Synthesis And Properties Of Cobaltite Thermoelectrical Materials

Because of the aggravated energy and environmental problems,the development of novel energy and energy conversion materials has triggered enormous interest.Among these novel materials,thermoelectric materials which can realize the conversion between thermal and electrical energy directly,are of particular interest due to the advantages such as environmental friendly,long service life,high conversion efficiency and so on.The layered sodium cobalt thermoelectric material is a typical thermoelectric material that exhibit high Seebeck coefficients,low thermal conductivity and thus have extensive application prospect in the field of thermoelectric generation.Moreover,the oxide thermoelectric materials possess high chemical stability and oxidation resistance,suggesting the great potential to be applied under high temperature conditions.In this paper,we focused on the sodium cobalt based thermoelectric materials.A systematic investigation is performed to study the composition,structure and thermoelectric properties of sodium cobalt thermoelectric materials based on XRD,SEM and XPS analysis.The growth mechanism and the influence of the composition,structure and ionic valence on the thermoelectric properties are elucidated,and the electrical transport mechanism is analyzed.First of all,the Na_xCoO₂ ceramics are prepared by solid state method.The variation of element content and crystal structure is revealed by TG-DTA,XRD and Raman method.The sintering mechanism of ceramics is established through in-situ XRD method.It is found that the Na content and crystal structure had a significant influence on the thermoelectric properties.Secondly,the layered Na_xCoO₂ crystals with high crystallinity have been prepared by one-step hydrothermal method.By adjusting the reaction conditions,the growth mechanism of sodium cobaltate crystals under hydrothermal conditions has been studied.The growth mechanism of Na_xCoO₂ crystals can be divided into two stages:dissolution recrystallization and exchange reactions.The textured ceramics with high oriented have been prepared by SPS?spark plasma sintering?rapid sintering technology,and the anisotropy of the thermoelectric property is also studied.Again,the SiC-Na_x CoO₂ nanocomposite ceramics have been prepared by solid state method.It is manifested that the grain growth is inhibited due to the chemical inertness of SiC nanoparticles.SiC-Na_xCoO₂ ceramics with multiscale structure have been prepared,and the influence of the crystal structure in multiscale ceramics on their thermoelectric properties has been studied.The results show that the enhanced phonon scattering and porosity can effectively reduce the thermal conductivity of the materials and increase the thermoelectric properties.Finally,based on the different ion valence,we choose Cu and Zn ions as the ion doping to replace the ions in Co-site.The influence of transition metal ion content on the composition,structure and thermoelectric performance has been discussed.The results show that the electrical conductivity of the materials is notably enhanced due to the increased carrier concentration caused by the incorporation of Cu ions.Despite the fact that the Seebeck coefficients reduce due to the enhanced carrier concentration,the resultant ZT value is still improved to a certain extent.It can be concluded that ion doping plays an important role in improving the electrical conductivity of Na_xCoO₂ materials.