二维CdO的低晶格热导率研究

寻求具有较小晶格热导率k_lat的高热电性能的二维材料具有重要意义。基于从头计算和声子玻耳兹曼输运理论,该研究首先对二维CdO结构进行优化,并通过计算声子谱验证了单层CdO的动力学稳定性。在此基础上详细研究了单层CdO的声子输运性质。计算表明在室温下单层CdO的晶格热导率k_lat约为5.7 W/(m·K),低于单层石墨烯、磷烯、黑磷和MoS₂等二维材料的晶格热导率。其中,Z方向声学模式(Z-direction acoustic, ZA),横声学支(transverse acoustic,TA),纵声学支(longitudinal acoustic, LA),Z方向光学模式(Z-direction optical, ZO),横光学支(transverse optical, TO),纵光学支(longitudinal optical, LO)对k_lat的百分比贡献分别为73.7％、13.9％、3.7％、2.8％、4.7％和1.2％。研究发现,ZA、TA、LA声学支和光学支之间的强散射是导致单层CdO低热导率的原因。本文计算结果可用于指导基于CdO的低维热电器件的设计。     

立方晶系Ca2Si声子色散和热力学性质的第一性原理计算

采用模守恒贋势和超软贋势两种方法分别计算立方晶系Ca2Si的声子色散关系、声子态密度和热力学特性.运用线性响应方法和有限位移方法确定立方晶系Ca2Si的声子色散关系和声子态密度,得到立方晶系Ca2Si的声子振动模式是各向异性,且立方晶系的Ca2Si是不稳定结构.在声子色散关系确定的基础上分别计算定容热容、焓、熵和自由能随温度的变化关系,得到Cv-T曲线在低温条件下符合德拜第三定律;当T=1000K时,立方晶系Ca2Si的热熔Cr=21J·mol-1 ·K-1;在高温条件下,随着温度的增加Cv→3R.通过分析声子色散、振动关系和热力学特性得到立方晶系Ca2Si的振动系统由声子和电子组成.     

Temperature dependence of Raman‐active mode frequencies and linewidths in TlGaSe2 layered crystals

Raman spectra of TlGaSe₂ crystal at different temperatures are discussed. The temperature dependence of frequency shifts and linewidths of the Raman peaks in the frequency region of 10‐320 cm^‐1 have been measured in the range from 50 to 320 K. The analysis of the experimental data showed that the temperature dependencies of phonon frequencies and linewidths are well described by considering the contributions from thermal expansion and lattice anharmonicity. The anharmonic contribution (phonon‐phonon coupling) is found to be due to three‐phonon processes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anharmonicity in GaTe layered crystals

The temperature dependencies (10‐300 K) of seven Raman‐active mode frequencies in layered semiconductor gallium telluride have been measured in the frequency range from 25 to 300 cm^‐1. Softening and broadening of the optical phonon lines are observed with increasing temperature. Comparison between the experimental data and theories of the shift of the phonon lines during heating of the crystal showed that the experimental dependencies can be explained by contributions from thermal expansion and lattice anharmonicity. Lattice anharmonicity is determined to be due to threephonon processes.     

X-ray study of debye-waller factors and debye characteristic temperatures in AgGa(S1−xSex)2 solid solutions

The Debye-Waller factors and Debye characteristic temperatures for two principal crystallographic directions in the AgGaS₂–AgGaSe₂ solid solutions with chalcopyrite structure were determined by an X-ray powder diffractometry technique as a function of composition and temperature in the range from 80 to 700 K. The anisotropy of both the dynamical characteristics is shown to exist and the character of anisotropy changes with composition. The composition dependences of the Debye-Waller factors and Debye temperatures are non-linear. From the experimental data some thermodynamic properties were calculated and the variation of all investigated dynamical characteristics during anion substitution was analyzed.     

Mg2Sn机械性质及热电输运性能的第一性原理计算

窄带隙半导体材料Mg₂Sn具有丰度大、密度低、无毒及环境友好等特点,是中低温热电材料的优秀候选者。本文基于密度泛函理论,结合不同形式的电子交换关联能系统分析了Mg₂Sn晶体的弹性系数、声子振动谱和电子能带结构,并基于非平衡玻尔兹曼输运理论计算了Mg₂Sn的热电性能。结果表明,GGA-PBE作为电子交换关联能可以很好地拟合立方相Mg₂Sn的力学性能(声子振动谱无虚频率),其体弹性模量为42.1 GPa且各向同性。同时,当工作温度高于300 K,Mg₂Sn的德拜温度开始趋于平缓且不高于315 K,表明Mg₂Sn在该温度区域内具有低声子热导率。使用B3LYP作为电子交换关联能可以估算Mg₂Sn费米能级附近的电子结构,发现价带顶附近存在三重简并态。同时计算结果表明,Mg₂Sn p型掺杂下的热电优值(ZT值)优于n型掺杂,可达1.05。本研究结果为进一步优化Mg₂Sn热电性能的研究提供借鉴。     

Structural and optical properties of CdO nanowires synthesized from Cd(OH)2 precursors by calcination

CdO nanowires were produced by calcination process using Cd(OH)₂nanowires as precursors. The Cd(OH)₂ nanowires were synthesized via arc discharge method submerged in de‐ionized water. Transmission electron microscopy (TEM) analysis of the as‐synthesized Cd(OH)₂ nanowires revealed that nanowire morphology was abundant form with the diameters range from 5 to 40 nm. In addition to the nanowire morphology, Cd(OH)₂ nanospheres and hexagonal shaped nanoparticles were also displayed. The Cd(OH)₂ nanostructures were used as precursors to produce CdO nanowires and calcinated in air at 400 °C for four hours. After calcination, the structural, morphological and optical properties of the as‐synthesized CdO nanowires were characterized by means of TEM, selected area electron diffraction (SAED), X‐ray diffraction (XRD) and UV‐vis spectroscopy. The XRD and SAED techniques showed that the as‐synthesized Cd(OH)₂ nanostructures could be transformed into CdO nanostructures after the calcination process. TEM results revealed that the as‐synthesized CdO nanowires were 5–30 nm in diameter and shorter than corresponding Cd(OH)₂ nanowires. In addition, the diameters of the spherical or irregular CdO nanoparticles ranged from 20 nm to 50 nm. UV‐vis spectroscopy analysis was showed that the direct gap of the CdO nanowires were found to be 2.60 eV which is slightly higher than the earlier reported values of the bulk CdO for direct band gaps (2.3 eV) due to quantum size effect. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-temperature heat capacity,thermodynamic properties and debye temperature of CuGe2P3

The heat capacity at constant pressure of CuGe₂P₃ is measured in the temperature range from 180 to 550 K. Standard enthalpies and entropies relative to 298.15 K are calculated from the heat capacity data. The Debye temperature in the high-temperature limit is estimated to be about 710 K.     

Thermal conductivity of FeS2 pyrite crystals in the temperature range 50–300 K

The thermal conductivity of two single-crystal samples of pyrite FeS₂ are investigated by the method of stationary longitudinal heat flux in the temperature range 50–300 K. The low electrical conductivity of the crystals with a small impurity content causes an identical value of experimental lattice thermal conductivity. The temperature dependence of the phonon mean free path is established.     

Temperature variation of the phonon frequencies in V3Si along the [110] direction by the X-ray inelastic scattering

The investigation of the intensity of X-ray diffuse scattering by the lattice vibrations along the second-order symmetry axis in V₃Si single crystals is performed at different temperatures from 300 to 8 K and the phonon dispersion curves in this direction are determined. It is found that the phonon frequencies of all modes decrease with lowering temperature.     

X-Ray Determination of the Mean Amplitude of Vibration and Debye Temperatures of KRS-5 (TIBr0.46I0.54)

The X-ray diffraction intensities of Bragg reflections have been measured for KRS-5 (TIBr_0.16I_0.54) at room temperature. From the observed intensities the mean Debye-Waller factor and the X-ray Debye temperature have been evaluated. The values of the Debye-Waller factor, mean square amplitude of vibration and the Debye temperature are 2.73 ± 0.06 Ų, 0.104 ± 0.002 Ų and 90 ± 1 K, respectively.     

Analyse de la Raie Zéro-Phonon et des Processus de Relaxation Excitoniques dans le Spectre d'Absorption Singulet-Triplet du Cristal de 4-4′-Dichlorobenzophenone

Abstract

In this paper we discuss the dynamic scattering information which can be obtained from an analysis, at high resolution, of the triplet exciton absorption band in the molecular crystal 4-4′-dichlorobenzophenone. The zero phonon lineshapes, widths and positions are investigated as a function of temperature. At temperatures below 30 K the broadening and the shift are analyzed in terms of exchange theory involving a torsional mode of the molecule. The lifetime in the phonon promoted state is 0.75 ps over the range 5 K-30 K. The difference between the excited and ground state energies of the torsional mode is δω = -8.45 cm^?1. Above 30 K the change on linewidth is discussed in a model which implies a momentarily localization of the exciton by lattice phonons. During the localization time a weak coupling between the trapped excitation and phonons is assumed. At very low temperatures the lineshape is asymmetric, gaussian on the high energy side, lorentzian on the low energy side. This experimental result may be due to the fact that the k = 0 level is at the bottom of the exciton band.     

Heat capacity of oxide glasses measured by AC calorimetry

Heat capacities of vitreous silica, and some binary and ternary silicate, borate, and phosphate glasses were measured in the temperature range from 300 to 840 K by AC calorimetry. In this method, the conditions of measurement, especially AC frequency and sample thickness, must be examined in order to measure the heat capacity with high accuracy, and thus, an optimum condition for measuring the heat capacity was investigated. The relationship between heat capacity and chemical composition was discussed in terms of the Debye model, which can be used for calculation of the heat capacity of atomic solids. We found that heat capacities of all the samples studied here scaled with the molar heat capacity at the Debye temperature have a similar magnitude and temperature dependence, from 300 K to the glass transition points. Based on this observation, we propose an empirical equation which is composed of separate contributions of the compositional and temperature dependence of the heat capacity.     

Estimation of the debye temperature of diamond-like semiconducting compounds from bulk modul and microhardness

The reliability of estimation of Debye temperature θ_D evaluated from bulk modul and microhardness is proved for semiconductors with diamond, sphalerite, and chalcopyrite structure. The known linear relations available from the literature do not yield a reasonable agreement between experimental and calculated Debye temperatures. As in the case of the Lindemann rule a better approximation is obtained by use of a more general relation θ_D = a₁x + a₂. The constants a₁ and a₂ are calculated for the substances under consideration. Unknown Debye temperatures are calculated.     

Temperature variation of the structural and thermal characteristics of YBa2Cu3O7−x in the range from 90 to 400 K

The X-ray studies of the tetragonal YBa₂Cu₃O_7−x compound are performed at different temperatures from 400 to 90 K and the temperature variation of the thermal expansion coefficients and the Debye characteristic temperature is determined. The anisotropy of thermal expansion is investigated. It is found that the two dynamical characteristics monotonously decrease with temperature lowering whereas for the superconducting orthorhombic modification their temperature dependences are anomalous. The mean thermal expansion coefficient as well as the overall Debye temperature for the tetragonal phase are smaller than those for the orthorhombic one.     

Effect of crystal disorder on linewidth of the Raman modes in GaS1‐xSex layered mixed crystals

The linewidths of Raman‐active intralayer compressional modes in GaS_1‐xSe_x layered mixed crystals (0 ≤ x ≤ 1) have been measured in the 10‐300 K temperature range to study the anharmonic effect as a function of compositional variation and temperature. It was found that the anharmonicity increases with an increase in substitutional disorder. The cubic (three‐phonon) processes with energy conservation is responsible for the anharmonic contribution to the broadening of the intralayer phonon lines with temperature. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron‐phonon short‐range interactions mobility and p‐ to n‐type conversion in TlGaS2 crystals

The conductivity type conversion from p ‐ to n ‐type at a critical temperature of 315 K in TlGaS₂ crystals is observed through the Hall effect measurements in the temperature range of 200–350 K. The analysis of the temperature‐dependent electrical resistivity, Hall coefficient and carrier concentration data reveals the extrinsic type of conduction with donor impurity levels that behave as acceptor levels when are empty. The data analysis allowed the calculation of hole and electron effective masses of 0.36m ₀ and 0.23m ₀, respectively. In addition, the temperature‐dependent Hall mobility is found to decrease with temperature following a logarithmic slope of ∼1.6. The Hall mobility in the n ‐region is limited by the electron‐phonon short‐range interactions scattering with an electron‐phonon coupling constant of 0.21. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

DC conduvtivity of V2O5TeO2 glasses

The dc conductivity of semiconducting vanadium tellurite glasses of compositions in the range 50 to 80 mol% V₂O₅ has been measured in the temperature region 77 to 400 K. Measurements have been made on annealed samples at different annealing temperatures. Annealing the samples at temperature of about 250°C causes the appearance of a complex crystalline phase resulting in an increase of conductivity. Results are reported for amorphous samples of different compositions. The conductivity of tellurite glasses is slightly higher than the corresponding composition of phosphate glasses, but the general trend of the increase of conductivity and decrease of high temperature activation energy with increasing V₂O₅ content is similar in the two systems. The data have been analysed in the light of existing models of polaronic hopping conduction. A definite conclusion about the mechanics of conduction (adiabatic or nonadiabatic) is difficult in the absence of a precise knowledge of the characteristic phonon frequency v₀. Adiabatic hopping is indicated for v₀～10¹¹ Hz, however this value leads to unreasonably low value for the Debye temperature θ_D, and higher values for v₀～10¹³ hz satifiies the conditions for nonadiabatic hopping which appears to be the likely mechanism of conduction in V₂O₅TeO₂ glasses. The low temperature data (< 100 K) can be fitted to Mott's variable range hopping, which when combined with ac conductivity data gives reasonable values of α, but a high value for the disorder energy.     

Thermoelastic properties of ULE® titanium silicate glass

The elastic moduli, and their temperature and pressure derivatives, for ULE titanium silicate glass, have been measured by the ultrasonic pulse superposition technique. From the extrapolated 0 K values of the elastic moduli and their pressure derivatives, the elastic 0 K Debye temperature and Grüneisen constant have been evaluated, and compared with the thermally measured values. The same anomalies in the dynamic propeties as found in other glasses, i.e. positive temperature and negative pressure derivatives, are also found for ULE glass. The correlation between the changes in the bulk modulus and additive in silica-rich glasses is discussed.