Changing thermal conductivity to improve the performance of silicon nitride components

Enhancing the thermal conductivity of â-Si 3N 4 ceramics

Silicon nitride ceramics are important engineering materials due to their excellent properties such as fracture toughness, wear resistance and high temperature strength. They were originally developed to compete with metallic parts and now find application in such areas as engine components, glow plugs for diesel engines, cutting tools, bearings, nozzles and kiln furniture.

Thermal conductivity is an important physical property of Si 3N 4 ceramics. As the thermal conductivity strongly influences the rate of heat dissipation, this determines the reliability and performance of components in many industrial applications. In vehicle engines, low thermal conductivity is desired for heat insulation components to decrease fuel consumption, while high conductivity is required for cooling components with good thermal shock resistance.

Thermal conductivities of â-Si 3N 4 ceramics are known to range from 10 to 162 W.m -1.K -1 at room temperature. The thermal conductivity is greatly affected by processing variables such as purity of raw powders, type and amount of sintering aids, forming and sintering conditions.

In this paper published in AZojomo*, Koji Watari, Kiyoshi Hirao, Manuel E. Brito, Motohiro Toriyama and Kozo Ishizaki from National Institute of Advanced Industrial Science and Technology and Nagaoka University of Technology , have reviewed previous works and summarized results of the thermal conductivity of â-Si 3N 4 ceramics obtained under various conditions

The experimental observations and theoretical calculations showed that the amount and type of crystal defects in grains as well as thermal anisotropy are significant factors influencing the thermal conductivity of â-Si 3N 4 ceramics. Removal of crystal defects in grains is an important factor in increasing thermal conductivity of â-Si 3N 4 ceramics. This is achieved by using high purity powders, selection of effective sintering aids and controlling grain growth. Increasing thermal anisotropy in â-Si 3N 4 ceramics is also achieved by grain orientation during forming. By combining these processing techniques, it is possible to produces â-Si 3N 4 ceramics with higher thermal conductivities than 150 W.m -1.K -1 at room temperature.