Effect Of Grain Alignment Research Articles

Effects of Grain Alignment with Magnetic Fields on Grain Growth and the Structure of Dust Aggregates

Abstract Dust grains drift through the interstellar medium and are aligned with the magnetic field. Here we study the effect of grain alignment and motion on grain growth in molecular clouds (MCs). We first discuss the characteristic timescales of alignment of the grain axis of maximum inertia ( a ˆ 1) with its angular momentum (J; i.e., internal alignment) and alignment of J with the magnetic field ( B ; i.e., external alignment). We determine the maximum grain size with efficient internal ( a max , aJ ) and external ( a max , JB ) alignment for composite grains. For the MC density of n H ∼ 103–108 cm−3, we find that external alignment can occur for very large grains, but internal alignment only occurs for grains smaller than a max , aJ ∼ 2 μ m . The presence of iron clusters within dust grains or suprathermal rotation increases a max , aJ to ∼10–50 μm. We then study the growth of aligned grains drifting through the gas. Due to the motion of aligned grains across the magnetic field, gas accretion would increase the grain elongation rather than decrease, as expected from the growth of randomly oriented grains. Coagulation by grain collisions also increases grain elongation, leading to the increase of elongation with the grain size. The coagulation of aligned grains forms dust aggregates that contain elongated binaries comprising a pair of grains with parallel short axes. Grains within dust aggregates in 67P/Churyumov–Gerasimenko obtained by Rosetta have the grain elongation increasing with the grain radius, implying that such dust aggregates might form from aligned grains.

Observations of Magnetic Fields Surrounding LkHα 101 Taken by the BISTRO Survey with JCMT-POL-2

Abstract We report the first high spatial resolution measurement of magnetic fields surrounding LkHα 101, part of the Auriga–California molecular cloud. The observations were taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 μm is found to be mostly associated with the redshifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis–Chandrasekhar–Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of B POS ∼ 115 μG. The mass-to-magnetic-flux ratio in critical value units, λ ∼ 0.3, is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkHα 101 region is subcritical, and the magnetic field is strong enough to prevent gravitational collapse. The inferred δ B/B 0 ∼ 0.3 implies that the large-scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power law with an index of α = 0.82 ± 0.03, which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkHα 101) in the region. Magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques can potentially explain such a decreasing trend.

The effects of microstructural properties and temperature on the mechanical behavior of Nextel 720 composite fibers: A novel multiscale model

A novel multiscale technique called the Bridging Cell Method (BCM) was used to model the mechanical behavior of Nextel 720 composite fibers across a range of temperatures. The previously validated room temperature Nextel BCM was used as a benchmark, and a temperature-dependent formulation was employed to account for the temperature effect. The model was tested under uniaxial tensile loads ranging from room temperature to 1700 K. The results were evaluated in terms of ultimate tensile strength (UTS), elasticity, and failure strain. The temperature-dependent Nextel BCM was validated via experimental data obtained from the literature. The high-temperature drop in composite's strength was predicted by the Nextel BCM. This was the result of implementing the fibers' microstructural evolution into the model, caused by phase transformation occurring at higher temperatures. The model was further validated with the fracture resistance variations of the composite that resulted from changes in crack length. The fracture behavior of the fibers at room and high temperatures were tested, by applying inter- and intra-granular cracks. Finally, the effect of grain alignment with respect to the applied load was investigated. The results show that the Nextel BCM is capable of predicting Nextel 720 composite's mechanical behavior across a range of temperatures.

Enhancement of magnetic relaxation behavior by texturing in Bi-2212 superconducting rods

Time decay of magnetization, known as magnetic relaxation, is crucial for both fundamental and applied point of view in bulk high temperature superconductors (HTS) by setting the limits to the HTS devices stability. Melt-processed Bi2Sr2Ca1Cu2−xGaxO8+δ rods (Bi-2212, x=0, 0.1) were used to study the effect of both grain alignment and substitution on the samples critical current density, relaxation and pinning behavior. The magnetic field has been applied both perpendicular and parallel to the rods growth axis to determine the effect of grain alignment. It has been found that Ga substitution reduces grains orientation and sizes leading to lower magnetic properties. The peaks of the curves, which indicate the temperature dependence of the samples magnetic relaxation rate (S), have been observed in the 7–35K temperature range. Characteristic pinning energy (Ue/kB) of samples was determined using the formalism developed by Maley. The change of pinning energy as a function of magnetization has been found to be exponential between 3 and 60K, which is in agreement with the collective creep theory.

Effect of grain alignment on lateral carrier transport in aligned-crystalline silicon films on polycrystalline substrates

We report the studies on the effect of grain alignment on lateral carrier transport in nominally 〈001〉-oriented aligned-crystalline silicon (ACSi) films on polycrystalline substrates. With improving grain alignment, energy barrier height at the grain boundaries was reduced from 150 to less than 1 meV, and both conductivity and Hall mobility became less sensitive to hydrogen passivation. This suggests that the dangling bonds in ACSi films are a major source of trapping sites, and that they become less dominant with improving grain alignment. These results demonstrate that improving grain alignment enhances the lateral carrier transport in small-grained (≤1 μm) polycrystalline silicon films, by reducing dangling bond density at the grain boundaries.

Effect of grain alignment on interface trap density of thermally oxidized aligned-crystalline silicon films

The authors report studies of the effect of grain alignment on interface trap density of thermally oxidized aligned-crystalline silicon (ACSi) films by means of capacitance-voltage (C-V) measurements. C-V curves were measured on metal-oxide-semiconductor (MOS) capacitors fabricated on ⟨001⟩-oriented ACSi films on polycrystalline substrates. From high-frequency C-V curves, the authors calculated a decrease of interface trap density from 2×1012to1×1011cm−2eV−1 as the grain mosaic spread in ACSi films improved from 13.7° to 6.5°. These results demonstrate the effectiveness of grain alignment as a process technique to achieve significantly enhanced performance in small-grained (⩽1μm) polycrystalline Si MOS-type devices.

Temperature dependence of pinning potentials in grain-aligned and Re-doped Hg-1223 superconductors

Effect of grain alignment and Re-doping on the flux-creep phenomena in Hg-1223 superconductors is investigated. Hg-based superconductors of HgBa 2Ca 2Cu 3Re x O y ( x = 0.15 and 0.18) were fabricated by high vacuum sealing and sintering method using a precursor. Grain alignment was formed magnetically under external magnetic field of 10 T. Connectivity between the superconducting CuO 2 layers is known to be enhanced with addition of Re. Flux creep characteristics and DC magnetizations were measured using a SQUID magnetometer at temperature range 5–150 K under magnetic fields up to 1 T. The pinning potentials U 0 ∗ estimated from the logarithmic time dependence of the magnetization for B∥ ab are generally larger than those for B∥ c. Non-linear relation of pinning barrier energy U( J) on current density J leads to apparently low energy in the high J region which is remarkable at low temperatures. Independence of the U 0 ∗ on the applied fields B∥ c in this region and increase of the U 0 ∗ with lowering B∥ ab indicate the difference of non-linearity. In addition, temperature dependence of the flux creep rate S is favorably compared with the Anderson–Kim model.

ルテチアを焼結助剤とする配向組織窒化ケイ素の作製と機械特性

Porous anisotropic silicon nitrides exhibiting superior mechanical properties at elevated temperatures were successfully fabricated. The silicon nitrides containing lutetia (Lu2O3) as sintering additive were fabricated using a partial sinter-forging technique in which uniaxial pressure was applied after heating up to the sinter-forging temperature. Rod-like silicon nitride grains were formed during the heating, and they were aligned by the subsequent forging. Strength of a specimen with porosity of about 23% was more than 600 MPa at room temperature (R.T.), which was achieved by choosing a suitable sinter-forging condition. It maintained more than 450 MPa even at 1500°C. Fracture energies at R.T. and 1500°C were 110 and 150 J•m-2, respectively. The concurrent achievement of high strength and fracture energy is very likely the result of the combined effects of grain alignment, presence of pores between grains, and the lutetia sintering additive. High strength at elevated temperatures is caused by the lutetia sintering additive.

High Temperature Mechanical Properties of Partially Sinter-Forged Porous Anisotropic Silicon Nitride

Mechanical properties of porous anisotropic silicon nitrides, fabricated by a partial sinter-forging technique, were investigated in the temperature range from room temperature (R.T.) to 1400°C. The fabricated specimens, with porosity of about 24%, exhibited a relatively high strength as well as a high fracture energy. These good properties were maintained even at temperatures as high as 1300°C. This achievement of good properties is very likely due to the combined effects of grain alignment, presence of pores between grains and yttria sintering aid refractoriness.

Effect of grain alignment on magnetic structure in nanoscale material

Intergranular exchange interactions act to bind nanograins in nanoscale magnetic materials into larger interaction domains with a magnetic structure considerably larger than that of a single grain. To investigate the effect of differing relative grain orientation on the scale of the interaction domains in melt-quenched Nd/sub 2/Fe/sub 14/B, a two-dimensional micromagnetic model has been used. The model consists of 18 grains characterized by the intrinsic properties of Nd/sub 2/Fe/sub 14/B. Two cases are considered; one with a narrow distribution of easy axis orientations centered along the applied field direction and one with a nearly isotropic easy axis distribution. Computational results confirm that the scale of the magnetic structure for well-aligned material is very large, while for isotropic material only a few grains can be coupled Into an interaction domain before the intergranular misalignment is large enough to pin domain walls and impede reversal.

Effects of Grain Alignment on Fabric Mechanical Properties

This study was designed to measure the effect of grain alignment on selected fabric physical and mechanical properties. Kawabata's Shear Tester and Pure Bending Tester were used to assess shear and bending properties of 19 apparel fabrics. Fabric samples were cut at six different grain alignment variations. Shear properties were more sensitive than bending properties to variations in grain alignment. Significant positive correlations were identified between tilt and shear stiffness and hysteresis measurements. Although bending modulus and hysteresis values were not significantly related to tilt, a trend toward decreased bending values with increased tilt angles was observed. Results confirm a previous study by Orzada, Moore, and Collier (1997).

Phase Assemblage in PBSSCO OPIT High-Temperature Superconductor Precursor Powders - Rietveld Analysis of X-ray Diffractograms

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Effect of grain-alignment on the levitation force of melt-processed YBCO bulk superconductors

Single-domain YBCO bulk superconductors have been fabricated by Top Seeded Melt Slow Cooling Growth(TSSCG) process. Two typical YBCO cylinder samples with differential grain-alignment were selected for the investigation of the relationship between the grain-alignment and the levitation force under the same testing condition at liquid nitrogen temperature. It is found that the levitation force values is much different for the two samples, the levitation force of the sample with H ∥ c-axis is more than two times higher than that of the samples with H ⊥ c-axis. So it is necessary to take account of this anisotropy in practical applications. The relationship between a magnet and a superconductor can be well described with a double exponential function. All the results are discussed in details.

The grain-alignment and its effect on the levitation force of melt processed YBCO single-domained bulk superconductors

Well textured single-domained YBCO bulk superconductors with differential grain-alignment have been fabricated. The effect of grain-alignment on the levitation force has been investigated at liquid nitrogen temperature. It is found that the levitation force values are highly anisotropic depending on the field direction, the levitation force with H‖ c-axis is about three times higher than that with H⊥ c-axis; so we have to take account of this anisotropy in the practical applications. The relation shape between a magnet and a superconductor can be well described with a double exponential function. All the results are discussed in detail.

Grain alignment: effects on fabric and garment drape

Undesirable garment drape often occurs because of the manufacturer’s desire to save fabric by rotating patterns to position them more closely in the marker, and thus cutting the garment off‐grain. This study was designed to subjectively and objectively measure the effect of grain alignment on fabric and garment drape. Data from an apparel industry survey were utilized to establish tilt values for quantitative analysis of drape and shear. Twelve tilt combinations were examined. No significant differences were found between drape values of control samples and those with tilt variations. Generally, shear stiffness and hysteresis values increased as tilt angles increased across all fabrics. Asymmetry of shear curves also increased. Twenty‐one apparel design students subjectively evaluated fabrics draped on a pedestal and skirts constructed in each tilt variation. Fabric drape amount evaluations were more highly correlated with drape values than were drape preference evaluations. Advanced design students were more sensitive to small differences in garment drape than were beginning students.

Effect of Grain Alignment on the Coercivity for Sintered NdFeB Magnets

Taking account of the effect of the degree of grain alignment on the coercivity, the angular dependence of the coercivity for sintered NdFeB magnets is discussed in terms of various coercivity mechanisms. The comparison between various theoretical results and experiments indicates that the misalignment of the grains makes coercivity of the magnets increase rather than decrease and the starting field theory of coercivity is in better agreement with experiments than either the nucleation or pinning mechanisms.

Particle size, shape and orientation distributions: Aeneral spheroid problem and application to deformed Si3N4microstructures

Abstract A general method is presented for solving stereological problems for particle systems consisting of spheroids having variable size, shape (aspect ratio) and orientation provided that the orientation has axial symmetry. The kernel function, which connects the probability density function (distribution) of the spheroids and that of the sectioning ellipses on the observation planes along the symmetry axis, is calculated theoretically. The spatial distribution of the spheroids can be calculated from the experimentally measured planar data of the ellipses through the inverse operation of the kernel function. Using this method, the spatial size, shape and orientation distributions of grains were calculated in as-sintered and unidirectionally elongated silicon nitride samples consisting of rod-shaped βSi3N4 grains. The calculated spatial distribution shows that the aspect ratio remains almost constant during the deformation although it appears elongated upon planar observation owing to a superficial effect of grain alignment along the tensile axis.

Silver-doped (Bi,Pb) 2Sr 2Ca 2Cu 3O 10/Ag high-temperature superconducting composites

The effect of silver doping on the microstructure and transport properties of Ag-sheated (Bi,Pb) 2Sr 2Ca 2Cu 3Ag x O 10 composite tapes has been investigated through scanning electron microscope (SEM) observation and electrical measurements including critical temperature ( T c), critical current density ( J c) and J c behaviour in magnetic field for a series of samples with varying dopant levels ( x = 0.0–3.5). The results show that silver doping has no noticeable effect on the T c of the samples, but influences the sample's J c, which decreases with increasing silver doping content when the samples are sintered at the same temperature. A slight improvement of J c behaviour in magnetic field is observed for the lightly doped samples, while higher-level doping shows a small degradation in the J c behaviour. Microstructural analyses reveal that silver exists as an isolated phase inside the tapes without visible reaction with and diffusion within the superconductor matrix, so has no influence on T c. But the undesirable shape and size of the silver particles cause grain misorientation, and hence lead to a decrease in J c. The influence of silver doping on the J c behaviour in magnetic field is a combined effect of grain alignment, grain conductivity and flux pinning.

Effect of grain alignment and processing temperature on critical currents in YBa2Cu3O7-δ sintered compacts

Magnetically aligned YBa2Cu3O7-δ ceramics show resistivities approaching that of single crystals and improved transport critical currents in a magnetic field. Reduced microcracking and increased transport along the ab-plane are believed responsible for the improved performance over nonaligned ceramics. Aligned and nonaligned samples were prepared in parallel using a range of sintering and annealing temperatures. A rapid rise in density for samples sintered above 900°C in oxygen is accompanied by rapid grain growth, improved alignment, a drop in the room temperature resistivity, and an increase in the critical current. The presence of low melting point (i.e., Ba–Cu–O-rich) phases at grain boundaries is believed responsible for the rapid densification. However, the presence of this phase does not appear to be the most important factor limiting Jc. A high temperature oxygen anneal at 900°C improved performance as compared to anneals at 500°C, possibly due to the removal of carbon. For aligned samples sintered at 980°C, critical currents are over 200 A/cm2 at 77 K, ¼ tesla, and room temperature resistivities are below 350 μΩ-cm.