288
8 Porous Nanostructured Materials
dissolution, especially those containing the so-called S phase of Al 2 CuMg composition [222–224]. The dealloying of these materials leads to a Cu-rich porous structure,
although exact composition has not been given in any of the relevant studies. The
Cu-rich porous structure can form from sputtered Al 2 CuMg layers [224] or isolated
grains of the same composition [222]. Nevertheless, the relatively high-rate dissolution of the intermetallic Al 2 CuMg grains and the formation of a Cu-rich porous
network is a major process in the corrosion of 2024-T3 Al alloys with <5 wt.% Cu
and <2 wt.% Mg content [223]. The dealloying studies of Al-based structural materials have no synthetic aspect for obtaining nanostructures (although the dealloyed
Cu-rich pore system fully complies such criteria), but are focused on the passivation
behaviour of alloys containing the intermetallic Al 2 CuMg phase and the reaction of
the Al alloys during the formation of conversion coatings.
Concerning the ternary systems containing noble metals, a number of studies were
published for the Ag–Au–Pt system [225–227]. The silver content of the ternary
master alloys varied between 65–80 at.%, and Pt was the minority component with
a concentration between 1 and 7 at.%. Pt was found to act as a stabilizing agent with
additional beneficial properties like the decrease of the ligament diameter (below
10 nm), the prevention of the fragmentation of the dealloyed material, the retention
of a part of the silver content of the dealloyed porous structure and the enhancement
of the catalytic properties of the dealloyed material. With Pt addition to the Ag–Au
system, the dealloying process could be performed in two steps with a mild heat
treatment in between [225]. With a two-step dealloying, a two-level porous system
could be created in which the already formed ligaments were retained and they hosted
a secondary pore system with much smaller pore size than that formed in the first
dealloying step. The residual silver content of the dealloyed pore system varied in a
wide range depending on the dealloying condition. With an increase of the Pt content
of the precursor mater alloy from 0 to 3 at.%, the residual silver content increased
form 28 to 48 at.% [227], while about 11 at.% was given for the two-step dealloying
process [225]. The accumulation of the Pt content of the dealloyed structure at the
surface of the ligaments was found, which contributed to the enhancement of the
catalytic properties of the ternary system.
8.4.4 Dealloying of Non-equilibrium Alloys
Already among the binary alloys discussed in Sect. 6.4.2, there were several
metastable specimens that were produced and/or processed by a non-equilibrium
method (e.g., mechanically alloyed and then spark-plasma-sintered Pd 20 Al 80 samples
[197]). In this chapter, a variety of alloys will be presented that can be manufactured
with non-equilibrium methods, hence widening the master alloy composition range
to be used for dealloying.
Concerning the preparation method of the metastable materials, co-sputtering is
a relatively easy method that was used for binary alloys, mostly for Pt–Si precursor
materials [228, 229]. The quenching of a well-homogenized binary alloy from a
8 Porous Nanostructured Materials
dissolution, especially those containing the so-called S phase of Al 2 CuMg composition [222–224]. The dealloying of these materials leads to a Cu-rich porous structure,
although exact composition has not been given in any of the relevant studies. The
Cu-rich porous structure can form from sputtered Al 2 CuMg layers [224] or isolated
grains of the same composition [222]. Nevertheless, the relatively high-rate dissolution of the intermetallic Al 2 CuMg grains and the formation of a Cu-rich porous
network is a major process in the corrosion of 2024-T3 Al alloys with <5 wt.% Cu
and <2 wt.% Mg content [223]. The dealloying studies of Al-based structural materials have no synthetic aspect for obtaining nanostructures (although the dealloyed
Cu-rich pore system fully complies such criteria), but are focused on the passivation
behaviour of alloys containing the intermetallic Al 2 CuMg phase and the reaction of
the Al alloys during the formation of conversion coatings.
Concerning the ternary systems containing noble metals, a number of studies were
published for the Ag–Au–Pt system [225–227]. The silver content of the ternary
master alloys varied between 65–80 at.%, and Pt was the minority component with
a concentration between 1 and 7 at.%. Pt was found to act as a stabilizing agent with
additional beneficial properties like the decrease of the ligament diameter (below
10 nm), the prevention of the fragmentation of the dealloyed material, the retention
of a part of the silver content of the dealloyed porous structure and the enhancement
of the catalytic properties of the dealloyed material. With Pt addition to the Ag–Au
system, the dealloying process could be performed in two steps with a mild heat
treatment in between [225]. With a two-step dealloying, a two-level porous system
could be created in which the already formed ligaments were retained and they hosted
a secondary pore system with much smaller pore size than that formed in the first
dealloying step. The residual silver content of the dealloyed pore system varied in a
wide range depending on the dealloying condition. With an increase of the Pt content
of the precursor mater alloy from 0 to 3 at.%, the residual silver content increased
form 28 to 48 at.% [227], while about 11 at.% was given for the two-step dealloying
process [225]. The accumulation of the Pt content of the dealloyed structure at the
surface of the ligaments was found, which contributed to the enhancement of the
catalytic properties of the ternary system.
8.4.4 Dealloying of Non-equilibrium Alloys
Already among the binary alloys discussed in Sect. 6.4.2, there were several
metastable specimens that were produced and/or processed by a non-equilibrium
method (e.g., mechanically alloyed and then spark-plasma-sintered Pd 20 Al 80 samples
[197]). In this chapter, a variety of alloys will be presented that can be manufactured
with non-equilibrium methods, hence widening the master alloy composition range
to be used for dealloying.
Concerning the preparation method of the metastable materials, co-sputtering is
a relatively easy method that was used for binary alloys, mostly for Pt–Si precursor
materials [228, 229]. The quenching of a well-homogenized binary alloy from a