Analysis of lattice thermal conductivities of rare-earth sulphides at high temperatures

The lattice thermal conductivities of rare-earth sulphides have been analyzed at high temperatures in the frame of the two-mode conduction of phonons for the first time by studying the total lattice thermal conductivities of GdS and LaS in the entire temperature range 100–1000 K. The temperature exponents for the three-phonon scattering relaxation rates are reported for the transverse as well as for the longitudinal phonons. The separate percentage contributions due to the transverse and longitudinal phonons towards the total lattice thermal conductivities of the above samples have similarly been studied. The role of the four-phonon processes too has been included in the present investigation.     

Degree of crystallinity and lattice thermal conductivity of polyethylene at low temperatures

The lattice thermal conductivity of a semicrystalline polymer was studied at low temperatures by calculating the total lattice thermal conductivities of four samples of polyethylene with different degrees of crystallinity between 0.43 and 0.81 and temperatures between 0.4 and 20 K. The contributions of the crystalline and noncrystalline natures and their percentage contributions were taken into account. The predicted lattice thermal conductivity of polyethylene was in fairly good quantitative agreement with the experimental value, and showed a strong crystallinity dependence, with a distinctive cross-over point at about 2 K.     

Low temperatures lattice thermal conductivity on non-crystalline polymers

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.     

Electrical conductivity of aqueous sodium hydroxide solutions at high temperatures

Electrical conductivities of dilute sodium hydroxide aqueous solutions have been determined at 75, 100 and 150°C at 1.6 MPa using a recently developed DC-measuring technique especially suited for the study of aqueous solutions above room temperature. The data were analyzed with modern theories to obtain the infinite dilution conductivity and the association constant at the three temperatures.     

The thermal dissociation of diatomic molecules at high temperatures

On the basis of a solution of gas-kinetic equations describing the population of molecules (cut-off harmonic oscillators) at various oscillatory levels, the process of thermal dissociation has been analyzed. It has been shown that thermal dissociation, in addition to disturbing the Boltzmann distribution, leads to a reduction in the vibrational temperature compared with the translational one. This affects to a considerable extent the rate of thermal dissociation, and also the process of vibrational relaxation. The question of the applicability of the well-known relation of statistical thermodynamics connecting the rate constants of forward and back reactions has also been analyzed. The results obtained agree qualitatively with the experimental data.     

Analysis of nitromethane thermal decomposition at low temperatures

The thermal decomposition of nitromethane (NM) over the temperature range from 580 to 700 K at pressures of 4 Torr to 40 atm was analyzed. On the basis of literature data, with the use of theoretical transitional curves of the modified Kassel integral, the rate constants k of NM decomposition at the upper pressure limit were determined. The values thus obtained are in good agreement with the results of extrapolation of the high-temperature (1000–1400 K) k _{1, ∞} values to lower temperatures. The reasons for which the NM decomposition rate constants differ by two orders of magnitude at low temperatures are considered. A general expression for the NM decomposition rate constant at the upper pressure limit over the 580–1400 K temperature range was determined: k _{1, ∞} = (1.8 ± 0.7) × 10¹⁶ exp((?58.5 ± 2)/R _T ) s^?1. These data disprove the hypothesis that a nitro-nitrite rearrangement takes place during the NM decomposition at low temperatures.     

Analysis of lattice thermal conductivity and phonon-phonon scattering relaxation rate: Application to Ge

The three-phonon scattering relaxation rates and their temperature exponents have been analysed in the frame of Guthrie's classification of the phonon-phonon scattering events as class I and class II events and as a result of this, a new expressionτ _3ph ^-1 =(B _N,I+B _U,Ie^-θ/αT) g(w)T ^m I ^(T)+(B _N,II+B _U,IIe^-θ/αT)g(w)T ^mII^(T) for the three phonon scattering relaxation rates has been proposed for the first time to calculate the lattice thermal conductivity of a sample. Using the expression proposed above, the lattice thermal conductivity of Ge has been analysed in the temperature range 2–1000K and result obtained shows a very good agreement with the experimental data. The percentage contributions due to three-phonon normal and umklapp processes are also reported. The role of four phonon processes is also included at high temperatures. To estimate an approximate value of the scattering strength and the phonon conductivity, the analytical expression is also obtained in the frame of the expression proposed above forτ _3ph ^-1 .     

Analysis of the lattice thermal resistivity due to the presence of electrons at low temperatures: Application to phosphorus-doped Ge

The temperature-dependence of the extra lattice thermal resistivity of a doped sample due to the presence of electrons has been studied at low temperatures for the first time by analysing the extra lattice thermal resistivity due to electrons of five samples of phosphorus-doped Ge having different carrier concentrations in the range 1.2×10²³–1.1×10²⁴ m^?3 in the temperature range 1–5 K. The variation of the extra lattice thermal resistivity of a doped sample due to electrons with the parameters η^* (the reduced Fermi energy),m ^* (the density of states effective mass),E _D (the deformation potential constant) andn (the carrier concentration) which are responsible for the electron-phonon scattering relaxation rate has also been analysed for the first time in the present study. A distinction has been made between non-peripheral and peripheral phonons in the present analysis. An analytical expression is reported for calculation of an approximate value of the extra lattice thermal resistivity of a doped sample due to the presence of electrons at low temperatures.     

Stabilization at high temperatures

A general detailed model of polymer oxidative degradation in the presence of a stabilizer-oxygen acceptor was considered. The motion of the degradation zone boundary and the sample lifetime were obtained for various conditions of the process. In particular, it is shown that within the given concentration immobile oxygen acceptor has the best stabilizing properties. The theoretical results are compared with the experimental data obtained by using two independent methods: polymer sample cracking and a change in the ESR signal. Good agreement between theory and experiment was obtained.     

Analysis of degradation products by thermal volatilization analysis at subambient temperatures

In the subambient thermal volatilization analysis (TVA) technique, degradation products initially at ?196°C are allowed to warm up to ambient temperature in a controlled manner under vacuum conditions, and volatilization from the sample tube to a trap at ?196°C is monitored by means of a Pirani gauge. The technique is discussed in relation to earlier TVA work in which volatilization from a heated polymer sample was followed. Design and operation of a subambient TVA system are described, and examples of the application of the technique to the study of the degradation products of seven polymers are considered.     

A new approach to lattice thermal conductivity: Application to Ge

The new expression τ _3ph ^?1 =g(ω) (B_N + B_UE^?Θ/αT)T^m 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.     

Band-state interpretation of lattice thermal conductivity and microhardness of ternary chalcopyrite semiconductors

A new approach utilising the concept of band states and periodicity has been used to explain the lattice thermal conductivity and microhardness of ternary chalcopyrite crystals. The experimental values agree quite well with the calculated values using our model. A single fitting parameter used in each case explains the uniqueness of the model.     

Role of the Debye temperature in the lattice thermal conductivity of silicon

This paper is a theoretical study of the effect of the variation in the Debye temperature _D with temperature on the lattice thermal conductivity of Si in the temperature range 2–300 K. Expressions for the three-phonon scattering relaxation rates previously proposed by Shermaet al. are used here. The percentage changes in the lattice thermal conductivity due to the Debye temperature for the transverse and the longitudinal phonons are studied separately.

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Zusammenfassung Dieses Manuskript ist eine theoretische Untersuchung des Einflusses der Veränderung der Debey-Temperatur D mit der Temperatur auf die Gitterwärmeleitfähigkeit von Si im Temperaturbereich 2–300 K. Unlängst von Shermaet al. vorgeschlagene Ausdrücke für die drei Phononen-Streuungsrelaxations-Geschwindigkeiten fanden dabei Anwendung. Die prozentuelle Änderung der Gitterwärmeleitfähigkeit in Abhängigkeit von der Debey-Temperatur wurde für transversale als auch für longitudinale Phononen separat untersucht.

     

Thermal expansion and impurity effects on lattice thermal conductivity of solid argon

Thermal expansion and impurity effects on the lattice thermal conductivity of solid argon have been investigated with equilibrium molecular dynamics simulation. Thermal conductivity is simulated over the temperature range of 20-80 K. Thermal expansion effects, which strongly reduce thermal conductivity, are incorporated into the simulations using experimentally measured lattice constants of solid argon at different temperatures. It is found that the experimentally measured deviations from a T(-1) high-temperature dependence in thermal conductivity can be quantitatively attributed to thermal expansion effects. Phonon scattering on defects also contributes to the deviations. Comparison of simulation results on argon lattices with vacancy and impurity defects to those predicted from the theoretical models of Klemens and Ashegi et al. demonstrates that phonon scattering on impurities due to lattice strain is stronger than that due to differences in mass between the defect and the surrounding matrix. In addition, the results indicate the utility of molecular dynamics simulation for determining parameters in theoretical impurity scattering models under a wide range of conditions. It is also confirmed from the simulation results that thermal conductivity is not sensitive to the impurity concentration at high temperatures.     

X-Ray diffraction at high temperatures

Experimental equipment for X-ray diffraction at high temperatures is introduced and the possibility to directly observe the evolution of the atomic (geometric) structure as a function of temperature is discussed. Recent results on crystallization investigations of amorphous iron-based alloys and on the structural development in thin molybdenum sulphide films are given. The kinetics of the process was followed by means of time resolved experiments in both cases.     

Heat capacity and thermal diffusivity of heavy oil saturated rock materials at high temperatures

Journal of Thermal Analysis and Calorimetry - In this paper, a typical combined cycle power generation unit in Iran is simulated by a mathematical method in order to perform sensitivity analysis on...     

A pyrometric differential thermal analyser for high temperatures

An apparatus for differential thermal analysis to a temperature of 3400°C is described. The specimen is heated inductively with a frequency of 1 MHz either without crucible at its upper end only or in a crucible which is one of the components of the system investigated. The differential principle is applied by comparing the radiation emitted from the sample with the power fed to the sample, which is registered by a bulb coupled to the high frequency circuit. The emission of the sample and bulb are measured by two photodiodes. The different behaviour of sample and bulb is compensated electronically. Investigations in metal ceramic eutectics are given as an example.     

Evolution of HCN by thermal oxidative degradation from nylon 66 at high temperatures including burning

Work on the evolution of HCN from nylon 66 was extended to temperatures from ca. 300 to 695°C. Below ca. 300°C the evolution of HCN is governed by chemical decomposition, and above 300°C the evolution is controlled by diffusion. Above 530°C oxidation of HCN becomes noticeable and ignition occurs at 590°C. The rate constants for all of the temperature ranges and for the oxidation of HCN are given in terms of Arrhenius equations. The activation energy for the oxidation of HCN before ignition (590°C) is reached is 47 kcal/mole, and beyond this point, the oxidation is controlled by diffusion. The rate constants increase linearly with oxygen concentration as long as HCN oxidation is negligible; however, they pass through maxima if HCN oxidation is appreciable (some HCN is evolved even in the absence of oxygen). A new flash degradation apparatus has been constructed for these high-temperature ranges and a degradation mechanism has been proposed which is in satisfactory agreement with the experimental results.