Theoretical Study On Thermal Transport Regulation And Thermoelectric Properties Of Graphyne

Thermoelectric conversion is a green new energy conversion technology,showing a great potential in the field of sustainable energy.Thermoelectric efficiency can usually be expressed by the figure of merit ZT,(5=²/,where S is the Seebeck coefficient,σis the electrical conductivity,andκis the thermal conductivity,T is absolute temperature,so excellent thermoelectric materials have the properties of high Seebeck coefficient,high electrical conductivity,and low thermal conductivity.The application of carbon-based nanomaterials in the field of thermoelectric is enormously restricted to their high thermal conductivity.Graphyne,as an allotrope of graphene,expresses low thermal conductivity and high carrier mobility.Therefore,it can be expected to expand the application of carbon-based materials in thermoelectric field.In this work,molecular dynamics and first-principles methods are used to calculate the thermal transport,electrical transport and thermoelectric properties of the graphyne family.And then we investigate the regulation methods of the thermal and electrical transport of graphyne to improve its thermoelectric properties.Some meaningful results and feasible schemes are provided for the thermoelectric research of graphyne.Firstly,by using the first-principles calculation combined with the Boltzmann theory,the thermoelectric properties of types of graphynes,i.e.,6,6,6-,6,6,12-,14,14,18-and 18,18,18-graphyne,are systematically investigated.We found that the thermal conductivity of four graphynes is much smaller than that of graphene,which is expected to become an excellent thermoelectric material;The ZT of 6,6,12-graphyne expresses extremely significant anisotropy,whose value along the zigzag direction is even 1～2 orders of magnitude higher than that along the armchair direction;The ZT of 18,18,18-graphyne shows completel y opposite anisotropy comparing with other three t ypes of graphynes,its ZT value along armchair direction is higher;Comparing with the ZT at high temperature range,14,14,18-graphyne exhibits abnormal higher ZT at low temperature,whose maximum value approaches to 1.6,which is caused by the abnormal temperature dependence of its Seebeck coefficient peak,therefore,it can be seen as an excellent carbon-based thermoelectric material.More importantly,its ZT at low temperature is abnormally higher than that at high temperature,breaking the traditional view that the higher ZT exists in high temperature range.Secondly,we explore the effects of polycrystallization and amorphization on the thermal transport properties of graphyne（suppressing thermal conductivity to enhance ZT）.Polycrystallization greatly reduces the thermal conductivity of graphyne,and the reduction rate even reaches 90%.Adjusting the crystal orientation of polycrystalline graphyne can further regulate its thermal conductivity;Compared with polycrystallization,amorphization is more prominent in reducing the graphyne’s thermal conductivity,and the reason for this phenomenon is that the amorphous graphyne has more low-frequency phonons excited to the mid-frequency phonon frequency range,so that the contribution of low-frequency phonons to thermal conductivity decreases.Finally,we deeply study the effects of adding strain on the electrical transport properties of graphyne（improve electrical transport performance to enhance ZT）.In terms of energy band,as the applied strain gradually increases（-2%～2%）,the conduction band of 14,14,18-graphyne moves upward as a whole,so that its direct band gap increases from 0.078 e V to 0.128 e V,the increase range even reaches 64%;We also found that the Seebeck coefficient of 14,14,18-graphyne increases gradually with the strain from compression to tension（-2%～2%）in the low carrier concentration range(10¹¹～10¹² cm^-2);Further,the PF value gradually increases with the increase of strain in the low carrier concentration range(10¹¹～10¹² cm^-2).