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Abstract

The Perturbed Angular Correlations technique was used to determine the configurations around Zirconium ions and their thermal behavior in non-aggregated sub-micron zirconia spherical particles. Three residues containing- Zr surroundings were determined for the non-crystalline starting particles, which were identified under the assumption of a certain chemical reactions sequence during synthesis. While the one made up mainly by hydroxyl groups was common to both samples, the two involving mainly organic residues were particle size dependent. Upon crystallization, both samples stabilized in the t'- and t- tetragonal forms and the Xc-cubic form but their amounts and temperatures of appearance were different. On heating, the structure of the smaller particles became gradually monoclinic achieving total degradation upon the subsequent cooling to RT.

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[2] [3] [4] [5] [6] [7] XIX Latin American Symposium on Solid State Physics
  • Tech E Torres-García
  • Paláiz
  • A Barranco
  • Vázquez
  • Ramos
  • G Fuentes
and Tech. 11 309-319. Torres-García E, Paláiz-Barranco A, Vázquez-Ramos C and Fuentes G A 2001 J. Mater. Res. 16 2209-2212. [2] [3] [4] [5] [6] [7] XIX Latin American Symposium on Solid State Physics (SLAFES XIX) Journal of Physics: Conference Series 167 (2009) 012041 IOP Publishing doi:10.1088/1742-6596/167/1/012041 5
  • E Torres-García
  • A Paláiz-Barranco
  • C Vázquez-Ramos
  • G Fuentes
Torres-García E, Paláiz-Barranco A, Vázquez-Ramos C and Fuentes G A 2001 J. Mater. Res. 16 2209-2212.
  • A M Rodríguez
  • M C Caracoche
  • P C Rivas
  • A F Pasquevich
  • S Mintzer
Rodríguez A M, Caracoche M C, Rivas P C, Pasquevich A F and Mintzer S R 2001 J. Am. Ceram. Soc. 84 188-192.
  • F Bondioli
  • V Cannillo
  • E Fabbri
  • M Messori
Bondioli F, Cannillo V, Fabbri E and Messori M 2006 Polimery 51 794-798.
  • S Figueroa
  • J Desimoni
  • P C Rivas
  • M C Caracoche
  • O De Sanctis
Figueroa S, Desimoni J, Rivas P C, Caracoche M C and de Sanctis O 2006 J. Am. Ceram. Soc. 89 3759-3764.