Diffusional and dislocation creep of NiO polycrystals

Microstructural and high-temperature mechanical characteristics of nickel oxide/zirconia composites for solid oxide fuel cells

“…This result agrees with the difference in creep strength reported for monolithic NiO [18] and 8YSZ [21]. The microstructure after deformation was investigated looking for modifications with respect to the as-fabricated materials.…”

“…The creep parameters n and Q were determined by stress and temperature changes during deformation; values of n =4.5± 0.8 and Q= 560± 70 kJ/mol were estimated for the 40NiO composite. The high stress exponent value suggests that an intragranular creep mechanism is the rate-controlling deformation process, as found in 8YSZ [21] and NiO [18] monoliths strained at high stresses. Regarding the activation energy for creep, the present value is close to that reported for monolithic NiO, 510 ± 60 kJ/mol [18].…”

“…The high stress exponent value suggests that an intragranular creep mechanism is the rate-controlling deformation process, as found in 8YSZ [21] and NiO [18] monoliths strained at high stresses. Regarding the activation energy for creep, the present value is close to that reported for monolithic NiO, 510 ± 60 kJ/mol [18]. There are no values for monolithic cubic zirconia deformed at high stresses; however, an activation energy of 530 ±30 kJ/mol was reported for 9 to 21 mol% Y 2 O 3 -stabilized cubic-ZrO 2 single crystals [22].…”

“…Cubic zirconia powders show an excellent sinterability, achieving nearly full densification at temperatures as low as 1300°C [16,17]. Nickel oxide, however, exhibits a reduced sinterability; densities lower than 92% of theoretical density have been systematically reported in the literature, even for sintering temperatures as high as 1873 and 1973 K (= 0.84-0.88T m , where T m = 2230 K is the melting temperature of NiO) [11,18].…”

High temperature creep behaviour of 4 mol% yttria tetragonal zirconia polycrystals (4-YTZP) with grain sizes between 0.38 and 1.15 μm

“…The conditions leading to any of the two values n = 1 or 2 for the superplastic response are unknown. There are examples showing that n = 1 in NiO polycrystals with grain sizes between 9 and 21 m, 22 and in UO 2 with grain size between 2 and 10 m 23 ; both of these ceramics deform by grain boundary sliding. In the case of YTZP ceramics, values of n = 1.5 ± 0.3 have been reported for a grain size of 1.8 m, 18 whereas, for the grain sizes between 0.3 and 0.6 m, probably the most widely studied YTZP ceramics, values of n between 2 and 5 have been reported.…”