Nanoscale Characterization of Lead-Free Piezoceramics Using Atomic Force Microscopy

Abstract

The last 35 years have seen a tremendous advancement in atomic force microscopy (AFM) in terms of its versatility and resolving power in exploring the functional properties of materials. Among them, the introduction of the piezoresponse force microscopy (PFM) technique, pioneered by Güthner and Dransfeld in 1992, has turned into a mainstream method for probing and controlling the static and dynamic properties of nanoscale ferroic structures and devices. PFM enables non-destructive visualization and manipulation of ferroelectric nanodomains and direct measurements of the local physical characteristics of ferroelectrics. Using the PFM technique, the work in this thesis is dedicated to studying ferroelectric domain structure in lead-free piezoceramics from two perspectives. On the one hand, the underlying mechanisms of measured functional properties in piezoceramics have been probed by direct observation with PFM. On the other hand, the domain structure evolution of piezoceramics under external stimuli has been visualized, thereby revealing their potential applications. Several different AFM techniques, including standard PFM, Kelvin probe force microscope (KPFM), and switching spectroscopy PFM, have been utilized for making the comprehensive study. Firstly, the evolution of the domain structure of a lead-free Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) piezoceramic under temperature and electric field stimulation on micrometer and nanometer scales was studied. The PFM results highlight the critical role of wedge-shaped domains in domain evolution. Transitional domain structures with an increased density of nanodomains appear in both the thermal and poling cycles. Interestingly, the electric-field-dependent domain structure evolution at different temperatures shows better domain structure reversibility at high temperatures than at temperatures close to the phase boundary, implying a slow rate of fatigue for the functional properties in this temperature range. Next, the unipolar fatigue behavior of three BZT-xBCT compositions with different crystallographic structures, i.e., 40BCT(R), 50BCT(O) and 60BCT(T), were evaluated. PFM studies were performed to relate the fatigue behavior to the different strain mechanisms of each of the three studied compositions. PFM domain maps indicate that the high amount of extrinsic contributions to strain made orthorhombic 50BCT(O) and rhombohedral 40BCT(R) compositions most susceptible to fatigue during unipolar cycling. Unlike them, the tetragonal composition 60BCT(T) has a high amount of intrinsic contributions to strain, making it more resistant to electric fatigue, resulting in relatively stable electromechanical properties. Na1/2Bi1/2TiO3-based compositions have been another promising candidate for lead-free piezoceramics. With the inclusion of ZnO, the depolarization temperature of 0.94Na1/2Bi1/2TiO3-0.06BaTiO3:0.1ZnO relaxor ferroelectric/semiconductor composites is enhanced. Room temperature PFM data directly demonstrate a long-range ferroelectric order induced by the ZnO inclusion in NBT-6BT:0.1ZnO composites. Also, PFM results show a slow rate of depolarization after poling in NBT-6BT:0.1ZnO composites. Compared to pure NBT-6BT ceramics, site-specific PFM hysteresis loops were acquired to reveal the modification of local ferroelectricities with the ZnO inclusion. In addition, I tried to map the domain structure of BT under different creep mechanisms with the PFM technique. Taking advantage of the unique advantages of AFM in terms of good spatial resolution and versatility, this thesis presents four studies on lead-free piezoceramics in terms of structural morphology, domain structure, domain wall dynamic, local hysteresis properties, and local potential. It provides an in-depth understanding of functional behaviors in piezoceramics from the structure point of view, primarily the ferroelectric domain (wall) structure