Tetragonal ceramics

The addition of yttria to zirconia not only stabilizes the cubic or tetragonal form but also lowers the temperature ofthe t m transformation. The practical consequence of this is that larger zirconia particles can be retained in the metastable tetragonal form, thus considerably easing any problems associated with the fabrication of a toughened ceramic, such as ZTA. One important feature of this system is the solubility of yttria in zirconia up to a concentration of approximately 2.5 mol% which, in conjunction with a low eutectoid temperature, will facilitate the formation of fully tetragonal ceramics which are referred to as tetragonal zirconia polycrystals (see Section 1.6.4). 

In all appHcations involving zirconia, the thermal instabiHty of the tetragonal phase presents limitations especially for prolonged use at temperatures greater than - 1000° C or uses involving thermal cycling. Additionally, the sensitivity of Y—TZP ceramics to aqueous environments at low temperatures has to be taken into account. High raw material costs have precluded some appHcations particularly in the automotive industry. 

Tetragonal 2inconia is a stmctural ceramic that exhibits ferroelasticity and the toughness enhancement has been estimated to be as high as 5. An example of a partial hysteresis loop for this material is shown in Figure 4 (35). Domains do not have to be present prior to the stress... 

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis St e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures fomied by symmetry changes driven by cation ordering. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transfomiation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly separated from those in the field of the martensites. 

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. 

A second type of behavior existing in the PLZT s is the linear (Pockels) effect which is generally found in high coercive field, tetragonal materials (composition 3), This effect is so named because of the linear relationship between An and electric field. The truly linear, nonhysteretic character of this effect has been found to be intrinsic to the material and not due to domain reorientation processes which occur in the quadratic and memory materials. The linear materials possess permanent remanent polarization however, in this case the material is switched to its saturation remanence, and it remains in that state. Optical information is extracted from the ceramic by the action of an electric field which causes linear changes in the birefringence, but in no case is there polarization reversal in the material. 

Hollandite does not contain long-lived ACTs and, therefore, it is undergoing p-y-irradiation from fission and corrosion products, but in multiphase ceramics it can also be a-irradiated from neighbouring ACT-bearing phases. Irradiation by a-particles from external 238PuOz sources and heavy ions results in a volume expansion of 2-2.5% and transformation of tetragonal to monoclinic symmetry. 

---