Papers by Author: Horst Beige

Abstract: The ultrasound pulse echo overlap method has been used to determine the elastic stiffness coefficients and the corresponding ultrasonic attenuation for a single crystal of the Ni40Ti50Cu10 alloy as a function of temperature. The elastic stiffness coefficients exhibit anomalies near the martensitic phase transition. In the shear stiffness coefficient, corresponding to C44 propagation mode in austenite, a large jump occurs from 36 GPa, above the transition, down to 15 GPa, below the transition. This jump is accompanied by a strong increase in the ultrasonic attenuation. The stiffness coefficients corresponding to C11 and C' = (C11 – C12)/2 modes in austenite show an anomaly at the phase transition, however, these are small effects compared to the one associated with C44 mode. The elastic behavior of this crystal has been characterised down to a temperature of 100 K.

Abstract: The phase diagram of the poly(vinylidene fluoride-trifluorethylene) (P(VDF-TrFE)) copolymer system shows for VDF contents of 50...85 mol% a ferroelectric (FE)-paraelectric (PE) phase transition below melting temperature. Investigations on P(VDF-TrFE) 75/25 samples revealed a slight anisotropic behaviour, which leads to a strongly anisotropic stretching effect both on the phase transition and on the amount and nature of the FE phase in samples subjected to mechanical stretching along the main directions of the film. In this work, both the refractive index n1,2 and the piezoelectric coefficient d33 of mechanically stretched P(VDF-TrFE) have been measured for samples with different levels of permanent deformation. These parameters are found to reflect the anisotropy of the permanently deformed samples. The stretching effect is most pronounced (n1,2) or limited (d33) to the vicinity of the yielding point of the material. Above the yielding point, almost the piezoelectric d33 coefficient of the non-deformed sample is observed for samples with large permanent deformation.

Abstract: In the field of applications of piezoelectric materials properties, functional gradient materials (FGM) are suitable for bending devices due to reduced internal mechanical stresses and lower production costs as well as for ultrasonic transducers because of their increased band width. This paper reports both on preparation, poling, characterization of FGM actuators, and on the description of suitable models of the poling and the bending processes. The calculations of the bending behavior show that the deflection at the end of the cantilever with a continuous gradient still reaches 2/3 of the deflection of a bimorph, whereas the maximum stress goes to zero, which is the main advantage of FGM compared to the commonly used bimorph devices. As a model system with well-defined electromechanical and dielectric properties of the homogeneous components the solid solution of BaTi1-xSnxO3 (BTS) with 0.075 £ x £ 0.15 was chosen. The FGMs approximated by a layered system with a one-dimensional gradient of the Sn-content were prepared both by successive uniaxial powder pressing and by tape casting with the doctor blade method. The chemical gradient was transformed into a gradient of the piezoelectric properties by a poling process. Several models were developed for the description of the non-trivial problem of the poling process in layered systems. The calculated data were compared with experimental results. It was shown that the very small electrical conductivity of the single layers generally cannot be neglected during the poling process and must be incorporated into more sophisticated models. The bending properties of several poled BTS structures with up to 4 layers were measured and discussed.