60
2 Methods for Investigating Electro-Chemo-Mechanical …
the remaining pinhole is sealed with silicone grease. The upper end of the wire is fixed
to one arm of a balance, the other arm of which is loaded with standard weights. The
total length of the wire is 50 mm, the portion in contact with the electrolyte solution
being 30 mm. The strain ε is equal to the relative change in wire length
dl
l
, while the
stress σ is given by a force per cross-sectional area of the wire, i.e., σ =
F
A
=
mg
A
(F: force, m: mass, g: acceleration of gravity, and A: cross-sectional area). The
electrochemical cell is housed in a Faraday cage. The potential difference between
the Ag wire and the reference electrode is measured with a voltmeter with a high
input resistance of 10
9
. The applied force is sufficiently low to keep proportionality
between stress and strain (within a limit of elastic deformation). The change of open
circuit potential E ocp is determined as a function of σ under quasi-static strain by
load on-load off cycles. The value of
dE ocp
dσ
= 2.6 × 10
−11 V Pa
−1 was obtained from
the linear relation between E ocp and σ , which was converted to
dE ocp
dε
= 1.04 V
by using the isothermal elastic compliance s =
dε
dσ
= 2.5 × 10
−11 Pa
−1 at room
temperature for the Ag wire [38].
Dynamic Electro-Chemo-Mechanical Analysis (DECMA)
As an alternative method, potential changes during cyclic elastic deformation of a
metal thin film on a polymer substrate can be directly measured by using a lockin technique [40–42]. Figure 2.15 shows (a) the scheme of the experimental setup
for the potential variation during cyclic elastic deformation and (b) the top view of
the working (Au thin film) electrode used for the experiment [41]. A (111)-textured
Au thin-film (20 nm) electrode was prepared by DC magnetron sputtering onto the
top of a thin titanium adhesion layer (2 nm) on a thick polyimide (Kapton) sheet
(125 µm) with Poisson’s ratio of ν = 0.35. The electrode geometry was achieved
by sputtering through a shadow mask. The polyimide substrate with the Au film
electrode is attached to a fixed grip (right) and a mobile grip (left) which can be
cyclic displaced by a computer-controlled piezoactuator equipped with a calibrated
displacement sensor. The response of the actuator limits the frequency range of cyclic
strain to ω ≤ 100 Hz. The Au film facing down is wetted from below by a meniscus
of solution.
The dashed line in Fig. 2.15b represents the wetting boundary so that the
wetted region is larger than the circular electrode section, assuring constant wetted
Lagrangian area (constant number of surface metal atoms in contact with solution)
throughout the strain cycles. When the axial strain of
dl
l
(l: gauge length) is imposed
on the substrate, the area strain of the substrate is given by dε =
(1−ν)dl
l
. This strain
is precisely transferred to the Au electrode, which has been confirmed by in situ
diffraction under load [43]. The reference electrode is separated from the main body
of the electrochemical cell by a Luggin capillary. Au wire as the counter electrode
is set in a compartment separated from the main reservoir by a channel. The entire
equipment is housed in a stainless steel chamber with fittings of high-vacuum grade.
Before experiments, the chamber is flushed repeatedly with a high-purity argon gas
and then sealed in argon gas at atmospheric pressure.
2 Methods for Investigating Electro-Chemo-Mechanical …
the remaining pinhole is sealed with silicone grease. The upper end of the wire is fixed
to one arm of a balance, the other arm of which is loaded with standard weights. The
total length of the wire is 50 mm, the portion in contact with the electrolyte solution
being 30 mm. The strain ε is equal to the relative change in wire length
dl
l
, while the
stress σ is given by a force per cross-sectional area of the wire, i.e., σ =
F
A
=
mg
A
(F: force, m: mass, g: acceleration of gravity, and A: cross-sectional area). The
electrochemical cell is housed in a Faraday cage. The potential difference between
the Ag wire and the reference electrode is measured with a voltmeter with a high
input resistance of 10
9
. The applied force is sufficiently low to keep proportionality
between stress and strain (within a limit of elastic deformation). The change of open
circuit potential E ocp is determined as a function of σ under quasi-static strain by
load on-load off cycles. The value of
dE ocp
dσ
= 2.6 × 10
−11 V Pa
−1 was obtained from
the linear relation between E ocp and σ , which was converted to
dE ocp
dε
= 1.04 V
by using the isothermal elastic compliance s =
dε
dσ
= 2.5 × 10
−11 Pa
−1 at room
temperature for the Ag wire [38].
Dynamic Electro-Chemo-Mechanical Analysis (DECMA)
As an alternative method, potential changes during cyclic elastic deformation of a
metal thin film on a polymer substrate can be directly measured by using a lockin technique [40–42]. Figure 2.15 shows (a) the scheme of the experimental setup
for the potential variation during cyclic elastic deformation and (b) the top view of
the working (Au thin film) electrode used for the experiment [41]. A (111)-textured
Au thin-film (20 nm) electrode was prepared by DC magnetron sputtering onto the
top of a thin titanium adhesion layer (2 nm) on a thick polyimide (Kapton) sheet
(125 µm) with Poisson’s ratio of ν = 0.35. The electrode geometry was achieved
by sputtering through a shadow mask. The polyimide substrate with the Au film
electrode is attached to a fixed grip (right) and a mobile grip (left) which can be
cyclic displaced by a computer-controlled piezoactuator equipped with a calibrated
displacement sensor. The response of the actuator limits the frequency range of cyclic
strain to ω ≤ 100 Hz. The Au film facing down is wetted from below by a meniscus
of solution.
The dashed line in Fig. 2.15b represents the wetting boundary so that the
wetted region is larger than the circular electrode section, assuring constant wetted
Lagrangian area (constant number of surface metal atoms in contact with solution)
throughout the strain cycles. When the axial strain of
dl
l
(l: gauge length) is imposed
on the substrate, the area strain of the substrate is given by dε =
(1−ν)dl
l
. This strain
is precisely transferred to the Au electrode, which has been confirmed by in situ
diffraction under load [43]. The reference electrode is separated from the main body
of the electrochemical cell by a Luggin capillary. Au wire as the counter electrode
is set in a compartment separated from the main reservoir by a channel. The entire
equipment is housed in a stainless steel chamber with fittings of high-vacuum grade.
Before experiments, the chamber is flushed repeatedly with a high-purity argon gas
and then sealed in argon gas at atmospheric pressure.