Mechanical applications, ferroelectric

M. Grossmann, Imprint An Important Failure Mechanism of Ferroelectric Thin Films in View of Memory Applications. Dissertation. RWTH-Aachen, 2001. published by vdi Verlag, Fortschritt-Berichte vdi Reihe 9 Elektronik/Mikro- und Nanotechnik. 

The rest of this chapter is structured as follows. Section 5.2 gives a brief historical overview together with a description of the synthesis, structure and preparation of ferroelectric polymers. Section 5.3 defines the properties required to evaluate these materials, and includes typical values and brief accounts of the measurement methods. Section 5.4 details the applications of ferroelectric polymers, covering sound transducers, biomedical, pyroelectric and mechanical applications. Conclusions are briefly described in section 5,5. 

Ferromagnetic and ferroelectric materials are only two examples of a wider group that contains domains built up from switchable units. Such solids, which are called ferroic materials, exhibit domain boundaries in the normal state. These include ferroelastic crystals whose domain structure can be switched by the application of mechanical stress. In all such materials, domain walls act as planar defects running throughout the solid. 

Approximately ten years ago, it was first reported by Haertling and Land (jj that optical transparency was achieved in a ferroelectric ceramic material. This material was, in reality, not just one composition but consisted of a series of compositions in the lanthanum modified lead zirconate-lead titanate (PLZT) solid solution region. The multiplicity of compositions, each with different mechanical, electrical and electrooptic properties has led to a decade of study in defining the chemical and structural nature of these materials in understanding the phenomena underlying their optical and electrooptic properties and in evaluating the practicality of the large number of possible applications (2-12),... 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

Ferroelectrics. Among the 32 crystal classes, 11 possess a centre of symmetry and are centrosymmetric and therefore do not possess polar properties. Of the 21 noncentrosymmetric classes, 20 of them exhibit electric polarity when subjected to a stress and are called piezoelectric one of the noncentrosymmetric classes (cubic 432) has other symmetry elements which combine to exclude piezoelectric character. Piezoelectric crystals obey a linear relationship P,- = gijFj between polarization P and force F, where is the piezoelectric coefficient. An inverse piezoelectric effect leads to mechanical deformation or strain under the influence of an electric field. Ten of the 20 piezoelectric classes possess a unique polar axis. In nonconducting crystals, a change in polarization can be observed by a change in temperature, and they are referred to as pyroelectric crystals. If the polarity of a pyroelectric crystal can be reversed by the application on an electric field, we call such a crystal a ferroelectric. A knowledge of the crystal class is therefore sufficient to establish the piezoelectric or the pyroelectric nature of a solid, but reversible polarization is a necessary condition for ferroelectricity. While all ferroelectric materials are also piezoelectric, the converse is not true for example, quartz is piezoelectric, but not ferroelectric. 

The practical application of ultrasonics requires effective transducers to change electrical energy into mechanical vibrations and vice versa. Transducers are usually piezoelectric, ferroelectric, or magnetostrictive. The application of a voltage across a piezoelectric crystal causes it to deform with an amplitude of deformation proportional to the voltage. Reversal of the voltage causes reversal of the mechanical strain. Quartz and synthetic ceramic materials are used. 

The physical limits and technological requirements of domain dimensions for a new generation of nanodomain-based devices were considered. It was shown that for both ferroelectric thin films and crystals the achievable domain size is in the range of 100 nm. It is shown that for 100 nm thick ferroelectric films, an application of nanosize electrodes does not make a big difference compared with conventional polarization reversal setups and physical mechanism. However, in the case of bulk ferroelectrics, the use of a switching afm tip electrode for generation of long domains with a nanometer size radius requires a new approach both for polarization reversal instrumentation and physics of domain inversion. 

Fig. 2.44 Schematic illustrating the changes accompanying the application of electrical and mechanical stresses to a polycrystalline ferroelectric ceramic (a) stress-free - each grain is non-polar because of the cancellation of both 180° and 90° domains (b) with applied electric field - 180° domains switch producing net overall polarity but no dimensional change (c) with increase in electric field 90° domains switch accompanied by small ( 1%) elongation (d) domains disorientated by application of mechanical stress. (Note the blank grains in (a) and (b) would contain similar domain structures.)...

Spin coated copolymer films show a decrease in remanent polarisation if the film thickness [504, 505] decreases. The application of piezoelectric materials in micro-electro-mechanical systems (MEMS) or sensors makes it often necessary to decrease the lateral dimensions of the elements. Recently, Alexe et al. [506, 507] fabricated freestanding microcells with lateral dimensions down to 100 nm and heights of 110 nm from a ferroelectric PZT by direct... 

Porosity of Ceramics, Roy W. Rice Intermetallic and Ceramic Coatings, edited by Narendra B. Dahotre and T. S. Sudarshan Adhesion Promotion Techniques Technological Applications, edited by K. L. Mittal and A. Pizzi Impurities in Engineering Materials Impact, Reliability, and Control, edited by Clyde L. Briant Ferroelectric Devices, Kenji Uchino Mechanical Properties of Ceramics and Composites Grain and Particle Effects, Roy W. Rice Solid Lubrication Fundamentals and Applications, Kazuhisa Miyoshi... 

Cady in World War II realized that such a mechanical resonance of a vibrating crystal could be used in frequency control. This discovery had an important influence on radio communications.Alternating electric fields, such as those generated by the radio tubes of the time, were applied to plates of piezoelectric crystals and the expansions and contractions of the plates were caused to react on electrical circuits. If the natural frequency of the mechanical vibration of the quartz plate coincided with the frequency of oscillation of the electric circuit, resonance between the two took place and energy was acquired by the mechanical oscillators. Later. Rochelle salt and barium titanate, which are each both ferroelectric and piezoelectric, were used. ° In ferroelectric crystals, the polarization or dipole moment is reversed or reoriented upon application of an electric field. Ferroelasticity is another property displayed by some crystals in which stress can cause the interconversion between two stable orientational states. These physical properties of crystals are of great use in modern technology. 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

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