Investigation of Ceramics and Ferroelectric Materials by Atomic Force Acoustic Microscopy

Abstract

In atomic force acoustic microscopy (AFAM) the cantilever is vibrating in one of its resonance frequencies while the sensor tip is continuously in contact with the sample surface. The radius of the contact area depends on the applied static force, the radius of the sensor tip, and the elastic constants of the tip and the surface. It ranges between several nanometers and several tens of nanometers. Polycrystalline materials which appear elastically isotropic on a macroscopic scale are therefore anisotropic on the length scale which is probed by an AFM. When ferroelectric ceramics are imaged, the acoustic image reveals a substructure within the grains indicating variations in contact stiffness. The ferroelectric domains in these materials are usually larger than a typical contact radius between the tip and sample surface. In ferroelectrics an important contribution to the forces results from the electrical polarization of the individual domains. The elastic constant which is probed by a contacting AFM tip is the indentation modulus. Measurements on thin films of nanocrystalline piezo-ceramics revealing their piezoactivity as a function of annealing temperature are discussed. These images show the orientation of the local polarization as well, in particular if the ultrasonic displacements are excited by an ac voltage applied between the tip and a counter-electrode placed below the sample. The ac frequency is adjusted so that it corresponds to the contact resonance. This mode is called by us the ultrasonic piezo-mode. Further applications of the AFAM technique are presented to study fracture mechanical properties of ceramics as well as the growth of the island structure of other thin ferroelectric films.