A characterization of thick-film PTC resistors

Thick-film PTC resistors (5093, Du Pont, 1 kΩ/sq.) with a high, linear and positive TCR were fired at temperatures between 750 and 950 °C. The development of the resistors' conductive phase and microstructure was investigated by X-ray powder-diffraction analysis and by scanning electron microscopy, respectively. Temperature coefficients of resistivity, sheet resistivities and noise indices were measured as a function of firing temperature. The 5093 resistor material is based on ruthenate, which decomposes during firing at temperatures over 800 °C into RuO2. As the needle-like RuO2 crystals form, the sheet resistivities decrease from very high values to a nominal resistivity of around 1 kΩ/sq. At firing temperatures higher than 850 °C the volume of the single-crystal RuO2 grains increases and therefore their number in a given volume of thick-film PTC layer decreases. The network of “needles ” starts to break, leading to increased sheet resistivities and increased noise indices.