Effect of Temperature on Piezoresistive Behaviour of Carbon Nanotube Filled Epoxy Nanocomposite Strain Sensors sQRS

Abstract

The incorporation of composite materials in structural applications is experiencing significant growth, with structures such as composite wind turbine blades now reaching 100 m in length and operating under increasingly demanding environmental conditions, including broad temperature ranges. Reliable structural health monitoring (SHM) based on quantum piezoresistive strain sensors (sQRS) therefore requires a thorough understanding of how temperature affects the sensor’s response. In this study, we investigate the effect of temperature, from 25 °C to 150 °C, on the electrical and piezoresistive behaviour of epoxy-based carbon nanotube (CNT) sensors intended for integration into composite structures. We show that below the glass transition temperature (Tg ≈ 95–100 °C, determined by dynamic mechanical analysis) of the epoxy matrix, the sensor’s resistance decreases reversibly and near-instantaneously with temperature, governed by enhanced tunnelling conduction competing with matrix thermal expansion. Under mechanical loading, this thermal sensitivity translates into a marked, though still usable, decrease in gauge factor (56% between −30 °C and 80 °C), attributed to reduced stress transfer and increasing matrix Poisson’s ratio. Above Tg, the sensor’s response becomes irreversible: matrix softening enables dynamic percolation and permanent reorganisation of the CNT network, leading to a substantial, non-recoverable loss of piezoresistive sensitivity. These findings identify Tg as the practical operational limit for reliable strain sensing with sQRS, and demonstrate that below this limit, temperature-induced resistance drift is linear and readily compensated, making the sensor’s thermal response itself a potential tool for in situ detection of the matrix’s physical state.