Preparation of Micro‐/Mesoporous SiOC Bulk Ceramics

Abstract

Micro-/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100°C-1400°C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano-SiO 2 filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO 2 fillers and SiO 2 -rich clusters in the SiOC matrix act as pore-forming sites and can be etched away by HF. At the same time, the SiO 2 filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7 m 2 /g and an average pore size of 2.61 nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.

Effect of additive structure and size on SiO2 formation in polymer‐derived SiOC ceramics

“…The yields of the PSO, 30P, and 30T samples after pyrolysis at 1100°C, 1300°C, and 1400°C are shown in Figure . The yield for the pure PSO sample decreases slightly between 1100°C and 1300°C, from 72.2% to 70.2%, likely due to the carbothermal reduction of SiO 2 to be discussed later, and is consistent with previous studies using the same PSO precursor . The ceramic yield for the PSO sample is then nearly the same between 1300°C and 1400°C, suggesting that there is no further carbothermal reduction.…”

“…For the 30P sample, the onset of SiC formation is delayed until 1400°C, and the SiC diffraction peaks are much smaller than for either the PSO sample or the 30T sample. The formation of SiC within SiOC is through either the phase separation of the amorphous SiOC: or the carbothermal reduction of SiO 2 : …”

“…The addition of the SiO 2 caused an increase in pore volume after etching with hydrofluoric acid. However, the samples with the added SiO 2 powder also had a greater volume of pores (<10 nm in size), which could not be attributed to the added powder but rather to more extensive phase separation of the SiOC phase . 10 wt% of either tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) was added to a base PSO and in situ hydrolyzed using water vapor injection during pyrolysis .…”

Phase evolution of SiOC‐based ceramic nanocomposites derived from a polymethylsiloxane modified by Hf‐ and Ti‐alkoxides

“…In this temperature range, predominantly hydrogen, methane, and other fragments of the organic groups attached to silicon and from residual OR‐groups of non‐reacted sites of the metal alkoxides are released . At temperatures above 900°C, the decomposition of PMS and modified PMS is completed . The final ceramic yields vary between 77 and 82 wt%.…”

Thermal stability and electrical conductivity of carbon-enriched silicon oxycarbide

“…This again agrees with our earlier observation of carbon hindering SiOC phase separation and crystallization. 25 With the homogeneous mixing of polymer precursors PHMS and DVB, this delayed SiO 2 crystallization seems to be especially obvious. With the higher carbon content in this study, SiC crystallization can be delayed beyond 1400 1C.…”