Study of Crystallization and Composition on Silicon Oxycarbide (SiOC) and Silicon Boron Oxycarbide (SiBOC) Ceramics Obtained from Alkoxysilanes Precursors

Mariana Segatelli,Emerson Ribeiro,Paulo Da Silva,César Tarley,Maria Silva,Patricia Sanchez,Lívia Silva

doi:10.21577/0103-5053.20210023

Abstract

This work deals on the preparation of silicon oxycarbide (SiOC) and silicon boron oxycarbide (SiBOC) ceramics from pyrolysis involving alkoxysilanes with different organic groups. Boron content, organic substituent and annealing time were evaluated on crystallization and composition of resulting ceramics. B-free precursors composed of methyltriethoxysilane (MTES), phenyltriethoxysilane (PTES) and vinyltrimethoxysilane (VTMS) were obtained by the sol-gel method, whilst B-containing precursors were prepared by adding to each alkoxysilane a proper amount of boric acid resulting in B/Si atomic ratios of 0.1 and 0.5. Precursors were pyrolyzed at 1500 °C for 1 and 3 h to produce respective SiOC and SiBOC ceramics. X-ray diffraction (XRD) patterns revealed enhanced SiC phase crystallization for PTES-derived ceramics, followed by those containing vinyl and methyl groups, which was also confirmed by X-ray photoelectron spectroscopy (XPS) from percentages of Si-C and Si-O bonds. Csp2 and Csp3 amounts varied among ceramics, indicating direct influence of organic substituent on conductive carbon phase development. Boron addition induced the growth of SiC crystallites, having more evident effect in matrices with higher proportions of amorphous fraction and lower residual carbon amounts. Organic group nature, boron content and annealing time played an important role for production and evolution of SiC and C phases into ceramics.

Highlights

silicon oxycarbide (SiOC) and silicon boron oxycarbide (SiBOC) ceramics were successfully prepared from pyrolysis route of MTES, PTES and VTMS precursors, with and without boric acid, at 1500 °C during 1 and 3 h isothermal annealing
Crystallization and composition on ceramics were evaluated in terms of organic substituent nature, boron content and annealing time
Enhanced SiC phase crystallization for PTES-derived ceramics, followed by those containing VTMS and MTES, confirmed by X-ray photoelectron spectroscopy (XPS) measurements from percentages of bands assigned to Si-C and Si-O bonds, was attributed to carbon content in each polymer precursor

Summary

Introduction

Silicon oxycarbide (SiOC) based materials have attracted great attention in Si-polymer derived ceramics (PDCs) field due to their excellent mechanical, thermal, electrical, optical, oxidation resistance and low densities properties.[1,2] These characteristics make them suitable in a wide application range including high resistance SiOC fibers and ceramic matrix composites,[3,4] high-temperature receivers and ceramics for thermal insulation,[5,6] Li‐ion batteries and voltammetric sensors,[7,8] platforms for integrated photonics and luminescent thin films,[9,10] ceramic coatings for anti-oxidation performance as well as protective layers for high-temperature corrosion,[11,12]SiOC materials are usually prepared by controlled pyrolysis of poly(organosiloxanes) precursors, under inert atmosphere.[15,16,17,18] Their micro/nanostructures widely change according to molecular architecture and composition of selected precursors together with pyrolysis conditions such as temperature, isothermal annealing, atmosphere, and heating/cooling rates.[17,19]In general, when poly(organosiloxanes) are thermally heated from 800 to 1000 °C, a mixed amorphous ceramic network composed mainly by random arrangements of Si sites tetracoordinate with O and C atoms, commonly described as SiOxC4-x, 0 ≤ x ≤ 4, is obtained. More intense bands at 818 cm-1 (νSi-C) with respect to those at 1086 cm-1 (νSi-O-Si) were mainly verified for the set of CPS ceramics, suggesting more effective phase segregation into ceramic matrix with the SiC formation, as expected after pyrolysis at 1500 °C.50,51 This trend, more evident for CPS ceramics, might be explained by the higher Cfree amounts produced during heating treatment upon inert atmosphere for phenyl groups-containing precursors when compared to methyl and vinyl groups.

Results

Conclusion