Grain Refinement And Properties Analysis Of Ca₃Co₄O₉Thermoelectric Materials

Thermoelectric materials as a kind of semiconductors which can do conevertion between thermal energy and electric energy can be made into thermoelectric power generation devices and thermoelectric cooling devices according to the Seebeck effect and Peliter effect. Thermoelectric devices have some characteristics such as simple and compact structures, high reliability, no noise or other pollution and so on. Oxide-based thermoelectric materials are considered the most friendly to the environment in the use of thermoelectric power generation, and p-type innocuous thermoelectric materials Ca3Co4O9with good stability have high thermoelectric power.Based on the citrate Sol-gel method, the difference of grain refining effect of Ca3Co4O9by physical methods (mechanical milling method) and chemical methods (Pechini method and dispersants method)were studied. The phase and microstructure were analyzed by XRD and SEM technologies. The results showed that grain refining Ca3Co409powders were synthesized by all the three method, and bulk materials with refining grains were sintered by SPS. What’s more, Pechini method using glycol as solvent to replace water reduced the agglomeration and obtained smaller grain powders than using dispersants methods, and finally the prepared Ca3Co4O9powder was around100nm, and the dense bulk material was with the tiniest grains after SPS sintered. Compared to the mechanical milling method, the preparation process was more convenient and energy saving.Ca3Co4O9materials with different grain sizes were prepared through Pechini method by adjusting the ratio of ethylene glycol and water in the solvent, and the effect of grain size on the thermoelectric properties of Ca3Co4O9materials were studied. The results was that the conductivity of Ca3Co4O9had some losses because of the reduced grain size, but the thermoelectric power was improved, and ultimately grain refinement improved the electrical properties of the material. Consequently, to reduce the grain bounding scattering caused by grain refinement, Ag with great electrical conductivity was doped into the material. Argenti nitras was used as the source of Ag during the Sol-gel process, and XRD results showed there was Ag precipitated phase in the sample, and samples with Ag phase had both higher conductivity and Seebeck coefficient, as a result, the power factor was improved26%, and the ZT value of Ag doped grain refining O9sample was improved33%.