Sintered Reaction‐Bonded Silicon Nitride with High Thermal Conductivity and High Strength

Abstract

Sintered reaction‐bonded silicon nitride (SRBSN) materials were prepared from a high‐purity Si powder doped with Y2O3 and MgO as sintering additives by nitriding at 1400°C for 8 h and subsequently postsintering at 1900°C for various times ranging from 3 to 24 h. Microstructures and phase compositions of the nitrided and the sintered compacts were characterized. The SRBSN materials sintered for 3, 6, 12, and 24 h had thermal conductivities of 100, 105, 117, and 133 W/m/K, and four‐point bending strengths of 843, 736, 612, and 516 MPa, respectively. Simultaneously attaining thermal conductivity and bending strength at such a high level made the SRBSN materials superior over the high‐thermal conductivity silicon nitride ceramics that were prepared by sintering of Si3N4 powder in our previous works. This study indicates that the SRBSN route is a promising way of fabricating silicon nitride materials with both high thermal conductivity and high strength.

A review of the processing, composition, and temperature-dependent mechanical and thermal properties of dielectric technical ceramics

“…Attainment of high thermal conductivity requires the use of high purity starting powders with low oxygen content and low cationic impurities, full densification of the sintered body, and the optimisation of many factors including sintering parameters [91] and sintering aid(s). Very high thermal conductivity Si 3 N 4 has been reported in many studies [94,191,[198][199][200], produced by careful control of purity and oxygen content of raw powders [94,201], selection of additives [202], control of microstructure morphology and grain size [109,178,194,197,203,204], and use of β -Si 3 N 4 seed crystals [205]. However, despite the large volume of studies reporting techniques for production of high thermal conductivity silicon nitride, typical commercially available silicon nitride [206] at present has a far lower thermal conductivity.…”

“…HIP results in the highest strength silicon nitrides, comparable to that of HPSN (800-1200 MPa), with that of SSN slightly lower (600-1000 MPa) [92]. SRBSN has shown the greatest potential for having simultaneously high thermal conductivity and strength [94]. The toughening effect of silicon nitride's bimodal microstructure, due to crack bridging, is controlled by the size and volume fraction of the large acicular grains [81].…”

“…The toughening effect of silicon nitride's bimodal microstructure, due to crack bridging, is controlled by the size and volume fraction of the large acicular grains [81]. Extensive research over the past four decades has significantly improved the thermal [91], creep [95,96], and mechanical properties of silicon nitrides [81], notably simultaneously improving strength, fracture toughness [81] and thermal conductivity [94]. Strong, high thermal conductivity Si 3 N 4 ceramics may be attainable with use of high-purity sub-micron powders, and careful selection of sintering aids.…”

“…SRBSN is selected as the nominal production route for Si 3 N 4 for the current work, due to its potential for having simultaneously high thermal conductivity and strength [94]. Data are also considered for SSN, HIPSN and HPSN materials representative of commercially available silicon nitrides.…”

“…Si 3 N 4 with thermal conductivities in excess of 45 W·m -1 ·K -1 are not commonly commercially available, typical values being in the range 15-42 W·m -1 ·K -1 . Typical high-purity commercial Si 3 N 4 powders contain ≈ 1 wt% oxygen [94], and metallic impurities [91], limiting the attainable thermal conductivity. The coarse microstructures typically present in high thermal conductivity Si 3 N 4 , resulting from promotion of grain growth, significantly impacts on the material's strength [94,110,207], reducing the suitability of these materials for structural applications.…”