Isothermal Segments Research Articles

Mechanistic insights into on-surface reactions from isothermal temperature-programmed X-ray photoelectron spectroscopy.

On-surface synthesis often proceeds under kinetic control due to the irreversibility of key reaction steps, rendering kinetic studies pivotal. The accurate quantification of reaction rates also bears potential for unveiling reaction mechanisms. Temperature-Programmed X-ray Photoelectron Spectroscopy (TP-XPS) has emerged as an analytical tool for kinetic studies with splendid chemical and sufficient temporal resolution. Here, we demonstrate that the common linear temperature ramps lead to fitting ambiguities. Moreover, pinpointing the reaction order remains intricate, although this key parameter entails information on atomistic mechanisms. Yet, TP-XPS experiments with a stepped temperature profile comprised of isothermal segments facilitate the direct quantification of rate constants from fitting time courses. Thereby, rate constants are obtained for a series of temperatures, which allows independent extraction of both activation energies and pre-exponentials from Arrhenius plots. By using two analogous doubly versus triply brominated aromatic model compounds, we found that their debromination on Ag(111) is best modeled by second-order kinetics and thus proceeds via the involvement of a second, non-obvious reactant. Accordingly, we propose that debromination is activated by surface supplied Ag adatoms. This hypothesis is supported by Density Functional Theory (DFT) calculations. We foresee auspicious prospects for this TP-XPS variant for further exploring the kinetics and mechanisms of on-surface reactions.

Optimization of DSC measurements for organic phase change materials

Phase Change Materials (PCM) have gained popularity for Thermal Energy Storage (TES) in several areas, like cold chain and building heating and cooling, etc. This work focuses on optimizing and later standardizing thermal analysis of organic PCM using Differential Scanning Calorimetry (DSC). For this, dodecane, and hexadecane were used as PCM, with melting points of −10 °C and 18 °C respectively. A set of experiments was planned using Design of Experiments (DOE), and accordingly, several DSC measurements using the heat flux method were employed to analyze the effect of parameters such as baseline shifts, ramp rates, operating temperatures, duration of isothermal segments, sample mass, and crucibles. The results show that precise enthalpy and onset values are achieved using a ramp rate of 0.5 K/min or less, duration of isothermal segments of 5 min, sample mass of <10 mg, and a temperature window of ±20 °C from melting points. Using appropriate parameters, DSC is an effective way to characterize a PCM.

Thermogravimetric studies, kinetic modeling and product analysis of the pyrolysis of model polymers for technical polyurethane applications

The pyrolysis of four kinds of common polyurethanes comprising a rigid and a flexible foam, a cast elastomer, and a thermoplastic polyurethane (TPU) was investigated by thermogravimetric analysis and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). All samples are based on methylene diphenyl diisocyanate (MDI), with polyether polyols as the soft segment of the foams and polytetrahydrofuran as the soft segment of the cast elastomer and TPU. Each polyurethane degrades in a two-step manner with a high correlation between the polymer structure and the mass loss in each step. Two kinetic modeling approaches based on either parallel or consecutive reactions were successfully applied to describe the pyrolytic degradation. The virgin polymers and pyrolysis intermediates obtained from thermogravimetry with isothermal segments at different temperatures of 350–450 °C were investigated by ATR-FTIR. Both foams exhibit retention of feedstock nitrogen in a carbonaceous intermediate while no significant residue formation and nitrogen retention are observable for cast elastomer and TPU pyrolysis. Aromatic amines in the foam pyrolysis intermediates point to the formation of a secondary polymer originating from the urethane segments in the course of the degradation. In Py-GC/MS, the foams yield nitrogen-carrying fragments of the MDI. MDI and 1,4-butanediol are regenerated from TPU pyrolysis, indicating different degradation pathways. CE pyrolysis also releases 1,4-butanediol but no nitrogen-carrying compounds are detectable. This work demonstrates the individuality of polyurethane materials in terms of their pyrolysis behavior and released products. Potential high-value products are identified in pyrolysis at laboratory scale. The findings of this study underline the need for a comprehensive examination of polyurethane pyrolysis with differentiation of polyurethane composition and morphology to optimize technical scale polyurethane pyrolysis.

Steady Two-Dimensional Conduction: Simple and Double Layer Potentials, Corner Singularities, and Induced Heat Flux

Abstract Two-dimensional steady-state heat conduction is possible outside closed boundaries on which two isothermal segments at different temperatures are separated by two adiabatic segments. Remarkably, previous research showed that the conduction shape factor for the region exterior to the boundary is equal to that for the interior region, despite the asymmetry and singularity of the boundary heat flux distributions. In this study, classical potential theory is used for the temperature and heat flux distributions as a combination of simple-layer and double-layer potentials, including the relationships between the values inside and outside the boundary curve. Isothermal boundaries exhibit an induced heat flux that varies from point-to-point on the boundary. The induced flux integrates to zero over each isothermal edge. Singularities of the heat flux are identified and resolved. Computations that validate the theory are provided for mixed boundary conditions on a disk and a square. Numerical fits to both the simple-layer and double-layer densities are given for the disk and the square. The analysis explains why the interior and exterior conduction shape factors are equal despite wildly differing heat flux distributions, and the results are compared to a previous study of this configuration. This paper also develops fundamental concepts of potential theory and can serve as a tutorial on the subject.

Hydrothermal carbonization enthalpy using differential scanning calorimetry: Assessing the accuracy of the exhaust sample method

High pressure differential scanning calorimetry (DSC) is suitable to study the thermodynamics and kinetics of hydrothermal carbonization (HTC). To isolate the thermal effect of the process from the heat required to increase the DSC system temperature, a recently proposed method uses an additional run on the already reacted sample as the blank. The method assumes that the blank system is in equilibrium during the whole run; it neglects the potential for secondary reactions in the blank run, which may affect the results. This work assesses the effect of such an assumption on the final enthalpy results by performing additional re-runs on the same specimen and adopting longer isothermal segments to push the system composition closer to equilibrium. The study of HTC of cellulose at 250°C demonstrates that the thermal contribution of reactions in the second run is negligible compared to the primary HTC effects in the first run and within the experimental error. A deeper investigation of the thermal effect during re-runs shows that the biggest effect of reactions in such runs is to change the system composition rather than a direct exothermicity. Further, the exothermic activity disappears after 12 hours, suggesting that the sample has reached thermal equilibrium.

Refolding of Lysozyme in Glycerol as Studied by Fast Scanning Calorimetry.

The folding of lysozyme in glycerol was monitored by the fast scanning calorimetry technique. Application of a temperature–time profile with an isothermal segment for refolding allowed assessment of the state of the non-equilibrium protein ensemble and gave information on the kinetics of folding. We found that the non-equilibrium protein ensemble mainly contains a mixture of unfolded and folded protein forms and partially folded intermediates, and enthalpic barriers control the kinetics of the process. Lysozyme folding in glycerol follows the same or similar triangular mechanism described in the literature for folding in water. The unfolding enthalpy of the intermediate must be no lower than 70% of the folded form, while the activation barrier for the unfolding of the intermediate (ca. 140 kJ/mol) is about 100 kJ/mol lower than that of the folded form (ca. 240–260 kJ/mol).

A mass-temperature decoupled discretization strategy for large-scale molecular-level kinetic model

The molecular conversion of complex mixture involves a large number of species and reactions. The corresponding kinetic model consists of a series of ordinary differential equations with severe stiffness, leading to an exponentially growing computational time. To reduce the computational time, we proposed a mass-temperature decoupled discretization strategy for a large-scale molecular-level kinetic model. The method separates the mass balance and heat balance calculations in the rigorous adiabatic reactor model and divided the reactor into several isothermal segments. After discretization, the differential equations for heat balance can be replaced by algebraic equations between nodes. We used a molecular-level diesel hydrotreating kinetic model as the case to validate the proposed method. A good agreement between the discretization model and rigorous model was observed while the computational time was significantly shortened. Moreover, the method was also extended to heavy oil fluid catalytic cracking and accurately calculated the product composition and temperature distribution.

Insights into CO2 hydrates formation and dissociation at isochoric conditions using a rocking cell apparatus

In order to study the hydrate-liquid–vapour-equilibria (HLVE), the isochoric pressure search method is one of the most sought-after methods through which the characteristics of the hydrate formation and dissociation processes of a given hydrate-former substance can be traced. This method contains three temperature driven segments that are known as: (i) fast cooling step, (ii) isothermal step and (iii) a slow heating step applied to a given water + hydrate-former system under closed conditions. In this work, we have carried out a study to determine the effect of various variables on the formation/dissociation process of CO2 + H2O hydrate systems such as cooling rate, heating rate, isothermal segment temperature and its length, rocking rate, initial pressure and operating temperature. These variables may influence the characteristic shape of the p-T loops and thereby might affect the precise determination of kinetics and thermodynamics parameters. The experiments have been conducted using a rocking cell apparatus where nearly 50 experiments using 10 different versions of the isochoric pressure search method have been carried out. This work primarily shows practicalities of how to obtain the hydrate equilibrium curve more efficiently and accurately. The large number of experiments provide qualitative indications of the combined mass transfer and kinetics. This information might have implications towards the use of hydrate technology for energy and environmental related applications.

Metamorphic evolution of high-grade granulite-facies rocks of the Mashan Complex, Liumao area, eastern Heilongjiang Province, China: Evidence from zircon U–Pb geochronology, geochemistry and phase equilibria modelling

The nature and age of the Mashan Complex (or Mashan Group) in northeast China is key for determining the Precambrian geological evolution and origins of the Jiamusi Massif. Here we present a detailed investigation of high-grade granulite-facies rocks of the Mashan Complex from the Liumao area, Heilongjiang Province, China. This includes zircon U–Pb dating, petrological and geochemical analysis, geothermobarometry, phase equilibria modelling, and a P–T path. The metamorphic rocks consist mainly of two groups: pelitic and felsic granulites; amphibolite-facies Sil–Grt–Bi, Sil–Grt, and Grt–Bi gneisses. The protoliths of the metamorphic rocks were clay rocks and sandstones, which were derived dominantly from silicic rocks in a continental island arc setting. In the pelitic granulites, four metamorphic assemblages (M1–M4) were recognised. Three metamorphic assemblages (M2–M4) were identified in the felsic granulites. Geothermobarometric data and phase equilibria modelling indicate that the M2, M3, and M4 assemblages in the pelitic granulite were metamorphosed at 9.2 kbar/845–865 °C, 4.7–5.1 kbar/820–865 °C, and 4.0–4.8 kbar/660–720 °C, respectively. These features imply a collisional orogenic process with a clockwise P–T path with nearly isothermal decompression segments. Similar results were obtained for the felsic granulite, with the metamorphic P–T conditions of the peak assemblage (M2) evaluated as 9.0–9.2 kbar/845–870 °C. The similar P–T paths indicate that the rocks underwent an evolutionary process that started with crustal thickening (M2), followed by rapid crustal uplift (M3), and then final cooling (M4). Based on our new zircon U–Pb age data, the peak of granulite-facies metamorphism in the Liumao area of the Jiamusi Massif occurred at ca. 530 Ma and final cooling occurred at ca. 490 Ma.

Sensitivity analysis and optimization of a membrane reactor for hydrogen production through methane steam reforming

Hydrogen is one of most studied sources for clean power generation in the near future. Nowadays, hydrogen is mainly produced through methane steam reforming in packed bed reactors, with a promising alternative to this technology being the implementation of hydrogen-selective membrane reactors. This work compares the isothermal mathematical models of both designs by assessing the effects of multiple design variables on methane conversion, while also providing recommended operating conditions for maximum efficiency of the membrane reactor over the packed bed technology. Additionally, an optimization study is carried by dividing the reactor length in isothermal segments to achieve higher efficiency. Results showed that the membrane technology considerably increases hydrogen production, with temperature being the most influential variable on methane conversion. While the temperature profile optimization provided similar conversions compared to the isothermal models, the membrane reactor’s efficiency was increased, further justifying its implementation.

Challenges in the experimental investigation of the caloric and thermomechanical behaviour of semi-crystalline polymers: A study on the example of polyethylene terephthalate (PET)

Investigations on industrial produced polyethylene terephthalate (PET) were carried out in the temperature range from the glassy to the liquid state. In differential scanning calorimetry (DSC) experiments, the sample passed through this temperature range several times at different speeds, interrupted by isothermal segments. Thereby the caloric behaviour observed in earlier publications could be confirmed. A closer look reveals variations in the behaviour, depending on the initial state of the sample. The thermal and mechanical boundary conditions seem to have a long-term influence on the semi-crystalline state. Independent of the initial state, a fundamental decrease in the crystallization tendency could be observed after each temperature loop. The most likely explanation for that is an increase in molecular weight in the higher temperature ranges. This leads to an increase in viscosity, as it occurs in the solid poly-condensation (SSP) process. Mechanical experiments were also carried out in the temperature range investigated, but only at different isothermal levels due to experimental limits. For these experiments, theoretical considerations were made on the basis of the viscoelastic generalization of Hooke’s law. These show the influence of the significant decrease of the ratio between shear- and bulk stiffness in the glass-transition range. While the uniaxial tensile mode fail, the uniaxial compression mode with suppressed transversal expansion needs the fluid like behaviour to determine the bulk modulus instead of the longitudinal modulus. The experimental investigations confirm these theoretical results. Concerning the actual purpose of the mechanical investigation, Struik’s protocol is used at higher isothermal levels. As a result, crystallization-related changes in the mechanical behaviour could be observed.

Reaction/crystallization kinetics studied via in situ XRD: experimental conditions versus methods of kinetic analysis

ABSTRACTTheoretically simulated kinetic data were used to evaluate the errors associated with the common issue of evaluating the in situ non-isothermal X-ray diffraction data, where the complex multi-step temperature program (alternating the non-isothermal heating steps with isothermal steps during which the diffraction patterns are collected) is for the purposes of evaluation replaced by a simple non-isothermal heating performed at the reduced/effective heating rate. The kinetic analysis has shown that, in general, best results are provided by the non-linear optimisation methods simultaneously evaluating the data-curves obtained for all the different heating rates. For the nucleation growth (KMJMA) kinetics the distortive influence of the temperature program parameters increases as follows: heating rate during non-isothermal segments < duration of the isothermal segment < temperature interval between the isothermal segments. The non-optimisation methods of kinetic analysis (integral isoconversional methods for evaluation of activation energy E and master plots for determining the appropriate kinetic model) were found to perform inaccurately, with large degree of randomness based on the selection of starting temperature, and are not recommended for evaluation of the in situ XRD data – the only exception seem to be the differential isoconversional methods that provided accurate E values. Generalisation of the present conclusions for all KMJMA processes is suggested and discussed.

The layered model of the Copahue geothermal reservoir, Argentina

An exhaustive review of the available data related to the reservoir of the volcano-hosted Copahue geothermal system is presented. Reservoir interpretation to date suggests a vapor-dominated nature. However, the review provided reveals the limitations of this model, regarding the transition from shallow argillic to deep propylitic alteration zones, gas geothermometers calibrated to near-neutral pH environment, production tests, and low recharge of the feed zone, among the other features. We propose that a layered model with a shallow vapor zone above a fluid pressurized reservoir better accounts for the known features of the geothermal reservoir. Temperature profiles define the 800-m-thick upper thermal boundary that includes the impermeable clay cap. An argillic alteration layer (smectite + chlorite + illite + kaolinite) with low electric resistivity response, consistent with the presence of smectite, composes the clay cap. A shallow steam cap develops immediately below the impermeable layer. This vapor zone has 200–215 °C, low seismic activity, and high electrical resistivity, and is highlighted by the isothermal segment of the well logs. The bottom part of the reservoir is subjected to controversy because of the lack of direct measurements. The deep liquid-dominated reservoir has a temperature of ~ 280 °C, develops below 1500 m depth under near-neutral pH condition, and probably has a pervasive propylitic alteration, and wairakite + laumontite + chlorite + epidote + prehnite + actinolite is the likely mineral assemblage. Pressure collapse might trigger the development of the shallow steam cap during deflation–inflation volcanic cycles or Pleistocene-to-recent tectonic deformation. This work points out the main weak points of the previous conceptual model, providing an alternative one based on the first comprehensive compilation of data of the Copahue reservoir. Further research, including a deep exploration drilling stage, is required to establish the physicochemical reservoir state precisely and to validate the proposed layered model.

An Indirect Method for Vapor Pressure and Phase Change Enthalpy Determination by Thermogravimetry

Vapor pressure is a fundamental property of a pure substance. This property is the pressure of a compound’s vapor in thermodynamic equilibrium with its condensed phase (solid or liquid). When phase equilibrium condition is met, phase coexistence of a pure substance involves a continuum interplay of vaporization or sublimation to gas and condensation back to their liquid or solid form, respectively. Thermogravimetric analysis (TGA) techniques are based on mass loss determination and are well suited for the study of such phenomena. In this work, it is shown that TGA method using a reference substance is a suitable technique for vapor pressure determination. This method is easy and fast because it involves a series of isothermal segments. In contrast to original Knudsen’s approach, where the use of high vacuum is mandatory, adopting the proposed method a given experimental setup is calibrated under ambient pressure conditions. The theoretical framework of this method is based on a generalization of Langmuir equation of free evaporation: The real strength of the proposed method is the ability to determine the vapor pressure independently of the molecular mass of the vapor. A demonstration of this method has been performed using the Clausius–Clapeyron equation of state to derive the working equation. This algorithm, however, is adaptive and admits the use of other equations of state. The results of a series of experiments with organic molecules indicate that the average difference of the measured and the literature vapor pressure amounts to about 5 %. Vapor pressure determined in this study spans from few mPa up to several kPa. Once the p versus T diagram is obtained, phase transition enthalpy can additionally be calculated from the data.

Synthesis of Large Mesoporous-Macroporous and High Pore Volume, Mixed Crystallographic Phase Manganese Oxide, Mn2O3/Mn3O4 Sponge.

The controlled synthesis of mixed crystallographic phase Mn2O3/Mn3O4 sponge material by varying heating rates and isothermal segments provides valuable information about the morphological and physical properties of the obtained sample. The well-characterized Mn2O3/Mn3O4 sponge and applicability of difference in reactivity of H2 and CO2 desorbed during the synthesis provide new developments in the synthesis of metal oxide materials with unique morphological and surface properties. We report the preparation of a Mn2O3/Mn3O4 sponge using a metal nitrate salt, water, and Dextran, a biopolymer consisting of glucose monomers. The Mn2O3/Mn3O4 sponge prepared at 1 °C·min-1 heating rate to 500 °C and held isothermally for 1 h consisted of large mesopores-macropores (25.5 nm, pore diameter) and a pore volume of 0.413 mL/g. Furthermore, the prepared Mn2O3/Mn3O4 and 5 mol %-Fe-Mn2O3/Mn3O4 sponges provide potential avenues in the development of solid-state catalyst materials for alcohol and amine oxidation reactions.

A modified kinetic analysis method of cellulose pyrolysis based on TG–FTIR technique

The pyrolysis characteristics of cellulose was investigated using TGA and TG–FTIR. An modified reaction model was proposed to study cellulose pyrolysis. In this model, the active cellulose (E=137 kJ/mol) and dehydrocellulose (E=91.3 kJ/mol) were formed by parallel mechanism. The degradation of the active cellulose was followed by two competitive pathways, this is, one that formed levoglucosan (E=159.2 kJ/mol) and another that formed furan and some low molecular weight products (E=203.5 kJ/mol). The random scission model was used to describe the kinetics of cellulose pyrolysis. The same kinetic parameters were used for different linear heating rates (10–100 °C/min) and isothermal segment conditions. By successfully modeling non-isothermal and isothermal segment data, the model was proved to be credible in describing cellulose pyrolysis.

Multi-step cure kinetic model of ultra-thin glass fiber epoxy prepreg exhibiting both autocatalytic and diffusion-controlled regimes under isothermal and dynamic-heating conditions

As packaging technologies are demanded that reduce the assembly area of substrate, thin composite laminate substrates require the utmost high performance in such material properties as the coefficient of thermal expansion (CTE), and stiffness. Accordingly, thermosetting resin systems, which consist of multiple fillers, monomers and/or catalysts in thermoset-based glass fiber prepregs, are extremely complicated and closely associated with rheological properties, which depend on the temperature cycles for cure. For the process control of these complex systems, it is usually required to obtain a reliable kinetic model that could be used for the complex thermal cycles, which usually includes both the isothermal and dynamic-heating segments. In this study, an ultra-thin prepreg with highly loaded silica beads and glass fibers in the epoxy/amine resin system was investigated as a model system by isothermal/dynamic heating experiments. The maximum degree of cure was obtained as a function of temperature. The curing kinetics of the model prepreg system exhibited a multi-step reaction and a limited conversion as a function of isothermal curing temperatures, which are often observed in epoxy cure system because of the rate-determining diffusion of polymer chain growth. The modified kinetic equation accurately described the isothermal behavior and the beginning of the dynamic-heating behavior by integrating the obtained maximum degree of cure into the kinetic model development.

Defining a procedure for predicting the duration of the approximately isothermal segments within the proposed drying regime as a function of the drying air parameters

One of the main disadvantages of the recently reported method, for setting up the drying regime based on the theory of moisture migration during drying, lies in a fact that it is based on a large number of isothermal experiments. In addition each isothermal experiment requires the use of different drying air parameters. The main goal of this paper was to find a way how to reduce the number of isothermal experiments without affecting the quality of the previously proposed calculation method. The first task was to define the lower and upper inputs as well as the output of the “black box” which will be used in the Box-Wilkinson’s orthogonal multi-factorial experimental design. Three inputs (drying air temperature, humidity and velocity) were used within the experimental design. The output parameter of the model represents the time interval between any two chosen characteristic points presented on the Deff – t. The second task was to calculate the output parameter for each planed experiments. The final output of the model is the equation which can predict the time interval between any two chosen characteristic points as a function of the drying air parameters. This equation is valid for any value of the drying air parameters which are within the defined area designated with lower and upper limiting values.

THE CORONAL LOOP INVENTORY PROJECT: EXPANDED ANALYSIS AND RESULTS

ABSTRACT We have expanded upon earlier work that investigates the relative importance of coronal loops with isothermal versus multithermal cross-field temperature distributions. These results are important for determining if loops have substructure in the form of unresolved magnetic strands. We have increased the number of loops targeted for temperature analysis from 19 to 207 with the addition of 188 new loops from multiple regions. We selected all loop segments visible in the 171 Å images of the Atmospheric Imaging Assembly (AIA) that had a clean background. Eighty-six of the new loops were rejected because they could not be reliably separated from the background in other AIA filters. Sixty-one loops required multithermal models to reproduce the observations. Twenty-eight loops were effectively isothermal, that is, the plasma emission to which AIA is sensitive could not be distinguished from isothermal emission, within uncertainties. Ten loops were isothermal. Also, part of our inventory was one small flaring loop, one very cool loop whose temperature distribution could not be constrained by the AIA data, and one loop with inconclusive results. Our survey can confirm an unexpected result from the pilot study: we found no isothermal loop segments where we could properly use the 171-to-193 ratio method, which would be similar to the analysis done for many loops observed with TRACE and EIT. We recommend caution to observers who assume the loop plasma is isothermal, and hope that these results will influence the direction of coronal heating models and the effort modelers spend on various heating scenarios.

Nonlinear and complex cure kinetics of ultra-thin glass fiber epoxy prepreg with highly-loaded silica bead under isothermal and dynamic-heating conditions

In the advent of the miniaturized mobile devices, the packaging technology is required utmost high performance of the thin composite laminates in such materials properties as the coefficient of thermal expansion (CTE) and stiffness. Accordingly, the composition of a thermosetting resin becomes extremely complicated often including multiple fillers, monomers and/or catalysts in thermoset-based glass fiber prepregs. The prepreg systems is so complicated that it is usually difficult to obtain a reliable kinetic description and methodology that could be used for the complex thermal cycles including both isothermal and dynamic-heating segments in a facile manner. In this investigation, we propose an isoconversional kinetic using an ultra-thin glass fiber epoxy prepreg with highly loaded silica filler (the ultra-thin glass fiber/silica bead epoxy prepreg) as a model system. The activation energy was determined as a function of the conversion of curing reactions, which was fitted to linear models. The kinetic prediction using the linear models showed an excellent agreement to isothermal experiments. The master curve of a conversion-dependent function which derived from activation energy dependency used to investigate the complex reactions of the ultra-thin glass fiber/silica bead epoxy prepreg.