Investigations On The Electrochemical Preparation, Characterization And Forming Mechanism Of Bi₂Te₃ Based Thermoelectric Thin Films And Nanowires

Low dimensional thermoelectric materials have attacted much attention due to their extensive application on pelter coolers and power generators. The purposes of this dissertation are to electrochemically fabricate Bi₂Te₃ based nanostructured thermoelectric materials. The reduction process for unitary, binary and ternary system of Bi³⁺, HTeO₂⁺, SbO⁺ in acid nitric medium was investigated by means of cathodic polarization, cyclic voltammetry and electrochemical impedance measurements. The mechanisms were developed to direct the production of Bi₂Te₃ and Bi_2-xSb_xTe₃ thermoelectric films and nanowire arrays. By using ESEM, FETEM, HREM, SEAD, XPS, XRD, EDS and EDX techniques, the morphology, structure, composition, and properties of the deposited materials were examined. And the factors influencing thermoelectric performances of the thin films and nanowire arrays were thoroughly studied.The electrochemical behaviors of Bi3+, HTeO2+ and SbO+ in Bi-Te, Sb-Te and Bi-Sb-Te system are different from that of monocomponent system because of ion interaction. HTeO2+ plays important role in the whole reduction process, which can be reduced firstly because of its strong adsorption on the surface of the Pt electrode, then react with other ions to form the proper compounds.The Bi₂Te₃ and Bi_2-xSb_xTe₃ thermoelectric thin films were prepared by potentiostatic electrodeposition. The composition of the film and its thermoelectric properties can be controlled through adjusting electrodeposition potential, so the synthesis of the materials can be easily performed according to the actual requirements. N-type semiconductive film of Bi₂Te₃ was prepared at the potential of -0.030V with the largest seebeck coefficient of -44μV·K^-1. The Bi_2-xSb_xTe₃ film is p-type semiconductor and when to be electrodeposited at -0.5V it possesses the largest seebeck coefficient of 213μV·K^-1, whose composition was proved to be Bi_0.5Sb_1.5Te₃. When the temperature increased in the scope of 300～350K, the seebeck coefficient of the Bi₂Te₃ and Bi_2-xSb_xTe₃ thermoelectric thin films increased while their resistance reduced, which means that they can be used under room temperature. In addition multidoping is the effective method for improving the thermoelectric properties of materials.The Bi₂Te₃ and Bi_2-xSb_xTe₃ thermoelectric nanowire arrays have beensuccessfully fabricated by direct current deposition respectively in porous anodic aluminum oxide template. The potentiostatic deposits of Bi₂Te₃ nanowires are polycrystalline with hexagonal structure. The compostion of the galvanostatic deposits of Bi_2-xSb_xTe₃ nanowires is determined by the current, so the thermoelectric properties of the nanowires can be controlled through adjusting electrodeposition current. The seebeck coefficient of nanowire arrays is much lower than that of bulk materials. The main factors to affect the thermoelectric performance are uneven distribution of the composition and the large diameter of the nanowires.