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Chapter 5
5.4.2.1 Oxide and needle formation
In the presence of HO and O, a MoO layer containing sodium is formed on top of
x
2
2
metallic molybdenum. Similar material is probably present on the surface of the
grain boundaries, as is described in reference. A schematic representation of the first
stages of the degradation is shown in Figure 5.31, while more information about the
exact nature of the (Na)MoO layer is described in chapter 5.4.2. It should be noted that
x
the information given in Figure 5.30 is not included in this figure in order to improve
readability.
It is proposed that the degradation of metallic molybdenum follows these steps:
1. Due to exposure to damp heat, a thin water film is formed on the surface
of metallic molybdenum. The NaO present in the glass can migrate via the
2
grain boundaries and dissolve in this water film. Furthermore O from the
2
environment is also present.
2. A: The corrosion of metallic molybdenum occurs due to the formation of an
oxide film by the reaction between oxygen and molybdenum on the interface
between the molybdenum film and surrounding water or air. The exact nature
of the formed product depends on the pH and possible potential (not relevant
here) of the water film, but in this case, probably MoO is formed via redox
3
reactions as for example defined in reference [35]:
B: The reaction of Na with various atmospheric species can result in the
formation of needles. These needles are likely formed as a drying effect
when the samples are removed from the climate chamber and have various
geometrical shapes.
Sodium can be built into the MoO , as is explained in chapter 5.4.2
3
3. Cracks occur in the (Na)MoO , probably due to a different in the Coefficient of
x
Thermal Expansion (CTE) of molybdenum and its oxides [33,34] which leads
o
to vulnerability when samples are removed from the 85C environment and
very quickly cooled to room temperature. The difference in density of the two
materials can also play a role in this cracking.
4. Due to cracking or other damage to the (Na)MoO, the degradation process
x
can start again on new freshly exposed metallic molybdenum.
5.4.2.2 The formation of molybdenum bronze by Na intercalation
+
It is mentioned above that sodium plays a role in the degradation of metallic
molybdenum films via the formation of (Na)MoO. It is proposed that this occurs via
x
+
the inclusion of Na in the MoO matrix. This is possible due to the structure of MoO ,
3
3
which has a MoO octahedron with molybdenum atom at the center surrounded by
6
166
5.4.2.1 Oxide and needle formation
In the presence of HO and O, a MoO layer containing sodium is formed on top of
x
2
2
metallic molybdenum. Similar material is probably present on the surface of the
grain boundaries, as is described in reference. A schematic representation of the first
stages of the degradation is shown in Figure 5.31, while more information about the
exact nature of the (Na)MoO layer is described in chapter 5.4.2. It should be noted that
x
the information given in Figure 5.30 is not included in this figure in order to improve
readability.
It is proposed that the degradation of metallic molybdenum follows these steps:
1. Due to exposure to damp heat, a thin water film is formed on the surface
of metallic molybdenum. The NaO present in the glass can migrate via the
2
grain boundaries and dissolve in this water film. Furthermore O from the
2
environment is also present.
2. A: The corrosion of metallic molybdenum occurs due to the formation of an
oxide film by the reaction between oxygen and molybdenum on the interface
between the molybdenum film and surrounding water or air. The exact nature
of the formed product depends on the pH and possible potential (not relevant
here) of the water film, but in this case, probably MoO is formed via redox
3
reactions as for example defined in reference [35]:
B: The reaction of Na with various atmospheric species can result in the
formation of needles. These needles are likely formed as a drying effect
when the samples are removed from the climate chamber and have various
geometrical shapes.
Sodium can be built into the MoO , as is explained in chapter 5.4.2
3
3. Cracks occur in the (Na)MoO , probably due to a different in the Coefficient of
x
Thermal Expansion (CTE) of molybdenum and its oxides [33,34] which leads
o
to vulnerability when samples are removed from the 85C environment and
very quickly cooled to room temperature. The difference in density of the two
materials can also play a role in this cracking.
4. Due to cracking or other damage to the (Na)MoO, the degradation process
x
can start again on new freshly exposed metallic molybdenum.
5.4.2.2 The formation of molybdenum bronze by Na intercalation
+
It is mentioned above that sodium plays a role in the degradation of metallic
molybdenum films via the formation of (Na)MoO. It is proposed that this occurs via
x
+
the inclusion of Na in the MoO matrix. This is possible due to the structure of MoO ,
3
3
which has a MoO octahedron with molybdenum atom at the center surrounded by
6
166
