Thermochimica Acta最新文献

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.

Crystallization transformation kinetics and fragility of (Cu₅₀Zr₄₃Al₇)₉₈Y₂ bulk metallic glass (BMG) were investigated at non-isothermal and isothermal conditions by differential scanning calorimetry. Activation energies for the BMG were calculated for the glass transition, onset crystallization, and crystallization peak using various methods of non-isothermal analysis. Results suggested that atomic rearrangement during glass transition is more complex than crystallization, and growth poses greater challenges than nucleation. Isothermal analysis conducted in the supercooled liquid region provides evidence of crystallization being controlled by diffusion, with a calculated mean Avrami exponent of 2.2. Additionally, the findings of fragility studies and kinetic studies demonstrated a strong correlation with the glass-forming ability (GFA), thereby validating the high GFA of the BMG analyzed in this study. Thus, this research results provide a detailed understanding of the complex crystallization kinetics, thermal behavior, and GFA of (Cu₅₀Zr₄₃Al₇)₉₈Y₂ BMG, emphasizing its potential in materials science applications.

Despite the growing popularity of phase change materials (PCM) and suspensions of micro-encapsulated phase change materials (mPCM) in both industrial and scientific applications, their properties characterization remains partial and mainly limited to thermal properties. The characterization of such suspension is a crucial aspect for comprehending their behavior and optimizing their performance. This paper is dedicated to a wide characterization of two suspensions containing micro-encapsulated phase change material. In pursuit of a thorough understanding, we also extend our characterization efforts to the bulk PCM encapsulated within the particles.

Our primary focus is on determining the materials thermal properties such as latent heat and specific heat capacity using differential scanning calorimetry. Different cooling and heating rates have been employed in our measurements. Regarding our experiments, the bulk phase change material is predominantly identified as octadecane. Results obtained from calorimetry are then compared between the bulk PCM and the mPCM suspensions. By comparing the magnitudes of latent heat obtained for suspensions with the bulk material, we can accurately determine the mass fraction of PCM within each suspension. Noteworthy, during the solidification process an additional latent heat peak is observed only for the encapsulated PCM.

The density of the materials is also measured. The phase change of PCM included in the capsules can be observed in densimetry: within the studied temperature range (283.15–306.15 K), the most significant density variations occur during intervals associated with phase change transitions. In ranges where the PCM, included in the capsules, is in a single-phase state (solid/liquid), we provide linear laws that account for density variations with temperature.

Finally, our characterization work focuses on the rheological properties of the materials. While the bulk PCM in liquid phase exhibits a Newtonian viscosity, mPCM suspensions present a non-Newtonian viscosity. Both shear-thinning and shear-thickening behaviors are observed depending on the suspension particle volume fraction $\varphi$ of mPCM. This paper concludes with a full comparison of our characterization results with models and correlations proposed in the literature.

Coal-to-liquid (CTL) base oil is a high-quality lubricant base oil made from coal. However, its poor oxidation stability and short service life limit its application. Pressurized differential scanning calorimetry and Rotary bomb oxidation test are utilized for the evaluation of oxidation stability. Thermogravimetric analysis is performed for the thermal stability assessment. The results showed the reasonable combination of commercial antioxidants can significantly improve the oxidative and thermal stability of two typical CTL base oils, CTL3 and CTL6. The synergistic mechanism of antioxidants in CTL base oil was studied from the perspective of pyrolysis kinetics and pyrolysis mechanism. For CTL3, adding the compound antioxidant 2,6-di-tert-butyl-4-methylphenol (T501) and n-phenyl-1-naphthylamine (T531) results in an almost eightfold increase in oxidation induction time (OIT) and a 33.4 % increase in activation energy compared to pure CTL3. For CTL6, adding the compound antioxidant dialkyl dithiophosphate (T203) and diphenylamine (L57) results in an almost tenfold increase in OIT and a 10.9 % increase in activation energy compared to pure CTL6. The designed commercial antioxidant compound systems had excellent synergistic effects and the synergistic mechanisms were illustrated.

In this work, alkali metal hydrates, barium hydroxide octahydrate (BHO) and sodium acetate trihydrate (SAT) were added into epoxy resin (EP) to prepare a series of flame-retardant EP composites. It showed that EP samples can pass the V-0 rating of UL-94 when the amount of hydrated salt exceeds 45 wt%. When the additional amount of SAT is 50 wt%, the LOI of the EP sample increases to as high as 39% compared to 19% of pure EP. In addition, the time to ignition (TTI) of EP with 50 wt% SAT was prolonged to 157 s from 63 s of pure EP, which can provide valuable time for fire escape. The peak heat release rate (PHRR) was reduced by 63.9% compared with that of pure EP (from 1392.4 kW/m² to 502.18 kW/m²), which indicated that phase change heat absorption plays a crucial role in reducing fire hazards of EP.