Substitutional Carbon Research Articles

Computational investigation of the effect of alkoxy carbon substitution on the mechanism of carbonyl group reduction by 1-hydridosilatranes

We undertook computational investigations to determine the effects of the electronic nature of the cage atoms on the energetics of the reduction of acetone by 1-hydridosilatranes. QTAIM analyses of the parent 1-hydridosilatrane 1H6 and hexasubstituted silatranes 1Me6 and 1F6 showed that the change in charge induced by the donor/acceptor properties of the substituents remained almost entirely localized on the alkoxy carbon; the hydridic nature of the silicon-bound hydrogen as gauged by the charge remained essentially unchanged. This shows resilience to the electronic effects of the substituents at this position. The energetics of acetone reduction to 2-propanol were mapped out for all three cases. Calculations showed favorable Gibbs free energies for each reduction, with considerable exergonicities and modest barriers. Reductions using 1H6 and 1Me6 follow hydride transfer mechanisms without involvement of carbonyl oxygen that lead to an ion/neutral pair, while reduction using 1F6 seems to involve a concerted σ-metathesis-like mechanism that leads directly to a dialkoxysilatrane.

Interaction Behavior of Biogenic Material with Electric Arc Furnace Slag

Electric arc furnaces (EAFs) are used for steel production, particularly when recycling scrap material. In EAFs, carbonaceous material is charged with other raw materials or injected into molten slag to generate foam on top of liquid metal to increase energy efficiency. However, the consumption of fossil carbon leads to greenhouse gas emissions (GHGs). To reduce net GHG emissions from EAF steelmaking, the substitution of fossil carbon with sustainable biogenic carbon can be applied. This study explores the possibility of the substitution of fossil material with biogenic material produced by different pyrolysis methods and from various raw materials in EAF steelmaking processes. Experimental work was performed to study the effect of biogenic material utilization on steel and slag composition using an induction melting furnace with 50 kg of steel capacity. The interaction of biogenic material derived from different raw materials and pyrolysis processes with molten synthetic slag was also investigated using a tensiometer. Relative to other biogenic materials tested, a composite produced with densified softwood had higher intensity interfacial reactions with slag, which may be attributed to the rougher surface morphology of the densified biogenic material.

Synthesis and Hydrolysis of Atmospherically Relevant Monoterpene-Derived Organic Nitrates.

The partition of gas-phase organic nitrates (ONs) to aerosols and subsequent hydrolysis are regarded as important loss mechanisms for ON species. However, the hydrolysis mechanisms and the major factors controlling the hydrolysis lifetime are not fully understood. In this work, we synthesized seven monoterpene-derived ONs and systematically investigated their hydrolysis in bulk solutions at different pH values. The hydrolysis lifetimes ranged from 12.9 min to 8.5 h for allylic primary ON and tertiary ONs, but secondary ONs were stable at neutral pH. The alkyl substitution numbers, functional groups, and carbon skeletons were three important factors controlling hydrolysis rates. Tertiary and secondary ONs were found to hydrolyze via the acid-catalyzed unimolecular (SN1) mechanism, while a competition of SN1 and bimolecular (SN2) mechanisms accounted for the hydrolysis of primary ONs. The consistency of experimental and theoretical hydrolysis rates calculated by density functional theory further supported the proposed mechanisms. Reversible reactions including hydrolysis and nitration were first reported to explain the hydrolysis of ONs, highlighting the possibility that particulate nitric acid can participate in nitration to generate new nitrogen-containing compounds. These findings demonstrate that ON hydrolysis is a complex reaction that proceeds via different mechanisms and is controlled by various parameters.

A machine learning approach to investigate the materials science of enamel aging

Understanding aging of tooth tissues is critical to the development of patient-centric oral healthcare. Yet, the traditional methods for analyzing the composition–structure–property relationships of hard tissues have limitations when considering aging and other factors. ObjectiveTo apply unsupervised machine learning tools to pursue an understanding of relationships between the composition and mechanical behavior of aging enamel. MethodsMolar teeth were collected from primary (age ≤ 8), young adult (24 ≤ age ≤ 46) and old adult (55 ≤ age) donors. The hardness and elastic modulus were quantified using nanoindentation as a function of distance from the Dentin Enamel Junction (DEJ) within the cervical, cuspal and inter-cuspal regions of the enamel crown. Similarly, a co-located analysis of the chemical composition and structure was performed using Raman spectroscopy. A Self-Organizing Maps (SOMs) algorithm was implemented to identify multi-dimensional composition–property relationships. ResultsThe hardness and elastic modulus are positively correlated to crystallinity and negatively correlated with carbonate substitution. Furthermore, the effects from fluoridation on the age-dependent properties of enamel is non-linear and depends on its location. The contributions of fluoridation to the enamel properties are different in the cervical and non-cervical regions and appear to be unique within primary and senior adult teeth. SignificanceBased on the findings, unsupervised learning methods can reveal complicated non-linear structure–property relationships in tooth tissues and help to understand the materials science of aging and its consequences.

Structure, elastic, and electronic properties of the [formula omitted] phases MAX: Ab initio calculations

The crystal structure, elastic, and electronic properties of the Nb2SnC1−xBx phases were investigated using first-principles calculations within the local density approximation. We found that the substitution of carbon by boron atoms induces an increase in the lattice parameters, and therefore, the unit cell volume increases. On the other hand, elastic constants (except C12) and hardness tend to increase as boron content increases. Thus, according to Born, Poisson’s ratio, and Pugh’s criterion, the Nb2SnC1−xBx phases are structurally stable, metallic, and show ductile behavio. The Phonon density of states of the Nb2SnC to Nb2SnB compounds showed no imaginary phonon frequency in the entire Brillouin zone; therefore, the P63/mmc space group is dynamically stable. Furthermore, the density of states at the Fermi energy increases due to the increase of the B 2p states.

Optical properties of fluorinated boron nitride monolayers

In the present work, the optical properties of fluorinated BN monolayers as well as the effect of typical point defects, namely, anti-sites, substitutional carbon impurities and fluorine vacancies, were investigated using first principles calculations. It has been considered fluorinated nanosheets in both chair-like and boat-like configurations. The obtained results indicate that the fully fluorinated monolayers (termed as BN-difluorine) are semiconductors with high transmissivity and just absorb and conduct in the ultraviolet range. Besides, chair-like monolayers have a transmissivity higher than their boat-like counterparts due to the fact that they absorb and reflect less. The introduction of a point defect on the BN-difluorine layer, for both configurations, significantly modifies its optical properties in the low energy range, where absorption and conduction is induced in the whole infrared and visible regions, depending upon the defect. By contrast, at higher energies, the optical spectra of the defective BN-difluorine monolayers are quite similar to the undefective ones.

Photoluminescent properties of the carbon-dimer defect in hexagonal boron-nitride: A many-body finite-size cluster approach

We study the carbon dimer defect in a hexagonal boron-nitride monolayer using the GW and Bethe-Salpeter many-body perturbation theories within a finite size cluster approach. While quasiparticle energies converge very slowly with system size due to missing long-range polarization effects, optical excitations converge much faster, with a $1/R^3$ scaling law with respect to cluster average radius. We obtain a luminescence zero-phonon energy of 4.36 eV, including significant 0.13 eV zero-point vibrational energy and 0.15 eV reorganization energy contributions. Inter-layer screening decreases further the emission energy by about 0.3 eV. These results bring support to the recent identification of the substitutional carbon dimer as the likely source of the zero-phonon 4.1 eV luminescence line. Finally, the GW quasiparticle energies are extrapolated to the infinite h-BN monolayer limit, leading to a predicted defect HOMO-LUMO photoemission gap of 7.6 eV. Comparison with the optical gap yields a very large excitonic binding energy of 3 eV for the associated localized Frenkel exciton.

Electrical and ferromagnetic properties of p-type ZnO thin films with and without the addition of graphene prepared by the sol-gel method

This study determines the effect of the addition of graphene on the electrical and ferromagnetic properties of the ZnO thin films that are prepared by the sol-gel method. ZnO is a typically oxygen-deficient metal oxide and exhibits intrinsic n-type conductivity. ZnO thin films to which graphene is added and those to which it is not, which are annealed at 500 °C for 30 min in oxygen, exhibit p-type behavior. There is an increase in the hole concentration of the graphene-doped ZnO thin film because of competition between the densities of donor-like defects such as oxygen vacancies and acceptor-like defects such as oxygen interstitials (Oi) and the defect complex that is related to the substitution of carbon for zinc (CZn) and Oi. There is ferromagnetism in a ZnO thin film at room temperature because of the presence of Oi. The intensity of the ferromagnetic signal is increased because CZn and Oi are formed in a graphene-doped ZnO thin film.

Separation of Isomers and Mechanisms of Inversion of Stereochemistry of Group 9 d6 Tris-Chelate Complexes of Hinokitiol.

Tris-chelate complexes of Co(III), Rh(III), and Ir(III) with 4-isopropyltropolone (hinokitiol or β-thujaplicin) form by the substitution of carbonate and chloride ligands from group 9 trivalent metal salts. The new complexes are neutral, are readily soluble in most organic solvents, and are brightly colored with strong charge transfer bands. The fac isomers of Co(hino)3 and Rh(hino)3 were isolated from the mixture by fractional recrystallization from ethanol. The remaining mixtures were respectively enriched by 5:3 and 4.4:3 for the mer isomer. The 1H NMR data show that the complexes exhibit remarkable stereochemical lability, which is unusual for diamagnetic d6 group 9 metals, with rotational barriers of 14.2 and 18.2 kcal/mol found for the inversion of stereochemistry of Co(hino)3 and Rh(hino)3. The low activation barriers, as well as the analysis of some key structural parameters, suggest that the inversion of stereochemistry occurs via a trigonal-twist (Bailar) mechanism. Facile substitution of a single hinokitiol ligand in the cobalt complex with ethylenediamine to form [Co(en)(hino)2]Cl also indicates that the tris-chelates are substitutionally and configurationally labile.

Hydrogen Storage in Pure and Boron-Substituted Nanoporous Carbons-Numerical and Experimental Perspective.

Nanoporous carbons remain the most promising candidates for effective hydrogen storage by physisorption in currently foreseen hydrogen-based scenarios of the world’s energy future. An optimal sorbent meeting the current technological requirement has not been developed yet. Here we first review the storage limitations of currently available nanoporous carbons, then we discuss possible ways to improve their storage performance. We focus on two fundamental parameters determining the storage (the surface accessible for adsorption and hydrogen adsorption energy). We define numerically the values nanoporous carbons have to show to satisfy mobile application requirements at pressures lower than 120 bar. Possible necessary modifications of the topology and chemical compositions of carbon nanostructures are proposed and discussed. We indicate that pore wall fragmentation (nano-size graphene scaffolds) is a partial solution only, and chemical modifications of the carbon pore walls are required. The positive effects (and their limits) of the carbon substitutions by B and Be atoms are described. The experimental ‘proof of concept’ of the proposed strategies is also presented. We show that boron substituted nanoporous carbons prepared by a simple arc-discharge technique show a hydrogen adsorption energy twice as high as their pure carbon analogs. These preliminary results justify the continuation of the joint experimental and numerical research effort in this field.

Suppression of charge trapping in ON-state operation of AlGaN/GaN HEMTs by Si-rich passivation

In this paper, we investigate the charge trapping in power AlGaN/GaN high electron mobility transistors which occurs in ON-state operation (V DS = 40 V, V GS = 0 V, I DS = 0.18 A mm−1). By analysing the dynamic ON-resistance (R ON) after OFF-state and ON-state stress in devices with different SiN x passivation stoichiometries, we find that this charge trapping can be largely suppressed by a high Si concentration passivation. Both potential probe and electroluminescence (EL) measurements further confirm that the stress can induce negative charge trapping in the gate–drain access region. It is shown that EL is generated as expected under the field plates at the gate edge, but is obscured by the field plates and is actually emitted from the device near the drain edge; hence care is required when using EL alone as a guide to the location of the high field region in the device. From temperature-dependent dynamic R ON transient measurements, we determine that the apparent activation energy of the measured ‘trap’ response is around 0.48 eV, and infer that they are located in the heavily carbon-doped GaN layer. Using the leaky dielectric model, we explain the response in terms of the hopping transport from the same substitutional carbon acceptor buffer dopants.

Towards ab initio identification of paramagnetic substitutional carbon defects in hexagonal boron nitride acting as quantum bits

Paramagnetic substitutional carbon (${\mathrm{C}}_{\text{B}},{\mathrm{C}}_{\text{N}}$) defects in hexagonal boron nitride are discussed as candidates for quantum bits. Their identification and suitability are approached by means of photoluminescence (PL), charge transitions, electron paramagnetic resonance, and optically detected magnetic resonance (ODMR) spectra. Several clear trends in these are revealed by means of an efficient plane wave periodic supercell ab initio density functional theory approach. In particular, this yields insight into the role of the separation between ${\mathrm{C}}_{\text{B}}$ and ${\mathrm{C}}_{\text{N}}$. In most of the cases, the charge transition between the neutral and a singly charged ground state of a defect is predicted to be experimentally accessible, since the charge transition level position lies within the band gap. A posteriori charge corrections are also discussed. A near-identification of an experimentally isolated single spin center as the neutral ${\mathrm{C}}_{\text{B}}$ point defect was found via comparison of results to recently observed PL and ODMR spectra.

Point Defects in Monolayer h-AlN as Candidates for Single-Photon Emission.

A single-photon emission (SPE) system based on a solid state is one of the fundamental branches in quantum information and communication technologies. The traditional bulk semiconductors suffered limitations of difficult photon extraction and long radiative lifetime. Two-dimensional (2D) semiconductors with an entire open structure and low dielectric screening can overcome these shortcomings. In this work, we focus on monolayer h-AlN due to its wide band gap and the successful achievement of SPE compared to its bulk counterpart. We systematically investigate the properties of point defects, including vacancies, antisites, and impurities, in monolayer h-AlN by employing hybrid density functional theory calculations. The -1 charged Al vacancy (VAl-) and +1 charged nitrogen antisite (NAl+) are predicted to achieve SPE with the zero-phonon lines of 0.77 and 1.40 eV, respectively. Moreover, the charged point-defect complex CAlVN+, which is composed of vacancies and carbon substitutions, also can be used for SPE. Our results extend the avenue for realizing SPE in 2D semiconductors.

Miscanthus to Biocarbon for Canadian Iron and Steel Industries: An Innovative Approach

Iron-based industries are one of the main contributors to greenhouse gas (GHG) emissions. Partial substitution of fossil carbon with renewable biocarbon (biomass) into the blast furnace (BF) process can be a sustainable approach to mitigating GHG emissions from the ironmaking process. However, the main barriers of using biomass for this purpose are the inherent high alkaline and phosphorous contents in ash, resulting in fouling, slagging, and scaling on the BF surface. Furthermore, the carbon content of the biomass is considerably lower than coal. To address these barriers, this research proposed an innovative approach of combining two thermochemical conversion methods, namely hydrothermal carbonization (HTC) and slow pyrolysis, for converting biomass into suitable biocarbon for the ironmaking process. Miscanthus, which is one of the most abundant herbaceous biomass sources, was first treated by HTC to obtain the lowest possible ash content mainly due to reduction in alkali matter and phosphorous contents, and then subjected to slow pyrolysis to increase the carbon content. Design expert 11 was used to plan the number of the required experiments and to find the optimal condition for HTC and pyrolysis steps. It was found that the biocarbon obtained from HTC at 199 °C for 28 min and consecutively pyrolyzed at 400 °C for 30 min showed similar properties to pulverized coal injection (PCI) which is currently used in BFs due to its low ash content (0.19%) and high carbon content (79.67%).

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.

A general approach for calculating melt–solid impurity segregation coefficients based on thermodynamic integration

The equilibrium segregation of impurities at the melt–solid interface during silicon crystallization is a key factor in determining the impurity concentration and distribution in the crystal. Unfortunately, this property is difficult to measure experimentally due to the presence of complex transport physics in the melt. Here, using the Tersoff family of empirical potential models, we describe a thermodynamic integration framework for computing the interstitial oxygen and substitutional carbon segregation coefficients in silicon. Thermodynamic integration using an ideal gas reference state for the impurity atoms is shown to be an efficient and convenient pathway for evaluating impurity chemical potentials in both solid and liquid phases. We find that the segregation coefficient is captured well for substitutional carbon impurity while it is significantly underestimated for interstitial oxygen. The latter discrepancy is partially attributed to the qualitatively incorrect silicon solid-to-liquid density ratio predicted by the empirical interatomic potential.

Early Osteoconductivity and Degradability of Sintered Porous Bodies of A- and B-Type Carbonated Apatites and Stoichiometric Hydroxyapatite

An artificial bone that mimics the chemical composition of biological apatite is one of the promising implants for the treatment of bone defects and is useful as a scaffold for tissue-engineered bone. There are three types of carbonate substitution in hydroxyapatite (HAp) at hydroxyl groups (A-type), phosphate groups (B-type), and both groups (AB-type). Less is known about the role of A-type carbonate apatite (CAp) in influencing the bone regeneration process and there are no comparative studies of these apatites. The present study aims to elucidate osteoconductivity and degradability of porous sintered bodies of stoichiometric HAp, A-type CAp (A-CAp), and B-type CAp (B-CAp) with a constant carbonate content at 5.0 wt%. Both CAp with 75% porosity showed similar ionic dissolution rates which were faster than HAp with 67% porosity. A-CAp had the largest affinity for albumin from time-dependent adsorption tests. The lowest attachment of MC3T3-E1 cells was observed for A-CAp; however, the proliferation rate was similar of both CAp, which was lower than HAp. Early osteoblastic marker (alkaline phosphatase activity) was significantly upregulated for CAp at day 3 relative to HAp. After two weeks of implantation, A-CAp and HAp showed scarce bone formation encapsulated by fibrous tissue. In contrast, B-CAp showed significant bone ingrowth and closely adjoined to the surrounding bone tissue. The rapid degradation of CAp was attributed to osteoclast activities, in which A-CAp showed better capacity in promoting osteoclast maturation. It is concluded that a specific carbonate substitution type significantly impacts osteoconductivity or degradability of artificial bones.

Titanium carbonitride fabricated by spark plasma sintering: Is it a ceramic model of carbon-induced Friedel-Fleisher strengthening effect?

This work is devoted to the analysis of the composition, microstructure and room-temperature mechanical properties of a carbonitride ceramics: the case of Ti(C,N) ceramics fabricated by spark plasma sintering (SPS). Particular attention has been given to know whether there is a possible composition change under electric field, finding all the computed lattice parameters of SPS-ed TiN1-xCx ceramics lay within tiny interval (∼TiC0.43N0.57), hence following Vegard’s law. The analysis of the strengthening effect of substitutional carbon, making use of our own results and those available in literature shows that a classical x1/2 dependence could account for experimental data. This fact reveals that a Friedel-Fleisher mode of strong localized carbon pinning points might describe the hardening effect of those impurities. However, the large scattering of experimental data and the fact that tiny deviations of stoichiometry might play a relevant role on the hardening force us to leave this as an open problem.

The effect of atomistic substitution on thermal transport in large phonon bandgap GaN

Recently, wide phonon bandgaps have been found to play an essential role in thermal transport in some semiconductors, such as GaN, where there is a sensitive dependence of the thermal conductivity on isotopes and lattice defects. We explore the effect of carbon and indium substitution on thermal transport in bulk and monolayer GaN from a first-principles approach. Our calculations reveal that the low-frequency phonon modes below the phonon bandgap make dominant contributions to thermal transport. Our phonon mode analysis demonstrates that after the atom substitution, the reduced phonon bandgap and localized vibrations expand the phonon scattering space, thus leading to a large suppression of phonon lifetimes, which is the primary reason for the significant reduction in thermal conductivity. Our findings provide insights on an atomistic scale into the physical mechanism of the effects of atom substitution on thermal transport in GaN, and our results may guide to enhanced thermal management.

Effect of Precursor Deficiency Induced Ca/P Ratio on Antibacterial and Osteoblast Adhesion Properties of Ag-Incorporated Hydroxyapatite: Reducing Ag Toxicity.

Ag-containing hydroxyapatite (HA) can reduce risks associated with bacterial infections which may eventually require additional surgical operations to retrieve a failed implant. The biological properties of HA in such applications are strongly affected by its composition in terms of dopants as well as Ca/P stoichiometry, which can be easily controlled by altering processing parameters, such as precursor concentrations. The objective of this in vitro study was to understand the effect of variations in HA precursor solutions on antibacterial properties against Escherichia coli (E. coli) and for promoting osteoblast (bone-forming cell) adhesion on Ag incorporated HA (AgHA) which has not yet been investigated. For this, two groups of AgHAs were synthesized via a precipitation method by adjusting precursor reactants with a stoichiometric value of 1.67, being either (Ca + Ag)/P (Ca-deficient) or Ca/(P + Ag) (P-deficient), and were characterized by XRD, FTIR, and SEM-EDS. Results showed that Ag+ incorporated into the Ca2+ sites was associated with a corresponding OH− vacancy. Additional incorporation of CO32− into PO43− sites occurred specifically for the P-deficient AgHAs. While antibacterial properties increased, osteoblast adhesion decreased with increasing Ag content for the Ca-deficient AgHAs, as anticipated. In contrast, significant antibacterial properties with good osteoblast behavior were observed on the P-deficient AgHAs even with a lower Ag content, owing to carbonated HA. Thus, this showed that by synthesizing AgHA using P-deficient precursors with carbonate substitution, one can keep the antibacterial properties of Ag in HA while reducing its toxic effect on osteoblasts.