Ceramic yield increasing

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

The pyrolysis of the coammonolysis products was studied. The 6 CH3SiHCl2/l HSiCl3 ammonolysis product would be the least cross-linked since it contains the least amount of trifunctional component and, as expected, low ceramic yields were obtained on pyrolysis of these products. Pyrolysis of the 3 1 products gives increased ceramic yields, while pyrolysis of the most highly cross-linked 1 1 ammonolysis products gives quite good ceramic yields, 72% for the product prepared in Et20 78% for that prepared in THF. 

The above work follows earlier and more comprehensive efforts by the Seyferth group25 on the use of base catalyzed crosslinking of cyclotetrasilazane, —[MeHSiNH]4—, to increase its ceramic yield and processability. The currently proposed mechanism is suggested to involve silaimine formation followed by an addition reaction across the reactive double bond, as illustrated in equation 1325,26. 

Based on the extensive work of Harrod and coworkers101, Seyferth and coworkers also examined metallocene (i.e. Cp2ZrH2) catalyzed dehydrogenative crosslinking of PMS as another method of increasing ceramic yields and carbon content during transformation... 

SCHEME 3. Modifying PMS to increase ceramic yields and SiC purity... 

The hardness of such a composite, for example, can be varied by control of the molar ratio of alkyl R groups to Si atoms, as is illustrated for PDMS in Figure 8.12.138 Low values of R/Si yield a brittle ceramic, and high values give a relatively hard elastomer. The most interesting range, R/Si = 1, can yield a relatively tough ceramic of increased impact resistance. 

SCHEME 18.16 Dehydrogenative cross-linking of oligosilazanes. This step increases molecular weight and inhibits depolymerization reactions dnring thermolysis. Dehydrogenative cross-linking of [SiH(CH3)-NH] increased ceramic yields from about 30 to 80%. ... 

After these preheat treatment processes, the two treated (cross-linked) PMS resins were pyrolyzed at 1273 K. Resin recovery after preheat treatment and ceramic yield of these cross-linked resins at 1273 K are summarized in Table 19.2. Use of the reflux system is effective to increase resin recovery and ceramic yield. Figure 19.4 shows the overall ceramic yield of the starting PMS with different thermal histories. The dotted line indicates the intrinsic ceramic yield from PMS by direct pyrolysis up to 1273 K (about 30%). The filled circle indicates the ceramic yield at 1273 K with two-step pyrolysis, in which reflux treatment is the first step. Overall ceramic yield begins to increase at 423 to 523 K, and is saturated (approximately 75%) at 623 K. Even when a preheat treatment step on PMS is a simple heat treatment in an open argon gas flow (open circle), the overall... 

After the preheat treatment, the obtained cross-linked PVS resins were pyro-lyzed at 1273 K. Resin recovery after preheat treatment and the ceramic yields of these cross-linked resins at 1273 K are summarized in Table 19.3. After simple heat treatment, the ceramic yield of the cross-linked PVS resin increased as compared with the starting PVS (ceramic yield 36 mass%). The increasing rate in the ceramic yield is, however, counterbalanced by low resin recovery. 

Figure 19.8 shows the overall ceramic yield of PVS. Simple heat treatment in an argon gas flow does not increase the resulting ceramic yield. Even in the case of reflux conditions, the reflux treatment is not effective to increase the... 

The 550 to 600 K temperature range of the PVS ceramic, where yield increases, is considerably higher than the required reflux temperature for PMS. This temperature range is consistent with the Si-CH2-Si bridge formation accompanied by silane, ethane, and ethylene evolution. 

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