Aqueous Combustion Synthesis and Characterization of Nanosized Tetragonal Zirconia Single Crystals
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NANOCRYSTALLINE ceramics are finding applications in several fields because of their excellent mechanical, electronic, and thermodynamic properties. These properties are very sensitive to the particle size. Also, new properties can be observed in such materials where a metastable phase may be found. Sometimes, the observed metastable phases of the systems are even more interesting from a technological point of view than the equilibrium ones.[1] Nanosized metastable tetragonal zirconia particle reinforcement is very interesting, because it contributes to the toughening of ceramics, an important class of engineering materials. Pure zirconia (ZrO2) exhibits three polymorphs: monoclinic, tetragonal, and cubic structure. The monoclinic phase is stable at room temperature and transforms to the tetragonal phase at 1170 C during heating, while the tetragonal phase transforms to the cubic one at 2370 C up on heating. Both transformations are reversible on cooling, although the tetragonal to monoclinic transition occurs at a lower temperature (about 950 C). This martensitic transformation, which has been extensively studied, is the basis of the transformation toughening mechanism exhibited by zirconiabased ceramics.[2] In all the known zirconia-based systems, the tetragonal phase is only stable at high temperature, even though the low-temperature boundary of the tetragonal B.S.B. REDDY, Graduate Student, INDRAJIT MAL, Undergraduate Student, SHANIDEEP TEWARI, Undergraduate Student, KARABI DAS, Associate Professor, and SIDDHARTHA DAS, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India. Contact e-mail: [email protected] Manuscript submitted August 28, 2006. Article published online July 18, 2007. 1786—VOLUME 38A, AUGUST 2007
phase field decreases with the addition of a dopant oxide. However, in pure and doped ZrO2 nanopowders and fine-grained ceramics, the tetragonal phase can eventually be retained in a metastable condition. In the case of nanopowders, the existence of a critical crystallite size has been determined, above which the material reverts to the stable monoclinic phase. This critical crystallite size is about 20 to 30 nm for pure ZrO2.[3] However, some exceptions have been reported in the literature. For example, the tetragonal phase can be retained in a powder with an average crystallite size of 45 nm; this has been attributed to the high agglomeration of the material.[4] The main reasons for these controversial opinions seem to be related to the extrinsic factors such as the presence of impurities, morphology of nanocrystallites, and existence of residual stresses in the nanoparticle agglomerates.[1] The metastable phases of zirconia, prepared by various wet-chemical processes, have attracted much attention due to their scientific and technological importance. The wet-chemical processes are advantageous over conventional solid-state synthesis in terms of homogeneity and powder characteristics. Wet-chemical proc
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