Si-C Bonds Research Articles

Co-pyrolysis of wood chips and bentonite/kaolin: Influence of temperatures and minerals on characteristics and carbon sequestration potential of biochar

Biochars have been highlighted as a means of carbon sequestration, which is significant for achieving carbon neutrality. Mixtures of wood chips and either bentonite or kaolin were co-pyrolysed at temperatures of 350 °C and 550 °C, and the microstructural characteristics and the carbon sequestration potential of the resultant biochar were explored in the study. The addition of minerals promoted the formation of a stable carbon structure in biochar, especially the proportion of SiC bonds in the high-temperature mineral-composited biochar increased by 3.56–3.82 times compared with the original biochar. After bentonite or kaolin was added to wood chips pyrolysed at 550 °C, the carbon loss after H2O2 oxidation was reduced to no more than 19.2%, and the Recalcitrance Index (R50) of biochar increased to no less than 0.89. The combined action of high temperature and minerals promoted the formation of highly aromatic structures of biochar (H:C < 0.4) and reduced the amount of dissolved organic carbon to 4.89 mg g−1. Furthermore, minerals directly covered the surface of biochar, and the content of SiC bond increased, thus strengthening the chemical and thermal stability of biochar. However, the addition of minerals had no significant effect on the biological stability of biochar. The study indicates that the pre-pyrolysis mineral addition is an effective way to increase the carbon sequestration potential of biochar.

Ab Initio Study of Octane Moiety Adsorption on H- and Cl-Functionalized Silicon Nanowires.

Using first-principles calculations based on density functional theory, we investigated the effects of surface functionalization on the energetic and electronic properties of hydrogenated and chlorinated silicon nanowires oriented along the <112> direction. We show that the band structure is strongly influenced by the diameter of the nanowire, while substantial variations in the formation energy are observed by changing the passivation species. We modeled an octane moiety absorption on the (111) and (110) surface of the silicon nanowire to address the effects on the electronic structure of the chlorinated and hydrogenated systems. We found that the moiety does not substantially affect the electronic properties of the investigated systems. Indeed, the states localized on the molecules are embedded into the valence and conduction bands, with no generation of intragap energy levels and moderated change in the band gap. Therefore, Si-C bonds can enhance protection of the hydrogenated and chlorinated nanowire surfaces against oxidation without substantial modification of the electronic properties. However, we calculated a significant charge transfer from the silicon nanowires to the octane moiety.

Enhancement of Oxidation of Silicon Carbide Originating from Stacking Faults Formed by Mode-Selective Phonon Excitation Using a Mid-Infrared Free Electron Laser.

Silicon carbide (SiC) is a promising material for wide applications due to its excellent material properties including high physical and chemical stability as well as great electronic properties of a wide bandgap. The high stability, however, makes its surface processing difficult. Especially, electrochemical processing is not well-established because of low electrochemical reactivity. Here, we show that selective phonon excitation by a mid-infrared free electron laser (MIR-FEL) enhances the anodic reactions. The selective excitation of two different vibration modes of the Si-C bond induces two different stacking faults, which act as a current path. As an application, we discovered that MIR-FEL irradiation enables Pt electroless deposition. This work reveals the interactions among phonons, lattice defects, and electrochemical reactions, encouraging further development of not only electrochemical surface processing but also a new application of MIR-FEL.

Thermal and Mechanical Properties of Amorphous Silicon Carbide Thin Films Using the Femtosecond Pump-Probe Technique

Nanoscale amorphous silicon carbide (a-SiC) thin films are widely used in engineering applications. It is important to obtain accurate information about their material properties because they often differ from those of the bulk state depending on the fabrication technique and process parameters. In this study, the thermal and mechanical properties of a-SiC thin films were evaluated using the femtosecond pump-probe technique, which provides high spatial and temporal resolutions sufficient to measure films that have a thickness of less than 300 nm. a-SiC films were grown using a plasma-enhanced chemical vapor deposition system, and the surface characteristics were analyzed using ellipsometry, atomic force microscopy, and X-ray reflectometry. The results show that the out-of-the-plane thermal conductivity of the films is lower than that of bulk crystalline SiC by two orders of magnitude, but the lower limit is dictated by the minimum thermal conductivity. In addition, a decrease in the mass density resulted in a reduced Young’s modulus by 13.6–78.4% compared to the literature values, implying low Si-C bond density in the microstructures. The scale effect on both thermal conductivity and Young’s modulus was not significant.

Surface microstructure and wetting of SiC ceramic modified by ion bombardment

Ion bombardment is one of the powerful methods to modify the surface characteristics of materials. In this study, SiC ceramic is bombarded under different fluences by 60 keV Ar-ion beam. The effects of Ar ion bombardment on the surface microstructure and subsequent wetting of SiC ceramic using AgCu-Ti filler metal are investigated. Ar ion bombardment can break the SiC bond of SiC cell, thus, SiCx(x < 1), SiCx(x < 1) twins and the amorphous layer are generated in the SiC surface with the increase of fluence from 1×1014/cm2 to 1×1017/cm2. With the increase of fluences, the contact angle decreases by the acceleration of interfacial reaction rate as a result of the break of SiC bond in SiC surface in the rapid spreading stage. The amorphous layer is gradually formed in the SiC surface, requiring more time to reach the equilibrium stage for droplets, while the bonding quality of the SiC/AgCu-Ti wetting interface is obviously improved. Significant modification in the microstructural and excellent wetting behavior are obtained by leveraging the ion bombardment technique.

Structural properties of the fiber –matrix interface in carbon-fiber/carbon-matrix composites and interfaces between carbon layers and planar substrates

Abstract The structure of the fiber –matrix interface region of carbon-fiber/carbon-matrix (C/C)-composites, which is important for the mechanical properties of a composite, was studied by transmission electron microscopy. The pyrolytic carbon matrix was obtained by chemical vapor infiltration of carbon fiber preforms with different fiber architecture and fiber types. While the texture of the carbon matrix can be controlled by the infiltration conditions, the carbon texture at the fiber –matrix interface differs from the bulk matrix and cannot be determined by the infiltration parameters. We often observe a high-textured layer with a thickness up to 100 nm at the interface prior to the formation of the bulk matrix with a lower texture. The fiber –matrix interface properties of C/C-composites are compared with the interface of pyrolytic carbon layers deposited on planar Si-, BN- and cordierite-substrates. In contrast to BN-substrates, thin high-textured pyrolytic layers are observed at the interface of carbon on planar Si- and cordierite-substrates where strong bonding between pyrolytic carbon and the substrate is achieved by the formation of Si-C bonds. Stress-induced ordering is discussed as a mechanism for the formation of the high-textured interface layers.

Enhanced mechanical properties of GNPs/6061Al composites by Si atoms repairing the graphene defects: The first-principle calculation and experiment

The mechanical properties of graphene are significantly affected by its in-plane holes defects. Studies have found that Si element was expected to spontaneously dope into the hole defects of graphene during the material processing process to increase the strength of graphene containing hole defects. First-principle calculated that when 2% and 10% Si doped into graphene, the strength was increased by 7% and 13% compared with graphene containing hole defects. In this paper, the Si atoms decomposed from PDMS at high temperature were used to realize the preparation of Si modified graphene/Al composites. Raman and XPS found that under the condition of long-term heat preservation at 600 °C, the content of hole defects in graphene decreased, and a new Si-C bond was produced. Under the joint action of graphene defect regulation and other strengthening mechanisms, the tensile strength of the composites has increased 9.4% from 338 MPa to 370 MPa, and the yield strength increased by 10.7%, while the plasticity did not decrease significantly. The performance improvement of the composites was due to the Si modification, which caused the strength of graphene to be improved compared with graphene containing holes.

The effects of the thickness of the sandwiched layer and of the annealing time on induced nanostructures during solid state dewetting of a metal-semiconductor-substrate triple layer structure

A structure made of a very thin Pd metal layer at the surface, followed by a SiC layer of varied thickness were deposited onto a c-Si substrate by e-beam evaporation at room temperature. Solid state dewetting of the top metal layer is achieved at near eutectic temperature of Pd/Si phase at 800 °C and we study its effect on the sandwiched SiC semiconductor layer of varied thicknesses. Scanning electron microscopy (SEM) images showed that the dewetting process accelerates with decreasing the SiC film thickness, whereas the nucleation and shape patterning were found to be less sensitive to the time of anneal in the studied time domain between 30 min – 3 hours. Focused ion beam scanning electron microscopy (FIB-SEM) and Transmission electron microscopy (TEM) analysis revealed the thermally initiated surface dewetting of the Pd metal at the SiC interface as well as an apparent diffusion into this intercalated layer. Large dewetted Pd nanostructures protruding into the c-Si substrate suggest a dewetting effect of the underlying SiC layer from the substrate surface. Rutherford Backscattering Spectrometry (RBS) analysis revealed the diffusion of silicon from the SiC sandwiched layer into the Pd layer during annealing which resulted in the formation of Pd2Si phase, this was also confirmed by X-ray Diffraction (XRD). While the absence of the 800 cm−1 absorption mode in the Fourier transform infrared (FTIR) spectrum of the as-deposited SiC layer suggests a disordered microstructure, a narrow absorption peak that appeared at 796 cm−1 in the annealed samples confirmed the crystallization of the network in SiC bonds.

A novel, simple, and quick plate assay to screen silicolytic bacteria and silicase production using different substrates

Silicase helps in dissolving silica by catalyzing the cleavage of SiO and SiC bonds to form silicic acid. It belongs to carbonic anhydrases owing to its highest structural homology. Potential applications of silicase are wide and diverse ranging from e-waste remediation to tailoring nanochips and bio-desilication of rice straw for its utilization as substrate for biofuels production. Herein, a soil bacterium was isolated and screened for its ability to produce silicase. A simple and quick screening method for silicolytic bacteria was established. Different agro-residues including rice straw, wheat straw and bagasse along with silicon dioxide and magnesium trisilicate were investigated for their use as substrate. Silicase activity was found to be the highest for rice straw as a substrate since it has the highest silica content among other agro-residues. FTIR analysis of all production media showed silica dissolution before and after inoculation.

Tuning mechanical and corrosion performance of SiOC glass coatings prepared by thermal MOCVD

In this study, silicon oxycarbide (SiOC) glass coatings were successfully deposited by thermal metal organic chemical vapor deposition (MOCVD). The composition, chemical bonding and mechanical properties of the prepared SiOC glass coatings were studied using X-ray photoelectron spectroscopy (XPS) and nano indentation techniques. The surface morphologies of SiOC coatings were characterized by field emission scanning electron microscopy (FESEM) before and after etching with HF acid for prolong time. The results showed that the percentage of Si-C bonds in SiOC glass coatings increases with increasing the content of HMDS in liquid precursors mixture. Raman spectroscopy results revealed the presence of amorphous carbon (with high content of sp3 bonds) in the prepared SiOC coatings. Consequently, the hardness and modulus of SiOC glass coatings increased from 8.99 GPa and 82.6 GPa to 17.93 GPa and 195.6 GPa, respectively. Further, the prepared SiOC glass coatings exhibited excellent corrosion resistance against HF acid.

Geminal C-Cl and Si-Cl bond activation of chloromethanes and chlorosilanes by gallanediyl LGa.

The activation of relatively inert E-X σ-bonds by low-valent main group metal complexes is receiving increasing interest. We here confirm the promising potential of gallanediyl LGa (L = HC[C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) to activate E-Cl (E = C, Si) σ-bonds of group 14 element compounds. Equimolar reactions of LGa with chloromethanes and chlorosilanes EHxCl4-x (E = C, x = 0-2; E = Si, x = 0, 1) occurred with E-Cl bond insertion and formation of gallylmethanes and -silanes L(Cl)GaEHxCl3-x (E = C, x = 2 (1), 1 (2), 0 (3); E = Si, x = 1 (4)). In contrast, consecutive insertion into a geminal E-Cl bond was observed with two equivalents of LGa, yielding digallyl complexes [L(Cl)Ga]2EHxCl2-x (E = C, x = 2 (5); E = Si, x = 1 (6), 0 (7)). Compounds 1-7 were characterized by heteronuclear NMR (1H, 13C, 29Si (4, 6)), IR spectroscopy and elemental analysis, and their solid-state structures were determined by single-crystal X-ray diffraction (sc-XRD).

A DFT study on the cyclization-degradation mechanism for phenylmethylsiloxanes in thermal vacuum

A series of DFT calculations has been performed on the cyclization-degradation mechanisms of phenylmethylsiloxanes in a thermal vacuum. Like dimethylsiloxanes, a four-membered cyclic motif is identified for all transition states. The flexibility of the backbone is a precondition for the back-biting reaction. The phenylmethylsiloxanes confer more stability than the dimethylsiloxanes because the replacement of methyl by phenyl reduces the flexibility of the backbone. The activation energy is mainly determined by the total energy effect in the process of bonds’ formation and scission. The depolymerization of the main chain for the phenylmethylsiloxanes is higher in the energy barrier than that for the dimethylsiloxanes by about 14.38 kcal/mol. The cyclization degradation mechanism of them little differs intrinsically. The effect of the end group on the activation energy of the depolymerization reduces rapidly as the main chain lengthens. Whether in phenylmethylsiloxanes or in dimethylsiloxanes, the effect of hydroxyl end-groups is to assist in the cleavage of Si-C bonds in the way of the hydrogen abstraction, hence, the replacement of the trimethylsilyl terminal group by the dimethylhydroxylsilyl group should decrease the thermal stability of siloxanes. The phenylmethylsiloxanes show slightly higher activation energy of intramolecular hydrogen abstraction than the dimethylsiloxanes, because the methyl has a stronger reducibility than the phenyl.

Comparative investigation on tetramethylsilane and neopentane combustion: Jet-stirred reactor pyrolysis and kinetic modeling

This work reports a comparative study on jet-stirred reactor (JSR) pyrolysis of tetramethylsilane (TMS) and neopentane at 1.04 bar and 770–1070 K. Gas chromatography (GC) was used to detect pyrolysis products and measure their mole fraction profiles versus heating temperature. TMS was observed to decompose about 70 K later than neopentane, showing a slower reactivity than its hydrocarbon counterpart. Hydrocarbon products such as methane, ethylene, ethane and propene were detected in the TMS pyrolysis, while besides TMS, no other Si-containing species was observed. Methane, acetylene, ethylene and isobutene were observed as dominant products in the neopentane pyrolysis. The pyrolysis models of TMS and neopentane were developed and validated against the present data, as well as the previous pyrolysis data of TMS. The available rate constants of SiC bond dissociation reaction were evaluated based on the mole fraction profiles of TMS in the present and previous pyrolysis work over a wide temperature region. The influence of methane-elimination reactions on the decomposition of TMS and neopentane was also discussed. Rate of production (ROP) analysis and sensitivity analysis were performed to provide insight into the fuel decomposition chemistry in both the TMS and neopentane pyrolysis. The SiC/CC bond dissociation reactions and methane-elimination reactions were concluded to be the crucial fuel consumption pathways in both the TMS and neopentane pyrolysis.

Reaction of Dichlorophenylborane with H-Si(100).

Traditional approaches to achieving dopant functionalized Si involve grafting the dopant to the Si substrates through O–Si or C–Si bonds, resulting in indirect attachment of the dopant to the Si. Recently, ultrahigh vacuum work has demonstrated that high densities of direct B–Si bonds enable unprecedented electronic behaviors in Si that make it possible for Si to be used as a next-generation electronic material. As solvothermal approaches are inherently amenable to scale-up, there is currently a push to develop solvothermal approaches for the formation of direct dopant-Si bonds. Thus far, B–Si chemistries for next-generation electronic materials have been demonstrated with boron trichloride and bis(pinacolatodiboron). In this work, we use a combination of experimental work and computational studies to examine the reactivity of a phenyl derivatized boron trichloride, namely dichlorophenylborane, with H–Si(100). We determine that despite the stability and ease for the formation of C–Si bonds, the organic component, the phenyl group remains attached to the B and does not yield competitive formation of products via a Si–C bond. This reaction proved a new solvothermal method for the formation of direct B–Si bonds that, with further work, can be leveraged in developing next-generation electronic materials.

Deep-Blue Room-Temperature Phosphorescent Carbon Dots/Silica Microparticles from a Single Raw Material.

Blue-emitting room-temperature phosphorescence (RTP) materials have always been one of the major challenges in the field of RTP materials. Deep-blue (430 nm) RTP emitting silica microparticles were prepared via a one-step hydrothermal reaction from a single raw material of 3-aminopropyltriethoxysilane (APTES). APTES provided amino groups and ethyl groups as a nitrogen and carbon source, which was further condensed to be nitrogen-contained carbon dots (CDs). Besides, the siloxane side of APTES was hydrolyzed and formed silica microparticles. The CDs with the potential chromophores (C═O and C-N etc.) were connected to SiO2 via Si-C bonds of APTES. The covalent bonds and the rigid silica network effectively restricted the motions of potential chromophores of the CDs and reduced the energy gap between the singlet state and triplet state, which was favorable to the RTP. It was believed that this work will guide researchers to realize other blue or deep-blue RTP materials.

Transition Metal Complexes of Heavier Vinylidenes: Allylic Coordination vs Vinylidene-Alkyne Rearrangement at Nickel.

Transition metal π-allyl complexes are key reagents/intermediates of various catalytic and stoichiometric allylation reactions. We now report the first transition metal complex of a heavier allylic π-system. The η3-Si2Ge allyl nickel complex is formally obtained by the oxidative addition of the Si-Cl bond of the heavier vinylidene [R2(Cl)Si-(R)Si═(NHC)Ge:] to [Ni(COD)2] (R = 2,4,6-triisopropylphenyl; NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; COD = 1,5-cyclooctadiene). Due to geometric constraints, the coordination to the Ni(II) center occurs through the formal Si═Ge double bond instead of the residual lone pair of electrons at germanium. In contrast, the Si-N bond of the analogous vinylidene [R2(Me2N)Si-(R)Si═(NHC)Ge:] (obtained by nucleophilic substitution of Cl by NMe2) does not oxidatively add to Ni(0), and a hydridosilagermene-η2-nickel complex is obtained instead. The formation of this complex necessarily implies the isomerization of the heavier vinylidene to the corresponding heteroalkyne with the Si≡Ge triple bond in the coordination sphere of nickel followed by the activation of a C-H bond of one of the isopropyl groups of an N-heterocyclic carbene (NHC) ligand.

Palladium-Catalyzed Enantioselective Carbene Insertion into Carbon-Silicon Bonds of Silacyclobutanes.

We report herein a highly efficient palladium-catalyzed carbene insertion into strained Si-C bonds with excellent enantioselectivity, which provides a rapid and distinct method to access silacyclopentanes with a three- or four-substituted stereocenter asymmetrically. Mechanistic studies using hybrid density functional theory suggest a catalytic cycle involving oxidative addition, carbene migratory insertion, and reductive elimination. In addition, roles of the chiral ligands in controlling the reaction enantioselectivity are also elucidated.

The Creation of True Two-Dimensional Silicon Carbide.

This paper reports the successful synthesis of true two-dimensional silicon carbide using a top-down synthesis approach. Theoretical studies have predicted that 2D SiC has a stable planar structure and is a direct band gap semiconducting material. Experimentally, however, the growth of 2D SiC has challenged scientists for decades because bulk silicon carbide is not a van der Waals layered material. Adjacent atoms of SiC bond together via covalent sp3 hybridization, which is much stronger than van der Waals bonding in layered materials. Additionally, bulk SiC exists in more than 250 polytypes, further complicating the synthesis process, and making the selection of the SiC precursor polytype extremely important. This work demonstrates, for the first time, the successful isolation of 2D SiC from hexagonal SiC via a wet exfoliation method. Unlike many other 2D materials such as silicene that suffer from environmental instability, the created 2D SiC nanosheets are environmentally stable, and show no sign of degradation. 2D SiC also shows interesting Raman behavior, different from that of the bulk SiC. Our results suggest a strong correlation between the thickness of the nanosheets and the intensity of the longitudinal optical (LO) Raman mode. Furthermore, the created 2D SiC shows visible-light emission, indicating its potential applications for light-emitting devices and integrated microelectronics circuits. We anticipate that this work will cause disruptive impact across various technological fields, ranging from optoelectronics and spintronics to electronics and energy applications.

Ionization-bombardment assisted deposition of MXene/SiC heterostructure for micro-supercapacitor with enhanced sodium storage

Improving the energy density of MXene micro-supercapacitors (MSCs) is still a challenge because the MXene nanosheets will easily agglomerate, leading to irreversible reduction of capacity. We present SiC/MXene heterostructure by ionization-bombardment assisted deposition synthesis. Under high voltage, C–C bond and Si-O bond are broken by electron bombardment. After arc sintering, C atom and Si atom form stable Si-C bond with lower bond energy, and then form SiC nanocrystals. Furthermore, SiC nanocrystals were patterned into mesoporous SiC nanonesh to enhance the specific surface area. The surface of SiC framework is full of charge, which makes the MXene nanosheets uniformly deposited and self-assembled to form heterostructure. SiC/MXene heterostructure has low barrier, strong bonding force and fast electron migration rate. SiC nanomesh provide a conductive network, and also serve as a framework to support the MXene film, increasing ion storage space, creating an interconnected path for the conduction of Na ions. The specific capacity of the MXene/SiC MSC is up to 97.8 mF cm−2 at a current density of 1 A cm−2. The fabrication of high-performance nanostructures leads to large-scale production and micro-energy storage devices.

Effect of nitrogen doping on the microstructure and thermal stability of diamond-like carbon coatings containing silicon and oxygen

Using hexamethyldisiloxane, acetylene, and N2 as the precursors, binary Si/O and ternary Si/O/N co-doped diamond-like carbon (DLC) coatings were deposited on 316 L stainless steel and single-crystal Si wafer substrates via high-bias voltage plasma-enhanced chemical deposition. The coated specimen was annealed in the temperature range 200–650 °C in a chamber maintained at 5 Pa vacuum. The microstructure and chemical bonds were characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The mechanical properties were investigated using a residual stress tester and a nano-indenter. It was found that approximately 50% of the Si in the binary Si/O co-doped coating were present as SiO bonds. Notably, N doping reduced the sp3 CC fraction, hindered the formation of O-containing bonds, and enhanced the SiC bonds in the as-deposited ternary Si/O/N co-doped coatings; however, the dominant chemical state of the N atom remained in the form of CN bonds. Furthermore, N doping was beneficial toward the stabilization of the SiC and SiO bonds and hindered graphitization during annealing. The thermal stability of the microstructure and mechanical performance of the ternary co-doped DLC coatings were superior to those of the binary Si/O-co-doped DLC coatings. After annealing at 500 °C, the ternary coatings retained a hardness of 16.5 GPa.