Yttria-stabilized hafnia: Thermochemistry of formation and hydration of nanoparticles
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The surface enthalpy of yttria-stabilized hafnia (YSH) (YxHf1 xO2 x/2) with different compositions was directly measured by a combination of high-temperature oxide-melt solution calorimetry and water adsorption calorimetry. The surface enthalpies for hydrated surfaces are 0.27 6 0.06 J/m2 for x 5 0.1, 0.77 6 0.09 J/m2 for x 5 0.17, and 1.30 6 0.09 J/m2 for x 5 0.24; and those for anhydrous surfaces are 0.51 6 0.06, 1.08 6 0.13, and 1.76 6 0.09 J/m2 respectively. The enthalpies of both hydrated and anhydrous surfaces increase approximately linearly (R2 . 0.93) with increasing yttrium concentration. The surface enthalpies of Y0.1Hf0.9O1.95 were used to approximate those for pure anhydrous cubic hafnia. Combining the data relating to surface energies for monoclinic hafnia from our previous work and estimated data for tetragonal hafnia, a tentative stability map of HfO2 polymorphs as a function of surface area (SA) was constructed.
I. INTRODUCTION
Hafnia has three polymorphs under atmospheric pressure. At room temperature, the stable phase is monoclinic, it transforms into the tetragonal phase with a transition enthalpy of ;8.4 kJ/mol when heated to ;1800 °C.1 The cubic phase forms above ;2500 °C.24 Neither tetragonal nor cubic bulk structures can be retained by quenching from high temperature. However, they can exist as metastable nanomaterials5–9 and bulk cubic phases can be stabilized by doping with Ca2+, Mg2+, Y3+ and other rare earth elements.4 Surface and interface energies play a significant role in stabilizing nanomaterials but such experimental data for yttria-stabilized hafnia (YSH) are not available. The surface enthalpies of different polymorphs of TiO2,10,11 Al2O3,12,13 Fe2O3,14,15 and ZrO216,17 have been measured by high-temperature oxide-melt solution calorimetry18 and enthalpy maps showing phase stability crossover at the nanoscale of these materials have been proposed. In this method, samples with the same bulk composition and structure but different surface areas (SA), are dropped from room temperature into an oxide melt and the corresponding heat effect (drop solution enthalpy, ΔHds) is measured. Providing that the final state is the same (dilute solution in oxide melt), and SA is the only variable, the difference in drop solution enthalpy between samples is directly related to surface enthalpy. Due to the small changes in PV (pressure volume) term, the surface enthalpy can be considered as the surface energy. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.31 1022
J. Mater. Res., Vol. 27, No. 7, Apr 14, 2012
We have reported surface enthalpy measurements for monoclinic hafnia.19 In the current work, we present results for Y2O3 stabilized cubic hafnia with x 5 0.10, 0.17 and 0.24 in YxHf1 xO2 x/2. The cubic YSH samples were prepared by annealing amorphous precursors synthesized by a coprecipitation method. The surface enthalpies of cubic YSH with different compositions were measured by high temperature solution calorimetry and water adsor
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