In situ dynamic mechanical analysis

Performing the loading-unloading measurement in a mechanical testing machine allows one to obtain the Young’s modulus of a nanoporous material. However, at these exper-imental conditions, the nanoporous materials are always compressed to a large extent (section 3.5.2.1) and only a few values of the elastic modulus are obtained at different strains. A dynamic mechanical analyzer overcomes this problem. When a sample is clamped in the device, a static force is applied to ensure good contact between the sample and the pushrod during the measurement. A sinusoidal force or strain oscilla-tion with a defined frequency is applied as the input, and the output displacement or force is recorded, as illustrated in Figure 3.10.

d y n a m i c f o r c e

d i s p l a c e m e n t

p h a s e s h i f t : δ

t i m e

s t a t i c f o r c e

Figure 3.10. Schematics of dynamic mechanical analysis: The sinusoidal force and displace-ment with a phase shift of δ.

The applied stress as a function of time is described as:

σ(t) =σ₀sin(ωt) (3.8)

where σ0 is the applied maximum stress and ω defines the frequency.

The resulting output strain of a viscoelastic material is observed with a phase shift δ, and the strain is:

ε(t) =ε₀sin(ωt+δ) (3.9) where ε₀ is the maximum actuation strain.

According to Hookes law, at uniaxial loading conditions, the input stress and output strain are associated with dynamic Young’s modulus, E^∗(ω):

σ(t) =E^∗(ω)ε(t) (3.10)

where the dynamic Young’s modulus is given by:

E^∗(ω) =E^′(ω) +iE^′′(ω) (3.11) where E^′= (σ₀/ε₀)cos(δ)is the real part known as storage modulus which features the energy stored per cycle; whereas iE^′′ = (σ₀/ε₀)sin(δ) is the imaginary part defined as loss modulus that characterizes the energy dissipated.

In this work,in situ DMA experiments were carried out using a dynamic mechanical analyzer (DMA/SDTA861^e, METTLER TOLEDO) equipped with a three-electrode electrochemical cell (in Figure3.1c) as illustrated in Figure3.11a. The tested specimen was placed in the middle of the cell and compressed by the pushrod (Figure 3.11b).

The electrochemical reactions were carried out in 0.1 M HClO₄ at RT, with NPG or NPG/PPy as WE, carbon cloth as CE, and homemade Ag/AgCl as RE. The cyclic

a b

Figure 3.11. (a) In situ DMA setup consisting of the dynamic mechanic analyzer and three-electrode electrochemical cell filled with 0.1 M HClO4. (b) Pushrod and WE at a higher magnification. WE: working electrode (NPG or NPG/PPy specimen), RE: reference electrode (homemade Ag/AgCl), CE: counter electrode (carbon cloth). A gold plate is attached to the pushrod for electrical contact. The dynamic and static forces passed via the pushrod and were applied to WE. Sample length change was detected by the displacement sensor marked in (a). The white scale bars at the lower right of (a) and (b) indicate 1 cm.

voltammetry was applied by the potentiostat. In a typical experiment, a dynamic force with a peak-to-peak amplitude of 0.5 N and a frequency of 1 Hz was applied. In order to keep good contact between DMA pushrod and working sample a, 5 N static force was imposed. When measuring the actuation under loads, a series of loads ranging from 1 to 18 N were exerted with other parameters unvaried.

Results

4.1 Microstructure characterization of NPG

Samples of mm-sized nanoporous gold (NPG) were successfully fabricated by mean of electrochemical dealloying of Ag₇₅Au₂₅under 1.265 V (vs. standard hydrogen electrode, SHE) and the subsequent 15 cycles electrochemical reduction in the potential range of 0.015 V∼1.515 V. As shown in Figure4.1a, a smooth fracture surface of the central part of a typical specimen suggests that no cracks were formed. As marked in Figure 4.1a, a series of sites from the outer to the middle of the fracture surface were selected to inspect the chemical composition of NPG. Energy dispersive X-ray spectroscopy (EDX) analysis was carried out on those spots and a typical EDX spectrum of the central part of the sample is shown in Figure4.1c. From the spectrum one can see that Ag cannot be completely removed by dealloying, which is in accordance with previous results [26, 42, 211]. The composition analysis reveals a residual Ag concentration of

∼ 2.7 at.%.

A typical microstructure of NPG is shown in Figure4.1b. A bicontinuous network is constructed by plenty of nanoscale ligaments with the mean characteristic width of ligaments L_D = 27 ± 4 nm. In this work, in situ mechanical experiments were performed on NPG/PPy electroactuators with the remaining pore space filled with electrolyte which supplies a pathway for ionic diffusion and conduction. The pore space in NPG network should be large enough for the accommodation of PPy film deposition on gold ligaments and the electrolyte flow in the remaining pores. Therefore, the initial ligament-pore network in NPG should be coarsened. Annealing of NPG in the air is a facile way to grow ligaments and pores [26, 131]. Figure 4.2 depicts the microstructure of the annealed NPG. Clearly, the small ligaments have grown to the larger ones with the mean characteristic width L_D = 160 ± 40 nm. The enlarged nanopores have enough space for PPy deposition and the diffusion of the electrolyte.

The solid fraction, ϕ (estimated using ϕ = ρ_NPG/ρ_Au = m_NPG/V_NPG/ρ_Au, with ρ_NPG, ρ_Au, m_NPG, and V_NPG respectively representing density of NPG and the monolith Au, and the measured mass and outer volume of NPG), of the annealed NPG is 0.31 ± 0.009.

41

a b

c

Figure 4.1. Microstructure and composition characterization of NPG. (a) Scanning elec-tron microscopy (SEM) picture of the fracture surface of NPG fabricated by electrochemical dealloying and electrochemical reduction. (b) SEM image showing the microstructure in the area (red mark in a) at the larger magnification, showing the diameter of the ligaments L_D=27±4 nm. (c) A representative energy dispersive X-ray (EDX) spectroscopy spectrum showing the composition of the central part of the sample shown in (a).

Figure 4.2. SEM picture showing the microstructure of the annealed NPG obtained by annealing at 300 ^○C for 30 min in the air. The mean ligaments diameter L_D=160±40 nm.