Flexible and patterned-free Ni/NiO-based temperature device on cylindrical PET fabricated by RF magnetron sputtering: Bending and washing endurance tests

Flexible Resistive Temperature Detectors (RTDs) based on metal thin film have broad application prospects in smart clothes, however, they are constructed with patterned metal films using complicated manufacturing processes. Herein, we report a simple radio frequency (RF) magnetron sputtering method to fabricate light-weight, inexpensive and flexible temperature sensors (TS), whose sensitive materials act as electrode itself composed of Ni, NiO and Ni/NiO bilayer thin films. The Ni/NiO bilayer TS could be bent in different radius with resistance change below 5% signifying high flexibility. Fabricated Ni/NiO bilayer TS exhibited superior sensing performance with temperature coefficient of resistance (TCR) of 3.8 × 10⁻³ °C⁻¹ compared to 3.2 × 10⁻³ °C⁻¹ and 3.1 × 10⁻³ °C⁻¹ for Ni and NiO thin films respectively. It showed a linear response with R²-value of 0.9852. The Ni/NiO bilayer TS showed TCR of 1.0 × 10⁻⁴ °C⁻¹ even after 16,000 repetitive mechanical bends. Ni/NiO temperature sensor maintains high TCR towards exposure to different chemical solutions (acid, alkaline, detergents). The easy-fabrication process with good surface condition, together with the demonstrated results, suggest Ni/NiO is a promising candidate for flexible RTD device.