On-axis Samples Research Articles

Assessing pseudo-ductile behavior of woven thermoplastic composites under tension and bending

Most fiber-reinforced composites are inherently brittle and fail suddenly at low strains without yielding and energy-absorbing capability. Still, under some conditions, they can demonstrate ductile like response known as pseudo-ductility. To investigate such a response, experimental analysis of carbon- and glass-fabric reinforced thermoplastic polymer (C/GFRP) composites was performed in on- and off-axis orientations under service loading conditions of tension and bending. Tensile tests of off-axis specimens were conducted with a full-field strain-measurement digital image correlation (DIC) technique. Cyclic bending tests of on- and off-axis C/GFRP specimens were performed to assess their ductility and damage behavior. The tests revealed that on-axis CFRP laminates failed due to fracture of brittle carbon fibers under tension, monotonic and cyclic bending. The on-axis GFRP samples demonstrated a linear-elastic brittle response under tension but a visco-elasto-plastic nonlinear behavior under monotonic and cyclic bending with hysteresis and energy absorption. This nonlinearity could be due to stress stiffening of thin laminates under large-deflection bending and mesoscopic effects in the woven fabrics. The off-axis C/GFRP specimens exhibited ductile behavior akin to metals, enduring high strains with permanent deformation before ultimate failure, and absorbing substantial amounts of energy. The pseudo-ductile response of the off-axis specimens is attributed to plasticity due to matrix cracking, fiber-matrix debonding as well as fiber trellising, whereas in the on-axis GFRP specimens, it is primarily due to visco-elasto-plastic behavior of glass fibers and the TPU matrix. It is concluded that material's response can be tailored for stiffness, strength and ductility for specific applications.

Development of an angle detection system for channeling implantation to the c-axis of SiC using birefringence phenomenon

We developed an angle detection system for channeling ion implantation in 4H-SiC using the birefringence phenomenon. Our optical method detects the c-axis direction in 4H-SiC due to its uniaxial optical properties. The system, consisting of a laser, polarizer, gonio stage, and analyzer, is simple and cost-effective. We conducted experiments on both on-axis and off-axis 4H-SiC (0001) samples, presenting angular dependence results around the [1–100] and [11–20] rotations. Despite the need for consideration of light incident angles, the performance was comparable to Rutherford backscattering spectrometry. These findings suggest the potential application of our system in channeling implantation to the c-axis of 4H-SiC.

In(Ga)N 3D Growth on GaN-Buffered On-Axis and Off-Axis (0001) Sapphire Substrates by MOCVD.

In(Ga)N epitaxial layers were grown on on-axis and off-axis (0001) sapphire substrates with an about 1100 nm-thick GaN buffer layer stack using organometallic chemical vapor deposition at 600 °C. The In(Ga)N layers consisted of a thin (~10–25 nm) continuous layer of small conical pyramids in which large conical pyramids with an approximate height of 50–80 nm were randomly distributed. The large pyramids were grown above the edge-type dislocations which originated in the GaN buffer; the dislocations did not penetrate the large, isolated pyramids. The large pyramids were well crystallized and relaxed with a small quantity of defects, such as dislocations, preferentially located at the contact zones of adjacent pyramids. The low temperature (6.5 K) photoluminescence spectra showed one clear maximum at 853 meV with a full width at half maximum (FWHM) of 75 meV and 859 meV with a FWHM of 80 meV for the off-axis and on-axis samples, respectively.

Electrical resistance changes of 3D carbon fiber/epoxy woven composites under short beam shear loading along different orientations

Electrical resistance under cyclic loading is important for designing intelligent structures of carbon-fiber reinforced composites. Here we report electrical resistance changes of 3D carbon fiber/epoxy woven composites under short beam shear loading along different orientations. The electric changes were measured in short beam shear tests combined with four-probe method and digital image correlation technology along five orientations, i.e., 0°, 30°, 45°, 60° and 90°. The on-axis (0° and 90°) samples have higher electrical conductivity, shear strength and resistance increase than other off-axis (30°, 45° and 60°) samples. Relative resistance changes become larger during monotonic loading and decrease under cyclic loading. The 45° and 60° samples show a higher relative resistance change compared with other samples under cyclic loading. Relationship between shear strength and relative resistance change was also revealed. This electric-mechanical coupling behavior provides a possible way for characterizing inner damages.

Structural and Optical Properties of InAsSbBi Grown by Molecular Beam Epitaxy on Offcut GaSb Substrates

Three InAsSbBi samples are grown by molecular beam epitaxy at 400 °C on GaSb substrates with three different offcuts: (100) on-axis, (100) offcut 1° toward [011], and (100) offcut 4° toward [011]. The samples are investigated using X-ray diffraction, Nomarski optical microscopy, atomic force microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The InAsSbBi layers are 210 nm thick, coherently strained, and show no observable defects. The substrate offcut is not observed to influence the structural and interface quality of the samples. Each sample exhibits small lateral variations in the Bi mole fraction, with the largest variation observed in the on-axis growth. Bismuth rich surface droplet features are observed on all samples. The surface droplets are isotropic on the on-axis sample and elongated along the [011¯] step edges on the 1° and 4° offcut samples. No significant change in optical quality with offcut angle is observed.

On-axis sample viewer with flexible working distance for an X-ray spectroscopy beamline.

Conducting research using micrometer-sized X-ray beams with small samples is common at modern synchrotron X-ray sources. Often, the relative alignment between the X-ray beam and sample is time consuming. An on-axis or coaxial camera system with a view of the sample in a direction along the path of the X-ray beam with its depth of field set to coincide with the location of the focal spot of the X-ray beam is preferred. Besides the use of a drilled mirror, I propose the use of a Pellicle mirror to create an on-axis viewer that allows various sample environments and X-ray beam sizes.

Development of \u03b2-Ga2O3 layers growth on sapphire substrates employing modeling of precursors ratio in halide vapor phase epitaxy reactor

Gallium oxide is a promising semiconductor with great potential for efficient power electronics due to its ultra-wide band gap and high breakdown electric field. Optimization of halide vapor phase epitaxy growth of heteroepitaxial upbeta-Ga2O3 layers is demonstrated using a simulation model to predict the distribution of the ratio of gallium to oxygen precursors inside the reactor chamber. The best structural quality is obtained for layers grown at 825–850 °C and with a III/VI precursor ratio of 0.2. Although the structural and optical properties are similar, the surface morphology is more deteriorated for the upbeta-Ga2O3 layers grown on 5 degree off-axis sapphire substrates compared to on-axis samples even for optimized process parameters. Cathodoluminescence with a peak at 3.3 eV is typical for unintentionally doped n-type upbeta-Ga2O3 and shows the appearance of additional emissions in blue and green region at ~ 3.0, ~ 2.8, ~ 2.6 and ~ 2.4 eV, especially when the growth temperatures is lowered to 800–825 °C. Estimation of the band gap energy to ~ 4.65 eV from absorption indicates a high density of vacancy defects.

High temporal and spatial resolution single-shot digital holography with Fresnel domain filtering using witch's hat illumination.

High-resolution, single-shot on-axis digital holography is proposed. Generally, an on-axis configuration samples carrier fringes with higher spatial resolution compared to an off-axis configuration. However, the reconstructed image is obtained with unnecessary images of a conjugate image and a zero-order beam. The proposed method uses a phase-modulated illumination beam and image processing to eliminate these unnecessary images. Since time-division and parallel phase-shifting methods are not required, the proposed method has higher temporal and spatial resolutions. During image processing, the conjugate image is removed by filtering on the Fresnel domain while keeping most of the information of the object image intact. The usefulness of the proposed method is confirmed by a numerical simulation and an optical experiment.

Off-axis bending behaviors and failure characterization of 3D woven composites

This paper presents the influence of the off-axis angles on the flexure behaviors of three-dimensional (3D) woven carbon/epoxy composites. Four kind of samples with different angles, 0 degree, 30 degree, 45 degree and 90 degree, are experimentally tested via three-point bending method. The immersion focused ultrasound imaging and micro-computed tomography (Micro-CT) techniques are employed to investigate the deformation and damage mechanisms in the specimens fractured in bending. Results indicate that the stress-deflections of on-axis (0 degree and 90 degree) samples exhibit obvious quasi-brittle behaviors, whereas those of off-axis (30 degree and 45 degree) samples show important ductile features. Furthermore, the dominant failure mechanism of on-axis samples are identified to be inter-ply delamination and fiber bundle fractures, whereas those of off-axis samples are matrix cracks and tows debonding.

Effects of off-axis angle on shear progressive damage of 3D woven composites with X-ray micro-computed tomography

This paper presents the influence of the off-axis angles on the shear progressive damage of three-dimensional (3D) woven carbon/epoxy composites. Five kind of samples with different angles, 0°, 30°, 45°, 60° and 90°, of the weft and warp yarn orientation are experimentally tested via short beam shear method. X-ray micro-computed tomography (Micro-CT) techniques are employed to identify the internal damage initiation/evolution of 3D woven composites. Results indicate that the off-axis angles have significant effects on the mechanical behaviors, damage change and final failure mechanisms. Furthermore, the ultimate strength of on-axis samples are mainly influenced by the yarn tensile fracture and delamination, whereas those of off-axis samples are largely governed by the debonding and yarn pull-out breakage.

Defect-related temperature dependence of THz emission from GaAs/AlGaAs MQWs grown on off- and on-axis substrates

Simultaneous molecular beam epitaxial growth of GaAs/AlGaAs multiple quantum wells on two different substrates (one on GaAs (100) and another on a GaAs substrate misoriented by 4° in the (111) direction) resulted in samples of similar structure, but having different defect profiles. The on-axis sample had a higher defect density and more types of electron traps than the off-axis counterpart. Temperature-dependent terahertz (THz) emission and temperature-dependent photoluminescence were measured; in both cases, an intensity quenching was observed between 75 K – 250 K for the on-axis sample, but not in the off-axis sample. We attribute the THz emission quenching to the electron traps present in the sample, which decreases the photocarriers participating in setting up the surface field.

Propagation of threading dislocations in heteroepitaxial diamond films with (111) orientation and their role in the formation of intrinsic stress

In the present study, systematic correlations were revealed between the propagation direction of threading dislocations, the off-axis growth conditions, and the stress state of heteroepitaxial diamond on Ir/YSZ/Si(111). Measurements of the strain tensor ε⃡ by X-ray diffraction and the subsequent calculation of the tensor of intrinsic stress σ⃡ showed stress-free samples as well as symmetric biaxial stress states for on-axis samples. Transmission electron microscopy (TEM) lamellas were prepared for plan-view studies along the [1¯1¯1¯] direction and for cross-section investigations along the [112¯] and [11¯0] zone axes. For samples grown on-axis with parameters which avoid the formation of intrinsic stress, the majority of dislocations have line vectors clearly aligned along [111]. A sudden change to conditions that promote stress formation is correlated with an abrupt bending of the dislocations away from [111]. This behaviour is in nice agreement with the predictions of a model that attributes formation of intrinsic stress to an effective climb of dislocations. Further growth experiments under off-axis conditions revealed the generation of stress states with pronounced in-plane anisotropy of several Gigapascal. Their formation is attributed to the combined action of two basic processes, i.e., the step flow driven dislocation tilting and the temperature dependent effective climb of dislocations. Again, our interpretation is supported by the dislocation propagation derived from TEM observations.

Utilization of AZO/Au/AZO multilayer electrodes instead of FTO for perovskite solar cells

Aluminum (3at%)-doped ZnO (AZO)/Au/AZO multilayer films with low resistivity and high transmittance are in situ deposited at room temperature via facing-target sputtering for AZO and direct current (DC) sputtering for Au. The optical and electrical properties of multilayer films are compared with conventionally (on-axis) sputtered and facing-target sputtered (FTS) AZO films. The FTS samples show better crystallinity than that of on-axis sputtered samples, and the lowest value for resistivity is 7.9×10−5Ω-cm at an Au layer thickness of 8nm, which is compared with that of a conventional FTO electrode. The photo-conversion efficiencies (PCE) of the perovskite solar cells based on the AZO multilayer electrodes deposited via FTS and on-axis sputtering are 9.5% and 8.2%, respectively. Although the PCE of AZO multilayer-based-solar cells exhibit a lower efficiency than that of the FTO based-devices (~12.3%), the perovskite solar cell performance based on the FTS AZO multilayer electrodes is very attractive for applications to flexible planar-structured solar cells.

Scanning near-field microscopy of carrier lifetimes in m-plane InGaN quantum wells

Time-resolved scanning near-field photoluminescence (PL) spectroscopy was applied to map carrier lifetimes in wide m-plane InGaN/GaN quantum wells grown on on-axis and miscut substrates. Both radiative and nonradiative lifetimes were found to be spatially nonuniform. Lifetime variations were smaller for quantum wells grown on miscut, as compared to on-axis substrates. Correlation with surface topography showed that largest deviations of recombination times occur at +c planes of pyramidal hillocks of the on-axis sample. Observed correlation between radiative lifetimes and PL peak wavelength was assigned to a partial electron localization.

Spatially incoherent off-axis Fourier holography without using spatial light modulator (SLM).

We present a spatially incoherent dual path Fourier holographic system. Conceptually it is similar to Fourier incoherent single channel holography (FISCH). Although our incoherent off-axis Fourier holographic (IOFH) system does not have the robustness of a single channel system, it has three advantages over FISCH, with two being quite obvious from setup. First, no SLM is required, thus making the system simple and cost-effective. Second, it is capable of high light throughput because in FISCH, the use of SLM reduces light intensity in half by splitting one beam into two; furthermore, an analyzer is required to create interference which also reduces light intensity. The third advantage, which makes this IOFH system applicable even for on-axis samples (as opposed to samples in a half plane as is necessary for FISCH), is achieved by tilting one mirror. Here we demonstrate our system with a sample in half plane as in FISCH for different axial positions, and then by placing the object on an optical axis and tilting one mirror. The reconstructed images demonstrate holographic capabilities of our IOFH system for both on-axis and half plane sample locations.

Release of subducted sedimentary nitrogen throughout Earth’s mantle

The dynamic process of subduction represents the principal means to introduce chemical heterogeneities into Earth's interior. In the case of nitrogen (N) - atmosphere's most abundant gas - biological-activity converts N2 into ammonium ions (NH4+), which are chemically-bound within seafloor sediments and altered oceanic crust that comprise the subducting slab. Although some subducted N re-emerges via arc-related volcanism (Sano et al., 1998; Fischer et al., 2002), the majority likely bypasses sub-arc depths (150-200 km) and supplies the deeper mantle (Li et al., 2007; Mitchell et al., 2010; Johnson and Goldblatt, 2015; Bebout et al., 2016). However, the fate of subducted N remains enigmatic: is it incorporated by the shallow convecting mantle - the source of ridge volcanism, or is the deeper mantle - nominally associated with mantle plumes - its ultimate repository? Here, we present N-He-Ne-Ar isotope data for oceanic basalts from the Central Indian Ridge (CIR)-Reunion plume region to address this issue. All on-axis samples with depleted MORB mantle (DMM) affinities (3He/4He = 8 ± 1 RA; Graham, 2002) have low N-isotopes (mean δ15N = -2.1 ‰), whereas those with plume-like 3He/4He display higher values (mean δ15N = 1.3 ‰). We explain these data within the framework of a new mantle reference model to predict a time-integrated net N regassing flux to the mantle of ~3.4 × 1010 mol/yr, with the plume-source mantle representing the preferential destination by a factor of 2-3. The model has implications for the present-day imbalance between N subducted at trenches and N emitted via arc-related volcanism, the N-content of Earth's early atmosphere, as well as relationships between N2 and the noble gases in mantle reservoirs, including 3He/4He-δ15N relationships in plume-derived lavas.

Deep level traps and the temperature behavior of the photoluminescence in GaAs/AlGaAs multiple quantum wells grown on off-axis and on-axis substrates

Photoluminescence (PL) spectroscopy from 10K up to 300K was performed on ∼50Å and ∼120Å GaAs/AlGaAs multiple quantum wells (MQWs) grown on on-axis and off-axis GaAs substrates. An anomalous quenching of the integrated PL in the 80–200K region was observed for the on-axis substrate-grown samples. X-Ray diffractometry (XRD) showed no significant structural difference between the on- and off-axis samples. Deep-Level Transient Spectroscopy revealed more defects for the on-axis layers. The growth of ∼50Ǻ MQW on-axis layers appears to be more susceptible to defect formation. The observed PL quenching is attributed to the presence of these traps.

Uniformity of Epitaxial Graphene on On-Axis and Off-Axis SiC Probed by Raman Spectroscopy and Nanoscale Current Mapping

Epitaxial graphene was grown on the surface of on-axis and off-axis SiC (0001) by solid state graphitization at high temperatures (2000 °C) in Ar ambient. The effect of the miscut angle on the lateral uniformity of the few layers of graphene (FLG) was investigated by combined application of micro-Raman spectroscopy and Torsion Resonance Conductive Atomic Force Microscopy, the latter method enabling a quantification of the FLG coverage on SiC with submicrometer lateral resolution. While the on-axis samples result in uniform coverage by thin (~ 3 monolayers) FLG, the coverage for off-axis samples is much less uniform, following closely the step bunching morphology of the SiC surface.

Deep defects in GaN/AlGaN/SiC heterostructures

Deep level transient spectroscopy (DLTS) measurements were carried out on GaN/AlGaN/SiC heterostructures prepared by low-pressure metalorganic vapor phase epitaxy. Si-doped n-GaN layers were grown using an n-AlGaN nucleation layer (8% and 30% of aluminum) on two kinds of p-type 4H-SiC substrates. The DLTS spectra of on-axis (0001) grown samples exhibit a dominant peak of a majority carrier trap with apparent activation energy close to 0.80 eV and capture cross section of about 5×10−14 cm2 regardless of the AlGaN composition. The energy of this deep level decreases with increasing electrical field due to Poole–Frenkel effect. Carrier capture kinetics indicates interacting point defects arranged along a line, probably a threading dislocation. Two additional traps (0.52 and 0.83 eV) were found in on-axis samples with 8% AlGaN composition. For 30% Al content, only a 0.83 eV level was detected. Majority carrier trap with activation energy of 0.66 eV was observed in the off-axis grown samples. This level is probably related to an interface defect or to a defect lying near the heterojunction interface.

In-situ surface preparation of nominally on-axis 4H-SiC substrates

A study of the in-situ surface preparation has been performed for both Si- and C-face 4H-SiC nominally on-axis samples. The surface was studied after etching under C-rich, Si-rich and under pure hydrogen ambient conditions at the same temperature, pressure and time interval using a hot-wall chemical vapor deposition reactor. The surfaces of all the samples were analyzed using optical microscopy with Normarski diffractional interference contrast and atomic force microscopy with tapping mode before and after in-situ etching. Polishing related damages were found to be removed under all etching conditions, also the surface step structure was uncovered and a few defect-selective etch pits were observed. For the Si-face sample, the best surface morphology was obtained after Si-rich etching conditions with more uniform and small macro-step height which resulted in the lowest surface roughness. For the C-face sample the surface morphology was comparable under all etching conditions and not much difference was found except for the etching rate. Si droplets were not observed under any etching conditions.