Electrically induced liquid infiltration for the synthesis of carbon/carbon–silicon carbide composite

The electrically induced liquid infiltration (EILI) method for the synthesis of carbon/carbon–silicon carbide (C/C–SiC) materials was developed. The method involves Joule preheating of a porous carbon/carbon preform surrounded by silicon media, followed by silicon infiltration into the pore structure, and its reaction with carbon to form pore-free C/C–SiC composite. This technique is characterized by high heating rates (10²–10³ K/s) and short processing times (5–20 s), which distinguish it from conventional approaches. The influence of maximum treatment temperature, as well as preheating rate on the depth of infiltration, reaction kinetics, and the material microstructure was investigated. C/C–SiC composite with a compressive strength which was twice that of the initial C/C material was synthesized.     

Spatial organization of the sp-hybridized carbon atoms and electronic density of states of hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films prepared by plasma decomposition of hydrocarbons exhibit a wide variety of electronic and mechanical properties depending on their deposition conditions, which makes them very interesting for applications in several domains. This versatility is essentially due to the presence of both sp²- and sp³-hybridized carbon atoms in variable proportions, and to the tendency of the sp² C atoms to gather into π-bonded clusters with different bonding configurations. The relationships between the film microstructure and their electronic density of states, as deduced from a detailed analysis of their optical properties over a large spectral range, are described and discussed, taking as reference materials the purely sp² (graphite) and purely sp³ (diamond) carbon crystalline phases, as well as the prototype hydrogenated amorphous tetra-coordinated semiconductor, hydrogenated amorphous silicon. It is shown that the type of clustering of the sp² C atoms is certainly more determinant for the electronic density of states, and especially for the optical gap value, than the proportion of these atoms in the material.     

Microstructural and mechanical characterization of sol gel-derived Si–O–C glasses

The mechanical properties of three silicon oxycarbide glasses pyrolysed under inert (Ar) atmosphere were studied as a function of the pyrolysis temperature. The silicon oxycarbide glasses were prepared from various alkyl substituted alkoxysilanes such as HSi(OEt)₃ and HMeSi(OEt)₂ in different ratios by using the sol-gel method. The Si–O–C-glasses obtained were respectively: (i) silicon oxycarbide network with excess carbon, (ii) stoichometric SiC_xO_2(1−x) where x=0.30 and (iii) silicon oxycarbide matrix with an excess of Si. Si–C bonds introduced in the starting silica gel network can be partially retained in the final glass after pyrolysis under inert atmosphere. After pyrolysis at temperatures between 600–1500 °C, the presence of tetracoordinated C atoms in the silica network results in an improvement of mechanical properties and thermal stability compared with silica glass. By using elemental analysis, density, SEM, BET and XRD (combined with Rietveld-analysis), the glass characterization was performed. Flexural strength (MOR), elastic modulus (E) and Vickers hardness (H_V) were measured and will be discussed in terms of glass composition and microstructure.     

Characteristics and surface energy of silicon-doped diamond-like carbon films fabricated by plasma immersion ion implantation and deposition

Diamond-like carbon (DLC) films doped with different silicon contents up to 11.48 at.% were fabricated by plasma immersion ion implantation and deposition (PIII-D) using a silicon cathodic arc plasma source. The surface chemical compositions and bonding configurations were determined by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results reveal that the sp³ configuration including Si–C bonds increases with higher silicon content, and oxygen incorporates more readily into the silicon and carbon interlinks on the surface of the more heavily silicon-doped DLC films. Contact angle measurements and calculations show that the Si-DLC films with higher silicon contents tend to be more hydrophilic and possess higher surface energy. The surface states obtained by silicon alloying and oxygen incorporation indicate increased silicon oxycarbide bonding states and sp³ bonding states on the surface, and it can be accounted for by the increased surface energy particularly the polar contribution.     

Mesoporous silicon nanocubes coated by nitrogen-doped carbon shell and wrapped by graphene for high performance lithium-ion battery anodes

Silicon materials have received widespread attention due to their inherent high theoretical specific capacity. However, large volumetric expansion and poor electrical conductivity hinder the large-scale application of silicon materials. To address these issues, we synthesize mesoporous silicon nanocubes coated by nitrogen-doped carbon shell (MSC@C) and wrapped by graphene (MSC@rGO) respectively. The ordered mesoporous silica nanocubes are obtained via a hydrolysis reaction of Tetraethyl Orthosilicate (TEOS) and further reduced by a magnesiothermic reduction to prepare mesoporous silicon nanocubes (MSC). The porous structure of MSC not only speeds up the transfer of ions and electrons, but also buffers the internal stress triggered by the volume expansion of the electrode material. Moreover, in addition to providing additional lithium storage sites and high conductivity, the graphene or nitrogen-doped carbon shell also effectively prevents aggregation and cracking of the mesoporous silicon, greatly promoting the stability of the entire electrode structure. Therefore, the electrochemical properties of composite materials are significantly enhanced by the combination of the mesoporous structure and the nitrogen-doped carbon shell or graphene. MSC@C can deliver the initial discharge specific capacity of 2852.7 mAh·g^?1 and the initial Coulombic efficiency (CE) of 83.74%. After 100 cycles, the MSC@C and MSC@rGO composite materials exhibit reversible specific capacities of 1070.5 mAh·g^?1 and 738.2 mAh·g^?1 at 0.1 A g^?1, respectively.     

Sandwich structure of carbon-coated silicon/carbon nanofiber anodes for lithium-ion batteries

For electrospun silicon/carbon nanofiber composites, the surface precipitation of silicon nanoparticles can cause poor cycle stability. To solve this, a carbon-coated silicon/carbon nanofiber (Si/C@C) composite with a ‘sandwich’ structure is constructed by hydrothermal reaction of glucose and an electrospun silicon/carbon nanofiber, followed by high-temperature carbonization. The effects of the thickness of the carbon coating layer and calcining temperature on the electrochemical performance are studied. The results showed that carbon is uniformly and continuously coated on the surface of the composite fibers, which avoid direct exposure of precipitated silicon on the surface of the nanofibers to the electrolyte, reduce the occurrence of side reactions and is conducive to the stable formation of SEI films. At the same time, the carbon shell inhibit the volume expansion of silicon to a certain extent and improve the conductivity of the composites. Consequently, the obtained Si/C@C exhibit good rate performance and cycle stability. With the optimised carbon coating thickness and calcination temperature, the obtained electrodes deliver a reversible capacity of 1120 and 683 mA h g^-1 at a current density of 0.1 and 2 A g^-1 respectively, and a specific capacity of 602 mAh∙g^-1 at a current density of 1 A g^-1 after 100 cycles, a capacity retention rate of 80%. The facilely synthesised Si/C@C composite shows potential applications in high-capacity silicon-based anode materials.     

Low-temperature reactive sintering of carbon vacant high-entropy carbide ceramics with in-situ formed silicon carbide

It is thought that the sintering of high-entropy (HE) ceramics is generally more difficult when compared to that of the corresponding single-component ceramics. In this paper, we report a novel approach to densify the HE carbide ceramics at relatively low temperatures with a small amount of silicon. Reactive spark plasma sintering (SPS) was used to densify the ceramics using powders of HE carbide and silicon as starting materials. Dense ceramics can be obtained at 1600 -1700°C. X-ray diffraction analysis reveals that only non-stoichiometric HE carbide phase with carbon vacancy and SiC phase exist in the obtained ceramics. The in-situ formed SiC phase inherits the morphology of the starting silicon powder owing to the slower diffusion of silicon atoms compared to that of the carbon atoms in HE carbide phase. The mechanical properties of the prepared ceramics were preliminarily studied.     

Bioactive layers based on black glasses on titanium substrates

Black glasses are amorphous materials based on silicon oxycarbide. The use of precursors in the form of ladder‐like silsesquioxanes enables the control of the amount of carbon ions in the glass network by adjusting ratios of T to D structural units in precursors. In this study, four different sols precursors of four different layers of black glasses on titanium substrates were prepared. The materials were analyzed with the use of various spectroscopic and microscopic methods. Formation of continuous and hermetic layers resistant to corrosion was proven. The black glasses layers were examined as materials for biomedical applications. Therefore, preliminary tests of their bioactivity and biocompatibility were performed. The best results were obtained for the material of lower contribution of carbon ions.     

SiCO ceramic microspheres produced by emulsion processing and pyrolysis of polysiloxanes of various structures

Crack-free silicon oxycarbide microspheres were synthesized from precursors obtained by a one-pot aqueous emulsion-process of modified polyhydromethylsiloxane. The process involved cross-linking by hydrosilylation and the advanced hydrolysis of polyhydromethylsiloxane SiH groups to SiOH. These species then participate in SiOH+SiH condensation, enhancing the cross-linking. The microspheres were additionally modified by SiH group-substitution in the initial polymer and by using various cross-linkers. The precursor powder particle structure was also modified by varying the stirring rate during emulsification. The modified preceramic microspheres, with average diameters from 7.6 to 56 µm, were subjected to pyrolytic processes at various temperatures. The chemical composition of the pyrolyzed microspheres and their precursors was studied by ²⁹Si and ¹³C MAS NMR, FTIR spectroscopy, and elemental analysis. The structures of the microspheres were examined by SEM. Selected samples were also investigated by XRD and Raman spectroscopy. All of the synthesized preceramic microspheres retained their regular spherical shapes during pyrolysis at temperatures of up to 1200 °C. Heating at 1000 °C and 1200 °C yielded amorphous silicon oxycarbide ceramic materials with segregated free carbon domains. The chemical structure and morphology of the obtained ceramic microspheres were significantly influenced by the modification of the preceramic materials.     

Effect of Ca and B incorporation into silicon oxycarbide on its microstructure and phase composition

Ca- and/or B-modified silicon oxycarbides were synthesized via pyrolysis of suitable polysilsesquioxane-based single-source precursors. Their polymer-to-ceramic transformation was investigated with thermogravimetric analysis, coupled with in situ evolved gas analysis. The prepared silicon oxycarbides were investigated with respect to their crystallization behavior, network architecture, and chemical compositions. The network connectivity in silicon oxycarbides can be affected/tuned upon using two different “tools”: (a) first, the use of network modifiers, such as Ca in our study, leads to a slight depolymerization of the network via generation of a small amount of Q³ sites; (b) second, the modification of silicon oxycarbide with B/Ca leads to a decrease of the carbon content in the network and thus to a significant decrease of its connectivity. Using these two different effects, the network connectivity in silicon oxycarbides can be finely tuned.     

Transformation of silicon nanoparticles on a carbon nanotube heater into hollow graphitic nanocapsules via silicon carbide

The changes in the structure and composition of silicon (Si) nanoparticles supported on a multiwall carbon nanotube (MWCNT) during Joule heating of the MWCNT were studied by in situ high-resolution transmission electron microscopy. The Si nanoparticles reacted with the outer layers of the MWCNT to form silicon carbide (SiC) nanoparticles with increasing temperature. At temperatures of up to approximately 1900 K, silicon atoms were entirely sublimated from the SiC nanoparticles, and the remaining carbon atoms formed hollow carbon nanocapsules consisting of multilayered graphene shells on the MWCNT.     

Pyrolysis Kinetics for the Conversion of a Polymer into an Amorphous Silicon Oxycarbide Ceramic

We present experimental and analytical results for the pyrolysis reactions underlying the conversion of a cross-linked polymer into an amorphous ceramic material. The activation energies, obtained from thermogravimetric data, and chemical analysis of the volatiles by mass spectroscopy are used to identify the reaction pathways. The reaction is determined to be first-order, which is consistent with its solid-state nature. The magnitude of the weight loss is analyzed to calculate the number of molecular sites in the polymer that participate in the reaction. The experiments were conducted on a polymer made from silsesquioxanes that convert into silicon oxycarbide ceramics on pyrolysis. The results show that <2.5% of the silicon atoms in the polymer are removed as volatile silanes, and less than one-half of the carbon atoms are lost as methane. These results are a first step in understanding the molecular basis for the ceramic yield, as well as the evolution of the nanostructure as the material changes from an organic into a ceramic state by reactions that can occur at <850°C.     

Mass production of three-dimensional hierarchical microfibers constructed from silicon–carbon core–shell architectures with high-performance lithium storage

In this work, a facile approach is reported to mass produce highly porous fibers constructed from silicon–carbon core–shell structures. The C–Si microfibers are prepared using a modified electrospinning deposition method (ESD), and subsequent calcination of the carbon shells. Benefited from the step of vacuum drying, the unnecessary solvent left in the precursor will volatilize, resulting in the uniform three-dimensional hierarchical microfibers constructed from silicon–carbon core–shell architectures. The uniform covering layers of carbon formed by decomposition of polymer contribute to the improvement of conductivity and alleviation of volume change. The pores in the microfibers are helpful for the diffusion of electrolyte. When evaluated as an anode material for lithium-ion batteries, the C–Si microfibers exhibit improved reversibility and cycling performance compared with the commercial Si nanoparticles. A high capacity of 860 mAh g⁻¹ can be retained after 200 cycles at a current rate of 0.3 C. The rate capability of the C–Si microfibers is also improved. The special structure is believed to offer better structural stability upon prolonged cycling and to improve the conductivity of the material. This simple strategy using the modified ESD method could also be applied to prepare other porous energy materials.     

Infiltration of macroporous carbon materials with silicon oxycarbide modified by carbon nanotubes

Two types of carbon, i.e. carbon electrode (CE) and carbon furnace lining (CFL) were modified with silicon oxycarbide (SiOC) or carbon nanotubes-containing SiOC (SiOC/fCNT) by means of polysiloxane impregnation and pyrolysis. The two carbon materials differed in pore size and in surface chemical state. The impact of these factors on the infiltration efficiency was investigated by comparing the physical, electrical and mechanical properties in samples before and after infiltration. It was shown that SiOC phase is formed in the CE macropores, leading to a reduction in the average pore size from 8.2 to 5.3 μm, and in porosity from 12.6 to 7.3%. For CFL, which contains both meso- and macropores, the observed changes in porosity are smaller. Introducing fCNT into the resin changes its surface nature from hydrophobic to hydrophilic. This modified solution better wets a CE surface containing functional groups and provides an enhanced interface contact between this carbon and SiOC. The fCNT-modified SiOC phase improves the compressive strength and modulus of both types of carbon. The electrical resistivity of CE modified with SiOC/fCNT is slightly higher, whereas for CFL it does not change. Oxidation tests in air up to 1000 °C showed a significant reduction in the mass loss of both carbon materials after their modification with pure SiOC and SiOC/fCNT. The proposed infiltration procedure can be applied to conventional carbon and graphite technology, in particular to porous carbons containing a macropore fraction.     

Electrical and thermal response of silicon oxycarbide materials obtained by spark plasma sintering

In order to obtain dense silicon oxycarbide (SiOC) materials that maintain the properties of glass, non-conventional spark plasma sintering was used to sinter SiOC powders from 1300 to 1700 °C and with 40 MPa of pressure. The concurrence of electrical current, high pressure and low vacuum while the material is being heating produces a dense SiOC-derived material composed of a SiO₂ glassy matrix reinforced with SiC nanowires grown in situ, graphene-like carbon and turbostratic graphite. SiOC materials with high electrical and thermal response are obtained as a result of this new processing technique. Electrical resistivity undergoes an extraordinary decrease of five orders of magnitude from 1300 (1.0 × 10⁵ Ω m) to 1700 °C (0.78 Ω m), ranging from insulate to semiconductor material; and thermal conductivity increases by 30%, for these sintering temperatures.     

Analyzing the effect of composition,density, and the morphology of the “free” carbon phase on elastic moduli in silicon oxycarbide ceramics

We perform atomistic simulations to model structures and calculate elastic properties of silicon oxycarbide ceramics. We explore individual parameters – composition, density, carbon content – to disentangle mutual dependencies that are difficult to separate in experimental studies. Each parameter is studied through dynamic simulations at finite temperatures for a wide range of temperatures. With multi-million atom models in simulation boxes as large as 40 nm, we reveal a hitherto “hidden” parameter: the morphology of the “free” carbon phase. Embedding, distribution, and interconnection of the carbon phase inside the amorphous matrix of SiCO severely impact the material's mechanical properties. As a consequence, we call for the development of new characterization techniques that will quantify the morphology of carbon in this and similar systems.     

The conformal silicon deposition on carbon nanotubes as enabled by hydrogenated carbon coatings for synthesis of carbon/silicon core/shell heterostructure photodiodes

One-dimensional heterostructures, based on functionalities of dissimilar materials within a monolithic structure, are promising building blocks for different applications. Herein, utilizing surface decoration of multiwalled carbon nanotubes (MWCNTs) with hydrogenated graphitic carbon layers (HGCLs), the realization of a vertically aligned MWCNT/amorphous-silicon (a-Si) core/shell heterostructure is reported. The proposed method enables the formation of conformal, continuous, and all-around silicon deposition on the carbon nanotube and eliminates any signature of line-of-sight deposition problem, even for thicknesses as low as a few nanometers. Precise elaboration using comparative Raman analysis reveals that the HGCLs play a major role in the construction of such structures. Evidence of direct binding between Si and C, a missing remarkable feature in previous reports, has been observed in high-resolution transmission electron microscope images and X-ray photoelectron spectroscopy. Monitoring time evolution during the formation of the silicon shell declares a diffusive mechanism for the deposition of Si on the surface of MWCNTs. Furthermore, the electro-optical proficiency of the MWCNT/a-Si heterostructure was studied by fabrication of a photodiode. Unlike previous attempts, a naturally formed Schottky junction at the high-quality a-Si/nanotube interface is exploited for charge separation in this photodiode, which provides a sensitivity of >10⁷% in the reverse saturation current for a wavelength of λ = 405 nm.     

Characterization of silicon- and carbon-based composite anodes for lithium-ion batteries

In recent years development of active materials for negative electrodes has been of great interest. Special attention has been focused on the active materials possessing higher reversible capacity than that of conventional graphite. In the present work the electrochemical performance of some carbon/silicon-based materials has been analyzed. For this purpose various silicon-based composites were prepared using such carbon materials as graphite, hard carbon and graphitized carbon black. An analysis of charging-discharging processes at electrodes based on different carbon materials has shown that graphite modified with silicon is the most promising anode material. It has also been revealed that the irreversible capacity mainly depends on the content of Si. An optimum content of Si has been determined with taking into account that high irreversible capacity is not suitable for practical application in lithium-ion batteries. This content falls within the range of 8-10 wt%.The reversible capacity of graphite modified with 8 wt% carbon-coated Si was as high as 604 mAh g⁻¹. The irreversible capacity loss with this material was as low as 8.1%. The small irreversible capacity of the material allowed developing full lithium-ion rechargeable cells in the 2016 coin cell configuration. Lithium-ion batteries based on graphite modified with silicon show gravimetric and volumetric specific energy densities which are higher by approximately 20% than those for a lithium-ion battery based on natural graphite.     

Poly(urethanes) containing silarylene and/or germarylene units

A series of 10 poly(urethanes) were synthesized by solution polymerization from bis(chloroformates) and aromatic diamines, containing both silicon or germanium as central atom. So, the polymers prepared contain two silicon atoms or two germanium atoms exclusively or combinations of both. Me, Et, and Ph groups were bonded to the central atoms according to the nature of the monomers employed. Poly(urethanes) were characterized by FTIR, ¹H, ¹³C, and ²⁹Si NMR spectroscopy and the results agreed with the proposed structures. Additionally, intrinsic viscosity values were established in DMSO solutions and thermal analyses were developed. In all cases, thermostable oligomers were obtained, which showed a degradation process beginning at ∼240–260°C. Polymers showed a thermal dependence with the nature of the heteroatom employed. Thus, in general, when germanium was used as central atom, the thermal stability was higher than the polymers containing silicon which agrees with the lower polarity and higher energy of the C Ge bond in comparison with the C Si one. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008.     

The use of plastic waste as carbon raw materials to obtain TiC-based powders

Plastic pollution is one of the most pressing environmental problems, and a huge amount of effort and money is directed towards solving it. The existing processing methods are ineffective. The main component of all plastic materials (C_xH_y) is carbon. High-purity fine titanium carbide was obtained using plastic waste (polyethylene terephthalate – C₁₀H₈O₄) as a carbon raw material. Combustion processes, phase composition and structure of the obtained materials were studied. A probable mechanism for the formation of titanium carbide during the combustion of the (Ti + C₁₀H₈O₄) mixture was proposed. During the synthesis, polyethylene terephthalate decomposes into carbon, hydrogen and oxygen. Carbon and oxygen react with titanium and form titanium oxycarbide. Titanium oxycarbide is saturated with carbon to form titanium carbide and carbon dioxide. The remaining hydrogen and carbon dioxide are released from the synthesis products, which leads to self-purification of the synthesis products. The obtained results will create the basis for the development of a fundamentally new, cost-effective technology for recycling plastic waste into carbides and carbide-containing materials.