Enhanced thermal conductivity in Si3N4 ceramic by addition of a small amount of carbon

Abstract

In order to fabricate Si3N4 ceramic with enhanced thermal conductivity, 93 mol%α-Si3N4-2 mol%Yb2O3-5 mol%MgO powder mixture was doped with 5 mol% carbon, and sintered firstly at 1500 °C for 8 h and subsequently at 1900 °C for 12 h under 1 MPa nitrogen pressure. During the first-step sintering, the carbothermal reduction process significantly reduced the oxygen content and increased the N/O ratio of intergranular secondary phase, resulting in the precipitation of Yb2Si4O7N2 crystalline phase, higher β-Si3N4 content and larger rod-like β-Si3N4 grains in the semi-finished Si3N4 sample. After the second-step sintering, the final dense Si3N4 product acquired coarser elongated grains, lower lattice oxygen content, tighter Si3N4-Si3N4 interfaces and more devitrified intergranular phase due to the further carbothermal reduction of oxynitride secondary phase. Consequently, the addition of carbon enabled Si3N4 ceramic to gain a significant increase of ∼25.5% in thermal conductivity from 102 to 128 W∙m−1 K−1.