Thermal conductive colored aluminum nitride sintered body and method of preparing the same

Abstract: Disclosed herein is an AℓN sintered body which is colored, excellent in thermal conductivity and useful as an electronic material, and a method of preparing the same. The thermal conductive colored AℓN sintered body contains 100 parts by weight of AℓN, 0.005 to 0.5 parts by weight of carbon, not more than 1 part by weight of a boron compound in terms of the simple substance of boron, 0.01 to 15 parts by weight of a rare earth aluminum oxide in terms of the simple substance of the rare earth element, and 0.01 to 15 parts by weight of a compound containing an element belonging to the groups IVB-VB-VIB of the periodic table in terms of the simple substance of the element, and its thermal conductivity is 100 to 270 W/m·K at the ordinary temperature. This AℓN sintered body is obtained by shaping a mixture into a prescribed configuration and thereafter firing the as-formed compact in a non-oxidizing atmosphere containing at least 10 percent by volume of nitrogen under a temperature of 1500 to 2100°C.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a ceramic sintered body which is applied to an electronic material such as an IC substrate material, a package material or the like, and more particularly, it relates to an aluminum nitride sintered body which is colored and has high thermal conductivity, and a method of preparing the same. Description of the Background Art In recent years, the technique related to a large-scale integrated circuit device (LSI) has been greatly developed with remarkable improvement of the degree of integration, in particular. Following such improvement of the degree of integration and increase of the IC chip size, the heating value of a package for carrying such an IC chip thereon is increased. Thus, importance has been attached to the heat radiation property of a substrate material which is applied to a package for a semiconductor device or the like. An alumina sintered body, which has been widely applied to an IC substrate, cannot cope with such increase of the heating value of the IC chip, since alumina has only a small thermal conductivity and hence the sintered body is insufficient in heat radiation property. As to a substitute for such an alumina sintered body, study has been made on beryllia, which has a higher thermal conductivity. However, beryllia is hard to handle due to its toxicity. On the other hand, a sintered body of aluminum nitride (AℓN) has been watched with interest as a material for a circuit board or a package for a semiconductor device since aluminum nitride is essentially superior in thermal conductivity as well as insulability, and has no toxicity. In general, an impurity content of an AℓN sintered body is so reduced as to obtain an AℓN sintered body which is excellent in thermal conductivity as well as electrical insulability. Such a sintered body is preferably applied to an optical filter or the like in practice, since the same is also excellent in transparency. When the AℓN sintered body is applied to a package or the like, however, transmission of ultraviolet rays leads to a malfunction of the IC, resulting in a significant problem. In order to cope with this problem, colored AℓN sintered bodies have been studied in various ways, and several successful cases have been reported. For example, Japanese Patent Laying-Open Gazette No. 63-233079 (1988) discloses a black aluminum nitride sintered body and a method of preparing the same. According to this literature, a specific amount of calcium tungstate powder and/or calcium molybdate powder is added to and mixed with AℓN powder, and the mixture is molded and then fired in a non-oxidizing atmosphere, to obtain a black AℓN sintered body. On the other hand, Japanese Patent Laying-Open Gazette No. 63-310772 (1988) discloses a black AℓN sintered body which is obtained by sintering AℓN powder with an additive of metal molybdenum or a molybdenum compound. However, the aluminum nitride sintered bodies disclosed in the aforementioned gazettes have only low thermal conductivities of about 100 W/m·K. Since AℓN easily incorporates an impurity such as oxygen, high-purity AℓN powder is generally employed in order to improve the thermal conductivity of an AℓN sintered body. If any one of the aforementioned materials is added to AℓN in order to color the AℓN sintered body, its thermal conductivity is reduced. Thus, it is impossible to obtain a colored AℓN sintered body which has a high thermal conductivity. In a substrate material for carrying an IC chip having a high output thereon, however, high thermal conductivity is required for improving the heat radiation property, in order to prevent the IC from a malfunction caused by a temperature rise of the IC chip. At the same time, the substrate must be colored in order to prevent the IC from a malfunction caused by transmission of ultraviolet rays. According to the prior art, however, it has been difficult to provide a ceramic substrate which can satisfy the two requirements for the substrate material for an IC chip, as hereinabove described. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a colored aluminum nitride sintered body having an excellent thermal conductivity of at least 100 W/mK and not more than 270 W/mK at the ordinary temperature, which is useful as an electronic material such as an IC package material or a substrate material, and a method of preparing the same. In consideration of the aforementioned circumstances, the inventors have made deep study on an AℓN sintered body which is provided with two incompatible characteristics of high thermal conductivity and coloring, and found the following three points: (a) Presence of carbon is effective for coloring of the sintered body. (b) If specific composition amounts of carbon, boron oxide, an oxide of a rare earth element and a compound of an element belonging to the group IVB, VB or VIB of the periodic table are added to AℓN and fired, the as-formed AℓN sintered body exhibits a high thermal conductivity of at least 100 W/m·K and not more than 270 W/m·K at the ordinary temperature, while the sintered body is colored. (c) When prescribed composition amounts of a rare earth aluminum oxide, which is a composite oxide of a rare earth oxide and aluminum oxide, and a compound of an element belonging to the group IVB, VB or VIB of the periodic table as well as a specific amount of carbon are contained, the as-formed AℓN sintered body exhibits a high thermal conductivity of at least 100 W/m·K and not more than 270 W/m·K at the ordinary temperature, while the sintered body is colored. In a specific composition according to the present invention, the aluminum nitride sintered body contains 100 parts by weight of AℓN, at least 0.005 parts by weight and not more than 0.5 parts by weight of carbon, not more than 1 part by weight of a boron compound in terms of the simple substance of boron, at least 0.01 parts by weight and not more than 15 parts by weight of a rare earth aluminum oxide in terms of the simple substance of the rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from a group of compounds containing elements belonging to the groups IVB, VB and VIB of the periodic table in terms of the simple substance of the element contained therein. The elements belonging to the group VIB of the periodic table are Ti, Zr and Hf, and those belonging to the group VB are V, Nb and Ta, while those belonging to the group VIB are Cr, Mo and W. The rare earth elements are Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu. The rare earth aluminum oxide is formed by chemical reaction between the added rare earth element oxide and aluminum oxide which is contained in AℓN as an impurity, to be present in the sintered body. While the effect of existence of the rare earth aluminum oxide is not varied with the type of the rare earth element, Sc, Y, La or Ce is preferably employed in order to obtain a reliable sintered body which is excellent in chemical resistance and the like. The carbon content in the aluminum nitride sintered body is restricted within a range of at least 0.005 parts by weight and not more than 0.5 parts by weight with respect to 100 parts by weight of aluminum nitride. If the carbon content is smaller than the lower limit of this range, it is impossible to obtain a sintered body which has a high thermal conductivity. If the carbon content exceeds the upper limit of the aforementioned range, on the other hand, carbon reduces the liquid phase of the rare earth aluminum oxide in firing to inhibit mass transfer through the liquid phase, and hence it is impossible to obtain a dense sintered body. Further, it has been recognized possible to obtain an AℓN sintered body which is colored and has excellent thermal conductivity by bringing AℓN crystal grains contained in the AℓN sintered body into sufficiently coarse states and finely dispersing particles of a compound of an element belonging to the group IVB, VB or VIB of the periodic table. This effect is remarkable when the AℓN crystal grains are at least 1.0 µm in mean particle diameter, while a further remarkable effect is attained when the AℓN crystal grains are within a range of 2 to 15 µm in mean particle diameter. If the mean particle diameter of the AℓN crystal grains exceeds 15 µm, the thermal conductivity is not much improved and the sintered body is not sufficiently colored. If the ratio of the mean particle diameter of the AℓN crystal grains to that of the particles of the compound of the element belonging to the group IVB, VB or VIB of the periodic table (AℓN particles/particles of the compound of the element belonging to the group IVB, VB or VIB of the periodic table) is small, the as-formed AℓN sintered body is inferior in thermal conductivity and insufficiently colored. Thus, the said ratio is at least 2.0. In a method of preparing a thermal conductive colored aluminum nitride sintered body according to one aspect of the present invention, at least 0.01 parts by weight and not more than 5 parts by weight of carbon, not more than 1 part by weight of boron oxide in terms of the simple substance of boron, at least 0.01 parts by weight and not more than 15 parts by weight of an oxide of a rare earth element in terms of the simple substance of the rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from a group of oxides, carbides and nitrides of elements belonging to the groups IVB, VB and VIB of the periodic table in terms of the simple substance of the element contained therein are added to 100 parts by weight of aluminum nitride, to obtain a mixture. This mixture is shaped into a prescribed configuration, to obtain a compact. This compact is fired in a non-oxidizing atmosphere containing at least 10 percent by volume of nitrogen under a temperature of at least 1500°C and not more than 2100°C. The amount of carbon is synthetically determined in consideration of the type of the added carbon, the oxygen content in the AℓN raw material powder, firing conditions, and the like. At least 0.01 parts by weight and not more than 5 parts by weight of carbon is added to 100 parts by weight of aluminum nitride, for a reason similar to the above. The lower limit of the amount of carbon is set at 0.01 parts by weight, so that the finally obtained aluminum nitride sintered body contains at least 0.005 parts by weight of carbon. The carbon material may be prepared from carbon black, coke, graphite powder, diamond powder or the like. Such a carbon material is preferably restricted to a specific grain size, a specific surface area, a specific pH value, a specific volatile constituent and the like, so that the same is homogeneously dispersed in an unsintered body of aluminum nitride. In particular, it is effective to employ pulverized carbon black, which has a BET value of at least 200 m²/g. The term "BET value" indicates one value of a specific surface area measured by an absorption process. The firing temperature is set to be at least 1500°C since the speed of densification of the sintered body is lowered to reduce the economic property if the temperature is less than 1500°C. If the firing temperature exceeds 2100°C, on the other hand, it is difficult to obtain a dense sintered body since aluminum nitride is remarkably decomposed and evaporated. The rare earth aluminum oxide is formed by reaction between the rare earth element oxide and aluminum oxide. Grain growth of AℓN progresses due to mass transfer through the liquid phase of the rare earth aluminum oxide. If a temperature for forming such a liquid phase, i.e., the melting point of the rare earth aluminum oxide, is in excess of 2100°C, therefore, it is difficult to obtain a dense sintered body due to decomposition/evaporation of AℓN. The thermal conductivity of the colored aluminum sintered body is improved by addition of carbon conceivably in the following mechanism: When a compound of an element belonging to the group IVB, VB or VIB of the periodic table is added to an unsintered body and fired in the conventional method, the liquid phase of the rare earth aluminum oxide has inferior wettability with respect to the aluminum nitride particles. Further, the compound of the element belonging to the group IVB, VB or VIB of the periodic table, which has inferior wettability with respect to the AℓN particles, remains in grain boundary portions of the sintered body, to contribute to coloring thereof. At the same time, the compound is partially dissolved in the AℓN particles with oxygen. If the compound is thus partially dissolved in the AℓN particles with oxygen and the AℓN particles are inferior in wettability with respect to the liquid phase of the rare earth aluminum oxide, it is difficult to trap oxygen from the AℓN particles into the liquid phase of the rare earth aluminum oxide. It is known that untrapped oxygen is dissolved in the particles of the AℓN sintered body and forms an oxynitride of Aℓ (Aℓ (8/3 + x/3) O 4-x N x ), spinel (Aℓ₉O₃N₇), α-Aℓ₂O₃ and the like, to significantly reduce the thermal conductivity of the sintered body, even if the sintered body is dense. On the other hand, the inventors have clarified that carbon has a function of reducing oxygen which is dissolved in the AℓN particles following partial dissolution of the compound of the element belonging to the group IVB, VB or VIB of the periodic table and removing the same from the system. It is amazing that the liquid phase is not deteriorated in wettability with respect to the AℓN particles in which the compound of the element belonging to the group IVB, VB or VIB of the periodic table is dissolved but oxygen is sufficiently trapped, whereby it is possible to obtain a sintered body having high thermal conductivity. Further, carbon contained in the sintered body scatters visible light which is transmitted through the sintered body, to effectively color the sintered body. In addition, it is possible to obtain a sintered body which is colored and has high thermal conductivity conceivably because the effect of such coloring is not lost by addition of the compound of the element belonging to the group IVB, VB or VIB of the periodic table. The aluminum nitride sintered body is colored by addition of boron oxide in the following mechanism: The added boron oxide partially reacts with the added compound of an element belonging to the group IVB, VB or VIB of the periodic table, to form a boride or a boron oxide of the element belonging to the group IVB, VB or VIB of the periodic table. Such a boride or boroxide of the element belonging to the group IVB, VB or VIB of the periodic table has a small band gap, and causes light absorption in a visible region. Thus, it is conceivable that the as-formed sintered body is colored due to presence of such a substance in the AℓN sintered body. The boron oxide material may be prepared from a compound such as boric acid or ester of boric acid such as triethoxyboron, for example, which forms boron oxide by firing, hydrolysis or the like, in place of an oxide. The effect is not varied with the type of the compound. The coloring effect for the AℓN sintered body is improved as the particle diameter of the compound is reduced. In particular, a compound having a particle diameter of about 0.05 to 1 µm is preferably employed. The amount of boron oxide is not more than 1 part by weight in terms of the simple substance of boron since boron oxide remarkably volatilizes during firing to inhibit densification of the sintered body and reduce the thermal conductivity if the amount exceeds 1 part by weight. The compound of an element belonging to the group IVB, VB or VIB of the periodic table may be prepared from a compound which forms an oxide, a carbide or a nitride by firing, such as an inorganic compound of an element belonging to the group IVB, VB or VIB of the periodic table such as a nitrate or an oxalate, or an organic metal compound such as an alkoxide, for example, in place of an oxide, a carbide or a nitride, to attain a similar effect. The effect of coloring is improved as the particle diameter of the compound is reduced. In particular, a compound having a particle diameter of about 0.05 to 1 µm is preferably employed. This compound can be used with no problem if the particle diameter thereof is about 20 times that of the AℓN powder. In a method of preparing a thermal conductive colored aluminum nitride sintered body according to another aspect of the present invention, at least 0.01 parts by weight and not more than 5 parts by weight of a compound liberating carbon in terms of the amount of free carbon, not more than 1 part by weight of boron oxide in terms of the simple substance of boron, at least 0.01 parts by weight and not more than 15 parts by weight of an oxide of a rare earth element in terms of the simple substance of the rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from oxides, carbides and nitrides of elements belonging to the groups IVB, VB or VIB of the periodic table in terms of the simple substance of the element are added to 100 parts by weight of aluminum nitride, to obtain a mixture. This mixture is shaped into a prescribed configuration, to obtain a compact. This compact is heated in a non-oxidizing atmosphere under a temperature of at least 150°C and not more than 1500°C, to liberate carbon. Thereafter the compact is fired in a non-oxidizing atmosphere containing at least 10 percent by volume of nitrogen under a temperature of at least 1500°C and not more than 2100°C. The compound liberating carbon may be prepared from an organic compound such as a fatty acid compound or an aromatic compound, or a high molecular compound such as styrene resin, acrylic resin, phenol resin or urethane resin. According to the present invention, it is preferable to employ one of polyacrylonitrile, polyvinyl alcohol, polyvinyl butyral, polyethylene terephthalate, glucose, fructose and sucrose, or a mixture thereof. While carbon is contained in an unsintered body in the aforementioned method according to the first aspect of the present invention, a compound liberating carbon is contained in an unsintered body in the method according to the second aspect of the present invention. According to this method, it is possible to homogenize distribution of carbon in the as-formed sintered body and to reduce ununiformity of the composition, the color tone etc. of the sintered body by appropriately selecting the compound liberating carbon to be homogeneously dispersed in the unsintered body. In a method of preparing a thermal conductive colored aluminum nitride sintered body according to still another aspect of the present invention, not more than 1 part by weight of boron oxide in terms of the simple substance of boron, at least 0.01 parts by weight and not more than 15 parts by weight of an oxide of a rare earth element in terms of the simple substance of the rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from a group of oxides, carbides and nitrides of elements belonging to the groups IVB, VB and VIB of the periodic table in terms of the simple substance of the element are added to 100 parts by weight of aluminum nitride, to obtain a mixture. This mixture is shaped into a prescribed configuration, to obtain a compact. This compact is fired in a non-oxidizing atmosphere containing at least 10 percent by volume and not more than 90 percent by volume of hydrocarbon gas and at least 10 percent by volume of nitrogen under a temperature of at least 1500°C and not more than 2100°C. The hydrocarbon gas is introduced in order to remove oxygen from the system by chemical reaction. While such an effect is reliably attained so far as the hydrocarbon gas is prepared from a reducing gas containing carbon, it is preferable to employ methane gas, ethylene gas, acetylene gas, propane gas or butane gas, according to the present invention. In the aforementioned method according to the first or second aspect of the present invention, the mixture is shaped into a compact and carbon or a compound liberating carbon is added to the compact, in order to remove oxygen, which follows partial dissolution of the compound of the element belonging to the group IVB, VB or VIB of the periodic table in the AℓN particles, from the system by solid phase reaction with carbon. On the other hand, the method according to the third aspect of the present invention is characterized in that oxygen, which follows partial dissolution of the compound of the element belonging to the group IVB, VB or VIB of the periodic table in the AℓN particles, is removed from the system by vapor phase reaction with the hydrocarbon gas. Such hydrocarbon gas has higher reactivity as compared with carbon or that liberated from the compound, and efficiently reduces oxygen which follows partial dissolution of the compound of the element belonging to the group IVB, VB or VIB of the periodic table in the AℓN particles within a short period, to remove the same from the system. Thus, it is possible to obtain a colored aluminum nitride sintered body having a high thermal conductivity also when a relatively short firing pattern is employed. Further, while the composition, the color tone and the like of the sintered body may be ununiformalized depending on the dispersed state of carbon or the compound liberating carbon in the method according to the first or second aspect of the present invention, such ununiformity is hardly caused in the method according to the third aspect of the present invention. At least 0.01 parts by weight and not more than 15 parts by weight of the rare earth element oxide is added in terms of the simple substance of the rare earth element so that the as-formed aluminum nitride sintered body contains the rare earth aluminum oxide within the same range in terms of the simple substance of the rare earth element, for the following reason: If the amount of the rare earth element oxide is smaller than the lower limit of this range, the absolute amount of the liquid phase of the rare earth aluminum oxide, which is a composite oxide, is so insufficient that a dense sintered body cannot be obtained. If the said amount exceeds the upper limit of the above range, on the other hand, an excess liquid phase exudes out on the surface of the sintered body, to instabilize the color tone of the as-formed sintered body with spots and the like. At least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from oxides, carbides and nitrides of elements belonging to the groups IVB, VB and VIB of the periodic table is added in terms of the simple substance of the rare earth element so that the as-formed aluminum nitride sintered body contains the compound containing the element within the same range for the following reason: If the amount of the compound is smaller than the lower limit of this range, the as-formed sintered body is insufficiently colored. If the amount exceeds the upper limit of the above range, on the other hand, the compound of the element is dissolved in the liquid phase of the rare earth aluminum oxide, which is a composite oxide, to extremely inhibit wettability with respect to the AℓN particles, and hence a dense sintered body cannot be obtained. The effect according to the present invention can be sufficiently attained by adding only the aforementioned components to aluminum nitride, while it is also possible to employ a well-known binder such as acrylic resin such as PVA, PVB or PMMA, paraffin or the like, in order to improve compactibility of the unsintered body. Further, 50 parts by weight of hexagonal BN may be contained in 100 parts by weight of AℓN at the maximum, in order to improve characteristics such as mechanical workability. In the inventive method of preparing an aluminum nitride sintered body, the step of shaping the mixture having a prescribed composition may be carried out by any method such as a doctor blade coater, press molding, CIP (cold isostatic pressing) or the like. The thermal conductive colored aluminum nitride sintered body obtained in the aforementioned manner simultaneously satisfies the two incompatible requirements for thermal conductivity and coloring, dissimilarly to the prior art. When an IC chip is carried on a substrate of the inventive aluminum nitride sintered body, it is possible to suppress a temperature rise caused by heat generation of the IC chip and transmission of ultraviolet rays from the exterior. Consequently, it is possible to prevent the IC from malfunctions. The substrate may be in the form of a widely known substrate such as DIP (dual-in-line package), LCC (leadless chip carrier), a flat package, PGA (pin grid array), a cerquad package, a hybrid package, a mother board, or the like. The characteristics of the substrate, i.e., high thermal conductivity and coloring, are not damaged in any form. When the inventive aluminum nitride sintered body is applied to a substrate material in the aforementioned form, it generally comprises a conductor layer which is formed of tungsten or the like, a sealing layer which is formed of glass or the like, or a thin film circuit. An effect of such a conductor layer, sealing layer or thin film circuit is not damaged in a substrate which is formed by the inventive aluminum nitride sintered body. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 1 part by weight of carbon black having a BET value of 500 m²/g, 0.5 parts by weight of B₂O₃ in terms of the simple substance of boron, 1 part by weight of yttrium oxide in terms of the simple substance of yttrium, and 1 part by weight of titanium oxide in terms of the simple substance of titanium were added to 100 parts by weight of AℓN powder having a mean particle diameter of 1 µm. A binder, a plasticizer, a deflocculant, a solvent, a wetting agent and an antistatic agent were added to this powder, and mixed with each other in a ball mill. The plasticizer was prepared from a mixture of dibutyl phthalate (D.B.P.) and benzyl butyl phthalate (B.B.P.). The deflocculant was prepared from menhaden fish oil. The solvent was prepared from a mixture of trichloroethylene, ethyl alcohol and methyl ethyl ketone. The wetting agent was prepared from monooleic acid glycerin. The as-formed slurry was defoamed, to prepare a green sheet of AℓN by the doctor blade coater. This green sheet was fired in a nitrogen atmosphere at a temperature of 1800°C for 3 hours, to obtain a black sintered body. The thermal conductivity of this sintered body was measured at the ordinary temperature, by a laser flash method. The result was 155 W/m·K. This sintered body contained 0.03 parts by weight of carbon, 0.36 parts by weight of boron, 0.89 parts by weight of yttrium and 0.83 parts by weight of titanium with respect to 100 parts by weight of AℓN respectively. The as-formed sintered body is listed as a sample No. 1 in Tables 1 and 2. Components were similarly blended in accordance with compositions shown in Table 1 to prepare compacts by the doctor blade coater, and these compacts were fired at temperatures within a range of 1500 to 2100°C. Table 2 shows color tones and thermal conductivities of the as-formed sintered bodies, as well as carbon contents, boron contents, yttrium contents, contents of elements belonging to the groups IVB, VB and VIB of the periodic table (additive contents) in the sintered bodies. In order to measure mean particle diameters of AℓN crystal grains and Ti compound particles contained in the AℓN sintered bodies, fracture surfaces of the AℓN sintered bodies were observed with a scanning electron microscope. The results were 5.4 µm and 0.3 µm, and the ratio (AℓN particles/Ti compound particles) was 18. Comparative Example 1 Components were blended in accordance with compositions shown in Table 3, to prepare compacts by the doctor blade coater in a similar manner to the above. The compacts were then fired at temperatures within a range of 1500 to 2100°C. Table 4 shows color tones and thermal conductivities of the as-formed sintered bodies, as well as carbon contents, boron contents, yttrium contents and contents of elements belonging to the groups IVB, VB and VIB of the periodic table (additive contents) in the sintered bodies. Example 2 A prescribed amount of polyvinyl alcohol serving as a compound liberating carbon, 0.5 parts by weight of B₂O₃ in terms of the simple substance of boron, 1 part by weight of yttrium oxide in terms of the simple substance of yttrium and 1 part by weight of titanium oxide in terms of the simple substance of titanium were added to 100 parts by weight of AℓN powder having a mean particle diameter of 1 µm. A binder, a plasticizer, a solvent and a wetting agent were mixed into this powder by a ball mill. The binder was prepared from a mixture of ethylene vinyl acetate (EVA) and polypropylene (PP). The plasticizer was prepared from a mixture of dibutyl phthalate (D.B.P.) and benzyl butyl phthalate (B.B.P.). The solvent was prepared from butyl alcohol. The as-formed slurry was granulated by spray drying, and press-molded under a pressure of 1 ton/cm². The as-formed compact was heat treated in a nitrogen atmosphere under a temperature of 1000°C for 2 hours, to liberate carbon. The amount of free carbon was 0.86 parts by weight with respect to 100 parts by weight of AℓN. Thereafter the compact was fired in a nitrogen atmosphere at a temperature of 2000°C for 5 hours, to obtain a black sintered body. Through a measurement similar to Example 1, this sintered body exhibited a thermal conductivity of 160 W/m·K at the ordinary temperature. Further, the sintered body contained 0.03 parts by weight of carbon, 0.43 parts by weight of boron, 0.87 parts by weight of yttrium and 0.86 parts by weight of titanium with respect to 100 parts by weight of AℓN respectively. This aluminum nitride sintered body is listed as a sample No. 35 in Tables 5, 6 and 7. Components were similarly blended in accordance with compositions shown in Table 5, to prepare compacts by press molding. The compacts were heat treated under conditions shown in Table 6, to liberate carbon. Table 6 shows carbon contents with respect to 100 parts by weight of AℓN, which were measured after the heat treatment. The heat treated compacts were fired at temperatures within a range of 1500 to 2100°C. Table 7 shows color tones and thermal conductivities of the as-formed sintered bodies, as well as carbon contents, boron contents, yttrium contents and contents of elements belonging to the groups IVB, VB and VIB of the periodic table (additive contents) in the sintered bodies. Example 3 1 part by weight of yttrium oxide in terms of the simple substance of yttrium and 1 part by weight of titanium oxide in terms of the simple substance of titanium were added to 100 parts by weight of AℓN powder having a mean particle diameter of 1 µm. A deflocculant of octadiene was added to this powder, and mixed therewith in methyl isobutyl ketone through a ball mill. The as-formed slurry was dried at a temperature of 230°C, and a compact was prepared by a CIP method. This compact was fired in a non-oxidizing atmosphere containing 30 percent by volume of butane gas and a rest of ammonia gas under a temperature of 1800°C for 1 hour. Thus, a black sintered body was obtained. The thermal conductivity of the as-formed sintered body was measured at the ordinary temperature in a similar manner to Example 1. The result was 155 W/m·K. Further, this sintered body contained 0.02 parts by weight of carbon, 0.36 parts by weight of boron, 0.83 parts by weight of yttrium and 0.87 parts by weight of titanium with respect to 100 parts by weight of AℓN respectively. This aluminum sintered body is listed as a sample No. 59 in Tables 8, 9 and 10. Components were similarly blended in accordance with compositions shown in Table 8, to prepare compacts by the CIP method. The compacts were fired in non-oxidizing atmospheres having compositions shown in Table 9 at temperatures within a range of 1500 to 2100°C. Table 10 shows color tones and thermal conductivities of the as-formed sintered bodies, as well as carbon contents, boron contents, yttrium contents and contents of elements belonging to the groups IVB, VB and VIB of the periodic table (additive contents) in the sintered bodies. Example 4 20 parts by weight of hexagonal BN, 1 part by weight of carbon black having a BET value of 250 m²/g, 1 part by weight of scandium oxide in terms of scandium, and 1 part by weight of titanium oxide in terms of the simple substance of titanium were added to 100 parts by weight of AℓN powder having a mean particle diameter of 1 µm. A binder, a plasticizer, a solvent and a wetting agent were added to the powder and mixed with each other in a ball mill. The binder was prepared from a mixture of ethylene vinyl acetate (EVA) and polypropylene (PP). The plasticizer was prepared from a mixture of dibutyl phthalate (D.B.P.) and benzyl butyl phthalate (B.B.P.). The solvent was prepared from butyl alcohol. The as-formed slurry was granulated by spray drying, and press-molded under a pressure of 1 ton/cm². The as-formed compact was fired in a nitrogen atmosphere at a temperature of 1900°C for 2 hours. Thus, a black sintered body was obtained. The thermal conductivity of the sintered body was measured at the ordinary temperature, similarly to Example 1. The result was 130 W/m·K. Further, this sintered body contained 0.03 parts by weight of carbon, 0.38 parts by weight of boron, 0.83 parts by weight of scandium and 0.82 parts by weight of titanium with respect to 100 parts by weight of AℓN respectively. This aluminum nitride sintered body is listed as a sample No. 86 in Tables 11 and 12. Titanium oxide having rutile type crystals was employed as a compound containing an element belonging to the group IVB, VB or VIB of the periodic table, and blended with components in accordance with compositions shown in Table 1, to prepare compacts by press molding. The compacts were fired at temperatures within a range of 1500 to 2100°C. Table 12 shows color tones and thermal conductivities of the as-formed sintered bodies, as well as carbon contents, boron contents, rare earth element contents and titanium contents in the sintered bodies. Example 5 Tungsten oxide was employed as a compound containing an element belonging to the group IVB, VB or VIB of the periodic table and blended with components in accordance with compositions shown in Table 13, to prepare compacts by the doctor blade coater, similarly to Example 1. The compacts were fired at temperatures within a range of 1500 to 2100°C. Table 14 shows color tones and thermal conductivities of the as-formed sintered bodies, as well as carbon contents, boron contents, rare earth element contents and tungsten contents in the sintered bodies. Comparative Example 2 Tungsten oxide was employed as a compound containing an element belonging to the group IVB, VB or VIB of the periodic table and blended with components in accordance with compositions shown in Table 15, to prepare compacts by the doctor blade coater, similarly to Example 1. The compacts were fired at temperatures within a range of 1500 to 2100°C. Table 16 shows color tones and thermal conductivities of the as-formed sintered bodies, as well as carbon contents, boron contents, rare earth element contents and tungsten contents in the sintered bodies. Comparing the thermal conductivities and color tones of the samples obtained in the aforementioned Examples and Comparative Examples, the inventive samples exhibited thermal conductivities of at least 100 W/m·K with gray or black colors. On the other hand, the comparative samples colored in black exhibited thermal conductivities of less than 100 W/m·K, while those showing thermal conductivities exceeding 100 W/m·K exhibited only white or spotted colors. Comparative Example Amount of Carbon (wt.part) Amount of Boron (wt. part) Amount of Yttrium (wt. part) Amount of Additive (wt. part) Added Compound 28 0.00 0.50 1.00 1.00 TiO₂ 29 1.00 0.50 0.00 1.00 TiO₂ 30 1.00 0.50 1.00 0.00 ZrO₂ 31 6.00 0.50 1.00 1.00 V₂O₅ 32 1.00 0.50 20.00 1.00 MoO₃ 33 1.00 0.50 1.00 20.00 WO₃ 3 4 1.00 1.50 1.00 1.00 WO₃ Compara tive Example Color Tone Thermal Conductivity W/m·K Carbon Content (wt. part) Boron Content (wt. part) Yttrium Content (wt. part) Additive Content (wt. part) 28 Black 65 0.00 0.35 0.85 0.79 2 9 White 50 0.02 0.37 0.00 0.78 30 Pale Gray 165 0.05 0.37 0.84 0.00 31 Whitened Not Densified 0.61 0.37 0.86 0.76 32 Spotted 130 0.05 0.32 17.95 0.80 33 Whitened Not Densified 0.05 0.33 0.92 17.67 3 4 Whitened Not Densified 0.04 1.16 0.81 0.88 Example Amount of Carbon (wt. part) Amount of Boron (wt. part) Amount of Rare Earth Element (wt. part) Amount of TiO₂ (in Terms of Ti) (wt. part) Added Rare Earth Element Compound 86 1.00 0.50 1.00 1.00 Sc₂O₃ 87 1.00 0.50 1.00 1.00 Y₂O₃ 88 1.00 0.50 1.00 1.00 La₂O₃ 89 1.00 0.50 1.00 1.00 Ce₂O₃ 90 3.00 0.50 3.00 1.00 Sc₂O₃ 91 3.00 0.50 3.00 3.00 Y₂O₃ 92 3.00 0.50 3.00 3.00 La₂O₃ 93 3.00 0.50 0.50 0.50 Ce₂O₃ 94 0.50 0.50 0.50 0.50 Sc₂O₃ 95 0.50 0.50 0.50 0.50 Y₂O₃ 96 0.50 0.50 0.50 0.50 La₂O₃ 97 0.50 0.50 0.50 0.50 Ce₂O₃ Example Color Tone Thermal Conductivity W/m·K Carbon Content (wt. part) Boron Content (wt. part) Rare Earth Element Content (wt. part) Ti Content (wt. part) 86 Black 130 0.03 0.38 0.83 0.82 87 Black 155 0.03 0.36 0.89 0.83 88 Black 125 0.03 0.39 0.84 0.86 89 Black 130 0.03 0.44 0.81 0.85 90 Black 180 0.11 0.39 2.51 2.52 91 Black 205 0.11 0.37 2.66 2.63 92 Black 190 0.10 0.42 2.64 2.51 93 Dark Gray 210 0.12 0.42 2.56 2.46 94 Dark Gray 105 0.01 0.43 0.37 0.40 95 Black 140 0.02 0.39 0.39 0.40 96 Black 130 0.01 0.40 0.40 0.39 97 Black 130 0.01 0.43 0.39 0.41 Example Amount of Carbon (wt. part) Amount of Boron (wt. part) Amount of Rare Earth Element (wt. part) Amount of WO₃ (in Terms of W) (wt. part) Added Rare Earth Element Compound 98 1.00 0.50 1.00 1.00 Sc₂O₃ 99 1.00 0.50 1.00 1.00 Y₂O₃ 100 1.00 0.50 1.00 1.00 La₂O₃ 101 1.00 0.50 1.00 1.00 Ce₂O₃ 102 3.00 0.50 3.00 3.00 Sc₂O₃ 103 3.00 0.50 3.00 3.00 Y₂O₃ 104 3.00 0.50 3.00 3.00 La₂O₃ 105 3.00 0.50 3.00 3.00 Ce₂O₃ 106 0.50 0.50 0.50 0.50 Sc₂O₃ 107 0.50 0.50 0.50 0.50 Y₂O₃ 108 0.50 0.50 0.50 0.50 La₂O₃ 109 0.50 0.50 0.50 0.50 Ce₂O₃ Example Color Tone Thermal Conductivity W/m·K Carbon Content (wt. part) Boron Content (wt. part) Rare Earth Element Content (wt. part) W Content (wt. part) 98 Black 140 0.02 0.91 0.87 0.85 99 Black 160 0.03 0.89 0.83 0.90 100 Black 155 0.03 0.88 0.80 0.83 101 Black 140 0.02 0.90 0.86 0.81 102 Black 195 0.11 2.70 2.67 2.60 103 Black 200 0.10 2.75 2.46 2.69 104 Black 185 0.10 2.68 2.41 2.45 105 Black 205 0.12 2.78 2.58 2.55 106 Black 130 0.01 0.39 0.39 0.39 107 Black 150 0.02 0.34 0.39 0.46 108 Black 130 0.01 0.37 0.38 0.36 109 Black 135 0.02 0.38 0.37 0.38 Comparative Example Amount of Carbon (wt. part) Amount of Boron (wt. part) Amount of Rare Earth Element (wt. part) Amount of WO₃ (in Terms of W) (wt. part) Added Rare Earth Element 110 0.00 0.50 1.00 1.00 Sc₂O₃ 111 0.00 0.50 1.00 1.00 Y₂O₃ 112 0.00 0.50 1.00 1.00 La₂O₃ 113 0.00 0.50 1.00 1.00 Ce₂O₃ 114 1.00 0.50 20.00 1.00 Sc₂O₃ 115 1.00 0.50 20.00 1.00 Y₂O₃ 116 1.00 0.50 20.00 1.00 La₂O₃ 117 1.00 0.50 20.00 1.00 Ce₂O₃ 118 1.00 0.50 1.00 0.00 Sc₂O₃ 119 1.00 0.50 1.00 0.00 Y₂O₃ 120 1.00 0.50 1.00 0.00 La₂O₃ 121 1.00 0.50 1.00 0.00 Ce₂O₃ 122 1.00 0.50 1.00 20.00 Sc₂O₃ 123 1.00 0.50 1.00 20.00 Y₂O₃ 124 1.00 0.50 1.00 20.00 La₂O₃ 125 1.00 0.50 1.00 20.00 Ce₂O₃ 126 1.00 1.50 1.00 1.00 Sc₂O₃ 127 1.00 1.50 1.00 1.00 Y₂O₃ 128 1.00 1.50 1.00 1.00 La₂O₃ 129 1.00 1.50 1.00 1.00 Ce₂O₃ According to the present invention, as hereinabove described, it is possible to obtain an aluminum nitride sintered body which is colored and has an excellent thermal conductivity. The inventive aluminum sintered body is useful as an electronic material such as an IC package material or a substrate material, with a high industrial value.

Claims: A thermal conductive colored aluminum nitride sintered body containing 100 parts by weight of aluminum nitride, at least 0.005 parts by weight and not more than 0.5 parts by weight of carbon, not more than 1 part by weight of a boron compound other than BN in terms of the simple substance of boron, at least 0.01 parts by weight and not more than 15 parts by weight of a rare earth aluminum oxide in terms of the simple substance of the rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from a group of compounds containing elements belonging to the groups IVB, VB and VIB of the periodic table in terms of the simple substance of the element contained in said compound, said thermal conductive colored aluminum nitride sintered body having a thermal conductivity of at least 100 W/mK and not more than 270W/mK at the ordinary temperature, the mean particle diameter of aluminum nitride crystal grains being 2 to 15 µm on a fracture of said aluminum nitride sintered body, the ratio of said mean particle diameter to the mean particle diameter of particles of said compound of said element belonging to the group IVB, VB and VIB of the periodic table being at least 2.0, and said sintered body having a color within the range of gray to black. A thermal conductive colored aluminum nitride sintered body in accordance with claim 1 wherein said rare earth element is any one of scandium, yttrium, lanthanum and cerium. A method of preparing a thermal conductive colored aluminum nitride sintered body comprising the steps of: adding at least 0.01 parts by weight and not more than 5 parts by weight of carbon, boron oxide, however, in an amount of not more than 1 part by weight in terms of the simple substance of boron, at least 0.01 parts by weight and not more than 15 parts by weight of an oxide of a rare earth element in terms of the simple substance of said rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compund selected from a group of oxides, carbides and nitrides of elements belonging to the groups IVB, VB and VIB of the periodic table in terms of the simple substance of the element contained in said compound to 100 parts by weight of aluminum nitride, thereby obtaining a mixture; shaping said mixture into a prescribed configuration for obtaining a compact; and firing said compact in a non-oxidizing atmosphere containing at least 10 percent by volume of nitrogen under a temperature of at least 1500°C and not more than 2100°C, said thermal conductive colored aluminum nitride sintered body having a thermal conductivity of at least 100 W/mK and not more than 270 W/mK at the ordinary temperature. A method of preparing a thermal conductive colored aluminum nitride sintered body having a thermal conductivity between 100 and 270 W/mK comprising the steps of: adding at least 0.01 parts by weight and not more than 5 parts by weight of a compound liberating carbon in terms of the amount of free carbon, boron oxide, however, in an amount of not more than 1 part by weight of boron oxide in terms of the simple substance of boron, at least 0.01 parts by weight and not more than 15 parts by weight of an oxide of a rare earth element in terms of the simple substance of said rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from a group of oxides, carbides and nitrides of elements belonging to the groups IVB, VB and VIB of the periodic table in terms of the simple substance of the element contained in said compound to 100 parts by weight of aluminum nitride, thereby obtaining a mixutre; shaping said mixture into a prescribed configuration for obtaining a compact; heating said compact in a non-oxidizing atmosphere under a temperature of at least 150°C and not more than 1500°C for liberating carbon; and firing said compact in a non-oxidizing atmosphere containing at least 10 percent by volume of nitrogen under a temperature of at least 1500°C and not more than 2100°C. A method of preparing a thermal conductive colored aluminum nitride sintered body in accordance with claim 2, wherein said compound liberating carbon is prepared from at least one compound selected from a group of polyacrylonitrile, polyvinyl alcohol, polyvinyl butyral, polyethylene terephtalate, glucose, fructose and sucrose. A method of preparing a thermal conductive colored aluminum sintered body having a thermal conductivity between 100 and 270 W/mK comprising the steps of: adding boron oxide, however, in an amount of not more than 1 part by weight in terms of the simple substance of boron, at least 0.01 party by weight and not more than 15 parts by weight of an oxide of a rare earth element in terms of said rare earth element, and at least 0.01 parts by weight and not more than 15 parts by weight of at least one compound selected from a group of oxides carbides and nitrides of elements belonging to the groups IVB, VB and VIB of the periodic table terms of the simple substance of the element contained in said compound to 100 parts by weight of aluminum nitride, thereby obtaining a mixture; shaping said mixture into a prescribed configuration for obtaining a compact; and firing said compact in a non-oxidizing atmosphere containing at least 10 percent by volume and not more than 90 percent by volume of hydrocarbon gas and at least 10 percent by volume of nitrogen under a temperature of at least 1500°C and not moe than 2100°C. A method of preparing a thermal conductive colored aluminum nitride sintered body in accordance with claim 4, wherein said hydrocarbon gas is prepard from any one of methane gas, ethylene gas, acetylene gas, propane gas and butane gas

Inventors: MAEDA TAKAO, MIYAKE MASAYA, SHIMODA KOUHEI, SOGABE KOUICHI, MAEDA, TAKAO, MIYAKE, MASAYA, SHIMODA, KOUHEI, SOGABE, KOUICHI, MAEDA, TAKAO, C/O ITAMI WORKS, MIYAKE, MASAYA, C/O ITAMI WORKS, Shimoda, Kouhei, c/o Itami Works, Sogabe, Kouichi, c/o Itami Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: C04B35
IPC codes: H01L23, C04B35