Grain Boundary Sites Research Articles

Базовые элементы структуры границ зерен наклона. Часть I. Ось разориентации [100

The computer simulation methods have been applied to calculate structure and energy of symmetric tilt grain boundaries with the misorientation axis [100]. The calculations have been carried out with the use of the structural-vacancy model previously developed by the authors. The misorientation angles of grain boundaries of common type have varied from 0º up to 90º while the increment has amounted 1º. The reverse density of coincidence sites in special grain boundaries has amounted Σ ≤ 53. The calculations have been carried out with the use of the Morse pair potential and the Cleri-Rosato many-body potential. It is shown that the dependence of the energy of grain boundaries on the misorientation angle when calculated with different potentials has a similar form, and their structure does not depend on the choice of potential and is in good agreement with high-resolution electron microscopic images. Only one special grain boundary is distinguished on the energy curve which is Σ5(013). It has been found that the structure grain boundaries may be represented by a limited number of atomic groups that have been named basic structural units. The structure of low-angle grain boundaries with the misorientation angle less than 8º is described by alternation of basic structural units of D types and ideal crystal, while the structure of low-angle grain boundaries with the misorientation angle ranging from 8º up to 13º is described by alternation of structural units of C and D types, from 14º up to 23º – B and C types. High-angle grain boundaries in the range of misorientation angles from 24º up to 37º contain only of basic structural units of B types, from 38º to 50º – A and B, more than 50º – only A.

Revealing the factors influencing grain boundary segregation of P, As in Si: Insights from first-principles

Phosphorous (P) and arsenic (As) segregation at grain boundaries (GBs) usually deteriorate the electrical performance of n-type doped Si-wafers. In order to probe the factors influencing P and As segregation behaviors along Si GBs, a systematic investigation upon the interactions between P, As atoms and a series of coincidence site lattice Si GBs, including Σ3 {111}, Σ9 {221}, Σ27 {552}, Σ3 {112}, was carried out via first-principles calculations. It is revealed that the segregation behaviors of P, As along different GBs are different, which is dependent on both GB characteristics, i.e. the intrinsic lattice distortion, number of dangling/extra bonds, deep levels in the bandgap of density of states, and also the impurity properties. It reveals that smaller P atoms tend to segregate to the compressed atomic sites in GBs, by which the intrinsic GB lattice distortion is reduced. However, GB lattice distortion hardly induce As segregation owing to the similar atomic size between As and Si atoms. Calculations also indicate that under-coordinated core sites with dangling bond along GBs are remarkably attractive for P and As segregation, as a result of the nature of group-V elements to be three-coordinated rather than four-coordinated. Furthermore, GBs having deep levels in the bandgap of density of states, produced either by dangling or extra bonds, show a strikingly high attractive strength for both P and As segregation. The present work is supposed to provide important insights on substitutional impurity segregation along GBs in multi-crystalline Si in the atomic and electronic level.

Grain-boundary segregation of 3d-transition metal solutes in bcc Fe: ab initio local-energy and d-electron behavior analysis

We present ab initio calculations of grain-boundary (GB) segregation of the series of 3d transition-metal (TM) solutes in bcc Fe, taking advantage of the local-energy analysis. For 11(3 3 2) and 3(1 1 1) symmetrical tilt GBs, the segregation behaviors of 3d-TM solutes can be classified into three groups. The early TMs (Sc, Ti and V) are preferentially segregated to the looser sites of GBs antiferromagnetically, the late TMs (Co, Ni and Cu) are preferentially segregated to the tighter sites of GBs ferromagnetically, and the middle TMs (Cr and Mn) are segregated antiferromagnetically without fixed site preference. TMs at both ends of the 3d series show larger segregation-energy gains, while Mn shows a cusp at the center, which is similar to the ab initio interaction energies between the 3d-TM solutes and a screw-dislocation core in bcc Fe. By the local-energy analysis combined with the local densities of states, the segregation of the early TMs is mainly attributed to the stabilization of surrounding Fe atoms by the TM solute at the looser sites of GBs, and that of the late TMs is mainly attributed to the stabilization of the TM solute itself from bulk Fe to GB sites and the destabilization of Fe atoms around the TM solute in bulk Fe. The cusp at Mn is mainly caused by the destabilization of Fe atoms around the Mn solute in bulk Fe, due to nearly-localized high-spin d states of Mn, in contrast to substantial d-d hybridization for Mn in GBs. For each group of 3d-TM solutes, the effects on the magnetic and mechanical properties of Fe GBs are also analyzed by the d-electron behavior in common with the segregation mechanism.

Bubble evolution in Kr-irradiated UO2 during annealing

Transmission electron microscopy observation of Kr bubble evolution in polycrystalline UO2 annealed at high temperature was conducted in order to understand the inert gas behavior in oxide nuclear fuel. The average diameter of intragranular bubbles increased gradually from 0.8 nm in as-irradiated sample at room temperature to 2.6 nm at 1600 °C and the bubble size distribution changed from a uniform distribution to a bimodal distribution above 1300 °C. The size of intergranular bubbles increased more rapidly than intragranular ones and bubble denuded zones near grain boundaries formed in all the annealed samples. It was found that high-angle grain boundaries held bigger bubbles than low-angle grain boundaries. Complementary atomistic modeling was conducted to interpret the effects of grain boundary character on the Kr segregation. The area density of strong segregation sites in the high-angle grain boundaries is much higher than that in the low angle grain boundaries.

Nanolines of transition metals ruled by grain boundaries in graphene: An ab initio study

We have performed an ab initio investigation of the energetic stability, and the electronic properties of transition metals (TMs = Mn, Fe, Co, and Ru) adsorbed on graphene, upon the presence of grain boundaries (GBs); where we found an energetic preference for the TMs lying on the GB sites (TM/GB). Further energy barrier calculations, of the transition metals in TM/GB, reveal that the GBs promote the formation of energetically favorable diffusion channels on graphene. By increasing the concentration of the TM adatoms, the energetic stability of the TM/GB systems has been strengthened; giving rise to TM nanolines (TM-NLs). The electronic properties of those TM-NLs were characterized through extensive electronic band structure calculations, where the energy bands of the TM/GB systems indicate the appearance of an anisotropic spin-polarized electronic current along the TM-NLs on graphene.

Effect of transition metal impurities on the strength of grain boundaries in vanadium

Effects of 3d (Ti-Ni), 4d (Zr-Pd), and 5d (Hf-Pt) transition metal impurities on strength of two representative vanadium grain boundaries (GBs), symmetric Σ3(111) and asymmetric Σ5(210), were studied by first-principles calculations within the framework of the Rice-Wang thermodynamic model and within the computational tensile test. The desirable elements to increase the GB cohesion were predicted based on their segregation and strengthening behaviors across the different GB sites. It reveals that the elements Ti, Zr, Hf, Nb, and Ta are good choices for the GB cohesion enhancers. In addition, the GB strengthening by solutes is sensitive to the GB structures. The elements Cr, Mn, Fe, Co, and Ni decrease the GB strength of the Σ3(111) GB but they can increase the cohesion of the Σ5(210) GB. Furthermore, the origin of Ti-induced change of the GB strength was uncovered by analyzing the atomic bonds and electronic structures as well as the tensile strength. This work provides a theoretical guidance to screen promising alloying elements in V-based materials with improved resistance to GB decohesion and also helps us to understand the formation mechanism of Ti-rich precipitates in the V-Cr-Ti alloys under neutron or ion irradiation environments.

First-principles study of the effect of boron on grain boundary in NiAl

The grain boundary (GB) is critical to the mechanical properties of NiAl. The addition of B in NiAl can lead to the increase of yield strength due to its segregation in GB. To elucidate the B-induced enhancement of GB, we investigate the segregation state and its strengthening mechanism of the additive B in the typical ∑5(310)[001] GB of NiAl at atomic scale using the first-principles method. Our calculations shown that the additive B atom prefers to segregate at the interstitial I1 site of GB, in which case it is surrounded by four first-nearest-neighboring (1NN) and two second-nearest-neighboring (2NN) Ni atoms. It is found the segregated B prefers to bond with its neighboring Ni instead of Al atoms. Our calculations further shown not only the intergranular cohesion is improved by the covalent bonding between the segregated B and its four 1NN Ni atoms, but also the intra-planar cohesion of GB is enhanced by the covalent bonding between the segregated B and its two 2NN Ni atoms. The covalent bonding between B and Ni atoms mainly stem from the contribution of B-p and Ni-d states, partly from the weak interactions of B-s and Ni-s states. Our first-principles tensile test shown that the yield stress and ductility of NiAl are indeed improved due to the segregation of B in GB.

First-principles study of Si and Mg segregation in grain boundaries in Al and Cu: application of local-energy decomposition

Segregation of Si and Mg at grain boundaries (GBs) in Al and Cu has been investigated using density-functional theory calculations combined with recently developed local-energy and local-stress schemes. The physics behind the impurity-segregation energy is effectively analyzed by the local-energy decomposition. For the \(\Sigma \)9 tilt and \(\Sigma \)5 twist GBs in Al and Cu, Si shows large segregation-energy gains only at tighter sites, where local configuration of remarkably short Si–Al or Si–Cu bonds with high charge densities of covalent-bonding features are formed, leading to the local-energy stabilization as the final-state effects. On the other hand, Mg shows large gains only at looser sites. For Mg in the Cu GBs, the formation of stable Mg–Cu bonds or Mg states at looser sites is the origin of the preferential segregation as the final-state effects. For Mg in the Al GBs, however, the local energies of Mg–Al bonds are not so stable at looser sites, while the instability of Al atoms at looser sites in pure GBs before substitution is the origin of the preferential segregation as the initial-state effects. The behaviors of Si and Mg in Al GBs are dominated by the difference in local sp bonding nature among Mg, Al and Si, while Si–Cu and Mg–Cu \(sp-d\) hybridization interactions dominate the behaviors of Si and Mg in Cu GBs.

Direct observation of electron emission from grain boundaries in CVD diamond by PeakForce-controlled tunnelling atomic force microscopy

A detailed investigation of electron emission from a set of chemical vapour deposited (CVD) diamond films is reported using high-resolution PeakForce-controlled tunnelling atomic force microscopy (PF-TUNA). Electron field emission originates preferentially from the grain boundaries in low-conductivity polycrystalline diamond samples, and not from the top of features or sharp edges. Samples with smaller grains and more grain boundaries, such as nanocrystalline diamond, produce a higher emission current over a more uniform area than diamond samples with larger grain size. Light doping with N, B or P increases the grain conductivity, with the result that the emitting grain-boundary sites become broader as the emission begins to creep up the grain sidewalls. For heavy B doping, where the grains are now more conducting than the grain boundaries, emission comes from both the grain boundaries and the grains almost equally. Lightly P-doped diamond samples show emission from step-edges on the (111) surfaces. Emission intensity was time dependent, with the measured current dropping to ∼10% of its initial value ∼30h after removal from the CVD chamber. This decrease is ascribed to the build-up of adsorbates on the surface along with an increase in the surface conductivity due to surface transfer doping.

Duplex nanocrystalline alloys: Entropic nanostructure stabilization and a case study on W–Cr

Abstract

Atomistic simulations of the interaction of alloying elements with grain boundaries in Mg

Quantum density functional theory (DFT) is used to compute binding energies of important alloying elements (Ag, Zn, Ti, Al, Cd, Zr, Y, Ca, Nd, Ce and La) to a Σ7 grain boundary (GB) in Mg. In particular, quantifying the interaction of rare earth (RE) elements with Mg GBs is of significance given the strong effect of these solutes on modifying the texture of Mg alloys. For most alloying elements studied, the binding energy scales with the size of the solute and the local GB site volumes. Based on these trends, a model for the solute–GB binding energy is developed which accurately captures the behavior of the technologically important RE solutes and Ca. This model is then employed in conjunction with molecular statics calculations to predict solute segregation to general GBs not accessible by DFT. The predicted trends are found to be in qualitative agreement with available experimental data for GB segregation in Mg.

First-principles investigation into the effect of Cr on the segregation of multi-H at the Fe Σ3 (1 1 1) grain boundary

First-principles calculations were carried out to investigate the effect of Cr on segregation of multiple H atoms in the Σ3 (111) grain boundary in bcc Fe. In the absence of Cr, four H atoms can be trapped at the interstitial site of the Fe grain boundary (areal density: 28nm−2), and no H2 molecules are formed. The presence of Cr, however, suppresses the segregation tendency of H and only two H atoms can be trapped at this grain boundary. Although the volume expansion associated with the segregation of Cr to the grain boundary promotes H segregation, such a booting effect is unable to remedy the repulsion of H resulted from charge density increase in the grain boundary core. As a consequence, Cr mitigates H aggregation at the Σ3 (111) grain boundary in bcc Fe.

Study of preferential localized degradation and breakdown of HfO2/SiOx dielectric stacks at grain boundary sites of polycrystalline HfO2 dielectrics

• The real-time observation of faster degradation at GB sites of HfO 2 dielectric . • C-AFM results show enhanced defect generation rate at the GB sites. • Preferential breakdown of SiO x IL occurs below the degraded GB region. • A breakdown sequence in polycrystalline HfO 2 /SiO x dielectric stacks was proposed. Grain boundaries (GBs) in polycrystalline high-κ (HK) dielectric films affect the performance and reliability metrics of HK based advanced metal–oxide-semiconductor (MOS) devices. A full understanding of device reliability can only be had with the knowledge of the detrimental role of GB in degradation and breakdown (BD) of polycrystalline HK/interfacial layer (IL) dielectric stacks. In this work, we present a nanoscale resolution study on how the polycrystalline microstructure affects the degradation and BD at GB sites of polycrystalline HfO 2 in HfO 2 /SiO x ( x ⩽ 2) dielectric stacks using conductive-atomic force microscopy (C-AFM), supported by a statistical failure distribution model and device level simulations. Results clearly show an enhanced trap generation and faster degradation of polycrystalline HfO 2 gate dielectrics at GB sites as compared to the bulk (grain) regions implying shorter time-dependent dielectric breakdown (TDDB) lifetime at the GB sites. The SiO x IL below the degraded GB experiences a BD event when an enhanced electric field across the SiO x IL reaches the critical BD field, eventually triggering the overall BD of the HfO 2 /SiO x dielectric stack.

Quantifying the energetics and length scales of carbon segregation to α-Fe symmetric tilt grain boundaries using atomistic simulations

Segregation of impurities to grain boundaries (GBs) plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to GBs. Molecular statics simulations are used to calculate the segregation energies for carbon within multiple substitutional and interstitial GB sites over a database of 125 symmetric tilt GBs in Fe. The simulation results show that there are two energetically favorable GB segregation processes: (1) an octahedral C atom in the lattice segregating to an interstitial GB site and (2) an octahedral C atom and a vacancy in the lattice segregating to a grain boundary substitutional site. In both cases, lower segregation energies than appear in the bulk lattice were calculated. Moreover, based on segregation energies approaching bulk values, the length scale of interaction is larger for interstitial C than for substitutional C in the GB (≈5 Å compared to ≈3 Å from center of the GB). A subsequent data reduction and statistical representation of this dataset provides critical information about the mean segregation energy and the associated energy distributions for carbon atoms as a function of distance from the grain boundary, which quantitatively informs higher scale models with energetics and length scales necessary for capturing the segregation behavior of alloying elements and impurities in Fe. The significance of this research is the development of a methodology capable of ascertaining segregation energies over a wide range of GB character (typical of that observed in polycrystalline materials), which herein has been applied to carbon segregation to substitutional and interstitial sites in a specific class of GBs in α-Fe.

Effects of transition metals on the grain boundary cohesion in tungsten

The cohesion effect of transition metals on W grain boundaries (GBs) was studied via first-principles calculations. Systematic d-band filling for a given atomic environment results in trends of cleavage energy as a function of the solute’s electronic valence across different GB sites. Various solutes were found to strengthen the GB. Furthermore, lower- and higher-valence elements enhance cohesion at different positions. This suggests that employing both lower-valence (Hf, Ta, Nb) and higher-valence solutes (Re, Os, Ru) may further increase intergranular cohesion.

Grain boundary assisted degradation and breakdown study in cerium oxide gate dielectric using scanning tunneling microscopy

The presence of grain boundaries (GBs) in polycrystalline high-κ (HK) gate dielectric materials affects the electrical performance and reliability of advanced HK based metal-oxide-semiconductor devices. It is important to study the role of GB in stress-induced-leakage current (SILC) degradation and time-dependent dielectric breakdown of polycrystalline HK gate stacks. In this work, we present nanoscale localized electrical study and uniform stressing analysis comparing the electrical conduction properties at grain and GB locations for blanket cerium oxide (CeO2)-based HK thin films using scanning tunneling microscopy. The results clearly reveal higher SILC degradation rate at GB sites and their vulnerability to early percolation, supporting the phenomenon of GB-assisted HK gate dielectric degradation and breakdown.

Microstructural evolution during aging of Ti-5Al-5Mo-5V-1Cr-1Fe alloy

The evolution of microstructure on aging of an (α+β) titanium alloy (Ti-5Al-5Mo-5V-1Cr-1Fe) in the β and (α+β) solution-treated and quenched conditions was investigated. The presence of very fine ω phase was detected by electron diffraction for samples aged below 400 °C. The fine α aggregates are uniformly formed within β grains by nucleating at the ω particles or β/ω interfaces. At higher temperature, the formation of ω phase is avoided and the α lamellae are precipitated at the preferred site of grain boundary and then within the matrix. The highest hardness values are found when the alloys are aged at 450 °C for β condition and 350 °C for (α+β) condition.

Mantle deformation and noble gases: Helium and neon in oceanic mylonites

In an effort to constrain the behavior of noble gases during mantle deformation, we present new helium and neon data in mylonites from subaerial St. Peter and St. Paul Archipelago (Mid-Atlantic Ridge) and the submarine Southwest Indian Ridge. Coupled vacuum crushing and melting experiments show that most of the helium and neon within the mylonites is contained in the mineral matrices rather than fluid or melt inclusions: only 5 to 18% of the total helium is released by crushing. The mylonites and ultramylonites have much higher total helium concentrations than expected, based on their small grain size. The St. Paul's Rocks mylonites have helium contents equivalent to gas-rich MORB glasses, ranging from 6 × 10 − 6 to 3.8 × 10 − 5 cc STP He/g. The submarine mylonites and ultramylonites have helium contents between 5 × 10 − 8 and 4.4 × 10 −7 cc STP He/g, compared to 6.2 × 10 − 9 to 3.6 × 10 − 8 for the protogranular/porphyroclastic peridotites. Although the dataset is small, it suggests a relationship between metamorphic texture and noble gas abundance, and that mylonitization introduces mantle helium into mineral matrices. The mylonites are extremely fine grained, with an average grain size of ~ 10 µm, so helium residence in grain boundaries is also plausible. St. Paul's Rocks have modal hornblende, extreme geochemical enrichments in incompatible elements, and high temperature alteration phases (e.g., talc) that are rare or absent in the other samples; mineralogy must also play an important role. The 3He/ 4He ratios in the peridotites are primarily mantle derived, based on comparison with MORB data, suggesting that peridotites reflect the source mantle isotopic compositions. Neon isotopes in St. Paul's Rocks are a mixture of air and normal mantle, and fall along the line defined by MORB glasses. The atmospheric neon signal is preferentially released by crushing in vacuum, suggesting it resides within weakly bound sites in cracks and grain boundaries. He/Ne ratios in St. Paul's Rocks vary widely (~ 20×) with deformation and mineralogy, with the highest He/Ne ratios (and helium concentration) found in the finest grained ultramylonite peridotite. The neon and helium isotopic data show that mantle gases are preserved in fine-grained mylonites at very high concentrations. The most likely mechanism is diffusive trapping within defects at pressure in the mantle. The relationship between texture and helium abundance in peridotites suggests that metamorphism is a potentially important control on noble gas distribution in the mantle and crust.

Availability of Liquid Crystalline Materials for Organic Polycrystalline Semiconductor Thin Films

ABSTRACTWe have investigated the charge carrier transport properties of polycrystalline films fabricated with discotic and calamitic liquid crystals. The polycrystalline films of these liquid crystals exhibiting particular textures under a polarized microscope, i.e., hexagonal and co-circular patterns for discotic and calamitic liquid crystals, respectively, which indicate that the molecular alignment in the liquid crystalline phase is well preserved exhibit clear hole transport along with molecular stacks even in a 15 μm thick film, judging from transient photocurrents by time of flight method. The hole mobilities are over 0.1 cm2/Vs at room temperature. These results indicate formation of fewer grain boundaries across conduction channels. On the other hand, the carrier transport properties in the polycrystalline films of non-liquid crystalline molecules with the same molecular shape exhibit no particular patterns as described and photocurrents are annihilated in the bulk as a function of time, indicating many deep trap sites in grain boundaries. The present results indicate that the liquid crystalline molecule is a good candidate for preparing quality organic polycrystalline semiconductor thin films in opto-electronic applications.

Relationship between structure, segregation and electrical activity in grain boundaries

Using the contactless microwave phase-shift technique (μ W-PS) and High Resolution Transmission Electron Microscopy (HRTEM), we show that the twist and mixed parts of a Σ = 51(θ = 16.10∘) grain boundary in germanium (Ge) are electrically active. We also show that we can passivate the electrically active grain boundaries by sulfur segregation which has been studied by energy filtering HRTEM. Atomistic simulations show that the most favorable places for this segregation are the high energy sites of grain boundary.