Abstract: An Si3N4 sintered body produced by reactive sintering of silicon, wherein a compound of at least one element selected from the group consisting of Y, Yb and Sm is contained by 0.6 to 13% by weight as Ln2O3 (wherein Ln=Y, Yb or Sm), an oxygen content in Si3N4 crystal grains is not more than 1% by weight, a ratio of Si and Ln in the Si3N4 sintered body is within a range of 0.1 to 0.8 in a molar ratio of SiO2/Ln2O3 of the Si in terms of SiO2 to the oxide Ln2O3, and the sintered body has a relative density of 85 to 99.9%, a thermal conductivity of at least 70 W/m.K or more and a three-point bending strength of at least 600 MPa. The Si3N4 sintered body is produced by mixing 80 to 99% by weight of silicon powder and 1 to 20% by weight of powder of oxide of at least one element of Y, Yb and SM, by nitriding a molded body of the powder mixture in an atmosphere containing nitrogen at 1400° C.

Abstract: A Si3N4 composite substrate which manifests no generation of cracking on the substrate even by mechanical shock or thermal shock, and is excellent in heat radiation property and heat-cycle-resistance property is obtained by using a Si3N4 substrate as a ceramic substrate. A Si3N4 substrate having a thermal conductivity of 90 W/m·K or more and a three-point flexural strength of 700 MPa or more is used, and the thickness tm of a metal layer connected on one major surface of the substrate and the thickness tc of the Si3N4 substrate are controlled so as to satisfy the relation formula: 2 tm≦tc≦20 tm. When metal layers are connected to both major surfaces of the Si3N4 substrate, the thickness tc and the total thickness ttm of the metal layers on both major surfaces are controlled so as to satisfy the relation formula: ttm≦tc≦10 ttm.

Abstract: An Si3N4 sintered body produced by reactive sintering of silicon, wherein a compound of at least one element selected from the group consisting of Y, Yb and Sm is contained by 0.6 to 13% by weight as Ln2O3 (wherein Ln=Y, Yb or Sm), an oxygen content in Si3N4 crystal grains is not more than 1% by weight, a ratio of Si and Ln in the Si3N4 sintered body is within a range of 0.1 to 0.8 in a molar ratio of SiO2/Ln2O3 of the Si in terms of SiO2 to the oxide Ln2O3, and the sintered body has a relative density of 85 to 99.9%, a thermal conductivity of at least 70 W/m.K or more and a three-point bending strength of at least 600 MPa. The Si3N4 sintered body is produced by mixing 80 to 99% by weight of silicon powder and 1 to 20% by weight of powder of oxide of at least one element of Y, Yb and SM, by nitriding a molded body of the powder mixture in an atmosphere containing nitrogen at 1400° C.