Anisotropy Index Research Articles

Group 14 semiconductor alloys in the P41212 phase: A comprehensive study

This study reported the structural properties, stability, mechanical anisotropy properties (anisotropy in Poisson's ratio, shear modulus, Young's modulus, and the universal elastic anisotropy index) and electronic properties (electronic band structure) of Si20-xGex, Ge20-xSnx, and Si20-xSnx alloy (x = 0, 4, 8, 12, 16, and 20) in the P41212 phase. The lattice parameters of semiconductor alloys in the P41212 phase increased with increasing x. The dynamic and mechanical stability of the Si20-xGex, Ge20-xSnx, and Si20-xSnx alloy (x = 0, 4, 8, 12, 16, and 20) in the P41212 phase were proven by phonon spectra and elastic constants. In terms of Young's modulus, the shear modulus, Poisson's ratio, and AU, Ge12Sn8 in the P41212 phase displayed the maximum anisotropy, while Si12Sn8 in the P41212 phase exhibited the minimum anisotropy. The electronic band structures with the HSE06 hybrid functional indicated that all Si20-xGex, Ge20-xSnx, and Si20-xSnx alloy (x = 0, 4, 8, 12, 16, and 20) in the P41212 phase were indirect band gap semiconductor materials. The Si-Ge alloy band gap modulation range in the P41212 phase was small compared to that of Si-Sn and Ge-Sn alloys in the P41212 phase, while Si-Sn and Ge-Sn alloys had a better band gap modulation effect.

Probing white matter microstructure in youth with chronic pain

Alterations in white matter (WM) have been reported in individuals with chronic pain, although findings are inconsistent, and mostly derived using the limited Diffusion Tensor Imaging (DTI) model. Going beyond DTI, one study reported lower WM complexity in corpus callosum, internal capsule and a hippocampus tract. In youth with chronic pain, no studies have examined WM changes, except in headache. The current study therefore examined potential WM alterations in youth with chronic pain using DTI as well as Neurite Orientation Dispersion and Density Imaging (NODDI), allowing more specific interrogation of WM microstructure. Multi-shell diffusion-weighted MR images were analyzed in 45 youth with chronic pain (age M=15.8±2.5, 41 females) and 25 controls (age M=16.0±3.2, 18 females). Preprocessing included estimation and correction of susceptibility-, eddy current-, and motion-induced distortions (FSL). DTI and NODDI models were fit, deriving indices of fractional anisotropy (FA/DTI), neurite density (NDI/NODDI) and orientation dispersion (ODI/NODDI). Data was spatially aligned using tensor-based alignment (DTI-TK), followed by tract-based spatial statistics (TBSS; TFCE-corrected). Group contrasts showed that while there were no group differences in FA and NDI, patients showed increased ODI in several tracts, including the corticospinal tract, corpus callosum and forceps minor (connecting medial and lateral frontal lobe), uncinate fasciculus (connecting frontal and temporal lobe), internal capsule (connecting subcortical structures) and fornix (a hippocampus tract). Our findings reveal specific WM alterations in several tracts that are implicated in (chronic) pain. In particular, the dispersion in fiber orientation was increased, reflective of less coherently organized fibers. This may indicate possible delayed pruning in these tracts. Neurite density indices were not different, reflective of similar amounts of fibers and myelin. Taken together, youth with chronic pain showed specific alterations in fiber dispersion of several WM tracts in cortico-limbic circuitry. Future work will examine associations with individual difference factors. NIH R01HD083270 to LE Simons.

Characterization of Hydrogen-Bonded Liquid Crystal Materials in the Millimeter-Wave Region

A novel liquid crystal (LC) materials consisting of hydrogen bonding are investigated in the millimeter-wave (MMW) frequency region of electromagnetic wave spectra. Relative permittivity (or refractive index) and loss tangent (or absorption coefficient) are evaluated using a rectangular waveguide test cell and are compared with those values of conventional LC materials. The most unusual result is that the refractive index anisotropy increases significantly in the MMW region comparing with that for visible light, which is contrary to what is observed in the conventional LC materials. Although the anisotropy, which determines the tunability of LC devices, is not so large in this stage, the loss tangent is as small as, or better than, those of the excellent conventional-type LC materials synthesized for the microwave applications. We find out a potential of the hydrogen-bonded LCs as one of the best classes of LC materials for MMW application.

Effect of terahertz laser field on anisotropic optical absorption and refractive index changes of coaxial square quantum well wires

The influence of terahertz laser field on nonlinear anisotropic optical properties, such as optical absorption and refractive index changes is studied. By solving the Schrödinger equation of the system in the framework of effective mass with the Kramers–Henneberger (KH) transformation, the eigenfunctions and energy eigenvalues of the system are obtained. The optical absorption and refractive index changes are deduced by the compact density matrix form and the scheme of iterative method. The results show that the polarization angle and intensity of terahertz laser field have a strongly influence on the peak of optical absorption and refractive index change in different directions respectively.

Exploration of D022-Type Al3TM(TM = Sc, Ti, V, Zr, Nb, Hf, Ta): Elastic Anisotropy, Electronic Structures, Work Function and Experimental Design.

The structural properties, elastic anisotropy, electronic structures and work function of D022-type Al3TM (TM = Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta) are studied using the first-principles calculations. The results indicate that the obtained formation enthalpy and cohesive energy of these compounds are in accordance with the other calculated values. It is found that the Al3Zr is the most thermodynamic stable compound. The mechanical property indexes, such as elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and Vickers hardness are systematically explored. Moreover, the calculated universal anisotropic index, percent anisotropy and shear anisotropic factors of D022-type Al3TM are analyzed carefully. It demonstrates that the shear modulus anisotropy of Al3La is the strongest, while that of Al3Ta is the weakest. In particular, the density of states at Fermi level is not zero, suggesting that these phases have metal properties and electrical conductivity. More importantly, the mechanisms of correlation between hardness and Young’s modulus are further explained by the work function. Finally, the experimental design proves that D022-Al3Ta has an excellent strengthening effect.

High frequency meta-ferroelectrics by inverse design

Composites with subwavelength features exhibit effective properties that depend on microstructure morphology and materials, which can be adjusted to obtain enhanced characteristics. We detail the systematic design of electromagnetic metamaterials composed of dielectric inclusions in a ferroelectric matrix that, under an applied voltage, present an optimized effective tunability higher than the bulk due to a nonlinear local electric field enhancement. The effect of volume fraction, losses, and biasing field on homogenized properties is investigated and the analysis of the photonic band diagram is carried out, providing the frequency dependence of the anisotropic effective index and tunability. Such metaceramics can be used in microwave antennas and components with higher reconfigurability and reduced power consumption.

A single grain boundary parameter to characterize normal stress fluctuations in materials with elastic cubic grains

A finite element analysis of intergranular normal stresses is performed in order to identify a possible statistical correlation between the intergranular normal stresses and the corresponding grain boundary type within a polycrystalline aggregate. Elastic continuum grains of cubic lattice symmetry are assumed in the analysis. Meaningful results are obtained by analyzing the first two statistical moments of grain boundary normal stresses obtained on several grain boundary types.Among the five macroscopic parameters (5D) describing a grain boundary, the orientation of the grain boundary plane relative to the two adjacent crystal lattices (4D) is found to be the most important property influencing the normal stresses. To account for it, a single new (1D) parameter E12 is introduced, which combines the geometrical aspect of grain boundary with its material properties and measures the average stiffness of grain boundary neighborhood along the grain boundary normal direction. It is demonstrated that E12, in combination with Zener elastic anisotropy index A, is able to accurately predict normal stress fluctuations on any grain boundary type in a material with cubic lattice symmetry. It is argued that largest normal stresses most likely form on grain boundaries whose normals are oriented along the stiffest direction in both adjacent grains (〈111〉 for crystals with A>1 or 〈001〉 for crystals with A<1). Moreover, it is shown that classification of grain boundaries according to their propensity to exhibit large normal stresses can be trivially reduced to the (analytical) calculation of the corresponding effective stiffness parameter E12.A few practical implications are discussed relevant to intergranular stress-corrosion cracking of Coincidence Site Lattice grain boundaries. For example, it is highlighted that in face-centered cubic materials the coherent Σ3 twin grain boundaries, which are known for their very high cracking resistance, nevertheless exhibit largest intergranular normal stresses, indicating that cracking resistance is associated with high grain boundary strength.

Simultaneous enhancement in thermal conductivity and flame retardancy of flexible film by introducing covalent bond connection

Owing to the high interface thermal resistance and aggregation in polymer-based materials, the thermal conductivity of multi-walled carbon nanotubes (MWCNTs) has been severely decreased in practical applications. In this work, an amino-functionalized black phosphorene (BP–NH 2 ) fabricated by ball milling has been employed to react with carboxylated multi-wall carbon nanotubes (MWCNTs–COOH) via covalent bonds to prepare a nanofiller (BP–MWCNTs) with high thermal conductivity and excellent flame retardancy. When incorporating 20.0 wt% BP–MWCNTs into cellulose nanofiber (CNF), the CNF/BP–MWCNTs 20.0 shows an in-plane thermal conductivity of 22.38 ± 0.39 W m −1 K −1 and a cross-plane thermal conductivity of 0.36 ± 0.03 W m −1 K −1 , resulting in an anisotropy index of 62.17. The effective medium theory (EMT) calculation demonstrates the interface thermal resistance is reduced to 1/39 that of pure MWCNTs, which is ascribed to the covalent bond between BP–NH 2 and MWCNTs–COOH. Simultaneously, the CNF/BP–MWCNTs 20.0 can pass the testing of UL-94 V-0 grade and the limiting oxygen index (LOI) value can be increased from 18.1% to 29.9%. Its peak of heat release rate (PHRR), total heat release (THR), smoke production rate (SPR), total smoke production (TSP), CO generated per second (COP) and CO 2 generated per second (CO 2 P) are decreased by 37.47%, 43.51%, 50.00%, 35.29%, 50.00% and 19.70%, respectively, which is due to the formation of an intumescent flame-retardant system rich in carbon source (MWCNTs), acid source (BP) and gas source (amino groups). This covalent linkage strategy offers a new thought for the preparation of flexible materials to enhance the thermal conductivity and fire resistance. • Aminated black phosphorene was successfully fabricated by ball milling. • Interface thermal resistance was significantly reduced. • Desirable thermal conductivity was obtained. • Efficient intumescent flame-retardant system was prepared.

In-plane anisotropic optical and mechanical properties of two-dimensional MoO3

Molybdenum trioxide (MoO3) in-plane anisotropy has increasingly attracted the attention of the scientific community in the last few years. Many of the observed in-plane anisotropic properties stem from the anisotropic refractive index and elastic constants of the material but a comprehensive analysis of these fundamental properties is still lacking. Here we employ Raman and micro-reflectance measurements, using polarized light, to determine the angular dependence of the refractive index of thin MoO3 flakes and we study the directional dependence of the MoO3 Young’s modulus using the buckling metrology method. We found that MoO3 displays one of the largest in-plane anisotropic mechanical properties reported for 2D materials so far.

New mechanism for optical super-resolution via anisotropic near-zero index metamaterials

In this paper, we propose a new mechanism for obtaining optical super-resolution based on anisotropic near-zero index metamaterials (AZIMs). It is found that AZIMs can not only achieve perfect transmission of electromagnetic energy but also realize a super-resolution lens. Moreover, we find the performance of super-resolution imaging is not sensitive to the loss and impedance mismatching, as analytically and numerically demonstrated. Our findings provide a new way to explore high-resolution optical imaging.

Noninvasive Assessment of Neurodevelopmental Disorders after In Utero Irradiation in Mice: An In Vivo Anatomical and Diffusion MRI Study.

In utero exposure to ionizing radiation can lead to cerebral alterations during adulthood. Using anatomical magnetic resonance imaging (MRI), it is possible to assess radiation-induced structural brain damage noninvasively. However, little is currently known about microstructure alterations in brain tissue. Therefore, the goal of this study was to establish, based on an original and robust pipeline of MRI image analysis, whether the long-term effects of in utero radiation exposure on brain tissue microstructure could be detected noninvasively. Pregnant C57BL/6N mice received a single dose of 1 Gy on gestation day 14.5, which led to behavioral impairments in adults. At 3 months old, in vivo MRI data were acquired from in utero irradiated and nonirradiated male mice. An MRI protocol was designed to assess the effects of radiation on the parameters of brain volume, non-Gaussian diffusion (ADC0, kurtosis and signature index) and anisotropic diffusion (fractional anisotropy and mean, axial, radial diffusivities and anisotropic signature index) in 10 key cerebral structures defined using an in-house atlas of the mouse brain. Based on the relative amplitude of these anatomical and microstructural changes, maps of the radiosensitivity of the brain to in utero irradiation were created. We observed microcephaly in irradiated mice with noticeably larger volume changes in the cortex and the corpus callosum. We also observed significantly lower ADC0, anisotropy fraction (sFA), radial diffusivity (sRD), as well as signature index (S-index and SI3) values, which are original markers sensitive to tissue microstructure alterations. All these changes together are in favor of a decreased cellular "imprint" and in some regions a reduced density in myelinated axons. A reduction in the number and complexity of myelinated axons was further revealed by myelin basic protein immunostaining. Combining anatomical and diffusion MRI is a promising approach to noninvasively investigate the radiosensitivity of local brain areas in adult mice after in utero irradiation in terms of microstructure.

New method for analysis of electrooptical response in liquid crystal devices with non-monotonous relaxation

ABSTRACT In the design and development of liquid crystal (LC) devices, it is crucial to determine the characteristic times of optical responses in devices driven with applied electric fields. These characteristic times associated with relaxation processes of the LC upon application (on) or removal (off) of an electric field are commonly calculated through measurement of the interval between 10% and 90% of the total change. Signals with non-monotonic oscillatory patterns originating either from the influence of backflow processes or from the variation in the phase shift between the anisotropic refractive indices in the reorientation dynamics complicate such a determination of the response times of LC devices. To obtain response times in LC devices with non-monotonous relaxation processes, we developed a novel numeric method that suppresses the oscillatory component of the signal and allows a direct determination of the characteristic times. The technique was validated with simulations of smooth and noisy data sets and was also applied to experimental data obtained from LC devices based on the LC 4-n-pentyl-4ʹ-cyanobiphenyl (5CB). The obtained results confirm that the method is reliable and applicable to data sets with complicated oscillatory patterns.

Dielectric and electro optical properties of 6CHBT nematic liquid crystals doped with MgO nanoparticles

ABSTRACT Recent progress has shown that a minor concentration of nanoparticles in liquid crystals can have an important effect on the electrical and optical properties of liquid crystals. In this study, the temperature-dependent dielectric and electro-optical properties of planar and homeotropic oriented 6CHBT nematic liquid crystals doped with MgO nanoparticles were investigated. MgO nanoparticles were dispersed in pure 6CHBT in various concentrations. It was established that the dielectric permittivity (ε|| and ε⊥) and the dielectric anisotropy (Δε) are decreased by adding nanoparticles up to a certain concentration, and then it increased. The effect of doping nanoparticles on the physical and optical parameters, namely, the ac electric conductivity, absorption, emission, threshold voltage, splay elastic constant, and anisotropic refractive index were investigated. In the frame of the new six-parameter dielectric permittivity model, the temperature-dependent dielectric properties were improved.

Early nutrition and white matter microstructure in children born very low birth weight.

Infants born at very low birth weight (<1500 g) are vulnerable to nutritional deficits during their first postnatal month, which are associated with poor neurodevelopmental outcomes. Despite this knowledge, the impact of early postnatal nutrition on white matter microstructure in children born with very low birth weight has not been investigated. In this prospective cohort study, we employed a whole-brain approach to investigate associations between precise estimates of nutrient intake within the first postnatal month with white matter microstructure at 5 years of age. Detailed information about breastmilk, macronutrient and energy intakes during this period were prospectively recorded for all participants. Multi-shell diffusion and T1-weighted MRIs were acquired in 41 children (21 males; mean scan age: 5.75 ± 0.22 years; mean birth weight: 1028.6 ± 256.8 g). The diffusion tensor imaging and neurite orientation dispersion and density imaging models were used to obtain maps of fractional anisotropy, radial diffusivity, orientation dispersion and neurite density indices. Tract-based spatial statistics was used to test associations between metrics of white matter microstructure with breastmilk, macronutrient (protein, lipids and carbohydrate) and energy intake. Associations between white matter microstructure and cognitive outcomes were also examined. Compared to children who did not meet enteral feeding recommendations, those who achieved enteral protein, lipid and energy recommendations during the first postnatal month showed improved white matter maturation at 5 years. Among the macronutrients, greater protein intake contributed most to the beneficial effect of nutrition, showing widespread increases in fractional anisotropy and reductions in radial diffusivity. No significant associations were found between white matter metrics with breastmilk or carbohydrate intake. Voxel-wise analyses with cognitive outcomes revealed significant associations between higher fractional anisotropy and neurite density index with higher processing speed scores. Lower radial diffusivity and orientation dispersion index were also associated with improved processing speed. Our findings support the long-term impacts of early nutrition on white matter microstructure, which in turn is related to cognitive outcomes. These results provide strong support for early postnatal nutritional intervention as a promising strategy to improve long-term cognitive outcomes of infants born at very low birth weight.

Anisotropies in Elasticity, Sound Velocity, and Minimum Thermal Conductivity of Low Borides VxBy Compounds

Anisotropies in the elasticity, sound velocity, and minimum thermal conductivity of low borides VB, V5B6, V3B4, and V2B3 are discussed using the first-principles calculations. The various elastic anisotropic indexes (AU, Acomp, and Ashear), three-dimensional (3D) surface contours, and their planar projections among different crystallographic planes of bulk modulus, shear modulus, and Young’s modulus are used to characterize elastic anisotropy. The bulk, shear, and Young’s moduli all show relatively strong degrees of anisotropy. With increased B content, the degree of anisotropy of the bulk modulus increases while those of the shear modulus and Young’s modulus decrease. The anisotropies of the sound velocity in the different planes show obvious differences. Meanwhile, the minimum thermal conductivity shows little dependence on crystallographic direction.

Insight into the structural stability and overall performances of V2REAl20 ternary phases

In order to expand the applications of aluminum alloy, the ternary aluminum-based intermetallics have attracted the attentions of scientists. In present work, the overall performances of V2REAl20 (RE = Ho, Gd, Ce, La, Y, and Sc) were explored by DFT investigations. The thermodynamic data, lattice vibrations and volume change curves illustrate that six ternary phases are structurally stable. The values of elastic modulus and elastic constants demonstrate that V2REAl20 compounds display the better mechanical properties than aluminum. The 3D and 2D curves constructions and anisotropic indexes explain that V2REAl20 compounds possess the weak anisotropy. Moreover, the thermodynamic curves indicate that ternary V2REAl20 compounds have the better physical properties than aluminum with the elevated temperatures. In the meantime, we provided the discussion in comparison to the results of ternary Ti2REAl20 compounds.

Association of Gyrification Pattern, White Matter Changes, and Phenotypic Profile in Patients With Parkinson Disease.

To investigate the cortical gyrification changes as well as their relationships with white matter (WM) microstructural abnormalities in the akinetic-rigid (AR) and tremor-dominant (TD) subtypes of Parkinson disease (PD). Sixty-four patients with the AR subtype, 26 patients with the TD subtype, and 56 healthy controls (HCs) were included in this study. High-resolution T1-weighted and diffusion-weighted images were acquired for each participant. We computed local gyrification index (LGI) and fractional anisotropy (FA) to identify the cortical gyrification and WM microstructural changes in the AR and TD subtypes. Compared with HCs, patients with the AR subtype showed decreased LGI in the precentral, postcentral, inferior and superior parietal, middle and superior frontal/temporal, anterior and posterior cingulate, orbitofrontal, supramarginal, precuneus, and some visual cortices, and decreased FA in the corticospinal tract, inferior and superior longitudinal fasciculus, inferior fronto-occipital fasciculus, forceps minor/major, and anterior thalamic radiation. Decreases in LGI and FA of the AR subtype were found to be tightly coupled. LGIs of the left inferior and middle frontal gyrus correlated with Mini-Mental State Examination and Hoehn & Yahr scores of patients with the AR subtype. Patients with the TD subtype showed no significant change in the LGI and FA compared with patients with the AR subtype and HCs. Our results suggest that cortical gyrification changes in PD are motor phenotype-specific and are possibly mediated by the microstructural abnormalities of the underlying WM tracts.

Exploration of the structure, properties, and anisotropy of (r, h)-TiAl2 under high pressure

In this study, we use the first principles calculation based on the density functional theory (DFT) to investigate the effects of high pressure on the structural, anisotropic, mechanical, and thermal properties and the phase stability of r-TiAl2 (HfGa2 type) and h-TiAl2 (ZrGa2 type). The calculation results of the formation enthalpy indicate that r-TiAl2 and h-TiAl2 remain stable under the 0–40 GPa. There is no phase transition, and the two phase structures are mechanically stable under the applied pressure. The Bulk modulus (B), Shear modulus (G), and Young's modulus (E) increase with increasing pressure from 0 to 40 GPa. The 3D surface structure of the elastic modulus and the universal elastic anisotropy index (AU) indicate that the elastic anisotropy of h-TiAl2 is higher than that of r-TiAl2. In addition, Sound velocity and Debye temperature are investigated from analysis of the elastic constants and moduli. Finally, the density of states and charge density difference are also discussed.

Birefringent whispering gallery cavities designed by linear transformation optics.

It was reported that whispering gallery cavities designed by conformal transformation optics can support high-Q resonant modes with emission directionality. Intrinsically, these cavities have gradient index profiles implementing conformal mappings in physical space. In this paper, using the linear coordinate transformation, we propose another design scheme of whispering gallery cavities with (piecewise-) homogeneous, anisotropic index profile. We numerically show that so-designed cavities are also able to support high-Q whispering gallery modes with directional far-field emission patterns. We verify such characteristics by using a phase space representation (called the Poincaré Husimi function) of the intracavity wave function.

MgO nanoparticles dispersion effects in electro optical parameters of 5CB nematic liquid crystal

MgO nanoparticles (NPs) have been dispersed in pure 5CB in three various concentrations. It was found that by incrementing NPs concentration, dielectric permittivities (ε ∥ and ε ⊥) and dielectric anisotropy (Δε) continuously decrease. Real dielectric permittivities of pure and doped liquid crystals in terms of frequency in the range 50–500Hz were obtained. The influence of nano doping on physical and optical parameters, i.e. threshold voltage, ac electric conductivity, phase transition temperature, light transmission, splay elastic constant and refractive index anisotropy was investigated. The temperature-dependent dielectric properties have been improved in the frame of a novel six-parameter dielectric permittivity model.