Academic literature on the topic 'Reaction enthalpies'

In the present paper, M05-2X/6-311+G(d,p) level of theory was used to investigate antiradical activity of cyanidin towards highly damaging radical species (.OH, .OCH3, .OOH and .OOCH3). The applied method successfully reproduces the values of reaction enthalpies (ΔHBDE, ΔHIP, and ΔHPA). These parameters are important to determine which of the mechanisms are preferred. Reaction enthalpies related to the antioxidant mechanisms of the investigated species were calculated in water and DMSO. The enthalpies of reactions indicate the preferred radical scavenging mechanisms in polar (water) and polar aprotic (DMSO) solvents. Single- electron transfer followed by proton transfer (SET-PT) is not a favorable reaction pathway under any conditions. Both remaining mechanisms, HAT and SPLET, are suitable for the reaction of cyanidin with •OH and •OCH3 in all solvents under investigation. On the other hand, in the reaction of cyanidin with •OOH and •OOCH3, the SPLET mechanism is possible in both solvents. Simulation of the reaction of the cyanidin anion with the hydroxy radical confirmed that position 3` of Cy‒O- is the most suitable for reaction with •OH through electron transfer mechanism (ET) in both solvents.

Activation enthalpies for a series of five 1,3,8-nonatriene intramolecular Diels–Alder (IMDA) reactions involving substrates 1–5 have been determined experimentally and Singleton’s natural abundance method has been employed to determine kinetic isotope effects in the IMDA reaction of fumarate 3. The activation enthalpies for the IMDA reactions of the systems possessing a –CH2OCH2– diene/dienophile tether are significantly smaller than their counterparts possessing the –CH2OC(=O)– tether. The experimental activation enthalpies have been used to benchmark computed values from four model chemistries, namely two density functional theory functionals, B3LYP and M06-2X, and two generally very accurate composite ab initio wave function methods, CBS-QB3 and G4(MP2). G4(MP2) outperformed the computationally more expensive CBS-QB3 method, but the vastly cheaper M06-2X/6-31G(d)//B3LYP/6-31G(d) method was sufficiently accurate to be the recommended method of choice for calculating activation parameters. Experimental 2H kinetic isotope effects for the IMDA reaction of fumarate 3 confirmed the computational predictions that this Diels–Alder reaction is concerted but asynchronous.

Using Transition-State Theory, experimental rate constants, determined over a range of temperatures, for reactions of vitamin E type antioxidants are analysed in terms of their enthalpies and entropies of activation. It is further shown that computational methods may be employed to calculate enthalpies and entropies, and hence Gibbs Free Energies, for the overall reactions. Within the Linear Free Energy Relationship (LFER) assumption, that the Gibbs Free Energy of activation is proportional to the overall Gibbs Free Energy change for the reaction, it is possible to rationalise, and even to predict, the relative contributions of enthalpy and entropy for reactions of interest, involving potential antioxidants.

Reaction energies calculated by quantum chemical methods are compared with reaction enthalpies obtained on the basis of experimental heats of formation. Fifty six ion-molecule reactions (X+ + H2, XH+ + H2, where X is boron, carbon, or nitrogen, and reactions involving C2H2.+, C2H4.+, and C2H6.+) have been arranged into structurally related sets. Moreover, nine processes important in connection with the C2H2 formation in interstellar clouds are treated.

A Molecular Electron Density Theory (MEDT) study is presented here for [3+2] cycloaddition (32CA) reactions of three trimethylsilyldiazoalkanes with diethyl fumarate. The presence of silicon bonded to the carbon of these silyldiazoalkanes changes its structure and reactivity from a pseudomonoradical to that of a zwitterionic one. A one-step mechanism is predicted for these polar zw-type 32CA reactions with activation enthalpies in CCl4 between 8.0 and 19.7 kcal·mol−1 at the MPWB1K (PCM)/6-311G(d,p) level of theory. The negative reaction Gibbs energies between −3.1 and −13.2 kcal·mole−1 in CCl4 suggests exergonic character, making the reactions irreversible. Analysis of the sequential changes in the bonding pattern along the reaction paths characterizes these zw-type 32CA reactions. The increase in nucleophilic character of the trimethylsilyldiazoalkanes makes these 32CA reactions more polar. Consequently, the activation enthalpies are decreased and the TSs require less energy cost. Non-covalent interactions at the TSs account for the stereoselectivity found in these 32CA reactions involving the bulky trimethylsilyl group.

In this paper, the study of various ortho and meta–substituted Sesamol derivatives is presented. The reaction enthalpies related to three antioxidant action mechanisms HAT, SET–PT, and SPLET for substituted Sesamols, have been calculated using the DFT/B3LYP method in gas phase and water. Calculated results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP), and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In the ortho position, substituents show a larger effect on reaction enthalpies than in the meta position. In comparison with the gas phase, water attenuates the substituent effect on all reaction enthalpies. In the gas phase, BDEs are lower than PAs and IPs, i.e., HAT represents the thermodynamically preferred pathway. On the other hand, the SPLET mechanism represents the thermodynamically favored process in water. Results show that calculated enthalpies can be successfully correlated with Hammett constants (σm) of the substituted Sesamols. Furthermore, calculated IP and PA values for substituted Sesamols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO).

Hydrocarbons, in particular polycyclic aromatic hydrocarbons (PAHs), have been long discussed to be carriers of interstellar infrared (IR) emission and ultraviolet (UV) absorption features. Yet, their origin in dense phases of the interstellar medium (ISM), such as molecular clouds, remains unclear. In this work, growth mechanisms based on ion-molecule reactions between cationic PAHs/hydrocarbons and methyne (CH) were investigated. The reaction type and the precursor were derived and selected from known chemical and physical properties of the ISM. These chemical reactions were characterised by calculating branching ratios (based on cross sections) and capture rate coefficients, minimum reaction paths, reaction enthalpies, thermal equilibrium constants, and microcanonic isomerisation and radiative deactivation rate coefficients. In order to cope with the variety of reaction parameters, a hierarchic workflow scheme was set up. First, the reaction potential energy surface was sampled by molecular dynamics simulations. Then, minimum energy paths of the most probable reaction channels were investigated. Finally, molecular and kinetic properties of stationary points were calculated. The quantum chemical level of theory was increased at each step from DFTB (tight-binding density-functional), to DFT, and finally to post-Hartree-Fock methods. Results on CH based hydrocarbon growth showed the transition from non-cyclic hydrocarbons to cyclic and aromatic structures and from cyclic to polycyclic aromatic hydrocarbons. Additionally, the reactive collisions between hydrocarbons and CH were found to produce sufficient energy for isomerisation and fragmentation processes even at ultra low temperatures. In all, the results indicate that methyne might be a proper precursor for the formation of large interstellar PAHs
Kohlenwasserstoffe, insbesondere polyzyklische Kohlenwasserstoffe (engl. PAHs), werden seit einigen Jahren als Mitverursacher interstellar IR-Emissions- und UV-Absorptionsbanden angesehen und diskutiert. Dabei ist die Herkunft dieser Moleküle in den dichten Phasen des interstellaren Mediums (ISM) aber noch nicht aufgeklärt. In dieser Arbeit wurden daher die Bildungsmechanismen, welche auf Ion-Molekül-Reaktionen zwischen kationischen PAHs und Kohlenwasserstoffen und dem Molekül CH beruhen, untersucht. Sowohl der Reaktionstyp als auch der Präkursor wurden anhand von bekannten physikalischen und chemischen Eigenschaften des ISM abgeleitet und ausgewählt. Die Analyse der chemischen Reaktionen basierte auf Berechnungen zur Produktzusammensetzung und Einfangsratenkoeffizienten (welche wiederum aus berechneten Reaktionsquerschnitten hervorgingen) Minimumenergiepfade (MEP), Reaktionsenthalpien, thermische Gleichgewichtskonstanten und mikrokanonische Isomerisierungs- und Strahlungsdeaktivierungs-Ratenkoeffizienten. Um der Vielzahl an Reaktionsparameter gerecht zu werden, wurden die Berechnungsmethoden entsprechend eines hierarischen Fließschemas kombiniert. Hierzu wurden zuerst durch Molekulardynamik-Simulationen die Reaktionspotentialenergieflächen abgerastert. Auf der nächsten Stufe wurden statistisch bedeutsame Reaktionskanäle bezüglich ihrer Minimumenergiepfade untersucht. Den Abschluss bildete die Berechnung molekularer und kinetischer Charakteristika stationärer Punkte auf einem MEP. Entsprechend dieses Schemas wurde die quantenchemische Genauigkeit auf jeder Stufe von approximativer DFT über DFT zu post-Hartree-Fock verändert. Die Ergebnisse des CH-basierten Kohlenwasserstoffwachstums zeigten einen Übergang von nichtzyklischen zu zyklischen and aromatischen Strukturen, sowie von zyklischen zu polyzyklischen Kohlenwasserstoffen. Außerdem zeigte sich, dass reaktive Kollisionen zwischen Kohlenwasserstoffen und CH auch bei Tiefsttemperaturen immer ausreichend Energie für Isomerisierungs- und Fragmentationsprozesse liefert. Die Ergebnisse dieser Arbeit lassen den Schluss zu, dass CH ein geeigneter Präkursor für die Bildung großer interstellarer PAH ist

Hydrocarbons, in particular polycyclic aromatic hydrocarbons (PAHs), have been long discussed to be carriers of interstellar infrared (IR) emission and ultraviolet (UV) absorption features. Yet, their origin in dense phases of the interstellar medium (ISM), such as molecular clouds, remains unclear. In this work, growth mechanisms based on ion-molecule reactions between cationic PAHs/hydrocarbons and methyne (CH) were investigated. The reaction type and the precursor were derived and selected from known chemical and physical properties of the ISM. These chemical reactions were characterised by calculating branching ratios (based on cross sections) and capture rate coefficients, minimum reaction paths, reaction enthalpies, thermal equilibrium constants, and microcanonic isomerisation and radiative deactivation rate coefficients. In order to cope with the variety of reaction parameters, a hierarchic workflow scheme was set up. First, the reaction potential energy surface was sampled by molecular dynamics simulations. Then, minimum energy paths of the most probable reaction channels were investigated. Finally, molecular and kinetic properties of stationary points were calculated. The quantum chemical level of theory was increased at each step from DFTB (tight-binding density-functional), to DFT, and finally to post-Hartree-Fock methods. Results on CH based hydrocarbon growth showed the transition from non-cyclic hydrocarbons to cyclic and aromatic structures and from cyclic to polycyclic aromatic hydrocarbons. Additionally, the reactive collisions between hydrocarbons and CH were found to produce sufficient energy for isomerisation and fragmentation processes even at ultra low temperatures. In all, the results indicate that methyne might be a proper precursor for the formation of large interstellar PAHs.
Kohlenwasserstoffe, insbesondere polyzyklische Kohlenwasserstoffe (engl. PAHs), werden seit einigen Jahren als Mitverursacher interstellar IR-Emissions- und UV-Absorptionsbanden angesehen und diskutiert. Dabei ist die Herkunft dieser Moleküle in den dichten Phasen des interstellaren Mediums (ISM) aber noch nicht aufgeklärt. In dieser Arbeit wurden daher die Bildungsmechanismen, welche auf Ion-Molekül-Reaktionen zwischen kationischen PAHs und Kohlenwasserstoffen und dem Molekül CH beruhen, untersucht. Sowohl der Reaktionstyp als auch der Präkursor wurden anhand von bekannten physikalischen und chemischen Eigenschaften des ISM abgeleitet und ausgewählt. Die Analyse der chemischen Reaktionen basierte auf Berechnungen zur Produktzusammensetzung und Einfangsratenkoeffizienten (welche wiederum aus berechneten Reaktionsquerschnitten hervorgingen) Minimumenergiepfade (MEP), Reaktionsenthalpien, thermische Gleichgewichtskonstanten und mikrokanonische Isomerisierungs- und Strahlungsdeaktivierungs-Ratenkoeffizienten. Um der Vielzahl an Reaktionsparameter gerecht zu werden, wurden die Berechnungsmethoden entsprechend eines hierarischen Fließschemas kombiniert. Hierzu wurden zuerst durch Molekulardynamik-Simulationen die Reaktionspotentialenergieflächen abgerastert. Auf der nächsten Stufe wurden statistisch bedeutsame Reaktionskanäle bezüglich ihrer Minimumenergiepfade untersucht. Den Abschluss bildete die Berechnung molekularer und kinetischer Charakteristika stationärer Punkte auf einem MEP. Entsprechend dieses Schemas wurde die quantenchemische Genauigkeit auf jeder Stufe von approximativer DFT über DFT zu post-Hartree-Fock verändert. Die Ergebnisse des CH-basierten Kohlenwasserstoffwachstums zeigten einen Übergang von nichtzyklischen zu zyklischen and aromatischen Strukturen, sowie von zyklischen zu polyzyklischen Kohlenwasserstoffen. Außerdem zeigte sich, dass reaktive Kollisionen zwischen Kohlenwasserstoffen und CH auch bei Tiefsttemperaturen immer ausreichend Energie für Isomerisierungs- und Fragmentationsprozesse liefert. Die Ergebnisse dieser Arbeit lassen den Schluss zu, dass CH ein geeigneter Präkursor für die Bildung großer interstellarer PAH ist.

L'adsorption des polymeres et particulierement des polyelectrolytes derive essentiellement des liaisons qui s'etablissent entre les groupements fonctionnels de la macromolecule et ceux de la surface. Les possibilites de liaisons, principalement par des ponts hydrogene, par des forces electrostatiques ou par la formation de complexes de surface, dependent des equilibres qui s'etablissent entre la surface ou la macromolecule et tous les composants du systeme. Le ph, ou un ion qui a une forte interaction avec la surface, peuvent ainsi changer de maniere significative la reaction d'adsorption. Nous consacrons cette etude a l'analyse de reactions qui se produisent dans l'interface lors de l'adsorption d'un polyelectrolyte sur un mineral. Nous rappelons d'abord les bases theoriques generales qui permettent de comprendre le phenomene d'adsorption et sa dependance des parametres experimentaux. Nous decrivons plus en detail le modele developpe par l'ecole neerlandaise car nous l'utilisons plusieurs fois dans ce travail pour discuter les resultats experimentaux. Les systemes etudies sont constitues de deux mineraux: le dioxyde de titane et le carbonate de calcium, comme adsorbants, et d'une serie de polymeres acryliques comprenant des polyacrylamides plus ou moins hydrolyses, des acides polyacryliques et un trimere de l'acide acrylique, comme adsorbats. Une premiere serie d'analyses met en evidence que l'adsorption n'est reproductible dans un systeme donne que si l'on respecte strictement le mode operatoire choisi. Ce phenomene est du a la fois au manque de reversibilite de la reaction et a la polydispersite des echantillons de polymere. Ayant adopte une methode de travail, nous pouvons analyser l'influence sur la quantite de polymere adsorbee des principaux parametres du milieu (concentration et nature des ions, ph, temperature) et de la nature de la surface ou du polymere. Les resultats sont compares de maniere qualitative a ceux deja obtenus dans notre laboratoire ou par d 'autres auteurs. La discussion montre que les variations d'adsorption ne s'expliquent pas simplement par des criteres electrostatiques. D'autres interactions jouent un grand role et en particulier celles qui mettent en jeu les ions du systeme. Les changements d'ionisation de la surface et du polyelectrolyte, ainsi que les deplacements d'ions qui accompagnent la reaction d'adsorption du polymere, sont analyses en detail. Dans notre systeme contenant un polymere anionique, il faut distinguer les sites positifs de surface ou le contre ion negatif doit etre deplace pour que la liaison polymere-surface s'etablisse et les sites negatifs ou il apparait que le contre ion (cation) demeure. On observe aussi que le polyelectrolyte entraine dans la couche adsorbee une partie de ses propres contre ions. Les mesures microcalorimetriques des chaleurs d'ionisation de surface et d'adsorption des acides polyacryliques montrent que l'energie d'adsorption est dominee par les reactions analysees precedemment. En milieu acide, la deprotonation de la surface explique la diminution de la chaleur d'adsorption avec l'augmentation de la force ionique, en milieu basique l'adsorption est une reaction endothermique qui se produit grace a l'augmentation d'entropie engendree par la desolvatation des isons coadsorbes. Le trimere de l'acide acrylique permet de valider les analyses precedentes. Dans ce cas en effet, l'enthalpie d'adsorption mesuree est compatible avec celle calculee a partir de l'evolution des isothermes avec la temperature. En conclusion cette etude explique comment les ions, y compris le proton, interviennent de maniere specifique dans la fixation des polyelectrolytes a la surface. Elle fournit des informations originales et importantes pour la modelisation ou le controle de l'adsorption de ces macromolecules

Ce travail porte sur le développement de nouvelles méthodes de mesure permettant la caractérisation de réactions chimiques très exothermiques dans des conditions de sécurité. Pour cela, nous souhaitons combiner l’analyse thermique des réactions et la technologie microfluidique. L’utilisation de la microfluidique rend possible l’utilisation de très faibles volumes réactionnels limitant ainsi tout risque lié à la dangerosité des réactions explosives. Le premier appareil développé est un microcalorimètre qui mesure le flux de chaleur global dégagé lors d’un écoulement co-courant ou gouttes. Plusieurs paramètres peuvent être déterminés : enthalpie de mélange et de réaction, concentration par dosage calorimétrique et cinétique. Le deuxième dispositif consiste à mesurer le champ de température du milliréacteur isopéribolique à l’aide d’une caméra InfraRouge et ainsi de suivre localement l’évolution de la réaction pour déterminer les paramètres thermocinétiques
This work deals with the development of new measurement methods in order to characterize high exothermic chemical reactions in safe conditions. Thus, we combine thermal analysis with microfluidic technology. The use of microfluidics allows to manipulate a very small amount of product safely. First, we have developed a microcalorimeter to measure the global heat flux produced in co-flow or droplet-flow configurations. Several parameters can be determined: reaction and mixing enthalpy, concentrations by calorimetric titration and kinetics. The second method uses an InfraRed camera to measure the temperature field of the isoperibolic millireactor. Then, the local evolution of the reaction is estimated by thermal processing. From such inverse methods, the thermokinetic parameters can be determined

L'objectif de cette thèse est de développer un nouveau code pour prédire l'équilibre final d'un processus chimique complexe impliquant beaucoup de produits, plusieurs phases et plusieurs processus chimiques. Des méthodes numériques ont été développées au cours des dernières décennies pour prédire les équilibres chimiques finaux en utilisant le principe de minimisation de l'enthalpie libre du système. La plupart des méthodes utilisent la méthode des « multiplicateurs de Lagrange » et résolvent les équations en employant une approximation du problème de Lagrange et en utilisant un algorithme de convergence pas à pas de type Newton-Raphson. Les équations mathématiques correspondantes restent cependant fortement non linéaires, de sorte que la résolution, notamment de systèmes multiphasiques, peut être très aléatoire. Une méthode alternative de recherche du minimum de l’énergie de Gibbs (MCGE) est développée dans ce travail, basée sur une technique de Monte-Carlo associée à une technique de Pivot de Gauss pour sélectionner des vecteurs composition satisfaisant la conservation des atomes. L'enthalpie libre est calculée pour chaque vecteur et le minimum est recherché de manière très simple. Cette méthode ne présente a priori pas de limite d’application (y compris pour las mélanges multiphasiques) et l’équation permettant de calculer l’énergie de Gibbs n’a pas à être discrétisée. Il est en outre montré que la précision des prédictions dépend assez significativement des valeurs thermodynamiques d’entrée telles l'énergie de formation des produits et les paramètres d'interaction moléculaire. La valeur absolue de ces paramètres n'a pas autant d’importance que la précision de leur évolution en fonction des paramètres du process (pression, température, ...). Ainsi, une méthode d'estimation cohérente est requise. Pour cela, la théorie de la « contribution de groupe » est utilisée (ceux de UNIFAC) et a été étendue en dehors du domaine d'interaction moléculaire traditionnel, par exemple pour prédire l'énergie de formation d’enthalpie libre, la chaleur spécifique... Enfin, l'influence du choix de la liste finale des produits est discutée. On montre que la prédictibilité dépend du choix initial de la liste de produits et notamment de son exhaustivité. Une technique basée sur le travail de Brignole et Gani est proposée pour engendrer automatiquement la liste des produits stable possibles. Ces techniques ont été programmées dans un nouveau code : CIRCE. Les travaux de Brignole et de Gani sont mis en œuvre sur la base de la composition atomique des réactifs pour prédire toutes les molécules « réalisables ». La théorie de la « contribution du groupe » est mise en œuvre pour le calcul des propriétés de paramètres thermodynamiques. La méthode MCGE est enfin utilisée pour trouver le minimum absolu de la fonction d'enthalpie libre. Le code semble plus polyvalent que les codes traditionnels (CEA, ASPEN, ...) mais il est plus coûteux en termes de temps de calcul. Il peut aussi être plus prédictif. Des exemples de génie des procédés illustrent l'étendue des applications potentielles en génie chimique
The objective of this work is to develop a new code to predict the final equilibrium of a complex chemical process with many species/reactions and several phases. Numerical methods were developed in the last decades to predict final chemical equilibria using the principle of minimizing the Gibbs free energy of the system. Most of them use the “Lagrange Multipliers” method and solve the resulting system of equations under the form of an approximate step by step convergence technique. Notwithstanding the potential complexity of the thermodynamic formulation of the “Gibbs problem,” the resulting mathematical formulation is always strongly non-linear so that solving multiphase systems may be very tricky and having the difficult to reach the absolute minimum. An alternative resolution method (MCGE) is developed in this work based on a Monte Carlo technique associated to a Gaussian elimination method to map the composition domain while satisfying the atom balance. The Gibbs energy is calculated at each point of the composition domain and the absolute minimum can be deduced very simply. In theory, the technique is not limited, the Gibbs function needs not be discretised and multiphase problem can be handled easily. It is further shown that the accuracy of the predictions depends to a significant extent on the “coherence” of the input thermodynamic data such the formation Gibbs energy of the species and molecular interaction parameters. The absolute value of such parameters does not matter as much as their evolution as function of the process parameters (pressure, temperature, …). So, a self-consistent estimation method is required. To achieve this, the group contribution theory is used (UNIFAC descriptors) and extended somewhat outside the traditional molecular interaction domain, for instance to predict the Gibbs energy of formation of the species, the specific heat capacity… Lastly the influence of the choice of the final list of products is discussed. It is shown that the relevancy of the prediction depends to a large extent on this initial choice. A first technique is proposed, based on Brignole and Gani‘s work, to avoid omitting species and another one to select, in this list, the products likely to appear given the process conditions. These techniques were programmed in a new code name CIRCE. Brignole and Gani-‘s method is implemented on the basis of the atomic composition of the reactants to predict all “realisable” molecules. The extended group contribution theory is implemented to calculate the thermodynamic parameters. The MCGE method is used to find the absolute minimum of the Gibbs energy function. The code seems to be more versatile than the traditional ones (CEA, ASPEN…) but more expensive in calculation costs. It can also be more predictive. Examples are shown illustrating the breadth of potential applications in chemical engineering

A calorimeter has been developed to study metal-polymer interfaces in ultrahigh vacuum (UHV) conditions. Metal atoms are pulsewise deposited on a polymer substrate, gradually forming a metallic overlayer, while the reaction heat is measured in-situ with a pyroelectric sensor. Differential enthalpies of interface formation are derived from the calorimetry data and reported as a function of metal coverage. The ability of the instrument to resolve the underlying chemical reactions and growth morphology that characterize these interfacial systems is confirmed by the widely different enthalpy curves observed for each of the three metals studied in the present work - calcium, chromium and copper - deposited on PMDA-ODA polyimide substrates and the correspondingly distinct conclusions about the interfacial reactivity that were made in each case: Strong binding was discovered at the interface between calcium and polyimide. A calcium-polyimide complex with a binding energy of 600 ± 20 kJ/mol is suggested from the high initial intensity and exponential decay of the observed enthalpies with increasing metal coverage. The surface density of calcium binding sites was estimated at 4.1 nm- 2 . A more complicated reactivity was observed for the chromium-polyimide interface. Reaction enthalpies of approximately 60 ± 20 kJ/mol were observed at low coverages, six times lower than the enthalpy of formation of bulk chromium metal. This indicates the creation of a thermodynamically unstable interface. Chromium deposition apparently induces an endothermic disruption of the polyimide surface, which both lowers the net reaction enthalpy and suppresses the formation of metal-metal bonds. The copper results, meanwhile, are consistent with the formation of spherical metal clusters. The calorimeter records the change in surface energy from which the physical properties of the metal clusters, such as the cluster size, can be determined as a function of coverage.

碩士
逢甲大學
航太與系統工程學系
105
Preparation of ternary carbide composites, Ta2AlC, Cr2AlC, and Zr–Al–C/Al2O3, which have low formation enthalpies, was conducted by self-propagating high-temperature synthesis (SHS) involving thermite reactions and PTFE (C2F4) additions. The effects of sample stoichiometry and PTFE content were investigated on the propagation mode of the combustion wave, flame-front velocity, reaction temperature, and phase composition and microstructure of the final products. Experimental results of the Ta2AlC synthesis showed that upon ignition a planar reaction front formed and traversed the entire sample in a self-sustaining manner. An excess amount of Al led to a reduction of the combustion velocity and an increment of the combustion temperature. The decrease of C caused an increase in the combustion velocity and a decrease in the combustion temperature. It was found that the increase of Al along with the decrease of C reduced the formation of binary carbides. The addition of PTFE was able not only to supply a part of carbon, but also to enhance the combustion process and evolution of Ta2AlC. Formation of Cr2AlC was studied by using different carbon sources, including carbon black (Cb), graphite (Cg), and Al4C3. In order to achieve self-sustaining combustion, PTFE was used as a reaction enhancer. Results of the Cr2AlC synthesis showed that the reaction zone was confined to a localized region and propagated in a spinning mode. The flame-front velocity increased with PTFE content due to the PTFE enhancement, but the combustion temperture decreased becuase of the endothermic decompostion of PTFE. Compared with carbon black and graphite, Al4C3 as the carbon source produced the optimal Cr2AlC/Al2O3 composite. Production of the Zr-Al-C compounds of different phases exhibited a spinning combustion wave. However, regardless of excess Al, PTFE addition, and different carbon sources, poor formation of Zr2AlC, ZrAlC2, and Zr2Al3C5 was obtained and the major carbide formed was ZrC. This was mainly attributed to their low reaction temperatures. Therefore, chemical oven SHS (COSHS) a modified SHS was adopted to increase the synthesis temperature. In COSHS, the sample was encapsulated into a Ti-C shell. Experimental results of COSHS indicated no formation of Zr2AlC, but confirmed the presence of ZrAlC2 and Zr2Al3C5 and a significant decrease of ZrC. Because ZrC and Zr2Al3C5 were considered as the intermediates for the production of ZrAlC2, Zr2Al3C5 was found to be the phase formed better than the other two Zr-Al-C compounds. Keywords: Self-propagating high-temperature synthesis (SHS); MAX ternary carbides; PTFE; XRD; SEM.

Definition and mathematics of enthalpy. Definition of heat capacity at constant pressure as CP = (∂H/∂T)V. Endothermic and exothermic reactions. Role of the change in enthalpy as regards the direction and spontaneity of a change in state. Enthalpy changes and phase changes. Measuring enthalpy changes by calorimetry. Hess’s law of constant heat formation. Chemical standards and standard states. Standard enthalpies of formation, ionic enthalpies and bond energies. How the change in enthalpy varies with temperature. Kirchhoff’s equations. Applications of thermochemistry to a variety of worked examples, including flames and explosions.

Electrochemical measurements have been playing an increasingly important role in the thermodynamic study of reactions in solution, not only because they provide data that are difficult (or even impossible) to obtain by other methods but also because these data can often be compared with the values determined for the analogous gas-phase reactions, thus yielding information on solvation energetics. Figure 16.1 was adapted from a scheme proposed by Griller et al. It summarizes the thermochemical information on the R–X bond that can be probed by electrochemical methods. The vertical arrows represent homolytic cleavages, and the horizontal arrows depict reduction or oxidation potentials. The authors have appropriately called the scheme in figure 16.1 a “mnemonic,” rather than a “thermochemical cycle,” because not all arrow combinations define thermochemical cycles. This can be made more clear by inspecting figure 16.2, where true thermochemical cycles are defined. For example, the enthalpy of reaction 7 is not the sum of the enthalpies of reactions 1 and 4 (as might be suggested by figure 16.1) but their sum minus the enthalpy of reaction 12. In fact, true thermochemical cycles in figure 16.1 can only be defined by considering parallelograms confined either to the upper or the lower part of the mnemonic. For instance, the enthalpy of reaction 7 is given by the enthalpy of reaction 4 plus the enthalpy of reaction 9 minus the enthalpy of reaction 3, but it is not equal to the enthalpy of reaction 6 minus the enthalpy of reaction 11 plus the enthalpy of reaction 10. Also, the enthalpy of reaction 1 (the homolytic dissociation of the R–X bond in the neutral molecule RX) can be given by the sum of the enthalpies or reaction 5 and 11 minus the enthalpy of reaction 3 or, for example, by the sum of the enthalpies of reactions 7 and 12 minus the enthalpy of reaction 4. The attractive feature of the mnemonic in figure 16.1 (or the thermochemical cycles in figure 16.2) is that it depicts the seven possible R–X cleavage reactions of RX, RX−, and RX+, as well as their relationships.

The determination of enthalpies of reaction in solution, using isoperibol reaction-solution calorimetry, is often the easiest and most accurate method of determining enthalpies of formation of compounds that cannot be studied by combustion calorimetry. The technique was pioneered by Thomsen who, between 1882 and 1886, performed thermochemical measurements involving the solution of various substances in liquids (e.g., diluted acids). Many types of isoperibol reaction-solution calorimeters have been developed since then. The designs vary according to the nature of the reactions of interest. One of the most widely used consists of a vessel, such as the one shown in figure 8.1, immersed in a thermostatic water bath. The sample is sealed inside a thin-walled glass ampule A, fixed to an ampule breaking system B in the calorimeter head C. The calorimeter head also supports the temperature sensor D, the stirrer E, and an electrical resistance F, used for calibration of the apparatus. The Dewar vessel G, containing the solution to be reacted with the sample, is adjusted to C. The assembled calorimetric vessel is transferred to the thermostatic bath, and from then on, the experimental procedure closely follows that already described in section 7.1 for isoperibol static-bomb combustion calorimetry. The reaction is initiated at the end of the fore period by pushing down the plunger H and breaking the ampule against a pin situated at the bottom of the ampule breaking system B. As a result of the calorimetric experiment, a temperature-time curve such as the one in figure 7.2 is obtained. Note that figure 7.2 is typical of an exothermic process. In the case of an endothermic process, a decrease of the temperature of the calorimetric system is observed during the reaction period. The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, ΔHICP, usually at the reference temperature of 298.15 K. This involves the determination of the corresponding adiabatic temperature change, ΔTad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1; the determination of the energy equivalent of the calorimeter in a separate experiment.

This part includes a discussion of the main experimental methods that have been used to study the energetics of chemical reactions and the thermodynamic stability of compounds in the condensed phase (solid, liquid, and solution). The only exception is the reference to flame combustion calorimetry in section 7.3. Although this method was designed to measure the enthalpies of combustion of substances in the gaseous phase, it has very strong affinities with the other combustion calorimetric methods presented in the same chapter. Most published enthalpies of formation and reaction in the condensed phase were determined by calorimetry (see databases indicated in appendix B). It is therefore not surprising that the discussion of calorimetric methods occupies a large fraction of part II. The heart of a calorimeter is the calorimeter proper (also called measuring system or sample cell), which contains the reaction vessel, where the chemical reaction or phase transition under study occurs. Sometimes the calorimeter proper coincides with the reaction vessel. For example, in the setup shown in figure 6.1a, which is typical of many combustion calorimeters, the reaction vessel is placed inside the calorimeter proper. In the arrangement of figure 6.1b, used in many reaction-solution calorimeters, the calorimeter proper is also the reaction vessel. Normally, a controlled-temperature jacket surrounds the calorimeter proper. Other parts besides thermometers, commonly found in calorimeters, are stirring, heating, cooling, and ignition devices. Some of these devices are placed inside the calorimeter proper or cross its boundaries and are also considered to be part of it. In modern instruments, the data acquisition and many steps of the calorimetric experiments are usually computer-controlled. Calorimeters of many different designs have been constructed and operated. However, these are all variations of a few basic categories. For example, based on the heat exchange mode between the calorimeter proper and the surrounding jacket, it is convenient to distinguish three main classes of calorimeters: adiabatic, heat conduction, and isoperibol. In a perfectly adiabatic calorimeter no heat is transferred between the calorimeter proper and the jacket (the corresponding heat flow rate Φ = dQ/dt = 0, where Q represents the heat exchanged and t is time).

Titration calorimetry is a method in which one reactant inside a calorimetric vessel is titrated with another delivered from a burette at a controlled rate. This technique has been adapted to a variety of calorimeters, notably of the isoperibol and heat flow types. The output of a titration calorimetric experiment is usually a plot of the temperature change or the heat flow associated with the reaction or physical interaction under study as a function of time or the amount of titrant added. A primary use of titration calorimetry is the determination of enthalpies of reaction in solution. The obtained results may of course lead to enthalpies of formation of compounds in the standard state by using appropriate thermodynamic cycles and auxiliary data, as described in chapter 8 for reaction-solution calorimetry. Moreover, when reactions are not quantitative, both the equilibrium constant and the enthalpy of reaction can often be determined from a single titration run. This also yields the corresponding ΔrGo and ΔrSo through equations 2.54 and 2.55. Extensive use has been made of titration calorimetry as an analytical tool. These applications, which are outside the scope of this book, have been covered in various reviews. The historical development of titration calorimetry has been addressed by Grime. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia from a plot of the observed temperature change against the volume of ammonia added. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol or heat-flow instruments. The titration procedures described in the literature can be classified as continuous or incremental, depending on the mode of titrant addition. In the first case the titrant is continuously introduced in the reaction vessel at a programmed (not necessarily constant) rate during a run.

Abstract Some applications of thermally sprayed coatings need a metallurgical bonding of substrate and coating. This can be reached by laser remelting of a thermally sprayed coating, which causes, on the other hand, a certain dilution of the substrate elements into the coating. This article discusses the influence of reaction enthalpies on the microstructure formation in the alloying systems Ni-Al and Ti-Al. Experimental work and simulation were done to examine the time constants of solidification influenced by laser dwell time and reaction enthalpy. It was observed that, for short dwell times, the reaction heat dominates the solidification process and the microstructure formation.

Abstract Cost-effective solar power generation in CSP plants requires the challenging integration of high energy density and high-temperature thermal energy storage with the solar collection equipment and the power plant. Thermochemical energy storage (TCES) is currently a very good option for thermal energy storage, which can meet the industry requirement of large energy density and high storage temperature. TCES specifically exploits reversible chemical reactions wherein heat is absorbed during the forward endothermic reaction and released during the reverse exothermic reaction. The associated enthalpic storage of energy (i.e., the heat of reaction) offers higher density and enhanced stability compared to sensible and latent heat storage. Metal oxide redox reactions are particularly well-suited for TCES given their characteristically high enthalpy of reaction and high reaction temperature. In addition, the air is suitable as both a heat transfer fluid (HTF) and reactant; thus, simplifying process design and eliminating the need for indirect HTF storage and any intermediate heat exchanger. Among the palette of available metal oxides, cobalt oxide is one of the most promising candidates for TCES given its high enthalpy of reaction with high reaction temperature. One of the critical design parameters for TCES reactors is the optimal heating and cooling rates during respective charging and discharging modes of operation. In order to study the effect of heating/cooling rate on cobalt oxide TCES performance, a constant 10°C/min rate was selected for both storage cycle heating and cooling. Considering the intrinsic redox kinetics of cobalt oxide at considered constant heating/cooling rate, we studied milligram scale quantities of cobalt oxide (99.9% purity, 40 μm average particle size) using a dual-mode thermogravimetric (TGA)/differential scanning calorimetry (DSC) system, which simultaneously measures weight change (TGA) and differential heat flow (DSC) as a function of TCES cycling under continuous air purge. In addition, we investigated the cyclic stability of cobalt oxide in the context of the redox kinetics and particle coarsening behavior, employing scanning electron microscopy (SEM). TGA/DSC tests were conducted for 30 successive cycles using pure cobalt oxide. It was shown that pure cobalt oxide in powder form (38μ particle size) could complete both forward and reverse reaction at the selected heating rate with little degradation between cycles. In parallel, SEM was used to examine morphology and particle size changes before and after heating cycles. SEM results proved grain growth occurs even after only five initial cycles.

Increased research into the chemistry, physics and material science of hydrogen cycling compounds has led to the rapid growth of solid-phase hydrogen-storage options. The operating conditions of these new options span a wide range: system temperature can be as low as 70K or over 600K, system pressure varies from less than 100kPa to 35MPa, and heat loads can be moderate or can be measured in megawatts. While the intense focus placed on storage materials has been appropriate, there is also a need for research in engineering, specifically in containment, heat transfer, and controls. The DOE’s recently proposed engineering center of expertise underscores the growing understanding that engineering research will play a role in the success of advanced hydrogen storage systems. Engineering a hydrogen system will minimally require containment of the storage media and control of the hydrogenation and dehydrogenation processes, but an elegant system design will compensate for the storage media’s weaker aspects and capitalize on its strengths. To achieve such a complete solution, the storage tank must be designed to work with the media, the vehicle packaging, the power-plant, and the power-plant’s control system. In some cases there are synergies available that increase the efficiency of both subsystems simultaneously. In addition, system designers will need to make the hard choices needed to convert a technically feasible concept into a commercially successful product. Materials cost, assembly cost, and end of life costs will all shape the final design of a viable hydrogen storage system. Once again there is a critical role for engineering research, in this case into lower cost and higher performance engineering materials. Each form of hydrogen storage has its own, unique, challenges and opportunities for the system designer. These differing requirements stem directly from the properties of the storage media. Aside from physical containment of compressed or liquefied hydrogen, most storage media can be assigned to one of four major categories, chemical storage, metal hydrides, complex hydrides, or physisorption. Specific needs of each technology are discussed below. Physisorption systems currently operate at 77K with very fast kinetics and good gravimetric capacity; and as such, special engineering challenges center on controlling heat transfer. Excellent MLVSI is available, its cost is high and it is not readily applied to complex shape in a mass manufacture setting. Additionally, while the heat of adsorption on most physisorbents is a relatively modest 6–10kJ/mol H2, this heat must be moved up a 200K gradient. Physisorpion systems are also challenged on density. Consequently, methods for reducing the cost of producing and assembling compact, high-quality insulation, tank design to minimize heat transfer while maintaining manufacturability, improved methods of heat transfer to and from the storage media, and controls to optimize filling are areas of profitable research. It may be noted that the first two areas would also contribute to improvement of liquid hydrogen tanks. Metal hydrides are currently nearest application in the form of high pressure metal hydride tanks because of their reduced volume relative to compressed gas tanks of the same capacity and pressure. These systems typically use simple pressure controls, and have enthalpies of roughly 20kJ/mol H2 and plateau pressures of at most a few MPa. During filling, temperatures must be high enough to ensure fast kinetics, but kept low enough that the thermodynamically set plateau pressure is well below the filling pressure. To accomplish this balance the heat transfer system must handle on the order of 300kW during the 5 minute fill of a 10kg tank. These systems are also challenged on mass and the cost of the media. High value areas for research include: heat transfer inside a 35MPa rated pressure vessel, light and strong tank construction materials with reduced cost, and metals or other materials that do not embrittle in the presence of high pressure hydrogen when operated below ∼400K. The latter two topics would also have a beneficial impact on compressed gas hydrogen storage systems, the current “system to beat”. Complex hydrides frequently have high hydrogen capacity but also an enthalpy of adsorption >30kJ/mol H2, a hydrogen release temperature >370K, and in many cases multiple steps of adsorption/desorption with slow kinetics in at least one of the steps. Most complex hydrides are thermal insulators in the hydrided form. From an engineering perspective, improved methods and designs for cost effective heat transfer to the storage media in a 5 to 10MPa vessel is of significant interest, as are materials that resist embrittlement at pressures below 10MPa and temperatures below 500K. Chemical hydrides produce heat when releasing hydrogen; in some systems this can be managed with air cooling of the reactor, but in other systems that may not be possible. In general, chemical hydrides must be removed from the vehicle and regenerated off-board. They are challenged on durability and recycling energy. Engineering research of interest in these systems centers around maintaining the spent fuel in a state suitable for rapid removal while minimizing system mass, and on developing highly efficient recycling plant designs that make the most of heat from exothermic steps. While the designs of each category of storage tank will differ with the material properties, two common engineering research thrusts stand out, heat transfer and structural materials. In addition, control strategies are important to all advanced storage systems, though they will vary significantly from system to system. Chemical systems need controls primarily to match hydrogen supply to power-plant demand, including shut down. High pressure metal hydride systems will need control during filling to maintain an appropriately low plateau pressure. Complex hydrides will need control for optimal filling and release of hydrogen from materials with multi-step reactions. Even the relatively simple compressed-gas tanks require control strategies during refill. Heat transfer systems will modulate performance and directly impact cost. While issues such as thermal conductivity may not be as great as anticipated, the heat transfer system still impacts gravimetric efficiency, volumetric efficiency and cost. These are three key factors to commercial viability, so any research that improves performance or reduces cost is important. Recent work in the DOE FreedomCAR program indicates that some 14% of the system mass may be attributed to heat transfer in complex hydride systems. If this system is made to withstand 100 bar at 450K the material cost will be a meaningful portion of the total tank cost. Improvements to the basic shell and tube structures that can reduce the total mass of heat transfer equipment while maintaining good global and local temperature control are needed. Reducing the mass and cost of the materials of construction would also benefit all systems. Much has been made of the need to reduce the cost of carbon fiber in compressed tanks and new processes are being investigated. Further progress is likely to benefit any composite tank, not just compressed gas tanks. In a like fashion, all tanks have metal parts. Today those parts are made from expensive alloys, such as A286. If other structural materials could be proven suitable for tank construction there would be a direct cost benefit to all tank systems. Finally there is a need to match the system to the storage material and the power-plant. Recent work has shown there are strong effects of material properties on system performance, not only because of the material, but also because the material properties drive the tank design to be more or less efficient. Filling of a hydride tank provides an excellent example. A five minute or less fill time is desirable. Hydrogen will be supplied as a gas, perhaps at a fixed pressure and temperature. The kinetics of the hydride will dictate how fast hydrogen can be absorbed, and the thermodynamics will determine if hydrogen can be absorbed at all; both properties are temperature dependent. The temperature will depend on how fast heat is generated by absorption and how fast heat can be added or removed by the system. If the design system and material properties are not both well suited to this filling scenario the actual amount of hydrogen stored could be significantly less than the capacity of the system. Controls may play an important role as well, by altering the coolant temperature and flow, and the gas temperature and pressure, a better fill is likely. Similar strategies have already been demonstrated for compressed gas systems. Matching system capabilities to power-plant needs is also important. Supplying the demanded fuel in transients and start up are obvious requirements that both the tank system and material must be design to meet. But there are opportunities too. If the power-plant heat can be used to release hydrogen, then the efficiency of vehicle increases greatly. This efficiency comes not only from preventing hydrogen losses from supplying heat to the media, but also from the power-plant cooling that occurs. To reap this benefit, it will be important to have elegant control strategies that avoid unwanted feedback between the power-plant and the fuel system. Hydrogen fueled vehicles are making tremendous strides, as can be seen by the number and increasing market readiness of vehicles in technology validation programs. Research that improves the effectiveness and reduces the costs of heat transfer systems, tank construction materials, and control systems will play a key role in preparing advanced hydrogen storage systems to be a part of this transportation revolution.