Porous Material

A new structure is proposed to improve the external quantum efficiency of porous silicon ͑PS͒ light emitting diodes ͑LED͒. It is based on a heterojunction between n-type doped silicon and PS. The heterojunction is formed due to the doping selectivity of the etching process used to form PS. The improvement of the proposed LED structure with respect to usual metal/PS LED is demonstrated. This is thought to be due to a different injection mechanism for which carriers are injected directly into conduction band states. Anodic oxidation experiments show further improvements in the LED efficiency.

During storage of solid animal manure, biological transformation of nitrogen and carbon may increase the temperature to 60 to 70 o C, i.e. composting. The rise in temperature may cause a substantial loss of ammonia, as the ammonium is transformed to ammonia at high temperatures and because the heat is generating a flow of air through the dung heap. In

Two-dimensional natural convective flow in a tilted rectangular porous material saturated with fluid is analyzed by solving numerically the mass, momentum and energy balance equations, using Darcy's law and the Boussinesq approximation. Isothermal boundary conditions are considered, where two opposite walls are kept at constant but different temperatures and the other two are thermally insulated. The external parameters considered are the tilt angle, the aspect ratio and the Darcy-Rayleigh number. Three main convective modes are found: conduction, single and multiple cell convection and their features described in detail. Local and global Nusselt numbers are presented as functions of the external parameters.

A combination of ultra small angle X-ray scattering (USAXS) and ultra small angle neutron scattering (USANS) is used to characterize porous materials. The analysis methods yield quantitative information, including the mean skeletal chord length, mean pore chord length, skeletal density, and composition. A mixed cellulose ester (MCE) membrane with a manufacturer-labeled pore size of 0.1 m was used as a model to elucidate the specifics of the method. Four approaches describing four specific scenarios (different known parameters and form of the scattering data) are compared. Pore chords determined using all four approaches are in good agreement with the scanning electron microscopy estimates but are larger than the manufacturer's nominal pore size. Our approach also gives the average chord of the skeletal solid (struts) of the membrane, which is also consistent for all four approaches. Combined data from USAXS and USANS gives the skeletal density and the strut composition.

Results are presented for laminar forced convection cooling of heat generating blocks mounted on a wall in a parallel plate channel. The eect on heat transfer of insertion of a porous matrix between the blocks is considered. The¯ow in the porous medium is modeled using the Brinkman±Forchheimer extended Darcy model. The mass, momentum and energy equations are solved numerically by a control-volume-based procedure. The local Nusselt number at the walls of the blocks, the mean Nusselt numbers and the maximum temperature in the blocks are examined for a wide range of Darcy number and thermal conductivity ratio. The computations are ®rst conducted for a single block, then for evenly mounted blocks. The results show that the insertion of a porous material between the blocks may enhance the heat transfer rate on the vertical sides of the blocks. Although the porous matrix reduces the heat transfer coecient on the horizontal face, signi®cant increases in the mean Nusselt number (up to 50%) are predicted and the maximum temperatures within the heated blocks are reduced in comparison with the pure¯uid case. Ó 1998 Elsevier Science Inc. All rights reserved.

Coir fiber from coconut husk is an important agricultural waste in Malaysia. Acoustic absorption coefficient of the fiber as a porous material is studied in this paper. Two types of fiber are investigated, fresh from wet market and industrial prepared mixed with binder. Moreover two analytical models, namely; Delany-Bazley and Biot-Allard are used for analysis. Experimental measurements in impedance tube are conducted to validate the analytical outcomes. Results show that fresh coir fiber has an average absorption coefficient of 0.8 at f > 1360 Hz and 20 mm thickness. Increasing the thickness is improved the sound absorption in lower frequencies, having the same average at f > 578 Hz and 45 mm thickness. Delany-Bazley technique can be used for both types of fiber while Biot-Allard method is compensated for the industrial prepared fiber considering the binder additive. This form generally shows poor acoustical absorption in low frequencies. Inevitably, fiber has to be mixed with additives in commercial use to enhance its characteristics such as stiffness, unti-fungus and flammability. Hence other approaches such as adding air gap or perforated plate should be used to improve the acoustical properties of industrial treated coir fiber.

The City of Adelaide suffers from rapid damage to historic building materials due to salts, rising damp, and damp-proof course failures. Adelaide City Council has partially funded repairs to over 400 buildings in the past 15 years. To begin to examine the scope of this problem and the effectiveness of various treatments, 24 historic buildings in Adelaide were examined, with a focus on the building materials, historic interventions and current treatments applied to treat rising damp and salt decay. Analysis of 90 samples found high levels of sodium sulfate, sodium chloride and sodium nitrate in cellars, ground water and building materials, suggesting a clear example of rising damp. Samples of disintegrating masonry, depth profiles acquired by drilling, poultices and damp proof courses (DPC) were analyzed by Ion Chromatography (IC) and Environmental Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (ESEM/EDS). Mercury Intrusion Porosimetry (MIP) and capillarity test were carried out in the building materials and DPCs showing that porous materials with high porosity, small pores and low strength are more prone to salt weathering. The City of Adelaide is a natural laboratory for comparing and analyzing different treatments of salt-laden masonry, with hundreds of treated buildings of the same age, in the same environment, and with similar building materials. induced environmental problem is caused by alterations in the water regime of many catchments following either land clearing and/or artificial irrigation. Salinization is a growing problem in Australia, affecting agricultural productivity, historical structures and archaeological sites [6]. Porous building materials, such as stone, brick and mortar are prone to the ingress of moisture and damage from salt crystallization. Rising damp occurs where ground moisture is drawn into mortar and masonry surfaces by capillary action. The extent of the rising damp is determined by the hygroscopic indices of the salts, the availability of moisture, and degree of evaporation [7]. Salts are precipitated and crystallized at different heights on the wall in a sequence according to their solubility [8], temperature and relative humidity. Above ground level, water evaporates from the wall and the solution becomes more and more concentrated while still being subject to capillary rise. As the solution becomes saturated during transport any further evaporation will cause crystallization and immobilization of the salt [9].

We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting. We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials). The most commonly used molecular simulation, theoretical and experimental methods are first presented. We also provide a brief description of the most widely used porous materials. The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surfacedriven and confinement-driven phases are then discussed. We also address how confinement affects the glass transition.