Grain Boundary States Research Articles

Computer simulation of the effect of ultrasound and annealing on the structure of a two-dimensional severely deformed nanocrystalline material

Qualitative comparison of the effect of ultrasonic treatment and annealing on the structural relaxation of a two-dimensional nanocrystalline material after severe plastic deformation has been performed by the molecular-dynamics method. The treatments of both types lead to a decrease in the dislocation density and to a more equilibrium state of grain boundaries and triple junctions but, unlike the temperature effect, ultrasonic treatment does not result in a considerable grain growth. Thus, the calculations conducted indicate potentialities of the ultrasonic treatment of bulk nanomaterials with the aim of relieving internal stresses without increasing grain size.

Ultra-fast diffusion channels in pure Ni severely deformed by equal-channel angular pressing

Grain boundary self-diffusion in Ni severely deformed by equal-channel angular pressing has been investigated. The radiotracer technique was applied by using the 63Ni isotope and high-precision mechanical grinding. Ultra-fast diffusion rates, which exceed those along general high-angle grain boundaries in annealed coarse-grained Ni by orders of magnitude, were observed. Such high diffusivities were attributed to a non-equilibrium state of grain boundaries produced by severe plastic deformation. A significant downward deviation from an Arrhenius plot for ultra-fast diffusivity for temperatures above 400 K was found. This was interpreted in terms of a partial relaxation of the non-equilibrium grain boundary structure. Additionally, the formation of ultra-fast diffusion channels, presumably in the form of percolating porosity as a result of stepwise annealing of the material, was observed. This phenomenon is discussed in terms of the free volume redistribution in grain boundaries. It is proposed to quantify the excess energy of interfaces in terms of the observed enhancement of diffusivity. This approach allows the excess energy of non-equilibrium grain boundaries to be determined from diffusion measurements.

Hydrogen passivation of Fe-related deep energy levels at a direct silicon-bonded (1 1 0)/(1 0 0) grain boundary

The influence of hydrogenation on the electrical characteristics of an Fe-contaminated grain boundary (GB) formed by direct silicon bonding technology has been investigated. Due to hydrogen passivation, the density of GB states is reduced by one order of magnitude, but the energy distribution of deep levels and corresponding carrier capture cross-sections cannot be significantly affected. It is believed that the efficiency of hydrogen passivation is strongly dependent on the form of Fe contaminants at the GB.

Electronic states of graphene grain boundaries

We introduce a model for amorphous grain boundaries in graphene, and find that stable structures can exist along the boundary that are responsible for local density of states enhancements both at zero and finite (~0.5 eV) energies. Such zero energy peaks in particular were identified in STS measurements [J. \v{C}ervenka, M. I. Katsnelson, and C. F. J. Flipse, Nature Physics 5, 840 (2009)], but are not present in the simplest pentagon-heptagon dislocation array model [O. V. Yazyev and S. G. Louie, Physical Review B 81, 195420 (2010)]. We consider the low energy continuum theory of arrays of dislocations in graphene and show that it predicts localized zero energy states. Since the continuum theory is based on an idealized lattice scale physics it is a priori not literally applicable. However, we identify stable dislocation cores, different from the pentagon-heptagon pairs, that do carry zero energy states. These might be responsible for the enhanced magnetism seen experimentally at graphite grain boundaries.

Severe plastic deformation of copper: The state of grain boundaries and their triple junctions

The results of electron microscopic studies of the structure and phase composition of nanocrystalline copper produced during severe plastic deformation by torsion under high quasi-hydrostatic pressure are presented. Particular attention was focused on grain triple junctions. It is found that the triple junctions contain partial disclinations and secondary-phase particles. The strain dependences of the fraction of these triple junctions during formation of nanocrystalline copper are measured. Phase analysis of the structure of secondary phases is performed, and the sizes volume fractions of the secondary-phase particles are determined. The torsion-curvature of the crystal lattice appearing near triple junctions is measured. The problem of screening of the long-range field of stresses forming in triple junctions is discussed.

Effect of iron contamination on grain boundary states at a direct silicon bonded (110)/(100) interface

AbstractThe effect of iron contamination on the electrical property of a (110)/(100) interfacial grain boundary (GB) in a p‐type direct‐silicon‐bonded wafer has been investigated by current–voltage and capacitance–voltage characteristics. It is found that iron contamination can change the charge property of this “model” GB. Based on proper physical modeling, it is found that compared to a “clean” grain boundary, iron contamination can significantly increase the density of GB states, and the corresponding GB neutral level has also been modified. These results show us a clear physical picture of Fe contamination influencing the GB states. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Au contamination on the electrical characteristics of a “model” small-angle grain boundary in n-type direct silicon bonded wafer

We have investigated the electrical characteristics of a “model” small-angle grain boundary (GB) in n-type direct silicon bonded wafers with intentional Au contamination. It is found that the Au aggregated at the GB can cause new acceptorlike states, developing a potential barrier. The density of Au-related GB states is about 1–2×1012 cm−2 eV−1 in the energy range of Ec-0.65–Ec-0.33 eV. With the energy level becoming deeper, the corresponding electron capture cross-section becomes larger, in the order of magnitude 10−16–10−15 cm2. It is believed that Au contamination has strong influence on the electrical properties of GB. These results are interesting for the GB engineering of n-type multicrystalline silicon solar cells for terrestrial application.

Structure and properties of grain boundaries in submicrocrystalline molybdenum prepared by high-pressure torsion

Transmission electron microscopy was used to study submicrocrystalline structure and thermal stability of molybdenum prepared by severe plastic deformation at 400°C by high-pressure (6 GPa) torsion using 5 revolutions of anvils. Emission Mossbauer spectroscopy was used to study the state of grain boundaries in submicrocrystalline Mo annealed at temperatures of 350–800°C.

Influence of the grain size and structural state of grain boundaries on the parameter of low-temperature and high-rate superplasticity of nanocrystalline and microcrystalline alloys

A model has been proposed for calculating the grain size optimum for the deformation of nanocrystalline and microcrystalline materials under superplasticity conditions. The model is based on the concepts of the theory of nonequilibrium grain boundaries in metals. It has been demonstrated that the optimum grain size dopt can be calculated as the size at which a high level of nonequilibrium of grain boundaries is combined with a high intensity of the accommodation of grain boundary sliding. The dependences of the quantity dopt on the rate and temperature of the strain and the thermodynamic parameters of the material have been derived. The results obtained have been compared with the experimental data on the superplasticity of nanocrystalline and microcrystalline aluminum and magnesium alloys.

Effect of grain growth on cation exchange between dunite and fluid: implications for chemical homogenization in the upper mantle

The effect of grain growth on the cation exchange between synthesized forsterite aggregates (i.e., dunite) and nickel-rich aqueous fluid was evaluated experimentally at 1.2 GPa and 1,200°C. The grain boundary (GB) migration caused nickel enrichment in the area swept by the GBs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to ~80 times that was calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected fluid networks was found to be 6.5 × 10−19–6.7 × 10−18 m3/s, which is 4–5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper mantle.

10.1007/s11455-008-2015-5

The depth-concentration profiles of nickel upon diffusion in submicrocrystalline (SMC) molybdenum processed by severe plastic deformation (SPD) have been studied by Auger electron spectroscopy. The coefficients (D b) and activation energies of the grain-boundary diffusion of nickel in SMC molybdenum were determined in a 973–1123 K temperature interval. The results indicate that a difference between the D b values in SMC and coarse-grained molybdenum is related to a nonequilibrium state of grain boundaries in the SPD-processed metal.

The effect of scale levels of rotational plastic deformation modes on the strain resistance of polycrystals

Experiments on polycrystalline lead alloys with different states of grain volumes and boundaries under tension in a wide range of test temperatures have been performed. The qualitative and quantitative characteristics of the grain size dependence of the strain resistance of the examined materials are shown to be substantially affected by the development of rotational plastic deformation modes at different scale levels. This effect must be accounted for in the interpretation of the parameters of the Hall-Petch equation in the framework of the multiscale approach employed in physical mesomechanics.

Deformation Behavior and Plastic Strain Localization of Nanostructured Materials Produced by Severe Plastic Deformation

The literature on the deformation behavior and plastic strain localization inherent to nanostructured metallic polycrystals produced by severe plastic deformation techniques is reviewed. The effects of the texture, structure heterogeneity and state of grain boundaries on the special features and evolution of mesoscopic and macroscopic localized deformation bands are investigated. The role of grain-boundary sliding in the development of mesoscopic plastic deformation bands is discussed.

Kinetic Monte Carlo simulation of electrodeposition of polycrystalline Cu

A two-dimensional cross-sectional poly-lattice kinetic Monte Carlo (2DCSP-KMC) model has been developed for simulation of the electrodeposition of polycrystalline copper on a copper and gold substrate. The mis-orientation and higher energetic state of grain boundaries are taken into account by introducing a ‘strange’ coordination number and a correction coefficient while determining the diffusion rate. The evolution of the microstructure, the average grain size, the variance of grain size and the distribution of grain-boundary mis-orientation of the deposit are also considered in simulations. The model has proven capable of capturing some key aspects of nucleation and growth mechanism including the nucleation type (e.g. homogeneous or heterogeneous), texture development, the growth of grains and higher energetic state of grain boundaries. The simulated microstructure qualitatively agrees with our experimental observation on the copper deposition on gold.

The effect of dispersion hardening on the regularities and mechanisms of the creep of copper with submicron grain sizes

Regularities and mechanisms of the creep of submicrocrystalline copper and Cu-1.1 vol % Al2O3 dispersion-hardened powder composite in a temperature range of (0.2–0.35) T m Cu have been investigated. The role that the grain-boundary state and dispersion-hardening material with Al2O3 nanosized (10–40 nm) particles plays in the development of plastic deformation during creep is analyzed.

Influence of current-induced recrystallization on piezoresistive properties of polysilicon nanosized thin films deposited at different temperatures

To improve the accuracy and stability of piezoresistive sensors based on polysilicon nanosized thin films (PNTFs), 80nm-thick PNTFs were deposited on Si substrates by LPCVD at different temperatures and fabricated into cantilever beams. The electrical trimming characteristics and dependences of gauge factor, TCR and TCGF on resistor trim were measured. Based on interstitialvacancy model, the electrical trimming is due to the mobility increase caused by current-induced recrystallization of grain boundaries (GBs). The changes in GF, TCR and TCGF with resistor trim are due to the GB state variation (including scattering center, GB width, tunneling current, localized and extended state conductions).

Minority carrier conductive channel formed at a direct silicon-bonded interfacial grain boundary

We have demonstrated that a direct silicon-bonded interfacial grain boundary (GB) acts as an electrically conductive channel for minority carriers. This conductive channel is attributed to the formation of an inversion layer, resulting in an anomalous bright electron-beam-induced current contrast of the GB observed outside the collection diode. The charging at GB states related to interfacial dislocations and oxygen precipitates is found to cause a large potential barrier, which is responsible for the formation of an inversion layer.

Tensile behaviour of submicrocrystalline ferritic steel processed by large-strain deformation

Mechanical tests have been carried out on Fe–15%Cr ferritic stainless steel with various microstructures. Ultrafine-grained microstructures with grain sizes of 0.2–0.3 µm were developed by large-strain cold-working and light annealing. The effects of severe deformation on the mechanical behaviour of as-processed and recovered steel were evaluated with reference to the same material having conventional work-hardened and recrystallised microstructures. Despite the low dislocation density in the fine grain interiors in the as-processed state, the samples with strain-induced submicrocrystalline structure were characterised by high internal stresses that resulted in a higher strength than could be expected from simple grain-size strengthening. These internal stresses were associated with a non-equilibrium state of strain-induced grain boundaries after severe deformation.

Hydrogen passivation of deep energy levels at the interfacial grain boundary in (110)/(100) bonded silicon wafers

This letter evaluates the density of grain boundary (GB) states before and after hydrogenation by J-V, C-V, and capacitance transient methods using gold/direct silicon-bonded (DSB) (110) thin silicon top layer/(100) silicon substrate junctions. The GB potential energy barrier in thermal equilibrium was reduced by 70 meV from 0.46 eV (before hydrogenation) to 0.39 eV (after hydrogen treatment). Whereas the clean sample had a density of GB states of ∼6×1012 cm−2 eV−1 in the range of Ev+0.54–0.64 eV, hydrogenation reduced the density of GB states to ∼9×1011 cm−2 eV−1 in the range of Ev+0.56–0.61 eV, which is about a sevenfold reduction from that of the clean sample.

Synthesis and Characterization of ZnO/Glass/ZnO Structures Showing Highly Nonlinear Current-Voltage Characteristics

As part of an effort to develop low-voltage surge filters, zinc oxide (ZnO) bicrystals were investigated to clarify the electronic state of grain boundaries in ZnO varistors. The varistors had a ZnO/glass/ZnO-type sandwich structure, and the interfacial glass layer was composed of oxide glass of the Bi-B-O system. A highly nonlinear current-voltage relationship was obtained for a sandwich structure composed of Co-doped single crystal ZnO, and the current through the junction was proportional to roughly the 30th power of the bias voltage at the breakdown stage. In contrast, the junction using undoped single crystal ZnO showed insignificant nonlinearity. Dielectric measurements revealed that a double-sided depletion layer was formed at the interface and the high nonlinearity was ascribed to the corresponding potential barrier formed at the interface.