Enhancement Of Thermoelectric Properties Of Bi₂Ca₂Co₂O₉ Through Doping And Combining

Oxide thermoelectric?TE?materials has a good application prospect in high temperature power generation field due to their excellent properties,such as high temperature resistance,oxidation resistance as well as their simple synthesis process and suitable for massive production.Bi₂Ca₂Co₂O_y,a misfit-layered Co-based oxide,can be described as an alternate stacking of conductive CdI₂-type CoO₂ layer and insulating rock-salt-type Bi₂Ca₂O₄ layer along c-axis.Because of its complex layered crystal structure,Bi₂Ca₂Co₂O_y has low thermal conductivity and high Seebeck coefficient,but it has poor power factor for its high resistivity.This paper tries to optimize TE properties of Bi₂Ca₂Co₂O_y by ion doping and compositing.First,effects of Cu doping and Cu-Fe co-doping on TE properties of Bi₂Ca₂Co₂O_y is studied,and the primary affecting mechanism of Cu doping is also explored.In Bi₂Ca₂Co₂O_y,when Co replaced by Cu,the resistivity and thermal conductivity are reduced,Seebeck coefficient,power factor and ZT values were enhanced,so TE properties optimized,accompanied with neither significant change for the valence and concentration of Co ions nor obvious change of the activation energy for polaron hoping.Effect of Cu-Fe co-doping on the thermoelectric properties of Bi₂Ca₂Co₂O_y is not as good as that of Cu doping.Secondly,effect of Sr substitution for Ca on TE properties of Bi₂Ca₂Co₂O_y is also explored.Sr doping induces the lattice expansion of rock salt layer due to its larger ion radius,induces the misfitness changing between the insulating layer and the conductive layer,and then induces the optimization for thermoelectric properties of Bi₂Ca₂Co₂O_y.With the increase of Sr doped,resistivity and thermal conductivity of materials are reduced,Seebeck coefficient is slightly enhanced and then decreased,all of them cause the increase of power factor and ZT valueAt last,TE properties of Bi₂Ca₂Co₂O_y/Ca₃Co₄O₉ composite and Cu-doped composite are investigated.Significant conductive percolation transition in misfit layered cobalt is observed for the first time,TE composite with large power factor is obtained,too.It is sure that making advantage of conductive percolation is an effective method to enhance TE properties of composites,and further improvement could be made by combining the doping and composite methods.