First-Principles Investigations On The Enhanced Thermoelectric Performance For PbTe Under High Pressure

The development of improved thermoelectric materials could lead to revolutionary advances in many important technologies such as, power generation of electron device, power generation of space flight, refrigeration of integrated circuit and cooling of superconducting. The search for high-performance thermoelectric materials is a hot subject of current research. The performance of thermoelectric materials depends on the dimensionless figure of merit (ZT) given by ZT = (S2σT/κ), where S,σ, T andκare the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity. High-performance of a thermoelectric material requires a high ZT with an effort to maximize the power-factor S2σand minimizeκ. PbTe is one of the best thermoelectric materials used for thermoelectric generators for temperatures ranging between 400 and 800 K. Previously, many attempts were made to improve its ZT value since the middle of the past century, such as alloys of other compounds, doping, building nanosized minority phase in the PbTe matrix, preparing nanowires of PbTe, quantum wells of PbTe/Pb1?xEuxTe, and PbSe0.98Te0.02/PbTe superlattices.High pressure is an effective method to change the positions and the distributions of atoms, and it is able to tune the electronic structure and lattice vibration. High pressure can have a very large effect on the chemical and physical properties of matter. It is a thermodynamic variable that is as fundamental as temperature. Recently, there have been revolutionary advances in high-pressure, and the static pressures about 400 GPa now achievable inside the diamond anvil cell. Under these conditions, such large changes occur in the density, electron structure. Extreme changes in chemical equilibria and material properties can result, allowing access to a wide range of new compounds and unusual states of matter. Furthermore, many solids synthesized at high pressure can be quenched to ambient conditions, where they can be thermodynamically metastable, yet remain indefinitely kinetically stable. Such routes to metastable solids have attracted much attention because many of the most interesting and useful materials are metastable.It is at the beginning to explore the high performance thermoelectric materials through the high pressure method. Recent experiments show that the thermoelectric performance of PbTe can be improved by high pressure. But there are no further researches to explain reasonably the physical reasons of the high ZT values for PbTe after press, so it is difficult to get PbTe with the highest ZT value. In this dissertation, we use the first principle method based on density functional theory (DFT) and the semiclassical Boltzmann theory to study extensively the transport properties of PbTe under high pressure. We analysis carefully the course of loading pressure, and explain the reasons of improving thermoelectric performance from the changing of electronic structure. Our calculations first predict that N-type PbTe within the orthorhombic Pnma phase at 6.5 GPa has nearly two times larger ZT than that for the B1 phase at zero pressure. The comparable researches of PbTe under the stress of hydrostatic and uniaxial pressure show that the ZT values can be improved largely by the uniaxial stress. The largest P-type ZT value tuned by the uniaxial stress is about two times larger than that at ambient pressure. We also predict the optimum doping ranges for the hihest ZT values, and the doping predictions can guide the scientists to do further experimental researches. The significance of our research is that high pressure can be regarded as a new method to improve the thermoelectric performance.From the calculations for both N- and P-doped B1 phase, it is obvious that the choice of band gap plays an important role in the calculation of S and ZT, but only at lower carrier concentration, while the effect is negligible at higher carrier concentration. Compared to the B1 phase, the Pnma phase shows strong anisotropic behaviors in transport coefficients, and the anisotropic behaviors in the electric conductivity for the Pnma phase is larger than that of hexagonal Bi2Te3. The N-type Pnma phase has larger Seebeck (S) coefficient and comparable electric conductivity (σ) along y axis than those of the N-type B1 phase, and the larger S2σis responsible for the higher ZT value of the Pnma phase. Further calculations show that the ZTs of the N-type Pnma at 300 and 600 K are approximately two times larger than those of the N-type B1 phase. The electronic structures near the band gap are responsible for the high ZT values of the Pnma phase. The total density of states (DOS) of the Pnma phase is higher than that of the B1 phase near the lowest conduction band (LCD). Since the materials with high S are usually associated with large DOS near the band gap. So the current DOS results are mainly responsible for the predicted higher S in the Pnma structure than those in the B1 phase for N-type materials. For the good thermoelectric materials the energy distribution of carriers should be narrow and have a high carrier velocity in the direction of the applied field, which is possible in a highly anisotropic system. The constant energy surface in this kind of highly anisotropic system should have a large area to increase the DOS and S, while a small area along other direction to increase the group velocity and conductivity. For the constant energy surface of the Pnma phase, there are small areas along kx in the highest valence band (HVB) and ky in the LCB for the P- and N-type compounds, respectively. There are larger areas of the constant energy surface along other directions. These special electronic structures are also illustrated in the band structure. There is larger energy dispersion along ky and kx directions in energy band plot for N- and P-doped samples, respectively. The strong anisotropy for the Pnma phase in the electronic structure is mainly responsible for the high thermoelectric performance.The research of transport properties for PbTe within the B1 structure under high pressure show that the ZT values are only increased a little by hydrostatic pressure. The highest ZT values increased by hydrostatic pressure are 1.3 and 1.1 times higher than that of B1 phase at zero pressure for P- and N- type samples, respectively. Hydrostatic pressure has a little effect on the change of band structure of PbTe, and it doesn't have enough effect on the transport properties. The uniaxial stress is applied along z direction in crystallography. The Seebeck coefficients along x and z directions are increased by the uniaxial pressure for both P- and N- type samples. The electrical conductivity along z axis can be improved about two times higher than the original values at zero pressure for P-type PbTe. The electrical conductivities along x axis are all smaller than that at ambient pressure after pressed along z directions. The highest P-type ZT values along z directions increased by the uniaxial pressure of 3.0 and 4.0 GPa are 1.82 and 1.84 times higher than that at ambient pressure.The analyses of electronic structure for PbTe under uniaxial stress suggest that uniaxial pressure can improve the Seebeck coefficients through increase the total DOS near the HVB. The uniaxial stress along z direction in crystallography change the cubic crystal lattice into quadradic lattice, and there is little distribution along the kz directions for the constant energy surface in the reciprocal space after such change of lattice. The anisotropy of the energy distributions in the reciprocal space is the main reason of high electrical conductivity along z direction.