Twist Grain Boundary Research Articles

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.

Electrical Behaviour of Twist Grain Boundary Phases of 4-n-dodecyloxy- 4'-(cholesteryloxycarbonyl-1-butyloxy) chalcone

There are only few compounds, which have shown wide temperature range twist grain boundary (TGB) phases. One such compound is the cholesterol derived, 4-n-dodecyloxy-4'-(cholesteryloxycarbonyl-1-butyloxy) chalcone, which shows a wide temperature range TGBA (~ 11 °C) and TGBC * (~ 19 °C). In the present work, we are reporting the temperature and frequency dependence of the dielectric parameters of the above compound for two different conditions of molecular anchoring i.e. planar and homeotropic. The dielectric studies have been carried out in the frequency range of 1 Hz to 35 MHz. In homeotropically aligned sample, the experimental dielectric spectra display a feeble relaxation mode in TGBA and TGBC * phases in MHz frequency range. Planar oriented molecules show a soft mode like relaxation due to the tilt fluctuation of molecules in MHz region for both the TGB phases and support theoretical formulation for the soft mode relaxation of the TGB phases. Various dielectric parameters have been determined for the two TGB phases of the above mentioned compound.

Molecular dynamical investigation on dislocation near twist-grain boundary of Ni under compression

The dislocations and the strength of a nickel bicrystal are investigated in present study. There are three kinds of crystalline orientations of the nickel bicrystal, namely (100), (110), and (111). Misfit dislocations are presented on the interface between two nickel grains when a compression process is utilized on the bicrystal. The misfit dislocations on a Ni(100) interface form a square-latticed network and those on a Ni(111) form a triangle-latticed network. When a large twist angle is applied to the Ni bicrystal, the square- and triangle-latticed unit of the misfit dislocation networks will shrink or even disappear. Thus, a plane defects distribute over the interface. If the bicrystal is compressed further, dislocations within each grain are developed from the defects on the interface between grains. The configuration of the dislocations within grains is more regular when the applied twist angle is smaller. A Ni(111) bicrystal owns the largest amount of maximal stress no matter what the twist angle is. Those of a Ni(110) bicrystal is the second and of a Ni(100) bicrystal is the smallest.

Synthesis and Phase Transitional Behavior of Dimer-like Optically Active Liquid Crystals

The results of our detailed studies pertaining to a relatively new class of low molar mass mesogens are presented. Four homologous series of optically active dimer-like mesogens, comprising cholesterol as the conventional pro-mesogenic core covalently tethered to a nonmesogenic salicylaldimine segment through a flexible spacer of varying length and parity, have been synthesized and evaluated for their thermal, electrical switching, electrochemical, and gelation properties. The thermal behavior, being the prime focus of this study, has been characterized by optical, calorimetric, X-ray diffraction, and electrical switching studies. In each series, the length of the even/odd-parity spacer is held constant, while the length of the terminal N-alkyl tail attached to nonmesogenic salicylaldimine core has been varied to gain insight into the fundamental correlation between structure and phase transitional properties. These compounds, with a few exceptions, exhibit liquid crystal phase(s); the identified phases are chiral nematic (N*), twist grain boundary (TGB), chiral smectic A (SmA), and chiral smectic C (SmC*) phases. The selective reflection property of the N* phase and ferroelectric behavior of the SmC* phase have been ascertained for some selected members. In general, the phase 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 found in the clearing temperatures; the even-parity dimer-like compounds belonging to three different series exhibit higher values when compared to members of an odd-parity series. Thus, our study demonstrates that these new class of low molar mass materials behave analogous to liquid crystal dimers. Electrochemical behavior and gelation ability have been demonstrated for some selected materials.

Tunable helium bubble superlattice ordered by screw dislocation network

Helium bubble nucleation at low-angle twist boundaries in gold has been investigated. It is found that the helium bubbles preferentially nucleate at screw dislocation nodal points and result in helium bubble superlattice formation, which is completely isomorphic with the screw dislocation network along the twist-grain boundary. Molecular statics calculations reveal that defect formation/solution energies along the screw dislocations, especially at the nodal points, are lower than their bulk counterparts. It is believed that this driving force is responsible for the helium bubble superlattice formation. Our study suggests that grain boundary engineering via adjustable twist angles in parallel boundaries to form tunable 3D bubble superlattices could afford a very promising approach for design of radiation tolerant materials.

The Chiral Pool as Valuable Natural Source: New Chiral Mesogens Made From Lactic Acid

Twenty new enantiopure chiral materials have been synthesized, showing liquid crystalline phases of smectic (SmA*), twisted grain boundary (TGB) and cholesteric (N*) type. The central ester-linkage as well as the length of the attached alkyl-chains were varied in a systematic way and the effects of these structural changes on the liquid crystalline properties have been studied. The liquid crystalline phases were characterized by means of differential scanning calorimetry, polarisation microscopy as well as small angle X-ray diffraction (SAXS).

Twisted Grain Boundary Phases in Binary Mixture of Smectic and Cholesteryl Compounds

A binary mixture of cholesteryl ethyl corbonate (CEC) and terephthal-bis-p-n-tetradecylaniline (TB14A) exhibits a twisted grain boundary (TGB) phase and smectic polymorphic phases. The mixture exhibits TGB-A and TGB-C phases at different concentrations and at different temperatures. Mixtures with higher concentrations of TB14A exhibit I – Ch – TGB-A – TGB-C – Sm A – Sm I – Sm F phases sequentially when the specimen is cooled from its isotropic melt. The existence of different TGB phases and polymorphic smectic phases are confirmed by differential scanning calorimetric (DSC) and optical microscopic studies. The phase diagram is in accordance with the Renn and Lubbensky model.

Wide thermal range frustrated liquid crystal phase in chiral dimers

The existence of a complex and highly frustrated mesophase, namely the twist grain boundary (TGB) phase possessing chiral smectic C (SmC*) blocks, denoted as the TGBC* phase, over an exceptionally wide (105–150 °C) thermal range in chiral dimers has been evidenced by optical, calorimetric, spectroscopic, X-ray and He–Ne laser diffraction studies.

Cholesterol-based unsymmetrical Schiff's base dimer terminated with 4-alkoxy-5-phenylthiophene unit: synthesis and characterisation

A series of unsymmetrical cholesterol based dimers have been synthesised and characterised. All the dimers possess a cholesterol unit connected with a 4-alkoxy-5-phenylthiophene unit through Schiff's base linkage. The length of the terminal alkyl and the alkyl spacer has been varied. All the dimers exhibit mesomorphism. Four dimers possess only the cholesteric (chiral nematic, N*) phase. One of the dimers showed smectic A (SmA) and N* phases whereas the other dimer displayed SmA, smectic chiral C (SmC*), N* and twist grain boundary (TGB) phases.

Ab initio calculations for industrial materials engineering: successes and challenges

Computational materials science based on ab initio calculations has become an importantpartner to experiment. This is demonstrated here for the effect of impurities and alloyingelements on the strength of a Zr twist grain boundary, the dissociative adsorption anddiffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grainboundary and in bulk Ni, and the dependence of the work function of a TiN–HfO2 junction on the replacement of N by O atoms. In all of these cases, computations provideatomic-scale understanding as well as quantitative materials property data ofvalue to industrial research and development. There are two key challenges inapplying ab initio calculations, namely a higher accuracy in the electronic energyand the efficient exploration of large parts of the configurational space. Whileprogress in these areas is fueled by advances in computer hardware, innovativetheoretical concepts combined with systematic large-scale computations will be neededto realize the full potential of ab initio calculations for industrial applications.

1H NMR Relaxometry Study of a Rod-Like Chiral Liquid Crystal in Its Isotropic, Cholesteric, TGBA*, and TGBC* Phases

The molecular dynamics of a chiral liquid crystal showing a rich variety of frustrated mesophases has been investigated by means of 1H NMR relaxometry. The interest in this lactate derivative, HZL 7/*, is related to a large range of thermal stabilities of the twist grain boundary (TGB) phases. Dispersions of the 1H spin-lattice relaxation times, T1, in the frequency range from 300 MHz to 5 kHz were measured and consistently analyzed in the isotropic, chiral nematic, TGBA*, and two TGBC* phases. In the isotropic and N* phases, a three-exponential magnetization decay was observed and assigned to three specific molecular groups of the HZL 7/* (molecular core, methyl, and methylene groups). In the TGB phases, all T1 components merge into a single one. The analysis of the T1 dispersion in the TGBA* phase shows that the translational self-diffusion relaxation mechanism dominates over a broad frequency range and that layer undulations are less relevant than the relaxation contribution associated with the diffusion process across the TGB structure. In the TGBC1* phase, the T1 dispersion presents a strong contribution of in-layer tilt direction fluctuations (T1(-1) proportional to ν(-1/2)), while, in the TGBC2* phase, the linear frequency dependence of T1 could be associated with a much stronger contribution of layer undulations than for the other TGB phases. This is at present the first molecular dynamics investigation on several TGB phases by means of 1H NMR relaxometry.

Stabilization of TGB Phase in the Binary Mixture of Rod-Shaped Compounds

A miscibility study of two cholesterol based rod-shaped compounds (R1 and R2) is presented. Both the compounds possess in common enantiotropic liquid crystalline phase sequence Cr → SmA → N* → Isotropic, which is generally exhibited by cholesterol-based compounds. Additionally, one of the rod-shaped compounds (R1) possesses a twist-grain boundary (TGB) phase over a short temperature range. In the binary mixture of these two rod-shaped compounds the TGB phase was stabilized over a long temperature range compared to that in the individual molecule R1. In the binary mixture up to 20% (w/w) of compound R1 possessing the TGB phase exhibited enhancement of the temperature range of the TGB phase compared to that of individual compound R1. Further gradual increase of percentage of R1 in the binary mixture did not show any enhancement of the TGB phase compared to that of individual R1.

Atomic scale modeling of {110} twist grain boundaries in α-iron: Structure and energy properties

Atomic scale modeling was used to study the structure and energy of {110} twist grain boundaries (TWGBs) with various coincidence-site-lattice misorientations in α-iron. The small angle {110} TWGB contains a hexagonal dislocation network of two sets of 1/2⟨111⟩ and one set of ⟨001⟩ dislocation segments. The ⟨001⟩ segments are almost pure screw dislocations and the angle between the two 1/2⟨111⟩ segments varies from 83 to 109° for the rotation angle from 0.25 to 5.40°. This TWGB dislocation structure agrees well with an experimental observation that was not explained adequately so far. The large-angle TWGBs consist of periodic patterns rather than a dislocation network. The variation of the boundary energy with the rotation angle can be well fitted to the Read–Shockley equation in the low-angle range. An apparent cusp in the curve of the boundary energy against the rotation angle has been found and discussed.

Molecular dynamical investigation on plastic behavior of Cu(100) twist-grain boundary under shear load

By using molecular dynamics with EAM potential, yield strength of copper column with (100) twist grain boundary and twist grain boundary effect are investigated under shear load. The results reveal that dislocations nucleate at the misfit dislocation network of low-angle twist grain boundary and dislocations stacking influence the yield strength of copper column. For high-angle twist grain boundary, twist grain boundary sliding decreases the yield stress of copper column. Meanwhile it is found that, with increasing twist angle, copper column yielding strength increases firstly. When twist angle reaches the critical value, the yielding strength decreases with increasing twist angle. It is concluded that different mechanisms mediate the yielding of copper column. Dislocation nucleation controls the yield stress of copper column for low-angle twist grain boundary and yielding of copper column is dominated by grain boundary sliding for high-angle twist grain boundary.

Atomistic Simulation Study of Cohesive Energy of Grain Boundaries in Alpha Iron under Gaseous Hydrogen Environment

Highly accurate prediction of material strength under a practical hydrogen environment and development of materials with minimal hydrogen effect are essential for safe use of hydrogen energy. Here, we estimated the relationship between grain boundary (GB) properties and GB cohesive energy under gaseous hydrogen environment for <110> symmetrical tilt GBs and <001> twist GBs in alpha iron using atomistic simulations. We employed the embedded-atom-method (EAM) potential developed by Wen et al. and conjugate gradient (CG) method for structure relaxation. First, we estimated the relationship between GB energy and GB free volume for various misorientation angles. After the estimation of GB properties, we incorporated a hydrogen atom into various occupation sites and obtained the distribution of hydrogen trap energy around the GBs. We found a good correlation among GB energy, GB free volume, and the number of trapped hydrogen atoms: high energy GBs have large gaps, and many hydrogen atoms are trapped in those spaces. Finally, we estimated the GB cohesive energy and found that there is negligible hydrogen influence on the low energy GBs. The reduction of the GB cohesive energy of Σ3{112} boundary was estimated to be only 6.61% under high-pressure hydrogen environment (T = 300K, p = 70MPa).

Optical Investigations of Twist Grain Boundary Phases Exhibiting Cylindrical and Cone-Like Domain Textures

Polarized optical microscopy observations performed on mixtures of Liquid crystals reveal developable domain textures for Twist Grain Boundary (TGB) phases. In these structures, the smectic blocks, the grain boundaries and the layers of helical structure of TGB phase are coiled in a double-twist manner. The energy considerations require that the blocks can just be folded in cones and cylinders, the only surfaces that may be unrolled without elongations. The cones need defects to build up; in particular, a disclination is necessary all along the cone axis. In the N* phase, the field of the χ-director that emanates from the center of the spherulite has the geometry of a hedgehog point defect. The disclination lines of strength k = 2 escape into third dimension with no singularity remaining and the χ-director is not materialized. However in TGBA the χ-singularities are materialized as singularities of the slabs.

Twisted smectics as the liquid crystal analogues of type II superconductors

In 1972, de Gennes pointed out the formal analogy between the Landau–Ginzburg Hamiltonians describing the phase transition from normal to superconductor on one hand and from nematic to smectic A liquid crystals on the other hand. This elegant analogy became a source of inspiration not only for theoreticians, as it revealed and emphasised the beauty and the richness of the physics of liquid crystals, but also for chemists in their constant search for novel liquid crystalline structures. The analogy with type I superconductors was first described and bore an appropriate educational value: the expulsion of twist in smectic liquid crystals can be viewed as equivalent to the well-known Meissner effect in superconductors. However, the investigation of the type II condition definitely opened new doors as it led to the discovery of fantastic new liquid crystal structures. In 1988, Renn and Lubensky ‘invented’ the Twist Grain Boundary (TGB) smectic phase as the direct analogue of the Abrikosov flux lattice. The twist penetrates the smectic layers of a TGB via a twisted lattice of screw dislocations analogous to magnetic vortices, just as the magnetic field penetrates the type II superconductor. We review in this paper the major steps of the experimental studies of the TGB phases and show that the observed behaviours closely follow the superconductor model, hence illustrating the depth of de Gennes' analogy.

Forming contacts and grain boundaries between MgO nanoparticles

The present article is concerned with how nanoparticles join: it considers MgO nano-cubes as a model, well-defined system. The development of grain boundaries (GBs) between cube particles has been re-examined using MgO smoke. In addition to the face-to-face contact which leads to the well-known low-∑ twist GBs, interactions are also found which initially involve point-to-face contact, edge-to-face contact, or contacts along the cube edges. It is proposed here that the point contact lead to a line contact through the requirement to balance charges, and rotation about such a line of contact leads to formation of the interface, i.e., the grain boundary. The atoms along the edges have lower coordination than the atoms in the bulk, which may contribute to the edge–edge and edge-face boundary formation. The inherently small size of nanoparticles makes transmission electron microscopy (TEM) an invaluable technique for characterizing the contacts between them without modifying them in any way. The present study uses TEM to characterize the types of boundaries formed, discusses the boundary structures, and considers how the particle morphology may determine the formation of low-∑ GBs.

Synthesis and mesomorphic behaviour of new mesogenic compounds possessing a cholesteryl ester moiety connected to a pyrimidine core

New mesogenic compounds containing a cholesteryl ester and a pyrimidine moiety connected through a polymethylene spacer have been prepared. The mode of linkage has been made via –C C– and –C N– to understand the structure-property relationship. Only two compounds with a pentamethylene spacer show mesomorphic behaviour. The mesomorphic behaviour has been investigated by polarizing optical microscopy, differential scanning calorimetry and HRXRD studies. Enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases are exhibited by the newly synthesized compounds.

Molecular dynamical investigation on plastic behavior of Cu(100) twist-grain boundary under uniaxial tension

Misfit dislocation structures at twist-grain boundary and its effect on grain boundary strength are investigated by using molecular dynamics simulation with EAM potential. The results reveal that the density of misfit dislocations which are formed on small angle twist-grain boundary is increased as the twist angle increased. Dislocations nucleate at each unit of the dislocation networks and the yield stress is increased as the density of misfit dislocations increases since grain boundary is strengthened by the interaction between dislocation networks. Face defect is formed at large angle twist-grain boundary, and the dislocations nucleate at the atomically sharp corner of the grain boundary. The yield stress is kept almost at the same value since there is little effect on the nucleation force of dislocation on face-defect grain boundary.