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Title:The Effects of Titania on Selected Electrical and Mechanical Properties of Hafnia Rich Mixed Oxides in the System Hafnia Zirconia Titania
Author(s):Staszak, Paul Russell
Doctoral Committee Chair(s):Brown, Sherman D.,
Department / Program:Ceramics Engineering
Discipline:Ceramics Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Materials Science
Abstract:A study of the effects of titania on selected properties of hafnia-rich mixed oxides in the system hafnia-zirconia-titania (HZT) was made in the region 5 to 20 mol% titania. The studied properties included electrical conductivity, thermal expansion, and fracture strength and toughness.
The effects of titania on the properties were studied for the reduced state as well as the oxidized state of the sintered mixed oxides. X-ray analyses showed that the materials were not always single phase. The oxidized compositions went from being monoclinic solid solutions at low titania additions to having three phases (two monoclinic and a titanate phase) at high additions of titania. The reduced compositions showed an increasing cubic phase presence mixed with the monoclinic phase as titania was added. The grains of all samples were nominally 1 $\mu$m. Microcracking regions varying from 16 $\mu$m to 3000 $\mu$m were also observed in the microstructures. The wide variability in phases and microcracking regions accounted for the majority of the observed property trends.
The electrical conductivity increased with temperature to approximately 10$\sp{-1}$ mhos/cm at 1700$\sp\circ$C for all compositions. For low temperatures at high oxygen partial pressures, higher concentrations of titania lowered the conductivity suggesting that the titanium ions go into interstitial positions thereby reducing oxygen ion conductivity. For all temperatures at low oxygen partial pressures, the materials became highly anion deficient leading to higher conductivities suggesting that Ti$\sp{4+}$ is being converted to Ti$\sp{3+}$ and Ti$\sp{2+}$ thereby enhancing electronic conductivity.
The thermal expansion coefficient decreased with increasing titania as did the monoclinic to tetragonal transformation temperature. A hysteresis between the heating and cooling curves in the 15 and 20 mol% compositions suggested extensive microcracking. The low thermal expansion coefficient of the second phase titanate partially accounted for the decrease in thermal expansion in these higher titania compositions.
The fracture strength of the oxidized bars tended to decrease with the addition of titania owing to the presence of the second phase titanate. The fracture strengths of the reduced bars exhibited a minimum corresponding to a two-phase region of monoclinic and cubic phases. When the second phases were suppressed, the titania tended to increase the fracture strength slightly in both the oxidized and reduced states. The fracture toughness followed similar trends. (Abstract shortened with permission of author.)
Issue Date:1988
Description:147 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.
Other Identifier(s):(UMI)AAI8823256
Date Available in IDEALS:2014-12-16
Date Deposited:1988

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