 6ec
In addition to the minimal increase in visco~ity for
the Alpha-alumina slurry with the passing time, there
wa~ little, if any, deviation in actual pH from the
theoretically calc~lated value. These values after 60
min for the respective slurriss (in .103M NaOH)of H20,
Tartaric Acid and HCl were:
Actual pH Calculated p~
2012.6 13.0
3.8 3.8
1.7 1.1
It iS apparent that the significant change in
viscosity of the Beta-type-alumina as compared to the
alpha-alumina demonstrates the chemically uncommon char-
; acter of these materials. Furthermore, the unexpected
reaction of the Beta-type-alumina slurry causing gro3s
deviation from the calculàted pH strengthens the con-
clusion that Alpha and ~eta alumina are of a different
nature. The chemical distinctions are also evidenced
by a comparison of the X-ray diffraction patterns of the
two materials,
~ brs which are both flowable and fillable
are needed for rapid production of solid electrolyte or
reaction zone separator green bodie~ with uniform wall
thicknesses by isostatic pressing. The action of auto-
matic isostatic pressing machinery involves allowing the
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iL~87374
--10--
powder to flow under the influence of gravity to the
isostatic pressing mold. This mold must be filled rapidly
and uniformly with very little vibration, or, more usually,
no vibration, Beta-type-alumina powders produced by the
prior art, involving no addition of binders or addition
of binders by means other than spray drying, have ex-
hibited poor characteristics of flowability and fillability.
Such prior art powders are found to have rough surface
textures and to contain large proportions of fine material.
These surface and size characteristics and others believed
to contribute to effectively large cohesive and frictional
forces between powder partioles make the prior art powders
unsuitable for rapid isostatic pressing and especially
unsuita~le for automatic isostatic pressing. It i8
believed, therefore, that a method for producing a highly
flowable and fillable powder suitable for use in auto-
matically-filled die cavity pressing machinery, the
pressed articles from said machinery to be sinterable
to dense Beta-type-alumina ceramic articles of suitably
low electrical resistivity and suitably high uniformity
of wall thickness would be an advancement in the art.
It is therefore an object of this invention
to prepare a stabilized slurry cf Beta-type alumina
powder which may be employed in a spray drying proces~
to produce minute spherical agglomerates of said Beta-
type alumina.
It is a further object of this invention to
provide a method for producing a highly flowable and
fillable powder consisting predominately of ~ and/or
~"-alumina suitable for use in automatically-filled die
cavity pressing machinery, the pressed articles from said
machinery to be sinterable to dense Beta-type-alumina
ceramic articles of suitably low electrical resistivity
and suitably high uniformity of wall thickness for use
as solid electrolytes or reaction zone separators in
energy conversion devices.
These and various other objects, features and
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~108737~
--ll--
advantages of the invention will be better understood
from the following description taken in connection with
the accompanying drawings and photographs in which:
Figure 1 is a photograph of spherical spray
dried powder particles of said invention,
Figure 2 iQ a plot of the volume of a four
gram mass of two powders versus time which indicates
the improved fillability characteristics of powders pre-
pared under said invention compared to those prepared
by prior art techniques.
The invention sought to be protected herein
comprises in part a stabilized slurry of Beta-type
alumina material and a method for stabilizing the same,
said stabilized slurry having a prolonged low to moderate
viscosity consistent with ceramic formation requirements
in processes, such as spray drying, where the visc~sity
of conventional beta-type alumina slurries is too high
to yield a wor~able slurry for the particular process,
In accordance with the invention the slurry comprises:
(a) A liquid phase constituent suitable for sus-
pending a Beta-type alumina powder without causing sub-
stantial adverse affect on chemical utility of said
powder as a Beta-type alumina ceramic forming material;
(b) Beta-type alumina powder which has been
calcined and appropriately deagglomerated and mixed in
said liquid phase to form a finely dispersed slurry
having a concentration of said powder in the range of
about 30 to about 70 weight percent; and
(c) sufficient dispersing agents to stabilize said
slurry to desired low to moderate viscosity for a period
of time adequate to enable the desired process to be
accomplished, said dispersing agent being seleated from
the class of polycarboxylic acids containing 3 or more
carbon atoms and being capable of chelating aluminum ion.
The Beta-type aIum~na included in the slurry
has a composition as discussed generally hereinbefore.
Thus, generally the composition comprises from about 0.1
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:~87374
-12-
to about 5 weight percent lithium oxide, from about 5
to about 15 weight percent sodium oxide, and a balance
of aluminum oxide.
Another embodiment of this invention relates
to the spray drying of the above discussed Beta-type
alumina slurries to form a highly flowable and fillable
powder consisting largely of smooth spherical part~cles,
It has been discovered that when preparing
slurries of seta-type alumina in accordance with this
invention polycarboxylic acids containing at least 3
carbon atoms and being capable of chelating aluminum
ion when employed in amounts ranging from at least about
0.010 mole acid function per 100 grams of the ceramic
powder, preferably in amounts ranging from about 0.010
to about 0.200 and most preferably in the range of 0.025
to 0.100 mole acid function per 100 gram~ of ceramic
powder, will depress the slurry viscosity by 2-3 times
for one to three hours,
A~ discussed above the polycarboxylic acids
useful as stabilizing agents in the slurries of this in-
vention contain at least 3 carbon atoms and must be
capable of chelating aluminum ion. It should be re-
cognized that the chelating nature of the polycarboxylic
acid is merely a parameter which defines the stabilizing
agent and that such chelating function does not necessarily
occur in the stabilized slurry of the invention. Although
not i~tended to be limiting, the polycarboxylic acids
found to be useful in the invention are those aluminum
ion chelating agents containing 3 to 15, preferably 3 to
8 carbon atoms. m ese polycarboxylic acids include both
aliphatic polycarboxylic acids, saturated or unsaturated,
as well as olefinic and are preferably hydroxy functional,
i.e., bear,at least one hydroxyl group, wi~h the most
preferred agents being mono-or di-hydr~xy functional poly-
carboxylic acids. Among the polycarboxylic acids whichfall within this general scope are tartaric acid, citric
acid, tartronic acid, malic acid, and ~
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~0~737~
-13-
y-trihydroxyglutaric acid. An example of a polycarboxylic
acid chelating agent useful in the invention and con-
taining greater than 15 carbons is polya~rylic acid
having a number average molecular weight ~Mn) between
4000 and 6000.
The most preferred polycarb~xylic acids falling
within the class discussed above are the hyd~oxy func-
tional (preferably mono-or ai hydroxy) saturated aliphatic
polycarboxylic acids capable of chelating aluminum. of
these acids the most preferred are citric and tartaric
acids.
While the described acid chelating agents serve
to reduce viscosity as discussed above, if the same
~uantity of nonchelating acid such as hydroxchloric or
}5 acetic acid is added, the viscosity is not depressed
significantly ~or several hours as reguired. ~his con-
trast between chelating and nonchelati~g acids was un-
expected, especially in view of the fact that the amount
of acid added was generally only sufficient to lower
the pH of the slurry to around ll.
It will be understood by those skilled in the
art that the other agents added to the slurry also help
to control the viscosity. It is unexpected, however,
that such specific chelating acids for aluminum as for
; 25 example tartaric and citric acids would have such a
dramatic effect in lowering the slurry viscosity and thus
making possible a more concentrated slurry. Such a con-
centrated slurry is advantageous in at least three
respects: 1) it lowers the oost of spray drying a given
quantity of powder since such cost is dependent on the
amount of water which must be evaporated per unit weight
of powder, 2) it increases the rate at which the powder
may be produced, and 3) it increases the density of each
individual spray dried agglomerate particle leading, in
turn, to a more fillable powder as defined above.
It will be understood by those skilled in the
art that the slurry viscosity is to be adjusted to some
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~l087374
-14-
convenient value sufficiently high to retard settling
out of the suspended particles during pumping from the
feed tank to the atomization device and sufficiently low
to allow convenient pumping from the feed tank to the
atomization device. I~ will also be understood that the
slurry viscosity and surface tension may be altered to
obtain optimum spray dried powder particle size dis-
tribution and optimally smooth-surfaced, spherical spray
dried powder particles.
The aspect of the invention as it relates to
obtaining a stable slurry of Beta-type-alumina powder
suitable for spray drying will now be further described
by way of example.
EXAMPLE 1
A powder of 3eta-type-alumina was prepared'
by firing an intimate mixture consisting of
Li2O~as LiNO3) - 0.7~
Na2O ~ag Na2CO3) - 8.7%
A12O3 - balance
at 1255C ~or 2 hours and the deagglomerating the same
by dry milling for 1 hour in a point shaker with Lucalox
balls. Several samples of this preparation of ~eta-
type-alumina were slurried in water at a concentration
of 52 percent by weight powder. Other constituents of
each slurry were present in the following amounts ex-
pressed as percents by weight of the Beta-type-alumina
powder: 2.8% polyvinyl alcohol and a trace of 2-octanol.
In addition to the above constituents, amount~ of various
acids equivalent to 0.02S mole acid function per 100g
powder were added. Each slurry was made as ~uickly as
possible with the binder constituent ~polyvinyl alcohol)
being the last added. Viscosities and pH values of each
slurry were obtained periodically. Viscositie~ are re-
ported as seconds taken for a standard amount of slurry,
about 10 milliliters, to flow under the influence of
gravity out of a standard vessel hav,ing an orifice of
about 1 millimeter, ~he results are shown in Table 1.
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~L~87374
-15-
The viscosity results obtained at 50 minutes remain sub-
stantially the same for up to three hours. The results
clearly sh~w a beneficial effect due to tartaric acid
as opposed to no acid or the non-chelating acid~
hydrochloric and acetic.
EXAMP~E II
The results of a similar experimental series
using a slurry concentration of 57 weight percent of the
Beta-type-alumina prepared as in Example I are shown in
Table II. The benefits of the chelating acids tartaric
and citric are clearly evident.
To further illustrate the subject invention,
the following detailed example of the preferred embodi-
ments is presented.. All percentages are by weight based
on total slurry weight Qr weight of Beta-type-alumina
powder as in the preceding example. All o~ percentages
are by weight based on total weight.
TABLE 1
52~ powder, 0.025 mole acid/lOOg powder
Time from
Binder Addition Viscosity
Acid lmin) pH ~ec)
; None 5 12.15 19
SO 12.51 25
25 dl-Tartaric 5 11.52 16
11.54 16
HCl 5 11.05 23
ll.S7 20
Acetic 5 9.85 21
SO 11.58 22
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` ~08737~
-16-
TA~LE II
57% powder, 0.025 mole acid/lOOg powder
Time from
Binder Addition Viscosity
5 Acid (min) E~ ~sec)
dl-Tartaric 5 11.42 37
11.57 33
Citric S 11.20 45
11.82 40
HCl 5 10.10 95
11.69 50
Acetic 5 9.83 1~7
11.65 74
EXAMPLE III
A powder of Beta-type-alumina was prepared by
firing an intimate mix consisting of: Li20 ~as LiN03) -0.7%
Na20 ~as Na2C03)--8 7
A1203 - balance
The mix was fired at 1250C. for 2 hours and then
deaggl~merated ~y sha~ing 500 grams of the fire~ mix 50
high purity aluminum oxide sphere6 of 1 cm diameter.
Flowability of this and subsequent powders was as6essed
by determining whether the powder would flow through sand
timers having the various orifice sizes 18, 12, 8, 5 and
2.5 millimeters as described previously. The smallest
orifice size, in millimeters, through which a given
powder will flow is defined as the flowability index. If
the powder will not flow through any of the aforementioned
~ sand timers it is assigned the flowability index "no flow".
; 30 The fired mix powaer, after deagglomeration,
was characterized as follows:
particle shape - irregular
particle surface - rough
approximate means size - 6~m ~by weight, Coulter
Counter)
flowability index - no flow
The fired mix powder was spray d~ied from an
aqueous slurry of 54.5~ fired mix powder, 2.8~ polyvinyl
. ~
108737~
-17-
alcohol, 1.2% polyethylene glycol, 2.1% tartaric acid,
and a trace of 2-octanol, The spray dryer used was of
the 2-fluid, concurrent type. The resulting spray dried
powder is shown in Figure 1. This powder was char-
acterized as follows:shape - predominately spherical
surface - smooth
approximate means size - lOO~m (by number,
microscopically)
flowability index - 2.5
By way of contrast, a sample of fired mix
powder which had been coated with 1% polyvinylbutyral
binder by evaporation of a slurry of the powder in an
acetone binder solution was characterized as follows:
shape - irregular
surface - rough
approximate mean size - 15~m (by number, micro-
scopically)
flowability index - 18
The last described powder is typical of the prior art.
The fillability, as previously defined, of
prior art powder and of powder made according to the
present invention was determined by noting the volume
of a bed of 4.0g of powder in a graduated volumetric
cylinder of lOcm total volume both after filling and
after application of a reproducible frequency and
amplitude of vibration to the cylinder. The results
for the two powders are shown in Figure 2. The powder
ma~e according to this invention is clearly superior in
fillability both initially and after any given time of
vibration.
Powder made according to the present invention
has been filled into an is~static pre~sing mold within
10 seconds, i~ostatically pressed at 55,000 psi, prefired
to 650C. to eliminate volatile organic constituents, and
sintered for 30 minutes at 1620C. to obtain a uniform-
walled body with density 96.7% of theoretical and an
,. . . - . . , ., .,
~ .; ... , . :
-l87374
electrical (sodium ion conduction) resistivity of 3.5
ohm-cm. (~t 3Q0~.)
.
EXAMPLE IV
The technique of slurry spray drying was also
extended to powder in which zeta ~lithium) alumina
(Li20.SA1203) is substituted for ~iNo3 as the source of
Li20. The stabilized slurry contained 46.5% by weight
of a powder in which the lithia-alumina ratio was 1 to
6 (i.e. Li20.5A1203 plus some excess A1203) plus
Na20.5A1203 soda component in a ratio tc give a nominal
9.0% Na20-0.8% Li20-90.2% A1203 by weight composition.
The ~lurry was stabilized with 2.1% tartaric acid, 1.2~
polyethylene glycol and 2.8% polyvinyl alcohol, all based
on the powder. The spray dried powder possQssed excellent
flowability characteristics ~index = 2.5) suitable for
i~ostatic pressing. This material, after isostatic
pre~sing at 55 kpsi, could be sintered to 2 density of
97% (1620C. - 30 min) and an electrical resistlvity
for sodium ion conduction of 4.3 ohm-cm ~300~C~.
While there has been shown and described what
are at present consi~ered the preferred embodiments of
the invention, it will be o~vious to those skilled in
the art that various changes and modifications may-be
made therein without departing from the scope o~ the
invention as defined by the appended claims. Likewise,
the slurry application defined herein is merely exemplary
of numerous conventional processes which require a
regulated viscosity within the defined range.
"
