Effect of nickel coating on the stress-dependent electric permittivity, piezoelectricity and piezoresistivity of carbon fiber, with relevance to stress self-sensing

Abstract

This paper unprecedentedly reports the effect of metal (nickel) coating on the stress-dependent electric permittivity, piezoelectricity and piezoresistivity of carbon fiber. Both permittivity (2 kHz) and DC conductivity of carbon fiber are increased by nickel coating. For 7-μm diameter carbon fiber, nickel coating (0.25-μm thickness) increases the relative permittivity from 12,200 to 63,200, and decreases the resistivity from 1.5 × 10−5 to 1.5 × 10−7 Ω.m. The relative permittivity of the nickel coating (Rule of Mixtures) is 404,600 - similar to 405,300 for nickel wire (160-μm diameter). The resistivity of the nickel coating (Rule of Mixtures) is 2.0 × 10−8 Ω.m - lower than 8.8 × 10−8 Ω.m for the nickel wire - probably because of the higher degree of preferred crystallographic orientation in the nickel coating. The nickel structure affects the conduction more than polarization. The piezoelectric and piezoresistive effects are diminished by the nickel coating, which governs these effects. The nickel coating changes the stress dependence of the permittivity (for capacitance-based self-sensing) from positive to negative and changes the piezoresistivity (for resistance-based self-sensing) from negative (gage factor −1830) to positive (gage factor +1650). The piezoelectricity of the nickel-coated carbon fiber and nickel wire are similar, but the piezoresistivity is weak for the latter (gage factor +30).