Silicon nitride sintered body having high heat conductivity and silicon nitride structural element

1. A silicon nitride sintered body exhibiting a high heat conductivity, the silicon nitride sintered body containing a rare earth element in an amount of 2 to 17.5 mass % in terms of the oxide thereof, Fe in an amount of 0.07 to 0.5 mass % in terms of the oxide thereof, Ca in an amount of 0.07 to 0.5 mass % in terms of the oxide thereof, Al in an amount of 0.1 to 0.6 mass % in terms of the oxide thereof, Mg in an amount of 0.3 to 4 mass % in terms of the oxide thereof, and Hf in an amount not larger than 5 mass % (including 0 mass %) in terms of the oxide thereof.

2. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , which has a heat conductivity not lower than 65 W/m·K, and a three point bending strength at room temperature not lower than 700 MPa, and has a porosity not higher than 2 volume %.

3. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , wherein a deterioration rate of a room temperature strength after an oxidizing treatment for 1000 hours within a stationary air of 800 to 1,000° C. is not higher than 10%.

4. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , which comprises a crystalline grain boundary phase in an amount of not smaller than 20% of a grain boundary phase.

5. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , which comprises a grain boundary phase containing a compound having a rare earth element, Si, Al, Fe, Ca, Mg and O and/or a compound having a rare earth element, Si, Al, Fe, Ca, Mg, O and N.

6. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 5 , wherein the rare earth element is at least one kind of element selected from the group consisting of Y, Er and Yb.

7. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , wherein the rare earth element is contained in the silicon nitride sintered body in an amount of 3 to 12.5 mass % in terms of the oxide thereof.

8. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , wherein Fe is contained in the silicon nitride sintered body in an amount of 0.07 to 0.45 mass % in terms of the oxide thereof.

9. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , wherein Ca is contained in the silicon nitride sintered body in an amount of 0.07 to 0.45 mass % in terms of the oxide thereof.

10. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , wherein Al is contained in the silicon nitride sintered body in an amount of 0.1 to 0.4 mass % in terms of the oxide thereof.

11. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , wherein Mg is contained in the silicon nitride sintered body in an amount of 0.5 to 3 mass % in terms of the oxide thereof.

12. The silicon nitride sintered body exhibiting a high heat conductivity according to claim 1 , wherein Hf is contained in the silicon nitride sintered body in an amount of 0.2 to 5 mass % in terms of the oxide thereof.

13. A silicon nitride structural element comprising a silicon nitride sintered body exhibiting a high heat conductivity, the silicon nitride sintered body containing a rare earth element in an amount of 2 to 17.5 mass % in terms of the oxide thereof, Fe in an amount of 0.07 to 0.5 mass % in terms of the oxide thereof, Ca in an amount of 0.07 to 0.5 mass % in terms of the oxide thereof, Al in an amount of 0.1 to 0.6 mass % in terms of the oxide thereof, Mg in an amount of 0.3 to 4 mass % in terms of the oxide thereof, and Hf in an amount not larger than 5 mass % (including 0 mass %) in terms of the oxide thereof.