Sol-Gel and CVD

S. Kitao and M. Asaeda, Gas separation performance of thin porous silica membranes prepared by sol-gel and CVD methods, in A.J. Burggraaf, J. Charpin and L. Cot (Eds.), Proceedings of the Second International Conference on Inorganic Membranes, 1-4 July 1991, Montpellier. Transtech Publ., Zurich, 1991, pp. 267-27 >. 

Irrespective of its implications on stability, the results reported by Tsuru et al. clearly indicate that He/H2 separation factor should be a guiding tool for the understanding of the structure of the membranes. We have previously reported the differences in He/H2 separation factors between sol-gel and CVD made silica membranes [36]. We proposed that the study of the difference in permeation behavior should give an understanding of the structural differences between apparently porous (sol-gel) and dense (CVD) membranes. Recent results have confirmed the importance of this understanding. It should be possible to extend the theoretical model of permeation reported by Oyama et al. [35], incorporating molecules like H2O with a lesser degree of vibrational freedom. 

Ceramic porous membranes can be used as supports for composite membranes using metal active layers. The following sections describe a few examples of multilayer ceramic membranes obtained via sol-gel and CVD techniques. 

Various strategies are used to produce electrode structures within the membrane pores, including sol—gel synthesis, CVD, eiectrodeposition, and electroless deposition. With careful control of the synthetic conditions, the pores are either filled completely or preferentially coated at the pore walls, producing hollow tubes (see Figure 10b). Following infiltration with the desired electrode material, the membrane is subsequently removed under conditions that do not disturb the active material, leaving an array of either solid nanofibers or nanotubes attached to a current collector like the bristles of a brush (Figure 11). In this case there is very limited interconnectedness between the nanofibers, except at the current collector base. 

In this study, the silica membranes to apply for HI decomposition reaction was investigated, and prepared by the sol-gel and the thermal chemical vapor deposition (CVD) methods. The objective of this work is to study the characteristics of the silica membrane preparation and the hydrogen permselectivity of the membrane reactor used for HI decomposition in the thermochemical water splitting IS process. 

The silica membrane to apply to the HI decomposition membrane reactor of the IS process was prepared by sol-gel and thermal CVD methods. 

Physical functionalization by the application of surface coatings, including APS (atmospheric plasma spraying), CVD (chemical vapor deposition), CGDS (cold gas dynamic spraying), PLD (pulsed laser deposition), MAO (micro-arc oxidation) or plasma electrolytic oxidation (PEO), sol-gel, and polyelectrolyte coatings and films. 

Among the bottom-up methods, sol-gel, hydrothermal, CVD, ultrasounds, and so on can be mentioned, but also scanning tunneling microscopy can be used If the tip is sufficiendy close to the surface, the interactions increase and the repulsive forces generated can push the molecules across the surface. 

The separation layer, either porous or dense, can be formed using different methods such as sol-gel and template routes, hydrothermal synthesis, chemical vapor deposition (CVD), or physical sputtering, depending on the membrane material and its application. These membrane preparation methods will be described in the following chapters of this book for different membranes and membrane reactors. We note that the preparation of inorganic membranes involves a multi-step high-temperature treatment process. Therefore, inorganic membranes are much more expensive than polymeric ones. 

A recent competitor to CVD in the planarization of silicon dioxide is the sol-gel process, where tetraethylorthosilicate is used to form spin-on-glass (SOG) films (see Appendix). This technique produces films with good dielectric properties and resistance to cracking. Gas-phase precipitation, which sometimes is a problem with CVD, is eliminated. 

Films for the DCC approach can be deposited by any conventional film deposition technique including CVD, evaporation, PVD, sol-gel, etc. By monitoring the rates and the deposition time for each of the constituents in a given sample, approximate compositions of the various samples can be tracked. However, in any thin-film sample the direct structural and compositional evaluation is problematic. 

The hybridizing component can also be formed directly on the surface of a pristine or modified nanocarbon using molecular precursors, such as organic monomers, metal salts or metal organic complexes. Depending on the desired compound, in situ deposition can be carried out either in solution, such as via direct network formation via in situ polymerization, chemical reduction, electro- or electroless deposition, and sol-gel processes, or from the gas phase using chemical deposition (i.e. CVD or ALD) or physical deposition (i.e. laser ablation, electron beam deposition, thermal evaporation, or sputtering). 

Recently we published a short review of the single source precursor concept in chemical vapor deposition (CVD) and in the sol/gel process [1]. hi this article we described in which way several constituent elements of a targeted material can be assembled in a precursor molecule and how this assembly has an effect on the properties of the hnal material. Three types of precursors have been distinguished (1) Precursors which contain the correct ratio of metallic elements (SSP-I), (2) precursors which besides the correct ratio of metallic elements also have ligands which interact with one another to form only few side-products (SSP-II), and (3) precursors with a surplus of one metallic element compared to a thermodynamically stable phase and which form biphasic mixed-materials on a nanometer scale (SSP-III) [1],... 

There are numerous variations to this MCVD process, including outside deposition on a mandrel, plasma CVD, and even fiber derived from sol-gel-processing, which will be described in the next section. 

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