Calorimetric and high-resolution transmission electron microscopy study of nanocrystallization in zirconia gel
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Calorimetric and high-resolution transmission electron microscopy study of nanocrystallization in zirconia gel Jiˇr´ı M´alek,a) Takefumi Mitsuhashi, Julio Ram´ırez-Castellanos, and Yoshio Matsui National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki 305, Japan (Received 19 October 1998; accepted 12 January 1999)
The formation of metastable tetragonal zirconia nanophase by thermal treatment of a zirconia gel derived from zirconyl chloride has been studied by high-resolution transmission electron microscopy (HRTEM) and differential scanning calorimetry (DSC). HRTEM observations revealed that a fully crystallized sample consists of nanocrystals, around 13 nm in size. This nanocrystalline t-ZrO2 has practically the same crystal structure as that of the high-temperature tetragonal zirconia phase. The nonisothermal crystallization rate is very fast in as-prepared zirconia gel. DSC data at various heating rates can be described by a two-parameter model which predicts the crystallization kinetics in isothermal conditions very well. The Johnson–Mehl–Avrami (JMA) model can be used, however, in partially crystalline samples (crystallinity .30%) where the rate of crystallization process is considerably slower. The kinetic exponent of the JMA model (m 1.0 6 0.1) then corresponds to linear dependence of the crystallization rate as a function of the fraction crystallized.
I. INTRODUCTION
Zirconia ceramics are one of the most promising materials for advanced technologies. Their mechanical properties such as fracture toughness, strength, and hardness are excellent.1 They can be fabricated by sintering superfine zirconia powders prepared by drying of a gel precipitated from solutions of zirconium salts.2 Knowledge of crystal growth kinetics and temperature of the tetragonal-monoclinic transformation is important in order to prepare zirconia ceramics with defined properties. It is known that the metastable tetragonal polymorph frequently crystallizes during the heating of zirconia gel even though the monoclinic phase is the thermodynamically stable phase at low temperatures.3–8 A mechanism of formation of t-ZrO2 in zirconia gel has been proposed by Clearfield.9 The existence of metastable t-ZrO2 at low temperatures was attributed to the effect of domain boundaries10 or to the stabilizing effect of oxygen vacancies found in crystallized zirconia gel.11–14 It is still not clear whether the structure of the metastable tetragonal phase at low temperatures is the same as the well-described high temperature form.15 The crystallization kinetics of pure zirconia gel and zirconia-yttria gel was studied by differential thermal analysis (DTA),16 differential scanning calorimetry (DSC),17 and by hot-stage x-ray diffraction (XRD).18 Some of these data were interpreted within the Johnson–
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Permanent Address: Joint Laboratory of Solid State Chemistry, Academy of Sciences of the Czech Republic & University of Pardubice, Studentsk´a 84, Pardubice 532 10, Czech Republi
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