Processing and characterization of polymer precursor derived silicon oxycarbide ceramic foams and compacts

Abstract

This work focused on the fabrication of silicon oxycarbide ceramic (SiOC) foams as well as dense compacts using poly(hydridomethylsiloxane) (PHMS) as a polymer precursor. The room-temperature cross-linking of PHMS was achieved by the addition of 1,4-diazabicyclo [2.2.2] octane (DABCO) with the release of hydrogen gas as a by-product. This resulted in self-blowing of the polymer precursor at room temperature and thereby offered the possibility of producing SiOC foams without the need of any external blowing agents. We also reported the fabrication of crack-free silicon oxycarbide compacts by cold compaction and pyrolysis route using polyvinyl alcohol (PVA) as a processing additive. Cylindrical-shaped pellets were pyrolysed at 1300 ℃ in argon atmosphere with a ceramic yield of approximately 85%. Increased resistance to phase separation and crystallization up to 1400 ℃ was attributed to the presence of large volume fraction of free carbon in the material which was confirmed by Raman spectroscopy.

Gradient-Hierarchic-Aligned Porosity SiOC Ceramics

“…As previously shown, the PHMS polymer can be crosslinked via hydrosilation when there are additional vinyl-containing precursors or via hydrosilylation/dehydrocoupling when there is water in the system24252627. The latter case results in the formation of hydrogen (H 2 ) gas that can be used as to “self blow” the system to obtain porous components23242829.…”

“…The latter case results in the formation of hydrogen (H 2 ) gas that can be used as to “self blow” the system to obtain porous components23242829. In the present experiments, the heat applied from the bottom of the aluminum mold initiated the crosslinking reactions, simultaneously the gas bubbles nucleated probably with the aid of LDH particles (both as a surface and water including source), expelled from the heated surface and moved into the cooler zone till reaching to the upper surface (atmospheric).…”

“…In the present experiments, the heat applied from the bottom of the aluminum mold initiated the crosslinking reactions, simultaneously the gas bubbles nucleated probably with the aid of LDH particles (both as a surface and water including source), expelled from the heated surface and moved into the cooler zone till reaching to the upper surface (atmospheric). Additional experiments (not shown for the brevity) in which the crosslinking was conducted in the oven (uniform heating) instead of hot-plate directional heating, foams with no aligned porosity were obtained222428. This implies that the formation of aligned channels was caused by the temperature gradient from bottom to top resulting in simultaneous curing and gas release together with the coalescence of gas bubbles during raise, yielding larger channels on the top.…”

Thermal conductivity of precursor derived Si–B–C–N ceramic foams using Metroxylon sagu as sacrificial template

“…Ceramic foams are well known for their low density, high specific strength, high thermal stability, low thermal conductivity and high dimensional stability [10,11]. However, ceramic foams currently available can be used only up to temperatures as high as 1400 1C without any phase change [12]. Hence, there exists a need to produce ceramic foams which can withstand temperatures beyond this limit without any phase change.…”

“…The replica technique results in the formation of open cell foams and hollow struts, resulting in poor mechanical properties [13]. Foams processed through direct foaming offer near-net shape fabrication with nonuniform and uncontrolled porosity and pore size [12]. The use of sacrificial template in processing ceramic foams results in tailored porosity, tailored pore size and the formation of a combination of open and closed cells along with inter-connected pores, all of which are controllable.…”