Research On Microstructure And Thermal Conductivity Of The Spark Plasma Sintered Si₃N₄ Ceramics

Silicon nitride ceramics processing excellent mechanical properties, are considered as structural materials for wide applications in many fields such as aerospace, mechanical industry and chemical industry. At the same time, the theoretical thermal conductivity of silicon nitride ceramics is so high that is comparable to aluminum nitride, so the ceramics are regarded as a promising candidate for amplification as ceramic IC substrates. For solving the heat dissipation problem of large scale integrated circuit used in future complicated environment, it is important to explore a suitable preparation processing for silicon nitride ceramics with both high thermal conductivity and good mechanical performance.In this work, silicon nitride ceramics were synthesized by spark plasma sintering method. The effects of sintering additives and sintering technology on microstructure and thermal properties were systematically investigated. The mechanism of additives with different composition and contents during SPS process were studied. By comparing densification process, phase transition and grain growth behavior of silicon nitride ceramics under diverse sintering processes, the influence factors of thermal conductivity were revealed. The feasibility of the heat treatment was investigated at the same time. Through heat- treatment,both microstructure and thermal properties of silicon nitride ceramics were optimized.The results showed that,compared with Y2O3, the use of additive MgSiN2 led to higher thermal conductivity by producing lower viscosity liquid and higher N/O ratio.With help from this, grain rearrangement was accelerated and the silicon nitride ceramics could be densified rapidly within three min under 1600℃, both αâ†'β-Si3N4 phase transformation and growth of Si3N4 grains were accelerated simultaneously.The experimental results reflected that only 2wt% sintering additive MgSiN2 is enough to finish densification of silicon nitride ceramics. When improving the content of MgSiN2, the starting densification temperature was reduced, because of the promoted wettability by plenty of the lower viscosity liquid, and the thermal conductivity was improved due to better phase transformation and growth of Si3N4 grains. By contrast, the effect of second phase distributed in triple grain boundary junction on thermal conductivity was not obvious.Silicon nitride ceramics were synthesized by using MgSiN2 as additive. The effects of sintering techniques, sintering temperature and holding time on microstructure and thermal properties of Si3N4 ceramics was elucidated. The results showed that SPS increased densification rate because of the promoted wettability by the external pressure in comparison to GPS, so the good comprehensive propertiescould be obtained. In SPS term, the densification of Si3N4 ceramics was realized between 1400 o C to 1600 oC. As sintering temperature improved, phase transformation and growth of Si3N4 grains were activated from sufficient energy and lower viscosity liquid, which is beneficial to improve the thermal conductivity. Thermal conductivity,flexural strength and fracture toughness of Si3N4 ceramics could reach 60.11W/m·K,931 MPa and 9.18MPa·m1/2 respectively when sintered under 1800 oC for5min. However there is no obvious improvement through extended holding time.Post heat-treatment on Si3N4 ceramics were held by spark plasma sintering furnace and gas pressure sintering furnace. The effects of heat treatment techniques on microstructure and thermal properties of Si3N4 ceramics was investigated. The results showed that no obvious optimized microstructure could be noticed through spark plasma heat-treatment. In contrast, thermal conductivity was improved after gas pressure heat-treatment, especially when there was a small amount β phase well dispersed in the matrix, which could be treated as the seeds of phase transition and further growing of grains. However the above positive action on thermal conductivity was weaken, because decrease of densification as a result of decomposition of secondary phases during heat treatment.