Self-diffusion Of Atoms Research Articles

Anomalous texture development induced by grain yielding anisotropy in Ni and Ni-Mo alloys

During the annealing of Ni and Ni-Mo films, a {110} texture, usually considered to be both of high surface energy and high strain energy, was found to develop. Quantitative thermodynamic model calculations showed that the anomalous formation of this {110} texture originates fundamentally from the grain yielding anisotropy of Ni. Based on extensive grain yielding anisotropy model calculations, a “texture map” based on {111}, {100}, and {110} textures was constructed to predict the texture transition temperatures for different film thicknesses. A kinetic analysis of the texture evolution in the films is further presented, revealing that the texture evolution is controlled by the self-diffusion of atoms at grain boundaries. These findings pave the way for the achievement of unusual surface orientations through the quantitative texture design of thin films.

Modeling and investigation for atomic diffusion and mechanical properties of TiAl/Ti3Al interface: temperature effect

The effect of temperature on the diffusivity and mechanical property of the TiAl/Ti3Al interface at nanoscale was investigated using molecular dynamics (MD). The diffusion mechanism is atomic self-diffusion in the TiAl or Ti3Al blocks at the temperatures of 673 ~ 1273 K. However, self-diffusion and asymmetric counter-diffusion determine the diffusion process. The self-diffusion of atoms in the Ti3Al block is much better than that in the TiAl block under the same temperature. The average atomic diffusive distance in the TiAl block is significantly farther than that in the Ti3Al block owing to its lower melting point and greater numbers of slipping system. The yield strength of the joint after diffusion welding at the temperatures of 673 ~ 1373 K is about 1.2 GPa higher than that at 1473 K. The temperature has little effect on the mechanical properties of elastic modulus, yield strength, yield strain and flow stress of the diffusion joint at the temperatures of 673 ~ 1273 K. However, the elastic modulus, yield strength and flow stress decrease gradually with the increase in temperature at 1273 ~ 1473 K.

Features of the formation of the structure and properties of powder steels with additives that activate diffusion processes during sintering

Effect of activating the sintering process of powder steel alloyed with nickel or chromium by grinding the initial powders and introducing alkali metal compounds was investigated. The kinetics of grinding the initial iron powders, Cr30, and a mixture of iron powders with 4 % nickel was studied. It is shown that, depending on the hardness of the powder, it is grinded in three or two stages. When grinding more hard powders, there is no stage of intensive deformation of particles and an increase in their size. Crystalline lattice defects resulting from grinding of powders accelerate diffusion processes. This reduces sintering temperature by 100–200 °С compared to the sintering temperature of steels from the initial powders, contributes to a homogeneous structure, reduces porosity by 4–17 %, and increase strength of powder steels by 1.5–1.6 times. The mechanism of the effect of sodium bicarbonate on the acceleration of diffusion of carbon, nickel and chromium into iron has been established. With the introduction of sodium bicarbonate under the action of water vapor, formed upon its decomposition to carbonate, thin oxide films are formed on iron particles, which are actively recovered in a protective-recovering atmosphere during sintering. This leads to formation of a metal contact between the particles, acceleration of the self-diffusion of iron atoms and the diffusion of alloying additives into iron by 5–7 times, depending on the sintering temperature and the amount of added additive. Sodium forms nanodispersed complex compounds of the ferritic type Na3Fe5O9along the grain boundaries of the iron base, which provide grain refinement and the formation of a homogeneous structure. Changes in the structure of powder steel with the introduction of sodium bicarbonate cause an increase in its strength by 1.5–1.7 times. The results can be used to obtain structural products from alloyed powder steels.

Study of grain boundary self-diffusion in iron with different atomistic models

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data.

Study of Grain Boundary Self-Diffusion in Iron with Different Atomistic Models

We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data.

Effect of hydrogenation on metal distribution in Ti–V–Cr alloys

The paper reports on the distribution of metal elements during aging after electrolytic hydrogenation in both (TiCr1.8)100-xVx and (TiCr1.8)100-xVx + 4 mol% Zr7Ni10 alloys. It is shown that insertion of hydrogen leads to self-diffusion of metals atoms. Moreover, after long time enough, stochastic aperiodic changes in the distribution of metal elements are observed.

Self-diffusion of Ti interstitial based point defects and complexes in TiC

To date, the mechanism of Ti atom self-diffusion is unproven. Prior theoretical work mostly focused on Ti vacancy based mediators, but these do not reproduce the experimental activation energy or entropy. In this work, in density functional theory calculations, Ti interstitials and related defect complexes are systematically considered as possible mediators of Ti self-diffusion. Among these defects, the defect complex of two C vacancies tightly bound to a Ti dumbbell is found to have the lowest formation energy. A sustainable migration of the complex, in a translational or rotational fashion, is enabled in the presence of another (free) carbon vacancy nearby the complex, and thus the rate of Ti self-diffusion by this mechanism is dependent on the concentration of carbon vacancies. The calculated activation energy of the complex agrees well with the experimental value in TiC0.97. Similar analyses of the Ti self-diffusion mechanisms mediated by Ti interstitials or dumbbells yield much higher activation energies, but the corresponding migration energies are evaluated to be less than 1 eV, which suggests they can be possible mediators of the radiation-enhanced Ti self-diffusion in TiC. To fully enable the comparison with experiments that are typically conducted at temperatures as high as 2500 K, we also consider the temperature dependent vibrational contribution to the activation energy of the defect complex. The vibrational contribution imposes an additive effect on the defect formation energy, while the migration energies are lowered due to the thermal expansion of the lattice. When combined, these factors give an excellent agreement with the experiments. This work gives strong support to the concept that Ti interstitial based defect complexes are likely diffusion mediators for Ti atom self-diffusion in TiC, further establishes a solid basis for large-scale modeling, and may eventually pave the way to accurately predicting defect-controlled diffusional processes.

Evaluation of the structure and properties for the high-temperature phase of zirconium from the atomistic simulations

We study peculiarities of phase transitions in zirconium and properties of the high-temperature β-Zr phase. To get a more detailed understanding of the structure and thermodynamic characteristics of zirconium, we perform atomistic simulations with two different interatomic potentials. Both potentials demonstrate an unstable behavior of β-Zr phase at low temperatures but explain this phenomenon by substantially different reasons. For one of the potentials, the mechanical instability takes place, and for the other potential the instability of β-Zr is purely dynamic. Review of the available experimental data shows that it is more correct to describe β-Zr through the low-temperature dynamic instability. The structure peculiarity discussed for β-Zr leads to a local non-cubic symmetry of this phase and low formation energy of the self-interstitial atoms. The latter leads to fast atomic self-diffusion that is consistent with existing data. We also perform deformation tests for the atomistic models of β-Zr-Nb alloys taking into account the studied details of α-β transition.

Aluminum metallization and wire bonding aging in power MOSFET modules

A limiting factor for the long-term reliability of power MOSFET-based devices is the electro-thermal and/or thermo-mechanical aging of the metallic parts. Here, we assess the bonding wire and source metallization degradation of power devices, designed for applications in the automotive industry. Our approach consists in characterizing the metal microstructure before and after accelerated aging tests, by scanning electron microscopy, ion milling and microscopy, focused ion beam tomography, transmission electron microscopy and grain structure mapping. To focus on the wire-metallization bonding interface, we have set up a dedicated sample preparation that allows us to disclose the metallization under the bonding wires. This critical location is significantly different from the naked metallization, as the bonding process induces plastic deformation prior to aging. The main mechanism behind the device failure is the generation and propagation of fatigue cracks in the aluminum metallization. Away and under the wire bonds, they run perpendicularly from the surface down to the silicon substrate following the grain boundaries, due to an enhanced self-diffusion of aluminum atoms. Moreover, initial imperfections in the wire-metallization bonding (small cavities and aluminum oxide residues) are the starting point for harmful cracks that propagate along the wire-metallization interface and can eventually cause the wire lift-off. These phenomena can explain the local increase in the device resistance occurring at failure.

Teaching the Growth, Ripening, and Agglomeration of Nanostructures in Computer Experiments

This article describes the implementation and application of a metal deposition and surface diffusion Monte Carlo simulation in a physical chemistry lab course. Here the self-diffusion of Ag atoms on a Ag(111) surface is modeled and compared to published experimental results. Both the thin-film homoepitaxial growth during adatom deposition onto a single-crystal surface and the agglomeration of grown nanostructures due to Ostwald ripening during a heating experiment are simulated. In addition, a Monte Carlo classroom experiment is presented that simulates the random walk of a single Ag atom on the Ag(111) plane. These two Monte Carlo simulations allow for the teaching of random walk, surface diffusion, island formation, and agglomeration. The universal Arrhenius law is evaluated for thermally activated processes.

Effect of Ca additions on the microstructure and creep properties of a cast Mg–Al–Mn magnesium alloy

The microstructure and creep properties of cast Mg–6Al–0.3Mn (AM60) alloys containing 0.5, 1.2, and 2.0wt% Ca were investigated by impression creep testing method in the stress range of 150–750MPa at temperatures from 423 to 523K, corresponding to 0.458 <T/Tm < 0.567. Results showed that calcium addition can substantially improve creep properties by substituting β-Mg17Al12 phase with an interconnected and thermally stable Al2Ca phase. Unlike β-Mg17Al12 particles that crack during creep, the network of lamellar Al2Ca phase withstands deformation at high temperatures, and thus, improves creep resistance. Two different mechanisms were proposed for creep deformation of all alloys at different temperatures and stresses. Under low stresses and at low temperatures, stress exponents of approximately n~5 and activation energies of 75–100kJ/mol that are close to that of the pipe diffusion are indicative of dislocation climb controlled by pipe diffusion creep mechanism. At higher stresses and temperatures, power-law breakdown leads to the stress exponent n > 8 and activation energies of about 129–165kJ/mol that are slightly higher than that of the self-diffusion of magnesium atoms.

Investigation of the abnormal Zn diffusion phenomenon in III-V compound semiconductors induced by the surface self-diffusion of matrix atoms

Zn diffusion in III-V compound semiconductorsare commonly processed under group V-atoms rich conditions because the vapor pressure of group V-atoms is relatively high. In this paper, we found that group V-atoms in the diffusion sources would not change the shaped of Zn profiles, while the Zn diffusion would change dramatically undergroup III-atoms rich conditions. The Zn diffusions were investigated in typical III-V semiconductors: GaAs, GaSb and InAs. We found that under group V-atoms rich or pure Zn conditions, the double-hump Zn profiles would be formed in all materials except InAs. While under group III-atoms rich conditions, single-hump Zn profiles would be formed in all materials. Detailed diffusion models were established to explain the Zn diffusion process; the surface self-diffusion of matrix atoms is the origin of the abnormal Zn diffusion phenomenon.

Atomic self-diffusion anisotropy of HCP metals from first-principles calculations

A plane wave pseudo-potential method based on density functional theory is employed to calculate the migration energy barrier for the atomic self-diffusion in HCP metals including Mg, Zn, Ti, Zr, and Hf. The influences of some key factors (plane-wave cutoff energy, k-mesh, supercell size, and geometric optimization scheme) on the calculated migration energy barrier and its anisotropy are systematically investigated. We show that the supercell size affects heavily the migration energy barrier and its anisotropy for the metals with valence d electrons (Ti, Zr, and Hf) but not for the ones with only valence s metals (Mg). In general, the anisotropy of the migration energy barrier reduces with increasing size of the supercell especially for Ti, Zr, and Hf. The optimization of the shape and volume of the supercell matters for the migration energy barrier calculated with the small size supercell but not for that calculated with the large supercell. With the calculated migration energy barrier, the self-diffusion coefficients are evaluated based on the transition state theory and compared with other first-principles calculations and the experimental measurements.

Structural and viscosity properties of CaO-SiO2-Al2O3-FeO slags based on molecular dynamic simulation

Since CaO-SiO2-Al2O3-FeO is one of the most important slag systems in metallurgic processes, it is important to explore its microstructural characteristics and viscosity. Molecular dynamics (MD) simulation was carried out to investigate the effect of basicity (R=CaO/SiO2 mole ratio) on both the structural and viscosity properties of CaO-SiO2-Al2O3-FeO slags. The viscosities were estimated based on the atomic self-diffusion coefficients obtained from MD simulations, and were compared with mathematical models and experimental data. The results showed that both Si-O and Al-O network depolymerized into simple structure with increasing basicity, while atomic self-diffusion coefficients increased and viscosities decreased. The increase in basicity has a lower influence on the structure of Al2O3 rich samples. In addition to the basicity effect, the CaO-SiO2-Al2O3-FeO slag with high Al2O3 content trended to form more complex structures. Finally, a correlation between structural properties and viscosity of CaO-SiO2-Al2O3-FeO slags was established.

One-dimensional diffusion of Sr atoms on Sr/Si(111)-3 × 2 reconstruction surface

The electronic and geometric structures of the Sr/Si(111)-3×2 surface were investigated by scanning tunnelling microscopy and scanning tunnelling spectroscopy. The honeycomb-chain (HCC) model may be used to describe the reconstruction structure of the Sr/Si(111)-3×2 surface.Furthermore, one-dimensional (1D) concerted motion of Sr atom chains on the Sr/Si(111)-3×2 surface was observed at room temperature. Three reasons contribute to this 1D self-diffusion: low metal coverage of the Sr/Si(111)-3×2 reconstruction surface, weak interaction between the Sr and Si substrate, and surface vacancies and thermal fluctuation energy at room temperature.From this study, the origin of the long-existing blurred low energy electron diffraction pattern of alkali-earth metal induced-Si(111)3×2 surface was clarified, and the self-diffusion of metal atoms at room temperature also explains the common phase transition phenomenon on these reconstructed surfaces.

Seedless Synthesis of Monodisperse Cuboctahedral Gold Nanoparticles with Tunable Sizes

Polyhedral gold nanoparticles are of great current interest because of their unique optical and chemical properties which are attributable to their well-defined facets, corners, and size. While various polyhedral gold nanoparticles of different sizes mostly synthesized by the seed-mediated method have been reported, synthesis of gold cuboctahedra with tunable sizes still remains challenging. Here, we report for the first time a seedless method of synthesizing monodisperse gold cuboctahedra with finely tunable sizes ranging from 40 to 80 nm using cetyltrimethylammonium 4-vinylbenzoate (CTAVB) as a selective capping and mild reducing agent in the presence of a high concentration of HCl in aqueous solution. The HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. This slow particle growth provides an easy and reliable way of tuning the particle size by stopping the reaction at different times and allowing sufficient time for the surface self-diffusion of Au atoms. Combined with the selective capping of {100} facets with CTA+, the surface self-diffusion of Au atoms from {111} to {100} facets is considered to be the key mechanism for the formation of Au cuboctahedra and their stable growth without morphological deformation.

Origin of the first-phase selection during thin film reactive diffusion: Experimental and theoretical insights into the Pd-Ge system

Pd-Ge reactive-diffusion in two different samples Pd/Ge(001) and Pd/PdGe/Ge(001) was studied experimentally and theoretically, in order to clearly identify the driving force controlling the first-phase selection in the sequential phase formation regime. The experimental and theoretical results both show that only the atomic transport kinetic in the phases controls the first-phase selection: the phase exhibiting the fastest atomic self-diffusion forms first, even if its formation energy is not the lowest.

The Effect of Self-Diffusion on the Zn Diffusion in III-V Compound Semiconductors

Zn diffusion in III-V compound semiconductors are commonly processed under group V-atoms rich conditions because the vapor pressure of group V-atoms is relatively high. In this paper, we found that group V-atoms in the diffusion sources would not change the shaped of Zn profiles, while the Zn diffusion would change dramatically under group III-atoms rich conditions. The Zn diffusions were investigated in typical III-V semiconductors: GaAs, GaSb and InAs. We found that under group V-atoms rich or pure Zn conditions, the double-hump Zn profiles would be formed in all materials except In As. While under group III-atoms rich conditions, single-hump Zn profiles would be formed in all materials. Detailed diffusion models were established to explain the Zn diffusion process; the surface self-diffusion of matrix atoms is the origin of the abnormal Zn diffusion phenomenon.

Diffusion in thorium carbide: A first-principles study

The prediction of the behavior of Th compounds under irradiation is an important issue for the upcoming Generation-IV nuclear reactors. The study of self-diffusion and hetero-diffusion is a central key to fulfill this goal. As a first approach, we obtained, by means of first-principles methods, migration and activation energies of Th and C atoms self-diffusion and diffusion of He atoms in ThC. We also calculate diffusion coefficients as a function of temperature.

Creep in solid 4He at temperatures below 1 K

Creep in solid 4He at temperatures of ∼100–1000 mK is studied experimentally by detecting the flow of helium through a frozen porous membrane under a constant external force. Creep curves are measured for different temperatures and mechanical stresses. This method has made it possible to detect low creep rates in helium down to the lowest temperatures in these experiments. It is found that throughout this temperature range, creep is thermally activated and the activation energy decreases with falling temperature and increasing mechanical stress. An analysis shows that for temperatures above ≈500 mK, Nabarro-Herring diffusive creep takes place in solid helium with mass transfer by self diffusion of atoms and a counterflow of vacancies. The experimental data have been used to obtain the self-diffusion coefficient as a function of temperature for different stresses. At temperatures below ≈500 mK creep takes place at a very low flow rate (∼10−13 cm/s) and a very low activation energy (∼0.5–0.7 K), while the creep mechanism remains unclear.