Beta-alumina composites and methods for producing them

A hot-pressed beta-alumina composition is provided which consists essentially of M.sub.2 O.nAl.sub.2 O.sub.3, wherein n is a positive integer between about 3 and about 12 and M is selected from Na and K. The new beta-alumina composition features a flexural strength of at least about 45,000 psi as measured by ACMA Test No. 2, and high transmissability to light. Methods for preparing the above-described beta-alumina composition are also provided.

FIELD OF THE INVENTION 
This invention relates to improved ionically conductive crystalline 
beta-alumina compositions and methods for preparing them. More 
specifically, the invention relates to a method for producing ionically 
conductive crystalline beta-alumina from a finely divided amorphous 
powder. 
PRIOR ART 
Beta-alumina is a well-known commercially available material which has 
found widespread use as a cation conductor in devices which are 
electrically or electrochemically actuated, and which is particularly 
useful for forming half-cell separators in batteries employing a molten 
alkali as a reactant. Crystalline beta-alumina has a hexagonal type 
structure consisting of spinel blocks of oxygen in which the aluminum is 
situated in the same positions as magnesium and aluminum in a magnesium 
aluminate spinel. The spinel blocks are separated by a NaO mirror plane. 
The distance between the two oxygen mirror planes distinguishes 
.beta.-alumina from .beta.'-alumina. In .beta.-alumina, the distance 
between the mirror planes is 11.23 A; in .beta."-alumina the distance is 
about doubled. 
Beta alumina is generally prepared commercially by heating an appropriate 
mixture of sodium carbonate and aluminum oxide to somewhere between 
1550.degree. and 1800.degree. C. In U.S. Pat. No. 3,475,225, issued to 
G.J. Tennenhouse, typical temperatures of about 1700.degree. C for 
sintering mixtures of sodium and aluminum oxides are reduced to 
temperatures between 1000.degree. and 1600.degree. C by using pressures 
ranging from about 5000 psi to about 110,000 psi. U.S. Pat. Nos. 3,131,238 
and 3,437,724 are typical of disclosures of hot-pressing techniques for 
forming crystalline compositions. However, none of these are used to form 
beta-alumina. 
The present invention results in improved crystalline beta-alumina which is 
produced at temperatures and pressures which are substantially lower than 
those employed in the prior art. In addition, the method of this invention 
utilizes standard equipment and readily available materials in the 
production of the crystalline beta-alumina which has improved physical 
characteristics when compared with beta-alumina produced by prior art 
methods. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a hot pressed beta-alumina composition 
is provided which consists essentially of M.sub.2 O.multidot.nAl.sub.2 
O.sub.3, wherein n is a positive integer between about 3 and about 12 and 
M is an alkali metal selected from Na and K. The novel beta-alumina 
composition of this invention features the following physical properties; 
ionic conductivity, high theoretical density, and a flexural strength of 
at least about 45,000 psi when measured by ACMA Test No. 2. The 
beta-alumina compositions of this invention also have a high 
transmissability to light. 
The ionically conductive crystalline beta-alumina composition of this 
invention is prepared and formed into a composite by a method which 
comprises: 
mixing and reacting together a solution aluminum alcoholate and an aqueous 
solution of an alkali metal salt; 
coprecipitating as a gelatinous mass aluminum hydroxide and the alkali 
metal salt; 
drying the gelatinous mass coprecipitated in the previous step; 
grinding the dried gelatinous mass into a fine amorphous powder; and 
hot pressing the resulting powder to yield a beta-alumina composite. 
In a preferred embodiment, this invention provides a hot-pressed ionically 
conductive crystalline beta-alumina composition consisting essentially of 
Na.sub.2 O.multidot.5Al.sub.2 O.sub.3. The hot pressed Na.sub.2 
O.multidot.5Al.sub.2 O.sub.3 beta-alumina composite has essentially 
theoretical density and is nearly transparent. 
Beta-alumina composites produced in accordance with the teachings of this 
invention possess the advantage that standard hot pressing equipment can 
be used to attain the temperatures and pressures required, thus 
eliminating the need for specially designed apparatus capable of 
withstanding the high temperatures and pressures necessary for the methods 
of the prior art.

DETAILED DESCRIPTION OF THE INVENTION 
The beta-alumina composites of this invention are made by vacuum hot 
pressing a finely divided homogeneous beta-alumina powder. The finely 
divided homogeneous beta-alumina powder is prepared by first 
coprecipitating a mixture of hydrated aluminum oxide and an alkali metal 
salt from solutions of components which, when heated, give rise to a 
volatile by-product. To a solution of an aluminum alcoholate is added an 
aqueous solution of the alkali metal salt. A gelatinous precipitate of 
hydrated aluminum oxide and the alkali metal salt results. After 
separating the gelatinous precipitate from the supernatent liquid and 
drying it, the dried precipitate is heated at a temperature of at least 
about 400.degree. C but which is less than about 1200.degree. C, for a 
time period of from 1 to 16 hours, thus removing undesired volatiles. The 
temperature and length of time used in any particular application is 
dependent upon the particular alkali metal salt being used. The resulting 
amorphous material is then ground to a fine powder. The powder should be 
ground to a particle size of less than about 50 micrometers, and 
preferably less than 1 micrometer or submicron. 
Various alkali metal salts can be used in the above-described process 
including, for example, the bicarbonate, acetate, hydroxide, nitrate and 
carbonate of sodium and potassium. The beta-alumina powders thus produced 
have the formula M.sub.2 O.multidot.nAl.sub.2 O.sub.3 wherein M is the 
alkali metal ion and n is an integer from about 3 to about 12 depending 
upon the ratio of reactants used. 
Before hot pressing, the powder can be placed in a pressing cylinder and 
cold pressed at a pressure of at least about 4000 psi and preferably in 
the range of from about 4000 to about 16,000 psi. This insures intimate 
contact between the powders and prevents vacuum removal of loose powder. 
Whether cold pressed or not, the powder is then placed in the pressing 
apparatus, and the apparatus is assembled and connected to a vacuum 
system. Thereafter the powder is heated to a first temperature of from 
about 1150.degree. C to about 1400.degree. C, and preferably at least 
about 1200.degree. C, while a vacuum is drawn and maintained. Upon 
reaching the selected temperature, an initial pressure of at least about 
4000 psi is applied while maintaining the vacuum. This pressure is 
maintained during a holding period, during which the powder can be further 
heated until it reaches a second temperature slightly higher than the 
first temperature by about 100.degree. C. The initial pressure is held for 
a time period of at least about 5 minutes, preferably at least about 10 
minutes. The pressure is then increased to a higher pressure of at least 
about 20,000 psi, advantageously at least about 25,000 psi. The compressed 
powder is maintained under vacuum at this temperature and pressure for a 
length of time of at least about 10 minutes, preferably at least about 20 
minutes. Then the compressed powder is cooled to a temperature below about 
1150.degree. C and preferably to about 1000.degree. C whereupon the vacuum 
is released and the pressing cylinder is backfilled with nitrogen. The 
compressed powder is then cooled further and the pressure is released, 
yielding an ionically conductive crystalline beta-alumina composite. 
Preferably the temperature is cooled to about 800.degree. C before the 
pressure is released. It should be noted that the particular temperatures, 
pressures, and time periods used for the hot pressing process are 
generally dependent upon the composition of the powder and the amount of 
powder being hot pressed. 
In another embodiment of the above-described process, after the compressed 
powder is heated to the first temperature, preferably about 1200.degree. 
C, and the initial pressure is applied, the compressed powder is further 
heated to a higher temperature, for example 1300.degree. C, whereupon the 
process is continued as described above. 
The beta-alumina composites produced in accordance with the teachings of 
this invention exhibit spinel crystal structure as determined by X-ray 
diffraction patterns. Although hot pressed from an amorphous powder, all 
compositions produced ionically conductive crystalline composites having 
conductivities in the range of from about the order of 10.sup.-3 to about 
the order of 10.sup.-6 (ohm-cm).sup.-1. The beta-alumina composites of 
this invention further exhibited a flexural strength of at least about 
45,000 psi as measured by ACMA Test No. 2 and a high transmissability to 
light. More specifically, the transmission of incident radiation is at 
least about 70% when the wavenumber of the incident radiation is between 
about 2200 and about 3800 cm.sup.-1. The density of the sample is also 
high, approaching that of the calculated or theoretical density for a 
single crystal of beta-alumina having the same chemical composition. 
Specific examples have a density at least as high as 93% that of 
theoretical. 
The invention will be further illustrated by the following examples. 
EXAMPLE 1 
In a 4 liter beaker, 408.4 g (2.0 mole) aluminum isopropoxide was dissolved 
in a mixture of 1750 ml of benzene and 850 ml of isopropanol. To this was 
added, with stirring, a solution containing 17.6 g (0.44 mole) sodium 
hydroxide dissolved in 500 ml water. The resulting gel was stirred for 5 
minutes, allowed to stand for 1 hour, then filtered by suction and dried 
at 180.degree. C for about 16 hours (overnight). The easily friable 
amorphous product weighed 191 g. 
Thirty grams of this dried gel, contained in a fused alumina crucible, was 
placed in a muffle furnace at 400.degree. C. The furnace temperature was 
raised to 900.degree. C and the mixture held at 900.degree. C for 4 hours. 
An amorphous beta-alumina having the composition Na.sub.2 
O.multidot.4Al.sub.2 O.sub.3 was produced. After cooling, the amorphous 
material was ground for 2 hours with a mullite mortar and pestle, using a 
Fisher Automatic Mortar Grinder. The material was then ready for hot 
pressing. 
It should be noted that about 10 percent excess sodium hydroxide was used 
in Example 1 because it was found that, when using the hydroxide of an 
alkali metal, approximately 10 percent of the hydroxide does not 
precipitate but remains in the filtrate. 
EXAMPLE 2 
Beta-alumina powder prepared according to the method described in Example 1 
was sieved through 270 mesh (U.S. Standard), and placed in the pressing 
cylinder between pyrolytic graphite discs and cold pressed at 5000 psi. 
The cylinder containing the powder was then placed in the pressing 
apparatus, and the apparatus was assembled and connected to a vacuum 
system. 
The powder was heated to 1200.degree. C for a time period of about 30 
minutes while the vacuum was maintained below 150 microns. No pressure was 
applied until the powder reached 1200.degree. C, at which time 4000 psi 
was applied. Heating continued until the temperature reached 1300.degree. 
C. The powder was then held at 1300.degree. C and 4000 psi for 10 minutes. 
The pressure was next increased to about 25,000 psi and held for 20 
minutes while the temperature remained at about 1300.degree. C. The heat 
was shut off and when the temperature had cooled to about 1000.degree. C 
the vacuum was shut off and the apparatus backfilled with N.sub.2. When 
the temperature was approximately 800.degree. C the applied load was 
removed. The apparatus was disassembled when the temperature approached 
ambient, and the pressed Na.sub.2 O.multidot.5Al.sub.2 O.sub.3 disc was 
then removed from the pressing cylinder. 
EXAMPLE 3 
In a manner similar to that of Example 1, beta-alumina powders were 
prepared using sodium bicarbonate, sodium acetate, sodium nitrate, 
potassium carbonate and potassium hydroxide. Except when using potassium 
hydroxide, the alkali metal salts were mixed in the exact proportion in 
which they were desired in the final product. When using potassium 
hydroxide, 10 percent excess hydroxide was used as in Example 1. 
EXAMPLE 4 
Sample Na.sub.2 O.multidot.5Al.sub.2 O.sub.3 discs were prepared according 
to the methods described in Examples 1 and 2 except using sodium carbonate 
as the alkali metal salt and hot pressing at a maximum pressure of 20,000 
psi. The resulting composite discs were then physically characterized. 
The infrared spectra were measured using a Beckman 21A spectrophotometer. 
The density of the samples was determined by a hydrostatic weighing 
technique. The Knoop hardness was measured using a Tukon testing machine 
with a 400-g load. The coefficient of thermal expansion was obtained using 
a Leitz dilatometer on samples 10 mm long .times. 3 mm square. The modulus 
of rupture was calculated from loads measured on an Instron testing 
machine by the procedure described in ACMA Test No. 2. Samples 0.18 cm 
square .times. 2.54 cm long were tested using a three-point bending 
fixture with a 1.8 cm span. A head speed of 0.13 cm/min was used. Tests 
were run both parallel and perpendicular to the pressing direction. The 
fracture surfaces were examined using a scanning electron microscope with 
the sample positioned at 45.degree.. 
A typical infrared transmission spectrum is shown in FIG. 1, for a 
thickness of 1.5 mm. As can be seen, the sample is free of impurity 
absorption bands frequently found in single crystals that have been 
exposed to moist air. Note particularly the absence of D(OH) absorption at 
3100 cm.sup.-1. X-ray diffraction analysis was made both parallel and 
perpendicular to the direction of hot pressing. FIG. 2A shows a typical 
X-ray diffraction pattern parallel to the direction of hot pressing and 
FIG. 2B shows a typical X-ray diffraction pattern perpendicular to the 
direction of hot pressing. Enhancement of the refraction peaks as seen in 
FIG. 2B indicate a preferred crystalline orientation. 
Other physical properties of the samples are tabulated below in Tables 1 
and 2. 
Table 1 
______________________________________ 
Properties of Na.sub.2 O . 5Al.sub.2 O.sub.3 
Knoop hardness 1120-1200 400-g load 
Coeff. of expansion 
7.03 .times. 10.sup.-6 
25-800.degree. C 
in./in./.degree. C 
Density 3.257 g/cc 
Ionic conductivity 
7.5 .times. 10.sup.-4 
at 25.degree. C 
(ohm cm) (parallel 
face) 
______________________________________ 
Table 2 
______________________________________ 
Modulus of Rupture (Flexural Strength) 
Load applied to surface 
Load applied to surface 
parallel in direction 
perpendicular in direction 
of pressing of pressing 
______________________________________ 
Sample No. Sample No. 
1 53,636 psi 2 58.252 psi 
4 46,120 3 58,463 
5 46,120 9 56,820 
6 52,075 
______________________________________ 
.3 
After the pressed discs of Na.sub.2 O.multidot.5Al.sub.2 O.sub.3 were 
ground and polished, they were highly translucent to visible light. In 
fact, they were nearly transparent in that overhead lights could clearly 
be seen through them. 
EXAMPLE 5 
Na.sub.2 O.multidot.nAl.sub.2 O.sub.3 powders wherein n=4, 6 and 11 were 
produced in a manner similar to to Example 1 except that after drying, 
samples were heated at 400.degree., 900.degree. and 1200.degree. C for 
various lengths of time before grinding and hot pressing. All powders made 
by heating at 400.degree. or 900.degree. C were amorphous. The 
hot-pressing steps of Example 2 were followed to make composites for 
measuring physical properties, except that the temperature was not 
increased above 1200.degree. C during the application of pressure. The 
results are tabulated below in Tables 3, 4 and 5. 
Table 3 
__________________________________________________________________________ 
Na.sub.2 O : 4Al.sub.2 O.sub.3 
Particle 
Time 
Temp. 
.sigma., (ohm-cm).sup.-1 
% Theor. BET Size 
Hrs. 
.degree.C 
Dry Wet Dens. 
Al/Na 
M.sup.2 /g 
.mu.m 
__________________________________________________________________________ 
1 400 1.7 .times. 10.sup.-4 
3.4 .times. 10.sup.-4 
98.3 3.90 62 0.03 
3 " 1.9 .times. 1.sup.-4 
3.2 .times. 10.sup.-4 
98.4 4.06 86 0.02 
8 " 3.5 .times. 10.sup.-4 
2.8 .times. 10.sup.-4 
97.6 4.13 83 0.02 
16 " 1.4 .times. 10.sup.-4 
3.1 .times. 10.sup.-4 
98.1 4.26 73 0.03 
1 900 1.6 .times. 10.sup.-4 
3.1 .times. -4 
98.2 4.12 41 0.05 
3 " 3.1 .times. 10.sup.-4 
2.4 .times. 10.sup.-4 
98.7 4.06 39 0.05 
8 " 2.1 .times. 10.sup.-4 
2.3 .times. 10.sup. -4 
98.2 4.16 30 0.06 
16 " 1.9 .times. 10.sup.-4 
2.2 .times. 10.sup.-4 
97.3 4.11 23 0.08 
1 1200 
1.7 .times. 10.sup.-4 
1.7 .times. 10.sup.-4 
95.7 4.23 3 0.62 
3 " 1.1 .times. 10.sup.-4 
2.7 .times. 10.sup.-4 
96.9 4.30 4 0.46 
8 " 1.4 .times. 10.sup.-4 
2.9 .times. 10.sup.-4 
96.9 3.80 7 0.26 
16 " 8 .times. 10.sup.-5 
2 .times. 10.sup.-4 
-- 4.31 10 0.19 
__________________________________________________________________________ 
Table 4 
__________________________________________________________________________ 
Na.sub.2 O:6Al.sub.2 O.sub.3 
Particle 
Time 
Temp 
.sigma., (ohm-cm).sup.-1 
% Theor. 
BET Size 
Hrs 
.degree.C 
Dry Wet Dens. 
Al/Na 
M.sup.2 /g 
.mu.m 
__________________________________________________________________________ 
1 400 1.6 .times. 10.sup.-4 
3.0 .times. 10.sup.-4 
97.3 5.67 95 0.02 
3 " 1.7 .times. 10.sup.-4 
3.7 .times. 10.sup.-4 
98.2 5.58 93 0.02 
8 " 2.7 .times. 10.sup.-4 
3.17 .times. 10.sup.-4 
97.8 5.46 96 0.02 
16 " 2.3 .times. 10.sup.-4 
3.2 .times. 10.sup.-4 
97.2 5.77 110 0.02 
1 900 2 .times. 10.sup.-4 
3.1 .times. 10.sup.-4 
97.8 5.78 62 0.03 
3 " 1.7 .times. 10.sup.-4 
1.9 .times. 10.sup.-4 
98.9 5.77 49 0.04 
8 " 1.5 .times. 10.sup.-4 
8.6 .times. 10.sup. -5 
96.2 5.90 51 0.04 
16 " 1.9 .times. 10.sup.-4 
1 .times. 10.sup.-4 
95.6 5.81 45 0.04 
1 1200 
5.9 .times. 10.sup.-5 
1.7 .times. 10.sup.-4 
95.4 5.76 6 0.31 
3 " 1.1 .times. 10.sup.-5 
2.7 .times. 10.sup.-4 
97.0 5.95 7 0.26 
8 " 1.5 .times. 10.sup.-4 
2.2 .times. 10.sup.-4 
96.4 5.60 6 0.31 
16 " 8.8 .times.10.sup.-5 
1.4 .times. 10.sup.-4 
95.2 6.41 6 0.31 
__________________________________________________________________________ 
Table 5 
__________________________________________________________________________ 
Na.sub.2 O:11Al.sub.2 O.sub.3 
Particle 
Time 
Temp 
.sigma., (ohm-cm).sup.-1 
BET Size 
Hrs. 
.degree.C 
Dry Wet Dens. 
Al/Na 
M.sup.2 /g 
.mu.m 
__________________________________________________________________________ 
1 400 2.7 .times. 10.sup.-4 
1.51 .times. 10.sup.31 5 
96.9 10.25 
167 0.01 
3 " 1.5 .times. 10.sup.-5 
1.6 .times.10.sup.-5 
96.2 10.20 
175 0.01 
8 " 3.8 .times. 10.sup.-5 
1.2 .times. 10.sup.-5 
97.4 11.17 
167 0.01 
16 " 6.7 .times. 10.sup.-5 
5.3 .times. 10.sup.-5 
93.0 10.89 
173 0.01 
1 900 1.0 .times. 10.sup.-5 
3.1 .times. 10.sup.-5 
98.4 11.48 
116 0.02 
3 " 8.3 .times. 10.sup.-6 
1 .times. 10.sup.-5 
97.4 10.79 
106 0.02 
8 " 8.9 .times. 10.sup.-5 
1.1 .times. 10.sup.-5 
96.2 10.72 
87 0.02 
16 " 8.5 .times. 10.sup.-6 
1.2 .times. 10.sup.-5 
96.6 10.84 
91 0.02 
1 1200 
7.4 .times. 10.sup.-6 
3.5 .times. 10.sup.-5 
97.6 10.93 
24 0.08 
3 " 6.1 .times. 10.sup.-6 
2.9 .times. 10.sup.-6 
93.0 11.80 
8 0.23 
8 " 9 .times. 10.sup.-6 
2 .times. 10.sup.-6 
92.8 10.68 
9 0.20 
16 " 1.3 .times. 10.sup.-5 
1.2 .times. 10.sup.-5 
94.7 11.46 
8 0.23 
__________________________________________________________________________ 
EXAMPLE 6 
Na.sub.2 O.multidot.nAl.sub.2 O.sub.3 powders wherein n was varied from 3 
to 12 were produced in a manner similar to that of Example 1. After hot 
pressing at 1200.degree. C, the conductivity of the resulting beta-alumina 
composite was measured and its X-ray diffraction pattern was examined. The 
results are tabulated below in Table 6. 
Table 6 
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Na.sub.2 O . Al.sub.2 O.sub.3 
Hot-Pressed at 1200.degree. C 
(Dry) X-ray diffraction 
Na.sub.2 O:Al.sub.2 O.sub.3 
(ohm-cm).sup.-1 
.alpha. .beta. 
______________________________________ 
1:3 1.6 .times.10.sup.-4 X 
1:4 1.7 .times. 10.sup.-4 X 
1:5 5 .times. 10.sup.-4 X 
1:6 5 .times. 10.sup.-4 X 
1:7 3.2 .times. 10.sup.-5 
X X 
1:8 1.6 .times. 10.sup.-5 
X X 
1:9 1.6 .times. 10.sup.-5 
X X 
1:10 6 .times. 10.sup.-5 
X X 
1:11 1 .times. 10.sup.-5 
X X 
1:12 2.8 .times. 10.sup.-5 
X X 
______________________________________ 
This invention has been fully disclosed with particular references to the 
preferred embodiments thereof. However, it is understood that variations 
and modifications can be made without departing from the spirit and scope 
of this invention.