All Screen Printed and Flexible Silicon Carbide NTC Thermistors for Temperature Sensing Applications

Abstract

In this study, Silicon Carbide (SiC) nanoparticle-based serigraphic printing inks were formulated to fabricate highly sensitive and wide temperature range printed thermistors. Inter-digitated electrodes (IDEs) were screen printed onto Kapton^® substrate using commercially avaiable silver ink. Thermistor inks with different weight ratios of SiC nanoparticles were printed atop the IDE structures to form fully printed thermistors. The thermistors were tested over a wide temperature range form 25 °C to 170 °C, exhibiting excellent repeatability and stability over 15 h of continuous operation. Optimal device performance was achieved with 30 wt.% SiC-polyimide ink. We report highly sensitive devices with a TCR of -0.556%/°C, a thermal coefficient of 502 K (β-index) and an activation energy of 0.08 eV. Further, the thermistor demonstrates an accuracy of ±1.35 °C, which is well within the range offered by commercially available high sensitivity thermistors. SiC thermistors exhibit a small 6.5% drift due to changes in relative humidity between 10 and 90%RH and a 4.2% drift in baseline resistance after 100 cycles of aggressive bend testing at a 40° angle. The use of commercially available low-cost materials, simplicity of design and fabrication techniques coupled with the chemical inertness of the Kapton^® substrate and SiC nanoparticles paves the way to use all-printed SiC thermistors towards a wide range of applications where temperature monitoring is vital for optimal system performance.

Keywords: negative temperature coefficient (NTC); printed electronics; printed temperature sensors; screen printing; silicon carbide; silver ink; temperature sensing; thermistors; wide band-gap semiconductor.

Figure A1

SEM micrographs of (a,b) 31 wt.%, (c,d) 32 wt.%, (e,f) 33 wt.%, (g,h) 34 wt.% SiC thermistor inks.

Figure A1

SEM micrographs of (a,b) 31 wt.%, (c,d) 32 wt.%, (e,f) 33 wt.%, (g,h) 34 wt.% SiC thermistor inks.

Figure A2

Current-voltage curve to determine contact resistance of Ag-SiC interface.

Figure A3

Electrical resistance versus temperature of three individual 30 wt.% SiC thermistors over 5 thermal cycles each.

Figure 1

SiC thermistor schematic: (a) top view, (b) side view.

Figure 2

(a–c) SiC thermistor fabrication process. (d) High-resolution optical microscopy image of printed SiC thersmitor.

Figure 3

Experimental setup.

Figure 4

Characterization of commercially sourced 3C−SiC particles. (a) XRD spectra, (b) Raman spectra, (c) UV-VIS spectra with Tauc plot as inset to determine electronic band gap, (d) SEM micrograph.

Figure 5

EDX imaging of (a) SiC particles representing: (b) Si, (c) C and (d) O species.

Figure 6

(a,b) TEM micrographs and (c) SAED pattern of SiC particles.

Figure 7

(a) Current–voltage characteristics and (b) electrical conductivity of 30 wt.%, 35 wt.% and 40 wt.% SiC thermistor inks.

Figure 8

SEM micrographs of (a,b) 30 wt.%, (c,d) 35 wt.% and (e,f) 40 wt.% SiC thermistor inks.

Figure 9

(a) Electrical resistance versus temperature of 30 wt.%, 35 wt.% and 40 wt.% SiC thermistors. (b) Long-term cycling, (c) thermal stability, (d) humidity stability, (e) change in baseline resistance after bend testing and (f) electrical resistance versus temperature plot after bend testing of 30 wt.% SiC thermistor.

Figure 10

Accuracy of printed 30 wt.% SiC thermistor calculated via the Steinhart’s equation.