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Y. Kharchenko et al.
faster than the YSZ–NiO ceramics, and its reoxidation is intensified by intermediate
degassing [15, 20, 28]. We estimated that in contrast to optimal reduction/oxidation
stage duration of 4 h for YSZ–NiO composite [28], its value for pure NiO ceramics
is 1 h.
After redox cycling of NiO ceramics according to mode 4 (Table 1), we revealed
that its microstructure has some resemblance to that formed by mode 3. Most of the
coarse particles have reduced fringes and unreduced cores. Tiny particles of metallic
nickel of sizes in a range of 0.2–0.5 μm and nanopores in a range of 0.1–0.8 μm
can be found on the grain boundaries (Fig. 4b). Besides, completely reduced fine
particles (less than 2.0–2.3 μm) can be seen.
The coarse particles have the hexagonal shape similar to the as-sintered, material
but their edges are smoothed and the contours are distorted. The particle surfaces
are covered with the nanoparticles of reduced nickel (Fig. 5c). For this treatment
mode, the transgranular cleavage is predominant (Fig. 5b). It is accompanied by
the formation of elongated nickel fragments surrounding each cleavage facet. Most
of the facets are oriented perpendicularly to the direction of the maximum tensile
stresses.
The changes in the nanostructure of the material during the high-temperature
(600 °C) reduction and reoxidation have been modeled and explained based on the
analysis of nano- and microstructure. For this purpose, a concept of stepwise outward
loss and inward gain of metallic Ni during a redox cycle has been proposed. A cermet
reduced in the first half-cycle mainly comprises particles which have non-reduced
NiO core (dark-gray area in Fig. 6a) with fringes of reduced nickel (light-gray areas).
We analyzed the ADF STEM image of the Ni/NiO nanostructure of the cermet
reduced in Ar–5 vol% H 2 mixture according to mode 3 that should be the same as
in the case of treatment according to mode 4 for 1 cycle (Fig. 6a). No discernible
shrinkage of the nickel phase was found that can be explained by its partial reduction.
Based on EELS and EDS analyses, it was confirmed that each particle of the size of
1 μm or larger consists of non-reduced NiO core (dark-gray area, Fig. 6a) with an
envelope of reduced nickel (light-gray area). We can see randomly oriented subgrains
as constituents of the Ni envelope, whereas NiO core looks like a single grain.
Fig. 6 ADF STEM image of the Ni/NiO nanostructure (a) and modeling (b, c) of changes in the
cermet (a, b) reduced in the gas mixture according to mode 3 and (c) after redox treatment in the
gas mixture/air according to mode 4 (Table 1)
Y. Kharchenko et al.
faster than the YSZ–NiO ceramics, and its reoxidation is intensified by intermediate
degassing [15, 20, 28]. We estimated that in contrast to optimal reduction/oxidation
stage duration of 4 h for YSZ–NiO composite [28], its value for pure NiO ceramics
is 1 h.
After redox cycling of NiO ceramics according to mode 4 (Table 1), we revealed
that its microstructure has some resemblance to that formed by mode 3. Most of the
coarse particles have reduced fringes and unreduced cores. Tiny particles of metallic
nickel of sizes in a range of 0.2–0.5 μm and nanopores in a range of 0.1–0.8 μm
can be found on the grain boundaries (Fig. 4b). Besides, completely reduced fine
particles (less than 2.0–2.3 μm) can be seen.
The coarse particles have the hexagonal shape similar to the as-sintered, material
but their edges are smoothed and the contours are distorted. The particle surfaces
are covered with the nanoparticles of reduced nickel (Fig. 5c). For this treatment
mode, the transgranular cleavage is predominant (Fig. 5b). It is accompanied by
the formation of elongated nickel fragments surrounding each cleavage facet. Most
of the facets are oriented perpendicularly to the direction of the maximum tensile
stresses.
The changes in the nanostructure of the material during the high-temperature
(600 °C) reduction and reoxidation have been modeled and explained based on the
analysis of nano- and microstructure. For this purpose, a concept of stepwise outward
loss and inward gain of metallic Ni during a redox cycle has been proposed. A cermet
reduced in the first half-cycle mainly comprises particles which have non-reduced
NiO core (dark-gray area in Fig. 6a) with fringes of reduced nickel (light-gray areas).
We analyzed the ADF STEM image of the Ni/NiO nanostructure of the cermet
reduced in Ar–5 vol% H 2 mixture according to mode 3 that should be the same as
in the case of treatment according to mode 4 for 1 cycle (Fig. 6a). No discernible
shrinkage of the nickel phase was found that can be explained by its partial reduction.
Based on EELS and EDS analyses, it was confirmed that each particle of the size of
1 μm or larger consists of non-reduced NiO core (dark-gray area, Fig. 6a) with an
envelope of reduced nickel (light-gray area). We can see randomly oriented subgrains
as constituents of the Ni envelope, whereas NiO core looks like a single grain.
Fig. 6 ADF STEM image of the Ni/NiO nanostructure (a) and modeling (b, c) of changes in the
cermet (a, b) reduced in the gas mixture according to mode 3 and (c) after redox treatment in the
gas mixture/air according to mode 4 (Table 1)