Simultaneous promotion of mechanical and electrical properties of hot-pressing halloysite-based mullite ceramics through carbon incorporation

Halloysite (HAL) based mullite/carbon composites were fabricated by hot-pressing, using cheap HAL clay and chitosan as starting materials. The effects of sintering temperatures and carbon on the phase evolution, microstructure, bulk density, flexural strength and fracture toughness of composites were investigated. With increasing of temperature, the loading carbon on the surface of HAL further pyrolyzed and graphitized. The incorporation of carbon into matrix promoted the formation of silica-rich liquid phase and the process of mullitization due to the excellent thermal conductivity of carbon. The morphology of mullite crystal changed from sparse needle-like structure to interlocking columnar structure after the incorporation of carbon. The mechanical properties and electrical conductivity were greatly improved. The highest values of flexural strength and fracture toughness of mullite/carbon were 162.0 MPa and 3.1 MPa·m^1/2, respectively. The best electrical conductivity reached 12.25 S/m in the case of mullite/carbon obtained at 1300°C.