One-dimensional Grain Research Articles

Generation of phonon quantum states and quantum correlations among single photon emitters in hexagonal boron nitride

Hexagonal boron nitride exhibits two types of defects with great potential for quantum information technologies: single-photon emitters (SPEs) and one-dimensional grain boundaries hosting topologically-protected phonons, termed as topologically-protected phonon lines (TPL). Here, by means of a simple effective model and density functional theory calculations, we show that it is possible to use these phonons for the transmission of information. Particularly, a single SPE can be used to induce single-, two- and qubit-phonon states in the one-dimensional channel, and (ii) two distant SPEs can be coupled by the TPL that acts as a waveguide, thus exhibiting strong quantum correlations. We highlight the possibilities offered by this material-built-in nano-architecture as a phononic device for quantum information technologies.

Spin/charge density waves at the boundaries of transition metal dichalcogenides

One-dimensional grain boundaries of two-dimensional semiconducting {\MX} (M= Mo,W; X=S,Se) transition metal di-chalcogenides are typically metallic at room temperature. The metallicity has its origin in the lattice polarization, which for these lattices with $D_{3h}$ symmetry is a topological invariant, and leads to one-dimenional boundary states inside the band gap. For boundaries perpendicular to the polarization direction, these states are necessarily 1/3 occupied by electrons or holes, making them susceptible to a metal-insulator transition that triples the translation period. Using density-functional-theory calculations we demonstrate the emergence of combined one-dimensional spin density/charge density waves of that period at the boundary, opening up a small band gap of $\sim 0.1$ eV. This unique electronic structure allows for soliton excitations at the boundary that carry a fractional charge of $\pm 1/3\ e$.

Modelling grain active ventilation process in the silos

Changes in grain raw material properties in a complex thermophysical process are presented in the article, and a multifactorial mathematical model for developing engineering solutions for active ventilation is obtained. A numerical study of non-stationary changes in temperature and moisture content in a one-dimensional grain layer along with the height of a metal silo depending on environmental conditions is carried out. The simulation of the temperature and moisture fields in siloses is important for assessing the state and determining the parameters of the active ventilation system. The results show a significant air velocity effect on cooling and the speed of the drying process. The overall duration of drying and cooling of the grain top layer depends slightly on the initial parameters of the air such as the temperature and the moisture content. The obtained model is reliable to predict the drying kinetics for different storage conditions and different grain types. With the help of the presented model, it is possible to substantiate the safe grain storage modes in conditions of heterogeneity and temporal variations of standard parameters (temperature, air humidity).

Controllable phase separation and improved grain growth mode of Mg-doped Sb7Te3 films

The effects of Mg on the structure and phase-change kinetics of Sb7Te3 were investigated for phase change memory application. The results revealed that the addition of Mg increased the crystallization temperature, and crystalline activation energy, while hindering grain growth and suppressing phase separation from Sb+Sb2Te, Sb2Te, to Sb phases. Laser-induced phase-change behavior showed four regions in the Mg-Sb7Te3 films: crystallization, transition toward stability, amorphization and ablation. A reversible switching between crystallization and amorphization occurred more rapidly in Mg-doped Sb7Te3 films than in conventional Ge2Sb2Te5. Evaluation of Johnson-Mehl-Avrami plots indicated that the crystallization mode changed from three, two to one-dimensional grain growth as the Mg-doping level increased. This phenomenon contributed to the rapid phase change of Mg-doped Sb7Te3 films.

Controllable crystal growth and fast reversible crystallization-to-amorphization in Sb2Te-TiO2 films

The structure evolution and crystallization processes of Sb2Te-TiO2 films have been investigated. The Sb2Te-rich nanocrystals, surrounded by TiO2 amorphous phases, are observed in the annealed Sb2Te-TiO2 composite films. The segregated domains exhibit obvious chalcogenide/TiOx interfaces, which elevate crystallization temperature, impede the grain growth and increase crystalline resistance. Compared with that in conventional Ge2Sb2Te5 film, the shorter time for onset crystallization (25 ns) and amorphization (100 ns) has been achieved in as-deposited (Sb2Te)94.7(TiO2)5.3 film under 60 mW laser irradiation. The corresponding recrystallization and re-amorphization can also be realized in the film. From Johnson-Mehl-Avrami (JMA) analysis, it is further found that the one-dimensional grain growth with controlled interface is dominant for the film during the fast phase-change process. Therefore, (Sb2Te)94.7(TiO2)5.3 film with improved crystallization mechanism is promising for high-stable and fast-speed memory applications.

Static Recrystallization Kinetics and Crystallographic Texture of Nb-Stabilized Ferritic Stainless Steel Based on Orientation Imaging Microscopy

In the present study, Nb-stabilized ferritic stainless steel was prepared with annealing (430-A) and without annealing (430-NA) annealing, and the microstructure of the resulting samples was examined. The steel was then subjected to cold rolling and isothermal annealing in order to analyze its recrystallization kinetics and texture evolution. Microstructural characterization was performed by scanning and transmission electron microscopies. Recrystallization kinetics were evaluated by measuring the microhardness of the samples, and analyzing their kernel average misorientation and grain orientation spread via electron backscatter diffraction. The Avrami exponent data revealed that one-dimensional grain growth occurred owing to the migration of high-angle grain boundaries. The mean activation energies for recrystallization for 430-NA and 430-A was found to be 365 and 419 kJ mol−1, respectively. The recrystallization texture was influenced by oriented nucleation and selected growth mechanisms, as well as by the Nb carbonitride distribution and grain boundary energy. The recrystallized and growing grains with the {554}〈225〉 orientation showed a dimensional advantage over the other recrystallized components. The coincident site lattice boundaries were attributed to the progression of recrystallization since the CSL numeric fraction increased as the temperature increased. The {554}〈225〉 component was associated with the ∑19a boundary, which exerted a significant control on the selective growth during the recrystallization.

Cold rolled tungsten (W) plates and foils: Evolution of the microstructure

This paper is the first part of our series on ultrafine-grained (UFG) tungsten produced by cold rolling. The aim of our project is to investigate the correlation between microstructure, mechanical properties and deformation mechanisms in UFG tungsten, a material interesting for future high-temperature applications. For this purpose, a batch of tungsten sheets with different thicknesses has been produced by subsequent cold rolling out of a single sintered compact.The aim of this paper is to characterise the microstructure of the as-received tungsten sheets. Quantitative grain size analysis by EBSD confirms a grain refinement by cold rolling well down into the UFG-regime, reaching 240nm in the S-direction for the 100μm foil. The grain refinement comes with an increase in the number of HAGBs, while LAGBs show no correlation with the degree of deformation introduced by cold rolling. All plates are distinctively textured, the main texture being the {001}〈110〉-orientation (rotated cube texture).Vickers microhardness measurements confirm a steady increase in hardness with advanced rolling up to 687kg/mm2 for the 100μm foil. This indicates that the one-dimensional grain refinement to UFG by cold rolling is sufficient to achieve properties comparable to other UFG tungsten specimens. Therefore, it can be concluded that the batch of samples introduced in this paper will be fit to investigate the evolution of mechanical properties and deformation mechanisms in correlation with the microstructure.

Finite Element Modeling of Diffusional Creep with Explicit Consideration of Enhanced Vacancy Diffusivity in a Finite Region Adjacent to the Grain Interface

We propose a finite element model to analyze diffusional creep and stress relaxation in grains with several atomic monolayers thick grain boundary region of enhanced diffusivity. Our model is motivated by the need to study deformations in submicron integrated circuit interconnect structures during the thermal annealing step of the fabrication procedure. On the length scale of modern interconnects, deformations do not conform to the assumptions of classical theories of diffusion creep. We employ a coupled formulation which relates vacancy concentration field and stress field through the creep strains in polycrystals. The approach has been implemented in the commercially available finite element software package with user-defined subroutines. We validate the model for the case of stress relaxation in one-dimensional grain array by comparing finite element results to the predictions of classical theories. We use our model to study the effect of grain size on creep rate in a polycrystal under external load.

A discrete nonlinear mass transfer equation with applications in solid-state sintering of ceramic materials

The evolution of grain structures in materials is a complex and multiscale process that determines the material's final properties. Understanding the dynamics of grain growth is a key factor for controlling this process. We propose a phenomenological approach, based on a nonlinear, discrete mass transfer equation for the evolution of an arbitrary initial grain size distribution. Transition rates for mass transfer across grains are assumed to follow the Arrhenius law, but the activation energy depends on the degree of amorphization of each grain. We argue that the magnitude of the activation energy controls the final (sintered) grain size distribution, and we verify this prediction by numerical simulation of mass transfer in a one-dimensional grain aggregate.

Crystallization kinetics of amorphous Ga–Sb–Te films: Part II. Isothermal studies by a time-resolved optical transmission method

Isothermal crystallization kinetics of amorphous Ga–Sb–Te films was studied by means of a time-resolved optical transmission method. Thin films with compositions along the pseudo-binary tie-lines Sb7Te3–GaSb and Sb2Te3–GaSb in the ternary phase diagram were prepared by the co-sputtering method. Crystallization of GaSbTe films reveals a two-stage process: an initial surface nucleation and coarsening (Stage 1) followed by the one-dimensional grain growth (Stage 2). The kinetic exponent (n) value in Stage 1 shows strong dependence on film compositions, while that of Stage 2 is less dependent. The activation energy in Stage 1 increases with increasing GaSb content and reaches the maximum values at compositions close to GaSb, but a decreasing trend was observed in Stage 2. Kinetics parameters between isothermal crystallization of thin films and non-isothermal crystallization of powder samples analyzed by differential scanning colorimetry [J. Mater. Res. 19, 2929 (2004)] are compared. The kinetic parameters in Stage 1 show much correspondence with those of non-isothermal cases in comparable kinetic exponents but with lower activation energies. The discrepancies between nonisothermal and isothermal kinetics are attributed to the sample morphology and the constraint effects.

The influence of spatial grain size correlation and topology on normal grain growth in two dimensions

In this paper two-dimensional grain growth has been simulated by the program Surface Evolver, and the influence of grain size correlation and topology on normal grain growth in two dimensions has been studied. The paper is an extension to two dimensions of the treatment in the accompanying paper discussing one-dimensional grain growth. Two-dimensional grain structures initially of 10 4 random Voronoi grains have been coarsened until approx. 2000 grains were left. It is demonstrated that a size correlation between neighbouring grains exists in a two-dimensional grain structure, and a spatial grain size correlation function relating the average size of all neighbours to grains of size R to R itself is defined. It is demonstrated that both the distribution function and the correlation function approaches a quasi stationary state when the mean grain size has roughly doubled compared to its initial value. The results are in agreement with corresponding results obtained for one-dimensional grain growth simulations. A modification of Hillert's mean field theory incorporating the correlation effect has also been developed and compared to the simulated results.

The influence of spatial grain size correlation on normal grain growth in one dimension

In this paper we demonstrate that a size correlation between neighbouring grains will exist in a grain structure. Such a correlation is shown to prevail during coarsening of the grain structure and this will in turn influence the kinetics of the process and the size distribution. In this paper the phenomena has been investigated numerically in a one-dimensional grain structure. In the accompanying paper the treatment is extended to the two-dimensional case. In the one-dimensional case we demonstrate that a quasi stationary grain size function as well as a quasi stationary size correlation function exists in the system after a transient which depends markedly upon the initial conditions. The kinetics of the coarsening process has also been analysed analytically by means of a modification of Hillert's mean field theory.

Rekristallisationsverhalten von zonengeschmolzenem nickel

The process of primary recrystallization and the subsequent grain growth were investigated in high purity zone melted nickel after 60 and 80% cold work.The formation of nuclei occurred predominantly at original grain boundaries by growth of subgrains which had already been formed during the deformation process.The rate of grain growth during and after primary recrystallization was determined by observing the time dependence of the largest and of the average grain diameters. The results may be interpreted in terms of the concept that the elementary process of grain boundary motion consists of a diffusion of single atoms across the grain boundary. Then the rate determining process is the grain boundary diffusion. The activation energy of grain growth was about 28–30 kcal/Mol and was equal to the activation energy of nuclei formation for large degrees of deformation.During the primary recrystallization the time dependence of the isothermally recrystallized fraction XR of the matrix may be described by the equation xr = 1 − exp(−Btκ). The K-value of 1.38 suggests a predominantly one-dimensional grain growth.The experimental data for the rate of grain growth after primary recrystallization indicate an influence of residual impurities still present in the crystal.