Piezoelectric materials Flashcards
How can a piezoelectric material be used?
1) Direct effect (sensors)
A piezoelectric material, when subjected to a stress, experiences a difference of tension on the material (ΔV) (mechanical-electrical coupling).
2) Reverse effect (actuator)
Applying a difference in tension (ΔV), a deformation occurs. (electro-mechanical coupling)
What is the Piezoelectric principle?
Without stress applied, the structure of the mateiral is in equilibrium and no electrical charges appear on the surface. When we apply a stress, the atoms of the structure move and there is the loss of equilibrium from the electrical point of view.
Negative charges appear on the top surface, while possitive on the bottom, and therefore a difference between tension exists.
What are some Piezoelectric materials?
Different materials in nature:
- Quarz
- Tourmaline
- Rochelles
- Salt
- Cane sugar
- Tartaric acid
Also
1) Piezo-ceramics (example: Lead-zirconate-titanates or PZTs)
How are Piezo-ceramic materials obtained?
They are obtained by sintering.
1) Heating to 800-1000 Celcius of mixture of powder of lead, zirconium and titanium oxides
2) Mix with binder
3) Sintering in desired shapes
4) Cooling (cubic unit cell becomes tetragonal)
What are Perovskites?
Perovskites are compount made of tetravalent metals (Titanium and Zirconium), divalent metals (lead) and oxygen. The structure of piezoelectric material is a Perovskite structure
The structure of a perovskite is characterized by a central atom. During the production, while the material is at high temperatures (higher than the Curie temperature), the shape of the material is cubic and the material doesn’t exhibit the piezoelectric effect.
During the cooling, when T<Tcurie, the structure becomes tetragonal and a movement of the central atom appears. In this way, the material is able to show an electrical dipole moment in the direction of displacement of the central atom. When we apply a mechanical force, we induce a movement around the equilibrium position of the central atom and electrical charge appers on the surface and ΔV can be measured.
Discuss the PZT ceramics structure.
PZT ceramics possess a polycrystalline structure.
Polycrystalline structure in imperfections in bulk material result in a larger number of domain randomly oriented. In the PZT ceramic we can observe the presence of sub-domains with crystals randomly oriented, however the resultant of the electrical dipole is equal to zero (inside the whole domain).
By applying an electrical fild, we can induce an orientation of the dipole moments and we can fix this configuration. In this way we can obtain a polarized material. THe poling process permits to obtain a polarized material starting from a non-polarized material. By applying this process we can obtain a material that exhibits a piezoelectric effect.
Describe/Write the constitutive laws of PZT materials.
The strain can be seen as the sum of the elastic (deformation due to mechanical stress), the platic one, the thermal one, and the “piezoelectric” one. We consider only the elastic and the piezoelectric strain and therefore we get these two equations:
ε = s^Eσ + d^cE (actuator converse/reverse effect)
D = d^dσ + ε^σ E (sensor equation)
E is the electric field, D is the electric displacement, d^c is theconverse/reverse coefficient and d^d is the direct coefficient.
How can the consititutive laws of PZT me simplified?
Typically we apply an electric field (by introducing a difference of tension to the electrodes, which are applied on the bigger surface) in the direction 3 ( thickness direction), so only E3 is applied. The polarization is in direction 3 (so up polarization). There will be an elongation is direction 3. This means that d33 is not zero. After an elongation there is a contraction in the other two directions (Poisson effect) and therefore also d31 and d32 is not zero.
If we consider an electric field applied in drection 1, we can observe a rotation of the central pole in the perovskite structure. This rotation induces a tangential deformation. The same effect happens if we apply E2. This is why d15 and d24 is not zero. All the othe rcomponents are equal to zero.
What are primary and secondary effects and which are used mostly?
THe primary effect is the deformation that the material shows in the same direction of the electric field. The secondary effects are the contraction of the other two directions. Since L»T, it is better to exploit the secondary effects, because ε = ΔL/L.
