Thermochemical Aspects Research Articles

Utilizing Multiple Objectives for the Optimization of the Pultrusion Process Based on a Thermo-Chemical Simulation

Pultrusion is one of the most effective manufacturing processes for producing composites with constant cross-sectional profiles. This obviously makes it more attractive for both researchers and practitioners to investigate the optimum process parameters, i.e. pulling speed, power and dimensions of the heating platens, length and width of the heating die, design of the resin injection chamber, etc., to provide better understanding of the process, consequently to improve the efficiency of the process as well the product quality. Numerous simulation approaches have been presented until now. However, optimization studies had been limited with either experimental cases or determining only one objective to improve one aspect of the performance of the process. This objective is either augmented by other process related criteria or subjected to constraints which might have had the same importance of being treated as objectives. In essence, these approaches convert a true multi-objective optimization problem (MOP) into a single-objective optimization problem (SOP). This transformation obviously results in only one optimum solution and it does not support the efforts to get more out of an optimization study, such as relations between variables and objectives or constraints. In this study, an MOP considering thermo-chemical aspects of the pultrusion process (e.g. cure degree, temperatures), in which the pulling speed is maximized and the heating power is minimized simultaneously (without defining any preference between them), has been formulated. An evolutionary multi-objective optimization (EMO) algorithm, non-dominated sorting genetic algorithm (NSGA-II [Deb et al., 2002]), has been used to solve this MOP in an ideal way where the outcome is the set of multiple solutions (i.e. Pareto-optimal solutions) and each solution is theoretically an optimal solution corresponding to a particular trade-off among objectives. Following the solution process, in other words obtaining the Pareto-optimal front, a further postprocessing study has been performed to unveil some common principles existing between the variables, the objectives and the constraints either along the whole front or in some portion of it. These relationships will reveal a design philosophy not only for the improvement of the process efficiency, but also a methodology to design a pultrusion die for different operating conditions.

New strategies for resolving oligosaccharide isomers by exploiting mechanistic and thermochemical aspects of fragment ion formation

Three complementary experimental approaches for elucidating human milk oligosaccharide (HMOs) isomers by Fourier Transform Ion Cyclotron Resonance mass spectrometry (FT-ICR) are described: tandem-MS disruption by double resonance to distinguish different fragmentation pathways, examination of fragment intensity ratios arising from differential alkali metal ion affinities and monitoring competitive fragmentation rates. The interpretation of the fragmentation pattern from a mechanistic and thermochemical point of view permits the assignment of not only pure isomers but, in some cases, mixtures of them. Methodologically the procedures are simple, reliable and rapid making unnecessary both the use of previous separation techniques and tedious chemical modifications of the HMOs. In principle, the rationale can be expanded to resolve other isomeric mixtures of biological nature.

Regioselectivity in Sonogashira synthesis of 6-(4-nitrobenzyl)-2-phenylthiazolo[3,2-b]1,2,4-triazole: a quantum chemistry study

In the present research, the experimentally observed regioselectivity in Sonogashira synthesis of 6-(4-nitrobenzyl)-2-phenylthiazolo[3,2-b]1,2,4triazole has been modeled by means of density functional theory (DFT) employed to investigate the structural and thermochemical aspects of this synthesis in the gas and solution phases. Comparison of our calculated structural parameters of the title compound with the available X-ray crystallographical data demonstrate a reliable agreement. Then, the effect of two different solvents, DMF and ethanol, are examined via polarized continuum model calculations, showing a significant decrease in the computed values of the reaction enthalpy and free energy changes compared with the gas phase results. We have also considered two tautomeric structures of the intermediate species that it seems the mode of its intermolecular cyclization has an important role in regioselectivity of the final products. Moreover, all obtained results in the gas and solution phases also confirm that the synthesis of the title compound is thermodynamically more favorable than the other regioisomeric product. We also discuss the thermodynamical feasibility of this reaction at higher temperatures. Finally, we concentrate on the survey of substituent effect by choosing electron-withdrawing and electron-donating groups on the aryl iodide. Our calculated thermochemical data in the gas and solution phases indicate that the use of electron-withdrawing moieties is more favorable thermodynamically than electron-donating ones which has been previously concluded via the experimental elucidations.

Environmental degradation of thermal-barrier coatings by molten deposits

Abstract

Thermochemical Aspects of High Plutonia (44%Pu) MOX Fuel

Use of high plutonia MOX as fuel is contemplated from the point of view of rapid disposition of plutonium and from economic power generation. As part of the fuel development programme, high plutonia MOX (44%) is undergoing test irradiation at FBTR, Kalpakkam. Several thermophysical and thermochemical properties need to be evaluated for this composition. This includes phase stability, melting temperature, vapour chemistry, oxygen potential, component redistribution under steep thermal gradient and compatibility studies between fuel-clad and fuel-coolant. Some of the thermochemical aspects are summarized in this paper.

Resonant dissociative electron capture by the simplest amino acids and dipeptides

The formation of ions from amino acids (glycine and alanine) and dipeptides (glycylglycine, alanylalanine, and glycylalanine) under the resonant electron capture conditions was studied by negative ion resonant electron capture mass spectrometry. The isobaric ions were found, their effective yield curves were experimentally separated, and the elemental composition was determined. The thermochemical aspect of ion formation was considered, and probable dissociative channels of fragmentation ion formation and their structures were established on the basis of this aspect. Bond cleavage reactions only and H-shift processes were revealed. The rearrangements occur presumably through the stage of formation of intramolecular hydrogen bonds. The cross-sections of formation of ions [M − H]− were measured in the energy range 1.1–1.3 eV. The metastable decay channels of ions [M − H]− and [M − COOH]− were found in the energy range 4.5–7.5 eV for dipeptides, which enabled establishing the genetic relationship between the parental and daughter ions and revealing hidden fragmentation pathways.

Thermo-chemical and rheological finite element analysis of the cure process of thick carbon-epoxy composite laminates

This paper deals with the development and the validation of a computational model for the analysis of the thermo-chemical and rheological aspects related to the autoclave cure process of thick carbon-epoxy composite laminate. The transient three dimensional distributions of temperature, reaction rate, degree of cure, and viscosity into the processing material have been evaluated, assuming the time-temperature curve and the heat generation rate due to resin reaction as thermal load and solving the energy balance using a finite element scheme. The rheological behaviour of the processing resin has been studied assuming a three parameters correlation models for the evaluation of the processing material viscosity. A good agreement between present results and data reported in the related literature has been found. The proposed model can be used as an effective tool for analysis and design of optimized thermal cycles.

Promoted hydrogen release from ammonia borane by mechanically milling with magnesium hydride: a new destabilizing approach

Ammonia borane (NH(3)BH(3), AB) is an intriguing molecular crystal with an extremely high hydrogen capacity and moderate thermal stability. In the present study, we show a simple but effective approach for destabilizing AB for promoted hydrogen release at moderate temperatures. It is found that mechanically milling with magnesium hydride (MgH(2)) can dramatically improve the dehydrogenation properties of AB, on both the kinetic and thermochemical aspects. For the mechanically milled AB/0.5MgH(2) material, over 8 wt% hydrogen can be released from AB within 4 h at around 100 degrees C without undesired volatile by-products. Moreover, the dehydrogenation reaction of the AB/0.5MgH(2) sample becomes significantly less exothermic than that of neat AB. In situ X-ray diffraction results demonstrate that the MgH(2) additive well maintains its phase stability during the ball-milling and the subsequent heating processes. Meanwhile, Raman spectroscopy and in situ(11)B NMR studies show that the MgH(2) additive exerts considerable influence on the chemical bonding state and decomposition process/products of AB. Combined phase/structure analyses results suggest that MgH(2) exerts effect via developing solid phase interaction with AB.

Degradation of Yttria Stabilized Zirconia Thermal Barrier Coatings by Molten CMAS (CaO-MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>) Deposits

In advanced gas turbine engines that operate in a dust-laden environment causing ingestion of siliceous debris into engines, thermal barrier coatings (TBCs) are highly susceptible to degradation by molten CMAS (calcium-magnesium alumino silicate) deposits. In this study, the degradation mechanisms other than the commonly reported thermomechanical damage are investigated with an emphasis on the thermochemical aspects of molten CMAS induced degradation of TBCs. Free-standing yttria stabilized zirconia (8YSZ) TBC specimens in contact with a model CMAS composition were subjected to isothermal heat treatment in air at temperatures ranging from 1200°C to 1350°C. Phase transformations and microstructural development were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Starting at 1250°C, the molten CMAS readily infiltrated and dissolved the YSZ coating followed by reprecipitation of zirconia with a different morphology and composition that depends on the local melt chemistry. Significant amount of Y2O3 depleted monoclinic ZrO2 phase evolved from CMAS melt that dissolved ť-ZrO2 was evident. Thus the mechanism of dissolution and reprecipitation due to molten CMAS damage resulted in destabilization of the YSZ with disruptive phase transformation (t’ f + m).

Some properties of explosive mixtures containing peroxides: Part II. Relationships between detonation parameters and thermal reactivity of the mixtures with triacetone triperoxide

This study concerns mixtures of triacetone triperoxide (3,3,6,6,9,9-hexamethyl-1,2,4,5,7,8-hexoxonane, TATP) and ammonium nitrate (AN) with added water (W), as the case may be, and two dry mixtures of TATP with urea nitrate (UN). Relative performances (RP) of the mixtures and their individual components, relative to TNT, were determined by means of ballistic mortar. Thermal reactivity of these mixtures was examined by means of differential thermal analysis and the data were analyzed according to the modified Kissinger method (the peak temperature was replaced by the temperature of decomposition onset in this case). The reactivity, expressed as the E a R −1 slopes of the Kissinger relationship, correlates with the squares of the calculated detonation velocities for the charge density of 1000 kg m −3 of the studied energetic materials. Similarly, the relationships between the E a R −1 values and RP have been found. While the first mentioned correlation (modified Evans–Polanyi–Semenov equation) is connected with the primary chemical micro-mechanism of the mixtures detonation, the relationships in the second case should be connected with the thermochemical aspects of this detonation.

Thermochemical aspects of interaction of polyimide composites with organic sorbents

Interaction of ITA-31 polyimide binder with a series of industrial sorbents based on styrene and vinylpyridine derivatives was studied.

Thermochemical Interaction of Thermal Barrier Coatings with Molten CaO–MgO–Al2O3–SiO2 (CMAS) Deposits

Thermal barrier coatings (TBCs) are increasingly susceptible to degradation by molten calcium–magnesium alumino silicate (CMAS) deposits in advanced engines that operate at higher temperatures and in environments laden with siliceous debris. This paper investigates the thermochemical aspects of the degradation phenomena using a model CMAS composition and ZrO2–7.6%YO1.5 (7YSZ) grown by vapor deposition on alumina substrates. The changes in microstructure and chemistry are characterized after isothermal treatments of 4 h at 1200°–1400°C. It is found that CMAS rapidly penetrates the open structure of the coating as soon as melting occurs, whereupon the original 7YSZ dissolves in the CMAS and reprecipitates with a different morphology and composition that depends on the local melt chemistry. The attack is minimal in the bulk of the coating but severe near the surface and the interface with the substrate, which is also partially dissolved by the melt. The phase evolution is discussed in terms of available thermodynamic information.

Single crystal synthesis of higher-order sulfohalogenides in the temperature gradient: Bi 19S 27Br 3, Bi 2Cu 3S 4Br, Pb 1− xBi 2+ xCu 4− xS 5I 2

The present paper reports on the single crystal synthesis of three sulfohalogenides (ternary, quaternary, and quinary) in the temperature gradient and focuses primarily on the thermochemical aspects of the synthesis.

Free radicals in catalytic oxidation of light alkanes: kinetic and thermochemical aspects

The kinetic and thermochemical analyses of the catalytic oxidation of C1C4 alkanes indicate that the overall reaction network includes both homogeneous and heterogeneous elementary reactions of free radicals. Two thermochemical parameters of oxide catalysts, the affinity of the surface active site E[OH] to a hydrogen atom and oxygen binding energy E[O], are the main factors determining the kinetic features of the overall process. The possible contribution of free radical reactions to some metal-catalyzed processes (steam reforming, partial and total oxidation of methane) is analyzed. Trends in the studies of catalytic chemistry of free radicals and in the development of innovative catalytic processes of light alkane transformation into valuable products are discussed.

CO oxidation on thin Pt crystals: Temperature slaving and the derivation of lumped models

We review the Langmuir–Hinshelwood model for the heterogeneous catalytic oxidation of carbon monoxide on thin (≈3000 Å) platinum catalysts. Our ultimate goal is to model observed thermo-mechano-chemical oscillations on a thin Pt (110) crystal under low pressure/vacuum conditions. Here we consider only the thermochemical aspects. We supplement the reaction-diffusion model of chemical kinetics by a heat balance equation for the catalyst surface, derived from the energetics of the reaction, adsorption, and desorption processes, and including radiation and conduction to the supporting environment. From the resulting distributed system a four-dimensional lumped ordinary differential equation is derived, which we study via dynamical systems theory, making use of time scale separation, and deriving reduced two-dimensional models. We show that key types of dynamics of the isothermal reaction persist, including multiple equilibria and periodic oscillations, but with the varying catalyst temperature “slaved” to the chemistry. We show how the stability of the periodic orbit branch changes as the catalyst’s area and thickness change, we verify that the lumped models capture the dominant dynamics of the distributed system, and we outline the major differences.

Phase Transitions in YBa2Cu3Ox AT 300–900 K: Thermochemical and Structural Aspects

The thermochemical and structural aspects of the occurrence of two anomalies in the temperature dependence of the specific heat of YBa2Cu3O6.91 are considered. The anomalies were found at T = 300–500 K and T > 600 K. The specific heat was measured by mixing calorimetry in the temperature range 300–900 K. Analytical expressions for the temperature dependences of enthalpy and specific heat were obtained. The observed anomalies were confirmed by structural data and measurements of the temperature dependence of the linear‐expansion coefficient. The appearance of the anomalies is explained by the presence of apical oxygen.

Book Review: Propellants and Explosives. Thermochemical Aspects of Combustion. By Naminosuke Kubota

Book Review: Propellants and Explosives. Thermochemical Aspects of Combustion. By Naminosuke Kubota

Tribology of coated tools in conventional and HSC machining

The aim of research of the tribological conditions in the tool–workpiece interface in cutting processes is to find the main causes for unpredicted tool life. As is shown in the paper there are many causes for their influences on the tool wear. Not only mechanical and chemical but also thermo-chemical aspects of wear is important. The high cutting temperature, which is a result of high-speed cutting (HSC), enhances diffusion and oxidation process. Diffusion processes between the chip and the top rake surface of the cutting edge result in crater wear, and oxidation reactions with the environment induce scaling of the cutting edge. The phenomenon of central wear can be used to define the tool life in the case of HSC milling of alloyed tool steels ( hardness>45 HRC) .

Thermochemical aspects of proton transfer in the gas phase.

The beginning of the twentieth century saw the development of new theories of acidity and basicity, which are currently well accepted. The thermochemistry of proton transfer in the absence of solvent attracted much interest during this period, because of the fundamental importance of the process. Nevertheless, before the 1950s, few data were available, either from lattice energy evaluations or from calculations using the emerging molecular orbital theory. Advances in mass spectrometry during the last 40 years allowed studies of numerous systems with better accuracy. Thousands of accurate gas-phase acidities or basicities are now available, for simple atomic and molecular systems and for large biomolecules. The intrinsic effect of structure on the Brønsted basic or acidic properties of molecules and the influence of solvents have been unravelled. In this tutorial, the basics of the thermodynamic principles involved are given, and the mass spectrometric techniques are briefly reviewed. Advances in the design and measurements of gas-phase superacids and superbases are described. Recent studies concerning biomolecules are also evoked.

The Polymorphism of Glycine. Thermochemical and structural aspects

X-ray, DSC and solution calorimetric investigations were carried out for α-, β- and γ-modifications of glycine. Particular attention was paid to kinetic and thermochemical aspects of γ- → α-phase transition. The temperature of this phase transition turned out to be sensitive to a) conditions under which the crystals of the γ-modification were grown, b) tempering of crystals c) form (geometry) of crystals. Kinetics of this phase transition of single crystals of γ-phase in rhomboedric form can be described by the equation for two-dimension nuclei growth, whereas for crystals of triangle geometry the equation for three dimension growth is valid. On the basis of energy parameters describing growth of α-form in γ- →α-phase transition, the kind of structure defects, which are responsible for this phase transition, was estimated. Taking into account the ΔsolHm, the absolute values of the lattice energies of the investigated polymorphs indescending order are follows: γ->α->β-modification. The obtained results are discussed with respect to the peculiarity of the crystal lattice structures, particularly the network of hydrogen bonds. The β-modification of glycine is monotropically related to the other forms, whereas γ-and α-polymorphs are enantiotropically-related phases.