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Development and characterization of yttria stabilized zirconia doped with erbia

University of British Columbia

Zirconia ceramics have high thermal resistance, chemical inertness and irradiation stability. One application where zirconia composites may find use is in supercritical water nuclear reactors (SCWR), which are expected to have improved thermal efficiency in comparison to current nuclear reactors. Zirconia ceramics may be useful as insulator materials in SCWR pressure tubes where high thermal resistance, chemical stability, irradiation resistance and supercritical water (SCW) degradation resistance are necessary. Additionally, some preliminary tests have found that specific zirconia composites, such as yttria stabilized zirconia (YSZ) outperform pure zirconia in SCW environments. YSZ is a commonly used zirconia composite, and has an excellent thermal resistance and chemical inertness. Due to the presence of yttrium, the high temperature brittle failure caused by the tetragonal to monoclinic crystal structure transformation frequently observed in zirconia is avoided. However, currently available YSZ does not fully satisfy the SCW degradation targets for a SCWR, thus motivating research into the possibility of doping YSZ to further improve YSZ’s SCW degradation resistance. One possible dopant is erbium oxide (erbia), a rare earth oxide that is used as a technical ceramic and has been studied for use in nuclear applications due to its ability to absorb neutrons. Erbia has not been studied in depth as a dopant in the zirconia system or in YSZ. Thus, this research was conducted in order to determine the effect of erbia doping on the densification, hardness, grain size, crystal lattice parameter and SCW degradation resistance of YSZ. Erbia-YSZ composites of 8mol% YSZ with 5, 10 and 15mol% erbia were fabricated by spark plasma sintering (SPS), a novel powder metallurgy process that is known to produce high density composites, at 1200, 1300 and 1400°C with 30 and 60MPa pressures for 5min. Densification during SPS was characterized based on ram position during sintering. Grain size analysis of the as-sintered composites was performed on fractured surfaces. A preliminary study of the SCW degradation resistance of the erbia-YSZ composites was conducted by exposing them to SCW for 2.5h in pressure vessels. The results suggest that erbia-YSZ composites with >95% theoretical density (TD) were successfully fabricated using SPS. Erbia doping into YSZ was found to inhibit densification, but promoted grain growth and resulted in crystal lattice expansion. Additionally, weight loss during SCW degradation revealed that erbia doping improved the SCW degradation resistance of YSZ, but additional work is necessary to further improve the degradation resistance to meet SCWR targets.

Text

2015-07-03

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/53996

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/