Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Thermoelectricity, as a substantial energy form alternate to the traditional fossil fuels, has attracted tremendous attentions nowadays. The energy conversion efficiency of the thermoelectric device is mainly governed by the dimensionless thermoelectric figure of merit (aka zT) of thermoelectric materials, which consists of both electrical and phonon transport properties. Nowadays, the exploration of high figure of merit thermoelectric materials still rely greatly on the experimental efforts due to the lack of first principles methods for calculating the thermoelectric transport properties. Comparing with the computational methods for the phonon transport properties (aka lattice thermal conductivity), which can be calculated considering the phonon-phonon interactions as the scattering term in the Boltzmann transport equation (BTE), the first principles methods for calculating the electrical transport properties fall behind. Till now, the most common methods for calculating the electrical transport properties usually employ the combination of BTE along with the relaxation time approximation. The human-adjustable and single-value nature of the relaxation time makes this calculation scheme for the electrical conductivity lack physical meaning and predictive power.
In this dissertation, we developed first principles algorithms for calculating the electrical transport properties using the electron-phonon interaction as the scattering term in the electron BTE, which can be combined with available methods for phonon transport properties to provide a full description of the thermoelectric figure of merit. The complete methodology is presented in Chapter 2. Although 3C-SiC possesses a simple structure, the polar nature of this material makes it a good candidate to examine the accuracy of our algorithms for calculating the electrical transport properties. The calculated charge carrier (both electron and hole) mobilities as a function of temperature agree well with the experimental results. Besides, a temperature dependent scattering mechanism is observed through our calculations in Chapter 3.
Despite the excellent thermoelectric performance of n-type Mg3Sb2, the low thermoelectric figure of merit of the p-type counterpart prevents this material from practical applications. In Chapter 4 of this dissertation, we presented our work on the anisotropic transport properties of both n- and p-type Mg3Sb2, which are hard to explore experimentally. Our calculated n-type thermoelectric figure of merit using the methods developed in Chapter 2 is in excellent agreement with the experimental value, showing the excellent predictive power of our methods. Most importantly, strong anisotropic thermoelectric figure of merit of the p-type Mg3Sb2 is observed, with the out-of-plane figure of merit beyond unity, making it possible for device applications. Moreover, we further proposed through highly oriented polycrystalline samples, it is possible to greatly improve the p-type performance of Mg3Sb2 experimentally.
Nanomaterials, especially the two-dimensional materials, have drawn great attentions these days after the discovery of graphene. Although it remains challenging to measure the thermoelectric transport properties of two-dimensional materials experimentally, it can be easily calculated using our algorithms developed in Chapter 2. In Chapter 5, we presented our work on the thermoelectric transport properties of two-dimensional α-Tellurium (α-Te). We found despite the thermoelectric figure of merits of both n-type and p-type two-dimensional α-Te are already promising compared with other two-dimensional materials, small tensile strain (less than 4%) could further boost the n-type thermoelectric performance. However, the tensile strain has a negative effect on the p-type thermoelectric properties. Lastly, in Chapter 6, we discussed possible future efforts following the vein of the first principles methods for calculating the thermoelectric transport properties.
Meng, Fanchen, "First Principles Methods for Calculating Thermoelectric Transport Properties" (2021). All Dissertations. 2811.