Silicon nitride bodies

Abstract: A silicon nitride body wherein a number of grain boundaries of Si₃N₄ per a length of 10 µm is not more than 20 when measured along a straight line drawn in an arbitrary section of the silicon nitride body. A process is disclosed for producing such a silicon nitride body, which comprises the steps of grinding and mixing a raw material of silicon nitride and not more than 0.3 wt% of Aℓ when calculated as Aℓ₂O₃, shaping the mixture, and firing the shaped body. The silicon nitride bodies may be not only silicon nitride sintered bodies, but also vapor deposited films of silicon nitride formed by CVD or the like, frame sprayed films of silicon nitride formed by flame spraying, etc. These silicon nitride bodies have an advantageously high thermal conductivity.

Description: The present invention relates to silicon nitride sintered bodies suitable for use as high temperature structural materials. Various ceramic materials have been tried as high temperature structural materials. High strength and high fracture toughness as well as good thermal shock resistance are cited as characteristics required for the high temperature structural ceramics. High heat conductivity, low coefficient of thermal expansion, low Young's modulus, etc. are required to improve the thermal shock resistance. BeO, AℓN, SiC, etc. are known as ceramic materials having high thermal conductivity. However, among the above-mentioned ceramic materials having high thermal conductivity, BeO and AℓN have low strength, and SiC has high strength, but its fracture toughness is low. On the other hand, although Si₃N₄ is known as a material having high strength and high toughness, but it generally has low thermal conductivity. Accordingly, known silicon nitride sintered bodies have a problem in thermal shock resistance. JP-A-63-021254 discloses a Si₃N₄ body made by molding and sintering Si₃N₄ powder of large particle diameter and containing a sintering accelerator, to improve high temperature mechanical strength of the product. JP-A-52-122299 discloses a Si₃N₄ body, containing at least 90% β-phase, made by molding and sintering a Si₃N₄ powder containing two or more of BeO, MgO and SrO as sintering additives and at least one rare earth metal oxide. The Si₃N₄ powder used is at least 30 wt% α-phase, of not less than 99% purity and has a grain size of not more than 10µm. The aim is to improve density, mechanical strength and resistance to thermal shock. The present invention aims to provide silicon nitride bodies favorably usable as high temperature structural materials by improving the defect in the thermal shock resistance of Si₃N₄ through increasing the thermal conductivity thereof. The sintered silicon nitride body according to the present invention is set out in claim 1. One process for producing the silicon nitride bodies according to the present invention comprises the steps of grinding and mixing a raw material of silicon nitride containing not more than 0.3 wt% of Aℓ when calculated as Aℓ₂O₃, shaping the mixture, and firing the shaped body. The present inventors have discovered that in the above-mentioned silicon nitride bodies, when the number of the grain boundaries of Si₃N₄ per the length of 10 µm is not more than 20 when measured along a straight line drawn in an arbitrary section thereof, heat dissipation at the grain boundaries becomes smaller, so that highly thermal conductive silicon nitride bodies having high thermal conductivity of not less than 63 W/m.k (0.15 cal/cm·sec.°C) can be obtained. For this purpose, it is necessary to make grains of Si₃N₄ larger. This can be accomplished, for example, by raising the sintering temperature and prolonging the sintering time in the case of the sintering process. Since the Aℓ component in the silicon nitride body such as the silicon nitride sintered body is solid solved into grains of Si₃N₄ to produce sialon having low thermal conductivity, the thermal conductivity of the silicon nitride body is deteriorated. For this reason, the content of Aℓ in the silicon nitride body is not more than 0.3 wt% when calculated as Aℓ₂O₃, because reduction in the thermal conductivity can be prevented in this case. Further, it is preferable to use a raw material of silicon nitride containing not more than 0.3 wt% of Aℓ in the producing process when calculated as Aℓ₂O₃. Furthermore, it is preferable to grind the raw material with grinding media made of silicon nitride. As to Si₃N₄ there are two kinds of an α type and a β type, and the heat conductivity of the α type is smaller than that of the β type. Therefore, the higher the β conversion rate, the higher the heat conductivity. For this reason, the Si₃N₄ is β-Si₃N₄. These and other objects, features and advantages of the invention will be appreciated upon reading of the following description of the invention. In the following, actual examples of the present invention will be explained. Experiment 1 : A sintering aid containing not more than 0.01 wt% of impurity Aℓ was added and mixed into a raw material of silicon nitride containing not more than 0.01 wt% of impurity Aℓ at ratios given in Table 1. The sintering aid was selected from raw materials of Y₂O₃, Yb₂O₃, SiC, MgO and ZrO₂. The mixture was ground with a vibration mill by using grinding media made of silicon nitride porcelain and a nylon resin vessel having an inner volume of 1.2 mℓ. To 200 g of the raw material mixture were added 1.8 kg of the grinding media and 300 mℓ of water, and the above grinding was effected at a vibration frequency of 1,200/minutes for 3 hours. Then, water was evaporated, and the mixture was granulated to a grain diameter of 150 µm in the form of a powder to be shaped. Next, the powder was isostatically pressed under a pressure of 7 metric tons/cm² to form a shaped body of 50 × 40 × 6 mm. The shaped body was fired in a nitrogen atmosphere under conditions of temperature, time and pressure shown in Table 1. Thereby, sintered bodies in Example Nos. 1-11 and Comparative Example Nos. 12-14 were obtained. With respect to the thus obtained sintered bodies, the number of grain boundaries per a length of 10 µm along a straight line drawn in an arbitrary section, thermal conductivity, β conversion rate, four point bending strength and thermal shock resistance were measured. Results are also shown in Table 1. The number of the grain boundaries was measured as follows: First, a microstructure of an arbitrary section of the silicon nitride body was photographed by a scanning type electron microscope at a magnitude by which grains of Si₃N₄ were able to be discerned one by one. Next, a straight line was drawn in the photograph, and the number of the grain boundaries through which the straight line passed was counted. The straight line was drawn over plural photographs continuously connected until the number of the grain boundaries exceeded 1,000, and that total distance L (µm) of the straight line which was required to count 1,000 grain boundaries was determined. Then, the number of the grain boundaries per 10 µm was determined by (1000/L) × 10. For example, if a straight line having a length of 500 µm is required to count 1,000 grain boundaries, the number of the grain boundaries per 10 µm is 20. The thermal conductivity was measured by using a heat constant measuring device according to a laser-flash process. The shape of the each of test pieces was 10 mm in diameter and 3 mm in thickness. One surface of the test piece was coated with carbon by sprayer, and a thermocouple was bonded to the opposite side by using a silver paste. The β conversion rate was measured by X-ray diffraction. Diffraction intensities α(210) and α(201) of a (210) plane and a (201) plane of α-Si₃N₄, respectively, and diffraction intensities β(101) and β(210) of a (101) plane and a (210) plane of β-Si₃N₄, respectively, were measured, and the β conversion rate was determined by the following equation. β conversion rate = [β(101) + β(210)]/[α(210) + α(201) + β(101) + β(210)] × 100 The four point bending strength was measured according to a "process for testing bending strengths of fine ceramics" prescribed in JIS R-1601. The thermal shock resistance was measured by an underwater quenching method in which a temperature difference that did not substantially provoke reduction in strength at room temperature after quenching was measured. From results in Table 1, it is seen that Example Nos. 1-11 in which the number of the grain boundaries per 10 µm was not more than 20 had better thermal shock resistance as compared with Comparative Example Nos. 12-14. Further, when Example Nos. 1-3 are compared, it is seen that the number of the grain boundaries per 10 µm is decreased by prolonging the firing time even at the same firing temperature, so that high thermal conductivity is attained. Further, when Example Nos. 2 and 4 are compared, it is seen that the number of the grain boundaries is decreased by raising the firing temperature even for the same firing time, so that high thermal conductivity is attained. To the contrary, the number of the grain boundaries per 10 µm is greater and the thermal conductivity is low in the case of Comparative Example No. 13 having a shorter firing time and Comparative Example No. 14 having a lower firing temperature. Experiment 2 : In order to examine influences of the content of Aℓ in silicon nitride sintered bodies, raw materials of Y₂O₃ and Yb₂O₃ containing not more than 0.01 wt% of impurity Aℓ were added to each of five kinds of powdery raw materials of silicon nitride in which Aℓ was contained in an amount of 0.01 wt%, 0.05 wt%, 0.07 wt%, 0.12 wt%, or 0.15 wt%. Then, a shaped body was formed from the thus obtained mixture, and fired at 1,900°C in a nitrogen atmosphere for 3 hours in the same manner as in Experiment 1, thereby obtaining sintered bodies in Example Nos. 21-25 shown in Table 2. With respect to the sintered bodies obtained above, the thermal conductivity, the number of grain boundaries per 10 µm, the β conversion rate and the content of Aℓ in the sintered body were measured in the same manner as in Experiment 1. Results are shown in Table 2. Run No. Content of Aℓ in raw material of silicon nitride (Wt%) Thermal conductivity (cal/cm·sec.°C) Number of grain boundaries per 10 µm β conversion rate (%) Content of Aℓ in sintered body when calculated as Aℓ₂O₃ (wt%) Example 21 0.01 0.17 12 100 <0.1 22 0.05 0.17 12 100 0.1 23 0.07 0.16 15 100 0.2 24 0.12 0.15 15 100 0.3 25 0.15 0.15 20 100 0.3 From the results in Table 2, it is seen that among the sintered bodies satisfying the number of the grain boundaries in the the present invention, the greater the content of Aℓ, the lower the thermal conductivity, and when the content of Aℓ is not more than 0.3 wt% as calculated in the form of Aℓ₂O₃, high thermal conductivity can be attained. Experiment 3 : In order to examine influences of grinding media used, a raw material of silicon nitride containing not more than 0.01 wt% of Aℓ as an impurity was formulated at the same compounding recipe with use of the same additives as in Example 2. Example Nos. 26-30 shown in Table 3 were obtained from the thus obtained mixture in the same manner as in Experiment 1 with use of Si₃N₄ grinding media containing MgO as additive, Si₃N₄ grinding media containing MgO and Aℓ₂O₃ as additive, Si₃N₄ grinding media containing Y₂O₃ and Aℓ₂O₃ as additive, or Aℓ₂O₃ grinding media, respectively. With respect to the sintered bodies obtained above, the thermal conductivity, the number of grain boundaries per 10 µm, the β conversion rate and the content of Aℓ in the sintered body were measured in the same manner as in Experiment 1. Results are shown in Table 3. Run No. Kind of grinding media Grinding time (hour) Thermal conductivity (cal/cm· sec.°C) Number of grain boundaries per 10 µm β conversion rate (%) Content of Aℓ in sintered body when calculated as Aℓ₂O₃ (wt%) Example 26 MgO base Si₃N₄ 3 0.19 9 100 <0.1 27 MgO-Aℓ₂O₃ base Si₃N₄ 3 0.18 9 100 0.1 28 Y₂O₃-Aℓ₂O₃ base Si₃N₄ 3 0.18 9 100 0.1 29 Aℓ₂O₃ 1 0.16 15 100 0.2 30 Aℓ₂O₃ 3 0.15 15 100 0.3 From the results in Table 3, it is seen that since Example Nos. 26-28 using the Si₃N₄ grinding media had the small mixing amount of Aℓ from the grinding media during the grinding, the thermal conductivity was not substantially lowered. On the other hand, it is seen that since Example Nos. 29 and 30 using the Aℓ₂O₃ grinding media had the great mixing amount of Aℓ, the thermal conductivity is lowered. The present invention is not limited to the above mentioned examples only, for example, although the above-mentioned examples are directed to the silicon nitride sintered bodies, thermal conductivity of vapor deposited films of silicon nitride formed by CVD or the like as well as flame sprayed films of silicon nitride can be increased as long as the requirements of the present invention are satisfied. As is clear from the foregoing explanation, according to the present invention, when the number of the grain boundaries of Si₃N₄ in the silicon nitride body per 10 µm is not more than 20 when measured along a straight line drawn in an arbitrary section, and preferably when the content of Aℓ is also not more than 0.3 wt% as calculated in the form Aℓ ² O ³, the silicon nitride body having high thermal conductivity of, for example, not less than 63 W/m.K (0.15 cal/cm·sec.°C) can be obtained. Thus, the silicon nitride bodies according to the present invention can favorably be used as high temperature structural bodies.

Claims: A sintered polycrystalline β-type silicon nitride body, wherein (i) the number of grain boundaries of Si₃N₄ per length of 10µm is not more than 20 when measured along a straight line drawn in an arbitrary section of the silicon nitride body, (ii) the content of Al in the silicon nitride body is not more than 0.3 wt% when calculated as Al₂O₃ , and (iii) the thermal conductivity of the sintered silicon nitride body is not less than 63 W/m.K (0.15 cal/cm/sec.°C).

Inventors: ISOMURA MANABU, SAKAI HIROAKI, ISOMURA, MANABU, SAKAI, HIROAKI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: C04B35
IPC codes: C04B35