Improvements in metrology of non-linear piezoelectrics

Abstract

Applications exist for piezoelectric, polycrystalline ceramic devices that subject them to an array of mechanical and electrical stresses. Manufacturers' data relating performance of their materials is becoming less relevant as the materials are employed under conditions far removed from the characterisation conditions. Current constitutive equations do not adequately predict the behaviour of ferroelectric devices. The continuing materials metrology project sponsored by the National Physical Laboratory within the Materials Testing and Standards programme supported by the U.K. Department of Trade and Industry aims to address and resolve the aforementioned problems by examining fundamental material behaviour and the influence of testing procedures on data, leading to improved product design and performance. PZT ceramics were subject to longitudinal, compressive impact loading ranging from 32 to 82 μs in duration and 2 to 40 MPa in magnitude. Values of the piezoelectric charge coefficient, d₃₃ were derived and compared with those obtained via the conventional low frequency (100 Hz), low stress (10 kPa) Berlincourt method. Results obtained from the impact rig gave d₃₃ values ranging between 50 and 200 % of those gained from the Berlincourt rig. The charge coefficient is a non-linear function of the stress magnitude and an inverse function of the stressing frequency. The degree of d₃₃ variation with stress is related to the “ferroelectric hardness” of the material^[1]. Fatigue of the piezo devices was also studied. The PZT materials displayed a drop in the values of d₃₃ ranging between ∼ 20 and 50 % when subject to 5000 impacts, 40 MPa in magnitude and 82 μs in duration. Samples of X-cut α quartz were subject to the same tests as a reference exercise. It is evident that the mobility of the ferroelectric domains and therefore the electric current they produce can be correlated with the stress to which it is subjected and that repeated impacting of the PZT materials leads to rapid depolarisation, microcracking and a deterioration of the piezoelectric properties. An examination using SEM was carried out to locate sites of microcracking and to determine the effects of microstructure on fatigue behaviour.