Method for preparing silicon nitride ceramic material

A method for preparing a silicon nitride ceramic material, comprising:

• (1) at least one of silicon powder and silicon nitride powder as original powder and Y2O3 powder and MgO powder as sintering aids are added with an organic solvent and a binder and mixed in a protective atmosphere to obtain a mixed slurry;
• wherein a molar ratio of the Y2O3 powder to MgO powder is (1.0 to 1.4):(2.5 to 2.9), and a proportion of the sintering aids does not exceed 5 wt % of a sum of weights of the original powder and the sintering aids;
• if the original powder is the silicon powder or a mixture of the silicon powder and the silicon nitride powder, the weight of the original powder refers to a sum of weights of the silicon nitride powder in the original powder and silicon nitride produced after nitridation of the silicon powder in the original powder;
• if the original powder contains silicon powder, a weight of the silicon powder is not less than 75 wt % of the original powder;
• the protective atmosphere is an inert atmosphere or a nitrogen atmosphere;
• and an amount of an added binder is 5 to 9 wt % of a sum of a weights of the original powder and the sintering aids;
• (2) the mixed slurry is tape-cast in a protective atmosphere to obtain a green body;
• (3) the green body is put into a reducing atmosphere and pretreated at 500° C. to 800° C. to obtain a biscuit;
• wherein the reducing atmosphere is a hydrogen/nitrogen mixed atmosphere with a hydrogen content not higher than 5 vol %;
• (4) the biscuit is put into a nitrogen atmosphere and subjected to low-temperature heat treatment at 1600° C. to 1800° C. and high-temperature heat treatment at 1800° C. to 2000° C. in sequence to obtain the silicon nitride ceramic material;
• wherein if the original powder contains silicon powder, the biscuit is nitrided after the pretreatment and before the low-temperature heat treatment;
• parameters of the nitridation include:
• atmosphere:
• hydrogen/nitrogen mixed atmosphere with a hydrogen content not higher than 5 vol %;
• pressure:
• 0.1 MPa to 0.2 MPa;
• temperature:
• 1350° C. to 1450° C.;
• and temperature preservation time:
• 3 to 6 hours;
• the silicon nitride ceramic material comprises a silicon nitride phase and a grain boundary phase;
• wherein a content of the silicon nitride phase is more than or equal to 95 wt %;
• wherein the grain boundary phase is a mixture containing at least three elements, Y, Mg and O;
• wherein the content of the grain boundary phase is less than or equal to 5 wt %, and the content of a crystalline phase in the grain boundary phase is more than or equal to 40 vol %;
• and wherein the silicon nitride ceramic material has a thermal conductivity of 90 W·m−1·K−1 or more, and a breakdown field strength of 30 kV/mm or more.