1. INTRODUCTION
Zirconia (ZrO2) has attracted the ongoing interest of researchers because the
combination of electrical, mechanical and thermal properties of this material has led
to its use in a wide range of applications, such as electronic materials, oxygen sensors,
humidity sensors, solid oxide fuel cells, catalyst support, thermal barrier coating,
tooth crowns and refractory [1]. Pure ZrO2 exhibits four primary polymorphs that are
stabilized at different temperature ranges and different pressures: A monoclinic
structure is stable from room temperature to 1175o
C, as baddeleyite, a tetragonal
structure is stable at the temperature range from 1175 to 2370o
C, a cubic structure is
stable above 2370o
C [2] and a rhombic structure is stable at a high pressure [3]. The
monoclinic form has remarkable properties that are excellent mechanical properties
and high thermal stability, but tetragonal and cubic forms have the properties different
from the monoclinic structure, which has properties of high ionic conductivity and
optical characteristics. Hence the monoclinic structure is used for mechanical and
thermal applications, but tetragonal and cubic forms are used for electrical and optical
applications [4-5].