Sacrificial aggregate

A foundry core is a sacrificial aggregate that produces the interior configurationy or cavityy of a cast metal part The main component of this core is usually silica sand but other particulate inert materials have been used Sand is bonded by a core bindery which may be an organic resin composition such as a phenolicy furany alkyd or isocyanate Inorganic compositions of phosphates and silicates have also been used as core binders ... 

Aqueous pools of reversed micelles have been fruitfully employed for the in situ generation of semiconductor particles. The first publication in this area described the formation of CdS in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in isooctane [611]. The preparation involved the addition of aqueous CdCl or Cd(N03)2 to isooctane solutions of AOT. Exposure to controlled ammeters of the CdS particles formed. Irradiation of degassed, AOT-reversed-micelle-entrapped, platinized CdS by visible light (450-W Xenon lamp X > 350 nm) in the presence of thiophenol (PhSH) resulted in sustained hydrogen formation. Sacrificial electron transfer occurred from thiophenol to positive holes in the colloidal CdS and, consequently, diminished undesirable electron-hole recombinations (Fig. 101) [611]. 

A broad variety of sacrificial colloidal cores have been used for hollow capsule fabrication. They are inorganic or organic particles from tens of nanometers and up to tens of micrometerss, like melamine formaldehyde (MF), polysterene spheres, CaCC>3 and MgCQ3 particles, protein and DNA aggregates, small dye... 

The CdS nanoclusters and their loose aggregates were also found to be effective catalysts for the photoreduction of aromatic ketones and olefins in methanol using triethylamine as sacrificial electron donor, yielding alcohols and dihydro compounds, respectively [152]. The use of CdS nanoclusters seems to afford more selectivity than the commercially available CdS powders. 

The study was also extended to CoO nanoparticles [73] employing a sacrificial starch template. This method provided uniform shape and size, and was easy to perform at bulk levels. The size distribution of the nanoparticles was in the range of 15-30 nm, with a crystallite size of 18 nm. The low size and aggregation free character of the nanopaiticles resulted in a NIR reflectance of above 75%, in spite of a black color (where NIR reflectance values are expected to be less than 30%). This provided feasibility for employing CoO nanoparticles as components of cool coatings on leather, automobile surfaces, and so oa... 

It can be seen from UV-visible characterization of the species that the tetraaryl porphyrinic chromophores 125(M) tend to form J-type aggregates (red-shifted absorptions). In these assemblies, the use of equimolar mixtures of 125 (Zn) and 125(H2) led to a very efficient (81%) ET process, via a postulated and reasonable Forster mechanism, with a rate constant of A et = 3.1 X 10 s-l. When iron(III) porphyrins 125(Fe) are used, free base protoporphyrin 124 bearing alkyphosphocholine side arms can be incorporated in the fluid part of the vesicles. In the presence of a sacrificial electron donor on the outside of the vesicle, a vectorial eT process occurs from the excited free base porphyrins to the iron(III) species, leading to photoreduction of the iron(III) to iron(II). Previous work had already shown that this could also be achieved between zinc porphyrins and iron porphyrins in vesicles ... 

A PPy-chitosan hollow nanosphere (core diameter 20 3 nm, shell thickness 15 4 nm) has been fabricated by using AgCl nanoparticle as a sacrificial core at 2 °C [221,222]. The core and shell were sequentially formed in the same reaction medium. During the synthetic process, chitosan stabihzed the AgCl nanoparticle and prevented the aggregation of PPy. In addition, the PPy hollow nanosphere was stable in acidic aqueous media and insoluble in basic media due to the presence of chitosan in the shell part. 

Mn02 colloids have been studied in light-driven systems, using [Ru(bpy)3] " " as photosensitizer and S20g or [Co(NH3)5Cl] as sacrificial agents,as well as in the presence of dark oxidants like Ce(IV). Unfortunately colloidal manganese oxides display poor performance in all these conditions, probably due to the aggregation of initially small particles (6 nm), to yield a physical separation of the catalyst from the reaction mixture. 

Fig. 27 Top SEM images of CaCOs particles grown on a glass slip in the early reaction stage, PEO-6-PMAA, [CaCb] = 10 mM, lgL , 5h. a CaCOs particles with either spherical or hollow structures, b Zoom showing the calcite rhombohedral subunits grown on the surface of the hollow structure and the inner part consisting of tiny primary nanocrystals with a grain size of about 320 nm as indicated by the arrow (sacrificial vaterite template). Bottom Proposed formation mechanism of the calcite hollow spheres a polymer-stabilized amorphous nanoparticles b Formation of spherical vaterite precursors c aggregation of the vaterite nanoparticles d vaterite-calcite transformation starting on the outer sphere of the particles e formation of calcite hollow spheres under consumption of the sacrificial vaterite precursors. Reproduced in part from [61] with permission of the American Chemical Society...

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