Elastoresistance Effects in Evaporated Bismuth Films

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Abstract
For the purpose to use bismuth films as strain gauges or strain transducers, about one hundred bismuth films were evaporated onto glass substrates in an ultra-high vacuum under various conditions, and their characteristics were measured. In carrying out measurements of the change in resistance with mechanical stress or strain (piezo- or elasto-resistance effect), the glass substrate was supported in the form of a cantilever and uniaxial stress was applied parallel to the film. The following characteristics were observed: The transverse elastoresistance is greater than the longitudinal effect; the gauge factor increases when the temperature is lowered; although relaxation phenomena exist in the elastoresistance in bismuth, they can be cancelled by means of the combination of both the transverse and longitudinal effects; inversion of the signs exists in such coefficients as the longitudinal gauge factor, Hall coefficient, and temperature coefficient of resistance at film thicknesses around 0.5∼1µ. Some further experimental investigations were carried out on films deposited on thin epoxy-resin substrates. They were used as surface strain gauges and showed a gauge factor of approximately 15 at room temperature, and no significant changes were observed under a long-run test of continuous vibrations of 106 repetitions with a strain of ±3.3×10-4. Response to an ultrasonic vibration of the frequency 20 kc/s was also found to be good. There is still a problem in determining the proper conditions for the production of the films as strain gauges.