Pyroelectric properties

11 Pyroelectric Ceramics and Thin Films Characterization, Properties and Selection 

As a final note on this section, whichever method is used, it is also necessary to consider how the pyroelectric is mechanically mounted in the system. Any mechanical clamping (such as bonding the material to a substrate of different thermal expansion coefficient) can lead to spurious currents due to changes in the stress on the sample, coupling via one or more of the piezoelectric coefficients. 

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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 ... 

Nowadays a large number of functional crystals belongs to the TTB family and presents electro-optic, ferro- piezo- pyroelectric properties (e g. the well-known barium sodium niobate Ba2NaNb50i5 (BNN)). In this work iron and manganese based Kx(lvr%(]Vr )i-xF3 fluorides (0.4solid state reaction and characterised by different diffraction techniques in order to investigate their crystalline stmcture. 

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. 

Table 11.2 Pyroelectric properties of several thin film ferroelectrics, c = 2.7 X 106 Jm 3K 1...

In conclusion, it has been demonstrated that the pyroelectric properties of polar materials can be compared relatively simply through the measurement of a few key physical parameters (pyroelectric,dielectric and thermal coefficients) and the judicious use of appropriate figures-of-merit. It is essential that the dielectric properties are measured in the frequency range appropriate for device use, and this is typically in the range of a few to 100 Hz. The properties of many pyroelectric ceramics and thin films have been compared and it has been shown that good pyroelectric properties can be obtained from this films manufactured at relatively low temperatures, a fact that bodes well for their future applications in fully-integrated arrays. 

Compounds belonging to the BaMF4 series display both ferroelectric (M = Co, Ni, Mg, Zn) and pyroelectric properties (M = Fe, Mn) but have estimated Curie temperatures above their melting points125,126. ... 

Ortho-lithiation of an O-protected m-fluorophenol, followed by treatment with phosphorus tribromide and aqueous acid deprotection, has given the phosphino-phenol 18. In the presence of potassium t-butoxide in an aprotic solvent, this is converted into the non-planar system 19, which exhibits pyroelectric properties. An organolithium route to the alkynylphosphine 20 has been developed. The same paper also reports a new route to the lithiated alkynylphosphine 21 and a study of its reactivity towards electrophiles. Diastereoselective lithiation of... 

Table 27.5 lists applications of some of the most commercially important mixed metal, perovskite-t5q)e oxides, and illustrates that it is the dielectric, ferroelectric, piezoelectric (see Section 13.9) and pyroelectric properties of these materials that are exploited in the electronics industry. 

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

The relaxational behaviour of polyvinylidene fluoride (PVDF) is of interest. Its p phase exhibits piezo- and pyroelectric properties and so it is used in various devices, such as sensors, transducers or actuators for different applications. Typically such materials are prepared through a technological process involving stretching and... 

By virtue of their symmetry, ferroelectric smectics are piezoelectric. Polarization can be induced by mechanical shear and, conversely, an electric field can produce shear flow. They also possess 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). 

Piezoelectricity is defined as an electric polarization that occurs in certain crystalline materials at mechanical deformation [69]. The polarization is proportionate to the deformation and the polarity changes with change in deformation. In reverse, electric polarization produces mechanical deformation in piezoelectric crystals. The piezoelectric materials also possess pyroelectric properties, i.e., electric polarization is generated at temperature change. 

A major advance was made in 1969 when a strong piezoelectric effect was discovered in poly(vinylidene fluoride) (PVDF). The effect is much greater than for other polymers. In 1971, the pyroelectric properties of PVDF were also first reported, and as a consequence, considerable research and development has continued during the last two decades. 

The third part deals with ferroelectric polymers and their pyroelectric properties. We describe the realization and performance of an IR pyroelectric sensor using copolymers of poly(vinylidene fluoride-ethylene trifluoride). 

ON THE PYROELECTRIC PROPERTIES OF SOME MATERIALS AND THEIR APPLICATION TO THE DETECTION OF INFRARED. 

Poling of Pyroelectric Materials After the manufacturing, pyroelectric materials usually do not possess pyroelectric properties, since the pyroelectric domains are randomly oriented. The poling process consists of the... 

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