Ferroelectric Dielectric And Piezoelectric Properties Of Ferroelectric Thin Films And Ceramics
Eliz Cisneroz
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Understanding the grain size effects that govern crystal structure and the functional properties of ferroelectrics is of vital importance in improving the performance of ferroelectric systems, which are embedded in a number of electronic devices, such as sensors, actuators, transducers and non-volatile memories1,2,3. Due to a growing demand for miniature devices, significant progress in the fabrication of micro-, meso- and nano-scale ferroelectric structures has been made4,5. A fundamental understanding of grain size effects on the dielectric and ferroelectric properties was achieved by studying low-dimensional ferroelectric structures6,7,8,9,10,11,12. Theoretical and experimental studies on thin/ultrathin films6,7,8, nanowires9,10 and other types of nano-dimensional systems11,12 have shown that ferroelectricity persists down to the nanoscale, thereby demonstrating their potential for use in miniature devices. Nevertheless, certain applications require bulk components with specific functional properties, which can be directly obtained from a specific grain size. Although grain size effects on the dielectric, piezoelectric and ferroelectric properties have been widely studied in several ferroelectric bulk systems, there are still a number of aspects which remain unclear. These are mainly related to the grain size dependency of the piezoelectric and ferroelectric properties, often showing discrepancies in the existing literature. In addition, there are a number of other factors that could influence the grain size dependency; their identification is the main aim of the present study. Barium titanate ceramics are chosen as a model ferroelectric system for this research.
These differences demonstrate that processing conditions can have a significant influence on the physical properties of ferroelectric ceramics. However, at present, the detailed mechanisms for the grain size dependence of the piezoelectric properties in the BaTiO3 ceramics prepared using different processing methods and different starting materials are still unclear.
It is well known that the dielectric and piezoelectric properties of ferroelectric ceramics include intrinsic and extrinsic contributions; the former originates from the deformation of the unit cell under an external electric or mechanical field, while the latter is mainly due to domain wall movement and point defects42,43,44,45,46,47,48. The domain wall contribution is determined by the domain wall density and domain wall mobility, which are also both influenced by many factors including grain size, back fields and defects42,43,44,45,46,47,48. The differences in the grain size dependence of the piezoelectric properties of the CS and SPS BaTiO3 ceramics can be interpreted based on the following aspects.
Regarding the ferroelectric properties, it was observed that the coercive field of SPS ceramics decreases with increasing grain size, while in CS ceramics it increases in ceramics with larger grains. The latter is attributed to an increased pinning effect on domain walls by point defects developed in ceramics sintered at high temperature. In ceramics where the point defects contribution is not dominant, the maximum and remnant polarization increase with increasing grain size.
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Barium strontium titanate (Ba0.8Sr0.2TiO3, BST) nanocrystalline ceramics have been synthesized by high energy ball milling. As the sintering temperature increases from 1200 C to 1350 C, the average grain size of BST ceramics increases from 86 nm to 123 nm. The X-ray diffraction (XRD) studies show that these ceramics are tetragonal. The phase and grain size of the sintered pellets have been estimated from the XRD patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The effect of grain size on dielectric and ferroelectric properties is studied. The dielectric and piezoelectric parameters are greatly improved at room temperature with increase in grain size. The Curie transition temperature is found to shift slightly towards higher temperatures as the grain increases from 86 nm to 123 nm. The coercive field decreases and the remnant polarization and spontaneous polarization increase as the grain size of BST nano ceramics increases. These ceramics are promising materials for tunable capacitor device applications.
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Susan Trolier-McKinstry is an Evan Pugh University Professor and Steward S. Flaschen Professor of Ceramic Science and Engineering, and Professor of Electrical Engineering. Her main research interests include thin films for dielectric and piezoelectric applications. She directs both the Center for Dielectrics and Piezoelectrics and the Center for Three-Dimensional Ferroelectric Microelectronics. She is a member of the National Academy of Engineering, a fellow of the American Ceramic Society, IEEE, and the Materials Research Society, and an academician of the World Academy of Ceramics. She currently serves as an associate editor for Applied Physics Letters. She was 2017 President of the Materials Research Society; previously she served as president of the IEEE Ultrasonics, Ferroelectrics and Frequency Control Society, as well as Keramos.
This faculty member is associated with the Penn State Intercollege Graduate Degree Program (IGDP) in Materials Science and Engineering (MatSE) where a multitude of perspectives and cross-disciplinary collaboration within research is highly valued. Graduate students in the IGDP in MatSE may work with faculty members from across Penn State.
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The existence of domain walls, which separate regions of different polarization, can influence the dielectric, piezoelectric, pyroelectric and electronic properties of ferroelectric materials. In particular, domain-wall motion is crucial for polarization switching, which is characterized by the hysteresis loop that is a signature feature of ferroelectric materials. Experimentally, the observed dynamics of polarization switching and domain-wall motion are usually explained as the behaviour of an elastic interface pinned by a random potential that is generated by defects, which appear to be strongly sample-dependent and affected by various elastic, microstructural and other extrinsic effects. Theoretically, connecting the zero-kelvin, first-principles-based, microscopic quantities of a sample with finite-temperature, macroscopic properties such as the coercive field is critical for material design and device performance; and the lack of such a connection has prevented the use of techniques based on ab initio calculations for high-throughput computational materials discovery. Here we use molecular dynamics simulations of 90 domain walls (separating domains with orthogonal polarization directions) in the ferroelectric material PbTiO3 to provide microscopic insights that enable the construction of a simple, universal, nucleation-and-growth-based analytical model that quantifies the dynamics of many types of domain walls in various ferroelectrics. We then predict the temperature and frequency dependence of hysteresis loops and coercive fields at finite temperatures from first principles. We find that, even in the absence of defects, the intrinsic temperature and field dependence of the domain-wall velocity can be described with a nonlinear creep-like region and a depinning-like region. Our model enables quantitative estimation of coercive fields, which agree well with experimental results for ceramics and thin films. This agreement between model and experiment suggests that, despite the complexity of ferroelectric materials, typical ferroelectric switching is largely governed by a simple, universal mechanism of intrinsic domain-wall motion, providing an efficient framework for predicting and optimizing the properties of ferroelectric materials.
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