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Title: Stress sensing in layered ceramic structures
Author: Castle, R. C.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2000
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When a ceramic laminate consisting of alternate dielectric and conducting layers is loaded, the dimensions of the laminae will change, leading to a corresponding change in capacitance across the dielectric layers. The effect of mechanical strain on the capacitance of individual dielectric layers was studied by measuring the capacitance changes in the outer layer of Al2O3/Pt and TiO2/Pt laminates loaded in three point bending. Metal conducting layers were used in this work to provide a simple conductor at room temperature, which would be compatible with both ceramic materials. It is shown that in both materials systems the measured capacitance at zero load agrees well with the predicted values for a parallel plate capacitor, and that on loading the capacitance varies linearly with strain. The rate of change of capacitance with strain agrees well with values predicted from dimensional changes in the dielectric layer, and is observed to change sign when the sign of the strain is reversed. The change in capacitance is also observed when the laminates are unloaded and reloaded, and it is shown that the rate of change of capacitance is independent of the loading rate. The measured capacitance is found to be a sensitive function of the a.c. frequency used in the measurement circuit, and it is shown that this is due to the presence of inductance in the circuit which originates from the connecting wires. The rate of change of capacitance with strain is also found to vary significantly with the a.c. frequency. This results from a variation of inductance with strain which is thought to be due to the small movement of the connecting wires when the laminate to which they are attached is deflected in bending. This necessitates the use of low a.c. frequencies at which inductance effects are negligible. The temperature of the dielectric is also found to influence the measured capacitance significantly, and it is shown that in A12O3 this can be overcome by comparing the capacitance of the strained body with another at the same temperature under zero strain. However this technique cannot be used in TiO2 because of the large temperature coefficient of capacitance of this material.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available