Papers by Keyword: Activation Energy

Abstract: Our investigations show that electrochemical corrosion of copper is faster than electrochemical corrosion of aluminium at temperatures below 100°C. Literature data analysis shows that the Al atoms diffuse faster than the Cu atoms at temperatures higher than 475°C, Al rich intermetallic compounds (IMCs) are formed faster in the Cu-Al system, and the Kirkendall plane shifts toward the Al side. Electrochemical corrosion occurs due to electric current and due to diffusion. An electronic devise working time, for example, depends on initial copper cover thickness on aluminium wire, connected to the electronic devise, temperature, and volume and dislocation pipe diffusion coefficients, so copper, iron, and aluminium electrochemical corrosion rates are investigated experimentally at room temperature and at temperature 100°C. Intrinsic diffusivities ratios of copper and aluminium at different temperatures and diffusion activation energies in the Cu-Al system are calculated by proposed here methods using literature experimental data. Dislocation pipe and volume diffusion activation energies of pure iron are calculated separately by earlier proposed method using literature experimental data. Aluminium dissolved into NaCl solution as the Al³⁺ ions at room temperature and at temperature 100°C, iron dissolved into NaCl solution as the Fe²⁺ (not Fe³⁺) ions at room temperature and at temperature 100°C, copper dissolved into NaCl solution as the Cu⁺ ions at room temperature and as the Cu⁺ and the Cu²⁺ ions at temperature 100°C. It is found experimentally that copper corrosion is higher than aluminium corrosion, and ratio of electrochemical corrosion rates, k_Cu/k_Al>1, decreases with temperature increasing, although iron electrochemical corrosion rate does not depend on temperature below 100°C. It is obvious, because the melting point of iron is more higher than the melting point of copper or aluminium. It is calculated that the copper electrochemical corrosion rate is approximately equal to aluminium electrochemical corrosion at temperature about 300°C, so copper can dissolve into NaCl solution mostly as the Cu²⁺ ions at temperature about 300°C. The ratio of intrinsic diffusivities, D_Cu/D_Al<1, increases with temperature increasing, and the intrinsic diffusivity of aluminium could be approximately equal to the intrinsic diffusivity of copper at temperature about 460oC. Intrinsic diffusivities ratios in the Cu-Zn system at temperature 400°C and in the Cu-Sn system at temperatures from 190°C to 250°C are analyzed theoretically using literature experimental data. Diffusion activation energies and pre-exponential coefficients for the Cu-Sn system are calculated combining literature experimental results.

Abstract: The δ-TRIP steel has attracted wide attention from researchers and industry, as it possesses the mechanical properties of the third-generation advanced automotive steels, and the lower density achieved by the addition of light element aluminum. However, there have been few studies on the hot deformation behavior of δ-TRIP steel at high temperature, which is critical for the design of the hot rolling process. In the present study, hot compression tests were carried out on a 980 MPa δ-TRIP steel to achieve a better understanding of the deformation behavior at high temperatures from 800°C to 1150°C. Combined with the electron backscatter diffraction (EBSD) analysis, the results showed that δ-TRIP steel was a positive strain-rate sensitive material, and its soften was contributed by dynamic re-crystallization of ferrite and austenite during deformation at high temperature. The hot deformation activation energy (Q = 327.485 kJ/mol) of the experimental steel was calculated, and the strain index was 5.65. Besides, the related constitutive equation and the flow stress equation were also established.

Abstract: The procedure for determining the activation energy of conductivity Е_κ is analyzed depending on the temperature step ΔT. It is shown that with increasing ΔT, the error in the calculation of E_κ decreases, but the calculated value of E_κ decreases. In order not to lose the temperature dependence of the activation energy, it is necessary to choose the optimal value of Δt. In our opinion, this value should not exceed 5 – 10 °C. Taking into account the decrease in concentration with increasing temperature due to a decrease in density has virtually no effect on the accuracy of determining E_κ, provided that ΔT is 5 – 10 °C. It has been shown that in the temperature range 20 – 80 °C, the activation energy of conductivity decreases with increasing temperature. This decrease is due to the rupture of intermolecular hydrogen bonds of the solvent with increasing temperature. It was suggested that the movement of ions in an aqueous solution may be accompanied by the breaking of hydrogen bond of the solvent.

Abstract: In this paper, trap levels around the glass transition temperature (T_g) of polymers have been characterized using Thermally Stimulated Current (TSC) technique. Deconvolution on α-peaks of the T_g for PE (-104 °C), plasticized PVC (-35 °C), PMMA (90 °C) and PET (96 °C) were carried out based on the first-order kinetic theory for non-Debye relaxation. Using temperature, T from TSC experimental data, we have successfully separated the α-peaks of the thermoplastic polymers. It is found that the complex curve of α-peaks can composed of four (4) to eight (8) sub peaks. Dominant sub peaks were identified at T_max = -105 °C, -34 °C, 89 °C and 92 °C for PE, pPVC, PMMA and PET, respectively. These peaks show activation energy, E_a of shallow and deep trap centers ranged from 0.3 eV to 4.6 Ev where they represent the depolarization of localized dipoles and space charges relaxations in the polymers.

Abstract: Crystallization among amorphous alloy is a crucial study since it generally affects it properties, which may detrimental or beneficial, depending in the intended application of the materials. Controlling crystallization is crucial for obtaining the desired properties. The crystallization study was performed using differential scanning calorimeter (DSC). Samples were heated at heating rate between 20 and 40 K·min^-1. Structural evolution during crystallization was studied under X-ray diffraction (XRD). Apparent activation energy for each temperature characteristics was determined using Kissinger’s equation. Local Avrami exponent was investigated using modified Johnson-Mehl-Avrami-Kolgomorov equation. Liquid fragility, which indicates the strength of the glass formation, was predicted using temperature characteristics instead of its viscosity. It was found that upon crystallization both as-cast samples crystallize to cubic-Al, Al₂CuMg and Al₂Cu and Al₃Ni. Alloy with composition of (Al₇₅Cu₁₇Mg₈)₉₅Ni₅ shows superior activation energy at every temperature characteristics than alloy with composition of Al₇₅Cu₁₀Mg₈Ni₇. Local Avrami exponent and local activation energy for (Al₇₅Cu₁₇Mg₈)₉₅Ni₅ show high values at the beginning and at the end of crystallization process. From liquid fragility, it was predicted that the samples are stronger glass former than previous studied Al-amorphous alloys.

Abstract: The effect of the composition on the weight loss of wood protected by a coating based on inorganic and organic substances in the process of thermal exposure, which is a feature of the study of the flame retardant effectiveness of the composition, is investigated. The solution of this problem is carried out by specially developed methods. The influence of fire protection under the influence of high-temperature heat flux on the change in the process of loss of mass of fire-protected wood is determined and the mechanism of kinetics of action of the composition is characterized, which is characterized by a decrease in the speed of flame propagation and mass loss. The results of thermogravimetric studies determined the weight loss of the coatings as a function of temperature, the results of which investigated the activation energy at the temperature decomposition of the coatings and found that for wood it was 36.56 kJ / mol, and in the case of fire protection it increased 2.3 times. which makes it possible to conclude that it is advisable to use lacquer varnish to improve the fire retardant efficiency of wood. Thus, for the specimen of fire-retardant lacquer wood, there is a gradual decrease in temperature, ie, the work of the coating is fixed, and, accordingly, the activation energy is increased during the decomposition of the wood. In order to establish the flame retardant efficiency in the application of high-temperature blowing lacquer, studies were conducted to determine the combustibility index of wood by mass loss, flame spread and temperature increase of flue gases and found that when processing wood goes to the group of combustible materials with a burning index.

Abstract: The hot deformation behavior of Ti-Al-Nb-Zr-Mo-Cr titanium alloy was studied by hot compressive method in this paper, and the flow stress constitutive equation and activation energy also calculated. The results show that the flow stress increased with deformation temperature dropping and strain rate increasing. The flow behavior of the sample hot-deformaed exhibited a peak value of stress in the α+β field, whereas, the true stress attained a steady state in the β field. According to the stress-strain curves of the alloy and its stress characteristics, the Arrhenius constitutive equation was obtained. The average activation energy was about 654.228 KJ/mol in the α+β field, and about 272.196 KJ/mol in the β field, respectively.

Abstract: In this paper, the reaction kinetic mechanism of Fe₂O₃ powder containing carbon was studied by microwave magnetizing roast. Based on the temperature-rise curve and weight loss curve of Fe₂O₃ powder by microwave magnetizing roast, the kinetic parameters of Fe₂O₃ powder microwave magnetizing roast were calculated by non-isothermal methods. The controlling steps of different temperature-rising periods in microwave magnetizing roast process of Fe₂O₃ powder were calculated by the Achar-Brindley-Sharp-Wendworth method. The results indicated that the controlling step of microwave magnetizing roast was phase boundary reaction control of contracted cylinder in 250~450°C, and it was three-dimensional diffusion control of spherical symmetry in 450~650°C. The results showed that the starting temperature of reduction roasting of Fe₂O₃ powder was 250°C, which was lower than that under electrical heating, thereby, it proved in theory that microwave heating can enhance reaction rate.

Abstract: The MMS-200 thermal simulation testing machine was used to study the static softening behavior of low carbon high niobium microalloyed steel. The effect of niobium to the static recrystallization softening behavior of the microalloy steel had been analyzed by establishing the kinetics model of static recrystallization and the micro-morphology of precipitates. The results indicated that: the static softening behavior of the tested steel significantly influenced by the deformation temperature and the interval pass time of the rolling processing. At relatively high deformation temperature and long interval pass time, the ratio of static softening was increased. Then the deformation temperature was lower to 950°C, and the static softening behavior of the test steel was ceased. But when the deformation temperature was higher than 1000°C, the static softening behavior of the test steel completely occurred. The activation energy of the test steel was 325·mol^-1 by the established model calculated.

Abstract: The positive effect of Sc,Zr-addition on mechanical properties in Al-based alloys preferred for automotive manufacture to produce lightweight vehicles is generally known. Microstructure, mechanical, electrical and thermal properties of the conventionally cast and homogenized (475 °C/60 min) Al-5.4wt.%Zn-3.1wt.%Mg-1.5wt.%Cu (7075) and Al-5.2wt.%Zn-3.0wt.%Mg-1.4wt.%Cu-0.2wt.%Sc-0.1wt.%Zr (7075-ScZr) alloys during isochronal annealing were characterized. Precipitation reactions were studied by microhardness, electrical resistivity and conductivity measurements, differential scanning calorimetry and positron annihilation spectroscopy. Microstructure observation by scanning and transmission electron microscopy proved the Zn,Mg,Cu-containing eutectic phase at grain boundaries in the alloys. The melting of this eutectic phase was observed at ~ 481 °C for the both alloys. The distinct changes in microhardness and electrical resistivity isochronal curves as well as in heat flow of the alloys studied are mainly caused by dissolution of the clusters/Guinier-Preston (GP) zones and by formation of the metastable phase particles of the Al–Zn–Mg–Cu system. Clusters/GP zones were formed during the cooling and/or in the course of the storage at room temperature. These clusters/GP zones were formed predominantly by Mg and Zn alloying elements. Hardening effect after isochronal annealing at temperatures above ~ 300 °C reflects the Sc,Zr-addition in both states of the 7075-ScZr alloy. Probably precipitation of the T-phase (Al₂Zn₃Mg₃) and S-phase (Al₂CuMg) particles took place during the annealing. The Sc,Zr-addition does not significantly influence precipitation of the particles formed in the Al–Zn–Mg–Cu system.