An official website of the United States government
Porous silicon oxycarbide (SiOC) is emerging as a much superior ultrahigh surface area material that can be stable up to high temperatures with great tailorability through composition and additive modifications. In this study, bulk SiOCs were fabricated from a base polysiloxane (PSO) system by using different organic additives and pyrolysis atmospheres followed by hydrofluoric acid (HF) etching. The additives modify the microstructural evolution by influencing the SiO2 nanodomain formation. The SiOC ceramics contain significantly less SiC and more SiO2 with Ar+H2O atmosphere pyrolysis compared to Ar atmosphere pyrolysis. Water vapor injection during pyrolysis also causes a drastic increase in specific surface areas. The addition of 10 wt% tetraethyl orthosilicate (TEOS) with Ar+H2O pyrolysis produces a specific surface area of 1953.94 m2/g, compared to 880.09 m2/g for the base PSO pyrolyzed in Ar. The fundamental processes for the composition and phase evolutions are discussed as a novel pathway to creating ultrahigh surface area materials. The ability to drastically increase the specific surface area through the use of pyrolysis atmosphere and organic additives presents a promising processing route for highly porous SiOC ceramics.
more »
« less
This content will become publicly available on May 1, 2026
Twice‐Functionalized Montmorillonite Nanosheets for Polymer‐Derived MMT‐SiOC Nanocomposites: Phase Formation and Porosity
Abstract In this study, montmorillonite (MMT) nanosheets are purified and exfoliated from a crude clay source and further twice‐functionalized with cetritrimethylammonium bromide and [3‐(2‐aminoethylamino)propyl]trimethoxysliane (AEAPTMS) to promote dispersion in the preceramic polymer. Phase profiles and compositions of MMT nanoflakes and MMT‐silicon oxycarbide (SiOC) are characterized with X‐ray diffraction, infrared spectroscopy, and thermogravimetric analysis. The microstructures are examined by scanning and transmission electron microscopy. MMT nanoflakes are randomly dispersed in the SiOC matrix with α‐quartz forming at the MMT‐SiOC interface. Pyrolysis to 1400 °C induced the formation of SiC nanowhiskers that are observed up to 20 µm in length and 200 nm in diameter. After selective etching of SiO2domains with HF, pore sizes and specific surface areas of MMT‐SiOC are analyzed with nitrogen adsorption. The study provided a new fundamental understanding of MMT‐SiOC interactions at different pyrolysis temperatures and also led to composites with specific surface areas reaching 120 m2 g−1 up to 1200 °C pyrolysis, and between 340 and 772 m2 g−1at 1400 °C pyrolysis and pore size distributions between 2 and 5 nm. The methodology and results presented improve the understanding and viability of 2D nanomaterial‐reinforced ceramic composites and MMT as a precursor for nanostructured SiC.
more »
« less
- Award ID(s):
- 2024546
- PAR ID:
- 10614946
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Small
- Volume:
- 21
- Issue:
- 19
- ISSN:
- 1613-6810
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract In this study, novel ferromagnetic Ni‐containing silicon oxycarbide (SiOC–Ni) was successfully fabricated from a base polysiloxane (PSO) with the addition of nickel 2,4‐pentanedionate. The resultant SiOC–Ni nanocomposite consists of in situ formed Ni nanocrystallites with a small amount of NiO uniformly dispersed in the amorphous SiOC matrix, and the corresponding nanocrystallite size increases with the increase of the pyrolysis temperature. The formation of nickel silicides (NixSiy) is completely suppressed by the effect of water vapor during the pyrolysis. The fundamental phase evolution process and mechanisms are explained. In an argon atmosphere, the SiOC–Ni materials pyrolyzed at 900°C are stable up to 1000°C with less than 6 wt% weight loss; they exhibit desirable electrical conductivity up to ~900°C with the highest electrical conductivity at ~247 S/m. This series of SiOC–Ni materials also demonstrates exciting ferromagnetic behaviors. Their new semiconducting behavior with soft ferromagnetism presents promising application potentials for magnetic sensors, transformers, actuators, etc.more » « less
-
We report here extracting SiO2 as spirosiloxane [(CH3)2C(O)CH2CH(O)CH3]2Si from rice hull ash (RHA) to carefully control the SiO2 : C mole ratios, allowing direct carbothermal reduction to SiC, Si3N4, or Si2N2O without the need to add extra carbon and as a mechanism to preserve the original nanocomposite structure. We can adjust SiO2 : C ratios from 2 : 15 to 13 : 35 simply by reacting RHA with hexylene glycol (HG) with catalytic base to distillatively extract SiO2 to produce silica depleted RHA (SDRHA) with SiO2 contents of 40–65 wt% and corresponding carbon contents of 60–35 wt% with specific surface areas (SSAs) of >400 m2 g−1. On heating SDRHA40–65 at 1400–1500 °C in an Ar, N2, or N2–H2 atmosphere, XRD patterns reveal formation of SiC, Si3N4, or Si2N2O as the major phase with some residual hard carbon. SEM studies reveal mixtures of particles and whiskers in the products, which show BET specific surface areas >40 m2 g−1 after oxidative removal of excess carbon. Dilute acid and boiling water prewashing of RHA with milling eliminates typical product impurities compared to those found using conventional carbothermal reduction of agricultural wastes, which qualifies the resulting composites as components for electrochemical energy storage devices among other applications, to be reported elsewhere.more » « less
-
null (Ed.)In this study, bulk and porous SiOC materials were synthesized via a polymer-derived ceramic (PDC) method from a base polysiloxane (PSO) precursor and an iron (Fe) catalyst under an inert pyrolytic atmosphere. Fe catalyzes not only the formation and nucleation of β-SiC at lower temperatures but also promotes phase separation of the amorphous SiO x C y phase, compared to PDCs without the Fe catalyst. Samples with Fe pyrolyzed at 1100 °C have an appreciable β-SiC content compared to a negligible/unobservable β-SiC content in the corresponding Fe-less samples. Selective etching of the SiO 2 phase shows that Fe also induces segregation of the amorphous SiO x C y phase, yielding larger specific surface areas and gas sorption capability below 1300 °C. At 1500 °C, the pore structure changes to form interconnected networks due to the highly phase separated SiO 2 and β-SiC microstructure. A Gibbs free energy minimization method was used to determine the relative phase content of the pyrolyzed samples, with the effect of Fe quantified with simplified vapor–liquid–solid (VLS), solid–liquid–solid (SLS), and classical nucleation theories.more » « less
-
Abstract Pore size distribution and surface chemistry of bio‐derived (milk) microporous dominated carbon “MDC” is synergistically tuned, allowing for promising carbon capture in a dry CO2atmosphere and in mixed H2O–CO2. The capture capacity is attributed to the synergy of a large total surface area with an ultramicroporous and microporous texture (e.g.,Stot1889 m2g−1,Smic1755 m2g−1,Sultra1393 m2g−1), and a high content of nitrogen and oxygen heteroatom moieties (e.g., 5 at% N, 10.5 at% O). Tailored two‐step low‐temperature pyrolysis‐chemical activation is employed to take advantage of the intrinsic properties of the precursor, allowing for this unusual textural properties‐heteroatoms combination. For example, tested at 1 bar and 295 or 273 K, MDCs adsorb up to 22.0 and 29.4 wt% CO2, respectively. MDCs are also tailored to be hydrophobic, with CO2/H2O adsorption selectivity even after prolonged cycling. Maximum working capacities of 10.8 wt% for pure CO2and 3.5 wt% for a flue gas simulant (15% CO2, 85% N2) are measured using temperature swing adsorption with dynamic purge gases, while being minimally affected by humid conditions. This work is directly aligned with the United Nation’s Sustainable Development Goal 13, take urgent action to combat climate change and its impacts.more » « less
