D.D.L. Chung

Electromagnetic interference shielding effectiveness of carbon materials

Carbon materials for electromagnetic interference (EMI) shielding are reviewed. They include composite materials, colloidal graphite and flexible graphite. Carbon filaments of submicron diameter are effective for use in composite materials, especially after electroplating with nickel. Flexible graphite is attractive for EMI gaskets.

Exfoliation of graphite

The exfoliation of graphite is a phase transition involving the vaporization of the intercalate in the graphite. Exfoliated graphite is an expanded graphite with a low density. This paper reviews the process of the exfoliation of graphite and the exfoliated graphite material. It surveys the applications of exfoliated graphite, covers both reversible and irreversible exfoliation and reviews the methods and mechanism of exfoliation. Other topics include the structure and properties of exfoliated graphite, graphite foils, exfoliated carbon fibres and composites.

Materials for electromagnetic interference shielding

Materials for the electromagnetic interference (EMI) shielding of electronics and radiation sources are reviewed, with emphasis on composite materials and resilient EMI gasket materials, which shield mainly by reflection of the radiation at a high frequency.

Materials for thermal conduction

Materials for thermal conduction are reviewed. They include materials exhibiting high thermal conductivity (such as metals, carbons, ceramics and composites), and thermal interface materials (such as polymer-based and silicate-based pastes and solder).

Carbon fiber reinforced concrete for smart structures capable of non-destructive flaw detection

Electrically conducting concrete, as provided by the addition of a short carbon fibers (0.2-0.4 vol.%) to concrete, can function as smart structure material that allows non-destructive electrical probing for the monitoring of flaws. The electrical signal is related to an increase in the concretes volume resistivity during crack generation or propagation and a decrease in the resistivity during crack closure. The linearity between the volume resistivity change and the compressive stress was good for mortar containing carbon fibers together with either methylcellulose or latex as dispersants. However, the linearity was poor for mortar containing carbon fibers together with both methylcellulose and silica fume, as this mortar required a minimum compressive stress for crack closure, whereas the other two mortars did not.

Electromagnetic interference shielding using continuous carbon-fiber carbon-matrix and polymer-matrix composites

A carbon-matrix composite with continuous carbon-fibers was found to be an excellent electromagnetic interference (EMI) shielding material with shielding effectiveness 124 dB, low surface impedance and high reflectivity in the frequency range from 0.3 MHz to 1.5 GHz. The shielding effectiveness of polymer-matrix composites with continuous carbon-fibers was less and that of polymer-matrix composites with discontinuous fillers was even less. The addition of 2.9 vol.% discontinuous 0.1 μm diameter carbon-filaments between the layers of conventional 7 μm diameter continuous carbon-fibers in a composite degraded the shielding effectiveness. The dominant mechanism of EMI shielding for both carbon-matrix and polymer-matrix continuous carbon-fiber composites is reflection.

Cement reinforced with short carbon fibers: a multifunctional material

This is a review of cement-matrix composites containing short carbon fibers. These composites exhibit attractive tensile and flexural properties, low drying shrinkage, high specific heat, low thermal conductivity, high electrical conductivity, high corrosion resistance and weak thermoelectric behavior. Moreover, they facilitate the cathodic protection of steel reinforcement in concrete, and have the ability to sense their own strain, damage and temperature. Fiber surface treatment can improve numerous properties of the composites. Conventional carbon fibers of diameter 15 μm are more effective than 0.1 μm diameter carbon filaments as a reinforcement, but are much less effective for radio wave reflection (EMI shielding). Carbon fiber composites are superior to steel fiber composites for strain sensing, but are inferior to steel fiber composites in the thermoelectric behavior.

Ozone treatment of carbon fiber for reinforcing cement

Abstract Ozone treatment of isotropic-pitch-based carbon fiber was found to increase the surface oxygen concentration and change surface oxygen from C–O to CO, thereby causing the contact angle between fiber and water to be decreased to zero. Thus, the bond strength between fiber and cement paste was increased and the tensile strength, modulus and ductility of carbon fiber reinforced cement paste were increased. Moreover, the degree of dispersion of fibers in mortar was increased and the effectiveness of the fibers for reducing the drying shrinkage was improved. As a consequence, the strain sensing ability of carbon fiber reinforced mortar was improved in terms of increased gage factor and better repeatability. The ozone treatment did not affect the morphology, tensile strength or volume electrical resistivity of the fiber itself.

Self-monitoring structural materials

Abstract Self-monitoring (or intrinsically smart) structural materials, including concrete containing short carbon fibers, and polymer-matrix and carbon-matrix composites containing continuous carbon fibers, were reviewed. Each material is capable of monitoring its own reversible strain and damage through the effects of these on the electrical resistance of the material. This capability is valuable for structural control and structural health monitoring. Among these three materials, the concrete gives the highest strain sensitivity or gage factor (up to 700), while the carbon-carbon composite gives the highest damage sensitivity (i.e., sensitivity even to the damage after the first cycle of tensile loading within the elastic regime). The origin of the self-monitoring ability differs among the three materials. For the concrete, it is related to slight fiber pull-out during strain and fiber and matrix fracture during damage. For the polymer-matrix composite, it is related to the increase in the degree of fiber alignment and reduction of fiber pre-stress during tension in the fiber direction and to fiber fracture and delamination during fatigue. For the carbon-carbon composite, it is related to dimensional changes during strain and fiber and matrix fracture during damage.

Increasing the electromagnetic interference shielding effectiveness of carbon fiber polymer–matrix composite by using activated carbon fibers

Electromagnetic interference (EMI) shielding is increaspaper is focused on continuous carbon fiber polymer– ingly needed for electronics and radio frequency sources, matrix composites. due to the interference of radio frequency waves (such as The mechanisms of EMI shielding are reflection, abthose from cellular telephones) to digital electronics [1]. In sorption and multiple reflections. Reflection is usually the particular, electronic enclosures, as well as rooms, vaults dominant mechanism, especially for carbon fibers. The and aircraft that house electronics, need to be able to contribution by multiple reflections is usually relatively provide EMI shielding. small for carbon composites. To improve the contribution Due to the desire for light weight for avionic electronics, by reflection, previous work has emphasized the use of laptop computers, aircraft and many other devices, polynickel-coated carbon fibers, as nickel is more conductive mer–matrix composites are increasingly important for EMI than carbon [5,6]. However, the enhancement of the shielding [2–4]. Furthermore, these composites are attraccontribution by multiple reflections by fiber modification tive due to their moldability and processability. has not received previous attention. In this work, we found The polymer matrix in the composites is typically that appropriate activation of the carbon fibers enhances electrically insulating, so it is not able to provide shielding. the contribution of multiple reflections without degrading However, the use of electrically conducting fillers renders the mechanical properties. the composites the ability to shield. Both discontinuous Activation is a method of fiber surface modification that and continuous fillers are used for this purpose. The involves a chemical reaction which causes increase of the discontinuous fillers are advantageous in that they are specific surface area through the formation of surface amenable to the use of extrusion, injection molding and pores. Activated carbon fibers with specific surface area 2 other conventional polymer processing methods for typically exceeding 1000 m /g are used for adsorption, composite fabrication. However, continuous fillers, namely which is relevant to fluid purification. They are also used fibers, are needed to provide high strength and high for the storage of hydrogen, natural gas and other fuels. In modulus, as required for structural applications, such as addition, they are used as electrodes for double-layer aircraft. capacitors and for industrial processes. However, they have Among the continuous fibers, carbon fibers are dominot been previously investigated for use in electromagnetic nant, due to their low density, high modulus, high strength applications such as EMI shielding. and wide availability. Compared to glass fibers, carbon Epoxy (a thermoset) is the dominant matrix used for fibers are attractive in their electrical conductivity, which carbon fiber polymer–matrix structural composites. The relates to EMI shielding effectiveness. Therefore, this epoxy used in this work was EPON Resin 862 together with EPI-CURE 3234 curing agent in the weight ratio 100:15.4 (Shell Chemical Co., Houston, TX). The carbon fiber used in this work was Thornel P-25 *Corresponding author. Tel.: 11-716-645-2593; fax: 11-716(without sizing or twist) from Amoco Performance Prod645-3875. E-mail address: ddlchung@acsu.buffalo.edu (D.D.L. Chung). ucts Inc., Alpharetta, GA. The diameter was 11 mm.