Thermal Conductivity of Evacuated Highly Transparent Silica Aerogel

The thermal loss coefficient k of evacuated load bearing transparent silica aerogel tiles is determined for temperatures between 280 K and 400 K. The measurements were performed with the large guarded hot plate high vacuum system LOLA I, the reference plates of which can be pressed onto the samples with an adjustable load (0... 1 bar). For a 22mm layer of aerogel (density ρ ≈ 100...110g/liter) the loss coefficient varied between about k = 0.36 W//m² K) at 280 K and k = 1.45 W/(m².K) at 400 K, corresponding to a pseudo-conductivity λ = 8. 10^?3 W/(m. K) and λ = 32. 10^?3 W/(m. K), respectively. Thus at typical winter temperatures (inside 20 °C, outside ?10 °C to 0 °C), evacuated transparent aerogel could provide excellent thermal insulation as a spacer in double pane windows. It also could be used in passive solar systems, for example as a cover for Trombe walls or as a transparent insulating structure around house walls. The dramatic increase of thermal losses at higher temperatures is due to a transmission window for infrared radiation between 3 and 7 μm, Additional data reveal the influence of water adsorption and of air pressure on the thermal conductivity of aerogel.     

On efficiency of both single fuel cells and stacks I

A method to estimate the efficiency of a stack of several identical cells is described on the basis of the electrochemical behavior of a single cell. Efficiency of fuel cell stacks is defined by means of a combination of semi-empirical models of polarization curves and dimensionless variables such as reaction extent and utilization. The connection of flows among the cells is basically divided in two extreme cases and one intermediate case. Higher efficiencies are obtained when the same main flow (both anodic and cathodic) passes consecutively through the stack cells (Chain Flow), because it is favored thermodynamically. It is less favored when the main flow is strictly divided among all the cells (Separate Flow). In the intermediate case, the main flow is divided among all the stack cells and all the outlets are collected in one flow. The latter can spontaneously evolve to the more thermodynamically stable behavior of Chain Flow.     

MAKING BETTER USE OF NATURAL LIGHT WITH A LIGHT-REDIRECTING DOUBLE-GLAZING SYSTEM

The subject of the present paper is a fixed daylighting system known as the LIF light guide. The system consists of a fixed arrangement for capturing sunlight incident at various angles and re-directing it by means of an optical wave-guide which provides diffuse ceiling illumination. The light guide is generally sandwiched between two glass panes and mounted above the windows to avoid obstructing visibility. Its intended function is to furnish offices and other working areas with a uniform level of natural illumination while avoiding glare.     

On efficiency of both single fuel cells and stacks II

The model developed in the first part of this work is used to predict the cell potentials and the irreversible Gibbs free energy of a stack of 15 cells. The model starts from a phenomenological equation of a polarization curve with the extent of reaction as the independent variable. Two extreme kinds of flow of reagents, defined as Chain and Separate Flows respectively, are considered. The cell potentials are obtained by a combination of the potential of the two extreme cases of flow. The stack cell potentials and the efficiencies, estimated by the model, reproduce the general characteristics obtained by the experiments.     

Determination of the Radiative Heat Transfer in Transparent Silica Aerogel

Radiative heat transfer in SiO₂ aerogel is determined from the spectral infrared extinction (absorption) E(λ). From E( λ) we derive a temperature dependent Rosse-land mean ē(T) for extinction. A suitable diffusion model for the infrared radiation allows to calculate the temperature dependent radiative loss coefficient k_raa(T). For a 1?cm thick aerogel board and a mean temperature within the insulation of T = 280?K, a value as low as k_rad,? 0.16W/m²K results. Between T = 250 and 450K a good approximation is_rad∞T_α, with α = 5.6.     

Effects of Immobilization and Permeabilization Procedures on Growth of Chenopodium rubrum Cells and Amaranthin Concentration

Cultures of Chenopodium rubrum were subjected to permeabilization and immobilization procedures to examine their potential for pigment production. Amaranthin content of culture media was highest for cultures treated with dimethylsulfoxide (DMSO) or dissolved chitosan. Labelling of the cells with L-(U-¹⁴C) tyrosine revealed that amaranthin was released from the cells but degraded rapidly. Product degradation in chitosan immobilized cells was delayed by 12–24 hr. Sufficient cell growth was observed in cultures treated with 0.77 mg chitosan per gram fresh biomass, with 0.42 ml.g^-1 DMSO, or when immobilized in a complex Ca-alginate-chitosan gel system. All other treatments resulted in inhibition of growth.     

Experimental study of MESFET travelling-wave structures

Wave impedances and propagation constants of waves travelling along the electrodes of a long FET structure were experimentally investigated. For this purpose, a GaAs MESFET with large gate width was fabricated. Connecting pads on both ends of the gate and drain electrodes make it possible to measure the four-port S-parameters of the structure. Using these results, it is shown that a great improvement in the gain is possible, if the normally open ends of the FET are connected to the optimum terminations.