PVDF films/transducers

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... 

A series of pol)rvinylidene fluoride (PVDF) piezoelectric transducers were fabricated. Thick and thin PVDF materials have been Investigated for transducer production. Thinner films were superior because they conform better to the surface of the samples. After initial evaluation, the PVDF transducers will be used to conduct drying studies. 

Figure 17.4 Process flow used to fabricate the prototype ultrasonic transducers (a) a silicon (Si) wafer is micromachined to create an array of holes with diameters between 0.75 and 2.00 mm, after which the wafer is then thermally oxidized to grow a 1.5 pm thick S1O2 layer (b) the wafer is then diced into 1 cm wide square die (c) the die is laid flat onto a piece of free-standing F DF film in a jig (the die is now viewed in cross-section through the hole) (d) the die and PVDF film are clamped into the jig against an O-ring forming an air-tight seal, and air pressure is applied to the face of the PVDF film to deflect it into the desired spherical shape (e) finally, conductive epoxy is injected into the hole and a 30 gage wire is potted into the epoxy the air pressure is maintained until the epoxy cures, then the transducer chip is removed from the jig.

Certain polymers, such as polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), possess special properties in the film form, caileApiezoelectricity and pyroelectricity. Piezoelectricity is electric polarization of a film produced by mechanical strain in some crystals. The polarization is proportional to the amount of strain and changes sign with it. The reverse is true and an electrical polarization induces a mechanical strain in piezoelectric sensors. Pyroelectricity is electric polarization of a film induced by thermal absorption in some polymer crystals. The induced polarization is proportional to the level of thermal change. These properties can be used in the manufacture of transducers, microphones, loudspeakers, pressure gauges, pickup heads, hydrophones, motion sensors, and other devices from biaxially oriented PVDF films. Table 13.37 gives the properties of a piezoelectric film of polyvinylidene fluoride. 

Bauer and oo-workers (64,117] have conducted extensive studies ot the piezoelectric behavior of PVDF films in shock enviroomenls up to 200 kbar and have shown that they can make excellent transducers for shock-wave phenomena. 

Such polymers as PVDF, in particular, have wide applications [3] sometimes its properties are advantageous for some reasons, e.g., low electric permittivity and small thickness, but limit ite application in other devices. When, for example, hydrophones, which are electroacoustic transducers used in a water environment, b use of the low transducer capacity made of PVDF, amplifiers should be placed very near. Moreover, vduge sensitivity in the open system given in dB in the relation 1/piPa determined by a product giz, (where g indicates hydrostatic piezoelectric voltage coefficient and Xj is transducer thickness), is low for transducers m of PVDF films. 

K. Kobsyariu and T. Yasud An appUcation of PVDF-film to medical transducers. Peno-electrics 52 181 (1981). 

Polymer electrets can be operated as sensors or actuators. Their operation is very similar to that of a piezoelectric material and their direct piezoelectric transducer coefficient (d33) is higher than that of solid PVDF ferroelectric polymers [97]. If a compressive force is apphed to the film, the pores will deform preferentially with respect to the polymer material. Unlike charges within the polymer will be pushed closer together and the potential measured at the contacts will change accordingly. Similarly, the application of a voltage across the electrodes will yield a change in thickness in the material. 

Wet chlorine is extremely corrosive, attacking most common metals except Hastel-loy C, titanium, and tantalum. Surface oxide films protect these metals from attack by the acids formed in chlorine hydrolysis. Tantalum is an ideal construction material for service with wet and dry chlorine. However it is expensive and normally used only in instruments such as transmitters, diaphragms, transducers, and thermowells. FRP is used for wet chlorine. Rubber-lined steel is also suitable for wet chlorine gas up to about 100°C. At low pressures and low temperatures, PVC, CPVC, and reinforced polyester resins are also used. Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene-hexafluoropropylene (FEP) are resistant at high temperatures. Other materials stable to moist chlorine include graphite and glass. 

Transducers presented in this paper were fabricated using free-standing 9 pm thick PVDF-TrFE films and 9 pm thick spun-on PVDF-TrFE films. The processes used to fabricate these two types of transducers are essentially the same and, for conciseness, the free-standing technique will be described first in detail. 

Lekkala and Paayanen (1999) proposed metalized porous PP as ElectroMe-chanical Film (EMFi). By utilizing the EMFi material, Reinhard et al. (2007) reported that the transmitter emitted a sound pressure level up to 90 dB at a distance of 1 m, and the ultrasonic receiver had a sensitivity of 500 pV/Pa. The sensitivity of an ultrasonic receiving transducer without an FET reached -226 dB re 1 V/pPa, which exceeds that of a PVDF-based MHz-range hydrophone (Horino etal. 2012). 

The electrical and mechanical properties trf piezoelectric polymers make them a possible alternative to ferroelectric ceramics such as lead zirconate titanate. For several reasons, they are attractive for transducer design. The mechanical flexibility and conformability of thin-film PVDF means that it can be configured into a wide range of transducer products. The low acoustic impedance of PVDF is companrf>le to body tissues, which makes it useful for acoustic imaging applications. Short impulse response and high axial resolution in acoustic imaging systems arc possible with PVDF-featured devices because of the robustness and broadband characteristics of the polymer. 

Yasuda [17]. A fetal phococardiographk transducer to monitor the fetal heart rale during pregnancy and labor has also been conceived and realized by Siccnkcsic ct aL in a prototype fonn [18], based on a 190-tun-tbick PVDF fthn. shaped as a portion of a sphere, 40 mm in (Uameter. The film operates in the transverse mode, and the transducer is connected to an amplifier enclosed In the box that supports the film. 

These properties are widely used in many applications due to its high permittivity and its low dielectric losses, PVDF is widely used for the insulation of electric and electronic devices. For example, it can be used as insulating layer in condensers. It is also useful as thin film electret in transducer technology. Its ferroelectric properties are used in microphones, loudspeakers, and so on. 

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