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Sonderforschungsbereich 595

Der Einfluss von Feld-Induzierten Phasenumwandlungen an der Blockierkraft in Blei-freien Ferroelektrika

J. Prof. Dr. K. G. Webber (FG Elektromechanik von Oxiden)
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The promising electromechanical behavior displayed by lead-free ferroelectrics has made their use in future commercial actuator applications inevitable.

In addition to producible strain, another figure of merit for electro-active materials to be used in an actuator is the blocking force and the operational range during mechanical loading. Current state-of-the-art blocking force measurement techniques typically measure the actuator free displacement under a given electric field and extrapolate the blocking force with the material compliance. This crudely defines the operational range of the actuator when operating against a spring load. During this simple analysis, it is assumed that the actuator compliance is both temperature and stress independent, which has been clearly shown to not be the case. A further complication is that in typical applications, the piezo-actuator is pre-loaded with a compressive stress to minimize crack growth, whereas the testing method assumes a stress-free state.

Despite the oversimplifications, this technique is commonly used because of its simplicity and low cost. However, the values obtained from such measurements are routinely used in high tech applications that could significantly benefit from a more detailed understanding of the stress- and temperature-dependent operational range of ferroelectric materials. Despite the numerous electrical structural investigations of lead-free materials, there have been to date very few studies on the effect of stress during electrical loading. It is therefore the primary aim of this investigation to understand the influence of temperature and stress on the constitutive behavior of lead-free ferroelectric materials for use in actuator applications.

To accomplish this work a new experimental arrangement will be constructed that will allow for the determination of the actual operating range of ferroelectric materials and will provide the real blocking force.