Transient Viscous Flow During the Evolution of a Ceramic (Silicon Carbonitride) from a Polymer (Polysilazane)

Abstract

The conversion of silicon‐based polymers into a ceramic occurs by the release of small organic molecules at intermediate temperatures, and hydrogen at the higher temperatures. The conversion is accompanied by significant densification. In order for the body to emerge without fracture damage, the shrinkage must be accommodated by viscous flow. In this paper we show that the viscosity changes during the conversion, going through a sharp minimum near 600°C. In the regime of hydrogen evolution, which begins near 700°C, fairly steady viscous flow leads to large shrinkage. The viscosity increases rapidly as 1000°C is approached, pointing to the exhaustion of hydrogen. The results presented here may be used to optimize the time–temperature protocol for achieving damage‐free specimens: heating quickly up to ∼700°C and then slowly at higher temperatures to allow time for viscous relaxation.

Template‐Free Synthesis of Porous Fe3O4/SiOC(H) Nanocomposites with Enhanced Catalytic Activity

“…Generally, in PDCs, the pores are generated by the evolution of small molecules like CH 4 and H 2 during the pyrolytic conversion process. However, the transient porosity spontaneously closes‐up at temperature above 600°C–800°C . Therefore, it is still challenging to fabricate porous PDCs without the assistance of templates …”

“…However, the transient porosity spontaneously closes-up at temperature above 600°C-800°C. 2,[17][18][19] Therefore, it is still challenging to fabricate porous PDCs without the assistance of templates. 2,3,20 Water pollution with organic dyes is an increasing challenging environmental issue worldwide.…”

On the onset of fracture as a silicon‐based polymer converts into the ceramic phase

“…The experiments are carried out with a CH 3 ‐SiO 1.5 gel derived from the methyltriethoxysilane (MTES) which converts into an SiCO ceramic. (b) The results are overlaid with earlier work on gas evolution, and the measurements of viscous flow in powder‐compacts to show that the onset of cracking coincides with the evolution of hydrogen and methane which begins at ~750°C.…”

“…In the following section, we relate the onset of fracture to the change an increase in the viscosity of the material, which coincides with evolution of hydrogen and methane…”

“…In Ref. experiments are described that measure the compressive strain rate in hot‐pressing type experiments with cross‐linked powders. The experiments were conducted at constant heating rates, at different applied loads.…”