Slight Anisotropy Research Articles

Bound-to-continuum intraband transition properties in InAs/GaAs quantum dot superlattice solar cells

We studied the interband and intraband excitation densities and the polarization dependent photocurrent in InAs/GaAs quantum dot superlattice (QDSL) solar cells with a built-in electric field of 5 kV cm−1. An almost polarization-insensitive intraband transition was observed for normal incident light, whereas slight anisotropy in the intraband transition resulted from the mixing between the conduction and valence bands. Furthermore, saturable two-color photocurrent was observed in the p–i–n solar-cell device because of electron transport in the QDSL miniband.

Sustained Self-Starting Orbital Motion of a Glass-Fiber “Nanoengine” Driven by Photophoretic Forces

Controllable optically driven rotation of microscopic objects is desirable in many applications, but is difficult to achieve. Here we report a sustained self-starting orbital motion of a clamped elongated nanostructure, a glass-fiber nanospike, when a CW laser beam is focused axially onto its tip. Analysis shows that photophoretic antitrapping forces, acting on the nanospike with a delayed response, introduce optomechanical gain into the mechanical motion, overcoming the intrinsic mechanical dissipation and resulting in growth from noise of oscillations at the resonant frequency of the nanospike. These photophoretic forces further enable phase-locking of the orthogonal fast and slow vibrations of the nanospike (induced by slight mechanical anisotropy), giving rise to a self-sustained orbital motion. The locked phase of orbital motion can be changed by tuning the gas pressure and adjusting the geometrical asymmetry of the system. This light-driven nanoengine opens up a new degree of freedom for controlling the rotational motion of elongated nano-objects.

Anisotropic fracture modeling of sheet metals: From in-plane to out-of-plane

Sheet metals usually exhibit varying degrees of anisotropy due to formation of texture during rolling process. The grain structures and distributions in sheet normal direction may also be different from that within the sheet plane. For example, martensite banding structures are usually observed in the middle of thickness for advanced high strength steels. The particular martensite morphology and inhomogeneous distribution will greatly affect the deformation and fracture behavior under out-of-plane direction. It is necessary to model the anisotropic fracture of sheet metals from in-plane to out-of-plane. In this work, a new stress invariant based ductile fracture criterion was developed by introducing a stress triaxiality and normalized third invariant dependent function h(η, ξ) and further extended to an anisotropic form through the linear transformation of stress tensor. Parametric study showed that the new anisotropic criterion can describe the direction dependency of ductile fracture in both strain and stress spaces. A series of in-plane fracture tests including tension with a central hole, notched tension, V-bending test, Nakajima test and in-plane shear as well as a new out-of-plane shear test were conducted to study the anisotropic fracture behavior of DP980 sheet metals over a variety of stress states. Hybrid experimental-numerical method is used to determine the fracture strains and the loading paths to fracture. The results show that slight in-plane anisotropy of fracture responses exists for this DP980 sheet, while the out-of-plane shear fracture stress is approximately 11% lower than that under the in-plane shear condition. The new anisotropic ductile fracture criterion is able to predict the anisotropic fracture behaviors of this DP980 sheet with high accuracy.

In- and through-plane conductivity of 8YSZ films produced at room temperature by aerosol deposition

Aerosol deposition is a unique spray coating process to deposit dense ceramic films directly at room temperature just by spraying a dry powder, without any binder or additional sintering step needed. Therefore, it is a very promising coating method for functional materials that may be used to produce a variety of electrochemical devices, solid oxide fuel cells or gas sensors. However, functional properties like the electrical conductivity significantly depend on the microstructure of the produced film. Consequently, anisotropic film properties may occur even for isotropic functional materials. In this work, films of oxide ion-conducting yttria-stabilized zirconia (YSZ) were produced by aerosol deposition. The orientation-dependent electrical conductivity was measured using two different electrode configurations: one for the in-plane conductivity and another one for through-plane measurements. A slight anisotropy was observed, attributed to the present film morphology. In particular, the grain boundaries influenced the total conductivity. Furthermore, a thermal post-deposition treatment significantly affected the total conductivity of aerosol-deposited YSZ films.

Pore Fabric Anisotropy of the Cambrian–Ordovician Nubia Sandstone in the Onshore Gulf of Suez, Egypt: A Surface Outcrop Analog

Integrated studies on pore fabric anisotropy have increased the general understanding of fluid flow patterns through reservoir rocks. In this study, pore anisotropy was studied based on measuring permeability and formation resistivity factors in vertical and horizontal directions for 130 plug samples from a total of 65 oriented block samples. These samples were representatively collected for the Nubia sandstones C and D in the western southern onshore of the Gulf of Suez. In addition, the porosity was measured using two techniques, namely water and helium injection. The effective pore radius r35 and the pore radius of the displacement pressure (rdp) were also measured. Petrographic studies of some representative thin sections and scanning electron microscope studies (SEM) were applied to study the mineral compositions of the studied samples to declare the most important porosity-reducing and porosity-enhancing diagenetic factors. The results show that the studied samples can be categorized into three rock types (RRTs), namely quartz arenite (RRT1), quartz wacke (RRT2) and mudstone (RRT3). The best storage and flow capacity was assigned to the RRT1 samples, whereas the least quality was assigned to the RRT3 samples. This could be attributed to wide pore throat distributions and the anisotropy of pore spaces due to the presence of a vertical subsidiary fracture system that dominated in many samples. This system was enhanced by introducing authigenic kaolinite as pore-filling clay minerals, causing the reduction in vertical permeability but supporting the vertical electric current flow. For the RRT3 samples, this vertical micro-fracture system was reduced by silica cementation which caused the reduction in both fluid and electric current flow. The pore anisotropy of most of the studied samples is in the range of slight anisotropy with some exceptions in the RRT3 samples which are characterized by moderate anisotropy.

Maltodextrin promotes calcium caseinate fibre formation through air inclusion

Commercial calcium caseinate is available as spray-dried and roller-dried powder. Shearing a dense spray-dried calcium caseinate dispersion gives rise to a fibrous material, whereas shearing dense roller-dried calcium caseinate yields a layered material with only slight anisotropy in mechanical strength. The addition of a polysaccharide phase in a continuous protein phase may lead to formation of fibrous structures after shearing, which is hypothesized to be a result of the elongation and orientation of the dispersed polysaccharide domains. We report the effect of the addition of maltodextrin to roller-dried calcium caseinate on structure formation. The strength of the material increased with the addition of maltodextrin, which is partly caused by the withdrawal of water from the caseinate phase towards the maltodextrin phase, leading to a higher local caseinate concentration. The anisotropy of fracture stress and fracture strain were enhanced with up to 5 wt% maltodextrin. The effect of maltodextrin on the mechanical anisotropy and fibrous appearance could be ascribed to the greater air incorporation as a result of the presence of maltodextrin.

Ultrasonic wave propagation predictions for polycrystalline materials using three-dimensional synthetic microstructures: Attenuation.

Ultrasonic attenuation plays a crucial role in inspection for heterogeneous materials such that theoretical models are critical for improved measurements. In this article, several assumptions often used in these models are examined with respect to their influence on attenuation. Here, dream.3d software is used to generate 10 ensembles with different volumes, each containing 50 realizations of equiaxed grains with cubic single-crystal symmetry, from which attenuations are calculated. Comparisons are then made with attenuation values derived from classical theories. These theories often decouple the spatial and tensorial components of the microstructure, assume statistical isotropy, and use a spatial correlation function that has a specific exponential form. The validity of these assumptions is examined by calculation of the spatial statistics to obtain the attenuations in their most general form. The results of Voigt-averaged results for nickel at 15 MHz show that the longitudinal and transverse attenuations are about one-third and one-fourth of those obtained from the theory, respectively. Such a difference is attributed to the relevant spatial correlation functions. The results also show a slight anisotropy in the attenuation. Finally, for microstructures with narrow grain size distributions and weak texture, the decoupling assumption is shown to be valid.

Creep and damage anisotropies of 9%Cr and 14%Cr ODS steel cladding

Two Oxide Dispersion Strengthened (ODS) steel tubes, obtained by means of hot extrusion and cold rolling, are studied as potential materials for cladding applications in sodium-cooled fast reactors. A 9%Cr martensitic tube (Fe9Cr1W-0.3Ti-0.25Y2O3) with isotropic microstructure is compared to a 14%Cr ferritic tube (Fe14Cr1W-0.3Ti-0.25Y2O3) with elongated grains and a strong texture. Tile and notched ring specimens are used to perform uniaxial creep tests at 650 °C in the longitudinal and transverse directions of the tubes. The 14%Cr ODS steel tube exhibits strong mechanical anisotropies, while the 9%Cr presents only a slight anisotropy in creep resistance, presumably due to the influence of specimen geometry on mechanical fields. Both intergranular and ductile zones have been observed on the fracture surfaces of the uniaxial creep specimens, suggesting the competition of two damage mechanisms. Damage for both tubes appeared anisotropic, with orientated cracks in the rolling directions on the ring specimens and the 14%Cr tile specimens. Internal pressure creep tests also performed at 650 °C show complementary results for both tubes in agreement with ring specimens’ ones, validating their use for hoop properties assessment. Overall, while the 14%Cr ODS steel tube is strong in the longitudinal direction, the 9%Cr tube proves almost no remarkable anisotropy in creep strength.

Whistler Wave Generation by Halo Electrons in the Solar Wind

Abstract We present an analysis of simultaneous particle and field measurements from the ARTEMIS spacecraft, which demonstrates that quasi-parallel whistler waves in the solar wind can be generated locally by a bulk flow of halo electrons (whistler heat flux instability). ARTEMIS observes quasi-parallel whistler waves in the frequency range ∼0.05−0.2f ce simultaneously with electron velocity distribution functions that are a combination of counter-streaming core and halo populations. A linear stability analysis shows that the plasma is stable when there are no whistler waves, and it is unstable in the presence of whistler waves. In the latter case, the stability analysis shows that the whistler wave growth time is from a few to 10 seconds at frequencies and wavenumbers that match the observations. The observations clearly demonstrate that the temperature anisotropy of halo electrons crucially affects the heat flux instability onset: a slight anisotropy T ∥/T ⊥ > 1 may quench the instability, while a slight anisotropy T ∥/T ⊥ < 1 may significantly increase the growth rate. These results demonstrate that heat flux inhibition is strongly dependent on the microscopic plasma properties.

Sintering evaluation of doped lanthanum gallate based on thermodilatometry

The sintering behavior of La0.9Sr0.1Ga0.8Mg0.2O3-δ oxide-ion conductor was systematically investigated by thermodilatometry. The shrinkage data obtained with heating rates of 4, 7, 10 and 12 °C min−1 were analyzed by the constant rate of heating model and by construction of the master sintering curve. Validation of the master sintering curve was carried out by measurements of density in conventionally sintered specimens. Slight anisotropy of shrinkage data was found and changes to the basic equation of density was proposed to account for this effect. Plotting the data determined by the constant rate of heating model versus density allowed an easy identification of the density range of constant activation energy. The activation energy (865 kJ mol−1) obtained from the master sintering curve correlates quite well with that (874 kJ mol−1) obtained by the constant rate of heating model.

Determination of the Anisotropic Rotational Diffusion Constant of Microcrystals Dispersed in Liquid Medium.

Microcrystals of ErBa2Cu4O8 suspended in a liquid medium were triaxially aligned by a frequency-modulated magnetic field and allowed a free rotational relaxation after the magnetic field was turned off. In situ X-ray diffraction measurements of the suspension were performed during relaxation, and the temporal change of the orientation fluctuation was monitored via broadening of the diffraction spots. The rotational diffusion constants were determined using the plot of the orientation fluctuation versus the elapsed time of rotational relaxation. The diffusion constants thus determined were in close agreement with those evaluated by the Stokes law but showed slight anisotropy, indicating that the microcrystals studied had shape anisotropy. The present method can provide a useful means for experimentally determining rotational diffusion constants of microcrystals suspended in viscous media. This paper shows that, due to the combination of the initial triaxial alignment and the subsequent monitoring of the relaxation process by means of X-ray diffraction, the diffusion constants along arbitrary crystallographic axes are determined separately.

Electromagnetic-radiation effect on alpha decay

The effect of the electromagnetic radiation on the spontaneous charge emission from heavy atomic nuclei is estimated in a model which may be relevant for proton emission and alphaparticle decay in laser fields. Arguments are given that the electronic cloud in heavy atoms screens appreciably the electric field acting on the nucleus and the nucleus “sees” rather low fields. In these conditions, it is shown that the electromagnetic radiation brings second-order corrections in the electric field to the disintegration rate, with a slight anisotropy. These corrections give a small enhancement of the disintegration rate. The case of a static electric field is also discussed.

Nature of the Debye-Process in Monohydroxy Alcohols: 5-Methyl-2-Hexanol Investigated by Depolarized Light Scattering and Dielectric Spectroscopy.

The slow Debye-like relaxation in the dielectric spectra of monohydroxy alcohols is a matter of long-standing debate. In the present Letter, we probe reorientational dynamics of 5-methyl-2-hexanol with dielectric spectroscopy and depolarized dynamic light scattering (DDLS) in the supercooled regime. While in a previous study of a primary alcohol no indication of the Debye peak in the DDLS spectra was found, we now for the first time report clear evidence of a Debye contribution in a monoalcohol in DDLS. A quantitative comparison between the dielectric and DDLS manifestation of the Debye peak reveals that while the dielectric Debye process represents fluctuations in the end-to-end vector dipole moment of the transient chains, its occurrence in DDLS shows a more local signature and is related to residual correlations that occur due to a slight anisotropy of the α relaxation caused by the chain formation.

Reactive molecular dynamics simulations of the mechanical properties of various phosphorene allotropes

Although various phosphorene allotropes have been theoretically predicted to be stable at 0 K, the mechanical properties and fracture mechanism at room temperature remain unclear for many of them. We investigate through reactive molecular dynamics simulations at room temperature the mechanical properties of phosphorene allotropes including: five sheets with hexagonal structures (β-, γ-, δ-, θ-, and α-phosphorene), one sheet with 4-8 membered rings (4-8-P), and two sheets with 5-7 membered rings. High, moderate and slight anisotropies in their mechanical properties are observed, depending on their crystal structures. Their Young’s moduli and tensile strength are approximately in the range from 7.3% through 25%, and from 8.6% through 22% of those of graphene, respectively. At the early stage of fracture, eye-shaped cracks are formed by local bond breaking and perpendicular to the tensile direction in hexagonal and 4-8-P sheets. Complete fractures take place with straight cracks in these hexagonal sheets under tension along the zigzag direction and under tension along the square edge direction in the 4-8-P sheet. Crack meandering and branching are observed during the tension of α-, β-, and γ-phosphorene along the armchair direction; and along the square diagonal direction in the 4-8-P sheet. Under uniaxial tension of two phosphorene sheets with 5-7 atom rings, 12 and 10 membered rings are formed by merging two neighbor heptagons, and a heptagon and its neighbor pentagon, respectively. These 12 and 10 membered rings coalesce subsequently, causing the failure of these two sheets. The results are of great importance in the design of these novel phosphorene allotropes.

Anisotropic properties of Na-\u03b2\u2033-alumina + YSZ composite synthesized by vapor phase method

Two α-alumina + YSZ samples were prepared by sintering for 3 h one at 1500 °C and the other at 1700 °C. The samples were then converted to Na-β″-alumina + YSZ by vapor phase conversion. Characterization techniques such as X-ray diffraction, scanning electron microscopy, and electrochemical impedance spectroscopy in addition to dimensional geometrical changes reveal the evolution of slight anisotropy in these samples during conversion. This results in an electrical conductivity anisotropy factor of about 5.5 and 1.8 for samples sintered at 1500 °C and 1700 °C, respectively. In all samples, the higher ionic conductivity was measured across the sample thickness as opposed to parallel to the disc faces. The ionic conductivity measurements show the conductivity of about 0.15 S/cm and 0.07 S/cm at 300 °C for samples sintered at 1500 °C and 1700 °C, respectively. The larger anisotropy in samples sintered at 1500 °C is explained by the higher aspect ratio of grains in this sample and by different Na concentrations.

Influence of strain rate and temperature on the mechanical behaviour of rubber-modified polypropylene and cross-linked polyethylene

In the present work, we investigate the effects of strain rate (e˙=0.01 s−1, 0.1 s−1, and 1.0 s−1) and low temperature (T=−30 °C, −15 °C, 0 °C, and 25 °C) on the mechanical behaviour in tension and compression of two materials: a rubber-modified polypropylene copolymer (PP) and a cross-linked low-density polyethylene (XLPE). Local stress-strain data for large deformations are obtained using digital image correlation (DIC) in the uniaxial tension tests and point tracking in the compression tests. Since both materials exhibit slight transverse anisotropy, two digital cameras are used to capture the strains on two perpendicular surfaces. Self-heating resulting from the elevated strain rates is monitored using an infrared (IR) camera. To enable the application of multiple digital cameras and an IR camera, a purpose-built transparent polycarbonate temperature chamber is used to create a cold environment for the tests. The mechanical behaviour of both materials, including the true stress-strain response and the volume change, is shown to be dependent on the temperature and strain rate. The dependence of the yield stress on the temperature and strain rate follows the Ree-Eyring flow theory for both materials, whereas Young’s modulus increases with decreasing temperature for PP and XLPE and with increasing strain rate for XLPE. Furthermore, a scanning electron microscope (SEM) study was performed on both materials to get a qualitative understanding of the volumetric strains.

Screening masses in strong external magnetic fields

We present results for the (color)magnetic and (color)electric screening masses of the Quark-Gluon Plasma in the presence of an external magnetic field. The screening masses are extracted from the correlators of Polyakov loops, determined by lattice QCD simulations at the physical point. We explore temperatures in the range $200\,\mathrm{MeV}\lesssim T\lesssim 330\,\mathrm{MeV}$ and magnetic field intensities up to $|e|B \sim 1.3\,\mathrm{GeV}^2$. We find that both screening masses are increasing functions of the magnetic field and that the dependence on $B$ becomes weaker for larger temperatures. In the case of the magnetic screening mass a slight anisotropy is also observable.

Theoretical prediction of new C–Si alloys in -20 structure**Project supported by the National Natural Science Foundation of China (Grant No. 61474089) and the Open Fund of Key Laboratory of Complex Electromagnetic Environment Science and Technology, China Academy of Engineering Physics (Grant No. 2015-0214.XY.K).

We study structural, mechanical, and electronic properties of C20, Si20 and their alloys (C16Si4, C12Si8, C8Si12, and C4 in structure by using density functional theory (DFT) based on first-principles calculations. The obtained elastic constants and the phonon spectra reveal mechanical and dynamic stability. The calculated formation enthalpy shows that the C–Si alloys might exist at a specified high temperature scale. The ratio of B/G and Poisson’s ratio indicate that these C–Si alloys in -20 structure are all brittle. The elastic anisotropic properties derived by bulk modulus and shear modulus show slight anisotropy. In addition, the band structures and density of states are also depicted, which reveal that C20, C16Si4, and Si20 are indirect band gap semiconductors, while C8Si12 and C4Si16 are semi-metallic alloys. Notably, a direct band gap semiconductor (C12Si8) is obtained by doping two indirect band gap semiconductors (C20 and Si20).

Shear induced weakening of the hydrogen bonding lattice of the energetic material 5,5′-Hydrazinebistetrazole at high-pressure

5,5′-Hydrazinebistetrazole (HBTA) has been studied by in-situ x-ray diffraction and vibrational spectroscopy to pressures near 25 GPa at room temperature. Analysis of the x-ray diffraction pattern of HBTA collected at ambient pressure and temperature revealed a monoclinic structure consistent with that previously reported. Under compression, the x-ray diffraction reveals little evidence of a phase transition over the pressure range studied. Slight anisotropy in response to compression was noted and the β angle decreased moderately, suggesting geometry modifications occur in the hydrogen bonding lattice and between neighboring HBTA molecules as a result of compression along the c axis. Blue shifts in the Infrared active NH stretching modes were observed, implying a weakening of the hydrogen bond with compression. The weakening of the hydrogen bonding lattice with pressure may lead to an increase in the bending angle of the CNNC bridge between the tetrazole rings and an increased overlap between the π-bonding orbitals. The Raman spectra showed a number of modes associated with HNNH motions of the bridge become more prominent in the spectra under compression. Additionally, the possibility that the increased bend in the angle of the CNNC bridge results from a shearing deformation is discussed.

Molecular ordering of PAH/MA-co-DR13 azopolymer layer-by-layer films probed by second-harmonic generation.

Molecular orientation within azopolymer thin films is important for their nonlinear optical properties and photonic applications. We have used optical second-harmonic generation (SHG) to study the molecular orientation of Layer-by-Layer (LbL) films of a cationic polyelectrolyte (poly(allylamine hydrochloride)) and an anionic polyelectrolyte containing azochromophore side groups (MA-co-DR13) on a glass substrate. The SHG measurements indicate that there is a preferential orientation of the azochromophores in the film, leading to a significant optical nonlinearity. However, both the signal strength and its anisotropy are not homogeneous throughout the sample, indicating the presence of large orientational domains. This is corroborated with Brewster angle microscopy. The average SHG signal does not increase with film thickness, in contrast to some reports in the literature, indicating an independent orientational order for successive bilayers. Analyzing the SHG signal as a function of the input and output polarizations, a few parameters of the azochromophore orientational distribution can be deduced. Fitting the SHG signal to a simple model distribution, we have concluded that the chromophores have an angular distribution with a slight in-plane anisotropy and a mean polar angle ranging from 45° to 80° with respect to substrate normal direction, with a relatively large width of about 25°. These results show that SHG is a powerful technique for a detailed investigation of the molecular orientation in azopolymer LbL films, allowing a deeper understanding of their self-assembling mechanism and nonlinear optical properties. The inhomogeneity and anisotropy of these films may have important consequences for their applications in nonlinear optical devices.