The Thermoelectric Properties Of Graphyne, Skutterudites And Alkalis Semiconductors From First-principle Theory

Since the second industrial revolution,the demand for electricity in human society has increased.However,the conversion of fossil fuel combustion to electrical energy has led to a series of environmental problems and regional conflicts.On the other hand,thermal energy is considered a ubiquitous renewable energy source.The heat process generates more than 90%of the energy we use,and the wasted energy is ultimately dissipated as heat.The thermoelectrics is the simplest technology applicable to direct heat-to-electricity energy conversion.In this work,utilizing the first-principle calculation combined with the Boltzmann transport equation?BTE?and semiclassical analysis,we have systematically investigated the electronic structure,lattice thermal conductivity ?_L,Seebeck coefficient S,and the dimensionless figure of merit ZT for skutterudites?CoSb₃ and IrSb₃?,alkalis semiconductors?LiH,NaH,Li₃Sb and Li₃Bi?.For skutterudites,the electronic structure and thermal transport properties as a function of hydrostatic pressure P are studied.Interestingly,as the pressure increases,the band gap and?_L have an approximate parabolic trend,which results in excellent thermoelectric properties,and ZT even exceeds 1.4?1.09?in IrSb₃?CoSb₃?at 54?58?GPa and at 1000 K.This anomalous behavior arises from the electron distribution and intrinsic scattering processes.Further analyses indicate that?i?unbonded electron pairs of Sb atoms are gradually transferred to the region between Co?Ir?and Sb atoms as the pressure increases,which leads to the formation of partial metallic bond and thus the band gap expands firstly and then shrinks;?ii?the change of the strength of anharmonic phonon scattering process results in the variation of?_L.As a results,these behaviors cause excellent thermoelectric?TE?properties.Combining the first principles calculations with the quasiharmonic approximation and BTE,we take the light alkalis semiconductors ionic crystals LiH and NaH as example to systematically investigate the lattice thermal properties.Remarkable,the thermal expansion effect plays a significant role in heat transport process,which makes thereduce about 40%compared with the value excluding the thermal expansion effect.As a result,the calculated?_L of LiH is 14.67?12.98?W/mK at 300?327?K,which is well consistent with the experimental value of 14.70?12.47?W/mK at the same temperature.Our analyses reveal that the thermal expansion effect lead to the reduction of phonon frequency and group velocity,the enhancement of phonon scattering processes and scattering rate,and consequently the reduction of?_L.In addition,we also present a systematic investigation of the TE properties of heavier alkalis semiconductors Li₃Sb and Li₃Bi.The?_L of 2.2 and 2.09 W/mK are obtained at room temperature in Li₃Sb and Li₃Bi systems with the band gap of 0.68 and 0.34 eV.The low?_L can induce excellent TE properties.Thus,the effect of doping on the transport properties has been judiciously researched and the maximum ZT of 2.42,1.54 is obtained at 900 K in the p-type doped Li₃Sb and p-type doped Li₃Bi with the stable structures.Up to date,the experimental finding of the maximum ZT is 2.6 at850 K,our results are very close to this value.Meanwhile,the lattice thermal transport properties of two-dimensions materials,?,?,?graphyne,a class of graphene allotropes,are studied.Strikingly,at room temperature,a low?_L of21.11,22.3 and 106.24 W/mk are obtained in?,?,?graphyne,which are much lower than that of graphene.We observe contributions from the phonon modes below the specified frequency and find that many optical phonon modes play critical roles in the phonon scattering process,thus,leading to the low?_L values.