Element Doping And CNTs/SiC Dispersion Effect On The Thermoelectric Properties Of Bi₂Ba₂Co₂O_y

Thermoelectric materials can realize direct conversion of heat and electricity relying on the carrier transport in solid material.Thermoelectric materials are mainly used in thermoelectric generation and thermoelectric refrigeration.Thermoelectric devices made by thermoelectric materials have the advantages of simple structure,reliable performance,and environment-friendly products,etc.Usually,the dimensionless figure of merit?ZT?is used to evaluation the thermoelectric performance,ZT=S²T/????,where S,?,? and T are seebeck coefficient,electrical resistivity,thermal conductivity and operating temperature.The power factor is defined as PF=S²/?.Oxide thermoelectric materials have better physics and chemistry stability,but their thermoelectric performance is much less than the alloy materials due to the large resistivity.NaCo₂O₄ material was found with high thermoelectric properties,which caused the researching upsurge of oxide thermoelectric materials.The Bi₂Ba₂Co₂O_y material with layered structure is one kind of Co-based oxide thermoelectric materials.Element doping and nanocrystallization are two important methods to improve the Co-base oxide thermoelectric performance.Appropriate element doping can improve the carrier concentration,thus improve the electrical transport properties.Especially some heavy elements doping can well reduce the material thermal conductivity.The quantum size effect and coulomb blockade of nanometer materials may improve the thermoelectric performance of materials.Samples in this thesis were prepared by the high temperature solid phase method,and were characterized by XRD,SEM and EDS etc.Electrical resistivity,Seebeck coefficient,thermal conductivity,power factor and ZT values of materials are measured and calculated.First,the influence of Na,K element replacing Bi partially on the Bi₂Ba₂Co₂O_y material is studied.The experimental results proved that K element doping is not conducive to improve Bi₂Ba₂Co₂O_y material thermoelectric performance.A small amount of Na doping can increase charge carrier concentration of Bi₂Ba₂Co₂O_y material,thus reduce the resistivity and reduced the Seebeck coefficient.The resistivity of excessive Na doped materials get increased instead.Na element doping reduces thermal conductivity,too.The ZT values of Bi_1.8Na_0.2Ba₂Co₂O_y sample reach 0.2 at 873 k,which 79% higher than BBCO sample.Then,the thermoelectric performance of the 1.25 at% Na doped and CNTs/C dispersed Bi₂Ba₂Co₂O_y composite materials are studied.CNTs enhance the interface scattering of composites,thus thermal conductivity reducessignificantly,power factors also fell.1.25 at%Na doping part compensates the increase of resistivity caused by CNTs.Nano point C has similar effect on Bi₂Ba₂Co₂O_y materials to CNTs.The ZT value of Bi_1.975Na_0.025Ba₂Co₂O_y+0.5 wt% CNT sample reaches 0.2 at 923 K,which is 53.3% higher than the Bi_1.975Na_0.025Ba₂Co₂O_y sample and 78.2% higher than the BBCO sample.Finally,the power factors of the nano SiC with Bi₂Ba₂Co₂O_y compound materials are studied.The nano SiC material itself has high Seebeck coefficient and low resistivity.The resistivity of composites decreases with small content nano SiC,while the resistivity of composites increases with excessive SiC.The introduction of the nano SiC particles also raises the Seebeck coefficient of material.The results show that the moderate amount of nano SiC mixed with Bi₂Ba₂Co₂O_y can increase the power factors of materials.