Overall Heat Of Reaction Research Articles

Fabrication and Characteristics of Al/PTFE Multilayers and Application in Micro-initiator

In this paper, a micro-initiator was designed and fabricated by integrating Al/PTFE multilayers with a Cu film bridge. The regularity layer structure and interface composition of Al/PTFE multilayers was analysed by transmission electron microscope and X-ray photoelectron spectroscopy, respectively. The heat release reaction in Al/PTFE multilayers can be triggered with reaction temperature of 430 °C, and the overall heat of reaction is 3192 J/g. Al/PTFE multilayers with bilayer thickness of 200 nm was alternately deposited on a Cu film bridge to improve the electric explosion performances. Compared to Cu film bridge, the Al/PTFE/Cu integrated film bridge exhibits improved performances with longer explosion duration time, more violent explosion phenomenon and larger quantities of ejected product particles.

Effect of dicumyl peroxide concentration on the polymerization kinetics of a polysilazane system

The effect of dicumyl peroxide (DCPO) concentration on the polymerization kinetics of a polysilazane system has been investigated through DSC analyses. DCPO acts as polymerization initiator by activating the polyaddition of vinyl groups. The overall heat of reaction is found to be a linear function of the DCPO concentration above 1.5 wt%, due to the addition of the heat of fully initiated polymerization and the heat of DCPO decomposition. Below 1.5 wt% of DCPO, the polymerisation of polysilazane is not fully initiated and only part of the reaction occurs. The resulting overall heat of reaction is therefore lower than that provided by the linear law valid for higher concentrations. A two‐reaction kinetic model built on this interpretation leads to a satisfactory representation of the DSC analyses performed for each DCPO concentration between 0.1 and 20 wt%. POLYM. ENG. SCI., 58:859–869, 2018. © 2017 Society of Plastics Engineers

Self-Organized Al2Cu Nanocrystals at the Interface of Aluminum-Based Reactive Nanolaminates to Lower Reaction Onset Temperature.

Nanoenergetic materials are beginning to play an important role in part because they are being considered as energetic components for materials, chemical, and biochemical communities (e.g., microthermal sources, microactuators, in situ welding and soldering, local enhancement of chemical reactions, nanosterilization, and controlled cell apoptosis) and because their fabrication/synthesis raises fundamental challenges that are pushing the engineering and scientific frontiers. One such challenge is the development of processes to control and enhance the reactivity of materials such as energetics of nanolaminates, and the understanding of associated mechanisms. We present here a new method to substantially decrease the reaction onset temperature and in consequence the reactivity of nanolaminates based on the incorporation of a Cu nanolayer at the interfaces of Al/CuO nanolaminates. We further demonstrate that control of its thickness allows accurate tuning of both the thermal transport and energetic properties of the system. Using high resolution transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry to analyze the physical, chemical and thermal characteristics of the resulting Al/CuO + interfacial Cu nanolaminates, we find that the incorporation of 5 nm Cu at both Al/CuO and CuO/Al interfaces lowers the onset temperature from 550 to 475 °C because of the lower-temperature formation of Al-Cu intermetallic phases and alloying. Cu intermixing is different in the CuO/Cu/Al and Al/Cu/CuO interfaces and independent of total Cu thickness: Cu readily penetrates into Al grains upon annealing to 300 °C, leading to Al/Cu phase transformations, while Al does not penetrate into Cu. Importantly, θ-Al2Cu nanocrystals are created below 63% wt Cu/Al, and coexist with the Al solid solution phase. These well-defined θ-Al2Cu nanocrystals seem to act as embedded Al+CuO energetic reaction triggers that lower the onset temperature. We show that ∼10 nm thick Cu at Al/CuO interfaces constitutes the optimum amount to increase both reactivity and overall heat of reaction by a factor of ∼20%. Above this amount, there is a rapid decrease of the heat of reaction.

ITC Methods for Assessing Buffer/Protein Interactions from the Perturbation of Steady-State Kinetics: A Reactivity Study of Homoprotocatechuate 2,3-Dioxygenase.

Isothermal titration calorimetry (ITC) can be used to study the thermodynamics of enzyme substrate binding or the kinetics of substrate turnover (or both). Substrate-binding interactions are observed in a typical ITC titration experiment in which the heat change for the addition of an aliquot of substrate to a solution containing the enzyme is determined for a number of titrant (i.e., substrate) injections and the data fit for the thermodynamic parameters (ΔG, ΔH, and -TΔS) for substrate binding. Of course, these measurements must be made under conditions where the substrate binds but does not turnover. In the ITC "kinetics" experiment, the power change observed after injection of an excess of substrate into a solution of the enzyme is a direct measure of the rate at which substrate is converted to product, and the ITC data can be analyzed for the kinetic parameters (Vmax, kcat, KM, and kcat/KM). The ITC technique is particularly versatile in that it can be applied to systems where there might not be a change in a spectroscopic signal for either substrate binding or the reaction of the substrate to form product. A complication is that if there are competing reactions, for example, buffer protonation, or product binding, to name just two, the enthalpy change measured for either substrate binding or for substrate turnover will be a summation of all of the reaction heats. Enzyme studies are typically done in buffered solutions at constant pH. The general, and often incorrect, assumption is that the buffer components are simply spectators and not participants in either substrate binding or substrate turnover. This chapter describes how we have used ITC measurements to identify problem buffers that impact the kinetics for an enzyme catalyzed reaction. Herein, we show the effects of several buffers on the steady-state kinetics for the conversion of the substrate, 3,4-dihydroxyphenyl acetate (homoprotocatechuate), to the ring-opened product, 5-carboxymethyl-2-hydroxymuconic semialdehyde by the nonheme iron(II) metalloenzyme, homoprotocatechuate 2,3-dioxygenase. Several buffers were observed to engage in buffer/enzyme interactions within the active site pocket. These enzyme-buffer interactions were shown to inhibit substrate turnover and to contribute additional enthalpy terms to the overall heat of reaction observed for substrate turnover (and for substrate binding).

A mathematical method for predicting heat of reaction of organic peroxides

The heat of reaction of organic peroxide is widely used to estimate the risk of fire and explosion in process industries. In this study, the quantitative relationship between the heat of reaction and the molecular structures of organic peroxides was established based on quantitative structure property relationship (QSPR). The genetic algorithm combined with partial least squares (PLS) was employed to select optimal subset of descriptors which had significant contribution to the overall heat of reaction. The best resulted model was consisted of seven variables which were Ss, Me, IVDM, HDcpx, ATS4m, MATS1e and MOR14m. Ss and Me are constitutional descriptors which are related to the electrical states of the atom, IVDM and HDcpx are topological descriptors which are related to the shape of the molecule, ATS4m and MATS1e are 2D autocorrelations descriptors which can reflect the topology structure of molecules and MOR14m is a 3D-MoRSE descriptor which can represent the spatial structure of the molecular. The correlation coefficient was 0.995 which mean the model had high fitting capacity. Model validation was also performed to check the stability and predictive capability of the presented model. The results showed that the presented model was a valid and predictive model. This study can provide a new way for predicting the heat of reaction of the organic peroxides.

Thermochemical investigation of the water uptake behavior of MgSO4 hydrates in host materials with different pore size

The reversible dehydration and re-hydration of salts is considered as a promising thermochemical process for the storage of heat from a renewable energy source such as solar energy. Such materials would help to overcome the gap between demand of energy and supply from renewable sources. So far however, attempts to make full use of the very high storage densities of salt hydrates were unsuccessful, largely due to problems with the kinetics of the hydration and dehydration reactions. In the present work, in order to overcome these problems, magnesium sulfate was embedded in various porous host materials with average pore diameters (dm) ranging from 1.7μm to 7nm. The dehydration and water uptake behavior of these composites was investigated by thermogravimetry and isothermal calorimetry. The heat of reaction of the composites at 85% RH and 30°C varies from 103MJ/m3 in glass frits (dm=1.7μm) to 556MJ/m3 in porous glass (dm=7nm) and increases with decreasing pore size. The contribution of the heat of hydration to the overall thermal effect decreases with decreasing pore size while the contribution of the heat of adsorption increases. In bulk samples and in glass frits with coarse, micron-sized pores, MgSO4·6H2O is the major hydration product. In host materials with submicron pores, water vapor uptake yields both a crystalline hydrated phase (most likely MgSO4·7H2O) and its saturated solution. Apart from pore size, the salt content of the porous substrate, i.e., the degree of pore filling, is another critical parameter affecting the overall heat of reaction. Pore size and pore filling as well as porosity are the most important parameters that need to be optimized in order to achieve high storage densities with composite materials of a porous host materials and embedded salt hydrates.

The Intrinsic Kinetics and Heats of Reactions for Cellulose Pyrolysis and Char Formation

The Intrinsic Kinetics and Heats of Reactions for Cellulose Pyrolysis and Char Formation

Contra‐thermodynamic Behavior in Intermolecular Hydrogen Transfer of Alkylperoxy Radicals

Quantum chemical investigation of bimolecular hydrogen transfer involving alkylperoxy radicals, a key reaction family in the free-radical oxidation of hydrocarbons, was performed to establish structure-reactivity relationships. Eight different reactions were investigated featuring four different alkane substrates (methane, ethane, propane and isobutane) and two different alkylperoxy radicals (methylperoxy and iso-propylperoxy). Including forward and reverse pairs, sixteen different activation energies and enthalpies of reaction were used to formulate structure-reactivity relationships to describe this chemistry. We observed that the enthalpy of formation of loosely bound intermediate states has a strong inverse correlation with the overall heat of reaction and that this results in unique contra-thermodynamic behavior such that more exothermic reactions have higher activation barriers. A new structure-reactivity relationship was proposed that fits the calculated data extremely well: E(A)=E(o)+alphaDeltaH(rxn) where alpha=-0.10 for DeltaH(rxn)<0, and alpha=1.10 for DeltaH(rxn)>0 and E(o)=3.05 kcal mol(-1).

Energetics of the lithium-magnesium imide–magnesium amide and lithium hydride reaction for hydrogen storage: An ab initio study

An ab initio study within the density functional theory of the recently described reversible hydrogen storage reaction Mg(NH 2) 2 + 2LiH ⇔ Li 2Mg(NH) 2 + 2H 2 has been conducted. The electronic structure, structural parameters, vibrational spectra, and enthalpies of formation of all the reactants and products as well as the heat of the overall reaction at zero and finite temperature have been calculated in the generalized gradient approximation (GGA) to the exchange correlation potential. The heat of the overall reaction is calculated to be 53.4 kJ/mol H 2 in contrast to the experimentally obtained overall heat of reaction of ∼44.1 kJ/mol H 2. The difference of ∼20% between the experimental and calculated values is discussed.

Influence of the initiating mechanism on the cationic photopolymerization of a cycloaliphatic epoxy resin with arylsulfonium salts

AbstractThe photocuring process of widely used 3,4‐epoxycyclohexylmethyl 3′,4′‐epoxycyclohexane carboxylate has been investigated with differential scanning photocalorimetry and attenuated total reflection/Fourier transform infrared. Mixed salts of triarylsulfonium hexafluoroantimonate have been employed as the photoinitiator. The photocuring of the biscycloaliphatic resins exhibits a complex behavior: the overall heat of reaction (including dynamic thermal postcuring) depends on the photocuring temperature, surprisingly high reaction rates are observed at lower photocuring temperatures, and the range of the glass transition of the fully cured material broadens and shifts to higher temperatures as the photocuring temperature increases. It is assumed that the balance between the initiation step and the propagation step is responsible for the changes in the reaction mechanism that produce the observed experimental results. This balance may depend on the amount of the photoinitiator, the irradiation intensity, and the photocuring temperature. The structure and final properties of the material may therefore depend on the adjustment of these parameters. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 16–25, 2007

Effects of Styrene Evaporation on the Cure Kinetics Behaviour of a Vinylester Resin System Suitable for Composite Pultrusion

The paper describes the effects of styrene evaporation on the curing reactions of a vinylester resin, explored by the use of differential scanning calorimetry. The thermosetting system under study is characterized by the presence of three distinct exothermic peaks, attributed to different cure reaction mechanisms. A chemical cure kinetics model is presented, incorporating effects of diffusion limitations on the later stages of the cure. Whilst the nature of the curing mechanism is unaffected by the loss of styrene, the expected reductions in the overall heat of reaction from the individual reaction components are noted and quantified. The glass to rubber transition temperature, Tg, of fully cured resin is shown to decrease from 124°C in the initial sample to 121°C in samples in which over 20% of the initial styrene content had evaporated.

A first principles analysis of CO oxidation over Pt and Pt 66.7%Ru 33.3% (111) surfaces

Nonlocal gradient corrected DFT slab calculations were carried out to determine the overall reaction energies along with the barriers for the activation of water and the oxidation of CO over well-defined Pt(111) and PtRu(111) surfaces. We attempt to elucidate the features that control the bifunctional mechanism proposed for the oxidation of CO in solution. The addition of Ru to Pt along with the presence of solution helps to enhance the elementary steps that comprise the bifunctional mechanism. The activation of water over Pt(111) in the vapor phase is energetically unfavorable with an activation barrier of +142 kJ/mol and overall heat of reaction of +53 kJ/mol. Water will desorb before it will react on Pt(111). The presence of solution reduces the barrier to about 75 kJ/mol. The addition of Ru lowers the barrier for water activation in the vapor phase. Alloying Pt with Ru lowers the barriers for the homolytic as well as the heterolytic activation of water in solution. The presence of solution, however markedly favors the heterolytic activation which leads to the formation of an adsorbed OH* surface intermediate, a proton which migrates into solution and an electron. The energetics for this path over the Pt 66.7%Ru 33.3%(111) surface were calculated to be quite favorable with an activation barrier of +27 kJ/mol and an overall energy of reaction of +26 kJ/mol. Ab initio molecular dynamics results indicate that water can be activated over PtRu at 300 K. The resulting surface Ru-OH group that forms induces the adsorption and subsequent activation of water at a neighboring Pt site. The surface hydroxyl intermediate continues to diffuse across the surface via a sequence of proton transfer steps. The barrier for the subsequent oxidation of adsorbed CO by surface OH groups over Pt(111) in the vapor phase is 86 kJ/mol with an overall reaction energy of 21 kJ/mol. The barrier is reduced to 71 kJ/mol when carried out over the Pt 66.7%Ru 33.3% alloy in the vapor phase. The barrier for the homolytic disproportionation of CO and OH in solution (over Pt 66.7%Ru 33.3%) is 90 kJ/mol with an overall reaction energy of +24 kJ/mol. The heterolytic path which involves the oxidation of CO by OH to form CO 2+H +(aq)+e− is more favorable than the homolytic path. The activation barrier for the heterolytic path over Pt 66.7%Ru 33.3% is +60 kJ/mol with an overall reaction of −6 kJ/mol (exothermic). These results indicate that at potentials which are less than or equal to the potential of zero total charge, water activation over Pt(111) may be difficult. CO oxidation, however is more likely limiting over the Pt 66.7%Ru 33.3% alloy at these potentials. These results only hold for the ideal conditions studied herein including ideal single crystal surfaces, an ideal liquid water solution phase and the absence of an applied potential.

Thermal hazard in a batch process involving hydrogen peroxide

Abstract Hydrogen peroxide is a versatile and interesting reagent for many industrial processes; nevertheless, it is very sensitive to impurities that can catalyze its decomposition, so that the desired reaction could be accompanied by undesired parallel and consecutive reactions. As an example, the butadiene free radical polymerization with hydrogen peroxide in the presence of an organic solvent was studied. Batch polymerization occurs in the liquid phase at about 120 °C. Because of the involved reactive compounds and the relatively high temperature, this is an intrinsically dangerous reaction. Therefore calorimetric data can give important information about safety and process optimization during the scale-up. The aim of this research project was to study the influence of impurities on the overall heat of reaction. The experiments were made in a high-pressure reaction calorimeter. The study has revealed that impurities do indeed affect the reaction course. Most importantly, the presence of carboxylic acids and/or ionic iron must be avoided and the recycle of unreacted reagents must be carefully controlled to minimize the build-up of these impurities.

Thermokinetic investigation of cellulose pyrolysis — impact of initial and final mass on kinetic results

Cellulose pyrolysis has been investigated by combined TGA/DTA applying constant heating rates between 0.14 and 105 K min −1. The experiments entailed variation of initial mass and initial bulk density of the cellulose samples. The final mass depended on the heating rate as well as on the initial mass and bulk density and was reproducible when these parameters were carefully controlled. Mass transport resistance is a dominating factor increasing the char yield even in samples of 1 mg significantly at all heating rates. At higher initial sample masses, the char yield is determined by meshed heat and mass transport phenomena. Exothermic secondary reactions form additional char and change the overall heat of reaction. The heat released by secondary reactions promotes the volatilization of primary tars which in turn reduces the char yield. Fitting global reaction models to the experimental TGA results assuming a single first-order reaction yielded apparent kinetic parameters which were strongly correlated, but varied in a broad range depending in a complicated manner on heating rate and initial sample mass. This, together with the inherent impossibility to predict changes in product distribution, results in the restricted applicability of a single first-order reaction as predictive model for reactor analysis and design. Multi-step models with a modified Broido-Shafizadeh mechanism are more successful in describing experimental results for small sample masses. Nevertheless, they can not be extended to conditions under which secondary char is formed due to the lack of kinetic data in the literature.

Phase separation induced by a chain polymerization: Polysulfone-modified epoxy/anhydride systems

The reaction-induced phase separation in a blend of a commercial polysulfone (PSu) with diepoxide-cyclic anhydride monomers, was studied. The diepoxide was based on diglycidylether of bisphenol A (DGEBA) and the hardener was methyl tetrahydrophthalic anhydride (MTHPA), used in stoichiometric proportion. Benzyldimethylamine (BDMA) was used as initiator. PSu had no influence on the polymerization kinetics, the gel conversion, and the overall heat of reaction per epoxy equivalent. A kinetic model including initiation, propagation, and termination steps was used to estimate the distribution of linear and branched species in the first stages of the chain-wise copolymerization. This distribution, together with the PSu distribution, were taken into account in a thermodynamic model of the blend. The interaction parameter was fitted from experimental determinations of conversions at the start of phase separation, obtained under different conditions. The thermodynamic model was used to explain the complex morphologies developed in materials containing different PSu concentrations as well as their dynamic mechanical response. The shift in glass transition temperatures was explained by the fractionation of different species during the phase separation process. Phase inversion produced a significant decrease of the elastic modulus in the glassy state and a thermoplastic-like behavior of the material in the rubbery region. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1349–1359, 1998

Cure behavior and properties of an epoxy resin modified with a bismaleimide resin

AbstractA bismaleimide (BMI) resin was added to an epoxy system composed of N,N′‐tetraglycidyldiaminodiphenyl methane (TGDDM) and diaminodiphenyl methane (DDM). Cure behavior of the BMI modified epoxy resins was studied by a dynamic differential scanning calorimetry (DSC) method. Dynamic DSC thermograms of the BMI modified epoxy resins indicated unimodal reaction exothermic peaks. The overall heat of reaction per unit mass decreased with BMI composition. The residual heat of reactions of the epoxy blends cured at 180°C for 3 h increased with BMI composition. Thermal stability of the epoxy system improved by incorporating BMI resin. Flexural strength and modulus increased with BMI composition.

Endothermic oxygenation of hemocyanin in the krill Meganyctiphanes norvegica

The oxygen affinity of the hemolymph of the krill Meganyctiphanes norvegica, increases with temperature in the pH range 7.4 to 8.1 reflecting an endothermic overall heat of reaction. This striking feature may be of adaptive significance with respect to the feeding excursions of the animal, which at night reaches the warmer phytoplankton-rich surface layers, where the oxygen availability is reduced due to increased temperature and photorespiration.

Reactions amines aromatiques-resines epoxydes: enthalpie et capacites calorifiques

The overall heat of reaction between aromatic amines and epoxy resins was measured for model and polymer systems. Heat capacities of model, polymer and non stoichiometric systems have also been determined as a function temperature. All data are in agreement with the idea of a complete reaction and, in some cases, secondary reactions.

Thermokinetics and chemorheology of the cure reactions of the tetraglycidyl diamino diphenyl methane–diamino diphenyl sulfone epoxy systems

AbstractThe cure behavior of commercial grade TGDDM–DDS mixtures of compositions ranging from 10 to 100 phr of hardener and the thermal polymerization of the epoxy component are analyzed by means of differential scanning calorimetry. The kinetic parameters and heats of reaction determined in isothermal and dynamic scans suggest that DDS primary amine addition and epoxide etherification dominate the cure reactions. The primary amine epoxide addition is characterized by overall heat of reaction (referred to the weight of the epoxy component) of 255 cal/g and by an activation energy of 16.6 kcal/mol. The corresponding values for the etherification reaction are, respectively, 170 cal/g and 41 kcal/mol. A method of derivation of the epoxide conversion from the heat evolved in DSC thermal scans of these systems is presented. The results are in good agreement with independent IR determinations. The steady shear and oscillatory viscosity measurements and the calorimetric analysis of the isothermal cure at 140°C, 160°C, and 180°C of a TGDDM–DDS mixture containing 35 phr of hardener indicate that gelation is principally governed by the primary amine addition. The gelation limits calculated in isothermal tests by combining the calorimetric analysis and the theory describing the nonlinear copolymerization of the tetrafunctional TGDDM with an essentially difunctional DDS were in good agreement with the values experimentally determined through rheological measurements.

Reaction of 1,1-diphenylpropene with stannic chloride and hydrogen chloride

SnCl4 and HCl have been found to react with 1,1-diphenylpropene to form a small equilibrium amount of the diphenylethylcarbenium ion. No further reaction is detectable, and the equilibrium between the three reactants may be determined from the uv absorption of the ion. The electrical conductances of the solutions have also been measured. The results are consistent with the formation of the salt of this carbenium ion with the SnCl5− anion. The salt in the concentrations formed in these experiments (ca. 10−5–10−4 M) was nearly all dissociated into free ions in methylene chloride solution. The equilibrium constant for formation of the salt was 2.0 M−2 at −30 °C, and the dissociation constant into ions 4.4 × 10−4 M. The overall heat of reaction to the carbenium ion salt was 8.8 kcal, most of which is attributed to the initial formation of the undissociated salt.