A set of 61 ion polarizabilities has been derived from the dielectric constants of 129 oxides and 25 fluorides using a least squares refinement technique in conjunction with the Clausius–Mosotti equation. These polarizabilities can be used to estimate mean dielectric constants of ‘‘well‐behaved’’ compounds. They should be particularly useful in calculation of mean dielectric constants of compounds whose dielectric constants have not been determined. They can also be used as a framework for distinguishing unusual dielectric behavior from normal dielectric behavior where deviations can frequently be attributed to piezo‐ or ferroelectricity, conductivity (ionic or electronic), ‘‘rattling’’ or ‘‘compressed’’ cations with correspondingly high or low polarizabilities, or the presence of dipolar impurities. Deviations observed from calculated dielectric constants can be used to search for unusual physical behavior.

Dielectric polarizabilities of ions in oxides and fluorides

Percolation and Bruggeman's effective media theories, as they apply to the electrical conductivity of composites, are reviewed, and a general effective media (GEM) equation, which combines most aspects of both percolation and effective media theories, is introduced. It is then shown that the GEM equation quantitatively fits electrical resistivity (conductivity) as a function of the volume fraction data for binary composites. The parameters used to fit the experimental data are the electrical resistivities of the two phases, the percolation threshold for the lower resistivity phase (oc), and an exponent t. Preliminary work, showing how the GEM equation can be used to model the piezoresistivity of composites by postulating that oc is a function of the independent variable, is also presented.

Electrical Resistivity of Composites

A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composite's thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composite's material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced. >

Modeling 1-3 composite piezoelectrics: thickness-mode oscillations

Advantages and Challenges of Relaxor-PbTiO3 Ferroelectric Crystals for Electroacoustic Transducers- A Review.

The vast majority of materials have a positive coefficient of thermal expansion and their volume increases on heating. There has been considerable recent interest in materials which display the unusual property of contracting in volume on heating; i.e. those with a negative coefficient of thermal expansion. This Perspective gives an overview of some of the physical phenomena that can give rise to this unusual effect. Recent insights into negative thermal expansion materials and some of their unusual structural properties are discussed.

Negative thermal expansion materials

The use of Pb(Zn1/3Nb2/3)O3 ceramics is restricted by the formation of a pyrochlore phase detrimental to both dielectric and piezoelectric properties. Recently it has been shown that a 6 mol% addition of BaTiO3 to PZN suppresses the formation of pyrochlore phase. Phase relations and dielectric properties of ceramics in the PZN-BT-PT system are reported here. Compositions with the perovskite structure, having high dielectric constant and low temperature coefficient of capacitance, have been identified.

Dielectric and Ferroelectric Properties of Ceramics in the Pb(Zn 1/3Nb2/3)O3‐BaTiO3‐PbTiO3 System

Crystal structure parameters have been refined from single crystal x-ray intensity data collected on Ba2TiSi2O8 at 24 and 300°C. Anisotropic refinements in space group P4bm yielded residuals of 0.035 and 0.042 at 24 and 300°C, respectively. Pyroelectric measurements give a room temperature pyroelcctric coefficient of + 10 μC m -2 K -1. A structural mechanism for the pyroelectric effect is discussed in terms of large oxygen displacements. A sign reversal in the pyroeleetric coefficient occurs at 160°C, perhaps caused by the cancellation of primary and secondary effects.

X-ray structure refinement and pyroelectric investigation of fresnoite, Ba2TiSi2O8

Grain-oriented glass-ceramics of Li2Si2o5, fresnoite (Ba2TiSi2O8), and its isomorphs Sr2TiSi2O8, and Ba2TiGe2O8, were prepared by recrystallizing glasses in a temperature gradient. Electromechanical and hydrostatic piezoelectric properties of these glass-ceramics were measured. Piezoelectric voltage coefficients g33 and hydrostatic voltage coefficient gh of these glass-ceramics are comparable to those of polyvinylidene fluoride and an order of magnitude higher than the corresponding values of lead zirconate-titanate. These glass-ceramics seem to be attractive candidate materials for hydrophones and several piezoelectric devices. Hydrostatic piezoelectric properties of Ba2TiSi2O8 and Ba2TiGe2O8 single crystals were also measured. The unusually high values of gh in fresnoite single crystals and glass-ceramics are supposed to be due to positive d31 in these materials. A composite model is proposed to explain the positive sign of d31 in fresnoite based on its crystal structure and internal Poisson's ratio stress.

Grain-Oriented Glass-Ceramics for Piezoelectric Devices

Flexible piezoelectric composites made from PZT spheres and polymers with 1-3 connectivity were fabricated using several types of polymers. The dielectric constants of the composites were 300 to 400 and the piezoelectric voltage coefficients g33 were 45 to 55×lO−3 V-m/N. The high-frequency properties of the composites were measured in both the thickness and radial mode of resonance. The frequency constants and the coupling coefficients of the composites for the thickness mode of resonance are comparable with the corresponding values for PZT. Possible applications of the composites are indicated.

Flexible Composite Transducers

Pyroelectric glass-ceramics of composition Ba2TiGe2O8 and Ba2TiSi2O8 were prepred by crystallizing the glasses in a temperature gradient. High pyroelectric responses up to 50% of the single-crystal values were observed because of the high degree of orientation of the crystallites in the glass-ceramic samples. The piezoelectric and dielectric properties of the glasses and the glass-ceramics are also consistent with the properties of the single crystals.

A. Halliyal

Papers

Dielectric and Ferroelectric Properties of Ceramics in the Pb(Zn 1/3Nb2/3)O3‐BaTiO3‐PbTiO3 System

X-ray structure refinement and pyroelectric investigation of fresnoite, Ba2TiSi2O8

Grain-Oriented Glass-Ceramics for Piezoelectric Devices

Flexible Composite Transducers

Ba2TiGe2O8 and Ba2TiSi2O8 pyroelectric glass-ceramics