Abstract
Materials of natural origin have been used for the production of cerment composite resistors. They are Fe-clay-based composite resistors of constant thickness (4.5 mm) and varying lengths ranging between 2.50 and 7.0 mm. The resistors were fabricated by a compaction method, employing a mould with a constant pressure of (1.00 ± 0.02) × 10 7 N/m 2. The aim of this work was the characterization of the resistors which involved the investigation of microstructural changes using scanning electron microscopy (SEM), energy dispersive X-ray fluorescence (EDAX), X-ray and other complementary techniques and measurements of electrical and physical properties with particular attention to electrical resistance, TCR, size effect, etc. The resistors of varying particle size and composition (75–90 wt% Fe) were subjected to varying peak firing temperature ranging between 140 and 900 °C for a period ranging between 1 and 4 h and then furnace cooled to room temperature after firing. The measurements of electrical properties showed that the electrical resistance varied markedly with iron content and also increased nonlinearly with the length of the resistor. The firing sequence (peak firing temperature, firing time and furnace atmosphere) was found to have considerable effect on the resistance of the resistors as this produces modification in the microstructure and chemical composition of the resistors. The resistance falls with increasing firing temperature, being very rapid at the early stage of firing, i.e. at T f between 140 and 600 °C. However, microstructural analyses do not give any significant difference between resistors heat treated at 140 and 600 °C. Fibre-like structures are seen to develop in these materials. EDAX analyses indicate that these fibres are rich in Si, Al, Ca, Na and Mg. All the resistors have negative TCRs with some of the resistors possessing TCR as low as −341.3 ± 13.4 ppm/ °C which is comparable with those of thermistors and other standard resistors.
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