The First-principles Computations Of Thermoelectric Materials SnSe And Half-heusler Alloy NbFeSb

Along with the rapid development of world economy and rapid growth of population,the consumption of coal,oil,and other fossil fuels is growing,which can cause the depletion of these energy sources on one hand,and on the other hand will cause serious environmental pollution problems.More worrying is that local wars for competing energy sources are either not uncommon.While human-being wants to develop in the current state of relative peace,we must first solve the problem of energy shortage.In 2014,the world's total primary energy consumption equals to about 12.928 billion tons of standard oil,and China's energy consumption accounts for about 23%of the total energy consumption,nearly six percentage points higher than the United States of America does.However,China only contributes to less than 15%of the total GDP global.In China,serious challenges are encountered for energy consumption and energy utilization efficiency.Statistical data show that more than 60%of energy is lost in vain worldwide and most likely in the form of waste heat.Therefore,if this part of waste heat can be used reasonably,it will improve energy efficiency and benefit mankind at the same time.Thermoelectric materials have special functionalities in conversing waste heat into electricity,promisingly contributing to the waste heat utilization.However,research on thermoelectric materials in our country has a large gap compared with the United States of America,Europe,and Japan.Thermoelectric materials can be widely used in co-generation entirely which is determined by the conversion efficiency.The conversion efficiency is scaled by the dimensionless figure of merit.Currently,the highest figure of merit reported experimentally is nor more than 3.Thus,any theoretical approach to predict thermoelectric materials with high figure of merit is highly favored.At the same time,an understanding of a number of physical and chemical problems becomes particular appealed,guiding experimental exploration experimentally.An establishment of novel theoretical models will be useful in predicting additional thermoelectric materials,which not only promotes the development of materials science,but also benefits to human and financial resources saving.The present thesis is organized as the following six chapters:Chapter one introduces the basic principle of thermoelectricity and applications of thermoelectric conversion technology,the state-of-arts on thermoelectric phsyics,and progress in syntheising novel thermoelectric materials,in which thermoelectric materials SnSe and half-Heusler NbFeSb are chosen as representative examples.In the last section of this chapter,the main contents of the thesis are put forward.In Chapter two,we introduce the basic knowledge of density functional theory mainly from the basic theory of quantum mechanics.A brief introduction to the density functional perturbation theory based computations is given too.In Chapter three,we first present the formulation on the Seebeck coefficient,electrical conductivity,and electronic thermal conductivity using the semi-classical Boltzmann theory.Then we evaluate the carrier mobility and relaxation time using the deformation potential theory.A detailed description of the calculation procedure for elastic constants using different deformation modes is presented.Finally,a brief introduction to the method for lattice thermal conductivity computation in a doped system is given.In Chapter four,the electronic properties of SnSe compound is comprehensively analyzed using the first-principles theory in combination with the semi-classical Boltzmann method.The results show that SnSe compound has multi-energy valley configuration and is an indirect bandgap semiconductor.Favorable bandgaps along the b-axis and c-axis are predicted,but the bandgap along the a-axis is big,whch cause similar electrical properties along the b-axis and c-axis but low conductivity along the a-axis.The band structure near the top valence band has the mixed feature,conducive to a higher Seebeck coefficient and high electrical conductivity.Meanwhile,the crystal structure of SnSe exhibits the coexisting ionic and covalent bond,resulting in good electrical properties.The reasons for very different figures of merit in monocrystalline and polycrystalline SnSe compounds are explained.Finally,the optimal concentrations for monocrystalline and polycrystalline SeSe materials are predicted.In Chapter five,the electronic properties of half-Heusler NbFeSb is comprehensively analyzed using the first-principles theory.The results show that NbFeSb is an indirect bandgap semiconductor with a bandgap of? 0.55 eV.The contribution to the electronic density of states near the top valence band from Sb atoms is nearly negligible,while Sb atoms have major contribution to the phonon density of states in the low-frequency range.A p-type doping at the Sb position,without prejudice to its electrical properties but remarkably reducing the lattice thermal conductivity is proposed.Carrier holes and electrons have different effective masses,resulting in the p-type NbFeSb of higher mobility and longer relaxation time than the n-type NbFeSb.The optimal concentration of NbFeSb at different temperatures is roughly 1.45×1021 cm-3.At 1000 K,the figure of merit can reach 0.86 at the optimal concentration.Therefore,half-heusler alloy NbFeSb,if synthesized using conventional methods,may not be a good thermoelectric material.In Chapter six are presented the conclusion and perspective.