The influence of carbon atom distribution in precursors with 1:1 carbon to silicon atoms ratio on structure and thermal evolution of obtained silicon oxycarbide based materials

In this work, a number of precursors with 1:1 silicon to carbon atoms ratio and various carbon atom distributions were synthesized and pyrolyzed in order to obtain silicon oxycarbide based materials. The different carbon atom distributions were obtained using both simple monomers with only one silicon atom, as well as large monomers containing either four or sixteen silicon atoms with predefined carbon atom positions. The silicon oxycarbide based materials were investigated using IR, XRD, ²⁹Si MAS NMR and elemental analysis after annealing at various temperatures, as well as TG. The research shows that carbon atom distribution has great impact on the structure of final material and can be used to tailor the material for its projected uses.     

Compressive thermal stress and microstructure-driven charge carrier transport in silicon oxycarbide thin films

This work correlates the charge carrier transport mechanism of silicon oxycarbide-based thin films with their morphology and thermal stress. Segregation of highly-graphitized carbon-rich, oxygen-depleted C/SiC areas homogeneously dispersed within an oxygen-rich C/SiOC matrix was seen on the 500 nm-SiOC thin films. Compressive biaxial stress induced by the mismatch with the Si-substrate thermal expansion coefficient was calculated at 109 MPa. Through Hall measurements, p-type carriers were shown dominating the SiOC film similar to monolithic samples. Thin films and monoliths have comparable carrier concentrations while the carrier mobility in SiOC thin films was 2 magnitudes higher than that of monolithic samples and is considered a consequence of the compressive thermal stress acting on the film. Improved conductivity of 16 S cm -1 is measured for the SiOC thin film sample which is assumed considering the enhanced carrier mobility alongside the reduced percolation threshold ascribed to the phase-separated morphology of the thin film.     

High-temperature behavior of silicon oxycarbide glasses in air environment

Silicon oxycarbide glasses (SiOC) have been produced by siloxane resin under flowing argon at 1000 °C and then their evolutions in air from 800 to 1700 °C were investigated. Those glasses annealed at various temperatures were characterized by X-ray diffraction, ²⁹Si MAS NMR, Raman spectroscopy, and chemical element analysis. It can be found that oxidation reactions of the SiOC glasses occurred at above 1000 °C; carbothermal reduction was indiscernible at temperature below 1600 °C but almost finished at 1700 °C; and the decomposition of SiO_xC_4−x network was complete at 1400 °C.     

In-situ formation of titanium carbide in carbon-rich silicon oxycarbide ceramic for enhanced thermal stability

Silicon oxycarbide (SiOC) ceramic has attracted great attention as fascinating candidate of high-temperature material, however, its thermal stability is significantly limited by the phase separation at high temperature. Here, a TiC/SiOC ceramic was prepared by pyrolysis of a tetrabutyl titanate modified carbon-rich polysiloxane (TBT/PSO) precursor. The TiC phase is in-situ formed by the carbothermal reaction of TBT-derived amorphous TiO₂ phase with excess free-carbon phase during pyrolysis, and its size and amount increase with the pyrolysis temperature. The SiC phase appears at a higher temperature than the TiC phase and is hindered by the increased Ti content in the TBT/PSO precursor. Thus, the TiC/SiOC ceramic exhibits better thermal stability and crystallization resistance than the TiC-free SiOC ceramic under the thermal treatment (1500 °C) in argon atmosphere. The in-situ formation of metal carbide into the carbon-rich SiOC ceramic would further expand its application at high temperature environments.     

Texture and micro-nanostructure of porous silicon oxycarbide glasses prepared from hybrid materials aged in different solvents

The influence of the aging conditions of the preceramic hybrid material on the microstructure of silicon oxycarbide (SiOC) glasses derived therefrom has been highlighted. The textural and structural properties of the glasses are modified by aging the hybrid precursor in different environments. Three solvents have been employed as aging media to produce macroporous SiOC ceramics with porosities in the range between 30 and 70 vol.%.It has been concluded that the polarity and chemical characteristics of the solvent plays an important role on the surface characteristics and structure of the obtained SiOC glass. Raman spectroscopy and Small Angle X-ray scattering reveal the presence of different nanodomain sizes depending on the polymeric fraction in the preceramic network. The free carbon phase developed during the hybrid-to-ceramic conversion turn out to have a high influence on the growth of the silica nanodomains and thus on the nanostructure of the obtained ceramic.     

Structure and crystallization behavior of Nylon 66/multi-walled carbon nanotube nanocomposites at low carbon nanotube contents

Multi-walled carbon nanotubes (MWNTs) were modified with poly(hexamethylene adipamide) (also known as Nylon 66) via a controlled polymer solution crystallization method. A “nanohybrid shish kebab” (NHSK) structure was found wherein the MWNT resembled the shish while Nylon 66 lamellar crystals formed the kebabs. These Nylon 66-functionalized MWNTs were used as precursors to prepare polymer/MWNT nanocomposites. Excellent dispersion was revealed by optical and electron microscopies. Nitric acid etching of the nanocomposites showed that MWNT formed a robust network in Nylon 66. Non-isothermal DSC results showed multiple melting peaks, which can be attributed to lamellar thickness changes upon heating. The crystallite sizes L₁₀₀ and L₀₁₀ of Nylon 66, determined by WAXD, decreased with increasing MWNT contents. Isothermal DSC results showed that crystallization kinetics increased first and then decreased with increasing MWNT contents in Nylon 66. This study showed that the effect of MWNTs on Nylon 66 crystallization is twofold: MWNTs provide heterogeneous nucleation sites for Nylon 66 crystallization while the tube network structure hinders large crystal growth.     

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.     

The preparation of multi-walled carbon nanotubes with a Ni-P coating by an electroless deposition process

The multi-walled carbon nanotubes (MWCNTs, mean diameter: 100-200 nm) with nickel-phosphorous (Ni-P) coatings were obtained by an electroless deposition process. To prepare the MWCNTs covered with continual Ni-P layers, a pre-treatment procedure comprised of acid-cleaning, sensitization and activation has been developed. The resulting MWCNTs have a uniform distribution of the Ni-P layers coated on the MWCNTs with the fibrous appearance maintained. X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations revealed that the as-coated MWCNTs were comprised of the amorphous Ni-P layers and inner carbon nanotubes covered with the Ni-P layers. These amorphous Ni-P-coated MWCNTs were used as precursors for preparing MWCNTs with nanocrystalline Ni-P(crystalline Ni and Ni₃P intermetallic compound) layers by the heat-treatment above 400 °C, which were determined by differential scanning calorimeter (DSC) and XRD studies. The results of this work provide an effective electrochemical method for preparing powdery MWCNTs with Ni-P layer as new composite materials from the aqueous solution.     

Effects of multi-walled carbon nanotubes on the crystallization behavior of PDCs-SiBCN and their improved dielectric and EM absorbing properties

Polymer derived siliconboron carbonitride ceramics (PDCs-SiBCN) containing multi-walled carbon nanotubes (MWCNTs-SiBCN) were fabricated and heat treated at 1350 °C and 1500 °C in nitrogen atmosphere. XRD patterns show the characteristic peaks of SiC appeared in MWCNTs-SiBCN treated at 1500 °C, which is 300 °C lower than the crystallization temperature of SiC in PDCs-SiBCN. The decrease of temperature can be ascribed to the heterogeneous nucleation promoted by MWCNTs as nucleating agent. Energy change during heterogeneous nucleation is analyzed to explain the acceleration of crystallization. The dielectric and electromagnetic (EM) absorbing properties of the as-prepared MWCNTs-SiBCN are investigated, which show a better wave-absorbing ability than PDCs-SiBCN treated at the same temperature, owing to a distinctive A + B + C microstructure in MWCNTs-SiBCN. This work makes it possible to fabricate the PDCs-SiBCN with good EM absorbing property at a lower temperature, granting them potential as a matrix material candidate in ceramic matrix composite field.     

Patterned growth of carbon nanotubes over vertically aligned silicon nanowire bundles for achieving uniform field emission

A fabrication strategy is proposed to enable precise coverage of as-grown carbon nanotube (CNT) mats atop vertically aligned silicon nanowire (VA-SiNW) bundles in order to realize a uniform bundle array of CNT-SiNW heterojunctions over a large sample area. No obvious electrical degradation of as-fabricated SiNWs is observed according to the measured current-voltage characteristic of a two-terminal single-nanowire device. Bundle arrangement of CNT-SiNW heterojunctions is optimized to relax the electrostatic screening effect and to maximize the field enhancement factor. As a result, superior field emission performance and relatively stable emission current over 12 h is obtained. A bright and uniform fluorescent radiation is observed from CNT-SiNW-based field emitters regardless of its bundle periodicity, verifying the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.     

Titania-doped multi-walled carbon nanotubes epoxy composites: Enhanced dispersion and synergistic effects in multiphase nanocomposites

Multi-walled carbon nanotube-epoxy composites are modified with titania nanoparticles in order to obtain multiphase nanocomposites with an enhanced dispersion of carbon nanotubes. The dispersion is monitored using rheological and electrical conductivity measurements. An increase in dispersion quality can be correlated to an increased storage shear modulus of the uncured suspensions and to a decreased electrical conductivity in the bulk nanocomposite. The newly formed microstructure is revealed using transmission electron and optical microscopies. Due to chemical interactions between both types of nanoparticles an attractive potential is generated leading to a significant rearrangement in the particle network structure. Besides an enhanced dispersion, the hybrid structure leads to synergistic effects in terms of the glass transition of the nanocomposites. Although a decrease of the glass transition temperature (T_g) is observed for the nanocomposites containing only one type of filler, the combination of titania and carbon nanotubes into a hybrid structure reduces the decrease of T_g, thus demonstrating the potential of such hybrid structures as fillers for multi-functional epoxy nanocomposites.     

Ablation resistant ZrC coating modified by polymer-derived SiC/TiC nanocomposites for ultra-high temperature application

To improve the ablation resistance of ZrC coating on SiC-coated carbon/carbon composites above 2000 °C, SiC/TiC nanocomposites (SiC/TiC-NCs) powders derived from single-source precursor were incorporated into ZrC coating, denoted as ZrC-SiC/TiC-NCs, via supersonic atmospheric plasma spraying (SAPS). After SAPS, the incorporated SiC/TiC-NCs evolved into TiC/(SiC and Zr_xTi_yC) embedded in amorphous SiC. The ablation resistance of the ZrC-SiC/TiC-NCs coating was evaluated by oxyacetylene flames with a heat flux of 4.18 MW/m². For comparison, the ZrC-SiC-NCs coating without Ti modification was seriously damaged due to rapid gas denudation. The good ablation resistance of ZrC-SiC/TiC-NCs coating is mainly attributed to the distinctive “capsule-like” multi-crystalline microstructure of SiC/TiC-NCs. During ablation, TiO₂ and Zr_xTi_yO₂, due to the oxidation of TiC and Zr_xTi_yC, contributed to the formation of Zr-Ti-Si-O glass with high viscosity and low evaporation pressure, improving the ablation resistance.     

In situ electropolymerization of conducting polypyrrole/carbon nanotubes composites on stainless steel: Role of carbon nanotubes types

In situ electropolymerization was used to prepare polypyrrole-oxidized multi-walled carbon nanotubes and polypyrrole-oxidized single-walled carbon nanotubes composites on a stainless steel surface from 0.1 M oxalic acid by using cyclic voltammetry. The electropolymerization process was investigated and discussed, and the results showed that the addition of the oxidized carbon nanotubes greatly enhanced the electropolymerization process, especially in the case of oxidized single walled carbon nanotubes. The results also showed that increasing the pyrrole monomer concentration leads to increasing the amount of polypyrrole electrodeposited, and this is more pronounced in the presence of the carbon nanotubes. The electropolymerization process was mainly under diffusion control as the process was inhibited by increasing the scan rate. In general, the presence of oxidized carbon nanotubes improved the electropolymerization of the polypyrrole and greatly enhanced its thermal and morphological properties.     

Different cellular response mechanisms contribute to the length-dependent cytotoxicity of multi-walled carbon nanotubes

To date, there has not been an agreement on the best methods for the characterisation of multi-walled carbon nanotube (MWCNT) toxicity. The length of MWCNTs has been identified as a factor in in vitro and in vivo studies, in addition to their purity and biocompatible coating. Another unresolved issue relates to the variable toxicity of MWCNTs on different cell types. The present study addressed the effects of MWCNTs'' length on mammalian immune and epithelial cancer cells RAW264.7 and MCF-7, respectively. Our data confirm that MWCNTs induce cytotoxicity in a length- and cell type-dependent manner. Whereas, longer (3 to 14 μm) MWCNTs exert high toxicity, especially to RAW264.7 cells, shorter (1.5 μm) MWCNTs are significantly less cytotoxic. These findings confirm that the degree of biocompatibility of MWCNTs is closely related to their length and that immune cells appear to be more susceptible to damage by MWCNTs. Our study also indicates that MWCNT nanotoxicity should be analysed for various components of cellular response, and cytotoxicity data should be validated by the use of more than one assay system. Results from chromogenic-based assays should be confirmed by trypan blue exclusion.     

Unique nucleation of multi-walled carbon nanotube and poly(ethylene 2,6-naphthalate) nanocomposites during non-isothermal crystallization

Multi-walled carbon nanotube (MWCNT) and poly(ethylene 2,6-naphthalate) (PEN) nanocomposites are prepared by a melt blending process. There are significant dependence of non-isothermal crystallization behavior and kinetics of PEN/MWCNT nanocomposites on the MWCNT content and cooling rate. The incorporation of MWCNT accelerates the mechanism of nucleation and crystal growth of PEN, and this effect is more pronounced at lower MWCNT content. Combined Avrami and Ozawa analysis is found to be effective in describing the non-isothermal crystallization of the PEN/MWCNT nanocomposites. The MWCNT in the PEN/MWCNT nanocomposites exhibits much higher nucleation activity than any nano-scaled reinforcement. When a vary small quantity of MWCNT was added, the activation energy for crystallization is lower, then gradually increased, and becomes higher than that of pure PEN above 1.0 wt% MWCNT content. The incorporation of MWCNT improves the storage modulus and loss modulus of PEN/MWCNT nanocomposites.     

Evolution of nanoporosity and electrochemical behavior in organosilicon polymer derived carbon hybrids

Nanoporous carbon hybrids with high specific surface area and pore volume have been prepared from inexpensive commercial precursors, such as nanocarbon and organosilicon polymers. The synthesized carbon hybrids were found to possess specific surface area from 916 to 1798 m² g^?1, pore volume in the range of 0.5–1.2 cc g^?1, and micropore volume up to 0.804 cc g^?1. Cyclic voltammetry in aqueous electrolyte indicated ideal supercapacitive behavior for certain samples. Specific capacitance in the range of 176–333 F g^?1for a moderate voltage scan rate of 20 mV s^?1 was observed for the carbon hybrids. The article explores a simple method for the fabrication of novel carbon hybrids with excellent porosity control and pore volume. The process can open new avenues for the fabrication of a series of novel carbon hybrids, where pore dimensions and specific surface area can be engineered with the careful selection of organosilicon polymers and process conditions.     

Sulfonated multi-walled carbon nanotubes for the removal of copper (II) from aqueous solutions

Sulfonated multi-walled carbon nanotubes (s-MWCNTs) was prepared from purified multi-walled carbon nanotubes (p-MWCNTs) by concentrated H₂SO₄ at elevated temperature. The structure was characterized by SEM, FTIR, Raman, XPS, and BET. It could be dispersed steadily in water at a dosage of 1.0 mg/mL for a week. The adsorption performance of s-MWCNTs toward Cu(II) was investigated including the effects of pH and ionic strength. Results indicated the adsorption was much dependent on pH but not on ionic strength. The adsorption capacity for Cu(II) was enhanced 58.9% via the sulfonation. Moreover, the adsorption mechanisms were carefully analyzed by Freundlich and D-R models.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Decorated carbon nanotubes by silicon deposition in fluidized bed for Li-ion battery anodes

Multi-walled carbon nanotubes Graphistrength^® were decorated with silicon by Fluidized Bed Chemical Vapor Deposition. The ability to fluidize of these nanotubes forming ball-shaped jumbles of several hundreds of microns in diameter and that of the final CNT-Si balls was first studied. These balls reveal to fluidize with characteristics of Geldart's group A particles, i.e. without bubbles and with high bed expansion. Coating experiments from silane SiH₄ were performed at 500 °C in the 30–60 wt.% range of silicon deposited. SEM and TEM imaging reveals that the nanotubes are coated by silicon nanoparticles uniformly distributed from the periphery to the center of the balls for the whole conditions tested. On-line acquisition of key process parameters evolution shows that the material remains fluidizable, even for large proportions of silicon deposited. The Sauter diameter and the tapped, untapped and skeleton densities of balls increase with the percentage of silicon deposited, whereas their specific surface area decreases due to the progressive filling of the pores by the deposit. This composite material is a promising candidate as anode to replace graphite in lithium-ion batteries.