Structural and thermal properties of the opal-epoxy resin nanocomposite

Thermal conductivity of the opal-epoxy resin nanocomposite is measured in the range 4.2–250 K, and the material is studied by electron microscopy at 300 K. An analysis of the electron microscope images permits a conclusion on the character of opal void filling by the epoxy resin. It is shown that the thermal conductivity of the nanocomposite within the range 40–160 K can be fitted fairly well by the corresponding standard expressions for composites. For T<40 K and T>160 K, the experimental values of the nanocomposite thermal conductivity deviate strongly from the calculated figures.     

Thermal conductivity of the amorphous and nanocrystalline phases of the beech wood biocarbon nanocomposite

Natural composites (biocarbons) obtained by carbonization of beech wood at different carbonization temperatures T _carb in the range of 800–2400°C have been studied using X-ray diffraction. The composites consist of an amorphous matrix and nanocrystallites of graphite and graphene. The volume fractions of the amorphous and nanocrystalline phases as functions of T _carb have been determined. Temperature dependences of the phonon thermal conductivity κ(T) of the biocarbons with different temperatures T _carb (1000 and 2400°C) have been analyzed in the range of 5–300 K. It has been shown that the behavior of κ(T) of the biocarbon with T _carb = 1000°C is controlled by the amorphous phase in the range of 5–50 K and by the nanocrystalline phase in the range of 100–300 K. The character of κ(T) of the biocarbon with T _carb = 2400°C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.     

Thermal conductivity of nanowires

Thermal conductivity of nanowires(NWs) is a crucial criterion to assess the operating performance of NWs-based device applications, such as in the field of heat dissipation, thermal management, and thermoelectrics. Therefore, numerous research interests have been focused on controlling and manipulating thermal conductivity of one-dimensional materials in the past decade. In this review, we summarize the state-of-the-art research status on thermal conductivity of NWs from both experimental and theoretical studies. Various NWs are included, such as Si, Ge, Bi, Ti, Cu, Ag, Bi_2Te_3, ZnO, AgTe,and their hybrids. First, several important size effects on thermal conductivity of NWs are discussed, such as the length,diameter, orientation, and cross-section. Then, we introduce diverse nanostructuring pathways to control the phonons and thermal transport in NWs, such as alloy, superlattices, core–shell structure, porous structure, resonant structure, and kinked structure. Distinct thermal transport behaviors and the associated underlying physical mechanisms are presented.Finally, we outline the important potential applications of NWs in the fields of thermoelectrics and thermal management,and provide an outlook.     

Thermal conductivity of the transition metals

The thermal conductivity of the transition metals is examined with an account of the features of their electronic structure. It is shown that the thermal conductivity is governed by the form of the stable d-electron configuration, their statistical weights, their energetic stability, and by the number of collectivized electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 5, pp. 41–47, May, 1969.     

Thermal conductivity of the diamond-paraffin wax composite

The thermal conductivity of diamond-paraffin wax composites prepared by infiltration of a hydrocarbon binder with the thermal conductivity λ _m = 0.2 W m⁻¹ K⁻¹ into a dense bed of diamond particles (λ _f ∼ 1500 W m⁻¹ K⁻¹) with sizes of 400 and 180 μm has been investigated. The calculations using universally accepted models considering isolated inclusions in a matrix have demonstrated that the best agreement with the measured values of the thermal conductivity of the composite λ = 10–12 W m⁻¹ K⁻¹ is achieved with the use of the differential effective medium model, the Maxwell mean field scheme gives a very underestimated calculated value of λ, and the effective medium theory leads to a very overestimated value. An agreement between the calculation and the experiment can be provided by constructing thermal conductivity functions. The calculation of the thermal conductivity at the percolation threshold has shown that the experimental thermal conductivity of the composites is higher than this critical value. It has been established that, for the composites with closely packed diamond particles (the volume fraction is ∼0.63 for a monodisperse binder), the use of the isolated particle model (Hasselman-Johnson and differential effective medium models) for calculating the thermal conductivity is not quite correct, because the model does not take into account the percolation component of the thermal conductivity. In particular, this holds true for the calculation of the heat conductance of diamond-matrix interfaces in diamond-metal composites with a high thermal conductivity.     

Thermal conductivity of superconducting niobium

The phonon scattering term in the superconducting state of the electronic conduction has been obtained for niobium which is regarded as an intermediate coupling superconductor. The limiting slope for the phonon scattering term of the reduced thermal conductivity against reduced temperature has been found to be 2.8, as compared with 1.5 for weak coupling superconductors.     

Thermal conductivity of liquid Ga

We present accurate values of the thermal conductivity of liquid Ga calculated from measurements of the Lorenz number and the electrical conductivity.     

Thermal conductivity of e-beam coatings

We present the results of the study on the thermal conductivity of different thin film materials produced by conventional thermal evaporation. The main features of the thermal pulse method employed for the measurement of the thermal conductivity are described. Thermal conductivity can be measured by determining the traveling time of a thermal wave propagating trough the film. A pump laser beam is directed onto a sample consisting of a thin transparent test layer and a totally absorbing substrate for the laser wavelength. As a consequence of the laser pulse, a temperature profile builds up at the substrate-film interface. A thermal pulse starts to diffuse from the substrate-film interface to the surface of the layer. Therefore, the temperature rise at the surface of the test layer starts with a time delay with respect to the laser pulse. The time delay depends on the propagation time of the thermal wave through the layer and is related to the thermal conductivity and the thickness of the layer. Measurements are evaluated by calculations based on the finite difference method. The results show that the analyzed thin films have lower thermal conductivity than the corresponding materials in bulk form.     

金属纳米颗粒的热导率

本文使用统计模拟方法对金属纳米颗粒的电子平均自由程进行了计算,并考察了纳米颗粒的晶格比热和声子平均群速度,最后应用动力学理论对纳米颗粒的电子热导率和声子热导率分别进行了求解.研究结果表明:具有相同特征尺寸的方形、球形纳米颗粒的无量纲电子(或声子)平均自由程比较接近.金属纳米颗粒的电子热导率远大于声子热导率;电子、声子热导率随着直径减小呈现降低趋势,而电子热导率的颗粒尺度依赖性比声子热导率更为明显;随着颗粒直径进一步减小,声子热导率与电子热导率趋于同一数量级.当纳米颗粒特征尺寸大于4倍块材电子(或声子)平均自由程,其电子(或声子)热导率的颗粒尺度依赖性将减弱.     

Thermal conductivity of bismuth tellurite

This paper reports on the results of investigations of the thermal conductivity along the three crystallographic directions in bismuth tellurite crystals. It is found that bismuth tellurite exhibits a low thermal conductivity inherent in glasses and disordered solid solutions. At temperatures below the Debye temperature, the thermal conductivity coefficients depend on the temperature as \(\sqrt T \), which is characteristic of disordered solid solutions. The temperature dependence of the thermal conductivity of bismuth tellurite is calculated in the framework of the Debye model.     

Thermal conductivity of gallium sulfide

The results of investigating of the thermal conducitivity of a GaS single crystal in directions parallel and perpendicular to the c axis in the temperature interval 5–300 K are reported. The investigations show that the degree of anisotropy of the thermal conductivity of GaS decreases with temperature. Fiz. Tverd. Tela (St. Petersburg) 41, 24–25 (January 1999)     

Thermal conductivity of YbInCu4

The electrical and thermal conductivities of an YbInCu₄ polycrystalline sample have been measured within the 4.2–300-K range. The behavior of the heat conductivity has been found to change sharply above and below T _v =70–75 K, the temperature corresponding to an isostructural phase transition from a state with an integral valence (T>T _v ) to a mixed-valence state (T<T _v ) of Yb ions. A preliminary qualitative analysis of the results is presented. Fiz. Tverd. Tela (St. Petersburg) 41, 1548–1551 (September 1999)     

Thermal conductivity of neutron-irradiated silica

Low temperature thermal conductivity Λ(T) of vitreous silica has been measured after exposure to various neutron fluences. It has been observed that the increase in Λ(T) saturates for approximately the same fluence for which the change in mass density ? saturates. A correlation between Λ(T) and the fictive temperature has been found.     

Thermal conductivity of Rochelle salt

The temperature dependence of the anisotropic thermal conductivity of a Rochelle salt single crystal was investigated in different crystallographic directions in the range from −35 to +40°C. No anomalous behaviour was found near the Curie points.     

Unusual behavior of thermal conductivity of a crystalline-NaCl-opal nanocomposite

An opal-based nanocomposite has been prepared with NaCl incorporated in its pores. The nanocomposite was produced by impregnating the opal with a NaCl solution at room temperature. Thermal conductivity of the nanocomposite has been measured in the temperature range 4.2–300 K. The effective heat conductivity of the nanocomposite was found to be equal to that of pure opal. The observed phenomenon can be explained by assuming that NaCl resides in opal pores in the form of noncontacting needles, thus precluding heat propagation through it. Fiz. Tverd. Tela (St. Petersburg) 40, 379–380 (February 1998)     

Low-temperature thermal conductivity of an opal + epoxy-resin nanocomposite

The thermal conductivity of an opal + epoxy-resin nanocomposite under 100% filling of first-order opal voids by epoxy resin was measured in the range 5–100 K. For T < T₀ (T₀ is the temperature at which the thermal conductivity of epoxy resin becomes equal to that of amorphous SiO₂ opal spheres, with inclusion of their porosity associated with second-and third-order voids), the thermal conductivity of the opal + epoxy-resin nanocomposite undergoes a sharp decrease, which is qualitatively accounted for by the appearance of Kapitsa heat resistance at the contacts between the amorphous opal spheres and epoxy resin.     

Thermal studies of the electrical conductivity of LiCoYb-ferrites

In the present work, the effect of temperature on the electrical properties of Li_0.5+zCo_zYb_xFe_2.5−2z−xO₄ where z=0.1 and 0.0≤x≤0.2 has been discussed. Both R_ac and R_dc increase with temperature up to the Curie-point after which R_ac decreases to 10% of its maximum value and levels off at ≈500 K while R_dc decreases to 19% of its maximum value and levels off at ≈400 K. A noticeable sharp increase appears on both resistances at x=0.1. The relation between relative density and relative volume which decreases as a result of sintering temperature was discussed for different Yb content. The dependence of thermal stimulation depolarization current (TSDC) on the polarized time and field for Li_0.6Co_0.1Yb_0.075Fe_2.225O₄, has been discussed. The present results point to the possibility of using the investigated samples for electronic applications.     

Thermal conductivity: Recent developments

The detailed spectral energy distribution of the radiation emitted by stars provides information on their composition and physical parameters. Many of the astpophysically important spectral lines of neutral and ionized atoms whose study can lead to a better understanding of the processes occurring in stellar atmosphere and in the interstellar gas lie in the ultraviolet region of the spectrum.

Since ultraviolet radiation from the stars is absorbed by the Earth's atmosphere, observations must be made from space vehicles. In this article some scientific objectives of a programme in ultraviolet astronomy are considered, and a review made of the types of space vehicles and attitude control systems employed. A brief discussion is made of some recent results and their interpretation.