Influence of chemical composition on martensitic transformation of MnNiIn shape memory alloys

The most extensively studied Heusler alloys are those based on the Ni–Mn–Ga system. However, to overcome the high cost of Gallium and the generally low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, two shape memory alloys, Mn₅₀Ni_50?xIn_x (x = 7.5 and 10), were obtained by rapid solidification. We outline their structural and thermal behaviour. The structural austenite–martensite transformation was checked by calorimetry. The transformation temperatures decrease as In content increases. The same pattern is reflected in entropy and enthalpy changes linked to transformation. The control of the valence electron by atom (e/a) determines the transformation temperatures range in this kind of alloys, and it is possible to develop alloys that can be candidates in applications such as sensors and actuators. In addition, X-ray diffraction was performed to verify the crystalline structure at room temperature.

     

Self consistent equations for calculating ideal gas heat capacity,enthalpy and entropy. III. Coal chemicals

Rehman, Z.U. and Lee, L.L., 1985. Self consistent equations for calculating ideal gas heat capacity, enthalpy and entropy. III. Coal chemicals. Fluid Phase Equilibria, 22: 21–31.Heat capacity, enthalpy and entropy correlations presented in previous studies (Aly and Lee, and Fakeeha et al.) are generalized here to include 37 coal chemicals and 29 additional hydrocarbons. We take into account one additional vibrational contribution in the formulae. The temperature range tested extends from 298.15 to 1000 K. The ideal heat capacity is predicted to within 0.07% for most substances.     

多晶Ni-Mn-Ga磁性记忆合金的相变行为及稀土元素铽的作用

研究了Ni50Mn25 xGa25-x 和Ni50Mn29Ga21-xTbx 2种成分系列磁性记忆合金的相变行为. 保持Ni含量不变, 增加Mn, 降低Ga含量会使马氏体相变温度明显提高, 同时相变滞后温区减小, 居里温度基本不变. 如果添加稀土元素铽, 相变温度继续升高, 居里温度仍然不变, 材料继续保持强的铁磁性及热弹性马氏体相变的特征.     

Entropic and enthalpic contributions to the chair-boat conformational transformation in dextran under single molecule stretching

The contribution of entropy and enthalpy to the chair-boat conformational changes (clicks) occurring during the force-extension of single molecules of an axially linked polysaccharide, dextran, was investigated. Experimental single molecule force-extension measurements were carried out by atomic force microscopy over the temperature range of 5-70 degrees C. This enabled the separation of the entropy and enthalpy components of the conformational change. The contribution of entropy to the Gibbs energy of the conformational transformation was found to be small (<12 J mol(-1) K(-1)), demonstrating that the click is largely (>89%) enthalpic in nature.     

Thermochemistry of dissolution of sodium alkylsulfonates

The effect of temperature on the solubility of ionic surfactants was interpreted in terms of standard enthalpy and entropy of dissolution at reference temperature by considering the change in the heat capacity. The significant value of the latter quantity causes the curvature of the function logarithm of equilibrium constant (or solubility) vs. the reciprocal thermodynamic temperature. The solubility data for several sodium n-alkylsulfonates, published by Saito, Moroi, and Matuura, were interpreted by nonlinear regression analysis. It was found that both the enthalpy and entropy of dissolution decrease with the chain length. The heat capacity increases in the course of the dissolution process.     

Effects of thermal history on enthalpy relaxation

The effects of the thermal history on enthalpy relaxation in polymethylmethacrylate (PMMA) have been studied by differential scanning calorimetry (DSC). The temperature dependence of specific heat capacity in the liquid and glassy states, that of relaxation time and the exponent of the Kohlrausch–Williams–Watts function have been obtained by the measurement of the response of heat flux to the sinusoidal temperature variation. The phenomenological model equation as an extension of linear rheology has been applied to enthalpy relaxation. The evolution of entropy under a given thermal history same as the experiment has been calculated and compared with the DSC results. The calculated results reproduce two peaks of specific heat capacity at lower and higher temperatures in the glass transition region: the former is characteristic of PMMA and the latter is observed in typical glassy polymers.     

THERMODYNAMICS-OF PRECIPITATION OF SILVER LAURATE FROM AQUEOUS SOLUTION*

The heat of precipitation of silver laurate was measured calorimetrically over the temperature range from 14 to 45°C, and the results compared with data derived from the dependence of silver laurate solubility on temperature. The standard enthalpy change was found to vary linearly with the thermodynamic temperature, whereas the standard entropy change was a linear function of the logarithm of the temperature. The slopes of both plots yielded the same value of the heat capacity change. Accordingly, the enthalpy of surfactant salts precipitation or dissolution cannot be obtained from the temperature dependency of solubility alone. The latter procedure is based on the assumption of negligible heat capacity change, which is not the case for precipitation reactions. The data on silver laurate also suggest that the heat capacity change does not vary much with temperature.     

Specific reversible melting of polymers

Temperature‐modulated differential scanning calorimetry can detect a certain amount of reversible latent heat in flexible macromolecules. In short, one can identify a reversible melting in such polymers earlier thought to exhibit only fully irreversible crystallization and melting. Details of the reversible melting of isotactic polypropylene and ethylene‐1‐octene copolymers of low and medium densities have newly been measured and linked to the crystallization, annealing, or melting temperature. It is possible to assign the experimental reversibility of melting to specific crystal fractions that ultimately melt irreversibly at higher temperatures; that is, it is suggested that reversible melting mainly occurs only between the temperatures of their formation and their zero‐entropy‐production melting temperature, at which they change to a melt of the same degree of metastability. This is supported by the almost complete absence of reversibility below the temperature of crystal formation and the observation of a distinct relationship between the amount of irreversibly by annealing reorganized material and reversibility in the case of isotactic polypropylene. A given crystal fraction, characterized by its formation temperature and zero‐entropy‐production melting temperature, has a specific reversibility of the melt‐to‐crystal transition, which is represented by the ratio of the reversible latent heat to the total enthalpy change when the crystal fraction of interest ultimately melts. This specific reversibility is, for ethylene‐1‐octene copolymers, at least 25% at temperatures in the primary crystallization range, and this indicates that the reversible contribution to the total of the melting processes is much larger than expected from simple calculations by the excess apparent reversible heat capacity being referred to the heat of fusion of the polymer, as is commonly done. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2039–2051, 2003     

Kinetic laws of thermal transformations in nickel nanofilms

Transformations in nickel nanofilms as a function of thickness (d = 3–60 nm) and temperature of heat treatment (T = 373–873 K) are studied via optical spectroscopy, microscopy, and gravimetry. It is found that, depending on the thickness of nickel films and temperature of heat treatment, the kinetic curves of the degree of transformation are satisfactorily described in terms of the linear, inverse logarithmic, cubic, and logarithmic laws. The contact potential difference for Ni and NiO films and the photovoltage for Ni-NiO systems are measured. An energy band diagram for Ni-NiO systems is constructed. A model of the thermal transformation of Ni films, including the stages of oxygen adsorption, charge carrier redistribution in a Ni-NiO contact field, and the formation of nickel(II) oxide, is proposed.     

Thermodynamic functions of water and ice confined to 2 nm radius pores

The heat capacity C(p) of the liquid state of water confined to 2 nm radius pores in Vycor glass was measured by temperature modulation calorimetry in the temperature range of 253-360 K, with an accuracy of 0.5%. On nanoconfinement, C(p) of water increases, and the broad minimum in the C(p) against T plot shifts to higher temperature. The increase in the C(p) of water is attributed to an increase in the phonon and configurational contributions. The apparent heat capacity of the liquid and partially frozen state of confined water was measured by temperature scanning calorimetry in the range of 240-280 K with an accuracy of 2%, both on cooling or heating at 6 K h(-1) rate. The enthalpy, entropy, and free energy of nanoconfined liquid water have been determined. The apparent heat capacity remains higher than that of bulk ice at 240 K and it is concluded that freezing is incomplete at 240 K. This is attributed to the intergranular-water-ice equilibrium in the pores. The nanoconfined sample melts over a 240-268 K range. For 9.6 wt % nanoconfined water concentration ( approximately 50% of the maximum filling) at 280 K, the enthalpy of water is 81.6% of the bulk water value and the entropy is 88.5%. For 21.1 wt % (100% filling) the corresponding values are 90.7% and 95.0%. The enthalpy decrease on nanoconfinement is a reflection of the change in the H-bonded structure of water. The use of the Gibbs-Thomson equation for analyzing the data has been discussed and it is found that a distribution of pore size does not entirely explain our results.     

Heat capacity and thermodynamic functions of thulium orthophosphate TmPO<Subscript>4</Subscript> in the range of 10–1350 K

The heat capacity of TmPO₄ in temperature ranges of 9.11–346.05 and 304.6–1344.6 K is measured via adiabatic and differential scanning calorimetry, respectively. The measurement data are used to calculate the temperature dependences of the heat capacity, entropy, change in enthalpy, and reduced Gibbs energy of TmPO₄ in the range of 10–1344 K. The Gibbs energy of formation of thulium orthophosphate from elements Δ_fG⁰(298.15 K) is determined.     

Temperature dependence of dimerization and dewetting of large-scale hydrophobes: a molecular dynamics study

We studied by molecular dynamics simulations the temperature dependence of hydrophobic association and drying transition of large-scale solutes. Similar to the behavior of small solutes, we found the association process to be characterized by a large negative heat capacity change. The origin of this large change in heat capacity is the high fragility of hydrogen bonds between water molecules at the interface with hydrophobic solutes; an increase in temperature breaks more hydrogen bonds at the interface than in the bulk. With increasing temperature, both entropy and enthalpy changes for association strongly decrease, while the change in free energy weakly varies, exhibiting a small minimum at high temperatures. At around T=Ts=360 K, the change in entropy is zero, a behavior similar to the solvation of small nonpolar solutes. Unexpectedly, we find that at Ts, there is still a substantial orientational ordering of the interfacial water molecules relative to the bulk. Nevertheless, at this point, the change in entropy vanishes due to a compensating contribution of translational entropy. Thus, at Ts, there is rotational order and translational disorder of the interfacial water relative to bulk water. In addition, we studied the temperature dependence of the drying-wetting transition. By calculating the contact angle of water on the hydrophobic surface at different temperatures, we compared the critical distance observed in the simulations with the critical distance predicted by macroscopic theory. Although the deviations of the predicted from the observed values are very small (8-23%), there seems to be an increase in the deviations with an increase in temperature. We suggest that these deviations emerge due to increased fluctuations, characterizing finite systems, as the temperature increases.     

Thermodynamics of glycine solution in aqueous urea. Rule \sqrt m

The solution heats of glycine in aqueous urea have been determined by calorimetry at 298 K (molality 0–13) and 313 K (molality 0–22). The solution heat of the amino acid is described by the linear dependence of this quantity on the square root of the urea molality. The enthalpy, entropy, and Gibbs energy of the transfer of glycine from water to aqueous urea, as well as the heat capacity, entropy variation, and Gibbs energy of glycine solution have been calculated for the temperature range 273–323 K. It is found that urea additions to water reverse the sign of the heat capacity of solution.     

Heat capacity and thermodynamic functions of YVO4 in the 13–347 K region

The heat capacity of YVO₄ was measured by adiabatic calorimetry in the region of 13.11–347.14 K. The values of thermodynamic functions (the entropy, enthalpy change and reduced Gibbs function) were calculated using smoothed heat capacity values. The value of the Gibbs energy of formation from simple compounds was calculated.     

Correlation between Entropy and Structural Characteristics of Metal tris-Acetylacetonates

The heat capacity of aluminum tris-acetylacetonate Al(C₅H₇O₂)₃ was measured over the temperature range 8–321 K by adiabatic calorimetry. The data obtained were used to calculate its thermodynamic functions (entropy, enthalpy, and reduced Gibbs energy). Correlation between entropy at 298.15 K and unit cell volume was observed for metal β-diketonates of the acetylacetonate group.     

Reversible and irreversible heat capacity of poly[carbonyl(ethylene‐co‐propylene)] by temperature‐modulated calorimetry

The heat capacity of poly[carbonyl(ethylene‐co‐propylene)] with 95 mol % C₂H₄? CO? (Carilon EP®) was measured with standard differential scanning calorimetry (DSC) and temperature‐modulated DSC (TMDSC). The integral functions of enthalpy, entropy, and free enthalpy were derived. With quasi‐isothermal TMDSC, the apparent reversing heat capacity was determined from 220 to 570 K, including the glass‐ and melting‐transition regions. The vibrational heat capacity of the solid and the heat capacity of the liquid served as baselines for the quantitative analysis. A small amount of apparent reversing latent heat was found in the melting range, just as for other polymers similarly analyzed. With an analysis of the heat‐flow rates in the time domain, information was collected about latent heat contributions due to annealing, melting, and crystallization. The latent heat decreased with time to an even smaller but truly reversible latent heat contribution. The main melting was fully irreversible. All contributions are discussed in the framework of a suggested scheme of six physical contributions to the apparent heat capacity. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1565–1577, 2001     

The Magnetocaloric Effect and Heat Capacity of Suspensions of High-Dispersity Samarium Ferrite

The magnetocaloric effect and specific heat capacity of an aqueous suspension of samarium ferrite were determined calorimetrically over the temperature range 288–343 K in magnetic fields of 0–0.7 T. The data obtained were used to calculate changes in the magnetic component of the molar heat capacity and entropy of the magnetic phase and changes in the enthalpy of the process under an applied magnetic field. The magnetocaloric effect was found to increase nonlinearly as the magnetic field induction grew. The corresponding temperature dependences contained a maximum at 313 K related to the second-order magnetic phase transition at the Curie point. The field and temperature dependences of heat capacity contained a maximum in fields of 0.4 T and a minimum at the magnetic phase transition temperature.     

Applications of low temperature calorimetry in material research

Low temperature calorimetry has been used not only to obtain heat capacity, entropy, enthalpy and Gibbs free energy, but also to investigate and understand lattice vibrations, metals, superconductivity, electronic and nuclear magnetism, dilute magnetic systems and structural transition involved in material research.     

The heat capacity and thermodynamic functions of EuPO<Subscript>4</Subscript> over the temperature range 0–1600 K

The heat capacity of EuPO₄ was measured by adiabatic calorimetry over the temperature range 9.81–298.48 K. The experimental and literature data were generalized to obtain the temperature dependence of the heat capacity of europium orthophosphate from 0 to 1600 K. This dependence was used to calculate thermodynamic functions (entropy, enthalpy, and reduced Gibbs energy). The data on the heat capacity of europium orthophosphate and diamagnetic lanthanum orthophosphate were used to estimate the noncooperative magnetic transition (Schottky anomaly) value.     

THERMODYNAMIC PROPERTIES OF THE DISSOLUTION OF FATTY ACID SALTS IN WATER*

The enthalpy, entropy and heat capacity change of the dissolution of calcium and barium laurates, myristates and palmitates were determined by reaction calorimetry and by solubility at different temperatures. Heat capacity change, due to water restructuring or "iceberg" formation, was found to increase with the chain length. Linearity was observed up to 12 C-atoms in the chain, while the dissolution of chains with 16 C-atoms was accompanied by a significantly higher increase in the heat capacity than it would be expected. This phenomenon was explained on the basis of synergistic effect, i.e. by mutual promotion in the water restructuring.