Highly heat-conductive silicon nitride-base sintered-body and method for fabricating the same

Abstract: A highly heat-conductive, mechanically strong Si 3 N 4 -base sintered body that can be fabricated by a highly productive method. The sintered body comprises crystal phases of Si 3 N 4 grains and grain-boundary phases containing a 1 to 20wt.% compound, in oxide equivalent, of at least one kind of rare-earth element selected from the group consisting of Sm, Yb, Y, Er and Gd. In the sintered body, half widths of X-ray diffraction peaks on the (500) and (330) planes of the Si 3 N 4 are 0.2° or less. To fabricate the sintered body, Si powders are mixed with oxide powders of the foregoing rare-earth elements to form a compact, the compact is heated at a rate of 0.2 to 0.6°C/min in the range of 1220 to 1320°C in a nitrogen atmosphere containing 5% or less He to be nitrided, and the nitrided compact is sintered at 1650 to 1950°C in a nitrogen atmosphere.

Description: The invention relates to a mechanically strong, highly heat-dissipative silicon nitride (Si 3 N 4 )-base sintered-body (sintered body mainly consisting of silicon nitride) suitable for not only insulation substrates and heat-dissipating plates of semiconductor devices but also structural members of automobiles, machines, and OA equipment. It also relates to a method for fabricating the sintered body. Ceramics mainly consisting of silicon nitride are superior to other ceramic materials in terms of heat resistance, mechanical strength, and toughness, and are suitable for structural members of automobiles, machines, and OA equipment. In exploiting their superior insulation property, application to insulating, heat-dissipating substrates of semiconductor devices has also been tried. Superior in thermal conductivity, AlN and SiC have been used for ceramic substrates for semiconductor devices. However, conventional, highly heat-conductive materials such as AlN or SiC are insufficient in terms of mechanical strength and toughness, imposing limitations on handling, operating environments, and shapes of the products. The circumstances, therefore, require the development of ceramic materials having not only high strength for withstanding external force but also high heat-dissipating power. To enhance the performance of products, the heat-dissipating properties of structural members, to which resistance to thermal shock is essential, as well as the mechanical strength of the sliding members must be improved. Because Si 3 N 4 innately has high mechanical strength, a wide range of its applications from structural members to heat-dissipative insulation substrates can be expected if its thermal conductivity is further increased. However, due to its comparatively low thermal conductivity, the conventional Si 3 N 4 -base sintered body is minimally applied to heat-dissipative insulation substrates and other uses. Al is known to reduce the thermal conductivity of Si 3 N 4 . When Al 2 O 3 is added as a sintering agent, for example, Al dissolves in Si 3 N 4 grains, becoming a factor that scatters phonons and reduces thermal conductivity. In order to eliminate this drawback, a sintering process has been developed, which does not use a sintering agent that dissolves in Si 3 N 4 grains. Findings reveal that the addition of only Y 2 O 3 as a sintering agent produces a dense Si 3 N 4 -base sintered body with thermal conductivity of 70 W/mK. (See J. Ceram. Soc. J. vol. 97 (1) pp. 56-62, 1989.) Another factor that reduces the thermal conductivity of Si 3 N 4 is the existence of the grain boundary phases composed of glass phases having low thermal conductivity. To counteract this effect, unexamined Japanese patent application Tokukaihei 7-149588 discloses a process that controls the cooling rate after sintering to slower than 100 °C/hr so that some of grain-boundary phases are crystallized, resulting in thermal conductivity higher than 60 W/mK. It is also known that when β-silicon nitride is used as material powders to be sintered at a high temperature of 2000 °C under a high pressure of 300 atm, the grain size becomes coarse, decreasing the effect of the grain-boundary phase on the thermal conductivity to produce a highly heat-conductive silicon nitride-base sintered body. (See J. Ceram. Soc. J. vol. 104 (1) pp. 49-.) As mentioned above, the dissolution of Al when used as a sintering agent in grains and the thermal conductivity of grain-boundary phases are studied to increase the thermal conductivity of an Si 3 N 4 -base sintered body. However, this approach alone does not fulfill the object. A constituent including Al must inevitably be added to obtain a dense Si 3 N 4 body with sufficiently high mechanical strength, and this reduces the thermal conductivity. To prevent this reduction and further increase the thermal conductivity, it is necessary to employ high temperature and high pressure at the cost of reduced productivity. The object of the invention, in consideration of the above-mentioned circumstances, is to offer a highly heat-conductive, mechanically strong Si 3 N 4 -base sintered body that can be produced with high productivity. To achieve the above object, the present inventors studied factors that affect the thermal conductivity of an Si 3 N 4 -base sintered body, and found that further increased thermal conductivity is possible by improving the crystallinity of Si 3 N 4 grains, in addition to the conventionally considered factors such as the thermal conductivity of grain-boundary phases and the grain size of Si 3 N 4 . More specifically, it was found that the crystallinity of Si 3 N 4 grains is substantially improved by the selection of a proper sintering agent and the conditions of the nitriding reaction, such as temperature and atmosphere, in the reaction-sintering method that uses Si powders as the starting material. Accordingly, the present invention provides a highly heat-conductive silicon nitride (Si 3 N 4 )- base sintered body, wherein half widths of X-ray diffraction peaks on the (500) and (330) planes of the Si 3 N 4 are 0.2° or less, and wherein the sintered body has a thermal conductivity of 80 W/mK or more and a three-point bending strength of 800 MPa or more. In addition, it is desirable that the measured results of the X-ray diffraction peaks on Si 3 N 4 satisfy the following formula: (W 5 - W 2 )/(tan  5 - tan  2 ) ≦ 0.001, where W 2 : half width of the diffraction peak on the (200) at a measured angle  =  2 , W 5 : half width of the diffraction peak on the (500) at a measured angle  =  5 . The above-described silicon nitride-base sintered body comprises the crystal phases of β-Si 3 N 4 grains and grain boundary phases, which preferably contain a 1 to 20 wt.% compound, in oxide equivalent, of at least one kind of rare-earth element selected from the group consisting of Sm, Yb, Y, Er, and Gd. The grain boundary phases having the foregoing preferable composition may further contain a 0.1 to 10 wt.% element, in oxide equivalent, of at least one kind selected from the group consisting of Mg, Ca, and Ti. To obtain an Si 3 N 4 -base sintered body having satisfactory crystallinity as mentioned above, annealing in a high-temperature, high-pressure atmosphere or gas-phase synthesis may be conceived. However, for achieving low-cost production and high mechanical strength (reduction in grain size), it is desirable that the starting material of such a fabricating method be low cost and highly pure, and the sintering process be controlled so that Si 3 N 4 grains may not contain impurities and defects. To meet these requirements, the invention employs the following fabricating method: According to the method for fabricating the silicon nitride-base sintered body of the invention, Si powders containing 0.1 wt.% or less Al are mixed with a sintering agent representing 1 to 20 wt.% of the Si powders in Si 3 N 4 equivalent to form a compact. The compact is then heated at a rate of 0.2 to 0.6 °C/min in the range of 1220 to 1320 °C in a nitrogen atmosphere containing 5 % or less He to be nitrided at 1220 to 1400 °C before sintering between 1650 and 1950 °C in a nitrogen atmosphere. In the method according to the present invention a sintering agent is used that comprises oxide powders of at least one kind of rare-earth element selected from the group consisting of Sm, Yb, Y, Er, Gd, Eu, Tb and Tm. These sintering agents form few solid solutions with Si 3 N 4 grains, give no uneven distortion to the crystal, and expedite the nitriding reaction of Si. According to the method for fabricating the silicon nitride-base sintered body, the above-mentioned mixture of Si powders and rare-earth elements' oxide powders may further be mixed with other oxide powders, representing 0.1 to 10 wt.% of the foregoing Si powders in Si 3 N 4 equivalent, of at least one kind of element selected from the group consisting of Mg, Ca, and Ti. It is also desirable that the sintered body be cooled at a rate of 100 °C/hr or more in the range of the sintering temperature to 1600 °C and at a rate of 100 °C/hr or less in the range of 1600 to 1500 °C. As explained above, the present invention offers a highly heat-conductive, mechanically strong Si 3 N 4 -base sintered body having improved crystallinity of Si 3 N 4 grains by the use of the silicon-powder reaction-sintering method, which is superior in productivity. According to the invention, which employs the reaction-sintering method that uses Si powders as the starting material, Si powders are available at a lower cost and purer quality than Si 3 N 4 powders, impurities such as Al included in the material can be reduced, and Si 3 N 4 -base sintered bodies can be fabricated with low-cost processes without relying on high temperature and high pressure. In the invention, attention was focused on the hitherto neglected distortion in Si 3 N 4 grains due to defects resulting from the nitriding reaction and sintering process. Reduction of this distortion was achieved in the method according to the present invention through the selection of sintering agents comprising oxides of Sm, Yb, Y, Er, and/or Gd as mentioned above, and through the optimization of temperature and atmosphere for the nitriding reaction. This approach enabled the embodiment of an Si 3 N 4 -base sintered body that has substantially high thermal conductivity in comparison with the conventional method without losing the intrinsic mechanical strength of Si 3 N 4 . It also enabled sintering at low temperature and low pressure compared to conventional methods without depending on sintering agents such as Al 2 O 3 that reduce thermal conductivity, so that a dense Si 3 N 4 -base .sintemd'body can be produced easily. More specifically, in the method for fabricating the silicon nitride-base sintered body of the invention, Si powders containing a 0.1 wt.% or less impurity Al are used as the principal material powders. Because Al in Si powders significantly affects the thermal conductivity of a sintered body as minute impurities in grains, it is necessary to reduce the Al content of Si powders to 0.1 wt.% or less in order to achieve sufficiently high thermal conductivity. The use of such Si powders suppresses the dissolution of Al into Si 3 N 4 grains in comparison with the ordinary Si 3 N 4 powders, which usually contain 0.1 to 1 wt.% Al in the form of a solid solution, and facilitates the production of highly purified Si 3 N 4 grains. In the method according to the present invention, a sintering agent is used that comprises oxide powders of at least one kind of rare-earth element selected from the group consisting of Sm, Yb, Y, Er, Gd, Eu, Tb and Tm. The use of sintering agents having such desirable composition enables easy fabrication of highly crystallized Si 3 N 4 grains through nitriding and sintering processes. Sintering agents of 3A-group elements other than those designated above tend to develop defects in Si 3 N 4 grains, to cause uneven distortion in the crystal, or to allow the added elements themselves to dissolve into the grains during the process of nitriding and sintering, making it difficult to produce Si 3 N 4 with satisfactory crystallinity. This leads to the difficulty of the embodiment of a highly heat-conductive Si 3 N 4 -base sintered body. In addition to this, the nitriding reaction of Si cannot be performed sufficiently, developing coarse molten blocks of Si, so that adequate mechanical strength may not be obtained. The added amount of a sintering agent should be in the range of 1 to 20 wt.% of the Si powders, the principal material powders, in Si 3 N 4 equivalent. If less than 1 wt.%, the nitriding reaction cannot be performed sufficiently, developing coarse molten blocks of Si, so that the mechanical strength of the sintered body decreases considerably. If more than 20 wt.%, the grain-boundary phases develop excessively, decreasing not only the mechanical strength and toughness but also the thermal conductivity of the sintered body. In addition to the above sintering agents, an additional additive may be added, which comprises oxide powders, representing 0.1 to 10 wt.% of Si powders in Si 3 N 4 equivalent, of at least one kind of element selected from the group consisting of Mg, Ca, and Ti. These oxides promote sintering without reducing thermal conductivity, enabling the production of high-strength sintered bodies at comparatively low temperature and low pressure. For instance, sintering is possible at 1750 °C or below in a nitrogen atmosphere under atmospheric pressure without relying on a process with high-pressure gas such as hot-isostatic pressing (HIP). The foregoing material powders of Si, a sintering agent, and an additional additive are mixed by a ball mill or other well-known means to form a compact having a given shape. As for the forming method, the die-pressing method, sheet-forming method, and other prevalent methods are applicable. The compact is nitrided at 1220 to 1400 °C in a nitrogen atmosphere containing 5 % or less He. If lower than 1220 °C, the nitriding reaction cannot be performed sufficiently. If higher than 1400 °C, the reaction heat melts Si, developing defects, so that the sintered body cannot have sufficient mechanical strength. The He gas, 5 % or less in the nitrogen atmosphere, facilitates homogeneous reaction of nitriding, suppressing the distortion in grains and the generation of large pores, so that the sintered body can be improved in thermal conductivity and mechanical strength. In the above nitriding process, it is essential that the compact be heated at a rate of 0.2 to 0.6 °C/min in the range of 1220 to 1320 °C, the temperature range where the nitriding reaction proceeds rapidly, entailing the possible formation of defects, and hence requires sufficient control of the reaction rate. If the temperature-rising rate is less than 0.2 °C/min, the prolonged reaction time not only reduces productivity but also may decrease crystallinity. If more than 0.6 °C/min, the Si 3 N 4 grain remarkably decreases in crystallinity, resulting in reduced thermal conductivity of the sintered body. The nitrided compact is then sintered between 1650 and 1950 °C in a nitrogen atmosphere. If the sintering temperature is lower than 1650 °C, the sintered body increases in porosity, reducing thermal conductivity and mechanical strength. If higher than 1950 °C, Si 3 N 4 decomposes easily, so that a high-pressure process is required, reducing the productivity undesirably. It is desirable that the sintering be executed to the extent that the completed Si 3 N 4 -base sintered body has a relative density (ratio of a measured density to the theoretical density) of 95 % or more. As explained above, the fabricating method of the invention characteristically performs (1) the proper selection of the sintering agent that expedites the nitriding reaction without impairing the crystallinity of forming Si 3 N 4 grains, (2) the inclusion of a small amount of He in the nitrogen atmosphere when the compact is heated and nitrided, and (3) the slow-rate heating in the temperature range where nitriding reaction proceeds rapidly. In addition, the present inventors' findings on the effect of the cooling rate from the sintering temperature on the crystallinity of Si 3 N 4 grains demonstrated that simple slow-rate cooling produces local distortion in the Si 3 N 4 grains, differing from the common knowledge that considerably slow cooling promotes crystallization of grain-boundary phases and favorably increases thermal conductivity. Based on the above study, in the fabricating method of the invention, the most desirable cooling process after the sintering is specified as follows: The sintered body is to be cooled at a rate of 100 °C/hr or more from the sintering temperature to 1600 °C, at which temperature the crystallization of grain-boundary phases is completed, and at a slower rate of 100 °C/hr or less from 1600 to 1500 °C. This cooling procedure further promotes the crystallization of grain-boundary phases, increases the crystallinity of Si 3 N 4 grains, and therefore provides the highest thermal conductivity. Thus, the fabricating method of the invention, as compared to the conventional means, enables the dense sintering at low temperature and low pressure, is superior in productivity, and produces an Si 3 N 4 -base sintered body with high mechanical strength and substantially high thermal conductivity. Specifically, the Si 3 N 4 -base sintered body has a relative density of 95 % or more, thermal conductivity of 80 W/mK or more, and three-point bending strength of 800 MPa or more. In other words, the Si 3 N 4 -base sintered body of the invention is superior in crystallinity to the conventional sintered body in that the half widths of X-ray diffraction peaks on the (500) and (330) of Si 3 N 4 are 0.2 ° or less. In addition, as an index for estimating the rate of variation in the spacing of lattice planes by means of the X-ray diffraction peak of Si 3 N 4 , it is desirable that Δd/d = (W 5 -W 2 )/(tan  5 - tan  2 ) be 0.001 or less, where d is the spacing of lattice planes, Δ d is the variation of the spacing of lattice planes, W 2 is the half width of the diffraction peak on the (200) at a measured angle  =  2 , and W 5 on the (500) at  =  5 . Incidentally, the conventional Si 3 N 4 -base sintered body usually has a half width of more than 0.2 ° and a Δd/d of 0.003 or more. It is preferable that the Si 3 N 4 -base sintered body of the invention be composed of the crystal phases made of β-silicon nitride grains and the grain-boundary phases containing a 1 to 20 wt.% compound, in oxide equivalent, of at least one kind of rare-earth element selected from the group consisting of Sm, Yb, Y, Er, and Gd. Yet, in the foregoing preferable composition, the grain-boundary phases may further contain a 0.1 to 10 wt.% element, in oxide equivalent, of at least one kind selected from the group consisting of Mg, Ca, and Ti. It is also desirable that the foregoing β-Si 3 N 4 grains have an average size of 0.5 to 20 µm. EXAMPLE 1 Si powders having an average grain size of 1 µm and containing 0.05 wt.% Al were mixed with a sintering agent composed of oxide powders of rare-earth elements, the kind and added amount of which are shown in Table 1 below. In the table, the added amount is shown by the weight ratio to the Si powders in Si 3 N 4 equivalent; the same is applied to the following tables. The mixing was carried out in an alcohol solvent by means of a ball mill having an Si 3 N 4 -lined pot and Si 3 N 4 balls to produce slurry. The slurry was dried with a spray drier to form granules about 120 µm in average diameter. The granules were dry-compacted to form samples for the three-point bending test (45 mm in length, 8 mm in width, and 5 mm in thickness) and samples for thermal conductivity measurement (12.5 mm in diameter and 5 mm in thickness). These compacts were placed in a BN-lined carbon refractory case to be nitrided in an N 2 gas atmosphere containing 3 % He (gas-pressure ratio: N 2 :He = 0.97:0.03, gas pressure: 3 atm). For nitriding the compacts, the temperature was raised at a rate of 0.3 °C/min in the range of 1220 to 1320 °C and kept at 1400 °C for one hour. Subsequently, the nitrided compacts were sintered at 1800 °C for three hours in an N 2 gas atmosphere at 3 atm. Of the completed Si 3 N 4 -base sintered bodies, samples for the three-point bending test were used to measure a relative density and three-point bending strength, and samples for thermal conductivity were used to measure thermal conductivity by the laser flash method. The crystallinity of Si 3 N 4 grains was measured by the X-ray diffraction method using Cu-K α-rays at a scanning rate of 1 ° /min, excitation potential of 50 kV, excitation current of 200 mA, and step width of 0.01° . These results are shown in Table 1 below, in which Δ d/d means (W 5 - W 2 )/(tan 5 - tan  2 ), where W 2 is the half width of the diffraction peak on the (200) at a measured angle  =  2 , and W 5 on the (500) at  =  5 . Sample Sintering agent Half width (° ) Δd/d Thermal conductivity Bending strength Kind wt.% (500) (330) W/mK MPa 1 Sm 2 O 3 10 0.08 0.1 0.000 140 1000 2 Yb 2 O 3 10 0.05 0.08 0.000 150 1000 3 Gd 2 O 3 10 0.12 0.14 0.001 120 800 4 Y 2 O 3 10 0.16 0.20 0.001 90 800 5 Er 2 O 3 10 0.12 0.14 0.001 120 800 6 Dy 2 O 3 10 0.12 0.14 0.001 80 600 7 Pr 6 O 11 10 0.25 0.25 0.002 65 600 8 Ce 2 O 3 10 0.3 0.3 0.002 60 600 9 La 2 O 3 10 0.3 0.35 0.003 50 700 10 Sm 2 O 3 0.5 0.21 0.23 0.002 60 600 11 Sm 2 O 3 1 0.14 0.14 0.001 80 800 12 Sm 2 O 3 12 0.08 0.1 0.000 140 1000 13 Sm 2 O 3 20 0.12 0.14 0.001 80 900 14 Sm 2 O 3 21 0.19 0.21 0.001 70 800 EXAMPLE 2 Si 3 N 4 -base sintered bodies were fabricated by the same means as Example 1 using the same Si powders except that 10 wt.% Sm 2 O 3 powders were added as a sintering agent, and that the temperature-rising rate in the range of 1220 to 1320 °C at the time of nitriding was varied as shown in Table 2 below. Measurements were carried out on the completed Si 3 N 4 -base sintered bodies on the same items as Example 1 to obtain the results shown in Table 2. Sample Temperature-rising rate in sintering Half width (° ) Δd/d Thermal conductivity Bending strength °C/min (500) (330) W/mK MPa 15 0.1 0.2 0.25 0.0014 50 800 16 0.2 0.1 0.12 0.0008 100 1000 17 0.5 0.08 0.1 0.0008 140 1000 18 0.6 0.12 0.14 0.001 90 800 19 0.7 0.25 0.35 0.0025 50 650 EXAMPLE 3 Si 3 N 4 -base sintered bodies were fabricated by the same means as Example 1 using the same Si powders except that 10 wt.% Yb 2 O 3 powders were added as a sintering agent, that the temperature was raised at a rate of 0.2 °C/min in the range of 1220 to 1320 °C at the time of nitriding, and that the gas-pressure ratio of N 2 to He in the atmosphere for nitriding was varied as shown in Table 3 below. Measurements were carried out on the completed Si 3 N 4 -base sintered bodies on the same items as Example 1 to obtain the results shown in Table 3. Sample Gas-pressure ratio in nitriding Half width (° ) Δd/d Thermal conductivity Bending strength N 2 He (500) (330) W/mK MPa 20 1 0 0.19 0.21 0.001 75 900 21 0.95 0.05 0.05 0.08 0.000 150 1000 22 0.9 0.1 0.2 0.25 0.002 70 800 EXAMPLE 4 Si 3 N 4 -base sintered bodies were fabricated by the same means as Example 1 except that Si powders having a varied Al content as shown in Table 4 below were mixed with 10 wt.% Sm 2 O 3 powders as a sintering agent, and that the temperature was raised at a rate of 0.2 °C/min in the range of 1220 to 1320 °C at the time of nitriding. Measurements were carried out on the completed Si 3 N 4 -base sintered bodies on the same items as Example 1 to obtain the results shown in Table 4. Sample Al content in Si powders Half width (° ) Δd/d Thermal conductivity Bending strength wt.% (500) (330) W/mK MPa 23 0.01 0.08 0.1 0.00 130 1000 24 0.1 0.08 0.1 0.00 130 1000 25 0.15 0.21 0.25 0.00 60 1000 EXAMPLE 5 Si 3 N 4 -base sintered bodies were fabricated by the same means as Example 1 using the same Si powders except that 10 wt.% Yb 2 O 3 powders were added as a sintering agent, that MgO powders were added as an additional additive with the amount shown in Table 5 below, and that the sintering was performed at 1700 °C. In Table 5, the added amount of the additional additive is shown by the weight ratio to the Si powders in Si 3 N 4 equivalent; the same is applied to the following tables. Measurements were carried out on the completed Si 3 N 4 -base sintered bodies on the same items as Example 1 to obtain the results shown in Table 5. Sample Additional additive Half width (° ) Δd/d Thermal conductivity Bending strength Kind Wt.% (500) (330) W/mK MPa 26 MgO 0.05 0.1 0.12 0.001 80 800 27 MgO 1 0.05 0.08 0.001 130 1000 28 MgO 5 0.05 0.08 0.001 150 1100 29 MgO 10 0.1 0.12 0.001 130 900 30 MgO 11 0.12 0.14 0.001 80 850 EXAMPLES Si 3 N 4 -base sintered bodies were fabricated by the same means as Example 1 using the same Si powders except that a 10 wt.% sintering agent and a 5 wt.% additional powder additive each of which are shown in Table 6 below were added, and that the sintering temperature was varied as shown in Table 6. Measurements were carried out on the completed Si 3 N 4 -base sintered bodies on the same items as Example 1 to obtain the results shown in Table 6. Sample Sintering agent Additional additive Sintering temperature Half width (° ) Δd/d Thermal conductivity Bending strength °C (500) (330) W/mK MPa 31 Er 2 O 3 CaO 1600 0.19 0.21 0.0012 60 600 32 Er 2 O 3 CaO 1650 0.12 0.14 0.001 120 800 33 Yb 2 O 3 MgO+CaO 1750 0.06 0.05 0.0005 150 1200 34 Er 2 O 3 CaO 1800 0.12 0.14 0.001 120 1000 35 Er 2 O 3 CaO 1900 0.12 0.14 0.001 120 800 36 Er 2 O 3 CaO 1960 0.12 0.14 0.001 90 600 EXAMPLE 7 Si 3 N 4 -base sintered bodies were fabricated by the same means as Example 1 using the same Si powders except that a 10 wt.% sintering agent composed of Sm 2 O 3 powders was added, that a 5 wt.% additional additive composed of MgO and CaO powders was added, and that the cooling rate after sintering (from the sintering temperature to 1600 °C and from 1600 to 1500°C) was varied as shown in Table 7 below. Measurements were carried out on the completed Si 3 N 4 -base sintered bodies on the same items as Example 1 to obtain the results shown in Table 7. Sample Cooling rate after sintering (°C/hr) Half width (° ) Δd/d Thermal conductivity Bending strength ∼1600°C 1600∼ 1500°C (500) (330) W/mK MPa 37 50 100 0.1 0.12 0.001 130 1200 38 150 50 0.08 0.08 0.001 160 1200 39 300 300 0.12 0.14 0.001 120 1200

Claims: A highly heat-conductive silicon nitride (Si 3 N 4 )-base sintered body, wherein half widths of X-ray diffraction peaks on the (500) and (330) planes of the Si 3 N 4 are 0.2° or less, and wherein the sintered body has a thermal conductivity of 80 W/mK or more and a three-point bending strength of 800 MPa or more. A silicon nitride-base sintered body as claimed in claim 1, wherein the measured results of the X-ray diffraction peaks on Si 3 N 4 satisfy the following formula: (W 5 -W 2 )/(tan 5 - tan 2 ) ≤ 0.001 where W 2 is half the width of the diffraction peak on the (200) plane at a measured angle  =  2 , and W 5 is the half width of the diffraction peak on the (500) plane at a measured angle  =  5 . A silicon nitride-base sintered body as claimed in claim 1 or claim 2, the sintered body comprising: a crystal phase comprising β-silicon nitride grains; and a grain-boundary phase comprising from 1 to 20 wt.% of a compound, in oxide equivalent, of at least one kind of rare-earth element selected from Sm, Yb, Y, Er, and Gd. A silicon nitride-base sintered body as claimed in claim 3, wherein the grain-boundary phase further comprises from 0.1 to 10 wt.% of an element, in oxide equivalent, of at least one kind selected from Mg, Ca and Ti. A silicon nitride-base sintered body as claimed in any one of claim 1 to 4, wherein the sintered body has a relative density of 95% or more, and the Si 3 N 4 grains have an average size of from 0.5 to 20 µm. A method for fabricating a highly heat-conductive silicon nitride-base sintered body as defined in claim 1, the method comprising: mixing Si powders containing 0.1 wt.% or less Al with a sintering agent representing from 1 to 20 wt.% of the Si powders in Si 3 N 4 equivalent to form a compact, wherein oxide powders of at least one kind of rare-earth element selected from Sm, Yb, Y, Er, Gd, Eu, Tb and Tm are used as the sintering agent; heating the compact at a rate of from 0.2 to 0.6°C/min at a temperature in the range of from 1220 to 1320°C in a nitrogen atmosphere containing 5% or less of He; nitriding the compact at a temperature in the range of from 1220 to 1400°C; and sintering the nitrided compact at a temperature in the range of from 1650 to 1950°C in a nitrogen atmosphere. A method as claimed in claim 6, wherein oxide powders, representing from 0.1 to 10 wt.% of the Si powders in Si 3 N 4 equivalent, of at least one kind of element selected from Mg, Ca and Ti are further mixed with the Si powders and the sintering agent. A method as claimed in claim 7, wherein sintering of the nitrided compact is carried out at a temperature in the range of from 1650 to 1750°C in a nitrogen atmosphere at atmospheric pressure. A method as claimed in any one of claims 6 to 8, wherein the sintered body is cooled at a rate of 100°C/hr or more in the range of the sintering temperature to 1600°C and at a rate of 100°C/hr or less in the range of from 1600 to 1500°C.

Inventors: ITOH AI, MIYANAGA MICHIMASA, ITOH, AI, MIYANAGA, MICHIMASA, Itoh, Ai, Itami Works of Sumitomo Elec. Ind., Ltd, Miyanaga, Michimasa, Itami Works of Sumitomo Elec.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1999
Main classification: C04B35
IPC codes: C04B35