Flexible Negative Temperature Coefficient Thermistors

Abstract

Flexible electronics have become the subject of industry focus, generating a market desire for fully flexible temperature sensors. To the best of our knowledge, there are no current systems that can adequately satisfy both the mechanical flexibility and sensing performance demanded for commercial applications. This work focused on a design for a flexible temperature sensor by designing a thermistor using functional granular composites using ceramic NTC fibre particles and a resistive polymer matrix. The objective was to achieve good thermistor performance (high sensitivity: B-value > 3000 K, and low resistivity: rho ˜ 10^1 - 10^2 Ocm) while being mechanically flexible (surviving bending radii = 1 cm over 100 cycles). To achieve this objective this thesis work was organised into 5 major phases. Due to the high commercial value of NTC ceramic thermistors, no reference composition with its exact processing conditions are available in literature that produce a thermistor with commercially desirable properties. Phase 1 was therefore to select a NTC ceramic composition and Phase 2 was to investigate the necessary processing conditions to be able to produce bulk ceramics with adequate temperature sensing performance. Phase 3 involved making NTC ceramic fibres by wet fibre spinning and modifying the processing conditions developed in Phase 2. Phase 4 involved the selection of an appropriate polymer matrix and to attempt to create thermistor composites. Phase 5 involved creating more optimal composite thermistors using stencil printing and to test its thermistor and bending performance. Using an isotropic mixture of Mn2.4 Ni0.5 Cu0.1 O4 NTC ceramic fibre particles in an Electrodag matrix, composite thermistors were created by stencil printing onto thin-film substrates of polyethylene terephthalate that have silver inkjet printed interdigitate electrodes. A percolated network is formed with as little as 10 wt% of fibres (˜ 2.86 vol%). The composite with 10 wt% of fibres has good thermistor performance at a low fibre content. The average resistivity at 25°C (rho_25) is low around 920 · 10^3 Ocm (with a standard deviation of 220 · 10^3 Ocm) and the sensitivity is high with a B-value = 3290 K and alpha = -3.7 %/K. The overall composite resistance and sensitivity can be tailored and customised by changing the gap width of the interdigitate electrodes. At an electrode gap width of 0.13 mm, with our interdigitate electrodes, R_25 = 280 kO and R_85 = 45 kO. The composite with 10 wt% fibre particle content and an electrode gap of 0.13 mm can survive 300 bends cycles (100 cycles with bending radius 10 mm (strain ˜ 0.621%), 100 cycles with bending radius 5 mm (strain ˜ 1.23%), 100 cycles with bending radius 2.5 mm (strain ˜ 2.44%) without any observable, aesthetic damage. The relative resistance (R/R0) does increase with bending, but after 20 bend cycles it appears to stabilise for every bending radius. In comparison to other flexible temperature sensors in recent literature, this thermistor composite is found to deliver a superior combination of thermistor performance and mechanical flexibility.