Ferroelectrics.pptx

This PPT gives introduction
to Dielectrics, Piezoelectrics & Ferroelectrics Materials, Methods and Applications. A quick glance at the dielectric phenomena, symmetry, classification, modelling, figures of merit and applications.
Comprehensive overview of the physics and applications of
ferroelectric

1. Dielectrics, Piezoelectrics & Ferroelectrics Materials, Methods and Applications

2. Outline Dielectrics and dielectric polarization Dielectrics -> Piezoelectrics -> Pyroelectrics -> Ferroelectrics Ferroelectric phenomena and properties Measurement of spontaneous polarization Classification of ferroelectrics Application of ferroelectrics

3. Dielectric s D ielectric is an electrical insulator which can be polarized on application of external electric filed. They do not conduct electricity due to the very low density of free charge carriers. Here, the electrons are bound to microscopic regions within the material, that is the atoms, molecules, or clusters. D   E  P   E    E   (1   ) E    E Polarization is defined as dipole moment per unit volume. It modifies dielectric function of a vacuum, for example to increase the capacity (i.e. the ability to store charge) of capacitors. Electronic Polarizability Ionic Polarizability Dipolar Polarizability Space Charge Polarizability

4. Applications

5. Polarization is the separation of a positive and a negative charge barycenter of bound charges. If this separation is induced by an applied electric field , it is called dielectric polarization. If the separation is induced by an applied strain field , it is called piezoelectric polarization . Several dielectric crystals exhibit a spontaneous polarization below a critical temperature which is related to a change in crystal symmetry. A spontaneous polarization in the material leads to pyroelectric effects under temperature changes. Ferroelectricity is obtained when the orientation of polarization can be reoriented between crystallographically equivalent configurations by an external field. Piezoelectrics, Pyroelectrics & Ferroelectrics

6. Ferroelectric Materials: A ferroelectric material is a material that exhibits, over some range of temperature, a spontaneous electric polarization that can be reversed or reoriented by application of an electric field Ferroelectricity Domain is a region in which all the individual dipoles point in a particular direction Barium titanate is a prototype perovskite ferroelectric Perovskite general formula: ABO3 In barium titanate Ba 2+ occupies A site (corner of cube) O 2- occupies face centre and Ti 4+ occupies body centre Dipole moment of a unit cell arises from Ti 4+ off-centering

7. Measurement of Spontaneous Polarization Sawyer-Tower circuit :

8. Temperature dependence of the spontaneous polarization and permittivity in a ferroelectric material. (a)–(f) indicate the temperature ranges for each application. Temperature dependence of electrical properties of a ferroelectric Night vision

9. Ferroelectricity: Two main classes of ferroelectrics Ex: NaNO 2 , KDP Ex: ionic crystals with perovskite , or ilmenite structure. Based on the origin of spontaneous Polarization

10. Ferroelectric Materials can be structurally categorized into 4 groups: Corner Sharing Octahedra: Perovskite-Type Compounds (such as BaTiO 3 , PT, PZT, PMN, and PLZT) Tungsten-Bronze-Type Compounds (such as PbNb 2 O 6 ) Bismuth Oxide Layer Structured Compounds (such as Bi 4 Ti 3 O 12 and PbBi 2 Nb 2 O 9 ) Lithium Niobate and Tantalate (such as LiNbO 3 and LiTaO 3 ) Compounds Containing Hydrogen Bonded Radicals (such as KDP, TGS, and Rochelle Salt) Organic Polymers (such as PVDF and co-polymers) Ceramic Polymer Composites (such as PZT-PE) Types of Ferroelectric Materials Perovskite (BaTiO3) LiNbO3 Polyvinylidene fluoride (PVDF)

11. Ferroelectric properties Most ferroelectric materials undergo a structural phase transition from a high-temperature nonferroelectric (or paraelectric ) phase into a low-temperature ferroelectric phase. The symmetry of the ferroelectric phase is always lower than the symmetry of the paraelectric phase . The temperature of the phase transition is called the Curie point, T C . Above the Curie point the dielectric permittivity falls off with temperature according to the Curie–Weiss law where C is the Curie constant, T (T ≤ T C ) is the Curie–Weiss temperature Some ferroelectrics, such as BaTiO 3 , undergo several phase transitions into successive ferroelectric phases .

12. Ferroelectrics have very high dielectric constants at relatively low applied field frequencies. Capacitors made from these materials can be significantly smaller than capacitors made out of other dielectric materials . The combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors some of the most useful technological devices in modern society. Ferroelectric capacitors are at the heart of medical ultrasound machines, high quality infrared cameras, fire sensors, sonar, vibration sensors, and even fuel injectors on diesel engines . Hysteresis means memory and ferroelectric capacitors are used to make ferroelectric RAM for computers and RFID cards. The electro-optic modulators that form the backbone of the Internet are made with ferroelectric materials . Applications of Ferroelectrics

13. Ferroelectricity: Landau- Ginzburg phenomenological theory Free energy Order parameter (polarization) Electric field F  1 aP 2  1 bP 4  1 cP 6  ...  EP 2 4 6 To find the equilibrium solution we need to find the minima of FP by solving the equation: Ignoring higher terms we can get the linear solution :  F   P  F  aP  E   P    P  1  E a All the coefficients of polarization have temperature dependence. The dependence of a on temperature can be approximated as : a =   Х =   Curie Weiss Law