Temperature Dependence Of Enthalpy Research Articles

A model of heterogeneous undercooled liquid and glass accounting for temperature-dependent nonexponentiality and enthalpy fluctuation.

Dynamic heterogeneity is a fundamental characteristic of glasses and undercooled liquids. The heterogeneous nature causes some of the key features of systems' dynamics such as the temperature dependence of nonexponentiality and spatial enthalpy fluctuations. Commonly used phenomenological models such as Tool-Narayanaswamy-Moynihan (TNM) and Kovacs-Aklonis-Hutchinson-Ramos fail to fully capture this phenomenon. Here we propose a model that can predict the temperature-dependent nonexponential behavior observed in glass-forming liquids and glasses by fitting standard differential scanning calorimetry curves. This model extends the TNM framework of structural relaxation by introducing a distribution of equilibrium fictive temperature (Tfe) that accounts for heterogeneity in the undercooled liquid. This distribution is then frozen at the glass transition to account for the heterogeneous nature of the glass dynamics. The nonexponentiality parameter βKWW is obtained as a function of temperature by fitting the Kohlrauch-Williams-Watts (KWW) equation to the calculated relaxation function for various organic and inorganic undercooled liquids and glasses. The calculated temperature dependent βKWW shows good agreement with the experimental ones. We successfully model the relaxation dynamics far from equilibrium for two silicate systems that the TNM model fails to describe, confirming that temperature dependent nonexponentiality is necessary to fully describe these dynamics. The model also simulates the fluctuation of fictive temperature δTf during isothermal annealing with good qualitative agreement with the evolution of enthalpy fluctuation reported in the literature. We find that the evolution of enthalpy fluctuation during isothermal annealing heavily depends on the cooling rate, a dependence that was not previously emphasized.

Influence of lithium on the temperature dependence of heat capacity and thermodynamic functions changes in AK1 alloy based on high-purity aluminum

The heat capacity of the AK1 alloy based on high-purity aluminum with lithium was determined in the cooling mode using the known heat capacity of the reference aluminum sample. By means of mathematical processing of cooling rate curves of samples from AK1 alloy with lithium and the reference sample, polynomials describing their cooling rates were obtained. Further, the polynomials of the temperature dependence of the heat capacity of alloys, described by a four-member equation, were established using the experimentally found values of the cooling rates of samples from alloys and the standard, taking into account their mass. Using integrals from specific heat capacity, models of temperature dependence of enthalpy, entropy and Gibbs energy changes were established. It was found that the heat capacity of the alloys increases with rising temperature. The addition of lithium when the temperature is up to 600 K significantly increases the heat capacity, and at temperatures above 600 K lithium in amounts of 0.5 wt. % reduces the heat capacity of the initial alloy AK1. It is shown that lithium addition increases the enthalpy and entropy of the initial AK1 alloy and decreases the Gibbs energy value.

Unconventional floppy network structures in titanate glasses

Annealed titanate glasses are important candidate materials for high-performance electronic components, but their structural response to temperature-induced properties upon thermal annealing remains elusive. Here, using high-energy synchrotron X-ray scattering, high-temperature Raman spectroscopy, and empirical potential structure refinement simulation, we track the structural evolution in situ of a barium dititanate (BaTi2O5) glass upon annealing around and below the glass transition temperature. We find that the network structure is intrinsically floppy, as the network units ([TiOm] polyhedra) themselves and their topological packing can be easily modulated by annealing temperature. The floppy nature of the glassy titanate network challenges the currently well-cognized Rigid-Unit Mode model explaining temperature-driven structural reorganization of prototypical network glasses (e.g., silicate/borate glasses). As temperature increases, the floppy network of BaTi2O5 glass undergoes a non-monotonic, two-stage network rearrangement. After cooling back to room temperature, the distortion of network units is found recoverable whereas the change of network connectivity in the intermedium-range order is irreversible. Simulation ensembles further show the irreversible connectivity change can be ascribed to the local development of crystal-like cationic motifs, which in turn induce excess enthalpy relaxation. Our findings provide an atomic-scale perspective on temperature-dependent structural and enthalpy relaxation of a model titanate glass upon annealing, which are crucial for understanding thermal-related changes in the properties of titanate glasses.

Temperature-dependent enthalpy parameters of the molecular interaction for urea and tetramethylurea as solutes in formamide, ethylene glycol and water

Enthalpies of dilution of urea (U) solutions in formamide (FA), ethylene glycol (EG) and water were measured calorimetrically in the temperature range between (288.15 and 318.15) K. Based on the results obtained, the enthalpy-homotactic coefficients of pairwise (h22) and triplet (h222) interactions between the solvated U molecules were computed and compared with the similar data for the respective solutions of tetramethylurea (TMU). An analysis was done of how temperature influences the solvophilic and solvophobic effects manifested in the h22 and h222 parameters for U and TMU in the studied organic solvents with a spatial H-bonding network.

Computational thermochemistry of oxygenated polycyclic aromatic hydrocarbons and relevant radicals

Oxygenated polycyclic aromatic hydrocarbons (OPAHs) have attracted growing attention due to their toxicological harmfulness and significant role in soot formation. This study comprehensively investigates the thermochemistry of OPAH species and relevant radicals via quantum-chemical calculations. Temperature-dependent enthalpy of formation, entropy, and heat capacity for C5C18 OPAHs (59 molecules and 33 radicals) are consistently determined at the M06–2X/6-311++G(d,p) level of theory. Considerable differences are found to be introduced by different methods when calculating electronic energies, and the G3 method outperforms the composite compound methods G3/G4/CBS-APNO. The calculated thermochemical properties from the G3 method show excellent agreement with literature data. An accurate thermochemistry database for OPAHs is thus developed. In addition, the existing group additivity (GA) method does not apply to OPAHs since the group additivity values (GAVs) derived from small hydrocarbons fail to predict large polycyclic species. Based on our dataset, GAVs are obtained from combinatorial considerations. The updated GAVs can be applied with enhanced confidence to estimate the thermochemical parameters at different temperatures for larger OPAHs where such high-accuracy quantum chemistry calculations are intractable. These thermodynamic properties and GAVs are crucial for the development of accurate kinetic models for OPAH formation chemistry and for achieving emission control.

Thermodynamic Re-assessment of the Ag–Ni–Sn system

The ternary Ag–Ni–Sn and the constituent binary Ag–Sn and Ni–Sn systems were critically re-assessed considering the available experimental data based on the CALPHAD (CALculation of PHAse Diagram) method. Thermodynamic descriptions of both Ag–Sn and Ni–Sn systems were obviously improved in comparison with the previous reports, especially in the heat capacity of Ag3Sn and enthalpy of mixing of Ag–Sn liquid, as well as the homogeneity range of Ni3Sn4 and the temperature dependent enthalpy of mixing of Ni–Sn liquid. Based on the updated thermodynamic description for the binary systems, a set of self-consistent thermodynamic parameters for the Ag–Ni–Sn system was then obtained. Comprehensive comparisons between the calculated results and experimental data on thermodynamic properties, isothermal and vertical sections as well as invariant reactions indicated the reliability and accuracy of the present thermodynamic parameters. Liquidus projection and the scheil reaction scheme of the entire composition range were also presented correspondingly.

A theoretical investigation of the effect of Ga alloying on thermodynamic stability, electronic-structure, and oxidation resistance of Ti2AlC MAX phase

We present a systematic investigation of thermodynamic stability, phase-reaction, and chemical activity of Al containing disordered Ti2(Al-Ga)C MAX phases using machine-learning driven high-throughput framework to understand the oxidation resistance behavior with increasing temperature and exposure to static oxygen. The A-site (at Al) disordering in Ti2AlC MAX (M=Ti, A=Al, X=C) with Ga shows significant change in the chemical activity of Al with increasing temperature and exposure to static oxygen, which is expected to enable surface segregation of Al, thereby, the formation of Al2O3 and improved oxidation resistance. We performed in-depth convex hull analysis of ternary Ti–Al–C, Ti–Ga–C, and Ti–Al–Ga–C based MAX phase, and provide detailed contribution arising from electronic, chemical and vibrational entropies. The thermodynamic analysis shows change in the Gibbs formation enthalpy (ΔGform) at higher temperatures, which implies an interplay of temperature-dependent enthalpy and entropic contributions in oxidation resistance Ga doped Ti2AlC MAX phases. A detailed electronic structure and chemical bonding analysis using crystal orbital Hamilton population method reveal the origin of change in phases stability and in oxidation resistance in disorder Ti2(Al1−xGax)C MAX phases. Our electronic structure analysis correlate well with the change in oxidation resistance of Ga doped MAX phases. We believe our study provides a useful guideline to understand to role of alloying on electronic, thermodynamic, and oxidation related mechanisms of bulk MAX phases, which can work as a precursor to understand oxidation behavior of two-dimensional MAX phases, i.e., MXenes (transition metal carbides, carbonitrides and nitrides).

Surface Tension and Thermal Conductivity of NIST SRM 1155a (AISI 316L Stainless Steel)

NIST SRM 1155a is an AISI 316L stainless steel (Cr18–Ni12–Mo2) and Standard Reference Material (SRM) intended for use with test methods for elemental analysis. In a previous paper “Measurement of thermophysical properties of solid and liquid NIST SRM 316L stainless steel”, we already published reliable thermophysical properties of high temperature solid and of the liquid phase of this material such as temperature dependent enthalpy, density and electrical resistivity, as well as specific heat capacity for the solid and the liquid phase. In this paper, we add additional thermophysical properties obtained by ohmic pulse-heating and by the laser flash method, namely thermal conductivity and thermal diffusivity as a function of temperature. Furthermore we report surface tension measurement results of liquid SRM 1155a obtained by means of electromagnetic levitation. Simulation of processes like additive manufacturing, laser welding, laser cutting or metal casting depend on the above named quantities as input data. Ohmic pulse-heating as well as electromagnetic levitation are so called “containerless” investigation techniques and no significant chemical reactions of the hot liquid alloy with its surrounding occur. The data presented here are compared to the available literature data and are accompanied by an uncertainty analysis according to the “Guide to the Expression of Uncertainty in Measurement”.

Dependencies of the parameters of vacancy formation and self-diffusion in a single-component crystal on temperature and pressure

An analytical method for calculating the parameters of electroneutral vacancy formation and self-diffusion of atoms in a single-component crystal is proposed. The method is based on the four-parameter pairwise Mie–Lennard-Jones interatomic interaction potential. The method allows calculation of all activation process parameters, that is, Gibbs energy, enthalpy, entropy, and volume, for both the vacancy formation process and the self-diffusion process. The method is applicable at any pressure (P) and temperature (T). The temperature dependences of the activation process parameters for gold have been calculated from T = 10 K–1330 K along two isobars, P = 0 and 24 GPa. It is shown that, at low temperatures, due to quantum regularities, activation parameters strongly depend on temperature, and the entropy of activation processes in this region has a negative value. In the high-temperature region, the probability of vacancy formation and the self-diffusion coefficient pass into classical Arrhenius dependences, with a weakly temperature-dependent enthalpy and a positive value of the activation process entropy. Good agreement has been obtained with estimated activation parameters for gold in the literature. It is shown that at T = 0 K the activation process parameters reach their minima: the Gibbs energy, enthalpy, and volume of the activation process all become zero, and the minimum of the activation process entropy lies in the negative region. The connection of the proposed calculation method with the Varotsos cBΩ model is discussed.

Changing the Parameters of Vacancy Formation and Self-Diffusion in a Crystal with Temperature and Pressure

An analytical method for calculating the parameters of the electroneutral vacancies formation and self-diffusion of atoms in a single-component crystal is proposed. The method is based on the 4-parameters pairwise Mie--Lennard-Jones interatomic interaction potential. The method allows calculating all the activation processes parameters: Gibbs energy, enthalpy, entropy and volume for both the vacancy formation process and the self-diffusion process. The method is applicable at any pressure (P) and temperature (T). The temperature dependencies of the activation processes parameters for gold are calculated from T=10 K to 1330 K along two isobars P=0 and 24 GPa. It is shown that at low temperatures, due to quantum regularities, activation parameters strongly depend on temperature, and the entropy of activation processes in this region has a negative value. In the high temperature region, the probability of vacancy formation and the self-diffusion coefficient pass into classical Arrhenius dependencies with a weakly temperature-dependent enthalpy and with a positive value of the activation process entropy. Good agreements were obtained with the estimates of activation parameters for gold known from the literature. The values of activation parameters at T=0 K were discussed. Keywords: vacancy, self-diffusion, interatomic potential, gold, state equation, thermal expansion.

Изменение параметров образования вакансий и самодиффузии в кристалле с температурой и давлением

An analytical method for calculating the parameters of the electroneutral vacancies formation and self-diffusion of atoms in a single-component crystal is proposed. The method is based on the 4-parameters pairwise Mie–Lennard-Jones interatomic interaction potential. The method allows calculating all the activation processes parameters: Gibbs energy, enthalpy, entropy and volume for both the vacancy formation process and the self-diffusion process. The method is applicable at any pressure (P) and temperature (T). The temperature dependencies of the activation processes parameters for gold are calculated from T = 10 K to 1330 K along two isobars P = 0 and 24 GPa. It is shown that at low temperatures, due to quantum regularities, activation parameters strongly depend on temperature, and the entropy of activation processes in this region has a negative value. In the high temperature region, the probability of vacancy formation and the self-diffusion coefficient pass into classical Arrhenius dependencies with a weakly temperature-dependent enthalpy and with a positive value of the activation process entropy. Good agreements were obtained with the estimates of activation parameters for gold known from the literature. The values of activation parameters at T = 0 K were discussed.

Effects of chain length, temperature, and ionic strength on association and dissociation thermodynamics of DNA

In this study, association of oligonucleotides of length 8-22 bases and their complementary oligonucleotides to form DNA was analyzed using isothermal titration calorimetry. Additionally, dissociation of DNA into its respective oligonucleotides was analyzed using differential scanning calorimetry. The effects of chain length, temperature, and ionic strength on the association and dissociation of DNA were evaluated using thermodynamic analyses. At 25 °C, the association of 10-base oligonucleotides largely depended on ionic strength, however, for oligonucleotides of 12-base and above, association was independent of ionic strength. In oligonucleotides of length 12-, 16-, and 22-bases, both binding enthalpy and entropy changes decreased with increase in chain length around 25 °C, providing similar binding affinity. The highest association constant was approximately 1 × 109 M–1. During association of DNA oligonucleotide of the same chain length, binding enthalpy change gradually decreased with increase in temperature, indicating a negative heat capacity change around 25 °C. As the temperature increased close to the melting temperature, the binding enthalpy change increased, indicating a positive heat capacity change. The temperature-dependent binding enthalpy change was apparently extrapolated to the calorimetric enthalpy change of DNA at the melting temperature. At the melting temperature, the effect of chain length and ionic strength could be observed clearly; the stability of DNA increased with increasing chain length and ionic strength.

Viscosity, enthalpy relaxation and liquid-liquid transition of the eutectic liquid Ge15Te85

The viscosity and enthalpy relaxation in the Ge15Te85 eutectic liquid are investigated using a combination of parallel plate rheometry and conventional and modulated differential scanning calorimetry (DSC). The results of these measurements provide evidence for similar temperature dependence of enthalpy and shear relaxation timescales in this liquid. However, the enthalpy relaxation is found to be more than an order of magnitude slower than the shear relaxation process. The temperature dependence of the viscosity of the Ge15Te85 liquid in the supercooled state and above its melting point can be described using a single fragility index, except over a narrow temperature region in the immediate vicinity of the melting point where the viscosity abruptly increases, signifying a liquid-liquid transition. This anomalous viscosity jump coincides with the onset of an exothermic event observed in the DSC cooling curve. Together, these results are consistent with a transition that involves incipient liquid-liquid immiscibility that can be bypassed via rapid supercooling.

Thermodynamics of Ca(OH)2/CaO reversible reaction: Refinement of reaction equilibrium and implications for operation of chemical heat pump

Thermochemical energy storage system based on Ca(OH)2/CaO reversible reaction can also be operated as a chemical heat pump to boost the temperature of the delivered energy stream. High energy density and potential to achieve a wide range of high delivery temperatures make this system attractive for application in many high temperature industrial processes. A refinement of the equilibrium relationship between pressure and temperature for the reaction is presented in this paper by incorporating temperature dependence of the thermodynamic properties and vapor phase non-ideality. The results show that incorporation of the temperature dependence of enthalpy leads to prediction of higher equilibrium pressures and the non-ideality in vapor phase can become significant at high pressures. Equilibrium relationships of greater accuracy that are based on sound theoretical foundation are highly desired, as they have significant impact on the design and operation of the chemical heat pump/thermochemical energy storage system.

Measurement and modelling of the thermodynamic properties of Ni51Fe36Co13 soft magnetic alloy

A soft magnetic alloy, Ni51Fe36Co13 is used as the sensor in temperature sensitive magnetic switch circuit of diverse safety rod drive mechanism of advanced fast breeder reactors. In the present work the magnetic and thermodynamic properties of this indigenously developed soft ferromagnetic alloy were measured using variety of experimental techniques. The alloy has a single phase face centered cubic structure from room temperature up to 1273 K with the Curie transition at 898 ± 1 K. Temperature dependent enthalpy increment, dilatation and thermal diffusivity were measured from 298 to 1273 K and the corresponding values for isobaric heat capacity, coefficient of linear thermal expansion and thermal conductivity were determined. In addition, dynamic elastic moduli and Poisson’s ratio were also estimated using impulse excitation technique. In the course of evaluation of thermodynamic properties, contribution due to magnetic ordering was treated separately using the Calphad approach to indicate the magneto-volume and magneto-caloric effect. To understand the low temperature behavior, calculations based on quasi harmonic theory were carried out using measured thermodynamic quantities. Based on the study, it is observed that magnetic ordering has significant effect on the thermodynamic properties of Ni51Fe36Co13 ferromagnetic alloy.

Characteristic fast H\u2212 ion conduction in oxygen-substituted lanthanum hydride

Fast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H−) ions are a unique species because of their natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H− conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen-doped LaH3 (LaH3−2xOx) has an optimum ionic conductivity of 2.6 × 10−2 S cm−1, which to the best of our knowledge is the highest H− conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H− trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H−, and the framework comprising densely packed H− in LaH3−2xOx are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H− conduction.

Thermophysical parameters of organic PCM coconut oil from the T-history method and its variation with the chemical dopant

The thermophysical parameters of organic phase change material (PCM) of coconut oil (co_oil) with chemical dopant have been studied by analyzing the temperature vs time data during liquid-solid phase transition (solidification process) based on T-history method. The dopant consist of nanoparticles of graphite, CuO, and ZnO, each of them added separately to co_oil in 1 wt% and 2 wt%. The data of pure co_oil are taken as the comparison. Analysis of the data reveals the values of solid and liquid specific heats and the heat of fusion related to phase transition of co_oil+dopant. In general, we found that the solid specific heat tends to decrease with dopant concentration, while liquid specific heat tends to increase with dopant concentration with a rather small variation for all type of dopant. The heat of fusion, on the other hand, decreases with the increase of dopant concentration. The temperature dependent enthalpy shows that the solid-liquid phase transition of co_oil occurred with a temperature change, which might be due to its mixture composition.

The enthalpy-temperature curve of new PCM Ca(NO3)2.4H2O and Co(NO3)2.6H2O for low-temperature latent TES

Enthalpy is one of the most important thermophysical parameters of phase-change materials (PCM). This parameter is particularly essential to predict the performance of PCM as latent heat energy storage for certain application since it determines the thermal energy storing capability around its phase change temperature. In this paper, the thermophysical parameters (solid and liquid specific heats and specific heat of fusion) and enthalpy-temperature curve of inorganic PCM of Ca(NO3)2.4H2O and Co(NO3)2.6H2O have been studied by analyzing the temperature vs time data during liquid-solid phase transition (solidification process) based on T-history method. The analysis of the data has been performed following the original method proposed by Zhang et al. and its modification by Hong et al. In addition, the method proposed by Marín et al. have been used to obtain the temperature-dependent enthalpy. We found that the two materials have different phase change behaviors. The obtained thermophysical parameters are then compared with the data from references.

Comment on “Solubilities of ammonia in basic imidazolium ionic liquids” [Fluid Phase Equilib. 297 (2010) 34–39

The enthalpies of solvation of ammonia in imidazolium-based ionic liquids (and other associated thermodynamic properties) are re-analyzed based on the data of [Fluid Phase Equilib. 297 (2010) 34–39], correcting numerical errors in the original analysis. In contrast to the original conclusions, the re-analyzed data demonstrates that the enthalpy of solvation is not a strong function of alkyl chain length. Errors and uncertainties of the analysis procedure are also discussed, suggesting that the strong observed temperature dependence of enthalpy may in fact be an artifact of the systematic neglect of the non-ideality of the ammonia solute.

High Temperature Heat Capacity and Thermodynamic Properties of Tm2Ge2O7 and TmInGe2O7 in the Region of 350–1000 K

Tm2Ge2O7 and TmInGe2O7 germanates are obtained via solid-phase synthesis from stoichiometric mixtures of initial oxides through multi-stage annealing in the temperature range 1273–1473 K. The heat capacity of oxide compounds is measured using differential scanning calorimetry. The temperature dependences of enthalpy, enthropy, and normalized Gibbs energy are calculated from the resulting experimental data.