Dielectric- Poled Ferroelectric Ceramics

Question 1

Direct (generator) effect

Answer 1

Apply a mechanical stress. Material distorts and generates piezoelectric response. Current flows from charge build up. Electricity generated

Question 2

Converse (motor) effect

Answer 2

Apply electric field. This generates a polarisation. Polarisation induces mechanical changes that can be detected. Generates strain

Question 3

Polarisation Vs applied field loop

Answer 3

Basically a magnetic hysteresis loop. Up to saturation polarisation. Crosses y-axis at remenant polarisation. Crosses x-axis at some finite value of field (coercive field)

Question 4

Strain Vs applied field graph

Answer 4

Butterfly curve. Initially material at origin. Curves up to maximum strain. Then curves down to y-intercept and keeps curving down to a minimum strain (still above 0) when strain has been removed by depoling. Then curves up to maximum strain on opposite side and so on. Should be like a reflection

Question 5

Why do poled ceramics have a limited working range?

Answer 5

Too big a strain causes fatigue and cracking in the material. Therefore Eapp must be much less than Ec and temperature much less than Tc and limited strain

Question 6

Ageing in poled ceramics

Answer 6

Slow decay of remnant polarisation Pr due to diffusion (domain wall movement). This occurs to relax the internal strain.

Question 7

What are oxygen vacancies associated with?

Answer 7

Many electrical degradation/conduction problems in perovskite-based dielectrics

Question 8

Hard piezoelectric over time

Answer 8

Slow reduction in Pr over time. Has high level of oxygen vacancies

Question 9

Soft piezoelectric material over time

Answer 9

Fast reduction of Pr followed by stable Pr. High level of metal vacancies

Question 10

Donor doping to make soft piezoelectrics

Answer 10

A-site doping of La3+. Favoured mechanism for neutrality is ionic compensation producing B-site metal vacancies (no Ti4+). Defective unit cell has dipole in direction towards more positive La3+. To align with a field of a different orientation would require movement of Ti vacancy and the movement of cations or A-site metal vacancies. This requires lots of energy. Ba-Ba or Ti-Ti distances are 4Å (1 unit cell) so migration of the vacancies on their respective metal sublattice is difficult

Question 11

Acceptor doping for making hard piezoelectrics

Answer 11

Dope with Fe3+ ions. Favoured mechanism for neutrality is to create oxygen vacancies. Fe replaces Ti. Oxygen vacancy is more positive and Fe3+ more negative so forms dipole towards O vacancy. O-O distance 2.3Å and are almost fully close packed so migration of oxygen vacancies easier than for Ba or Ti. During poling the vacancies can align their dipole moment with applied fields and can act as pinning centres to restrict domain wall movement.

Question 12

Hard piezoelectrics ageing

Answer 12

Eapp on. During poling Eapp can facilitate oxygen vacancy migration to promote alignment of Fe3+-Vo defect dipoles with applied field. Eapp off. Ageing starts. The defect dipoles act as pinning centres for domains after poling. Pr changes more slowly

Question 13

Soft piezoelectric ageing

Answer 13

Eapp on. During poling Ti vacancies don’t move to align with applied field. Eapp off ageing starts. The VTi disrupts domain structure making domains smaller so easier to move and loose pole. Pr changes more slowly

Question 14

Hard Vs soft piezoelectrics

Answer 14

Hard has lower tanδ, higher Ec, more difficult to depole, lower d (strain induced) values. Hard has high voltage and high load applications. Soft has high sensitivity applications

Question 15

Why does hard have lower tanδ

Answer 15

Lower losses due to pinning

Question 16

Directionality in piezoelectrics

Answer 16

The direction of applied stress and strain leads to an electrical response in particular directions and vice versa. Relationship based on the crystal structure of the material and the elements linked to the piezoelectric effect.

Question 17

Directionality for BaTiO3

Answer 17

Squashing in (001) direction gives a different response to the (110) direction for example. It is an anisotropic unit cell with no centre of symmetry

Question 18

Energy and efficiency for piezoelectrics

Answer 18

Amount of energy (field/stress) put in relates to how much energy (strain/field) results. This defines the efficiency in the response and some materials are more efficient than others and have different butterfly curves

Question 19

Advantages and disadvantages of poled ferroelectric ceramics

Answer 19

Simplicity, small size, low cost, high reliability.

Ageing, depoling.

Question 20

Applications of piezoelectric ceramics

Answer 20

High voltage generators
Ultrasonic generators
Loud speakers
Sensors
Resonators
Micropositioners
Electro-optic applications
Piezoelectric composites

Question 21

Piezoelectric micro-positioners

Answer 21

Devices that move very small distances in response to electrical signals. Used for actuators on electrical equipment. Require large displacement from small applied field and lots of sensitivity.

Question 22

How to increase change in thickness for piezoelectric micro-positioners

Answer 22

Increase the voltage across it (depole).

Use multilayer device so that each Δt is added to give greater sensitivity and movement

Question 23

Type of piezoelectric used in micropositioners

Answer 23

Soft (e.g PZT- donor doped). They produce a larger strain for smaller fields and can more easily move around their butterfly curve than hard piezoelectrics

Question 24

How does gas ignitor work?

Answer 24

Ceramic element connected to open circuit (small gap) on a mounting base. When compressed the ceramic creates a field so that a spark can form across the small gap. This ignites the gas

Question 25

Type of piezoelectric used for gas ignitor

Answer 25

Hard (e.g acceptor doped PZT). Need to generate large field and material to be robust an reusable without loss of function. Need to retain ferroelectric/piezoelectric response over time with no repole

Question 26

How do transducers work?

Answer 26

Sound waves generate compression in piezoelectric. This generates a charge which produces current in the circuit (cannot discharge through insulator piezoelectric). Acts like a capacitor releasing charge.
Or
Apply field and generate compression/tension in the piezoelectric to generate sound waves. Material shrinks/grows in confined volume to generate sound response.
Can be used as a sensor (sonar) or noise making device (speaker).

Question 27

Type of piezoelectric used in transducers

Answer 27

Fatigue is an issue. Compliance is desirable. Often make polymer-ceramic composites. Polymer is more compliant and the ceramic gives a larger response