Xiaoping Shui

Submicron diameter nickel filaments and their polymer-matrix composites

Discontinuous nickel filaments of diameter 0.4 μm and having a carbon core of diameter 0.1 μm were fabricated by electroplating nickel on discontinuous carbon filaments. They exhibited a grain size of 0.016 μm and electrical resistivity of about 5 × 10−6 Ω·cm. In an amount as low as 7 vol.% in a polymer (polyether sulfone) matrix, they resulted in a composite exhibiting electromagnetic interference shielding effectiveness of 87 dB and reflection coefficient 0.95 at 1–2 GHz, tensile strength 52 MPa, tensile ductility 1.0%, and density 1.87 g/cm3.

Carbon filaments and carbon black as a conductive additive to the manganese dioxide cathode of a lithium electrolytic cell

Abstract Carbon filaments, when surface treated and used in stead of carbon black as the conductive additive to MnO 2 cathodes in lithium cells, produced a more gently sloping discharge curve (desirable for applications requiring end-of-life indication). These filament composite cathode plates also occupied less volume (higher packing density) and were handleable without the use of a binder, thus resulting in higher volumetric energy density than the carbon black counterpart. The Li/MnO 2 discharge capacity increased with the cathodes electrolyte absorptivity and rate of electrolyte absorption, as opposed to the cathodes electrical conductivity, whether carbon filaments or carbon black was used. The cathodes electrolyte absorption characteristics and packing density and the carbons electron transfer rate were enhanced by surface treatment of the carbon. For carbon filaments, solvent cleansing, followed by either surfactant treatment or chopping plus drying, was effective; solvent cleansing also decreased the volume resistivity of both the carbon compact and the MnO 2 /filament compact. For carbon black, surfactant treatment was effective and resulted in increases in test cell discharge capacity, open- and closed-circuit voltages (OCV and CCV), and cathode packing density. The volume electrical resistivity of the filament compact was lower than that of the carbon black compact, but the volume resistivity of the composite cathode was higher using carbon filaments instead of carbon black; the latter is due to the spreadability of carbon black between the MnO 2 particles.

Submicron nickel filaments made by electroplating carbon filaments as a new filler material for electromagnetic interference shielding

Short nickel filaments of diam 0.4 μm and containing 94 vol% Ni and 6 vol% C were fabricated by electroplating with nickel 0.1 μm diam catalytically grown carbon filaments. The use of these filaments in polyether sulfone in amounts of 3, 7,13, and 19 vol% gave composites with electromagnetic interference shielding effectiveness at 1–2 GHz of 42,87,84, and 92 dB, respectively, compared to a value of 90 dB for solid copper. Less shielding was attained when 0.1 μm diam carbon filaments or 2 or 20 μm diam nickel fibers were used instead.

Early fatigue damage in carbon-fibre composites observed by electrical resistance measurement

Early fatigue damage during the first tenth (or less) of the fatigue life was observed in carbon fibre composites by d.c. electrical resistance measurement. The damage was most severe in the first loading cycle and the incremental damage in each subsequent cycle diminished cycle by cycle. For the continuous carbon fibre carbon-matrix composite, the resistance increased irreversibly during early fatigue due to matrix damage and possibly fibre fracture as well. For the short carbon fibre polymer-matrix and cement-matrix composities, the resistance decreased irreversibly during early fatigue due to matrix damage near the junction of adjacent fibres and the resulting increase in the chance that adjacent fibres touched one another.

Electrochemical behavior of porous carbons

Abstract The electrochemical behavior of porous carbons based on carbon fibers and those based on phenolic was evaluated by cyclic voltammetry via the Fe 2+ Fe 3+ redox couple. An irreversible electrochemical response obtained from the fiber-based porous carbons was made reversible by coating of the porous carbons with graphite flakes or carbon black; the electron-transfer rate constant ks and the capacitance were increased, while the electrochemical area was decreased. Porous carbons based on phenolic exhibited variation from batch to batch in their electrochemical behavior, due to contamination by long-chain aliphatic hydrocarbons (comprising ≤ 10% of the porous carbon weight). Their electrochemical performance was rendered consistent from batch to batch and also enhanced (with increased ks) by cleansing in methylene chloride. The electrochemical areas obtained for phenolic-based porous carbons were larger than those of the fiber-based porous carbons, even though their specific geometric surface areas (per unit volume) were much lower. The electrochemical area decreased with increasing specific geometric surface area (per unit volume) for phenolic-based porous carbons, while the specific electrochemical surface area did not vary much; at a pore density of 30 ppi, the electrochemical area was even higher than that of carbon black, due to the large pore size and the consequent penetrability by the electrolyte. Among the fiber-based porous carbons, both electrochemical area and specific electrochemical area decreased with decreasing specific geometric surface area (increasing fiber length). The 500 ppi phenolic-based porous carbon, after methylene chloride cleansing, exhibited higher ks, than conventional glassy carbon. The fiber-based porous carbons exhibited higher apparent density, lower resistivity and higher compressive strength than those based on phenolic.

Hairy carbon electrodes studied by cyclic voltammetry and battery discharge testing

Abstract Hairy carbon is a new material developed by growing submicron carbon filaments on conventional carbon substrates. Typical substrate materials include carbon black, graphite powder, carbon fibers and glassy carbon. A catalyst is used to initiate hair growth with carbonaceous gases serving as the carbon source. To study the electrochemical behavior of hairy carbons, cyclic voltammetry and discharge testing were conducted. In both cases, hairy carbon results surpassed those of the substrate materials alone.

Effect of chemisorbed oxygen on the electrochemical behavior of graphite fibers

Abstract The effect of chemisorbed oxygen on the electrochemical behavior of radially structured graphite fibers (Amoco Thornel P100) was studied. The outer graphite skin was removed by heating in air, allowing the edge plane sites to be exposed and increasing the concentration of chemisorbed oxygen. Mass spectrometry, conducted while heating the oxidized fibers, detected primarily carbon monoxide, suggesting that the chemisorbed oxygen was in the form of phenol, carbonyl, and/or quinone functional groups. Cyclic voltammetry showed that the electron transfer rate and reversibility of the iron cyanide redox species increased, the voltammetric peak separation decreased, and the cathodic peak current density approached the anodic peak current density as the burn-off level increased from 0% to 17%. Moreover, a decrease in surface tension accompanied by an increase in wettability of the fiber by the electrolyte was observed upon burn-off by 17%. Subsequent reduction in hydrogen resulted in a large increase in surface tension, a decrease in the surface oxygen concentration, an increase in the oxygen-binding energy and a dramatic loss of electrochemical activity. The investigation demonstrates that the domination of edge sites produced by thermally removing the basal plane surface skin of the fibers resulted in the formation of oxygencontaining surface functional groups that reduced the fiber surface tension (increasing wettability), thereby improving the electron transfer rate and electrochemical reversibility.

Electrical resistivity of submicron-diameter carbon-filament compacts

The electrical resistivity of submicron-diameter carbon-filament compacts was decreased by increasing the filament diameter from 0.05 to 0.16 μm, and also by graphitization of the filaments. The use of nickel-coated carbon filaments gave even lower resistivity. The compacts exhibited lower resistivity than the corresponding polymer-matrix composites, such that the difference between compact and composite diminished with increasing filament volume fraction.

Magnetic properties of nickel filament polymer-matrix composites

The magnetic properties of polyethersulfone-matrix composites with 3-19 vol.% polycrystalline nickel filaments (0.4 (im diam) were investigated. These filaments were found to exhibit hysteresis energy loss 10800 J/m3 of nickel and coercive force 16.9 kA/m, compared to corresponding values of 4930 J/m3 and 4.7 kA/m for 2 μ.m diam polycrystalline nickel fibers, 1020 J/m3 and 0.5 kA/m for 20 μm diam polycrystalline nickel fibers, and 1280 J/m3 and 2.3 kA/m for solid polycrystalline nickel.

Electrochemical behavior of hairy carbons

Abstract Hairy carbons were prepared by catalytic growth of carbon hairs (diameter 0.05–0.2 μm, at least partly crystalline) on carbon black, graphite particle and carbon fiber substrates. Of the substrates used, the most abundant hair growth was achieved using carbon black, due to the confinement of the catalyst size by the pores in the carbon black. The cyclic voltammetric response of all hairy carbons was superior to that of the corresponding substrate materials. In the case of hairy carbon black, reversibility was achieved and capacitance, electrochemical area and specific surface area were greatly decreased. In the case of hairy graphite particles and hairy carbon fibers, the electron transfer rate ks (as studied via the Fe 2+ Fe 3+ redox couple) was increased, but, in contrast to the hairy carbon black, capacitance and electrochemical area were increased. Hair growth, followed by an oxidation heat-treatment, resulted in oxidized hairy carbon black which exhibited higher ks and better electrochemical reversibility than as-received carbon black, oxidized carbon black, hairy (but not oxidized) carbon black, carbon filaments (i.e. hair by itself) and hairy carbon fibers. This is partly due to the enhancement of the packing density by both hair growth and oxidation. The oxidized hairy carbon black exhibited much lower capacitance and much lower electrochemical area compared to as-received carbon black. The combination of high ks and low capacitance makes oxidized hairy carbon black technologically attractive for battery and analytical electrodes. The particulate nature of oxidized hairy carbon black is in contrast to the fibrous nature of carbon fibers or carbon filaments, which require secondary operations for dispersion. Additionally, hairy carbon black allows good compaction and electrochemical reversibility without the need for a binder.