Enthalpy of formation of cubic yttria-stabilized hafnia
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The enthalpy of formation of cubic yttria-stabilized hafnia from monoclinic hafnia and C-type yttria was measured by oxide melt solution calorimetry. The enthalpies of formation fit a function independent of temperature and quadratic in composition. The enthalpies of transition from m-HfO2 and C-type YO1.5, to the cubic fluorite phase are 32.5 ± 1.7 kJ/mol and 38.0 ± 13.4 kJ/mol, respectively. The interaction parameter in the fluorite phase is strongly negative, −155.2 ± 10.2 kJ/mol, suggesting even stronger short range order than in ZrO2–YO1.5. Regular solution theory or any other model assuming random mixing on the cation and /or anion sublattice is not physically reasonable. A more complex solution model should be developed to be consistent with the new calorimetric data and observed phase relations. I. INTRODUCTION
Hafnium and zirconium are similar elements with nearly identical ionic radii. Because of this, hafnium compounds are often nearly identical in structure and chemistry to zirconium compounds and have similar applications. Both HfO2 and ZrO2 are candidates for solid electrolytes, ceramic toughening agents, nuclear waste forms, and alternative gate dielectrics.1–4 In these applications, specific crystallographic phases and the transitions between them are of principal interest. The three polymorphs of HfO2 and ZrO2 at atmospheric pressure are monoclinic (m), tetragonal (t), and cubic (c). The transitions between these phases are very similar in terms of structure in both oxides. Transition temperatures are, however, notably higher in HfO2. m-HfO2 (m-ZrO2) is the stable polymorph at room temperature and transforms to t-HfO2 (t-ZrO2) at about 1700 °C (1200 °C); t-HfO2 (t-ZrO2) remains stable to about 2600 °C (2370 °C) where it transforms to c-HfO2 (c-ZrO2).1,2 Oxides with the cubic fluorite structure, such as UO2, ThO2, and CeO2 have high oxygen mobilities and have been studied for applications as ionic conductors.5 The cubic fluorite phases of pure HfO2 and ZrO2 are only stable at very high temperatures but can be stabilized to much lower temperatures by doping. Stabilized cHfO26–8 and stabilized c-ZrO25 phases have also been studied extensively for applications as ionic conductors.
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Present address: Nuclear Materials and Technology Division, NMT-16, MS G721, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0234 J. Mater. Res., Vol. 19, No. 6, Jun 2004
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In addition to stabilizing the cubic phases, doping HfO2 and ZrO2 with lower valent cations such as yttrium increases the oxygen vacancy (the charge carrier) concentration, resulting in further increases in ionic conductivity. Conductivities of stabilized c-HfO2 and stabilized c-ZrO2 phases do not increase monotonically with doping level, but rather reach a maximum and decrease with further doping.7,9 At these higher doping levels, point defects introduced by doping interact, oxygen vac
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