Grain Boundary Model Research Articles

Effect of Pt on agglomeration and Ge out diffusion in Ni(Pt) germanosilicide

The effect of Pt alloy in Ni(Pt∼5 and 10 at. %) on the agglomeration and Ge out diffusion in nickel germanosilicide formed on Si0.75Ge0.25(100) has been studied. A remarkable improvement in the agglomeration behavior with increasing Pt atomic percentage is observed by sheet resistance measurements while still maintaining Ni(Pt) monogermanosilicide phase between 400 and 800 °C. Ge out diffusion from the monogermanosilicide grains has been suppressed up to a temperature of 700 °C with the addition of Pt, evident by x-ray diffraction and micro-Raman spectroscopy. In addition, that improvement of surface morphology and suppression of Ge out diffusion with increasing Pt atomic percent is also confirmed by Rutherford backscattering and cross-sectional transmission electron microscopy. The improved morphology and agglomeration are explained by a grain-boundary model which includes kinetic effects. The suppression of Ge out diffusion from the germanosilicide grains is attributed to reduced atomic diffusion and the presence of stronger Pt–Si and Pt–Ge bonds due to the addition of Pt.

Atomistic Monte Carlo Simulations of Homogeneous and Heterogeneous Precipitation on Grain Boundaries of NbC in Steels

The precipitation of niobium carbides in industrial steels is commonly used to control the recrystallization process or the amount of interstitial atoms in solid solution. It is then important to understand the precipitation kinetics and especially the competition between homogeneous and heterogeneous precipitation, since both of them have been observed experimentally, depending on the alloy composition, microstructure and thermal treatments. We propose Monte Carlo simulations of NbC precipitation in α-iron, based on a simple atomic description of the main parameters which control the kinetic pathway : - realistic diffusion properties, with a rapid diffusion of C atoms by interstitial jumps and a slower diffusion of Fe and Nb atoms by vacancy jumps - a simple model of grain boundaries which reproduces the equilibrium segregation properties of Nb and C - a point defect source which drives the vacancy concentration towards its equilibrium value. Depending on the precipitation conditions, MC simulations predict different kinetic behaviours, including homogeneous and heterogeneous NbC precipitation, early segregation of C atoms and its importance as a first stage for NbC precipitation, wetting phenomena on grain boundaries and transient precipitation of metastable carbides.

Experimental observation and computer simulation on non-equilibrium grain-boundary segregation kinetics of phosphorus

An experimental study and computer simulation on non-equilibrium grain-boundary segregation kinetics and the critical time for phosphorus in 12 CrlMoV steel (which is used in steam pipeline of ships) are put forward in this paper. The segregation level of phosphorus with solution temperature 1050°C at the isothermal holding temperature of 540°C, have been measured at grain-boundaries. A non-equilibrium grain-boundary segregation kinetics curve of phosphorus is given. The critical time for phosphorus non-equilibrium grain-boundary segregation is about 500 h at 540°C for the experimental steel. When the holding time is longer than 1500 h, non-equilibrium segregation disappears and the level of segregation reaches full equilibrium. The simulation using the kinetic equations of non-equilibrium grain-boundary segregation is in good accordance with the experimental observation for phosphorus in steel 12Cr1MoV. The non-equilibrium grain-boundary segregation kinetic model is therefore proved.

Effect of interstitials on tensile strength and creep in nanostructured Ni

The tensile, creep and anelastic behavior of nanostructured nickel doped and un-doped with boron was investigated. Specimen material with an average grain size of around 30 nm produced by the pulse electrodeposition method contains impurities such as carbon, sulfur and boron. The interstitials content does not have notable impact on the tensile strength at room temperature and 373 K. But, at 473 K, the minor change in sulfur content from 0.03 to 0.061 at.% raises the ultimate strength by 150 MPa while the boron doping further improves the tensile strength. On the other hand, with increasing sulfur content in nanostructured Ni, the ductility decreases. All the specimens exhibit significant anelastic relaxation from room temperature to 473 K. The creep test results show that both minimum creep rate and creep strain significantly decrease with increasing sulfur or by doping boron in nanostructured nickel. The stress exponent in the expression of Coble-type creep increases to around five at 373 and 473 K from two at room temperature. A model for grain boundary sliding, in which grain boundary dislocations and back stress are introduced, has successfully explained the large stress exponents. The calculated back stress indicates that the interstitials in grain boundaries effectively retard the sliding of grain boundary dislocations.

Ab initio theoretical tensile test on Y-doped Σ = 3 grain boundary in α-Al 2O 3

It has been known for a long time that Y ions segregate to the grain boundaries (GBs) in polycrystalline alumina with the beneficial effects of enhanced mechanical properties and increased creep resistance. No detailed microscopic theory exists to explain this so called “Y-effect”. We provide a quantum mechanical explanation for this effect through a series of carefully designed large-scale computations. The results of our theoretical tensile experiments show that the maximum stresses in pure crystalline α-Al 2O 3, undoped Σ = 3 GB, and Y-doped Σ = 3 GB models are, respectively 55, 31 and 39 GPa at the uni-axial strains of 17%, 14% and 16% in the direction perpendicular to the GB. The participation of the Y-4d and Y-4p orbitals enhances the covalent character of the Y–O bond, making it stronger than the Al–O bond it replaces. This could be the major reason for the improved mechanical properties of Y-doped α-Al 2O 3.

Investigation of electrical properties of tantalum doped SnO 2 varistor system

Ta 2O 5 doped SnO 2 varistor systems containing 0.5 mol% ZnO and 0.5 mol% CoO were prepared by mixed oxide method. Considering that ZnO and CoO oxides are densification additives only the SnO 2·ZnO·CoO ceramics cannot exhibit electrical nonlinearity. A small amount of Ta 2O 5 improves the nonlinear properties of the samples greatly. The height and width of the defect barriers were calculated. It was found that samples doped with 0.05 mol% Ta 2O 5 exhibit the highest density (98.5%), the lowest electric breakdown field ( E b = 1100 V/cm) and the highest coefficient of nonlinearity ( α = 11.5). The effect of Ta 2O 5 dopant could be explained by the substitution of Ta 5+ by Sn 4+. A grain-boundary defect barrier model for the SnO 2·ZnO·CoO·Ta 2O 5 varistor system was also introduced.

Scanning probe microscopy study of grain boundary migration in NiAl

A post-mortem scanning probe microscopy (SPM) study of grain boundary migration in Ni-rich NiAl at 1400 °C is presented. The migration of grain boundaries during annealing is quantified using the SPM measurements of surface topography of the regions swept by migrating grain boundaries. It is shown that the quantitative conclusions about the dynamics of grain boundary motion can be drawn from the study of surface topography in the vicinity of both individual migrated boundaries and migrated triple junctions. In the case of individual boundaries, the curvature of the blunted root of the grain boundary groove formed at original boundary position provides the information about the beginning of migration process. In the case of triple junctions that moved along one of three boundaries forming the junction, the variable width of the grain boundary groove allows to recover the dynamics of migration process. Using the Mullins model of grain boundary grooving and its modifications we estimated that the grain boundary migration rate is 0.52 ± 0.09 μm/s. This is much higher than the average migration rate obtained by dividing the migrated distance by total annealing time. It is concluded that in the near-surface region of NiAl the grain boundaries migrate in jerky, spasmodic fashion.

Grain-boundary sliding and separation in polycrystalline metals: application to nanocrystalline fcc metals

In order to model the effects of grain boundaries in polycrystalline materials we have coupled a crystal-plasticity model for the grain interiors with a new elastic–plastic grain-boundary interface model which accounts for both reversible elastic, as well irreversible inelastic sliding-separation deformations at the grain boundaries prior to failure. We have used this new computational capability to study the deformation and fracture response of nanocrystalline nickel. The results from the simulations reflect the macroscopic experimentally observed tensile stress–strain curves, and the dominant microstructural fracture mechanisms in this material. The macroscopically observed nonlinearity in the stress–strain response is mainly due to the inelastic response of the grain boundaries. Plastic deformation in the interior of the grains prior to the formation of grain-boundary cracks was rarely observed. The stress concentrations at the tips of the distributed grain-boundary cracks, and at grain-boundary triple junctions, cause a limited amount of plastic deformation in the high-strength grain interiors. The competition of grain-boundary deformation with that in the grain interiors determines the observed macroscopic stress–strain response, and the overall ductility. In nanocrystalline nickel, the high-yield strength of the grain interiors and relatively weaker grain-boundary interfaces account for the low ductility of this material in tension.

Modeling of grain boundary barrier modulation in ZnO invisible thin film transistors

We model grain boundaries (GBs) for polycrystalline ZnO thin film transistors (TFTs). Experimental result shows a non-linear increase of drain current and gradual enhancement of field effect mobility with increasing gate bias. Our initial single GB model was unable to explain the experimentally obtained results, where we considered the peak defect distribution at the mid gap. Realizing from the experimentally obtained results, we remodeled the grain boundary considering the peak distribution close to the conduction band, which then better replicates the experimental observation. We describe here the transfer characteristic of experimental ZnO TFT in linear region with calculated potential profiles. Appropriate grain boundary modeling signifies that the slower decrease in potential barrier in grain boundary with applied gate voltage is responsible for such non-linear changes in drain current and gradual enhancement of mobility.

Anomalous grain boundary physics in polycrystalline CuInSe2: the existence of a hole barrier.

First-principles modeling of grain boundaries (GB) in CuInSe2 semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB. Consequently, the absence of holes inside the GB prevents GB electrons from recombining. At the same time, the GI is purer in polymaterials than in single crystals, since impurities segregated to the GBs. This explains the puzzle of the superiority of polycrystalline CuInSe2 solar cells over their crystalline counterpart. We identify a simple and universal mechanism for the barrier, arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction. This discovery opens up possibilities for the future design of superior polycrystalline devices.

Roles of Alumina in Zirconia for Functional Applications

The Al2O3 addition to stabilized ZrO2 has been studied for more than 20 years. In this article, literature and new results on the positive and negative effects of Al2O3 additions on the electrical properties of ZrO2 are summarized and analyzed. In particular, a comprehensive grain‐boundary conduction model is proposed. The Al2O3 addition always increases the bulk resistivity, mainly because of the formations of defect associates and insulating Al2O3 second‐phase particles. The Al2O3 addition within the solubility limit increases the grain‐boundary resistivity, as a result of increased grain‐boundary space‐charge potential; the Al2O3 addition above the solubility limit, however, scavenges the silicon‐rich second phase from the grain boundaries, thereby decreasing the grain‐boundary resistivity.

Carrier transport characteristics in PbSrSe thin films grown by molecular beam epitaxy

The dark-current characteristics of PbSrSe thin films grown by molecular beam epitaxy on BaF2 substrates with Sr composition from 0.066 to 0.276 have been measured systematically under different temperatures from 77 K to 300 K. The carrier-transport characteristics have been explained on the basis of a grain-boundary barrier model. The barrier height is found to be strongly related to the Sr composition. The different conductance behavior among the PbSrSe thin films is due to the variation of the grain-boundary barrier. Both the experimental barrier height, determined from the temperature-dependent conductance, and the theoretical results, deduced from the Poisson equation, reveal that the barrier height decreases with increasing applied bias. Furthermore, the success in explaining the observed negative-capacitance phenomenon gives further evidence that the accumulation of electrons at the grain boundaries plays a key role in the carrier transport of the PbSrSe thin films.

Thickness dependence of resistivity for Cu films deposited by ion beam deposition

The thickness dependence of the resistivity for Cu films deposited by ion beam deposition (IBD) was evaluated using Fuchs–Sondheimer (F–S) model for electron surface scattering and Mayadas–Shatzkes (M–S) model for electron grain boundary scattering. For fitting the F–S and M–S models to the experimental data, the approximate equations proposed in both models were discussed and it was confirmed that the experimental resistivity of the Cu films could be described well by a simple form combined of the approximate equations for both models. By means of the simple form in this work, the most reasonable fit to the experimental data could be obtained under the conditions of the surface scattering coefficient p=0 and the reflection coefficient at grain boundary R=0.40.

Modeling of transport in polycrystalline organic semiconductor films

We propose a grain-boundary barrier model with an energy distribution of interfacial traps to describe charge transport in polycrystalline organic thin films. The model is applied to the interpretation of charge transport in unintentionally doped pentacene films. It gives an acceptable explanation for the concomitant increase in threshold voltage and mobility, and allows an understanding of the difference between the dopant-concentration and gate-voltage dependences of the mobility.

FIR and near-millimetre dielectric response of SrTiO3, BaTiO3 and BST films and ceramics

Abstract The dielectric response of Ba 1− x Sr x TiO 3 ceramics and films ( x =0, 0.9, 1) was studied with a particular attention to the soft mode behaviour in the far IR and near-mm range and its comparison with the single crystal behaviour. To analyse the response quantitatively, we used commercial Fourier transform IR and monochromatic backward-wave-oscillator spectroscopy in both transmission and reflection modes and a rigorous evaluation. The soft-mode behaviour is extremely sensitive to the sample quality, particularly in films, and correlates with the lower low-frequency response. The most important factors for this behaviour in the polycrystalline films are grain boundaries and porosity and nano-cracks, which often appear in thicker polycrystalline films along some of the grain boundaries. The brick-wall model for grain boundaries and cracks discussed within the generalized effective medium approximation appears to be appropriate for describing the observed phenomena. In quasi-epitaxial films the macroscopic tensile stress, which influences the phase diagram very sensitively and induces ferroelectricity even in the pure SrTiO 3 , seems to play the most important role. From the soft-mode behaviour and its splitting, it appears that in the studied BaTiO 3 films all the phase transitions, as in single crystals, seem to be present, but smeared with phase coexistence regions down to 10 K. In the quasi-epitaxial BST-0.9 film a smeared ferroelectric transition appears near 150 K.

多結晶体の変形に及ぼす結晶粒と粒界すべりの影響

Polycrystalline materials are composed of great amount of crystal grains, each of which is joined with neighboring grains. Since the slip system of each crystal grain is randomly distributed over the domain, the materials show mechanical inhomogeneity. This paper discusses the effects of grain size, the resistance of crystal grain, as well as the grain boundary sliding on the deformation behavior of polycrystal. Finite element analyses are carried out with reference to the combined model of crystal plasticity and grain boundary sliding. It is shown that both the size and the flow stress parameters are dominant on the inhomogeneity and that the deformation around the grain boundary is relaxed by the grain boundary sliding.

Nonlinear electrical characteristics and dielectric properties of Ca,Ta-doped TiO 2 varistors

Ca,Ta-doped TiO2 varistors with high nonlinear coefficients are obtained by a ceramic sintering. The nonlinear electrical and dielectric properties of the samples doped with 0.5mol% Ca and various concentrations of Ta (0.05∼2.0mol%) were investigated. The samples sintered at 1350 °C have nonlinear coefficients of α=5.1∼42.1 and high relative dielectric constants approach 105. The effects of Ta-doping on the nonlinear and dielectric properties of the Ca,Ta-doped TiO2 varistors are studied in greater detail. When the concentration of Ta is 0.5mol%, the sample possesses the highest nonlinear coefficient and a comparatively lower dielectric constant. The effects of Ta and the nonlinear electrical behavior of the TiO2 system are explained by analogy to a grain-boundary atomic defect model.

Nonlinear electrical characteristics and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO2 ceramics doped with 1.0 mol% Ca and different concentrations of Ta were obtained by sintering processing at 1450°C. The microstructures, nonlinear electrical behavior and dielectric properties of the ceramics were investigated. The samples have nonlinear coefficients of α = 2.0–5.0 and ultrahigh relative dielectric constants which is up to 105. Especially, the effects of Ta dopant on the nonlinear electrical characteristics and dielectric properties of the (Ca, Ta)-doped TiO2 ceramics were studied in detail. When the concentration of Ta is 2.0 mol%, the sample exhibits the highest nonlinear coefficient and a comparatively lower dielectric constant. By analogy to a grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO2 system were explained.

Nonlinear Electrical and Dielectric Properties of (Ca, Ta)-Doped TiO2 Varistors

(Ca, Ta)-doped TiO2 varistors with high nonlinear coefficients were obtained by ceramic sintering process. The nonlinear electrical and dielectric properties of the samples doped with 1.0 mol% Ca and different concentrations of Ta (0.05-2.0 mol%) were investigated. Especially, the effects of Ta dopant on the nonlinear and dielectric properties of the (Ca, Ta)-doped TiO2 varistors were studied in detail. When the concentration of Ta is 0.5 mol%, the sample possesses the highest nonlinear coefficient (16.6). By analogy to a, grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO2 system were explained.

Experimental study on non-equilibrium grain-boundary segregation kinetics of phosphorus in an industrial steel

The non-equilibrium grain-boundary segregation level of phosphorus in an industrial steel, 12Cr1MoV, was measured. A non-equilibrium grain-boundary segregation kinetics curve of phosphorus is given, which provides direct evidence of the non-equilibrium grain-boundary segregation kinetic model proposed by Xu Tingdong.