Papers by Keyword: Diffusion Mechanism

Abstract: In this paper, the diffusion isotope effect and diffusion mechanism are investigated by means of molecular dynamics simulations in two liquid alloys, Ni-Ag and Ni-Cu. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms which are moving cooperatively in a basic diffusion event as experienced by a given atomic species. It is shown that the composition dependence of N_D is typically very small. However, the temperature dependence of this parameter is much more pronounced. In addition, it is shown that, on average, in these alloys and temperatures considered, N_D is limited to the range: 5<N_D<17. This is consistent with results of molecular dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, depending on temperature, the neighbouring atoms are all involved in the basic diffusion event.

Abstract: In previous work, perturbed angular correlation spectroscopy (PAC) was used to determine jump rates of ¹¹¹Cd, the daughter of the ¹¹¹In radiotracer, in the series of phases RIn₃ (R = rare-earth element) through nuclear quadrupole relaxation. Greater relaxation, indicating faster Cd jump rates, was observed in heavy rare-earths for compositions more deficient in indium, as would be expected for diffusion mediated by vacancies on the In sublattice. On the other hand, greater relaxation was observed for light rare-earths (R = La, Ce, and Pr) for compositions with excess indium, suggesting Cd diffusion is mediated there by a different mechanism. In this work, computer simulations were carried out to better understand the nature of the relaxation observed for the light rare-earths and the origin of the change in behavior across the rare-earth series. As a first step, formation enthalpies of intrinsic defects were calculated using density functional theory (DFT) for series end-members LaIn₃ and LuIn₃. Both compounds were found to exhibit Schottky thermal disorder. Additional DFT simulations show that the binding enthalpy between In-and R-vacancies is larger in LaIn₃ than in LuIn₃, suggesting that diffusion in LaIn₃ might be mediated by divacancies. Site enthalpies of Cd also were calculated, and it was found more favorable energetically for Cd to occupy the In sublattice than the R sublattice in both end-member phases.

Abstract: Using the method of perturbed angular correlation of gamma rays, diffusional jump-frequencies of probe atoms can be measured through relaxation of the nuclear quadrupole interaction. This was first shown in 2004 for jumps of tracer atoms that lead to reorientation of the local electric field-gradient, such as jumps on the connected a-sublattice in the L1₂ crystal structure. Studies on many such phases using the ¹¹¹In/Cd PAC probe are reviewed in this paper. A major finding from a 2009 study of indides of rare-earth elements, In₃R, was the apparent observation of two diffusional regimes: one dominant for heavy-lanthanide phases, R= Lu, Tm, Er, Dy, Tb, Gd, that was consistent with a simple model of vacancy diffusion on the In a-sublattice, and another for light-lanthanides, R= La, Ce, Pr, Nd, that had no obvious explanation but for which several alternative diffusion mechanisms were suggested. It is herein proposed that the latter regime arises not from a diffusion mechanism but from transfer of Cd-probes from In-sites where they originate to R-sites as a consequence of a change in site-preference of ¹¹¹Cd-daughter atoms from In-sites to R-sites following transmutation of ¹¹¹In. Support for this transfer mechanism comes from a study of site-preferences and jump-frequencies of ¹¹¹In/Cd probes in Pd₃R phases. Possible mechanisms for transfer are described, with the most likely mechanism identified as one in which Cd-probes on a-sites transfer to interstitial sites, diffuse interstitially, and then react with vacancies on b-sites. Implications of this proposal are discussed. For indides of heavy-lanthanide elements, the Cd-tracer remains on the In-sublattice and relaxation gives the diffusional jump-frequency.

Abstract: The problem of reliability prediction and assurance is characteristic of wireless devices based on nanoscale multilayer heterostructures because of the sensitivity of heterostructures’ parameters to the degradation processes due to the thinness of layers. In the current work, the degradation of the nanoscale AlAs/GaAs resonant-tunneling heterostructures due to the diffusion of the constituent elements was investigated. Analysis and comparison of data on Al and Si diffusion coefficients in GaAs shows that they strongly vary depending on the conditions of heterostructure fabrication. This happens while the defect density of the grown heterostructures depends on a large number of technological factors such as the substrate temperature during molecular beam epitaxy, chamber pressure, annealing temperature and time, defect density in the initial substrate, and many others. The values of the diffusion coefficients obtained by the authors of this article by IR spectral ellipsometry are consistent with the data of foreign researchers. This allows their use to predict the reliability of resonant tunneling diodes and nonlinear radio transmitters based on them.

Abstract: An overview of current understanding of diffusion properties of grain boundariesin severely deformed materials is presented. The results are evaluated with respect to thetypes of severe plastic deformation, defects introduced by such processing, and processing pa-rameters. An extended hierarchy of diffusion paths is shown to exist in ultra- ne grainedmaterials, with relative diffusion rates being different by orders of magnitude. Properties ofdeformation-modi ed grain boundaries, such as width, diffusivity, diffusion mechanism and freevolume excess, are examined. A model of deformation-modi ed state of general high-angle grainboundaries is discussed with respect to the underlying concept of free volume accumulation.The relaxation stages of the deformation-modi ed state of grain boundaries are analyzed.

Abstract: Secondary ion mass spectroscopy (SIMS) has been adopted to study the diffusion of Al and Si in Cu_44.25Ag_14.75Zr₃₆Ti₅ bulk metallic glass (BMG). It has been found that around the transition temperature of metallic glass, the relation between its diffusion coefficient and the temperature satisfy the same Arrhenius relation, which means the metallic transition has not caused change to the diffusion mechanism. In addition, the radius of Al atom is close to that of Si atom, but under the same temperature and time condition, the diffusion coefficient of Si atom in bulk metallic glass (BMG) is twice that of the Al atom, while there is not a big difference in diffusion activation energy. This is because as non-metallic element, the radius of Si atom has a strong binding force with the metal atoms in the base material, which also has a bigger diffusion coefficient.

Abstract: The diffusion processes taking place in heterogeneous catalytic systems have been discussed. Various diffusion mechanisms such as Knudsen diffusion, molecular diffusion, configurational diffusion and surface diffusion sensitivity in catalytic systems were investigated. The concentration gradients inside the catalyst pellet were obtained for various Thiele modulus. The Knudsen number was calculated and discussed for large and small pores. The transitional diffusion coefficient was determined for diethylbenzene. The experimental pore size distribution carves for an industrial and synthesized catalyst was obtained and the effect of pore size distribution on diffusion coefficient was discussed.

Abstract: Diffusion of both titanium and nickel was measured in the near stoichiometric Ni-49.4at.%Ti alloy with the B2 ordered structure. The radiotracer technique and the 44Ti and 63Ni isotopes were applied in the temperature interval from 900 to 1300 K. The penetration profiles were determined by precision parallel grinding or by ion beam sputtering at larger and smaller penetration depths, respectively. Titanium and nickel diffusivities were found to follow linear Arrhenius dependencies with the pre-exponential factors of 2.710-7 and 4.710-9 m2/s and the activation enthalpies of 205 and 143 kJ/mol, respectively. A vacancy mediated diffusion mechanism is suggested to provide diffusion of both nickel and titanium in the compound NiTi.

Abstract: Based on the X-ray method an experimental research has been carried out aimed to estimate the pulsed magnetic field influence on the heterodiffusion of Al in -Fe within the temperature interval of 700-820 °С and at the amplitude intensity of the magnetic field pulse equal to 0–557.2 kА/m and its frequency ranging from 0 to 8 Hz. It is established that the pulsed magnetic field changes noticeably the value of the measured coefficient of Al diffusion in -Fe. Possible mechanisms of the pulsed magnetic field influence on the diffusion process are considered on taking into account the interaction of moving domain walls, dislocations and impurity atoms.

Abstract: The paper is devoted to the research of diffusion mechanism in Ni3Al intermetallide over grain boundaries of tilt <111> and <100> by the method of molecular dynamics. It was found that grain boundary diffusion represented the combination of three main mechanisms: migration of atoms along the nuclei of grain boundary dislocations, cyclic mechanism near the nuclei and the formation of the chain of displaced atoms from one nucleus of dislocation to the nucleus of the other one. Diffusion mobility of Ni atoms was essentially bigger than the mobility of Al atoms. Grain boundary slipping and intergrain slipping were also observed during the deformation of the bicrystal.