Fabrication And Optimization Of Sb₂Te₃ Based Low Dimensional Thermoelectric Composites

Thermoelectric materials (TE) can realize the direct transformation of heat and electricity based on Seebeck effect and Peltier effect, which has a promising application in the field of power generation and solid state refrigerators. This device has a lot of advantages beyond the traditional facilities, such as noiseless, non-axial type gera, light, pollution-free, small, portable, safe and stable, which has significant potential utilization in auto industry?medical treatment? industrial manufacture? biomedicine and national military. At present the highest thermoelectric efficiency maintains around 10%, however, the dimensionless figure of merit must be improved up to 3 to compare with the efficiency of traditional apparatus. There are a lot of approaches to improve the thermoelectric performance, such as nanostructure engineering, band engineering, which could optimize the transport of carriers and phonons.Bi2Te3 is a commercial room temperature thermoelectric materials, which is widely studied by the researchers all over the world. Nevertheless, a complete TE device must equip several TE couples, which contain at least a pair of N type and P type TE materials. is a P type TE materials, that matches the N type Bi2Te3. The researches on pure materials are rare, so further study on TE materials is on investigation.We synthesized nanostructured Sb2Te3 platelets via solvothermal method. This fabricated sample has a edge distance of 1-2 ?m with thickness of several ?m. We further explored the influence of different conditions, such as reaction temperature? time and the amount of PVP and NaOH, on the the morphology and purity of Sb2Te3. The rhombohedral crystalline structure constructed the hexagonal framework of single crystal and the growth speed is adjusted by PVP on the surface. The inner crystalline orientated to grow into nanostructured Sb2Te3 platelets.Organic conducting polymer possesses the advantages of excellent electrical conductivity and low thermal conductivity, which attracts great interests of researchers. Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the promising organic TE. Designing semiconductor-organic composite is one of the effective ways to improve the thermoelectric materials. We inserted the organic conducting polymer PEDOT into the matrix of Sb2Te3, which integrated the high thermoelectric properties of both materials. The Seebeck coefficient of the composite was improved, while the electrical conductivity was not significantly decreased. The Sb2Te3 nanoparticles and the added PEDOT conducting polymer acting as the phonon scattering center, decreased the thermal conductivity from 0.398 Wm-1K-1 for the pure phase to 0.148 Wm-1K-1 for the composites at 523 K. The ZT value of Sb2Te3-PEDOT composites reached 1.18 at 523 K, which was 60% higher than the pure sample. The thermal conductivity of the composites maintained a low value after 50 thermal cycling, indicating the good stability of Sb2Tes-PEDOT composite.In order to improve the electrical property of the composites, we successfully designed and synthesized the Sb2Te3/PEDOT/Sb2Te3 structure. We inserted PEDOT into Sb2Te3 bulk, forming the anisotropy three layered TE materials. The correlationship between the electrical conductivity parallel to the layers and thermal conductivity perpendicular to the layers are decoupled. Through modifying the thickness of PEDOT layer, we obtained enhanced thermoelectric property. Owing to the mutual contribution of electrical conductivity and Seebeck coefficient, the power factor reached 1.64 mWm"'K'2, which was 5 tines of the pure phase. The thermal conductivity perpendicular to the layers varied between 0.31 Wm-1K-1 and 0.34 Wm-1K-1 at 523 K, which was far below the pure Sb2Te3 (0.448 Wm-1K-1 ). The composite with 50 um PEDOT increased from 0.74 at 298 K to 1.3 at 523, which was twice of the sample.In addition, we synthesized novel Au@Sb2Te3 heterostructures through coating Au nanoparticles on Sb2Te3 nanoplatelets. The measurement of thermoelectric performance indicated the enhancement of electrical conductivity as Au content increased. The electrical conductivity for composite with 4% Au content was up to 3.89×104 S/m, compared to the 2.04×104 S/m for pristine Sb2Te3. The Seebeck coefficient increased at low content of Au nanoparticles, which achieved the maximum value 175.8?V/m. Owing to effective scattering of the nanoparticles and Au-Sb2Te3 interface, the thermal conductivity of the composite decreased dramatically. The composite with 1% Au concentration declined to 0.51849 W/(mK). With the mutual coordination of the three parameters and optimization of electrical and thermal properties on 1% Au composites, the maximum of ZT reached 0.8, which was twice of the pure phase. The percolation theory indicated the composite with proper concentration of Au nanoparticles can effectively improve the thermoelectric performance.We further investigated the ternary P type TE materials Sb2Te3-xSex. The carrier concentration had great influences on electrical and thermal conductivity. The analysis of positron annihilation studied the relationship between point defects and thermoelectric performance. As the increase of the substitution of Se atoms to Te, the average lifetime of positron decreased, revealing the decline of point defects and hole concentration, thus leaded to the decrease of electrical and thermal conductivity.