Nanostructural study of sol-gel-derived zirconium oxides
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Ricardo Caruso, Edgardo Benavi´dez, Oscar de Sanctis Laboratorio de Materiales Cerámicos, FCEIA, IFIR, UNR, 2000 Rosario, Argentina
Sergio R. Mintzer Departamento de Materiales, Gerencia de Desarrollo, Comisión Nacional de Energía Atómica, 1429 Buenos Aires, Argentina (Received 19 June 2002; accepted 28 October 2002)
Two sol-gel derived zirconia powders were prepared at pH ⳱ 0.5 and pH ⳱ 5.5. They were investigated as a function of temperature using mainly perturbed angular correlation spectroscopy. The aim was to elucidate the relationship between the nanoscopic configurations around Zr4+ ions and the morphology and structure of the powders. The highly porous material resulting from the solution at higher pH could be described mainly by defective and disordered, very hydrolyzed tetragonal arrays. As temperature increased, the amount of these arrays decreased while they became increasingly asymmetric, thus suggesting their superficial localization. The easy removal of hydroxyls led to the early appearance of the monoclinic phase. The gel obtained from the precursor at pH ⳱ 0.5 was entirely described by configurations still involving organic residues. After their calcination, the powder underwent a well-defined two-step hydroxyl removal thermal process leading to the crystallization of the tetragonal and the monoclinic phases. The thermal stability of the metastable tetragonal phase in the investigated powders seems to be controlled by their different capability to absorb oxygen.
I. INTRODUCTION
Zirconia-based ceramics have gained recent attention as engineering materials because of their diverse commercial applications. Research on the metastable phases of these materials and their transitions have been a subject of great scientific and technological importance during recent decades. A number of explanations have been offered for the interpretation of the presence of metastable tetragonal as well as cubic zirconia structures at low temperature. Garvie attributed it to crystallite size effects,1 Wu et al. to strain and lattice defects,2 and other authors to the existence of a certain number of oxygen anion vacancies.3 Precipitation and sol-gel methods are the most-used chemical methods to prepare zirconia-based ceramics.4,5 Although at first sight both methods are similar, the molecular structure configurations of the resulting zirconia precursor gels obtained through them are not. The precipitation-derived product is indeed more hydrous, and consequently, its complex molecular structure contains varying amounts of water in the form of hydrous oxide ZrO2 · H2O, metazirconic acid H2ZrO3, or zirconyl hydroxide ZrO(OH)2. Furthermore, it exhibits a tendency to polymerize. On the other hand, the product obtained by the sol-gel method consists of randomly arranged 208
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J. Mater. Res., Vol. 18, No. 1, Jan 2003 Downloaded: 16 Mar 2015
discrete nanoscopic sol particles with voids within them. It is also known that the resulting molecular structure configuration is strongly dependent on proce
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