Preparation And Properties Of Ca₃Co₄O₉ Matrix Composite Thermoelectric Materials

Oxide thermoelectric materials have important application value in the field of new energy because of their oxidation resistance properties.Ca3Co4O9,an excellent thermoelectric materials,is very difficult to be sintered into ceramics under atmospheric pressure due to its decomposition above 926?.At present,hot pressed sintering and plasma sintering method are two main methods for sintering of this ceramic,which are complex and difficulty to increase the density of Ca3Co4O9 ceramics.For instance,Ca3Co4O9 ceramics sintered by SPS can be fused partly between particles,and then much solid-gas interface(S-G interface)remained,which causes the significant difference of thermoelectric properties between Ca3Co4O9 ceramics and single crystal.In this paper,Bi2O3 is employed as sintering aids to achieve the interface fusion under the decomposition temperature of Ca3Co4O9 in order to further improve the performance of the Ca3Co4O9 ceramics.In the experiment,the Ca3Co4O9 powder were firstly synthesized by using sol-gel method,cosolvent method and solid-state reaction method respectively,then mixed with Bi2O3,and sintered in order to prepare the Bi-Ca-Co-O/Ca3Co4O9 composite thermoelectric material.Effects of the amount of Bi2O3 and Ca3Co2O6 as well as the hot-pressing process factors on the properties of composite materials were studied,The thermoelectric properties of materials were tested by using the self-made thermoelectric power test equipment and commercial ZEM-3 Seebeck coefficient instrument.As a result,Bi2O3 melts and fills up the gaps among the Ca3Co4O9 particles,then the S-G interface reduced and the Ca3Co4O9 particles contacted better.And further more,Bi2O3 diffuses into the Ca3Co4O9 grain boundary and reacts with Ca3Co4O9,as a resultant,new thermoelectric phase Bi2Ca3Co2O9 can be obtained among the Ca3Co4O9 particles,so the Bi-Ca-Co-O/Ca3Co4O9 composite thermoelectric materials is formed,in which the influence of grain boundary is reduced or eliminated effectively,the thermoelectric properties of Ca3Co4O9 sample are improved:At 600?,the power factor of as prepared materials is 2×10-4 W/m·K2 and comparative with that of SPS sintered samples.