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

Raj, Rishi; Pederiva, Luca; Narisawa, Masaki; Sorarù, Gian Domenico

doi:10.1111/jace.16089

Cited by 17 publications

(18 citation statements)

References 22 publications

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“…Skipping the second and third thermogravimetric events for a moment, we found that the fourth and last thermogravimetric events, approximately in a range of 610–840 °C, originated from the release of methane only (see graph for m / z = 16 in Figure , right). Indeed, in this temperature range, SiCH 3 and any residual SiH moieties can react to yield new SiCH 2 Si bridges and gaseous CH 4 and H 2 . As stated in the method section, H 2 evolution is not monitored by the MS due to the low sensitivity of our equipment for m / z signals below 4.…”

Section: Resultsmentioning

confidence: 99%

Novel Sulfur‐Containing Cross‐Linking Agent for Si‐Based Preceramic Polymers

Taheri

Asgari

Jeon

et al. 2019

Macro Chemistry & Physics

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Cross‐linking polymethylhydrosiloxane (PMHS) with divinylthiophene (DVT) via hydrosilylation in highly dilute conditions and subsequent supercritical drying in CO2 yield a polymeric aerogel containing aromatic sulfur integrally and uniformly distributed throughout the monolith. Fourier‐transform infrared (FT‐IR) spectroscopy indicates almost complete consumption of vinyl groups and SiH bonds in the product. Both FT‐IR and Raman spectroscopic analyses support loss of conjugation of vinyl groups with the retained double bonds of the thiophene ring. Scanning electron microscopy (SEM) indicates a condensed colloidal structure with characteristic particulate diameters of about 165 nm. SEM coupled with energy dispersive X‐ray spectroscopy elemental mapping shows that sulfur is distributed homogeneously in the polymeric aerogel. Porosimetry of the mesoporous aerogel indicates the effective average pore diameters are about 12 nm. Thermogravimetric analysis (TGA) establishes greater thermal stability of the PMHS‐DVT product than either of the pure unreacted components. TGA coupled with mass spectrometric (TG‐MS) identification of the volatiles released during pyrolysis shows that sulfur is driven from the cross‐linked polymer as thiophene and its derivatives. Recorded mass spectra support the hypothesis that cross‐linking DVT bridges between PMHS chains in the polymeric aerogel, and that this results in a more thermally stable monolith.

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Section: Resultsmentioning

confidence: 99%

Novel Sulfur‐Containing Cross‐Linking Agent for Si‐Based Preceramic Polymers

Taheri

Asgari

Jeon

et al. 2019

Macro Chemistry & Physics

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“…But on the other hand, since H 2 has to diffuse out of the sample this could lead to a properties gradient, the regions closer to the outer surface containing less hydrogen compared to the inner parts. 53 EDXS profiles of the SiOC-CO 2 sample show that the surface is oxygen-rich and carbon-deficient compared to the bulk. These results agree with the H and E profiles of the same samples showing that the hardness and elastic modulus are lower at the surface.…”

Section: Discussionmentioning

confidence: 97%

“…As hydrogen in the amorphous PDCs network acts as a “chain breaker,” its release leads to a more interconnected structure thus increasing E and H (the release of hydrogen is accompanied by formation of Si–C, Si–O, and Si–N bonds). But on the other hand, since H 2 has to diffuse out of the sample this could lead to a properties gradient, the regions closer to the outer surface containing less hydrogen compared to the inner parts 53 …”

Section: Discussionmentioning

confidence: 99%

Effect of the pyrolysis atmosphere on the mechanical properties of polymer‐derived SiOC and SiCN

et al. 2020

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Mechanical properties of polymer‐derived ceramics are usually measured on samples pyrolyzed in inert atmosphere. Here, we report the hardness and elastic modulus of SiOC and SiCN pyrolyzed in both inert (Ar) and reactive (CO2) atmosphere. The external surface of the specimens exposed to the pyrolysis gas was characterized by Vickers microhardness measurements and infrared spectroscopy. The elastic modulus was evaluated by three‐point bending tests on thin (150‐200 µm) and dense specimens. Polished sections of the SiOC samples were prepared to study, by energy‐dispersive X‐ray spectroscopy (EDXS) and nanoindentation, how the elemental composition, hardness, and elastic modulus vary from the surface toward the bulk. For both compositions, pyrolysis in CO2 leads to a strong decrease in the hardness and elastic modulus. The hardness of both the samples pyrolyzed in CO2 approaches the typical value for fused silica, suggesting that CO2 selectively breaks the Si–C and Si–N bonds and leads to the formation of a silica‐like network. EDXS and nanoindentation reveal that the modification induced by the CO2 flow extends below the surface at least for a thickness of about 30 µm.

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“…Only thin cross sections pyrolyzed at a slow rate, which reduces gradients in hydrogen from the surface inwards, pre-empt fracture. 5 In a recent development nanolayer films, less than 100 nm thick, deposited on the fiber preform were shown to pyrolyze in less than a minute into a ceramic that was free of defects. 6,7 The process was cycled for building up a full ceramic matrix in 40-80 cycles in about 2-4 hours.…”

Section: Introductionmentioning

confidence: 99%

“…The inorganic phase, the ceramic, is highly refractory; it cannot creep to relax the gradients in shrinkage. Only thin cross sections pyrolyzed at a slow rate, which reduces gradients in hydrogen from the surface inwards, pre‐empt fracture 5 …”

Section: Introductionmentioning

confidence: 99%

Thin coatings of hafnon abate oxidative recession of SiC fibers

Azarnoush

Raj

2020

J. Am. Ceram. Soc.

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The oxidation behavior of SiC fibers coated with (a) undoped polysilazane and (b) precursors containing a mixture of polysilazane and hafnium butoxide in equal weight fractions, is reported. The coatings were prepared by repetitive cycles of nanolayer depositions, as reported in recent publications. The oxidation experiments were carried out at 1400°C in ambient air (Boulder, CO) for up to 100 hours. The extent of degradation of SiC was measured by the recession in the diameter of the fibers as a function of time. The fibers with undoped polymer precursor recessed significantly, whereas the fibers coated with HfSiCNO remained essentially unchanged. These results are in agreement with earlier work from our laboratory where the resilience of hafnon and zircon, as well as hafnia and zirconia, against high-temperature corrosion in streaming humid environments had been highlighted.

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

Cited by 17 publications

References 22 publications

Novel Sulfur‐Containing Cross‐Linking Agent for Si‐Based Preceramic Polymers

Novel Sulfur‐Containing Cross‐Linking Agent for Si‐Based Preceramic Polymers

Effect of the pyrolysis atmosphere on the mechanical properties of polymer‐derived SiOC and SiCN

Thin coatings of hafnon abate oxidative recession of SiC fibers

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