Study On The Effect Of Diverse Ion Doping In Ca₃Co₄O₉ Thermoelectric Material And Relevant Structure Calculation

Ca₃Co₄O₉-based thermoelectric materials have been obtained by different synthesis and preparation method with diverse ion doping in this dissertation. Via testing of thermoelectric property and XRD, SEM, etc, the relationship between structural feature and correlation property of sample has been dicussed, combining quantum chemistry calculation by discrete variation method which is based on Density Function Theory. The results indicated:Compared with solid state reaction synthesis method under normal pressure, combining sol-gel method can prepare Ca₃Co₄O₉ sample which has moderate grain size and ideal layer structure with high efficiency. Compared with firing under normal pressure, samples fired combining hot pressing have clearer layer structure and higher density. F and Zn-doping both have futher enhancement on the synthesis of Ca₃Co₄O₉.Proper amount of Zn doping is highly advantageous to improve electric conductivity and Seebeck coefficient and decrease thermal conductivity. That is Zn doping can improve thermoelectric property. At high temperature(900K) 2# sample with x=0.10 has the biggest ZT. Making quantum chemistry calculation of Ca₃Co₄O₉ models with Zn separately substituting Co(l) in CoO₂ layer and Co(2) in Ca₂CoO₃ layer with results indicating: After Zn substituting Co, the covalent bond strength of Zn-O is evidently below Co-O, while the covalent bond strength of Co-O also decrease compared with pure sample. This is the reason for the increase of electric conductivity and decrease of thermal conductivity after Zn doping. The clean electric charge of Zn is higher than Co,while the clean electric charges of Co and Ca both decrease after doping which may also have certain function on the electric conductivity of materials.Between the temperature 400K and 800K, F doping increases the electric conductivity; At low temperature under 500K, F-doping is beneficial for the increase of Seebeck coefficient. Above 500K, as the temperature goes up, the Sccbeck coefficient of samples then near to each other gradually. Be contrary with traditional knowledge of doping effect, the thermal conductivity increases after F doping in the research and also increase along with the aggrandizement of the doping quantity. At high temperature(900 K), the doping sample b₁ has the biggest ZT value. Make quantum chemistry calculation of Ca₃Co₄O₉ model with F substituting O by DFT-DVM method. After F substituting O, the covalent bond strength of Co-F is evidently below Co-O, while the covalent bond strength of Co-O also decreases. This is the reason for the increase of electric conductivity after F doping. After F replace O, the clean electric charge of F is obviously lower than O and the clean electric charge of Ca decreases slightly as well. Thus the decrease of ionic bond strength may also have certain function on the electric conductivity of materials.