The thermokinetic modeling of substrate rapid solidification

A boundary problem arising from rapid solidification with changing conditions at the moving boundary for the classical Stefan problem was solved by combining the heat transfer with crystallization kinetics using a front-tracking finite-difference scheme. At each time step, the conditions at the moving boundary were determined by an iterative process.     

Resistance degradation in donor-doped PZT ceramic stacks with Ag/Pd electrodes: I. Phenomenology of processes

The resistance degradation behaviour of Nd-doped PZT layers with Ag/Pd electrodes was investigated under electric fields of 250–1250 V/cm at temperatures between 425 and 520 °C. Degradation processes caused by high humidity (water films) could thus be avoided. Several phenomena were found in the course of degradation: conductivity variations on a short-time scale are interpreted in terms of field induced oxygen vacancy concentration changes (stoichiometry polarization). The corresponding diffusion coefficient was estimated. Increase of the current by several orders of magnitude during long-time load was associated with cation motion. Moreover, formation of surface precipitates was found on the field-stressed ceramics. The main occurrence of these precipitates close to anodes is in contrast to a conventional electrochemical interpretation. However, precipitates (mostly consisting of silver) are not directly responsible for the high conductivity of degraded PZT layers. Rather, regions exhibiting metal-like conductivity are irreversibly formed in the bulk.     

Resistance degradation in donor-doped PZT ceramic stacks with Ag/Pd electrodes: II. Distribution of conduction paths

Neodymium-doped PZT layers with Ag/Pd inner electrodes showed a metal-like conductive behaviour after degradation under electric fields of 250–1250 V/cm at 420–520 °C. This conductive state was studied by means of impedance spectroscopy. Depth profiling by mechanical removal of material revealed a strongly inhomogeneous distribution of conduction paths within a degradation zone located in the interior of the ceramic layers, probably along grain boundaries. The thickness of the degradation zone (several tens to hundreds of micrometres) increased with degradation time and DC load. After complete mechanical removal of the degradation zone, virgin PZT was again found. Spatially resolved conductivity measurements (microelectrodes) and conductive atomic force microscopy (C-AFM) investigations gave further information on inhomogeneities in conductivity and the connectivity of surface precipitates. A mechanistic model is provided suggesting formation of silver paths along grain boundaries according to a mechanism which strongly differs from conventional electrochemical dendrite formation.     

Forecasts and outcomes

The forecasts used by planners often overstate the outcomes that are eventually observed. Many forecasts prepared for planning purposes project changes that turn out to have been exaggerations of the changes that actually take place. This article explores some of the reasons forecasts of social changes may overstate the eventual outcomes and considers the role of interests as well as some of the technical factors that may encourage exaggeration and overstatement. The nature of the models often used as a basis for preparing forecasts may lead to the overstatements of impacts. The article concludes by recognizing the homeostatic nature of the forces that forge city structures and presents some implications regarding power relations and ethics.     

Gelation of semiflexible polyelectrolytes by multivalent counterions

Filamentous polyelectrolytes in aqueous solution aggregate into bundles by interactions with multivalent counterions. These effects are well documented by experiment and theory. Theories also predict a gel phase in isotropic rodlike polyelectrolyte solutions caused by multivalent counterion concentrations much lower than those required for filament bundling. We report here the gelation of Pf1 virus, a model semiflexible polyelectrolyte, by the counterions Mg(2+), Mn(2+) and spermine(4+). Gelation can occur at 0.04% Pf1 volume fraction, which is far below the isotropic-nematic transition of 0.7% for Pf1 in monovalent salt. Unlike strongly crosslinked gels of semiflexible polymers, which stiffen at large strains, Pf1 gels reversibly soften at high strain. The onset strain for softening depends on the strength of interaction between counterions and the polyelectrolyte. Simulations show that the elasticity of counterion crosslinked gels is consistent with a model of semiflexible filaments held by weak crosslinks that reversibly rupture at a critical force.     

Canada's increasing housing affordability burdens

This study uses national survey and census data on shelter costs and income to describe changes in the proportion and the number of low‐income households spending more than half of their income on shelter. While affordability problems increased consistently over the last two decades for almost all classes of households, the problems are highly concentrated among those with low‐incomes. Women household maintainers are significantly more likely to experience problems and the number of income recipients in a household is a key indicator of a potential problem. While all regions and major cities had increasing problems, the data show major differences across regions and urban centres. No correlation is found between the growth of cities or the growth in rent levels and the growth of the proportion of low‐income households with severe affordability problems. Housing prices were remarkably stable during the 1990s and cannot be claimed as the main cause of the escalating problem. However, strong correlations relate the growth of affordability problems to city size and to the prevailing rent level, suggesting that land rent is a factor in determining the problem's spatial incidence and that continued concentration of the population in major cities will continue to fuel the growth of the problem. The most disturbing finding is that, for the most vulnerable groups, the prevalence and severity of affordability problems worsened during the 1990s, reflecting the consequences of a larger and longer trend toward increasing income inequality in Canadian society. The paper points to other research which links the affordability issue to homelessness and argues that the trends in affordability burdens be considered as ripe for serious policy intervention at all three levels of government. While specific policy conclusions cannot be based on this study, the results do point to the growing need for a change in Canadian housing policy.