Investigations On The Electrochemical Preparation, Characterization And Dposition Mechanism Of Bi₂Te₃ Based P-type Thermoelectric Materials

Thermoelectric generators and materials have attracted much attention due to therequirements of environmental protection and energy crisis. In this dissertation, thepurposes of the experiments were to prepare high performance p-type Bi₂Te₃ basedthermoelectric materials using electrochemical deposition method. Sb, Se, Cu and Agwere chose as doping elements according to the results of chemical calculation, and theelectrochemical behaviors of Bi-Sb-Te and Bi-Sb-Te-Se systems were investigated byelectrochemical measurement. The morphology, structure, composition andthermoelectric properties of the deposits were characterized through ESEM, XRD, EDSand Seebeck measurement techniques. On the basis, the preparation technology ofBi₂Te₃ based thermoelectric materials was studied and the optimized preparationtechnology was obtained. Besides, the preparation techniques of Bi₂Te₃ basedthermoelectric materials in the micro-zone were also investigated.The electrochemical processes of Bi-Sb-Te and Bi-Sb-Te-Se systems wereinvestigated using cyclic voltammetry, electrochemical impedance spectroscopy, andcathodic polarization. The results indicated that the reduction processes of the ions ofBi3+, SbIII, HTeO2+ and H2SeO3 in unitary solution systems were irreversible andmulti-step. The reduction processes of HTeO2+ and H2SeO3 were more complicated,including adsorption of ions, reduction of adsorbed ions and reduction of free ions. Theresults also shown that in the quarternary Bi-Sb-Te-Se solution, the adsorbed H2SeO3and HTeO2+ were firstly reductive and then reacted with Bi3+ to form Bi-Te-Se alloyunder comparative positive potential. With the potential moving to the range of SbIII/Sbreaction, the electrodeposits contained Bi-Sb-Te and Bi-Sb-Te-Se. The Sb2Te3 wasobtained when the potential was more negative than the redox potential of the SbIII/Sbreaction.Quarternary Bi2-xSbxTe3-ySey and MxBiySb2-x-yTez (M=Cu, Ag) doped materialswere prepared by potentiostatic electrodeposition method. The results shown that theelectrodeposition technologies, such as solution compositions and electrodepositingpotentials, had great effect on the morphology, composition, structure and performanceof the electrodeposited materials. P-type Bi2-xSbxTe3-ySey compounds with highthermoelectric properties could be obtained by adjusting the electrodepositiontechnology parameters. Doping of copper and silver had different effect on theBi_2-xSb_xTe_y material. Cu-doped Bi_2-xSb_xTe_y materials have fine morphology and low electrical resistivities. Doping of silver can decrease electrical resistivity more thandoping of copper and effectively enhance the thermoelectric performance ofBi_2-xSb_xTe_y material. Doping of copper and silver can reduce the Seebeck coefficient ofBi_2-xSb_xTe_y material. The crystal structures of electrodeposits were changed fromamorphous state to polycrystalline after annealing, as well as the thermoelectricproperties was improved. With the increase of annealing temperature, the powerfactors of the doped materials were enhanced.The results of preparing thermoelectric materials in micro-zones indicated thatdifferent micro-zone sizes affected the diffusion of ions onto the electrode surface, andthus affected the composition and performance of the doped Bi_2-xSb_xTe_y materials.