Sintered silicon nitride and circuit board and electronic device using the same

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PROBLEM TO BE SOLVED : To provide a silicon nitride sintered compact which has high dielectric strength and a superior heat-dissipating property and mechanical property, and a circuit board and an electronic device using the same.SOLUTION : A silicon nitride sintered compact includes silicon nitride as the major constituent, magnesium, a rare earth element and aluminum in an amount not less than 2 mass% and not more than 6 mass%, not less than 4 mass% and not more than 10 mass%, and not less than 0.1 mass% and not more than 0.5 mass% respectively, includes β-SiNas a main crystal phase and a grain boundary phase containing a component expressed by the compositional formula : RESiNO(RE is a rare earth element). The ratio of the first peak intensity Iof RESiNOat a diffraction angle of 30°-35° to the first peak intensity Iof β-SiNat a diffraction angle of 27°-28° (I/I) as determined by X-ray diffractometry is not greater than 20% (excluding 0%).