Flash synthesis of zirconia nanoparticles by microwave forced hydrolysis
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ed hydrolysis preparation of zirconia sols and powders by microwave heating of zirconium tetrachloride solutions at temperatures equal to 180 °C leads in a few minutes to monodispersed nanoscale zirconia particles. Synthesis was performed in a microwave reactor called the RAMO system. This microwave reactor was designed by the authors. This flash-synthesis process combines the advantages of forced hydrolysis (homogeneous precipitation) and microwave heating (very fast heating rates). The sols and powders were characterized by x-ray diffraction,photon correlation spectroscopy (PCS), small-angle x-ray scattering, and transmission electron microscopy. Sols are colloidally stable, which means that after 6 months no sedimentation is observed and the size distribution given by PCS measurements has not changed. For all synthesis conditions (with or without HCl, zirconium salt concentration, and synthesis time), zirconia polycrystalline particles were produced. According to the different analyses, these zirconia polycrystalline particles were constituted of aggregates of small primary clusters. I. INTRODUCTION
Monodispersed nanometric zirconia sols are used for coatings (few molecular layers).1,2 Due to their specific physical properties (optical and electrical), such thin-film zirconium oxides are a technologically important class of materials with a wide range of applications.3–7 Zirconia powders having fine spherical particles with narrow size distribution are intensively sought as raw materials for advanced ceramics.8–11 One of the methods reported in the literature for producing ultrafine monodispersed oxides is forced hydrolysis: heating of acidified metal salts solutions at temperatures up to 100 °C for a few hours to a few days. According to Matijevic,12 the interest in forced hydrolysis is the easy deprotonation of hydrated metal ions in aqueous solutions induced by increasing temperature. The limitations of this process are the operating conditions (salt concentration, pH, nature of the anions, and temperature), which are very restrictive. Several authors studied synthesis of zirconia nanoparticle by forced hydrolysis. Clearfield13 showed that the reflux heating of zirconyl salt solutions induces a hydrolytic polymerization. The first particles appeared after
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Address all correspondence to this author. e-mail: [email protected]; [email protected]; [email protected] J. Mater. Res., Vol. 16, No. 9, Sep 2001
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20 h of heating and the final product was a colloidal sol. Murase and Kato14 boiled a zirconyl chloride solution for 120 h. They obtained zirconia particles of 100 nm composed of much smaller crystallites (4.5 nm). Morgan15 synthesized a fine 6-nm zirconia by heating zirconyl nitrate solutions mixed with nitric acid for 14 days at 150 °C. Blesa et al .16 produced monodispersed spherical zirconia particles by heating a solution containing ZrOCl2, K2SO4, and HCl at roughly 100 °C. Hu et al.17 synthesized nanocrystalline zirconia
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