Method of preparing a ceramic porous body

Abstract

A ceramics porous body having high porosity as well as high strength is especially suitable for use as a filter for removing foreign matter from a fluid or as a catalytic carrier. The porous body has a porosity of at least 30% and comprises columnar ceramic grains having an aspect ratio of at least 3. In particular, the porous body comprises Si.sub.3 N.sub.4 grains, of which at least 60% are hexagonal columnar .beta.-Si.sub.3 N.sub.4 grains. The porous body further comprises at least one compound of a rare earth element in an amount of at least 1 volume % and not more than 20 volume % in terms of an oxide of the rare earth element, and optionally at least one compound of elements of the groups IIa and IIIb of the periodic table and transition metal elements in an amount of not more than 5 volume % in terms of an oxide of each element. A compact of mixed powder obtained by adding the compound powder of the rare earth element to silicon nitride powder is heat treated in a nitrogen atmosphere at a temperature of at least 1500.degree. C., to prepare the silicon nitride ceramic porous body.

Description

FIELD OF THE INVENTION

The present invention generally relates to a ceramics porous body which is useful as a filter material for removing foreign matter from a fluid or as a catalytic carrier, and more specifically, it relates to a silicon nitride ceramics porous body and a method of preparing the same.

BACKGROUND ART

Known porous bodies employed for filter materials or catalytic carriers, include those consisting of various materials such as resin, metals or ceramics. Among these, a filter or a catalytic carrier consisting of a ceramics material is generally employed in high temperature or strongly corrosive environments which cannot with stand other materials. A filter or a catalytic carrier consisting of oxide ceramics such as alumina (Al.sub.2 O.sub.3) has already been put into practice.

As to a porous body consisting of nonoxide ceramics, on the other hand, only small examples have been put into practice while Japanese Patent Laying-Open No. 63-291882 discloses a silicon nitride based or silicon carbide based porous body prepared by a heat treatment. Further, Japanese Patent Laying-Open No. 1-188479 discloses a method of compacting mixed powder of silicon powder and silicon nitride powder of relatively coarse particles and thereafter nitriding the same thereby preparing a porous body as a solid target.

As hereinabove described, it is difficult to use a porous body consisting of resin or a metal in a high temperature or corrosive atmosphere. It is inevitably necessary to employ a porous body made of ceramics for a filter for removing foreign matter from a high-temperature exhaust gas or for a carrier serving as a catalyst for decomposing a harmful matter.

As an example of such porous bodies made of ceramics, porous bodies made of alumina have been put into practice. While the porous bodies of alumina are varied in pore size, porosity and bending strength, a porous body having a porosity of 35 to 40% and a mean pore size of 25 to 130 .mu.m has a bending strength of 20 to 35 MPa, whereby the strength of the porous body is insufficient depending on its use.

In the silicon nitride based porous body disclosed in the aforementioned Japanese Patent Laying-Open No. 63-291882, the porosity is less than 30% and fluid permeability is insufficient. In general, the strength of ceramics tends to be reduced following an increase in the porosity, and it has been extremely difficult to attain compatibility between the porosity and the strength.

SUMMARY OF THE INVENTION

In view of the above, the present invention has been proposed in order to solve the aforementioned problems, and an object thereof is to provide a ceramics porous body having high porosity as well as high strength.

The present inventors have deeply studied the aforementioned subject, and discovered that it is possible to prepare a silicon nitride ceramics porous body that is mainly composed of columnar .beta.-Si.sub.3 N.sub.4 (.beta.-silicon nitride) crystal grains and capable of maintaining high strength also when its porosity is high, by heat treating a compact of mixed powder of silicon nitride (Si.sub.3 N.sub.4) powder and prescribed additive powder at a high temperature.

Namely, a ceramic porous body according to the present invention is generally characterized in that it has a porosity of at least 30% and is mainly composed of columnar ceramics grains having an aspect ratio of at least 3. Specifically, it is a porous body having a mean pore size of at least 0.05 .mu.m and not more than 12 .mu.m. Further, the crystal grains preferably have the shape of hexagonal poles or columns, i.e. rod-like crystal shape with a hexagonal cross-section

More specifically, the ceramics porous body according to the present invention is a silicon nitride ceramics porous body that is mainly composed of silicon nitride with a ratio of at least 60%, preferably at least 90%, of .beta.-Si.sub.3 N.sub.4 columnar grains with respect to the entire silicon nitride grains, contains at least one compound of a rare earth element with at least 1 volume % and not more than 20 volume % of an oxide of the rare earth element, and has a porosity of at least 30%.

The aforementioned silicon nitride ceramics porous body may contain at least one compound of elements of the groups IIa and IIIb of the periodic table and transition metal elements, with not more than 5 volume % of an oxide of each element. Further, the silicon nitride ceramics porous body according to the present invention preferably has bending strength of at least 80 MPa at an ordinary temperature i.e. room temperature, and bending strength of at least 50 MPa at a temperature of 1000.degree. C.

In summary, further, a method of preparing a silicon nitride ceramics porous body according to the present invention comprises the following steps.

A first step involves adding at least one compound powder of a rare earth element in an amount corresponding to at least 1 volume % and not more than 20 volume % of an oxide of the rare earth element, or further adding at least one compound of elements of the groups IIa and IIIb of the periodic table and transition metal elements in an amount corresponding to not more than 5 volume % of an oxide of each element, to silicon nitride powder, thereby preparing a mixed powder.

A subsequent step involves preparing a compact from the aforementioned mixed powder.

A further subsequent step involves heat treating the compact in a nitrogen atmosphere at a temperature of at least 1500.degree. C. and not more than 2100.degree. C.

In the present invention, the compound of a rare earth element acts to react with SiO.sub.2 existing on the surface of the raw material of the silicon nitride (Si.sub.3 N.sub.4) powder during the heat treatment for forming a liquid phase and solidly dissolving Si.sub.3 N.sub.4, thereby precipitating columnar .beta.-Si.sub.3 N.sub.4 crystal grains. Further, the compound of the rare earth element acts to exist outside the .beta.-Si.sub.3 N.sub.4 grains as a grain boundary phase after the heat treatment, for joining the .beta.-Si.sub.3 N.sub.4 grains and maintaining strength. The rare earth element indicates an Sc, Y or lanthanoid element. The ratio of the added compound of the rare earth element is suitably in the range of 1 to 20 volume % of an oxide of the rare earth element, and more preferably 2 to 10 volume %. The form of the grain boundary phase is a silicate such as Y.sub.2 O.sub.3.SiO.sub.2, or an oxynitride such as Y.sub.2 O.sub.3.Si.sub.3 N.sub.4. Columnarization of the .beta.-Si.sub.3 N.sub.4 crystal grains is not sufficient if the added quantity of the compound of the rare earth element is less than 1 volume %, while oxidation resistance and strength at a high temperature are reduced if the amount exceeds 20 volume %, which further leads to increase in preparation cost since the rare earth element is generally high-priced.

The compound(s) of the element(s) of the group(s) IIa and/or IIIb of the periodic table and/or the transition metal element(s) is/are added when a sintered body is prepared, in general. The aforementioned compound of the rare earth element acts to reduce a liquid phase forming temperature, facilitate densification and improve strength when the same is employed with the compound(s) of the element(s) of the group(s) IIa and/or IIIb of the periodic table and/or the transition metal element(s). The elements of the group IIa of the periodic table are Be, Mg, Ca, Sr and the like, the elements of the group IIIb are B, Al, Ga and the like, and the transition metal elements are Fe, Ti, Zr and the like.

In order to prepare a porous body having high porosity, the added or additional ratio of the compound of such an element is preferably small. The added amount is suitably not more than 5 volume % of an oxide of each element, preferably not more than 2 volume %, and more preferably not more than 1 volume %.

Due to addition of the compound(s) of the element(s) of the group(s) IIa and/or IIIb of the periodic table and/or the transition metal element(s), on the other hand, the liquid phase is formed in a lower temperature region, whereby grain growth also takes place in a low temperature region. This is conceivably because the grain growth is caused by re-precipitation of Si.sub.3 N.sub.4 which is dissolved in the liquid phase, to reduce a grain growth starting temperature. When the compound(s) of the element(s) of the group(s) IIa and/or IIIb and/or the transition metal element(s) is/are added, therefore, it is possible to obtain a high-strength porous body at a low temperature, thereby attaining an advantage in view of the preparation cost. Further, such grain growth in a low temperature region tends to form fine crystal grains, whereby it is possible to prepare a porous body having a small pore size.

When the additional ratio of the compound(s) of the element(s) of the group(s) IIa and/or IIIb and/or the transition metal element(s) exceeds 5 volume %, densification is disadvantageously caused before columnar grain growth takes place to reduce porosity of the porous body while oxidation resistance is reduced due to a high densification effect from the low temperature region.

Particularly when a compound of a IVa group element such as Ti among the transition metals is added, the compound reacts with .beta.-Si.sub.3 N.sub.4 at a high temperature of at least 1600.degree. C. and it is possible to increase the bonding strength between the crystal grains, whereby a porous body of high strength can be obtained.

While the Si.sub.3 N.sub.4 powder employed as a raw material is mainly composed of .alpha.-Si.sub.3 N.sub.4 in general, .beta.-Si.sub.3 N.sub.4 or amorphous silicon nitride may alternatively be employed as the raw material. A mean grain size of the silicon nitride powder is preferably at least 0.1 .mu.m and not more than 20 .mu.m. If the mean grain size of the silicon nitride powder is less than 0.1 .mu.m, agglomeration of the powder materials is so intensely caused that the relative density of the compact as obtained is not more than 30%, and the handling strength of the compact as well as the strength of the porous body after the heat treatment are insufficient. When the mean grain size of the silicon nitride powder exceeds 20 .mu.m, on the other hand, the degree of sintering by the heat treatment is reduced and the porous body cannot attain a strength of at least 80 MPa.

Most generally the aforementioned compound of the rare earth element and the compound(s) of the element(s) of the group(s) IIa and/or IIIb of the periodic table and/or the transition metal element(s) is/are added as oxide powder materials, but it is also possible to add the same as compounds such as hydroxides or alkoxides which are decomposed to form powder materials of hydroxides or oxides. It is also possible to add these compounds in the form of nitride powder materials or the like.

These powder materials are mixed with each other by a prescribed method such as a ball mill method, and thereafter compacted. Also as to the compacting method, it is possible to employ a prescribed method such as die pressing or CIP (cold isostatic pressing). The compact density varies with the characteristics of the powder materials and the target porosity of the porous body.

In order to facilitate growth of columnar grains as well as to attain a high porosity, the compact density is preferably low. In order to ensure attainment of the strength that is required for handling the compact, and to improve the strength of the porous body after the heat treatment, however, it is necessary to prepare the compact with a compact density exceeding a certain constant level. When commercially available .alpha.-Si.sub.3 N.sub.4 powder is employed, it is preferable to set the compact density at 30 to 60% of theoretical density, more preferably at 35 to 50%. If only the compound of the rare earth element is added, the porosity after the heat treatment exceeds 30% when the compact density is less than 30% in relative density, while the pore size is also increased and a porous body having high bending strength cannot be obtained even if columnar crystals are formed. When the compact density exceeds 60% in relative density, on the other hand, it is possible to attain sufficiently high bending strength in the porous body, while porosity is less than 30% and the pore size is also reduced.

The compact as obtained is heat treated in a nitrogen atmosphere at a temperature of at least 1500.degree. C. after a compacting assistant (resin or the like) is removed by thermal decomposition or the like. Transition to .beta.-Si.sub.3 N.sub.4 (in a case of employing .alpha. powder) and grain growth (columnarization) proceed by the heat treatment, so that the compact is converted to a porous body mainly consisting of .beta.-Si.sub.3 N.sub.4 columnar grains. The heat treatment temperature varies with the composition of the additive, the grain size of the raw material powder, and the mean pore size and the porosity of the target porous body.

When only a compound of a rare earth element such as Y.sub.2 O.sub.3 is added, for example, it is necessary to carry out the heat treatment in a high temperature region of at least 1700.degree. C. In this case, no remarkable densification proceeds even if the heat treatment is carried out at a higher temperature, and hence it is also possible to carry out the heat treatment in a temperature region extremely increasing the pore size. When the compound(s) of the element(s) of the group(s) IIa and/or IIIb of the periodic table and/or the transition metal element(s) is/are added in addition to the compound of the rare earth element, on the other hand, a liquid phase is formed in a low temperature region and Si.sub.3 N.sub.4 which is dissolved in this liquid phase is precipitated as columnar .beta.-grains as described above, whereby it is possible to prepare a porous body of high strength also by a heat treatment in a low temperature region. However, a heat treatment which is carried out at a high temperature is improper as a method of preparing a porous body due to progress of densification. The densification is readily facilitated and the porosity is readily reduced as the additional amount of the compound(s) of the element(s) of the group(s) IIa and/or IIIb and/or the transition metal element(s) is increased.

Therefore heat treatment temperature for the compact is preferably in the range of 1600.degree. to 1900.degree. C. amount of addition of the compound(s) of the element(s) of the group(s) IIa and/or IIIb and/or the transition metal element(s) is in excess of 0 volume % and not more than 1 volume %, 1600.degree. to 1850.degree. C. if the amount of addition of the compound is in excess of 1 volume % and not more than 2 volume %, and 1500 to 1700.degree. C. if the amount of addition is in excess of 2 volume % and not more than 5 volume %. In general, grain growth is not sufficient if the heat treatment temperature for the compact is less than 1500.degree. C.

Since silicon nitride is increased in decomposition pressure at a high temperature, it is necessary to increase a nitrogen partial pressure with the heat treatment temperature. The atmosphere of the heat treatment may be an inert atmosphere containing nitrogen, and a mixed atmosphere of argon (Ar) or the like may be employed. A temperature of at least 1700.degree. C. is required when no compound(s) of the element(s) of the group(s) IIa and/or IIIb of the periodic table is added, but a heat treatment at a temperature exceeding 2100.degree. C. is advantageous for preparation of a porous body having a large pore size due to extreme grain growth. However, the nitrogen partial pressure must be at least several 100 atm. in order to control the porosity and the bending strength to be in the inventive ranges, i.e. at least 30% and at least 80 MPa at room temperature and at least 50 MPa at a temperature of 1000.degree. C. respectively, and the cost is disadvantageously increased in view of the apparatus required to achieve this. When the heat treatment is carried out at a temperature exceeding 2100.degree. C., further, the use of the porous body is also disadvantageously restricted due to a tendency that the bending strength of the porous body is also reduced. Thus, the heat treatment temperature is preferably not more than 2100.degree. C.

The porous body thus obtained has a structure in which .beta.-Si.sub.3 N.sub.4 columnar crystal grains are joined with each other by a grain boundary phase that is formed from the compound of the rare earth element, the compound(s) of the element(s) of the group(s) IIa and/or IIIb of the periodic table and/or the transition metal element(s), or a Si substance derived from the Si.sub.3 N.sub.4 powder, and that exhibits high strength also when the porosity is high. It is conceivable that the reasons why such a high strength is achieved are that the inventive porous body has a structure in which the columnar crystal grains are entangled with each other dissimilarly to a generally employed Al.sub.2 O.sub.3 porous body having a polycrystalline network structure consisting of spherical crystal grains, and that the columnar grains have extremely high strength (several GPa) since they are single crystals having substantially no defects.

In this porous body, it is possible to arbitrarily control the mean pore size within the range of at least 0.05 .mu.m and not more than 12 .mu.m by the grain size of the raw material powder and the compact density. If the mean pore size is less than 0.05 .mu.m, development of the columnar grains is not sufficient and the aspect ratio thereof is less than 3. As a result, the porosity is disadvantageously reduced. When the mean pore size exceeds 12 .mu.m, on the other hand, the sizes of the crystal grains exceed 36 .mu.m in length and 12 .mu.m in breadth, which reduces the strength. Therefore, it is possible to employ the inventive porous body in the field of microfiltration etc. in a higher temperature environment or in an arrangement in which it receives a load, by controlling the mean pore size within the aforementioned range.

The ratio of .beta.-Si.sub.3 N.sub.4 forming the columnar grains is preferably at least 60% of the entire Si.sub.3 N.sub.4, and more preferably at least 90% thereof. The ratio of .beta.-Si.sub.3 N.sub.4, is thus defined at an extremely high value, since .alpha.-Si.sub.3 N.sub.4, which is another crystal form of Si.sub.3 N.sub.4, exhibits a spherical shape and causes a reduction in strength. When .beta.-silicon nitride columnar grains are at least 60% and less than 90% of the entire silicon nitride grains, the crystal structure thereof is in such a form that .alpha.-silicon nitride grains and .beta.-silicon nitride columnar grains are composed with each other. In this case, the .beta.-columnar grains couple portions where .alpha.-crystal grains exist with each other, whereby it is possible to attain higher strength than that having a .beta.-transition ratio of less than 60%. Further, growth of such columnar grains also serves to prevent densification. Since Si.sub.3 N.sub.4 exhibits high oxidation resistance, the silicon nitride ceramics porous body can be employed without breakage even when a high load is applied at a high temperature. Further, the silicon nitride ceramics porous body according to the present invention has high strength and a low coefficient of thermal expansion, whereby it also has excellent characteristics as to a thermal shock.

While specially a silicon nitride ceramics porous body has been described above, the inventive body is not limited thereto because both high porosity and high strength essentially result from a structure in which, the columnar grains are entangled with each other, regardless of the specific ceramic composition. Thus, another material having such a structure in which columnar grains are entangled with each other also provides a similar effect. For example, such behavior is recognized also in aluminum nitride containing Si and a sintering assistant such as an oxide of a rare earth element as impurities. In general, therefore, it is possible to attain the aforementioned effect in a ceramics porous body having a porosity of at least 30% when the same is mainly composed of columnar ceramics grains having an aspect ratio of at least 3. The porous body is excellent in the aforementioned effect when the aspect ratio, which indicates a ratio of the length to the breadth of the columnar grains, is high in general, while the effect of improvement in strength is small if the aspect ratio is less than 3. In many samples according to the invention, the aspect ratio is at least 10. The highest aspect ratio for any of the inventive samples is 20 (see e.g. the eighth and ninth samples in Table 4.1).

Further, the columnar grains of the silicon nitride ceramics porous body have a hexagonal pole or rod shape having a hexagonal cross-section. In this case, the pores are formed by side surfaces of the hexagonal poles. It has been proved as the result of study by the inventors that, when the side surfaces, which are planes, are covered with a metal (platinum, for example) serving as a catalyst, the metal can uniformly adhere onto the surfaces and is thereby to be improved in performance as a catalyst.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE OF THE INVENTION

EXAMPLE 1

An yttrium oxide powder material of 0.5 .mu.m mean grain size (specific surface area: 7 m.sup.2 /g) was added to a silicon nitride powder material mainly composed of .alpha.-silicon nitride (.alpha.-Si.sub.3 N.sub.4) of 0.3 .mu.m mean grain size (specific surface area: 11 m.sup.2 /g), and mixed with ethanol as a solvent in a ball mill for 72 hours. Amounts of addition of the yttrium oxide powder material are shown in Table 1.

Mixed powder materials obtained in the aforementioned manner were dried and thereafter compacted using a metal die of 100 mm.times.100 mm dimensions under a pressure of 20 kg/cm.sup.2 with addition of a compacting assistant. Compacts as obtained were about 15 mm in thickness and about 35% in relative density in every composition. The relative density was determined by dividing the compact density, which was calculated from measurements of the weight and dimensions, by the theoretical density, which was a weighted mean of silicon nitride and the additive.

The compacts as obtained were heat treated under conditions shown in Table 1, thereby obtaining porous bodies. Test pieces of 3 mm.times.4 mm.times.40 mm in size for a three-point bending test in accordance with JIS 1601 were cut out from the porous bodies. The test pieces were employed for measuring bending strength values at an ordinary room temperature and at 1000.degree. C. Further, porosity values were calculated from the relative density values (porosity (%)=100 - relative density (%)). In addition, .beta.-transition ratios were obtained from X-ray diffraction peak intensity ratios, by carrying out X-ray diffraction through the porous bodies as obtained. The calculation expression is shown below.

(.beta.-transition ratio) (%)={A/(A+B)}.times.100 where A represents X-ray diffraction peak intensity of .beta.-silicon nitride, and B represents an X-ray diffraction peak intensity ratio of .alpha.-silicon nitride.

A scanning electron microscope (SEM) was employed to observe broken-out sections, thereby obtaining mean crystal grain sizes. Mean pore sizes were measured with a mercury porosimeter. These measurement results are shown in Table 1.

TABLE 1__________________________________________________________________________ Heat Treatment Conditions Porous Body CharacteristicsAdditive Retention Pressure of Mean Crystal Grain Size Bending Strength .beta.-Transition 3 Y.sub.2 O.sub.3 Temperature Time Atmosphere Porosity Pore Size Length Breadth Room Temperature 1000.degree. C. RatioNo. (Vol %) (.degree.C.) (H) (atm) (%) (.mu.m) (.mu.m) (.mu.m) (MPa) (MPa) (%)__________________________________________________________________________ 1 0 1800 2 4 60 0.5 -- 0.5 7 1 30 2 0.5 1800 2 4 45 0.8 1 0.5 80 80 100 3 1 1800 2 4 39 1.5 3 0.8 150 150 100 4 2 1800 2 4 48 1.8 12 0.8 130 100 100 5 4 1800 2 4 48 0.8 15 1.0 120 100 100 6 8 1800 2 4 58 3.5 20 1.5 100 85 100 7 12 1800 2 4 57 3.0 20 1.6 110 70 100 8 20 1800 2 4 55 4.0 18 1.8 100 60 100 9 30 1800 2 4 50 3.0 25 2.0 90 40 10010 4 1500 2 4 61 0.3 -- 0.4 5 0.7 1511 4 1600 2 4 60 0.4 1.5 0.4 6 0.8 2012 4 1700 2 4 58 1.0 3 0.5 85 55 7013 4 1700 2 4 56 2.0 10 0.8 100 80 9014 4 1800 2 4 55 2.5 15 1.2 120 100 10015 4 1900 2 10 55 3.5 20 1.5 110 100 10016 4 2000 2 40 54 8.0 35 2.0 90 80 10017 4 2100 2 100 54 12.0 50 3.0 80 60 10018 4 1800 1 4 54 2.5 12 1.2 120 90 10019 4 1800 5 4 55 3.5 20 1.5 110 90 10020 4 1800 2 10 57 3.0 20 1.5 110 100 10021 4 1650 2 4 53 0.8 2.0 0.6 61 38 5022 4 1700 2 4 52 1.0 2.3 0.7 80 50 6023 4 2100 10 100 25 13.0 42 3.8 40 32 10024 2 1700 20 10 28 0.04 0.11 0.04 65 28 75__________________________________________________________________________

EXAMPLE 2

Porous bodies were prepared by a method similar to that in Example 1 except that oxide powder materials of respective rare earth elements shown in Table 2 were employed as compounds of rare earth elements in place of yttrium oxide powder materials, and evaluated. The results are shown in Table 2. It is understood from the results that similar silicon nitride porous bodies are obtained also when rare earth oxides other than yttrium oxide are employed.

TABLE 2__________________________________________________________________________ Porous Body CharacteristicsAdditive Heat Treatment Condition Bending StrengthAdditional Retention Pressure of Crystal Grain Size Room .beta.-Transition 1A Group Ratio Temperature Time Atmosphere Porosity Pore Size Length Breadth Temperature 1000.degree. C. RatioAdditive (Vol %) (.degree.C.) (H) (atm) (%) (.mu.m) (.mu.m) (.mu.m) (MPa) (MPa) (%)__________________________________________________________________________La.sub.2 O.sub.3 4 1800 2 4 50 2.0 18 1.4 130 100 100CeO.sub.2 4 1800 2 4 52 2.2 20 1.4 100 80 100Nd.sub.2 O.sub.3 4 1800 2 4 48 2.2 15 1.4 130 90 100Gd.sub.2 O.sub.3 4 1800 2 4 52 2.4 15 1.1 120 80 100Dy.sub.2 O.sub.3 4 1800 2 4 53 2.5 16 1.3 110 90 100Yb.sub.2 O.sub.3 4 1800 2 4 55 2.8 20 1.5 100 80 100Y.sub.2 O.sub.3 4 1800 2 4 55 2.5 15 1.2 120 100 100__________________________________________________________________________

EXAMPLE 3

Porous bodies were prepared by a method similar to that in Example 1 except that yttrium oxide, being an oxide of a rare earth element, was added as an A group additive, and aluminum oxide, magnesium oxide and titanium oxide, being compounds of elements of the groups IIa and IIIb of the periodic table and a transition metal element, were added as B group additional compounds, and evaluated. The results are shown in Table 3.

As can be seen From Table 3 (which comprises Tables 3.1, 3.2, 3.3, and 3.4), it is understood that it is possible to prepare silicon nitride porous bodies at lower temperatures in the present Example than in Examples in which only rare earth oxides were added.

TABLE 3__________________________________________________________________________ Porous Body CharacteristicsAdditive Heat Treatment Condition Bending .beta.-hA Addi- B Addi- Pressure Crystal Room Trans-Group tional Group tional Temper- Retention of Atmo- Pore Grain Size Temper- ition Addi- Ratio Addi- Ratio ature Time sphere Porosity Size Length Breadth ature 1000.degree. RatioNo. tive (Vol %) tive (Vol %) (.degree.C.) (H) (atm) (%) (.mu.m) (.mu.m) (.mu.m) (MPa) (MPa) (%)__________________________________________________________________________ 1 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 0 1800 2 4 55 2.5 15 1.2 120 100 100 2 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 0.5 1800 2 4 45 2 15 1.5 150 100 100 3 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 1.2 1800 2 4 28 1.9 15 1.5 170 120 100 4 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 2 1800 2 4 12 1.5 15 1.5 220 150 100 5 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 5 1800 2 4 2 1 12 1.5 540 350 100 6 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 10 1800 2 4 4 1 10 2 350 210 100 7 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 0.5 1500 2 1 58 0.5 1.5 0.5 50 40 40 8 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 0.5 1600 2 1 54 1.5 7 0.7 80 40 90 9 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 0.5 1700 2 4 48 1.8 12 1 120 100 10010 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 0.5 1750 2 4 44 2.2 15 1.2 130 100 10011 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 0.5 1900 2 10 40 2.5 20 2.2 130 110 10012 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 2 1700 2 4 35 1 10 1.2 110 80 10013 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 2 1750 2 4 31 1.3 15 1.3 140 80 10014 Y.sub.2 O.sub.3 4 Al.sub.2 O.sub.3 5 1700 2 4 20 0.8 10 1.2 160 120 10015 Y.sub.2 O.sub.3 4 MgO 0.5 1500 2 4 53 0.9 2 0.5 70 50 7016 Y.sub.2 O.sub.3 4 MgO 1.2 1500 2 4 50 1 2.5 0.6 90 60 8017 Y.sub.2 O.sub.3 4 MgO 2 1500 2 4 42 1 3 0.7 100 60 9018 Y.sub.2 O.sub.3 4 MgO 5 1500 2 4 32 0.9 3 0.6 100 50 9519 Y.sub.2 O.sub.3 4 MgO 10 1500 2 4 26 0.8 4 0.8 130 40 10020 Y.sub.2 O.sub.3 4 MgO 0.5 1600 2 4 50 1.2 10 1.2 100 60 9021 Y.sub.2 O.sub.3 4 MgO 1.2 1600 2 4 42 1.2 10 1 110 70 9522 Y.sub.2 O.sub.3 4 MgO 2 1600 2 4 38 1.2 12 1 120 70 10023 Y.sub.2 O.sub.3 4 MgO 5 1600 2 4 30 1 12 1.3 150 70 10024 Y.sub.2 O.sub.3 4 MgO 10 1600 2 4 15 0.9 15 1.5 200 50 10025 Y.sub.2 O.sub.3 4 MgO 0.5 1800 2 4 42 1.8 20 1.8 140 100 10026 Y.sub.2 O.sub.3 4 MgO 1.2 1800 2 4 20 1.2 22 2 210 150 10027 Y.sub.2 O.sub.3 4 MgO 2 1800 2 4 2 -- 25 2 500 300 10028 Y.sub.2 O.sub.3 4 MgO 5 1800 2 4 1 -- 25 2.5 550 300 10029 Y.sub.2 O.sub.3 4 MgO 10 1800 2 4 1 -- 20 2.5 450 270 10030 Y.sub.2 O.sub.3 4 MgO 0.5 1400 2 4 55 0.8 1 0.5 40 20 3031 Y.sub.2 O.sub.3 4 MgO 0.5 1700 2 4 45 1.5 15 1.6 130 80 10032 Y.sub.2 O.sub.3 4 MgO 0.5 1800 2 4 42 1.8 20 1.8 140 100 10033 Y.sub.2 O.sub.3 4 MgO 0.5 1900 2 10 35 2.3 25 2 120 80 10034 Y.sub.2 O.sub.3 4 MgO 0.5 2000 2 100 35 3 30 2.5 70 40 10035 Y.sub.2 O.sub.3 4 TiO.sub.2 0.5 1800 2 4 45 0.6 12 1.0 150 120 10036 Y.sub.2 O.sub.3 4 TiO.sub.2 1.2 1800 2 4 42 0.6 10 0.7 200 150 10037 Y.sub.2 O.sub.3 4 TiO.sub.2 2 1800 2 4 40 0.5 8 0.5 225 170 10038 Y.sub.2 O.sub.3 4 TiO.sub.2 5 1800 2 4 35 0.5 8 0.5 315 180 10039 Y.sub.2 O.sub.3 4 TiO.sub.2 10 1800 2 4 28 0.2 4 0.3 421 350 10040 Y.sub.2 O.sub.3 4 TiO.sub.2 0.5 1600 2 4 52 0.3 7 0.5 72 38 9041 Y.sub.2 O.sub.3 4 TiO.sub.2 0.5 1700 2 4 50 0.7 8 0.8 180 110 10042 Y.sub.2 O.sub.3 8 Al.sub.2 O.sub.3 3.5 1650 10 10 18 0.03 0.09 0.04 79 42 7243 Y.sub.2 O.sub.3 8 Al.sub.2 O.sub.3 0.5 2100 20 100 25 12.5 45 13 62 48 10044 Y.sub.2 O.sub.3 8 MgO 4.5 1600 10 10 10 0.01 0.02 0.01 66 18 6845 Y.sub.2 O.sub.3 8 MgO 0.2 2100 15 100 27 15.0 38 5 55 35 10046 Y.sub.2 O.sub.3 8 TiO.sub.2 4.5 1700 10 10 5 0.04 0.08 0.03 85 41 7547 Y.sub.2 O.sub.3 8 TiO.sub.2 0.8 2100 20 100 28 12.8 29 8 72 40 100__________________________________________________________________________

EXAMPLE 4

Silicon oxide powder (20.6 volume %) and yttrium oxide powder (1.2 volume %) of 0.5 .mu.m mean grain size were added to aluminum nitride powder of 0.5 .mu.m mean grain size, and mixed with an ethanol solvent in a ball mill for 72 hours.

Mixed powder thus obtained was dried and thereafter compacted using a metal die of 10 mm.times.10 mm dimensions under a pressure of 20 kg/cm.sup.2 with addition of a compacting assistant. Density of the compact as obtained was 37% in relative density.

This compact was heat treated in the atmosphere at a temperature of 600.degree. C. for 1 hour for removing the compacting assistant, and thereafter heat treated in nitrogen at atmospheric pressure at a temperature of 1700.degree. C. for 1 hour, to obtain a porous body. Porosity, a mean pore size and a mean aspect ratio of crystal grains of this porous body were 35%, 1.6 .mu.m and 4 respectively. Three-point bending strength values at an ordinary room temperature and at 1000.degree. C. were 90 MPa and 60 MPa respectively.

EXAMPLE 5

.alpha.-silicon nitride raw powder materials of 0.3 .mu.m, 7.0 .mu.m and 12.0 .mu.m mean grain size were employed to prepare mixed powder materials so that yttrium oxide powder contents were 4 volume % in the case of the powder of 0.3 .mu.m and 5 volume % in the cases of 7.0 .mu.m and 12.0 .mu.m by a method similar to that in Example 1, thereby preparing compacts having relative density values shown in Table 4. Compact density values were adjusted by changing uniaxial compacting pressures in the range of at least 1 kg/cm.sup.2 and not more than 2000 kg/cm.sup.2. The compacts as obtained were treated and evaluated under the same conditions as those in Example 1 except that heat treatments after decomposition of a compacting assistant were carried out under the same conditions in nitrogen of 4 atm. at a temperature of 1800.degree. C. for 2 hours. The evaluation results are shown in Table 4.

From these results, it is understood that it is possible to control the mean pore sizes of the porous bodies obtained after the heat treatments by controlling the mean grain sizes of the raw powder materials and the density values of the compacts.

TABLE 4__________________________________________________________________________ Characteristics CrystalRaw Compact Heat Treatment Conditions Mean Grain Size Bending Strength .beta.-Material Additive Relative Retention Pressure of Pore Major Minor Room TransitionGrain Size Y.sub.2 O.sub.3 Density Temperature Time Atmosphere Porosity Size Axis Axis Temperature 1000.degree. Ratio(.mu.m) (Vol. %) (%) (.degree.C.) (H) (atm) (%) (.mu.m) (.mu.m) (.mu.m) (MPa) (MPa) (%)__________________________________________________________________________0.3 4 20 1800 2 4 72 1.6 22 1.7 40 35 1000.3 4 25 1800 2 4 70 1.6 20 1.7 60 50 1000.3 4 27 1800 2 4 67 1.5 20 1.5 70 50 1000.3 4 30 1800 2 4 60 1.2 18 1.2 100 80 1000.3 4 35 1800 2 4 48 0.8 15 1.0 120 100 1000.3 4 40 1800 2 4 42 0.6 10 0.8 150 130 1000.3 4 45 1800 2 4 40 0.5 6 0.5 180 150 1000.3 4 50 1800 2 4 38 0.2 4 0.2 210 180 1000.3 4 55 1800 2 4 35 0.1 2 0.1 280 230 1000.3 4 60 1800 2 4 31 0.05 1 0.07 350 280 1000.3 4 65 1800 2 4 27 0.03 1 0.06 400 350 1000.3 4 70 1800 2 4 20 0.02 1 0.05 450 400 1007.0 5 20 1800 2 4 50 5.1 22 2.5 50 40 1007.0 5 28 1800 2 4 47 3.8 20 2.1 60 50 1007.0 5 30 1800 2 4 43 2.4 18 1.7 88 70 1007.0 5 40 1800 2 4 40 1.8 15 1.2 130 100 1007.0 5 50 1800 2 4 38 1.2 14 1.1 210 150 1007.0 5 60 1800 2 4 32 0.7 12 0.8 220 180 1007.0 5 65 1800 2 4 19 0.3 10 0.5 250 200 10012.0 5 20 1800 2 4 60 6 28 3.0 50 30 10012.0 5 28 1800 2 4 60 4 25 2.5 82 65 10012.0 5 30 1800 2 4 58 3.5 16 1.8 105 88 10012.0 5 40 1800 2 4 53 3.1 12 1.7 170 103 10012.0 5 50 1800 2 4 50 2.0 8 1.4 190 120 10012.0 5 60 1800 2 4 37 1.5 7 1.3 210 180 10012.0 5 65 1800 2 4 28 1.2 5 1.1 240 200 10012.0 5 20 2100 2 100 25 13.2 45 11 43 18 100__________________________________________________________________________

EXAMPLE 6

Silicon nitride ceramics porous bodies of 0.1 to 5.0 .mu.m in mean pore size which were prepared by the inventive preparation method were worked into the form of discs of .phi.25 mm.times.0.5 mm thickness. These porous bodies were employed to carry out permeation experiments through isopropyl alcohol (20.degree. C.) and pure water (20.degree. C.). The results are shown in Table 5. Table 5 shows flow rate results in a case of employing .alpha.-alumina ceramics porous bodies having the same pore sizes as comparative examples.

It is understood from the results that the silicon nitride porous bodies have higher performance with regard to liquid permeation than flow rates the alumina porous bodies.

TABLE 5______________________________________ Grain Size Porosity IPA Flow Rate Pure Water Flow RateMaterial (.mu.m) (%) (ml(min/cm.sup.2) (ml/min/cm.sup.2)______________________________________Silicon Nitride 0.1 45 0.82 1.97Silicon Nitride 0.2 48 2.01 4.82Silicon Nitride 0.5 60 4.11 9.86Sliicon Nitride 1.0 60 14.1 33.8Silicon Nitride 2.0 55 22.5 54.0Silicon Nitride 5.0 50 40.3 96.7.alpha.-Alumina 0.1 40 0.43 1.02.alpha.-Alumina 0.2 40 1.06 2.55.alpha.-Alumina 0.5 40 1.78 4.25.alpha.-Alumina 1.0 40 4.96 11.9.alpha.-Alumina 2.0 40 8.85 21.25.alpha.-Alumina 5.0 40 17.7 42.5______________________________________ IPA (isopropyl alcohol) flow rates and pure water flow rates are permeation flow rates in pressurization at 20.degree. C. and 1.0 kg/cm.sup.2.

According to the present invention, as hereinabove described, it is possible to obtain a ceramics porous body having high porosity and high strength. This porous body, which is excellent in high temperature characteristics and chemical resistance, is useful as a filter which is employed at a high temperature or a catalytic carrier which is employed in an atmosphere having high corrosiveness.

Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims.

Claims

1. A method of producing a ceramic porous body comprising the steps:

(a) preparing a mixed powder by mixing together a silicon nitride powder and at least one compound powder of a rare earth element in an amount of at least 1 volume % and not more than 20 volume % in terms of an oxide of said rare earth element;

(b) pressing said mixed powder to form a compact; and

(c) heat treating said compact in a nitrogen atmosphere at a temperature of at least 1700.degree. C. and not more than 2100.degree. C.;

so as to produce said ceramic porous body to contain silicon nitride grains including columnar .beta. silicon nitride grains having a hexagonal cross-section in a content ratio of at least 60% of said columnar .beta. silicon nitride grains relative to a total of all of said silicon nitride grains, to have a porosity of at least 30%, and to have an aspect ratio of said columnar .beta. silicon nitride grains in the range of at least 3 and not more than 20.

2. The method of claim 1, wherein said silicon nitride powder used in said step (a) has a mean grain size in the range of at least 0.1 .mu.m and not more than 20 .mu.m, and said step (b) comprises controlling a density of said compact to be in a range of at least 30% and not more than 60%.

3. The method of claim 1, wherein said silicon nitride powder used in said step (a) contains mainly .beta. silicon nitride.

4. The method of claim 1, wherein said silicon nitride powder used in said step (a) is .beta. silicon nitride powder.

5. The method of claim 1, wherein said silicon nitride powder used in said step (a) contains mainly amorphous silicon nitride.

6. The method of claim 1, wherein said silicon nitride powder used in said step (a) is amorphous silicon nitride powder.

7. The method of claim 1, wherein said compound powder of a rare earth element used in said step (a) comprises at least one compound selected from the group consisting of alkoxides, hydroxides and nitrides of said rare earth element.

8. The method of claim 1, carried out so that said ceramic porous body has pores with a mean pore size of at least 0.05 .mu.m and not more than 12 .mu.m.

9. The method of claim 8, wherein said mean pore size is not more than 4 .mu.m and said porosity is greater than 50%.

10. The method of claim 8, wherein said porosity is at least 40%, and a bending strength of said porous body is at least 100 MPa.

11. A method of producing a ceramic porous body comprising the steps:

(a) preparing a mixed powder by mixing together a silicon nitride powder, at least one first compound powder of a rare earth element in an amount of at least 1 volume % and not more than 20 volume % in terms of an oxide of said rare earth element, and at least one second compound powder of an element selected from the group IIa, the group IIIb and the transition metal elements of the periodic table in a selected volume percentage expressed in terms of an oxide of said element and selected from the group consisting of a first percentage of more than 0 volume % and not more than 1 volume %, a second percentage of at least 1 volume % and not more than 2 volume %, and a third percentage of more than 2 volume % and not more than 5 volume %;

(b) pressing said mixed powder to form a compact; and

(c) heat treating said compact in a nitrogen atmosphere at a selected temperature in a first temperature range of at least 1600.degree. C. and not more than 1900.degree. C. when said mixed powder prepared in said step (a) contains said first percentage of said second compound powder, in a second temperature range of at least 1600.degree. C. and not more than 1850.degree. C. when said mixed powder prepared in said step (a) contains said second percentage of said second compound powder, and in a third temperature range of at least 1500.degree. C. and not more than 1700.degree. C. when said mixed powder prepared in said step (a) contains said third percentage of said second compound powder;

so as to produce said ceramic porous body to contain silicon nitride grains including columnar .beta. silicon nitride grains having a hexagonal cross-section in a content ratio of at least 60% of said columnar .beta. silicon nitride grains relative to a total of all of said silicon nitride grains, to have a porosity of at least 30%, and to have an aspect ratio of said columnar .beta. silicon nitride grains in the range of at least 3 and not more than 20.

12. The method of claim 11, wherein said mixed powder prepared in said step (a) contains said first percentage of said second compound powder, and said selected temperature for said heat treating of said step (c) is in said first temperature range.

13. The method of claim 11, wherein said mixed powder prepared in said step (a) contains said second percentage of said second compound powder, and said selected temperature for said heat treating of said step (c) is in said second temperature range.

14. The method of claim 11, wherein said mixed powder prepared in said step (a) contains said third percentage of said second compound powder, and said selected temperature for said heat treating of said step (c) is in said third temperature range.

15. The method of claim 11, wherein said silicon nitride powder used in said step (a) is .beta. silicon nitride powder.

16. The method of claim 11, wherein said silicon nitride powder used in said step (a) is amorphous silicon nitride powder.

17. The method of claim 11, wherein said compound powder of a rare earth element used in said step (a) comprises at least one compound selected from the group consisting of alkoxides, hydroxides and nitrides of said rare earth element.

18. The method of claim 11, carried out so that said ceramic porous body has pores with a mean pore size of at least 0.05 .mu.m and not more than 12 .mu.m.

19. The method of claim 18, wherein said mean pore size is not more than 4 .mu.m and said porosity is greater than 50%.

20. The method of claim 18, wherein said porosity is at least 40%, and a bending strength of said porous body is at least 100 MPa.