Ferroelectric particles

For ferroelectric particles (the same size as in a magnetic colloid) with the spontaneous polarization F 5 x 10 6 C/cm2, we get p = PV 7.5 x 10 15 CGSE units. Hence, the electric held amplitude that satishes the condition 0 > 1 is Eq > 5 CGSE units or 1.5 kV/cm. 

A simple quantitative model, with no adjustable parameters, is developed for the finite size effect of ferroelectric particles on the Curie transition temperature. As the size of ferroelectric particles decreases, the Curie temperature decreases. Based on our model for lattice contraction and the Landau-Ginsburg-Devonshire (LGD) phenomenological theory, size effects on Curie temperature of lead zirconate titanate PbZri. Ti Oj (PZT, x > 0.6) are considered. It is shown that Curie temperature of PZT (x > 0,6) decreases with decreasing particle size. The predictions of our equation are in agreement with the experimental results. 

Ferroelectric composites are alternatives to standard piezoelectric and pyroelectric ceramics such as lead zirconate titanate (PZT) and BaHOs (BT). They combine the strong ferroelectric and dielectric properties of ceramics with the easy processing and good mechanical properties of polymers. Dispersion of micrometer-sized ferroelectric particles in an electrically passive epoxy matrix was first published by Furukawa et al. [1976] and later extended to ferroelectric matrices such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-3-fluoroethylene) (PVDF-TrFE) [Hsiang et al., 2001 Hilczer et al., 2002 Gimenes et al., 2004 Lam et al., 2005 Beloti et al., 2006]. However, the necessity of miniaturization of electronic components and... 

The introduction of ferroelectric nanoparticles in polymeric matrices seems attractive for improving the dielectric permittivity of polymeric-based materials. New pyro-and piezoelectric systems have been proposed, and better efficiency has been obtained by using ferroelectric particles with d 700 nm. However, the understanding of correlations between structure and macroscopic properties needs to be improved. Nevertheless, it is interesting to note that smart hybrid materials have been prepared by combining organic and inorganic phases, which illustrates the versatility of such nanocomposites. 

The final section Part IV is concerned with physical properties of polymeric nanocomposites (PNCs). Two types of nanoparticles, leading to two different characters and applicabilities of PNC, are discussed layered silicates (with natural or synthetic clays), used in structural-type PNCs and the others used in functional PNCs. Sender et al. in Chapter 13 describe the performance of PNCs with acicular ferroelectric particles producing PNCs with good electroactive (dc conductivity) and mechanical properties. In Chapter 15, Nicolais and Carotenuto focus on metal clusters in polymeric matrices, which combine optical transparency with magnetism, luminescence. Ultraviolet-visible absorption, thermochromism, and so on. 

To summarize, let us formulate the conditions of superparaelectric phase appearance in an ensemble of noninteracting ferroelectric particles of spherical shape. 

Lynch and Shelby (1984) examined the reasons for the high dielectric properties of lead titanate glass-ceramics. They defined the matrix phase around the ferroelectric crystals. They also determined a special effect they called clamping. This phenomenon occurs when the glass matrix contracts and produces compressive stress within the ferroelectric particles, causing paraelectric-to-ferroelectric phase transition. As a result, the crystal size must be controlled for electro-optic applications. The crj tals should preferably be smaller than 0.1 pm. 

Piezo/pyroelectric composite (inorganic ferroelectric particles in polymer matrix) Ions in crystalline particles form ionic dipoles upon symmetry breaking Ionic dipoles deform/ rotate with mechanical (electrical) stress Ferroelectric matrix polymer may compensate piezo-or pyroelectric effect... 

B. Jiang, L.A. Bursill, Phenomenological theory of size effects in ultrafine ferroelectric particles of lead titanate. Phys. Rev. B 60(14), 9978-9982 (1999)... 

The structure formation in an ER fluid was simulated [99]. The characteristic parameter is the ratio of the Brownian force to the dipolar force. Over a wide range of this ratio there is rapid chain formation followed by aggregation of chains into thick columns with a body-centered tetragonal structure observed. Above a threshold of the intensity of an external ahgn-ing field, condensation of the particles happens [100]. This effect has also been studied for MR fluids [101]. The rheological behavior of ER fluids [102] depends on the structure formed chainlike, shear-string, or liquid. Coexistence in dipolar fluids in a field [103], for a Stockmayer fluid in an applied field [104], and the structure of soft-sphere dipolar fluids were investigated [105], and ferroelectric phases were found [106]. An island of vapor-liquid coexistence was found for dipolar hard spherocylinders [107]. It exists between a phase where the particles form chains of dipoles in a nose-to-tail... 

CVD-derived powders may prove very useful and profitable in the production of bulk ferroelectric materials which are produced by hot-pressing or sintering. These powders offer great uniformity, small particle size, and high reactivity (see Ch. 19). 

Some piezoelectric crystals are electrically polarised in the absence of mechanical stress one example is gem-quality tourmaline crystals. Normally, this effect is unnoticed because the crystal does not act as the source of an electric field. Although there should be a surface charge, this is rapidly neutralised by charged particles from the environment and from the crystal itself. However, the polarisation decreases with increasing temperature and this can be used to reveal the polar nature of the crystal. If tourmaline is heated its polarisation decreases and it loses some of its surface charges. On rapid cooling it has a net polarisation and will attract small electrically charged particles such as ash. Such crystals are known as pyroelectric, and ferroelectric crystals are a special subclass of pyroelectric crystals. 

Microcrystals exhibit properties distinctly different from those of bulk solids. The fractional change in lattice spacing has been found to increase with decreasing particle size in FejOj. Magnetic hyperfine fields in a-FejOj and FejO are lower in the microcrystalline phase compared to those of the bulk crystalline phases. The tetra-gonality (i.e. the departure of the axial ratio from unity) of ferroelectric BaTiOj decreases with decrease in particle size in PZT, the low-frequency dielectric constant decreases and the Curie temperature increases with decreasing particle size. The small particle size in microcrystals cannot apparently sustain low-frequency lattice vibrations. 

The piezoelectric effect is an electromechanical effect in which mechanical evoke and reverse an electric reaction in a ferroelectric material and vice versa. The word piezo has been derived from the Greek piezein which means press . Compounds are composed of positive and negative ions and are electrically neutral as a whole. The fact that electrically charged particles are still present in the crystal can for example be demonstrated by means of the electric... 

BaTiC>3 particles are another very attractive and intensively studied type of nanoparticles in nematic liquid crystals. Cook et al. reported on an asymmetric Freedericksz transition, where doping nematic TL205 with single domain ferroelectric BaTiC>3 nanoparticles (9 nm in diameter) reduced or increased the threshold voltage by 0.8 V depending on the polarity of the applied voltage [149]. 

In addition to lowering V th, ferroelectric nanoparticles such as BaTi03 or Sn2P2S6 [144, 156, 318-323] have also been shown to increase the nematic-to-isotropic phase transition temperature (TN/Iso) and the order parameter of the nematic host [142, 320, 324-326], which are thought to have their origin in a coupling of the electric dipole moment of the particles with the orientational order of the surrounding nematic molecules (Fig. 6). 

Particle Size and Surface Effects on Critical Thickness for Ferroelectricity of BaTi03 by ab initio Calculations... 

The effects of particle size on the critical thickness for ferroelectricity of nanoscale BaTiCh particles were investigated using a first-principles method and population analysis. Models composed of a small Ba8Ti706 cluster and a point charge array surrounding the cluster were used for the calculations, with the size of each... 

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