Stability, instability, metastability and grain size in nanocrystalline ceramic oxide systems

Giora Kimmel ,  Jacob Zabicky 

Abstract

The following nanocrystalline binary oxide systems were studied: Mg-Ti, Zn-Ti, Fe-Al, Fe-Ti, Ni-Ti, Mg-Ti, Zr-Al, Zr-Mg and Y-Ln (Ln = rare earth element). Some pure or doped unary oxides were also studied. The xerogels were heated at constant T (200 to 1600 °C) for 3 to 6 hours. There was a threshold T for oxide formation and in many cases the products were metastable nanocrystalline phases, depending on grain size and composition, including doping. The oxide phases of Ni-Ti, Mg-Ti, Zr-Al, and Ln-Y formed at T < 900 °C are different from those formed at high T. New ranges of solid solution and formation of higher temperature structures were found. A transition phase can be defined as a structure formed at relative low T, irreversibly transforming higher T into an equilibrium phase of the same elemental composition. Some low T transition phases have a structure similar to that of a high T equilibrium phase, e.g., (equilibrium phase given in parentheses) tetragonal ZrO2 (monoclinic) and low-T qandilite-like solid solutions (qandilite + geikielite). Others are unique with no representation in the equilibrium phase diagram, e.g., gamma-like aluminas (corundum) and anatase (rutile), which are formed as nanocrystalline oxides due to small growth rate caused either by low-T of calcination or due to additives. To asses the importance of crystal size in the stabilization of transition phases, we conducted the following studies: (a) XRPD analysis of all unary, doped and binary compositions; (b) evolution of transition phases in HT XRPD of the Mg titanates; (c) at temperatures were mixtures of transition and equilibrium phases were found, phase evolution was studied with time; (d) the retention of pure tetragonal ZrO2 on quenching Al-Zr oxides after calcinations at high T; (e) additional evidence from HRTEM, SEM and DTA experiments is also being collected. A model correlating the size effect with the unusual phases and structures is proposed.

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Presentation: Invited at E-MRS Fall Meeting 2007, Acta Materialia Gold Medal Workshop, by Giora Kimmel
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-04-19 16:13
Revised:   2009-06-07 00:44
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