Thermochimica Acta最新文献

Organic compounds can be used as temperature calibrants in fast scanning calorimetry. Their advantages include ease of surface cleaning of the calorimetric chip and good thermal contact with the chip surface. Among several compounds tested, benzoic acid was identified as a convenient and reliable calibrant for temperatures below approximately 130 °C. However, organic calibrants often exhibit unusual heating rate dependencies of the onset temperatures of melting. This phenomenon can be semi-quantitatively explained by considering different heat flows within the sensor. Notably, the thermal resistance between the heater and thermopile, often overlooked, introduces an additional time constant that can sometimes result in a negative apparent thermal lag. In addition, the onset temperatures are influenced by factors such as sample position, thickness, surface wetting, and spreading. These factors limit the accuracy of transition temperature determinations to approximately ±1 K below 130 °C and ±5 K up to 220 °C.

This study aimed to clarify the secondary crystallization process of low-isotacticity polypropylene (LT-PP). LT-PP demonstrates an exceptionally low crystallization rate at room temperature, which is approximately 1/5000 lower than that of isotactic PP (iPP). During the secondary crystallization of LT-PP at 30 °C, the thickness of lamellar (c-axis) and a- and b-axes of crystallite size remained constant. In addition, no significant change was observed in the CC-C bending vibration. It seems that the direction of the CC-C molecular order is similar to the thickness direction. This vibration mode may be associated with changes in the thickness of the lamellae. To explain the log(t) dependence of crystallinity, the Seto–Frank model was employed.

The primary objective of this research is to explore the feasibility of synthesizing phase-pure perovskite SrSnO₃ doped with transition metals and to evaluate the potential of these products as high-temperature inorganic pigments. The initial step in preparing perovskite powders with the general formula SrSn_0.95M_0.05O_3-δ (M = Mn, Fe, Co, Ni) involved synthesizing SrSn_0.95M_0.05(OH)₆ followed by its thermal decomposition. The thermal decomposition processes and the reaction pathway for perovskite formation were analyzed using thermal analysis and X-ray diffraction analysis. Single-phase products of beige SrSn_0.95Fe_0.05O_3-δ and brown SrSn_0.95Co_0.05O_3-δ were successfully obtained by calcining the precursors at 1,100 °C. In contrast, brown SrSn_0.95Mn_0.05O_3-δ contained a phase impurity of SnO₂ and doping with Ni ions resulted in a phase mixture of SrSnO₃ and NiO. The pigment quality of the powders was assessed based on their color parameters, described using the CIE Lab system.

Epoxy/multilayer graphene (ML-graphene) composites were prepared using different curing agents to control the graphene dispersion by changing the curing reactivity. With increasing initial reactivity, the aggregation size of the ML-graphene decreased and their thermal conductivity increased. In particular, the thermal conductivity of the composite prepared with p-phenylenediamine showed a maximum value of 1.46 W/(m·K) at 25 wt% ML-graphene loading because of the highest initial curing reactivity. The application of a magnetic field led to graphene alignment along the applied field, resulting in two times higher thermal conductivity than that of the corresponding system without magnetic field. The relationship between the interfacial affinity for epoxy/graphene and thermal conductivity was also investigated. As a result, resulting in a biphenyl epoxy composite showed higher thermal conductivity (6.17 W/(m·K)) than that of the bisphenol-A epoxy composite. This is derived that the π-conjugated and planar structure of biphenyl epoxy can easily interact with the surface of graphene.

Ethanol is a promising sustainable fuel for its environmental friendliness and renewability. Due to the association effect in ethanol molecules, the particular behavior in isobaric heat capacity was explored by combining experimental and theoretical methods. Experimental isobaric heat capacity measurements of ethanol were performed over the temperature range from (298.15 to 573.15) K and at pressures up to 15 MPa in both liquid and vapor phases by a flow calorimeter. Different association schemes were combined respectively with PC-SAFT equation of state and SAFT-VR Mie equation of state to compare their accuracy in isobaric heat capacity prediction, and it could be concluded that two-site (2B) model was better than three-site (3B) model. It was also found that PC-SAFT equation of state was able to yield good results in predicting the isobaric heat capacity far from the saturated state and critical region, however, SAFT-VR Mie equation of state showed better prediction performance near the saturated state and critical region.

The thermotropic phase behavior, molecular structure and supramolecular organization of a homologous series of N,O-diacyl-β-alaninols (DABAOHs) with matched acyl chains (C9-C18) are reported. The C9-C11 DABAOHs showed a single thermotropic transition in DSC studies, whereas the longer chainlength compounds gave two transitions. Transition temperatures, enthalpies and entropies of the DABAOHs exhibited odd-even alternation, suggesting minor differences in the packing of odd- and even chain length compounds. Crystal structure of N,O-didecanoyl-β-alaninol revealed a bent geometry, with several N-H···O and C-H···O hydrogen bonds stabilizing the molecular packing. Powder X-ray diffraction studies suggested that all DABAOHs are packed in a tilted bilayer mode. These results provide a thermodynamic and structural basis for investigating the structure-function relationships of N,O-diacyl-β-alaninols.

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.