JPH0580427B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing a ceramic dielectric,
Specifically, it contains particles with a particle size of 1 μm or less made of a substantially cubic perovskite compound,
By firing at a low temperature of 1100°C or less, it is made of a substantially cubic perovskite compound sintered body, has virtually no piezoelectricity, has very small temperature change in capacitance, and has an insulating property. The present invention relates to a method for producing ceramic dielectrics that have excellent electrical properties such as resistance and low dielectric loss tangent. In general, a perovskite compound is a compound that has a crystal structure similar to calcium titanate ore (perovskite), and by molding and sintering such a compound, it has dielectricity, piezoelectricity, and semiconductivity. Dielectric ceramics are obtained, and these have recently been used in capacitors, radio wave filters,
It is used in large quantities as ignition elements, thermistors, etc. in electronic equipment such as communication devices and computers. Traditionally, perovskite compounds include Mg, Ca,
Carbonates or oxides such as Sr, Ba, Pb, Ti, Zr,
It is manufactured by mixing oxides such as Hf and Sn, calcining at a temperature of 1000°C or higher, wet-pulverizing, filtering and drying. However, according to this method,
Since the perovskite compound is agglomerated by calcination, it is difficult to reduce the particle size to 1 μm or less even by wet pulverization, and the shape after pulverization is similar to that of a crushed product. Therefore, when forming and sintering perovskite compound particles by the calcining method to make a dielectric material, not only are the sintering properties poor, but the particles also have a particle size of about 5 to 10 μm due to sintering. Crystal growth occurs, making it impossible to obtain a sintered body consisting of fine particles. In particular, a multilayer ceramic capacitor is integrally formed by alternately laminating a ceramic dielectric material made of a sintered body of a perovskite compound and an electrode metal, and the ceramic dielectric material is typically barium metatitanate (BaTiO ₃ ). However, as mentioned above, conventional perovskite compound sintered bodies have sintered particles with large particle sizes, and are said to be optimal as dielectric materials for capacitors. There is a large gap between the particle size and the particle size of about 1 μm. Furthermore, conventionally, perovskite compounds such as barium metatitanate as described above have large particle sizes, so in order to increase the degree of sintering and obtain a dense sintered body, for example, in the case of the above-mentioned multilayer ceramic capacitor, , it is necessary to heat it to a high temperature of about 1300 to 1400℃ together with the electrode metal, so as an internal electrode of a multilayer ceramic capacitor,
For example, expensive high melting point noble metal electrode systems such as platinum and palladium must be used. Therefore, in order to use a relatively low melting point and inexpensive metal material such as silver as the internal electrode of a multilayer ceramic capacitor, a low melting point glass composition powder and perovskite compound particles were mixed and compared. Although methods of sintering at relatively low temperatures have been proposed (e.g., KRChowdary et al.
Ferroelectrics, 1981 , Vol. 37, pp. 689-692, Japanese Unexamined Patent Publication No. 54-66450, etc.), the performance of the obtained multilayer ceramic capacitor is still not satisfactory due to the large grain size of the perovskite sintered body. do not have. As a result of intensive research to solve the above-mentioned problems in the production of ceramic dielectrics, the present inventors found that fine particles with a particle size of 1 μm or less made of a substantially cubic perovskite compound have excellent sintering characteristics. Excellent, use this as a liquid phase with a low melting point substance at 1100
It has been discovered that by firing at a low temperature below 10°C, a dielectric ceramic consisting essentially of a sintered cubic perovskite compound and having excellent electrical properties can be obtained. This makes it possible to reduce the cost of manufacturing the electrodes of multilayer ceramic capacitors, and also because the dielectric material made of the sintered body is substantially free from piezoelectric phenomena, and the temperature change in capacitance is very small. The present invention was developed based on the discovery that it is small and has excellent dielectric properties such as insulation resistance and dielectric loss tangent. Accordingly, the present invention aims to provide a method for producing ceramic dielectrics, in particular a method for producing ceramic dielectrics containing substantially cubic perovskite compounds.
It is made of fine particles of 1μm or less and a low melting point glass substance, and by firing at a low temperature of 1100℃ or less, it has excellent dielectric properties, in particular, very little temperature change in capacitance, and virtually no change in capacitance. An object of the present invention is to provide a method for manufacturing a ceramic dielectric material that does not have piezoelectric properties, has high insulation resistance, and has a low dielectric loss tangent. The method for producing a ceramic dielectric according to the present invention comprises: (a) a hydroxide of at least one element selected from Group A consisting of Mg, Ca, Sr, Ba and Pb;
A mixture of at least one element selected from group B consisting of Ti, Zr, Hf, and Sn with a hydroxide is hydrothermally treated to form a substantially cubic perovskite compound with a particle size of 1 μm or less. The method is characterized in that spherical particles are prepared, and (b) a powder of a low melting point substance having a melting point of 1100° C. or less is added to the spherical particles, followed by firing. The perovskite compound particles used in the method for manufacturing a ceramic dielectric according to the present invention are the above-mentioned A
at least one element selected from the group and the above B
These are fine particles having a particle size of 1 μm or less and are made of a perovskite compound having a substantially cubic crystal structure and containing at least one element selected from the group. As is already known, particles of a fine perovskite compound having a substantially cubic crystal structure are hydroxides of at least one element selected from Group A (hereinafter referred to as A hydroxide). .)
and a hydroxide of at least one element selected from Group B (hereinafter referred to as B hydroxide), and hydrothermally treats the mixture. It is known that a perovskite compound is partially produced by mixing hydroxide A and hydroxide B, so in the present invention, the hydroxide mixture is
It may contain a part of a perovskite compound. Further, the mixture may be a mixture of perovskite compounds having a low degree of crystallinity as a whole. The above hydroxide mixture can be prepared, for example, by mixing hydroxide A and hydroxide B as a simple method. As another method, for example, a mixture of a salt of a group A element and a salt of a group B element may be reacted with an alkali, or a hydroxide (or salt) of a group A element and a salt of a group B element ( or hydroxide) may be reacted with an alkali. Furthermore, as another method, a hydroxide (or alkoxide) of a group A element and an alkoxide (or hydroxide) of a group B element may be reacted, or an alkoxide of a group A element and an alkoxide of a group B element may be reacted. You may also hydrolyze a mixture with. Next, by hydrothermally treating the hydroxide mixture as described above, it is possible to obtain fine particles having a particle size of 1 μm or less and consisting of a substantially cubic perovskite compound that can be suitably used in the present invention. As already known, hydrothermal treatment refers to heat treatment in an alkaline aqueous medium, and in the present invention, hydrothermal treatment refers to
The hydroxide mixture, which is alkaline in nature, may be heated at 100° C. or the critical temperature of the aqueous medium, after adding an alkali if necessary. When the hydrothermal treatment temperature is lower than 100°C, the reaction between hydroxide A and hydroxide B does not proceed sufficiently, making it impossible to obtain the desired perovskite compound in high yield. On the other hand, a higher reaction temperature is preferable from the viewpoint of accelerating the reaction rate, but on the other hand, the higher the reaction temperature, the higher the equipment cost and thermal energy cost, so from a practical standpoint, a temperature of 300°C or less is preferable, and usually,
A range of 120 to 300°C is suitable. After this hydrothermal treatment, by filtering the slurry-like reaction mixture and drying the solid content, fine perovskite compound particles with a particle size of 1 μm or less can be obtained. In the above-mentioned hydrothermal treatment, the degree of alkalinity of the aqueous medium, that is, the excess degree and concentration of alkali, may be adjusted as appropriate, but in general, the higher the excess degree of alkali, the better the perovskite compound particles obtained. Particle size becomes smaller. Furthermore, the higher the concentration of A hydroxide and B hydroxide in the aqueous medium, the smaller the particle size of the obtained perovskite compound. Therefore, the degree of excess of alkali and the concentration of each hydroxide in the hydrothermal treatment may be selected depending on the required particle size. As described above, perovskite compound particles obtained by hydrothermal synthesis are different from perovskite compound particles obtained by conventional calcination methods, and are composed of a perovskite compound whose crystal structure is substantially cubic, and the particle size is is less than 1μm, usually 0.01~1μm
Since the spherical fine particles fall within the range of 1 to 2, they have a large surface energy and a uniform particle size distribution, and also have excellent filling properties, making it possible to obtain a dense sintered body. In particular, when producing a sintered dielectric material having a matrix of a low melting point material according to the method of the present invention,
It has the advantage that the sintered particles of the perovskite compound are uniformly dispersed in the matrix. On the other hand, for example, according to a method of hydrothermally synthesizing a mixture of barium hydroxide and hydrous titanium oxide to obtain fine and spherical cubic barium titanate (Kuichiro Kubo et al., Industrial Chemistry Magazine, Vol. 71, No. 1) No. 114-118
(1968)), it is generally difficult to complete the reaction, and unreacted barium salt is eluted when the reaction mixture is washed with water and filtered after the hydrothermal reaction. Even after firing, it may not be easy to obtain a sintered body having the required Ba/Ti ratio. Therefore, in the present invention, when obtaining perovskite compound particles by a hydrothermal synthesis method, an insolubilizing agent is used as an additive to insolubilize a group A element such as barium after the hydrothermal treatment of the hydroxide mixture. By adding , it is made of a substantially cubic perovskite compound, has a particle size of 1 μm or less, is fine and spherical, and has a predetermined A.
It is preferable to use perovskite compound particles having a ratio of group elements to group B elements. Generally, when a perovskite compound is obtained by hydrothermal synthesis, if the reaction is not completed, the aqueous medium after the reaction is completed contains elements of group B, such as
Ti exists as a solid compound, and group A elements, such as Ba, exist as water-soluble compounds. Therefore, barium compounds are eluted when the reaction product is filtered and washed with water, and as a result, the required Ba/Ti ratio is It is not possible to obtain perovskite compounds with Therefore, as previously proposed by the present inventors (Japanese Patent Application No. 59-154289), by fixing the Group A elements remaining in the aqueous medium after hydrothermal treatment as water-insoluble compounds, the reaction product The elements of group A can remain in the reaction product even during filtration and water washing, and thus, by drying and firing the reaction product, a perovskite compound sintered body having the desired element ratio can be obtained. can. Such insolubilizers include insolubilized products of group A elements that have low solubility in aqueous media;
In addition, compounds and resins that are thermally decomposed during firing and do not remain in the sintered body are preferably used. Specifically, examples include carbon dioxide gas, carbonic acid compounds such as sodium carbonate and ammonium carbonate, alkali metal salts of fatty acids such as lauric acid, myristic acid, palmitic acid, and stearic acid, oxalic acid, malonic acid, ketomalonic acid, and stone liquor. Examples include acids, polybasic acids such as succinic acid or alkali metal salts thereof, and cation exchange resins such as Amberlite IRC-50. Further, as the insolubilizing agent, it is also possible to use an agent that remains in the sintered body but does not have a harmful effect on the properties of the sintered body. Examples of such an insolubilizer include silica such as zeolite, an aluminum-based inorganic ion exchanger, and the like. According to the present invention, after insolubilizing the Group A elements in the water washing medium after hydrothermal treatment, the reaction product is filtered, washed with water, and dried in accordance with a conventional method to obtain the required amount. Perovskite compounds having an A/B ratio can be obtained. As mentioned above, the cubic perovskite compound particles obtained by insolubilizing group A elements after hydrothermal treatment have a fine spherical shape with a particle size of 1 μm or less and have a predetermined A/B ratio. It has excellent sinterability and dispersibility, and forms dense and strong sintered particles by sintering at low temperatures, and compared to conventional ceramic dielectrics, it has stable performance even when made into a thin film. Obtainable. In the present invention, when a perovskite compound is obtained by a hydrothermal synthesis method, it contains a small amount of the above-mentioned insolubilizing agent, a compound of a group A element insolubilized by the insolubilizing agent, an unreacted oxide of a group B element, etc. That is allowed. Furthermore, it is generally known that when sintering perovskite compound particles, the particle growth and electrical properties of the sintered body can be controlled by the action of additives, but in the present invention, it is possible to control the particle growth and the electrical properties of the sintered body. Various known additives can be used. Examples of such additives include B, Bi, Li,
Examples include compounds of alkali metals such as Na and K, rare earth elements such as Y, Dy, Ce, and Sm, transition metals such as Fe, Mn, Co, Ni, and Nb, and further elements such as Si and Al. Such additives may be added at any stage of the preparation of the perovskite compound and its calcination. According to the method for manufacturing a ceramic dielectric according to the present invention, a powder of a low melting point substance having a melting point of 1100° C. or lower is added to particles substantially consisting of a perovskite compound as described above, and the particles are fired. It is particularly preferable that the low melting point substance has a melting point in the range of 400°C to 1000°C. Therefore, it is typically an oxidation of at least one element selected from the group consisting of Bi, B, Pb and W. I can name things. As described above, according to the method of the present invention, a mixture of fine particles consisting essentially of a perovskite compound and a low-melting substance powder is preferably formed into, for example, granules in order to improve sinterability, and then this is formed into granules. 1100
By firing at a temperature of .degree. C. or lower, a ceramic dielectric material can be obtained as a dense and strengthened sintered body. Thus, in general, when a perovskite compound is sintered in the presence of a low melting point substance, the higher the ratio of the low melting point substance to the perovskite compound, the higher the degree of sintering of the sintered body, while the relative dielectric constant known to decrease. Therefore, the proportion of the low melting point substance to the perovskite compound is appropriately selected in consideration of the degree of sintering and relative permittivity of the sintered particles, but usually the proportion of the low melting point substance is 1 to 30% by weight based on the perovskite compound. The content is preferably 3 to 15% by weight. As described above, according to the method of the present invention, perovskite compound particles have a substantially cubic particle size.
Since they are fine particles of 1 μm or less, a ceramic dielectric can be quickly obtained as a dense and strengthened sintered body by firing at a low temperature of 1100° C. or less using a low melting point substance as a liquid phase. Further, the ceramic dielectric material thus obtained according to the method of the present invention has substantially no piezoelectricity, since the sintered particles of the perovskite compound are fine particles consisting essentially of cubic crystals. , temperature change in capacitance is very small, and furthermore, it has high insulation resistance and low dielectric loss tangent. Moreover, according to the method of the present invention, it is possible to obtain a ceramic dielectric material that is dense and strong and has the above characteristics by firing at a low temperature of 1100°C or less. can be used as internal electrodes and can also save thermal energy, making it possible to significantly reduce electrode production costs for multilayer ceramic capacitors. Furthermore, according to the method of the present invention, after hydrothermal treatment A
The perovskite compound particles obtained by insolubilizing the group elements have the ratio of group A elements/group B elements regulated to a predetermined value, have excellent sinterability and dielectric properties, and are suitable for sintering of perovskite compounds. Since the particles are finely and uniformly dispersed in a matrix of low melting point material, the resulting ceramic dielectric can be made much thinner than conventional ceramic dielectrics, making it possible, for example, to make ceramic capacitors more compact. and stable performance can be ensured. The present invention will be specifically described below with reference to Examples. Example 1 Partially hydrated titanium chloride (TiCl _2.36 (OH) _1.64 , Ti 16.5% by weight and chlorine 28.8% by weight) aqueous solution (manufactured by Osaka Titanium Co., Ltd., hereinafter referred to as titanium chloride) in which titanium tetrachloride is partially hydrated. Add 1250 ml of water to 139 g (0.48 mol of Ti) and add 1250 ml of water to this aqueous solution.
483 ml of 5.0% by weight ammonia water was added over 30 minutes to obtain titanium hydroxide. After washing this titanium hydroxide with water, it was separated by filtration, and 151.4 g of barium hydroxide octahydrate (Ba(OH) _{2.8H 2} O) was added to it under a nitrogen atmosphere.
(0.48 mol as Ba) and added water,
A slurry having a BaTiO ₃ concentration of 0.8 mol/was obtained. 600 ml of this slurry was charged into a 1-capacity autoclave made of Hastelloy C, and the temperature was raised to 200° C. after 90 minutes while stirring at 700 to 900 rpm, and the slurry was heated at 200° C. for 5 hours for hydrothermal treatment. After this, the slurry was washed with water and filtered to obtain _BaTiO3 . This _BaTiO3 is
As a result of observation using an electron microscope, the average particle size was 0.1 μm.
It was a spherical object, and X-ray diffraction showed peaks characteristic of cubic BaTiO ₃ . Figure 1 shows the results obtained in this example.
An electron micrograph (20000x) of _BaTiO3 particles is shown. Separately, 3.95 g of boron oxide (B ₂ O ₃ ) and 94.20 g of bismuth oxide (Bi ₂ O ₃ ) were thoroughly mixed in a mortar and then heated to a temperature of 700° C. in a platinum crucible to melt them. This melt was poured into water, rapidly cooled and solidified, and then ground in a mortar and ball mill.
A powder of B ₂ O ₃ −Bi ₂ O ₃ low melting point substance was obtained. Next, based on the previously obtained BaTiO ₃ , the above-mentioned low melting point substances were added to 5% by weight, 10% by weight and 10% by weight, respectively.
After adding at a ratio of 15% by weight and thoroughly mixing in a mortar, an 8% by weight polyvinyl alcohol aqueous solution was added to this mixture, granulated to form granules, and further pressure-treated.
Green pellets with a diameter of 20 mm were obtained by pressure molding at 1000 Kg/cm ² . The green pellets were heated at 400°C for 3 hours to thermally decompose and volatilize the polyvinyl alcohol, and then fired at a predetermined temperature for 1 hour to obtain a sintered ceramic body, and the sintered density was measured. . Next, both sides of this sintered ceramic body were polished to a thickness of about 1 mm, both sides were coated with silver using an ion coater, and the electrical properties were measured. The relative dielectric constant and dielectric loss were measured using an LF impedance analyzer manufactured by Yokogawa Huuret Pat Card Co., Ltd., and the insulation resistance was measured using a PA meter manufactured by Yokogawa Huuret Pat Card Co., Ltd. The table shows the sintered density, dielectric constant, dielectric loss tangent, and resistivity of the sintered body obtained above. From these results, it is clear that the dielectric ceramic according to the present invention has excellent sinterability and electrical properties. Furthermore, the X-ray diffraction pattern of the sintered ceramic body of Experiment No. 4 in the table thus obtained is shown in FIG. Further, temperature changes in relative permittivity of typical ceramic dielectrics according to the present invention for Experiment Nos. 1, 4 and 7 in the table are shown in FIG. 3, respectively. Furthermore, a DC voltage of 3kV/mm is applied to each of these ceramic dielectrics.
was applied for 30 minutes to polarize, and the electromechanical coupling coefficient
The kp value was measured. In either case, the kp value was almost 0, and piezoelectricity was not exhibited. Comparative Example 1 High-purity barium carbonate and titanium oxide (both manufactured by Sakai Chemical Industry Co., Ltd.) were mixed in an equimolar ratio, and 2
After being calcined for an hour, it was wet-pulverized in a ball mill to obtain BaTiO ₃ with an average particle size of 1.6 μm. FIG. 4 shows an electron micrograph of this perovskite compound particle. 5% by weight of the low melting point substance powder obtained in Example 1 was added to this, and after thoroughly mixing in a mortar, Example 1
A sintered body was produced in the same manner as above, and its physical properties and electrical properties were evaluated. The results are shown in the table. Comparative Example 2 5% by weight of the low melting point substance powder obtained in Example 1 was added to commercially available high purity BaTiO ₃ having an average particle size of 1.3 μm. After thorough mixing in a mortar, a sintered body was produced in the same manner as in Example 1, and its physical properties were measured. The results are shown in the table. Example 2 Obtained by hydrothermal treatment in the same manner as in Example 1
The BaTiO ₃ slurry was washed with water and filtered, and the barium in the entire filtrate was analyzed by atomic absorption spectrophotometry, and the reaction rate was 98.3%. However, the reaction rate is defined as [(number of moles of barium charged - number of moles of barium Ba in the filtrate)/number of moles of Ba charged]×100. After the above filtration, the solids were dispersed in the filtrate again at a temperature of 110°C, and carbon dioxide gas was blown into the filtrate until the pH reached 6.5, followed by washing with water, filtration, and drying until _no chlorine was detected. Obtained. When the Ba/Ti ratio was analyzed using fluorescent X-rays, it was 1.00, and the particle size was
It was 0.1 μm. After adding 5% by weight of the low melting point substance powder obtained in Example 1 to this BaTiO ₃ and thoroughly mixing in a mortar,
A sintered body was prepared in the same manner as in Example 1, and its physical properties and electrical properties were evaluated. The results are shown in the table. Example 3 Barium isopropoxide 76.7 under nitrogen atmosphere
(0.30 mol as Ba) and 85.3 g (0.30 mol as Ti) of titanium isopropoxide were dissolved in 320 ml of isopropyl alcohol and heated under reflux for 2 hours. Add 65ml of decarboxylated ion-exchanged water to this solution.
to hydrolyze the alcoholate, and then, after cooling to room temperature, further
Water was added to adjust the slurry concentration to 0.5 mol/BaTiO ₃ . Thereafter, a sintered body was obtained in the same manner as in Example 1, and its sintering properties and electrical properties were evaluated. The results are shown in the table. Example 4 31.7 g (0.06 mol as Sr) of 30% strontium chloride (SrCl ₂ ) aqueous solution under nitrogen atmosphere, 26
Weight% barium chloride dihydrate salt (BaCl ₂ 2H ₂ O)
Mix 255.5 g of aqueous solution (0.24 mol as Ba) and 87.1 g of titanium chloride aqueous solution (0.30 mol as Ti), and add this mixed aqueous solution to 184 g of 35% by weight caustic soda aqueous solution kept at 60°C over a period of 1 hour. death,
After that, water is added to make the slurry concentration Ba _0.8 Sr _0.2
The amount of TiO ₃ was adjusted to 0.5 mol/. Thereafter, a sintered body was obtained in the same manner as in Example 1, and its sintering properties and electrical properties were measured. The results are shown in the table. Example 5 12 g of germanium oxide (GeO ₂ ) and 42.7 g of lead oxide (PbO) were thoroughly mixed in a mortar and then heated to 800° C. using a platinum crucible to melt them.
This melt was poured into water, rapidly solidified, and then ground in a mortar and ball mill to form Pb ₅ Ge ₃ O ₁₁
A low melting point substance was obtained. After this, 10% by weight of the low melting point substance powder obtained above was added to the BaTiO ₃ obtained in Example 1, and after thoroughly mixing in a mortar, green pellets were obtained in the same manner as in Example 1. . The obtained green pellets were heated at 400° C. for 3 hours to decompose and volatilize the polyvinyl alcohol, and then baked at a predetermined temperature for 30 minutes. Thereafter, a sintered body was obtained in the same manner as in Example 1, and the sintering properties were measured. The results are shown in the table.

[Table] Comparative Examples 3 and 4 10% by weight of the Pb ₅ Ge ₃ O ₁₁ low melting point substance powder obtained in Example 4 was added to each of BaTiO ₃ obtained in Comparative Examples 1 and 2, and the mixture was sufficiently mixed in a mortar. After mixing with
Green pellets were obtained in the same manner as in Example 1. The obtained green pellets were fired in exactly the same manner as in Example 4, and then a sintered body was obtained in the same manner as in Example 1, and its sintering properties and electrical properties were measured.
The results are shown in the table.

[Brief explanation of the drawing]

FIG. 1 is an electron micrograph (20,000 times) showing the structure of BaTiO ₃ particles obtained by hydrothermal treatment according to the present invention. FIG. 2 is an X-ray diffraction diagram of the sintered ceramic body according to the present invention. Figure 3 is a graph showing the temperature change in dielectric constant of experiment numbers 1, 4, and 7, which are typical ceramic dielectrics obtained according to the present invention, and Figure 4 is a graph showing the temperature change of the dielectric constant of typical ceramic dielectrics obtained by the conventional firing method.
This is an electron micrograph (20,000x magnification) showing the particle structure of BaTiO ₃ .

Claims (1)

[Claims] 1 (a) A hydroxide of at least one element selected from Group A consisting of Mg, Ca, Sr, Ba and Pb and at least one element selected from Group B consisting of Ti, Zr, Hf and Sn. (b) spherical particles of a substantially cubic perovskite compound having a particle size of 1 μm or less are prepared by hydrothermally treating a mixture of one element with a hydroxide; (b) the spherical particles have a melting point; A method for producing a ceramic dielectric, characterized by adding a powder of a low melting point substance having a temperature of 1100°C or less and firing it. 2 (a) Hydroxide of at least one element selected from group A consisting of Mg, Ca, Sr, Ba and Pb and water of at least one element selected from group B consisting of Ti, Zr, Hf and Sn. The mixture with the oxide is hydrothermally treated, and then (b) treated with an insolubilizing agent that makes the Group A element insolubilized in water, followed by filtration, washing with water, and drying, (c) a perovskite compound having a substantially cubic crystal structure. (d) producing spherical particles having a particle size of 1 μm or less; (d) adding a powder of a low melting point substance having a melting point of 1100° C. or less to the spherical particles; and firing the mixture. Method.