Drying Effect on Thermal Behavior and Structural Modifications of Hydrous Zirconia Gel Gianfranco Dell’Agli, Giuseppe Mascolo,* ,w Maria Cristina Mascolo, and Concetta Pagliuca Laboratorio Materiali, Dipartimento di Meccanica, Strutture, Ambiente e Territorio, Universita` degli Studi di Cassino, Cassino (Frosinone) 03043, Italy Adopting four different drying modalities on three single stocks of hydrous zirconia gel, 12 samples of pure zirconia xerogels were obtained. In particular, the following drying modalities were adopted: drying in air at room temperature, drying in air oven at 601 or 1101C, and, finally, drying by freeze-drying. The resulting xerogels showed, on heating, very different reactivity, affecting the structural phase transformations of pure zirconia. The kinetics feature could be the key to explain the formation mechanism of the metastable phases of zirconia at room tem- perature. The experimental results show, in fact, that the meta- stable phase is favored by a fast crystallization rate, whereas the stable one, by a slow crystallization rate. These findings could resolve the conflicting results on the metastability of zirconia reported in the literature. I. Introduction A T atmospheric pressure and at increasing temperature, pure zirconia, ZrO 2 , crystallizes in three different polymorphs: monoclinic phase (MZ), which is thermodynamically stable up to 11701C, tetragonal phase (TZ), which exists in the 11701– 23701C temperature range, and cubic phase (CZ), which is stable between 23701 and 27001C. 1 Several and controversial models still propose to explain the formation mechanism of both metastable tetragonal and cubic zirconia at room temperature from based-pure zirconia precur- sors. These models concern crystallite size or surface energy difference between polymorphs, 2–7 lattice defects (oxygen va- cancies), 8,9 structural similarity between precursor and metasta- ble phases, 10 impurities. 11,12 The formation of the metastable phases at room temperature has been extensively justified with the crystal sizes below a crit- ical value of 15–20 nm. However, MZ crystallites of 6 nm in sizes were synthesized by hydrothermal treatment. 13 In addition, MZ particles of about 3.6 nm in diameter resulted by sponta- neous oxidation of crystallized ZrAu ingots at room tempera- ture. 14 On the other hand, TZ monocrystals of 200 nm in sizes were obtained by microwave treatment of ZrOCl 2 solutions. 15 These findings do not agree with the existence of critical size values for the formation of metastable phases of pure zirconia. According to, Valmelette and Isa 14 MZ appears to be the true stable phase at room temperature. The formation of metastable phases could be explained by the nonequilibrium conditions adopted to produce these powders or as a consequence of the slow kinetics conversion of metastable phases into the stable MZ phase. 16 Structural transformations of zirconia also take place during ball milling. Such an evolution strongly depends on both the type of milling assembly used and the nature of the precursor. An almost complete amorphization of MZ with a volume- averaged domain size of 2.7 nm after 15 h by milling was observed with corundum. A complete transition from MZ into TZ phase was detected followed by another one into CZ by stainless-steel ball milling. 17 In, Bailey et al., 18 pure TZ phase, obtained by thermal decomposition of ZrOCl 2 at 5301C, formed MZ after 3 h and reformed TZ after 48 h by milling. Nearly pure TZ or MZ polymorph were obtained after ca- lcination at 6001C of precursor gel precipitated in ammonia so- lution with controlled pH. 19 Zirconium hydroxides precipitated at pHo7 had composition and structure significantly different from those formed in alkaline conditions. The exothermic effect in DTA or differential scanning calorimetry (DSC) at 4601C on calcination was attributed to the fast transition from an amor- phous phase to TZ. 20 Pure TZ was also obtained by heating amorphous powder precipitated in NH 3 in air at 4101C or at 2151–2451C under 1 kbar with distilled water. 15 In addition to the pH of the precipitating solution, the crystallization of a sol- derived zirconia precursor is also affected by the chemical spe- cies present in the solution where it is aged. 21,22 The hydrothermal treatment of precipitated zirconia in am- monia solution (pH 5 9), in the presence of chelating agents, favored the formation of TZ, whereas alkyl halides promoted the formation of MZ. 23 It is evident the difficulty to formulate a model capable of explaining all the mentioned and conflicting results. In order to give a contribution to this unresolved question, a study has been carried out on samples of zirconia xerogels obtained from three stocks of hydrous zirconia gel dried with four different modal- ities. The structural evolution of the products, owing to different thermal treatments, has been investigated. II. Experimental Procedure Three stocks of zirconia gel were precipitated by adding ZrCl 4 (98% purity, Acros Organic, Geel, Belgium) solution to ammo- nia 4M (GR grade, Carlo Erba, Rodano (MI), Italy) under continuous stirring. The gelatinous precipitates were vacuum filtered and washed repeatedly with distilled water, until no re- action for Cl À ions (with AgNO 3 ) was observed. Each wet pre- cipitate was divided in four portions, which were differently dried up to constant weight. The first portion, Za sample, was dried in air at room temperature for several weeks; the second one, Z60 sample, was dried in an air oven at 601C for 20 h; the third one, Z110 sample, was dried at 1101C for 16 h; and, finally, the fourth one, Zfd sample, was dried by freeze-drying for 24 h (Lio Cinquepascal Apparatus, Meda, Italy). The 12 dried xerogels were calcined in air in a muffle furnace at various temperatures up to 6001C adopting a heating rate of 101C/min. The calcined products were characterized by X-ray diffraction (XRD) analysis (Philips diffractometer—CuKa radi- ation, X’Pert, Almelo, the Netherlands). The average crystallite sizes of the polymorphs have been estimated by the Scherrer K. Bowman—contributing editor This work was financially supported by COFIN 2006, ‘‘Synthesis of hydrated Ti-Zr oxides with macro- and meso-porosity and their proton conductivity.’’ *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: mascolo@unicas.it Manuscript No. 23872. Received October 16, 2007; approved July 9, 2008. J ournal J. Am. Ceram. Soc., 91 [10] 3375–3379 (2008) DOI: 10.1111/j.1551-2916.2008.02635.x r 2008 The American Ceramic Society 3375