Landau theory of thin ferroelectric films

Abstract

Landau-Devonshire theory¹ is a useful phenomenological model to describe the properties of ferroelectric phase transitions. Below the transition temperature, the Landau model can be generalized to describe the thermodynamic stability of a ferroelectric crystallite in a bistable polarized configuration, and to predict the response of the crystallite to external fields and charges. The three primary elements to be considered in modeling thin-film ferroelectric devices are the polar response of the ferroelectric itself, the contribution of electrode interfaces, and the interaction of mobile and immobile charged defects and carriers with the ferroelectric and the electrodes. First, the hysteresis properties of a single domain or crystallite are derived. This result is generalized to find the polar response in a polycrystalline film where there may be variations in the size and orientation of the crystallites and in the coercivity, remanence and offset of the domains. After postulating that metal electrodes form Schottky barriers with respect to the ferroelectric, we can then calculate the electric fields and potentials throughout the ferroelectric film. These calculations show that space charges form near the electrodes and the magnitude of the electric field in these regions is large. A further examination of the space charges results in a model for the C-V response of the ferroelectric capacitor, as the C-V response is dominated by space charge effects. The charge concentrations, contact potentials, high-field permittivity, and space charge widths can be extracted from the C-V data. Finally, the interactions between defects and domains leading to domain pinning and fatigue are investigated.     

Tuning PEG-DA hydrogel properties via solvent-induced phase separation (SIPS)()

Poly(ethylene glycol) diacrylate (PEG-DA) hydrogels are widely utilized to probe cell-material interactions and ultimately for a material-guided approach to tissue regeneration. In this study, PEG-DA hydrogels were fabricated via solvent-induced phase separation (SIPS) to obtain hydrogels with a broader range of tunable physical properties including morphology (e.g. porosity), swelling and modulus (G'). In contrast to conventional PEG-DA hydrogels prepared from an aqueous precursor solution, the reported SIPS protocol utilized a dichloromethane (DCM) precursor solution which was sequentially photopolymerized, dried and hydrated. Physical properties were further tailored by varying the PEG-DA wt% concentration (5 wt%-25 wt%) and M(n) (3.4k and 6k g mol (-1)). SIPS produced PEG-DA hydrogels with a macroporous morphology as well as increased G' values versus the corresponding conventional PEG-DA hydrogels. Notably, since the total swelling was not significantly changed versus the corresponding conventional PEG-DA hydrogels, pairs or series of hydrogels represent scaffolds in which morphology and hydration or G' and hydration are uncoupled. In addition, PEG-DA hydrogels prepared via SIPS exhibited enhanced degradation rates.     

Resilience to maternal depression in young adulthood.

Using a prospective longitudinal design, this study investigated factors associated with resilience in 20-year-old offspring of depressed mothers (n = 648). Resilient youth were operationally defined as those whose mothers were depressed but who themselves had no history of recurrent depression and currently evidenced adequate academic or work and romantic functioning, no Axis I psychopathology, and no clinically significant internalizing behavior problems. Low levels of perceived maternal psychological control (p = .02) and high child IQ (p p = .02), high maternal warmth (p p p p p = .03). Interventions focused on these 2 protective factors might yield the strongest benefits for offspring of depressed mothers as they transition to early adulthood. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Improved Passive Treatment of Acid Mine Drainage in Mushroom Compost Amended with Crab-Shell Chitin

Crab-shell chitin, which is inherently high in calcium carbonate and nutrients, was tested as a multifunctional, fractional amendment to improve the effectiveness of spent mushroom compost (SMC), which is a low-cost, frequently used, but often underperforming substrate for treating acid mine drainage (AMD). Batch and continuous-flow column tests were used to evaluate different crab-shell/SMC mixtures for their ability to neutralize acidity, reduce sulfate, and remove metals in field-collected AMD. Alkalinity generation and the removal of manganese and sulfate were strongly correlated to the fraction of crab shell in the substrate: the treatment capacity increased from 36.7 L/kg for the traditional 90% SMC/10% limestone substrate up to 428 L/kg for 100% crab shell. The costs associated with adding crab shell to SMC were found to be minimal relative to the resulting improvement in water quality. Based on these data, it appears that a small fraction of crab shell (5–15%) does not provide a significant benefit over traditional compost and limestone substrates, but that larger fractions (50–100%) are much more efficient than traditional SMC substrates, especially for the removal of metals.     

An artificial neural network approach to the problem of wireless sensors network localization

One of the imperative problems in the realm of wireless sensor networks is the problem of wireless sensors localization. Despite the fact that much research has been conducted in this area, many of the proposed approaches produce unsatisfactory results when exposed to the harsh, uncertain, noisy conditions of a manufacturing environment. In this study, we develop an artificial neural network approach to moderate the effect of the miscellaneous noise sources and harsh factory conditions on the localization of the wireless sensors. Special attention is given to investigate the effect of blockage and ambient conditions on the accuracy of mobile node localization. A simulator, simulating the noisy and dynamic shop conditions of manufacturing environments, is employed to examine the neural network proposed. The neural network performance is also validated through some actual experiments in real-world environment prone to different sources of noise and signal attenuation. The simulation and experimental results demonstrate the effectiveness and accuracy of the proposed methodology.     

Parallel implementation of isothermal and isoenergetic Dissipative Particle Dynamics using Shardlow-like splitting algorithms

A parallel implementation of the Shardlow splitting algorithm (SSA) for Dissipative Particle Dynamics (DPD) simulations is presented. The isothermal and isoenergetic SSA implementations are compared to the DPD version of the velocity-Verlet integrator in terms of numerical stability and performance. The integrator stability is assessed by monitoring temperature, pressure and total energy for both the standard and ideal DPD fluid models. The SSA requires special consideration due to its recursive nature resulting in more inter-processor communication as compared to traditional DPD integrators. Nevertheless, this work demonstrates that the SSA exhibits stability over longer time steps that justify its regular use in parallel, multi-core applications. For the computer architecture used in this study, a factor of 10–100 speedup is achieved in the overall time-to-solution for isoenergetic DPD simulations and a 15–34 speedup is achieved for the isothermal DPD simulations.     

Spatial proximity is more than just a distance measure

In order to design computer systems that are intuitive to use, the way humans reason about their “real world” surroundings needs to be taken into consideration. Geographic Information Systems (GIS) focus on spatial reasoning. Over the last decades, many advances have been made in GIS interfaces and functionality; however the concept of proximity or nearness, which is essential in many forms of human reasoning, is still being addressed insufficiently.This paper provides a thorough and comprehensive synthesis of the disparate literature that pertains to the subject of proximity. It offers insights into why existing methods for reasoning with proximity work, or do not work, and analyses their strengths and weaknesses. Finally, the paper provides the derivation of new proximity measures, and their evaluation, backed by experiments and reflections. New measures are formally described in a unifying and compelling framework. This framework acknowledges that while distance is one factor that influences proximity perception, proximity is much more than just a distance measure.     

Effect of inclusion of soluble and insoluble fibres into extruded breakfast cereal products made with reverse screw configuration

Five dietary fibre rich ingredients were used at 5%, 10% and 15% replacement levels in a white flour cereal base to produce an extruded cereal product. The inclusion of the dietary fibres into the flour bases had no significant effect on the expansion ratio of the products. However, the bulk density of the extruded products increased with inulin addition. The pasting properties of the raw flour and fibre base as well as the extruded products were altered with the incorporation of dietary fibre, with guar gum enriched products showing elevated peak and final viscosity readings. This appeared to be related to moisture manipulation and hence the regulation of gelatinisation. In vitro starch hydrolysis of the raw bases and the extruded samples illustrated that the extrusion process significantly increased the availability of carbohydrates for digestion. Additionally, the inclusion of dietary fibres in the raw bases significantly reduced the rate and extent of carbohydrate hydrolysis of the extruded products. As such the addition of dietary fibres to extruded products reduced the amount of readily digestible starch components of breakfast products, and increased the amount of slowly digestible carbohydrates.     

Determination of the similarity between gonads recovered from single sea urchins (Evechinus chloroticus) compared to the similarity of gonads recovered from different urchins

Inherent variability in the New Zealand sea urchin, Evechinus chloroticus gonads quality poses a significant hurdle in the design and implementation of processing and shelf-life trials. This study investigated if the five gonads collected from an individual animal were sufficiently similar to act as replicates in handling and processing trials. For gonads collected from a single urchin, the average standard deviation (SD) and coefficient of variation (CV) of the moisture content (SD: 059; CV: 0.01), protein (SD: 2.45; CV: 0.30), glucose (SD: 0.05; CV: 0.23), glycogen (SD: 1.40; CV: 0.24), alanine (SD: 0.05; CV: 0.23), glycine (SD: 40.74; CV: 0.13); asparagine (SD: 0.27; CV: 0.14), glutamate (SD: 1.93; CV: 0.14), methionine (SD: 0.30; CV: 0.07), valine (SD: 2.20; CV: 0.07), IMP (SD: 2.29; CV: 0.08) and K-value (SD: 1.73; CV: 0.06) were greater than the SD and CV of gonads recovered from different urchins for moisture content (SD: 2.85; CV: 0.04), protein (SD: 7.89; CV: 0.60), glucose (SD: 0.09; CV: 0.39), glycogen (SD: 2.09; CV: 0.30), alanine (SD: 0.09; CV: 0.39), glycine (SD: 239.98; CV: 0.77), asparagine (SD: 01.09; CV: 0.52), glutamate (SD: 12.17; CV: 1.02), methionine (SD: 3.09; CV: 0.80), valine (SD: 313.93; CV: 0.80), IMP (SD: 7.41; CV: 0.22) and K-value (SD: 7.47; CV: 0.21). It was therefore concluded gonads from a single urchin could be used as a valid means of reducing the initial inherent variability in the starting raw material when conducting shelf-life and processing trials.