First-principles Study Of Thermoelectric Properties Of BN Nanoribbons

The thermoelectric effect uses the Seebeck effect to directly convert thermal energy into electrical energy.The thermoelectric energy converter manufactured by this principle can convert the heat generated by the operation of factories,power plants,vehicles and electronic equipment into electrical energy,thereby providing a new solution for recycling heat treatment and waste heat.In order to take full advantage of the advantages of thermoelectric materials,researchers have done a lot of work in finding new thermoelectric materials and improving the performance of existing thermoelectric materials,including hexagonal boron nitride and its nanoribbons.Hexagonal boron nitride(h-BN)has a layered hexagonal honeycomb structure like graphene,but due to the polarity of boron nitride bonding,the two have distinct properties in many respects.Both graphene and hexagonal boron nitride(h-BN)can be made into nanoribbons from their sheet structure or nanotube structure,and are divided into zigzag nanoribbons and armchair nanoribbons according to the edge type.Excellent thermoelectric performance requires materials with high conductivity,high Seebeck coefficient and low thermal conductivity.Although graphene nanoribbons(GNR)have high thermal conductivity,their quantum effects as low-dimensional materials have great potential for thermoelectric applications.The thermoelectric properties of boron nitride nanoribbons(BNNR)are the main research purposes of this paper.By modeling the boron nitride nanoribbons,the electronic structure of the nanoribbons was simulated by the first-principles-based VASP software,and the energy band structure and density of states of the nanoribbons were obtained.Then,by the BoltzTrap program based on the Boltzmann equation,the self-consistent result file in VASP is processed to obtain the Seebeck coefficient,conductivity ratio relaxation time and electron thermal conductivity ratio relaxation time curve of the nanoribbon.At the same time,we calculated the relaxation time and obtained the values of conductivity and electron thermal conductivity.After using Phonopy and ShengBTE to generate a file for recording the displacement of the atomic coordinates,the calculation of the second and third order atomic force constants is carried out by means of VASP,and the lattice thermal conductivity of the nanoribbons is calculated by ShengBTE.Finally,the calculation based on the ZT value is obtained.The formula gives the thermoelectric figure of merit of the nanoribbon.In this paper,four different temperature conditions of 100K,300K,600K and 800K are selected to analyze the thermoelectric performance of BNNR.The results show that the zigzag boron nitride nanoribbon(4ZBNNR)and the armchair boron nitride nanoribbon(4ABNNR)Seebeck coefficient with a width of 4 decrease with the increase of temperature,and the maximum value can reach 0.003V/K;The thermoelectric power factor of 4ZBNNR reaches a maximum of 0.218 W·m-1·K-2 at 800K,while the 4ABNNR achieves a maximum of 0.00233 W·m-1·K-2 at 300K.Finally,the ZT value of 4ABNNR is close to 1,and it is less affected by temperature.The ZT value of 4ZBNNR increases with increasing temperature,and the maximum ZT value reaches 0.975.