Carbothermal Reaction of Silica–Phenol Resin Hybrid Gels to Produce Silicon Nitride/Silicon Carbide Nanocomposite Powders

Abstract

A carbothermal reaction of silica–phenol resin hybrid gels prepared from a two‐step sol–gel process was conducted in atmospheric nitrogen. The gels were first pyrolyzed into homogeneous silica–carbon mixtures during heating and subsequently underwent a carbothermal reaction at higher temperatures. Using a gel‐derived precursor with a C/SiO2 molar ratio higher than 3.0, Si3N4/SiC nanocomposite powders were produced at 1500°–1550°C, above the Si3N4–SiC boundary temperature. The predominant phase was Si3N4 at 1500°C, and SiC at 1550°C. The Si3N4 and SiC phase contents were adjustable by varying the temperature in this narrow range. The phase contents could also be adjusted by changing the starting carbon contents, or by its combination with varying reaction temperature. A two‐stage process, i.e., a reaction first at 1550°C and then at 1500°C, offered another means of simple and effective control of the phase composition: the Si3N4 and SiC contents varied almost linearly with the variation of the holding time at 1550°C. The SiC was nanosized (∼13 nm, Scherrer method) formed via a solid–gas reaction, while the Si3N4 has two morphologies: elongated microsized crystals and nanosized crystallites, with the former crystallized via a gaseous reaction, and the latter formed via a solid–gas reaction. The addition of a Si3N4 powder as a seed to the starting gel effectively reduced the size of the Si3N4 produced.

Decoration of Ceramic Foams by Ceramic Nanowires via Catalyst‐Assisted Pyrolysis of Preceramic Polymers

“…In the alumina foam, the formation of SiC was never confirmed by XRD, but it was assumed to be present, in accordance to what observed in numerous studies for bulk materials . Because the gas evolution of SiO or CO occurs in a limited fashion in bulk materials, the reaction with lower free energy change can occur.…”

“…The SiO, CO, and N 2 gases could directly react with each other to form Si 2 N 2 O via a gas phase reaction, according to Eqs. () (with free energy changes at 1450°C), reported in other studies …”

Growth of One‐Dimensional Nanostructures in Porous Polymer‐Derived Ceramics by Catalyst‐Assisted Pyrolysis. Part I: Iron Catalyst

“…However, previous investigations showed that this reaction is not thermodynamically favored over the entire pyrolysis temperature range of the present study (1250°–1400°C) 62 . Therefore, formation of SiC proceeds rather via a solid–gas process, according to reaction () 2,61,62 . Further evidences for reaction () being active in the system are provided by (i) the fact that SiC was almost always found in close proximity to the graphitic regions (see Fig.…”

“…However, this reaction by itself is not sufficient to explain the growth mechanism of the nanowires, due to increased CO partial pressure, resulting from the precursor decomposition and from the byproduct of the reactions, which would make reaction () difficult to proceed 60 . Accordingly, the growth of the Si 3 N 4 nuclei formed should rather progress via a gas‐phase process following reaction (), as observed in other studies 52,54,57,60,61 …”

“…Increases in pyrolysis temperature promoted a depression of the amorphous halo, a better β‐SiC crystallization and the transformation of the iron silicide phases to FeSi (JCPDS #38‐1397), above 1300°C. Nanowires became visible by SEM, indicating the formation of SiC at the expense of the amorphous carbon and silica‐rich phase (producing SiO gas) 61 . Pyrolysis at 1400°C resulted in a ceramic including only β‐SiC and minor amounts of FeSi.…”