1 Piezoelectric effect in cigarette lighters.

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1 BQ2 Group Notes Write very brief notes (maximum 3 of a page figures can exceed this size limit, 4 should definitely not take you more than an hour) on one of the following topics. E- mail me your ranked choices on Topics A to H from 1 (highest preference) to 8 (lowest preference); it s first come first serve. Hand draw suitable diagrams to aid your explanations and me the text / word processed document if possible. These notes will be combined and sent around to everyone at the end, so this is mainly an exercise in drawing very clear, labelled diagrams while making good use of a ruler. During the supervision you will then take five minutes to explain to your supervision partner how the use of your chosen topic s effect has lead to its respective application. 1 Piezoelectric effect in cigarette lighters. Application of a force to the button on the lighter compresses a spring loaded hammer. When the button reaches the bottom of its travel, the hammer is released and propelled by the spring to strike the piezoelectric crystal (usually quartz). This produces a large stress in the crystal. Quartz (SiO 2 ) has tetrahedral coordination of Si around O and O around Si. The tetrahedra are corner sharing so each O is part of two tetrahedra. The tetrahedra form spirals (3 or 6 fold depending on temperature) so there is a unique direction and no centre of symmetry.

2 Upon the application of the stress to the tetrahedra (compression along [1 10]), oxygens 3 and 4 move inwards. To remain equidistant from the O 2- the Si 4+ moves upwards [00 1]. As the centre of charge of the oxygen ions has not changed and the position of the Si 4+ has, there is now polarisation of the tetrahedra in the [001] direction. Figure 1: A diagram showing tetrahedral coordination of O 2- ions around Si 4+ ion As there is a unique direction and no centre of symmetry, the polarisation of the tetrahedra sum to form a polarisation across the whole crystal. Therefore there is a large potential difference across the crystal, high enough to breakdown air and form a spark across a small gap to ignite the gas which is released as the button is pressed. V = d κɛ 0 In actual quartz as the electronegativity of Si and O are quite similar, the bond is closer to covalent however as O is more electronegative than Si, Si does have a partial positive charge (and oxygen has partial negative). Conor Williams 2 Piezoelectric effect in MRI scanners. In MRI scanners, piezoelectric materials are used as motors to move either the patient, or surgical robots around. Due to the extremely high magnetic fields needed to make the MRI function, traditional electromagnetic motors cannot be used while the imager is functioning. Thus piezoelectric materials, which change shape under small voltages, are used. One benefit these motors have is that patients can be moved, in order to scan a different part of them, without having to shut down the MRI. Clearly this increases the efficiency of the MRI - it can treat more patients each day which is a very attractive feature given that MRI scanners cost 100s of thousands of pounds. page 2 of 12

3 BQ1 Additional Questions Additionally, biomedical firms are now looking into using these motors to create surgical robots capable of functioning within strong magnetic fields. Doctors can use these robots in conjunction with real time images from the MRI scanner to perform precision surgery; this is very useful for brain tumours or other areas which are not easily accessed or viewed with the naked eye. Figure 2: A voltage applied to the biomorphic legs causes them to bend and extend or retract, allowing them to move a drive rod forward. Clearly there are some problems with the electric, and therefore magnetic, fields created by the tiny currents across the piezoelectric structures however these can be mediated. The piezoelectric motors are very sensitive to voltages and can therefore be used near enough (from about 1 metre away) to the MRI to function for patient translocation. For the surgical robots, as well as magnetic insulation, external magnetic noise removal filters are used. In addition to being non-magnetic, piezoelectric substances provide very high breaking and torque resistance when subject to a steady voltage. This allows them to, once positioned, be incredibly stable, another clear benefit in precision imaging and surgery. Above are the uses of piezoelectric motors in MRI. Since they are also much smaller than normal electromagnetic motors when compared by force output, piezoelectric motors are used to dispense very small, exact volumes. They can take precise steps when exact voltages are used. Similarly they can position large masses on a nanometre scale useful for anything from lasers in synchrotrons to high end military weapons and surveillance page 3 of 12

4 cameras. Bibliography Luke Baines 3 Piezoelectric effect in producing ultrasound waves. The piezoelectric effect can be used to convert an alternating current signal into ultrasound. A piezoelectric crystal sits in-between two electrodes, as shown in figure 1. The crystal oscillates at the same frequency as the AC signal frequency (around the natural frequency of the crystal). For large frequencies (>18kHz) the pressure wave created by the oscillation is considered to be ultrasound. The ultrasound waves are transmitted through the end of the transducer, though a material with a smaller acoustic impedance. Around the transducer is a material with a very large acoustic impedance which ensures the ultrasound is only emitted from one direction. Usually lead zirconate titanate (PZT) is used as the piezoelectric crystal, figure 2 shows page 4 of 12

5 the unit cell. When a voltage is applied across two faces the crystal polarizes. This means the octahedral is distorted as the central ion moves into the new center of charge. This results in a shape change of the crystal. When the opposite potential difference is applied the polarization flips allowing the crystal to oscillate hence producing ultrasound waves. Henry Calver 4 Piezoelectric effect in sensing ultrasound waves. The piezoelectric effect can be used in devices known as ultrasound transducers in order to generate and detect ultrasound waves. The diagram above shows the essential components of such a device, which is intended for sensing pulses of ultrasound in an ultrasound scan. The incoming ultrasound wave is absorbed by the thin membrane at the front of the device (usually made out of a flexible plastic). The oscillations of the membrane are transferred to the electrodes/ piezoelectric crystal. The rest of the exterior of the transducer is enclosed in metal and an acoustic insulator (such as rubberised cork), ensuring that the only signal which passes through to the piezoelectric crustal is along the length axis of the device (thus increasing the clarity of the signal). The pressure wave from the plastic membrane cause the piezoelectric crystal to be periodically put under tensile and compressive stress at a frequency equal to that of the ultrasound. The amplitude of these oscillations is greatest when the ultrasound frequency is equal to the natural resonant frequency of the piezoelectric disc (and so this page 5 of 12

6 frequency of ultrasound produces the clearest output signal). The resonant frequency can be varied by varying the thickness of the piezoelectric disc, and so each sensor is tuned to a particular frequency. As the crystal is put under stress, dipoles form in the unit cells of the crystal. These dipoles are in the same orientation, and so there is a net voltage across the length of the material when it is under stress. This is known as the piezoelectric effect, described by: V = d κɛ 0 V σ While the transducer is being struck by an ultrasound wave, the thickness of the piezoelectric crystal oscillates, and so an alternating voltage is created on the electrodes on either side of the crystal. For safety reasons, the electrode nearest the plastic membrane is earthed via a connection to the metal casing of the device. The live electrode carries the output signal to a coaxial cable which is also connected to the casing for reference. The output signal is then connected to an amplifier and other signal processors. In order that the electrodes do not reduce the amplitude of the piezoelectric crystal oscillations, they must be very light (usually made out of silver layers). In this particular design of an ultrasound sensor, which is used for medical and industrial scanning, the input signal is discrete pulses of ultrasound. The time of arrival of these page 6 of 12

7 pulses can be used to determine useful information about the medium through which the sound is passing (blood flow rate, material boundaries, etc.) In order to allow for the time for arrival of these pulses to be effectively distinguished from the output signal, the oscillations of the piezoelectric crystal must be damped between each of the pulses. This is achieved by a backing material which absorbs the energy of the crystal and dissipates it. the damping coefficient can be tuned by varying the elasticity of the backing material such that the oscillations are dissipated in as short a time as possible (ζ = 1). Craig Ferguson 5 Ferroelectric effect in nanogenerators for energy harvesting. Ferroelectric effect in nanogenerators for energy harvesting The basic concept of these generators is using the energy in mechanical vibrations that would otherwise be wasted to change the polarisation of the ferroelectric material and thus be used to generate current. And it is thought that it may one day be possible to make self-sustaining microelectronics. Barium Titanate BaTiO3 has been used as the ferroelectric material in most studies so far, and this has a curie temperature of 120, meaning it has a tetragonal structure at room temperature. Each part of a ferroelectric material can become polarised in any of the 100 directions and they tend to arrange themselves in domains with regions that all have the same polarisation because the polarisation leads to a lattice distortion. page 7 of 12

8 It is the change in surface polarisation that can be used to generate current using electrodes at the surface. When the material is deformed (by the vibrations) the position of these domains changes accordingly as the material is stretched and compressed. This figure shows the effect of the vibrations that compress the material, causing a change in polarisation of the material, which causes a change in polarisation and so creates an e.m.f in the top electrode (grey) and bottom electrode (yellow) A small electric field is used to set the initial polarisation of the material. One drawback of these is that it requires a relatively constant frequency of vibrations to create a usable current. Jose Gray 6 Ferroelectric effect in Sony PS2 The Sony Playstation 2 had a RAM chip in it which used Ferroelectric RAM rather than the more conventional Dynamic RAM, or DRAM. RAM means Random Access Memory, and basically means computer memory that a computer can reach all parts of equally quickly. This is opposed to something like a hard disk where you have to wait for an arm to move to the data to read or write. FeRAM is very similar to DRAM, except that FeRAM uses a ferroelectric material in its capacitors rather than a dielectric. In DRAM, each bit of data (either a 0 or a 1) is represented by a cell containing a capacitor and a transistor in a rid laid out like this: page 8 of 12

9 BQ1 Additional Questions These horizontal wires are called word lines and the vertical ones are called bit lines. The bit lines are in pairs, with a sense amplifier on each pair. To write to a bit, the corresponding word line is powered, opening the transistor, and the corresponding bit-line is set to either 0V or a specified voltage, which will write to the capacitor either a 0 or 1. Reading the bits is a bit more complicated, but basically involves putting about half the output voltage on all the bit-lies, opening one word-line, and letting charge flow into (if 0) or out of (if 1) the capacitor. The sense amplifier defeats this and amplifies it and then outputs it. In FeRAM, this is slightly different. Instead of each capacitor being either charged or uncharged, the 0 and 1 states of the capacitors are the two direction of polarisation. page 9 of 12

10 When reading FeRAM, a voltage is placed across the capacitor. If it is the same direction as the polarisation, nothing will happen, but if it is opposite then there will be a current from the capacitor and most regularly be topped up. This is not the case in FeRAM. However, FeRAM is harder to make as small as we can make DRAM today, so DRAM dominates the commercial market. Kevin Fletcher 7 Pyroelectric effect in pollution control Pyroelectric samples, being samples which can produce an electric potential when they are heated or cooled due to the movement of charges at a unit cell level which would not be symmetric across the unique direction and would therefore produce a polarisation. The Situation A source of Infrared can be emitted to pass through a body of gas (the sample gas), in terms of pollutants this could be air or gases being emitted from potentially harmful sites. Then since certain gases absorb specific wavelengths of infrared radiation due to their corresponding vibrational energy we can use specific wavelengths to look for certain gases. By passing the infrared source through the gas, where there will be an intensity loss as the IR photons are absorbed and re-emitted randomly, and through a reference a comparison can be made as to how much IR has been absorbed and as such the proportion of the specific gaseous pollutant in the sample. The Use of the Pyroelectric Effect The key part of the analysis technique is to quantitatively measure how the intensity of the IR has dropped, and through this how much page 10 of 12

11 IR has been absorbed. This can be determined by the IR (after having passed through the reference and the gaseous sample) heating the pyroelectric compound, which in turn induces a voltage which will quantify the change in intensity of the IR and hence the proportion of the pollutant in the sample can be measured. Analysis This technique can: Confirm whether a pollutant is inside a sample, for example if gases from a manufacturing plant were being tested for harming the environment. Be used to calculate a general trend over a period for a sample through repeated measurement and whether reduction techniques are successful. As the absorption of IR is a key factor in the greenhouse effect it can be used to determine the effect of a particular pollutant. Kieran Heal 8 Pyroelectric effect in disaster relief. Post-Disaster relief robot system: Most urgent task after a major disaster is to search for survivors who could be trapped in the ruins. A current development is the design of ant search robots using pyroelectrics. The robot can search a site and provide feedback to rescue workers about possible survivors. What are pyroelectrics? If the temperature of a sample changes, there will be a change in the relative positions of positive and negative charge centres as a result of the thermal expansion/ contraction. This leads to a change in the electrical polarisation which is pyroelectricty. As the temperature changes, there is some relative movement of charge which cannot be symmetric along a unique direction, hence resulting in a net polarisation. Further detail on the recovery system The robots often use an IR pyroelectric detector which is cheap, doesn t depend on external light and can detect a person, several metres away. The pyroelectric IR sensor receives the IR radiation given off by the human and the IR signal processors gives a high-level output. The sensor will produce a weak signal, but this is passed through page 11 of 12

12 Figure 3: Diagram of the Ant robot recovery system an electronic circuit to amplify and filter the signal. in his way, the rescue worker are given information about where to search which saves vital time. References: Emerging research in artifical intelligence and computational intelligence Post-disaster detection rescue robot system design Will Sutcliffe IA Materials, Supervision 4, page 12 of 12