A calorimetric study of LiClO4 · 3H2O

The heat capacity of LiClO₄ · 3H₂O was measured from 17 to 343 K, in order to determine the relationship between its phase diagram and the phase diagrams of divalent metal perchlorate hexahydrates The heat capacity curve of pure LiClO₄ · 3H₂O was found to be smooth, indicating an absence of polymorphism, although a small amount of excess perchloric acid in one sample gave rise to a eutectic point at 232 8 ± 0 3 K The absence of transitions in LiClO₄ · 3H₂O is discussed in terms of its known structure, and some conclusions are drawn concerning the differences in H-bonding in this salt and in divalent metal perchlorate hexahydrates.     

A method for determining the static conductivity of semiconductor transparent heat mirrors

A simple method for determining the static conductivity of semiconductor transparent heat mirrors is developed that is based on the relationships of Drude theory and high-frequency optical measurements. The method consists of two steps. In the first step, the dielectric permittivity of the lattice ε_L and the plasma wavelength λ_p of the semiconductor film (a part of the heat mirror) are determined graphically using the reflectance measured at extreme points of interference fringes that are observed in the transparency regions of the reflectance spectra. The second step consists in the numerical computation of relaxation times and the static conductivity using the derived values of ε_L and λ_p and an experimental value of total emittance. The method was tested with transparent heat mirrors based on films of tin-doped indium oxide.     

Heat capacity study of double perovskite-like compounds BaTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>x</Subscript>O<Subscript>3</Subscript>

The temperature dependence of the heat capacity of two compositions in the solid solution system BaTi_1?xZr_xO₃ (x = 0.25, 0.35) was measured using adiabatic calorimetry. In the T-x phase diagram, these compounds occupy positions near the crossover from conventional ferroelectric behavior to the relaxor state. Both compounds reveal diffuse heat capacity anomalies: two anomalies in the temperature ranges 250–350 and 150–200 K at x = 0.35 and one anomaly within the range ～150–320 K at x = 0.25. The results obtained are discussed together with structural and dielectric measurements.     

Correlative atomic scale characterisation of secondary carbides in M50 bearing steel

Correlative atom probe tomography (APT) and transmission electron microscopy (TEM) are used to characterise the microstructure and chemistry of carbide precipitation in M50 bearing steel. This is a prerequisite in establishing relationships between the microstructure and hydrogen embrittlement (HE) resistance. Secondary carbides are the focus of this study, as they play a major role in improving HE-resistance. Secondary carbides are observed in APT, with compositions close to M₄C_3, M₂C and M₃C. Correlative TEM measured orientation relationships between the martensite matrix and carbides, enabling the confirmation of M₃C cementite precipitates in the corresponding APT reconstruction. Additionally, other precipitates observed in TEM were correlated to the M₂C carbides in APT data. The M₄C₃ carbides are found to have a significantly lower volume fraction than the M₂C carbides.     

Caloric characteristics of PbTiO3 in the temperature range of the ferroelectric phase transition

The heat capacity and thermal expansion of the PbTiO₃ ceramic sample have been measured in the temperature range 80?C970 K. The electrocaloric and barocaloric efficiencies of lead titanate in the ferroelectric phase transition range have been investigated by analyzing the experimental data in terms of the thermodynamic theory of phase transitions, the electrical equation of state P(T, E), the Pippard equation, and the S(T, p) diagram.     

Phase diagram of lithium-doped copper oxide, Cu1−xLixO

Heat capacities of lithium-doped samples of CuO have been measured below room temperature by adiabatic calorimetry. The antiferromagnetic ordering transition to incommensurately modulated state was detected as a step in the heat capacity. Its concentration dependence was compatible with existing reports based on Li-NMR. The incommensurate-commensurate transition of lithium-doped copper oxide was clearly detected for the first time. The magnetic phase diagram of Cu_1−xLi_xO was thus constructed. The suppression of both transition temperatures by the Li doping is nearly twice as strong as that expected from mean-field and percolation theories.     

Heat capacity of a Cr2O3 antiferromagnet near the critical temperature

The heat capacity of a Cr₂O₃ antiferromagnet near the critical temperature is precisely measured by ac calorimetry. The critical behavior of the heat capacity is examined. The regularities of variations in the universal critical parameters near the critical point are determined, and their values are calculated. A crossover from the Heisenberg (n=3) to the Ising (n=1) critical behavior is revealed.     

Tuning anisotropy by impurities: magnetocaloric experiments on CsNi 0.9 Fe 0.1 Cl 3

The magnetic phase diagram of the Fe²⁺ doped hexagonal ABX₃ compound CsNi_0.9Fe_0.1Cl₃ is investigated by heat capacity and magnetocaloric experiments. In spite of the high doping concentration, some phase boundaries appear surprisingly well-defined, while others are broadened significantly. The discussion of this behaviour clarifies the potentials and limitations of doping as a means to manipulate the effective anisotropy in quasi one-dimensional ABX₃ compounds. Received 20 April 2001     

Anisotropy-tuning by doping: phase diagram of CsNi0.98Fe0.02Cl3 measured by caloric experiments

The magnetic phase diagram of the Fe²⁺ doped hexagonal ABX₃ compound CsNi_0.98Fe_0.02Cl₃ is investigated measuring the specific heat capacity and the magnetocaloric effect. In contrast to earlier specific heat, ultrasound and NMR experiments our measurements identify transitions between three ordered phases with the field applied in the hexagonal plane. Since the pure compound, a Heisenberg antiferromagnet with weak Ising anisotropy, has only two ordered phases for this field direction, while three are predicted for weak easy-plane anisotropy, our data provide the unambiguous proof that the anisotropy of CsNiCl₃ can be effectively modeled by doping with Fe²⁺.     

Heat capacity of La1−x SrxMnO3 single crystals in different magnetic states

The heat capacity of three single-crystal samples of La_1?x Sr_xMnO₃ (x=0, 0.2, and 0.3) is measured in the temperature range 4–400 K. It is found that the heat capacity undergoes abrupt changes due to the transitions from the antiferromagnetic phase to the paramagnetic phase (x=0) and from the ferromagnetic phase to the paramagnetic phase (x=0.2 and 0.3). The phonon contribution to the heat capacity and the Debye characteristic temperatures for the La_0.7Sr_0.3MnO₃ sample are determined over a wide range of temperatures. The electronic density of states at the Fermi level is evaluated. It is demonstrated that an increase in the strontium concentration x brings about an increase in the electronic density of states at the Fermi level. The contributions of spin waves to the heat capacity and the entropy are estimated under the assumption that the phonon spectrum remains unchanged upon doping with Sr.     

Heat capacity study of the first monolayer of adsorbed methane on grafoil

The heat capacity of CH₄ adsorbed on Grafoil below monolayer completion has been determined experimentally for the first time. The specific heat at constant density shows two characteristic features, very broad, weak anomalies at 47.6 K (around one third of a monolayer) and (for occupation numbers below two thirds of a monolayer) very narrow, strong peaks at 56.35 K. The anomalies at 47.6 K are interpreted as being due to a registry — out-of-registry transition of the two dimensional (2D) solid CH₄ adsorbate into an incommensurate expanded structure. The anomalies at 56.35 K define the 2D triple point of adsorbed CH₄. For higher occupation numbers the liquid-solid phase boundary shifts to higher temperatures. The present data make partial re-definition of the 2D-CH₄ phase diagram necessary.     

Heat capacity of LuZnCu<Subscript>4</Subscript>

The heat capacity at constant pressure C _p for LuZnCu₄ is measured in the temperature 3–250 K. The parameter γ, i.e., the coefficient of the linear-in-temperature term of the electronic component of heat capacity, is determined. The contribution of the magnetic component of the heat capacity to C _p(T) of YbZnCu₄ is estimated from the available data on the heat capacity of YbZnCu₄ and the experimental data obtained for LuZnCu₄.     

Heat capacity and the p-T phase diagram of Pb2MgTeO6 elpasolite

The heat capacity of Pb₂MgTeO₆ is measured in the temperature range 80–300 K. It is found that the heat capacity exhibits an anomaly associated with the phase transition at T ₀=186.9 K. The thermodynamic parameters of the structural transformation are determined. The effect of hydrostatic pressure up to 0.5 GPa on the phase transition temperature is examined.     

Laves Phases in Ferrite-Martensite EC-181 Steel

The microstructure and phase composition of EC-181 ferrite-martensite steel were studied in dependence on their heat treatment (temperature and aging time after quenching from 1080°C). It was established that Laves phases are formed only at the boundaries of ferrite grains, whereas Me ₂₃C₆ and V₂C carbides are formed in the ferrite and martensite components of the structure.     

High-temperature heat capacity of La2CuO4

The heat capacity of La₂CuO₄ has been measured in the temperature range 400–950 K. The temperature dependence of the heat capacity has been found to exhibit an extremum at 526 K.     

In situ measurements of the spectral emittance of coal ash deposits

The spectral emittance of deposits left by bituminous and sub-bituminous coals under oxidizing conditions have been measured in situ. Pulverized coal is injected into a down-fired entrained-flow reactor. Ash accumulates on a probe in the reactor effluent and radiation emitted by the ash layer is recorded using a Fourier transform infrared (FTIR) spectrometer. Values for the spectral emissive power emitted by the ash and the surface temperature of the ash are extracted from these data. These results are then used to calculate the spectral emittance of the deposit. The spectral emittances of ash deposits formed by burning Illinois #6 (bituminous) coal and Powder River Basin (sub-bituminous) coal were measured between 3000 and 500 wavenumbers. The spectral emittance of the deposit left by the bituminous coal has a constant value of approximately 0.46 between 3000 and 2400 wavenumbers. Between 2200 and 1200 wavenumbers, the spectral emittance of the deposit increases from approximately 0.47 to approximately 0.61. Between 1200 and 500 wavenumbers, the spectral emittance is relatively constant at 0.61. The spectral emittance of the deposit left by the sub-bituminous coal is also relatively constant between 3000 and 2400 wavenumbers at a value of 0.29. Between 2200 and 500 wavenumbers, the spectral emittance of deposits from the sub-bituminous coal increases from approximately 0.29 to 0.55. Differences between these spectral emittance measurements and those measured ex situ illustrate the importance of making in situ measurements. Band emittances were calculated using the measured spectral emittances, and band emittances of the deposits are reported as functions of temperature.     

Critical properties of the anisotropic Ising model with competing interactions

The critical properties of the anisotropic Ising model with competing interactions have been investigated by Monte Carlo methods. The region of localization of the Lifshitz point on the phase diagram has been computed. Relations of the finite-size scaling theory are used to calculate the critical exponents of the heat capacity, susceptibility, and magnetization at various values of the competing interaction parameter J ₁. A crossover to a critical behavior characteristic of a multicritical point with increasing parameter J ₁ is shown to be present in the system.     

Heat capacity of Bio-SiC and SiC/Si ecoceramics prepared from white eucalyptus, beech, and sapele tree wood

This paper reports on measurement of the heat capacity at constant pressure C _p of silicon bio-carbide prepared within the 5–300 K temperature interval from beech tree wood (bio-SiC(BE)), and within 80–300 K, from tree wood of sapele (bio-SiC(SA)), as well as SiC/Si ecoceramics of beech, sapele, and white eucalyptus wood. It has been shown that in bio-SiC(BE) the measured heat capacity contains a significant contribution of surface heat capacity, whose magnitude decreases with increasing temperature. Of the ecoceramics, only SiC/Si(SA) characterized by a high enough porosity has revealed a small contribution to the heat capacity coming from its surface component. The experimental results obtained are discussed.     

Enhancement of the positive emittance-switching performance of thermochromic VO2 films deposited on Al substrate for an efficient passive thermal control of spacecrafts

This work involves the study of the positive emittance-switching (i.e. emittance that increases with increasing the temperature) of thermochromic VO₂ films deposited using reactive pulsed laser deposition (RPLD) on Al substrates. The temperature dependence of the emittance of a 260 nm-thick VO₂ film on Al substrate revealed a maximum of the emittance of 0.29 around 68 °C. It is attributed to an increase in the infrared radiation absorption by the VO₂ film due to the coexistence of both insulating and metallic phases in the vicinity of the transition temperature of VO₂. The emittance tunability between 25 °C and 68 °C is 0.21. Since practical SRD application requires both high emittance at high temperature and large tunability, we demonstrate, by both simulation and fabrication, that these goals can be accomplished to some extent by a top dielectric a-Si:H/SiO₂ λ/4 stack layer. In fact, the addition of a-Si:H/SiO₂ λ/4 overlayer results in an increase of the maximum value of the emittance by 114% (from 0.29 to 0.62) as well as an increase of the tunability by 81% (from 0.21 to 0.38). This work reports an important improvement of the positive emittance-switching efficiency of the VO₂-based structures and holds promise for a new generation of smart radiator devices (SRDs) for a passive thermal control of spacecrafts.     

Magnetic phase diagram of Tb3−xHoxCu4Sn4 system

Magnetic phase diagram of Tb_3−xHo_xCu₄Sn₄ was determined on the basis of magnetic and heat capacity data. X-ray diffraction data proved that these compounds crystallize in the orthorhombic Gd₃Cu₄Ge₄-type structure. The compounds are antiferromagnets at low temperatures and the reciprocal magnetic susceptibility obeys the Curie-Weiss law. The paramagnetic Curie temperatures are negative and their absolute values decrease with increasing Ho content. An anomalous concentration dependence of the Néel temperature is observed.