Linear, branched, or cyclolinear polymers or oligomers can be fabricated easily by solution or melt-fabrication techniques. If a polymeric material that has been shaped and fabricated in this way is then cross-linked and pyrolyzed in an inert atmosphere to drive off the organic components (typically the side groups), the resultant residue may be a totally inorganic ceramic in the shape of fabricated article. Thus, ceramic fibers, film, coatings, and shaped objects may accessible without recourse to the ultra high temperatures needed for melting of the ceramic materials itself.
Note, however, that although the final shape of the object may be retained during pyrolysis, the size will be diminished due to the loss of volatile material. If the pyrolisis takes places too quickly, this contraction process may cause cracking of the material and loss of strength. Moreover, too rapid a rate of heating before or during the initial cross-linking step may cause the polymer intermediate to melt and lose its shape, or the polymer may depolymerize and the product volatilize. Thus, the temperature program or the sequence and rate at which the temperature is raised can have a profound influence on the final properties.
THE SOL-GEL PROCESS TO OXIDE CERAMICS(Note: the use of techniques for the low temperature preparation of oxide ceramics such as silica, this process can also be used to produce alumina, titanium oxide, or other metal oxides)
Starting in the 1950s a process was developed that leads from small-molecule silicon alkoxides such as tetraethoxysiloxane (tetraethyl orthosilicate), to organosiloxan oligomers and low polymers, and eventually to silica via a “low temperature” synthesis route. A simplified outline of the basic chemistry is shown in reaction (1)-(3), where R is an ethyl or higher alkyl unit. Any or all of the Si-OR bonds can be hydrolyzed to Si-OH functional groups, and these are able to condense to form Si-O-Si linkages. The linear polymers formed in reaction (1) can undergo further alkoxide hydrolysis to give the cross-linked species, also formed in reaction (2), and these cross-linked polymers may further cross-link to give an ultra structure similar the one formed in reaction (3).
Moreover, step (1) could generate six-, eight-, or higher membered rings instead of chains and these rings may couple to yield clusters of ring (reactions sequence (4)). Although cyclic trimeric siloxane rings are shown for simplicity in reaction (4), the most probable products are cyclic tetramers and higher cyclics. Reaction (2) could also generate cage structures and these too could become linked to formed clusters. Subsequently, the alkoxy groups in the other boundaries of the ring or cage clusters can hydrolyze and couple to yield clusters of clusters, and so on. Ring clusters may, in principle, react with chain clusters or linier polymers to increase the structure complexity. Eventually a catastrophic gelation of the system will occur, and what was originally a solution or a colloid sol become solvent-swollen solid in the shape of the original reaction vessel or mold. Subsequent heating to drive off water and alcohol will complete the condensation process and leave an amorphous form of silica.
The complexity of this reaction mechanism is legendary. The reaction pathways and types of product change with variations in pH, the nature of the alkoxy group OR, the rate which water and alcohol are removed during heating, the type of solvent, the present of other metal alkoxides, or the existence of functional organic molecules that can enter into the condensation process. The addition of transition metal akoxides provides a means for the introduction of color into the final ceramic, an option that is useful for the fabrication of optical fillers, colored coating, or ceramic art object.
Variations in pH, concentration, and temperature have a profound effect in the condensation pathway. Acidic media, high concentrations of reagents and lower temperatures favor the formation of chains or loosely cross-linked chains. Basic media, dilute solutions, or higher temperatures favor the formation of rings, cages, and cluster networks. (Figure 9.4)
Figure 9.4. Reaction conditions exert a strong influence on the course of sol-gel polymerization reaction. Basic pH, higher temperatures, and grater dilutions favor the formation of rings and rings clusters, as shown in the pathway on the left. Acidic pH, lower temperatures, and higher concentrations favor the formation of chains and dendritic structures.
Processing conditions also determine the types of products that are formed, as shown in figure 9,5. For example, fibers can be pulled from a system that contains colloidal clusters that have not yet gelled. Evaporation of a colloidal suspension or a solution may give a “xerogel” coating. Once the system has gelled, extraction of the remaining small molecules using a volatile solvent will leave an “aerogel” which has volume and shape, but is mostly unfilled space. Such materials have been used in flotation devices, lightweight structural materials, or filters. Evaporation of the solvent from a gel allows contraction of the system to a xerogel, and subsequent heating above the melting point gives a dense glass. This a way to produce lens performs. Composite materials called “creamer” are accessible if water-or alcohol-soluble organic polymer is included in the original reaction mixture. The organic polymer raises the impact resistance of the final ceramic.
This article is taken from :
Mark, James E., Allcock, Hary R., and West, Robert, 2005, Inorganic Polymers, second edition, Oxford University Press Inc., New YorkNote : this page is incomplete, there are some pictures not uploaded, please wait for further improvement
