JP2002308678A - Porous ceramics and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain porous ceramics having uniform and fine closed pores and to provide a method for manufacturing the same. SOLUTION: The porous ceramics having uniform and fine closed pores are obtained by mixing metal powder as the precursor of the ceramics with a sintering aid, heat treating the mixture by microwave heating to nitride or oxidize the metal powder from its surfaces, and diffusing the metals into the nitrides or oxides formed as the shells of the metals. Because the porous ceramics obtained by this method have a high proportion of the closed pores and a uniform dispersion state, the ceramics show excellent characteristics when used for electronic circuit boards, or the like, requiring resistance against moisture absorption, a low dielectric constant, low dielectric loss and high mechanical strength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous ceramic which is an electric insulating material used for various printed circuit boards and the like and a lightweight and moisture-resistant structural material, and a method for producing the same.

[0002]

2. Description of the Related Art Ceramics are materials used as various structural materials and electronic component materials. In recent years, ceramics have been required to have further improved properties such as lighter weight, higher strength, and improved electrical properties. For example, in a wafer transfer stage or a drawing stage used as a part of a semiconductor manufacturing apparatus, further reduction in the weight of the stage material is required for high precision and high speed driving. In addition, as for the circuit board and the insulating material used for the electronic equipment, materials having lower dielectric constant and lower dielectric loss have been strongly demanded with the recent increase in the frequency.

[0003] For this purpose, it is considered effective to use ceramics in a porous state. For example, if the relative density of ceramics is reduced to 50%, the weight is reduced to 5%.
It can be reduced by 0%. In addition, since air has a dielectric constant of about 1 and a dielectric loss of 0, which indicates excellent electrical insulation, porous ceramics can have desirable characteristics as a material requiring a low dielectric constant and a low dielectric loss.

However, it is difficult to obtain a porous sintered body in which fine pores are uniformly dispersed simply by controlling the sintering process of the ceramic sintered body. In the normal case, problems such as a decrease in strength due to generation of coarse atmospheric pores and non-uniform characteristics occur. In addition, since most of the pores of the obtained porous sintered body are open pores, the inherent moisture resistance of the ceramic is impaired, and the electrical properties (dielectric constant and dielectric loss) due to moisture are remarkably deteriorated and variations in various properties are caused. For example, there is a problem that characteristics desired in practical use cannot be obtained.

[0005] Therefore, various techniques for obtaining a porous material having fine closed pores have been devised. For example, JP-A-3-
Japanese Patent No. 177372 discloses that S containing a phase having a different coefficient of thermal expansion to improve the toughness has a volume ratio of 0.07 to 27.5% when the closed pores are totalized.
An iC-based porous sintered body is disclosed. However, in this method, if an attempt is made to obtain a SiC-based porous sintered body having 27.5% or more closed pores, there is a problem that oxidation resistance decreases and pore diameter increases.

Japanese Unexamined Patent Publication No. Hei 5-310469 discloses a high-purity calcia sintered body having a diameter of 2 to 10 μm and a closed porosity of 5 to 15%. This sintered body is obtained by mixing a foaming agent such as phenol aldehyde or a combustible fine powder such as carbon black into a slurry of calcium carbonate and water, followed by firing.
However, in this method, there is a problem that a foaming agent or a residue of flammable fine powder is present in the closed pores, and it is difficult to maintain the shape when the foaming agent is increased, so that the closed porosity cannot be increased. is there.

Further, Japanese Patent Application Laid-Open No. 6-157157 discloses a lightweight and high-strength ceramic having closed pores formed by balancing the pressure of the closed pores inside the ceramic and the pressure in the firing furnace. . However, this method has a problem that it is difficult to control the pore diameter.

Japanese Patent Application Laid-Open No. 11-116333 discloses that borosilicate glass is phase-separated by heat treatment, a soluble phase is eluted, pulverized, and then only the surface is melted with a flame to form closed pores. Prepare porous glass with closed pores on the order of nanometers. A method is disclosed in which a mixture of glass / aggregate / resin spheres is prepared using a porous aggregate obtained by subjecting the glass to crystallization heat treatment, and a ceramic circuit board is manufactured by a green sheet laminating method. The relative permittivity of the ceramic circuit board obtained by this method is 2 or less, and the coefficient of thermal expansion is 13 to 17 ppm / ° C. In this method, the material is phase-separated by heat treatment and is limited to a material that elutes a soluble phase. Further, not only is the process complicated, but also it is necessary to use the composite in different phases, so that the original mechanical and electrical characteristics cannot be obtained. Furthermore, once the open pores are exposed to the atmosphere and moisture is adsorbed, it is difficult to completely dissociate and control this.

[0009]

In the prior art for forming closed pores as described above, it is necessary to add a foaming agent, a melt, or a second phase different from a matrix phase such as a phase having a different coefficient of thermal expansion. Therefore, there is a problem that the electrical and mechanical properties are greatly reduced by the second phase or the residue of the second phase. In addition, when the porosity is increased, the porosity and the pore diameter that can be formed are limited, for example, the matrix skeleton cannot be formed or the pore diameter cannot be controlled. Therefore, the present invention has been made to solve the above problems. That is, the present invention provides a porous ceramic having uniform and fine closed pores and a method for producing the same.

[0010]

The porous ceramic of the present invention has a relative density of less than 70% and a ratio of closed pores among all the pores is 50% or more. Furthermore, when the relative density is 5
It is less than 0%, and the ratio of closed pores in all the pores is 90% or more. In the case of ordinary porous ceramics, pores are formed between the particles as schematically shown in FIG. 2, whereas the porous ceramics of the present invention have a hollow particle as schematically shown in FIG. Because of this structure, the dense portion (skeleton portion) has a network-like structure. In addition, since it does not contain coarse pores, it has better mechanical strength than conventional porous ceramics, and also has high thermal conductivity depending on conditions. In particular, in porous ceramics having a structure in which pores of uniform diameter are dispersed for hollowing out particles, in any cross section of the porous ceramics,
The radii r1 and r2 of two adjacent holes and the width b of the ceramic portion can be set to (r1 + r2) / b> 1. More preferably, (r1 + r2) / b> 2.
Further, the porous ceramic is composed of a ceramic and an oxynitride phase. Further, the ceramic is any one of silicon nitride, silicon oxide, aluminum nitride and aluminum oxide.

Further, there is provided a ceramic circuit board, wherein at least a part of the insulating layer is made of the porous ceramic material.

Further, the porous ceramics of the present invention can be obtained by a manufacturing method in which a molded body made of a metal powder which is a precursor of the porous ceramics is prepared and heat-treated in a reaction gas. Further, a porous ceramic comprising hollow ceramic particles can be obtained by subjecting the molded body to heat treatment under microwave irradiation.
The metal powder is silicon, and the porous ceramic is silicon nitride or silicon oxide. Alternatively, the metal powder is aluminum, and the porous ceramic is aluminum nitride or aluminum oxide.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The porous ceramic of the present invention will be described in detail below, including its manufacturing method. The porous ceramic of the present invention comprises the steps of preparing a metal powder and a sintering aid powder, mixing these powders to form a mixed powder, and molding the mixed powder to form a compact. Sintering the compact in an atmosphere in which nitrogen or oxygen is present to form a sintered body of metal nitride or metal oxide. The closed pores are obtained by hollowing a metal powder which is a precursor of ceramics. The relative density and the ratio of closed pores in all pores can be controlled by the particle size of the starting metal powder. As the metal powder, a commercially available high-purity metal powder can be used. However, a natural oxide film or a thermal oxide film is formed on the surface of the metal powder by a subsequent heat treatment. In the case other than oxide ceramics, the degree of hollowing changes remarkably depending on the amount of these oxide films, and therefore, control of the amount of oxygen in the metal powder is important. The amount of oxygen is 0.4 m
It is desirable to select one having a range of not less than ol% and not more than 1.5 mol%.

The average particle size of the metal powder is 0.1 μm or more and 1 μm or more.
5 μm or less is preferable. If it is less than 0.1 μm, the specific surface area is large, so that it is difficult to control the amount of oxygen. If it is 15 μm or more, the reaction time for completely hollowing out becomes long, which is not economical.

A rare earth oxide is added to the metal powder as a sintering aid. Rare earth oxides include Yb ₂ O ₃ , Sm ₂ O ₃ ,
At least one selected from Er ₂ O ₃ , Gd ₂ O ₃ , and Y ₂ O ₃ is 0.2 mol% or more to 2.5 mol% with respect to the metal powder.
It is preferable to add 1% or less. If it is less than 0.2 mol%, diffusion of metal is not promoted and hollowing is not sufficiently performed. When the content is 2.5 mol% or more, the total porosity tends to decrease. Conventionally, Fe ₂ O ₃ , Al ₂ O _{3, and the} like, which are known as sintering aids for ceramics, are not preferable in the case of the present invention because hollowing is not sufficiently performed. Further, the average particle size of the sintering aid to be added is preferably 0.1 μm or more and 1 μm or less. If the thickness is less than 0.1 μm, it becomes difficult to handle because agglomeration or the like is likely to occur, and if it is 1 μm or more, the nitridation or oxidation reaction of the metal powder hardly proceeds. When the oxide film on the surface of the metal powder hinders the reaction, an alkali metal or an alkaline earth metal or an oxide of such a metal is added as a second sintering aid in addition to the sintering aid. Is preferred. The addition amount of the second sintering aid is 0.
1 mol% or more and 1.5 mol% or less are preferable, and the average particle diameter is preferably 0.1 μm or more and 2 μm or less.

The metal powder, a sintering aid and, if necessary, an organic binder are added and mixed by a conventional method such as ball milling or ultrasonic mixing. After mixing, dry.
Then, it is molded into a predetermined shape to obtain a molded body. Molding is
Known molding methods such as ordinary dry press molding, extrusion molding, doctor blade molding and injection molding can be used, and if the most desirable molding method in terms of quality and production is selected according to the desired shape, Good. It is also possible to granulate the mixed powder after mixing prior to molding to increase its bulk density in advance to enhance moldability. The organic binder is added when the moldability is further improved.

By heat-treating the compact in an atmosphere gas containing nitrogen or oxygen to promote the nitridation or oxidation reaction of the metal, the individual metal powders are hollowed and the reacted adjacent metal powders are nitrided. It is possible to obtain a porous ceramic having fine closed pores by integrating substances or oxides. As shown schematically in FIGS. 3 and 4, first, the surface of the metal powder is nitrided or oxidized. When the heat treatment is advanced, it appears that the metal diffuses toward the nitride or oxide on the outer periphery during the nitridation or oxidation reaction, and the nitridation or oxidation reaction proceeds, and the metal is hollowed out. For this reason, the portion which was finally the metal powder becomes a void. The degree of hollowing varies depending on the amount of oxygen contained in the starting metal powder, the type of sintering aid, or the heat treatment method. Since the size of each closed pore basically depends on the particle size of the metal powder as a starting material, if the particle size of the metal powder is uniform, the size of the closed pore is uniform. No coarse closed pores are included.

The heat treatment can be performed in a carbon heater furnace or the like. In order to promote diffusion of the metal powder and suppress disappearance of the hollow structure due to grain growth, heat treatment using microwaves is preferable. In particular, heating by irradiating a microwave having a frequency of 20 GHz or more can promote the diffusion of the metal into the metal nitride or metal oxide formed on the outer shell of the metal powder. This is preferable because it becomes easier.

Since the preferable temperature range of the heat treatment varies depending on the starting metal powder, the case of obtaining a porous ceramic of Si ₃ N ₄ by nitriding Si will be described in detail below. The heat treatment temperature for nitriding Si is 1200
C. or higher is preferred. If the temperature is lower than 1200 ° C., the progress of the nitriding reaction of the metal powder becomes slow, which is not economical. Further, the temperature is preferably 1500 ° C. or less for carbon heater heating, and 1750 ° C. or less for microwave heating. At a temperature higher than this, phase transformation and grain growth of the metal nitride occur, so that the hollowed structure changes and it becomes difficult to obtain the porous ceramic of the present invention.

It is preferable that the temperature is raised to the maximum temperature in two or more steps and the temperature is raised stepwise. This is because the nitriding reaction of the metal is an exothermic reaction, and if the temperature is raised to the final sintering temperature at once, the temperature of the metal exceeds the melting point of the metal due to its own heat generation, and the melting of the metal occurs. When the melting of the metal occurs, it becomes an unreacted molten mass, and coarse pores are generated or eluted from the molded body, thereby deteriorating the mechanical and electrical properties of the porous ceramic. Even when other metal powders are used as a starting material or when an oxidation reaction is performed, the temperature condition changes, but it is still preferable that the temperature be increased stepwise in two or more stages.

The atmosphere during the heat treatment is a non-oxidizing atmosphere containing N ₂ or NH ₃ in order to obtain a nitride. When an oxide is to be obtained, an oxidizing atmosphere containing O ₂ is used. In either case, the pressure is not limited, but is preferably 1 atm (101 kPa) or more and 5 atm (507 kPa) or less.

The porous ceramic of the present invention obtained as described above has a structure in which pores having a uniform diameter are dispersed by hollowing out individual particles of the metal powder. It is a porous ceramic having substantially a single layer of inorganic ceramics. Therefore, it is a porous ceramic having excellent moisture absorption resistance, low dielectric constant, and low dielectric loss. The relative density is less than 70%, and the proportion of closed pores in all the pores is 50% or more.
Furthermore, if the average particle size of the raw metal powder, the amount of oxygen on the surface, the type of sintering aid, and the sintering conditions are selected, the relative density is less than 50% and the ratio of closed pores in all pores is 90% or more. be able to.

In an arbitrary cross section of the porous ceramic of the present invention, as shown in FIG. 1, if the radii of adjacent holes are r1 and r2 and the thickness of the ceramic portion is b, (r1 + r2) / b> 1 Can be made. That is, if the average particle size of the raw metal powder, the amount of oxygen on the surface, the type of sintering aid, and the sintering conditions are selected, the diameter of the pores can be made twice or more the thickness of the ceramic. More preferably,
(R1 + r2) / b> 2.

With such a structure, dielectric loss can be further reduced. The dielectric loss of the porous ceramic of the present invention is about 10 ^-4 or less.
As mechanical properties, the bending strength by three-point bending is 150
It is a porous ceramic having an electrical and mechanical property of not less than MPa.

Although the material system and manufacturing method of the porous ceramic of the present invention are not limited, in particular,
_{3 N 4, SiO 2, AlN} , it is useful as structural materials or electronic materials in the material such as Al ₂ O _3. As a precursor of ceramics, a metal powder of Si or Al is used as a starting material. In the reaction process of nitriding or oxidizing this metal powder, uniform metal particles are promoted to diffuse into the outer shell, so that uniform pores are formed. It is possible to easily obtain porous ceramics dispersed in the ceramics.

[0026]

Example 1 Example 1 Si powder having an average particle diameter of 1 μm and an average particle diameter as a sintering aid
0.8 μm of the rare earth oxides listed in Table 1 with respect to Si powder
0.8 mol% was prepared. Each powder is commercially available
It is. The amount of oxygen on the surface of the Si powder depends on the inert gas
Solution, measured by infrared detection method,_Two0.7mo in conversion
What was previously confirmed to be 1% was prepared. Prepare
Each powder was dissolved in ethyl alcohol for 24 hours.
Ball mill mixed. After mixing, air dry and dry press
Using φ23 × 3mm and 4.5 × 7 × 45mm
It was molded into a size. This molded body is placed in a nitrogen atmosphere at atmospheric pressure.
1200 ° C. by microwave heating at a frequency of 28 GHz
And hold for 3 hours, then raise the temperature to 1400 ° C and hold for 3 hours
Was. The reason for the temperature rise in two stages is that the nitridation reaction of silicon
Exothermic reaction at 1400 ° C. (Si + 2 / 3N_Two= 1 /
3Si_ThreeN_Four+64 kJ), so 1400 ° C at a time
When the temperature rises to 1400 ° C due to its own heat generation
This is because Si has been melted as described above. Nature
After cooling, a φ20x1mm and 3x4x40mm
Finishing using a peripheral grinding machine and a surface grinding machine
Was. Using the finished sintered body,
The properties were measured. Incidentally, the sintered body is made of metal by X-ray diffraction.
No Si remains, and all Si _ThreeN_FourIt has become
And confirmed.

The total porosity was obtained by calculating the apparent density from the size and weight of the sintered body, and calculating the theoretical density from the amount of the sintering additive added in accordance with the mixing rule. (1
-Apparent density / theoretical density) x 100 (%).

The closed pore ratio was calculated by the following equation by measuring the open pore volume with a mercury porosimeter. (Total pore volume−open pore volume) / total pore volume × 100 (%)

The radii r1 and r2 of the adjacent holes and the thickness b of the ceramic portion are determined by cutting the sintered body and polishing the cross section.
SEM observation was performed. Based on the SEM photograph, the center point of the hole is defined as a point which becomes the position of the center of gravity in two dimensions, and as shown in FIG. The thickness b of the part was measured. 50
Table 1 shows the average value of the results of measuring the locations.

As an electrical characteristic, a dielectric loss (tan δ) at 1 GHz was measured by a measuring method specified in JIS C2141. Also, as mechanical properties, J
Finished to the strength test piece shape specified in ISR1601,
The three-point bending strength was measured based on the same rules. Table 1 shows the results. Also, for reference, the characteristics of a dense sintered body using Si ₃ N ₄ as a raw material are referred to 1, and a porous body prepared by using a method disclosed in JP-A-9-249457 using Si ₃ N ₄ as a raw material. Table 2 shows the characteristics of the porous body of SiO ₂ as Reference 3 with the characteristics of No. 2 as reference. In addition, the addition amount of Y ₂ O ₃ as a sintering aid of Reference Examples 1 and 2 was 5 m
was ol%, Al ₂ O ₃ in the dense body of the ginseng 1 in addition to Y ₂ O ₃
Is contained at 3 wt%. The SiO ₂ of Reference No. 3 contains no additive and elutes a soluble phase from borosilicate glass.

[0031]

[Table 1] * Marked comparative example

As can be seen from Table 1, the sintered body obtained by adding the sintering aid of the present invention has a porosity of 50% or more, that is, a relative density of 50% or less, and the ratio of closed pores is 5
0% or more. Further, the dielectric loss is as low as 1 × 10 ⁻⁴ or less as compared with the conventional porous ceramics, and the transverse rupture strength is 1 ×.
It can be seen that it has excellent electrical and mechanical properties of 50 MPa or more. In addition, the value of (r1 + r2) / b is a porous ceramic having a value of 1 or more, that is, a pore diameter equal to or more than that of the ceramic portion, if a sintering aid is selected. The diameter of the hole is, for example, 0.7 μm
Met.

Example 2 Three kinds of Si powder having an average particle diameter of 1, 10 and 15 μm and Sm ₂ O ₃ having an average particle diameter of 0.8 μm as a sintering aid were prepared in an amount of 1.0 mol% based on the Si powder. . Each powder is commercially available. Mixing, molding, sintering, and finishing were performed in the same manner as in Example 1. Table 2 shows the results of measuring the surface oxygen content, total porosity, closed porosity, and dielectric loss of the Si powder in the same manner as in Example 1.

[0034]

[Table 2]

As can be seen from Table 2, the amount of oxygen (in terms of SiO ₂ ) on the surface of the Si powder decreases as the average particle diameter of the powder increases. The total porosity, closed porosity and dielectric loss are
It can be seen that it differs depending on the average particle size of the starting material Si powder.

Example 3 A Si powder having an average particle diameter of 1 μm and Yb ₂ O ₃ having an average particle diameter of 0.8 μm as a sintering aid were prepared at a ratio shown in Table 3 with respect to the Si powder. The amount of oxygen on the surface of the Si powder is 0.1% in terms of SiO ₂ .
7 mol%. Each powder is commercially available. Using these powders, mixing, molding, sintering, and finishing were performed in the same manner as in Example 1. Table 3 shows the results of measuring the total porosity, closed porosity, and dielectric loss of each sintered body in the same manner as in Example 1.

[0037]

[Table 3] * Marked comparative example

As can be seen from Table 3, when the added amount of the sintering aid is less than 0.2 mol%, the total porosity is 65%, that is, the relative density is less than 35% and less than 50%. The ratio is as low as 30%, and the hollowization of the Si particles in the sintering step is not sufficiently performed. In addition, the addition amount is 2.
If it exceeds 5 mol%, the total porosity is 42%, that is, the relative density is 58% and less than 70%, but the ratio of closed pores is 4%.
It is as low as 5%, which indicates that the ratio of closed pores was reduced by grain growth.

Example 4 Si powder having an average particle diameter of 1 μm and an average particle diameter
0.8 μm Er_TwoO_Three0.6 mol% with respect to Si powder
Was prepared. The amount of oxygen on the surface of the Si powder is SiO _TwoIn conversion
Those described in Table 4 were used. Using these powders,
Mixing, molding, sintering and finishing in the same manner as in Example 1.
Was done. Total porosity, closed porosity and dielectric loss of each sintered body
Table 4 shows the measurement results of the loss in the same manner as in Example 1.

[0040]

[Table 4] * Marked comparative example

As can be seen from Table 4, the starting material S
The characteristics of the resulting sintered body differ depending on the amount of oxygen on the surface of the i-powder. That is, the amount of oxygen on the surface of the Si powder is
_When converted to ₂ and less than 0.4 mol%, the total porosity is 6
7% or relative density of 33% and less than 50%,
The ratio of closed pores is as small as 48%, and the hollowization of the Si particles is not sufficiently performed. Further, when it exceeds 1.5 mol%, the ratio of closed pores is less than 50%, and it can be seen that the structure of hollowing is changed by grain growth.

Example 5 Si powder having an average particle diameter of 1 μm and a sintering aid as an average particle diameter
0.8 μm Er_TwoO_Three0.8 mol% based on Si powder
Was prepared. The amount of oxygen on the surface of the Si powder is SiO _TwoIn conversion
The thing of 0.7 mol% was used. Using these powders
Then, mixing and molding were performed in the same manner as in Example 1. Success
In the atmosphere of nitrogen atmosphere at atmospheric pressure, 28GHz microwave
By heating, sintering was performed under the conditions shown in Table 5. Here, 1150
* 6 means holding at 1150 ° C for 6 hours.
0 * 3 + 1400 * 3 means hold at 1200 ℃ for 3 hours
Shows that the temperature was raised to 1400 ° C. and held for 3 hours.
You. Finishing of the sintered body was performed in the same manner as in Example 1. Each ware
Total porosity, closed porosity, value of (r1 + r2) / b
Table 5 shows the results of measuring the dielectric loss and the dielectric loss in the same manner as in Example 1.
Show. The value of (r1 + r2) / b is measured at 50 points.
This is the average of the results.

[0043]

[Table 5] * Marked comparative example

As can be seen from Table 5, the sintering temperature was 120
When the temperature is lower than 0 ° C., the hollowing of the Si particles is not sufficiently performed when the sintering time is about 6 hours. Therefore, the ratio of closed pores is
28%, the value of (r1 + r2) / b is 0.5, that is, the structure is such that the diameter of the pores is 誘 電 of that of the ceramic part, and the dielectric loss is as high as 120 × 10 ⁻⁵ . Also, when the sintering temperature is 1800 ° C., it can be seen that the hollow structure changes due to the grain growth and the phase transformation, and the structure becomes denser. When the sintering temperature is 1200 ° C to 1750 ° C,
The value of (r1 + r2) / b is 2 or more, and the dielectric loss is
It turns out that it is 12 * 10 < ^-5 > or less, and is excellent.

Example 6 The same Si powder and Er ₂ O ₃ powder as in Example 5 were prepared.
Using these powders, mixing and molding were performed in the same manner as in Example 1. The molded body was sintered under a nitrogen atmosphere at atmospheric pressure by heating with a carbon heater under the conditions shown in Table 6.
The sintering conditions are the same as in Example 5. Finishing of the sintered body was performed in the same manner as in Example 1. Table 6 shows the results of measuring the total porosity, closed porosity, (r1 + r2) / b value, and dielectric loss of each sintered body in the same manner as in Example 1. still,
The value of (r1 + r2) / b is an average value of the results of measuring 50 points.

[0046]

[Table 6] * Marked comparative example

As can be seen from Table 6, the sintering temperature was 120
When the temperature is lower than 0 ° C., the hollowing of the Si particles is not sufficiently performed when the sintering time is about 6 hours. Further, the sintering temperature is 1800
In the case of ° C., it can be understood that the structure of hollowing was changed due to grain growth and phase transformation, and the structure was densified. In addition, comparing Tables 5 and 6, it can be seen that heating by microwaves has a higher ratio of closed pores and lower dielectric loss. This is presumably because microwaves can be more efficiently heated, so that the diffusion reaction of silicon into the outer shell is further promoted.

Example 7 Al powder having an average particle diameter of 5 μm and Y ₂ O ₃ having an average particle diameter of 0.8 μm and MgO having an average particle diameter of 0.5 μm were used as sintering aids in the ratio shown in Table 7 with respect to the Al powder. Got ready. Each powder is commercially available. The amount of oxygen on the surface of the Al powder was measured by the method of Example 1 and was as shown in Table 7 in terms of Al ₂ O ₃ . Each prepared powder was ball-milled for 24 hours using ethyl alcohol as a solvent. After mixing, the mixture was air-dried, and φ23 × 3 mm and 4.5 were dried using a dry press.
It was molded to a size of x7x45 mm. The molded body was maintained at 900 ° C. for 3 hours by microwave heating at a frequency of 28 GHz in a nitrogen atmosphere at atmospheric pressure, then heated to 1250 ° C. and maintained for 3 hours. When the obtained sintered body was measured by X-ray diffraction, it was confirmed that metallic Al did not remain and that all were AlN. Finishing of the sintered body was performed in the same manner as in Example 1. The total porosity, closed porosity, (r
Table 7 shows the result of measuring the value of 1 + r2) / b and the dielectric loss in the same manner as in Example 1. Note that the value of (r1 + r2) / b is an average value of the results of measuring 50 points.

[0049]

[Table 7]

As can be seen from Table 7, No. 32, which is an AlN porous ceramic of the present invention, has a total porosity of 60%, that is, a relative density of 40% or less than 70%, and a ratio of closed pores of 70% or 50%. % Or more. Further, it is an excellent porous ceramic having a low dielectric loss.

Example 8 Al powder having an average particle size of 5 μm and Sm ₂ O ₃ having an average particle size of 0.8 μm and Li _{2 having an} average particle size of 0.5 μm were used as sintering aids.
O was prepared at a ratio shown in Table 8 with respect to the Al powder. Each powder is commercially available. Each prepared powder was ball-milled for 24 hours using ethyl alcohol as a solvent.
After mixing, air dry, using a dry press, φ23x3
mm and a size of 4.5 × 7 × 45 mm. After holding this molded body at 800 ° C. for 3 hours by microwave heating at a frequency of 28 GHz in an oxygen atmosphere at atmospheric pressure,
The temperature was raised to 00 ° C. and maintained for 3 hours. When the obtained sintered body was measured by X-ray diffraction, metal Al did not remain, and all were Al ₂ O ₃ . Example 1 Finishing of sintered body
The same was done. Total porosity, closed porosity,
Table 8 shows the result of measuring the value of (r1 + r2) / b and the dielectric loss in the same manner as in Example 1. Note that (r1 + r2) / b
Is the average of the results of measuring 50 locations.

[0052]

[Table 8]

As can be seen from Table 8, the Al ₂ O ₃ porous ceramic No. 35 of the present invention has a total porosity of 55%.
The ratio of closed pores is 60%, and the dielectric loss is low. .

[0054]

According to the present invention, it is possible to obtain a porous ceramic in which the ratio of closed pores is high and the closed pores are uniformly dispersed as compared with other materials and the conventional method. Since the porous ceramics of the present invention has a high ratio of closed pores and excellent electrical and mechanical properties, an electronic circuit board that requires moisture absorption resistance, a low dielectric constant, a low dielectric loss, and mechanical strength is also required. When used for such purposes, it exhibits excellent properties.

[Brief description of the drawings]

FIG. 1 is a schematic view of a cross-sectional structure of a porous ceramic of the present invention.

FIG. 2 is a schematic view of a cross-sectional structure of a conventional porous ceramic.

3A and 3B show a sintering process of the porous ceramic of the present invention, in which FIG. 3A shows a molded state, FIG. 3B shows an initial state of sintering, and FIG. .

4A and 4B are diagrams schematically illustrating a change of one metal particle in a sintering process of the porous ceramic of the present invention, wherein FIG. 4A shows a state before sintering, and FIG. Status,
(C) shows a state where sintering has progressed, and (d) shows a state where sintering has been completed.

[Explanation of symbols]

 1 metal powder particles 2 metal nitride or oxide 3 vacancy

Claims (10)

[Claims] 1. A porous ceramic having a relative density of less than 70% and a proportion of closed pores in all pores of 50% or more. 2. A porous ceramic having a relative density of less than 50% and a proportion of closed pores in all pores of 90% or more. 3. In an arbitrary cross section, the radii r1 and r2 of two adjacent holes and the width b of the ceramic portion are (r1
3. The porous ceramic according to claim 1, wherein + r2) / b> 1. 4. The porous ceramic according to claim 1, wherein the constituent phase comprises a ceramic and an oxynitride phase. 5. The method according to claim 1, wherein the ceramic is silicon nitride, silicon oxide,
The porous ceramic according to any one of claims 1 to 4, wherein the porous ceramic is one of aluminum nitride and aluminum oxide. 6. A ceramic circuit board, wherein at least a part of the insulating layer is made of the ceramic material according to claim 1. 7. A method for producing a porous ceramic, which comprises forming a molded body made of a metal powder which is a precursor of the porous ceramic, and heat-treating the molded body in a reaction gas to obtain the porous ceramic. 8. The method for producing porous ceramics according to claim 7, wherein said molded body is heat-treated under microwave irradiation to obtain porous ceramics comprising hollow ceramic particles. 9. The method according to claim 7, wherein the metal powder is silicon and the porous ceramic is silicon nitride or silicon oxide. 10. The method according to claim 7, wherein the metal powder is aluminum and the porous ceramic is aluminum nitride or aluminum oxide.