Ceramic yields

Preceramic polymer precursors (45,68) can be used to make ceramic composites from polymer ceramic mixtures that transform to the desired material when heated. Preceramic polymers have been used to produce oxide ceramics and are of considerable interest in nonoxide ceramic powder processing. Low ceramic yields and incomplete burnout currently limit the use of preceramic polymers in ceramics processing. 

Typical characterization of the thermal conversion process for a given molecular precursor involves the use of thermogravimetric analysis (TGA) to obtain ceramic yields, and solution NMR spectroscopy to identify soluble decomposition products. Analyses of the volatile species given off during solid phase decompositions have also been employed. The thermal conversions of complexes containing M - 0Si(0 Bu)3 and M - 02P(0 Bu)2 moieties invariably proceed via ehmination of isobutylene and the formation of M - O - Si - OH and M - O - P - OH linkages that immediately imdergo condensation processes (via ehmination of H2O), with subsequent formation of insoluble multi-component oxide materials. For example, thermolysis of Zr[OSi(O Bu)3]4 in toluene at 413 K results in ehmination of 12 equiv of isobutylene and formation of a transparent gel [67,68]. 

The solid-state decomposition of OV[OSi(O Bu)3]3 occurs with a precipitous weight loss at ca. 200 °C (as observed by TGA) and a final ceramic yield that is 10% less than the expected ceramic yield [79]. This discrepancy results from volatihzation and loss of HOSi(O Bu)3. However, solution thermolyses of OV[OSi(O Bu)3]3 in n-octane produce xerogels with an approximate composition of V2O5 6Si02 (after drying) with a quantitative ceramic yield (i.e., with no loss of HOSi(0 Bu)3) that have a BET surface area of 320 m g ... 

Thermal decomposition of Fe[0Si(0 Bu)3]3(THF) occurs at ca. 140 °C (by TGA) to provide a material with a lower ceramic yield (25.9%) than that calculated for FeOi.5 3Si02 (30.7%), suggesting potential loss of HOSi(0 Bu)3... 

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 Marshall group has optimized reaction (14) to obtain a poly-si lazane with Mw A000 Daltons which can be hand drawn to give 10-20 pm preceramic fibers. These fibers are then rendered Infusible by exposure to humid air and pyrolyzed to give fibers with the same ceramic yields, 55+%, as found by Verbeek et al. The ceramic products are mainly amorphous SiC and SijN with some Si02 (a consequence of the humidity treatments). 

Seyferth and Wiseman find that cyclomers and oligomers of the type -[MeSiHNH]x-, produced by ammonolysis of MeSiHCl2, will react with a strong base, eg KH, to undergo dehydrocycllzation , reaction (24), akin to reaction (23). The resulting products are soluble, tractable, sheetlike polymers that can be spun into fibers and give extremely high ceramic yields upon pyrolysis. 

Table I lists the molecular weights and viscoelastic properties for the precursors and selected polymers produced in reaction (34). It also contains the ceramic yields obtained on pyrolysis to 900°C and the composition of the ceramic product.

Oligomer Mn (GPC) Viscosity (poise) Ceramic Yield (% at 900°C) Si3N4 (Percent)... 

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. 

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%). 

While these silylamide-catalyzed reactions provided a good way to solve the problem of the low ceramic yield in the pyrolysis of [(CH3SiH)x (CH3Si)y]n, the problem of the elemental composition of the ceramic product remained (i.e., the problem of Si/C ratios greater than one) since only catalytic quantities of the silylamide were used. 

Further experiments showed that the "combined" polymers may be converted to black ceramic fibers. Pyrolysis of pressed bars of the "combined" polymer to 1000°C gave a black product of irregular shape (74-76% ceramic yield). In other experiments, SiC powder was dispersed in toluene containing 20% by weight of the "combined" polymer. The solution was evaporated and the residue, a fine powder of SiC with the "combined" polymer binder, was pressed into bars and pyrolyzed at 1000°C. A ceramic bar (6% weight loss, slightly shrunk in size) was obtained. 

We have described new routes to useful preceramic organosilicon polymers and have demonstrated that their design is an exercise in functional group chemistry. Furthermore, we have shown that an organosilicon polymer which seemed quite unpromising as far as application is concerned could, through further chemistry, be incorporated into new polymers whose properties in terms of ceramic yield and elemental composition were quite acceptable for use as precursors for ceramic materials. It is obvious that the chemist can make a significant impact on this area of ceramics. However, it should be stressed that the useful applications of this chemistry can only be developed by close collaboration between the chemist and the ceramist. 

The introduction of small amounts of boron into precursors that produce silicon nitride have been known to improve the ceramic yields of silicon nitride and Si—B—C—N ceramics as first reported in 1986.110 Several reports have appeared in the past couple of years alone that utilize borazine precursors such as 2,4-diethylb-orazine and other cyclic boron precursors, such as pinacolborane, 1,3-dimethyl-1, 3-diaza-2-boracyclopentane, for their reactions with silanes, polysilazanes, and polysilylcarbodiimides for the high-yield production of Si—B—N—C ceramics.111... 

Table 1 Glass-Transition Temperatures and Ceramic Yields Measured for Poly[f -(alkylamino)borazines] 10, 9, 8, and 7... 

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