SiBOC foams from methylvinylborosiloxane using urea crystals as a pore template and ethylenediamine as a gelling agent

Abstract

A simple urea crystal templating method is reported for the preparation of SiBOC ceramic foams, with rectangular rod-like pores, from methylvinylborosiloxane (MVBS) preceramic polymer solution. Pastes of urea crystals in concentrated MVBS solution were filled in open rectangular mold and set by cross-linking of MVBS using ethylenediamine (EDA) at room temperature. The cross-linking is due to the formation of adduct between electron-deficient boron in the polymer backbone and lone pair of electrons in amino groups of EDA. The gelation time of 60 wt.% MVBS solution in ethanol is modulated in the range of 240 to 3.25 h using EDA concentration in the range 0.33 to 10 wt.%. The gelled body is water-washed to leach out urea, followed by inert heat treatment at 1500 °C to produce SiBOC foam. The SiBOC ceramic foams of density 0.739 to 0.215 g cm⁻³, compressive strength 2.75 to 0.207 MPa, and thermal conductivity 0.262 to 0.073 W m⁻¹ K⁻¹ were achieved with urea concentrations of 50 to 88 vol.%. SEM micrographs show rectangular rod-like pores resulting from the urea particle template, which increases with an increase in urea concentration. XRD and TEM confirm SiBOC as amorphous, with the presence of a few nanocrystals of β-SiC and turbostratic carbon layers. The low thermal conductivity, excellent oxidation resistance, and reasonably high compressive strength make the prepared foams suitable candidate for high temperature thermal protection in aerospace.