Peripheral phonons and phonon conductivity of a metal at low temperatures

The role of the peripheral and non-peripheral phonons in the estimation of the lattice thermal conductivity of a metal has been studied at low temperatures by calculating their separate contributions towards the total lattice thermal conductivity. The study is made in the temperature range 0.4–2.5 K with the help of the Ziman expression for the scattering of phonons by the charge carriers and the Callaway expression of the phonon conductivity, and Sb is taken as an example. The separate percentage contributions due to peripheral and non-peripheral phonons have also been studied and it is found that the percentage contribution due to peripheral phonons increases with increasing temperature while the percentage contribution due to non-peripheral phonons decreases with increasing temperature. The percentage contributions of the lattice thermal resistivities due to electrons and holes towards the total lattice thermal resistivity of Sb have also been reported in the present note.     

Contribution to the lattice thermal conductivity due to the correction term in the frame of the generalized callaway integral: Application to Ge

The contribution to the lattice thermal conductivity due to the correction term (due to the three phonon normal processes) has been studied at very low temperatures in the frame of the generalized Callaway integral by deriving analytical expressions for it. The contribution of the correction term towards total phonon conductivity of Ge has been calculated in the temperature range 1–5 K and negligible contribution is found due to it.     

Thermal conductivity of GaAs／A1As superlattices： The Umklapp process

The thermal conductivity of GaAs/AlAs superlattices limited by the three-phonon Umklapp process and boundary scattering has been studied theoretically based on the model of lattice dynamics with force constant matrix. It was found that the Umklapp relaxation rate approximates BTω^2/n with a fitting parameter B. The thermal conductivity increases with the increase of temperature at low temperatures, and would show a peak behaviour at about 60K before falling off at high temperatures. In addition, the thermal conductivity increases with the increase of period thickness of the superlattices.     

Heat transfer at low temperatures: Application to sapphire

It is found that at low temperatures, the Callaway expression for the lattice thermal conductivity of a sample having dislocations reduces to the expression obtained by Dubey based on the nonlinear heat transfer theory. The obtained expression is used to explain the measurements of Wolfimeyer and Dillinger of phonon conductivity of sapphire in the temperature range 0.4–4 K.     

Molecular-dynamics calculation of the thermal conductivity coefficient of the germanium single crystal

The thermal conductivity coefficient of the germanium crystal lattice has been calculated by molecular dynamics simulation. Calculations have been performed for both the perfect crystal lattice and the crystal lattice with defects such as monovacancies. For the perfect germanium single crystal, the dependence of the thermal conductivity coefficient on the lattice temperature has been obtained in the temperature range of 150–1000 K. The thermal conductivity coefficient of the germanium lattice has been calculated as a function of the monovacancy concentration.     

Low temperature specific heat and thermal conductivity of bulk metallic glass (Cu50Zr50)94Al6

The low temperature specific heat and thermal conductivity of (Cu₅₀Zr₅₀)₉₄Al₆ bulk metallic glass have been studied experimentally. A low temperature anomaly in the specific heat is observed in this alloy. It is also found that in addition to Debye oscillators, the localized vibration modes whose vibration density of state has a Gaussian distribution should be considered to explain the low temperature phonon specific heat anomaly. The phonon thermal conductivity dependence on temperature for the sample does not show apparent plateau characteristics as other glass materials do; however, the influence of the resonant scattering from the localized modes on the lattice thermal conductivity is prominent in the bulk metallic glass at low temperatures.     

基于量子修正的石墨烯纳米带热导率分子动力学表征方法

本文提出了基于量子修正的非平衡态分子动力学模型,可用于石墨烯纳米带热导率的表征.利用该模型对不同温度下,不同手性及宽度的石墨烯纳米带热导率进行了研究,结果发现:相较于经典分子动力学模型给出的热导率随温度升高而单调下降的结论,在低于Debye温度的情况下,量子修正模型的计算结果出现了反常现象.本文研究还发现,石墨烯纳米带的热导率呈现出明显的边缘效应及尺度效应:锯齿型石墨烯纳米带的热导率明显高于扶手椅型石墨烯纳米带;全温段的热导率及热导率在低温段随温度变化的斜率均随宽度的增加而增大.最后,文章用Boltzmann声子散射理论对低温段的温度效应及尺度效应进行了阐释,其理论分析结果说明文章所建模型适合在全温段范围内对不同宽度和不同手性的热导率进行精确计算,可为石墨烯纳米带在传热散热领域的应用提供理论计算和分析依据.     

Thermal conductivity of (La0.25Pr0.75)0.7Ca0.3MnO3 under giant isotope effect conditions

The thermal conductivity of (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ manganite has been studied. The isotope substitution of ¹⁸O for ¹⁶O in this compound leads to a ferromagnetic-antiferromagnetic phase transition at low temperatures. It has been found that the thermal conductivity in the ferromagnetic state is approximately two times higher than in the antiferromagnetic state. It has been shown that the small value of thermal conductivity and its temperature dependence can be due to strong phonon scattering from crystal lattice defects, which are thought of as Jahn-Teller distortions. The parameters of this scattering can be determined within the Debye model of thermal conductivity from a comparison of samples differing in their isotope composition.     

Electrical conductivity of the semiconducting phase in vanadium dioxide single crystals

The electrical conductivity of the semiconducting phase of vanadium dioxide single crystals is studied over a wide range of temperatures. It is shown that the electrical conductivity varies with temperature as log σ ～ T in the range 340–170 K and as log σ ～ 1/T at temperatures below 120 K. The experimental results are described in the framework of the model in which the temperature dependence of the hopping conductivity of small-radius polarons is determined by the dependence of the resonance integral on the amplitude of thermal lattice vibrations.     

Thermal transport in CuCl2 intercalated pitch-derived carbon fibers

The effect of intercalation with CuCl₂ on the temperature variation of the thermal conductivity, thermopower and electrical resistivity of pitch-derived carbon fibers in the temperature range 2<T<300 K is studied. At high temperature, the lattice thermal conductivity is smaller than that of the pristine fibers while at low temperature an enhancement of both the lattice and the electronic thermal conductivities is observed. The thermopower is positive and exhibits a temperature dependence very similar to that generally reported in acceptor-type graphite intercalation compounds.     

Effect of thermal conductivity on ultrasonic attenuation in praseodymium monochalcogenides

The ultrasonic attenuation in intermetallic praseodymium monochalcogenides are evaluated in the temperature interval 100–500 K along the crystallographic directions 〈100〉, 〈110〉, and 〈111〉 for longitudinal and shear waves. A comparison has been made with lanthanum monochalcogenides and other similar materials. Ultrasonic attenuation at different temperatures is mainly affected by the lattice thermal conductivity values of the materials at these temperatures. Thermoelastic loss is very small in comparison to the attenuation due to phonon-phonon interaction mechanism at higher temperatures.     

具有本征低晶格热导率的硫化银快离子导体的热电性能

硫化银(Ag_2S)是一种典型的快离子导体材料,前期关于Ag_2S的研究主要集中在光电和生物等领域.最近的研究表明, a-Ag_2S具有和金属一样的良好延展性和变形能力.但是, Ag_2S的热电性能尚无公开报道.本工作合成了单相Ag_2S化合物,系统研究了其在300—600 K范围的物相变化、离子迁移特性和电热输运性质.研究发现, Ag_2S在300—600 K温度区间表现出半导体的电输运性质.由于单斜-体心立方相晶体结构转变, Ag_2S的离子电导率、载流子浓度、迁移率、电导率、泽贝克系数等性质在455 K前后出现急剧变化.在550 K, Ag_2S的功率因子最高可达5μW·cm~(–1)·K~(–2). Ag_2S在300—600 K温度区间均表现出本征的低晶格热导率(低于0.6 W·m~(–1)·K~(–1)). S亚晶格中随机分布的类液态Ag离子是导致b-Ag_2S体心立方相具有低晶格热导率的主要原因.在573 K, Ag_2S的热电优值可达0.55,与Ag_2Se, Ag_2Te, CuAgSe等已报道的Ag基快离子导体热电材料的性能相当.     

Calculation of the thermal conductivity and the specific heat of the lattice for a spin-boson system at low temperatures

The thermal conductivity of the lattice is calculated for a spin-boson system using the method of inverse linear response theory. The spin-boson Hamiltonian consists of coupled two-level systems (TLS) represented by pseudospin operators, acoustical phonons and an interaction between both systems. The lattice specific heat is determined in the framework of perturbation theory with respect to spin-boson coupling. For the calculation of both quantities results for spin-correlation functions derived within self-consistent random phase approximation are applied. At low temperatures the influence of spin-boson coupling and of interacting TLS on the temperature dependence of the lattice thermal conductivity and of the lattice specific heat is discussed and compared with corresponding experimental investigations.     

First principle investigation of the electronic and thermoelectric properties of Mg_2C

In this paper, electronic and thermoelectric properties of Mg_2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters,bulk modulus, band gap and thermoelectric properties(Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg_2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor(PF) increases with temperature. The thermoelectric properties of Mg_2C have been calculated in a temperature range of 100 K–1200 K.     

Soliton microdynamics of heat conduction in plutonium and uranium in the temperature range of martensitic phase transitions

The microdynamics of large-amplitude nonlinear lattice vibrations of plutonium and uranium has been investigated at high reactor temperatures in the ranges of martensitic phase transitions. Solutions of nonlinear dynamic equations have been obtained using the Lennard-Jones interatomic potential for the soliton generation and the energy transfer by solitons between the crystal boundaries in the shells. The synchronism of the soliton trajectories and peaks of energy fluxes demonstrates an analog of the shot effect. The temperature dependences of thermal conductivity coefficients are consistent with the experiment and exhibit local maxima in the ranges of phase transitions. A spectral analysis has revealed that the dominant heat transfer is provided by rarefaction solitons. The martensitic phase transitions are accompanied by a reorganization of the spectral density in the phase plane with a sharp increase in the high-frequency range. The spectral density has maxima of the quasi-biphonon type. The obtained data in dimensionless form, apart from plutonium and uranium, can be used for other monatomic crystals. The specific features of the thermal conductivity and microdynamics of the formation of vacancies and pores in crystals without shells have been discussed.     

Temperature and concentration dependences of thermal conductivity of solid solutions of gadolinium and dysprosium sulfides

Thermal conductivity of a number of solid solutions of gadolinium and dysprosium sulfides has been studied experimentally within the temperature range 80-400 K. The work offers the data on thermal conductivity coefficient and lattice thermal conductivity of the studied samples. It was found that replacement of gadolinium ions by dysprosium ions leads to significant decrease of the samples?? thermal conductivity and changes its temperature dependence character due to the resonance scattering of phonons by paramagnetic ions of dysprosium. Influence of this mechanism of phonon scattering conditions the area of anomalous change observed on the concentration dependence of thermal conductivity coefficient.     

Nb掺杂对还原性烧结的TiO2-δ陶瓷的晶体结构及热电性能的影响

利用传统的固相反应法在还原性气氛1200 ℃下分别制备出不同Nb掺杂量的Ti_1-xNb_xO_2-δ陶瓷样品.样品的粉末X射线衍射(XRD)结果显示:Nb掺杂量x较低时样品为多相混合,当Nb掺杂量x>0.02时样品为单一的四方相金红石型结构.在室温到900 K的测试温区,测试了单相样品(x=0.02,0.03,0.04)的电导率、Seebeck系数和热导率.测试 关键词： 氧化钛陶瓷 热电性能 氧空位     

尿素夹层二硫化钛化合物的合成及其热、电输运性质研究(英文)

我们成功的制备出了含不同成分尿素的夹层化合物(NH2CONH2)xTiS2,并研究了它们在低温(5～310K)下的热、电输运性质.夹层后晶格常数、热、电输运行为的变化也证实了尿素分子成功的夹层到TiS2的范德华尔斯层间.研究结果表明:尿素分子的夹层成功的降低了材料的热导率,特别是在浓度为12.5mmol/L尿素的苯溶液中合成的样品的热导率大约1.5Wm-1 K-1(仅为纯TiS2的50%),而且尿素分子的夹层使得材料在低温下的导电行为从金属性转变为半导体性.     

Thermal conductivity of UC1 − xNx from 100 to 1000 °K

The thermal diffusivities of UC_{1 ? x}N_x of several compositions were measured from 100 to 1000 °K by a laser flash method. The thermal conductivity was separated into electronic and phonon components by assuming the constant Lorenz number. The phonon conductivity showed an anomalous behaviour against composition at low temperatures. The total thermal conductivity of UC_{1 ? x}N_x showed a minimum above 300 °K at an intermediate composition which moved to higher carbon content with increasing temperature. This behaviour was explained by the temperature dependence of the lattice and electronic components.     

Heat transfer in solid HD at low temperatures

The lattice thermal conductivity of solid HD has been calculated in the temperature range 0.2–4°K. The important scatterers of phonons are found to be boundary walls of the crystal, isotopic impurities, phonons and molecules of ortho hydrogen and para deuterium. The presence of molecules of ortho hydrogen and para deuterium in solid HD which act as rotational impurities, are responsible for one and two phonon scattering processes in the system. The entire study is based on the Callaway model of the lattice thermal conductivity of an insulator. Excellent agreement is found between calculated and experimental values of phonon conductivity. The extra lattice thermal resistivity due to the presence of the ortho hydrogen and para deuterium is also calculated.