Piezoelectric and pyroelectric properties

In general, there are two characteristics to be considered when conducting an initial assessment of a material s suitability for a device first the appropriate piezoelectric or pyroelectric coefficients to determine the degree of sensitivity and optimum mode of operation (e.g. thickness or hydrostatic) and secondly, the effect of temperature on these properties, thus defining the maximum and minimum operating temperatures at which the material can be used efficiently. 

Piezoelectric materials generate an electric charge in response to a mechanical stress. This is known as the direct piezoelectric effect, where the charge per unit area or electric displacement D is proportional to the applied stress T. There is also a converse effect, since an applied electric field E produces a proportional strain S in the material, resulting in either expansion or contraction, depending on the field s polarity. For both effects, the proportionality constant is termed the piezoelectric charge constant d  

There are three additional piezoelectric constants the g constant gives the field produced by a stress and is useful in determining the voltage output of a stressed material the other constants, e (which relates T to E) and h (which relates S to E), are only occasionally used. As T and S are related by the elastic constants c of the material, and E and D are related by the dielectric constant, the piezoelectric constants are related to one another. Therefore 

The formal definition and derivation of these piezoelectric relationships may be found in [26]. 

The piezoelectric constants are tensor quantities hence it is necessary to assign a coordinate system to produce a matrix of coefficients related to the symmetry of the crystal axis. 

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Dielectric, piezoelectric and pyroelectric properties of LiTa03 derived ceramics containing additives of LiF and MgF2 were investigated and reported on in [407]. The materials were prepared at 900°C by means of two methods Reaction sintering, yielding powdered polycrystalline material ... 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

The unique piezoelectric and pyroelectric properties of semicrystalline films of PVDF arise from changes in the polarization imparted to the overall film by the crystalline P-phase. The polar nature of the P-phase is, in turn, a direct result of the parallel alignment of the dipole moment of the repeat units in the unit cell (Figure 11.1). The crystal polarization is defined as the dipole moment density of the crystal ... 

From Eq, (1) it is clear that a model of crystal polarization that is adequate for the description of the piezoelectric and pyroelectric properties of the P-phase of PVDF must include an accurate description of both the dipole moment of the repeat unit and the unit cell volume as functions of temperature and applied mechanical stress or strain. The dipole moment of the repeat unit includes contributions from the intrinsic polarity of chemical bonds (primarily carbon-fluorine) owing to differences in electron affinity, induced dipole moments owing to atomic and electronic polarizability, and attenuation owing to the thermal oscillations of the dipole. Previous modeling efforts have emphasized the importance of one more of these effects electronic polarizability based on continuum dielectric theory" or Lorentz field sums of dipole lattices" static, atomic level modeling of the intrinsic bond polarity" atomic level modeling of bond polarity and electronic and atomic polarizability in the absence of thermal motion. " The unit cell volume is responsive to the effects of temperature and stress and therefore requires a model based on an expression of the free energy of the crystal. 

Figure 11.12 The relationship between point group and piezoelectric and pyroelectric properties...

The high dielectric loss and high dielectric constant of PVDF (8-9) both restrict its use in some electrical applications and provide superior performance in others. PVDF has very unusual piezoelectric and pyroelectric properties which are opening up many new applications with a very high value in use (see Chapter 5). 

Polyvinylidene fluoride is used extensively in the electronics industry because of its piezoelectric and pyroelectric properties. Only two room temperature solvents, n-methylpyrrolidone and dimethyl acetamide, have been found. Still-wagon (37) used n-methylpyrrolidone as the solvent for low-angle laser light-scattering measurement because of the larger refractive index increment value (dn/dc) in that solvent. No SEC work in these solvents was mentioned. 

Kreher, W. and Rodel, J. 1998. Ferroelectric ceramics and composites statistical models for effective piezoelectric and pyroelectric properties. Applications of Ferroelectrics, 1998, lASF 98, Proceedings of the Eleventh IEEE International Symposium of Ferroelectrics, Edited by E. Colla, D. Damjanovic, and N. Setter, pp 455-458. 

Kaminskii AA, Belokoneva EL, Mill BV, Pisarevskii YuV, Sarkisov SE, Silvestrova IM, Butashin AV, Khodzhabagyan GG (1984) Pure and Nd -Doped Ca3Ga2Ge40i4 and Sr3Ga2Ge40i4 single erystals, their strueture, optical, spectral, luminiscence, electromechanical properties and stimulated emission. Phys Stat Sol (a) 86 345-362 Kar Lai Ng, Chan HLW, Choy ChL (2000) Piezoelectric and Pyroelectric Properties of PZT/P(VDF-TrFE) Composites with constituent phases poled in parallel or antiparallel directions. IEEE Trans UFFC 47 1308-1315... 

Relatively few applications have utilized the ferroelectric effect in ceramics. Ferroelectric ceramics have been widely employed because of the other properties that they display, however. Their dielectric, piezoelectric, and pyroelectric properties have led to their use in capacitor, actuator and other piezoelectric applications, and infrared detection devices. Again, the most widely used materials are the lead-based ABO perovskite compounds. 

Cellulose EAPap material is composed of molecular chains with a dipolar nature. In particular, the crystal structure of cellulose II is monoclinic, which is noncentro-symmetric and exhibits piezoelectric and pyroelectric properties. To investigate the dipole effects in EAPap, thermally stimulated current (TSC) measurement was conducted (Hongo et al. 1996). The classical procedure in TSC includes (1) heating the sample to a given temperature (200°C) (2) applying... 

In summary, chiral smectic-C phases lack a center of symmetry. Hence they can be used as materials for second-order nonlinear optics [120-124], and possess piezoelectric and pyroelectric properties. Pyroelectric measurements have been performed on LC polymers [125] as well as on LCEs [126-128]. Irradiation of an FLCE sample with light usually leads to a temperature increase resulting in a pyroelectric signal [129]. More interesting are systems in which dye molecules like azobenzenes lead to a shift of the phase transition temperature upon isomerization [19]. 

Mauzac M, Nuyen HT, Toumilhac FG, Yablonsky SV (1995) Piezoelectric and pyroelectric properties of new polysiloxane smectic C elastomers. Chem Phys Lett 240(5-6) 461-466. doi 10.1016/0009-2614(95)00574-n... 

It is important to note that all ferroelectric crystals possess piezoelectric and pyroelectric properties. It was also found that all pyroelectric materials are piezoelectric, but the opposite is not true. For example, quartz is piezoelectric but not pyroelectric. At the same time zinc oxide with its wurtzite... 

Kawai s (7) pioneering work almost thirty years ago in the area of piezoelectric polymers has led to the development of strong piezoelectric activity in polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene and tetrafluoroethylene. These semicrystalline fluoropolymers represent the state of the art in piezoelectric polymers. Research on the morphology (2-5), piezoelectric and pyroelectric properties (6-70), and applications of polyvinylidene fluoride 11-14) are widespread in the literature. More recently Scheinbeim et al. have demonstrated piezoelectric activity in a series of semicrystalline, odd numbered nylons (75-77). When examined relative to their glass transition tenq>erature, these nylons exhibit good piezoelectric properties (dai = 17 pCTN for Nylon 7) but have not been used commercially primarily due to the serious problem of moisture uptake. In order to render them piezoelectric, semicrystalline polymers must have a noncentrosynunetric crystalline phase. In the case of PVDF and nylon, these polar crystals cannot be grown from the melt. The polymer must be mechanically oriented to induce noncentrosynunetric crystals which are subsequently polarized by an electric field. In such systems the amorphous phase supports the crystalline orientation and polarization is stable up to the Curie temperature. 

Pyroelectric sensors have recently been increasingly employed for sensing purposes. Excellent piezoelectric and pyroelectric properties of pyroelectric materials, combined with design flexibility and miniaturization offered by thin-film geometry and also developments in micro-and nanoscale fluids technologies have generated great... 

A ferroelectric is formally defined as a pyroelectric material that has a reversible, or switchable , polarisation. Ferroelectricity was discovered in ca 1920 by Valasekt in Rochelle salt (NaKC4H406 4H2O) - a material that was known at the time for its piezoelectric and pyroelectric properties. For years after this discovery, ferroelectricity was... 

At present, piezoelectric and pyroelectric materials are of great interest It is enough to mention that in his Guide to the Literature of Piezoelectricity and Pyroelectricity Lang [2a] died 834 references on piezoelectric and pyroelectric properties and their applications published just during the period 1991-1992. Over 60 papers of this number refer to composite materials. Some of these papers will be cited below. 

Now we will focus our attention predominantly on properties involved in ekctrooptic applications, and will cover NLO, piezoelectric, and pyroelectric properties in detail in later sections. 

Electroactive polymers have piezoelectric and pyroelectric properties that are lower than those of ceramics materials. However, they have low permittivity as well as other advantages that enable their use in applications (Lang, 2008) such as acmators, vibrational control, ultrasonic transducers, and others such as shock sensors, health monitoring, tactile sensors, and energy conversion devices. 

For all device applications, it is necessary to know the effects of temperature on the piezoelectric and pyroelectric properties first to ensure that the material has sufficient thermal stability to withstand elevated temperatures without significant loss in activity, and secondly to determine if the level of activity varies with changes in operating temperature. 

Providing the temperature does not go above T ax/ this effect of temperature on piezoelectric and pyroelectric properties is reversible, particularly for pre-aged film. Rises in temperature from 20 to 50 °C result in approximately 60% and 40% increases in and 33 respectively [33]. Similarly, for 56 44 VDF TrFE, p is raised from 20 to 40 yC [28]. 

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