Several boron-modified polysilazanes of general type {B[C2H4Si(R)NH]3}
n
(C2H4 = CHCH3
or CH2CH2) were synthesized and their thermal behavior studied. In contrast to the known
derivatives with R = alkyl or aryl, we describe ceramic precursors in which the bulky moieties
R are substituted with lower weight groups and/or reactive entities. Reactive units enable
further cross-linking of the polymeric framework and therefore minimize depolymerization
during ceramization. The polymer-to-ceramic conversion of all synthesized polymers was
monitored by thermogravimetric analysis. Both low molecular weight substituents and/or
cross-linking units increase the ceramic yield from 50% (R = CH3) to 83−88%. High-temperature thermogravimetric analysis in an inert gas atmosphere indicates the ceramics
obtained are stable up to ∼2000 °C. XRD studies of the fully amorphous materials point out
that, with increasing temperature, formation of α-SiC or α-SiC/β-Si3N4 crystalline phases
occurs at 1550−1750 °C, depending on the material's composition. The resistance of these
novel materials toward oxidative attack was investigated by TGA in air up to 1700 °C and
SEM/EDX, indicating that the materials efficiently self-protect toward oxidation.
The synthesis, detailed spectroscopic characterization, polymer-to-ceramic conversion, and high-temperature behavior of a new class of polymeric precursors for Si-B-C-N composites are discussed. The title compoundsCH 2 -CH 2 ; 5a: R 1 , R 2 ) H; 5b: R 1 ) H, R 2 ) CH 3 ; 5c: R 1 , R 2 ) CH 3 ) were designed especially for the preparation of ceramic films and fiber-reinforced ceramic composite matrixes. They are obtained in quantitative yields by the reaction of oligovinylsilazane [(H 2 CdCH)SiH-NH] n (4) with tris(hydridosilylethyl)boranes of general type B(C 2 H 4 SiHR 1 R 2 ) 3 (C 2 H 4 ) CHCH 3 , CH 2 CH 2 ; 3a: R 1 , R 2 ) H; 3b: R 1 ) H, R 2 ) CH 3 ; 3c: R 1 , R 2 ) CH 3 ) in a thermally induced hydrosilylation reaction without catalyst and/or solvent and without the formation of byproducts. Ceramic yields are 83% for 5a, 82% for 5b, and 63% for 5c as shown by thermogravimetric analysis (TGA). High-temperature TGA of the as-obtained amorphous ceramic materials, carried out in an argon atmosphere, reveals a thermal stability toward degradation of the 5b-derived material 6b up to 2000 °C. In contrast, the 6a material, which was obtained from 5a, decomposes around 1850 °C. The least stable is the 6c ceramic, which decomposes at 1450 °C. The microstructure development of 6a-6c was investigated in the temperature range of 1400-2000 °C by X-ray diffraction (XRD), indicating that preferentially crystalline R-silicon carbide is formed at 1700 °C for 6a, 1500 °C for 6b, and 1600 °C for 6c. In addition, there are less intensive reflections observed in the XRD patterns of 6a, caused by the formation of β-silicon nitride.
The preparation of silicon nitride-and carbidebased ceramics by solid-state thermolysis of polysilazanes and polysilylcarbodiimides is described. Results on the ceramization of the preceramic compounds and the architecture of the corresponding amorphous states obtained by spectroscopic means and by X-ray and neutron scattering are reviewed. Fundamental correlations between the composition and structure of the preceramic compounds and the architecture of the amorphous state are revealed. Furthermore, the crystallization behavior of the amorphous precursor-derived Si-C-N ceramics is treated. Moreover, the influence of boron on the thermal stability of the amorphous state is described. The high-temperature behavior of these Si-B-C-N solids can be correlated with their phase composition. Ceramic materials with compositions located close to the three-phase equilibrium SiC BN C exhibit a high temperature stability up to 2000°C. This effect is accompanied by the formation of a metastable solid consisting of Si 3 N 4 and SiC nanocrystals that are embedded in a turbostratic B-C-N matrix phase. Based on thermodynamic considerations, a model for the high-temperature stability effect is proposed.
The potential barrier at the apex of a single-wall carbon nanotube emitter is found to be strongly and nonlinearly dependent on the external applied field, due to a quantum mechanical mechanism instead of the correction of image potential in Fowler-Nordheim theory. The field enhancement factor depends on the applied field and is much smaller than that predicted by the classical theory. The field induced apex-vacuum barrier lowering is confirmed to be the essential mechanism for efficient field electron emission from capped carbon nanotubes.
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