Monolith fabrication

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

An intriguing recent development has been the discovery that porous ceramic monoliths (fabricated for use as catalyst supports) can be lamination-coated with microporous or ultramicroporous barrier layers to yield high-area membrane modules [58]. It is reducing fabrication costs of ceramic membrane devices to levels comparable to (or less than) those of polymeric membrane devices, and it opens the door to development of large-area laminate devices with utility for particulate, macromolecular, or smaU-molecule separations. 

Figure 2 SEM images of MIP monoliths fabricated inside capillary columns. Image a represents a polymer which is too dense and cannot undergo hydrodynamic pumping. Image b illustrates the other side of the spectrum where only a thin him of polymer remains, thereby compromising the capacity of the polymer. (Reproduced with permission from Ref 35.)...

Kaanta, B. Chen, H. Zhang, X. (2010). A Monolithically Fabricated Gas Chromatography Separation Column with an Integrated High Sensitivity Thermal Conductivity Detector. /, Micromech. Microeng. Vol.20, N°5, pp. 1-6, ISSN 09601317. 

Liu, M.C. Ho, D. Tai, Y.C. Monolithic fabrication of three-dimensional microfluidic networks for constructing cell culture array with an integrated combinatorial mixer. Sens. Actuators B, 2008,129 (2), 826-833. 

Stacked planar optics, a new concept in integrating optical circuits, has been proposed. By using stacked planar optics, we not only make possible the monolithic fabrication of optical circuits, such as the directional coupler and wavelength demultiplexer, but we can also construct three-dimensional optical circuits by allowing coupling among individual components in the array with a suitable design. 

The first LOV system was a single, monolithic fabricated Poly(methyl methacrylate) (PMMA) component mounted atop a six-port selection valve, including a flow through cell furnished with optical fibers (Figure 1.21). Nowadays, there are different designs of LOV systems, fabricated from methylmethacrylate (Perspex), ultem (polyetherimide) or kel-F as monolithic stractures encompassing integrated microchannels. 

At present, corrugated metal foils fabricated by rolling are already being applied in the field of automotive exhaust gas systems. The metallic monoliths fabricated this way achieve channel dimensions in the sub-millimetre range and are actually microtechnology . 

Hayase, M., T. Kawase, and T. Hatsuzawa. 2004. Miniature 250 im thick fuel cell with monolithically fabricated silicon electrodes. Electrochemical and Solid-State Letters 7 A231-A234. 

Figure 17.2 Schematic view of miniature fuel cell design. A PEM sheet is hot pressed with monolithically fabricated Si electrodes. Catalyst metals are supported by porous Si formed by anodization. (From Hayase et al, 2004, with permission from the Electrochemical Society.)...

Hayase et al. (2011) describe a nfini-fuel cell with monolithically fabricated electrodes. A porous Pt layer on an Si substrate was formed by immersing porous Si into a Pt plating bath containing HF. After formation of the porous catalyst layer, fuel channels were opened by applying dry etching from the opposite side of the Si wafer. Two such Si electrodes were hot-pressed on either side of a polymer electrolyte membrane. By feeding H2 and O2 to the corresponding electrodes, a peak power density of 145 mW/cm was obtained. 

For the current ESA X-ray Evolving Universe Spectroscopy (XEUS) mission and for the NASA Constellation-X mission, components made from Zerodur are being tested. The increasing effective areas of the telescopes require nested thin-walled mirrors of large diameters. The requested sizes of the mirror shells and the corresponding manufacturing tools almost exceed the reasonable technical limits of monolithic fabrication and handling. Mirror shells with diameters of more than Im are therefore manufactured... 

In this chapter we review the principles of monolith fabrication, routes for febrication of monoUths such as xerogel monoliths, nanocomposite monoliths, ORMOSIL monohths, hybrid monoliths, and aerogel monohths. In addition, we will present several characterization methods of monohths, encapsulation or doping in monohths, and examples of apphcations of monohths. 

The term aerogel refers to solid foams with porosities in the range of over 90%. The sol-gel technology allows fabrication of aerogel monoliths [94,95]. A detailed review of aerogel science will be presented in the chapter 18. In this section, the main issues of aerogel monolith fabrication will be reviewed briefly. 

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