Xiangcheng Luo

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.

Carbon-fiber/polymer-matrix composites as capacitors

Abstract A continuous carbon-fiber/epoxy-matrix composite with a paper interlayer (0.04 mm thick after composite fabrication) was found to exhibit a capacitance of 1.2 μF/m2 at 2 MHz, in contrast to a value of 0.21 μF/m2 for epoxy-impregnated paper (0.10 mm thick). The high capacitance is partly a consequence of the large area of the surface of a fiber layer sandwiching the paper interlayer. This area is twice the flat area. Without a paper interlayer, the composite failed to serve as a capacitor, because of the conductivity in the through-thickness direction.

VIBRATION DAMPING USING FLEXIBLE GRAPHITE

Vibration damping is valuable for structures, as it material for high temperature or chemically harsh environmitigates hazards (whether due to accidental loading, wind, ments. ocean waves or earthquakes), increases the comfort of Due to its resilience, in addition to its thermal resistance, people who use the structures, and enhances the reliability chemical resistance, low thermal expansion and high and performance of structures. The basic concept about thermal conductivity, flexible graphite was investigated in damping involves the absorption of external energy this work for use in vibration damping. The investigation through internal motion or friction [1,2]. A layered strucinvolved simultaneous measurement of the loss tangent ture is attractive for damping due to the large internal (tan d, i.e. damping capacity) and storage modulus (stiffsurface area involved [3,4]. The relative motion between ness) under dynamic flexure (three-point bending at a very layers produces extra shear, which means more energy small deflection amplitude) at fixed frequencies. The dissipation [5]. For instance, a viscoelastic material or a product of loss tangent and storage modulus is the loss fluid layer is sandwiched within a beam for both passive modulus. As high values of both loss tangent and storage and active damping [6–8]. However, fluid and viscoelastic modulus are desired for vibration reduction, a high value materials suffer from their poor stiffness, limited resistance of the loss modulus is desired. Rather low frequencies are to heat and chemicals, in addition to high thermal expanused in this study due to their relevance to the vibration of sion and poor thermal conductivity, which aggravate large structures and due to the fact that the loss tangent thermal stresses. decreases with increasing frequency and hence becomes Flexible graphite is a flexible sheet made by compreshard to measure at a high frequency. For the sake of sing a collection of exfoliated graphite flakes without a comparison, the loss tangent and storage modulus of binder [9–18]. Due to the exfoliation, flexible graphite has rubber were also measured. 2 21 a large specific surface area (e.g. 15 m g [19]). As a Flexible graphite sheet (Grade GTB) was provided by result, flexible graphite is used as an adsorption substrate. EGC Enterprises, Inc. (Mentor, Ohio). The specific surface 2 21 Due to the absence of a binder, flexible graphite is area was 15 m g , as determined by nitrogen adsorption essentially entirely graphite (other than the residual amount and measurement of the pressure of the gas during of intercalate in the exfoliated graphite). As a result, adsorption using the Micromeritics (Norcross, GA) ASAP flexible graphite is chemically and thermally resistant, and 2010 instrument. This specific surface area corresponds to low in coefficient of thermal expansion (CTE). Due to its a crystallite layer height of 0.18 mm within a sheet. microstructure involving graphite layers that are preferenAccording to the manufacturer, the ash content of flexible 23 tially parallel to the surface of the sheet, flexible graphite is graphite is ,5.0%; the density is 1.1 g cm ; the tensile high in electrical and thermal conductivities in the plane of strength in the plane of the sheet is 5.2 MPa; the the sheet. Due to the graphite layers being somewhat compressive strength (10% reduction) perpendicular to the connected perpendicular to the sheet (i.e. the honeycomb sheet is 3.9 MPa; the thermal conductivity at 10938C is 43 21 21 microstructure of exfoliated graphite), flexible graphite is W m K in the plane of the sheet and 3 W m K electrically and thermally conductive in the direction perpendicular to the sheet; the coefficient of thermal 26 perpendicular to the sheet (although not as conductive as expansion (CTE) (21–10938C) is 20.4310 / 8C in the the plane of the sheet). These in-plane and out-of-plane plane of the sheet. microstructures result in resilience and impermeability to Dynamic mechanical testing (ASTM D4065-94) at fluids perpendicular to the sheet. The combination of controlled frequencies (0.2, 1.0 and 5.0 Hz) and room resilience, impermeability and chemical and thermal resisttemperature (208C) was conducted under flexure using a ance makes flexible graphite attractive for use as a gasket Perkin-Elmer Corp. (Norwalk, CT) Model DMA 7E dynamic mechanical analyzer. Measurements of tan d and storage modulus were made simultaneously at various frequencies. The specimens were in the form of beams of *Corresponding author. Tel.: 11-716-645-2593; fax: 11-716length at least 25 mm under three-point bending, with the 645-3875. E-mail address: ddlchung@acsu.buffalo.edu (D.D.L. Chung). span being 20 mm. The width of the flexible graphite

Graphite-graphite electrical contact under dynamic mechanical loading

Due to its electrical conductivity, thermal conductivity, with wear or abrasion studies stems from the fact that wear oxidation resistance and wear resistance, graphite is used or abrasion involves one element sliding against another, as an electrical contact material, particularly in sliding so that different points in a contact are not subjected to conditions, as encountered by brushes for electric motors dynamic stress in an in-phase manner. In contrast, dynamic and other devices and by sliding electrical contacts for compression does not involve sliding, so that each point in trams and other electric vehicles [1–8]. To further improve a contact is subjected to dynamic compression in an the conductivity, copper impregnated graphite may be used in-phase manner, i.e. all points experience the maximum [9,10]. Because of this application, the quality of graphite– compressive stress in a cycle simultaneously and all points graphite electrical contacts over time under dynamic experience the minimum stress in a cycle simultaneously. mechanical loading is of interest. Relevant questions As a result, correlation is possible between the effect (say concern how elastic and plastic deformations at the contact the contact electrical resistance at the contact) and the interface (particularly at the asperities) affect the quality of dynamic stress during dynamic loading. This correlation the electrical contact under mechanically loaded and allows identification of the point in a stress cycle at which unloaded conditions, and how these effects depend on the certain effect occurs, and moreover allows distinction stress amplitude and the number of loading cycles. between reversible effects (such as elastic deformation, Wear or abrasion involves subjecting each point of a which vanish upon unloading) and irreversible effects surface to dynamic shear. Studies of wear or abrasion are (such as plastic deformation, which remain upon unloadcommonly conducted by monitoring the effect over an area ing). Therefore, by studying the effect of dynamic comrather than that at a fixed point. For example, in wear pression rather than dynamic shear, this paper provides testing using the pin-on-disk configuration, the tip of the new information on the dynamic mechanical behavior of pin is continuously moved against the surface of the disk, contacts, i.e. the behavior of contacts under dynamic so that different points on the disk are subjected to stress at loading. different times and the effect of dynamic shear and the The technique used in this work for studying the stress variation within a cycle of dynamic shear at a dynamic mechanical behavior of contacts is contact electriparticular point of the surface are not monitored. Even if cal resistance measurement during dynamic compression the effect of wear or abrasion is monitored in real time, say below the yield stress. It involves simultaneous electrical by measuring the contact electrical resistance at the sliding and mechanical measurements. The technique requires that contact between the pin and the disk, the monitoring does the elements in contact are electrically conducting. The not allow correlation of the effect (say the electrical contact resistance of the interface between contacting resistance) at a point with the dynamic stress at the point elements can be conveniently measured by using the within a stress cycle. (The dynamic stress is to be elements as electrical leads — two for passing current and distinguished from the stress amplitude.) This difficulty two for voltage measurement (i.e. the four-probe method), as provided by two elements (beams) that overlap at 908 (Fig. 1). The volume resistance of each lead was negligible *Corresponding author. Tel.: 11-716-645-2593; fax: 11-716compared to the contact resistance of the junction, so the 645-3875. E-mail address: ddlchung@acsu.buffalo.edu (D.D.L. Chung). measured resistance (i.e. voltage divided by current) was

Flexible graphite under repeated compression studied by electrical resistance measurements

Flexible graphite (a gasket material) under repeated compression was studied by real-time measurement of the electrical resistance perpendicular to the flexible graphite sheet, which was sandwiched by copper. The resistance decreased reversibly upon compression perpendicular to the sheet, due mainly to the reversible conformability of flexible graphite and the consequent reversible decrease of the contact resistivity between flexible graphite and copper. Two cycles of compression largely eliminated the irreversible resistance and strain changes. A low stress amplitude (<4 MPa) and a low strain amplitude (<25%) were necessary in order to minimize irreversible deformation of the flexible graphite itself.

Tribology of Graphite and Concrete, Studied by Contact Electrical Resistance Measurement During Cyclic Compression

The tribology of graphite and cement mortar was studied by contact electrical resistance measurement during cyclic compression. Elastic deformation and plastic deformation at asperities were distinctly observed through the reversible and irreversible decreases, respectively, of the contact resistance upon loading. Elastic deformation was dominant at the maximum stress. Plastic deformation progressed and then saturated upon stress cycling.