Sol infiltration

In Section 3.4.2, we introdnced the concept of chemical vapor infiltration, CVI, in which a chemical vapor deposition process is carried out in a porous preform to create a reinforced matrix material. In that section we also described the relative competition between the kinetic and transport processes in this processing technique. In this section we elaborate npon some of the common materials used in CVI processing, and we briefly describe two related processing techniques sol infiltration and polymer infiltration. 

Polymer Infiltration. Polymer infiltration is similar to sol infiltration. The polymer is introduced in liqnid form nnder vacuum and coats the fibers. The polymer is then dried and pyrolyzed to yield the desired composition. Multiple infiltration cycles... 

A number of other strategies have been investigated in order to improve the quality (selectivity and fluxes) or the synthesis efficiency of zeolite membranes by pre-treatment of the support. Most of these treatments are listed in [49], and include laser ablation, utilisation of diffusion barriers to limit sol infiltration, oxidation of stainless-steel supports, deposition of iron oxide to favour heterogeneous nucleation on the support and the use of acid attacks to activate the surface. 

Marple, B. R. Voyer, J. Becharde, P. (2001). Sol Infiltration and Heat Treatment of Alumina-Chromia Plasma-Sprayed Coatings. Journal of the European Ceramic Society, Vol. 21, pp. 861-868, ISSN 09552219... 

Michael D. Sacks and ShaUesh D. Vora Preparation of SiO2 glass from model powder compacts. Ill Enhanced densification by sol infiltration, J. Am. Ceram. Soc., 71 (4) (1988) 245 249... 

Similarly to the case of direct-oxidation anode materials, sulfur-tolerant anode materials based on sulfides [6, 7] or double-perovskite oxides have special requirements for their processing into SOFC layers. For example, nickel sulfide-promoted molybdenum sulfide is tolerant to high sulfur levels [7], However, it has a low melting temperature [6] that has resulted in the development of cobalt sulfide as a stabilizer of the molybdenum sulfide catalyst [6], CoS-MoS2 admixed with Ag has an even higher performance in H2S-containing fuels than in pure H2 [6]. However, processing methods such as PS, infiltration, or sol-gel techniques that can process... 

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. 

Figure 12. Inverse opal of vanadium oxide ambigel. The pores are formed by packing -fim styrene beads and infiltrating a vanadium sol. (Reproduced with permission from ref 100. Copyright 2002 The Royal Society of Chemistry.)...

Some of the earliest sol-gel hybrids were achieved by monomer infiltration into previously formed silica gel. Starting with a dried silica gel (xerogel), the porous shape is filled with monomer. The monomer is polymerized in situ. This is shown schematically in Figure 5. Interpenetration is achieved when the impregnating monomer polymerizes in the open pores of the rigid silica matrix. 

Sol-gel processing (a) infiltration of a preform (b) mixing reinforcement in a sol or a gel (reproduced by permission of Woodhead Publishing Limited)74. 

Liquid-phase infiltration of preforms has emerged as an extremely useful method for the processing of composite materials. This process involves the use of low-viscosity liquids such as sols, metal- or polymer-melts. Using this infiltration process, it is possible to design new materials with unique microstructures (e.g. graded, multiphase, microporous) and unique thermomechanical properties (graded functions, designed residual strains and thermal shock). 

Colloidal zeolites have been used as building blocks to fabricate hierarchical porous materials. Infiltrating ethanol sol of zeolite nanoparticles into an ordered array of polystyrene spheres resulted in macroporous zeolites, which involves a self-assembly process. After ethanol evaporation, zeolite nanoparticles were aggregated by capillary forces. High concentration of external silanol groups favored the formation of hydrogen bonds between particles and eventually Si-O-Si bonds after calcination. The method has been further developed to produce transparent and self-standing zeolite membranes with controlled mesoporosity. Concurrently, the preformed zeolite-coated polystyrene spheres have been... 

FIGURE 12.11 Improvements of the mechanical properties of three-dimensional reinforced CMCs by hybrid infiltration routes (a) R.T. flexural stress-strain plots for a three-dimensional carbon fiber reinforced composite before and after cycles of infiltration (comparison between eight cycles with zirconium propoxide and fonr cycles pins a last infiltration with aluminum-silicon ester (b) plot of the mechanical strength as a fnnction of the final open porosity for composites and matrix of equivalent porosity, before and after infiltration (Reprinted from Colomban, R and Wey, M., Sol-gel control of the matrix net-shape sintering in 3D reinforced ceramic matrix composites, J. Eur. Ceram. Soc., 17, 1475, 1997. With permission from Elsevier) (c) R.T. tensile behavior (d) comparison of the R.T. mechanical strength after thermal treatments at various temperatures. (Reprinted from Colomban, R, Tailoring of the nano/microstructure of heterogeneous ceramics by sol-gel routes, Ceram. Trans., 95, 243, 1998. With permission from The American Ceramic Society.)... 

Within the last decade several novel processing methods have been developed to overcome the limitations of traditional methods. For CMCs, these include chemical vapor infiltration [7-10] repeated infiltration/ pyrolysis of sols or polymers [77], crystallization of glass matrices [12-14] and the direct formation of a ceramic matrix by the directed oxidation of a... 

---