Thermochimica Acta最新文献

The solution phase behavior of the binary systems of supercritical carbon di-oxide (SU-CO₂) + benzyl acetoacetate and SU-CO₂ + benzyl acetate was investigated in a synthetic high-pressure apparatus at five temperatures from 313.2 to 393.2 K and pressure up to 33.53 MPa for the industrial benefit of food, pharmaceutical and cosmetics application. The solubility of benzyl acetoacetate and benzyl acetate in the SU-CO₂ + benzyl acetoacetate and SU-CO₂ + benzyl acetate systems were increased with increasing temperature at constant pressure, respectively. Both system isotherms were exhibited in the simple Type-I category phase behavior. Besides, the Peng-Robinson equation of state has been successfully applied to predict the phase behavior of the SU-CO₂ + benzyl ester systems using adjustable molecular interaction parameters (k_ij and η_ij). Neither system shows a three-phase behavior at any point of temperature and pressure. A one-fluid-phase locale was ascertained above and throughout the solubility curve whereas a two-phase locale was exhibited inside the critical curve for both binary systems. The critical mixture curve provides the fingerprint for the phase behavior study of any binary system since it is used to understand and calculate thermodynamic properties effectively. The accuracy of the studied model was tested by evaluating the percentage of root mean square deviation utilizing optimized temperature-dependent mixture parameters. Indeed, this is the first reference point for the prediction of phase transition behavior for benzyl acetoacetate and benzyl acetate in SU-CO₂ and the findings make a remarkable impression on industrial applications.

Vapour pressure measurements in the Ce-Te system were carried out employing the isopiestic method. Four sets of isopiestic experimental runs were performed for the temperature range 805 – 1035 K for the composition span from ∼65 to ∼75 at% Te covering the Ce-Te phase diagram region encompassing the intermetallic compounds CeTe₂, Ce₂Te₅ and CeTe₃. Activity measurements, partial enthalpy of mixing of tellurium for CeTe₂ intermetallic compound and Gibbs energy of the reaction between CeTe₂ and Ce₂Te₅ are reported. A few phase boundaries of the Ce-Te phase diagram were redefined with the measured data and reported.

The crystallization behavior and miscibility of blends between poly(glycolic acid) (PGA) and minority poly(vinyl alcohol) (PVA) with different saponification degrees have been studied. It was found that the melting point and crystallization ability of PGA in blends were remarkably depressed. During isothermal crystallization, introduction of PVA led to a decrease in both the Avrami index and the crystallization rate of PGA. The observation of spherulite morphology further revealed that the addition of PVA inhibited the growth of PGA spherulites, but increased the density of nucleation. Besides, PVA1788 with lower saponification degree displayed a stronger impact than PVA1799 on the crystallization of PGA. All blends exhibited a single composition-dependent glass transition temperature (T_g), characteristic of miscible systems. The T_gs fitted the Kwei equation well, and the calculated interaction parameters demonstrated the formation of intermolecular interactions between PGA and PVA and revealed the stronger interactions presenting in PGA/PVA1788 blends. FTIR investigation directly confirmed the effect of PVA on the carbonyl groups of PGA and PVA1788 played more roles than PVA1799. The interactions mainly form between carbonyl groups in PGA and hydroxyl groups in PVA1799, while latter ones change to carbonyl and hydroxyl groups in PVA1788.

This study aims to investigate the effects of external positive and negative static electric fields (E₊ and E_- respectively) on thermal conductivity (K) and electrical conductivity (σ) in Al-33 wt. % Cu, Al-6.4 wt. % Ni and Al-12 wt. % Si eutectic alloys. For this purpose, the solidifications of Al-Cu, Al-Ni, and Al-Si eutectic alloys were directionally done under E₊ and E_-. The directions of E were chosen to be parallel (E₊) and antiparallel (E_-) to the solid-liquid (S-L) growth direction and the magnitudes of E were approximately (+10) and (−10) kV cm⁻¹ and (+16) and (-16) kV cm⁻¹ for the Al-Cu, Al-Ni, and Al-Si eutectic alloys, respectively. The effects of E₊ and E₋ on the K and σ were determined by the longitudinal heat flow and the four-point probe methods, respectively. While the K and σ values decreased with increasing temperature, the K and σ were increased and decreased with E₊ and E₋, respectively.

Boron (B) powder has been considered a promising high-energy material due to its high calorific value. Nevertheless, the low combustion efficiency and the difficulty in ignition restrict its application. To solve the problems, in this study, an energetic metal-organic framework (EMOF) was used as a modifier for the nano-sized B powder, and its effect on the ignition and burning performance of B powder was examined. EMOF can significantly increase the heat release of B powder and lower its initial oxidation temperature. The best improvement is achieved with 10% EMOF contents in air, while the highest heat release is obtained with 25% EMOF contents in pure oxygen. Furthermore, EMOF can also reduce the ignition delay of B powder, enhance the flame intensity, and increase the flame propagation rate. This study offers new perspectives on modifying B powder with incorporating EMOF to develop multifunctional energetic particles with improved ignition and combustion characteristics.

The article examines the curing of epoxy resin, based on diglycidyl ether Bisphenol-A, with the amine hardener Aramin (a mixture of aliphatic and aromatic amines). It was found that curing at 20, 40 and 60 °C is accompanied by microgelation. Coefficients of viscosity increase $k_{\eta }$ rise by 2, 3 and 4 times respectively. The possibility of using second order equation, Kamal catalytic equation, auto-acceleration and auto-inhibition equations and describing the curing process was considered. It was shown that no one of the equations describes the entire process with a high degree of accuracy. Equations, which could adequately describe distinct stages of the process, have been found. The conditions (time and degree of conversion) for the onset of microgelation, gelation and transition from a kinetic to a diffusion-controlled mechanism were established.

Investigating precipitation processes in aluminium alloys during cooling from the solutionising temperature is important because the level of solute supersaturation and the presence of pre-precipitated solutes determine the response to the subsequent age hardening step. Differential scanning calorimetry has been developed to a suitable method to follow precipitation over a wide range of cooling rates. We develop a device that allows us to measure electrical resistivity in-situ during the quenching of alloy samples from the solutionising temperature. A procedure is formulated that allows us to separate the signal related to precipitation from the large background caused by the temperature dependence of electrical resistivity. Application to an aluminium alloy 6014 reveals a two-stage precipitation reaction during cooling at rates between 1 and 20 K min^-1, the first related to precipitation of the stable β phase, the second due to the formation of various metastable phases. Comparison between resistivity and DSC signals measured at the same cooling rate shows very close correspondence between the two. Thus, in the future, both methods could be used in a complementary way.

Predicting the thermodynamic properties of multicomponent solution based on binary data is highly desirable. However, traditional methods face many challenges in practical applications due to the unclear mechanisms for obtaining the molar composition of sub-binary terms. In this article, a new extrapolation model is suggested, which derived from the assumption of the Kohler model. It determines the molar composition points of each sub-binary system through a clear mechanism by introducing the contribution coefficient, defined by the property differences between two components. Moreover, the new model can mathematically obtain all potential molar composition points of sub-binary systems. Additionally, a simple and effective method for calculating the property difference between two components is recommended. The performance of this new extrapolation model is demonstrated in several multicomponent alloy systems with different properties.

The determination of biomass composition via thermogravimetric analysis (TGA) has been a subject of considerable interest for many years. The current work proposes a revised workflow for determining the amounts of cellulose, hemicellulose, and lignin in biomass by combining TGA under an inert atmosphere with analyses of extractives and ash. An independent parallel reaction (IPR) model used for the deconvolution of the derivative thermogravimetry data was improved by constraining model parameters, i.e., thermal decomposition kinetic parameters and char fractions of cellulose, hemicellulose, and lignin, with values compiled from the literature using statistical analysis. The workflow is developed and demonstrated using cellulose and starch mixtures and then applied to biomass with varying levels of ash, including pine, birch, and oak wood, switchgrass, and pine bark. Using extractive-free biomass in the new TGA-IPR workflow improved the composition results compared with untreated biomass. The compositions determined by this method agreed well with values reported in the literature (within approx. 8 wt%) for the tested samples. The results demonstrate improved biomass composition accuracy using an accessible and rapid TGA-based approach.

A visualized macro thermogravimetric analyzer was utilized to gather data on sample weight, temperature, and image signals of centimeter-scale PET. The PET was subjected to both fast (above 300 K/min) and slow (below 25 K/min) heating rates. The experimental findings revealed that weight loss mainly occurred at different temperature ranges under fast (above 610 °C) and slow (400–520 °C) heating rates. The isoconversional method (ICM) and the distributed activation energy model (DAEM), both assuming single-step reactions, were employed separately to predict the conversion and rate of PET pyrolysis. However, the prediction error was considerable. To address this issue, a discrete distributed activation energy model (DDAEM) was developed, incorporating both single-step and double-step parallel reactions. The DDAEM yielded a prediction error within 10 %, which is better than ICM and DDAEM. Furthermore, all three models (ICM, DAEM, and DDAEM) indicated significant discrepancies in activation energies between fast and slow heating rates.