Entrapped porosity

Figure 19 shows a microstructure of Al203 sintered with 0.05 wt % Y203 and 0.05 wt % MgO above the ternary eutectic temperature. The lack of entrapped porosity reported by Rossi and Burke [5S] is confirmed. The microstructure has a distinct shift in grain size distribution if one compares this structure with Fig. 13. It is hypothesized that MgO remains an effective grain growth inhibitor in the presence of the ternary eutectic...

The post-HIP of materials with closed porosity - that is, >93% density after pressureless or uniaxial pressure-assisted heat treatment - allows for the closure of entrapped porosity, thus improving the mechanical properties. The four-point bending strength of485 MPa for samples with 2% carbon content could be improved to 580 MPa by HIP at 2000 °C with 200 MPa [409, 420]. 

Chemical Applications. Courtaulds has developed a series of acryHc-based fibers for controUed release of chemical reagents. The trade name of these fibers is Actipore. The reagents are entrapped within the fiber and slowly released at a rate dependent on the exact porosity of the fiber (102). 

Critical factors. In general, porosity is caused by the entrapment of gas during the welding process or during solidification of the weld metal. Surface contamination may provide a gas source during the welding operation. 

The porosity of refractory bricks has a direct bearing on the thermal conductivity. The densest and the least porous bricks have the highest thermal conductivity owing to the absence of air voids. On the other hand, in porous bricks the entrapped air in the pores acts as a nonconducting material. 

Sol-gel glass is ion-permeable due to its residual hydroxy groups and its porosity, and can be used to optically measure pH by entrapping (caging) conventional pH indicator dyes in the material. Often, water soluble indicator dyes do not have to be chemically modified for immobilisation (see chapter 4) because, due to their size, they are retained in the pores of the sol-gel glass and are not washed out. 

General Aspects and Heat Treatment Effects. After him formation, for most of the hlms aimed at electronic applications (other than amorphous oxides, such as Si02), the hlms are subjected to a heat treatment process for removal of residual organic species (entrapped solvent as well as the organic constituents associated with the precursor species), densihcation (elimination of residual porosity and structural free volume in the him), and crystallization. 

For example, whereas the solid oxidation catalyst MCM-41-entrapped perruthenate can be used for the conversion of benzyl alcohols only, a similarly perruthenated-doped amorphous ORMOSIL is equally well suited for a variety of different alcohol substrates.35 On the other hand, a uniform pore structure ensures access to the active centres, while in an amorphous material made of non-regular porosity hindered or even blocked sites can well exist (Figure 1.16), rendering the choice of the polycondensation conditions of paramount importance. 

Indeed, the mobility of the entrapped dopant is crucial in promoting the reactivity of the final materials. Thus, provided that the dopant molecules are at the surface and enjoy enough freedom, high porosity will certainly promote reactivity by limiting intraparticle diffusion but that will not be the case if microporous xerogels of different HLB are compared (c/. entrapped lipase and tetra-//-propy 1 am monium perruthe-nate (TPAP) where ORMOSIL with the smaller pores are more reactive). 

Human life, furthermore, certainly benefits from a less polluted world, and here, again, sol-gel entrapped catalysts are, literally, able to have transferred to within their large inner porosity the whole chemistry of fine chemicals production. Think for instance of an innocuous easily handled orange powder called SiliaCat TEMPO (Chapter 5) that added to a mixture of alcohols at 0 °C with a modest excess of aqueous, cold bleach rapidly converts them into all those fragrances, vitamins, hormones and drugs made of carbonyl compounds. 

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