Nanostructural Changes in a Ni/NiO Cermet During …
227
Subgrain sizes are in a range of 50–250 nm. We did not found any pore inside the
Ni grain or shrinkage (Fig. 6a). Such a microstructural peculiarity can be used to
explain quite high strength of the cermet.
A cermet oxidized in the next half-cycle mainly comprises particles that are characterized by completely oxidized nickel subgrain envelope formed in the previous
half-cycle. Due to the degassing, partial pressure inside the cermet is lowered what
is a cause of intense inward diffusion of oxygen [29–31].
The closer the subgrains are to the surface of a grain, the more their boundaries
are distinguished with the increasing number of redox cycles, because a network of
nanopores is formed in the external layer on the boundaries [20]. Therefore, tiny
pieces consisting of the subgrains of reduced Ni are being separated stepwise. The
gradual outward Ni losses occur after each reduction half-cycle. On the other hand,
a stepwise inward Ni gain occurs due to the decrease in Ni particle size, intensifying
the particle reduction (Fig. 6c).
Tiny pieces of reduced nickel are separated stepwise and coagulate in the places
of former boundaries forming “bridges” between nickel phase particles (Fig. 6c).
This is also a cause of some decrease in material porosity. The “bridges,” along
with the nickel fringes, form the cermet structure of an electrically conducted nickel
network. The measured value of specific electrical conductivity (1.85 × 10
5 S/m) is
satisfactory for SOFC applications and agreed well with the values presented in [20,
32]. Besides, it may be improved because there is a possibility of further reduction
of the anode in a hydrogen-containing atmosphere during SOFC operation.
The abovementioned peculiarities of fracture surface morphology (Fig. 5b) show
that the adhesion strength between nickel phase particles is higher than the ultimate
cleavage stresses. The characteristics of relative stiffness and strength (see Table 1),
which reflect the rearrangement of structural components on the boundaries of the
particles, also confirmed this hypothesis. We can characterize this phenomenon as
the clustering of particles with the formation of nanopores and “bridges” at the
sites of former boundaries (Fig. 5c). Such structural rearrangement followed by the
“bridging” effect is due to the intermediate degassing before starting the oxidation
stage. The “bridges” play an important role in providing high bond strength between
the particles. If cleavage planes of a nickel phase particle are perpendicular to the
direction of the maximum tensile stresses, a clear transgranular cleavage is observed,
but if they are not then a stepwise alternation of the transgranular cleavage and
transverse crack occurs till the particle boundary is reached (Fig. 4d). In any case,
the bond strength between the particles is high enough to provide such fracture
micromechanism.
Therefore, the following structural changes in the course of redox cycling may be
marked for mode 4: (1) a network of nanopores was formed in the external layer of
particles of the nickel phase; (2) thin pieces of reduced Ni subgrains were separated
causing a reduction in the size of particles of Ni-phase; (3) tiny Ni pieces were
coagulated causing partial reduction of the porosity and formation of “bridges”; (4)
the initial particles of the nickel phase were united in clusters with the formation of
nanopores at the sites of former boundaries; and (5) a nickel network was formed that
227
Subgrain sizes are in a range of 50–250 nm. We did not found any pore inside the
Ni grain or shrinkage (Fig. 6a). Such a microstructural peculiarity can be used to
explain quite high strength of the cermet.
A cermet oxidized in the next half-cycle mainly comprises particles that are characterized by completely oxidized nickel subgrain envelope formed in the previous
half-cycle. Due to the degassing, partial pressure inside the cermet is lowered what
is a cause of intense inward diffusion of oxygen [29–31].
The closer the subgrains are to the surface of a grain, the more their boundaries
are distinguished with the increasing number of redox cycles, because a network of
nanopores is formed in the external layer on the boundaries [20]. Therefore, tiny
pieces consisting of the subgrains of reduced Ni are being separated stepwise. The
gradual outward Ni losses occur after each reduction half-cycle. On the other hand,
a stepwise inward Ni gain occurs due to the decrease in Ni particle size, intensifying
the particle reduction (Fig. 6c).
Tiny pieces of reduced nickel are separated stepwise and coagulate in the places
of former boundaries forming “bridges” between nickel phase particles (Fig. 6c).
This is also a cause of some decrease in material porosity. The “bridges,” along
with the nickel fringes, form the cermet structure of an electrically conducted nickel
network. The measured value of specific electrical conductivity (1.85 × 10
5 S/m) is
satisfactory for SOFC applications and agreed well with the values presented in [20,
32]. Besides, it may be improved because there is a possibility of further reduction
of the anode in a hydrogen-containing atmosphere during SOFC operation.
The abovementioned peculiarities of fracture surface morphology (Fig. 5b) show
that the adhesion strength between nickel phase particles is higher than the ultimate
cleavage stresses. The characteristics of relative stiffness and strength (see Table 1),
which reflect the rearrangement of structural components on the boundaries of the
particles, also confirmed this hypothesis. We can characterize this phenomenon as
the clustering of particles with the formation of nanopores and “bridges” at the
sites of former boundaries (Fig. 5c). Such structural rearrangement followed by the
“bridging” effect is due to the intermediate degassing before starting the oxidation
stage. The “bridges” play an important role in providing high bond strength between
the particles. If cleavage planes of a nickel phase particle are perpendicular to the
direction of the maximum tensile stresses, a clear transgranular cleavage is observed,
but if they are not then a stepwise alternation of the transgranular cleavage and
transverse crack occurs till the particle boundary is reached (Fig. 4d). In any case,
the bond strength between the particles is high enough to provide such fracture
micromechanism.
Therefore, the following structural changes in the course of redox cycling may be
marked for mode 4: (1) a network of nanopores was formed in the external layer of
particles of the nickel phase; (2) thin pieces of reduced Ni subgrains were separated
causing a reduction in the size of particles of Ni-phase; (3) tiny Ni pieces were
coagulated causing partial reduction of the porosity and formation of “bridges”; (4)
the initial particles of the nickel phase were united in clusters with the formation of
nanopores at the sites of former boundaries; and (5) a nickel network was formed that