Structure and Thermodynamics of Silicon Oxycarbide Polymer-Derived Ceramics with and without Mixed-Bonding

Silicon oxycarbides synthesized through a conventional polymeric route show characteristic nanodomains that consist of sp2 hybridized carbon, tetrahedrally coordinated SiO4, and tetrahedrally coordinated silicon with carbon substitution for oxygen, called “mixed bonds.” Here we synthesize two preceramic polymers possessing both phenyl substituents as unique organic groups. In one precursor, the phenyl group is directly bonded to silicon, resulting in a SiOC polymer-derived ceramic (PDC) with mixed bonding. In the other precursor, the phenyl group is bonded to the silicon through Si-O-C bridges, which results in a SiOC PDC without mixed bonding. Radial breathing-like mode bands in the Raman spectra reveal that SiOC PDCs contain carbon nanoscrolls with spiral-like rolled-up geometry and open edges at the ends of their structure. Calorimetric measurements of the heat of dissolution in a molten salt solvent show that the SiOC PDCs with mixed bonding have negative enthalpies of formation with respect to crystalline components (silicon carbide, cristobalite, and graphite) and are more thermodynamically stable than those without. The heats of formation from crystalline SiO2, SiC, and C of SiOC PDCs without mixed bonding are close to zero and depend on the pyrolysis temperature. Solid state MAS NMR confirms the presence or absence of mixed bonding and further shows that, without mixed bonding, terminal hydroxyls are bound to some of the Si-O tetrahedra. This study indicates that mixed bonding, along with additional factors, such as the presence of terminal hydroxyl groups, contributes to the thermodynamic stability of SiOC PDCs.

In-Situ Manufacturing of SiC-Doped MgB2 Used for Superconducting Wire

Magnesium diboride (MgB2) is a highly potential superconducting material, in substitution of Nb3Sn, which has a critical temperature of ~ 39 K. This synthesis and manufacturing of MgB2 wire were conducted by in-situ powder in tube (PIT). The method doped with silicon carbide (SiC) was aimed to study the effect of phase formation on carbon substitution and morphological characteristics with the motivation to improve superconductivity properties. Magnesium, boron, and SiC powders were synthesized and functionally processed with stainless Steel 304 tube. Heat treatment was conducted at 750℃, 800℃, and 850℃ for 2 hours followed by furnace cooling. Characterization was carried out by x-ray diffractometer (XRD), scanning electron microscopy (SEM), and cryogenic magnet testing. The results showed that 1% SiC optimally increased the zero critical temperature of MgB2 ~ 37.18 K along with the sintering at 750℃ for 2 hours.

Wood product carbon substitution benefits: a critical review of assumptions

Background There are high estimates of the potential climate change mitigation opportunity of using wood products. A significant part of those estimates depends on long-lived wood products in the construction sector replacing concrete, steel, and other non-renewable goods. Often the climate change mitigation benefits of this substitution are presented and quantified in the form of displacement factors. A displacement factor is numerically quantified as the reduction in emissions achieved per unit of wood used, representing the efficiency of biomass in decreasing greenhouse gas emissions. The substitution benefit for a given wood use scenario is then represented as the estimated change in emissions from baseline in a study’s modelling framework. The purpose of this review is to identify and assess the central economic and technical assumptions underlying forest carbon accounting and life cycle assessments that use displacement factors or similar simple methods. Main text Four assumptions in the way displacement factors are employed are analyzed: (1) changes in harvest or production rates will lead to a corresponding change in consumption of wood products, (2) wood building products are substitutable for concrete and steel, (3) the same mix of products could be produced from increased harvest rates, and (4) there are no market responses to increased wood use. Conclusions After outlining these assumptions, we conclude suggesting that many studies assessing forest management or products for climate change mitigation depend on a suite of assumptions that the literature either does not support or only partially supports. Therefore, we encourage the research community to develop a more sophisticated model of the building sectors and their products. In the meantime, recognizing these assumptions has allowed us to identify some structural, production, and policy-based changes to the construction industry that could help realize the climate change mitigation potential of wood products.

Diverse Properties of Carbon-Substituted Silicenes

The theoretical framework, which is built from the first-principles results, is successfully developed for investigating emergent two-dimensional materials, as it is clearly illustrated by carbon substitution in silicene. By the delicate VASP calculations and analyses, the multi-orbital hybridizations are thoroughly identified from the optimal honeycomb lattices, the atom-dominated energy spectra, the spatial charge density distributions, and the atom and orbital-decomposed van Hove singularities, being very sensitive to the concentration and arrangements of guest atoms. All the binary two-dimensional silicon-carbon compounds belong to the finite- or zero-gap semiconductors, corresponding to the thoroughly/strongly/slightly modified Dirac-cone structures near the Fermi level. Additionally, there are frequent π and σ band crossings, but less anti-crossing behaviors. Apparently, our results indicate the well-defined π and σ bondings.

Forest Contribution to Climate Change Mitigation: Management Oriented to Carbon Capture and Storage

Today, climate change is assumed by many researchers and scholars as a certainty and is presented as the biggest challenge humanity has ever faced. It is commonly accepted that anthropogenic greenhouse gas emissions are the main cause that is accelerating the process. Therefore, it is urgent to find solutions to mitigate climate change, mainly because the intense effects have already been felt, in many cases in the form of the occurrence of extremely violent weather events. Forests are undoubtedly one of the most effective and easiest ways to provide the function of carbon sinks. However, it is essential and convenient to analyze the permanence time of this carbon in forests, because this permanence time depends directly on the forest management model used. This article aims to analyze forest management models from the perspective of carbon residence time in temperate forests, dividing the models into three types, namely carbon conservation models, carbon storage models, and carbon substitution models, according to their ability to contribute to functioning as carbon sinks, thereby contributing to the mitigation of climate change.

Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core

Understanding the effect of carbon on the density of hcp (hexagonal-close-packed) Fe-C alloys is essential for modeling the carbon content in the Earth’s inner core. Previous studies have focused on the equations of state of iron carbides that may not be applicable to the solid inner core that may incorporate carbon as dissolved carbon in metallic iron. Carbon substitution in hcp-Fe and its effect on the density have never been experimentally studied. We investigated the compression behavior of Fe-C alloys with 0.31 and 1.37 wt % carbon, along with pure iron as a reference, by in-situ X-ray diffraction measurements up to 135 GPa for pure Fe, and 87 GPa for Fe-0.31C and 109 GPa for Fe-1.37C. The results show that the incorporation of carbon in hcp-Fe leads to the expansion of the lattice, contrary to the known effect in body-centered cubic (bcc)-Fe, suggesting a change in the substitution mechanism or local environment. The data on axial compressibility suggest that increasing carbon content could enhance seismic anisotropy in the Earth’s inner core. The new thermoelastic parameters allow us to develop a thermoelastic model to estimate the carbon content in the inner core when carbon is incorporated as dissolved carbon hcp-Fe. The required carbon contents to explain the density deficit of Earth’s inner core are 1.30 and 0.43 wt % at inner core boundary temperatures of 5000 K and 7000 K, respectively.