We study the acoustomagnetoelectric (AME) effect in two-dimensional graphene with an energy bandgap using the semiclassical Boltzmann transport equation within the hypersound regime, (where represents the acoustic wav...We study the acoustomagnetoelectric (AME) effect in two-dimensional graphene with an energy bandgap using the semiclassical Boltzmann transport equation within the hypersound regime, (where represents the acoustic wavenumber and is the mean free path of the electron). The Boltzmann transport equation and other relevant equations were solved analytically to obtain an expression for the AME current density, consisting of longitudinal and Hall components. Our numerical results indicate that both components of the AME current densities display oscillatory behaviour. Furthermore, geometric resonances and Weiss oscillations were each defined using the relationship between the current density and Surface Acoustic Wave (SAW) frequency and the inverse of the applied magnetic field, respectively. Our results show that the AME current density of bandgap graphene, which can be controlled to suit a particular electronic device application, is smaller than that of (gapless) graphene and is therefore, more suited for nanophotonic device applications.展开更多
Understanding thermal transport at the submicron scale is crucial for engineering applications,especially in the thermal management of electronics and tailoring the thermal conductivity of thermoelectric materials.At ...Understanding thermal transport at the submicron scale is crucial for engineering applications,especially in the thermal management of electronics and tailoring the thermal conductivity of thermoelectric materials.At the submicron scale,the macroscopic heat diffusion equation is no longer valid and the phonon Boltzmann transport equation(BTE)becomes the governing equation for thermal transport.However,previous thermal simulations based on the phonon BTE have two main limitations:relying on empirical parameters and prohibitive computational costs.Therefore,the phonon BTE is commonly used for qualitatively studying the non-Fourier thermal transport phenomena of toy problems.In this work,we demonstrate an ultra-efficient and parameter-free computational method of the phonon BTE to achieve quantitatively accurate thermal simulation for realistic materials and devices.By properly integrating the phonon properties from first-principles calculations,our method does not rely on empirical material properties input.It can be generally applicable for different materials and the predicted results can match well with experimental results.Moreover,by developing a suitable ensemble of advanced numerical algorithms,our method exhibits superior numerical efficiency.The full-scale(from ballistic to diffusive)thermal simulation of a 3-dimensional fin field-effect transistor with 13 million degrees of freedom,which is prohibitive for existing phonon BTE solvers even on supercomputers,can now be completed within two hours on a single personal computer.Our method makes it possible to achieve the predictive design of realistic nanostructures for the desired thermal conductivity.It also enables accurately resolving the temperature profiles at the transistor level,which helps in better understanding the self-heating effect of electronics.展开更多
In this paper, a novel model is proposed to investigate the neutron transport in scattering and absorbing medium. This solution to the linear Boltzmann equation is expanded from the idea of lattice Boltzmann method(LB...In this paper, a novel model is proposed to investigate the neutron transport in scattering and absorbing medium. This solution to the linear Boltzmann equation is expanded from the idea of lattice Boltzmann method(LBM) with the collision and streaming process. The theoretical derivation of lattice Boltzmann model for transient neutron transport problem is proposed for the first time.The fully implicit backward difference scheme is used to ensure the numerical stability, and relaxation time and equilibrium particle distribution function are obtained. To validate the new lattice Boltzmann model, the LBM formulation is tested for a homogenous media with different sources, and both transient and steady-state LBM results get a good agreement with the benchmark solutions.展开更多
This article presents an extensive review of the status of the search of the dark matter. The first eight sections are devoted to topics in dark matter and its experimental searches, and the rest to selected topics in...This article presents an extensive review of the status of the search of the dark matter. The first eight sections are devoted to topics in dark matter and its experimental searches, and the rest to selected topics in astrophysics and cosmology, which are intended to supply some of the needed background for students in particle physics. Sections 9 and 13 are introductory cosmology. The three astrophysical topics, Big Bang nucleosynthesis Section 10, Boltzmann transport equation and freeze out of massive particles Section 11, and CMB anisotropy Section 12 can all be studied in analytical approaches when reasonable approximations are made. Their original analytically forms, to which this article follows very closely, were given by particle physicists. Dark matter is an evolving subject requiring timely update to stay current. Hence a review of such a subject matter would undoubtedly have something wanting when it appears in print. It is hoped that this review can form a humble basis for those graduate students who would like to pursue the subject of dark matter. The reader can use the extensive table of contents to see in some details the materials covered in the article.展开更多
We propose an optimized scheme to determine the smearing parameter in the Gaussian function that is used to replace the Dirac δ function in the first Brillouin zone sampling. The broadening width is derived by analyz...We propose an optimized scheme to determine the smearing parameter in the Gaussian function that is used to replace the Dirac δ function in the first Brillouin zone sampling. The broadening width is derived by analyzing the difference of the results from the phase-space method and Gaussian broadening method. As a demonstration, using the present approach,we investigate the phonon transport in a typical layered material, graphite. Our scheme is benchmarked by comparing with other zone sampling methods. Both the three-phonon phonon scattering rates and thermal conductivity are consistent with the prediction from the widely used tetrahedron method and adaptive broadening method. The computational efficiency of our scheme is more than one order of magnitude higher than the two other methods. Furthermore, the effect of fourphonon scattering in phonon transport in graphite is also investigated. It is found that four-phonon scattering reduces the through-plane thermal conductivity by 10%. Our methods could be a reference for the prediction of thermal conductivity of anisotropic material in the future.展开更多
The performance of double gate GaSb nMOSFETs with surface orientations of（100） and（111） are compared by deterministically solving the time-dependent Boltzmann transport equation（BTE）.Results show that the on-sta...The performance of double gate GaSb nMOSFETs with surface orientations of（100） and（111） are compared by deterministically solving the time-dependent Boltzmann transport equation（BTE）.Results show that the on-state current of the device with（111） surface orientation is almost three times larger than the（100） case due to the higher injection velocity.Moreover,the scattering rate of the（111） device is slightly lower than that of the（100） device.展开更多
In order to consider quantum transport under the influence of an electron-electron （e-e） interaction in a mesoscopic conductor,the Boltzmann equation and Poisson equation are investigated jointly.The analytical expr...In order to consider quantum transport under the influence of an electron-electron （e-e） interaction in a mesoscopic conductor,the Boltzmann equation and Poisson equation are investigated jointly.The analytical expressions of the distribution function for the Boltzmann equation and the self-consistent average potential concerned with e-e interaction are obtained,and the dielectric function appearing in the self-consistent average potential is naturally generalized beyond the Thomas-Fermi approximation.Then we apply these results to the tunneling junctions of a metal-insulator-semiconductor （MIS） in which the electrons are accumulated near the interface of the semiconductor,and we find that the e-e interaction plays an important role in the transport procedure of this system. The electronic density,electric current as well as screening Coulombic potential in this case are studied,and we reveal the time and position dependence of these physical quantities explicitly affected by the e-e interaction.展开更多
Let u = u(t, x, p) satisfy the transport equation ?u/?t+p/p0 ?u/?x= f, where f =f(t, x, p) belongs to L~p((0, T) × R~3× R~3) for 1 < p < ∞ and ?/?t+p/p0 ?/?x is the relativisticfree transport operator...Let u = u(t, x, p) satisfy the transport equation ?u/?t+p/p0 ?u/?x= f, where f =f(t, x, p) belongs to L~p((0, T) × R~3× R~3) for 1 < p < ∞ and ?/?t+p/p0 ?/?x is the relativisticfree transport operator from the relativistic Boltzmann equation. We show the regularity of ∫u(t, x, p)d p using the same method as given by Golse, Lions, Perthame and Sentis. This average regularity is considered in terms of fractional Sobolev spaces and it is very useful for the study of the existence of the solution to the Cauchy problem on the relativistic Boltzmann equation.展开更多
We have studied the axial resistivity of chiral single-walled carbon nanotubes (SWCNTs) in the presence of a combined direct current and high frequency alternating fields. We employed semiclassical Boltzmann equations...We have studied the axial resistivity of chiral single-walled carbon nanotubes (SWCNTs) in the presence of a combined direct current and high frequency alternating fields. We employed semiclassical Boltzmann equations approach and compared our results with a similar study that examined the circumferential resistivity of these unique materials. Our work shows that these materials display similar resistivity for both axial and circumferential directions and this largely depends on temperature, intensities of the applied fields and material parameters such as chiral angle. Based on these low-temperature bidirectional conductivity responses, we propose chiral SWCNTs for design of efficient optoelectronic devices.展开更多
Gallium nitride（GaN）, the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In th...Gallium nitride（GaN）, the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In this paper, we investigate the surface scattering effect on the thermal properties of GaN nanofilms. The contribution of surface scattering to phonon transport is involved in solving a Boltzmann transport equation（BTE）. The confined phonon properties of GaN nanofilms are calculated based on the elastic model. The theoretical results show that the surface scattering effect can modify the cross-plane phonon thermal conductivity of GaN nanostructures completely, resulting in the significant change of size effect on the conductivity in GaN nanofilm. Compared with the quantum confinement effect, the surface scattering leads to the order-of-magnitude reduction of the cross-plane thermal conductivity in GaN nanofilm. This work could be helpful for controlling the thermal properties of Ga N nanostructures in nanoelectronic devices through surface engineering.展开更多
This paper presents a self-contained description on the configuration of propagator method(PM)to calculate the electron velocity distribution function(EVDF) of electron swarms in gases under DC electric and magnetic f...This paper presents a self-contained description on the configuration of propagator method(PM)to calculate the electron velocity distribution function(EVDF) of electron swarms in gases under DC electric and magnetic fields crossed at a right angle. Velocity space is divided into cells with respect to three polar coordinates v,θ and f. The number of electrons in each cell is stored in three-dimensional arrays. The changes of electron velocity due to acceleration by the electric and magnetic fields and scattering by gas molecules are treated as intercellular electron transfers on the basis of the Boltzmann equation and are represented using operators called the propagators or Green’s functions. The collision propagator, assuming isotropic scattering, is basically unchanged from conventional PMs performed under electric fields without magnetic fields. On the other hand, the acceleration propagator is customized for rotational acceleration under the action of the Lorentz force. The acceleration propagator specific to the present cell configuration is analytically derived. The mean electron energy and average electron velocity vector in a model gas and SF6 were derived from the EVDF as a demonstration of the PM under the Hall deflection and they were in a fine agreement with those obtained by Monte Carlo simulations. A strategy for fast relaxation is discussed, and extension of the PM for the EVDF under AC electric and DC/AC magnetic fields is outlined as well.展开更多
Using quasi time dependent semiclassical transport theory, within relaxation time approximation, we obtained coupled electronic current equations in the presence of time varying field, and based on general scattering ...Using quasi time dependent semiclassical transport theory, within relaxation time approximation, we obtained coupled electronic current equations in the presence of time varying field, and based on general scattering mechanism,. In the vicinity of Dirac points, we find that a characteristic exponent?corresponds to acoustic phonon scattering,?long range Coulomb scattering mechanism and?is short range (delta or contact potential) scattering in which the conductivity is constant of temperature. The?case is the ballistic regime. In the low energy dynamics of Dirac electrons in graphene, the effect of the time dependent electric field is to alter just the electron charge by?making electronic conductivity nonlinear. The effect of constant magnetic field at finite temperature is also considered.展开更多
Hafnium diboride(HfB_(2))is an important metallic ceramic that works in harsh environments,due to its high strength and thermal conductivity.Although the thermal conductivity of HfB_(2) has been measured,the experimen...Hafnium diboride(HfB_(2))is an important metallic ceramic that works in harsh environments,due to its high strength and thermal conductivity.Although the thermal conductivity of HfB_(2) has been measured,the experimental results are scattered.Also,the thermal transport mechanism of HfB_(2) is not well understood.In this work,we study the thermal transport in both pristine and defective HfB_(2) from first-principles calculations.For the pristine HfB_(2),the room-temperature thermal conductivities are 175.0 and 157.7 W·m^(-1)·K^(-1)on a-and c-axes,respectively,where the contributions from electron and phonon are comparable.The Lorenz number is significantly smaller than the Sommerfeld value and shows a temperature dependence,which demonstrates that the Wiedemann-Franz law cannot be used to estimate electronic thermal conductivity.The phonon-isotope and the phonon-electron scattering are non-negligible compared to the phonon-phonon scattering.For the defective HfB_(2),the grain size effects are negligible with length scales larger than 1μm.The pore can limit thermal conductivity when its occupancy is larger than 10%.The vacancy is found to induce scattered results in experiments.The phonon thermal conductivity significantly reduces even with only 1%vacancy,while the electronic thermal conductivity is not sensitive to the vacancy.Our study provides an in-depth understanding of the thermal transport in HfB_(2),and the revealed mechanisms provide important guidance on the design of HfB_(2)-based materials.展开更多
Significant advancements in nanoscale material efficiency optimization have made it feasible to substantially adjust the thermoelectric transport characteristics of materials.Motivated by the prediction and enhanced u...Significant advancements in nanoscale material efficiency optimization have made it feasible to substantially adjust the thermoelectric transport characteristics of materials.Motivated by the prediction and enhanced understanding of the behavi-or of two-dimensional(2D)bilayers(BL)of zirconium diselenide(ZrSe_(2)),hafnium diselenide(HfSe_(2)),molybdenum diselenide(MoSe_(2)),and tungsten diselenide(WSe_(2)),we investigated the thermoelectric transport properties using information generated from experimental measurements to provide inputs to work with the functions of these materials and to determine the critical factor in the trade-off between thermoelectric materials.Based on the Boltzmann transport equation(BTE)and Barden-Shockley deformation potential(DP)theory,we carried out a series of investigative calculations related to the thermoelectric properties and characterization of these materials.The calculated dimensionless figure of merit(ZT)values of 2DBL-MSe_(2)(M=Zr,Hf,Mo,W)at room temperature were 3.007,3.611,1.287,and 1.353,respectively,with convenient electronic densities.In ad-dition,the power factor is not critical in the trade-off between thermoelectric materials but it can indicate a good thermoelec-tric performance.Thus,the overall thermal conductivity and power factor must be considered to determine the preference of thermoelectric materials.展开更多
文摘We study the acoustomagnetoelectric (AME) effect in two-dimensional graphene with an energy bandgap using the semiclassical Boltzmann transport equation within the hypersound regime, (where represents the acoustic wavenumber and is the mean free path of the electron). The Boltzmann transport equation and other relevant equations were solved analytically to obtain an expression for the AME current density, consisting of longitudinal and Hall components. Our numerical results indicate that both components of the AME current densities display oscillatory behaviour. Furthermore, geometric resonances and Weiss oscillations were each defined using the relationship between the current density and Surface Acoustic Wave (SAW) frequency and the inverse of the applied magnetic field, respectively. Our results show that the AME current density of bandgap graphene, which can be controlled to suit a particular electronic device application, is smaller than that of (gapless) graphene and is therefore, more suited for nanophotonic device applications.
基金We thank Minhua Wen,Shenpeng Wang,and Yongzhi Liu from Shanghai Jiao Tong University for valuable help with parallelization.We thank Dr.Chuang Zhang from Southern University of Science and Technology for valuable discussions on the synthetic iterative method.We thank Dr.Saeid Zahiri from Petrosazan Pasargad Asia,Yucheng Shi from the University of Chicago,Xinyue Han from Carnegie Mellon University and Ziyou Wu from the University of Michigan for valuable help in developing the code.Y.H.and H.B.acknowledge the support by the National Natural Science Foundation of China(52122606).The computations in this paper were run on theπ2.0 cluster supported by the Center for High Performance Computing at Shanghai Jiao Tong University.
文摘Understanding thermal transport at the submicron scale is crucial for engineering applications,especially in the thermal management of electronics and tailoring the thermal conductivity of thermoelectric materials.At the submicron scale,the macroscopic heat diffusion equation is no longer valid and the phonon Boltzmann transport equation(BTE)becomes the governing equation for thermal transport.However,previous thermal simulations based on the phonon BTE have two main limitations:relying on empirical parameters and prohibitive computational costs.Therefore,the phonon BTE is commonly used for qualitatively studying the non-Fourier thermal transport phenomena of toy problems.In this work,we demonstrate an ultra-efficient and parameter-free computational method of the phonon BTE to achieve quantitatively accurate thermal simulation for realistic materials and devices.By properly integrating the phonon properties from first-principles calculations,our method does not rely on empirical material properties input.It can be generally applicable for different materials and the predicted results can match well with experimental results.Moreover,by developing a suitable ensemble of advanced numerical algorithms,our method exhibits superior numerical efficiency.The full-scale(from ballistic to diffusive)thermal simulation of a 3-dimensional fin field-effect transistor with 13 million degrees of freedom,which is prohibitive for existing phonon BTE solvers even on supercomputers,can now be completed within two hours on a single personal computer.Our method makes it possible to achieve the predictive design of realistic nanostructures for the desired thermal conductivity.It also enables accurately resolving the temperature profiles at the transistor level,which helps in better understanding the self-heating effect of electronics.
基金supported by the Foundation of National Key Laboratory of Reactor System Design Technology(No.HT-LW-02-2014003)the State Key Program of National Natural Science of China(No.51436009)
文摘In this paper, a novel model is proposed to investigate the neutron transport in scattering and absorbing medium. This solution to the linear Boltzmann equation is expanded from the idea of lattice Boltzmann method(LBM) with the collision and streaming process. The theoretical derivation of lattice Boltzmann model for transient neutron transport problem is proposed for the first time.The fully implicit backward difference scheme is used to ensure the numerical stability, and relaxation time and equilibrium particle distribution function are obtained. To validate the new lattice Boltzmann model, the LBM formulation is tested for a homogenous media with different sources, and both transient and steady-state LBM results get a good agreement with the benchmark solutions.
文摘This article presents an extensive review of the status of the search of the dark matter. The first eight sections are devoted to topics in dark matter and its experimental searches, and the rest to selected topics in astrophysics and cosmology, which are intended to supply some of the needed background for students in particle physics. Sections 9 and 13 are introductory cosmology. The three astrophysical topics, Big Bang nucleosynthesis Section 10, Boltzmann transport equation and freeze out of massive particles Section 11, and CMB anisotropy Section 12 can all be studied in analytical approaches when reasonable approximations are made. Their original analytically forms, to which this article follows very closely, were given by particle physicists. Dark matter is an evolving subject requiring timely update to stay current. Hence a review of such a subject matter would undoubtedly have something wanting when it appears in print. It is hoped that this review can form a humble basis for those graduate students who would like to pursue the subject of dark matter. The reader can use the extensive table of contents to see in some details the materials covered in the article.
基金support from the National Natural Science Foundation of China (Grant No. 51706134)。
文摘We propose an optimized scheme to determine the smearing parameter in the Gaussian function that is used to replace the Dirac δ function in the first Brillouin zone sampling. The broadening width is derived by analyzing the difference of the results from the phase-space method and Gaussian broadening method. As a demonstration, using the present approach,we investigate the phonon transport in a typical layered material, graphite. Our scheme is benchmarked by comparing with other zone sampling methods. Both the three-phonon phonon scattering rates and thermal conductivity are consistent with the prediction from the widely used tetrahedron method and adaptive broadening method. The computational efficiency of our scheme is more than one order of magnitude higher than the two other methods. Furthermore, the effect of fourphonon scattering in phonon transport in graphite is also investigated. It is found that four-phonon scattering reduces the through-plane thermal conductivity by 10%. Our methods could be a reference for the prediction of thermal conductivity of anisotropic material in the future.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61674008,61421005,and 61404005)
文摘The performance of double gate GaSb nMOSFETs with surface orientations of（100） and（111） are compared by deterministically solving the time-dependent Boltzmann transport equation（BTE）.Results show that the on-state current of the device with（111） surface orientation is almost three times larger than the（100） case due to the higher injection velocity.Moreover,the scattering rate of the（111） device is slightly lower than that of the（100） device.
基金Project supported by the National Natural Science Foundation of China (Grant No 10404037)
文摘In order to consider quantum transport under the influence of an electron-electron （e-e） interaction in a mesoscopic conductor,the Boltzmann equation and Poisson equation are investigated jointly.The analytical expressions of the distribution function for the Boltzmann equation and the self-consistent average potential concerned with e-e interaction are obtained,and the dielectric function appearing in the self-consistent average potential is naturally generalized beyond the Thomas-Fermi approximation.Then we apply these results to the tunneling junctions of a metal-insulator-semiconductor （MIS） in which the electrons are accumulated near the interface of the semiconductor,and we find that the e-e interaction plays an important role in the transport procedure of this system. The electronic density,electric current as well as screening Coulombic potential in this case are studied,and we reveal the time and position dependence of these physical quantities explicitly affected by the e-e interaction.
文摘Let u = u(t, x, p) satisfy the transport equation ?u/?t+p/p0 ?u/?x= f, where f =f(t, x, p) belongs to L~p((0, T) × R~3× R~3) for 1 < p < ∞ and ?/?t+p/p0 ?/?x is the relativisticfree transport operator from the relativistic Boltzmann equation. We show the regularity of ∫u(t, x, p)d p using the same method as given by Golse, Lions, Perthame and Sentis. This average regularity is considered in terms of fractional Sobolev spaces and it is very useful for the study of the existence of the solution to the Cauchy problem on the relativistic Boltzmann equation.
文摘We have studied the axial resistivity of chiral single-walled carbon nanotubes (SWCNTs) in the presence of a combined direct current and high frequency alternating fields. We employed semiclassical Boltzmann equations approach and compared our results with a similar study that examined the circumferential resistivity of these unique materials. Our work shows that these materials display similar resistivity for both axial and circumferential directions and this largely depends on temperature, intensities of the applied fields and material parameters such as chiral angle. Based on these low-temperature bidirectional conductivity responses, we propose chiral SWCNTs for design of efficient optoelectronic devices.
基金supported by the National Natural Science Foundation of China(Grant Nos.11302189 and 11321202)the Doctoral Fund of Ministry of Education of China(Grant No.20130101120175)
文摘Gallium nitride（GaN）, the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In this paper, we investigate the surface scattering effect on the thermal properties of GaN nanofilms. The contribution of surface scattering to phonon transport is involved in solving a Boltzmann transport equation（BTE）. The confined phonon properties of GaN nanofilms are calculated based on the elastic model. The theoretical results show that the surface scattering effect can modify the cross-plane phonon thermal conductivity of GaN nanostructures completely, resulting in the significant change of size effect on the conductivity in GaN nanofilm. Compared with the quantum confinement effect, the surface scattering leads to the order-of-magnitude reduction of the cross-plane thermal conductivity in GaN nanofilm. This work could be helpful for controlling the thermal properties of Ga N nanostructures in nanoelectronic devices through surface engineering.
文摘This paper presents a self-contained description on the configuration of propagator method(PM)to calculate the electron velocity distribution function(EVDF) of electron swarms in gases under DC electric and magnetic fields crossed at a right angle. Velocity space is divided into cells with respect to three polar coordinates v,θ and f. The number of electrons in each cell is stored in three-dimensional arrays. The changes of electron velocity due to acceleration by the electric and magnetic fields and scattering by gas molecules are treated as intercellular electron transfers on the basis of the Boltzmann equation and are represented using operators called the propagators or Green’s functions. The collision propagator, assuming isotropic scattering, is basically unchanged from conventional PMs performed under electric fields without magnetic fields. On the other hand, the acceleration propagator is customized for rotational acceleration under the action of the Lorentz force. The acceleration propagator specific to the present cell configuration is analytically derived. The mean electron energy and average electron velocity vector in a model gas and SF6 were derived from the EVDF as a demonstration of the PM under the Hall deflection and they were in a fine agreement with those obtained by Monte Carlo simulations. A strategy for fast relaxation is discussed, and extension of the PM for the EVDF under AC electric and DC/AC magnetic fields is outlined as well.
文摘Using quasi time dependent semiclassical transport theory, within relaxation time approximation, we obtained coupled electronic current equations in the presence of time varying field, and based on general scattering mechanism,. In the vicinity of Dirac points, we find that a characteristic exponent?corresponds to acoustic phonon scattering,?long range Coulomb scattering mechanism and?is short range (delta or contact potential) scattering in which the conductivity is constant of temperature. The?case is the ballistic regime. In the low energy dynamics of Dirac electrons in graphene, the effect of the time dependent electric field is to alter just the electron charge by?making electronic conductivity nonlinear. The effect of constant magnetic field at finite temperature is also considered.
基金financially supported by the National Natural Science Foundation of China (NSFC) (No.52122606)support by Shanghai Municipal Natural Science Foundation (No.22YF1400100)the Fundamental Research Funds for the Central Universities (No.2232022D-22)。
文摘Hafnium diboride(HfB_(2))is an important metallic ceramic that works in harsh environments,due to its high strength and thermal conductivity.Although the thermal conductivity of HfB_(2) has been measured,the experimental results are scattered.Also,the thermal transport mechanism of HfB_(2) is not well understood.In this work,we study the thermal transport in both pristine and defective HfB_(2) from first-principles calculations.For the pristine HfB_(2),the room-temperature thermal conductivities are 175.0 and 157.7 W·m^(-1)·K^(-1)on a-and c-axes,respectively,where the contributions from electron and phonon are comparable.The Lorenz number is significantly smaller than the Sommerfeld value and shows a temperature dependence,which demonstrates that the Wiedemann-Franz law cannot be used to estimate electronic thermal conductivity.The phonon-isotope and the phonon-electron scattering are non-negligible compared to the phonon-phonon scattering.For the defective HfB_(2),the grain size effects are negligible with length scales larger than 1μm.The pore can limit thermal conductivity when its occupancy is larger than 10%.The vacancy is found to induce scattered results in experiments.The phonon thermal conductivity significantly reduces even with only 1%vacancy,while the electronic thermal conductivity is not sensitive to the vacancy.Our study provides an in-depth understanding of the thermal transport in HfB_(2),and the revealed mechanisms provide important guidance on the design of HfB_(2)-based materials.
文摘Significant advancements in nanoscale material efficiency optimization have made it feasible to substantially adjust the thermoelectric transport characteristics of materials.Motivated by the prediction and enhanced understanding of the behavi-or of two-dimensional(2D)bilayers(BL)of zirconium diselenide(ZrSe_(2)),hafnium diselenide(HfSe_(2)),molybdenum diselenide(MoSe_(2)),and tungsten diselenide(WSe_(2)),we investigated the thermoelectric transport properties using information generated from experimental measurements to provide inputs to work with the functions of these materials and to determine the critical factor in the trade-off between thermoelectric materials.Based on the Boltzmann transport equation(BTE)and Barden-Shockley deformation potential(DP)theory,we carried out a series of investigative calculations related to the thermoelectric properties and characterization of these materials.The calculated dimensionless figure of merit(ZT)values of 2DBL-MSe_(2)(M=Zr,Hf,Mo,W)at room temperature were 3.007,3.611,1.287,and 1.353,respectively,with convenient electronic densities.In ad-dition,the power factor is not critical in the trade-off between thermoelectric materials but it can indicate a good thermoelec-tric performance.Thus,the overall thermal conductivity and power factor must be considered to determine the preference of thermoelectric materials.
