Polymer-matrix composites containing a high proportion of continuous aligned carbon fibers as the reinforcement are the dominant advanced lightweight structural materials for aircraft, satellites, sporting goods, etc. Although their structural performance is well established, the multifunctionality of these materials is a topic of active research. Multifunctionality means the ability to provide both structural and nonstructural functions. It allows the structure to be inherently smart, without the need to embed or attach devices. Compared to the use of embedded or attached devices, a multifunctional structural material is advantageous in the low cost, high durability, large functional volume and absence of mechanical property loss. Nonstructural functions addressed include the conversion of heat to electricity (i.e., thermoelectricity), heat dissipation (i.e., thermal conduction) and strain/damage monitoring (i.e., sensing). The energy conversion allows the structure to be self-powered. The heat dissipation is important due to the increasing thermal load of aircraft. The monitoring is needed for structural health monitoring, load monitoring and vibration sensing. The attainment of these functions requires the exploitation of thermoelectric, thermal conduction, electrical conduction and piezoresistive properties, which are aspects that have received relatively little attention in relation to structural materials. The focus is on the thermoelectric and conduction behavior in the through-thickness direction and the piezoresistive behavior in the in-plane and through-thickness directions. Through the use of combinations of interlaminar fillers, the thermoelectric power is increased, the thermal conductivity is decreased and the electrical coductivity is increased, so that the dimensionless thermoelectric figure of merit is increased by four orders of magnitude. By using interlaminar filler and increasing the curing pressure during composite fabrication, the thermal conductivity is increased significantly. The unmodified interlaminar interface and the unmodified laminate are effective for electrical-resistance-based sensing of strain and damage. The contact electrical resistivity of the unmodified interlaminar interface is highly sensitive to impact, even impact at only 0.8 mJ. The interlaminar interface allows spatially resolved sensing, due to the two-dimensional array of interface sensors in an interlaminar interface of the laminate. The surface electrical resistance of the unmodified laminate is sensitive to the flexural strain, due to the effect of flexure on the depth of current penetration from the surface. This paper also addresses the materials science of the multifunctionality, particularly in relation to the thermoelectric power and the thermal conductivity of the laminates in the through-thickness direction, with decoupling of the contributions by the laminae and the interlaminar interfaces in the laminate.
University at Buffalo, State University of New York
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