Twist Grain Boundary Research Articles

Effect of grain boundary segregations of Fe, Co, Cu, Ti, Mg and Pb on small plastic deformation of nanocrystalline Al

The paper studies the effect of grain boundary (GB) segregation of impurity atoms (Fe, Co, Cu, Ti, Mg and Pb) on the stress–strain relations of nanocrystalline Al and its alloys subjected to both shear plastic deformation and hydrostatic pressure at room temperature using molecular dynamics (MD) simulations. It is found that the GB segregations of Fe and Co atoms in the nanocrystalline Al–Fe and Al–Co alloys, respectively, strengthen their GBs, while those of Cu and Pb atoms in GBs of the nanocrystalline Al–Cu and Al–Pb alloys have the opposite effect. Furthermore, the GB strengthening effect of the mentioned impurity atoms in both twist and tilt GBs of Al bicrystals are analyzed via molecular statics (MS) simulations. Similar to the MD simulations, the results show that the GB segregations of Co atoms have a noticeable positive effect on the strength of the GBs while those of Pb atoms have a negative effect. Moreover, both MD and MS studies reveal that the Co, Mg and Pb atoms tend to occupy GBs of polycrystalline Al, while Ti, Fe and Cu atoms do not.

ChemInform Abstract: Synthesis and Characterization of Novel Chiral Dimers Exhibiting Highly Frustrated Liquid Crystal Phases.

Abstract Two novel series of optically active dimers comprising cholesterol and biphenyl-4-yl 4-(n-alkoxy)benzoate cores interlinked though either an odd-parity/even-parity spacer have been prepared and characterized. They stabilize an extremely complex, frustrated liquid crystalline state viz., the twist grain boundary (TGB) phase with chiral smectic C structure, denoted as TGBC∗ phase, over a wide (50–110 °C) temperature range. Notably, the dimers with an odd-parity spacer show an additional frustrated liquid crystal phase namely, the blue phase (BP). The presence of such frustrated phases suggests that the synthesized dimers are characterized by high enantiomeric excess and strong molecular chirality. Thus, 12 new optically active, nonsymmetric dimers reported herein constitute new examples of rarely found strongly chiral, optically pure dimers showing frustrated liquid crystal phase over an adequately wide thermal range.

Study on optical characterization of TGB and reentrant smectic-A phases in binary mixture of two liquid crystals

In this work, our investigation is to study the optical and thermal properties of the binary mixture of cholesteric and nematic compounds, namely, cholesteryl nonanoate and p-methoxybenzylidene-p-ethylaniline, which exhibits a very interesting liquid crystalline twisted grain boundary (TGB) phase and reentrant smectic-A phase. The chiral liquid crystalline TGB phases and reentrant smectic-A phases have been observed at different concentrations and at different temperatures. The existence of TGB and reentrant smectic-A phases is confirmed by differential scanning calorimetry and optical microscopic studies. The variation of optical anisotropy has been discussed. The helical pitch of the cholesteric phase has also been discussed.

Tensile response of bi-crystalline Si nanofilms with twist and tilt grain boundaries

This present work studies the effects of grain boundaries (GBs), notches and temperature on the mechanical properties of silicon (Si) nanofilms via molecular dynamics simulations. Both ∑25 (710)〈010〉 tilt and ∑25(001)〈001〉 twist GBs are considered. It is found that the single-crystalline Si nanofilm has relatively low fracture strength of ∼11.6GPa at room temperature, and fracture occurs through the dislocation sliding mechanism regardless of the existence of notches. At the same temperature, the bi-crystalline Si generally has lower fracture strength than that of the single-crystalline due to the presence of GBs, which changes the crystallographic orientation of the Si nanofilms. Moreover, strength of the bi-crystalline Si depends on the type of the GBs, and it fractures through either the dislocation sliding or cleavage mechanism, depending on the presence of notches. The temperature reduces the fracture strength of the nanofilms but does not change the failure mechanism.

Nanoparticle-induced twist-grain boundary phase.

By means of high-resolution ac calorimetry and polarizing optical microscopy, it is demonstrated that surface-functionalized spherical CdSSe nanoparticles induce a twist-grain boundary phase when dispersed in a chiral liquid crystal. These nanoparticles can effectively stabilize the one-dimensional lattice of screw dislocations, thus establishing the twist-grain boundary order between the cholesteric and the smectic-A phases. A Landau-de Gennes-Ginzburg model is used to analyze the impact of nanoparticles on widening the temperature range of molecular organizations possessing a lattice of screw dislocations. We show that in addition to the defect-core-replacement mechanism, the saddle-splay elasticity may also play a significant role.

Synthesis and characterization of novel chiral dimers exhibiting highly frustrated liquid crystal phases

Two novel series of optically active dimers comprising cholesterol and biphenyl-4-yl 4-(n-alkoxy)benzoate cores interlinked though either an odd-parity/even-parity spacer have been prepared and characterized. They stabilize an extremely complex, frustrated liquid crystalline state viz., the twist grain boundary (TGB) phase with chiral smectic C structure, denoted as TGBC∗ phase, over a wide (50–110 °C) temperature range. Notably, the dimers with an odd-parity spacer show an additional frustrated liquid crystal phase namely, the blue phase (BP). The presence of such frustrated phases suggests that the synthesized dimers are characterized by high enantiomeric excess and strong molecular chirality. Thus, 12 new optically active, nonsymmetric dimers reported herein constitute new examples of rarely found strongly chiral, optically pure dimers showing frustrated liquid crystal phase over an adequately wide thermal range.

Chiral HPLC for a study of the optical purity of new liquid crystalline materials derived from lactic acid

New liquid crystalline (LC) materials were prepared by derivatization of lactic acid. First compound possesses the lactic acid unit as the only chiral center and the second group of LC materials contains two chiral centers. Mesomorphic properties of both the newly synthesized LC materials were studied and the presence of the SmA*–SmC* or exhibit the twist grain boundary (TGB) phases, namely TGBA and TGBC, in a wide range of temperatures down to the room temperature was established. The potential of high-performance liquid chromatography (HPLC) applying chiral stationary phases to separate enantiomers or diastereoisomers of the synthesized LC compounds was evaluated. Two different brands of commercial chiral sorbents, Lux Amylose-2 and Chiralpak AD-3, both based on modified silica with derivatized polysaccharide, were employed in the development of separation procedures. The optimized chiral HPLC method provided a baseline separation of the individual enantiomers for the LC material containing one chiral center. In the case of the more complex compound with two asymmetric carbon atoms, where four isomers exist, partial separation was reached only using the current chiral HPLC.

Atomistic view of thin Ni/Ni3Al (0 0 1) under uniaxial tension of twist grain boundaries

Atomic motion and the structure response of grain boundaries (GBs) are essential to the plastic deformation of small-volume polycrystal systems, especially for thin materials that exhibit some dramatic characteristics. Here, the microstructure and properties of thin Ni/Ni3Al (0 0 1) with the uniaxial tension of twist GBs are investigated using molecular dynamics (MD) simulations with an embedded atom (EAM) potential. We find that low angle twist GB dislocations propagate mainly from the corners to the center of the interface, and for high angle twist GBs they originate from the edge of the interface but do not extend to the center. In the process of plastic deformation, both low angle and high angle twist GB fractures occur in the center of the interface, which is completely different from the situation of thick materials reported by other references. More interestingly, the fracture patterns between low angle and high angle twist GBs exhibit extremely different performance due to the different characteristics of the initial dislocation structures in the interfaces.

Atomic simulations of the effect of twist grain boundaries on deformation behavior of nanocrystalline copper

The effects of twist grain boundary (GB) on the mechanical properties of two types of nanocrystalline copper (Cu) under tensile loading are investigated by molecular dynamics simulation. The results indicate that the plasticity of bicrystalline Cu with a high twist angle is much better than that of structure with low twist angle, due to the blockage of dislocations’ motion by twist GBs. However, the plasticity of trifurcate crystal Cu is better in low twist angle structure. The studies demonstrate that these different deformation behaviors are associated with the nucleation and propagation of dislocations. The results also show that regardless of crystalline types, the Young’s modulus of nanocrystalline Cu slightly increases with increasing twist angle. The general conclusions derived from this work may be of importance in devising high-performance face-center cubic metal with different GBs.

Electronic and atomic structures of KFe2Se2 grain boundaries

The electronic and atomic structures of the twist and tilt grain boundaries (GB) of the iron-based superconductor KFe2Se2 are studied based on the simulations of the first principles density functional theory. Our results have clarified that the Σ5[001] twist grain boundary of KFe2Se2 with layered structure has the lower grain-boundary energy. The local structure and the main features of the basic electronic structure within the [001] twist grain-boundary region have small differences compared with those in KFe2Se2 crystal. The large fluctuations of the charges and magnetic moments are found in the [001] tilt grain-boundary regions, especially the former are more prominent. The bi-collinear anti-ferromagnetic order is the most stable magnetic order even with grain boundaries in the bulk. The 5a×5a superstructure of Fe-vacancies in K2Fe4Se5 phase is intrinsically related to the coincident-site lattice of Σ5[001] twist grain boundary.

MULTI-SCALE SIMULATION OF GRAIN BOUNDARY STRUCTURE EFFECTS IN POLY-Si THIN FILM SOLAR CELL

The atomic structures of grain boundary (GB) and their effect on the performance of poly- Si thin film solar cell are studied by multi-scale simulations. First, the atomic structures of various GBs are calculated using molecular dynamics. Subsequently, the energy band diagram are obtained by ab-initio calculations. Then, finite difference method is performed to obtain solar cell performance. The results show that the Σ5 (twist) GB can greatly enhance the carriers recombination and results in small short-circuit current density (J SC ) and open-circuit voltage (V OC ). However, the Σ17 (twist and tilt) GBs have little influence on the cell performance. Also revealed in the simulations is that the GB near the p–n junction leads to very small J SC and V OC . When the distance between GB and p–n junction increases from about 1.10 μm to 3.65 μm, the conversion efficiency increases by about 29%. The thickness effect of solar cell containing the Σ5 (twist) GB on the cell performance is also studied. The results show that the conversion efficiency and J SC increase rapidly as the thickness increases from about 5.2 μm to 40 μm. When the thickness ranges from about 40 μm to 70 μm, the efficiency and the J SC both increase gradually and reach their own peak values at about 70 μm. When the thickness exceeds 70 μm, the efficiency and J SC both decrease gradually. However, the V OC keeps increasing with increase in thickness. The effects of GB on the carrier transport and recombination processes are discussed to understand the above results.

Twisted Grain Boundary Phase in Binary Mixture of Liquid Crystals

In the present work, our investigation is to study the optical and thermal properties of the binary mixture of cholesteric and nematic compounds, namely, cholesteryl chloride and 4,4′-hexyloxy azoxy benzene, which exhibits a very interesting liquid crystalline twisted grain boundary (TGB) phase and induced smectic phases like SmA, SmC*, SmE, and SmB phases. The chiral liquid crystalline TGB phases have been observed at different concentrations and at different temperatures. The mixture with concentration from 26% to 37% of shows I-Ch-TGB-SmA-SmC*-SmE-SmB phases sequentially when the specimen cooled from isotropic phase. The existence of TGB phase is confirmed by DSC and optical microscopic studies. The temperature variation of optical anisotropy is also discussed. X-ray studies have been carried out to understand the intermolecular interactions in the mixture. With the help of phase diagram, the phase behavior has also been discussed.

Cholesterol-based dimeric liquid crystals: synthesis, mesomorphic behaviour of frustrated phases and DFT study

Chiral unsymmetrical dimeric liquid crystals consisting of a cholesterol moiety as chiral entity and a substituted salicylidene imine core (with the substituent being butyl or fluoro or chloro group) interconnected through an even methylene spacer have been synthesised and their mesomorphic properties are characterised. All the dimers exhibit enantiotropic mesophases. The butyl homologue exhibited N* phase only, the fluoro- and chloro-substituted compound exhibited frustrated blue phases (BPs), N* phase and SmC* or twisted grain boundary (TGB) phases. The occurrence of a fluid frustrated phase, the BP, in particular, observed in compounds with a polar moiety and bent optimised conformation by density functional theory (DFT) study, indicates the importance of polar structures and bent shape of the compounds. Theoretical calculation was performed in order to study the optimised conformation, polarity and electron density distribution of the synthesised cholesterol derivatives using DFT. Time-dependent density functional theory (TD-DFT) calculation also had been carried out to investigate the absorption spectra and HOMO–LUMO energies. The experimental and theoretical absorption spectra are also presented.

Modulated phase of liquid crystals: Covariant elasticity in the context of soft, achiral smectic-Cmaterials

Ginzburg-Landau-de Gennes-type covariant theories are extensively used in connection with twist grain boundary phases of chiral smectogens. We analyze the stability conditions for the linear, covariant elasticity theory of smectic-C liquid crystals in the context of achiral materials, and predict an equilibrium modulated structure with an oblique wave vector. We suggest that a previous experimental observation of stripes in smectic-C is consistent with the predicted structure.

Different modulated structures of topological defects stabilized by adaptive targeting nanoparticles

It is demonstrated that interactions between nanoparticles and topological defects induce a twist-grain boundary phase in a chiral liquid crystal. The occurrence of this phase, the analogue of the Shubnikov phase in type-II superconductors, is driven by direct interactions between surface-functionalized CdSe quantum dots and screw dislocations. It is shown that, within an adaptive-defect-core-targeting mechanism, nanoparticles of appropriate size and functionalization adapt to qualitatively different cores of topological defects such as disclination lines and screw dislocations. This mechanism enables the effective reduction of the energetically costly, singular defect core volume, while the surrounding phase ordering remains relatively weakly affected. The findings suggest new pathways towards the controlled assembly of superstructures in diverse, symmetry-broken, condensed-matter systems, ranging from nanoparticle-decorated liquid crystals to superconductors.

Atomistic simulations of shock induced melting of bicrystal copper with twist grain boundary

We investigate the shock induced melting of Cu bicrystals with [001] twist grain boundary (GB) under various shock intensities using atomistic dynamics simulations. Two representative types of twist GBs are studied: ∑=5/(001)/36.87° and ∑=181/(001)/6.03° with relatively high and low GB energies, respectively. The properties of the shocked GB region evolving with time are characterized in terms of order parameters and mean-squared displacement. In the case of ∑=5/(001)/36.87°, it is found that prior to the homogeneous melting of the whole sample, continuous partial melting with considerable premelting of the shocked GB region precedes bulk melting with negligible superheating of GB, while solid state disordering may precede the partial melting. In contrast, for ∑=181/(001)/6.03°, the melting process is analogous to that of a shocked perfect crystal except that a limited superheated partial melting state occurs before homogeneous melting of the whole bicrystal.

Molecular Dynamic Dimulation of Energy and Structural Stability on Twist Grain Boundary in Bicrystal Copper

The energies and microstructures of <001> and <111> twist grain boundary (GB) planes in bicrystal copper with different twist angles were simulated by molecular dynamic (MD). The simulation results indicated that Σ5 and Σ3 twist GBs were stable GB structures of (001) and (111) planes respectively. Compared energies of both kinds of boundaries, <111> twist GBs were more stable. Furthermore, Σ3 GB had the lowest GB energy of all <111> twist GBs. Therefore, Σ3 <111> twist GB had the priority to form on (111) plane and was the most stable GB structure in this simulation that partly explained the reason why Σ3 GBs had high probability to appear in polycrystalline materials. The two more stable GB structures of <001> and <111> twist GBs are both low Σ value coincidence site lattice (CSL) special GBs, which directly proved that lower energy and more stable GBs could provide more GB failure resistance.

First-principles predictions for the tensile strength of Al metal with dislocations of twist grain boundaries

By the first-principles calculations based on the density functional theory, the tensile strength of Al metal with dislocations of twist grain boundaries (GBs) is predicted from its electronic structure to its essential mechanical properties. The results show that the theoretical tensile strength for Al twist GB is about 8.73GPa and it is less than that for Al glide GB(9.5GPa) (Phys. Rev. B 75, 174101 (2007)). However, its fracture strain for Al twist GB is 24% and 16% more than that for Al glide GB. It suggests that the mechanical properties of the metal can be greatly improved by experimentally modulating its defect or dislocation. Furthermore, the physics of the fracture of Al twist GB is analyzed by the distributions of charge density and the changes of bond length, and it is found that the facture appears in the GB. Our theoretical predictions can play an important role in guiding the improvement of mechanical properties and structural designs for Al metal.

Synthesis and thermal behavior of chiral dimers: occurrence of highly frustrated and cholesteric liquid crystal phases

A number of nonsymmetric, optically active, liquid crystal dimers belonging to three different series have been synthesized by condensing cholesteryl ω-(4-hydroxyphenoxy)alkanoates with cinnamic acids bearing mono-, di- and tri-alkoxy tail/s; these are in turn prepared by Knoevenagel condensation of corresponding benzaldehydes with malonic acid. The length and (odd–even) parity of the spacer, which covalently tethers cholesterol and phenyl cinnamate entities, have been varied; the length of the terminal tail, attached to a cinnamate core, has also been varied to understand structure–property correlation. The synthesized compounds have been fully characterized in terms of molecular structure and liquid crystal properties. The vast majority of the dimers exhibit enantiotropic liquid crystal behavior involving mesophases such as chiral nematic (N*), twist grain boundary (TGB) and some unknown mesophases. All the dimers with monoalkoxy tails, except one, display TGB phase possessing chiral smectic C (SmC*) blocks, denoted as the TGBC* phase, over an exceptionally wide thermal range. In fact, in one of the members, the phase occurs for more than 100 °C thermal range, persisting closer to room temperature; this is remarkable, given the fact that such a behavior has been scarcely reported owing to the complex and highly frustrated nature of the TGBC* phase. Compounds with dialkoxy tails and even-parity spacer display N* and/or TGBC* phase/s, where the latter phase exists over a relatively lesser temperature interval when compared to mono-alkoxy materials, but the dimer with dioctyloxy tails exhibits supercooled TGBC* phase up to −60 °C; the analogous dimers with odd-parity spacer display unknown mesophase/s. Trialkoxy-substituted dimers display the N* phase existing over a 30–40 °C thermal range with lower melting and clearing transition temperatures; these features are quite remarkable in view of their various technological applications. The phase transitional behavior shows a dependence on the length and parity of the central spacer as well as the length of the terminal tail. The odd–even effect has been prominently seen in the clearing temperatures, a feature strongly reminiscent to that observed for cholesterol-based dimers reported hitherto.

Recent Investigations on Grain Boundary Relaxation

In recent years, we have investigated the grain boundary (GB) internal friction in high-purity Al bicrystals with a wide variety of misorientations of tilt and twist GBs. The main results are reviewed and divided into the following sections: (1) the origin of the GB internal friction peak; (2) the individual behaviors of different types of GBs; and (3) the coupling effect and compensation effect involved in GB relaxation.