The change in resistance with respect to temperature was measured for the sample. Fig. 4 shows a plot of resistance (R) as a function of temperature (T) for the fired thermistor films. It can be seen from the graph that, the resistance of the thermistor decreases al- most linearly (i.e. quasi linear behavior) with the increase of temperature. This change in R–T characteristics of thermistor from exponential to quasi linear is mainly due to the contribution of RuO2 in the composition. As stated earlier, RuO2 has relatively low specific resistivity with high positive, linear metallic like dependence of resistivity with respect to temperature. These characteristics of RuO2 in conjunction with spinel phases are responsible for the quasi linear nature which is an important behaviour of thermistors [15].
The data on electrical properties pertaining to the prepared lead free thermistor paste composition along with the data available on ‘lead’ based commercial thermistor paste compositions reported by some researchers [16] is given in Table 1. It is seen from the table that the electrical properties of the prepared thermistor paste composition is compatible with the commercially available lead based NTC thick film compositions. The R–T characteristics of the commercially available thermistor compositions are exponential in nature which may be due to the absence of conductor phase (RuO2) in the reported commercial samples.