Piezopermittivity for capacitance-based strain/stress sensing

Abstract

Piezopermittivity refers to the reversible change of the electric permittivity (the main material property that describes the dielectric behavior) with the elastic strain. This paper addresses the emerging field of piezopermittivity, which provides the basis for capacitance-based strain/stress sensing, including self-sensing in case of structural materials. Piezopermittivity differs from piezoresistivity, which allows resistance-based strain/stress sensing. It also differs from the direct piezoelectric effect, which does not require the permittivity to change. In order to establish piezopermittivity, this paper presents the piezopermittivity theory, sensing methodology, and piezopermittivity-related materials science. Piezopermittivity stems from the effect of strain on the microstructure, which affects the permittivity. It is exhibited by electrical conductors (metals, carbons, and carbon fiber polymer-matrix and carbon-matrix composite) and nonconductors (perovskite ceramic and 3D-printed polymer), which are comparably effective for capacitance-based sensing, as shown at frequencies ≤2 kHz. All materials are unpoled. Poling of the perovskite ceramic does not alter the piezopermittivity behavior. The magnitude of the fractional change in permittivity much exceeds the strain magnitude, as expected for piezopermittivity. The sensing effectiveness, as described by the fractional change in permittivity per unit strain, is positive for positive piezopermittivity and negative for negative piezopermittivity. The majority of the materials exhibit positive piezopermittivity. The sensing effectiveness is +1.99×103 and -4.81×102 for uncoated and nickel-coated carbon fibers, respectively. The value for the uncoated carbon fiber is close to the value of +1.21×103 for a perovskite ceramic. For a 3D-printed polymer, the value is -2.87×106 in the direction perpendicular to the printed layers. Capacitance-based sensing is advantageous to the widely reported resistance-based sensing in that it does not require intimate contact of the electrodes with the specimen. The measurement of the capacitance of a conductive material using an LCR meter requires a dielectric film between the electrode and specimen.