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School of Metallurgy and Materials

2FCM Continuous Assessment Questions Feb 2012

(Prof T W Button)

Section A

Sensors and Actuators

1. Explain with the aid of sketches and equations the direct and indirect piezoelectric effects,being careful to distinguish between them. How do these effects differ from

electrostriction? [13 Marks]

2. Describe the effects of doping in perovskite-based piezoelectric materials, showing how theimportant properties can be optimised for different applications. [15 Marks]

.

The perovskite materials are ABO3 structure. Both the A and B site can be used for the dopingto modify the properties of the materials. The substitution ion in the perovskite-based materialscan charge carrier or the charge accepter which are the donor dopants and accepter dopants

The donor substitution normally has one more valence than the based material. In that way,

the cation vacancies are introduced in the material with surrounding oxygen ions. With the largeoxygen anion, the mobility in the domain is lowered but the domain wall mobility in the materials

is higher. For the acceptor dopants, they have lower charge than the based materials, which means

the anion vacancies are introduced. Then with the oxygen vacancies like the framework, themobility is increased in this type of dopants.

Normally the difference in charge between the dopants and based elements is only one.

Hence, with two dopants impurities one vacancy is introduced.

The perovskite-based materials with donor dopants are more stable than the one with accepterdopants which affected by the applied field and temperature.

Basically, the acceptor dopants produce hard PZT while the donor dopants produce soft PZT.

Apart from the stability of the two materials, they differ in the constant of the piezoelectric

properties. Since the higher mobility of the domain wall, the donor doping soft PZT has higherpiezoelectric constant like coupling coefficients , which means it generate more electric field

under mechanical strain or vice versa. But since the movement of the domain wall means more

dielectric and mechanical losses. Hence, the soft PZT is used for the sensors and actuatorsapplications to have more conversion between the electric energy and mechanical energy even

with higher losses.

The acceptor doping hard PZT is less stable and with lower piezoelectric constants due to the

limited movement of the domain wall. However the coercive field is increasing and the dielectricand mechanical losses is lower. So that, for the application operating in the lower power and in

the micro scale like acoustic and ultrasonic vibrations, the hard PZT is a desirable option.

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3. Describe the principles of operation of a Bimorph actuator and describe a fabricationprocess suitable for its manufacture. [10 Marks]

For the Bimorph actuator, there are two thin piezoelectric material layers bonded together and theelectrode in the center and the surfaces of the Bimorph actuator. With the different

compliance or the piezoelectric constant the mechanical strain in these two layers differ inthe deformation of under the same voltage electric field as shown in the Fig.1.

Fig.1 Fig.2

In general, there are two type of Bimorph, parallel and series as shown in the Fig.2. As for thearrangement of the parallel type, the two layers are in the opposite direction of electric field.

The upper layer deforms in the 31 mode while the lower layer contracts, which cause the

Bimorph to bend. In the series Bimorph the applied electric field is across the both layer.

With de different degree of deformation, the Bimorph would bend.

For a possible the fabrication process, firstly, following the powder-based ceramic process route,

the PZT powder is formed. Then, by tape casting and calendaring, there are two layers ofplates is needed. The third step is sintering the two layers with electroding and poling as

well as machining. Then the two layers are sintered together or bonded with the metal

electrodes between the two plates. Finally for the outer surface metal electrodes, the metalplate can glued to bond with the ceramic layer.

http://www.physikinstrumente.com/en/products/piezo/piezo_selection.php?table=allhttp://www.physikinstrumente.com/en/products/piezo/piezo_selection.php?table=all

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4. Describe the concept of connectivity in a piezoelectric-polymer composite illustrating youranswer with sketches of 0-3, 1-3 and 2-2 geometries. What are the advantages of using

composite geometries in, for example, medical imaging or SONAR applications?

[12 Marks]

For a piezoelectric-polymer composite material, there are two phases in the material, ceramic andpolymer. The properties of the composite are related to the distribution of the ceramic andpolymer phases. The connectivity in the composite is to distinguish different ways of mix the two

phases. Under the measurement of the dimension, one phase in the composite has four possible

value of the self connection from 0 to 3.

Fig.1 Fig.2

Fig.3

The 0-3 composite is like the little dispersed particles surrounded by the matrix, as shown in the

Fig.1. The particle is randomly distributed and not connected to each other while the matrix is a

continuous phase over the materials, which means there is connectivity of 0 for the particles andthe connectivity for the matrix is 3.

The 1-3 composite is like many parallel bars along the material over the matrix, as shown in the

Fig.2. The rod shape phase extents to the end of the surface but do not connect each other whichhave the connectivity of 1. And the matrix is still a continuous phase in the composite which has

the connectivity of 3.

The 2-2 composite is like the plates of two phases stacked alternatively over the whole cross area,as shown in the Fig.3. Each phase has the self connection in two dimensions which means they all

have the connectivity of 2.

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In the medical imaging and SONAR applications, there are many advantages for the 1-3

composite. First, since the piezoelectric materials component is arrange in the same direction, if

the electric field is also applied along the rod direction, the introduced ultrasonic wave in thatdirection would rise no less than 20%. Also, with the easily change of the volume fractions of

piezoelectric and polymer, the acoustic impedance can be modified to more desirable valuesimilar to the body tissue and fluids. Thirdly, the polymer in the matrix has the damping effect

witch lowers the effect of lateral vibration and cross coupling as well as spurious activity and

makes the application much more accurate. Finally, the composite materials can be fabricated into

different shape like spherical or curved shape, which means the focused and unfocused in the

sonar application is available.

Section B - Superconductivity

1. Explain the significance of the following terms in relation to superconducting materials:

a) critical temperatureb) Meissner effectc) critical fieldd) critical current.

Use diagrams or sketches where appropriate. [10 Marks]

2. With the aid of diagrams describe the essential differences between Type I and Type IIsuperconductors, giving one example of each type of material. [10 Marks]

3. Explain why Type II superconductors are technologically more attractive than those of

Type I. Use diagrams and sketches to illustrate your answer. [10 Marks]

Total tutorial sheet out of 80 marks.

Submit answers via WebCT and hard copy to the Undergraduate Office on or by 4.30pm

Tuesday 3 March 2012.

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Please read the following notes regarding the electronic submission via WebCT

T W ButtonFeb 2012