8.4 Electrochemical Dealloying
289
temperature at which the miscibility of the components is unlimited proved to be
successful for Pd–Co alloys [230]. In the Pd–Co system, quenching of a homogeneous alloy is a prerequisite for the application of dealloying because the slow
cooling would lead to a phase segregation, which results in the dissolution of some
grains of high LN metal content instead of bulk dealloying. The big majority of
the electrochemical dealloying experiments with non-equilibrium alloys uses amorphous metals obtained with the rapid solidification (also named as melt quenching)
of a molten mixture [179, 180, 231–240]. The quenching rate in this sample preparation procedure often achieves 10
5 –10
6 K s
–1 , which means that the solid ribbon
obtained conserves essentially the same atomic arrangements that prevail in the melt.
The number of the components of such alloys can be as high as five. An increase in
the number of the components leads to a decrease in the eutectic temperature of the
melt and also leads to an impediment of the crystallization process upon quenching.
The components of the electrolyte solution used for electrochemical dealloying
are often the same as used for equilibrium alloys. Dilute solutions of strong acids are
suitable to dissolve various metals, while for aluminium dissolution from non-noble
alloys, alkaline solutions or sodium chloride solutions are preferred. An important
difference arises when a metalloid component is to be removed during the dealloying
process. In this case, the application of HF is often indispensable, especially when
only silicon has to be dissolved [228, 229].
The composition of the dealloyed non-equilibrium alloys shows a great variety,
and no general trend can be outlined. From some binary alloys of noble metals, the
accompanying component can be removed quantitatively during dealloying (e.g.,
Au–Al [231, 233] and Pd–Co [230] alloys can be transformed to pure porous Au
and Pd, respectively). The dealloying of other binary alloys like Pt 0.1 Si 0.9 proceeds
with the selective dissolution of Si, but the Pt:Si atomic ratio increases to 1:1 only
in accord with both EDS and XPS observations [228].
Dealloying of ternary Mg 90-x Cu x Y 10 alloys leads to pure Cu porous structure
since there is a sufficient difference in the nobility of the constituents and the surface
diffusion rate of Cu is high enough to expose the non-noble atoms to the surface
[179]. For another ternary alloy family, Pd 80-x Ni x P 20 , dealloying led to a Pd-rich
porous structure up to 89 at.% Pd even if the Ni molar percentage in the master
alloy was 60%, and a measurable amount of Ni and P remained in the dealloyed
material [235]. For a slightly more phosphorous-rich Pd–Ni–P alloy, Pd 32 Ni 48 P 30 ,
the final Pd enrichment upon electrochemical dealloying was between 70 and 80
at%, the larger value being achieved with more positive dealloying potential [239].
An interesting dataset in the latter study shows that the release of Ni and P is not
proportional during the dealloying process (see Fig. 2f in Ref. [239]). In the early
stage of the dealloying process, Ni is released first, which leads to a temporary
increase in the phosphorous mole fraction in the partially dealloyed material. Later,
the composition changes monotonously. Although such time-resolved composition
data are not available for the majority of the dealloying studies, it can be assumed
that such sequential dealloying may take place in various other alloys, which is
likely to be accompanied with a concomitant change of the surface composition of
the intermediate-stage ligaments. Partial dealloying of ternary materials under mild
289
temperature at which the miscibility of the components is unlimited proved to be
successful for Pd–Co alloys [230]. In the Pd–Co system, quenching of a homogeneous alloy is a prerequisite for the application of dealloying because the slow
cooling would lead to a phase segregation, which results in the dissolution of some
grains of high LN metal content instead of bulk dealloying. The big majority of
the electrochemical dealloying experiments with non-equilibrium alloys uses amorphous metals obtained with the rapid solidification (also named as melt quenching)
of a molten mixture [179, 180, 231–240]. The quenching rate in this sample preparation procedure often achieves 10
5 –10
6 K s
–1 , which means that the solid ribbon
obtained conserves essentially the same atomic arrangements that prevail in the melt.
The number of the components of such alloys can be as high as five. An increase in
the number of the components leads to a decrease in the eutectic temperature of the
melt and also leads to an impediment of the crystallization process upon quenching.
The components of the electrolyte solution used for electrochemical dealloying
are often the same as used for equilibrium alloys. Dilute solutions of strong acids are
suitable to dissolve various metals, while for aluminium dissolution from non-noble
alloys, alkaline solutions or sodium chloride solutions are preferred. An important
difference arises when a metalloid component is to be removed during the dealloying
process. In this case, the application of HF is often indispensable, especially when
only silicon has to be dissolved [228, 229].
The composition of the dealloyed non-equilibrium alloys shows a great variety,
and no general trend can be outlined. From some binary alloys of noble metals, the
accompanying component can be removed quantitatively during dealloying (e.g.,
Au–Al [231, 233] and Pd–Co [230] alloys can be transformed to pure porous Au
and Pd, respectively). The dealloying of other binary alloys like Pt 0.1 Si 0.9 proceeds
with the selective dissolution of Si, but the Pt:Si atomic ratio increases to 1:1 only
in accord with both EDS and XPS observations [228].
Dealloying of ternary Mg 90-x Cu x Y 10 alloys leads to pure Cu porous structure
since there is a sufficient difference in the nobility of the constituents and the surface
diffusion rate of Cu is high enough to expose the non-noble atoms to the surface
[179]. For another ternary alloy family, Pd 80-x Ni x P 20 , dealloying led to a Pd-rich
porous structure up to 89 at.% Pd even if the Ni molar percentage in the master
alloy was 60%, and a measurable amount of Ni and P remained in the dealloyed
material [235]. For a slightly more phosphorous-rich Pd–Ni–P alloy, Pd 32 Ni 48 P 30 ,
the final Pd enrichment upon electrochemical dealloying was between 70 and 80
at%, the larger value being achieved with more positive dealloying potential [239].
An interesting dataset in the latter study shows that the release of Ni and P is not
proportional during the dealloying process (see Fig. 2f in Ref. [239]). In the early
stage of the dealloying process, Ni is released first, which leads to a temporary
increase in the phosphorous mole fraction in the partially dealloyed material. Later,
the composition changes monotonously. Although such time-resolved composition
data are not available for the majority of the dealloying studies, it can be assumed
that such sequential dealloying may take place in various other alloys, which is
likely to be accompanied with a concomitant change of the surface composition of
the intermediate-stage ligaments. Partial dealloying of ternary materials under mild