Time Reversal Aided Bidirectional OFDM Underwater Cooperative Communication Algorithm with the Same Frequency Transmission

In underwater acoustic channel, signal transmission may experience significant latency and attenuation that would degrade the performance of underwater communication. The cooperative communication technique can solve it but the spectrum efficiency is lower than traditional underwater communication. So we proposed a time reversal aided bidirectional OFDM underwater cooperative communication algorithm. The algorithm allows all underwater sensor nodes to share the same uplink and downlink frequency simultaneously to improve the spectrum efficiency. Since the same frequency transmission would produce larger intersymbol interference, we adopted the time reversal method to degrade the multipath interference at first; then we utilized the self-information cancelation module to remove the self-signal of OFDM block because it is known for sensor nodes. In the simulation part, we compare our proposed algorithm with the existing underwater cooperative transmission algorithms in respect of bit error ratio, transmission rate, and computation. The results show that our proposed algorithm has double spectrum efficiency under the same bit error ratio and has the higher transmission rate than the other underwater communication methods

Nanoscale Bandgap Tuning across an Inhomogeneous Ferroelectric Interface

We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovolatic effects and the spectral response of the photocurrent were explore to illustrate the reversible bandgap variation (~0.3eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the opticallyrelated behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters

Polyimides Crosslinked by Aromatic Molecules and Nanocomposites for High Temperature Capacitive Energy Storage

High temperature polymer-based dielectric capacitors are crucial for application in electronic power systems. However, the storage performance of conventional dielectrics polymer dramatically deteriorates due to the thermal breakdown under concurrent high temperatures and electric fields, and there are hardly reports on the causes of thermal breakdown from the aspects of the high temperature conduction loss and Joule heat dissipation. Herein, a combined strategy of crosslinking and compositing for polyimide-based nanocomposites is proposed, which minimizes the thermal breakdown by significantly inhibiting the high-temperature conduction loss and enhancing the high thermal conductivity. Furthermore, the rationale of the strategy was theoretically and experimentally verified from multiple perspectives. The charge-trapping effect is directly observed and quantitatively probed by Kelvin probe force microscopy with nano level resolution, indicating that the crosslinking network introduces local deep traps and effectively suppresses the charge transport. The thermal conductivity of the nanocomposites inhibits the high temperature thermal breakdown, which is confirmed by phase field simulations. Consequently, the optimized nanocomposites possess an ultra high discharge energy density(Ud) of 5.45 J/cm3 and 3.54 J/cm3 with a charge discharge efficiency, respectively, which outperforms the reported polyimide based dielectric nanocomposites. This work provides a scalable direction for high temperature polymer based capacitors with excellent performance

Prominent Size Effects without a Depolarization Field Observed in Ultrathin Ferroelectric Oxide Membranes

The increasing miniaturization of electronics requires a better understanding of material properties at the nanoscale. Many studies have shown that there is a ferroelectric size limit in oxides, below which the ferroelectricity will be strongly suppressed due to the depolarization field, and whether such a limit still exists in the absence of the depolarization field remains unclear. Here, by applying uniaxial strain, we obtain pure in-plane polarized ferroelectricity in ultrathin SrTiO3 membranes, providing a clean system with high tunability to explore ferroelectric size effects especially the thickness-dependent ferroelectric instability with no depolarization field. Surprisingly, the domain size, ferroelectric transition temperature, and critical strain for room-temperature ferroelectricity all exhibit significant thickness dependence. These results indicate that the stability of ferroelectricity is suppressed (enhanced) by increasing the surface or bulk ratio (strain), which can be explained by considering the thickness-dependent dipole-dipole interactions within the transverse Ising model. Our study provides new insights into ferroelectric size effects and sheds light on the applications of ferroelectric thin films in nanoelectronics

Multiple local symmetries result in a common average polar axis in high strain BiFeO3 based ceramics

For the first time, the origin of large electrostrain in pseudocubic BiFeO3-based ceramics is verified with direct structural evidence backed by appropriate simulations. We employ advanced structural and microstructural characterisations of BiFeO3 based ceramics that exhibit large electrostrain (>0.4%) to reveal the existence of multiple, nanoscale local symmetries, dominantly tetragonal/orthorhombic, which have a common, averaged direction of polarisation over larger, meso/micro-scale regions. Phase-field simulations confirm the existence of local nanoscale symmetries, thereby providing a new vision for designing high-performance lead-free ceramics for high strain actuators

Design of super-elastic freestanding ferroelectric thin films guided by phase-field simulations

Understanding the dynamic behavior of domain structures is critical to the design and application of super-elastic freestanding ferroelectric thin films. Phase-field simulations represent a powerful tool for observing, exploring and revealing the domain-switching behavior and phase transitions in ferroelectric materials at the mesoscopic scale. This review summarizes the recent theoretical progress regarding phase-field methods in freestanding ferroelectric thin films and novel buckling-induced wrinkled and helical structures. Furthermore, the strong coupling relationship between strain and ferroelectric polarization in super-elastic ferroelectric nanostructures is confirmed and discussed, resulting in new design strategies for the strain engineering of freestanding ferroelectric thin film systems. Finally, to further promote the innovative development and application of freestanding ferroelectric thin film systems, this review provides a summary and outlook on the theoretical modeling of freestanding ferroelectric thin films

A comparison study of solving diffusion equations with different algorithm methods

A comparison study for solving diffusion equations with different algorithm methods is studied to understand the oxygen vacancy defect transport under the electric field. We compare computational efficiency and numerical accuracy with different algorithm methods, including finite difference, finite element (COMSOL), and Fourier-Chebysev spectral methods. All the results of oxygen vacancy distribution under an electric field from different algorithm methods are compared with the analytical solution results. Two kinds of boundary conditions are used in solving diffusion equations and the absolute error of different methods are discussed. The main purpose of these results is to provide guidance for studying the role of point defect transport in the degradation and breakdown of devices

Thickness Dependence of Switching Behavior in Ferroelectric BiFeO3 Thin Films: A Phase-Field Simulation

A phase-field approach to the analysis of the thickness effects in electric-field-induced domain switching in BiFeO3 thin films has been formulated. Time evolutions of domain switching percentage for films with different thicknesses were explored to reveal the primary switching path and its dependence on film thickness. In addition, hysteresis loop for these films were calculated to obtain their coercive fields. Results show a nonlinear thickness dependence of coercive field for ultrathin films. A parametric study of the interactions between film thickness, coercive field, current-voltage (I-V) response, and polarization switching behavior is herein discussed, which could provide physical insights into materials engineering