Ab initio kinetics predictions for H-atom abstraction from diethoxymethane by hydrogen, methyl, and ethyl radicals and the subsequent unimolecular reactions

Diethoxymethane (DEM) is a promising oxygenated fuel and fuel additive, which has similar positive combustion characteristics as dimethoxymethane. DEM contains C-C bonds and can form ethylene via beta-scission, which potentially increases its sooting tendency. Since DEM is rarely studied, however, kinetic modeling attempts are forced to rely on rate constant analogies. Therefore, we employ high level CCSD(T)/aug-cc-pV(T+D)Z//B2PLYPD3BJ/6-311++(d,p) theory along with transition state theory to predict reaction rate constants for H-abstraction by H and CH3 and the subsequent unimolecular reactions. We further prove that the DLPNO approximation to CCSD(T) leads to a deviation of less than 0.25 k/mol in barrier heights for the presently studied open-shell electronic structures and use it for the prediction of reaction rate constants for H-abstraction by C2H5 radicals. We find that H-abstraction by ethyl radicals might denote a significant pathway, which should not be neglected in kinetic modeling studies of DEM. It is also shown that reaction pathways leading to ethylene formation are of minor importance and give thereby a first insight into the fate of the C-C bonds. To the best of our knowledge, this study represents the first high-level ab-initio study of DEM, which makes the reaction kinetics and thermochemistry data provided by this study vital for future comprehensive kinetic modeling of DEM. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Peer reviewe

Experimental and theoretical investigations of methyl formate oxidation including hot beta-scission

Recently the possibility of hot beta-scission pathways gained attention. These reactions give a shortcut during the important fuel consumption phase in combustion processes leading from H-atom abstraction directly to the beta-scission products without fuel radical thermalization. Methyl formate (MF) was shown to be prone to hot beta-scission due to a low beta-scission barrier height. Furthermore, MF as smallest methyl ester can be considered as biodiesel surrogate and it is an important intermediate product during combustion of various ethers. In this work a predominantly ab-initio derived detailed kinetic model of MF combustion is developed including hot beta-scission pathways and compared to a sophisticated literature model based on classical estimation methods. For this, new stoichiometric MF in air ignition delay time measurements in a shock tube and a rapid compression machine over a wide temperature range (790 K-1250 K) and pressures of 10, 20 and 40 bar served as validation targets. The experimental ignition delay times (IDT) show Arrhenius type behavior in both facilities at all conditions. The newly developed quantum-based model catches the pressure dependency and low-temperature reactivity well although overpredicting the IDT at higher temperatures. It was found that hot beta-scission is the major depletion pathway of formate group-centered MF radicals. This, however, does not change the overall reactivity of MF combustion due to the low stability of the alkyl peroxide (RO2) at the formate group. For species with competing thermal beta-scission and RO2 formation, however, hot beta-scission may have a significant impact. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Peer reviewe

Action of different treatments based on the mal e effect on the restart of the seasonal reproductive activity of Churra Galega Bragançana ewes

Este ensaio foi realizado com o intuito de estudar a acção do "efeito macho" sobre o restabelecimento da actividade reprodutora sazonal das ovelhas da raça Churra Galega Bragançana. Por outro lado, procurou-se averiguar do possível interesse da aplicação coordenada do "efeito macho" e de tratamentos com acetato de fluorgesterona (FGA) e gonadotropina sérica da égua gestante (PMSG) no restabelecimento desta mesma actividade reprodutora. Neste sentido, um grupo de 135 ovelhas da raça Churra Galega Bragançana, com idades compreendidas entre os 2 e os 4 anos e que haviam parido pela última vez há cerca de 6 meses, foi preparado para a realização deste estudo. Depois de retirar do ensaio todas as ovelhas que apresentavam actividade ovárica completa, aplicaram-se às restantes fêmeas quatro tratamentos distintos: EM • "efeito macho" (n=26); EM+FGA • "efeito macho" + esponjas vaginais com 30 mg de FGA (n=10); EM+PMSG • "efeito macho" + 600 UI de PMSG (n=10) e EM+FGA+PMSG ·"efeito macho" + esponjas vaginais com 30 mg de FGA + 600 UI de PMSG (n=10). No mês de Junho do ano estudado, 58,5% das ovelhas da raça Churra Galega Bragançana inicialmente seleccionadas para a realização deste trabalho apresentavam actividade ovárica completa. Das que se encontravam anéstricas, 87,0% respondeu à aplicação dos tratamentos. Esta resposta não variou em função do tratamento aplicado. De Igual modo, a resposta ovárica das ovelhas estudadas não foi afectada pelo tipo de tratamento empregue. Os tratamentos com FGA melhoraram a resposta comportamental das ovelhas aquando da primeira ovulação. Por seu turno, a adição de PMSG ao " efeito macho" permitiu elevar a resposta comportamental das ovelhas no momento da segunda ovulação. Neste ensaio, a taxa ovulatória média total apresentada pelas ovelhas Churras Bragançanas foi de 1,2±0,9

Simultaneous direct measurement of the electrocaloric and dielectric dynamics of ferroelectrics with microsecond temporal resolution

A contactless technique for direct time-resolved measurements of the full dynamics of the adiabatic temperature change in electrocaloric materials is introduced. The infrared radiation emitted by the electrocaloric sample is sensitively detected with $\mu$s time resolution and mK temperature resolution. We present time-resolved measurements of the electrocaloric effect up to kHz frequencies of the driving electric field and down to small field strengths. The simultaneous recording of transients for applied electric field and induced polarization gives a comprehensive view on the correlation of electrocaloric and ferroelectric properties. The technique can further be applied to the continuous measurement of fatigue for $> 10^6$ electric field cycles.Comment: 12 pages, 11 figure

SIMS-Untersuchungen an modernen Halbleitern : GaN und VO2

Für einen industriellen Einsatz der technologisch hochinteressanten Halbleiter Galliumnitrid (GaN) und Vanadiumdioxid (VO2) ist die Beherrschung der Herstellung und das Verständnis der Materialeigenschaften unerläßlich. Das erfordert zwangsläufig eine intensive Analytik an den Materialien und deren Bauelementen. In der vorliegenden Arbeit wird demonstriert, daß die Sekundärionenmassenspektrometrie (SIMS) eine dafür hervorragend geeignete Methode ist. Dabei wird das gesamte Spektrum der Meßmöglichkeiten von SIMS eingesetzt. Im Rahmen der vorliegenden Arbeit wurden Empfindlichkeitsfaktoren für verschiedene Elemente in GaN und VO2 bestimmt. Darüber hinaus wurde eine Methode entwickelt, die die Kontrolle der Stöchiometrie von VO2 erlaubt. Es sind damit in weiten Bereichen quantitative Analysen der Materialzusammensetzung möglich. Neben diesen Vorarbeiten für zukünftige SIMS-Untersuchungen und einer Vielzahl wertvoller Informationen für die Schichthersteller wurden SIMS-Analysen erfolgreich eingesetzt, um wissenschaftliche Fragen der aktuellen Forschung zu klären. Die Untersuchung der Magnesiumdotierung von GaN mit SIMS bildet einen Schwerpunkt der Arbeit. Bearbeitet wurden Fragen zur Einbaugrenze von Mg in c-GaN, zum Zusammenhang von Mg-Konzentration und Ladungsträgerkonzentration und zu den Gitterkonstanten Mg-dotierten GaN\u27s. Mit Hilfe der Ergebnisse des letzten genannten Punktes kann man den Zusammenhang von Mg-Konzentration und den g-Faktoren des Mg-Akzeptors in GaN erklären. Daneben konzentrieren sich die Messungen auf GaN/InGaN Mehrschichtstrukturen. Mit einer Tiefenauflösung von bis zu 2 nm wurden tiefliegende Probenbereiche gezielt hinsichtlich ihrer chemischen Zusammensetzung untersucht. Die SIMS-Untersuchungen dieser Arbeit an VO2 umfassen die Stöchiometriekontrolle, für die eine neuartige Methode auf der Basis des Valenzmodells der Sekundärionenemission entwickelt wurde, und die Dotierung der Proben. Daraus hervor gingen eine Reihe von Optimierungsmöglichkeiten die Herstellungstemperatur, die Methoden der Dotierung und die Handhabung betreffend. Desweiteren wurden VO2-Schichten auch eingehend optisch charakterisiert, um ihr Potential für den Einsatz als Architekturglasbeschichtung zu prüfen. Besonderer Wert wurde dabei auf die Optimierung der Schichteigenschaften hinsichtlich dieser Anwendung durch Fluor- und/oder Wolframdotierung und Antireflexschichten gelegt. Für Vanadiumdioxid wurde gezeigt, welches Potential es als Beschichtungsmaterial für intelligente Fensterverglasungen hat. Es zeichnet sich durch den einfachen Aufbau, das Fehlen von externen Regelungen und Versorgungen und den Erhalt der klaren Durchsicht gegenüber alternativen schaltenden und schaltbaren Architekturgläsern aus. Durch die Kombination der in dieser Arbeit aufgezeigten Optimierungsansätze wie Titandotierung und Antireflexschichten können VO2-beschichtete Fenster auch hinsichtlich der bautechnischen Kenngrößen zu einem marktfähigen Produkt entwickelt werden

Oxidation Kinetics and Thermodynamics of Resonance-Stabilized Radicals : The Pent-1-en-3-yl + O-2 Reaction

The kinetics and thermochemistry of the pent-1-en-3-yl radical reaction with molecular oxygen (CH2CHCHCH2CH3 + O-2) has been studied by both experimental and computational methods. The bimolecular rate coefficient of the reaction was measured as a function of temperature (198-370 K) and pressure (0.2-4.5 Torr) using laser photolysis-photoionization mass-spectrometry. Quantum chemical calculations were used to explore the potential energy surface of the reaction, after which Rice-Ramsperger-Kassel-Marcus theory/master equation simulations were performed to investigate the reaction. The experimental data were used to adjust key parameters, such as well depths, in the master equation model within methodological uncertainties. The master equation simulations suggest that the formation rates of the two potential RO2 adducts are equal and that the reaction to QOOH is slower than for saturated hydrocarbons. The initial addition reaction, CH2CHCHCH2CH3 + O-2, is found to be barrierless when accounting for multireference effects. This is in agreement with the current experimental data, as well as with past experimental data for the allyl + O-2 reaction. Finally, we conducted numerical simulations of the pent-1-en-3-yl + O-2 reaction system and observed significant amounts of penta-1,3-diene being formed under engine-relevant conditions.Peer reviewe

Direct kinetics study of CH2OO + methyl vinyl ketone and CH2OO + methacrolein reactions and an upper limit determination for CH2OO + CO reaction

Methyl vinyl ketone (MVK) and methacrolein (MACR) are important intermediate products in atmospheric degradation of volatile organic compounds, especially of isoprene. This work investigates the reactions of the smallest Criegee intermediate, CH2OO, with its co-products from isoprene ozonolysis, MVK and MACR, using multiplexed photoionization mass spectrometry (MPIMS), with either tunable synchrotron radiation from the Advanced Light Source or Lyman-alpha (10.2 eV) radiation for photoionization. CH2OO was produced via pulsed laser photolysis of CH(2)l(2) in the presence of excess O-2. Time-resolved measurements of reactant disappearance and of product formation were performed to monitor reaction progress; first order rate coefficients were obtained from exponential fits to the CH2OO decays. The bimolecular reaction rate coefficients at 300 K and 4 Torr are k(CH2OO + MVK) = (5.0 +/- 0.4) x 10(-13) cm(3) s(-1) and k(CH2OO + MACR) = (4.4 +/- 1.0) x 10(-13) cm(3) s(-1), where the stated +/- 2 sigma uncertainties are statistical uncertainties. Adduct formation is observed for both reactions and is attributed to the formation of a secondary ozonides (1,2,4-trioxolanes), supported by master equation calculations of the kinetics and the agreement between measured and calculated adiabatic ionization energies. Kinetics measurements were also performed for a possible bimolecular CH2OO + CO reaction and for the reaction of CH2OO with CF3CHCH2 at 300 K and 4 Torr. For CH2OO + CO, no reaction is observed and an upper limit is determined: k(CH2OO + CO) <2 x 10(-16) cm(3) s(-1). For CH2OO + CF3CHCH2, an upper limit of k(CH2OO + CF3CHCH2) <2 x 10(-14) cm(3) s(-1) is obtained.Peer reviewe

Analytic energy-level densities of separable harmonic oscillators including approximate hindered rotor corrections

Energy-level densities are key for obtaining various chemical properties. In chemical kinetics, energy-level densities are used to predict thermochemistry and microscopic reaction rates. Here, an analytic energy-level density formulation is derived using inverse Laplace transformation of harmonic oscillator partition functions. Anharmonic contributions to the energy-level density are considered approximately using a literature model for the transition from harmonic to free motions. The present analytic energy-level density formulation for rigid rotor-harmonic oscillator systems is validated against the well-studied CO+O˙H system. The approximate hindered rotor energy-level density corrections are validated against the well-studied H2O2 system. The presented analytic energy-level density formulation gives a basis for developing novel numerical simulation schemes for chemical processes