Biaxial Alignment Research Articles

Vapor-to-glass preparation of biaxially aligned organic semiconductors.

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180K below the clearing point TLC-iso (and 50K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility.

Influence of Substrate Temperature and Deposition Rate on the Texture and Surface Morphology of ISD–MgO

In this study, MgO thin films were grown on Hastelloy metal tapes by the inclined substrate deposition (ISD) method for coated conductors. The effects of substrate temperature and deposition rate on the biaxial texture alignment, texture angle (defined as the tilt angle of the MgO (002) plane from the substrate normal) as well as surface morphology of MgO films were investigated systematically. For the substrate inclination angle of 30°, as the increase of substrate temperature from room temperature to 600 °C, the texture quality of the MgO films degrades significantly, meanwhile the texture angle decreases from 27° to 19°. For depositions under substrate inclination angles of 30°, 35° and 55°, the full-width-at-half-maximum (FWHM) value of the in-plane and out-of-plane alignment of MgO films exhibits a “V” shape relationship with the deposition rate. Scanning electron microscopy (SEM) images show that enhanced surface random diffusion causes the film to become dense and reduces the intergrain voids in the columnar structure.

Orientation degrees in Dy123 powders by using Linear-drive type Modulated Rotating Magnetic Field system

Bi-axial magnetic alignment of DyBa2Cu3Oy (y ~ 7) grains with high in-plane orientation degrees has been achieved using only permanent magnets as magnetic field sources. When compared to electromagnets such as a superconducting magnet, arrayed magnets offer a lower cost and greater versatility as magnetic field sources for magnetic alignment. In this study, permanent magnets were arranged in such a way that they were able to create a rotating magnetic field with a strength of 0.8 T-class. This helps with the magnetic alignment of functional materials like cuprate superconductors.

A constitutive model of liquid crystal elastomers with loading-history dependence

Liquid crystal elastomers (LCEs) are a class of active polymers of increasing interest, because of their excellent actuation performances. To quantify the actuation performances and understand the thermomechanical behavior, many theoretical models were developed, mainly based on the free energy framework with uniaxial order tensor, which, however, cannot well capture the biaxial liquid crystal alignment under biaxial stretching or shearing. The LCE with the shape memory cycle effect exhibits an evident loading-history dependence of the thermomechanical process, which cannot be described without introducing the evolution condition of the alignment. In this work, a constitutive model of LCE is established by introducing a triaxial order tensor to describe the complex alignment under uniaxial/biaxial loading conditions. Inspired by the classical theory of plasticity, an equivalent order parameter is proposed to characterize the order degree of the alignment, and its evolution condition is introduced to specify the condition of the alignment evolution. Experiments of LCE under a variety of thermomechanical loading conditions were carried out to validate the developed model, showing the capability of the model in capturing complex thermomechanical responses of the polydomain LCE with shape memory cycle effect under uniaxial/biaxial stretching. Furthermore, the constitutive model can be extended to predict nonlinear stress-strain curves and actuation performances of LCE lattices, which hold promising potential for applications in soft actuators and flexible robots.

Observation of Non-Uniform Twin Microstructures in Dy123 Superconductor for Magnetic Biaxial Alignment

Despite the formation of twinned microstructures in grains of REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> with y ∼ 7, their grains can be biaxial aligned under modulated rotating magnetic fields (MRFs). In the present study, direct observation of twinned microstructures by transmission electron microscopy (TEM) was examined for a magnetically biaxial aligned grain of DyBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> with y ∼ 7. Use of the biaxial aligned grain enabled us to obtain planar views by TEM easier from a technical viewpoint. Furthermore, an inhomogeneous abundance ratio of two different types of domains (domain ratio) was clarified from the planar view of the grain. The present study strongly suggested that triaxial magnetic anisotropy of RE123 was remained in the grain level due to the inhomogeneous domain ratio, and it led to the achievement of the biaxial alignment of RE123 grains under MRFs.

One step “growth to spinning” of biaxially multilayered CNT web electrode for long cycling Li–O2 batteries

Li–O2 batteries have attracted great attention because of their high theoretical energy density, which exceeds the highest possible value of currently used Li-ion batteries. The operation of Li–O2 batteries include the formation and decomposition of Li2O2 at the cathode, and cell degradation occurs because such discharge products accumulate on the pores and cause pore clogging. For reversible charge–discharge process, in this study, a biaxially multilayered high crystalline carbon nanotube (CNT) web was designed and fabricated by a modified direct spinning method, which was the first demonstration that realize the biaxially multilayered structure in one-step process. The high crystallinity of the CNT surface reduced its decomposition and the biaxial alignment of CNT enabled the web to be opened for O2 gas and electrolytes without pore clogging. The pore size distribution verified that effective pores were better developed in the biaxially multilayered CNT web, which facilitated the flow of O2 gas and electrolytes. The use of this specially designed CNT as a cathode improved the cyclability by 75% and dramatically reduced the overpotential. For the practical use of Li–O2 batteries, a 3D folded cell with biaxially multilayered CNT web electrode is fabricated and demonstrated 30 cycles at 100 W h/kgcell cutoff.

Promotion of Epitaxial Growth and Enhanced &lt;italic&gt;J&lt;/italic&gt;c by Coaddition of Br and Metals (Zr, Sn, Hf) to the Fluorine-Free MOD Processed YBCO Films

Coaddition of Br and metals (M), where M = Zr, Sn, Hf, was attempted for YBCO films prepared by the fluorine-free metal-organic decomposition method. It was revealed that Br-addition, which subsequently forms textured Ba 2 Cu 3 O 4 Br 2 oxybromides in the film, promotes the biaxial alignment of the YBCO crystals. Although M-addition did not improve Jc owing to the serious degradation of crystallinity, (Br, M)-coaddition was found to largely enhance flux pinning properties by the suppression of the random crystal growth. The pinning force density at 40 K in 4.8 T reached ~62 GN/m 3 for (Br, M)-coadded YBCO films. This value is more than three times large compared to that of additive-free YBCO films.

Single crystal texture by directed molecular self-assembly along dual axes.

Creating well-defined single-crystal textures in materials requires the biaxial alignment of all grains into desired orientations, which is challenging to achieve in soft materials. Here we report the formation of single crystals with rigorously controlled texture over macroscopic areas (>1 cm2) in a soft mesophase of a columnar discotic liquid crystal. We use two modes of directed self-assembly, physical confinement and magnetic fields, to achieve control of the orientations of the columnar axes and the hexagonal lattice along orthogonal directions. Field control of the lattice orientation emerges in a low-temperature phase of tilted discogens that breaks the field degeneracy around the columnar axis present in non-tilted states. Conversely, column orientation is controlled by physical confinement and the resulting imposition of homeotropic anchoring at bounding surfaces. These results extend our understanding of molecular organization in tilted systems and may enable the development of a range of new materials for distinct applications.

Biaxial growth of pentacene on rippled silica surfaces studied by rotating grazing incidence X-ray diffraction

Pentacene is known to grow on isotropic silicon oxide surfaces in a substrate-induced phase with fiber textured crystallites. This growth study reports on the growth of pentacene crystallites on uniaxially oriented surfaces. Silica substrates have been treated by ion beam sputtering so that ripples with a lateral corrugation length of 38 nm and a surface roughness of 1.3 nm are formed. Pentacene thin films with a nominal thickness in the range from 20 nm up to 300 nm are deposited on top of the rippled surfaces. The films are characterized by atomic force microscopy and grazing incidence X-ray diffraction. Bi-axially oriented crystals are formed due to the grooves of the substrate surface opening up the possibility of a defined in-plane alignment of the crystals. In a first stage of thin film growth, the thin film phase (TFP) of pentacene is formed, while in the later stage the bulk crystal structure (C, Campbell phase) also appears. Due to the bi-axial alignment of the crystallites the transition from the thin film phase to the bulk crystal structure can be directly investigated. An epitaxial relationship with (1 2 0)TFP || (2 1 0)C and [−2 1 0]TFP ||[1 −2 0]C is observed which can be explained by an adaption of the herringbone layers of both crystal structures. This work reveals one possible microscopic mechanism for the transition from the metastable substrate-induced phase of pentacene to its equilibrium bulk structure.

Relationship between biaxial orientation degrees and grain in magnetically aligned (Y1−xErx)Ba2Cu3Oy powders with twin microstructures

We report the degrees of biaxial orientation for (Y1−xErx)Ba2Cu3Oy [(Y1−xErx)123] powder samples aligned in epoxy resin under a modulated rotating magnetic field of 10 T at room temperature. Although the triaxial magnetic anisotropy of ErBa2Cu3Oy was found to be approximately 20 times higher than that of YBa2Cu3Oy, the orientation degrees for (Y1−xErx)123 powder samples decreased with x. In the fabrication of (Y1−xErx)123 powders, sintering temperature is one of the parameters that predominantly affect orientation degree, and the sintering at temperatures close to the peritectic temperature leads to improvement of the orientation degree. In the case of the biaxial magnetic alignment of REBa2Cu3Oy (RE123: RE, rare-earth element) powders, the reduction of in-plane magnetic anisotropy induced by twin microstructures at the RE123 grain level should be taken into account. Therefore, in this study, we clarified that the fabrication processes of RE123 powders are important in addition to the control of the chemical composition using RE ions with high magnetic anisotropy.

Possibility of material cost reduction toward development of low-cost second-generation superconducting wires

Two approaches to reducing the material cost of second-generation superconducting wires are proposed in this paper: (1) instead of the electrical stabilizing layers of silver and copper presently used on the superconducting layer, a Nb-doped SrTiO3 conductive buffer layer and cube-textured Cu are proposed as an advanced architecture, and (2) the use of an electromagnetic (EM) steel tape as a metal substrate of coated conductors in a conventional architecture. In structures fabricated without using electrical stabilizing layers on the superconducting layer, the critical current density achieved at 77 K in a self-field was approximately 2.6 MA/cm2. On the other hand, in the case of using EM steel tapes, although the critical current density was far from practical at the current stage, the biaxial alignment of YBa2Cu3Oy (YBCO) and buffer layers was realized without oxidation on the metal surface. In this study, the possibility of material cost reduction has been strongly indicated toward the development of low-cost second-generation superconducting wires in the near future.

Biaxial magnetic alignment in twinned REBa2Cu3Oy superconductors

Biaxial magnetic alignment of REBa2Cu3Oy (RE123, RE = Y, Nd, Sm, Dy, and Er) superconductor powders containing twin microstructures was demonstrated. Appropriate choice of RE effectively improved the degrees of in-plane and c-axis orientation in RE123 powder samples aligned under modulated rotating magnetic fields at room temperature. From the relationship between the magnetic field strength and the degrees of orientation, it is concluded that heavy RE ions induced the improvement of the in-plane magnetic anisotropies in RE123 grains with twin microstructures.

Collagen biomaterials for cornea regeneration – how does it work

SummaryWe have previously shown in animal models and in early clinical studies that collagen‐based biomaterials promoted functional regeneration of corneal epithelium, stroma and nerves. Recently, we showed that these fabricated implants were made of collagen fibrils that were fine and aligned, like those in human corneas. There were noticeable differences such as the uniaxial alignment of the implant fibrils compared to the biaxial alignment in the cornea, and the lack of D‐banding of collagen fibrils in the implants. Nevertheless, the aligned fibrils facilitated an orderly in‐growth of corneal stromal cells to form a neo‐stroma. TEM examination showed the presence of extracellular vesicles (EVs) in the regenerating corneas. Immunohistochemistry showed that the EVs were positively stained for collagen amongst other cargoes. In implants made from short peptide analogs of collagen, alignment of short fibrils was observed. However these implants had a much higher number of EVs. This suggests that a combination of a scaffold comprising highly aligned fibrils mimicking the highly ordered corneal ECM together with elaboration of collagen and other ECM macromolecules is required for regeneration of a functional neo‐cornea and not scar tissue.

Development of bi-axial preferred orientation in epitaxial NiMnGa thin films and its consequence on magnetic properties

In the present work, the crystal structure, the micro-structure and the crystallographic orientation of NiMnGa thin films fabricated on a Si (100) substrate using magnetron sputtering at elevated temperatures have been evaluated. X-ray diffraction based measurements confirm both in-plane and out of plane (bi-axial) alignment of the unit cells in the films. Three levels of hierarchy in the microstructure, and the presence of primary nano-twinned martensitic variants and secondary nano-twins within the primary nano-twins due to adaptive modulation are some of the important microstructural features observed in the films using a transmission electron microscope. The results of temperature-dependent magnetization measurements and in-situ high temperature X-ray diffraction experiments indicate the presence of a pre-martensite phase above room temperature. Low temperature X-ray diffraction measurements reveal the evolution of a non-modulated (NM) martensite phase at 123 K. High resolution scanning electron microscopy (FE-SEM) and high resolution X-ray diffraction measurements have been carried out for an in-depth examination of the microstructure, growth morphology and crystalline quality of the films.

回転変調磁場を用いた希土類系高温超伝導体の3次元結晶配向

Modulated rotating magnetic field (MRF) enables bi-axial grain arrangement of substances with tri-axial magnetic anisotropy. Even feeble magnetic (paramagnetic and diamagnetic) substances become target materials for the magnetic grain alignment by using a 10 telsa-class strong magnet. In this review article, the authors introduce high potential of the magnetic alignment technique using MRF as a fabrication process of bi-axially grain-oriented rare-earth(RE)-based cuprate superconductors. In the former topic, the bi-axial magnetic alignment of RE-based cuprate superconductors without twin microstructures at room temperature are reported for the clarification of RE-dependent magnetization axes and tri-axial magnetic anisotropies. In the latter topic, the authors show the bi-axial magnetic alignment of practical RE-based superconductors, REBa2Cu3Oy (RE123), with twin microstructures. The inplane orientation degrees in RE123 were lowered compared with those in RE-based cuprates without twin microstructures; however, bi-axial magnetic alignment has been successfully achieved even in RE123 with twin microstructures.

The structural and optical properties of type III human collagen biosynthetic corneal substitutes

The structural and optical properties of clinically biocompatible, cell-free hydrogels comprised of synthetically cross-linked and moulded recombinant human collagen type III (RHCIII) with and without the incorporation of 2-methacryloyloxyethyl phosphorylcholine (MPC) were assessed using transmission electron microscopy (TEM), X-ray scattering, spectroscopy and refractometry. These findings were examined alongside similarly obtained data from 21 human donor corneas. TEM demonstrated the presence of loosely bundled aggregates of fine collagen filaments within both RHCIII and RHCIII-MPC implants, which X-ray scattering showed to lack D-banding and be preferentially aligned in a uniaxial orientation throughout. This arrangement differs from the predominantly biaxial alignment of collagen fibrils that exists in the human cornea. By virtue of their high water content (90%), very fine collagen filaments (2–9nm) and lack of cells, the collagen hydrogels were found to transmit almost all incident light in the visible spectrum. They also transmitted a large proportion of UV light compared to the cornea which acts as an effective UV filter. Patients implanted with these hydrogels should be cautious about UV exposure prior to regrowth of the epithelium and in-growth of corneal cells into the implants.

ZnO microrod/PDMS composites promote biaxially oriented growth of ZnO films on glass substrates

This work presents a facile approach for the fabrication of ordered hybrid structures comprising ZnO microrods within polymer matrices. After infiltrating polymers into epitaxial ZnO microrod arrays on the sapphire substrates, we used a peel-off process to transfer the resulting ZnO microrod/polymer composites on glass substrates. Next, through hydrothermal processing, we grew [0001]-oriented ZnO posts, with biaxial alignment, on the composites. Eventually, through coalescence overgrowth of the ZnO posts, we obtained a layer of biaxially oriented ZnO featuring a continuous and flat surface morphology. X-ray diffraction and transmission electron microscopy revealed the microstructure and polarity of the growing ZnO crystals.

Using the macroscopic scale to predict the nano-scale behavior of YSZ thin films

In this work, Yttria-stabilized zirconia (YSZ) thin films were deposited using dual reactive magnetron sputtering. By varying the deposition conditions, the film morphology and texture of the thin films are tuned and biaxial alignment is obtained. Studying the crystallographic and microstructural properties of the YSZ thin films, a tilted columnar growth was identified. This tilt is shown to be dependent on the compositional gradient of the sample. The variation of composition within a single YSZ column measured via STEM–EDX is demonstrated to be equal to the macroscopic variation on a full YSZ sample when deposited under the same deposition parameters. A simple stress model was developed to predict the tilt of the growing columns. The results indicate that this model not only determines the column bending of the growing film but also confirms that a macroscopic approach is sufficient to determine the compositional gradient in a single column of the YSZ thin films.

Magnetic Alignment in $\hbox{REBa}_{2}\hbox{Cu}_{3-{\rm x}}\hbox{Co}_{\rm x}\hbox{O}_{\rm y}$ Compounds

The effects of doping of the cobalt ion on magnetic anisotropy in twinned and orthorhombic YBa2Cu3Oy (Y123) were clarified from their magnetic alignment behaviors under a modulated rotating field of 10 T. The cobalt-free Y123 shows two different types of inplane orientation due to the twin microstructure, and the cobalt-doping induced a structural change into a tetragonal phase and conversion of the magnetic hard axis from the b-axis ([100] grain) into a [110] direction ([110] grain). Furthermore, magnetic alignment under a modulated rotating field of 5 T revealed that the inplane magnetic anisotropy of the [110] grains is superior to that of the [100] grain. It is likely that crystallochemical control at the chain site in Y123 promotes the bi-axial orientation using the [110] grains as a grain alignment type for the bi-axial magnetic alignment of twined Y123.

Mitigating the geometrical limitations of conventional sputtering by controlling the ion-to-neutral ratio during high power pulsed magnetron sputtering

High power pulsed magnetron sputtering has been used to grow thin chromium layers on substrates facing and orthogonal to the target. It is demonstrated that at low peak target current density, j T < 0.6 A/cm 2 corresponding to a low ion-to-neutral flux ratio, films grown on substrates facing the target exhibit in-plane alignment. This is due to the rectangular shape of the target that yields an asymmetry in the off-normal flux of sputtered species. With increasing j T the biaxial alignment degrades, as the major portion of the incoming flux (ions) can be effectively steered by the electric field of the substrate to remove asymmetry imposed by geometrical restrictions. Eventually, at j T = 1.7 A/cm 2 a fiber texture is obtained. For films grown on substrates orthogonal to the target, the large column tilt characteristic for growth at low j T , decreases with increasing ion content in the flux and almost disappears at the highest value of j T . The latter indicates that material flux to the substrate is highly ionized so that deposition takes place along substrate normal despite the high nominal inclination angle. Thus, in the limit of high j T the artifacts of conventional physical vapor deposition, resulting from the line-of-sight deposition, are effectively eliminated and the film growth proceeds more or less unaffected by the substrate orientation. Samples mounted orthogonally thus possess a similar texture, morphology, and topography as those facing the target.