The synthesis and electrical conductivity of a polyacrylamide/Cu conducting hydrogel

A polyacrylamide/Cu superabsorbent composite was synthesized by aqueous solution polymerization. Based on the electrical conductivity of Cu and the water absorbency of polyacrylamide, a novel conducting hydrogel with a conductivity of 1.08 mS m⁻¹ was prepared. The effect of crosslinker, initiator, monomer, neutralization degree, Cu amount, water absorbency and reaction temperature on the electrical conductivity of the hydrogel was investigated. An appended network structure model of the polyacrylamide/Cu conducting hydrogel is proposed.     

Pre-treatment of adsorbents for waste water treatment using adsorption coupled-with electrochemical regeneration

With the aim to address waste water treatment problems, a novel and economic water treatment technology was introduced at the University of Manchester. It comprised of a unique combination of adsorption and electrochemical regeneration in a single unit. This process successfully eliminated a number of organic pollutants by using an electrically conducting adsorbent material called Nyex? which was a modified form of synthetic graphite. To expand the scope of other graphite types in waste water treatment applications, natural vein and recycled vein graphite materials were selected for electrochemical surface treatment (pre-treatment) in order to evaluate their adsorptive and electrical properties. New graphite based adsorbents were developed and characterized using a laser diffraction particle size analyser, BET surface area, SEM analysis, X-ray (EDS) elemental analysis, X-ray powder diffraction, Boehm surface titration, Zeta potential electrical bed conductivity and bulk density measurements. Boehm surface titration and EDS (X-ray) elemental analysis showed a significant increase in oxygen containing surface functional groups. Although, no significant improvement in bed electrical conductivity was found to occur after electrochemical surface treatment, however, natural vein and recycled vein graphite materials presented highest bed electrical conductivity amongst competing graphite materials. Aqueous solution of acid violet 17 as a standard pollutant was used to evaluate the comparative performance of these adsorbents. The investigations revealed that electrochemical surface treatment contributed to an increase in the adsorption capacity by a factor of two only for natural vein graphite. Un-treated recycled vein graphite adsorbent delivered the same adsorptive capacity (3.0 mg g^?1) to that of electrochemically treated natural vein graphite. The electrochemical regeneration efficiency at around 100% was obtained using a treatment time of 60 and 30 min, current density of 14 mA cm^?2, charge passed of 36 and 18 C g^?1 for synthetic graphite, natural and recycled vein graphite materials respectively. Relatively a small consumption of electrical energy, 24 J g^?1 for regenerating natural vein graphite adsorbent versus 36 J g^?1 for synthetic graphite adsorbent, was found to be required for destruction/oxidation of adsorbed acid violet 17. Multiple adsorption/regeneration cycles presented no loss in adsorptive capacity over 5 adsorption/regeneration cycles. The use of natural and recycled vein graphite adsorbents offered some advantages over graphite intercalation based adsorbents with reduced electrical energy consumption during regeneration and simpler separation of particulate adsorbent.     

Amorphous silica-coated graphite particles for thermally conductive and electrically insulating resins

A thermally conductive and electrically insulating composite filler was produced by surfactant assisted sol–gel coating of amorphous silica on flake graphite. Amorphous silica-coated graphite (a-Si coated grp) obtained using a cationic surfactant showed the best enhancement of the insulating coating. The resulting a-Si coated grp/boehmite/polybutylene terephthalate polyester resin composite exhibited a high volume resistivity, exceeding 1.0 × 10¹⁴ Ω cm at an applied voltage of 500 V, and a thermal conductivity of 3.3 W/m K at 22.9 vol.% a-Si coated grp loading. The heat releasing performance of the developed resin composite in actual light-emitting diodes bulb housings was compared with conventionally used thermally and electrically conductive resin. This comparison revealed that the new composite released heat more effectively. This innovative technology, which may solve the trade-off between material properties and cost, will be available for a broad range of thermally conductive resin applications that simultaneously require thermal conduction and electrical insulation.     

Application of nitrogen-doped graphene nanosheets in electrically conductive adhesives

Nitrogen-doped graphene nanosheets (N-GNSs) were used as a conductive filler for a polymer resin adhesive and as a performance improver for a silver-filled electrically conductive adhesive (ECA). The N-GNS samples were prepared by the chemical-intercalation/thermal-exfoliation of graphite followed by a thermal treatment in NH₃. Only 1 wt.% of N-GNSs was required for the adhesive to reach a percolation threshold, and the performance using N-GNSs was much better than that obtained using carbon black or multi-walled carbon nanotubes (MWCNTs). The effect of N-GNS or MWCNT additives on reducing the electrical resistivity of Ag-particle filled ECAs at low Ag loading ratios was also investigated. With 30 wt.% of Ag filler, the polymer resin was still non-conducting, while a resistivity of 4.4 × 10⁻² Ω-cm was obtained using an Ag/N-GNS hybrid filler fortified with only 1 wt.% of N-GNSs due to large specific surface area, high aspect ratio, and good electrical conductivity of the doped graphene.     

Graphene/poly(3,4-ethylenedioxythiophene) hydrogel with excellent mechanical performance and high conductivity

We report a novel and easy route to synthesize a mechanically strong hydrogel composed of graphene and poly (3,4-ethylenedioxythiophene) (PEDOT). 3,4-Ethylenedioxythiophene played the role of reducing agent to convert a highly oxidative graphite oxide (h-GO) to graphene and in situ polymerized itself synchronously on the active sites of the graphene to construct the hydrogel. The content of the carbonyl groups in h-GO was found to have a major impact on the generation of the hydrogel. Also the morphology and the quantity of PEDOT formed in the hydrogel were considered to be the key factors for improving the mechanical performance of the hydrogel. As-prepared enhanced graphene/PEDOT hydrogel displayed a compressive fracture stress as high as 29.6 MPa, a storage modulus about 2.1 MPa at 10 rad/s, a good electrical conductivity of 0.73 S/cm and a high specific capacitance of 174.4 F/g, which make it a potential candidate for a number of technologies such as electrochemical sensor and supercapacitor.     

Aqueous slurry of S-doped carbon nanotubes as conductive additive for lithium ion batteries

S-doped carbon nanotubes (SCNTs) obtained by a post treatment approach are used as conductive additive for LiFePO₄ (LFP) cathodes in Lithium ion batteries (LIBs). The SCNTs exhibit higher specific surface area, higher conductivity and better hydrophily as compared to the pristine CNTs because of S doping. Thus the SCNTs can be stably dispersed in water, forming an aqueous conductive slurry. The LFP cathode using the aqueous SCNTs slurry as conductive additive exhibits excellent electrochemical performances in terms of capacity (143 mA h g⁻¹ at 2 C), rate capability and cycling stability (99.6% of initial capacity after 200 cycles) due to the uniform dispersibility of SCNTs in the bulk of electrodes forming a continuous conductive network. The full cell configuration with graphite as anode, affords a high reversible capability (150 mA h g⁻¹ at 0.2 C), good cycling stability (capacity retention of 87.6% at 2 C), ultrahigh energy density of 163.7 W h kg⁻¹ and power density of 296.8 W kg⁻¹. Our results provide an easy approach to prepare high performance LIB cathodes using water as solvent, thus leading to lower cost and more secure for the electrode production.     

Graphene nanoplatelet paper as a light-weight composite with excellent electrical and thermal conductivity and good gas barrier properties

Paper forms (i.e. thin free-standing films) of carbon-based materials have received increasing attention. Here we present a novel approach to fabricating a binder free, self-standing flexible paper consisting of exfoliated graphite nanoplatelets (GNPs). It is found that the electrical conductivity of the GNP paper can be as high as 2200 S cm^?1 and the thermal conductivity reaches 313 W m^?1 K^?1. Both thermoset and thermoplastic matrices were used to impregnate the porous GNP paper and an extremely high tensile modulus was attained. Even with 30 vol.% polymer, the GNP paper composite can still exhibit ～700 S cm^?1 electrical conductivity thanks to the highly continuous GNP network formed in the paper making process. The impregnated GNP paper was also investigated as a component in carbon fiber composite. It is found that when inserted into a layered laminate composite construction, gas permeability can be severely reduced and electrical and thermal conductivity can be greatly enhanced.     

Scalable and high-yield production of exfoliated graphene sheets in water and its application to an all-solid-state supercapacitor

One of the most critical issues in graphite exfoliation is realizing efficient, low-cost, eco-friendly, and scalable production of graphene for energy storage applications. The most promising strategies for exfoliating graphite to single-layer graphene sheets in scalable quantities with nearly non-oxidized content is the exfoliation of graphite by using an environmentally friendly solution. Herein, we demonstrate a universal exfoliation principle that uses imidazole, which has an anionic nature and π-conjugated heterocyclic coplanar structure, as the exfoliant for the successful production of large quantities of graphene suspensions in water. The exfoliant interacts with both surfaces of the exfoliated graphene sheets and the aqueous solution, significantly improving graphene dispersion (1 mg/mL) in water. Exfoliation in aqueous solutions produced graphene in high yield (>90%, ⩽3 layers), with a large lateral size (μm) and high quality (I_D/I_G ratio ≈ 0.1). The electrical conductivity of the graphene paper is 131.7 S/cm, which is superior to the values reported from exfoliated graphene prepared using solution processes. An all-solid-state supercapacitor with a new design fabricated using the atomically thin and flat conductive exfoliated graphene sheets delivered an ultrahigh area capacitance (∼71.9 mF/cm²). Therefore, imidazole -assisted exfoliation has great potential for scalable preparation of graphene suspensions for supercapacitor applications.     

Preparation of cellulose-based conductive hydrogels with ionic liquid

Conductive hydrogel composed of microcrystalline cellulose (MCC) and polypyrrole (PPy) was prepared in ionic liquid; and the resulting hydrogel was characterized with FT-IR, SEM, XRD and TGA. By doping with TsONa, the MCC/PPy composite hydrogels showed relatively high electrical conductivity, up to 7.83 × 10⁻³ S/cm, measured using a four-probe method. The swelling kinetics of the composite hydrogels indicated that the swelling process was mainly influenced by the cellulose content; and the equilibrium swelling ratio decreased as the increasing of MCC content in the hydrogels. In addition, the MCC/PPy composite hydrogels exhibited significantly enhanced mechanical property in contrast to MCC hydrogel.     

Preparation of flat porous carbon films from paper-thin wood shavings and control of their mechanical,electrical and magnetic properties

A method for preparing flat porous carbonized films (FPCFs) by carbonizing paper-thin wood shavings was developed, and their structures and properties were investigated. The FPCFs were binder-free and had horizontal honeycomb structures consisting of macropores of 2–50 μm arising from tracheids and pittings. The FPCFs with approximate dimensions of 120 mm × 104 mm × 113 μm showed some flexibility. They were effectively reinforced and became twistable by introducing styrene–butadiene rubber (SBR) into the macropores via impregnation. The SBR-modified FPCFs carbonized at 1023 K retained electrical conductivity on the order of 10⁻³ S/m with the inclusion of SBR concentrations up to 15 mg/cm². The electrical conductivity of FPCFs carbonized at 773 K changed from nonconductive to 10⁻² S/m by impregnating the macropores with electrically conductive carbon paste or to 10⁵ S/m with Cu chemical plating. The FPCFs became responsive to a magnetic field by impregnating the macropores with a magnetic fluid or by creating ferrites through chemical reactions within the macropores.     

Exfoliated graphite as a flexible and conductive support for Si-based Li-ion battery anodes

Exfoliated graphite (EG) was found to be a flexible and conductive support of anode materials for lithium ion batteries through the preparation of the composite of pyrolytic carbon-coated nano-sized silicon nanoparticles supported by exfoliated graphite (pC-Si-EG). Electrochemical analyses revealed that pC-Si-EG composite delivered a high capacity of 902.8 mAh g⁻¹ at a current density of 200 mA g⁻¹ and an excellent cycling stability with 98.4% capacity retention after 40 cycles. It was found that the polycrystalline silicon nanoparticles went through some very interesting changes and broke up into smaller Si nanoparticles dispersing onto the surface of EG after charging/discharging cycles. The results demonstrated that EG plays an important role in the superior electrochemical performances of pC-Si-EG anode due to its high porosity, excellent electronic conductivity and good flexibility.     

Thermal fatigue and hypothermal atomic oxygen exposure behavior of carbon nanotube wire

In low earth orbit (LEO), components of space systems are exposed to damaging hypothermal atomic oxygen and thermal fatigue. Carbon nanotube (CNT) wires are candidate materials for different applications in space systems. Thirty-yarn CNT wire’s behavior was evaluated when exposed to hypothermal atomic oxygen and thermal fatigue. CNT wire specimens were exposed to a nominal fluence of hypothermal atomic oxygen of 2 × 10²⁰ atoms/cm². The erosion rate due to hypothermal collision between atomic oxygen and CNT wires was calculated to be 2.64 × 10⁻²⁵ cm³/atom, which is comparable to highly ordered pyrolytic graphite. The tensile strength of CNT wire was not affected by this exposure, and a minor reduction of electrical conductivity (2.5%) was found. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy analysis showed erosion of surface layer with depleted carbon and increased oxygen. Thermal fatigue excursion of 5000 cycles from 70 to −50 °C at a rate of 55 °C/min showed no loss in tensile strength; however a large decrease in conductivity (18%) was seen. SEM analysis showed that the thermal fatigue created buckling of yarn and fracture of individual CNTs bundles. These reduced the effective area and electrical conductivity of CNT wire.     

SDS-stabilized graphene nanosheets for highly electrically conductive adhesives

We report sodium dodecyl sulfate (SDS) stabilization of graphene nanosheets, with two different sizes as auxiliary fillers inside the conventional electrically conductive adhesive (ECA) composite. Using this non-covalent modification approach we were able to preserve the single-layer structure of graphene layers and prevent their re-stacking inside the composite, which resulted in a significant electrical conductivity improvement of ECAs at noticeably low filler content. Addition of 1.5 wt% small and large SDS-modified graphene into the conventional ECAs with 10 wt% silver flakes led to low electrical resistivity values of 5.5 × 10³ Ω cm and 35 Ω cm, respectively, while at least 40 wt% of silver flakes was required for the conventional ECA to be electrically conductive. A highly conductive ECA with very low bulk resistivity of 1.6 × 10⁻⁵ Ω cm was prepared by adding 1.5 wt% of SDS-modified large graphene into the conventional ECA with 80 wt% silver flakes which is less than that of eutectic lead-based solders.     

Three-dimensional MnO/reduced graphite oxide composite films as anode materials for high performance lithium-ion batteries

MnO/reduced graphite oxide (MnO/RGO) composite films with three dimensionally porous structures have been synthesized by an improved electrostatic spray deposition setup and their microstructure and electrochemical properties have been characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric, Raman spectrometry and galvanostatic cell cycling. The results show that the structure and electrochemical performance of the electrode film are influenced significantly by the RGO content. The three dimensionally porous structure collapse does not occur in the MnO/RGO thin films for a RGO content lower than 16.58 wt%, the 16.58 wt% reduced graphite oxide content being optimal. Such an improvement in the cycling performance (772 mAh g⁻¹ after 100 cycles at 1 C) and rate capability (425 mAh g⁻¹ at 6 C) might be attributed to the excellent microstructure and electrical conductivity of MnO/reduced graphite oxide composite film electrodes.     

Effect of the compression molding parameters on the in-plane and through-plane conductivity of carbon nanotubes/graphite/epoxy nanocomposites as bipolar plate material for a polymer electrolyte membrane fuel cell

The main challenges for commercialization of a single-filler graphite (G) polymer-matrix composite as bipolar plates are its low electrical conductivity and flexural strength. The minimum requirements set by the US Department of Energy (DOE) are the electrical conductivity and flexural strength to be greater than 100 S/cm and 25 MPa, respectively. In this study, the electrical conductivity of a G/epoxy (EP) composite (single filler) is only 50 S/cm (in-plane conductivity) at 80 wt% G. However, flexural strength is greater than 25 MPa. Using carbon nanotubes (CNTs) as the second filler at a concentration of 5 wt% in a CNTs/G/EP nanocomposite resulted in the in-plane and through-plane electrical conductivity and flexural strength being 180 S/cm, 75 S/cm, and 45 MPa, respectively. The density of the CNTs/G/EP nanocomposite is also less than that of G/EP composite, which demonstrates that a total weight reduction is achievable.     

Electrostatic self-assembly of graphene–silver multilayer films and their transmittance and electronic conductivity

By alternating deposition of graphene oxide (GO) sheets and silver nitrate by means of an electrostatic self-assembly method, a GO–Ag⁺ film was prepared. After thermal annealing, a graphene–silver nanoparticle (GE–Ag) multilayer film, with high transparency and electrically conductivity, was obtained. The transmittance of a film with four assembly cycles was 86.3%, at a wavelength of 550 nm, better than that of a pure GE film (73.8%). While the surface resistance was 97 kΩ □^?1, much lower than that of a pure GE film (430 kΩ □^?1). The Ag nanoparticles play a crucial role in improving the properties of the GE–Ag film, acting as conductive paths and light-trapping nanoparticles, which not only reduces the reflection of the film, but also prevents the GE sheets from aggregation and provides conductive paths between sheets, improving the electrical conductivity.     

Microstructure and thermophysical properties of B4C/graphite composites containing substitutional boron

B₄C/graphite composites (BGC) containing substitutional boron were fabricated by pressureless sintering of powder mixtures of petroleum coke, coal tar pitch and B₄C. After sintering at 900 °C and graphitizing at 2200 °C, the microstructure of BGC was characterized by SEM, TEM, XRD, Raman spectroscopy and optical microscopy. XPS measurements revealed the formation of BC₃, and the matrix carbon contained around 6 wt.% substitutional boron. The thermal conductivity of the BGC at room temperature is 52.7 W/m K and the flexural strength is up to 35.1 MPa. The bulk density and electrical resistivity are 1.72 g/cm³ and 13.4 μΩ m, respectively. The correlation between microstructure and properties was investigated. The results showed that the microstructure improvement of the BGC has obvious effect on the thermal conductivity, flexural strength, and electrical resistivity.     

Graphite foam from pitch and expandable graphite

Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to “foam” the pitch. The resulting foam was subjected to a series of heat treatments with the objective of first crosslinking the pitch, and thereafter carbonizing and graphitizing the resulting foam. XRD confirmed that the graphitization at 2600 °C resulted in a highly graphitic material. The porosity of this foam derives from the loose packing of the vermicular exfoliated graphite particles together with their internal porosity. During the foaming process the pitch tends to coat the outside surface of the expanding graphite flakes. It also bonds them together. The graphite foam prepared with 5 wt.% expandable graphite had a bulk density of 0.249 g cm⁻³, a compressive strength of 0.46 MPa and a thermal conductivity of 21 W m⁻¹ K⁻¹. The specific thermal conductivity (thermal conductivity divided by the bulk density) of this low-density carbon foam was 0.084 W m² kg⁻¹ K⁻¹ which is considerably higher than that of copper metal (0.045 W m² kg⁻¹ K⁻¹) traditionally used in thermal management applications.     

Pulse thermometers based on synthetic single crystal boron-doped diamonds

A time delay of electrical resistance of boron-doped semiconductor diamond sensor at heating pulses was investigated. We used a symmetric scheme with 2 equivalent 2 × 2 × 1.5 mm³ single crystal elements connected via thermal conductive electrical insulating glue. One of elements served as pulse heater and the other one as a sensor. About 40 ms characteristic time delay of diamond sensor was achieved. It is supposed, that using smaller crystals for sensors and by optimization of their electrical and heat conductivity properties, a regular value of time delay less than 1 ms will be attained. The potential application of diamond thermometers for temperature control in combustion engines is shown.     

Low cost preparation of high thermal conductivity carbon blocks with ultra-high anisotropy from a commercial graphite paper

A carbon block with ultra-high anisotropy was produced from a commercial graphite paper as the thermal reinforcement and a thermosetting phenolic resin as the binder. Hot-pressing at a maximum temperature of 200 °C was used to densify and integrate the graphite paper stacks. It has been found that the graphite paper blocks have high thermal conductivities in the paper direction and low ones perpendicular. An anisotropy of 98.8% and a thermal conductivity of 197.8 W m^?1 K^?1 in the paper direction were achieved when the density was 1.1 g cm^?3. The thermal conductivity increased to 284.8 W m^?1 K^?1 with a decrease of anisotropy to 98.3% with a density of 1.56 g cm^?3.