Thermochimica Acta最新文献

Phase change materials (PCMs) have a wide range of applications in latent heat storage and thermal management. However, their practical use is hindered by high leakage rates and low thermal conductivity. To address these issues, polyvinyl alcohol/carboxylated carbon nanotubes/graphene hybrid aerogels (PCG) were carbonized at high temperatures to obtain polyvinyl alcohol/carboxylated carbon nanotubes/graphene carbon aerogels (cPCG). Polyethylene glycol (PEG) was then encapsulated within cPCG to form cPCG@PEG shape-stabilized PCMs (SSPCMs). These cPCG@PEG SSPCMs demonstrated excellent thermal conductivity (0.843 W•m^-1•K^-1) and superior solar-thermal conversion performance (91.8%). Additionally, the latent heat of cPCG@PEG showed a minimal decrease even after 100 melt-crystallization cycles. An experimental setup was designed to regulate the temperature of solar photovoltaic (PV) panels using cPCG@PEG. The results indicated that cPCG@PEG effectively managed the temperature of solar PV panels under varying light conditions. This study presents a novel approach for enhancing the application of porous PCMs in solar energy utilization and thermal management of equipment.

Curing kinetics are crucial for designing and optimizing the process parameters of a resin. This study examines the non-isothermal curing kinetics of acrolein-pentaerythritol (APE) resins, focusing on the impact of molecular weight (MW) and molecular weight distribution (MWD) on their cure behavior. Kinetic parameters were determined using isoconversional and combined kinetic analysis methods through microcalorimeter measurements. The findings suggest that the cure process follows the nucleation and growth models (Avrami−Erofeev equation), with an activation energy of 72.2 kJ/mol. A comparison of two APE resins with different molecular weights and molecular weight distributions reveals that higher MW components expedite the initial curing reaction but impede the main curing process, leading to extended curing durations. This study provides valuable insights into the curing kinetics of APE resin and the influence of MW and MWD, contributing to the reliable and reproducible production of composite parts.

The catalytic effect of cobalt tall oil and cobalt sunflower oil catalysts on the oxidation kinetics of heavy crude oil was investigated in this study. Comprehensive kinetic analyses, employing differential scanning calorimetry, thermogravimetric analysis, and kinetic modeling techniques, revealed that the presence of these catalysts significantly influenced the oxidation behavior of heavy oil. The catalysts exhibited pronounced shifts in the DSC and TG curves towards lower temperatures, indicating facilitated initiation of oxidation reactions at lower onset temperatures. Quantitative kinetic parameters, including activation energies and frequency factors, were determined using the Friedman and Kissinger-Akahira-Sunose analyses. The cobalt tall oil catalyst demonstrated superior performance, effectively lowering the activation energy barrier and increasing oxidation rates, particularly at higher conversion degrees. Catalyst characterization techniques, including X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy, revealed the formation of highly crystalline cobalt oxide nanoparticles with optimal dispersion and size distribution, as well as the presence of favorable functional groups for surface interactions. The results elucidated the role of these catalysts in facilitating the oxidation process through the provision of active sites, altered reaction pathways, favorable steric environments, and efficient oxygen activation capabilities. These findings contribute to the development of efficient catalytic systems for heavy oil upgrading processes and offer insights for further optimization of catalyst properties to achieve desired oxidation kinetics behavior.

The present work re-evaluates previously published in-situ high-energy x-ray diffraction (HEXRD) and differential scanning calorimetry (DSC) data on EN AW-6082, which were used to study the precipitation kinetics of stable β-Mg₂Si. Here, we address hitherto unattended information in the diffraction patterns. The revised analysis considers metastable precipitates and thermodynamically stable Fe-containing phases in addition to stable β-Mg₂Si investigated in the previous studies. Furthermore, we utilize mean-field simulations to convert the evolution of individual phases obtained from HEXRD data into an equivalent excess specific heat $c_p^{\text{ex}}$ signal. This methodology allows us to partly separate cooling and heating DSC data into the contributions of individual phases and make a quantitative comparison between results from HEXRD and DSC. This significantly improves our current understanding of DSC data and demonstrates, for instance, the difference in complexity between interpreting cooling and heating experiments in aluminum alloys.

The study of the system formed by ortho- and pyrophosphoric acid was resumed in order to understand the crystallization conditions of these two compounds and to highlight the existence of their possible polymorphism. To this end, the solid-liquid equilibria (SLE) of pyrophosphoric acid was studied in depth. Contradictions in literature data were resolved through systematic experimentation: solubility measurements, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Calorimetric measurements confirmed the existence of two crystalline forms of pyrophosphoric acid, and their stability domains were determined. Furthermore, thermodynamic modeling of the SLE has led to a consistent and refined representation of the observed phenomena. In particular, the transition temperature from low-temperature (form I) to high-temperature form (form II) of pyrophosphoric acid was determined at 298.4 K and the coordinates of the eutectic point common between H₃PO₄ and H₄P₂O₇ (I) were precisely determined. Modeling also confirms the non-negligible quantity of triphosphoric acid in the liquid state throughout virtually the entire compositional range. Finally, X-ray powder diffraction data were used to determine the cell parameters and space group of pyrophosphoric acid using EXPO 2014 software.

In this study, we introduced a novel technique that factorizes the reaction rate of complex reactions into a temperature-dependent rate constant and a conversion function using multiple linear regression on isoconversional kinetic data. In simulated reactions, our method demonstrated higher accuracy compared to model-free approaches. Furthermore, the new method exhibited satisfactory accuracy in non-isothermal predictions of polyethylene thermal decomposition, all without necessitating the computation of Arrhenius parameters. However, the new method enables the assessment of the Arrhenius parameters, the activation energy and pre-exponential factor, in complex reactions as well. The accuracy of this method is confined to the experimentally explored temperature range, necessitating cautious extrapolation for temperatures beyond this interval.

Two n-alkyl azides, n-tetradecyl- and n-octadecyl azides, were synthesized and characterized by differential scanning calorimetry and fast scanning calorimetry. Based on the experiments performed, the enthalpies and temperatures of solid-solid transitions and fusion, condensed phase heat capacities, vapor pressures above the liquids, and vaporization enthalpies were obtained, and temperature limits of stability were determined. Combining the newly obtained and existing literature data on the vaporization characteristics of n-alkyl azides, the linear dependences of the vaporization enthalpy and the natural logarithm of vapor pressure on the chain length were derived at 298.15 K and the p-T properties of unstudied members of homologous series were predicted.

In this study, Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 metallic glass is annealed at T_g and its impact on crystal growth is demonstrated with nanocalorimetry. With annealing following the rapid quenching, an amorphous phase free of nuclei, a relaxed amorphous phase, an amorphous phase with nuclei, and an amorphous phase with crystals are sequentially produced. With the reheating at rates ranging from 100 to 50,000 K/s, these four stages are quantitatively distinguished. Additionally, crystal growth behaviors of these four stages are demonstrated by the Kissinger and Mauro-Yue-Ellison-Gupta-Allan model. For the quenched and relaxed amorphous phases, the apparent crystallization activation energy (E_a) decreases with increasing heating rate, with a noticeable upward deviation at ultrahigh heating rates. When nuclei and crystals form in the amorphous phase, E_a keeps decreasing with the heating rate. As the annealing time increases, the maximum growth rate (u_max) exhibits a monotonic increase while the temperature corresponding to u_max displays a maximum.

Hydrate-based cold storage and refrigeration offer notable efficiency and safety advantages. By leveraging the high cold storage density of CO₂ hydrate and the low phase equilibrium pressure and GWP of DME and C₃H₈ hydrates, a gas mixture hydrate (50 mol.% CO₂, 33 mol.% DME, 17 mol.% C₃H₈) was used as a refrigerant for phase equilibrium assessment. Results showed phase equilibrium temperatures of 274.5 to 278.9 K at pressures from 0.4 to 0.86 MPa, aligning with conventional air conditioning pressures. Additives like THF, CP, TBAB, and TBAC improved conditions. Liquid promoters (THF, CP) increased temperatures by 4.5 to 7.9 K, while solid promoters (TBAB, TBAC) raised them by 5.3 to 11.1 K. Liquid promoters shifted the phase equilibrium curve for CO₂-DME-C₃H₈ hydrate almost parallel to the right, while solid promoters steepened the curve slope. A predictive model was developed, showing good molar phase change enthalpy.

Optimizing a practical polymerization strategy for poly(m-xylylene adipamide) (PA MXD6) requires regulating the high-temperature residence time and preventing the solidification of the reaction mixture. Dynamic heating strategies have shown promise in addressing this issue. However, conventional polycondensation kinetics are not optimal for characterizing nonisothermal processes due to continuous changes in the reactant state. This study employed thermal analysis as a continuous monitoring method to comprehensively investigate the effects of pressure, temperature, and diffusion on polymerization. The results indicate that high heating rates lead to faster reaction rates, as evidenced by the evolution of the kinetic parameters throughout the reaction process. Nevertheless, excessively high heating rates increase the solidification risk. To resolve this contradiction, a low-rate heating process with pressure was developed for efficient polymerization and scale-up, resulting in superior products. This study provides new insights into polyamide polymerization and offers practical guidelines for enhancing polymerization efficiency and process stability.