Synergy Of Two-phase Coexistence And Quantum Confinement Effect In Thermoelectric Composites

Thermoelectric materials can directly convert waste heat energy into electrical energy.Due to the practical requirements of energy saving and environmental protection,they have attracted widespread attention in recent years.Thermoelectric conversion has the advantages of no noise,no pollution,and high reliability.It is a green and environmentally friendly power generation technology.Thermoelectric materials can be used to convert waste heat energy from automobile exhaust and industrial exhaust gas into electricity.This will not only effectively increase the efficiency of fossil fuel use,but also reduce greenhouse gas emissions and protect the environment.Therefore,thermoelectric conversion has broad prospects for industrialization.At present,the bottleneck of the large-scale application of thermoelectric conversion is poor performance of thermoelectric materials,that is,low energy conversion efficiency.Therefore,the exploration of high-performance thermoelectric materials has become a widespread concern for researchers around the world.Composite materials are considered as a viable way to obtain high performance thermoelectric materials.One possible way to improve the thermoelectric properties of a material is to reduce the lattice thermal conductivity.The other way is to increase the power factor of the material.At present,the lattice thermal conductivity of nanocomposites prepared experimentally is close to the lowest value predicted by Cahill theory.Therefore,a significant increase in the power factor of a material is a better choice to further improve its thermoelectric performance.Researchers use multiple mechanisms to increase power factors in composite materials,including quantum confinement,energy filtering,percolation,and two-phase coexistence effects.However,there are few studies on the synergy of multiple effects.This dissertation focused on the in-plane heterojunctions of atomic-scale two-dimensional nanosheet.The synergistic effects of quantum confinement and two-phase coexistence effects were also explored.Firstly,in chapter 3,a simple series-parallel model is used to study the conditions at which the two-phase coexistence effect can increase the power factor of the composite material.The criteria are,the power factor values of the two phases need to be close,but the conductivity values need to have large differences.The physical mechanism for the increase of power factor was pointed out.The difference in thermal conductivities of the two phases makes the composite material have a non-uniform temperature distribution,so it can simultaneously have high electrical conductivity and high Seebeck coefficient.Then,in chapter 4,using density functional theory and Boltzmann transport equation,quantum confinement effect on thermoelectric power factor in Ti S2 two-dimensional nanosheet was studied.It was found that the Seebeck coefficient of Ti S2 monolayer increases by about 40% compared to that of the bulk.The increasement comes from larger density of states near the conduction band minimum caused by the quantum confinement effect.Finally,in Chapter 5,based on the work of the previous two chapters,Ti S2/Mo S2 two-dimensional in-plane heterojunction was considered as a low-dimensional composite material.The coexistence of two phases and the quantum confinement effect was investigated.The ratio of the two phases and the respective carrier concentration was obtained for when the maximum power factor was achieved.The calculation results show that the quantum confinement effect and two-phase coexistence effect can act synergistically to increase the power factor of atomic-level thickness two-dimensional in-plane heterojunctions.The physical mechanism behind them was also analyzed.The results have a positive significance for deepening the understanding of the physical properties of two-dimensional composite materials and designing new two-dimensional composite materials.