Silicon nitride powder, silicon nitride sintered body, sintered silicon nitride substrate, and circuit board and thermoelectric module comprising such sintered silicon nitride substrate

A method for producing silicon nitride powder comprising the step of heat-treating starting silicon nitride powder comprising 0.02-1.0 weight %, as converted to SiO2, of oxygen and having a specific surface area of 0.5 m2/g or more at a temperature of 1,800° C. or higher in a non-oxidizing atmosphere of nitrogen or nitrogen and hydrogen.

A silicon nitride sintered body comprising Mg and at least one rare earth element selected from the group consisting of La, Y, Gd and Yb, the total oxide-converted content of said elements being 0.6-7 weight %, with Mg converted to MgO and rare earth elements converted to rare earth oxides RExOy.

A silicon nitride sintered body, in which nano-size, fine particles composed of Mg, at least one rare earth element selected from the group consisting of La, Y, Gd and Yb, and O and having an average particle size of 100 nm or less are observed in silicon nitride grains, in a transmission electron micrograph having a magnitude of 10,000 times or more.

A method for producing a silicon nitride sintered body comprising the steps of mixing 1-50 parts by weight of a first silicon nitride powder having a β-particle ratio of 30-100%, an oxygen content of 0.5 weight % or less, an average particle size of 0.2-10 μm, and an aspect ratio of 10 or less, with 99-50 parts by weight of α-silicon nitride powder having an average particle size of 0.2-4 μm;

• and sintering the resultant mixture at a temperature of 1,800° C. or higher and pressure of 5 atm or more in a nitrogen atmosphere.

A high-strength, high-thermal conductivity circuit board comprising a silicon nitride substrate and a metal circuit plate bonded to at least one surface of said silicon nitride substrate, said substrate having a surface texture having a centerline average surface roughness Ra of 0.2-20 μm, and said silicon nitride substrate being constituted by a silicon nitride sintered body comprising Mg and at least one rare earth element selected from the group consisting of La, Y, Gd and Yb, the total oxide-converted content of said elements being 0.6-7 weight %, with Mg converted to MgO and rare earth elements converted to rare earth oxides RExOy.

A circuit board comprising a metal circuit plate bonded to an electrically insulating substrate, wherein said electrically insulating substrate is a silicon nitride substrate constituted by a silicon nitride sintered body comprising Mg and at least one rare earth element selected from the group consisting of La, Y, Gd and Yb, the total oxide-converted content of said elements being 0.6-7 weight %, with Mg converted to MgO and rare earth elements converted to rare earth oxides RExOy;

• an as-sintered surface layer being removed from said electrically insulating substrate in at least bonding areas of said electrodes to said electrically insulating substrate surface;
• and an electrically insulating substrate surface, from which said as-sintered surface layer is removed, having a centerline average surface roughness Ra of 0.01-0.6 μm.

A thermoelectric module comprising an electrically insulating substrate, electrodes bonded to said electrically insulating substrate, and p-type and n-type thermoelectric semiconductor elements connected in series via said electrodes;

• said electrically insulating substrate being the silicon nitride substrate constituted by a silicon nitride sintered body comprising Mg and at least one rare earth element selected from the group consisting of La, Y, Gd and Yb, the total oxide-converted content of said elements being 0.6-7 weight %, with Mg converted to MgO and rare earth elements converted to rare earth oxides RExOy;
• an as-sintered surface layer being removed from at least bonding areas of said electrodes to said electrically insulating substrate surface;
• and an electrically insulating substrate surface, from which said as-sintered surface layer is removed, having a centerline average surface roughness Ra of 0.01-0.6 μm.