Ceramics, transformations

The simplest way to follow the polymer-to-ceramic transformation process is by solid state MAS 29Si NMR spectroscopy. Thus, Figure 7 shows the MAS 29Si NMR spectra of samples of copolymer heated to selected temperatures in N2. Figure 8 provides an integration of the various species and how the relative amounts of each change on pyrolysis. 

Precursors must have different properties " " " (1) a high content of the final elements (mostly aluminum, silicon, zirconium, titanium, phosphorus), (2) a low content of health hazardous elements and elements that corrode the equipment (e.g., chlorine, sulfur), (3) a viscosity adapted to the process low viscosity for preform infiltration, medium viscosity for spinning and coating, (4) a controlled precursor-ceramic transformation (bubbling is researched for foams but not for dense parts), (5) the ability to be mixed with other precursors or to be processed ( good hydrolysis rate), and (6) low cost. 

Colomban, P, Raman studies of inorganic gels and of their sol-to-gel, gel-to-glass and glass-to-ceramic transformation, J. Raman Spectrosc., 27, 747, 1996. 

Polymer to Ceramic Transformation Processing of Ceramic Bodies and Thin Films... 

For some years now, glass-to-ceramic transformation could be confined using a focused laser beam. This allows for patterning a glass object in three dimensions (Lehmann, 2005). The nature of the local transformation (cavity, crystallite, composition change) is however still under debate. 

Polymer-derived ceramics (PDCs) represent a rather novel class of ceramics which can be synthesized via cross-linking and pyrolysis of suitable polymeric precursors. In the last decades, PDCs have been attaining increased attention due to their outstanding ultrahigh-temperature properties, such as stability with respect to decomposition and crystallization processes as well as resistance in oxidative and corrosive environments. Moreover, their creep resistance is excellent at temperatures far beyond 1000 °C. The properties of PDCs were shown to be strongly related to their microstructure (network topology) and phase composition, which are determined by the chemistry and molecular structure of the polymeric precursor used and by the conditions of the polymer-to-ceramic transformation. 

Against the pores formation as well as shrinkage during polymer-to-ceramic transformation, active and passive fillers can be used. Moreover, the fillers can have a strong influence on the mechanical, electrical and thermal conductivity, oxidation and thermal shock resistance, etc of the ceramics. 

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