Function Of Crystal Orientation Research Articles

Micropillar compression deformation of single crystals of Co3(Al,W) with the L12 structure

The compression deformation behavior of single-crystal micropillars of L12–Co3(Al,W) has been investigated at room temperature as a function of pillar size and crystal orientation. The critical resolved shear stress (CRSS) obeys an inverse power-law scaling against pillar size and is independent of orientation. With the bulk CRSS value of Co3(Al,W) at room temperature estimated by extrapolating the size dependence of CRSS for micropillars, comparison of high-temperature strength is made on the CRSS values for Co3(Al,W) and Ni3Al-based compounds. Co3(Al,W) has turned out not to be strong enough to provide excellent high-temperature strength for Co-based superalloys, at least for ternary composition.

Theoretical and experimental investigation of optical absorption anisotropy in β-Ga2O3

The question of optical bandgap anisotropy in the monoclinic semiconductor β-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk β-Ga2O3 was found to be a function of light polarization and crystal orientation, with the lowest onset occurring at polarization in the ac crystal plane around 4.5–4.6 eV; polarization along b unambiguously shifts the onset up by 0.2 eV. The theoretical analysis clearly indicates that the shift in the b onset is due to a suppression of the transition matrix elements of the three top valence bands at Γ point.

Theoretical Prediction of Transition Metal Alloying Effects on the Lightweight TiAl Intermetallic

The structural, mechanical properties and Debye temperature of doped intermetallic Ti7Al8X (X = Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W) have been investigated by employing the pseudo-potential plane-wave approach based on density functional theory, within the generalized gradient approximation (GGA) function. The calculated lattice constants of TiAl are found to be within 1 pct error, compared with the experimental values. The stability of calculated structures of Ti7Al8X at 0 GPa is measured by studying mechanical stability conditions and formation energy. All the single crystals are proved to be elastically anisotropic. The Young’s modulus as a function of crystal orientations has been systematically investigated. Mechanical properties of polycrystals are computed from values of shear modulus (G), bulk modulus (B), Young’s modulus (E), Poisson’s ratio (υ), and microhardness parameter (H) for Ti7Al8X. It is indicated that addition of alloying elements reduces the brittleness and microhardness of TiAl intermetallic. Debye temperature of TiAl calculated using elastic data of the present work is found to be influenced by the addition of alloying elements, which is further confirmed by the phonon dispersions of Ti8Al8, Ti7Al8Zr, and Ti7Al8Hf.

Investigations of orientation and length scale effects on micromechanical responses in polycrystalline zirconium using spherical nanoindentation

Here we investigate the elastic and plastic anisotropy of hexagonal materials as a function of crystal orientation using a high-throughput approach (spherical nanoindentation). Using high purity zirconium as a specific example, we demonstrate the differences in indentation moduli, indentation yield strengths and indentation post-elastic hardening rates over multiple grain orientations. These results are validated against bulk single crystal measurements, as well as data from cubic materials. By varying the indenter size (radius), we are also able to demonstrate indentation size effects in hexagonal materials, including possible signatures of strain hardening due to twin formation in the nanoindentation stress–strain curves.

Impact of the Crystallite Orientation Distribution on Exciton Transport in Donor-Acceptor Conjugated Polymers.

Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

Elastic anisotropy and thermodynamic properties of chromium tetraboride from first‐principles calculations

First‐principles calculations are performed to investigate the elastic anisotropy, and thermodynamic properties of chromium tetraboride (CrB4) under extreme P–T conditions. The calculated equilibrium lattice and the normalized crystal parameters as a function of pressure are in good agreement with the available experimental and theoretical data. Elastic constants Cij are calculated in the pressure range of 0–50 GPa, and the bulk modulus B, shear modulus G, and Young's modulus E are also obtained according to the Voigt–Reuss–Hill (VRH) approximation. Anisotropic indexes are employed to characterize the mechanical anisotropy. The bulk modulus and Young's modulus as a function of crystal orientations are also investigated. The obtained results suggest that CrB4 exhibits a pronounced elastic anisotropy. It is strongest along the b‐axis in response to compression loading, and the {100} and {001} shear planes might be the cleavage planes. The elastic anisotropy of CrB4 may possibly limit its applications. Using the quasiharmonic Debye model, thermodynamic properties like Debye temperature, Grüneisen parameter, heat capacity, and expansion coefficient are also systematically explored under extreme P–T conditions.

Mechanisms of template-assisted selective epitaxy of InAs nanowires on Si

A comprehensive investigation of InAs epitaxy on silicon using template-assisted selective epitaxy is presented. The variation in axial growth rate of InAs nanowires inside oxide nanotube templates is studied as function of nanotube diameter (20–140 nm), growth time (0–30 min), growth temperature (520–580 °C), V/III ratio (40–160), nanotube spacing (300–2000 nm), and substrate crystal orientation. It is found that the effective V/III ratio is reduced at least by a factor of two within the nanotube templates compared to the outside, detectable by changes in the growth facet morphology. The reduced V/III ratio originates from the different transport mechanisms for the As and In precursor species; As and In species are both transported by Knudsen diffusion in the vapor, but an additional contribution of In surface diffusion reduces the V/III ratio. The results reveal the interplay of growth parameters, crystal facets and template geometry and thus are generally applicable for nanoscale selective epitaxy.

Micropillar Compression of MoSi2 Single Crystals

ABSTRACTDeformation behavior of MoSi2 has been studied by micropillar compressions of single crystalline specimens prepared by focused ion beam (FIB) technique as a function of crystal orientation at room temperature. Activation of the {011}<100> and {01$\overline 3$}<331> slip systems were observed in the micropillars compressed along [$\overline 1$10] and [0 15 1], respectively. The CRSS values for each slip system exhibit an approximate power law relationship with the edge length of micropillar. The {01$\overline 3$}<331> slip exhibit much stronger size-dependence than the {011}<100> slip system.

Photodegradation activity and stability of porous silicon wafers with (1 0 0) and (1 1 1) oriented crystal planes

Hydrogen-terminated porous Si wafers with (100) and (111) oriented crystal planes were fabricated through a photo-electrochemical etching. It is found that the porosity of silicon wafers and their etch rates are determined as a function of crystal orientation. Due to the anisotropic etching behavior of single-crystal silicon, the hydrogen-terminated porous Si (100) wafers exhibit not only more excellent photodegradation activity but also stronger stability for methyl orange degradation than hydrogen-terminated porous Si (111) wafers under visible light irradiation. For the unetched Si wafers, however, the photodegradation activities of methyl orange exhibit a contrary conclusion.

Crystallographic controlled dissolution and surface faceting in disordered face-centered cubic FePd

Abstract

Broadband inelastic light scattering study on relaxor ferroelectric Pb(In1/2Nb1/2)-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The broadband inelastic light scattering spectra of ternary Pb(In1/2Nb1/2)-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals were investigated as a function of temperature and crystal orientation by combining Raman and Brillouin spectroscopies. The angular dependence of the strong Raman peak located at ∼50 cm−1 was investigated at 300 °C. The intensity variation of this mode with rotation angle was compatible with the F2g mode of Fm3¯m symmetry, suggesting that this mode arises from the 1:1 chemical order at the B-site in this perovskite structure. The temperature evolution of the polar nanoregions was associated with the growth of two central peaks and the change in the intensity of some Raman peaks, which were known to be sensitive to the rhombohedral symmetry. Both relaxation processes exhibited partial slowing-down behaviors with a common critical temperature of ∼160 °C. Poling the crystal along the [001] direction induced abrupt changes in some of the Raman bands at the rhombohedral–tetragonal phase transition. On the other hand, the diffuse tetragonal–cubic phase transition was not affected by the poling process. This high-temperature phase transformation seems to be smeared out by the inherent disorder and strong random fields enhanced by the addition of Pb(In1/2Nb1/2) into Pb(Mg1/3Nb2/3)O3-PbTiO3.

Transformation strains and temperatures of a nickel–titanium–hafnium high temperature shape memory alloy

A combined experimental and theoretical investigation of the transformation temperature and transformation strain behaviors of a promising new Ni50.3Ti29.7Hf20 high-temperature shape memory alloy was conducted. Actuation behavior of single crystals with loading orientations near [001]B2, [1¯10]B2, and [111]B2, as well as polycrystalline material in aged and unaged conditions was studied, together with the superelastic, polycrystalline torsion response. These results were compared to analytic calculations of the ideal transformation strains for tension, compression, and torsion loading of single crystals as a function of single crystal orientation, and polycrystalline material of common processing textures. H-phase precipitates on the order of 10–30nm were shown to increase transformation temperatures and also to narrow thermal hysteresis, compared to unaged material. The mechanical effects of increased residual stresses and numbers of transformation nucleation sites caused by the precipitates provide a plausible explanation for the observed transformation temperature trends. Grain boundaries were shown to have similar effects on transformation temperatures. The work output and recoverable strain exhibited by the alloy were shown to approach maximums at stresses of 500–800MPa, suggesting these to be optimal working loads with respect to single cycle performance. The potential for transformation strain in single crystals of this material was calculated to be superior to binary NiTi in tension, compression, and torsion loading modes. However, the large volume fraction of precipitate phase, in part, prevents the material from realizing its full single crystal transformation strain potential in return for outstanding functional stability by inhibiting plastic strain accumulation during transformation. Finally, calculations showed that of the studied polycrystalline textures, [001]B2 fiber texture results in superior torsion performance, while [011]B2 fiber texture results in superior tensile behavior, and both [011]B2 and random textures will result in the best possible compression performance.

Anisotropy of the molecular magnetV15spin Hamiltonian detected by high-field electron spin resonance

The molecular compound ${K}_{6}$[V${}_{15}^{\mathrm{IV}}$As${}_{6}^{\mathrm{III}}{\mathrm{O}}_{42}$(H${}_{2}$O)] $\ifmmode\cdot\else\textperiodcentered\fi{}$ 8H${}_{2}$O, in short ${V}_{15}$, has shown important quantum effects such as coherent spin oscillations. The details of the spin quantum dynamics depend on the exact form of the spin Hamiltonian. In this study, we present a precise analysis of the intramolecular interactions in ${V}_{15}$. To that purpose, we performed high-field electron spin resonance measurements at 120 GHz and extracted the resonance fields as a function of crystal orientation and temperature. The data are compared against simulations using exact diagonalization to obtain the parameters of the molecular spin Hamiltonian.

Elastic anisotropy and thermodynamic properties of iron tetraboride under high pressure and high temperature

First principles calculations are performed to investigate the elastic anisotropy and thermodynamic properties of the recently synthesized iron tetraboride (FeB4) under high pressure and high temperature. The obtained normalized crystal parameters dependence of the resulting pressure are in excellent agreement with experimental data. Young׳s modulus and shear modulus as a function of crystal orientations have been systematically investigated. The obtained results reveal that the FeB4 exhibits a pronounced elastic anisotropy and it is stiffest along [110] and the most compliant along [100] in response to tension or compression loading. Using a set of total energy versus volume obtained with the first-principles calculations, the quasiharmonic Debye model is applied to the study of the thermal and vibrational effects. The dependences of Debye temperature, Grüneisen parameter, heat capacity, and expansion coefficient on the temperature and pressure are systematically explored in the whole pressure range from 0 to 30GPa and temperature range from 0 to 1800K.

Elastic and thermodynamic properties of Re2N at high pressure and high temperature

First principles calculations are preformed to systematically investigate the elastic and thermodynamic properties of Re2N at high pressure and high temperature. The Re2N exhibits a clear elastic anisotropy and the elastic constants C11 and C33 vary rapidly in comparison with the variations in C12, C13 and C44 at high pressure. In addition, bulk modulus B, elastic modulus E, and shear modulus G as a function of crystal orientations for Re2N are also investigated for the first time. The tensile directional dependences of the elastic modulus obey the following trend: E[0001] <E[▪2▪1] >E[10▪0] >E[10▪1]. The shear moduli of Re2N within the (0001) basal plane are the smallest and greatly reduce the resistance of against large shear deformations. Based on the quasi-harmonic Debye model, the dependences of Debye temperature, Grüneisen parameter, heat capacity and thermal expansion coefficient on the temperature and pressure are explored in the whole pressure range from 0 to 50 GPa and temperature range from 0 to 1600 K.

Shock-induced plasticity in tantalum single crystals: Interatomic potentials and large-scale molecular-dynamics simulations

We report on large-scale nonequilibrium molecular dynamics simulations of shock wave compression in tantalum single crystals. Two new embedded atom method interatomic potentials of Ta have been developed and optimized by fitting to experimental and density functional theory data. The potentials reproduce the isothermal equation of state of Ta up to 300 GPa. We examined the nature of the plastic deformation and elastic limits as functions of crystal orientation. Shock waves along (100), (110), and (111) exhibit elastic-plastic two-wave structures. Plastic deformation in shock compression along (110) is due primarily to the formation of twins that nucleate at the shock front. The strain-rate dependence of the flow stress is found to be orientation dependent, with (110) shocks exhibiting the weaker dependence. Premelting at a temperature much below that of thermodynamic melting at the shock front is observed in all three directions for shock pressures above about 180 GPa.

Probing Symmetry Properties of Few-Layer MoS2 and h-BN by Optical Second-Harmonic Generation

We have measured optical second-harmonic generation (SHG) from atomically thin samples of MoS2 and h-BN with one to five layers. We observe strong SHG from materials with odd layer thickness, for which a noncentrosymmetric structure is expected, while the centrosymmetric materials with even layer thickness do not yield appreciable SHG. SHG for materials with odd layer thickness was measured as a function of crystal orientation. This dependence reveals the rotational symmetry of the lattice and is shown to provide a purely optical method of determining the orientation of crystallographic axes. We report values for the nonlinearity of monolayers and odd-layers of MoS2 and h-BN and compare the variation as a function of layer thickness with a model that accounts for wave propagation effects.

Simulations of the polarisation-dependent Raman intensity of [formula omitted]-carotene in photosystem II crystals

In order to clarify possibilities to identify the β-carotene (β-Car) radicals in secondary electron transfer (ET) reactions in the photosystem II core complex (PSIIcc), Raman intensities of all 96 β-Car cofactors in the unit cell of PSIIcc-dimer crystals as a function of polarisation and crystal orientation were simulated based on the 2.9Å resolution structure. The Raman-active symmetry Ag in the C2h group is assigned to the β-Car modes ν66 and ν67. Simulations are in agreement with experiment for off-resonant excitation at 1064nm. Resonant measurements at 476 and 532nm excitation can not be explained, which is attributed to mode mixing in the excited state and the existence of different spectral pools. The identity of the β-Car oxidised in secondary ET can not be resolved by Raman measurements on PSIIcc-dimer crystals. Additional simulations show that similar measurements on PSIIcc-monomer crystals could provide a possible route to solve this issue.

Hexagonal high-pressure phase of tantalum mononitride predicted from first principles

Based on the particle swarm optimization algorithm on crystal structural prediction, we first predict that TaN undergoes a phase transition from the experimental θ-TaN to a hexagonal P63/mmc structure at 87.5 GPa with volume drop of 1.6%. This hexagonal P63/mmc structure is isostructural with anti-NiAs and can be quenchable to ambient pressure by further phonon dispersions calculations. The Young's modulus E and shear modulus G as a function of crystal orientation for TaN have thus been systematically investigated. The calculated mechanical properties suggest that the P63/mmc-TaN is ultra-incompressible and hard due to its high bulk modulus (336 GPa), large shear modulus (214 GPa), originating from a staking of “N-Ta-N” sandwiches layers linked by strong covalent Ta-N bonding.

Modeling dislocation nucleation strengths in pristine metallic nanowires under experimental conditions

The nature of dislocation sources in small-scale metals is critical to understanding and building models of size-dependent crystalline strength at the nanoscale. Pre-existing dislocations with one pinning point can be readily described as truncated Frank–Read sources, and modeling their operation strengths is straightforward. In contrast, simple and accurate models describing surface dislocation nucleation processes remain elusive. Here, we develop a computationally simple model of heterogeneous dislocation nucleation from free surfaces by combining a continuum description of the nucleation process with the atomistic inputs where accuracy is critical. This model is used to derive the upper strength limits of single-crystalline face-centered cubic nanopillars as a function of size, material and crystal orientation for uniaxial compression and tension. The output parameter space of this model is critically compared against direct atomistic simulations, as well as experiments on tension of pristine gold nanowires and compression of copper nanopillars to highlight its virtues and limitations.