Magnesia-based beta alumina sintered bodies and a process for producing the same

A magnesia-based beta alumina sintered body having the following features: (1) the average crystalline particle diameter of beta alumina crystals as calculated assuming that the beta alumina crystals are of a circular section is in a range of 1-4 .mu.m; (2) the amount of the beta alumina crystals having particle diameters not more than 5 .mu.m is 85% to 98% when measured in a plane; (3) the maximum crystalline particle diameter is not more than 300 .mu.m, and the number of coarse particles having diameters falling in a range of 100 .mu.m to 300 .mu.m is not more than 1 as counted in an area of 10 mm.times.10 mm; and (4) the content of crystals of sodium aluminate is 0.5 wt % to 6.0 wt %. A process for producing such a magnesia-based beta alumina sintered body is also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to magnesia-based beta alumina sintered 
bodies and a process for producing the same. Such a magnesia-based beta 
alumina sintered body is utilized as a solid electrolyte to be used in 
sodium-sulfur cells. 
The beta alumina has four kinds of crystal systems: .beta.-alumina, 
.beta."-alumina, .beta.'"-alumina, and .beta.""-alumina. .beta.-Alumina 
and .beta."-alumina are mainly precipitated by ordinary producing 
processes. In the specification and claims of this application, the beta 
alumina sintered body is composed mainly of .beta.-alumina and 
.beta."-alumina. The term of the "magnesia-based" beta alumina sintered 
body means a beta alumina sintered body in which magnesia is used as a 
stabilizer. 
2. Related Art Statement 
The beta alumina sintered bodies have been heretofore used as the solid 
electrolytes of the sodium-sulfur cells. From the standpoint of 
performance, service life and operation reliability of the cell, the solid 
electrolyte is required to have large mechanical strength and low electric 
resistance. 
It has been formerly known that beta alumina having high strength and low 
electric resistance can be obtained by controlling the contents, ratios 
and crystalline particle diameters of .beta.-alumina crystals and 
.beta."-alumina crystals. For example, "Journals of MATERIALS Science 19 
(1984), pp 695-715" describes that the electric resistance varies with the 
ratios of the .beta.-alumina crystals and .beta."-alumina crystals, and 
that the larger the .beta."-alumina, the lower is the electric resistance 
(See p 703, FIG. 12). 
GB-B-1558305 discloses a process for producing a beta alumina sintered body 
having high electric conductivity, a fine crystalline structure and high 
durability. In this patent, having noted that it is difficult to obtain a 
uniform crystalline structure having high electric conductivity due to 
densification occurring in a single heating/cooling cycle when firing is 
effected along a single heating/cooling curve as done conventionally, a 
beta alumina sintered body having high electric conductivity and a uniform 
crystalline structure is obtained by repeating heating/cooling twice or 
more such that not more than 95% of the overall linear shrinkage takes 
place during any one cycle of heating/cooling. 
Further, U.S.P. discloses a process for producing a beta alumina sintered 
body having high strength and low electric resistance. In this pores, 
special starting compounds composed alumina, a sodium compound and a 
lithium-aluminum compound (Li.sub.2 O.nAl.sub.2 O.sub.3) are used, and a 
beta alumina sintered body is obtained by firing at 
1500.degree.-1600.degree. C. for a short time period such as less than 10 
minutes. 
However, the above Journals of MATERIALS Science discloses a process for 
producing the beta alumina sintered body having low electric resistance, 
but has no mentioning about a process for producing a beta alumina 
sintered body having low resistance and high strength. 
On the other hand, the two-stage peak firing process in GB-B-1558305 has 
various problems in practical application. That is, while the temperature 
distribution widely varies inside a large-scale furnace for mass 
production, it is necessary to extremely sharply vary the temperature 
distribution inside the furnace so that heating/cooling may be repeated 
along a given heat curve with the lapse of time. However, it is difficult 
to control the temperature distribution inside the furnace in such a 
manner particularly in the case of the mass production type large-scale 
furnace. As a result, it was extremely difficult to mass produce products 
having less variation in quality at a high yield. 
On the other hand, U.S. Pat. No. 4,113,928 describes the process for 
producing the lithia-based beta alumina sintered bodies, but does not 
specifically disclose anything about a process for producing 
magnesia-based beta alumina. In this process for producing the 
lithia-based beta alumina sintered bodies, the firing is effected at 
1500.degree.-1600.degree. C., and the molded body is kept at this firing 
temperature for an extremely short period, e.g., less than 10 minutes. 
Therefore, this process has also various problems in the practical 
application. That is, since the temperature distribution widely varies in 
the large-scale furnace for mass production, it takes a long time to make 
a temperature in a lower temperature zone follow that in a higher 
temperature zone. Therefore, the short firing temperature requires that 
even a large-scale furnace has an extremely excellent temperature 
distribution. However, it is difficult to produce such a large-scale 
furnace. 
Therefore, the above processes are inappropriate as a producing process for 
the mass production of the beta alumina sintered bodies. 
SUMMARY OF THE INVENTION 
The present invention is aimed at the provision of a magnesia-based beta 
alumina sintered body having both high strength and low electric 
resistance as well as a producing process suitable for the mass production 
of such sintered bodies. 
Conventionally, there is a problem that if high strength is to be obtained, 
electric resistance becomes larger, whereas if the electric resistance is 
to be reduced, strength decreases. The present invention is based on the 
knowledge that sintered bodies having high strength and small electric 
resistance can be obtained by employing a magnesia-based beta alumina 
system and making it sintered bodies have a specific microstructure. The 
present inventor has found that the producing condition suitable for the 
mass production of the beta-alumina sintered bodies having such a specific 
microstructure is that 1 a heat curve in the firing step is simple, that 
is, the firing is effected in a single cycle of heating/cooling with a 
single temperature peak, 2 the firing temperature is low and the 
acceptable width of the firing temperature zone is wide, and 3 the firing 
time needs to be neither extremely short nor extremely long. The present 
invention is based on the knowledge that such a producing process suitable 
for the mass production can be realized by appropriately setting the 
composition of the beta-alumina sintered body, an allowable diameter range 
of coarse particles and their existing amount and a cooling condition from 
the firing temperature. 
In order to realize the above object, the magnesia-based beta alumina 
sintered body according to the present invention has the following 
features: 
(1) the average crystalline particle diameter of beta alumina crystals as 
calculated assuming that the beta alumina crystals are of a circular 
section is in a range of 1-4 .mu.m; 
(2) the amount of the beta alumina crystals having particle diameters not 
more than 5 .mu.m is 85% to 98% when measured in a plane; 
(3) the maximum crystalline particle diameter is not more than 300 .mu.m, 
and the number of coarse particles having diameters falling in a range of 
100 .mu.m to 300 .mu.m is not more than 1 as counted in an area of 10 
mm.times.10 mm; and 
(4) the content of crystals of sodium aluminate is 0.5 wt % to 6.0 wt %. 
Further, the process for producing a magnesia-based beta alumina sintered 
body according to the present invention involves the steps of: weighing 
and mixing spinel, alumina and a sodium compound as starting materials in 
such respective amounts that the resulting magnesia-based beta alumina 
sintered body may be composed of 85.5 to 87.5 wt % of Al.sub.2 O.sub.3, 
3.5 to 4.5 wt % of MgO and 9.0 to 10.0 wt % of Na.sub.2 O; obtaining 
calcined beta alumina by calcining the resulting mixture; milling and 
granulating the calcined beta alumina; molding the granulated product; 
firing the molded product, wherein the firing temperature is set at 
1580.degree.-1650.degree. C., the molded product is fired while being kept 
at this firing temperature for a time period from 30 minutes to 60 
minutes, and the fired product is rapidly cooled in a temperature range 
from a maximum firing temperature to 1,450.degree. C. by setting a cooling 
rate at 300.degree. C./hr to 800.degree. C./hr. 
In the present invention, the microstructure is so controlled that the 
average crystalline particle diameter of beta-alumina crystals as 
calculated assuming that the beta alumina crystals are of a circular 
section may be in a range of 1-4.mu.. The average particle diameter is 
unfavorably less than 1 .mu.m, because in such a case electric resistance 
increases although no problem occurs with strength. If the average 
particle diameter is more than 4 .mu.m, strength decreases. Further, the 
particle size distribution is so controlled that the ratio of the beta 
alumina crystals having particle diameters of not more than 5 .mu.m may be 
85% to 98% when measured in a plane. If the the ration of the beta alumina 
crystals having particle diameters of not more than 5 .mu.m is less 85%, 
strength decreases. On the other hand, if this ratio is more than 98%, 
electric resistance unfavorably increases. 
Furthermore, when a large particle having particle diameters more than 300 
.mu.m exists in the tissue of the sintered body, cracking proceeds from 
such a coarse particle as a starting point so that the sintered body may 
be likely to be broken. Moreover when the number of coarse particles 
having diameters falling in a range of 10 to 300 .mu.m is not more than 1 
as counted in an area of 10 mm.times.10 mm on the average, the beta 
alumina sintered body can exhibit high diametral pressure strength of not 
less than 250 MPa. 
It is ideal that no coarse particles exist in the sintered body. However, 
even if fine raw material are uniformly mixed, a locally non-uniform 
distribution of an alkaline component, which is considered to be caused by 
granulating, cannot be actually avoided. As a result, coarse particles 
having sizes forming gaps between surrounding particles exists. Existence 
of a few to several particles smaller than 100 .mu.m does not largely 
influence strength, but coarse particles greater than 100 .mu.m give large 
influence upon strength. Thus, controlling must be so made that 
substantially no coarse particles greater than 300 .mu.m may be produced. 
Furthermore, it is important to make controlling such that the content of 
crystals of sodium aluminate remaining in the sintered body may be 0.5 wt 
% to 6.0 wt. %. It is preferable to formulate the starting mixture so that 
the crystals of sodium aluminate may remain in the sintered body, because 
electric resistance of the sintered body is small in this case. This is 
considered that the crystals of sodium aluminate precipitate in a 
temperature range of 800.degree.-900.degree. C. during the firing step, 
the precipitated sodium aluminate crystals are converted to 
.beta."-alumina through reaction with .beta.-alumina at temperature not 
less than 1,400.degree. C. to increase the ratio of .beta."-alumina, and 
consequently electric resistance of the sintered body decreases. If the 
residual amount of the crystals of sodium aluminate is less than 0.5%, the 
entire sintered body cannot be appropriately uniformly converted to 
.beta."-alumina. Thus, the residual amount of less than 0.5% is not 
preferred. On the other hand, the residual amount of more than 6% is not 
preferred, because strength of the sintered body decreases and electric 
resistance increases in this case. 
The following are preferred as the magnesia-based alumina sintered body 
according to the present invention. 
(1) The content of ZrO.sub.2 is 0.1 wt % to 3.0 wt % relative to the total 
weight of the sintered body. 
(2) The magnesia-based alumina sintered body has a diametrical compression 
strength of not less than 250 MPa, and a four-terminal electric resistance 
of not more than 5 .OMEGA.cm on at 350.degree. C. 
The following are preferred as the producing process of the present 
invention. 
(1) The starting materials are mixed in such respective amounts that 
residual crystals of sodium aluminate may be 0.5 to 6.0 wt % relative to 
the total weight of the magnesia-based beta alumina sintered body. 
(2) When the starting materials are mixed, ZrO.sub.2 is so added that the 
content of ZrO.sub.2 may be in a range of 0.1 to 3.0 wt % relative to the 
total weight of the sintered body. 
(3) The sintered body has a diametrical compression strength of not less 
than 250 MPa, and a four-terminal electric resistance of not more than 5 
.OMEGA.cm on at 350.degree. C. 
(4) The firing temperature is 1,600-1,630.degree. C. 
(5) The cooling rate in the range from the maximum firing time to 
1,450.degree. C. is 400-500.degree. C./hr. 
(6) The content of sodium aluminate remaining in the resulting beta alumina 
sintered body is 3-5 wt. %. 
(7) The addition amount of ZrO.sub.2 0.1-0.5 wt %. 
The above (6) and (7) are preferable in the case of the magnesia-based 
alumina sintered body. 
The following is another aspect of the producing process of the present 
invention. 
A process for producing a magnesia-based beta alumina sintered bodies, 
includes the steps of: weighing and mixing spinel, alumina and a sodium 
compound as starting materials in such respective amounts that the 
resulting magnesia-based beta alumina sintered body may be composed of 
85.5 to 87.5 wt % of Al.sub.2 O.sub.3, 3.5 to 4.5 wt % of MgO and 9.0 to 
10.0 wt % of Na.sub.2 O; obtaining calcined beta alumina by calcining the 
resulting mixture; milling and granulating the calcined beta alumina; 
molding the granulated products; and firing the molded products, wherein 
the firing temperature at which the molded products are fired on a low 
temperature side in a firing furnace is set at not less than 1580.degree. 
C., while the molded product being kept at the firing temperature for a 
time period not less than 30 minutes, whereas the firing temperature at 
which the molded products are fired on a higher temperature side in the 
firing furnace is set at not more than 1650.degree. C., while the molded 
product is fired while being held at the firing temperature for a time 
period of not more than 60 minutes, and the fired products are rapidly 
cooled in a temperature range from a maximum firing temperature to 
1,450.degree. C. by setting a cooling rate at 300.degree. C./hr to 
800.degree. C./hr. 
These and other objects, features and advantages of the invention will be 
appreciated upon reading the following description of the invention when 
taken in conjunction with the attached drawings, with the understanding 
that some modifications, changes and variations could be made by the 
skilled person in the art to which the invention pertain.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The producing process of the present invention suitable for the mass 
production of the magnesia-based beta alumina sintered body having the 
above micro-structure will be explained. First, the producing steps of 
this process will be explained along those shown in FIG. 1. 
(1) Production of spinel 
In order to first produce spinel, alumina and magnesia are mixed at molar 
ratios of 1:1, and the resulting mixture is calcined along a 
spinel-calcining curve shown in FIG. 2, thereby producing spinel. 
(2) Production of calcined beta alumina 
The thus obtained spinel, alumina and a sodium compound (e.g., sodium 
carbonate) are weighed and mixed ratios as starting materials at given 
ratios, and the mixture is milled to obtain a raw material for beta 
alumina. The raw material is dried and calcined to produce a calcined beta 
alumina raw material. Each of the starting materials used is preliminarily 
milled to a fine powder having the diameters of not more than 1 .mu.m. The 
starting materials are formulated in such mixing ratios that the 
composition of the beta alumina sintered body may be 85.5 to 87.5 wt % of 
Al.sub.2 O.sub.3, 3.5-4.5 wt % of MgO and 9-10.0 wt % of Na.sub.2). 
(3) Milling, granulating and molding 
The thus calcined beta alumina is milled, and a binder such as polyvinyl 
alcohol is mixed into the milled product. The resulting mixture is 
granulated with a spray dryer. The granulated material as a molding 
material is press molded in a bottomed tubular form by cold static ISO 
press. 
(4) Firing 
As shown in FIG. 5, a bottomed tubular molding thus molded is set in a 
container made of sense magnesia having high purity, which is placed in a 
furnace. The molding is fired along a heat curve in FIG. 3. The firing is 
effected under the condition that the firing temperature is set at 
1580.degree.-1650.degree. C., and the keeping time at the firing 
temperature is 30-60 minutes, and the cooling rate in a temperature range 
from the maximum firing temperature to 1450.degree. C. is 300.degree. to 
800.degree./hr. 
The producing process of the present invention will be further explained in 
detail along the above-mentioned steps. 
The reason why MgO is used as a stabilizer is that the firing temperature 
range can be made wider, and the sintered bodies having stabilized 
physical properties can be obtained in the large-scale furnace even if the 
temperature distribution is slightly bad in the furnace. The reason why 
spinel obtained by reacting MgO with alumina is used as a starting 
material is that a small addition amount of MgO is uniformly mixed to 
prevent local deviation in mixing. By so doining, the formation of coarse 
particles can be suppressed. The reason why the above-mentioned starting 
materials are mixed at the above ratios is that 0.5 to 6.0 wt % of the 
crystals of sodium aluminate is retained in the beta alumina sintered 
body. Thereby, the ratio of .beta." in the sintered body increases to 
lower electric resistance. Since each of the starting materials is milled 
to not more than 1 .mu.m, they are homogeneously mixed to make the 
microstructure of the sintered body homogeneous and fine. That is, 
strength of the sintered body increases. The reason why the calcined beta 
alumina is preliminarily produced is that if the above starting materials 
are directly subjected to granulation, molding and firing, while omitting 
the calcining step, it is likely that the sintered body has an extremely 
large shrinkage factor and contains an interior defect such as a crack. It 
is preferable that 0.1 to 3.0 wt % of ZrO.sub.2 is added relative to the 
total weight of the sintered body in the above step (2), because coarse 
particles are unlikely to be formed due to the action of ZrO.sub.2. 
Accordingly, the sintering time can be advantageously prolonged. If the 
addition amount ZrO.sub.2 is less than 1 wt %, the above effect is small. 
If it is more than 3.0 wt %, electric resistance exceeds 5.0 .OMEGA.cm (at 
350.degree. C.). Thus, less than 0.1 wt % and more than 3 wt % are not 
preferable when ZrO.sub.2 is added. The reason why the cooling rate in the 
temperature range from the maximum firing temperature to 1450.degree. C. 
is set at 300.degree.-800.degree. C./hr in the step (4) is that since the 
crystals of beta alumina are being formed at the firing temperature and 
the particle growth is occurring and since the average particle diameter 
of the beta alumina crystals is controlled to 1-4 .mu.m under the firing 
condition given in the step (4), the growth of the crystals is stopped in 
this state by rapidly cooling so as to control the microstructure. It is 
inpreferable that the cooling is effected more slowly than at 300.degree. 
C./hr, because the crystal growth proceeds to disable control of the 
particle size distribution. On the other hand, if is inpreferable that the 
cooling is effected at a speed faster than 800.degree. C./hr, because the 
sintered body and the magnesia container are damaged by thermal impact. 
The reason why the number of coarse particles being 100 .mu.m to 300 .mu.m 
is not more than 1 as measured in an area of 10.times.10 mm is that it was 
found out that influence upon strength of the sintered body is small and 
acceptable. As a result, the firing temperature-keeping time can be 
prolonged, and the firing temperature range can be made wider. 
The variations in the furnace-interior temperature distribution cannot be 
avoided in the case of the mass-production type large-scale furnace. As 
shown in FIG. 5, the molded body located at the heat curve A on a higher 
temperature zone in the furnace is fired in a temperature range not less 
than 1580.degree. C. for a t.sub.1 minutes. On the other hand, the molded 
body located at the heat curve B on a lower temperature zone in the 
furnace is fired in a temperature range not less than 1580.degree. C. for 
a t.sub.2 minutes. The molded body located on the higher temperature zone 
and that on the lower temperature zone need to acquire strength 250 MPa or 
more and electric resistance of 5 .OMEGA.cm or less (at 350.degree. C.) as 
the physical properties of the sintered body. 
The longer the acceptable firing time t.sub.1 and the shorter the minimum 
necessary firing time T.sub.2, the advantageous are the results for the 
mass production. In the present invention, the firing time t.sub.1 and the 
minimum necessary firing time t.sub.2 are 60 minutes and 30 minutes, 
respectively. As to the firing temperature, the lower the acceptable 
firing temperature and the wider the firing temperature range, the 
advantageous are the results for the mass production. In the present 
invention, the firing temperature is in a range of 
1580.degree.-1650.degree. C. The reason why the cooling rate from the 
maximum firing temperature M is important is that since the grain growth 
at this maximum firing temperature is most active, it is difficult to 
control the particle size unless redid cooling is effected from the 
maximum temperature. 
The reason why the acceptable firing temperature can be increased to 60 
minutes and the acceptable firing temperature can be set in a wide range 
of 1580.degree.-1600.degree. C. in the production of the sintered body of 
the present invention through firing, which sintered body has the 
microstructure, the combination of the following factors. That is, 1 the 
formation of abnormal crystals is suppressed, more specifically, the local 
deviation of the MgO is prevented by using MgO in the form of spinel as a 
starting material, and the starting materials are made as fine as not more 
than 1 .mu.m; 2 the cooling rate is set at an extremely high value of 
300.degree.-800.degree. C./hr in a range of the maximum firing temperature 
to 1450.degree. C.; 3 the existence of the coarse particles up to 300 
.mu.m can be allowed; 4 MgO is used as a stabilizer; and 5 beta alumina 
is calcined, and then molded and fired. The firing condition suitable for 
the mass production is attained by the above factors 1 through 5. In 
addition, 6 the formation of the coarse particles are preferably further 
prevented by adding 0.1-3.0 wt % of ZrO.sub.2 is added to the starting 
materials. Moreover, besides the above factors, when the composition of 
the sintered body is so set that the content of the crystals of sodium 
aluminate remaining in the sintered body may be 0.5-6.0 wt %, the sintered 
body having the microcrystals with a high percentage of .beta."-alumina 
and low electric resistance can be obtained. Therefore, the magnesia-based 
beta alumina sintered body having high strength and low electric 
resistance can be obtained by the producing process suitable for the mass 
production. 
(Experiment) 
In the following, a specific example of the process for producing the 
magnesia-based alumina sintered body according to the present invention 
will be described. 
(1) Production of spinel 
.alpha.-Alumina and magnesia were so weighed and mixed that the total mixed 
weight might be 50 kg and .alpha.-alumina and magnesia might be 1:1 in 
terms of molar ratio. In a trommel charged with grinding media and lined 
with .alpha.-alumina having a high purity of 99.7 wt %, water was so added 
to the resulting mixture that the content of the water might be 65 w %. 
Then, the mixture was mixed for 6 hours. The resulting mixture was dried 
in an electric dryer at 120.degree. C. for 6 hours. Then, the dried 
mixture was fused and crushed in a granular form smaller than about 5 mm, 
which was charged into a sheath made of .alpha.-alumina with a high purity 
of 99.9 wt %, and calcined in the electric furnace along the 
spinel-calcining curve in FIG. 2. Thereby, spinel was produced. The 
calcined product was analyzed by an X-ray quantitative diffraction 
analyzer, which revealed that the calcined product was composed of 100% 
spinel. 
(2) Production of calcined beta alumina 
The thus obtained spinel, .alpha.-alumina and sodium carbonate were so 
weighed as starting materials that the total starting materials might be 
50 kg and the composition of the resulting beta alumina sintered body 
might be 87.5 wt % of Al.sub.2 O.sub.3, 3.5 wt % of MgO and 9.0 wt % of 
Na.sub.2 O. Then, water was so added into the resulting mixture inside the 
trommel charged with grinding media and lined with .alpha.-alumina having 
high purity 99.7 wt % that the content of water might be 65 wt %. The 
resulting mixture was mixed and milled to prepare a raw material for beta 
alumina. The average particle diameter of the beta alumina raw material 
was measured to be 0.8 .mu.m by using a laser type particle size 
distribution meter. A slurry of the alumina raw material was poured into a 
spray dryer with an inlet temperature of 180.degree. C., thereby obtaining 
a granulated powder having the water content of 2.0 wt %. The granulated 
powder was charged into a sheath made of alumina with a high purity of 
99.0 wt %, and calcined along the calcining heat curve of FIG. 2 in the 
electric furnace at a calcining temperature of 1250.degree. C. for a 
calcining time of 3 hours. Thereby, calcined beta alumina was produced. 
The heat curve in FIG. 2 is also applicable to the production of spinel as 
well as to the production of the calcined beta alumina product. 
(3) Milling, granulating and molding 
Into the above-calcined beta alumina was so added water inside the trommel 
charged with grinding media made of .alpha.-alumina with high purity of 
99.7 wt % and lined with .alpha.-alumina having the same purity that the 
content of water might e 50 wt %. The resulting mixture was milled for 6 
hours, which was added with 2.0 wt % of polyvinyl alcohol (PVA) as a 
molding aid. Then, the mixture was mixed for 15 minutes. A slurry of the 
resultant mixture was granulated in the spray dryer with the inlet 
temperature of 190.degree. C. The thus granulated powder had the average 
particle diameter of 75 .mu.m and the water content of 2.2 wt %. 
The above granulated powder was molded in the form of a tube having an 
outer diameter of 30 mm, an entire length of 280 mm and a thickness of 2.5 
mm with one end closed under molding pressure of 1.5 tons by using a dry 
back type static pressure molding machine. 
(4) Firing 
The molded tube 1 was fired by using a firing container 2 made of magnesia 
as shown in FIG. 4. In this figure, 3 is a green setter made of beta 
alumina, and 4 is a firing table. The reason why the tube was fired in the 
firing container 2 is that scattering of sodium in beta alumina might be 
prevented. The tube 1 was fired along the heat curve shown in FIG. 3. The 
heat curve in FIG. 3 is a firing condition under which the diametral 
compression strength and the four-terminal electric resistance of the 
resulting sintered body might be not less than 250 MPa and not more than 
5.0 .OMEGA.cm at 350.degree. C., respectively. 
(Examples 1-5 and Comparative Examples 6 and 7) 
The starting materials and the producing condition given in the above 
experiment were employed, and firing was effected at various firing 
temperatures shown in the following Table 1. Thereby, magnesia-based beta 
alumina sintered bodies in Examples 1 through 7 were obtained. Examples 1 
through 7 were produced under the same condition except that 1.0 wt % of 
ZrO.sub.2 was added relative to the entire weight of the sintered body in 
Example 7. Example 6 is considered best among the sintered bodies added 
with no ZrO.sub.2. In Example 7, the growth of coarse particles was 
suppressed by the addition of ZrO.sub.2 to make the maximum crystal 
particle diameter smaller. Example 7 is considered best among those added 
with ZrO.sub.2. 
TABLE 1 
__________________________________________________________________________ 
Examples Comparative Examples 
1 2 3 4 5 6 7 8 9 10 11 12 
__________________________________________________________________________ 
1 Firing 1,580 
1,600 
1,620 
1,650 
1,650 
1,620 
1,610 
1,560 
1,580 
1,650 
1,650 
1,670 
temperature (.degree.C.) 
2 Firing time (min.) 
30 40 50 60 60 60 60 30 20 70 60 60 
3 Cooling rate (.degree.C./hr) 
300 500 700 800 300 500 500 300 300 800 250 800 
Ti .fwdarw. 1450.degree. C. 
4 Average crystal 
1.0 1.4 1.7 3.0 4.0 1.8 1.5 0.7 0.8 4.3 4.2 7.0 
particle diameter 
(.mu.m) 
5 Percentage of 
98 97 93 90 87 94 95 99 99 85 84 80 
crystalline par- 
ticle smaller than 
5 .mu.m (%) 
6 Maximum crystal 
40 60 110 200 300 115 70 20 30 350 360 400 
particle diameter 
(.mu.m) 
7 Diametral com- 
270 280 320 305 250 330 350 215 230 200 205 180 
pression strength 
(MPa) 
8 Four-terminal electric 
5.0 4.8 4.7 4.7 4.5 4.7 4.7 6.0 6.0 4.3 4.2 4.5 
resistance (.OMEGA.-cm at 
350.degree. C.) 
9 Amount of .beta."-alumina 
92.1 
93.2 
94.5 
95.5 
95.8 
94.0 
93.8 
90.2 
91.5 
96.3 
96.3 
95.8 
crystals (wt %) 
10 Amount of sodium 
5.0 4.8 4.5 4.1 4.0 4.5 4.7 5.2 5.3 3.7 3.8 3.7 
aluminate crystals 
(wt %) 
11 Addition amount of 
not not not not not not 1.0 not not not not not 
ZrO.sub.2 (wt %) 
added 
added 
added 
added 
added 
added added 
added 
added 
added 
added 
__________________________________________________________________________ 
Note: Ti -- Maximum firing temperature 
As is seen in Table 1, the magnesia-based alumina sintered bodies in 
Examples 1 through 7 realized the diametral compression strength and the 
four-terminal electric resistance aimed at by the present invention. On 
the other hand, when the firing was effected at less than the firing 
temperature range of the present invention as in Comparative Example 8, 
the crystal growth can be suppressed to make the maximum crystal diameter 
smaller. To the contrary, high strength is not obtained due to 
insufficient sintering, and electric resistance increases due to 
insufficient conversion to .mu."-alumina. Further, as in Comparative 
Example 9, if the firing temperature falls within the range of the present 
invention and if the firing time is as short as less than 30 minutes, 
since the sintering is insufficient, high strength is not obtained and 
electric resistance increases. That is, in order to realize the 
diametrical compression strength and the four-terminal electric resistance 
aimed at by the present invention, the minimum necessary firing 
temperature and the fixing time are 1580.degree. C. and 30 minutes, 
respectively. Comparative Example 10 is a case where the firing 
temperature was 1650.degree. C. and the firing time was more than 60 
minutes. In this case, the crystal growth proceeds, the average particle 
diameter increases, the maximum crystal particle diameter also increases, 
and the diametrical compression strength decreases. In Comparative Example 
11, the firing temperature was 1650.degree. C. and the firing time was 60 
minutes, which fall within the scope of the present invention. However, in 
Comparative Example 11, the sintered body was slowly cooled at a cooling 
rate of 250.degree. C./hr in the range of 1650.degree.-1450.degree. C. In 
this case, the crystal growth and the crystal-coarsing proceeded during 
the cooling step, thereby lowing the diametrical compression strength. 
Comparative Example 12 is a case where the firing temperature was more 
than 1650.degree. C. In this case, the crystal growth extremely rapidly 
proceeded to make crystal coarse. Consequently, the diametrical 
compression strength decreases. That is, the acceptable firing temperature 
is not more than 1650.degree. C. and the acceptable firing time is not 
more than 60 minutes, and the acceptable cooling rate in the temperature 
range Of 1650.degree.-1450.degree. C. is not less than 300.degree. C./hr. 
The physical properties of the magnesia-based alumina sintered bodies in 
Examples and Comparative Examples were measured as follows: 
(1) Measurement of the particle diameter of the .beta.-alumina crystals and 
.beta."-alumina crystals: 
A section of the beta alumina tube was mirror polished, and etched with hot 
phosphoric acid. Then, the thus etched surface was observed with a 
scanning type electron microscope, and the particle diameter was measured 
with respect to an obtained microstructure photograph by using an image 
analyzer. 
(2) Diametrical compression strength: 
A loop having a length of 10 mm was cut from the beta alumina tube, and 
compressed in a radial direction of the cut loop at a cross head speed of 
0.5 mm/min. by using an autograph manufactured by Shimazu Manufacturing 
Co., Ltd. The diametrical compression strength was calculated from a 
broken value. 
(3) Four-terminal electric resistance: 
A sample piece having a size of 2.times.2.times.40 mm was prepared form the 
beta alumina tube. Opposite ends of the sample piece were coated with 
carbon, and connected together via a platinum wire. Electric resistance in 
an axial direction of the sample piece was measured at an elevated 
temperature of 350.degree. C. 
(4) Analysis of components: 
The amount of .beta."-alumina crystals and that of the crystals of sodium 
aluminate were quantitatively analyzed by using a powder X-ray diffracting 
analyzer. 
As shown in the results in FIG. 1, the magnesia-based beta alumina sintered 
bodies according to the present invention can satisfy high strength (the 
diametrical compression strength of 250-350 MPa) and low electric 
resistance (the four-terminal electric resistance of 4.5-5.0 .OMEGA.cm (at 
350.degree. C.)). To the contrary, it is seen that Comparative Examples 8 
through 12 exhibit low strength and/or high electric resistance. The 
acceptable firing temperature range can be made wider by taking the 
maximum crystal diameter as not more than 300 .mu.m to sufficiently cope 
with the mass production.