The Investigation On Thermoelectric Conversion Material Of Rare Earth Doped Ca-Co-O System

With the rapid economic development, the problems of energy shortages and environment pollution have become increasingly serious. Developing a new environment-friendly energy source has become a global energy trend. Thermoelectric material is an advanced energy conversion material, which can achieve the energy conversion between the heat and electricity directly through the migration of carriers. It has been attached great importance from scientists because of its environmental protection and energy utilization advantages.In this paper, the precursor powders of the rare-earth single-element doped Ca3-xAxCo4O9 (A=La or Sm) and complex doped Ca3₂xLaxSmxCo4O9 materials were synthesized by sol-gel method, and the bulk thermoelectric materials were obtained through high sintering. The impact to the material structure and performance, from the preparation process of powders and bulk materials as well as the rare-earth ion substitution for the Ca-bit, has been studied. DTA-TG analysis of the gel powders obtained by pretreatment was made by using differential thermal analysis instrument, the microcosmic appearances of precursor powders and bulk materials were observed by using scanning electron microscope (SEM), and the composition of the materials was analyzed by using X-ray diffraction (XRD). The resistivity of thermoelectric materials was measured through four-probe method, thus obtained the electrical conductivity, and got the thermal conductivity of thermoelectric materials by estimating. The main results are as follows:1. Through the sol-gel method, the precursor powders of materials are synthesized respectively by conventional heating method and microwave heating method. Compared with the conventional heating method, the microwave heating method can make pure precursor powders get faster, and can obtain powders with a higher crystalline degree and better crystal growth. The powders are pressed under 20 MPa, sintered 5 hours at 900℃, and have the second sintering by microwave. Ultimately the bulk thermoelectric materials are obtained.2. XRD results show that, after the single-element doping and complex doping, the phase of materials has not changed, and there is no impurity formation. Rare-earth ions can successfully replace Ca-bit, and disperse in the lattice structure of the Ca-Co-O system, composing a single compound.3. The results of conductivity test show that, the conductivity of thermoelectric materials appears an upward trend with increasing temperature; the conductivity of the materials, after single-element doping and complex doping La and Sm, is higher than the conductivity of the material without doping; the conductivity increases with increasing doped content, and when it reaches a maximum it decreases with increasing doping. In this study, when the amount of single-element doping La and Sm is 0.7 and 0.5 respectively, the complex doped amount of La and Sm are 0.35 (the total doping amount is 0.7), the conductivity of doped materials reaches maximums respectively.In the same doping amount, the conductivity of the single-element doping La materials is greater than the Sm doped materials, and the conductivity of complex doped thermoelectric material is between the two single-element doped materials, but not the average of them. It is close to single-element doping Sm materials in low temperatures, and close to La doped materials in high temperatures.4. The estimation results of the electronic thermal conductivity show that the electronic thermal conductivity of the rare-earth doped Ca3-xAxCo4O9 and Ca3-2xLaxSmxCo4O9 system are not decrease, and increase with increasing temperature, but the values are very small; rare-earth doping changes the lattice structure, and strengthens the phonon scattering, then the phonon thermal conductivity substantially lowers, which plays a dominant role in the thermal conductivity. Therefore, comprehensive consideration of the changes of electronic thermal conductivity and phonon thermal conductivity, the overall thermal conductivity of doped materials is reduced.After rare-earth doping, the thermoelectric parameters have been some improvement. The single-element doping rare-earth elements La, Sm and complex doping of them are conducive to optimizing the thermoelectric property of materials.