Study on the properties of Ca9Co12O28 under high pressure

The pure Ca9Co12O28 thermoelectric (TE) material powder is synthesized by solid state sintering method. The powder is pressed into compact tablets under pressure of 200 MPa, 2.0 GPa, 2.5 GPa, 3.0 GPa, 3.5 GPa and 4.0 GPa, respectively. The electrical conductivity, Seebeck coefficient, thermal conductivity and hardness are tested. The properties of specimens under different high pressures are compared and analyzed. It is found that the relative density of the specimens under different high pressures can reach 95% approximately. The highest power factor of the specimens is 2.171 mu W/(cm.K2) at 3.5 GPa, which is 2.4 times that at 200 MPa. At the same time, the maximum Vickers hardness of the specimens under 3.5 GPa is 127.7 MPa, while that under 200 MPa is only 16.8 MPa, indicating that the hardness under 3.5Gpa is increased by about 6.6 times. More importantly, it is found that the thermal conductivity of the specimens decreases with pressure in the range of 2.5Gpa-4.0Gpa, which proves that high pressure can refine the heat transfer performance effectively. This study lays the foundation for the synthesis of new-type Ca9Co12O28 TE materials with high machinability, high power factor, low thermal conductivity and high TE figure of merit with high-pressure technology.