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P. 140
Chapter 5
between different but comparable experiments. Various experiments (carried out, not
reported in this thesis) also showed large volume expansion, but no unambiguous
correlation between the absolute numbers of the volume change and the deposition
parameters could be observed. Therefore, no conclusions can be drawn for the volume
expansion as a function of selenisation and sputter pressure. However, an increase of
the total layer thickness of at least 20% is observed in all cases.
A correlation between the ratio between the top and the bottom layer and the sputter
pressure was observed: more porous layers showed thicker top layers.
The composition of the top layer was studied with EDX. The main elements measured
after degradation were molybdenum and oxygen, while sodium, nitrogen and carbon
as well as elements from the glass were present in small quantities. For the selenised
samples, selenium was naturally also found. The oxygen content in the top layer was
very high, leading to Mo/O ratios that varied between 0.36 and 0.66. This indicates that
the top layer consists of molybdenum oxide. Other SEM figures showed the cracking
of the top layer, exposing a new metallic molybdenum surface layer to a degrading
environment, as is shown in Figure 5.7.
Another phenomenon observed by the SEM is the appearance of needle-like
structures on the surface. Their lengths vary from hundreds of nanometres to tens of
micrometres, as is shown in Figure 5.8.
100 μm
Figure 5.7
o
SEM pictures of the cracked surface of non-selenised Mo15 after 105 hours exposure to 85 C/85% RH.
When the needle-like structures were studied separately, molybdenum, oxygen
and nitrogen were measured in these structures and in similar concentrations in the
background material. Carbon and sodium, on the other hand, were found in these
structures in higher concentrations than in the surroundings, indicating that the
needle-like structures are rich in this element. This might indicate the formation of a
carbonate, for example Na CO .
3
2
138
between different but comparable experiments. Various experiments (carried out, not
reported in this thesis) also showed large volume expansion, but no unambiguous
correlation between the absolute numbers of the volume change and the deposition
parameters could be observed. Therefore, no conclusions can be drawn for the volume
expansion as a function of selenisation and sputter pressure. However, an increase of
the total layer thickness of at least 20% is observed in all cases.
A correlation between the ratio between the top and the bottom layer and the sputter
pressure was observed: more porous layers showed thicker top layers.
The composition of the top layer was studied with EDX. The main elements measured
after degradation were molybdenum and oxygen, while sodium, nitrogen and carbon
as well as elements from the glass were present in small quantities. For the selenised
samples, selenium was naturally also found. The oxygen content in the top layer was
very high, leading to Mo/O ratios that varied between 0.36 and 0.66. This indicates that
the top layer consists of molybdenum oxide. Other SEM figures showed the cracking
of the top layer, exposing a new metallic molybdenum surface layer to a degrading
environment, as is shown in Figure 5.7.
Another phenomenon observed by the SEM is the appearance of needle-like
structures on the surface. Their lengths vary from hundreds of nanometres to tens of
micrometres, as is shown in Figure 5.8.
100 μm
Figure 5.7
o
SEM pictures of the cracked surface of non-selenised Mo15 after 105 hours exposure to 85 C/85% RH.
When the needle-like structures were studied separately, molybdenum, oxygen
and nitrogen were measured in these structures and in similar concentrations in the
background material. Carbon and sodium, on the other hand, were found in these
structures in higher concentrations than in the surroundings, indicating that the
needle-like structures are rich in this element. This might indicate the formation of a
carbonate, for example Na CO .
3
2
138
