Effect of particle size and oxygen content of Si on processing, microstructure and thermal conductivity of sintered reaction bonded Si3N4

In the ceramic processing, use of the fine powders is more desirable in order to enhance densification and eventually the properties of ceramics. However, during the particle size reduction (using ball milling), the powders are also prone to surface oxidation. In this perspective, the present contribution systematically investigates the influence of Si particle size and its oxygen content on nitridation, densification, microstructure and thermal conductivity of sintered reaction bonded Si3N4 (SRBSN) mixed with 3.5% Y2O3-1.5% MgO (sintering additive). Despite it's relatively low density, the SRBSN sample with Si as received powders exhibited high thermal conductivity when compared to SRBSN with planetary ball milled samples. A maximum thermal conductivity of ∼90 W/m K was measured for the SRBSN sample (using high purity Si as received powders) after gas pressure sintering at 1950 °C for shorter sintering time of 3 h. It is a promising result as far as the industrial applications are concerned. Normally in the literature very high sintering temperatures and extensive sintering times were reportedly used for obtaining high thermal conductivity of Si3N4 ceramics. In the present work, the improvement in thermal conductivity of the SRBSN with Si as received powders can be attributed to its coarse grain microstructure, large elongated β-Si3N4 grains, relatively small amount of grain boundary phase (mainly because of low amount of oxygen in starting raw Si powders when compared to Si planetary ball milled powders) and large phonon scattering distance. As far as the thermal transport properties are concerned, the present research reveals that the use of Si powders with moderate particle size and low amount of oxygen is beneficial to improve the thermal conductivity of SRBSN.