Method of preparing silicon nitride porous body

A silicon nitride ceramic porous body having excellent acid and alkali resistance, mechanical strength, and durability can be employed as a filter or a catalytic carrier. The silicon nitride porous body contains a plurality of silicon nitride crystal grains with pores formed in grain boundary parts thereof, or includes a body part and a pore part wherein the body part is formed by a plurality of silicon nitride crystal grains and the pore part forms a three-dimensional network structure. The body part is formed by at least 90 vol. % of silicon nitride crystal grains, which are directly bonded to each other. In order to prepare the finished ceramic porous body, a porous body compact which is mainly composed of silicon nitride, is brought into contact with an acid and/or an alkali so that a component other than silicon nitride is partially or entirely dissolved and removed from the compact. The compact is prepared from a mixed powder of silicon nitride powder and at least one of a rare earth compound powder, a transition metal compound powder, and a bismuth compound, which is heat treated in the temperature range from 1600.degree. C. to 2100.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a silicon nitride porous body and a method 
of preparing the same, and more particularly, it relates to a silicon 
nitride porous body which can be employed as a filter or a catalytic 
carrier in an environment in coexistence with an acid or an alkali of high 
concentration, and a method of preparing the same. 
DESCRIPTION OF THE BACKGROUND ART 
Silicon nitride ceramic is excellent in heat resistance and thermal shock 
resistance, and has high strength. Due to these characteristics, the 
silicon nitride ceramic is worked into a porous body, to be used as a 
filter or a catalytic carrier which is employed in a high temperature 
range, in particular. 
Japanese Patent Laying-Open No. 1-188479 (1989) discloses a porous body 
which is prepared by molding a mixed powder of relatively coarse silicon 
powder and silicon nitride powder and thereafter nitriding the same. On 
the other hand, Japanese Patent Laying-Open No. 61-222966 (1986) discloses 
a technique of adding gypsum to silicon powder, nitriding and sintering 
the mixture, and thereafter removing the gypsum part with an acid. 
Further, International Patent Laying-Open No. WO94/27929 discloses a 
silicon nitride porous body which is formed by columnar silicon nitride 
grains and a method of preparing the same. 
A technique of bringing ceramics into a porous state with an acid or an 
alkali is widely employed in relation to other ceramics and glass 
materials. For example, Japanese Patent Laying-Open No. 6-183780 (1994) 
discloses a method of preparing porous glass by eluting a soluble 
component from crystallized glass by an acid treatment. On the other hand, 
Japanese Patent Publication No. 5-72355 (1993) discloses a mullite porous 
body and a method of preparing the same by eluting a matrix of a ceramics 
sintered body containing needlelike and columnar mullite crystals with an 
aqueous alkaline solution and forming pores. 
In addition, a material prepared by bringing oxide ceramics such as alumina 
or cordierite, for example, into a porous state is also known and has been 
used in practice. 
Applied products employing such ceramics porous bodies include a filter for 
separating substances which are larger than the maximum pore diameter of 
the porous body from a gas or a liquid. Such a filter consisting of the 
ceramics porous body is superior in heat resistance to a generally 
employed organic filter, and is sterilizable using steam. 
As an applied product other than the filter, it is possible to employ the 
ceramics porous body as a catalytic carrier by coating its surface with a 
metal catalyst such as platinum, for example. 
Oxide ceramics having excellent acid resistance and alkali resistance are 
applied to a filter or a catalytic carrier, as hereinabove described. 
However, the oxide ceramics may be sintered during employment at a high 
temperature, such that the porosity or pore diameter thereof is changed. 
In the conventional filter or catalytic carrier consisting of oxide 
ceramics, further, sufficient strength may not be attained. 
On the other hand, a porous body which is formed by columnar grains of 
silicon nitride has high strength and a sharp pore distribution, and the 
pore diameters can be controlled. In a step of preparing such a porous 
body, however, an additive which is added for facilitating crystal growth 
remains in the porous body. When the silicon nitride porous body is 
employed in a strong acid or alkaline solution, therefore, the additive is 
disadvantageously eluted as an impurity. This type of porous body, which 
is mainly formed by columnar grains of silicon nitride, may have a similar 
pore structure to a conventional ceramics porous body which is formed by 
spherical crystal grains, due to a grain boundary phase resulting from the 
additive. 
In case of preparing a porous body by reaction sintering from silicon 
powder, on the other hand, it is difficult to control the pore diameters, 
and only low strength can be attained. 
Porous glass which is mainly composed of quartz is formed by silicon oxide. 
Therefore, the silicon oxide is disadvantageously eluted when subjected to 
an alkali. 
In case of employing mullite crystal grains, it is possible to obtain a 
porous body having higher strength and a sharper pore distribution as 
compared with the porous glass mainly composed of quartz, depending on the 
grain shape and the structure. In a step of preparing this porous body, 
however, it is necessary to perform elution from a matrix which is a glass 
phase. Depending on the size of the porous body, this elution may be 
insufficient. Under an environment in coexistence with a strong aqueous 
acid solution, there is a possibility that junctions between the crystal 
grains are deteriorated, which reduces the strength. Further, the strength 
is reduced from the initial value by an eluting operation, for a reason 
similar to the above. 
When using the aforementioned ceramic porous body as a filter, the maximum 
pore diameter defines the minimum diameter of the filterable substance, as 
hereinabove described. Particularly when the ceramic porous body is 
employed for filtering a liquid, a large quantity of liquid other than the 
filtered substance must permeate the porous body in a unit time. In order 
to cope with this, it is necessary to increase the porosity or make the 
pore diameter distribution as sharp as possible so that the mean pore 
diameter approaches the maximum pore diameter. When the pore diameter 
distribution is sharpened in such a ceramic porous body, however, the 
porosity is generally reduced particularly when the filtered substance has 
a small diameter. Due to the aforementioned problems and a high cost, the 
filter made of the ceramic porous body still falls behind the organic 
filter in the present circumstances. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a silicon 
nitride porous body which has excellent heat resistance and thermal shock 
resistance, as well as acid resistance and alkali resistance. 
Another object of the present invention is to provide a silicon nitride 
porous body having excellent mechanical strength and durability in 
application to a filter or a catalytic carrier. 
Still another object of the present invention is to provide a silicon 
nitride porous body which can be employed as a filter having both 
permeability and separatability. 
The inventors have deeply studied the aforementioned problems, and have 
discovered that it is possible to partially or entirely dissolve and 
remove an additional component other than silicon nitride grains from a 
porous body, by preparing a compact from mixed powder of silicon nitride 
powder and a prescribed additive powder, heat-treating the compact at a 
high temperature for forming a porous body, and treating the porous body 
with an acid or an alkali. They have also discovered that a body part of 
the porous body is thus formed by at least 90 vol. % of silicon nitride 
crystal grains. 
A silicon nitride porous body according to an aspect of the present 
invention contains a plurality of silicon nitride crystal grains, with 
pores formed in grain boundary parts thereof. 
A silicon nitride porous body according to another aspect of the present 
invention comprises a body part and a pore part, wherein the body part is 
formed by a plurality of silicon nitride crystal grains and the pore part 
forms a three-dimensional network structure. 
The body part is preferably formed by at least 90 vol. % of silicon nitride 
grains, and more preferably at least 99 vol. % of silicon nitride grains, 
while the silicon nitride crystal grains are directly bonded to each other 
for forming the body part. 
Further, it is preferable to form at least 50 vol. % of the silicon nitride 
crystal grains in the fabricated porous body by .beta.-silicon nitride 
crystal grains, so that the body part is mainly composed of silicon 
nitride crystal grains for improving the mechanical strength of the pore 
part of the three-dimensional network structure and attaining a sharper 
pore diameter distribution. In this case, it is more preferable to control 
the .beta.-silicon nitride crystal grains so that at least 80 vol. % 
thereof are columnar grains having an average aspect ratio of at least 3 
and not more than 50. 
Assuming that d represents the mean width of the columnar grains in the 
minor axis direction, the mean pore diameter r of the porous body is 
preferably controlled in the range of d/10.ltoreq.r.ltoreq.10.times.d. 
The volume of the pore part is preferably at least 20 vol. % and not more 
than 75 vol. % with respect to the overall porous body. 
Preferably, the surfaces of the silicon nitride crystal grains are 
hydrophilic. 
A fine filter is formed by the inventive silicon nitride porous body. 
A method of preparing a silicon nitride porous body according to the 
present invention involves preparing a porous body mainly composed of 
silicon nitride and bringing the porous body into contact with an acid, 
thereby at least partially dissolving and removing a component other than 
the silicon nitride. In this case, the porous body which is brought into 
contact with an acid may be further brought into contact with an alkali, 
so that an alkali-soluble component is also removed. 
The method of preparing a porous body that is mainly composed of silicon 
nitride and that serves as a target treated with an acid and/or an alkali 
as described above comprises the steps of preparing a mixed powder by 
adding a prescribed powder to silicon nitride powder, preparing a compact 
from the mixed powder, and heat treating the compact in a 
nitrogen-containing atmosphere in a prescribed temperature range. 
In this method, the blending composition of the mixed powder and heat 
treatment conditions for the compact may be in three combinations, as 
shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Blending Composition of Mixed Powder 
(numerical value: volume percentage of specified element 
in terms of oxide) 
powder of compound of 
bismuth 
Heat Treatment 
rare earth element 
transition metal element 
compound 
Conditions for 
compound powder 
other than rare earth element 
powder 
Compact 
__________________________________________________________________________ 
(i) 
1 to 20 -- -- in nitrogen-containing 
atmosphere 
1700 to 2100.degree. C. 
(ii) 
1 to 20 in exces of 0 and not more 
-- in nitrogen-containing 
than 10 atmosphere 
1600 to 2100.degree. C. 
(iii) 
1 to 20 -- in excess of 
in nitrogen-containing 
0 and not atmosphere 
more than 10 
1600 to 1800.degree. C. 
__________________________________________________________________________ 
The silicon nitride powder serving as raw material powder preferably 
contains at least 90 vol. % of silicon nitride powder in the form of any 
of .alpha.-silicon nitride, .beta.-silicon nitride or amorphous silicon 
nitride. The most preferable is the use of .alpha.-silicon nitride and 
amorphous silicon nitride powders. A whisker-like raw material is not 
preferable, because it is not possible to control pore diameter, porosity, 
pore distribution and strength of the porous body, in case of using 
silicon nitride whiskers. Also, the replacement of silicon nitride 
component during liquid phase sintering does not occur in case of using 
silicon nitride whiskers. In this case, the oxygen content in the silicon 
nitride powder is more preferably at least 1 wt. % and not more than 8 wt. 
%. 
The porous body which is treated with an acid and/or an alkali is 
preferably further heat treated in the atmosphere at a temperature of at 
least 200.degree. C. and not more than 1500.degree. C. The heat treatment 
is preferably performed in the atmosphere at a temperature of at least 
200.degree. C. and not more than 1000.degree. C. 
In the method of preparing the porous body mainly composed of silicon 
nitride serving as the target that is treated with an acid and/or an 
alkali, the aforementioned step of preparing the mixed powder preferably 
includes a step of further adding a carbon source to the aforementioned 
silicon nitride powder so that not more than 1.0 wt. % of carbon remains 
in the compact before the heat treatment step. 
The silicon nitride porous body according to the present invention contains 
a plurality of silicon nitride crystal grains, and the pores are formed in 
the grain boundary parts thereof. Alternatively, the silicon nitride 
porous body according to the present invention comprises a body part and a 
pore part, so that the body part is formed by a plurality of silicon 
nitride crystal grains and the pore part forms a three-dimensional network 
structure. In this case, the abundance ratio of the silicon nitride 
crystal grains forming the body part is preferably at least 90 vol. %, 
more preferably at least 99 vol. %, in the part other than the hole or 
pore part, i.e., in the body part. Silicon nitride crystals are so stable 
that the same stably exist even under an environment containing an acid 
and/or an alkali. This leads to an advantage that a filter or a catalytic 
carrier prepared from the inventive porous body exerts no influence on a 
gas or liquid being filtered or treated. If the abundance ratio of the 
silicon nitride crystal grains is less than 90 vol. %, however, elution or 
chemical reaction is easily caused under such an environment, such that a 
catalytic carrier prepared from the porous body may act as catalytic 
poison to reduce reaction, for example. 
In this case, at least 50 vol. % of the silicon nitride crystal grains 
forming the body part are preferably .beta.-silicon nitride crystal 
grains. If the ratio of the .beta.-silicon nitride crystal grains is less 
than 50 vol. %, the number of .alpha.-silicon nitride crystal grains is 
increased, leading to a tendency that bonding between the crystal grains 
is weakened. In that case, when the additive existing in the bonded 
portions is treated with an acid or an alkali, therefore, the strength of 
the porous body may be so reduced that a filter or a catalytic carrier 
prepared from the porous body is inferior in durability if particularly 
high strength is required thereof. 
More preferably, at least 80 vol. % of the .beta.-silicon nitride crystal 
grains are formed by columnar grains. In addition to the columnar grains, 
grains having shapes close to polygonal and spherical ones may be formed 
as the .beta.-silicon nitride crystal grains. These shapes weaken the 
bonding between the crystal grains, also in case of the .alpha.-silicon 
nitride crystal grains. 
Further, it is more preferable that the average aspect ratio is at least 3 
and not more than 50 in the columnar grains of the .beta.-silicon nitride 
crystals forming the body part. If the average aspect ratio is less than 
3, the structure approaches that of a pore part formed by grains having 
crystal shapes other than those of columnar grains, and hence it is 
difficult to obtain a porous body structure mainly composed of columnar 
grains. In this case, it is particularly difficult to obtain a structure 
provided with a hole or pore part of a three-dimensional structure having 
high strength and a sharp pore diameter distribution. If the average 
aspect ratio is in excess of 50, on the other hand, target shapes may not 
be attained unless a heat treatment is performed under a high temperature 
and a high pressure for a longer time in order to achieve crystal growth 
by heat treating the compact, and hence the cost may be increased. 
In order to obtain a more preferable functional structural body, it is 
necessary to control the pore diameters as follows, in addition to the 
aforementioned conditions: Assuming that d represents the mean width of 
the columnar grains of the .beta.-silicon nitride crystals in the minor 
axis direction and r represents the mean pore diameter of the porous body, 
the relation d/10.ltoreq.r.ltoreq.10.times.d is preferably satisfied. More 
preferably, the relation d/10.ltoreq.r.ltoreq.2.times.d is satisfied. When 
the mean width d and the mean pore diameter r satisfy the aforementioned 
expression(s), the pores formed on the surface and in the interior of the 
silicon nitride porous body are in the form of long slits or wedges. Due 
to this shape effect, grains smaller than an equivalent diameter (e.g., a 
result of measurement with a mercury porosimeter) converted from circles 
of the same areas can be filtrated (slit effect). When the inventive 
silicon nitride porous body is used as a filter, the permeation flow rate 
is important as a factor of the performance of the filter. This permeation 
flow rate is proportionate to the sectional area of the pores: 
EQU (permeation flow rate)=K.multidot.(sectional area of pores)/(thickness) 
The conventional filter has substantially circular pores, and the size of 
permeable grains is substantially identical to the sectional area of the 
pores. On the other hand, a filter consisting of the inventive silicon 
nitride porous body can collect grains smaller than a pore diameter 
calculated by area conversion. In other words, it is possible to filtrate 
or filter out grains using pores having a larger sectional area than the 
filtrated grains, thereby attaining high permeability. 
FIG. 1 illustrates the relation between mean widths of columnar grains and 
mean pore diameters. A slit effect can be attained in a hatched region in 
FIG. 1. In regions out of this range, the sectional shapes of the pores 
approach circles, and the aforementioned slit effect cannot be attained. 
The volume of the pore part is preferably at least 20 vol. % and not more 
than 75 vol. % with respect to the overall porous body. In other words, 
the porosity is preferably at least 20 vol. % and not more than 75 vol. %. 
More preferably, this ratio is at least 40 vol. % and not more than 60 
vol. %. If the porosity is smaller than 20 vol. %, then pores, called 
closed pores, that are not directly continuous with the remaining pores 
may be formed and the function of the filter may not be sufficiently 
attained. If the porosity is in excess of 75 vol. %, on the other hand, 
then the distances between the columnar grains of the .beta.-silicon 
nitride crystals are so increased that it is difficult to attain the 
aforementioned slit effect. 
The porous body according to the present invention may contain an 
unavoidable impurity which is insoluble in an acid or an alkali in its 
interior. 
In the silicon nitride porous body according to the present invention, an 
unavoidable impurity may be formed under any of the following conditions 
(1) and (2): 
(1) A nitride or a carbide is formed as a transition metal compound: 
In this case, a carbide or a nitride is employed as a transition metal 
compound which is added for controlling the crystal structure of the 
silicon nitride porous body. Such a compound is hardly eluted in an acid, 
and is present between the columnar grains of the silicon nitride 
crystals. If the ratio of this compound is high, then formation of the 
columnar grains is inhibited. Particularly when this compound is contained 
to an extent of at least 8 vol. %, the ratio of the columnar grains of the 
silicon nitride crystals is reduced and the target structure of the 
present invention cannot be obtained. If the difference between thermal 
expansion coefficients of the silicon nitride crystal grains and the 
compound is large, thermal shock resistance is disadvantageously reduced. 
If the ratio of this compound is less than 1 vol. %, on the other hand, 
formation of the columnar grains of the silicon nitride crystals is hardly 
influenced and the ratio of the columnar grains in the .beta.-silicon 
nitride crystal grains is almost 100%. 
If the additive powder has smaller grain diameters than the raw material 
powder of silicon nitride, then the additive powder may be partially 
nitrided such that nitrided grains are dispersed as fine crystals in the 
interior of the crystal-grown columnar grains of the silicon nitride 
crystals. In this case, the dispersed fine crystals exert no direct 
influence on the structure of the silicon nitride crystal grains forming 
the porous body. Therefore, the presence of the fine crystals will not 
deteriorate the performance of the inventive silicon nitride porous body. 
(2) A carbide or a nitride of B or Si is mixed as an impurity: 
Following diffusion from a BN setter which is employed for firing the 
compact or formation of SiC in a carbon reducing atmosphere, a nitride or 
a carbide of any of the elements belonging to the groups IIIA and IVA of 
the periodic table may be mixed into the porous body. Similarly to the 
compound in the above item (1), this compound is hardly eluted in an acid, 
and is present between the columnar grains of the silicon nitride 
crystals. If the ratio of such a mixed compound is high, formation of the 
columnar grains of the silicon nitride crystals is inhibited. Particularly 
when such a nitride or carbide is contained to an extent of at least 8 
vol. %, the ratio of the columnar grains of the silicon nitride crystals 
is reduced and the target structure of the present invention cannot be 
obtained. If the difference between thermal expansion coefficients of the 
carbide or nitride and the silicon nitride crystal grains is large, 
thermal shock resistance is disadvantageously reduced. If the ratio of the 
carbide or nitride is less than 1 vol. %, on the other hand, then 
formation of the columnar grains of the silicon nitride crystals is hardly 
influenced by the carbide or nitride and the ratio of the columnar grains 
in the .beta.-silicon nitride crystal grains is almost 100%. 
In the method of preparing a silicon nitride porous body according to the 
present invention, a grain boundary phase part other than a skeleton 
formed by the silicon nitride crystal grains can be eluted by treating the 
porous body mainly composed of silicon nitride with an acid. In this case, 
the acid can be hydrochloric acid, sulfuric acid or nitric acid, or an 
acid prepared by combining these acids. The pH value of the acid is set to 
be not more than 4. If a weak acid having a pH value exceeding 4 is 
employed, there is a possibility that a long time is required for eluting 
the grain boundary phase part. While it is also possible to apply heat or 
pressure in the acid treatment, the grain boundary phase part can be 
sufficiently eluted under conditions of room temperature and atmospheric 
pressure. 
When a silicon oxide is formed on the surfaces of the silicon nitride 
crystal grains forming the porous body treated in the inventive method, 
the grain boundary phase part is eluted by an alkali treatment. In this 
case, the employed alkali can be potassium hydroxide or sodium hydroxide. 
As to the alkalinity, the pH value is preferably at least 13. If a weak 
alkali having a pH value of less than 13 is employed, there is a 
possibility that a long time is required for eluting the grain boundary 
phase part. While it is also possible to apply heat or pressure in the 
alkali treatment, the grain boundary phase part can be sufficiently eluted 
under conditions of room temperature and atmospheric pressure. 
It is possible to partially or entirely remove the grain boundary phase 
part formed by the additive through the aforementioned acid or alkali 
treatment. Thus, it is possible to obtain a silicon nitride porous body in 
which silicon nitride crystal grains are directly bonded to each other for 
mainly forming a skeleton part. In this case, the ratio of the silicon 
nitride grains occupying the body part of the silicon nitride porous body 
is preferably at least 90 vol. %, more preferably at least 99 vol. %. 
When the porous body treated with the acid and/or the alkali is further 
heat treated in the atmosphere in the temperature range of at least 
200.degree. C. and not more than 1000.degree. C., an oxy-nitride remaining 
after the acid and/or alkali treatment can be homogeneously distributed on 
the surfaces of the silicon nitride crystal grains. Due to the heat 
treatment, an element inhibiting the slit effect can be removed. Further, 
an Si-O-N film formed on the surface is hydrophilic, whereby pure water 
permeability of the porous body is improved as compared with that before 
the heat treatment. 
If the aforementioned heat treatment is performed at a temperature lower 
than 200.degree. C., however, the oxy-nitride remains unchanged and the 
aforementioned effect cannot be attained. If the heat treatment is 
performed at a temperature higher than 1000.degree. C., on the other hand, 
oxidation of the silicon nitride crystal grains progresses and reduces the 
strength. When the decrease in strength is permitted, the temperature can 
exceed 1000.degree. C., up to 1500.degree. C. 
In the aforementioned heat treatment, the heating time is not particularly 
defined. However, the heat treatment is preferably performed in the 
atmosphere for not more than five hours, in consideration of the 
preparation cost. 
Before the acid treatment, the porous body preferably has porosity of at 
least 19 vol. % and not more than 74 vol. %. If the porosity is not more 
than 19 vol. %, the pores may be partially blocked in the interior. Thus, 
the acid solution may hardly infiltrate into the interior of the porous 
body during the acid treatment. If the porosity is larger than 74 vol. %, 
on the other hand, the porous body to be treated is so easy to break that 
the same is hard to handle in case of performing the acid treatment. 
In the method of preparing the porous body to be subjected to the acid or 
alkali treatment, a compound of a rare earth element, a transition metal 
element or bismuth is added to the raw material powder. 
The compound of the rare earth element is adapted to react with an oxide 
layer which is present on the surface of the silicon nitride powder during 
the heat treatment of the compact for forming a liquid phase, dissolving 
silicon nitride and depositing columnar .beta.-silicon nitride crystal 
grains. After the heat treatment, further, the compound of the rare earth 
element is present as a grain boundary phase in the exterior of .alpha.- 
and .beta.-silicon nitride crystal grains. The term "rare earth element" 
indicates any of scandium (Sc), yttrium (Y) and lanthanoid elements. The 
compound of the rare earth element is suitably added in the range of 1 to 
20 vol. %, and more preferably in the range of 2 to 15 vol. % in terms of 
an oxide. The grain boundary phase may be in the form of silicate or 
oxy-nitride. If the content of the compound of the rare earth element is 
less than 1 vol. %, then the .beta.-silicon nitride crystal grains are not 
sufficiently brought into columnar shapes. If the content exceeds 20 vol. 
%, on the other hand, then bonding between the silicon nitride crystal 
grains is so inhibited that a large amount of oxy-nitride is formed to 
suppress formation of the columnar grains of the silicon nitride crystals. 
When the obtained porous body is treated with the acid or the alkali, 
therefore, its strength is disadvantageously reduced. If the compound of 
the rare earth element is added in a large amount, then the preparation 
cost is increased since the rare earth element is generally high-priced. 
On the other hand, it is possible to facilitate sintering of the silicon 
nitride powder and reduce the formation temperature for the liquid phase 
by adding a compound of a transition metal element. The compound of the 
transition metal element is suitably added in the range in excess of 0 
vol. % and not more than 10 vol. %, and more preferably in the range of at 
least 2 vol. % and not more than 5 vol. % in terms of an oxide of each 
element. If the content of the compound of the transition metal element 
exceeds 10 vol. %, bonding between the silicon nitride grains is so 
inhibited that the strength of the obtained porous body is 
disadvantageously reduced if the porous body is treated with an acid. 
Further, crystal growth is inhibited in the process of bringing the 
silicon nitride grains into columnar shapes, and the aspect ratio is 
disadvantageously reduced below 3. 
Further, it has been discovered that phase transition (from .alpha. to 
.beta.) or crystallization (from amorphous to .beta.) of silicon nitride 
is facilitated by adding a bismuth compound in place of the compound of 
the transition metal element so that dissolution of the grain boundary 
phase part is simplified in the acid or alkali treatment on the obtained 
porous body, as a result of the inventors' study. Namely, it is possible 
to prepare a silicon nitride porous body on which an acid and/or alkali 
treatment can be readily performed by adding a bismuth compound in the 
range in excess of 0 vol. % and not more than 10 vol. %, more preferably 
in the range of at least 1 vol. % and not more than 5 vol. %, in place of 
the compound of the rare earth element added to the raw material powder of 
silicon nitride. 
The silicon nitride powder employed as the raw material can be prepared 
from .alpha.-silicon nitride powder, amorphous silicon nitride powder 
and/or .beta.-silicon nitride powder. It is preferable that any single 
silicon nitride powder occupies at least 90 vol. % of the overall silicon 
nitride powder. If different types of silicon nitride powder are contained 
in excess of 10 vol. %, formation of the columnar grains of the silicon 
nitride crystals or the sintering speeds for the silicon nitride powder 
may be dispersed to result in formation of large pores or abnormally grown 
grains. 
The most preferable is the use of .alpha.-silicon nitride and amorphous 
silicon nitride powders. A whisker-like raw material is not preferable, 
because it is not possible to control pore diameter, porosity, pore 
distribution and strength of porous body, and replacement of silicon 
nitride component during liquid phase sintering does not occur, in case of 
using silicon nitride whiskers. 
In the silicon nitride powder employed as the raw material, it is 
preferable to employ .alpha.-silicon nitride powder, .beta.-silicon 
nitride powder or amorphous silicon nitride powder having an oxygen 
content of at least 1 wt. % and not more than 8 wt. %. If the oxygen 
content is less than 1 wt. %, the amount of formation of the liquid phase 
is so insufficient that columnar grains are hard to grow. If the oxygen 
content exceeds 8 wt. %, on the other hand, bonding between the silicon 
nitride crystal grains is so weakened that desorption of the crystal 
grains or the like is disadvantageously caused. Further, a large amount of 
oxynitride is generated to reduce the purity of silicon nitride to below 
90 vol. %. 
It is most general to add the aforementioned compound of the rare earth 
element, the transition metal element or the bismuth compound to the raw 
material powder as oxide powder. However, it is also possible to add a 
compound such as a hydroxide or an alkoxide which is decomposed to form 
powder of a hydroxide or an oxide to the raw material powder in the form 
of a liquid or a solid such as powder. 
The aforementioned raw material powder and the additive powder are mixed 
with each other by a prescribed method such as a ball mill method, and 
thereafter molded. The molding can be performed by a prescribed method 
such as pressing or extrusion molding. The molding density is preferably 
at least 30% and not more than 70%, and more preferably set in the range 
of at least 35% and not more than 60%. Increase of the porosity of the 
porous body can be expected through the later acid treatment, and hence a 
molding density of not more than 70% is sufficient. If the molding density 
is less than 30% however, strength of the compact is reduced to cause a 
problem in handling. If the molding density exceeds 70% on the other hand, 
the liquid phase generated on the basis of the additive is so hard to 
diffuse that it is difficult to form the columnar grains of the silicon 
nitride crystals. 
The obtained compact is heat treated in a nitrogen containing atmosphere at 
a temperature of at least 1600.degree. C. after a molding assistant such 
as resin is removed by thermal decomposition or the like. Phase transition 
to .beta.-silicon nitride (in case of employing .alpha.-silicon nitride 
powder or amorphous silicon nitride powder) or crystal growth (in case of 
employing .beta.-silicon nitride powder) progresses due to this heat 
treatment, whereby the porous body is converted to that mainly consisting 
of columnar grains of .beta.-silicon nitride crystals. The heat treatment 
temperature is varied with the composition of the additive, the grain 
diameters of the raw material powder, and the mean pore diameter and the 
porosity of the target porous body. 
When only a compound of a rare earth element such as yttrium oxide is added 
to the silicon nitride powder serving as the raw material powder, for 
example, the heat treatment must be performed in a high temperature region 
of at least 1700.degree. C. In this case, no further densification 
progresses even if the heat treatment is performed at a higher 
temperature, and hence the heat treatment can be performed in a 
temperature region extremely increasing the pore diameters. 
When a compound of a transition metal other than the rare earth element is 
added in place of the compound of the rare earth element, on the other 
hand, a liquid phase is formed from a low temperature region of at least 
1600.degree. C., and silicon nitride dissolved in this liquid phase is 
deposited as columnar .beta.-silicon nitride crystal grains. Therefore, it 
is possible to prepare a silicon nitride porous body which is mainly 
formed by columnar grains by a heat treatment even in the low temperature 
region. In this case, sufficient grain growth is not attained if the heat 
treatment temperature for the compact is less than 1600.degree. C. While 
densification progresses when the heat treatment is performed at a high 
temperature, the additive which is present in the grain boundary phase is 
partially or entirely removed by the acid and/or alkali treatment after 
the heat treatment, whereby the compact can be used as a porous body. 
If the mean grain diameter of the added compound of the transition metal 
element is smaller than that of the silicon nitride powder serving as the 
raw material powder, the widths of the columnar grains in the minor axis 
direction are increased as the amount of the additive is increased. 
Namely, the widths of the formed columnar grains of the silicon nitride 
crystals in the minor axis direction can be controlled by the amount of 
addition of the added compound. 
In case of any additive, densification so progresses that the porosity 
cannot be improved by the acid and/or alkali treatment and the compact 
cannot be used as a porous body if the compact is heat treated at a 
temperature exceeding 2100.degree. C. In the heat treatment at such a high 
temperature, the nitrogen partial pressure must be at least several 100 
atm. and the preparation cost is disadvantageously increased in view of 
the apparatus. 
Further, the decomposition pressure for silicon nitride is increased under 
a high temperature, and hence the nitrogen partial pressure must be 
increased depending on the heat treatment temperature. The heat treatment 
atmosphere may be prepared from an inactive atmosphere containing 
nitrogen, such as a mixed atmosphere of an inert gas such as argon (Ar) 
and nitrogen. 
In the step of preparing the mixed powder, it is possible to increase the 
porosity of the finally obtained porous body by adding a carbon source 
such as phenol to the silicon nitride powder for leaving not more than 1.0 
wt. %, more preferably at least 0.1 wt. % and not more than 0.5 wt. % of 
carbon in the compact before the heat treatment step, as compared with the 
case of adding no carbon source. If the carbon source is added to the 
silicon nitride powder, the carbon remaining in the compact suppresses 
rearrangement behavior of the silicon nitride raw material powder caused 
during the heat treatment, thereby suppressing reduction of the porosity 
following growth of crystal grains. 
The carbon conceivably has the following effect, although this effect has 
not yet been clarified or confirmed: 
The residual carbon has an action of reducing SiO.sub.2 existing on the 
surface of the silicon nitride powder. Thus, a ratio Y.sub.2 O.sub.3 
/(SiO.sub.2 +Y.sub.2 O.sub.3) in a liquid phase of SiO.sub.2 --Y.sub.2 
O.sub.3, which is formed in case of adding Y.sub.2 O.sub.3 powder as an 
assistant, for example, increases due to reduction of SiO.sub.2. Columnar 
crystals are readily deposited as this value is increased, and hence 
densification is inhibited and the porous body has high porosity. 
If the carbon content before the heat treatment is less than 0.1 wt. %, the 
ratio of the carbon occupying the compact is so small that no effect is 
attained. If the carbon content exceeds 1.0 wt. %, on the other hand, the 
ratio Y.sub.2 O.sub.3 /(SiO.sub.2 +Y.sub.2 O.sub.3) in the liquid phase 
exceeds the upper limit. In this case, the liquid phase formation 
temperature is so increased that the liquid phase is hardly formed or a 
viscous liquid phase is formed and hence silicon nitride is hardly 
dissolved in the liquid phase, the speed of movement of the dissolved 
component in the liquid phase is reduced, spherical Si.sub.3 N.sub.4 
grains are hardly converted to columnar crystals and high strength cannot 
be attained as the result, although columnar crystals are readily 
deposited. 
There are several methods of mixing in the carbon. In general, a binder 
containing carbon is employed for preparing a compact. The compact is 
subjected to a debindering treatment in the atmosphere and finally 
sintered, while the amount of residual carbon after the debindering 
treatment can be controlled by controlling the amount of the added binder 
and the debindering conditions. The amount of residual carbon is reduced 
as the debindering temperature or the debindering time is increased. Under 
the same debindering conditions, the binder is less removed and the amount 
of residual carbon is readily increased as the amount of the binder added 
for the molding is increased. 
While the amount of residual carbon is reduced as the debindering 
temperature is increased, the silicon nitride powder is oxidized and hence 
the amount of SiO.sub.2 existing on the surface of the silicon nitride 
powder is increased, in addition to such reduction of the amount of 
residual carbon, if the temperature is excessively increased. In this 
case, the ratio Y.sub.2 O.sub.3 /(SiO.sub.2 +Y.sub.2 O.sub.3) in the 
liquid phase which is present in sintering is decreased, and hence silicon 
nitride columnar crystal grains are hardly grown and densification readily 
progresses, as hereinabove described. Consequently, no three-dimensional 
entangled structure of silicon nitride columnar grains intended by the 
present invention can be attained but the porous body disadvantageously 
has low strength and small porosity. If the debindering temperature 
exceeds 1000.degree. C., oxidation of the silicon nitride powder 
unpreferably abruptly progresses. If the debindering temperature is not 
more than 200.degree. C., on the other hand, the binder is hardly removed 
and the amount of residual carbon is disadvantageously increased. 
If the concentration of the carbon source remaining in the compact before 
the heat treatment is less than 0.1 wt. %, however, the ratio of carbon 
occupying the overall compact is so small that the effect of suppressing 
rearrangement is small. If the concentration of the carbon source 
remaining in the compact is in excess of 1.0 wt. %, on the other hand, the 
remaining carbon inhibits growth of the silicon nitride crystal grains and 
bonding between the crystal grains, and hence it is difficult to prepare a 
silicon nitride porous body in the scope of the present invention. 
In case of adding a bismuth compound in place of the compound of the rare 
earth element or the transition metal element, the bismuth compound is 
evaporated at a temperature around 1850.degree. C. and hence the compact 
is preferably heated in a lower temperature range. In this case, the heat 
treatment temperature is set to be not more than 1800.degree. C. in 
general. 
The porous body obtained after the heat treatment has a structure 
consisting of a grain boundary phase formed from the compound of the rare 
earth element, the transition metal element or bismuth, or a silicon 
material derived from the silicon nitride powder, and a skeleton formed by 
silicon nitride crystal grains, mainly formed by .beta.-silicon nitride 
crystals, which are bonded to each other, depending on the type of the 
additive. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
(Example 1) 
Yttrium oxide powder of 0.55 .mu.m in mean grain diameter was added to 
.alpha.-silicon nitride powder of 0.5 .mu.m in mean grain diameter 
(.alpha.-ratio: 99%) in amounts shown in Table 2, and these powder 
materials were mixed with each other in a ball mill for 72 hours, with a 
solvent of ethanol. The oxygen content of the .alpha.-silicon nitride 
powder was 2.0 wt. %. 
The mixed powder materials obtained in the aforementioned manner were dried 
and thereafter molded in a metal mold of 100 mm by 100 mm under a pressure 
of 35 kg/cm.sup.2, with addition of molding assistants. The obtained 
compacts were about 15 mm in thickness and about 42% in relative density 
in all compositions. The relative density of each compact was obtained by 
dividing a compact density, which is calculated by measuring the weight 
and the dimensions, by a theoretical density, which is the weighted mean 
of silicon nitride and the additive. The compacts were heat treated under 
conditions shown in Table 2, thereby obtaining silicon nitride porous 
bodies. 
These porous bodies were held in hydrochloric acid of 10N concentration for 
2 hours, whereby yttrium chloride was generated and the colors of the 
solutions turned pale yellow. The temperature of the hydrochloric acid was 
22.degree. C. at this time. When these solutions were analyzed, yttrium 
ions were detected in amounts substantially identical to those of the 
additives. Thus, it was conceivably possible to substantially completely 
remove the additives by the acid treatments. 
Test pieces of 3 mm by 4 mm by 40 mm for a three-point bending test in 
accordance with JIS 1601 were prepared from these porous bodies. These 
test pieces were employed for measuring values of bending strength 
(strength) at ordinary, e.g. room, temperature. Further, porosity values 
were calculated from relative densities, as follows: 
EQU porosity (%) 100-relative density (%) 
In addition, X-ray diffraction was performed for obtaining ratios 
(.beta.ratios) of .beta.-silicon nitride crystal grains from X-ray 
diffraction peak intensity ratios, as follows: 
EQU (ratio of .beta.-silicon nitride) (%)={a/(A+B)}.times.100 
where A represents X-ray diffraction peak intensity of .beta.-silicon 
nitride, and B represents X-ray diffraction peak intensity of 
.alpha.-silicon nitride. 
Cutting planes were observed with a scanning electron microscope (SEM), 
thereby obtaining ratios (ratios of columnar grains) of columnar grains of 
.beta.-silicon nitride crystals relative to silicon nitride crystal grains 
of other shapes. Measurements were also taken of the mean widths (crystal 
grain widths) of the columnar grains of the .beta.-silicon nitride 
crystals in the minor axis direction and aspect ratios. The aspect ratios 
shown in Table 2 are arithmetic means of the aspect ratios of the columnar 
grains observed on the cutting planes. On the other hand, mean pore 
diameters were measured with a mercury porosimeter (AUTOSCAN-60 by 
Quantachrome Co.). Table 2 shows these results too. 
TABLE 2 
__________________________________________________________________________ 
Additive Porous Body Characteristics 
Heating 
Atmosphere 
Mean Pore 
Crystal Ratio of 
Y.sub.2 O.sub.3 
Temperature 
Time 
Pressure 
Porosity 
Diameter 
Aspect 
Grain Width 
Strength 
.beta. ratio 
Columnar 
No. 
(vol. %) 
(.degree.C.) 
(H) (atm) (%) (.mu.m) 
Ratio 
(.mu.m) 
(MPa) 
(%) Grains 
__________________________________________________________________________ 
1 0 1800 2 4 60 1 -- 1 1.4 28 0 
2 1 1800 2 4 41 0.6 3.8 0.65 62 100 94 
3 2 1800 2 4 52 0.71 15 0.5 99 100 95 
4 4 1800 2 4 53 0.8 15 0.5 167 100 100 
5 12 1800 2 4 71 1.2 13 0.59 83 97 100 
6 20 1800 2 4 74 2.5 8 0.57 71 96 99 
7 30 1800 2 4 83 12.5 4 1.2 35 100 97 
8 4 1500 2 4 66 0.11 -- 0.5 0.3 18 0 
9 4 1600 2 4 63 0.26 3.8 1.5 1.1 24 0 
10 4 1700 2 4 58 0.31 18 0.7 128 96 96 
11 4 1800 2 4 53 0.8 15 0.5 167 100 100 
12 4 1900 2 4 49 0.95 10 1.5 106 100 99 
13 4 2000 2 4 43 0.49 6.9 1.7 63 100 97 
14 4 2100 2 4 36 0.39 3.7 2 47 100 98 
15 4 2200 2 100 16 0.24 2 2.5 10 100 100 
16 4 1800 1 4 58 0.25 10 0.41 87 96 92 
17 4 1800 5 4 56 0.3 13 0.36 157 100 100 
18 4 1800 10 4 50 0.3 17 0.44 178 100 100 
19 4 1800 20 4 46 0.18 16 1.4 63 100 100 
__________________________________________________________________________ 
Changes of the characteristics of the porous bodies in case of changing the 
amounts of the additives and the heating temperatures are understood from 
Table 2. In the sample No. 1 containing no additive, no columnar grains 
were generated and no progress of phase transition to .beta.-silicon 
nitride was attained. On the other hand, the sample No. 7 contained the 
additive in excess of 20 vol. %. It is understood that the pores formed by 
the acid treatment were increased in diameter beyond the range capable of 
attaining a preferable slit effect. In the samples Nos. 8 and 9, the 
heating temperatures were lower than 1700.degree. C. In this case, 
columnar grains were hardly formed and preferable structures were not 
obtained although the samples were brought into porous states. In the 
sample No. 15, the heating temperature was in excess of 2100.degree. C. In 
this case, the porosity was reduced below 20 vol. %, with formation of 5 
vol. % of closed pores. The ratio of the closed pores was calculated from 
apparent porosity and a cumulative pore volume measured with the mercury 
porosimeter. 
As to the sample No. 4, three-point bending strength measured in case of 
performing no acid treatment was 173 MPa, and it was proved that the 
strength was hardly deteriorated when the acid treatment was performed. 
(Example 2) 
Porous bodies were prepared by a method similar to that in Example 1 except 
that oxide powder materials of various rare earth elements shown in Table 
3 were employed as compounds of rare earth elements in place of yttrium 
oxide powder, and evaluated. Table 3 shows the results. It is understood 
from these results that similar silicon nitride porous bodies can be 
obtained also when compounds of rare earth elements other than yttrium 
oxide are employed. 
TABLE 3 
__________________________________________________________________________ 
Additive Mean Pore 
Crystal Grain Ratio of 
Ratio 
Porosity 
Diameter 
Aspect 
Width Strength 
.beta. Ratio 
Columnar Grains 
Additive 
(%) (%) (.mu.m) 
Ratio 
(.mu.m) 
(MPa) 
(%) (%) 
__________________________________________________________________________ 
La.sub.2 O.sub.3 
4 53 1.8 12 1.4 135 100 98 
CeO.sub.2 
4 54 2.4 9 1.4 114 100 99 
Nd.sub.2 O.sub.3 
4 57 1.7 11 1.3 106 100 100 
Gd.sub.2 O.sub.3 
4 55 2.3 14 1.5 127 100 96 
Dy.sub.2 O.sub.3 
4 59 2.7 12 1.6 145 100 97 
Yb.sub.2 O.sub.3 
4 57 2.4 13 1.5 136 100 99 
__________________________________________________________________________ 
(Example 3) 
Silicon nitride porous bodies were prepared by a method similar to that in 
Example 1 except that 4 vol. % of yttrium oxide which is an oxide of a 
rare earth element serving as an additive A and titanium oxide (mean grain 
diameter: 0.5 .mu.m) or zirconium oxide (mean grain diameter: 0.65 .mu.m), 
which is a compound of a transition metal element other than the rare 
earth element, for serving as an additive B were added in each sample, and 
evaluated. Table 4 shows the results. 
TABLE 4 
__________________________________________________________________________ 
Porous Body Characteristics 
Additive Mean Pore 
Crystal Ratio of 
Heating 
Porosity 
Diameter 
Aspect 
Grain Width 
Strength 
.beta. ratio 
Columnar 
No. 
Additive B 
(vol. %) 
Temperature 
(%) (.mu.m) 
Ratio 
(.mu.m) 
(MPa) 
(%) Grains (%) 
__________________________________________________________________________ 
1 TiO.sub.2 
0.5 1800 45 0.61 12 1 120 98 84 
2 TiO.sub.2 
1.2 1800 42 0.63 14 0.7 150 99 96 
3 TiO.sub.2 
2 1800 40 0.54 16 0.5 175 100 100 
4 TiO.sub.2 
5 1800 35 0.52 11 0.5 225 100 100 
5 TiO.sub.2 
10 1800 28 0.21 8.9 0.3 345 97 96 
6 TiO.sub.2 
15 1800 15 4.3 3.3 0.33 320 96 94 
7 TiO.sub.2 
2 1500 47 0.3 -- 0.3 0.3 11 0 
8 TiO.sub.2 
2 1600 44 0.3 4.6 0.5 75 65 88 
9 TiO.sub.2 
2 2100 31 0.21 15 1.5 287 100 100 
10 TiO.sub.2 
2 2200 18 0.02 12 2.6 396 100 100 
11 ZrO.sub.2 
0.5 1800 43 0.57 13 1.2 149 99 88 
12 ZrO.sub.2 
1.2 1800 40 0.67 12 0.8 164 99 96 
13 ZrO.sub.2 
2 1800 38 0.49 14 0.6 168 100 100 
14 ZrO.sub.2 
5 1800 30 0.39 11 0.7 241 100 100 
15 ZrO.sub.2 
10 1800 24 0.24 9.1 0.42 321 96 97 
16 ZrO.sub.2 
15 1800 11 5.1 3.1 0.41 445 94 95 
17 ZrO.sub.2 
2 1500 48 0.3 -- 0.36 0.16 
13 0 
18 ZrO.sub.2 
2 1600 47 0.3 6 0.44 102 71 83 
19 ZrO.sub.2 
2 2100 22 0.18 4 1.4 246 100 100 
20 ZrO.sub.2 
2 2200 8 0.03 2 1.9 369 100 100 
__________________________________________________________________________ 
From Table 4, it is clearly understood that it is possible to prepare a 
silicon nitride porous body at a lower temperature (1600.degree. C.) than 
that employed in Example 1 with addition of only the compound of the rare 
earth element. In the samples Nos. 8 and 18, it was possible to prepare 
target porous bodies of the present invention at the heat treatment 
temperature of 1600.degree. C. Further, it was possible to remove not only 
the compound of the rare earth element but also grain boundary phase parts 
resulting from the compounds of the transition metal elements by acid 
treatments. 
The samples Nos. 6 and 16 contained the additives B in excess of 10 vol. %. 
In this case, the mean pore diameters of the porous bodies obtained by the 
acid treatments were increased beyond the range capable of attaining a 
preferable slit effect. In the sample No. 7, the heating temperature was 
lower than 1600.degree. C. In this case, columnar grains were hardly 
formed and a preferable structure was not obtained although the sample was 
brought into a porous state. 
(Example 4) 
Porous bodies were prepared by a method similar to that in Example 1 except 
that 4 vol. % of yttrium oxide, which is an oxide of a rare earth element, 
serving as an additive A and bismuth oxide, which is one of bismuth 
compounds, serving as an additive B were added to each sample, and 
evaluated. Table 5 shows the results. 
TABLE 5 
__________________________________________________________________________ 
Porous Body Characteristics 
Additive Mean Pore 
Crystal Grain Ratio of 
Heating 
Porosity 
Diameter 
Aspect 
Width Strength 
.beta. ratio 
Columnar Grains 
No. 
Additive B 
(vol. %) 
Temperature 
(%) (.mu.m) 
ratio 
(.mu.m) 
(MPa) 
(%) (%) 
__________________________________________________________________________ 
1 Bi.sub.2 O.sub.3 
0.5 1700 57 0.45 18 0.69 110 96 96 
2 Bi.sub.2 O.sub.3 
1.2 1700 59 0.61 16 0.71 126 97 100 
3 Bi.sub.2 O.sub.3 
2 1700 61 0.75 13 0.73 137 100 100 
4 Bi.sub.2 O.sub.3 
5 1700 65 0.99 10.3 
0.88 150 100 100 
5 Bi.sub.2 O.sub.3 
10 1700 69 1.2 4.9 0.91 110 100 100 
6 Bi.sub.2 O.sub.3 
15 1700 78 10.3 2.4 0.93 1.9 100 100 
7 Bi.sub.2 O.sub.3 
2 1500 65 0.28 -- 0.37 3.1 21 0 
8 Bi.sub.2 O.sub.3 
2 1600 63 0.39 16 0.58 76 89 96 
9 Bi.sub.2 O.sub.3 
2 1800 55 1 15 0.51 148 100 100 
__________________________________________________________________________ 
As clearly understood from Table 5, it was possible to reduce the formation 
temperatures for the columnar grains by adding bismuth, without changing 
the structures of the silicon nitride porous bodies. Further, the added 
bismuth oxide existed as grain boundary phases, which were removable by 
acid treatments. In particular, reaction was readily attained at room 
temperature using hydrochloric acid of pH 1, and it was possible to 
perform the acid treatments in short times. 
The sample No. 6 contained the additive B in excess of 10 vol. %. In this 
case, the pores formed by the acid treatment were increased in diameter 
beyond the range capable of attaining a preferable slit effect. In the 
sample No. 7, the heating temperature was lower than 1600.degree. C. In 
this case, columnar grains were hardly formed and a preferable structure 
was not obtained although the sample was brought into a porous state. 
(Example 5) 
When the sample No. 4 prepared in Example 1 was further treated in sodium 
hydroxide having a pH value of 14, a small amount of silicon was eluted in 
the solution. The amount of elution was 4 to 10 ppm. 
When this sample was further placed without agitation in sodium hydroxide 
having a pH value of 14 and sodium hypochlorite having a pH value of 12 
for 24 hours at 20.degree. C. and 110.degree. C. respectively, elution of 
silicon was not more than a measurement limit (several ppm). 
It is understood from the aforementioned results that the inventive porous 
body can be used as an extremely stable filter or catalytic carrier with 
no elution of silicon or the like when it is used in an environment in 
coexistence with an alkali. 
(Example 6) 
Flat plates of 25 mm in diameter and 0.5 mm in thickness were prepared from 
the samples Nos. 1, 4 and 7 (hereinafter referred to as samples Nos. 1-1, 
1-4 and 1-7 respectively) of Example 1 and the sample No. 6 (hereinafter 
referred to as a sample No. 3-6) of Example 3 respectively. These flat 
plates were employed for allowing permeation therethrough of water in 
which latex standard grains having homogeneous grain diameters were 
dispersed, thereby measuring the minimum grain diameters of collectable 
grains. This is a method of estimating or judging whether or not grains of 
prescribed sizes permeate the pores, based on concentration changes before 
and after permeation, thereby measuring the filter performance under the 
same conditions as actually exist when employing the porous bodies as 
filters. This is in comparison to a method employing a mercury porosimeter 
for calculating the pore areas from a pressure necessary for press fitting 
and from volume changes and consequently measuring diameters of circles 
having equivalent areas as pore diameters. Results obtained by the 
presently described measurement also serve as means for measuring the 
maximum pore diameters of the porous bodies. 
Table 6 shows the results. 
TABLE 6 
______________________________________ 
Pore Diameter 
Minimum Filterable 
Sample (.mu.m) Grain Diameter (.mu.m) 
______________________________________ 
1-1 1 1.5 
1-4 0.8 0.2 
1-7 12.5 15.0 
3-6 4.3 6.0 
______________________________________ 
From the aforementioned results, it is understood that it is possible to 
collect grains smaller than the pore diameters in the sample No. 1-4, 
which is capable of attaining a slit effect. In the remaining samples, 
however, it was possible to collect only grains larger than the pore 
diameters, since the pore diameters had distributions such that the 
maximum pore diameters were larger than the mean pore diameters. 
Table 7 shows pure water permeation flow rates of the sample No. 1-4 and a 
comparative .alpha.-alumina filter having non-slit pores capable of 
collecting grains of 0.2 .mu.m. 
TABLE 7 
______________________________________ 
Pure Water 
Pore Sectional Area (per pore) 
Permeation Flow Rate 
Sample (.mu.m.sup.2) (Ml/min/cm.sup.2) 
______________________________________ 
1-4 1.0 32.8 
.alpha.-alumina 
0.04 2.49 
______________________________________ 
The sample No. 1-4 has a high permeation flow rate since the permeation 
flow rate is generally proportionate to the sectional area of the pores. 
It is empirically known that the permeation flow rate of a porous body 
having the same structure as this is proportionate to the plane area of 
the pores. However, the results obtained this time are different, 
conceivably because the pores of the alumina porous body are close to 
circular shapes and the alumina grains forming the porous body have 
polygonal shapes close to spherical shapes in the comparative 
.alpha.-alumina filter, while columnar grains are entangled with each 
other to define a three-dimensional space and holes or pores are in the 
form of wedges or slits in the sample No. 1-4 , and hence the difference 
between the hole shapes influenced the results. 
While the alumina porous body had a porosity of 30 to 40%, that of the 
silicon nitride porous body (sample No. 1-4 ) was 53%. This difference is 
also reflected in the difference between the permeation flow rates. The 
porosity can be maximized up to 75% through the amount of the additive and 
by the acid/alkali treatment. In another porous body (alumina or the like) 
having similar pore diameters, on the other hand, the porosity is about 
40% in general, and the porous body is so reduced in strength that the 
same cannot withstand actual use as a filter or a catalytic carrier if the 
porosity exceeds this value. 
(Example 7) 
In the sample No. 4 of Example 1, silicon nitride porous bodies were 
prepared through a process similar to that of Example 1, with 
.alpha.-silicon nitride powder serving as a raw material powder having 
oxygen contents of 0.5 wt. % and 10 wt. % respectively. In case of the 
oxygen content of 0.5 wt. %, the ratio of columnar grains among the 
.beta.-silicon nitride grains was 1.3 vol. %. This is conceivably because 
the amount of liquid phase formation was so insufficient that the columnar 
grains were hard to grow. In case of the oxygen content of 10 wt. %, on 
the other hand, crystal grains fell out due to an acid treatment. Further, 
an oxy-nitride was formed to reduce purity of the silicon nitride to 88%. 
The crystal grains fell out since the oxy-nitride and the grain boundary 
phase were present between the columnar grains, which inhibited direct 
bonding between the silicon nitride crystal grains (particularly the 
columnar grains). Such inhibitors result from reaction between an oxide 
layer on the surface of the raw material powder and the additive. 
Therefore, it is possible to obtain a porous body which is formed by 
directly bonded silicon nitride crystal grains by setting the oxygen 
content in the range of 1 to 8 wt. %. 
(Example 8) 
In the sample No. 4 of Example 1, powder containing 85 vol. % of 
.alpha.-silicon nitride powder (mean grain diameter: 0.5 .mu.m) and 15 
vol. % of .beta.-silicon nitride powder (mean grain diameter: 0.55 .mu.m) 
was employed to prepare a silicon nitride porous body through a process 
similar to the above. In this case, heterogeneous crystal growth or 
contraction resulted in the interior of the silicon nitride porous body, 
such that the mean pore diameter was 4.0 .mu.m and the mean width of the 
columnar grains in the minor axis direction was 0.33 .mu.m. Thus, it was 
impossible to obtain a porous body having a preferable slit effect. 
(Example 9) 
Table 8 shows results obtained from silicon nitride porous bodies prepared 
under the same conditions as those for the samples Nos. 11 to 16 of 
Example 3 except that zirconium oxide having a mean grain diameter of 0.2 
.mu.m was employed. 
TABLE 8 
__________________________________________________________________________ 
Porous Body Characteristics 
Additive Mean Pore 
Crystal Grain Ratio of 
Heating 
Porosity 
Diameter 
Aspect 
Width Strength 
.beta. ratio 
Columnar Grains 
No. 
Additive B 
(vol. %) 
Temperature 
(%) (.mu.m) 
Ratio 
(.mu.m) 
(MPa) 
(%) (%) 
__________________________________________________________________________ 
1 ZrO.sub.2 
0.5 1800 43 0.69 16 0.5 136 99 89 
2 ZrO.sub.2 
1.2 1800 40 0.56 14.2 
0.56 169 98 94 
3 ZrO.sub.2 
2 1800 38 0.49 12.6 
0.75 173 100 100 
4 ZrO.sub.2 
5 1800 30 0.39 13 0.89 251 100 100 
5 ZrO.sub.2 
10 1800 24 0.33 9.1 1.2 336 97 96 
6 ZrO.sub.2 
15 1800 11 5.1 2.9 0.48 462 91 87 
__________________________________________________________________________ 
From the aforementioned results, it is understood that the powder of 
zirconium oxide was more homogeneously dispersed to influence growth of 
the columnar grains such that the mean widths (crystal grain widths) of 
the columnar grains in the minor axis directions were increased as the 
added amounts of zirconium oxide were increased. The sample No. 6 
contained zirconium oxide in excess of 10 vol. %. In this case, pores 
formed by the acid treatment were increased in diameter beyond the range 
of the mean pore diameter and the mean width of the columnar grains in the 
minor axis direction capable of attaining a preferable slit effect. 
(Example 10) 
In the sample No. 15 of Example 1, the porosity was 13 vol. % before the 
acid treatment. Since the porosity before the acid treatment was not more 
than 19 vol. %, it is understood that the grain boundary phase was not 
completely removed by the acid treatment. 
(Example 11) 
The sample No. 4 in Example 1 was heated in the atmosphere at a temperature 
of 1000.degree. C., and thereafter thrown into ice water of 0.degree. C. 
to be subjected to a thermal shock. The measured three-point bending 
strength of this sample was 165 MPa, which was substantially identical to 
that before application of the thermal shock. Thus, it is understood that 
this sample can withstand a thermal shock of 1000.degree. C. 
(Example 12) 
In the method of Example 1, 5 vol. % of yttrium oxide and titanium nitride 
were added to samples which were then molded and heat treated, thereby 
preparing silicon nitride porous bodies. Table 9 shows the amounts of 
added titanium nitride. The obtained porous bodies were treated in aqueous 
hydrochloric acid solutions of 10N concentration, to measure the ratios of 
columnar crystal grains in .beta.-silicon nitride. Table 9 shows these 
results too. 
TABLE 9 
______________________________________ 
Sample No. TiN (vol. %) 
Ratio of Columnar grains (%) 
______________________________________ 
1 0.5 100 
2 1.5 95 
3 7.0 75 
4 12.0 36 
______________________________________ 
When the amount of titanium nitride (TiN) was less than 1.0 vol. %, the 
ratio of the columnar crystal grains was 100%. On the other hand, it was 
understood that a preferable structure was not obtainable when the amount 
of titanium nitride was in excess of 8 vol. %, although the sample was 
converted to a porous state with formation of a small amount of columnar 
grains. 
(Example 13) 
FIGS. 2 and 3 show pore diameter distributions measured with a mercury 
porosimeter (AUTOSCAN-60 by Quantachrome Co.) as to silicon nitride porous 
bodies (samples 1 and 2) having mean pore diameters of 1.2 .mu.m and 1.0 
.mu.m and ratios of columnar grains of 20% and 95% respectively. In the 
porous body having a low ratio of columnar grains, the pore diameter 
distribution tends to be broad. When the ratio of columnar grains exceeds 
80%, on the other hand, a sharp pore diameter distribution is attained as 
shown in FIG. 3. Permeability for a gas or a liquid is improved and higher 
strength can be attained as the pore diameter distribution is narrowed. 
(Example 14) 
Porous bodies prepared from the sample No. 4 obtained in Example 1 were 
heat treated in the atmosphere for two hours, at temperatures of 
200.degree. C., 500.degree. C. and 800.degree. C. respectively. When the 
surfaces of these porous bodies were analyzed with FT-IR (Fourier 
transform infrared spectroscopic analysis), peaks of Si-O-N and Si-N were 
observed. These porous bodies were worked into disks of 25 mm in diameter 
and 0.5 mm in thickness, for measuring pure water permeation flow rates. 
Table 10 shows the results. 
TABLE 10 
______________________________________ 
Heat Treatment Temperature 
Pure Water Permeation Flow Rate 
(.degree.C.) (ml/min/cm.sup.2) 
______________________________________ 
untreated 32.8 
200 37.5 
500 51.9 
800 64.1 
______________________________________ 
The pure water permeation flow rates were increased as the heat treatment 
temperatures were increased, such that the sample heat treated at the 
temperature of 800.degree. C. attained pure water permeability which was 
twice that of an untreated product. 
As to pore diameter distributions and filterability, on the other hand, no 
differences were observed between the heat treated samples and the 
untreated product. 
While the reason why the pure water permeation flow rate is increased when 
the silicon nitride porous body is heat treated in the atmosphere is not 
clear, it is supposed that the surface states of the crystal grains 
forming the silicon nitride porous body are changed to those having higher 
hydrophilicity, considering the results of the FT-IR. If the heat 
treatment temperature exceeds 1000.degree. C., oxidation of the silicon 
nitride porous body so abruptly progresses that the strength may be 
reduced, and hence the heat treatment temperature is preferably not more 
than 1000.degree. C. When the silicon nitride porous body is employed in 
an application that does not require particular strength, however, the 
heat treatment may be performed at a temperature exceeding 1000.degree. C. 
While water permeability of the silicon nitride porous body is improved as 
the heat treatment temperature is increased in this case, the degree of 
reduction of the strength of the silicon nitride porous body is so 
increased that the silicon nitride porous body cannot be used for a filter 
if the heat treatment temperature exceeds 1500.degree. C. 
A silicon nitride porous body which is treated in the aforementioned manner 
can be used for a filter such as a membrane filter, to exhibit extremely 
high permeability. 
(Example 15) 
Table 11 shows changes of porosity and collection ratios for latex grains 
of 0.2 .mu.m in grain diameter, when phenol is added as a carbon source in 
the step of preparing the mixed powder of the sample No. 4 in Example 1, 
and when the residual carbon contents are varied in the stage of forming 
the compact. 
TABLE 11 
______________________________________ 
Residual Carbon 
Porosity 0.2 .mu.m Latex Grain Collection Ratio 
(wt. %) (%) (%) 
______________________________________ 
0 53 99.9 
0.1 53.5 99.9 
0.2 55 99.9 
0.5 57 99.9 
1.0 61 99.9 
1.5 65 25 
______________________________________ 
It is understood that the porosity is increased as the residual carbon 
content in the compact is increased. It is also understood that the 
collection ratio for the latex grains is reduced and no slit effect can be 
attained if the residual carbon content in the compact is more than 1.0 
wt. %. It is understood that the porosity of the porous body does not so 
improve if the residual carbon content in the compact is less than 0.1 wt. 
%. 
When the carbon contents of the prepared silicon nitride porous bodies were 
measured, it was understood that carbon remained in an amount of not more 
than 0.1 wt. % in the finished porous body because the carbon was removed 
in the final heat treatment step, as to the sample that had the residual 
carbon content of not more than 1.0 wt. % in the non-heat-treated compact 
stage. 
Although the present invention has been described with reference to certain 
Examples, variations and modifications are possible within the scope of 
the present invention, in addition to the above described Examples. 
The silicon nitride porous body obtained according to the present invention 
contains a plurality of silicon nitride crystal grains so that holes or 
pores are formed in the grain boundary parts thereof, or comprises a body 
part and a hole or pore part so that the body part is formed by a 
plurality of silicon nitride crystal grains and the hole part forms a 
three-dimensional network structure. Therefore, it is possible to provide 
a porous body which can be effectively employed as a filter or a catalytic 
carrier. 
Further, the body part of the inventive silicon nitride porous body is 
formed by at least 90 vol. % of silicon nitride crystal grains, whereby 
the porous body is stable against an acid or an alkali and exerts no 
influence on a filtered substance when the same is used as a filter of a 
chemical apparatus, for example. 
Also when the inventive porous body is employed as a catalytic carrier, the 
porous body will not react with a catalyst in a manner that would suppress 
catalytic reaction. 
Further, at least 50 vol. % of the silicon nitride crystal grains forming 
the inventive silicon nitride porous body are .beta.-silicon nitride 
crystal grains, whereby the porous body has excellent mechanical strength 
and durability. When at least 80 vol. % of the .beta.-silicon nitride 
crystal grains are formed by columnar grains having an average aspect 
ratio of at least 3 and not more than 50, further, it is possible to 
obtain a porous body structure in which the columnar grains are directly 
bonded to each other. When the porous body is employed as a filter or a 
catalytic carrier, therefore, it is possible to attain excellent 
mechanical strength and superior durability for allowing long-term use. 
In addition, a slit effect can be attained by controlling the widths of the 
columnar grains in the minor axis direction and the mean pore diameter, 
whereby the inventive silicon nitride porous body can be employed as a 
filter having both of permeability and separability. 
According to the inventive method, a grain boundary phase part containing 
an impurity such as the additive is removed by an acid and/or alkali 
treatment to leave stable silicon nitride crystal grains, whereby it is 
possible to provide a filter which can be used in an environment in 
coexistence with an acid or an alkali. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.