Theoretical Investigation And Performance Improvement On The Low-Cost Ternary Thermoelectric Materials

The impending energy crisis and environmental pollution have been becoming a critical problem around the world.Thermoelectric materials are recognized as novel clean energy materials,which enable direct conversion between thermal and electrical energy.Thermoelectric performance is quantified by a dimensionless figure of merit,ZT:the high electrical transport properties and the low thermal conductivity will lead a large ZT value,and the larger ZT value is the higher thermoelectric conversion efficiency.However,the current promising thermoelectric materials still face the following problems:low thermoelectric conversion efficiency,containing precious or toxic elements,and the poor thermodynamic stability,which limits the large-scale commercial application.In this dissertation,we foucs on low-cost thermoelectric materials,including three natural minerals(CuSbS2,CuBiS2,and AgBiSe2)with low thermal conductivity and half-Heusler compounds with high electrical transport properties.Using the first-principles calculation method combined with semi-classical Boltzmann theory,we scrutinize their electrical and heat transport properties,study the physical mechanisms behind the high-performance thermoelectric materials,explore methodologies to improve their thermoelectric performance and predict new high-performance thermoelectrics.This is essential for the developments and applications of thermoelectric technologies in the future.The followings are the brief descriptions of our studies.1.Understanding the ultralow lattice thermal conductivity of CuBiS2:CuBiS2 and CuSbS2 are isostructural natural mineral compounds,while the experimentally measured lattice thermal conductivities of CuBiS2(0.5 W/mK)is only one-third of CuSbS2(1.5 W/mK)at room temperature.From the calculated electronic structures and phonon properties,we find that the stereochemically active lone-pair electrons at the Sb sites are major contributors to the large Gruneisen parameters and strong anharmonicity in CuSbS2.However,in CuBiS2,in addition to the lone-pair electrons at the Bi sites,the Cu ion rattling further raises the Gruneisen parameters and strength the anharmonicity.The larger Gruneisen parameters in CuBiS2 lead to an ultralow thermal conductivity.The studies of ultralow lattice thermal conductivity of CuBiS2 not only indicates the possibility of using sulfosalt systems as efficient thermoelectric materials,but also offer new insights for discovering and designing high-ZT thermoelectric materials with lone-pair electrons combined with the rattler in the future.2.Improving the thermoelectric properties of AgBiSe2 at room temperature by band engineering:The hexagonal phase of AgBiSe2 has been attracting increasing attention for its room-temperature applications due to the intrinsic low lattice thermal conductivity(0.45 W/mK at 300 K).However,its ZT value is very low(less than 0.1)at 300K.Besides,the basic intrinsic semiconductor conducting behavior of AgBiSe2 is not clear yet,even in the experimental measurements.This will seriously hinder its thermoelectric property tuning using carrier concentrations.We then study the defect formation energies of different intrinsic charged point defects and optimize its thermoelectric properties through defect engineering.From the calculated defect formation energies of different intrinsic charged point defects,we clarify the intrinsic AgBiSe2 is a p-type conduction behavior semiconductor.Based on scrutinizing the band structure of AgBiSe2,we propose two kinds of methodologies to modify its band structures,and to achieve high band degeneracy:(i)shifting the Fermi level into the valence band using intrinsic defects,and(ii)converging several valence-band maxima by introducing extrinsic defects.We find that the two methods all can increase the band degeneracy and improve the thermoelectric properties of AgBiSe2.Moreover,we find that the intrinsic Ag vacancy is helpful to significantly increase the power factor,leading to a large ZT:the maximum ZT value increased to 0.3-0.5 at near room temperature.Based on analyzing the bonding characters and atomic energy levels in the compound,we predict that the band structure and thermoelectric properties of AgBiSe2 could be improved by introducing Cu,Rh,and Pd.Our work not only suggests that p-type AgBiSe2 is a promising room-temperature thermoelectric material,but also provides new methodologies to increase the band degeneracy by introducing intrinsic and extrinsic point defects.3.Screening high-efficiency half-Heusler thermoelectric materials via high-throughput computations:half-Heusler compounds exhibit high mechanical robustness and thermal stability.In addition,they have excellent electrical transport properties.A variety of half-Heusler materials have been proved to be high-performance thermoelectric materials.However,due to a large number of materials in the half-Heusler family,many promising half-Heusler thermoelectric compounds remain unexplored and unutilized.Using the high-throughput computational methodology,focusing on 75 thermodynamically stable half-Heusler compounds,we screen the bunch of half-Heusler compounds with excellent electrical properties using the electronic structure based electronic fitness function(EFF),which focuses on the decoupling of Seebeck coefficient and electrical conductivity.For the half-Heusler compounds with high EFF,we carry out the detailed calculations of electrical transport properties and thermal conductivity.This approach identifies several new high-performance half-Heusler thermoelectric compounds,such as BLiSi.Our work provides not only new high-performance half-Heusler thermoelectric materials,but also a new and effective method for screen potential high-performance thermoelectric candidates.