Size Of Twin Domains Research Articles

Microstructures and Mechanical Properties of Cu–38 mass%Zn Alloy Fabricated by Different Rolling Pass Schedules

The heterogeneous-nano (HN) structure consisting of twin domains, shear bands, and lamellar grains developed by heavy cold rolling of FCC metals with low stacking fault energy is known to provide extremely high strength. In the present study, the effects of the rolling pass schedule on the development of HN structure and the mechanical properties in a Cu–38 mass%Zn alloy were investigated. Rolling was conducted on two different pass schedules; one was conventional unidirectional rolling (1-DR) up to 90% reduction in thickness, and the other was the two-directional rolling (2-DR), in which the specimen was unidirectionally rolled to 25% reduction, then rotated 90° around the rolling direction, and subsequently, unidirectionally rolled up to the total reduction of 90%. At a total reduction of 50%, the number of grains with deformation twins in the 2-DR specimen was higher than that in the 1-DR specimen. Further cold rolling up to 90% produced the HN structure in both specimens. The size of twin domains in the 2-DR specimen was finer and their volume fraction was larger than those in the 1-DR specimen. Also, the 2-DR specimen exhibited a better strength-elongation balance than the 1-DR specimen. It can be concluded that the finer size and larger volume fraction of the twin domains effectively improved the strength-elongation balance in the 2-DR specimen.

Determination of the phase coherence length of PdCoO2 nanostructures by conductance fluctuation analysis

The two-dimensional layered compound PdCoO$_2$ is one of the best oxide conductors, providing an intriguing research arena opened by the long mean free path and the very high mobility of ~51000 cm2/Vs. These properties turn PdCoO$_2$ into a candidate material for nanoscale quantum devices. By exploring universal conductance fluctuations originating at nanoscale PdCoO$_2$ Hall-bar devices, we determined the phase coherence length of electron transport in c-axis oriented PdCoO$_2$ thin films to equal ~100 nm. The weak temperature dependence of the measured phase coherence length suggests that defect scattering at twin boundaries in the PdCoO$_2$ thin film governs phase breaking. These results suggest that phase coherent devices can be achieved by realizing the devices smaller than the size of twin domains, via refined microfabrication and suppression of twin boundaries.

Unraveling the Twin and Tunability of the Crystal Domain Sizes in the Medium-Pore Zeolite ZSM-57 by Electron Crystallography.

Tailoring the morphology of a specific crystalline material through distinct crystal growth mechanisms (classical and nonclassical) is challenging. Herein, we report the two unique morphologies of a medium-pore (10×8-ring) zeolite, ZSM-57, prepared by employing an identical organic structure-directing agent (OSDA) and different inorganic cations, namely Na+ and K+ , denoted as ZSM-57-Na (pentagonal nanoplates) and ZSM-57-K (pentagonal nanoprisms), respectively. The tunable twin domain size and twin boundaries in both samples have been unraveled at the atomic level by electron crystallography. It is of significance to note that the 10-ring pore openings run perpendicular to the pentagonal nanoplates and nanoprisms. Moreover, the distinct crystal growth mechanisms, which result in the different unique morphologies and tunable twin domains, were further determined by electron crystallography combined with other techniques. Nonclassical growth involving the aggregation of amorphous aluminosilicate nanoparticles to the smooth ZSM-57-Na crystal surface dominates the ZSM-57-Na crystallization process. For the ZSM-57-K sample, the classical layer-by-layer growth through the addition of silica molecules to advancing steps on the crystal surface dominates the ZSM-57-K crystallization process. The different morphologies of both samples result in the distinct catalytic lifespan of the methanol conversion and selectivity of lower olefins.

Twin domain imaging in topological insulator Bi2Te3 and Bi2Se3 epitaxial thin films by scanning X-ray nanobeam microscopy and electron backscatter diffraction.

The twin distribution in topological insulators Bi2Te3 and Bi2Se3 was imaged by electron backscatter diffraction (EBSD) and scanning X-ray diffraction microscopy (SXRM). The crystal orientation at the surface, determined by EBSD, is correlated with the surface topography, which shows triangular pyramidal features with edges oriented in two different orientations rotated in the surface plane by 60°. The bulk crystal orientation is mapped out using SXRM by measuring the diffracted X-ray intensity of an asymmetric Bragg peak using a nano-focused X-ray beam scanned over the sample. By comparing bulk- and surface-sensitive measurements of the same area, buried twin domains not visible on the surface are identified. The lateral twin domain size is found to increase with the film thickness.

Change in the phase composition, structure and mechanical properties of directed melt crystallised partially stabilised zirconia crystals depending on the concentration of Y2O3

The structure of partially stabilised zirconia (PSZ) crystals has been studied as a function of the Y2O3 stabilising oxide concentration using X-ray diffraction and transmission electron microscopy. Hardness and fracture toughness were measured using microhardness indentation. We demonstrate that directed melt crystallisation grown PSZ crystals contain two tetragonal phases (t and t′) that differ in the degree of tetragonality, and an increase in the Y2O3 concentration in the crystals increases the content of the non-transformable t′ phase. Experiments have demonstrated that an increase in the concentration of the stabilising oxide increases the concentration of positively charged oxygen vacancies, the F++ centres, which increase the lattice parameter and stabilise the structure of the crystals. An increase in the Y2O3 concentration changes the type and size of twin domains. The strength (fracture toughness) of the crystals was demonstrated to be proportional to the content of the transformable t phase.

Vortex pinning in YBa2Cu3O7−x films

Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa2Cu3O7−x (YBCO) films is provided by complex impedance measurements at temperature 8 K<T<T c and magnetic field 0<B<6 T. Below the vortex lattice melting transition Bg(T) but above a threshold field Bp≈8(1-T/T c ) T, the inductance of vortices increases as B2, much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region Bg(T<B<B*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ*≡ξ(B=B*)≈450A. The value of ξ* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at Bg.

Structural effects of element substitution on superconducting properties in 1-2-3 YBCO : an electron microscopy study

Changes in the crystal structure or the microstructure, as a result of different substitutions in YBa 2Cu 3O 7 have been studied by electron microscopy and have been related to the physical properties of these substituted materials. Oxygen ordering has been investigated in superconducting ABa 2Cu 3O 7−δ ceramic materials with A=Er, Nd, Sm, Y and Yb, as well as in samples of this type with the rare earth A partially substituted by Pr. The critical temperature T c was determined as a function of the oxygen deficiency of the compound. A distinct relationship exists between the width of the 60K plateau and the OrthoII ordering. Our results show that the OrthoII ordered phase is the superconducting phase with a characteristic Tc of 60K. Small Fe substitutions for Cu have a dramatic impact on T c as well as on the microstructure. The twin domain size decreases and a “tweed” pattern results. Randomly dispersed Fe-ions or Fe-clusters act as pinning centers for twin interfaces. Substituting Ga or Co foe Cu changes the structure of the “chain” layer. A model for a new superstructure ordering in this chain layer of Ga and Co substituted 1-2-3 YBCO is proposed.

Direct observation of the flux line lattice in Al-doped YBa2Cu3O7-δ

There has been much interest in the effect of partial substitution of Cu atoms in YBa2Cu3O7-δ (YBCO) by dopant atoms such as Fe, Al and Co. It has been found that an increase in the doping level alters the structure of YBCO - the twin domain size drops to 30-100Å for x&gt;0.025 and the domains form the so-called 'tweed' structure. HREM has confirmed that within the domains the structure remains orthorhombic but on a macroscopic scale is quasi-tetragonal. This, in turn, leads to a change in superconducting properties: a rapid decrease in the transition temperature Tc, critical current, etc.In the present contribution the first results are reported on the direct observation of the magnetic flux structure in YBa2(Cu1-xAlx)3O7-δ single crystals (x=0.04) which was also expected to be influenced by Al doping. The flux line (vortex) distribution was observed using the high resolution Bitter pattern technique which involves evaporation of fine ferromagnetic particles (both Fe and Co were used in our experiments) to ‘decorate’ vortices at the surface of a superconductor.

Twin-domain size and bulk oxygen in-diffusion kinetics of YBa 2Cu 3O 6+x studied by neutron powder diffraction and gas volumetry

We report experimental results of twin-domain size and bulk oxygen in-diffusion kinetics of YBa 2Cu 3O 6+ x , which supplement a previous and simultaneous study of the structural phase diagram and oxygen equilibrium partial pressure. Analysis of neutron powder diffraction peak broadening show features which are identified to result from temperature independent twin-domain formation in to different orthorhombic phases with domain sizes and 250 and 350Å, respectively. The oxygen in-diffusion flow shows simple relaxation type behaviour J=J 0 exp( −t τ ) despite a rather broad particle size distribution. At higher temperatures, τ is activated with activation energies 0.55 and 0.25 eV in the tetragonal and orthorhombic phases, respectively. Comparison between twin-domain sizes and bulk oxygen in-diffusion time constants indicates that the twin-domain boundaries may contribute to the effective bulk oxygen in-diffusion. All our results may be interpreted in terms of the 2D ASYNNNI model description of the oxygen basal plane ordering, and they suggest that recent first principles interaction parameters should be modified.