Preparation And Thermoelectric Properties Of ?-Na_xCoO₂ Materials By Sodium Polyacrylate Sol-gel Method

Thermoelectric material is a kind of new energy material which can mutually convert the heat into electricity via Seebeck and Peltier effect,because of no noise,no pollution and can recover low quality waste heat,in recent years thermoelectric material become a research hotspot in the field of new energy materials.Oxide based materials are considered as one of the most environmental economical materials in the application areas of thermoelectric materials,the P type oxide thermoelectric material Na_xCo₂O₄ with environment friendly,low cost,high temperature stability and thermoelectric potential,which has aroused the concern of the researchers.In this paper,we prepared Na_xCoO₂ polycrystalline samples,Na_xCoO₂?x=0.8¹.1?samples,Na_0.8-xCa_xCoO₂?x=0.05⁰.3?and Na_0.8Co_1-xCu_xO₂?x=0.01⁰.05?samples by sodium polyacrylate sel gel method combined with spark plasma sintering,the effects of the orientation,the content of Na and the doping of Ca and Cu element on the phase composition,microstructure and electrothermal transport performance were systematically studied.The main reaserch contents and results are as follows:In this research,we focus on Na_xCoO₂ materials,a sodium polyacrylate sol-gel method was developed for the preparation of NaxCoO2 single-phase powders,the effects of the molar ratio of Na⁺/Co²⁺,the concentration of raw materials and the calcination temperature on the phase composition and microstructure of the products were studied,then the proper preparation conditions were determined.The NaxCoO2bulk was prepared by SPS,the thermoelectric transport properties of samples were measured both parallel and perpendicular to the SPS pressure axis.The results showed that the Na_xCoO₂ samples prepared by sodium polyacrylate sol gel method with SPS were preferentially oriented along the c axis,the highest ZT value of 0.12 was measured at 900 K within the direction perpendicular to the SPS pressure axis,which is 71%higher than that of ZT value along the direction parallel to the SPS pressure axis.Using the above determined preparation methods,the effects of Na content on the phase composition,microstructure and thermoelectric properties of the materials were systematically studied.The results showed the thermoelectric properties of NaxCoO2bulk materials improved with sodium content,because the Seebeck coefficient and electrical conductivity increase with sodium content,however,the thermal conductivity decrease,So the ZT value increased significantly.At 900 K,Na1.1CoO2sample has the greatest ZT value of 0.39,which is almost three times of the maximum ZT of Na_0.8CoO₂ sample.Secondly,we dope Ca and Cu elements into the Na and Co sublattice,respectively,because of the similar electronegativities and atomic radius of Ca,Cu compared to those of Na and Co,and the effects of element doping on the thermoelectric properties of the material were studied,the results showed that the Ca was successfully incorporated into the Na_0.8CoO₂ lattice,the electrical resistivity and Seebeck coefficient increased with Ca content,it shows that the carrier concentration of system decreased after doping Ca into Na0.8CoO2,the thermal conductivity decreased with the increase of x simultaneously,finally,Na_0.5Ca_0.3CoO₂ has the largest ZT value of 0.25 at 900 K,which is increased by 108%compared with undoped sample.The Co site of Na0.8CoO2 substituted by Cu can improve the Seebeck coefficient and electrical conductivity of the system with decreasing the thermal conductivity,finally,the thermoelectric properties have been greatly improved,when x=0.05,Na_0.8Co_0.95Cu_0.05O₂ samples has the largest thermoelectric figure of merit 0.32at 900 K,which is increased by 167%compared with undoped sample.