Ceramics residues

Pyrolysis of the white solids obtained in these KH-catalyze< dehydrocyclodimerization reactions (under argon from 50-950°C) produced black ceramic residues, with the exception of the 1 1 THF ammonolysis-derived solid which left a brown residue. The ceramic yields were excellent (all greater than or equal to 82%, with the highest being 88%). 

To produce a ceramic material containing only Si3N4, the white solid polysilazane derived from DHCD of the oil obtained by ammonolysis of 6 1 CH3SiHCl2/CH3SiCl3 was pyrolyzed in a stream of ammonia (to 1000°C). A white ceramic residue containing only 0.36% by weight C resulted. 

On heating in air at 10°C per min, poly(m-carborane-siloxane) shows typically only 4% mass loss at 450°C and 7% mass loss at 600°C (see Fig. 4). In comparison, siloxanes without carborane units, show an approximate 50% mass loss at 450°C. As a consequence of the relatively high boron and carbon content of these materials, pyrolysis is expected to generate ceramic residues of boron carbide/silicon carbide. 

In halogen-free polymers, the zinc borate in combination with ATH at high loadings can also function as a flame retardant and smoke suppressant by releasing water and forming a porous ceramic residue, which acts as a thermal insulator. 

Figure 12. Micrograph of the ceramic residue obtained on pyrolysis of 7.

Insoluble solid polymers were isolated in 82% yield for Ti, 95% yield for Zr, and 80% yield for Hf. TGA ceramic residue yields were 72% for Ti, 73% for Zr, and 74% for Hf. The weight average molecular weights of the oily polymers were 4120 for Ti, 9020 for Zr, and 5010 for Hf. The TGA ceramic residue yields of the soluble oily polymers were ca. 14%. The dehydrocoupling mechanism of 4 should be similar to the sigma-bond metathesis for the dehydrocoupling of phenylsilane.11,12... 

These polymers may be used in the preparation of high-purity silicon nitride if the pyrolysis is carried out in a steam of ammonia (a reactive gas) rather than under nitrogen or argon. Thus pyrolysis to 1000 °C of the DHCD product of the ammonolysis product (THF) of the 1 1 mixture in a stream of ammonia gave a white ceramic residue in high yield, which contained only 0.29% C, with the remainder being silicon nitride. 

Routes via polysilanes or polycarbosilanes favor products with comparably low molecular weight in many cases (Fig. 1). During calcination, high quantities of silicon compounds are evaporated and subsequently low ceramic yields are obtained. A notable weight increase in the ceramic residue is sometimes found if the pyrolysis is carried out under either reflux or pressure (see, e.g.. Ref. 2). 

Its pyrolysis gives a high ceramic residue yield (>70%). 

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