Structural development in ultrafine particles of zirconia produced from sol-gel
The pH of the solution that hydrous zirconia is precipitated from appears to define the crystal structure formed after calcination of the material at 400-600{degree}C. A metastable tetragonal structure was obtained for either low (less than about 5) or high (greater than about 13) pH. The tetragonal phase formed at high pH is much more stable than that obtained at a low pH at a calcination temperature of 500{degree}C. Detailed structural studies were undertaken using transmission electron microscopy, X-ray diffraction and BET surface area. The results obtained from these analyses do not support the crystallite size theory, which states that the tetragonal particles of less than 300 {angstrom} are stabilized against the t {yields} m transformation. It was observed that the time taken to attain the final pH of the solution in contact with the precipitate in the pH range 7-11, plays a dominant role in determining the crystal structure of the zirconium oxide after calcination at 500{degree}C. The dependence of the crystal structure on the rate of precipitation is observed only in the pH range 7-11. Rapid precipitation in this pH range yields predominantly monoclinic zirconia, whereas slow (8 hours) precipitation produces the tetragonal phase. At a pH of approximately 13.0, only the tetragonal phase is formed from both slowly and rapidly precipitated hydrous oxide. The results indicate that both the pH of the supernatant liquid and the time taken to attain this pH play dominant roles in determining the crystal structure of zirconia formed after calcination of the hydrous oxide. The factors that determine the crystal phase are therefore imparted in a mechanism of precipitation that depends upon the pH, and it is inferred that it is the hydroxyl concentration that is the dominant factor.
- Research Organization:
- Kentucky Univ., Lexington, KY (USA)
- OSTI ID:
- 5964518
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360202* -- Ceramics
Cermets
& Refractories-- Structure & Phase Studies
CALCINATION
CHALCOGENIDES
CHEMICAL REACTIONS
COHERENT SCATTERING
CRYSTAL STRUCTURE
DECOMPOSITION
DIFFRACTION
DISPERSIONS
ELEMENTS
HAFNIUM
HEAT TREATMENTS
HIGH TEMPERATURE
METALS
MIXTURES
OXIDES
OXYGEN COMPOUNDS
PH VALUE
PRECIPITATION
PYROLYSIS
QUANTITY RATIO
SCATTERING
SEPARATION PROCESSES
SOL-GEL PROCESS
SOLUTIONS
THERMOCHEMICAL PROCESSES
TIME DEPENDENCE
TRANSITION ELEMENT COMPOUNDS
TRANSITION ELEMENTS
X-RAY DIFFRACTION
ZIRCONIUM COMPOUNDS
ZIRCONIUM OXIDES