Silica fabrication processes

The strong interest in biomineralization is due to its high effidency and the superiority of the properties of biosilica over those of silica fabricated in geological processes and industrially. It proceeds at mild, ambient conditions in... 

It is obvious from Equation (6) that the processes are accompanied by sodium chloride produdion. The salt content reaches a significant amount that sometimes poses a problem. The NaCl is withdrawn by dialysis that makes the silica fabrication inconvenient owing to this additional time consuming procedure. As a further disadvantage of sodium metasilicate, it is believed to be not very flexible in regulation of the silica morphology [61]. 

Fig. 12.9 Graphene-oxide-based mesoporous silica (GM-silica) sheets, (a) Fabrication process for GM-silica sheets, (b), (c) Typical SEM and (d), (e) TEM images reveal the flat GM-silica sheets with sizes from 200 nm to several micrometers having a mesoporous structure, (f) Representative atomic force microscopy image and (g) corresponding thickness analysis taken around the white line in (f) reveal a uniform thickness of 28 nm for GM-silica sheets. Reprinted with permission from [90]. Copyright 2010, John Wiley 8i Sons, Inc.

High-purity silica-based glasses are used as the fiber material, with fiber diameters ranging between about 5 and 100 ptm. The fibers are carefully fabricated to be virtually free of flaws and, as a result, are extremely strong and flexible. We will examine this unique fabrication process in more detail in the next chapter. 

Fig. 4.21k. Representation of the steps involved in the column fabrication processes (A) introducing with wet silica material into the end of the capillary, (B) the silica material is ready to fabricate the temporary frit, (C) fabrication of the temporary frit with a heating element, and (D) excess of silica material is flushed out after temporary frit is made.

In addition to utilization of monoliths as a column material, two reports describing respectively silicate and synthetic organic polymer based monolithic frits were published recently [85,86], The conventional method of frit fabrication for a particle packed column usually involves thermal sintering of a section of the packing material, such as bare or octadecyl silica, using a heating device. This approach has several weaknesses such as the lack of control of the temperature and porous properties of the frit that decreases reproducibly of the fabrication process. 

Silica can exist in many crystalline forms such as quartz, cristobalite, and tridymite. Fumed silica on the contrary tends to be amorphous, which could be attributed to the fabrication process of the abrasive. The amorphous nature is probably caused by the rapid cooling employed in the process [83]. Colloidal silica, which is usually synthesized via wet chemical methods, is highly amorphous as well. In addition, colloidal silica particles are usually spherical and highly hydrated in nature, which makes them far less likely to cause scratches on metal substrate surface. 

Considerable amount of research has gone into the development and characterization of surfactant templated silica, in order to produce various mesophases and nanowire geometries with well-controlled pore diameters within the quantum confinement regime (i.e. 2-20 nm). Lu and coworkers have characterized one of the more promising methods, a sol-gel dip-coating method for rapid (tens of seconds) template generation. The fabrication process involves evaporation-induced self assembly of liquid crystal domains dominated by inward growth from the solid-liquid... 

Fig. 12 Microphotograph of an analyte concentrator fabricated with FAb antibody fragments immobilized to controlled-pore glass silica. The irregularly shaped beads were housed between two frit structures. The analyte concentrator device was connected to two separation capillaries by a Teflon sleeve. The plastic connector was glued to the separation capillaries by an epoxy resin. The entire fabrication process was monitored by an stereo microscope. (For details of experimental conditions, see Ref. 120.)...

The fabrication process for metal silicate nanotubes is depicted in Fig. 32A. HRTEM image of the as-prepared SNT template (Fig. 32B) showed its hollow structure and hierarchical pore structure mesopores at the wall with about a 30 nm thickness and macropores at the center. This hierarchical pore structure is veiy suitable to prepare silicate materials. Under hydrothermal conditions, metal ions and other ions in water solution could easily diffuse into the pores of the SNT template and react with silica species to form metal silicates in situ (Fig. 32C). The original silica mesopores, where the reaction occurs, are uniformly dispersed in the walls, and the metal ions in water solution could easily diffuse into the pores of the SNT template. The whole silica wall with about 30 nm thickness can be readily converted to metal silicates under the reaction conditions. 

The fourth process which yieids high siiica giass fibers relies on acid leaching of borosilicate or aluminosiiicate precursor fibers in fabric form (Chapters 4 and 6). This is the oldest and least expensive process. Acid ieaching removes most of the compositionai oxides other than silica from a precursor fabric, individual fibers can be leached also, but they are not satisfactorily converted into sliver, braids of woven fabrics. Process details and properties of melt spun and acid leached silica substrate processes have been discussed in Chapter 4. Commercial applications of all four silica glass fibers will be discussed in Chapter 6. 

Figure 18.15 shows the schematic of the fabrication process. After oxidation of the double-side polished silicon substrate wafer, a first lower poly-Si layer with a thickness of 45 pm is deposited by means of an epi-poly process. In order to remove spikes and obtain a smooth surface, 5 pm of poly-Si has to be removed by poly-Si CMP. This polishing is a two-step process, consisting of a 5 pm bulk removal by means of a fiimed-silica slurry and a subsequent final polish of several 10 nm with a haze-firee slurry. After deposition and stmcturing of some intermediate layers, a second upper poly-Si layer, again with a thickness of 45 pm, is deposited and subsequendy polished with the same two-step poly-Si CMP process. As this will be the surface of the evaporated silver mirror, a smooth as well as flat surface has to be achieved. After a backside silicon etch and the removal of the sacrificial layer, the scanning mirror device is released, see Figure 18.16(a) and (b). 

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