Characteristics of elementary acts in the mechanical fracture kinetics of polymers

The main characteristics of elementary acts in the mechanical fracture kinetics of oriented polymers, namely, the initial potential barrier U ₀ and the activation volume V _A , are determined. The values of U ₀ And V _A are obtained from analyzing the temperature and force dependences of the life of polymers with allowance for the actual stresses (overstresses) applied to chain molecules, which are found by IR spectroscopy. The values obtained (U ₀ ≈ 4 eV, V _A ≈ 0.03 nm³) agree satisfactorily with the dissociation energy and activation volume of the fluctuation breaking of covalent skeleton bonds in polymer molecules.     

Possible manifestations of the quantum effect (Tunneling) in elementary events in the fracture kinetics of polymers

An elementary event in the kinetics of fracture of polymers, i.e., breaking of a stressed skeletal bond in a chain molecule, has been simulated by the decay of a loaded quantum anharmonic oscillator. The probability and the average time of expectation of the escape of a particle from the potential well in the Morse potential under the action of a tensile force have been calculated over a wide range of temperatures. It has been demonstrated that the escape of the particle occurs predominantly through the tunneling mechanism at low and medium temperatures and through a combination of the tunneling (under-barrier) and over-barrier (thermal-fluctuation) mechanisms with comparable contributions at high temperatures. The calculations have revealed that the participation of the tunneling mechanism in the kinetics of fracture of polymers manifests itself in a low-temperature athermal plateau in the temperature dependence of the breaking strength. A comparison between the calculated and experimental temperature dependences of the breaking strength for the oriented polymer polycaproamide has shown that the calculated and experimental results are in qualitative and quantitative agreement, which allows the conclusion that the tunneling mechanism can contribute to the fracture of polymers.     

On elementary events in the electrical failure kinetics of polymers

The kinetics of electrical failure of poly(ethylene terephthalate) under different experimental conditions is studied. Process variables are the film thickness, electrode configuration and material, presence of partial discharges, and field application conditions. It is found that the potential barrier for elementary events, which controls the failure kinetics, is nearly the same irrespective of the process, 1.1–1.3 eV. The control process is discussed.     

Statistical kinetics of quasi-brittle fracture

The statistical laws of fracture under conditions of static fatigue and loading at a constant rate are discussed. Phenomenological crack-growth models are used to theoretically analyze the statistics of quasi-brittle fracture of solids that undergo static fatigue and whose experimentally measured strength is controlled by kinetic factors. The strength and longevity distributions that correctly reflect the main empirical dependences are constructed. It is shown that the often detected specific features of the statistics of quasi-brittle fracture are likely to be caused by the qualitative similarity between the asymptotic behaviors of the statistics of thermal fluctuations and “dangerous” structural defects, namely, by the exponential distribution of the strong-thermal-fluctuation expectation time, a large number of statistically equivalent dangerous defects in a sample, and a sharp decrease in the probability of defects whose sizes are larger than a certain limiting size. The results obtained suggest that the generally accepted theoretical interpretation of the Weibull distribution is likely to require revision, since the assumption regarding the power asymptotics of the size distribution of cracklike defects during quasi-brittle fracture is in conflict with the empirical data on static fatigue.     

Mechanisms of elementary events in the kinetics of electrical breakdown of polymer and ceramic dielectrics

The kinetics of electrical breakdown of thin (15–70 μm) layers of polymers and ceramics in a constant-sign field at 77–480 K has been investigated. The temperature dependences of the longevity (breakdown waiting time) of both dielectrics have been found to be similar to each other. At elevated temperatures, the longevity of the dielectrics varies exponentially with increasing temperature, and at reduced temperatures, it is temperature-independent (there is an athermal plateau). The mechanisms of elementary events controlling the process of preparation of the dielectrics for breakdown at elevated and reduced temperatures are the thermal-fluctuation over-barrier electron transition from trap to trap and the tunneling (under-barrier) transition, respectively. The hopping electron transport in the field direction gives rise to critical space charges causing breakdown of the dielectrics. The transition barrier heights (trap depths) have been determined. The low-temperature longevities of the polymer and the ceramic have been found to be similar, whereas the transition barrier for the ceramic is much higher than that for the polymer and the applied field in the former case is significantly (by a factor of tens) lower than that in the latter case. Electron traps in the polymer are adequately described by the Coulomb center model, whereas this is not the case for the ceramic.     

Observation of laser speckle effects and nonclassical kinetics in an elementary chemical reaction

An experimental demonstration is provided for memory-based, nonclassical reaction kinetics in a homogeneous system with an elementary reaction, A+B-->C. A new reaction-kinetics regime is observed which is a direct consequence of speckles in the laser beam. However, in spite of the nonrandom, speckled initial distribution of reactant B, the long-time regime gives the first experimental demonstration of the asymptotic self-segregation ("Zeldovich") effect. Monte Carlo simulation results are consistent with the experiments.     

Modeling the kinetics of large sets of reactions on metal surfaces

We are concerned with constructing and solving kinetic models for large-scale catalytic mechanisms involving hundreds of reactions. We need reliable estimates for hundreds of rate constants for each project we undertake. In order to obtain reliable estimates of activation barriers with co-adsorbate effects, we employ the UBI-QEP method whenever applicable. We also bring quantum mechanical electronic structure methods to bear when necessary. Herein we describe a new model for obtaining reasonable estimates for rate constants for reactions on metal surfaces that yields rate constants and rate constant parameters without large amounts of computer time. The model is based on a novel approach to surface-phase chemistry and a re-thinking regarding the phases from which surface adsorbates react. We will also discuss recent literature regarding trends in Arrhenius pre-exponential factors (prefactors).     

Influence of the spacing between metal particles on the kinetics of reaction with spillover on the supported metal catalyst

The influence of the spacing between active metal particles placed on the supported metal catalyst on the kinetics of the catalytic reaction with spillover was investigated. The 2A+B₂→2AB reaction, modelling the CO oxidation on Pd/Al₂O₃ catalyst, was studied using Dynamic Monte Carlo simulations. It was shown that there exists an optimal spacing, that provides the maximum reaction rate. It was postulated that this optimum is a consequence of both competition and cooperative effects occurring between metal particles.     

Statistical kinetics of rock fracture and forecasting of seismic phenomena

Based on kinetic concepts of the strength of solids and a related hierarchical model of rock fracture, physically justified criteria for the appearance of the fracture source stage of the process are formulated. A technique for forecasting the region, time, and energy of seismic phenomena is developed, and its algorithm is described. Examples of forecasts based on an analysis of seismic conditions on the Kamchatka Peninsula and SUBR company mines are given.     

On the role of electron-hole pair excitation in the kinetics of atom recombination at metal surfaces

The dynamics of energy relaxation of the adspecies in exoergic processes at metal surfaces has been modeled by means of the master equation approach. The effect of energy disposal to the solid via electron-hole (e-h) pair excitation on the rate of adatom recombination, has been investigated in the case of a discrete set of vibrational levels of the adspecies. The kinetics is solved, analytically, for two recombination channels and by taking into account two energy dissipation pathways of the adspecies. It is shown that dissipation pathways, characterized by a sizable energy transfer per scattering event, affect the kinetics leading to enhanced recombination rates. The kinetic model has been applied to describe experimental data on H(D)-adatom abstraction and recombination at metal surfaces. The rate coefficient of the process is shown to be proportional to the energy power transferred to the solid, owing to the reaction exothermicity, and correlates to the surface electron density.     

Investigation of the kinetics of a metal vapor laser by an interferometric method

The strong visible transition at 553.5 nm was used to study the ground state depletion of Ba I by a discharge pulse at different temperatures. At all temperatures the depletion was found to be larger than 70%.     

纳秒尺度金属熔化相变数值模拟及实验验证

本文基于非均匀成核相变动力学理论和热传导方程研究了在极快速升温(10¹²K/s)条件下金属达到过热状态后发生熔化相变时,通过金属/光学窗口界面温度的演化历史研究了金属内部温度的变化情况.分析结果表明,当金属内部无熔化发生时,界面温度将很快达到平衡,此变化满足理想接触界面模型;而当金属内部有熔化发生时,界面温度受熔化速率的影响不能立即达到平衡而是经历了一个下降的弛豫过程.计算结果与实验观测结果进行比较后发现两者符合得较好,由此可以获得金属在高压下的熔化速率,熔化温度等相关信息.本文的研 关键词： 相变动力学 热传导 金属铁 冲击波     

Modeling of reactive kinetics in the metal surface contaminant cleaning using atmospheric pressure plasma arc

Atmospheric pressure plasma arc (APPA) cleaning is a newly developed method of metal surface cleaning. In this paper, a mathematical model of reactive kinetics in the metal surface contaminant cleaning using APPA has been developed. Based on the analysis of APPA cleaning mechanism and the feature of cleaning interface, a governing equation was established with heat transfer equation and energy conservation on the moving interface. Using fourth-order Rounge-Kutta method, above equation was solved and removal percentages of the cleaning contaminant at different time were obtained. In virtue of reactive kinetics theory, a reactive kinetics model of metal surface cleaning using APPA was established on the base of above calculation results. Afterwards, reactive kinetics parameters such as activation energy and pre-exponential factor were calculated. Cleaning lubricant was taken as an example, the results indicated that predictive values of lubricant removal percentages gotten from this established reactive kinetics model show good consistent with experimental data at the same time. Furthermore, the ambient temperature on the cleaning lubricant surface affects the removal rate strongly. The removal rate increases with the increase of the ambient temperature. To avoid the damage of metal substrate surface because of higher temperature and ensure the removal rate of the lubricant, the appropriate temperature which lies between the lubricant decomposition temperature and damage temperature of metal substrate under given calculation conditions should be determined.     

Sonolysis induced decomposition of metal carbonyls: kinetics and product characterization

The decomposition kinetics of Fe(CO)₅ and Mo(CO)₆ induced by sonolysis in hexadecane solvent was studied as a function of temperature (303–343 K) under an inert atmosphere. The decomposition data, obtained over at least two half lives in most of the runs, yielded first-order rate constant (k) values with correlation co-efficient (R²)>0.95. The products were characterized by various spectroscopic techniques. The transmission electron microscopy (TEM) yielded images from which the mean particle diameter (MPD) of 10 nm for Fe and <3 nm for Mo were estimated. The generation of amorphous Fe and semi-crystalline Mo particles was determined from line broadening and corresponding d-spacing values in the X-ray diffraction (XRD) spectra. The XAFS/XANES data were consistent with the production of Fe(0) metal but carbided Mo (Mo₂C). The one-step production of high-yield pyrophoric products demonstrated the applicability of sonolysis to effectively produce gram-quantity of zero-valent metals.     

Characteristics of the kinetics of photoreactions of polymethene dyes with radical polymerization

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 49, No. 4, pp. 573–580, October, 1988.     

Critical crack size for fracture during hot metal plastic deformation

During hot plastic deformation cracks originate at grain boundaries after critical deformation. Their propagation is, however, decelerated by the occuring dynamic recrystallization leading to the necessity of new cracks originating along boundaries of recrystallized grains. A model has been elaborated for determining the critical size of these cracks and its validity verified by hot torsion tests accomplished on low-carbon steel.     

The effect of reactions at the metal/sulfide scale interface on the silver sulfidation kinetics

This paper presents the results of kinetic studies on silver sulfidation at temperatures 470–670 K and at sulfur vapor pressure of 101 kPa obtained with the use of a modified Wagner's pellet method. It has been stated that the kinetics of silver sulfidation is influenced by sulfur dissolution in the surface layer of metal. This process lowers the rate of silver sulfidation in the case of the classical pellet method because of the lack of local thermodynamic equilibrium at the Ag/Ag₂S interface.