Studies On The Thermal Transport Properties Of Two-dimensional Hexagonal Nitrides And Thermoelectric Properties Of Strontium Titanate

Thermal conductivity is an important parameter for materials.Seeking for materials which meet specific needs is always an import subject.With the increasing of power density in electronic devices,materials with high thermal conductivity used as heat spreader have gained much attention.On the other hand,low conductivity is one of the key factors for high conversion efficiency in thermoelectric materials.In the present work,we studied the thermal conductivity of hexagonal boron nitride laminated and SrTiO3.Using first principle density functional theory combined with Boltzmann transport theory,we also investigated the lattice thermal conductivity for monolayer hexagonal nitrides and thermoelectric power factor for(SrO)m(SrTiO3)n superlattice.The main results are as below:(1)Hexagonal boron nitride(hBN)laminates were produced using hBN ink.These films consisted of hBN flaked with lateral dimensions of 1μm and an average thickness of about lOnm.The thermal conductivity of the hBN laminate films can be tuned by varying the density through rolling compression.The hBN films exhibit a high thermal conductivity of up to 20W/mK at 100℃,which is larger than that for materials used in thermal management.Being electrically insulating,hBN laminates has potential application in thermal management.(2)The intrinsic lattice thermal conductivity of the monolayer hexagonal nitrides(hBN,hAlN,hGaN,hInN)have been investigated using first-principle calculations combined with Boltzmann transport theory.The phonon dispersions indicate the studied materials are dynamically stable.With the increasing of the atomic number,the thermal conductivity of the monolayer hexagonal nitrides decreases.Compared to the wurtzite counterpart,hAIN remains a relatively high thermal conductivity of 111.9W/mK at 300K,while for hInN this value decreased to 6.1W/mK,which is only 1/20 of that of wurtzite InN.To analyzing the size dependence of thermal conductivity,we calculated the cumulative thermal conductivity as a function of phonon mean free path.The characteristic lengths at which the thermal conductivity can be significantly reduced by nanostructure is 6 μm for hBN and 190nm for hInN.(3)We showed that the thermal conductivity of SrTiO3 can be reduced by both vacuum annealing and SCCO2 treatment.An integrated study of Fourier transform infrared spectroscopy,scanning electron microscopy and X-ray absorption fine structure revealed that the increasing and segregation of oxygen vacancies is responsible for the reduction of thermal conductivity for the annealed SrTiO3,while for the SCCO2 treated sample,the reduced thermal conductivity is caused by the point defect related to the H ions.(4)The electronic structures and thermoelectric properties of(SrO)m(SrTiO3)n superlattices have been investigated using first-principles calculations combined with Boltzmann transport theory.Due to the much reduced dispersion along the c-axis,the thermoelectric properties for n-typesuperlattice are found to be highly anisotropic with the in-plane electrical conductivitywith respect to relaxation time much higher thanthe out-of-plane one.The reduction of in-plane Seebeck coefficient compared with SrTiO3 results in a slightly reduced power factor with respect to relaxation time for n-type doped(SrO)m(SrTiO3)n·However,both Seebeck coefficient and electrical conductivity with respect to relaxation time are relatively maintainedfor p-type doping,leading to a comparable power factorwith respect to relaxation time.If the reduced thermal conductivity is taken into account,an improved ZT value can be expected for(SrO)m(SrTiO3)n superlattice.