A Direct Method for the Fabrication of Macro‐Porous SiOC Ceramics from Preceramic Polymers

Abstract

Cellular ceramics, possessing both open or closed porosity, find use in several demanding engineering applications because of their favorable set of properties.[1] Several processing methods have been proposed for their fabrication, including the replication of the structure of polymeric foams, direct blowing, the use of sacrificial fillers, extrusion through special dies (for honeycombs), solid freeform techniques, the mimicking of natural templates (e.g. wood) or the assemblage of fibers or hollow bodies.[2,3] Preceramic polymers, in particular silicones, have been successfully used for obtaining ceramic components (such as foams and membranes) possessing a large amount of porosity, in the micro-, meso- and macro-size scale.[4, and references therein] However, some of the fabrication methods have some limitations: for instance, direct foaming techniques often lead to a gradient in the porosity amount and pore size along the main expansion axis;[5,6] the infiltration of a silicone resin within organic sacrificial fillers requires a burn out step that has to be carried out in a very controlled fashion in order to produce components without defects (besides often requiring warm pressing – depending on the rheological characteristics of the preceramic polymer – to obtain a well controlled morphology), thus limiting the size and shape of the component that can be produced;[7] the use of supercritical CO2 is regulated by the diffusion within the solid polymer, and works well only for components of limited thickness.[8] In recent years, it has been shown that blending preceramic polymers with different characteristics (molecular weight, molecular architecture, ceramic yield) allows to produce cellular ceramics.[9,10] This paper further explores this possibility, with the specific aim of directly developing a large amount of porosity within the resulting ceramic body during a one-step pyrolysis treatment

Gradient-Hierarchic-Aligned Porosity SiOC Ceramics

“…The latter case results in the formation of hydrogen (H 2 ) gas that can be used as to “self blow” the system to obtain porous components23242829. In the present experiments, the heat applied from the bottom of the aluminum mold initiated the crosslinking reactions, simultaneously the gas bubbles nucleated probably with the aid of LDH particles (both as a surface and water including source), expelled from the heated surface and moved into the cooler zone till reaching to the upper surface (atmospheric).…”

“…NMR spectra were acquired with single pulse (SP) sequence under the following conditions29: Si frequency: 59.60 MHz, π/4 pulse length: 2.25 μs, recycle delay: 150 s, 4 k scans. Samples were packed in 4 mm zirconia rotors, which were spun at 5 kHz under air flow.…”

Enhanced hydrogen and methane gas storage of silicon oxycarbide derived carbon

“…In order to produce SiOC ceramics with a low carbon content, polymethylsilsesquioxane ([(CH 3 SiO 1.5 )] n with n = 130-150, Silres MK, Wacker GmbH, Burghausen, Germany; total carbon after pyrolysis: $13 wt.%) was used denoted as PMS in the remainder of the text. For synthesis of SiOC with a higher carbon content, a polymethylphenylsilsesquioxane ([(C 6 H 5 ) 0.62 (CH 3 ) 0.31 (OR) 0.07 SiO 1.5 ] n with n % 20, Silres H44, Wacker GmbH, Burghausen, Germany; total carbon after pyrolysis: $33 wt.%) was used, denoted as PMPS in the remainder of the text [23][24][25]. Both kinds of polymeric precursors were thermally cross-linked in air at 250°C for 5 h and subsequently pyrolyzed under N 2 atmosphere (99.99%) at 1200°C for 2 h, with a 2°C/min heating/cooling rate.…”

Ceramic foams and micro-beads from emulsions of a preceramic polymer

“…2 In particular, highly porous silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) ceramics were obtained using polysiloxane and polysilazane precursors, respectively, through several processing strategies. [3][4][5][6][7][8] Among these methods, emulsion processing of preceramic polymers to produce ceramic components has so far received relatively little attention. [9][10][11][12][13] Recently, this approach was applied to the fabrication of SiOC ceramics with different shapes, e.g.…”