TY - GEN
T1 - Stability, instability, metastability and grain size in nanocrystalline ceramic oxide systems
AU - Kimmel, Giora
AU - Zabicky, Jacob
N1 - Publisher Copyright:
© (2008) Trans Tech Publications, Switzerland.
PY - 2008/1/1
Y1 - 2008/1/1
N2 - The following nanocrystalline binary oxide systems were studied: Mg-Ti, Ni-Ti, Zr-Al, as well as some pure and doped unary oxides. The xerogels were heated at a constant T (200 to 1600°C) for 3 to 6 hours. There was a threshold tempearture for oxide formation and in many cases the products were metastable nanocrystalline phases, depending on the grain size and composition, including doping. The oxide phases of Ni-Ti, Mg-Ti, and Zr-Al, formed at 900 °C are different from those formed at higher temperature. New ranges of solid solutions and the formations of higher temperature structures were found. A transition phase can be defined as a structure formed at relative low tempearture, irreversibly transforming at higher temperature into an equilibrium phase of the same elemental composition. Some low temperature transition phases have a structure similar to that of a high temperature equilibrium phase, e.g., (the equilibrium phase is 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 alumina (corundum) and anatase (rutile), which are formed as nanocrystalline oxides due to a low growth rate caused either by a low temperature of calcination or due to additives. To asses the importance of crystal size in the stabilization of transition phases, the following studies were undertaken: (a) XRPD analysis of all unary, doped and binary compositions; (b) the evolution of transition phases in HT XRPD of the Mg titanates; (c) the phase evolution was studied with time at temperatures were mixtures of transition and equilibrium phases were found; (d) the retention of pure tetragonal ZrO2 on quenching Al-Zr oxides after calcinations at high tempetature; (e) additional evidence from HRTEM, SEM and DTA experiments was also collected. A model, correlating the size effect with the unusual phases and structures is proposed.
AB - The following nanocrystalline binary oxide systems were studied: Mg-Ti, Ni-Ti, Zr-Al, as well as some pure and doped unary oxides. The xerogels were heated at a constant T (200 to 1600°C) for 3 to 6 hours. There was a threshold tempearture for oxide formation and in many cases the products were metastable nanocrystalline phases, depending on the grain size and composition, including doping. The oxide phases of Ni-Ti, Mg-Ti, and Zr-Al, formed at 900 °C are different from those formed at higher temperature. New ranges of solid solutions and the formations of higher temperature structures were found. A transition phase can be defined as a structure formed at relative low tempearture, irreversibly transforming at higher temperature into an equilibrium phase of the same elemental composition. Some low temperature transition phases have a structure similar to that of a high temperature equilibrium phase, e.g., (the equilibrium phase is 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 alumina (corundum) and anatase (rutile), which are formed as nanocrystalline oxides due to a low growth rate caused either by a low temperature of calcination or due to additives. To asses the importance of crystal size in the stabilization of transition phases, the following studies were undertaken: (a) XRPD analysis of all unary, doped and binary compositions; (b) the evolution of transition phases in HT XRPD of the Mg titanates; (c) the phase evolution was studied with time at temperatures were mixtures of transition and equilibrium phases were found; (d) the retention of pure tetragonal ZrO2 on quenching Al-Zr oxides after calcinations at high tempetature; (e) additional evidence from HRTEM, SEM and DTA experiments was also collected. A model, correlating the size effect with the unusual phases and structures is proposed.
KW - Amorphous oxides
KW - HTXRD
KW - Magnesium titanates
KW - Metastable ceramic oxides
KW - Nanocrystalline oxides
KW - Stability of ceramic oxides
KW - Synthesis of ceramic oxides by sol-gel technique
KW - Thermodynamics of nano structural materials
KW - Titania
KW - X-ray powder diffraction
KW - Zirconia-alumina
UR - https://www.scopus.com/pages/publications/84955167251
U2 - 10.4028/www.scientific.net/SSP.140.29
DO - 10.4028/www.scientific.net/SSP.140.29
M3 - Conference contribution
AN - SCOPUS:84955167251
SN - 9783908451570
T3 - Solid State Phenomena
SP - 29
EP - 36
BT - Perspectives of nanoscience and nanotechnology - Acta Materialia Gold Medal Workshop
A2 - Lojkowski, Witold
A2 - Blizzard, John R.
PB - Trans Tech Publications Ltd
T2 - European Materials Research Society, Fall Meeting, E-MRS 2007
Y2 - 17 September 2007 through 21 September 2007
ER -