Grain Boundary Position Research Articles

Phase-Field Simulation of Spinodal Decomposition in U-50Zr Metallic Nuclear Fuel.

During the γ phase-δ phase transition, U-50Zr fuel experiences spinodal decomposition, which has a significant effect on fuel properties. However, little is known about the spinodal decomposition of U-50Zr. The spinodal decomposition behavior in U-50Zr is studied in this research using the phase-field approach. The mechanism of spinodal decomposition from a thermodynamic perspective is studied, and the effects of temperature and grain boundary (GB) on spinodal decomposition are analyzed. It is found that the concentration of U atoms in the U-rich phase formed during spinodal decomposition is as high as 90%. The U-rich phase first appears at the GB position, and precipitation phases appear inside the grain later. Ostwald ripening occurs when larger precipitation phases on the GB absorb isolated smaller precipitation phases inside the grain. The coarsening rate of precipitation phases and the time it takes for spinodal decomposition to achieve equilibrium are both influenced by temperature.

Effects of grain boundaries on the evolution of radiation-induced bubbles in polycrystalline tungsten: A phase-field simulation

Tungsten (W) is widely recognized as a promising material for plasma-facing components in nuclear fusion reactors, and the evolution of bubbles in polycrystalline tungsten significantly impacts its properties. This study developed a phase-field model considering elastic inhomogeneity to investigate the evolution of radiation-induced bubbles. The Gibbs free energy of helium bubbles was derived using the Van der Waals equation. An explicit nucleation algorithm accounting for the position of grain boundaries (GBs) was coupled to the model. Furthermore, the model also considered the inequality between the internal pressure of the bubble and the surface tension. The growth kinetics of a single bubble with elastic heterogeneity were examined, and based on these findings, the evolution of bubbles under irradiation was simulated, yielding results consistent with experimental observations. Furthermore, the influence of GBs on radiation-induced bubbles was investigated using the developed model. The simulation results demonstrated that both GBs, as preferential nucleation regions and as sinks for vacancy absorption, had an impact on bubble nucleation and growth. The superior radiation resistance observed in nanocrystalline tungsten was dominantly attributed to the absorption of vacancies by GBs.

DC Performance Variations by Grain Boundary in Source/Drain Epitaxy of Sub-3-nm Nanosheet Field-Effect Transistors

Grain boundary (GB) at the source/drain (S/D) epitaxy was investigated using fully-calibrated TCAD. Because the S/D epi is grown separately at the bottom and the NS channels, nanosheet field-effect transistors (NSFETs) have unwanted GB within the S/D epi which fully relaxes the channel stresses. This GB changes the doping profiles and the stress values, which thus degrade the DC performances. We focused on single GB with different inclined angles and positions. N-type NSFETs have similar DC performances regardless of the GB since the changes of doping and stress were small. P-type NSFETs suffer from DC performance degradations but different depending on the GB positions. As the GB splits the p-type S/D epi into two, lower S/D below the GB has tensile stress and upper S/D above the GB has compressive stress. Since tensile stress increases boron diffusivity, more boron dopants diffuse into the NS channels as the device has lower S/D region, thus suffering the short channel effects greatly. The device having upper S/D region loses the channel stress much, so it degrades the on-state performance. This study provides clear understanding of the GB effects of NSFETs.

Grain boundary heredity from Cu/Al solid–liquid interface via diffusion during the solidification processes

• The grain boundary heredity of Cu/Al solid–liquid system is found after the cooling. • The geometric structure information of Cu GB can be reflected in CuAl regions. • The cooling rate has a great influence on the the position of new formed GB. • The phase angle of GB has a great influence on the final crystal structure. Using molecular dynamics (MD) simulations with the embedded atomic method (EAM), we explored the solidification process of Cu/Al solid–liquid interface where Cu has complex grain boundary structures. The simulation results show that if there is a vertical grain boundary in solid Cu, the Cu atoms near the grain boundary diffuse into the liquid Al region and form a horizontal grain boundary of CuAl after cooling, that is grain boundary heredity. The underlying physics of such grain boundary heredity is attributed to different diffusion rate of Cu atoms close to the grain boundary, and the geometric structure information of Cu grain boundary can be reflected in CuAl regions. The position of horizontal grain boundary can be dramatically influenced by the cooling rate. Our work offers insight into the defect and structural evolution during alloy solidification process, which can provide new approach to designing desirable alloy compounds.

Stabilizing effects of Ag doping on structure and thermal stability of FeN thin films

In this work, we investigated the effect of Ag doping (2–20 at.%) on the phase formation of iron mononitride (FeN) thin films. Together with deposition of FeN using reactive dc magnetron sputtering, Ag was also co-sputtered at various doping levels between 2–20 at.%. We found that doping of Ag around 5 at.% is optimum to not only improve the thermal stability of FeN but also to reduce intrinsic defects that are invariably present in (even in epitaxial) FeN. Conversion electron Mössbauer spectroscopy and N K-edge x-ray near edge absorption measurements clearly reveal a reduction of defects in Ag doped FeN samples. Moreover, Fe self-diffusion measurements carried out using secondary ion mass spectroscopy depth-profiling and polarized neutron reflectivity in 57Fe enriched samples exhibit an appreciable reduction in Fe self-diffusion in Ag doped FeN samples. Ag being immiscible with Fe and non-reactive with N, occupies grain-boundary positions as nanoparticles and prohibits the fast Fe self-diffusion in FeN.

A Method to Quantify the Collective Impact of Grain Boundaries on the Internal Quantum Efficiency of Multicrystalline Silicon Solar Cells

Herein, a method to quantify the amount of reduction in the internal quantum efficiency (IQE) of multicrystalline silicon solar cells that can be attributed to grain boundaries is presented. By correlating the IQE maps obtained via light beam induced current (LBIC) topography with optical images of Secco‐etched samples, the distribution of IQE values at the positions of grain boundaries can be compared with the distribution of IQE values for all other positions where other defects may exist. The segmentation of IQE at 826 nm maps of the samples shows grain boundaries to be more detrimental compared with the combined effects of all other defects that may exist in other regions of the cell. The grain boundaries reduce the average IQE by up to 4.07% absolute for the cell from the bottom of the ingot.

A New Method for Defect Prediction of Polycrystalline Silicon TFTs with Realistic Grain Boundary Model

We investigate the deviation of threshold voltage (V<SUB>th</SUB>) of p-channel polycrystalline silicon thin film transistors (poly-Si TFTs) using a new GB (grain boundary) model that better reflects reality. And we proposed new method for defect prediction using by this new GB model. The new GB model reflects a gaussian distribution of grain size and GB position. And by introducing a new parameter (β) which represents the defect ratio between GB and grain, the Vth deviation that increases as the channel gets shorter can be matched with the actual measurement results. And we found that the increase in the Vth deviation in the short channel becomes larger as the number of defects increases in GB (as the β increases). And we found that the β is an important physical parameter to explain why the Vth deviation of the short channel is rapidly increased. In this way, using this β value in our GB model, it is possible to predict the relative density of defect states of GB in the poly-Si by monitoring the Vth deviation in short channel TFTs in the process of developing polycrystalline TFTs.

The effect of grain boundaries on instability at the crystal/melt interface during the unidirectional growth of Si

The instability of crystal/melt interfaces including numerous Σ3, small-angle and large-angle grain boundaries during the unidirectional growth of Si was investigated, using an in situ observation system. Planar crystal/melt interfaces including Σ3 or small-angle grain boundaries were found to undergo local morphological transformation at the grain-boundary positions during high-velocity growth. The data also confirmed that the critical growth velocity for the appearance of instability at a crystal/melt interface including grain boundaries is significantly lower than that for a single crystal Si/melt interface. The ambient temperature field at the crystal/melt interface was calculated, taking into account the effective thermal conductivities of grain boundaries and the results show that a negative temperature gradient readily forms in the case that grain boundaries are present.

The influence of grain boundary interface traps on electrical characteristics of top select gate transistor in 3D NAND flash memory

The electrical characteristics of top select gate transistor (TSG) has been investigated in vertical channel three dimensional NAND flash memory. TSG shows wider initial Vth distribution as compared with memory cells, and even worse after erase. By experimental analysis and TCAD simulation, a physical model based on grain boundary (GB) interface traps is proposed to explain the mechanism. Grain boundary traps in offset region between bit line contact and TSG can induce a higher local potential barrier in channel, which results in higher TSG initial Vth. Besides, random GB position within offset region, leads to worse variation of TSG initial Vth. Furthermore, the local potential barrier in offset region of TSG cannot be reduced by erase operation, leading to worse Vth distribution after erase. According to proposed model, two methods with optimization of offset doping energy and poly-Si GB trap passivation condition are proposed to achieve tight distribution and improved erase uniformity.

Improved Laplacian Photoluminescence image evaluation regarding the local diode back voltage distribution

Laplacian photoluminescence-based local diode voltage evaluation was shown recently to lead to correct mean values of the local saturation current density in mm-sized regions, but local maxima in the positions of recombination-active grain boundaries appear overestimated. It is shown here by 2-D device simulations that this effect is at least partly due to the influence of the local diode back voltage, which is caused by the voltage drop at the bulk and back contact resistances. Visually the image of this back voltage appears like a blurred copy of the local diode current density. It is shown in this work that indeed the diode back voltage can be simulated in good approximation by blurring the diode current image, which comes out of the Laplacian evaluation, multiplied with an effective vertical bulk resistance. The corresponding point spread function can be obtained e.g. by device simulation. An iterative procedure is proposed leading to self-consistent results for the diode current density and the diode back voltage. If this method is applied to simulated local cell data, the assumed distribution of the saturation current density is retrieved accurately. Applying this method to measured photoluminescence images leads to a better correspondence to non-linear Fuyuki PL evaluation results than the previously performed direct evaluation of the local diode voltage data. Remaining differences will be discussed.

Implementing Transmission Electron Backscatter Diffraction for Atom Probe Tomography

There are advantages to performing transmission electron backscattering diffraction (tEBSD) in conjunction with focused ion beam-based specimen preparation for atom probe tomography (APT). Although tEBSD allows users to identify the position and character of grain boundaries, which can then be combined with APT to provide full chemical and orientation characterization of grain boundaries, tEBSD can also provide imaging information that improves the APT specimen preparation process by insuring proper placement of the targeted grain boundary within an APT specimen. In this report we discuss sample tilt angles, ion beam milling energies, and other considerations to optimize Kikuchi diffraction pattern quality for the APT specimen geometry. Coordinated specimen preparation and analysis of a grain boundary in a Ni-based Inconel 600 alloy is used to illustrate the approach revealing a 50° misorientation and trace element segregation to the grain boundary.

Mechanical properties of interface layers between cosintered alumina and spinel compacts

The main aim of this study is to investigate the mechanical and structural properties of the interface layer (interlayer) between alumina and spinel materials after high temperature thermal treatment (1600°C). A microstructurally distinct layer with columnar grains of up to 50 μm length was detected by SEM. Mechanical properties, such as indentation hardness and elastic modulus of the interface layer, parent alumina and spinel parts, were measured and compared by nanoindentation method. According to the results, several microstructural factors including initial alumina raw powders, the intrinsic microstructural inhomogeneity of the interlayers, such as presence of porosities, unresolved hard alumina phase in the microstructures, size of columnar grains and position of grain boundaries were proposed to affect the mechanical properties. Different micromechanical results between the regions of the samples were also attributed to generation of residual compressive stress during grinding and polishing of the surfaces.

Strain Accumulation in TiAl Intermetallics via High-resolution Digital Image Correlation (DIC)

Novel manufacturing process are more and more used to produce advanced structural materials such as the gamma titanium aluminide alloys (γ-TiAl). In this work we examine a Ti-48Al-2Cr-2Nb alloy obtained with an additive manufacturing technique by Electron Beam Melting (EBM) by conducting monotonic and cyclic loading experiments both on tension and compression samples to investigate the influence of the microstructure in strain accumulation process by fatigue loading. The residual strain maps corresponding to different applied stress levels, number of cycles and microstructures are obtained through the use of high- resolution Digital Image Correlation (DIC). The strain maps were overlaid with the images of the microstructure and detailed analyses were performed to investigate the features of the microstructure where high local strain heterogeneities arise. Such experiments, conducted ex-situ at room temperature, allow to characterize the effect of different microstructures on the strain accumulation process, providing additional information into the effect of the lamellar and equiaxed grains and also to capture the evolution of the local deformation process for TiAl. The measure of the residual strains provides further information on the role of the intermetallic phases on the fatigue behavior of γ-TiAl alloys. The comparison with the strain accumulation in fully lamellar microstructure with larger grain size permits to highlight the influence of the position of grain boundaries and the orientation of the lamellae for the onset of fatigue cracking. The analysis and comparison of the strain maps provide information for the selection of the microstructural parameters during material design (i.e. grain size and lamellar grains volume fraction).

Cellular Mechanisms of Enhanced Osteoblasts Functions via Phase‐Reversion Induced Nano/Submicron‐Grained Structure in a Low‐Ni Austenitic Stainless Steel

AbstractWe elucidate here the fundamental principles underlying the modulation of osteoblasts functions in stainless steel biomedical devices achieved by nanoscale/submicron grain structure obtained through the novel concept of phase reversion in a low Ni bearing 15Cr–9Mn–1.7Cu steel. Interestingly, a comparative investigation of nano/submicron (N‐SM) and coarse‐grained (CG) structure under identical conditions indicated that cell attachment, proliferation, and viability are favorably enhanced in N‐SM grained structure and significantly different from the CG structure. These observations were further confirmed by expression levels of vinculin and associated actin cytoskeleton. Computational analysis of immunofluorescence micrographs suggested increased vinculin concentration associated with actin stress fibers in the outer regions of the cells and cellular extensions, implying enhanced cell–substrate interactions on the N‐SM grained substrate. The favorable enhancement of osteoblasts functions and cellular attachment on N‐SM grained surface is attributed to ultrafine grain size, i.e., the availability of greater open lattice in the position of high angle grain boundaries, and high hydrophilicity. The integration of cellular and molecular biology with material science and engineering as described here provides a route to modulate cellular and molecular reactions in promoting osteoinductive signaling of surface adherent cells. The end outcome of the study is that stainless steels with low Ni contents in comparison to the conventionally used bioimplant with 10–13 wt%Ni, as specially processed, exhibit desired, enhanced cell functions, and bulk properties.

Numerical study on the dependence of ZnO thin-film transistor characteristics on grain boundary position

The dependence of transistor characteristics on grain boundary (GB) position in short-channel ZnO thin film transistors (TFTs) has been investigated using two-dimensional numerical simulations. To simulate the device accurately, both tail states and deep-level states are taken into consideration. It is shown that both the transfer and output characteristics of ZnO TFTs change dramatically with varying GB position, which is different from polycrystalline Si (poly-Si) TFTs. By analysing the mechanism of the carrier transportation in the device, it is revealed that the dependence is derived from the degrees of carrier concentration descent and mobility variation with GB position.

Grain-Boundary Fluctuations in Two-Dimensional Colloidal Crystals

We study grain-boundary fluctuations in two-dimensional colloidal crystals in real space and time using video microscopy. The experimentally obtained static and dynamic correlation functions are very well described by expressions obtained using capillary wave theory. This directly leads to values for the interfacial stiffness and the interface mobility, the key parameters in curvature-driven grain-boundary migration. Furthermore, we show that the average grain-boundary position exhibits a one-dimensional random walk as recently suggested by computer simulations [Z. T. Trautt, M. Upmanyu, and A. Karma, Science 314, 632 (2006)]. The interface mobility determined from the mean-square displacement of the average grain-boundary position is in good agreement with values inferred from grain-boundary fluctuations.

Biological significance of nanograined/ultrafine-grained structures: Interaction with fibroblasts

Given the need to develop high strength/weight ratio bioimplants with enhanced cellular response, we describe here a study focused on the processing–structure–functional property relationship in austenitic stainless steel that was processed using an ingenious phase reversion approach to obtain an nanograined/ultrafine-grained (NG/UFG) structure. The cellular activity between fibroblast and NG/UFG substrate is compared with the coarse-grained (CG) substrate. A comparative investigation of NG/UFG and CG structures illustrated that cell attachment, proliferation, viability, morphology and spread are favorably modulated and significantly different from the conventional CG structure. These observations were further confirmed by expression levels of vinculin and associated actin cytoskeleton. Immunofluorescence studies demonstrated increased vinculin concentrations associated with actin stress fibers in the outer regions of the cells and cellular extensions on NG/UFG substrate. These observations suggest enhanced cell–substrate interaction and activity. The cellular attachment response on NG/UFG substrate is attributed to grain size and hydrophilicity and is related to more open lattice in the positions of high-angle grain boundaries.

Influence of grain boundaries on current collection in Cu(In,Ga)Se 2 thin-film solar cells

Electron backscatter diffraction (EBSD) in combination with electron beam-induced current (EBIC) measurements in a scanning electron microscope were used to investigate grain boundaries in Cu(In,Ga)Se 2 thin-film solar cells. The measurements were performed on polished cross sections of working devices. EBIC maps enable the analysis of charge-carrier collection with a high spatial resolution and the extraction of the local minority charge-carrier diffusion length. EBSD images reveal the microstructure of the Cu(In,Ga)Se 2 absorber-layer. A combination of these techniques makes it possible to investigate the influence of the microstructure of the Cu(In,Ga)Se 2 absorber-layer and especially of grain boundaries on charge-carrier collection. It is shown that collection properties are grain specific and that there are positions of grain boundaries exhibiting a reduced EBIC signal. These grain boundaries might be regions of enhanced recombination with a recombination velocity of about 1 × 10 4 cm/s.

Analysis and visualization of multiply oriented lattice structures by a two-dimensional continuous wavelet transform

The phase-field-crystal model [K. R. Elder and M. Grant, Phys. Rev. E 70, 051605 (2004)] produces multigrain structures on atomistic length scale but on diffusive time scales. Since individual atoms are resolved but are treated identically it is difficult to distinguish the exact position of grain boundaries and defects within grains. In order to analyze and visualize the whole grains a two-dimensional wavelet transform has been developed, which is capable of extracting grain boundaries and the lattice orientation of a grain relative to a laboratory frame of reference. This transformation makes it possible not only to easily visualize the multigrain structure, but also to perform exact measurements on low- and high-angle boundaries, grain size distributions and boundary-angle distributions, which can then be compared to experimental data. The presented wavelet transform can also be applied to results of other atomistic simulations, e.g., molecular dynamics or granular materials.

Simulation Study of the Dependence of Submicron Polysilicon Thin-Film Transistor Output Characteristics on Grain Boundary Position

We investigate the impact of varying the grain boundary (GB) position on the output (Id–Vd) characteristics of submicron single GB polysilicon thin film transistors (TFTs), by two-dimensional (2D), drift-diffusion based, device simulation. We employ a localized GB trapping model with a distribution of both donor-like and acceptor-like trap states over the forbidden energy gap of the GB region. We show that for devices with channel lengths in the deep submicron regime, significant variations in output conductance (gd) occur as the GB position is varied. Specifically, we find that output conductance increases as the GB approaches the drain edge. Furthermore, the sensitivity of output conductance to the GB position increases as channel length decreases. The findings have important implications for any future analogue three-dimensional (3D) IC design that uses polysilicon as a device material.