Piezoelectric and piezoresistive behavior of unmodified carbon fiber

This work reports the piezoelectric and piezoresistive behavior of unmodified unpoled continuous 12 K/24 K carbon fiber (polyacrylonitrile-based, tensile modulus 240 GPa, electrical resistivity 1.6 X 10(-5) Omega m). The first report of the effect of stress on the permittivity of carbon fiber is provided. Under elastic tension (strain <= 0.041%, stress <= 100 MPa), the piezoelectric coupling coefficient d(33) is 5 X 10(-8) pC/N, with the stress-induced electric field increase (<= 103%) contributing 2 X 10(-8) pC/N, the stress-induced permittivity increase also contributing 2 X 10(-8) pC/N, and the stress-induced field-permittivity increase contributing 9 X 10(-9) pC/N. Due to the stress, the relative permittivity (2 kHz)increases by <= 150% (<= 31700) and the DC resistivity decreases by <= 44% (>= 9 X 10(-6) Omega m). The strong negative piezoresistivity, with stress-dependent gage factor ranging from -410 (< 0.02% strain) to -1900 (> 0.02% strain), is attributed to (i) the low modulus and consequent feasibility of preferred orientation increase as the strain/stress increases, and (ii) the low strain and the tendency for high strain (prior work) to cause damage that increases the resistivity, thereby reducing the negative piezoresistivity. Permittivity increase and resistivity decrease result from the increased movement of the charge carriers. All effects are monotonic and reversible, enable piezoelectricity/piezoresistivity-based self-sensing, and are independent of the sizing or the number of fibers per tow. (c) 2019 Elsevier Ltd. All rights reserved.