Chapter 4 - Piezoelectric materials for energy conversion Flashcards
Give a definition of the direct piezoelectric effect. What is the origin of the piezoelectric effect? What are the physical constraints for the design of piezoelectric materials?
The direct piezoelectric effect is the change of electric polarization proportional to the strain applied to the piezoelectric. The inverse piezoelectric effect is the opposite, that is a change in strain in the system due to an applied potential.
The origin of the piezoelectric effect is a change in the dipole moment as the structure is compressed or stretched.
Physical constraints for designing the piezoelectric materials are the piezoelectric coefficient, which tells us how strong this effect is, and its ability to sustain an applied stress or repeatedly undergo a recoverable strain.
Polycrystalline piezoelectric ceramics: what are the design principles and why is the polling process necessary?
It is usually a lot easier to synthesize these materials. In these materials, the domains are distributed quasi-randomly. When a stress is applied to the material, all dipoles rotates from original orientation to an orientation that minimizes the overall electrical and mechanical energy stored in the dipole. Thus, the piezoelectric ceramic is not actually piezoelectric, but we can use poling to make it so.
When a piezoelectric ceramic undergoes poling, a DC voltage is applied across the material at elevated temperature, directing the dipoles so that they are all more or less directed the same way. Now the piezoelectric effect can be exploited. This is fairly hard to model though, and one example of modeling it is to model the Pb-atoms (in PZT-materials) as 12 split sites.
Name the state of the art piezoelectric material for energy applications. What is the main parameter defining the attractiveness of a piezoelectric for energy harvesting devices?
The most used piezoelectric material are PZT ceramics, that is Pb[Zr,Tn]O3 structures - based on cubic pervoskite strucutre.
Other state of the art materials that show promise is ZrO and BiFeO3, and also the polymer polyvinylidene fluoride (PVDF).
The main parameter defining the attractiveness of a piezoelectric for energy harvesting is the piezoelectric coefficient, which tells us how big the piezoelectric effect is (the ratio of applied stress to voltage produced). Also important are their mechanical stability and abundance (we therefore want to change PZT with other materials, but the far superior piezoelectric coefficient has thus far prevented this).
Analyze pros and cons for the application of monocrystalline and polycrystalline piezoelectric materials for energy applications.
Monocrystalline: they are easy to model, but not always straightforward to synthesize.
Polycrystalline: these are harder to model, but normally much easier to synthesize, and thus use for large-scale applications.
What are the current principles in designing piezoelectric generators for larger-scale energy applications?
Some principles of of designing piezoelectric generators:
- cantilever based harvester (unimorph, bimorph)
- Piezo-leaf (based on cantilever, uses wind to bend the element).
- Roadway/railway generators
- city street piezoelectric generators
- hybrid materials where piezoelectric works as both piezoelectric and semiconductor with self-junction.
- nanowire arrays
Analyze perspectives of application of e.g. hybrid piezoelectric/semiconductor energy harvesters.
Due to non-centrosymmetry, there is produced an internal field in ferroelectrics that exhibits a spontaneous polarization that acts like a pn-junction = self-junction.
At one face electrons accumulate, other side holes accumulate- We get because of this band bending with a distinct redox-chemistry. We get a reduction at the face where the electrons are accumulated, and get an oxidation on the face where the holes are accumulated.
This means that it can work as either a photocatalyst or a single-material photovoltaic.
The problem with this is that most perovskites have too big band-gap to be useful absorbers (absorb too little of the solar spectrum). BiFeO3 shows some promise in this respect.
Why are piezoelectric wind generators so intriguing for use in cities?
Because normal windmills can’t be used in cities. As time progresses, only more people will live in cities, and the importance of being able to produce energy where it is consumed (in the cities) will be larger. Making artificial trees with piezoelectric leaves can therefore be a source of energy using wind power, where wind mills cannot be placed.
What are Weiss domains?
Regions of dipoles that are similarly aligned.
Show the family tree of dielectrics.
Dielectrics - piezoelectrics - pyroelectrics - ferroelectrics.
What is the general requirement for a material to exhibit the piezoelectric effect?
That is has a non-centrosymmetric crystallographic symmetry. This corresponds to 20 of the 32 crystal classes.
What is the presence and degree of the piezoelectric effect dependent on?
Presence is dependent on crystal symmetry, and the degree is material specific.
What is the piezoelectric coefficient defined as? What are the units?
d_xy = strain development / applied electric field [pm/V].
What does the subscripts of the piezoelectric coefficient denote?
The direction of the polar axis to the direction of the applied stress/strain.
d_33 = stress applied in the same direction as polar axis. d_31 = stress applied at right angles to the polar axis.
Name some of the oscillation modes that can appear in for example a quartz crystal. Draw them.
Longitudnial mode.
Flexural mode.
Thickness shear mode.
Face shear mode.
See slide 17 for figures.
How can one determine the resonant frequency of a crystal?
By measuring the impedance of the crystal as a function of frequency. At the resonant frequency, the amount of energy lost would be at its minimum.
