Controlled phase transformations in Al 3+ stabilized ZrO 2 nanoparticles via forced hydrolysis of metal cations in water

Abstract

Al 3+ stabilized nanoparticles are obtained in a metastable c-ZrO 2 phase by a reconstructive thermal decomposition of an amorphous hydroxyl gel in air at moderate temperature as low as 200 °C. Average crystallite size ( d) lies in the 5–8 nm range as per the Al 3+ content. A transparent colorless Zr 1−2 x Al 2 x O(OH) 2−2 x · αH 2O gel, synthesized by a forced hydrolysis of a mixed aqueous solution of aluminum chloride and zirconium oxychloride with an ammonia solution at ∼5 °C, has been used. The phase formation and improved thermal stability in Al 3+ modified c-ZrO 2 nanoparticles are studied with X-ray diffraction of the specimen before and after selected annealing at subsequent temperatures as high as 1200 °C. A small x=0.02–0.20 Al 3+ content is found to inhibit a promptly controlled growth during the processing in nanoparticles, d≤36 nm. Practically no grain growth occurs in d above 36 nm as long as the temperature lies below 1200 °C. A reconstructive c-ZrO 2→t- or m-ZrO 2 phase transformation appears at 1000 or 1200 °C extended temperature in successive steps. No independent Al 2O 3 recrystallizes in a perfect Al 3+:ZrO 2 solid solution. Virgin c-ZrO 2 grows rapidly and appears in m-ZrO 2 directly as early as at 800 °C.