Thermochimica Acta最新文献

In this work Photopyroelectric Techniques for thermal and optical characterization are introduced for measuring thermal diffusivity and optical absorption coefficient of colloidal suspensions of SiO₂. Thermal diffusivity and Optical absorption coefficients at 904 nm, 940 nm, 980 nm and 1550 nm were measured for spherical SiO₂ colloidal suspensions of 61 and 673 nm average size, in a range of 1 mg/mL to 26 mg/mL. It was found a linear increment of this optical property with SiO₂ nano-spheres concentration, which was not possible to obtain by means of absorbance measurements with conventional UV-Vis spectroscopy; in relation with thermal diffusivity it was found a slightly increment of this thermal property with increasing concentration, however this increment was not as large as the one reported for similar samples but using experimental techniques involving the optical properties of the samples. The direct measurement of optical absorption coefficients of colloidal suspensions allowed the knowledge of their absorptivity at the selected wavelengths. The presented methodology could be useful for measuring this optical property for colloidal suspensions in general at other wavelengths and as a function of other variables of interest, such as size particle.

With the rising use of metallic implants worldwide, bacterial infection has become a major concern due to its significant impact on patient health and national healthcare budgets. This study aimed to evaluate the sensitivity of Pseudomonas aeruginosa, a common bacterium responsible for implant surgery infections, to different concentrations of titanium tetrachloride (TiCl₄) solutions. Microcalorimetry was used to analyze the heat output produced by bacteria metabolism when exposed to TiCl₄ at concentrations ranging from 0 to 10 mM, as well as a saturated solution. The inner chamber of the calorimeter was set to the physiological temperature of the human body (309.65 K), and the heat output produced by bacteria metabolism was collected at intervals of 22.2 s for 48 h using a data acquisition and processing system. The data was then represented as thermograms. The results showed that higher concentrations of TiCl₄ had a bactericidal effect on the growth of Pseudomonas aeruginosa. This study supports the potential use of higher concentrations of this compound in surgical implants to reduce the risk of infection development due to its bactericidal properties.

As a type of renewable and biodegradable polymer, poly(lactic acid) (PLA) has been widely used in various fields. However, its weak crystallization ability deteriorates the properties of the final product. In this work, a novel nucleating agent (MWCNTs@CeO₂), multi-walled carbon nanotubes (MWCNTs) decorated with ceria (CeO₂), was synthesized using the hydrothermal method and subsequently used to manipulate the non-isothermal crystallization kinetics and nucleation behavior of PLA. The results show that MWCNTs@CeO₂ can act as an effective nucleating agent to accelerate the crystallization rate of PLA by reducing the half-crystallization time. In non-isothermal crystallization kinetics, the Avrami equation modified by Jeziorny cannot accurately describe the non-isothermal crystallization behavior of PLA and PLA/MWCNTs@CeO₂ composite films, whereas the Mo method can. Furthermore, it is found that the addition of MWCNTs@CeO₂ does not change the crystal structure of PLA, but increases the nucleation and growth rate of PLA, thereby promoting its crystallization.

To study the phase behavior and role of defects in the ion conduction of a well-investigated plastic crystal electrolyte, we measured the low-temperature heat capacities of quinuclidinium hexafluorophosphate ([HQ]PF₆) plastic crystal and the defects of pure [HQ]PF₆ and its mixtures containing lithium ions and chloride ions. The heat capacities of the pure material show two solid‒solid phase transitions at room temperature and a low temperature. Positron annihilation lifetime spectroscopy (PALS) revealed the phase dependence of the defect size for both the pure material and the mixtures. The defects in these two mixtures expand largely to approximately 2 (with Li⁺) and 2.5 (with Cl⁻) times larger than those in the pure material. The relationships between the ionic conductivity and vacancy volume of the pure material and the mixtures obey the Cohen−Turnbull free volume model, which is also phase dependent. This work help elucidate defect-assisted ion transfer in organic ionic plastic crystals.

The process of pyrolysis of liquid crystalline polyurethane (LCPU) composites with polyhedral oligomeric silsesquioxanes (POSS) has been studied using pyrolysis - gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry/Fourier transform infrared spectroscopy (TG/FTIR) coupled thermoanalytical techniques. LCPU elastomers modified with POSS molecules with different architectures, bearing aromatic, isobutyl and cycloaliphatic moieties, yield various volatile products of pyrolysis that were collected and characterized. The TG study showed significant differences in the thermal behavior of composites studied, evidencing the influence of the silsesquioxane type on the polyurethane pyrolysis paths. FTIR study of volatile products of thermal decomposition showed the presence of characteristic bands originated from carbonyl, amine, ether, methylene, and unsaturated hydrocarbon linkages. GC/MS investigation confirmed the existence of compounds identified by the IR method. The application of both thermoanalytical methods made it possible to postulate the thermal decomposition routes of PU/POSS hybrids and to point out the main factors responsible for changes in the thermal stability.

The recrystallization and grain growth activation energies of the hybrid AZ31/Mg-0.6Gd (wt.%) alloy were calculated using differential scanning calorimetry analyses and scanning electron microscopy, respectively, after fabricating by high-pressure torsion up to 20 turns and then subjecting to an isochronal annealing treatment from 423 to 723 K for 1 h. The DSC results show one exothermic peak belonging to the static recrystallization of the AZ31 region with an activation energy of 112 ± 10 kJ/mol. The grain growth kinetics for the AZ31 and Mg-0.6Gd regions were described by the Arrhenius equation. The calculation with a grain growth exponent equal to 4 gave values for the activation energies in both the AZ31 (146.2 ± 8.4 kJ/mol) and Mg-0.6Gd (90.9 ± 13.5 kJ/mol) regions. The present results reveal the heterogeneity of the thermal stability of the AZ31/Mg-0.6Gd hybrid material.

In this work, the pyrolysis behaviors and kinetics of two bamboo (Bambusa texlitis) wastes were investigated via thermogravimetric analysis. The weight loss behaviors of pyrolysis were studied by curvature of the conversion (α) curves. The curvature of α can depict the characteristic temperatures of pseudo component (hemicellulose, cellulose and lignin) pyrolysis. Model-free model was applied to estimate the apparent activation energies of the pure components for comparison with other kinetic models. Three parallel reaction (TPR) and parallel finite number first-order reaction (FNFOR) models were used for kinetic studies. The first-order TPR model underestimates the apparent activation energies of pseudo components. The nth-order TPR model underestimates the apparent activation energy of lignin and obtains characteristic temperatures that are inconsistent with those of pure lignin. The parallel FNFOR model is more reliable for simulating the kinetics of lignocellulosic biomass pyrolysis. The apparent activation energies of pseudo components depend on the variety of lignocellulosic biomass.

Non-isothermal thermogravimetry was used to study the thermal decomposition of the nifedipine drug. The dominant process of thermal molecular degradation (as confirmed by Raman spectroscopy) starts very slowly at ∼ 150 °C, which is below the melting point of T_m ≈ 170 °C. The decomposition kinetics was described in terms of the autocatalytic Šesták-Berggren kinetics; a novel approach denoted as single-curve multivariate kinetic analysis (sc-MKA) was used to determine the following set of parameters: activation energy of decomposition E_d = 115.5 ± 2.4 kJ mol^-1, decomposition pre-exponential factor log(A_d/s^-1) ≈ 8.5–8.9, Šesták-Berggren kinetic exponents M ≈ 0–0.2 and N ≈ 0.3–0.5. The consequent kinetic predictions based on these results have confirmed the good thermal stability of nifedipine, which allows for the melt-quenching preparation of the amorphous phase as well as for the processing via hot melt extrusion and 3D printing. An in-depth analysis of the relevant performance of the sc-MKA approach was done based on the akin literature data for indomethacin, nimesulide, and griseofulvin. The comparison has revealed more than sufficient performance of the sc-MKA method with regard to its application to the thermal decomposition data of active pharmaceutical substances. The most critical aspect of the sc-MKA procedure was demonstrated to be the accurate determination of the model-free parameters (activation energy E_d and pre-exponential factor A_d) and their temperature trends. Considering the specificity of the sc-MKA method, i.e., the fixed value of E_d(T), the determination of the temperature dependence of A_d(T) is of utmost importance and worth extensive repeatability checks.

This study investigated the kinetics of the multistep thermal dehydration/decomposition of the metakaolin-based geopolymer paste. The component two reaction steps were characterized by the evolution of water vapor and the simultaneous evolution of water vapor and CO₂, respectively. In a stream of dry N₂, the kinetics of the first and second reaction steps were characterized by the apparent activation energy (E_a) values of 92 and 166 kJ mol⁻¹, respectively. Both reaction steps exhibited a diffusion-controlled rate behavior. In a stream of wet N₂, the mass loss curves systematically shifted to higher temperatures with an increase in the water vapor pressure (p(H₂O)). The first reaction step was significantly influenced by p(H₂O), and the apparent E_a increased to 175 kJ mol⁻¹ at p(H₂O) = 11.4 kPa. The second reaction step was less sensitive to the atmospheric water vapor, as characterized by its E_a of ∼165 kJ mol⁻¹, irrespective of the p(H₂O).