Activation Energy For Self-diffusion Research Articles

Atomistic simulation of the self-diffusion in Fe (1 1 1) surface

The formation energies, the intra- and inter-layer self-diffusion activation energies of a single vacancy in Fe (1 1 1) surface have been investigated with the modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface is down to the sixth layer for the formation and intra-layer migration of the vacancy. It is easier for a vacancy to form and to migrate in intra-layer in the first (especially), the second and the third layer. For inter-layer migration, a single vacancy in each of the first six layers is favorable to migrate to the upper layers. On the contrary, a single vacancy in the seventh, the eighth and the ninth layers is favorable to migrate to the lower layers.

Transfer of glycols through an MA-41 ion-exchange membrane from aqueous and salt-aqueous solutions

Experiments on the molecular diffusion of glycols through an MA-41 ionite membrane in the SO 4 2− form demonstrated that the flux of the glycols decreases with increasing sodium sulfate concentration in the feed solution and with increasing molecular mass of the glycol. For ethylene glycol, as an example, it was demonstrated that its flux through the membrane nonlinearly depends on its concentration in the feed solution. Based on the observation that the fluxes of the nonelectrolyte and electrolyte differ significantly, we proposed a method for separating these components by means of dialysis through an ionite membrane. Computer simulations of the structure and IR spectra of the ethylene glycol-nine water molecules and ethylene glycol-nine water molecules-M+ (M+ = Na+, K+) systems were performed. It was revealed that, for the latter system, the activation energy of self-diffusion of ethylene glycol is lower than that for the former.

Computer simulation study of self-diffusion in Pd(0 0 1) surface

Both the formation energies and the intra- and inter-layer activation energies of self-diffusion of a single vacancy in the first six planes of Pd(0 0 1) surface have been investigated by means of molecular dynamics (MD) in conjunction with the semi-experiential many-body potential of the modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface on the vacancy is only down to the fifth-layer. It is easer for a single vacancy to form and to migrate in the first layer. Furthermore, the vacancy in the second layer is favorable to migrate to the first layer. This is in agreement with the experimental results that the first layer has the highest concentration of the vacancy.

Quantum chemical calculation of cation interaction with water molecules and ethylene glycol

A computer modeling is carried out for the structure and IR spectra of ethylene glycol-9 water molecules and ethylene glycol-9 water molecules-M+ systems, where M+ = Na+, K+. The presence of cations changes the structure of the glycol hydrate shell, which leads to a decrease in the activation energy of glycol self-diffusion in the solution with the addition of salts in comparison with its value in the water-glycol solution.

Creep Parameters in a Die-Cast Mg-Al-Ca Alloy

Tensile creep tests were conducted to determine the creep parameters for a die-cast Mg-Al-Ca alloy AX52 (X representing calcium) in a temperature range from 423 to 498 K. The stress exponent of the minimum creep rate, n, increases at the yield stress of the alloy, and it lowers at higher temperatures. The activation energy for creep, Qc, decreases with increasing applied stress typically below the yield stress. The change in the creep parameters, n and Qc, is associated with the decreased creep strength caused by the collapse of the eutectic intermetallic phase covering the primary � -Mg grains during creep. The thermally activated component of Qc is evaluated to be 143 kJ/mol below the yield stress, which is very close to the activation energy for the lattice self-diffusion of magnesium. It is deduced that the creep for the alloy is controlled by the hightemperature climb of dislocations. [doi:10.2320/matertrans.MAW200701]

Assessment of creep behaviour of the die-cast cylinder-head alloy AlSi6Cu4-T6

Abstract Mechanical properties of a T6-AlSi6Cu4 cylinder-head alloy are investigated at 4 stress levels up to 545 K through tensile and compressive test to assess creep damage. The effect of over-ageing for 200 h at comparable temperatures was studied through RT hardness tests. Creep behaviour was characterised by a minimum creep rate and an extended steady state was not observed. No threshold stresses were apparent from minimum creep rate vs. stress plots at any of the temperatures studied here. Apparent stress exponents were both temperature and stress dependent and the apparent creep activation energy was 2–3 times the lattice selfdiffusion activation energy for aluminium. The Norton plot was best described by an exponential relation. The total strain to failure exhibited a maximum for a particular applied stress and decreases for both higher and lower applied stress. Creep life is well described by the Monkman– Grant relation at the highest temperature, but exhibits deviations from the idealised behaviour at the lower test temperatures.

Analysis of high-temperature deformation and microstructure of an AZ31 magnesium alloy

High-temperature plastic deformation and dynamic recrystallization of AZ31 extruded (EX) and heat treated (FA) alloy was investigated in the temperature range between 200 and 400 °C. High-temperature straining resulted in partial dynamic recrystallization above 250 °C; in the EX alloy recrystallization was complete at 300 °C, while a moderate grain growth was observed at 400 °C. The peak flow stress dependence on temperature and strain rate are described by means of the conventional sinh equation; the calculation of the activation energy for high temperature in the whole range of temperature deformation gives Q = 155 kJ/mol, i.e. a value that was reasonably close but higher than the activation energy for self diffusion in Mg. The microstructure resulting from high-temperature straining was found to be substantially different in EX and FA alloys; in particular, the EX alloy was characterized by a lower flow stress, a higher ductility and by a finer size of the dynamically recrystallized grains. These results are then discussed on the basis of the “necklace” mechanism of dynamic recrystallization.

Self-diffusion of iron inL10-ordered FePt thin films

Diffusion of iron atoms in a thin FePt film of L1{sub 0} structure was investigated in the low temperature region where iron diffusivities are between 10{sup -22} m{sup 2} s{sup -1} and 10{sup -24} m{sup 2} s{sup -1}. A new method using nuclear resonant scattering of synchrotron radiation in grazing incidence geometry was used, providing higher accuracy and sensitivity than the conventional tracer methods. Isotopical FePt multilayer samples Pt(20 A)/[{sup 57}FePt(20 Angst )/FePt(30 A)]{sub 10}/MgO(001) produced by molecular beam epitaxy were annealed at four temperatures between 773 and 873 K for times between 60 and 120 min. The nuclear reflectivity was measured at room temperature after each annealing step and the decrease of the nuclear superstructure Bragg-peak intensities was observed. From the intensity loss, the diffusion coefficient and the activation energy for iron self-diffusion in the FePt thin film along the c axis were determined as D{sub 0}=(3.45{+-}0.44)x10{sup -13} m{sup 2} s{sup -1} and Q=(1.65{+-}0.29) eV. The value for the activation energy is the same as found by residual resistivity measurements in the same system.

Computer simulation study of self-diffusion in Pd(1 1 1) surface

Both the formation energies and the intra- and inter-layer activation energies of self-diffusion of a single vacancy in the first five planes of Pd(1 1 1) surface have been investigated by means of molecular dynamics (MD) in conjunction with the semi-empirical many-body potential of the modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface on the vacancy is only down to the fourth layer. It is easier for a single vacancy to form and to migrate in the first layer. Furthermore, the vacancy in the second layer is favorable to migrate to the first layer. This is in agreement with the experimental results that the first layer has the highest concentration of the vacancy.

Self-diffusion of Ni in B2 type intermetallic compound NiAl

Combining molecular dynamic (MD) simulation with modified analytic embedded-atom method (MAEAM) potential, the formation, migration and activation energies of Ni self-diffusion in intermetallic compound NiAl have been calculated for five diffusion mechanisms, NNN jump, [1 1 0] 6JC, straight [1 0 0] 6JC, bent [1 0 0] 6JC and triple-defect diffusion. The results show that the Ni self-diffusion is dominated by the triple-defect diffusion mechanism since it requires essentially the lowest migration or activation energy ( Q = 2.769 eV) in the five diffusion mechanisms. This is consistent with the conclusion of Frank et al.

The characteristic of damping peak in Mg–9Al–Si Alloys

The damping capacity of as cast Mg–9Al–Si alloys was investigated. A damping peak exhibits at around 420 K for a frequency of about 1 Hz and it was correlated to the characteristic grain boundary peak of magnesium. The activation energy of damping peak is lower than self-diffusion activation energy of pure magnesium, but higher than activation energy of AZ91 reported. The effect of measurement conditions including strain amplitude, heating rate and consecutive measurement number on damping peaks has also been discussed.

Nitrogen Vacancies as Major Point Defects in Gallium Nitride

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.

Growth dynamics and cell migration in ferroelectric thin films

Growth dynamics and unit cell migration of ferroelectric (Ba,Sr)TiO3 thin films were systematically studied with in situ reflective high-energy electron diffraction and atom force microscopy. By measuring the amplitudes of slow surface recovery oscillations of SrTiO3 film grown on (001) SrTiO3 surface, the activation energy of unit cell surface self-diffusion was surprisingly found to be only 0.29±0.01eV. A “unit-cell migration” model was developed to understand the epitaxial growth dynamics of ferroelectric thin films, which is a critical issue in oxide thin film growth and crucial in controlling the atomic structures.

Electrochemical properties of a network polymer based on tetra(sulfonatophenyl)-meta-cyclophanoctol

Conductivity of the network polymer based on tetra(sulfonatophenyl)-meta-cyclophanoctol in the H+, Na+, Cu2+, Zn2+, and Ni2+ forms is studied, and the self-diffusion coefficients and activation energies of diffusion of the metal cations in the polymer phase are estimated.

Creep Behaviour in Ti-based Alloys

Simulation of different mechanical properties in titanium base alloys is described in the review to study the relative competencies with service exposure at high temperature. Precipitation and solution hardening of matrix, as well as solutes that increase activation energy for self diffusion and that increase twinning propensity, increase creep life. A reduction of creep properties appears on strain induced plate formation or relative instability of interfaces of lamella. An increase in beta-phase in microstructure by fast cooling reduces fracture toughness properties, whereas a transverse texture of thermomechanical treatment reduces impact properties as compared to solution treated and annealed specimen. Coarsening of phases' at high temperature degradation appears to be rectified in many cases by Zener pinning from either indigenous or exogenous particulates. Microstructural and orientation sensitivity rankings have marked material performance under shear plugging during ballistic limit test.

Compressive Behavior of Extruded SiCw/AZ91 at Temperatures close to and above the Solidus of the Matrix Alloy

Compressive deformation behaviors of extruded SiCw/AZ91 were investigated in Gleeble-1500 thermal simulator at temperatures from 743 K to 783 K and strain rates from 6.4×10-2 s-1 to 1.0×101s-1. Results showed that high strain rate sensitivity (~0.5) occurred during compression; deformation activation energy normalized by threshold stress was higher than the lattice self-diffusion activation energy of magnesium. Dynamic recovery (DRV) and dynamic recrystallization (DRX) took place during compression, which refined the grains. The increase of deformation energy was attributed to non-basal planes slip and climbing of dislocations and also the presence of liquid phase.

Diffusion of gold and native defects in mercury cadmium telluride

The diffusion of defects is of great importance in nanoscale device fabrication, making it essential to understand theoretically the microscopic mechanisms governing how native and dopant defects diffuse. To gain this insight, we have performed, for the first time, ab initio density functional calculations to determine the diffusion barriers for multiple pathways of Au impurities and native species in mercury cadmium telluride (MCT). We consider interstitial and vacancy-mediated diffusion mechanisms and calculate the corresponding activation energies using ab initio pseudopotential total energy calculations. Depending on the stoichiometry, the activation energies for Hg self-diffusion are calculated to range from 1.35 eV to 1.60 eV (interstitial mechanism) and from 1.60 eV to 1.85 eV (vacancy mechanism). These theoretical values suggest that Hg self-diffusion is predominantly interstitial mediated, and are in good agreement with existing experimental estimates, which suggest possible activation energies ranging from 1.05 eV to 1.75 eV. For Te self-diffusion via the interstitial mechanism, the calculated activation energy ranging from 2.35 eV to 2.60 eV is also in good agreement with the experimental estimates near 2.30 eV. For Au impurities, whether they are incorporated into the interstitial or Hg site, we find the interstitial mediated mechanism to be the dominant one. Our calculated barrier of 0.35 eV for Au interstitials is also in good agreement with the experimental estimate of 0.45 eV.

Study of diffusion coefficients in liquid noble metals

Molecular dynamic simulation technique is employed to determine self-diffusion coefficients and activation energies in liquid noble metals due to their importance in the industrial applications. The embedded atom method (EAM) potentials for silver and gold, developed for solid-state simulations, are shown to produce plausible results for liquid state. The simulated values of diffusion coefficients in the liquid state compare very well with the experimental and the calculated reported results. The calculated diffusion data follows Arrhenius equation which enables to find the prefactor and the activation energy for liquid state diffusion. The deduced activation energies of self-diffusion are found close to the experimental values.

Grain boundary self-diffusion in Ni: Effect of boundary inclination

We examined the influence of the boundary plane on grain-boundary diffusion in Ni through a series of molecular dynamics simulations. A series of 〈010〉 ∑5 tilt boundaries, including several high symmetry and low symmetry boundary planes, were considered. The self-diffusion coefficient is a strong function of boundary inclination at low temperature but is almost independent of inclination at high temperature. At all temperatures, the self-diffusion coefficients are low when at least one of the two grains has a normal with low Miller indices. The grain boundary self-diffusion coefficient is an Arrhenius function of temperature. The logarithm of the pre-exponential factor in the Arrhenius expression was shown to be nearly proportional to the activation energy for diffusion. The activation energy for self-diffusion in a (103) symmetric tilt boundary is much higher than in boundaries with other inclinations. We discuss the origin of the boundary plane density–diffusion coefficient correlation.

Effect of the carbon isotope composition on the properties of diamond

Parameters of the interatomic interaction potential for 12C and 13C carbon isotopes in the crystal lattice of diamond have been determined. Based on these data, the isotope dependence of the properties of diamond such as the Debye temperature, molar heat capacity, thermal expansion coefficient, vacancy formation energy, self-diffusion activation energy, surface energy, and longitudinal sound velocity is described. This approach is used for estimating a change in the bulk compression modulus of lithium crystals upon the passage from 7Li to 6Li. As the temperature increases, the isotope dependence of the heat capacity at constant volume vanishes; at a certain temperature, the isotope dependence of the thermal expansion coefficient changes from growth to decay. The expected isotope dependence of the parameters of phase transitions is predicted. It is shown that carbon condensates formed upon deposition from the gas phase must be enriched with the heavy isotope.