Structure and Thermal Stability of Nanostructured Iron-doped Zirconia Prepared by High-energy Ball Milling
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Structure and thermal stability of nanostructured iron-doped zirconia prepared by high-energy ball milling J. Z. Jianga) Department of Physics, Building 307, Technical University of Denmark, DK-2800, Lyngby, Denmark
F. W. Poulsen Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
S. Mørup Department of Physics, Building 307, Technical University of Denmark, DK-2800, Lyngby, Denmark (Received 28 May 1998; accepted 19 October 1998)
Fully stabilized cubic zirconia doped with iron oxide has been synthesized by high-energy ball milling from powder mixtures of monoclinic zirconia and hematite. It is found that the iron ions dissolved in cubic ZrO2 are in substitutional positions with a maximum solubility of approximately 18.5 mol% a –Fe2 O3 . The unit-cell volume of the cubic ZrO2 phase decreases with increasing iron content. During heating the cubic-to-tetragonal transition occurs at approximately 827 ±C and the tetragonal-to-monoclinic transition seems to be absent at temperatures below 950 ±C. During cooling the tetragonal-to-monoclinic transition occurs at 900–1100 ±C.
I. INTRODUCTION
At ambient pressure, pure zirconia, ZrO2 , possesses three crystallographic forms: monoclinic (P21yC, Z 4) at temperatures below 1173 ±C, tetragonal (P42ynmc, Z 2) stable between 1173 and 2370 ±C, and cubic (Fm3m, Z 4) at temperatures from 2370 ±C to the melting point, approximately 2680 ±C.1 The cubic polymorph adopts the CaF2 structure and has a simple cubic packing of the oxygen ions with the zirconium cations in half of the available sites with eightfold-oxygen coordination. The tetragonal phase may be regarded as slightly distorted oxygen and zirconium arrays in the cubic phase. The zirconium cations are still coordinated by eight oxygen ions. The monoclinic polymorph, which is usually referred to as the Baddeleyite structure, contains zirconium cations in sevenfold-oxygen coordination. With the transformation from the monoclinic to the tetragonal phases, zirconia undergoes a considerable volume contraction of the order of 3–4%.2 This volume change is sufficient to exceed the elastic limit of the ZrO2 grains and to cause cracking on cooling. Thus the fabrication of large, pure zirconia bodies is impossible. ZrO2 materials may have a variety of interesting properties, e.g., high melting temperature and chemical inertness, high strength and high fracture toughness, low thermal conductivity, and good ionic conductivity. However, most applications require that the structure should be fully or partially stabilized tetragonal or cubic phases (hereafter S-ZrO2 ) to avoid the displacive tetragonal-toa)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 4, Apr 1999
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monoclinic transition during thermal cycling.3 Therefore, preparation of room-temperature stabilization of ZrO2 materials with a tetragonal or cubic structure has become a classic problem
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