Thermochimica Acta最新文献

This study provides insight into benefits of thermo-chemical conversion of coal slag as recovery process into value-added products. This research involves kinetic analysis of process conducted through non-isothermal thermal analysis measurements, with additional raw material characterization. Kinetic results showed that decomposition proceeds through two consecutive reactions steps (first one, including anorthite P1̅ → I1̅ phase transition, and then production of incongruent melting product (ternary system: CaO·Al₂O₃·2SiO₂ (CAS₂), where viscosity of slag changes), and second one including dehydration and formation of meta-muscovite, and subsequently, thermal disruption of muscovite de-hydroxylated phase, which proceeds with breaking of octahedral Al–O bonds), and one single-step reaction (attributed to CO-reduction of hematite to magnetite). Thermodynamic results showed an existence of physically meaningful isokinetic temperature (T_iso), which corresponds to active vibrational frequency of surroundings of SiO₂ reaction site, manifested through Si‒O bond weakening by catalytic reaction of freed hydroxide ion (OH⁻). It was concluded that at temperature T = T_iso, the course of process loses its dependence on temperature and pressure, regulating changes between thermodynamic parameters, through enthalpy-entropy compensation (EEC) effect.

To improve the combustion characteristics of micron-sized aluminum (Al) powder, a composite powder was prepared by doping a small amount (15 wt%) of magnesium fluoride (MgF₂), polytetrafluoroethylene (PTFE), and fluorinated graphite (FG) into the aluminum powder through solution dispersion method. The physical phase composition, micro morphology, thermal reactivity, and combustion performance of the composite powders were also characterized. The findings suggest that the promoting effect of the three fluorides on aluminum combustion can be ranked in the following descending order: FG > PTFE > MgF₂. The fluorides can to erode the Al₂O₃ shell layer directly, thus creating cracks and reaction pathways for the Al core enclosed within the shell. Moreover, the gasification of AlF₃ within the combustion products can escape from the surface of Al particles and promote the oxidation combustion of the exposed aluminum, which enhances the diffusion reaction on the active aluminum surface.

The experimental study of the kinetics and modes of the thermal degradation of polymethyl methacrylate (PMMA) in an argon flow was carried out. During thermogravimetric analysis the sample heating rates were 2, 5, 8, 20 and 35 K/min. Based on the integral isoconversional method the values of the kinetic rate constants of the PMMA thermal degradation were determined. When modeling the decomposition process of PMMA for low conversion degrees, it is advisable to use the reaction rate constant obtained for the conversion degree equal to 20 %, and for modeling the whole process – 50 %. Therefore, for evaluation calculations, it is possible to describe the process of PMMA decomposition with one gross reaction. Also, the investigation of the thermal degradation of PMMA particles under conductive heating conditions (680, 700, 720 K) in an argon and air was carried out. Based on the analysis of the data obtained, a scheme for the decomposition of PMMA, consisting of four stages, was established.

Ambient temperature fluctuations can affect the thermal noise and sensitivity of a large-volume Seebeck calorimeter. This paper proposes that a reference cell considerably smaller than the sample cell in the apparatus can effectively neutralize this noise. It is found that the thermal signals from both the reference and sample cells exhibit a convolution relationship. By deconvolving two distinct thermal pulses generated by abrupt changes in cooling fluid temperature, one from the reference cell and one from the sample cell, a corresponding response function is derived. And using this function, the noise is canceled out by subtracting the convoluted signal of the reference cell from that of the sample cell during isothermal calorimetry. Experimentally, utilizing this technique with two calorimeters, one with a 17.6-liter capacity and the other with a 27-liter capacity, has been shown to reduce noise by at least 5 % and 6 % from their initial values, respectively.

Herein we present a novel ‘polymerization strategy’ aimed at synthesizing flavor precursors, with a focus on enhancing their thermal stability and release properties. Three volatile flavors, menthol, methyl cyclopentenolone, and ethyl maltol, are reacted with acryloyl chloride to produce corresponding vinyl monomers. These monomers undergo radical polymerization to form homopolymers (P1−P3). To improve solubility in alcohol solvents, a hydrophilic oligoethylene glycol monomer is introduced, copolymerized with the flavor containing monomers, resulting in copolymers (P4−P6). An important advantage of these polymers lies in their significantly enhanced thermal stability, exhibiting an increase of approximately 100 to 200 °°C compared to small molecular flavors. Furthermore, our methodology enables efficient release of the target flavors upon heating, as evidenced by online analyses of volatiles from pyrolysis using fixed-bed reactor combined with single photoionization mass spectrometry (FBR-SPIMS) and pyrolysis-gas chromatography-mass spectrometry (Py-GC–MS). This work represents a practical and innovative approach to improving the thermal stability of volatile flavors and elevating their release temperature, offering promising applications in high-temperature food processing and tobacco industries.

In this study, the isothermal oxidation kinetics of magnetite (OM), high-Mg magnetite (MM), titanomagnetite (TM), and chromite (CM) were investigated by applying thermogravimetry (TG) analysis at temperatures ranging from 1073 K to 1223 K. The results show that different high-FeO spinels possess distinct oxidizability. The oxidation process of OM in the temperature range from 1073 K to 1223 K is faster than others, followed by MM and TM. While CM exhibits the poorest oxidizability, and generally undergoes complex phase transitions. In the initial stage of oxidation, high FeO spinels have a higher oxidation rate due to the surface oxidation of spinel particles. However, the oxidation rate gradually declines in the later stages of oxidation due to increased internal diffusion resistance. The results of oxidation kinetics indicate that the initial oxidation stage of four spinels can be described as random nucleation and subsequent growth mechanism. The average apparent activation energies of the initial oxidation stage of OM, MM, TM, and CM are 25.09 kJ/mol, 32.39 kJ/mol, 58.10 kJ/mol, and 82.42 kJ/mol, respectively.

The study examined the oxidative pyrolysis of raw and residual coals extracted with tetrahydrofuran, as well as the CO/CO₂/H₂/CH₄ emissions at high temperatures (200∼900 °C). Tetrahydrofuran was found to extract aromatic and aliphatic hydrocarbons from lower metamorphic coal, as well as hydroxyl, increasing the specific surface area of char formed by oxidative pyrolysis of residual coal. The CO₂/CO ratio was temperature dependent, and the increase period was fit by a polynomial and a linear function, with critical temperature of around 570 °C. The greater CO₂/CO ratio of raw coal suggested that oxidative pyrolysis produced more CO₂. The kinetic analysis showed that the change in activation energy is inconsistent with the gas production, and the gases were found to be proportionate to the specific surface area of the coal chars, implying that the char structure has a significant influence on gaseous product emissions and the coal oxidative pyrolysis at high temperatures is a diffusion-controlled heterogeneous reaction. The findings of this study could be useful for determining the status of coal fire.

The Friedman and Ortega isoconversional methods typically apply linear regression to the isoconversional kinetic data for calculation of activation energy solely as a function of extent of conversion. However, in complex reactions, activation energy depends on both conversion and temperature. Our modification involves quadratic curve fitting instead of linear regression, resulting in the determination of activation energy as a function of conversion and temperature. The new technique enables the calculation of the temperature dependence of activation energy for different heating rates, making it a valuable addition to isoconversional analysis. The conventional and modified approaches were utilized on the isoconversional kinetic data concerning polyethylene thermal degradation. The results provided a more detailed representation of the variations in activation energy when nonlinear regression was used.

Waste printed circuit board (WPCB) is an indispensable component in any waste electrical and electronic equipment (WEEE). A promising method for recycling WPCB is pyrolysis. To understand the effect of metals and brominated flame retardants (BFR) on WPCB pyrolysis, four samples were prepared, namely (a) raw WPCB (RW), (b) non-metallic WPCB (NM), (c) bromine extracted WPCB (RW_{BFR_ext}) and (d) bromine and metal extracted WPCB (NM_{BFR_ext}). The degradation kinetics study using the isoconversional methods: Friedman, Ozawa-Flynn-Wall (OFW), Kissinger–Akahira–Sunose (KAS), and Starink showed that the extraction of metals exhibited an increase in final degradation temperature and activation energy (E_α), and altered the reaction mechanism (f(a)) whereas, the reduction of BFR reduced initial degradation temperature. The identified kinetic triplets were verified using the reconstruction profiles. The parameters can be used in designing applications and developing an energy-efficient pyrolysis process.

Hollow porous carbon nanospheres (HPCS) are ideal scaffolds for phase change materials in thermal energy storage. However, their synthesis traditionally relies on template-based routes, involving tedious procedures and high costs. This study presents a facile method for preparing HPCS through one-step carbonization of phenolic resin using CuCl₂ as the activation agent. This mild activation agent not only helps create a rich porous structure, but also maintains the hollow spherical architecture of the polymer precursor. More importantly, copper ions are reduced to copper nanoparticles during the carbonization process and are in-situ loaded into porous carbon, enhancing the thermal conductivity of the scaffold. After incorporating paraffin, the resulting composite exhibits a high phase change enthalpy of 104.4 J g⁻¹, improved thermal conductivity of 0.95 W m⁻¹ K⁻¹, and excellent thermal cycling stability (100.5 J g⁻¹ after 50 heating-cooling cycles), indicating significant potential for thermal energy storage and management.