ScienceWeek

MATERIALS SCIENCE: ON PIEZOELECTRIC CERAMICS

The following points are made by Eric Cross (Nature 2004 432:24):

1) Piezoelectricity is a time-honored field in crystal physics, initiated by the brothers Jacques Curie (1856-1941) and Pierre Curie (1859-1906)(1) in 1880. When a piezoelectric crystal is subjected to a suitably oriented elastic stress, the crystal polarizes electrically in proportion to the applied stress; conversely, when a suitably oriented electric field is applied, the crystal changes shape (strains) in proportion to the field level. Such properties lead naturally to possibilities for sensing and actuating elastic changes, but in simple piezoelectrics the effects are too small. Compositions of lead zirconate titanate -- the "PZT" family of ceramics -- show much stronger piezoelectric effects, and have come to dominate the field. Their lead content, however, raises environmental concerns. Saito et al(2) have created lead-free piezoceramics with properties that closely match those of PZT.

2) In a normal ceramic, the random orientation of the individual crystallites imparts an infinite degree of rotational symmetry within the ceramic texture; as a result, irrespective of the crystallite symmetry, piezoelectricity is forbidden. What makes piezoelectricity possible in some ceramics, and markedly raises the level of piezoactivity in some single crystals, is the phenomenon of ferroelectricity. In the ferroelectric crystal or crystallite, there is a substructure of electrically polar domains that can be reoriented by a strong applied electric field. This domain reorientation is demonstrated by the appearance of electric hysteresis and significant shape change.

3) In spite of the fact that hysteresis in ceramic ferroelectrics was well known to the physics community, it was an engineer, R.B. Gray, working at Erie Technological Products in Erie, Pennsylvania, who in 1949 realized that ferroelectric capacitors under test there were humming at the common 60-Hz power-line frequency. From this he deduced their piezoelectricity, made for himself a piezoceramic phonograph pick-up and pinned down the master piezoceramic patent(3) for Erie. The patent, subsequently licensed to the Clevite Corporation, established an early lead for the United States in the piezoceramic business, and this was strongly reinforced by Clevite's role in the development of the now-dominant PZT family of piezoceramics.

4) PZT compositions had first been explored in Japan(4,5). But it was Jaffe et al(1954), at the US Bureau of Standards (now the National Institute of Standards and Technology), who uncovered the vertical "morphotropic phase boundary" and realized its likely role in enhanced piezoelectric activity. Depending on the proportions of lead zirconate and lead titanate it contains, and the temperature, a PZT composition has a particular crystal structure. The morphotropic phase boundary, positioned where the zirconate-to-titanate ratio is 52:48, marks the transition from rhombohedral to tetragonal ferroelectric crystallites.

References (abridged):

1. Curie, J. & Curie, P. Bull. Soc. Fr. Mineral. 3, 90 (1880)

2. Saito, Y. et al. Nature 432, 84-87 (2004)

3. Gray, R. B. US Patent No. 2, 486, 560 (1949)

4. Shirane, G. & Takeda, A. J. Phys. Soc. Jpn 7 (1), 5-XX (1952)

5. Sawaguchi, E. J. Phys. Soc. Jpn 8 (5), 615-629 (1953)

Nature http://www.nature.com/nature

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MATERIALS SCIENCE: ON SIZE LIMITS IN FERROELECTRICITY

The following points are made by Nicola A. Spaldin (Science 2004 304:1606):

1) With the continued demand for portability in consumer electronics, it is becoming increasingly important to understand the effects of miniaturization on the properties of the active components in electronic devices. In many cases, however, the basic physics of such size reduction is poorly understood and can be difficult to characterize, because competing effects such as surface properties, strain effects from substrates, and fundamental size quantization complicate the behavior. This is particularly true in the case of ferroelectrics -- materials that have a spontaneous electric polarization that can be switched by an applied electric field.

2) Indeed, it has long been believed, on the basis of empirical evidence, that there is a critical size on the order of hundreds of angstroms below which a spontaneous electric polarization cannot be sustained in a material (1). Such behavior would render ferroelectrics useless for applications at sizes below this cutoff, thereby limiting their importance in future technologies. Recent first-principles theoretical work (2-4), however, has indicated that the critical size is orders of magnitude smaller than previously thought, and this view has been corroborated by some measurements (5). Fong et al (Science 2004 304:1650) provided the first unambiguous experimental evidence that these theoretical predictions and recent experimental indications are indeed correct by confirming that ferroelectricity persists down to vanishingly small sizes.

3) Ferroelectrics find three main technological niches based on three related physical characteristics. First, as a result of their spontaneous electric polarization, they can be used as binary data storage media in which opposite directions of polarization represent the 1 or 0 data bits. In addition, because the electric polarization is coupled to the structure of the material, ferroelectrics can convert mechanical energy to electrical energy and vice versa. This leads to their widespread use in transducer applications such as piezoelectric actuators and sonar detectors. Finally, they have very large dielectric permittivities leading to applications in capacitors. In all cases, it is crucial to understand the size dependence of the ferroelectric behavior as ever smaller devices are produced.

4) Consider why the existence of a critical size might be expected intuitively. Most technologically important ferroelectrics are perovskite-structure oxides in which the polarization arises from the displacement of the negatively charged oxygen anions relative to the positively charged cations to create a net electric dipole moment. If the cations move in the "down" direction, then a net positive charge will accumulate on the bottom surface, and a negative charge will accumulate on the top surface. This will, of course, produce an electric field, called the depolarizing field, which acts to push the ions back toward the centrosymmetric state. The depolarizing field can be almost totally screened in large samples by using metallic electrodes. However, at some critical size the screening becomes insufficient, and the electrostatic energy associated with the residual depolarizing field becomes larger than the energy gained by ferroelectric ordering. The ferroelectric state is then unstable.

References (abridged):

1. T. M. Shaw, S. Trolier-McKinstry, P. C. McIntyre, Annu. Rev. Mater. Sci. 30, 263 (2000).

2. Ph. Ghosez, K. Rabe, Appl. Phys. Lett. 76, 2767 (2000)

3. B. Meyer, D. Vanderbilt, Phys. Rev. B 63, 205426 (2001)

4. J. Junquera, Ph. Ghosez, Nature 422, 506 (2003)

5. C. H. Ahn, K. M. Rabe, J.-M. Triscone, Science 303, 488 (2004)

Science http://www.sciencemag.org

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MATERIALS SCIENCE: ON NANOSCALE FERROELECTRICITY

The following points are made by C.H. Ahn et al (Science 2004 303:488):

1) The movement and storage of electrical charges and the manipulation of the electric fields they produce are the basis of the operation of computer processors and memories. The modern electronics industry demands an ever greater decrease in switching time and length scales, approaching the level of individual electrons and atoms. Although continued improvements in conventional semiconductor designs can to some extent address these needs, there is increasing motivation to consider alternative paradigms. In ferroelectric oxides (1-3), electric polarization, bound charges, and large electric fields are produced by displacements of individual atoms, and devices based on ferroelectric materials therefore can be made in principle to operate on atomic scales.

2) It is helpful to look at the crystal structure of the prototypical perovskite ferroelectric, BaTiO3. In the room-temperature ferroelectric tetragonal structure, the Ti and Ba sublattices are shifted relative to the negatively charged oxygens, producing a polarization (net dipole moment per unit volume) of 26 micro-C/cm^(2) (1). This shift breaks the cubic symmetry, resulting in six symmetry-equivalent variants with polarizations along the x, y, and z axes. The polarization of a single variant, regarded within macroscopic electrostatics, produces a bound surface charge on the order of 1.5 x 10^(14) electrons/cm^(2) and an internal electric field that if unscreened approaches 300 MV/cm. Domain walls separate variants with polarization in different directions (1,4). An applied electric field can be used to switch between the symmetry-related variants, and the direction of the polarization is stable when the field is removed. For bulk ferroelectrics, the coercive field (the electric field required to switch the polarization) is generally relatively modest, on the order of 10 to 100 kV/cm, much smaller than that estimated for uniform switching through the high-symmetry structure from first-principles calculations. This small value reflects the fact that switching generally occurs through nucleation and growth of domains (5), which are processes with lower energy barriers.

3) In summary: Ferroelectric oxide materials have offered a tantalizing potential for applications since the discovery of ferroelectric perovskites more than 50 years ago. Their switchable electric polarization is ideal for use in devices for memory storage and integrated microelectronics, but progress has long been hampered by difficulties in materials processing. Recent breakthroughs in the synthesis of complex oxides have brought the field to an entirely new level, in which complex artificial oxide structures can be realized with an atomic-level precision comparable to that well known for semiconductor heterostructures. Not only can the necessary high-quality ferroelectric films now be grown for new device capabilities, but ferroelectrics can be combined with other functional oxides, such as high-temperature superconductors and magnetic oxides, to create multifunctional materials and devices. Moreover, the shrinking of the relevant lengths to the nanoscale produces new physical phenomena. Real-space characterization and manipulation of the structure and properties at atomic scales involves new kinds of local probes and a key role for first-principles theory.

References (abridged):

1. M. E. Lines, A. M. Glass, Principles and Applications of Ferroelectrics and Related Materials (Clarendon Press, Oxford, 1977)

2. J. F. Scott, Ferroelectric Memories (Springer-Verlag, New York, 2000)

3. O. Auciello, J. F. Scott, R. Ramesh, Phys. Today 51, 22 (1998)

4. B. Meyer, D. Vanderbilt, Phys. Rev. B 65, 104111 (2002)

5. W. J. Merz, Phys. Rev. 95, 690 (1954)

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