High Performance Flexible Temperature Sensors via Nanoparticle Printing

Deformable temperature sensors are required for applications such as soft robotics, biometric sensing, cryopreservation of organs, and flexible electronics. In this paper, we demonstrate Cu–CuNi temperature sensors on flexible Kapton substrates by a novel method consisting of rapid aerosol jet printing of nanoparticles followed by laser sintering at low powers of 100 mW and 400 mW under a shroud of an inert gas to minimize oxidation. The sensors showed a highly linear response as a function of the temperature and the highest sensitivity among film-based sensors yet reported in literature (Rajagopal, M. C.; Sens. Actuators, A 2018, 272, 253; Yang, F.; Sci. Rep. 2017, 7 (1), 1721; Murakami, R.; J. Cryst. Growth 2018, 487, 72–77). The sensor film microstructure was investigated using scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS), transmission electron microscopy (TEM), and selective area electron diffraction (SAED). The Cu and CuNi film morphology consisted of fused nanoparticles with varying degrees of coalescence and porosities ranging from 9% to 24% through the thickness of the films. No surface oxidation was observed for CuNi films but oxide phase was detected for the Cu films, which did not affect the sensor performance after repeated tests up to a temperature of 140 °C. The sensor performance was independent of the manufacturing conditions of the aerosol jet printing process. Flexibility tests showed a stable device performance (variation of Seebeck coefficient within 2.5%) after 200 bending cycles at three different radii and 200 twisting cycles. The superior performance of the sensor films to the bending and twisting tests was attributed to the porosity of the sintered nanoparticles that allows significant strain without a proportional build-up of the stresses in the film. These results demonstrate the suitability of nanoparticle-based bottom-up fabrication methods for a range of deformable high-performance electronic devices.