Investigation Of Capacitive Thin Film Strain Gauges

Strain gauges play an important role in the research of the mechanical properties and mechanical behavior of the various parts. At present, these gauges rely on the piezoresistive effect, which means resistance is proportional to the change of strain. Resistance strain gauges is easily oxidized at high temperatures and its stability and reliability is poor, so it can’t meet the application requirements of aerospace, aviation, nuclear power and other fields in high temperature environment. However, the capacitive strain gauge strain’s sensitive media commonly used dielectric material, so it have advantages of high temperature stability and better immunity to noise. In this study, barium strontium titanate(Ba0.5Sr0.5TiO3, BST) as the strain-sensitive media of capacitive strain gauge, it is possible to increase the strain sensitivity coefficient since the dielectric constant of BST is sensitive to strain.In this paper, we contained rf magnetron sputtering deposition and micro machining process technology to develop BST capacitive strain gauges which made BST ceramic thin film as the strain layer, and compared to the Al2O3 capacitance strain gauges which made alumina as a strain-sensitive layer. We studied their gauge factors, dielectric properties and temperature characteristics. The main research contents and results are as follows:1. We designed and fabricated planar structure interdigital thin film BST capacitive strain gauge. The BST thin film was prepared by magnetron sputtering method and PdCr electrode was prepared by DC sputtering method, then through the lithography process and lift-off technology to achieve the interdigital electrodes’ graphics.2. We tested the frequency properties and temperature properties of BST capacitive strain gauge. The results showed that when the frequency in the vicinity of 100 kHz, the capacitance value of the strain gauge was more stable, and the dielectric loss was small, therefore we chose 100 kHz as the measurement frequency. With temperature increased, the capacitance of the BST strain gauges increased before decreased and the temperature coefficient of capacitance(TCC) changed from negative to positive.3. We studied the strain sensing properties of BST capacitive strain gauges and Al2O3 capacitance strain gauges. The flexural strain sensing properties of devices were investigated using a cantilever beam arrangement firstly. Test results indicated that for the BST capacitive strain gauge, due to the dielectric of BST increased with strain increased, thus the capacitance value increased. Through computing, the GF is 3.13 and the hysteresis is 8.9%. For the Al2O3 capacitance strain gauges, due to the electrode spacing increased with strain increased, thus the capacitance value decreased. The GF is-3.4 and the hysteresis is 11.5%. Secondly, we tested the shear strain sensing properties of gauges by Zwick tensile machine, the GF of BST capacitive strain gauge and Al2O3 capacitance strain gauges are 2 and-10.8 respectively. GF factor trend of two testing methods is the same but the value is different. The reason may be the direction of flexural strain and shear strain is different, dielectric constant of BST which due to its anisotropy is more conductive to the flexural strain, so the GF of BST capacitive strain gauge which tested by cantilever beam is bigger. Shear strain is more sensitive to the change of interdigital electrode spacing, so the GF of Al2O3 capacitance strain gauges which tested by Zwick tensile machine is bigger.