Optical transmittance and electrical conductivity of silica glass with biserial and hierarchical network structures made of carbon nanofibers

Silica glass composites, with biserial and hierarchical percolative network made of carbon nanofibers (CNFs), was fabricated using a layer-by-layer technique and spark plasma sintering to obtain high optical transmittance and electrical conductivity. Owing to the network, the critical volume fraction, V_c, for the CNF percolation in the silica glass-matrix composite (0.5–0.7 vol%), when the electrical conductivity of the composite drastically increased with change from insulator (～10^?10 S/m) to conductor (～10^?1 S/m), is smaller than theoretical V_c predicted for the three-dimensional random orientation of CNFs (2.6 vol% for the CNF aspect ratio of 30). The conductivity of the composite with above the V_c of CNFs (～10 S/m) is higher than that reported for the polymer-matrix composite (～10^?5–～10^?3 S/m). Furthermore, high optical transmittance was observed for the electrically conductive composite with V_c of CNFs.     

Desirable electrical and mechanical properties of continuous hybrid nano-scale carbon fibers containing highly aligned multi-walled carbon nanotubes

The continuous highly aligned hybrid carbon nanofibers (CNFs) with different content of acid-oxidized multi-walled carbon nanotubes (MWCNTs) were fabricated through electrospinning of polyacrylonitrile (PAN) followed by a series of heat treatments under tensile force. The effects of MWCNTs on the micro-morphology, the degree of orientation and ordered crystalline structure of the resulting nanofibers were analyzed quantitatively by diversified structural characterization techniques. The orientation of PAN molecule chains and the graphitization degree in carbonized nanofibers were distinctly improved through the addition of MWCNTs. The electrical conductivity of the hybrid CNFs with 3 wt% MWCNTs reached 26 S/cm along the fiber direction due to the ordered alignment of MWCNTs and nanofibers. The reinforcing effect of hybrid CNFs in epoxy composites was also revealed. An enhancement of 46.3% in Young’s modulus of epoxy composites was manifested by adding 5 wt% hybrid CNFs mentioned above. At the same time, the storage modulus of hybrid CNF/epoxy composites was significantly higher than that of pristine epoxy and CNF/epoxy composites not containing MWCNTs, and the performance gap became greater under the high temperature regions. It is believed that such a continuous hybrid CNF can be used as effective multifunctional reinforcement in polymer matrix composites.     

Multifunctional carbon nanofiber-reinforced Ge25Sb10S65 chalcogenide glassy composites

Carbon nanomaterials have wide applications in sensors, batteries, electromagnetic shielding, and mechanical reinforcement. Here, carbon nanofiber (CNF)-reinforced Ge₂₅Sb₁₀S₆₅ chalcogenide glassy composites with excellent mechanical and electrical properties were obtained. These glassy composites maintained the amorphous properties of glass. Thermodynamic parameters, microscopic morphology, and structural characteristics were further studied. Benefiting from the remarkable high strength and conductivity of CNFs, as well as the great interface connection between CNFs and glass, the electrical and mechanical properties of glassy composites were greatly enhanced. The Vickers hardness improved by 36% (from 200 kg/mm² to 272 kg/mm²), the tensile modulus increased from 45.9 GPa to 57 GPa, and the shear modulus increased from 22.2 GPa to 23.7 GPa when the CNF concentration increased from 0 wt% to 3.0 wt%. Furthermore, DC conductivity was raised by several orders of magnitude compared with bulk glass at 293 K (from 4.55 × 10⁻¹⁰ S/cm to 3.15 × 10⁻⁴ S/cm) owing to the formation of a continuous conductive network. Thus, these CNF-reinforced glassy composites provide a new way for realizing multifunctional composites.     

Biomimetic synthesis and characterization of carbon nanofiber/hydroxyapatite composite scaffolds

Three dimensional electrospun carbon nanofiber (CNF)/hydroxyapatite (HAp) composites were biomimetically synthesized in simulated body fluid (SBF). The CNFs with diameter of ∼250 nm were first fabricated from electrospun polyacrylonitrile precursor nanofibers by stabilization at 280 °C for 2 h, followed by carbonization at 1200 °C. The morphology, structure and water contact angle (WCA) of the CNFs and CNF/HAp composites were characterized. The pristine CNFs were hydrophobic with a WCA of 139.6°, resulting in the HAp growth only on the very outer layer fibers of the CNF mat. Treatment in NaOH aq. solutions introduced carboxylic groups onto the CNFs surfaces, and hence making the CNFs hydrophilic. In the SBF, the surface activated CNFs bonded with Ca²⁺ to form nuclei, which then easily induced the growth of HAp crystals on the CNFs throughout the CNF mat. The fracture strength of the CNF/HAp composite with a CNF content of 41.3% reached 67.3 MPa. Such CNF/HAp composites with strong interfacial bondings and high mechanical strength can be potentially useful in the field of bone tissue engineering.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Effect of amine functionalization of CNF on electrical, thermal, and mechanical properties of epoxy/CNF composites

This paper presents experimental results of the effect of amine functionalization of carbon nanofibers (CNF) on the electrical, thermal, and mechanical properties of CNF/epoxy composites. The functionalized and non-functionalized CNFs (up to 3 wt%) were dispersed into epoxy using twin screw extruder. The specimens were characterized for electrical resistivities, thermal conductivity (K), UTS, and Vicker’s microhardness. The properties of the nanocomposites were compared with that of neat epoxy. The volume conductivity of the specimens increased by E12 S/cm and E09 S/cm in f-CNF/epoxy and CNF/epoxy, respectively, at 3 wt% filler loading. The increase in K for former was 106% at 150 °C, while for the latter it was only 64%. Similarly, UTS increased by 61% vs. 45% and hardness 65% vs. 43%. T _g increased with increase in filler content. SEM examinations showed that functionalization resulted in better dispersion of the nanofibers and hence greater improvement in the studied properties of the nanocomposites.     

Piezoresistive effect of individual electrospun carbon nanofibers for strain sensing

Piezoresistive behavior of individual electrospun carbon nanofibers (CNF) was studied for the first time via a microelectromechanical systems platform. The gage factor of CNFs was found to vary from 1.96 to 2.55, not correlating with nanofiber diameter. The measured strain sensitivity of electrical resistance of individual CNFs could not be solely explained based on strain induced dimensional changes of CNFs, pointing to piezoresistivity in nanofibers. The microstructure of CNFs was studied via TEM imaging and Raman spectroscopy, suggesting the presence of sp² and sp³ hybridized carbon atoms in CNFs. The piezoresistivity of CNFs was explained in light of their hybrid structure. A one-dimensional model was adopted to relate CNFs piezoresistivity to their microstructure and electron tunneling between sp² hybridized regions through sp³ hybridized regions. The calibrated model revealed tunneling distances of 0.15–0.3 nm between sp² hybridized atoms. Moreover, our study pointed to the degree of graphitization and elastic mismatch between differently hybridized carbon atom regions in CNFs as critical parameters controlling CNFs’ piezoresistivity. This study sets the stage for the utilization of CNFs, not just as load bearing elements, but also as multifunctional nanoscale components with strain sensing capabilities, for instance in Nanoelectro-mechanical systems applications.     

Tribological and electrical properties of ceramic matrix composites with carbon nanotubes

Tribological behaviur of carbon fibrous phases (nanofibers and nanotubes) containing composites with Si₃N₄, ZrO₂ and Al₂O₃ matrices was studied by pin-on-disk technique in conditions of dry sliding. Coefficients of friction and wear rates were measured, wear damage mechanisms were observed and identified. The resulting tribological behaviur was related to microstructure and mechanical properties of respective materials. Electrical conductivity was measured in wide range of frequencies by two-point method and effect of volume fraction and distribution of CNTs and CNFs on percolation threshold was evaluated. Both coefficient of friction and electrical resistivity decreased with increasing amount of carbon phases, in both cases the nanofibers were more efficient than the nanotubes. The wear resistance in most cases decreased but for Si₃N₄–CNT composite a certain optimum (～5 wt.% CNT) was found.     

Dielectric properties of carbon nanofibre/alumina composites

Carbon nanofibre (CNF)/Al₂O₃ composites with concentrations between 1 and 9 vol.% of CNF were prepared by the traditional ceramic processing route followed by spark plasma sintering. The dielectric properties of these composites have been studied in a broad frequency range from mHz to the infrared range. Unlike conventional composites, the percolation threshold in this system is more complex depending on the particles topology. Positive and negative variations by several orders of magnitude in the low frequency AC conductivity have been detected for concentrations near the threshold at ∼2 vol.% of CNF. To explain these results, a modified percolation model has been proposed which takes into consideration the effect of the concentration of the filler on the microstructure of the composite.     

Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

Mechanical and electrical properties of carbon nanofibers reinforced aluminum nitride composites prepared by plasma activated sintering

High density carbon nanofibers (CNFs) reinforced aluminum nitride (AlN) composites were successfully fabricated by plasma activated sintering (PAS) method. The effects of CNFs on the microstructure, mechanical and electrical properties of the AlN composites were investigated. The experimental results showed that the grain growth of AlN was significantly inhibited by the CNFs. With 2 wt.% CNFs added into the composites, the fracture toughness and flexural strength were increased, respectively to 5.03 MPa m^1/2 and 354 MPa, which were 20.9% and 13.4% higher than those of monolithic AlN. The main toughening mechanisms were CNFs pullout and bridging, and the main reason for the improvements in strength should be the fine-grain-size effect caused by the CNFs. The DC conductivity of the composites was effectively enhanced through the addition of CNFs, and showed a typical percolation behavior with a very low percolation threshold at the CNFs content of about 0.93 wt.% (1.51 vol.%).     

Characterization of carbon nanofiber composites synthesized by shaping process

Catalytically grown carbon nanofibers (CNFs) are shaped into pellets in desired size and configuration by a conventional molding process so as to extend the potential applications of CNFs in industrial heterogeneous catalysis. After shaping, a novel carbon nanofiber composite with sufficient mechanical strength is produced, in which isolated CNFs are connected by a carbon network formed through polymer binder carbonization. Characterization of the synthesized CNF composite is performed by using HRTEM, XRD, Raman, N₂ physisorption, TPD and TGA. A comparison of the textural and structural properties, as well as the surface chemistry is made amongst the CNFs, the CNF composite, and a commercial coal-based activated carbon, in order to attain a comprehensive understanding of the CNF composite. The results show that the CNF composite preserves the mesoporous texture of the CNFs which will be beneficial to those reactions of mass transfer control. The modification effect of oxidative treatments on physico-chemical properties of the CNF composite is also investigated. More surface oxygen-containing groups are introduced to the composite by treating the material either in boiling HNO₃ solution or in static air at 400 °C.     

Highly dispersed and electrically conductive polycarbonate/oxidized carbon nanofiber composites for electrostatic dissipation applications

The influence of low cost, commercially oxidized carbon nanofibers (ox-CNFs) on the morphological, thermal, mechanical and electrical properties of polycarbonate (PC) composites was examined. Using a simple solution mixing process leads to good dispersion and high packing density of CNFs in the resultant composites. The composite materials exhibit a dramatic improvement in the DC conductivity; for example, increasing from 2.36 × 10⁻¹⁴ S/m for PC to ca. 10⁻² S/m for the composite at only 3.0 wt.% of CNFs, and exhibits a very fast static charge dissipation rate. Dynamic mechanical analysis showed a remarkable increase in storage modulus (282%) at 165 °C, compared to pure PC. Thermogravimetric analysis showed that thermal stability of the composites increased by 54 °C compared to the pure PC. To our knowledge, the measured electrical conductivity and thermal properties for PC/CNF are the highest values yet reported for PC/CNF composites at comparable loadings. The AC/DC conductivity is shown to play an important role to predict the state of dispersion.     

Suppression of AC conductivity by crystalline transformation in poly(vinylidene fluoride)/carbon nanofiber composites

Poly(vinylidene fluoride) (PVDF) is an important ferroelectric semi-crystalline polymer with multiple-phase behavior. In this study, remarkable effects of the various crystalline structures of PVDF nanocomposites on alternating current (AC) conductivity were discovered using carbon nanofibers (CNF). It was found that the transformation from α-phase to β-phase in PVDF, induced by the addition of CNFs, had a surprisingly suppressive effect on the AC conductivity of the nanocomposites. These unexpected results indicate that the decline in conductivity occurs after re-crystallization treatment (annealing) of the nanocomposites, and the reduction levels increase with increasing amounts of CNFs. Interestingly, the AC conductivity of annealed 5 wt% CNF/PVDF composites becomes even lower than that of re-crystallized nanocomposites with 3 wt% CNFs. These findings are believed to be very significant for fabrication and long-term service of PVDF composites in industry, which often involves exposure to repeated thermal cycling.     

Rapid densification mechanism and properties of h-BN/ZrO2 composites with oxide additives by spark plasma sintering

Large size high density h-BN/ZrO₂ composites (ø = 110 mm, h = 15 mm) were rapidly prepared by spark plasma sintering (SPS) with sintering cycle 20 min. The effects of additives on the mechanical properties, microstructure evolution, and corrosion resistance of the h-BN/ZrO₂ composites were studied. The Al₂O₃, MgO, SiO₂, La₂O₃ and ZrO₂ rapidly formed heterogeneous eutectic crystals under the action of SPS. The low eutectic compounds significantly promoted the diffusion of h-BN or ZrO₂, also increased the density. The flexural strength of h-BN/ZrO₂ composites could reach 196.31 MPa, and the apparent porosity was only 0.42 %. The additives are combined with zirconia to form a high viscosity eutectic, whose corrosion resistance to molten steel is obviously better than that of single ZrO₂. The corrosion depth of h-BN/ZrO₂ composites was only 114 µm after corroded in molten steel at 1550 °C for 80 min. The comprehensive properties of h-BN/ZrO₂ composites were obviously improved by appropriate additives.     

Processing of ultrafine ZrO2 toughened WC composites

The interrelationships between the dispersion of the secondary ZrO₂ phase and the material properties of WC-based composites with up to 10 vol% of ZrO₂ are investigated. The homogeneity of the ultrafine WC–nanometric ZrO₂ powder mixtures was optimized by means of multidirectional milling and bead milling. In an alternative route, zirconium butoxide was used as a ZrO₂ source. The composites were fully densified by means of pulsed electric current sintering (PECS), also known as spark plasma sintering, within a few minutes at 1700 °C allowing to maintain an ultrafine grained microstructure combining a hardness of 2600 kg/mm² with an indentation toughness of 6 MPa m^1/2. The ZrO₂ content and Y₂O₃ stabilization were found to strongly influence the mechanical properties and especially strength of the WC–ZrO₂ composites.     

Electrochemically deposited Fe2O3 nanorods on carbon nanofibers for free-standing anodes of lithium-ion batteries

Fe₂O₃ nanorod/carbon nanofiber (CNF) composites were prepared by the electrochemical deposition of Fe₂O₃ on a web of CNFs, which was then used as a free-standing anode. The conductive, three-dimensional structure of the CNF web allowed for the electrodeposition of the Fe₂O₃ nanorods, while its high conductivity made it possible to use the composite as a free-standing electrode in lithium-ion batteries. In addition, it was easy and cheap to fabricate by a simplification of a process of cell preparation. The nanorod-like Fe₂O₃ structures could only be electrodeposited on the CNFs; flake-like Fe₂O₃ was formed on flat conductive glass substrates. It can be attributed to the different growth mechanism of Fe₂O₃ on the CNFs because of the large number of reaction sites on the CNFs, differences in the precursor concentration and diffusivity within the CNF web. The formation of aggregates of the Fe₂O₃ particles on thicker CNFs also indicated that the CNFs had determined the Fe₂O₃ growth mechanism. The synthesised Fe₂O₃/CNF composite electrode exhibited stable rate capacities at different current densities. This suggested that CNF-based composite did not exhibit the intrinsic disadvantages of Fe₂O₃. Finally, carbon coatings were deposited on the Fe₂O₃/CNF composites to further improve their electronic conductivity and rate capability.     

Improvement of electrical conductivity of silicon nitride/carbon nano-fibers composite using magnesium silicon nitride and ytterbium oxide as sintering additives

In order to find out the influence of sintering additives on the electrical conductivity of Si₃N₄-based ceramics composites with dispersed carbon nano-fibers (CNFs) two different mixtures of sintering additives were tested – Al₂O₃/Yb₂O₃ and MgSiN₂/Yb₂O₃, respectively. Optimization of hot-pressing conditions was performed for each mixture. The results show that the electrical conductivity can be effectively increased up to 1315 S/m by replacement of traditional sintering aid – alumina, with magnesium silicon nitride, while the mechanical properties remained on the same level. Other advantages of using MgSiN₂ instead of alumina are the preservation of higher amounts of CNFs in the ceramic composite and lower densification temperature (1500 °C) compared to samples sintered with alumina-based sintering aids (1550 °C).     

Novel polyimide-based electrospun carbon nanofibers prepared using ion-beam irradiation

This paper describes a novel study focused on preparing carbon nanofibers, with a narrow fiber diameter distribution, from a fluorinated polyimide using both electrospinning and ion-beam irradiation. We specifically focused on the effects of ion species and ion fluences on the electrical conductivity of the nanofiber. The nanofibers were successfully prepared in the diameter range from 340 nm to 1500 nm by varying the concentration of polyimide solution using electrospinning. The Raman spectrum of the ion-irradiated nanofiber included the two well-known D (1360 cm^?1) and G (1580 cm^?1) peaks, indicating that the nanofiber surface changed to a carbon-enriched material. The carbon nanofibers underwent a more ordered graphitic carbon structure with an increase in the ion fluence and the electrical conductivity of the nanofiber irradiated at 1 × 10¹⁶ ions/cm² of Ar⁺ was 0.18 S/cm. In addition, the electrical conductivities of the ion-irradiated nanofibers increased in the order, He⁺ < Ne⁺ < Ar⁺, which indicated that the amount of nuclear energy in the ion species had the most influence on the electrical conductivity. However, the higher electrical conductivity of the carbon nanofibers is required to realize their industrial applications. This paper is the first to address the properties of the electrical conductivity of the carbon nanfibers prepared by electrospinning and ion irradiation as a new approach.