Preparation, crystallization, electrical conductivity and thermal stability of syndiotactic polystyrene/carbon nanotube composites

A homogeneous dispersion of multi-walled carbon nanotubes (MWCNTs) in syndiotactic polystyrene (sPS) is obtained by a simple solution dispersion procedure. MWCNTs were dispersed in N-methyl-2-pyrrolidinone (NMP), and sPS/MWCNT composites are prepared by mixing sPS/NMP solution with MWCNT/NMP dispersion. The composite structure is characterized by scanning electron microscopy and transmission electron microscopy. The effect of MWCNTs on sPS crystallization and the composite properties are studied. The presence of MWCNTs increases the sPS crystallization temperature, broadens the crystallite size distribution and favors the formation of the thermodynamically stable β phase, whereas it has little effect on the sPS γ to α phase transition during heating. By adding only 1.0 wt.% pristine MWCNTs, the increase in the onset degradation temperature of the composite can reach 20 °C. The electrical conductivity is increased from 10^{−10∼−16} (neat sPS) to 0.135 S m⁻¹ (sPS/MWCNT composite with 3.0 wt.% MWCNT content). Our findings provide a simple and effective method for carbon nanotube dispersion in polymer matrix with dramatically increased electrical conductivity and thermal stability.     

Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes

A new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with well-dispersed SWCNTs. For example, a SWCNT/PS composite with 0.5 wt% nanotubes made by this coated particle process (CPP) has an electrical conductivity of ∼3 × 10⁻⁴ S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ∼10⁻⁸ S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that the composites made by this new CPP are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common approach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT/polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymers and nanoparticles.     

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.     

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.%).     

Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO₂ and ZrO₂–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10⁻¹⁰ S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO₂/ZrO₂ boundaries.     

Characterizations of expanded graphite/polymer composites prepared by in situ polymerization

Poly(styrene-co-acrylonitrile)/expanded graphite composite sheets with very low in-plane (8.5 × 10⁻³ Ω cm) and through-thickness (1.2 × 10⁻² Ω cm) electrical resistivities have been prepared. The expanded graphite was made by oxidation of natural graphite flakes, followed by thermal expansion at 600 °C. Microscopic results disclosed that the expanded graphite has a legume-like structure, and each “legume” has a honeycomb sub-structure with many diamond-shaped pores. After soaking the expanded graphite with styrene and acrylonitrile monomers, the polymer/expanded graphite composite granules were obtained by in situ polymerization of the monomers inside the pores at 80 °C. The functional groups and microstructures of the oxidized graphite, expanded graphite and composites in the forms of particles or sheets were carefully characterized using various techniques, including X-ray powder diffraction, thermogravimetry, optical and electron microscopy. It was found that the honeycomb sub-structure survived after hot-pressing, resulting in a graphite network penetrating through the entire composite body, which produces a composite with excellent electrical conductivity.     

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.     

Microstructural and mechanical characterization of fly ash cenosphere/metakaolin-based geopolymeric composites

Novel fly ash cenosphere (FAC)/metakaolin (MK)-based geopolymeric composites were prepared by adding FAC to the MK-based geopolymeric slurry. Microstructure, mechanical property, thermal conductivity, and bulk density of the FAC/MK-based geopolymeric composites were investigated. It was confirmed by the scanning electron microscope (SEM) and transmission electron microscopy (TEM) that the FAC did not dissolve in alkaline condition, but element diffusion took place around the interface between geopolymeric matrix and FAC. The compressive strength, thermal conductivity and bulk density of FAC/MK-based geopolymeric composites decreased monotonically with the increase of the FAC content from 15 vol.% to 40 vol.%, and the minimum values for the 40 vol.% FAC/MK-based geopolymeric composite reached 36.5 MPa, 0.173 W m⁻¹ K⁻¹ and 0.82 g cm⁻³, respectively, in the range of FAC content from 15 vol.% to 40 vol.%. The results showed that the FAC could lower thermal conductivity effectively and bulk density of FAC/MK-based geopolymeric composites at a cost of slight decrease of mechanical properties. The 40 vol.% FAC/MK-based geopolymeric composite was a promising candidate material for intermediate-temperature thermal insulation applications due to its low thermal conductivity and low density.     

The preparation of a novel Si–CNF composite as an effective anodic material for lithium–ion batteries

The suggested pyrolytic carbon (PC) coated Si-carbon nanofiber (CNF) composites can be a solution to provide higher discharge capacity and cycle-ability facilitating the 1st cycle coulombic efficiency of cheap metallic Si particles as an appropriate anodic material for Li-ion battery. The CNFs on the surface of Si particle can provide flexible space to relieve volumetric expansion during charge. Well-controlled PC coating on the surface of Si particle can improve the coherence of CNFs on the Si particle, thereby to enhance the role of CNF as all the more effective electrode material. The additional PC coating on the Si–CNF composite can accomplish the lower surface area and afford the improvement of the 1st cycle coulombic efficiency. Ingenious combinations of PC coating and CNF compositeness successfully made the novel type Si–CNF composite to achieve a remarkable discharge capacity (1115 mA h g⁻¹), an excellent cycle-ability (77% retention rate after 20th cycle), and a good 1st cycle coulombic efficiency (79%) for the effective application as an anode material.     

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 °C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m °C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4×10⁻⁴ m²°C/W at 0.7 bar to 0.6×10⁻⁴ m²°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m °C) and 0.52 W/(m °C) at 70 °C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6×10⁻⁴ m²°C/W at 0.7 bar to 0.9×10⁻⁴ m²°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.     

Nafion/Analcime and Nafion/Faujasite composite membranes for polymer electrolyte membrane fuel cells

The Nafion/zeolite composite membranes were synthesized for polymer electrolyte fuel cells (PEMFCs) by adding zeolite in the matrix of Nafion polymer. Two kinds of zeolites, Analcime and Faujasite, having different Si/Al ratio were used. The physico-chemical properties of the composite membranes such as water uptake, ion-exchange capacity, hydrogen permeability, and proton conductivity were determined. The fabricated composite membranes showed the significant improvement of all tested properties compared to that of pure Nafion membrane. The maximum proton conductivity of 0.4373 S cm⁻¹ was obtained from Nafion/Analcime (15%) at 80 °C which was 6.8 times of pure Nafion (0.0642 S cm⁻¹ at 80 °C). Conclusively, Analcime exhibited higher improvement than Faujasite.     

LaBaCuFeO5+δ-Ce0.8Sm0.2O1.9 as composite cathode for solid oxide fuel cells

The performance of the LaBaCuFeO_5+δ-Ce_0.8Sm_0.2O_1.9 (LBCF-SDC) composite cathodes was studied in this paper. Electrical conductivity, thermal expansion and electrochemical properties were investigated by four probing DC technique, dilatometry, AC impedance and polarization techniques, respectively. The thermal expansion coefficients of the LBCF-SDC were between (16.3 and 13.4) × 10⁻⁶ K⁻¹ from 30 to 850 °C, which was lower value than LBCF (17.0 × 10⁻⁶ K⁻¹). AC Impedance spectroscopy measurements of LBCF-SDC/SDC/LBCF-SDC test cell were carried out. Polarization resistance values for the LBCF-SDC10 cathode was as low as 0.097 Ω cm² at 750 °C.     

Dielectric spectroscopy on melt processed polycarbonate—multiwalled carbon nanotube composites

Complex permittivity and related AC conductivity measurements in the frequency range between 10⁻⁴ and 10⁷ Hz are presented for composites of polycarbonate (PC) filled with different amounts of multiwalled carbon nanotubes (MWNT) varying in the range between 0.5 and 5 wt%. The composites were obtained by diluting a PC based masterbatch containing 15 wt% MWNT by melt mixing using a Micro Compounder. From DC conductivity measurements it was found that for samples processed at a mixing screw speed of 150 rpm for 5 min, the percolation occurs at a threshold concentration (p_c) between 1.0 and 1.5 wt% MWNT. For concentrations of MWNT near the percolation threshold, the processing conditions (screw speed and mixing time) were varied. The differences in the dispersion of the MWNT in the PC matrix could be detected in the complex permittivity and AC conductivity spectra, and have been explained by changes in p_c. The AC conductivity and permittivity spectra are discussed in terms of charge carrier diffusion on percolation clusters and resistor-capacitor composites.     

Effects of crosslinkers on semi-interpenetrating polymer networks of Nafion and fluorine-containing polyimide

A series of reinforced composite membranes were prepared from Nafion^®212 and crosslinkable fluorine-containing polyimide (FPI) with various crosslinkers. The crosslinkable FPI reacts with the crosslinkers and forms semi-interpenetrating polymer networks (semi-IPN) structure with Nafion^®212. The water uptake, swelling ratio, mechanical properties, thermal behavior, proton conductivity, and chemical oxidation stability of the composite membranes are studied. The degree of crosslinking is characterized by gel fraction of the composite membranes. Compared to pure Nafion^®212, the composite membranes exhibit excellent thermal stability, improved mechanical properties and dimensional stability. The tensile strength of the composite membranes is in the range of 37.3-51.2 MPa. All the composite membranes exhibit high proton conductivity which ranges from 1.9 × 10⁻² to 9.9 × 10⁻² S cm⁻¹. The proton conductivity of the composite membrane with 2-propene-1-sulfonic acid sodium salt (SAS) as the crosslinker is 9.9 × 10⁻² S cm⁻¹ at 100 °C which is similar to that of Nafion^®212 under the same condition.     

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte via ethylene glycol route and its characterizations for IT-SOFC

Strontium and magnesium doped lanthanum gallate (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ), known as LSGM, was first prepared via ethylene glycol method. This route of preparation showed improved electrical conductivity, better surface area and high density. X-ray diffraction patterns of LSGM sintered at different temperatures indicated that pure LSGM phase was formed after sintering at 1400 °C. X-ray Rietveld refinement confirmed the formation of pure perovskite orthorhombic phase of the LSGM. The sintered sample showed 99% relative density. Scanning electron microscopic study of LSGM also depicted fairly densed grain morphology. X-ray photoelectron spectroscopy measurement confirmed the stability of the sintered sample in air and the existence of constituent elements in their characteristic valence states. A surface without porosity was observed in BET measurement. Average thermal expansion coefficient was found to be 9.78×10⁻⁶/°C in the measured temperature range (RT–1000 °C). The frequency dependent electrical conductivity of the sample was measured in the temperature range 400–800 °C. Total electrical conductivity of the LSGM pellet was found to be 0.056 S cm⁻¹ at 800 °C.     

Synthesis of composite materials of carbon nanofibres and ceramic monoliths with uniform and tuneable nanofibre layer thickness

Composite materials consisting of ceramic monoliths and carbon nanofibres (CNFs) have been synthesized by catalytic growth of CNFs on the γ-alumina washcoating layer covering the walls of a ceramic monolith. The composites possess a relatively uniform mesoporous layer of CNFs of relatively small diameter. The thin alumina washcoating (ca. 0.1 μm) prevents the CNFs from being trapped inside the alumina pores and hence the CNFs grow freely throughout the washcoating layer to form a uniform layer of CNFs that completely covers the surface of the monolith walls. The growth temperature is found to control the thickness of the CNF layer (2-4 μm), the growth rate of the nanofibres, and the mechanical strength of the resulting CNF-monolith composite. At ideal conditions, a complete adhesion of the CNF layer and higher mechanical strength than the original cordierite monolith can be obtained. The CNF layer has an average pore size of 17 nm with absence of microporosity which renders these monoliths promising candidates for the use as catalyst supports, especially for liquid phase reactions. The CNFs have small diameters (5-30 nm) due to the high dispersion of Ni particles in the growth catalyst and the CNFs exhibit an unusual branched structure.     

Electrical conductivity of graphite/polystyrene composites made from potassium intercalated graphite

Composites between graphite and polystyrene have been synthesized starting from potassium intercalated graphite and styrene vapor. This in situ polymerization process can be used to make electrically conductive composites containing well-dispersed thin graphite sheets. The conductivities of the composites increase as the number of ordered carbon layers increases. With only 10% graphite in a polystyrene matrix, an electrical conductivity up to 1.3 × 10⁻¹ S/cm can be obtained. The key is synthesizing a material with at least four ordered graphite layers (a stage IV complex) separated by polystyrene. This composite shows an improvement in conductivity over a control composite made by radical polymerization of styrene containing the same amount of dispersed graphite which had a conductivity of 5.0 × 10⁻³ S/cm. Characterization of the complexes by powder X-ray diffraction, scanning electron microscopy and electrical conductivity is presented.     

Synthesis and electrochemical performance of carbon nanofiber-cobalt oxide composites

Carbon nanofiber (CNF) supported cobalt oxide composites as high-capacity anode materials were prepared through a facile, effective method for potential use in rechargeable lithium-ion batteries. The effects of the calcining temperature on the crystallinity, grain size, specific surface area of Co₃O₄ and phase transformation from Co₃O₄ to CoO were studied in detail. Both the specific surface area and CNF content in CNF-cobalt oxide composites strongly affect the electrochemical performance of these series composites. The CNF-Co₃O₄ composite with 24.3% CNF pyrolyzed at 500 °C in Ar shows an excellent cycling performance, retaining a specific capacity of 881 mAh g⁻¹ beyond 100 cycles. Homogeneous deposition and distribution of nanosized Co₃O₄ particles on the surface of CNF can stabilize the electronic and ionic conductivity as well as the morphology of Co₃O₄ phase, which may be the main reason for the markedly improved electrochemical performance.     

Synthesis and thermal behavior of Cu/Y2W3O12 composite

Cu metal matrix composite with Y₂W₃O₁₂ as a thermal expansion compensator was fabricated by high energy ball milling followed by compaction and sintering, and its thermal properties were explored for the potential applications as heat sinks in electronic industries, high precision optics, and space structures. The volume fraction of reinforcement was varied from 40% to 70% in order to tailor the composite for the simultaneous accomplishment of low thermal expansion and high thermal conductivity. The synthesis technique was optimized by varying the parameters like milling time from 1 to 20 h and sintering temperature from 600 to 1000 °C in order to achieve densified composites. The relative density of the composites is found to be around 90% for the 10 h milled powders followed by compaction at a pressure of 700 MPa and sintering at a temperature of 1000 °C. The thermal expansion of the composites exhibits linear behavior in the temperature range 200 to 800 °C and the low coefficient of thermal expansion (CTE) is found to be for Cu–70%Y₂W₃O₁₂ composite whose value, 4.32±0.75×10⁻⁶/°C, matches with that of Si substrate. The thermal conductivities are found to increase with a decrease in the volume fraction of the reinforcement and decrease with an increase in the temperature for all the samples. The experimentally determined CTE and thermal conductivity values are found to be comparable to those predicted by the thermal expansion based Kerner and Turner model and the thermal conductivity based Maxwell model, respectively.