Dielectrics- Pyroelectricity and Piezoelectricity Flashcards
Pyroelectric materials
There is spontaneous polarisation of a crystal lattice in the absence of an applied field. But cannot switch the direction of polarisation with an applied field. They are also piezoelectric. Only exhibit the property up to Tc. Beyond this the spontaneous polarisation is lost and the material becomes paraelectric.
How is polarisation controlled in pyroelectric materials?
Using temperature
Application of pyroelectric materials
IR sensors. Incident heat induces a change in polarisation. The new polarisation releases charge generating a current that is detected.
How does polarisation of pyroelectric materials change with temperature?
ΔP=πΔT
Where π is the pyroelectric coefficient units Cm^-2K^-1
1st order phase transitions
Show a discontinuity in state functions (e.g cell volume) at the transition temperature. All atoms move and change phase at the same temperature and time. The discontinuity can lead to large stresses in the material and fatigue in regular cycling
2nd order phase transitions
More gradual change in state functions with temperature meaning smaller stresses and less wearing on the material. 2nd order often preferred
Piezoelectricity
No spontaneous polarisation of a crystal lattice. Polarisation can’t be reversed with an applied field. Materials only exhibit property up to Tc. A surface electric charge develops on the solid when it is subjected to a mechanical stress. Effect is reversible. Voltage applied causes a change in shape. All ferroelectric and pyroelectric materials exhibit piezoelectricity
What are we interested in for piezoelectric materials?
Controlling the polarisation with mechanical processes or generating mechanical responses with electric fields
Effects of loading metal-oxygen tetrahedra in 2 different ways
Loaded along direction of one of the bonds- that bond will not move, the other 3 have a moment applied to them so the bend slightly so the charge on the O2- ions shifts, this distortion gives rise to a dipole.
Loaded perpendicularly to a tetrahedron edge- all bonds are deformed equally so no resultant dipole is created
What affects whether a material’s crystal based on tetrahedra develops a dipole under stress?
Need to add the effect from all tetrahedra in the crystal. If unit cell has a centre of symmetry then net polarisation will be 0. For non-centrosymmetric unit cells a net polarisation will exist. All piezoelectric crystals are non-centrosymmetric
Structure and dipoles in quartz
Dipoles exist in each distorted tetrahedron. They are connected to make an irregular hexagon but all the dipoles add to give 0 over a unit cell. When a load is applied, the structure is distorted so the dipole moments rotate slightly and don’t cancel each other out and a net polarisation is produced
What do piezoelectric polymers use?
Permanent dipoles on the polymer chains. Generally from strong C-X polar bonds and hydrogen bonds.
Poly(vinyl fluoride) and poly(vinylidene fluoride)
PVF and PVDF both piezoelectric polymers. Dipoles on tetrahedral unties. PVF has one F per tetrahedron and PVDF has 2. Piezoelectric effect greatest for isotactic form. Atactic shows reduced or no piezoelectric effect. Careful synthesis required to ensure monomer units link in regular manner during polymerisation and for the polymer to partially or fully crystallise into a non-centrosymmetric structure
How do polyamides work as piezoelectric materials?
Hydrogen bonds produce the dipole moments. Relative configurations of H-bonded dipoles depend on spacing between amide groups along polymer chain. Even polymers- dipoles opposed along chain and no piezoelectric effect observed. Odd polymers- dipoles align to give net polarisation and do exhibit piezoelectricity.
Advantage and disadvantage of polyamides as piezoelectric materials
Only small piezoelectric coefficients. But can be thin films of low density and have flexibility (useful for repetitive shape change). Used for sensors, transducers (like microphones, keyboards, flat-panel speakers)
