Opaque precursors

This section examines the role of the molecular- and intermediate-scale structures of the sols in influencing the microstructure of the corresponding thin films formed by dip-coating. We consider the cases of mutually transparent and mutually opaque precursors. 

The coalescence of atoms into clusters may also be restricted by generating the atoms inside confined volumes of microorganized systems [87] or in porous materials [88]. The ionic precursors are included prior to irradiation. The penetration in depth of ionizing radiation permits the ion reduction in situ, even for opaque materials. The surface of solid supports, adsorbing metal ions, is a strong limit to the diffusion of the nascent atoms formed by irradiation at room temperature, so that quite small clusters can survive. 

Troilite (FeS) is also a common mineral in chondrites. Chondrules sometimes have rims of troilite, which may be recondensates of evaporated sulfur from chondrule precursors. The recondensation behavior of sulfur onto chondrules would be different from that of moderately volatile alkali elements described above. During cooling, sulfur would not re-enter the melt because the chondrule melt would have solidified by the time sulfur began to recondense. Instead, sulfur would recondense as sulfide veneers around chondrules (Zanda et al. 1995) or as opaque assemblages... 

The application of absolute reaction rate theory to a chemical change at an interface is only useful if the calculations refer to an identified, or at least reliably inferred, model of the controlling bond redistribution step. This is a problem, because it is particularly difficult to characterize the structures of the immediate precursors to reaction in many solid state rate processes of interest. The activated species are inaccessible to direct characterization because they are usually located between reactant and product phases. The total amount of reacting material present within this layer, often of molecular dimensions, is small and irreversible chemical and textural changes may accompany opening of such specialized structures for examination or analysis. Moreover, the presence of metallic and/or opaque, ill-crystalUzed product phases may prevent or impede the experimental recognition of participating intermediates or essential textural features. 

The well-known demonstrations by metallurgists that heterogeneous nucleation in metals and alloys (e.g., nucleation of pearlite) frequently occurs at dislocations (67, 68), were the precursors of the remarkable experiments of Mitchell aZ. (69-75) who decorated dislocations in transparent crystals of silver halides by exposure to light. The photo-lytic silver was precipitated along dislocations which were thereby rendered visible in the optical microscope. An extension of this technique was effected by Dash (76-80), who succeeded in decorating dislocations in opaque silicon by preferential precipitation of copper which could be detected under infrared illumination. Dash strikingly demonstrated... 

The tubular-film process is unsuitable for polymers with a very low melt strength such as polyethylene terephthalate. It is also not suitable for polypropylene films for packaging because the films are too crystalline, opaque and brittle due to too slow cooling. However, these films are often used as a precursor for fibrillated film fibres. 

For catalytic reactions of type B experiments conducted in opaque vessels, the following calculations should be performed to ensure that the solids are suspended. Indeed, many homogeneous catalyzed reactions use precursors with very high densities and frequently limited solubilities. A good example might be Ir4(CO)i2, which has a density of 4.01 g/mL (68). 

An oxalato complex can also be regarded as a source of the aqueous precursor. The oxalato ion, Ti(0H)2(C204)2, was reported to be present in aqueous solution at low concentrations of <0.02 mol/1 and imder acidic conditions (pH 1—4) (Van de Velde, 1977). A concentrated and acidic aqueous solution could be prepared by directly adding titanium tetraisopropoxide into double molar oxalic acid solution. An opaque colloidal... 

PI composite materials are particularly suitable for this process because they can be produced from polyamic acid (PAA) precursors, which can tolerate the addition of water to accomplish the hydrolysis of the alkoxide. The traditional sol-gel route for preparation of silica/PI composites can be briefly described as follows. First, PAA is prepared and silica precursor such as tetraethoxysilane (TEOS) reacts with water and acidic catalyst and changes into sol. The sol is added to the PAA solution, and the resulting homogeneous mixture after being stirred is then used to prepare silica/PAA precursor films. S-silica/PI composites are finally obtained by heating silica/PAA precursor films. However, the compatibility between PI and silica was not as good as expected hence, most of the reported silica/PI composites quickly became opaque with the increase of silica content. This is partially because the size of silica particles increases rapidly with the increase of silica content in silica/PI composites prepared via the sol-gel process. This is obviously an unfavorable factor to prepare transparent silica/PI composites with excellent properties, especially at high silica content. Therefore, it is important to improve the compatibility between the PI and silica phases and reduce the silica size. 

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