Piezoelectricity bending

The market outlook appears to be particularly favomable for microvalves, driven by piezoelectric bending transducers or thermal principles. Such devices are already being produced and sold by several companies. The possible applications of these valves will increase when they can be mass-produced inexpensively and operated with very low power consmnption. Stimulus can be expected particularly from valves with 3/2-way functionahty, to be introduced as pilot valves in many areas of automation. 

Piezoelectricity Bending Discovery Principles of Torsional Splay Spring model Strain Twist Two-domain Heckmann diagram... 

Figure 7.14 Experimental set-up for atomic force microscopy. The sample is mounted on a piezoelectric scanner and can be positioned with a precision better than 0.01 nm in the x, y, and z direction. The tip is mounted on a flexible arm the cantilever. When the tip is attracted or repelled by the sample, the deflection of the cantilever/tip assembly is measured as follows. A laser beam is focussed at the end of the cantilever and reflected to two photodiodes, numbered 1 and 2. If the tip bends towards the surface, photodiode 2 receives more light than 1, and the difference in intensity between 1 and 2 is a measure of the deflection of the cantilever and thus of the force between the sample and the tip. With four photodiodes, one can also measure the sideways deflection of the tip, for example at an edge on the sample surface.

In situations where absorption of the incident radiation by the transducing gas is troublesome a piezoelectric transducer (made from barium titanate, for example) can be attached to the sample (or sample cuvette in the case of liquids) to detect the thermal wave generated in the sample by the modulated light (8,9). The low frequency, critically damped thermal wave bends the sample and transducer thus producing the piezoelectric response. The piezoelectric transducer will also respond to a sound wave in the solid or liquid but only efficiently at a resonant frequency of the transducer typically of the order of 10 to 100 KHz (see Figure 4). Thus neither in the case of microphonic nor piezoelectric detection is the PA effect strictly an acoustic phenomenon but rather a thermal diffusion phenomenon, and the term "photoacoustic" is a now well established misnomer. 

Piezo. This is the same basic print head technology as in continuous inkjet. The ink jet droplets are forced out through the nozzle after an electrical signal to the piezoelectric crystal causes a pressure wave to be set up in the ink (Figure 2.35b). The wave can be produced in the bend mode, as in Figure 2.35b or in a push or shear mode. An important variation on piezo technology is the Xaar... 

When a polymer film is deformed sinusoidally with time with an angular frequency to, an open-circuit voltage or a short-circuit current of the same frequency is observed across the electrodes on both surfaces of the film. This phenomenon is called the piezoelectricity of the film. The deformation is usually the elongational vibration along an axis in the film plane and sometimes the bending vibration. 

To measure bending piezoelectricity, it is merely necessary to modify the specimen clamp as indicated in Fig. 6. 

As stated in the Introduction, as-cast polymer films in general show a weak piezoelectric effect in both elongation and bending. The results by Furukawa, Uematsu, Asakawa and Wada (1968) for five kinds of polymer films are illustrated in Figs. 18 and 19. The effect is ascribed to space charges embedded in the film. 

The bending piezoelectricity in drawn and polarized polymer films was studied in detail by Kawai (1) (1970). Kitayama and Nakayama (1971) reported a very high piezoelectricity in composite films of polymer (PVDF, nylon 11, PVC) and powdered ceramics (barium titanate, PZT) after poling. In the case of PVDF and nylon, the piezoelectric constant increase by a factor of 102 when the ceramics make up 50% of the volume. The pyroelectricity and optical nonlinearity of polarized PVDF films have been studied by Bergmann, McFee, and Crane (1971). 

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

Figure 2 shows a schematic of a typical AFM instrument that consists of a cantilever-mounted tip, a Piezoelectric scanner, four position-sensitive photo detectors, a laser diode and a control unit. The process of operation of an AFM is relatively simple. The beam from the laser is directed against the back of the cantilever beam onto the quadrants of the photo detector. As the tip is moved across a sample, this causes the cantilever beam to bend or be twisted in manner that is proportional to the interaction force. This bending or twisting of the cantilever causes the position of the laser on the photo detector to be altered. The deflection of the cantilever beam can then be converted into a 3D topographical image of the sample surface (Gaboriaud and Dufrene, 2007 Kuznetsova et al., 2007 Lim et al., 2006). 

There exist a number of readout techniques based on optical beam deflection, variation in capacitance, piezoresistance, and piezoelectricity. Piezoelectricity is more suited for a detection method based on resonance frequency than the method based on cantilever bending. The capacitive method is not suitable for liquid-based applications. The piezoresistive readout has many advantages, and it is ideally suited for handheld devices. 

Fig. 2 Schematic representation of the basic detection elements of the scanning force microscope and of the piezoelectric transducers generating the displacement modulations for purposes of dynamic mechanical measurements. The dynamic components of the tip-sample forces resulting from the normal/lateral displacement modulations are detected via the torsion/bending of the microscopic cantilever and the deflection of the laser beam reflected off the rear side of the cantilever. The positional shift of the latter is registered by means of a segmented photo-diode...

Electrocapillary curves of various metals under different conditions have been determined with the bending plate technique 1158,159], The deflection of the plate is usually measured by an optical lever. Pangarov and Kolarov used an interferometric method to detect the bending of the plate I60]. Recently the method was improved by using microfabricated cantilevers 1161-163). Bard et al. glued flat metal 1164,165] or semiconductor 1166] electrodes onto piezoelectric disks. When applying a potential to the electrode the resultant bending caused a potential on the piezo, which was detected. 

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