Lecture handout including QS - Department of Materials Science ...

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BH26 Course B: Materials for Devices BH26 A material which can demonstrate a dielectric polarisation (e.g. due to charge separation) in the absence of a field is a polar material. It must contain a unique direction. A centrosymmetric crystal is non-polar, but a non-centrosymmetric crystal is not necessarily polar: (a) Non-polar; non-centrosymmetric: (b) Polar; non-centrosymmetric -Q -Q -Q +4Q -Q → -Q +4Q -Q … displacement of central ion … -Q +ve charge sits at centre of balance +ve charge is offset ⇒ charge imbalance of –ve charge, ⇒ there is charge ⇒ dipole moment & spontaneous balance, & zero dipole moment polarisation If a stress is applied to sample (a), inducing a shape change (e.g. if it is squashed or extended in one direction), the positive and negative charge centres will move relative to each other, i.e. there will be a change in polarisation. This phenomenon is called piezoelectricity. Conversely, application of an electric field will lead to a change in the positions of the ions, and hence to a shape change. If the temperature of sample (b) changes, there will be a change in the relative positions of positive and negative charges (e.g. due to thermal expansion / contraction), and this will lead to a change of electrical polarisation. This is called pyroelectricity. All polar crystals are, to some extent, pyroelectric: there is some relative movement of charges as temperature changes, which cannot be symmetric along a unique direction; therefore there is a net change in polarisation. In some materials the electrical polarisation of a sample such as (b) may be switched by an external electric field. This is ferroelectricity: on application of an electric field the crystal can become permanently polarised, and this polarisation can be reversed by applying the field in the opposite direction. Ferroelectric materials are a subset of pyroelectrics which, in turn, are a subset of piezoelectrics: non-centrosymmetric & polar Piezojust needs to be non-centrosymmetric Ferro- Pyro- non-centrosymmetric, polar & switchable
BH27 Course B: Materials for Devices BH27 Piezoelectrics Change in polarisation on application of a stress: Polarisation due to stress, P = Q A = dσ d is the piezoelectric coefficient σ is the stress (tension or compression) Resultant voltage across piezoelectric, V = Q C € Examples of Piezoelectric materials: (units of C N –1 ) (units of N m –2 ) = QL κε 0 A = dσ L κε 0 (see BH24) quartz (SiO 2 – see AH61); & many other crystals containing tetrahedral groups, e.g. ZnO, ZnS – zinc blende (application of stress distorts the tetrahedra). All pyroelectrics & ferroelectrics are piezoelectric. Piezoelectric Transducers: Generator effect - application of stress changes P and leads to a voltage change. Generator effect e.g. igniters (pressure on a thin sheet induces a high voltage (~kV), which is enough to cause electrical breakdown across a small (~1 mm) air gap); energy harvesting, & flashing lights on trainers; guitar pick-up. applied stress Motor effect - application of an electric field leads to a shape change. applied electric field Motor effect e.g. watches (a quartz crystal is made to vibrate at a precise, fixed frequency); medical devices (e.g. an array of quartz resonators focus sound waves onto a tumour to destroy it, or reflect off an object and therefore image it); displacement transducers (the ability to make very small, very precise mechanical movements leads to actuators with very high spatial resolution).
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Lecture handout including QS - Department of Materials Science ...

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