Silica films

Since the formation of the silica film does not depend on any particular property of the corrosive environment, high-silicon irons can resist attack by a very wide range of environments. Solutions which are capable of dissolving silica, even in a small degree, are, however, inimical to silicon irons, and there are also a few ions capable of penetrating the silica film, which can cause relatively serious corrosion of the metal. The presence of chromium... 

Before the silica film can form, some corrosion of the metal must necessarily take place, and it follows that initial corrosion rates are high. Fig. 3.62 illustrates this point and suggests that uniform rates of corrosion are not reached until at least 100 h after the onset of the attack. As a result, useful data on the corrosion of high silicon irons can be obtained only from tests of at least this duration. 

Since the corrosion resistance of the high-silicon alloys depends upon the permanence and impermeability of a thin silica film on the surface of the metal, it is obvious that any reagent which can damage the film will cause accelerated corrosion of the metal. For this reason all solutions containing hydrofluoric acid must be regarded as incompatible with the alloys. 

Attempts made to produce an alloy more resistant to hydrochloric acid have resulted in alloys containing 17-18% silicon or 14-5% silicon and chromium plus 3% molybdenum. The first is produced in Britain, and the second in the United States. The reason for the increase in resistance to hydrochloric acid of the Fe-18 Si alloy is thought to lie primarily in the increased density of the silica-rich film left on the metal by initial corrosion. The addition of 6% chromium with some molybdenum to Fe-14-5 Si causes the formation of extremely stable complex carbides with the consequent complete elimination of graphite plus the formation of a more penetration-resistant silica film, probably containing chromium in substantial quantity. 

The evidence at present available concerning the corrosion of high-silicon irons by sulphurous acid is insufficient to allow the formation of any theory about the mechanism by which the silica film barrier is broken down in the presence of this acid. As far as is known, this acid is corrosive to all types of high-silicon iron. 

Nitric acid is also withstood by high-silicon iron. The concentrated acid is believed to reinforce the silica film by the formation of a passive iron oxide... 

The irons are not recommended even for so weak a base as ammonium hydroxide, if the liquid temperature is greater than 20°C. The alternate handling of acids (for which the alloy is normally resistant) and alkalis may also prove troublesome since the alkali will normally prevent the formation of the protective silica film on which its acid resistance depends. 

The focusing of radiation within the instrument was formerly done by means of lenses, but these suffer from chromatic aberration and particularly in respect of the relationship between the visible and ultraviolet parts of the spectrum. Focusing is now usually carried out by means of suitably curved mirrors having a reflecting surface coated with aluminium which is protected by a silica film. 

Kauffman, A. M., A Simple Immersion Method to Determine the Refractive Index of Thin Silica Films, Thin Solid Films, Vol. l,No. 1,1967,pp. 131-136. 

The ACM-silica film looks darker after aging than the ENR-silica fihn. Better dispersion and interaction with the silica provides better aging resistance in the latter than in the former. The observations on the high temperature degradation of these composites are quite similar [69]. 

FIGURE 3.25 Visual appearance of the pre- and post-aged cross-linked rubber-silica hybrid nanocomposites synthesized from 30 wt% tetraethoxysilane (TEOS) (a) acrylic rubber (ACM)-silica and (b) epoxidized natural rubber (ENR)-silica films of average thickness 0.25 mm. (From Bandyopadhyay, A. and Bhowmick, A.K., Plastic Rubber Comp. Macromol. Eng., 35, 210, 2006. Courtesy of Maney Publishers.)... 

Similarly, monometallic Rh, Pd, and Au and bimetallic Pt-Rh and Pt-Pd nanowires were prepared in FSM-16 or HMM-1 by the photoreduction method [30,33,34]. The bimetallic wires gave lattice fringes in the HRTEM images, and the EDX analysis indicated the homogeneous composition of the two metals. These results show that the wires are alloys of Pt-Rh and Pt-Pd. Mesoporous silica films were also used as a template for the synthesis of uniform metal particles and wires in the channels [35,36]. Recently, highly ordered Pt nanodot arrays were synthesized in a mesoporous silica thin film with cubic symmetry by the photoreduction method [37]. The... 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

T.M. Butler, B. MacCraith, and C. McDonagh, Leaching in sol-gel derived silica films for optical sensing. J. Non-Cryst. Solids 224, 249-258 (1998). 

Grosso, D. Babonneau, F. Albouy, P. A. Amenitsch, H. Balkenende, A. R. Brunet-Bruneau, A. Rivory, J. 2002. An in situ study of mesostructured CTAB-silica film formation during dip coating using time-resolved SAXS and interferometry measurements. Chem. Mater. 14 931-939. 

Dourdain, S. Gibaud, A. 2005. On the capillary condensation of water in mesoporous silica films measured by x-ray reflectivity. Appl. Phys. Lett. 87 223105. 

Supplit, R Husing, N. Gross, S. Bernstorff, S. Puchberger, M. 2007. Hafnium oxide doped mesostructured silica films. Eur. J. Inorg. Chem. 2797-2802. 

Fattakhova-Rohlfing, D. Wark, M. Rathousky, J. 2007. Electrode layers for electrochemical applications based on functionalized mesoporous silica films. Sens. Actual B-Chem. 126 78-81. 

Fattakhova-Rohlfing, D. Rathousky, J. Rohlfing, Y. Bartels, O. Wark, M. 2005. Functionalized mesoporous silica films as a matrix for anchoring electrochemically active guests. Langmuir 21 11320-11329. 

Figure 3.7 Comparison of the monochromatic Mo 3d XPS spectra of M0O3 in an insulating silica-supported catalyst and in a conducting, thin silica film-supported model catalyst, showing the effect of inhomogeneous charge broadening (courtesy of H. Korpik, Eindhoven).

Ozensoy et al.127 also used PM-IRAS to study the CO adsorption behavior on Si02-supported Pd clusters. As mentioned above, these crystalline, ultrathin silica films possess the structural and electronic properties of the bulk analogues, but are thin enough to permit the use of vibrational and electronic spectroscopic techniques (and tunneling microscopy) without charging.39 40 As with the... 

Colloidal silica films spin-coated onto a glass substrate constitute another method to increase the surface area and therefore number of capture sequences which can be available for hybridization [43]. In this context, 0.3 p,m layers have been deposited from 20 wt % colloidal silica suspensions (particle size 16 to 65 nm). Heating to 350 °C for four hours is required to render the films sfable (fhrough parfial sinfering of fhe film with the underlying substrate). The particles pack randomly with no noticeable short- or long-range order. 

Synthesis ofPt and Au Nanoparticle Arrays in Mesoporous Silica Films 631... 

Synthesis ofPt and Au Nanoparticle Arrays in Mesoporous Silica Films and their Electric/Magnetic Properties in Terms of the Quantum-Size Effect... 

Figure 15.28 TEM images of (a) Au nanoparticle arrays in mesoporous silica film (b) superlattice of extracted Au nanoparticles stabilized with 1-dodecanethiol.

From above experimental results, Pt nanoparticle arrays were similarly formed in the mesoporous silica film by the photoreduction of H2PtCl6/mesoporous silica film/Si in the presence of vapors of water and methanol. As shown in Figure 15.29, Pt nanoparticles (diameter 3 nm) are packed close to each other in the one-dimensional mesopores, and in some part the arrays of Pt nanoparticles show an ordered structure [27]. 

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