COMPUTATIONAL INVESTIGATION OF THERMOELECTRIC PROPERTIES IN SELECTED TWO- AND THREE-DIMENSIONAL MATERIALS

Abstract

Thermoelectricity has emerged as one of the potential green energy harvesting technologies that provide clean and sustainable energy solutions. This technology relies on the materials that efficiently convert waste heat into useful electricity, known as thermoelectric (TE) materials, which have a wide span of applications ranging from wearable devices to space applications. In recent years, the search for novel TE materials has been extended to the classes of materials that simultaneously have decoupled transport parameters with inherently low lattice thermal conductivity. We have discussed four such classes of materials, i.e., Zintl phases, half-Heusler (HH) compounds, Li-transition-metal nitrides, and pnictide monolayers, and conducted extensive first-principles calculations followed by semiclassical Boltzmann transport theory to explore their thermoelectric properties of these materials. In the first problem, we realized that the carrier concentration optimization is an effective way to enhance the TE performance of a material. We have explored the TE properties of the p- type RbZn4P3 and n-type RbGaSb2 Zintl compounds at different hole and electron doping concentrations, respectively. A peak ZT value of 0.78 (0.87) at 700 K (900 K) for optimal hole (electron) doping concentration of 2×1020 cm-3 (2×1019 cm-3) has been obtained for RbZn4P3 (RbGaSb2). The key advantages of using Zintl compounds are their complex crystal structure, potential rattling of Rb cations, and presence of heavy elements. We have obtained low lattice thermal conductivities of both compounds, RbZn4P3 and RbGaSb2. This low thermal conductivity allows for the efficient conversion of heat into electricity, making them potential candidates for TE applications. In the second problem, we analysed and compared the electronic, transport, phononic, and thermal properties of 8 valence electron count (VEC) Li- based HH compounds LiCaX (X = As, Sb). The value of the Seebeck coefficient has been found to be higher in p-type LiCaX than that of n-type due to flat VB edges. TE performance is slightly enhanced with an increase in atomic weight of X atom owing to lowl  and significant power factor (PF). The remarkably lowl  of 8-VEC HH compounds LiCaX (X = As, Sb) has been understood in terms of different phonon modes and optimization of carrier concentration, resulting in an improved ZT at higher temperatures. The optimized carrier concentration (~1020 cm-3 for p-type carriers and ~1019 cm-3 for n-type carriers) of the investigated compounds was found to be comparable to the experimentally estimated value for other 8-VEC Li-based HH. Our study has predicted that the p-type HH alloys LiCaX are promising TE materials. In the third problem, we understood the role of lattice dynamics in ix realizing high TE performance in an already experimentally synthesized layered materials Li2MN2 (M= Zr and Hf). We have analyzed how replacing Zr with the relatively heavy element Hf in Li₂MN₂ leads to an increase in thermal conductivity instead of a decrease, which is against the common notion. We have obtained a lower lattice thermal conductivity (1.52 Wm-1K-1), compared to Li₂HfN₂, along the a-axis at 1000 K for Li2ZrN2 which has been attributed to the rattling behaviour of Zr that leads to shorter phonon lifetimes. Moreover, the anisotropic character owing to the layered structure of Li2MN2 has enabled tuning their transport properties. Therefore, a high PF has been obtained along the a-axis for both compounds, which resulted in high ZT in this direction. The high figure of merit (1.07) of Li2ZrN2 along the a-axis has unravelled its potential for high-temperature TE application. These calculations have provided valuable insights into the vibrational properties, including the phonon group velocities, phonon lifetime, and phonon frequencies that govern the thermal conductivity and the phonon-mediated heat transport. In the fourth problem, we explored structural, electronic, and TE properties of 2D YX (X = N, P, As) rare-earth pnictide monolayers. The monolayers are found to exhibit high anisotropy in the electronic transport properties, which is attributed to their unique crystallographic and electronic structure. The dimensionality reduction in these semiconducting materials with strong p-d hybridization led to high electron conductivities, thereby resulting in high ZT. The phonon band structure shows enhanced coupling between the low-frequency optical and acoustic modes. Accordingly, lowl  obtained, with values 3.335, 1.779, and 1.648 Wm-1K-1 for YN, YP, and YAs monolayers, respectively, at 500 K. For the p-type materials, the highest ZT is achieved in the order YN < YP < YAs in the y-direction, while for n-type materials, it is in order YN >YP > YAs in the x-direction. The monolayers YN, YAs, and YP are found to exhibit a maximum figure of merit of 2.02, 1.39, and 1.18, respectively, at 500 K, showing excellent TE performance. Our study outlines some effective approaches for enhancing the TE performance of materials, which include carrier concentration optimization, phonon engineering, and dimensional reduction. It may open new avenues for future experimental realization of these materials for energy harvesting applications in a wide temperature range. This thesis underlines the effective use of computational techniques to screen efficient materials for desired applications. These techniques save valuable resources and help experimentalists to synthesize the proposed materials without resorting to trial-and-error methodology in the wet lab.