Lattice Aspect Ratio Research Articles

Formation of anisotropic grains and modified ferroelectric properties in Cr‐doped Bi5FeTi3O15 thin films

AbstractAurivillius phase Bi5CrxFe1−xTi3O15 (0≤ x ≤1) thin films are prepared by the chemical solution deposition method, and the effect of Cr content on the microstructure, ferroelectric property, and electric transport behavior of Bi5CrxFe1−xTi3O15 films is investigated. X‐ray diffraction analysis shows that all of Bi5CrxFe1−xTi3O15 films are complete solid solution and maintain the Aurivillius structure. The replacement of Fe3+ with smaller Cr3+ decreases anisotropy and lattice aspect ratio in a‐b plane, which is minimized at the composition of Bi5Cr0.5Fe0.5Ti3O15. This changes the grain shape from sphere to plate, and Bi5Cr0.5Fe0.5Ti3O15 film consists of only plate‐like grains. Cr doping increases saturated polarization (Pm) and decreases coercive field (Ec). Cr doping increases Pm of Bi5CrxFe1−xTi3O15 film to 35 μC/cm2, but decreases Ec down to 125 kV/cm. A decrease in the lattice aspect ratio of a‐b plane promotes the alignment of ferroelectric dipoles under electric field. The frequency‐dependent dielectric property and the leakage current show that the plate‐like grains of Cr‐rich Bi5CrxFe1−xTi3O15 films suppress the transport of carriers from grains to grains and prevents a dramatic leakage current increase. The results of this study provide a design rule to control the ferroelectricity of Aurivillius phase Bi5CrxFe1−xTi3O15 thin films by modifying the composition and lattice aspect ratio.

Multiscale modelling of soft lattice metamaterials: Micromechanical nonlinear buckling analysis, experimental verification, and macroscale constitutive behaviour

Soft lattice structures and beam-metamaterials made of hyperelastic, rubbery materials undergo large elastic deformations and exhibit structural instabilities in the form of micro-buckling of struts under both compression and tension. In this work, the large-deformation nonlinear elastic behaviour of beam-lattice metamaterials is investigated by micromechanical nonlinear buckling analysis. The micromechanical 3D beam finite element model uses a primary linear buckling analysis to incorporate the effect of geometric imperfections into a subsequent nonlinear post-buckling analysis. The micromechanical computational model is validated against tensile and compressive experiments on a 3D-printed sample lattice structure manufactured via multi-material jetting. For the development and calibration of macroscale continuum constitutive models for nonlinear elastic deformation of soft lattice structures at finite strains, virtual characterization tests are conducted to quantify the effective nonlinear response of representative unit cells under periodic boundary conditions. These standard tests, commonly used for hyperelastic material characterization, include uniaxial, biaxial, planar and volumetric tension and compression, as well as simple shear. It is observed that besides the well-known stretch- and bending-dominated behaviour of cellular structures, some lattice types are dominated by buckling and post-buckling response. For multiscale simulation based on nonlinear homogenization, the uniaxial standard test results are used to derive parametric hyperelastic constitutive relations for the effective constitutive behaviour of representative unit cells in terms of lattice aspect ratio. Finally, a comparative study for compressive deformation of a sample sandwich lattice structure simulated by both full-scale beam and continuum finite element models shows the feasibility and computational efficiency of the effective continuum model.

Tricriticality for dimeric Coulomb molecular crystals in ground state

We study the ground-state properties of a system of dimers. Each dimer consists in a pair of equivalent charges at a fixed distance, immersed in a neutralizing homogeneous background. All charges interact pairwisely by Coulomb potential. The dimer centers form a two-dimensional rectangular lattice with the aspect ratio and each dimer is allowed to rotate around its center. The previous numerical simulations, made for the more general Yukawa interaction, indicate that only two basic dimer configurations can appear: either all dimers are parallel or they have two different angle orientations within alternating (checkerboard) sublattices. As the dimer size increases, two second-order phase transitions, related to two kinds of the symmetry breaking in dimer’s orientations, were reported. In this paper, we use a recent analytic method based on an expansion of the interaction energy in Misra functions which converges quickly and provides an analytic derivation of the critical behaviour. Our main result is that there exists a specific aspect ratio of the rectangular lattice which divides the space of model’s phases onto two distinct regions. If the lattice aspect ratio , we recover both types of the second-order phase transitions and find that they are of mean-field type with the critical exponent . If , the phase transition associated with the discontinuity of dimer’s angles on alternating sublattices becomes of first order. For , the first- and second-order phase transitions meet at the tricritical point, characterized by the different critical index . Such phenomenon is known from literature about the Landau theory of one-component fields, but in our two-component version the scenario is more complicated: the component which is already in the symmetry-broken state at the tricritical point also interferes and exhibits unexpectedly the mean-field singular behaviour.

Impact of Aggregation on the Percolation Anisotropy on a Square Lattice in an Elongated Geometry

The Monte Carlo simulation is applied to study the impact of the aggregation on the percolation anisotropy on a square lattice in the elongated Lx × Ly geometry. An interactive cluster-growth model, in which the probability of occupying a site on a lattice fz is dependent on the number of occupied neighboring sites z is used. The value of fz is 1/r at z = 0 and is equal to 1 in other cases. The degree of the aggregation parameter r ≥ 1 controls the morphology of aggregates. The transition from r = 1 to r → ∞ corresponds to the transition from the ordinary random percolation to the percolation of compact Eden clusters. The effects of the lattice aspect ratio a = Lx/Ly (Lx > Ly) on the finite-size scaling and the electrical conductivity are studied. The data evidence that the percolation threshold pc goes through the minimum, and the finite-size effects are enhanced with increase in r. The dependence of the electrical conductivity on the measuring direction (x or y) at different values of r and a is discussed.

Impact of thermal expansion of substrates on phase transition temperature of VO2 films

Non-epitaxial, (010)M1-oriented VO2 thin films were grown on various substrates [amorphous SiO2, Si (001), Al2O3 (0001), and CaF2 (001)] with Pt (111)/SiO2 buffer layers. Phase transition from MoO2-type monoclinic to rutile-type tetragonal structures of these VO2 layers was investigated with temperature-controlled micro-Raman spectroscopy. It was confirmed that substrates with larger thermal expansion coefficient cause larger out-of-plane lattice spacings of both Pt and VO2, and thus lower transition temperatures of VO2 films, as a result of higher in-plane shrinkage during cooling from the deposition temperature. The transition temperatures and aM1/2 lengths, estimated from bM1 lengths, of present samples were compared with previous reports in a strain—temperature phase diagram. The present results fit with previous reports better by assuming that in-plane lattice aspect ratio of VO2 films is not clamped by the substrates but is flexible during the temperature change. Thermal expansion of substrates is an essential parameter to be taken into account when one considers device application of the phase transition properties of VO2 films, especially thick or non-epitaxial.

Windows and Doors Lattice Structure for Improving Energy Efficiency

This paper relates generally to windows and doors for traditional Korean houses (Hanok). Especially, we focus on the minimizing the thermal losses on the windows and doors where the highest heat loss come to pass. The door includes a structure of a lattice door framed with vertical lattice frames and horizontal lattice frames which are arranged in a regular periodic pattern. The suggested method uses the convection flows to reduce thermal losses by structural formation of window and door's lattice without damaging the original aesthetic characteristic of windows and doors in Korean traditional wooden house. Based on computational analysis, we propose specific structures like as lattice aspect ratio of windows. A lattice aspect ratio of the lattice door ranges from 0.07 to 0.1 so that the vortex, air layer, induced by natural convection flow prevents thermal loss of a room from going out toward the outside air. Therefore, energy consumption can be reduced.

Inertial stabilization of flexible polymer micro-lattice materials

Soft micro-lattice materials with different lattice geometries were fabricated using a self-propagating photopolymer waveguide process. The parent polymer was characterized by dynamic mechanical analysis and the glass transition temperature shifted with equivalent strain rate. Quasi-static and dynamic compression tests were subsequently carried out to investigate the inertial stabilization of lattice member buckling as a function of strain rate and structural geometry (e.g. relative density and lattice aspect ratio). A high-speed digital camera was used to record the progression of deformation and failure events during compression. The micro-lattice structures exhibited super compressibility and increased strength. The observed strength increase, particularly for high aspect ratio and high strain rate, was attributed to inertial stabilization.

Buckled colloidal crystals with nonspherical bases for two-dimensional slab photonic band gaps

Theoretical modeling of the photonic band gap forming properties is reported for the buckled phase of anisotropic particles. These exist between the first and second particulate layers of confined colloidal suspensions. Inspired by the range of non-spherical mushroom-cap building blocks for self-assembly that have been synthesized using seeded emulsion-polymerization, we explore in particular the band structures as a function of toroid shape parameter. The parameter is adjusted to incrementally transform hemispheres to spheres. Additionally, corrugation heights that systematically modulate the slab photonic crystal unit cell from rectangular monolayer to square bilayer are investigated. Polarization independent gaps in the guided modes are determined for direct and inverted structures that exhibit bifurcation in the particle orientation perpendicular to the slab plane. Gaps in the guided modes are observed between the fourth and fifth, twelfth and thirteenth, as well as higher band locales as the particle morphology and lattice aspect ratio vary.

Study of large area high density magnetic dot arrays fabricated using synchrotron radiation based x-ray lithography

Large area arrays of dots have been patterned in Au/Co/Au(111) sandwiches with ultrathin Co layers (0.6 to 2 nm) and a perpendicular easy magnetization axis. Dot dimensions down to 0.2 μm have been achieved using x-ray lithography, with either positive resist and direct ion beam etching or a lift-off process with aluminum mask. Both processes have been tested against the damages they induce to the fragile structure of the samples. The magneto-optical effects and magnetization reversal processes in the arrays have been characterized versus Co thickness, dot dimension, and lattice aspect ratio. For high quality samples, the domain walls propagation mechanism that drives magnetization reversal in as-grown films is drastically modified in dot arrays, leading to a large increase of the coercive field with dot diameter reduction, together with changes in the shape of the hysteresis loops.