Effect of particle contact on the electrical performance of NTC-epoxy composite thermistors

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As demand rises for flexible electronics, traditionally prepared sintered ceramic sensors must be transformed into fully new sensor materials that can bend and flex in use and integration. Negative temperature coefficient of resistance (NTC) ceramic thermistors are preferred temperature sensors for their high accuracy and excellent stability, yet their high stiffness and high temperature fabrication process limits their use in flexible electronics. Here, a low stiffness thermistor based on NTC ceramic particles of micron size embedded in an epoxy polymer matrix is reported. The effect of particle-to-particle contact on electrical performance is studied by arranging the NTC particles in the composite films in one of three ways: (1) Low particle contact, (2) Improved particle contact perpendicular to the electrodes and (3) dispersing high particle contact agglomerated clumps throughout the polymer. At 50 vol.% of agglomerated NTC particles, the composite films exhibit a β-value of 2069 K and a resistivity, ρ, of 3.3 · 105 Ωm, 4 orders of magnitude lower than a randomly dispersed composite at identical volume. A quantitative analysis shows that attaining a predominantly parallel connectivity of the NTC particles and polymer is a key parameter in determining the electrical performance of the composite film.