Large Activation Energy Research Articles

Influence of heat conducting substrates on explosive crystallization in thin layers

Crystallization in a thin, initially amorphous layer is considered. The layer is in thermal contact with a substrate of very large dimensions. The energy equation of the layer contains source and sink terms. The source term is due to liberation of latent heat in the crystallization process, while the sink term is due to conduction of heat into the substrate. To determine the latter, the heat diffusion equation for the substrate is solved by applying Duhamel’s integral. Thus, the energy equation of the layer becomes a heat diffusion equation with a time integral as an additional term. The latter term indicates that the heat loss due to the substrate depends on the history of the process. To complete the set of equations, the crystallization process is described by a rate equation for the degree of crystallization. The governing equations are then transformed to a moving co-ordinate system in order to analyze crystallization waves that propagate with invariant properties. Dual solutions are found by an asymptotic expansion for large activation energies of molecular diffusion. By introducing suitable variables, the results can be presented in a universal form that comprises the influence of all non-dimensional parameters that govern the process. Of particular interest for applications is the prediction of a critical heat loss parameter for the existence of crystallization waves with invariant properties.

Mechanical properties, electronic structure and alkali-ion diffusion of Eldfellite-type AFe(SO4)2 (A = Li, Na, K) as potential cathode materials comparing with LiFePO4

To elucidate the structure-performance relationship of the new potential cathode materials AFe(SO4)2 ([Formula: see text], Na, K) compared with LiFePO4, first-principles calculations are performed to investigate the structure, mechanical stabilities and electronic properties of them. The calculated results show that AFe(SO[Formula: see text] ([Formula: see text], Na, K) compounds are mechanically stable and exhibit strong anisotropy. LiFe(SO[Formula: see text], NaFe(SO[Formula: see text] and KFe(SO[Formula: see text] are more brittle and have higher elastic modulus than LiFePO4, which is attributed to the strong chemical bonding of them. Electronic structure are predicted by HSE06 functional and the band gaps are 5.006, 4.996, 5.146 and 3.711[Formula: see text]eV for LiFe(SO[Formula: see text], NaFe(SO[Formula: see text], KFe(SO[Formula: see text] and LiFePO4, respectively. Full ab initio molecular dynamics simulations are performed to calculate the mean square displacements and diffusion coefficients of the alkali-ion. NaFe(SO[Formula: see text] has large diffusion coefficient as [Formula: see text][Formula: see text]m2/s at 1273[Formula: see text]K, as well as larger activation energy as 0.167[Formula: see text]eV. All the results contribute to understand the microscopic origin of the different behaviors of intercalation cathode used in rechargeable battery.

Behaviors of transmutation elements Re and Os and their effects on energetics and clustering of vacancy and self-interstitial atoms in W

We investigate the behaviors of rhenium (Re) and osmium (Os) and their interactions with point defects in tungsten (W) using a first-principles method. We show that Re atoms are energetically favorable to disperse separately in bulk W due to the Re–Re repulsive interaction. Despite the attractive interaction between Os atoms, there is still a large activation energy barrier of 1.10 eV at the critical number of 10 for the formation of Os clusters in bulk W based on the results of the total nucleation free energy change. Interestingly, the presence of vacancy can significantly reduce the total nucleation free energy change of Re/Os clusters, suggesting that vacancy can facilitate the nucleation of Re/Os in W. Re/Os in turn has an effect on the stability of the vacancy clusters (Vn) in W, especially for small vacancy clusters. A single Re/Os atom can raise the total binding energies of V2 and V3 obviously, thus enhancing their formation. Further, we demonstrate that there is a strong attractive interaction between Re/Os and self-interstitial atoms (SIAs). Re/Os could increase the diffusion barrier of SIAs and decrease their rotation barrier, while the interstitial-mediated path may be the optimal diffusion path of Re/Os in W. Consequently, the synergistic effect between Re/Os and point defects plays a key role in Re/Os precipitation and the evolution of defects in irradiated W.

Thermal characterization of crude oils in the presence of limestone matrix by TGA-DTG-FTIR

In present work oxidation of two heavy oils in limestone matrix was studied using simultaneous thermogravimetry (TGA), derivative thermogravimetry (DTG) and FTIR-spectroscopy techniques in the temperature range from 25 to 900°C. Before the measurements, the composition and properties of crude oils and limestone were evaluated. Obtained TG and DTG curves shows four different reaction regions: low temperature oxidation (LTO), fuel deposition (FD), high temperature oxidation (HTO) and decomposition of limestone. LTO reactions were accompanied by evaporation of light hydrocarbons, which was confirmed by appearance of stretching vibration bands of C-H groups in FTIR-spectra of evolved gases. Formation of carbon dioxide was observed for all oxidation reaction regions according to spectroscopic data. At the same time, CO was formed only in HTO region for both studied crude oils. Despite the different composition two crude oils have practically the same reactions intervals and peak temperatures. However, crude oil with higher API-gravity has a greater mass loss at the LTO and evaporation regions. The conversion of heavier oil with higher content of asphaltenes is larger during the high-temperature oxidation step.Three different kinetic models (Arrhenius, Coats & Redfern and Ingraham & Marrier) were used for analysis of TGA-DTG curves in LTO and HTO regions. Activation energy values of the crude oil samples were varied between 6.9–10.6kJ/mol in low temperature oxidation and 91.8-.0–181.9kJ/mol in high temperature oxidation regions. For two crude oils activation energies are similar in low temperature oxidation region. In high temperature oxidation region, crude oil with higher content of asphaltenes has larger activation energy.

Large-activation-energy analysis of gaseous reacting flow in pipes

This paper analyzes the exothermic reaction of an initially cold gaseous mixture flowing with a moderately large Reynolds number along a cylindrical pipe with constant wall temperature. An overall irreversible reaction with an Arrhenius rate having a large activation energy is used for the chemistry description. The flow is chemically frozen in the cold entrance region, where the velocity evolves towards the Poiseuille profile as the gas temperature increases towards the wall value, ushering in a reaction stage during which the rate of heat transfer from the wall changes from positive to negative. The subsequent downstream evolution of the flow depends critically on the competition between the heat released by the chemical reaction and the heat-conduction losses to the wall, as measured by the Damköhler number δ, first introduced by Frank-Kamenetskii in his seminal analysis of thermal explosions in cylindrical vessels. For values of δ below the critical value δ=2 corresponding to the quasi-steady explosion limit, heat losses to the wall keep the gas temperature close to the wall value, so that the chemical reaction occurs slowly along the pipe in a flameless mode, which is analyzed to give an implicit expression for the streamwise reactant distribution. By way of contrast, for δ > 2 the slow reaction rates occur only in an initial ignition region, which ends abruptly when very large reaction rates cause a temperature runaway, or thermal explosion, at a well-defined location, whose computation must account for the temperature found at the end of the entrance region. The predictions of the large-activation-energy analyses, including ignition distances for δ > 2 and flameless reactant consumption rates for δ ≤ 2, show good agreement with numerical computations of the reactive pipe flow for finite values of the activation energy.

A mathematical approach to the property of one-dimensional steady solution of reverse smolder waves

The mathematical property of one-dimensional steady solution for reverse smolder waves in the context of a model that permits both fuel-rich and fuel-lean has been studied using the method of analysis. Based on the equations and the boundary conditions some asymptotic properties of the solution at infinity are proved. It is shown that the value of oxygen or the mass of fuel (corresponding to the fuel-rich case and the fuel-lean case, respectively) tends to zero, and the temperature approaches to a fixed value. This is confirmed by other authors using large activation energy asymptotic methods.

High-performance giant-dielectric properties of rutile TiO2 co-doped with acceptor-Sc3+ and donor-Nb5+ ions

Giant dielectric properties of materials have widely been reported in recent years. However, a simultaneously low dielectric loss tangent (tanδ) and high dielectric permittivity (ɛ′) with a low temperature coefficient (Δɛ′(T)/ɛ′RT) have been very difficult to achieve. In this work, high performance of giant dielectric properties were achieved in rutile–TiO2 ceramics by co–doping with acceptor–Sc3+ and donor–Nb5+ ions. The important role of the former dopant was to capture free electrons at internal insulating layers, playing as an acceptor. The latter dopant was used to produce free electrons. Ceramics that were 10% (Sc3++Nb5+) co–doped TiO2 (10%ScNTO) exhibited very low tanδ≈0.016–0.035 and high ɛ′≈103–104 values. By optimizing the sintering conditions, excellent temperature stability of ɛ′ at 1 kHz with Δɛ′(T)/ɛ′RT < ±15% was obtained over a wide temperature range, meeting the requirement for use in X8R capacitors. Furthermore, the dielectric properties were slightly dependent on applied DC bias. Impedance spectroscopy analysis revealed that the ScNTO ceramics were electrically heterogeneous, consisting of semiconducting grains and very high insulating layers. The high-performance giant-dielectric properties of ScNTO ceramics were attributed to interfacial polarization at insulating layers with very high resistivity and large activation energy.

Properties of combustion waves in a model with competitive exothermic reactions

We study the properties of travelling combustion waves in a diffusional thermal model with a two-step competitive exothermic-exothermic reaction mechanism. This investigation considers the system in one spatial dimension under adiabatic conditions. Based on the notion of the crossover temperature, the model is examined analytically using the activation energy asymptotic method to predict travelling combustion wave behaviour in the limit of large activation energies. The model is then studied numerically using a shooting-relaxation method over a wide range of parameter values, such as those describing the ratios of enthalpies, pre-exponential factors and activation energies. It is demonstrated that the flame speed as a function of these parameters is a single-valued monotonic function and there are two flame regimes identified—each region representing parameter values when one reaction dominates the other.

N2O + SO2 reaction over Si- and C-doped boron nitride nanotubes: A comparative DFT study

Density functional theory calculations are performed to investigate the mechanisms of N2O reduction by SO2 over Si- and C-doped (6,0) boron nitride nanotubes (BNNTs). According to our results, the Si or C adatom can be strongly stabilized over the vacancy defect of the BNNT. The adsorption energy of Si and C atoms over defective BNNT is calculated to be −297.3 and −333.7kcal/mol, respectively, indicating a strong interaction between these dopant atoms and the tube surface. The N2O reduction reaction includes the decomposition of N2O (i.e. N2O→N2+O*), followed by the reduction of O* by SO2 molecule (i.e. SO2+O*→SO3). The calculated energy barrier of the SO2+O*→SO3 reaction on Si- and C-doped BNNTs is 2.4 and 5.4kcal/mol, respectively. Moreover, the effects of tube diameter and length on the N2O reduction are studied in detail. The disproportionation of N2O molecules (2N2O→2N2+O2) over both surfaces needs a quite large activation energy, which indicates the impossibility of this reaction at ambient condition. The results show that both Si- and C-doped BNNTs can be viewed as an effective green catalyst for the reduction of N2O.

Large activation energy in aged Mn-doped Sr0.4Ba0.6Nb2O6 ferroelectric ceramics

Large activation energy and a different migration path of oxygen vacancy diffusion in Mn-doped SBN ferroelectric ceramics are revealed.

Equilibrium between inactive ready Ni-SIr and active Ni-SIa states of [NiFe] hydrogenase studied by utilizing Ni-SIr-to-Ni-SIa photoactivation.

Previously, the Ni-SIr state of [NiFe] hydrogenase was found to convert to the Ni-SIa state by light irradiation. Herein, large activation energies and a large kinetic isotope effect were obtained for the reconversion of the Ni-SIa state to the Ni-SIr state after the Ni-SIr-to-Ni-SIa photoactivation, suggesting that the Ni-SIa state reacts with H2O and leaves a bridging hydroxo ligand for the Ni-SIr state.

Catalytic reduction of SO2 by CO over Au4Pt2(CO)n and Au6Pt(CO)n clusters: a first-principles study.

The catalytic properties of the magic gold-platinum bimetallic clusters (Au4Pt2 and Au6Pt) for the reduction of SO2 by CO, without or with preadsorbing CO molecules, are firstly investigated using density functional theory calculations. We find that the catalytic activities improve effectively with the preadsorption of CO onto the catalysts and that the catalytic activities of Au6Pt(CO)n are better than those of Au4Pt2(CO)n as more CO molecules are adsorbed onto the catalysts. During the reaction process, the Au4Pt2(CO)n clusters always keep two-dimensional morphologies except for when n = 5 and the Au6Pt(CO)n clusters have three-dimensional geometries except for when n = 0. The most stable adsorption site for SO2 molecules on the catalysts is the site of preadsorbing the next CO molecule on the corresponding catalysts. The largest activation energy (E) is related to the metal 5d (M-5d) band center and the charge transfer (Ct) as well as the bond length (Rb) between COS and the catalyst contribute to the desorption energy (Ed) of COS corporately. We propose that Au6Pt(CO)6 is a cost-effective gold-platinum bimetallic catalyst for the reduction of SO2 by CO.

Ab initio study of the structure and stability of CaMg(CO3)2at high pressure

Dolomite is one of the major mineral forms in which carbon is subducted into the Earth’s mantle. End-member CaMg(CO_3)_2 dolomite typically breaks down upon compression into two carbonates at 5–6 GPa in the temperature range of 800–1200 K (Shirasaka et al. 2002). However, high-pressure X-ray diffraction experiments have shown that dense high-pressure polymorphs of dolomite may be favored over single-cation carbonates (Santillan et al. 2003; Mao et al. 2011; Merlini et al. 2012). Here we compare calculated dolomite structures to experimentally observed phases. Using density functional theory interfaced with a genetic algorithm that predicts crystal structures (USPEX), a monoclinic phase with space group C2/c was found to have lower energy at pressures above 15 GPa than all previously reported dolomite structures. It is possible that this phase is not observed experimentally due to a large activation energy of transition from dolomite I, resulting in the observed second-order phase transition to a metastable dolomite II. Due to the complex energy landscape for candidate high-pressure dolomite structures, several structurally unique metastable polymorphs exist. We calculate the equation of state of a set of lowest-energy dolomite polymorphs with space groups P1, P2/c, and C2/c up to 80 GPa. Our results demonstrate a need for calculations and experiments on Fe-Mn bearing high-pressure carbonate phases to extend our understanding of Earth’s deep carbon cycle and test whether high-pressure polymorphs of double-cation carbonates represent the main reservoir for carbon storage within downwelling regions of Earth’s mantle.

Influence of ferroelectric domain texture on the performance of multilayer piezoelectric actuators

Lead zirconate titanate (PZT)-based multilayer piezoelectric actuators (MAs) present a maximum in their piezoelectric properties when pre-stressed with a low mechanical load (ca. 50MPa) along the longitudinal axis of the actuator. In this work, we investigate this phenomenon by combining polarised Raman spectroscopy with a Reverse Monte Carlo (RMC) method in order to quantify the domain orientation distribution of PZT-based MAs, both in-situ and at the remanent state, under combined applied electric field and mechanical load. Our study shows that the performance maximum of MAs is the result of two competing effects: (i) an increasing reservoir for non-180° domain switching for increasing applied compressive pre-stress and (ii) a large activation energy for domain switching at high pre-stresses. Hence, our study uncovers structure-property relationships in piezoceramic components that could be used to finely tune the electromechanical response of actuators.

Atomistic simulations of dislocations in a model BCC multicomponent concentrated solid solution alloy

Molecular statics and molecular dynamics simulations are presented for the structure and glide motion of a/2〈111〉 dislocations in a randomly-distributed model-BCC Co16.67Fe36.67Ni16.67Ti30 alloy. Core structure variations along an individual dislocation line are found for a/2〈111〉 screw and edge dislocations. One reason for the core structure variations is the local variation in composition along the dislocation line. Calculated unstable stacking fault energies on the (110) plane as a function of composition vary significantly, consistent with this assessment. Molecular dynamics simulations of the critical glide stress as a function of temperature show significant strengthening, and much shallower temperature dependence of the strengthening, as compared to pure BCC Fe as well as a reference mean-field BCC alloy material of the same overall composition, lattice and elastic constants as the target alloy. Interpretation of the strength versus temperature in terms of an effective kink-pair activation model shows the random alloy to have a much larger activation energy than the mean-field alloy or BCC Fe. This is interpreted as due to the core structure variations along the dislocation line that are often unfavorable for glide in the direction of the load. The configuration of the gliding dislocation is wavy, and significant debris is left behind, demonstrating the role of local composition and core structure in creating kink pinning (super jogs) and/or deflection of the glide plane of the dislocation.

Room temperature multiferroicity in Aurivillius compounds Bi6Fe2−xNixTi3O18 (0≤x≤1)

We investigate the structural, magnetic, ferroelectric, and dielectric properties of Bi6Fe2−xNixTi3O18 (0≤x≤1). The coexistence of ferroelectricity and ferromagnetism were observed at room temperature for the Ni-doped samples. The ferromagnetism in Bi6Fe2−xNixTi3O18 can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Ni-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, the substitution of Ni for Fe was effective for the enhancement of ferroelectric properties. The x=0.6 sample exhibits a maximum remnant polarization Pr of 37.8μC/cm2 because of a lower leakage current. The rather large activation energy in the x=0 and 0.2 samples implies that the relaxation process may be not associated with the thermal motion of oxygen vacancies inside the bulk.

Growth rate of crystalline ice and the diffusivity of supercooled water from 126 to 262 K

Understanding deeply supercooled water is key to unraveling many of water's anomalous properties. However, developing this understanding has proven difficult due to rapid and uncontrolled crystallization. Using a pulsed-laser-heating technique, we measure the growth rate of crystalline ice, G(T), for 180 K < T < 262 K, that is, deep within water's "no man's land" in ultrahigh-vacuum conditions. Isothermal measurements of G(T) are also made for 126 K ≤ T ≤ 151 K. The self-diffusion of supercooled liquid water, D(T), is obtained from G(T) using the Wilson-Frenkel model of crystal growth. For T > 237 K and P ∼ 10-8 Pa, G(T) and D(T) have super-Arrhenius ("fragile") temperature dependences, but both cross over to Arrhenius ("strong") behavior with a large activation energy in no man's land. The fact that G(T) and D(T) are smoothly varying rules out the hypothesis that liquid water's properties have a singularity at or near 228 K at ambient pressures. However, the results are consistent with a previous prediction for D(T) that assumed no thermodynamic transitions occur in no man's land.

Impact of alkyl chain and bay-phenoxy unit on the sensing performance of perylenediimide derivatives

In this contribution, three perylenediimide derivatives (PDIs) substituted on the 1,6,7,12-positions with phenoxy groups were prepared, on which the substituted groups are tetraphenoxy (pH), para-tertbutylphenoxy (TpH) and para- tetramethylbutylphenoxy (OpH). Studies on their sensing properties in hydrazine vapor (5ppm) suggested these materials have good sensing performance. In these three PDIs, OpH shows the best sensing performance owing to its better crystalline structure, smallest torsion angle and activation energy with face-to-face stacking mode. A well-ordered crystalline structure with smaller interplanar spacing and modest activation energy leads TpH exhibiting middle-grade of sensing performance in these PDIs. While the largest activation energy, a greater torsion angle with poor crystalline structure induce the least-efficiency sensing performance of pH sensors. This study indicates the aromatic groups, the length and branched point of alkyl chain on bay region have significant impact on sensing performance of PDIs devices by modulating crystalline structure, π-π overlap and energy level.

The slowly reacting mode of combustion of gaseous mixtures in spherical vessels. Part 1: Transient analysis and explosion limits

Frank-Kamenetskii's analysis of thermal explosions is revisited, using also a single-reaction model with an Arrhenius rate having a large activation energy, to describe the transient combustion of initially cold gaseous mixtures enclosed in a spherical vessel with a constant wall temperature. The analysis shows two modes of combustion. There is a flameless slowly reacting mode for low wall temperatures or small vessel sizes, when the temperature rise resulting from the heat released by the reaction is kept small by the heat-conduction losses to the wall, so as not to change significantly the order of magnitude of the reaction rate. In the other mode, the slow reaction rates occur only in an initial ignition stage, which ends abruptly when very large reaction rates cause a temperature runaway, or thermal explosion, at a well-defined ignition time and location, thereby triggering a flame that propagates across the vessel to consume the reactant rapidly. Explosion limits are defined, in agreement with Frank-Kamenetskii's analysis, by the limiting conditions for existence of the slowly reacting mode of combustion. In this mode, a quasi-steady temperature distribution is established after a transient reaction stage with small reactant consumption. Most of the reactant is burnt, with nearly uniform mass fraction, in a subsequent long stage during which the temperature follows a quasi-steady balance between the rates of heat conduction to the wall and of chemical heat release. The changes in the explosion limits caused by the enhanced heat-transfer rates associated with buoyant motion are described in an accompanying paper.

The slowly reacting mode of combustion of gaseous mixtures in spherical vessels. Part 2: Buoyancy-induced motion and its effect on the explosion limits

This paper investigates the effect of buoyancy-driven motion on the quasi-steady ‘slowly reacting’ mode of combustion and on its thermal-explosion limits, for gaseous mixtures enclosed in a spherical vessel with a constant wall temperature. Following Frank-Kamenetskii's seminal analysis of this problem, the strong temperature dependence of the effective overall reaction rate is taken into account by using a single-reaction model with an Arrhenius rate having a large activation energy, resulting in a critical value of the vessel radius above which the slowly reacting mode of combustion no longer exists. In his contant-density, convection-free analysis, the critical conditions were found to depend on the value of a Damköhler number, defined as the ratio of the time for the heat released by the reaction to be conducted to the wall, to the homogeneous explosion time evaluated at the wall temperature. For gaseous mixtures under normal gravity, the critical Damköhler number increases through the effect of buoyancy-induced motion on the rate of heat conduction to the wall, measured by an appropriate Rayleigh number . In the present analysis, for small values of , the temperature is given in the first approximation by the spherically symmetric Frank-Kamenetskii solution, used to calculate the accompanying gas motion, an axisymmetric annular vortex determined at leading order by the balance between viscous and buoyancy forces, which we call the Frank-Kamenetskii vortex. This flow is used in the equation for conservation of energy to evaluate the influence of convection on explosion limits for small , resulting in predicted critical Damköhler numbers that are accurate up to values of on the order of a few hundred.