Nigel Philip Harold Fawcett

The electrical response of thick-film resistors to hydrostatic pressure and uniaxial stress between 77 and 535 K

Abstract The longitudinal gauge factor for a thick-film resistor material (Heraeus 8241) printed on an alumina substrate is found to be 12.6 at 295 K. The piezoresistive coefficient G , the unit change in resistivity per unit change in strain is 19.5, with a negative temperature coefficient of −0.00335 K −1 , measured between 77 and 535 K. Thick-film resistors of different geometry were subject to hydrostatic pressure, and by the use of the piezoresistive equation based on elastic theory, and elastic modulus data for Heraeus 8241, G was calculated and found to be 19.7 at 295 K, thus validating the piezoresistive equations and the elastic modulus value for Heraeus 8241. Hydrostatic pressure tests at elevated temperature were believed to be subject to some error, 10% at 500 K, due to adiabatic heating effects. However, it is apparent that resistance change can be predicted with a knowledge of strain in the x , y and z axes. This will prove useful for thick-film strain sensor design, where the thick-film resistor is simultaneously stressed in more than one direction.

A contribution to the debate on the resistance-temperature characteristics of thick-film resistor materials

The almost parabolic resistance–temperature characteristic shown by a thick-film resistor material (Heraeus 8241), printed on an alumina substrate, is explained in terms of thermal expansion effects in the x, y and z directions between 77 and 535 K. The piezoresistive equation is used to strip the thermal expansion contribution from the almost parabolic resistance–temperature characteristic, leaving a near linear resistance–temperature characteristic. It is suggested that the point of inflection on the resistance–temperature characteristic is primarily due to the non-linear temperature coefficient of expansivity of alumina