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Low temperatures lattice thermal conductivity on non-crystalline polymers
Application to polycarbonate
Abstract
The density fluctuation model is used to analyze the lattice thermal conductivity data of two samples of polycarbonate between 0.04 and 1K. The study is carried out by calculating the latice thermal conductivity of a noncrystalline polymer as the sum of two contributions asK=K BM+K Em, whereK BE is attributed to phonons which interact with the crystal boundaries,K EM is due to phonons which interact with the empty spaces. The relative importance of each contribution has also been examined by estimating their percentage contributions to the lattice thermal conductivity. An excellent fit to the experimental data was obtained over the whole temperature range.
Abstract
Effective thermal conductivity of fireworks raw materials and their mixture have been measured by the temperature modulated DSC and the hot wire method, in order to predict spontaneous ignition properties precisely. As a result, an excellent linear correlation has been obtained between the density and the λe by the TMDSC method. Moreover, the low-density data by the hot wire method lie on the extrapolated point of the linear correlation. Thus, the λe within the ordinary limit of fireworks composition can be measured by the TMDSC method. Krupiczka’s estimation method shows a good agreement with the experimental values.
Abstract
The temperature dependence of the Debye temperature θD(T) was applied to analyze the lattice thermal conductivity of Si between 2 and 300 K. The analysis of experimental data in terms of the Dubey model of the two modes of conduction has been carried out by combining the relaxation time for phonon-phonon scattering, point defect scattering and boundary scattering. The relative importance of the contribution of each mode was examined by estimating their percentage contribution to the phonon conductivity. Agreement between theory and experiment is achieved over the whole temperature range of study.
A simple, low-cost apparatus has been designed and constructed for measurement of the thermal conductivities of samples with low cross-sections (∼10−7 m2). This apparatus has been used to determine variations in the thermal conductivity of the metallic glass Fe80B20 (Metglas 2605) in the crystallization process induced by thermal treatment.
The new expression τ3ph −1=g(ω) (BN + BUE−Θ/αT)Tm is proposed for the three-phonon scattering relaxation rate, considering contributions due to three-phonon normal and umklapp processes, which give a new approach to the lattice thermal conductivity. With use of the above expression, the lattice thermal conductivity of Ge has been calculated in the entire temperature range 2–1000 K: good agreement is found between the experimental and calculated values of the phonon conductivity in the entire temperature range of investigation. Analytical expressions are also obtained to calculate an approximate value of the lattice thermal conductivity. The role of four-phonon processes is also included in the present study.
Abstract
Photothermal Techniques are based on the conversion of the modulated light energy into heat within the sample. Using the Photothermal Probe Beam Technique, where the analysis of a laser beam deflected by the mirage effect near the sample leads to the thermal properties of this sample, we have determined the three components of the thermal conductivity tensor of an orthorhombic polydiacetylene single crystal. A numerical simulation of the probe beam deflection is also presented and compared to the experimental data.
The lattice thermal conductivity of GaAs has been analysed in the entire temperature range 100–800 K in the frame of the Sharma-Dubey-Verma (SDV) model of phonon conductivity, and very good agreement has been found between the calculated and experimental values of the lattice thermal conductivity in the entire temperature range of study. The temperature exponentm(T) for the three-phonon scattering relaxation rate for GaAs has also been calculated in the above temperature range. The separate percentage contributions due to transverse and longitudinal phonons have also been studied.
Abstract
One of the greatest challenges in the application of organic phase change materials (PCMs) is to increase their thermal conductivity while maintaining high phase change enthalpy. 1-Tetradecanol/Ag nanowires composite PCM containing 62.73 wt% (about 11.8 vol%) of Ag nanowires showed remarkably high thermal conductivity (1.46 W m−1 K−1) and reasonably high phase change enthalpy (76.5 J g−1). This behavior was attributed to the high aspect ratio of Ag nanowires, few thermal conduct interfaces, and high interface thermal conductivity of Ag nanowires in the composite PCM. These results indicated that Ag nanowires might be strong candidates for thermal conductivity enhancement of organic PCMs.
