Download Ferroic Materials - Lecture Slides | CHEM 584 and more Study notes Chemistry in PDF only on Docsity!
Ferroic Materials - References
Ferroic Materials - Intro
“ Ferroics ” is the generic name given to the study
of ferromagnets, ferroelectrics, and ferroelastics.
All are cooperative solid state phenomena.
Ferromagnet:
a material able to retain a permanent magnetic moment.
Ferroelectric:
a material able to retain a permanent dipole moment.
Ferroelastic:
a material able to retain a permanent shape after deformation (e.g. shape memory alloys); retains a strain; e.g., crystal structure spontaneously changes from a tetragonal structure (a square prism shape) to a monoclinic structure (a general parallelepiped).
Ferroic Materials - Intro
Piezoelectricity:
generating an electric charge in a material when subjecting it to a mechanical stress (direct effect); conversely, generating a mechanical strain in response to an applied electric field.
Pyroelectricity:
migration of positive and negative charge (and therefore establishment of electric polarization) to opposite ends of a crystal's polar axis as a result of a change in temperature.
Ferroelectricity:
The property of some materials to store a permanent electric field, i.e., retain a permanent dipole moment.
Ferroic Materials - Intro
Piezoelectricity:
Crystals that lack center of symmetry of ion distribution are electrically polarized (i.e. they develop surface charges) when they are mechanically stressed.
Pyroelectricity:
Some of crystals that lack center of symmetry of ion distribution, (i.e. piezoelectrics) can also spontaneously develop electric dipoles (polarize), with the degree of polarization dependent on temperature.
Ferroelectricity:
A subgroup of pyroelectrics has an unusual property of spontaneous alignment of the electric dipoles within domains (~1 μm), and the polarization direction can change under the influence of an electric field.
Dielectric Constant: Frequency Dependence
important for Ferroelectrics
Absorbance by Dielectrics
Ferroelectricity and Ionic Polarization
Ferroic Materials - Symmetries
Ferroelectric Ceramics
Ferroelectric Ceramics are dipolar below Curie TC.
Cooled below Tc in strong electric field: “poling” makes material with strong permanent dipole moment.
FRAM:
Ferroelectric random access memory
non-volatile RAM, much faster than Flash memory
2 nd^ Order Jahn-Teller Distortions (SOJT)
Octahedral d^0 Metal Ions: SOJT
BaTiO 3 : Ferroelectric
BaTiO 3 : Ferroelectric
Perovskite Diversity
s^2 p^0 Main Group Cations
Piezoelectric Materials - Intro
Piezoelectricity:
generating an electric charge in a material when subjecting it to a mechanical stress (direct effect); conversely generating a mechanical strain in response to an applied electric field.
Discovered in 1880 by Pierre & Jacques Curie
Types
Natural and Synthetic Crystals: Quartz, Rochelle Salt (Natural), Lithium Sulfate, Ammonium Dihydrogen Phosphate (Synthetic) Piezoceramic elements Lead Zirconate Titanate (PZT) Barium Titanate , Cadmium Sulfide Piezoelectric Polymer Polyvinylidene Fluoride (PVDF)
Piezoelectric Materials
at rest
compression induces voltage
applied voltage induces expansion Adapted from Fig. 18.36, Callister 7e.
Piezoelectricity – application of pressure produces current
Piezoelectric Materials
Applications of Piezoelectrics
A Piezoelectric material converts mechanical (strain) Energy to electrical energy and vice-versa. i.e., it transduces mechanical & electrical energy = “Transducer”
Piezoelectric Materials: Weiss Domains
Groups of dipoles with parallel orientation are called Weiss domains.
The Weiss domains are randomly oriented in the raw ceramic material, before the poling treatment has been finished. For this purpose an electric field (> 2000 V/mm) is applied to the (heated) piezo ceramics.
When an electric voltage is applied to a poled piezoelectric material, the Weiss domains increase their alignment proportional to the voltage.
The result is a change of the dimensions (expansion, contraction) of the PZT material.
Piezoelectric Materials (PZT)
Unpolarized Crystal
Polarized Crystal
After poling the zirconate-titanate atoms are off-center: unit cell becomes elongated and polarized
(1) Unpolarized- Random Weiss Domains (2) During Polarization (3) After polarization: Remnant Polarization exits even after removal of E field.
Pyroelectric Materials
Electric polarization due to Temp change.
Theophrastus in 314 BC noted that tourmaline
becomes charged when heated.
Sir David Brewster gave the effect the name in 1824.
Occurs in non-centrosymmetric crystals.
All pyroelectrics are piezoelectrics, but not vice versa.
Examples: ZnO, PZT, PbTiO 3
Similar to Ferroelectricity, but the ionic shifts
that give rise to polarization cannot be reversed by an external field. The Polarization changes gradually as temperature is increased.
Pyroelectric Materials
“Motion” Detector: heat changes in NIR
Applications:
Mechanical to Electrical
Force, Pressure, and acceleration sensors: pressure gauges, car impact & skid (yaw rate) sensors High Voltage - Low Current Generators, i.e., Piezo-spark ignition: stoves, grills, small engines, cell phone bombs Platform Stabilization Sensors Echo Imaging SONAR (Sound Navigation And Ranging) Echo Sounding Medical Imaging (Sonography, Echocardiography) Non-destructive testing and imaging (NDT/NDI) Measurement Transducers Audio Transducers (microphones)
Applications of Piezoelectrics
Applications:
Electrical to Mechanical
Ultrasonic motors Small Vibration Shakers Microactuators and High Precision Macro Actuators Inkjet Print Heads Audio Transducers (microphones & speakers) Piezoelectric Refrigerators (NASA) Drilling and Machining Welding (plastics especially, but also metals)
Applications of Piezoelectrics
Piezoelectric Materials
Actuator Types
Actuators are sometimes called motors or transducers
Sensors are called generators
Bimorph (extension) Bimorph (bending)
Longitudinal Wafer
Transverse Wafer
Stack Actuator (vertical extension)
