CMB226 - Dielectrics

Name the key 3 features of dielectrics, with values

All are electrical insulators at 298K: resistivity>10^8ohm.cm. All belong to non-centro-symmetric point groups. All possess or develop polarisation

Give the equation for the dipole moment vector, p

p=qd (d=charge separation)

Name the 4 basic polarisation mechanisms in an applied electric field

Atomic polarisation, Ionic polarisation, Dipole polarisation, and Space charge/diffusional polarisation

Explain atomic/electronic polarisation and say where it's present. Draw diagram separately.

The movement of the electron cloud around the nucleus, small distance so polarisation small. Present in all materials

Explain ionic polarisation and say where it's present. Draw diagram separately.

The small movement of charged particles (ions) in the crystal. Present in ionic materials but not covalent materials.

Explain dipolar polarisation. Draw diagram separately.

The alignment of pre-existing dipoles to form net dipole/polarisation

Explain space charge polarisation. Draw diagram separately.

Localised movement of charge across system caused by defects (impurities, vacancies, interstitials, etc) will generally be immobile. But under a field may move small distances to create dipoles over a grain. Not conduction - no long range movement.

Permittivity

Macroscopic property that measures polarisation.

Dielectric losses, tan o

Polarisation changes due to the energy of the electric field. As the charge moves within the material some of that energy is lost. Low loss = material responds efficiently to field. (This can be from the energy being converted into heat, small electric fields, localised currents etc.)

When do piezoelectrics polarise?

Only when subjected to mechanical stress

When do pyroelectrics polarise?

Spontaneously

When do ferroelectrics polarise?

Spontaneously polarise and the polarisation can be reversed by an electric field.

Curie Temp (re ferroelectrics)

Above this temp spontaneous polarisation is lost and the material become paraelectric

Explain the effect of ferroelectricity in hydrogen bonds and above Tc

H bond formed when H sits between 2 e-neg atoms, creates polarity. H can move and bond to alternative e-neg atom. Flips direction of polarisation. In ferroelectric systems an e field can do this. Above Tc H moves freely, no polarisation.

Explain the effect of ferroelectricity in the orientation of polar molecules and above Tc

Polar molecules normally randomly distributed = no net dipole. Below Tc E field aligns orientation, creating a collective dipole that can be moved. Above Tc, all rotating too much to retain organisation.

Explain the effects of ferroelectricity on medium sized metal cations and above Tc

In some crystals cations too small and can move around in site. By going off centre can end up with more pos charge in one half of cell and neg in the other - a dipole, e field can dictate direction. Above Tc cation moves too much, effectively in middle, no dipole.

What temperatures do the different phases of BaTiO3 occur at?

Rhombohedral T<-90, orthorhombic -90<T<0, tetragonal 0<T<130, cubic T>130

As the phases of BaTiO3 change where is Ti located.

Rhombohedral [111] towards a corner of the cell, between 3 O. Orthorhombic [110] towards an edge of the cell, between 2 O. Tetragonal [001] towards a face of the cell, nearer 1 O. Cubic Ti sits centrally

Ferroelectric Domains

Domain formed when many cells align, large dipole forms, generates counteracting e field that encourages individual dipoles to swap direction. But want to maintain local alignment. Twinning occurs - alternative direction domains

Ferroelectric domain walls strain

Occur between domains. Polarisation of cell changes lattice size, this means cell is no longer commensurate with another cell at 90 degrees. Creates mechanical strain

Explain the Hysteresis of polarisation

1. No net polarisation, P = 0 (to reduce electrostatic energy)

2. Field applied and gradually shift cells to align with field

3. Maximum level of 90 + 180o domain wall motion Psat

4. Removal of applied field results in domain relaxation to remove internal strain. Pr

5. Reverse field is required to return to P = 0. Ec

What is poling

Application of a large dc electric field in an oil bath (to ensure charge can't dissipate) at a temperature just below Tc. Used to align the dipoles in similar orientations and thus induce net polarisation within a ceramic.

Capacitor

Current only flows when the charge on one side becomes large enough to breakdown the barrier and cross it

Uses of capacitors

Charge storage (build up of charge on one place so it can be rapidly released eg camera flash), block DC current (DC only travels one way so will get stuck at capacitor), AC-DC conversion

Why are ferroelectric materials beneficial in capacitors?

Insulator. Polarise in the field created by plates further stabilising the charge and allowing a greater charge to build

How do you increase capacitor function?

Shrinking gap (d), increasing area of plates (A), increasing permittivity (E)

Why do you want to increase capacitor permittivity

Greater permittivity, better ability to polarise - material can store far larger charge on surface

How do you increase permittivity?

Permittivity varies directly with volume efficiency and 1/d^2. To decrease d: Use thin layers (1-10 um), oxidise the surface of a metal foil (electrolytic capacitors) (0.1-1um), tape cast ferroelectric BaTiO3 based ceramics (0.8-10um) stack layers to form a Multi Layer Capacitor

What is a Multi-Layer Ceramic Capacitors (MLCCs)? Give equation for volume efficiency.

Lots of alternating layers of electrodes separated by ceramic. Each layer contributes to overall area of electrodes while separation is only the very small gap between each electrode. Volume efficiency of MLCC = [Capacitance/Volume] x n

Temperature Coefficient of Capacitance (TCC)

How much capacitance varies with temp, due to polarisation changing with temp.

Define vacancy

Missing atom on lattice site

Define impurity

Atom replaced by foreign atom

Define interstitial

Extra atom not on lattice site - can be native or impurity

What is a Negative-Positive Oxide (NPO) ceramic

Mixture of 2 dielectric materials: one with a neg permittivity change with temp (-TCE) and one with a pos permittivity change with temp (+TCE). These cancel out and lead to a stable permittivity with temp

Effect of size of A-site dopant on ferroelectricity

Changing size of A-site cation changes the space for B-site cation. This changes how easily it goes off centre. Changes ferroelectric stability

Explain a core-shell structure and how it affects capacitance re structure

Where material has a single heterogeneous grains with mixed chemical composition. A core that is pure undoped material and a shell which is doped. These are mixed together with a central core & outer shell.

Explain how a core-shell structure affects capacitance re E-T

The different compositions have different E-T profiles which will merge together to give a flatter E-T

Explain how pyroelectricity is used in IR sensors

Heat induces change in polarisation, releases charge generating current that is detected. Heat=current.

Explain a pyroelectric first order phase change (could draw graph)

Show a discontinuity in state functions (e.g. cell volume) at the transition temperature. Less sensitive to heat changes. Discontinuity leads to large stresses and fatigue in regular cycling

Explain a pyroelectric second order phase change (could draw graph)

Don't show a discontinuity in state functions at the transition temperature. Better than first.

Piezoelectric solid

A surface electric charge develops when it is subjected to a mechanical stress. The effect is reversible. A voltage applied to a solid causes a change in shape.

Are piezoelectric crystals centrosymmetric or non-centrosymmetric and why?

Non-centrosymmetric as in a tetrahedron, force along a bond gives rise to a dipole as a result of distortion, if it were applied perp to an edge all bonds are deformed equally so does not. Net polarisation is non-centrosymmetric.

Explain dipole containing crystals

Deformation of crystal causes dipole moments to rotate slightly and therefore net polarisation is produced.

Why are piezoelectric polymers often useful?

Due to compliance (ability to adjust to strain with no long term failure). Which means they can be cycled a lot

Explain piezoelectricity in polyamides (H-bonds)

H bonds produce dipole moments in amides. For even polymers dipoles are opposed along chain and thus no piezoelectric effect. For odd polymers dipoles align to give net polarisation thus piezoelectricity

What are the advantages and disadvantages of piezoelectric polyamides?

Adv: Thin films of low density and have flexibility. Dis: small piezoelectric coeff.

What are the two effects for poled ferroelectric ceramics

Direct (generator) effect where stress is applied and electricity generated. Converse (motor) effect where electric field is applied and strain generated

Explain ageing in poled ceramics

Ageing is the slow decay of Pr due to diffusion, (ie domain wall movement), happens to relax internal strain. Often associated with oxygen vacancies

What is a hard piezoelectric

High level of oxygen vacancies, holds polarisation. Lower tan o, more difficult to depole, low d (strain induced values), high voltage and high load applications

What is a soft piezoelectric

Fast reduction of Pr followed by stable Pr, small Ec, higher tan o, easier to depole, high d (strain induced) values, high sensitivity applications

Advantages and disadvantages of poled ferro electric ceramics

Adv: simplicity, small size, low cost, high reliability. Dis: ageing and depoling

Explain piezoelectric micro positioners and explain whether soft or hard piezoelectric is used

Devices that move very small distances in response to electrical signals. They require large displacement from small applied field and lots of sensitivity, thus use soft piezo electric. Accurate and fast movement, inc t by inc V, inc t by inc number of layers

Explain gas ignitors and explain whether soft or hard piezoelectric is used

Generate spark by creating a field from compressed piezoelectric material, spark ignites gas. Hard piezoelectric used, as want to generate a large field, need material to be robust and reusable without loss of function

Explain transducers

Soundwaves generate compression in piezo, generates charge which produces current in circuit, cannot discharge through insulator piezo, effectively like a capacitor. Or vise versa. Can be used as sensor or speaker