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This paper deals with a new quasi-isothermal method for the measurement of heat capacities of pure solids and liquids in an LKB 2277 heat flow microcalorimeter TAM (thermal activity monitor). Deviations of less than 1% from litera...
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This paper deals with a new quasi-isothermal method for the measurement of heat capacities of pure solids and liquids in an LKB 2277 heat flow microcalorimeter TAM (thermal activity monitor). Deviations of less than 1% from literature data can be achieved. It is shown that even excess heat capacities of binary mixtures can be determined. With a quasi-isothermal method, no heating up or cooling down of the calorimetric unit is needed; thus the temperature of the thermostat directly represents the test temperature. [References: 7]
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A brief review is presented of contemporary ways of estimating heat capacity and determining their main advantages and disadvantages. Incremental schemes that predict the temperature dependences of heat capacity are considered in ...
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A brief review is presented of contemporary ways of estimating heat capacity and determining their main advantages and disadvantages. Incremental schemes that predict the temperature dependences of heat capacity are considered in detail. Results of estimating the heat capacity of (InAs)_(1– x )(GaAs)_( x )solid solutions using specially selected mixing rules are presented.
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The present work aims at the investigation of the combined space distribution effects of heat capacity and thermal resistance on the transient thermal behavior of a wall, seen as a continuum of distributed parameters. The physical...
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The present work aims at the investigation of the combined space distribution effects of heat capacity and thermal resistance on the transient thermal behavior of a wall, seen as a continuum of distributed parameters. The physical system is compared with the idealized wall lumped parameter model and its thermal time constant is related to its effective wall heat capacity, defined as the fraction of the wall heat capacity which participates in a transient thermal process. The effect of the space distribution of heat capacity and thermal resistance on the effective wall heat capacity is investigated for a wide range of homogeneous and multilayer thermally insulated walls. It is derived that the decrease of thermal resistance in homogeneous walls leads to an increase of their effective heat capacity. However, the effects are remarkably stronger on the effective heat capacity of thermally insulated multilayer walls, in which when the thermal insulation layer is at the ambient side, it leads to a significant increase of effective heat capacity, although when it is installed at the room side it leads to very low effective heat capacity, irrespective of the wall thermal resistance. Based on the first order results from a simplified room model, it was subsequently found that the influence of these parameters on the effective heat capacity of the building envelope leads to significant effects on the transient thermal behavior, thermal time constant and stability of structures.
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The thermal conductivity of wurtzite and zinc blende indium arsenide nanowires was measured using a microfabricated device, with the crystal structure of each sample controlled during growth and determined by transmission electron...
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The thermal conductivity of wurtzite and zinc blende indium arsenide nanowires was measured using a microfabricated device, with the crystal structure of each sample controlled during growth and determined by transmission electron microscopy. Nanowires of both phases showed a reduction of the thermal conductivity by a factor of 2 or more compared to values reported for zinc blende indium arsenide bulk crystals within the measured temperature range. Theoretical models were developed to analyze the measurement results and determine the effect of phase on phonon transport. Branch-specific phonon dispersion data within the discretized first Brillouin zone were calculated from first principles and used in numerical models of volumetric heat capacity and thermal conductivity. The combined results of the experimental and theoretical studies suggest that wurtzite indium arsenide possesses similar volumetric heat capacity, weighted average group velocity, weighted average phonon-phonon scattering mean free path, and anharmonic scattering-limited thermal conductivity as the zinc blende phase. Hence, we attribute the differing thermal conductivity values observed in the indium arsenide nanowires of different phases to differences in the surface scattering mean free paths between the nanowire samples.
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The temperature dependence of the molar heat capacities of the tellurites Fe-2(TeO3)(3), Fe2TeO5 and Fe2Te4O11 were determined. By statistical manipulation of the values obtained, the parameters in the equations for the correspond...
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The temperature dependence of the molar heat capacities of the tellurites Fe-2(TeO3)(3), Fe2TeO5 and Fe2Te4O11 were determined. By statistical manipulation of the values obtained, the parameters in the equations for the corresponding compounds showing this dependence were determined using the least-squares method. These equations together with the standard molar entropies were used to determine the thermodynamic functions Delta S-T(0)m(0) , Delta(T)(T), H-m(0) and (Phi(0)(m) + Delta H-T'(0)m(0)/T) for T'=298.15 K.
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Specific heat capacity at constant pressure cp (J K?1 g?1) is an important thermodynamic property that helps material scientists better understand molecular structure and physical properties. Engineers control temperature (through...
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Specific heat capacity at constant pressure cp (J K?1 g?1) is an important thermodynamic property that helps material scientists better understand molecular structure and physical properties. Engineers control temperature (through heat transfer) in physical systems. Differential Scanning Calorimetry (DSC) is an analytical technique that has been used for over fifty years to measure heat capacities with milligram size samples. For existing procedures, such as ASTM E1269?11 (2018), the accuracy of molar heat capacity measurements is typically ±2–5% relative to the literature values, even after calibration for both heat flow and heat capacity. A comparison of different DSC technologies is beyond the scope of this paper, but the causes of these deviations are common to all DSC instruments, although the magnitude of the deviation (observed and accepted) varies with instrument design. This paper presents a new approach (Heat–Cool) for measuring more accurate and reproducible specific heat capacities of materials. In addition to better performance, the proposed method is faster and typically requires no additional calibration beyond the routine calibration of temperature and heat flow, with melting point standards common to all applications of DSC. Accuracy, as used throughout this paper, means deviation from the literature. The estimated standard deviation of repeated measurements of the cp values obtained with the Heat–Cool technique typically falls in the ±1–2% range.
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? 2022 Elsevier B.V.In this study, we evaluated the temperature dependence of the absolute configurational entropy of tri-O-methyl-β-cyclodextrin (TMCD), a molecule with many degrees of freedom. We calculated the configurational ...
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? 2022 Elsevier B.V.In this study, we evaluated the temperature dependence of the absolute configurational entropy of tri-O-methyl-β-cyclodextrin (TMCD), a molecule with many degrees of freedom. We calculated the configurational entropy from the configurational heat capacity obtained by subtracting the calculated heat capacity due to lattice vibrations and intramolecular vibrations from the heat capacity measured above the glass-transition temperature. We fitted the configurational heat capacity using both a power law and a logarithmic function based on Landau's critical-point theory. The configurational entropy increases more gradually as the temperature increases. We found that the temperature at which the configurational entropy vanishes (the so-called Kauzmann temperature TK) lies between 280 K and 298 K for TMCD. This is about 50 K lower than the glass-transition temperature of TMCD, which is consistent with the expectations.
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During the past five years the low-temperature heat capacity of simple semiconductors and insulators has received renewed attention. Of particular interest is its dependence on isotopic masses and the effect of spin-orbit coupling...
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During the past five years the low-temperature heat capacity of simple semiconductors and insulators has received renewed attention. Of particular interest is its dependence on isotopic masses and the effect of spin-orbit coupling in ab initio calculations. Here we concentrate on the lead chalcogenides PbS, PbSe, and PbTe. These materials, with rock salt structure, have different natural isotopes for both cations and anions; a fact that allows a systematic experimental and theoretical study of isotopic effects, e.g., on the specific heat. Also, the large spin-orbit splitting of the dp electrons of Pb and the 5p of Te, using a computer code which includes spin-orbit interaction, allows an investigation of the effect of this interaction on the phonon-dispersion relations and the temperature dependence of the specific heat, and on the lattice parameter. It is shown that agreement between measurements and calculations significantly improves when spin-orbit interaction is included.
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The magnetic, electric, and thermal properties of the (Lrc_(1-y)Y_y)_(0.7)Ca_(0.3)jCoC_3 perovskites (Ln=Pr,Nd) were investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator (M-I) tran...
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The magnetic, electric, and thermal properties of the (Lrc_(1-y)Y_y)_(0.7)Ca_(0.3)jCoC_3 perovskites (Ln=Pr,Nd) were investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator (M-I) transition, which is observed in the praseodymium based samples with y=0.075 and 0.15 at T_(m-I)=64 K and 132 K, respectively. The study suggests that the transition, reported originally in Pr_(0.5)Ca_(0.5)CoO_3, is not due to a mere change in cobalt ions from the intermediate to the low-spin states but is associated also with a significant electron transfer between Pr~(3+) and Co_(3+)/Co_(4+) sites, so that the praseodymium ions occur below T_(M-I) in a mixed Pr~(3+)/Pr~(4+) valence. The presence of Pr~(4+) ions in the insulating phase of the yttrium doped samples (Pr(1-y)Y_y)_(0.7)Ca_(0.3)CoO_3 is evidenced by Schottky peak originating in Zeeman splitting of the ground-state Kramers doublet. The peak is absent in pure Pr_(0.7)Ca_(0.3)CoO_3 in which metallic phase, based solely on non-Kramers Pr~(3+) ions, is retained down to the lowest temperature.
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We present ab initio phonon dispersion relations for the three lead chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good agreement with inelastic neutron-scattering data and calculations of the specific heat g...
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We present ab initio phonon dispersion relations for the three lead chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good agreement with inelastic neutron-scattering data and calculations of the specific heat give good agreement with experimental data. The pronounced minimum of the transverse-optical branch at Γ due to the near ferroelectricity of the lead chalcogenides is qualitatively reproduced. In addition, we find a pronounced dip in the longitudinal-optical branch at Γ. This dip was previously explained as the effect of "free carriers" (due to the presence of impurities). The calculations demonstrate that it persists also in the case of pure lead chalcogenides. We explain the dip as a "near Kohn anomaly" which is associated with the small electronic band gap at the high-symmetry point L.
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