Characterization of thermal conductivity and mechanical property of pressureless sintered silicon nitride ceramics

Abstract

In this study, the Si3N4 was pressureless sintered with various sintering additives and its thermal and physical properties were investigated. The sintering additive systems consisted of MgO as a basic fixed additive, Y2O3, Yb2O3, La2O3 as rare-earth oxide additives, and SiO2 as a non-rare-earth oxide additive, whose properties were evaluated. It was confirmed that the density of the sintered body was increased as the amount of Y2O3 added was increased, and the maximum relative density of that additive system was 98.3%. The 3-point bending strength of MgO-Y2O3 additive system was also measured, and it was confirmed that the bending strength was also increased in proportion to the density, showing a maximum value of 963 MPa. The thermal conductivity of each additive system was measured by laser flash method. As the amount of Y2O3 got higher, the thermal conductivity decreased. In the case of Yb2O3, even though the density was lower than that of Y2O3, it showed a maximum thermal conductivity of 76.9 W/mK which is similar or higher than that of MgO-Y2O3 additive system. In order to analyze the factors affecting the thermal conductivity, the existence and the distribution of the secondary phase in grain boundary were investigated through the microstructure analysis using X-ray diffraction and scanning electron microscope. In the later part of this study, the investigation of thermal conductivity was approached by using Raman spectroscopy instead of the conventional laser flash method. This approach was based on a research of AlN ceramics that the oxygen-related defect in lattice and the FWHM (Full-Width of Half Maximum) of specific Raman peaks have a proportional relationship. For this, the relationship between the thermal conductivity of sintered specimens and the FWHM of the Raman peaks was shown, and the inverse relationship was found at 229 cm-1 and 451 cm-1.