Piezoelectric pressure sensors

There are several applications of ZnO that are due to its excellent piezoelectric properties [28,164]. Examples are surface-acoustic wave (SAW) devices and piezoelectric sensors [28,165-167]. Typically, SAW devices are used as band pass filters in the tele-communications industry, primarily in mobile phones and base stations. Emerging field for SAW devices are sensors in automotive applications (torque and pressure sensors), medical applications (chemical sensors), and other industrial applications (vapor, humidity, temperature, and mass sensors). Advantages of acoustic wave sensors are low costs, ruggedness, and a high sensitivity. Some sensors can even be interrogated wirelessly, i.e., such sensors do not require a power source. 

Tab. 7.3.1 provides a summary of the principles and features of the pressure sensors. The pressure sensors are broadly divided into three groups according to which principle they employ piezoresistive sensors, capacitive sensors, and piezoelectric sensors. 

Figure 2.19 An experimental preparation with left anterior descending coronary pneumatic occluder, subendocardial piezoelectric dimensional sensors in ischemic and non-ischemic zones and left ventricular and pleural pressure sensors. (From D D Glower, J A Spratt, J S Kabas, J W Davis and J S Rankin Quantification of regional myocardial dysfunction after acute ischemic injury, 1988 Am. J. Physiol. 255 Heart Circ. Physiol. 24) H85-H93 1988.)... 

Figure 6.3 The Mark II cavity spectrometer viewed through its Perspex cover. The spatial filter is visible in the centre of the case. The translation stage and piezoelectric actuator are seen in the foreground and operate via the bellows to move the near mirror. The coupling iris is in the centre of the far mirror. The waveguide couplings holding the mica vacuum windows can be seen either side of the case in the background. Sample inlet is on the left wall of the case not visible and outlet is on the right via the pressure sensor...

Quartz and piezoelectric ceramic crystals have more temperature independent constants than PVDF, so they are used for force and acceleration transducers. However, PVDF films can be used for large area flexible transducers. Their sensitivity to stress or strain allows the construction of pressure sensors (using the J33 coefficient), and accelerometers by mounting a seismic mass on the film. PVDF electrets are particularly suited for large area hydrophones (Fig. 12.21) that detect underwater signals. Their... 

Mandal, D., Yoon, S., Kim, K.J., 2011. Origin of piezoelectricity in an electrospun polyfvinylidene fluoride-trifluoroethylene) nanofiber web-based nanogenerator and nano-pressure sensor. Macromol. Rapid Commun. 32,831-837. 

Electroactive polymers (EAP) n. Polymers that respond to electrical and magnetic stimulus with a significant change in shape and size. An example of an electroactive polymer is poly(vinylidene fluoride) (PVDF) that has been used for pressure sensitive (piezoelectricity) devices such as pressure sensors. Also, some polymers respond to photonic and thermal stimulus. Bar-Cohen Y (ed) (2001) Electroactive polymer (EAP) actuators as artificial muscles. SPIE Press, Bellingham, Washington. 

Another possibly applicable crystal is Lithium sulfate (Li2S04.H20). It could be used below 90° C for its relatively high piezoelectric coefficients (especially for hydrostatic coefficient dh = 16.4 x 10 CN ) as a hydrostatic pressure sensor. Especially high hydrostatic piezoelectric coefficient (1000-2000 x 10 CN ) exhibits also the semiconductive ferroelectric crystal SbSI (see Fig. 7.18). Its hydrostatic piezoelectric coefficient is extremely high, but strongly temperature dependent especially at room temperature 22° C. 

Sensor performance for different applications is defined by various features of the ceramic. For example, the electrical output of most pressure sensors is dependent on the bulk piezoelectric properties of a PZT ceramic. Oxygen gas sensor performance is defined by the conductivity behavior of Zr02 ceramics, which is in turn dependent on the oxygen vacancy content of the material. The performance of still other sensors, for example, ceramic thermistors, is dependent on the grain boundary characteristics of doped BaTi03 ceramics. For humidity sensors based on NiO/ZnO, the p—n junction characteristics of the interface define sensor performance. 

Piezoelectric crystal sensors are passive solid-state electronic devices, which can respond to changes in temperature, pressure, and most importantly, to changes in physical properties at the interface between the device surface and a foreign fluid or solid. Such changes in physical properties include variations in interfacial mass density, elasticity, viscosity, and layer thickness. 

Besides the strain gauge type transducer, other pressure transducers may employ variable capacitance, variable inductance, or piezoelectric crystal sensors to detect pressure changes. The low level output pressure transducer can be used in systems controlling flammable and other hazardous gases if the system has been designed such that the electronic power levels being used to excite the transducer remain very low. They can even be used in Class I, Division II areas if the proper intrinsic safety barriers are used in conjunction. Like the pressure gauge, the transducer may have a... 

Pressure sensor selection guidelines can be found in Ref. 3. Pressure sensors have various transducers connected to them to convert the measurement into a usable signal. Strain gauges and piezoresistive and piezoelectric pressure transducers are commonly used to produce the measured values. 

Fig. 4 (a) Scheme of a flexible ferroelectric ceramic polymer nanocomposite layer, laminated on a flexible transistor backplane. In the piezoelectric sensor pixel, the polarization in the ceramic nanoparticles and polymer are antiparallel, while in the pyroelectric, pixel polarizations are parallel, (b) Equivalent circuit of a sensing pixel, (c) Photograph of a simple prototype with one temperature and one pressure sensor (Reprinted with permission (Graz et al. 2009))... 

Piezoelectric cavity pressure sensors are suitable for direct use in the cavity due to their physical properties. They are usually installed flush with the mold wall and can be adjusted to the surface by spark erosion or grinding of the sensor front. If installed correctly while maintaining the required drilling tolerances, an error-free measurement of the cavity pressure can be assumed. 

A pressure sensor which is well suited to measure dynamic pressure changes is a quartz crystal (piezoelectric measurement). A charge across the crystal is proportional to the force on the crystal. This force results in a deformation which causes a subsequent short lasting change in flow of electric charge. These sensors ate therefore not suited to measure static pressures. 

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