484
B. D. Vasyliv et al.
The observed tendency to reduce the amount of oxygen in the near-surface layers
indicates that hydrogen sulfide, at its high concentration, improves the reduction of
the nickel phase. It is likely that both of these processes (more active reduction and
interaction of nickel with high-temperature hydrogen sulfide gas) occur mainly on
the three-phase boundaries (pores–nickel phase–zirconium phase), resulting in the
concentration of sulfur as a product of the reaction in these places, i.e., in the pores
and near them.
First of all, the degradation of the structure is manifested through the discontinuity of the “zirconium phase-nickel phase” and “nickel phase-nickel phase”
bonds. This does not violate the integrity of the YSZ ceramic skeleton in the
cermet at concentrations up to 7 vol% H 2 S. With the increase of the hydrogen
sulfide concentration followed by the improvement of the nickel phase reduction,
a larger amount of completely reduced small nickel particles is formed (Fig. 30.8c
as compared to Fig. 30.8a). These particles interact actively with hydrogen sulfide
and weaken their bonds with the particles of the zirconium phase. Such structural
transformations are followed by a change of fracture micromechanisms prevailing
in a cermet: from the mixed one containing both the elements of the cleavage
and intergranular fracture along the cracked boundaries (Fig. 30.8b) to completely
intergranular fracture (Fig. 30.8d). As a result, the cermet strength is reduced to 61%
of the initial state (Table 30.1).
The arrow along the deviated branch of curve 2 (see Fig. 30.6) indicates a general
tendency of change of the relative strength and stiffness of the YSZ–Ni anode
cermet while hydrogen sulfide content in high-temperature hydrogenous atmosphere
increases. It can be marked that the arrow direction differs from that for porous Ni.
Unlike mechanical behavior, the electrical conductivity of YSZ–Ni cermet after
aging in a hydrogenous atmosphere at a concentration of 7 and 18 vol% H 2 S is (5.5–
6.5)·10 5 S/m, which does not differ from the value for one-time reduced cermet
(Table 30.1). Thus, the YSZ–Ni anode material is not sensitive to high-temperature
hydrogenous atmosphere with an admixture of 7 vol% H 2 S. The little drop of the
relative stiffness (see Fig. 30.6) is probably connected with a slight improvement
of the reduction of the nickel phase by hydrogen sulfide, as compared to pure
Ar–5 vol% ° 2 atmosphere. This concentration of hydrogen sulfide in the SOFC
operating environment does not lead to a significant decrease in its strength at a
stable high electrical conductivity.
30.4 Conclusions
The certain features of changes in the structure and physical and mechanical properties of porous nickel and the YSZ–Ni anode cermet caused by high-temperature
hydrogenous atmospheres with and without an admixture of H 2 S, were revealed.
It was found that exposition of porous Ni for 4 h in a hydrogenous atmosphere at
the temperature 600 ◦ ´ causes its significant plasticization. The relative strength
and stiffness of the material decrease with increasing hydrogen concentration and
B. D. Vasyliv et al.
The observed tendency to reduce the amount of oxygen in the near-surface layers
indicates that hydrogen sulfide, at its high concentration, improves the reduction of
the nickel phase. It is likely that both of these processes (more active reduction and
interaction of nickel with high-temperature hydrogen sulfide gas) occur mainly on
the three-phase boundaries (pores–nickel phase–zirconium phase), resulting in the
concentration of sulfur as a product of the reaction in these places, i.e., in the pores
and near them.
First of all, the degradation of the structure is manifested through the discontinuity of the “zirconium phase-nickel phase” and “nickel phase-nickel phase”
bonds. This does not violate the integrity of the YSZ ceramic skeleton in the
cermet at concentrations up to 7 vol% H 2 S. With the increase of the hydrogen
sulfide concentration followed by the improvement of the nickel phase reduction,
a larger amount of completely reduced small nickel particles is formed (Fig. 30.8c
as compared to Fig. 30.8a). These particles interact actively with hydrogen sulfide
and weaken their bonds with the particles of the zirconium phase. Such structural
transformations are followed by a change of fracture micromechanisms prevailing
in a cermet: from the mixed one containing both the elements of the cleavage
and intergranular fracture along the cracked boundaries (Fig. 30.8b) to completely
intergranular fracture (Fig. 30.8d). As a result, the cermet strength is reduced to 61%
of the initial state (Table 30.1).
The arrow along the deviated branch of curve 2 (see Fig. 30.6) indicates a general
tendency of change of the relative strength and stiffness of the YSZ–Ni anode
cermet while hydrogen sulfide content in high-temperature hydrogenous atmosphere
increases. It can be marked that the arrow direction differs from that for porous Ni.
Unlike mechanical behavior, the electrical conductivity of YSZ–Ni cermet after
aging in a hydrogenous atmosphere at a concentration of 7 and 18 vol% H 2 S is (5.5–
6.5)·10 5 S/m, which does not differ from the value for one-time reduced cermet
(Table 30.1). Thus, the YSZ–Ni anode material is not sensitive to high-temperature
hydrogenous atmosphere with an admixture of 7 vol% H 2 S. The little drop of the
relative stiffness (see Fig. 30.6) is probably connected with a slight improvement
of the reduction of the nickel phase by hydrogen sulfide, as compared to pure
Ar–5 vol% ° 2 atmosphere. This concentration of hydrogen sulfide in the SOFC
operating environment does not lead to a significant decrease in its strength at a
stable high electrical conductivity.
30.4 Conclusions
The certain features of changes in the structure and physical and mechanical properties of porous nickel and the YSZ–Ni anode cermet caused by high-temperature
hydrogenous atmospheres with and without an admixture of H 2 S, were revealed.
It was found that exposition of porous Ni for 4 h in a hydrogenous atmosphere at
the temperature 600 ◦ ´ causes its significant plasticization. The relative strength
and stiffness of the material decrease with increasing hydrogen concentration and