Synthesis and Thermal Behavior of Novel Si−B−C−N Ceramic Precursors

Abstract

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.

Progress of a novel non-oxide Si-B-C-N ceramic and its matrix composites

“…Fig. 18 Thermogravimetric curves of Si-B-C-N ceramic powders tested in air [10] (a), and surface morphology of the Si-B-C-N bulk ceramic oxidized at 1500 ℃ in air for 24 h (b) [67].…”

“…During the pyrolyzing process, side-chain groups break; organic molecules with low molecular weight volatilize; crosslinking and curing take place in the polymer precursor. When pyrolyzing is finished, the remaining substance is an inorganic ceramic, mainly consisting of Si, B, C and N. The as-pyrolyzed ceramic generally has an amorphous structure [6], a high structural stability [7,8], a large viscosity and a nice oxidation resistance [9,10]. When the amorphous ceramic is annealed at 1800 ℃-2000 ℃ in an inert atmosphere for several hours, its structure will change into a thermodynamically stable one [8,11].…”

Borazine Based Molecular Single Source Precursors for Si/B/N/C Ceramics

“…The occurrence of silicon most probably results from thermal decomposition of intermediate silicon nitride under evolution of nitrogen as indicated by the aforementioned abrupt weight loss at 1840 °C. The presence of free carbon in direct contact to silicon nitride can be excluded for thermodynamic reasons: Si 3 N 4 (or Si-N units in an amorphous state) would decompose at lower temperatures and would form silicon carbide [38][39][40]. Furthermore, it can be suggested that the borazine rings are connected by SiN 4 units preventing the formation of boron nitride phases at T < 1800 °C.…”

Preparation of Si-Al-N-C Ceramic Composites by Pyrolysis of Blended Precursors

“…15,16 The advantage of singlesource precursors is the atomic-level homogeneity of their elemental compositions. In addition, a blended precursor approach to fabrication of composites can also be proposed.…”