Ferroelectric Materials
Ferroelectric materials offer a wide range of useful properties. These include ferroelectric hysteresis (used in nonvolatile memories), high permittivities (used in capacitors), high piezoelectric effects (used in sensors, actuators and resonant wave devi
- PDF / 1,851,311 Bytes
- 27 Pages / 547.146 x 686 pts Page_size
- 60 Downloads / 228 Views
Ferroelectric M 27. Ferroelectric Materials
27.0.1 Definitions and Background Ferroelectric materials offer a very wide range of useful properties for the electronic engineer to exploit. As we will see, they are also a class of materials that is hard to define accurately in a single sentence. It is useful to start from the class of insulating materials that form dielectrics; in other words materials that will sustain a dielectric polarisation under the application of an electric field. There exists a set of these materials for which the crystal structure lacks a centre of symmetry. (If a crystal structure has a centre of symmetry, it means that for every atom in the structure there is a point in the unit cell through which inversion will bring one to the same type of atom.) A list of the non-centrosymmetric, or acentric, point groups is given in Table 27.1. All of the crystalline materials whose structures possess these point groups (with the exception of group 432) exhibit the phenomenon of piezoelectricity, which means that stress will generate a charge separation on the faces of
27.0.1 Definitions and Background ....... 27.0.2 Basic Ferroelectric Characteristics and Models .............................. 27.1 Ferroelectric Materials.......................... 27.1.1 Ferroelectric Oxides ................... 27.1.2 Triglycine Sulphate (TGS) ............ 27.1.3 Polymeric Ferroelectrics ............. 27.2 Ferroelectric Materials Fabrication Technology ......................................... 27.2.1 Single Crystals ........................... 27.2.2 Ceramics .................................. 27.2.3 Thick Films ............................... 27.2.4 Thin Films ................................ 27.3 Ferroelectric Applications ..................... 27.3.1 Dielectrics ................................ 27.3.2 Computer Memories .................. 27.3.3 Piezoelectrics............................ 27.3.4 Pyroelectrics ............................. References ..................................................
597 599 601 601 607 607 608 608 609 613 613 616 616 616 617 620 622
affect those of the devices in which they are exploited.
the crystal (the direct piezoelectric effect) and will undergo mechanical strain when subjected to an electric field (the converse piezoelectric effect). Both effects are widely exploited in electronic devices. A well-known example of a non-centrosymmetric material is the mineral α-quartz, which is used for the piezoelectric resonators employed for frequency filtering and electronic clocks. Table 27.1 Polar and acentric (non-centrosymmetric) point
groups Crystal system
Polar (acentric)
Nonpolar (acentric)
Triclinic Monoclinic Orthorhombic Trigonal Hexagonal Tetragonal Cubic
1 2, m mm2 3, 3m 6, 6mm 4, 4mm None
222 32 6¯ , 6¯ m2 4¯ , 422, 4¯ 2m 23, 4¯ 3m, 432
Part C 27
Ferroelectric materials offer a wide range of useful properties. These include ferroelectric hysteresis (used in nonvolatile memories), high permittivities (used in capacitors), high piezoelectric effects (used in sensors, a
Data Loading...
