Microstructural and compositional aspects of ZnO-based varistor ceramics prepared by direct mixing of the constituent phases and high-energy milling

ZnO-based varistor samples were prepared by the direct mixing of the constituent phases (DMCP) and sintering at 1100 °C for 2 h. The influence of the starting powder mixture's composition – the amounts of the pre-reacted varistor compounds and their composition – and its preparation, either with or without mechano-chemical activation (MCA), on the microstructure, phase composition and electrical characteristics of the varistor samples was studied. It showed that MCA improved the density and microstructural homogeneity of the varistor samples. MCA strongly affected the grain growth: it enhanced the nucleation of inversion boundaries (IBs) in the ZnO grains and the IBs-induced grain-growth mechanism resulted in uniform grain growth and hence a microstructure with smaller ZnO grains and a narrower grain size distribution. The final phase composition of the samples prepared by the DMCP method mainly depended on the presence of varistor dopants that can prevent the formation of the pyrochlore phase, especially Cr₂O₃, while MCA can affect it mostly by providing a homogeneous distribution of those dopants. The DMCP varistor samples prepared with MCA had much better current–voltage characteristics than the samples of the same composition prepared from unactivated powders.     

Use of recycled copper slag for blended cements

Copper slag is a by‐product generated during smelting to extract copper metal from the ore. The copper slag obtained may exhibit pozzolanic activity and may therefore be used in the manufacture of addition‐containing cements. In this paper the effect of the incorporation of the copper slag in cement is measured. Blends of copper slag with Portland cement generally possess properties equivalent to Portland cement containing fly ash, but very different to the silica fume incorporation. Copper slag and fly ash reduce the heat of hydration more effectively than silica fume in mortars. The replacement of 30% cement by copper slag reduces the flexural and compressive strength in a similar way to fly ash; however, after 28 days, the reduction is less than the percentage of substitution. Hydrated calcium aluminate phases were analysed using scanning electron microscopy (SEM) and X‐ray diffraction (XRD) techniques. The pozzolanic activity of copper slag is similar to that of fly ash and higher than silica fume. In the presence of low water/cement ratios, certain pozzolanic materials produce a very compact cement paste that limits the space available for hydration products, a determining factor in the formation of hydrated calcium aluminates. SEM was found to be a useful analytical technique when aluminates are formed and can be clearly detected by XRD. Copyright © 2008 Society of Chemical Industry     

Time Decay of the Activity of the Reduction Reaction of NO by CO on a Pd/Al2O3 Catalyst

An experimental study is made of the time decay of activity of the CO–NO reaction on a Pd/Al₂O₃ looking at the effect on reaction order and apparent activation energy. The optimum kinetics parameters fitting the steady state data at moderate pressures are determined. The time decay curves are analyzed through various catalyst deactivation models.     

Manifold mapping: a two-level optimization technique

In this paper, we analyze in some detail the manifold-mapping optimization technique introduced recently [Echeverría and Hemker in space mapping and defect correction. Comput Methods Appl Math 5(2): 107—136, 2005]. Manifold mapping aims at accelerating optimal design procedures that otherwise require many evaluations of time-expensive cost functions. We give a proof of convergence for the manifold-mapping iteration. By means of two simple optimization problems we illustrate the convergence results derived. Finally, the performances of several variants of the method are compared for some design problems from electromagnetics.     

Modelling biological processes under anaerobic conditions through integrating titrimetric and off-gas measurements-applied to EBPR systems.

An innovative method for modelling biological processes under anaerobic conditions is presented and discussed. The method is based on titrimetric and off-gas measurements. Titrimetric data is recorded as the addition rate of hydroxyl ions or protons that is required to maintain pH in a bioreactor at a constant level. An off-gas analysis arrangement measures, among other things, the transfer rate of carbon dioxide. The integration of these signals results in a continuous signal which is solely related to the biological reactions. When coupled with a mathematical model of the biological reactions, the signal allows a detailed characterisation of these reactions, which would otherwise be difficult to achieve. Two applications of the method to the enhanced biological phosphorus removal processes are presented and discussed to demonstrate the principle and effectiveness of the method.     

Emission of trace toxic metals during pulverized fuel combustion of Czech coals

A study of the trace elements emission (As, Se, Cd, Co, Cr, Cu, Zn, Hg, Tl, Pb, Ni, Sn, Sb, V, Mn and Fe) from pulverized coal combustion has been made at six heating and power stations situated in the Czech Republic. The amount of chlorine in coal has considerable influence on volatilization of some elements such as Zn, Cu, Pb, Hg and Tl, which is explained by the formation of thermodynamically stable compounds of these elements with chlorine. Generally, the affinities for Cl follows the order Tl > Cu > Zn > Pb > Co > Mn > Sn > Hg. The experimental data indicates enrichment of some of the trace toxic elements in the emissions (Cu, Zn, As, Se, Cd, Sn, Sb, Hg and Pb) and good agreement was obtained by thermodynamic equilibrium calculations with a few exceptions. In the case of Fe, Mn, Co, Cr and Sn calculated values are overestimated in the bottom ash and there are zero predicted amounts of these elements in the fly ash. In comparison, the results from experiments show up to 80% of these elements retained in fly ash. This implies that there exist additional steps leading to the enrichment by Fe, Mn, Co, Cr and Sn of small particles. Such mechanisms could include the ejection during devolatilization of small inorganic particles from the coal of bottom ash particles, or disintegration of the char containing these metals to small particles of fly ash. On the other hand, there are slightly overestimated or similar values of relative enrichment factors for As, V, Cu, Cd, Sb, Tl and Pb in the fly ashes and zero predicted values for bottom ashes. Our experimental results show about 5% or less of these elements are retained in bottom ashes, so they probably remain in the bottom ash inside unburned parts of coal. Copyright © 2003 John Wiley & Sons, Ltd.     

Influence of aminophylline and 8-(p-sulfophenyl)theophylline on the anticonvulsive action of diphenylhydantoin, phenobarbital, and valproate against maximal electroshock-induced convulsions in mice

Time variation of drinking is substantial and has an effect on aggregate estimates of consumption. In this article it is shown that because of a considerable seasonal variation in consumption (+/- 20%) a serious bias in annual consumption estimates can be expected in surveys with a limited time frame. The present study analyzes drinking data collected in the general population of the Netherlands from March 1985 through December 1985 (including Christmas and New Year's Eve). Since it was expected that sensitivity to temporal fluctuations might not be equal for different methods of measurement, several indices of consumption were compared. Although the assessed seasonal effect varies indeed across types of measurement, across male and female subsample and across types of alcoholic beverage, the general tendency is for consumption to be highest in the spring season and lowest in the autumn. Sales figures fluctuate accordingly. It is evident that the risk of biased estimates is larger the shorter the time frame of the survey. Seasonal variation was highest in the frequency domain. Furthermore, exclusion from the time frame of collective holidays, during which people drink more often and more per occasion (viz., Christmas), increases the risk of biased estimates. Even estimates of abstention, but also regular heavy drinking among women, appear to vary considerably over the three seasons in this study. The main conclusion is that results of comparisons of survey data on drinking, particularly those over time, are more or less invalid if the respective time frames of the surveys do not correspond.