Ferroelectric/piezoelectric ferroelectricity

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

At the temperatures of the phase transitions, maxima of the dielectric constant up to 10,000 are found. Moreover, ia the ferroelectric state below T the material becomes pyroelectric and shows high piezoelectric activity. 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

Piezoelectric and Electrostrictive Device Applications. Devices made from ferroelectric materials utilizing their piezoelectric or electrostrictive properties range from gas igniters to ultrasonic cleaners (or welders) (72). 

Polymer Ferroelectrics. In 1969, it was found that strong piezoelectric effects could be induced in the polymer poly(vinyhdene fluoride) (known as PVD2 or PVDF) by apphcation of an electric field (103). Pyroelectricity, with pyroelectric figures of merit comparable to crystalline pyroelectric detectors (104,105) of PVF2 films polarized this way, was discovered two year later (106.)... 

Ferroelectric—polymer composite devices have been developed for large-area transducers, active noise control, and medical imaging appHcations. North American Philips, Hewlett-Packard, and Toshiba make composite medical imaging probes for in-house use. Krautkramer Branson Co. produces the same purpose composite transducer for the open market. NTK Technical Ceramics and Mitsubishi Petrochemical market ferroelectric—polymer composite materials (108) for various device appHcations, such as a towed array hydrophone and robotic use. Whereas the composite market is growing with the invention of new devices, total unit volume and doUar amounts are small compared to the ferroelectric capacitor and ferroelectric—piezoelectric ceramic markets (see Medical imaging technology). 

Approximately 40% of the U.S. electronic ceramics industry is represented by ferroelectrics. Table 3 shows U.S. consumption of ceramic capacitors and piezoelectric materials (109). 

Japanese suppHers generally dominate the electronic ceramic business. Japanese production of ferroelectric devices in the first nine months of 1990 was valued at 711 x 10 for ceramic capacitors and 353 x 10 for piezoelectric devices, representing growths of 7 and 10.8%, respectively, over the previous year. 

Principal producers of ferroelectric capacitors are (110) Murata Manufacturing Co., Ltd., Kyoto, Japan Kyocera Corp., Kyoto, Japan Philips Electronics, N.V., Eindhoven, the Netherlands and NEC Corp., Tokyo, Japan. Principal piezoelectric ceramic component producers are (110) Murata Manufacturing Co., Ltd., Kyoto, Japan Motorola, Inc., Schaumburg, lU. EDO Corp., College Point, N.Y. Morgan Matroc, Inc., Bedford, Ohio Kyocera Corp., Kyoto, Japan and Philips Electronics, N.V., Eindhoven, the Netherlands. 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Alkaline-Earth Titanates. Some physical properties of representative alkaline-earth titanates ate Hsted in Table 15. The most important apphcations of these titanates are in the manufacture of electronic components (109). The most important member of the class is barium titanate, BaTi03, which owes its significance to its exceptionally high dielectric constant and its piezoelectric and ferroelectric properties. Further, because barium titanate easily forms solid solutions with strontium titanate, lead titanate, zirconium oxide, and tin oxide, the electrical properties can be modified within wide limits. Barium titanate may be made by, eg, cocalcination of barium carbonate and titanium dioxide at ca 1200°C. With the exception of Ba2Ti04, barium orthotitanate, titanates do not contain discrete TiO ions but ate mixed oxides. Ba2Ti04 has the P-K SO stmcture in which distorted tetrahedral TiO ions occur. 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

Ferroelectrics. Ferroelectrics, materials that display a spontaneous polarization ia the abseace of an appHed electric field, also display pyroelectric and piezoelectric behavior. The distinguishing characteristic of ferroelectrics, however, is that the spontaneous polarization must be re-orientable with the appHcation of an electric field of a magnitude lower than the dielectric breakdown strength of the material. 

In the broad range of ceramic materials that are used for electrical and electronic apphcations, each category of material exhibits unique property characteristics which directiy reflect composition, processing, and microstmcture. Detailed treatment is given primarily to those property characteristics relating to insulation behavior and electrical conduction processes. Further details concerning the more specialized electrical behavior in ceramic materials, eg, polarization, dielectric, ferroelectric, piezoelectric, electrooptic, and magnetic phenomena, are covered in References 1—9. 

The main categories of electrical/optical ceramics are as follows phosphors for TV, radar and oscilloscope screens voltage-dependent and thermally sensitive resistors dielectrics, including ferroelectrics piezoelectric materials, again including ferroelectrics pyroelectric ceramics electro-optic ceramics and magnetic ceramics. 

Other strongly ferroelectric crystals have been discovered and today, PZT -Pb(Ti, Zr)03 is the most widely exploited of all piezoelectric (ferroelectric) ceramics. 

Due to their high piezoelectric response, electrostriction in ferroelectrics, induced by an applied electric field, can be used as strain-inducing components (just as ferromagnetic materials can be exploited for their magnetostriction). Thus barium... 

The effect of shock-induced conduction is less distinct in ferroelectrics than in piezoelectrics but is nevertheless apparent from a number of studies. (See Davison and Graham [79D01] and Novitskii [79N03].) Differences in conduction with sample polarity, such as those seen in quartz but of opposite sign, are observed in ferroelectrics. 

In this chapter piezoelectric crystals and polymers ferroelectric and ferromagnetic solids resistance of metals shock-induced electrical polarization electrochemistry elastic-plastic physical properties. 

In order to anticipate problems and to interpret observations under the extreme conditions of shock compression, it is necessary to consider structural and electronic characteristics of PVDF. Although the phenomenological piezoelectric properties of PVDF are similar to those of the piezoelectric crystals, the structure of the materials is far more complex due to its ferroelectric nature and a heterogeneous mixture of crystalline and amorphous phases which are strongly dependent on mechanical and electrical history. 

In this book those ferroelectric solids that respond to shock compression in a purely piezoelectric mode such as lithium niobate and PVDF are considered piezoelectrics. As was the case for piezoelectrics, the pioneering work in this area was carried out by Neilson [57A01]. Unlike piezoelectrics, our knowledge of the response of ferroelectric solids to shock compression is in sharp contrast to that of piezoelectric solids. The electrical properties of several piezoelectric crystals are known in quantitative detail within the elastic range and semiquantitatively in the high stress range. The electrical responses of ferroelectrics are poorly characterized under shock compression and it is difficult to determine properties as such. It is not certain that the relative contributions of dominant physical phenomena have been correctly identified, and detailed, quantitative materials descriptions are not available. 

The contrast in knowledge is a result of the degree of complexity of materials properties elastic piezoelectric solids have perhaps the least complex behaviors, whereas ferroelectric solids have perhaps the most complex mechanical and electrical behaviors of any solid under shock compression. This complexity is further compounded by the strong coupling between electrical and mechanical states. Unfortunately, much of the work studying ferroelectrics appears to have underestimated the difficulty, and it has not been possible to carry out careful, long range, systematic efforts required to develop an improved picture. 

Fig. 5.19. Shock-induced volume polarizations have been observed in a wide range of solids due to a number of different physical phenomena, including piezoelectricity and ferroelectricity. The signals observed from polymers and ionic crystals are not due to established phenomena, and are described as due to shock-induced polarization effects.

In this chapter studies of physical effects within the elastic deformation range were extended into stress regions where there are substantial contributions to physical processes from both elastic and inelastic deformation. Those studies include the piezoelectric responses of the piezoelectric crystals, quartz and lithium niobate, similar work on the piezoelectric polymer PVDF, ferroelectric solids, and ferromagnetic alloys which exhibit second- and first-order phase transformations. The resistance of metals has been investigated along with the distinctive shock phenomenon, shock-induced polarization. 

The piezoelectric polymer investigations give new physical insight into the nature of the physical process in this class of ferroelectric polymers. The strong nonlinearities in polarization with stress are apparently more a representation of nonlinear compressibility than nonlinear electrical effects. Piezoelectric polarization appears to be linear with stress to volume compressions of tens of percent. The combination of past work on PVDF and future work on copolymers, that have quite different physical features promises to provide an unusually detailed study of such polymers under very large compression. 

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