White Si–O–C Ceramic: Structure and Thermodynamic Stability

Abstract

While pyrolysis of a polysiloxane precursor in argon typically produces a black amorphous Si–O–C ceramic containing “free” carbon (sp2 carbon), pyrolyzing the same precursor in hydrogen leads to a white amorphous ceramic with a negligible amount of sp2 carbon and a considerable hydrogen content. 29Si magic‐angle‐spinning nuclear magnetic resonance (MAS NMR) spectroscopy confirms the existence of very similar bonding environments of Si atoms in the Si–O–C network for both samples. In addition, 1H NMR spectroscopic measurements on both samples reveal that the hydrogen atoms are bonded mainly to carbon. For the thermodynamic analysis, the enthalpies of formation with respect to the most stable components (SiO2, SiC, C) of the black‐and‐white Si–O–C samples obtained after the pyrolysis at 1100°C are determined using high‐temperature oxidative drop‐solution calorimetry in a molten oxide solvent. The white ceramic is 6 kJ/g‐atom more stable in enthalpy than the black one. Although the role of hydrogen in the thermodynamic stability of the white sample remains ambiguous, the thermodynamic findings and structural analysis suggest that the existence of sp2‐bonded carbon in the amorphous network of polymer derived Si–O–C ceramics does not provide additional thermodynamic stability to the ceramic.

Thermodynamic Stability of Low‐k Amorphous SiOCH Dielectric Films

“…Statistically reliable data (within 2% error) were obtained by dropping a number of pellets. This methodology is well‐established and similar to that in recent studies …”

“…Thus, there is a great need to understand the thermochemistry of low‐ k a‐SiOCH materials to assess their ultimate stability in highly scaled and integrated nano‐electronic products. Examination of the thermochemistry of low‐ k a‐SiOCH materials will also broaden the understanding of the energy landscape in the Si–O–C and Si–O–C–H systems …”

Boria Effects on the High Temperature Oxidation of Silicon Carbide

“…Volatilization of the organics in oxygenated and water vapor gas environments was found to reduce carbon retention in the heat treated glasses [78][79][80] . A preferable approach to reducing carbon retention in sol-gel coatings that would avoid SiC substrate oxidation during glass conversion is exposure of the coatings to hydrogen during glass conversion, which was found to decrease carbon concentration SiOC ceramics derived from silicone resin by the evolution of methane gas [81][82][83] . Determining the carbon bonding to the glass structure as part of the glass network or segregation in elemental forms 84 is an area of active investigation 85 , and it can have notable effects on the microstructure and high temperature properties of silica 86 and borosilicate sol-gel derived glasses 75 .…”

“…The synthesis also does not replicate binary SiO2-B2O3glasses, primarily due to carbon retention from partial organic volatilization during the gel-to-glass conversion 75 . The retained carbon is expected to oxidize in the high temperature oxygen environment 80,82,83 and might affect oxygen transport through the glass coating during exposure. Boria volatilization is also expected to occur during high temperature exposure 57 , thus changing the glass composition over the duration of the test.…”

“…In addition, substantial carbon incorporation in the glass network may affect oxidant transport rates through the glass 77, 83 and might impede observation of boria effects on SiC oxidation. The incorporation of the carbon phase in the glass network is unknown.…”