Zircon composition dependence

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

Fig. 9.4. Dependence of piezoelectric properties of PbZrOj-PbTiOj on composition. The zirconate-rich phase is rhombohedral, whereas the titanate-rich phase is tetrahedral. The piezoelectric coefficients reach a maximum near the morphotropic phase boundary, approximately 45% PbZrOj and 55% PbTiOj. (After Jaffe et al., 1954.)...

The crystallographic and piezoelectric properties of the ceramics depend dramatically on composition. As shown in Fig. 9.4, the zirconate-rich phase is rhombohedral, and the titanate-rich phase is tetragonal. Near the morphotrophic phase boundary, the piezoelectric coefficient reaches its maximum. Various commercial PZT ceramics are made from a solid solution with a zirconate-titanate ratio near this point, plus a few percent of various additives to fine tune the properties for different applications. 

The investigations of SOEs with oxygen ion conductivity (mainly based on zirconia, ceria and gallates) and with protonic conductivity (mainly cerates and zirconates) included detailed studies of their ceramic and transport properties, contact resistance of grains in SOE, ageing processes depending on the mode of fabrication, temperature, composition of material, impurities, gas atmosphere, and other factors. 

Lead zirconate titanate (PhTij Zr e03 with x 0.5), also abbreviated as PZT, is a ferroelectric mixed perovskite with a Curie temperature that depends on the composition x (Figure 4.28). It also has a very high value of k on the so-called morphotropic boundary (the value of x at the boundary between the tetragonal and rhombohedral structures). It is a material of choice for making sensors and actuators, e.g., in medical echoscopes and sonar equipment. 

AP ion (rAi3+/ro2- = 0.38), which sits inside oxygen tetra-hedra in many aluminosilicates but inside octahedra in others (as is the case for AI2O3). The AT ion has a CN of both 4 and 6 in sillimanite, both 5 and 6 in andalusite, but only 6 in kyanite, even though all three are stable minerals and all have the composition A SiOs. Another example is the ion rzv /ro - = 0.51), which is octahe-drally coordinated in several crystals, e.g., CaZrOs (iso-morphous with perovskite), but has a CN of 8 in zircon, ZrSi04. As Pauling said, the size of the ion depends on the site that it occupies. 

An aerosol is a colloidal dispersion of liquid droplets in a vapor. Aerosols can be used to make oxide powders in a variety of ways. For example, an aqueous sol can be sprayed into alcohol, where the aerosol droplets gel to produce spheres [110]. Subsequent firing produces perfect dense oxide spheres whose size depends on the concentration of the initial sol. Schwartz et al. [4] prepared powders of lead lanthanum zirconate titanate by spraying a salt solution into ammonium hydroxide and then spray-dried the resulting powder to produce uniform spheres 0.5-2/rm in diameter. In such complex compositions, aerosols are advantageous, because each droplet is a small reaction vessel, and heterogeneity cannot occur on a scale larger than the size of the droplet. 

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