LECETURE 4. Piezoelectric sensor. Part 1

Transcription

1 LECETURE 4 Piezoelectric sensor Part 1 Prof. Dr. YU GU GU@chemie.uni-Frankfurt.de Office Room: N160/517

2 Piezoelectricity

3 The word piezoelectricity means electricity resulting from pressure and latent heat. Piezoelectricity was discovered in 1880 by French physicists Jacques and Pierre Curie Jacques ( , left) with his brother Pierre ( ) and his parents

4 Piezoelectricity Piezoelectricity is the electric charge that accumulates in certain solid materials (such as crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress. A piezoelectric disk generates a voltage when deformed

5 Piezoelectricity is exploited in a number of useful applications, such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, et cetera.

6 It forms the basis for a number of scientific instrumental techniques with atomic resolution, the scanning probe microscopies, such as STM, AFM, MTA, and SNOM. It also finds everyday uses such as being used as the time reference source in quartz watches.

7 Piezoelectric effect

8 The piezoelectric effect is understood as the electromechanical interaction between the mechanical and the electrical state in crystalline. The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect also exhibit the reverse piezoelectric effect.

9 Oscillation of the crystal excited by an alternating voltage (black), unreformed quartz (grey);

10 Direct piezoelectric effect, The internal generation of electrical charge resulting from an applied mechanical force. Reverse piezoelectric effect, The internal generation of a mechanical strain resulting from an applied electrical field.

11 an applied mechanical force DPE the internal generation of electrical charge the internal generation of a mechanical strain RPE an applied electrical field

12 Oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states.

13 Oscillatory system An undamped spring mass system is an oscillatory system Two pendulums with the same period fixed on a string act as pair of coupled oscillators. The oscillation alternates between the two.

14 Oscillation current Oscillation circuit Oscillation current LC circuit

15 LC circuit Inductor LC circuit diagram Capacitor

16 Quartz Crystal Microbalance QCM

17 Quartz crystal microbalance is a very sensitive mass deposition sensor based on the piezoelectric properties of the quartz crystal.

18 The QCM is a widely used acoustic sensor. The QCM is applied for the analysis of surface attached polymers, adsorbates, biomolecules, and cells. It is a noninvasive tool to measure interfacial processes insitu.

19 When the QCM was first developed, natural quartz was harvested, selected for its quality and then cut in the lab. The crystals are cut and polished into hair-thin discs which support thickness shear resonance in the 1-30 MHz range. The AT-cut are widely used in applications.

20 This technique uses the changes in resonance frequency of the crystal to measure the mass on the surface because the resonance frequency is highly dependent on any changes of the crystal mass.

21 A QCM measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. A QCM is capable of measuring mass deposition down to 0.1 nanograms.

22 Sauerbrey equation The Sauerbrey equation was developed by Prof. Dr. Günter Sauerbrey from Tiefenort, Germany, in It is a method for correlating changes in the oscillation frequency of a piezoelectric crystal with the mass deposited on it.

23 Sauerbrey equation The Sauerbrey equation is defined as:

24 Photograph of typical quartz crystal resonators as used for QCM, metallized with gold electrodes (left: front electrode, right: back electrode) by vapor deposition.

25 Economic ways of driving a QCM make use of oscillator circuits.

26 Modes of operation The only design criterion of thickness shear mode resonators for frequency control is frequency stability. The AT-cut is most appropriate. AT-cut quartz crystals are also typically used as sensor elements, although the requirements for sensor applications are more complex.

27 Only maximum frequency shift is not an appropriate measure. A better value is the limit of detection, which depends on the signal-to-noise ratio. Temperature dependence is small for AT-cut crystals. In liquid applications, the most temperature-sensitive value is the liquid viscosity.

28 Sensitivity to mechanical perturbations is smaller for thicker crystals, i.e., lower resonance frequencies. Noise and systematic errors introduced by the electronic circuitry must be also taken into account.

29 Homework (Materials for the Seminar course) Please select a piezoelectric material and illustrate its use with a example. You need to report them in the seminar course.

30 THANK YOU!