Activation Energy Spectrum Research Articles

Nuclear magnetic resonance analysis and activation energy spectrum of the irreversible structural relaxation of amorphous zirconium tungstate

Zirconium tungstate undergoes a sequence of phase transitions from cubic ($\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) to orthorhombic ($\ensuremath{\gamma}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) to amorphous ($a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) upon increasing pressure at room temperature. The amorphous phase is known to undergo anomalous endothermic recrystallization into a high-temperature $\ensuremath{\beta}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ phase above ${600}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}\mathrm{C}$ at ambient pressure (and back to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ when brought to room temperature). The endothermic recrystallization of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is preceded by an irreversible exothermic structural relaxation. New W-O bonds are formed upon amorphization, continuing a tendency of increasing W coordination number in going from $\ensuremath{\alpha}$ to $\ensuremath{\gamma}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$. In fact, contrarily to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$, in which one-quarter of the oxygen atoms are bonded only to one W (terminal oxygens), previous works found no evidence of single-bonded oxygen atoms in $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$. It thus could be argued that the irreversible character of the structural relaxation of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is due to W-O bond breaking upon annealing of the amorphous phase. To test this hypothesis, x-ray diffraction, $^{17}\mathrm{O}$ magic-angle spinning NMR, Raman, and far-infrared analyses were performed on samples of amorphous zirconium tungstate previously annealed to increasingly higher temperatures, looking for any evidence of features that could be assigned to the presence of terminal oxygen atoms. No evidence of single-bonded oxygen was found before the onset of recrystallization. Furthermore, the kinetics of the structural relaxation of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is consistent with a continuous spectrum of activation energy, spanning all the range from 1 to $2.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. These findings suggest that the structural relaxation of amorphous zirconium tungstate, however irreversible, is not accompanied by W-O bond breaking, but most probably characterized by a succession of (mostly) irreversible local atomic rearrangements.

Reactivity of Alkyl Halides in Concerted Molecular Decomposition Reactions (Review)

Results of theoretical description of the reactions of concerted molecular decomposition of alkyl halides RX to olefin and HX have been analyzed. Data on quantum-chemical calculation of the transition state of these reactions are given. The results obtained by analyzing experimental data in terms of the model of intersecting parabolas have been compared. The following factors determining the reactivity of RX (X = F, Cl, Br, I) in the decomposition reactions: enthalpy of decomposition, force constants of reacting bonds, triplet repulsion, electronegativity of the reaction-center atoms, the π-bond adjacent to the reaction center, dipole–dipole interaction, and elongation of the R–X bond win the transition state, have been distinguished. The energy spectrum of partial activation energies of the concerted molecular degradation of RX has been constructed.

Evolution of the activation energy spectrum and defect concentration upon structural relaxation of a metallic glass determined using calorimetry and shear modulus data

We performed high precision shear modulus and calorimetry measurements on a model Pd-based glass subjected to different annealing treatments below the glass transition temperature Tg. Applying the Interstitialcy theory, we found that structural relaxation-induced heat release/absorption below and near Tg can be precisely described using shear modulus data confirming thus the generic relationship between the heat and elastic relaxation effects in metallic glasses. The same approach is used for the reconstruction of underlying activation energy spectra. It is found that the spectra derived from calorimetry and shear modulus data are very close. Integration of these spectra gives the change of the concentration of frozen-in defects and its absolute value for different stages of structural relaxation.

Structurally Oriented Nano-Sheets in Co Thin Films: Changing Their Anisotropic Physical Properties by Thermally-Induced Relaxation.

We show how nanocrystalline Co films formed by separated oblique nano-sheets display anisotropy in their resistivity, magnetization process, surface nano-morphology and optical transmission. After performing a heat treatment at 270 °C, these anisotropies decrease. This loss has been monitored measuring the resistivity as a function of temperature. The resistivity measured parallel to the direction of the nano-sheets has been constant up to 270 °C, but it decreases when measured perpendicular to the nano-sheets. This suggests the existence of a structural relaxation, which produces the change of the Co nano-sheets during annealing. The changes in the nano-morphology and the local chemical composition of the films at the nanoscale after heating above 270 °C have been analysed by scanning transmission electron microscopy (STEM). Thus, an approach and coalescence of the nano-sheets have been directly visualized. The spectrum of activation energies of this structural relaxation has indicated that the coalescence of the nano-sheets has taken place between 1.2 and 1.7 eV. In addition, an increase in the size of the nano-crystals has occurred in the samples annealed at 400 °C. This study may be relevant for the application in devices working, for example, in the GHz range and to achieve the retention of the anisotropy of these films at higher temperatures.

Ultrasonic study of elastic creep in piezoceramics

Ultrasonic method and experimental setup for study the elastic creep of piezoelectric materials are described. The results of experimental studies of time behavior of the Young's modulus and the internal friction are presented as well as the longitudinal and transversal piezomoduli and the electromechanical coupling factor. Four compositions of piezoelectric ceramics both soft and hard, based on BaTiO3 and PZT, were compressed for a long time (0÷107s) by significant static loads (0÷120MPa). The possible physical mechanisms that cause the creep are briefly considered. The concept of a spectrum of activation energies of the corresponding processes is introduced. The upper and the lower boundaries of the relaxation times spectrum were measured and corresponding activation energies were found.

Parabolic model of the concerted molecular decomposition of chloroalkanes

The intersecting parabolas model (IPM) is used to analyze the measured kinetic parameters for the concerted molecular decomposition of chloroalkanes RCl to olefin and HCl. According to this model, the configuration of the transition state is formed by three atoms: C…H…Cl. The activation energy E and the rate constant k for 12 previously unstudied reactions of concerted molecular decomposition of RCl are calculated based on the enthalpy of reaction by using the IPM algorithms. The factors that influence the activation energy E for RCl decomposition are established: the enthalpy of reaction, energy of stabilization of radical R•, presence of a π bond adjacent to the reaction center, and dipole–dipole interaction for the decomposition of polychloroalkanes. The values of E and k for reverse reactions of addition of HCl to olefins are evaluated. The energy spectrum of partial activation energies for the concerted molecular decomposition of RCl is constructed.

Relaxation dynamics of Fe55Cr10Mo14C15B6 metallic glass explored by mechanical spectroscopy and calorimetry measurements

In this work, the mechanical relaxation dynamics of Fe55Cr10Mo14C15B6 metallic glass is explored by mechanical spectroscopy. The temperature-dependent loss modulus E″(T) shows the features of β relaxation well below glass transition temperature T g. This β relaxation can be well described in the framework of anelastic theory by a thermal activated process with activation energy of 165 kJ mol−1. Structural relaxation, also known as physical aging, has a large effect on the glass properties. The activation energy spectrum of structural relaxation is characterized by differential scanning calorimetry measuring the heat flow difference between as-quenched and relaxed states. The obtained energy spectrum is well described by a lognormal distribution with maximum probability activation energy of 176 kJ mol−1. The obtained activation energy of structural relaxation is similar to that of β relaxation observed from mechanical spectroscopy. Both values are also close to the Johari–Goldstein β relaxation estimated by the empirical rule E β = 26RT g.

The role of boundary variability in polycrystalline grain-boundary diffusion

We investigate the impact of grain-boundary variability on mass transport in a polycrystal. More specifically, we perform both numerical and analytical studies of steady-state diffusion in prototypical microstructures in which there is either a discrete spectrum of grain-boundary activation energies or else a complex distribution of grain-boundary character, and hence a continuous spectrum of boundary activation energies. An effective diffusivity is calculated for these structures using simplified multi-state models and, for the case of a continuous spectrum, employing experimentally obtained grain-boundary energy data. We identify different diffusive regimes for these cases and quantify deviations from Arrhenius behavior using effective medium theory. Finally, we examine the diffusion kinetics of a simplified model of an interfacial layering (i.e., complexion) transition.

Determination of the activation energy spectrum of structural relaxation in metallic glasses using calorimetric and shear modulus relaxation data

We consider two methods for reconstruction of the activation energy spectra of structural relaxation from calorimetric and shear modulus relaxation data. The methods are based on the assumption that structural relaxation of metallic glasses can be understood as a relaxation change of the concentration of frozen-in defects similar to dumbbell interstitials (interstitialcies) in simple crystals. For three Pd- and Zr-based glasses taken as an example, it is shown that both methods give nearly the same results. We calculated a decrease of the defect concentration and showed that the heat of structural relaxation per defect for all three glasses practically coincides with the theoretical value of the interstitialcy formation enthalpy.

Kinetics of Xe‐P3 release during pyrolysis of the coarse‐grained fractions of Orgueil (CI) meteorite nanodiamonds

AbstractThe kinetics of the release of the Xe‐P3 component from coarse‐grained fractions of Orgueil (CI) meteorite nanodiamonds has been investigated using stepped and isothermal pyrolysis. It has been shown that a first‐order chemical reaction diffusion model with a single activation energy cannot provide a satisfactory explanation for the observed retention of Xe‐P3 during parent body thermal metamorphism and the kinetics of Xe‐P3 release from nanodiamonds during isothermal pyrolysis. Using the activation energy and frequency factor calculated according to this model, it is shown that in the course of thermal metamorphism of the Orgueil meteorite almost the entire Xe‐P3 component must have been lost in a very short time (<4 yr at approximately 100 °C). However, the calculated retention of Xe‐P3 increases significantly if a diffusion model with a spectrum of activation energies is used. In this case, the model can explain not only a high retention of Xe‐P3 in the Orgueil nanodiamonds but also the release pattern of the Xe‐P3 from Semarkona and Bishunpur nanodiamonds that have experienced a significant gas loss during parent body metamorphism as well as the release of Xe‐P3 during isothermal pyrolysis of the Orgueil nanodiamonds. The energetically complicated Xe‐P3 distribution is most likely caused by structural damage to the nanodiamond grains or a complex phase composition of carbon in the surface layer of the diamond grains. It is supposed that the structural damage of the diamond grains can have a radiation origin, while the variations of the carbon phase composition in the grain's mantle can be caused by the radiation‐induced reactions and/or a thermal effect.

Annealing of UV Ar^+ and ArF excimer laser fabricated Bragg gratings: SMF-28e fiber

Fiber Bragg gratings fabricated in pristine SMF-28e fibers using pulsed ArF-excimer and cw 244-nm Ar+ laser were annealed using tempering rates from 0.0038 to 0.25 K/s. Demarcation energy mapping allowed for the determination of the frequency factors and the master curves for the SMF-28e fiber under different irradiation conditions. A Gaussian decomposition of the underlying energy distribution revealed several individual activation energy distributions characteristic for the fiber with peak energies and widths that were independent of the laser used. From a fit of the integrated Gaussian distributions to the master curves the relative contributions of the individual energy distributions that appeared in both irradiation conditions were calculated. The difference in the activation energy spectra obtained from the two laser irradiations is explained by the relative contributions of the individual distributions that differ. Using the analytical description of the master curve, thermal stability maps were obtained.

Relaxations in relation to Liquid Crystalline Formation and Glass Transition for Comb-like Polymer: Structural Relaxation as expressed by Entropy Model and Activation Energy Spectrum

This study demonstrates that the modelling approach for the kinetic phenomenon of glass transition is effective to examine the relaxation. In the enthalpy relaxation of poly(cyanobiphenyl-yloxy) alkyl acrylate, the decrease in enthalpy was measured as a function of ageing time and ageing temperature. Obtained data was analysed at first with the activation energy spectrum (AES) model, and next, with the framework of the entropy model which follows the evolution of the configurational entropy of the sample during the whole thermal history in the experiment. The prediction of DSC curve based on the result of analysis for the glass transition will be presented. AES model gives information about thermally activated processes which are available to contribute to observed enthalpy changes against the thermal history. The activated processes are relevant to the rearrangement of molecules. The rearrangement corresponds to the structural changes of polymer chain which can be observed in our study as the change in optical texture, known as the coarsening dynamics of nematic domain for the liquid crystalline materials. The analysis for the coarsening is also shown to elucidate the specific feature of physical parameter.DOI: http://dx.doi.org/10.3126/jncs.v30i0.9377Journal of Nepal Chemical Society Vol. 30, 2012 Page: 107-110 Uploaded date: 12/19/2013

Transiently suppressed relaxations in metallic glass

We show suppressed relaxation behavior and an apparent hardening effect under repeated loading in metallic glasses through cyclically tensile stress relaxation experiments. The effects are attributed to the gradual exhaustion of deformation units via activation energy spectra analysis. We find the suppression is partly alleviated and rapidly saturates with increasing the time interval between the cyclic loadings, reflecting the observed phenomena are associated with the un-recovered activated deformation units. These results may shine light on the long-standing issues such as the fatigue damage, cyclic hardening, and the deformation mechanism of metallic glasses.

Creep Modified Structural Relaxation of Fe-Based Amorphous Alloy

Structural relaxation an creep strain recovery of the amorphous metallic glass Fe40Ni41B19 after the longtime loading at different annealing temperatures below the glass transition temperature have been studied using anisothermal differential scanning calorimetry (DSC) and dilatometry (TMA). It has been demonstrated that structural relaxation effects depend on the stress-annealing temperature of the amorphous ribbon. The influence of the stress-annealing temperature on the rearrangement of deformation defects in the amorphous alloy Fe-Ni-B was analyzed. The properties of local disordering in flow defects were analyzed by means of their activation energy spectra. In the samples stress-annealed at higher temperatures, only the relaxation centers favourably oriented to the external stress contribute to the creep strain recovery process.

Structural Relaxations in Liquid Crystalline Formation and Glass Transition for Comb‐Like Polymer: Activation Energy Spectrum Analysis

SummaryThis study demonstrates that the modelling approach for the kinetic phenomenon of glass transition is effective to examine the relaxation. In the enthalpy relaxation of poly(cyanobiphenyl‐yloxy) alkyl acrylate, the decrease in enthalpy was measured as a function of ageing time and ageing temperature. Obtained data was analysed at first with the activation energy spectrum (AES) model, and next, with the framework of the entropy model which follows the evolution of the configurational entropy of the sample during the whole thermal history in the experiment. The prediction of DSC curve based on the analysis for the glass transition is presented. AES model gives information about thermally activated processes which are available to contribute to observed enthalpy changes against the thermal history. The activated processes are relevant to the rearrangement of the polymer chain. The effect of molecular weight on the peak maximum appeared in AES is reported.

Transient photoresponse in amorphous In-Ga-Zn-O thin films under stretched exponential analysis

We investigated transient photoresponse and Hall effect in amorphous In-Ga-Zn-O thin films and observed a stretched exponential response which allows characterization of the activation energy spectrum with only three fit parameters. Measurements of as-grown films and 350 K annealed films were conducted at room temperature by recording conductivity, carrier density, and mobility over day-long time scales, both under illumination and in the dark. Hall measurements verify approximately constant mobility, even as the photoinduced carrier density changes by orders of magnitude. The transient photoconductivity data fit well to a stretched exponential during both illumination and dark relaxation, but with slower response in the dark. The inverse Laplace transforms of these stretched exponentials yield the density of activation energies responsible for transient photoconductivity. An empirical equation is introduced, which determines the linewidth of the activation energy band from the stretched exponential parameter β. Dry annealing at 350 K is observed to slow the transient photoresponse.

ChemInform Abstract: Concerted Molecular Decomposition of Unsaturated and Oxygen‐Containing Compounds

AbstractReview: 62 refs.

Enthalpy relaxation near the glass transition for comb-like polymer: Power law relaxation revealed by DSC experiment

A series of enthalpy relaxation experiments at different ageing temperatures was conducted on a sample of poly(cyanobiphenyl hexylacrylate) with variation in the molecular weight using a DSC technique. The value of the relaxation function (ϕ) was obtained as a function of the ageing time (tA) from the experimental results using the excess enthalpy in the limit of infinity of tA and that of the given tA. The double logarithmic plot of −lnϕ and tA was constructed to determine the functional form of the relaxation function. Two possible forms are found, the stretched exponential and the power law forms, which suggests that the functional form varies continuously between these two according to the ageing experiment. The activation energy spectrum was derived for the two functional forms, which show the distribution of the activation energy of the molecular process controlling the decrease in enthalpy. It was revealed that, for the spectrum based on the power law relaxation, the overwhelming fraction of the relaxation processes is at the lower energy side and the population of the process decreases rapidly toward the higher energy side.

On the low-temperature absorption of longitudinal ultrasound in Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass

Ultrasonic absorption and attenuation in Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass have been investigated at frequencies of 20, 50 and 150 MHz and temperatures from 80 to 300 K. Using the obtained data the activation energy spectrum of defects responsible for the ultrasonic attenuation is estimated. The results are discussed in the framework of the Debye relaxation model.

Enthalpy relaxation of comb-like polymer analysed by combining activation energy spectrum and TNM models

In the enthalpy relaxation of poly(cyanobiphenyl ethylacrylate) (PCB2A), the decrease in enthalpy was measured as a function of ageing time and ageing temperature with DSC technique. Obtained data was analysed at first with the activation energy spectrum (AES) model, then compared with the prediction of a multiparameter phenomenological model which follows the evolution of the configurational entropy (SC) of the sample during the whole thermal history. In AES model, the decrease in enthalpy is controlled by molecular processes whose energies are distributed over a continuous spectrum. The spectrum gives the relation between the number and energy of processes. In consequence, AES of a single peak was obtained. The energy at which the peak maximum was located decreased with the increase in ageing temperature, which is a well-known behaviour as a distribution of relaxation processes. Furthermore, cp(T), the heat capacity data obtained by the calculation based on SC model reproduced the experimental cp(T) curve. Model parameters found in the calculation was discussed in relation to the energy value of AES maximum.