Nanotechnology 30 (2019) 505303
L Zhang et al
modified regions were calculated by analyzing the pixels with
graphics processing software.
Figure 1(n) presents the SEM images of the entire sin-
tered Ag layer when d=20 μm, D=90 μm, and
T=120 °C. Ag NPs could be sintered into a highly con-
tinuous Ag layer without other modifications because the
sintering temperature was considerably higher than the pre-
vious treatment. Therefore, the B-type region showed an Ag
layer with low porosity, as shown in figure 1(o).
3
. Results and discussion
3
.1. Characterization and structure of the electrodes
The electrical properties of the prepared electrodes are
shown in figure 1(p). The initial sheet resistance (R ) of all
0
The composition and structure of a flexible electrode sintered
at low temperatures are shown in figure 1. The high-temp-
erature-resistant paper (figure 1(a)) was used as the electrode
substrate; its cross-sectional structure is shown in figure 1(b).
The top layer and the middle layer mainly consisted of carbon
and titanium, respectively, and the bottom layer consisted of
polyimide. The ignition point of this layered structure reached
electrodes were lower than 7 Ω, suggesting the superior
conductivity of flexible electrodes in this study. However, the
size of the patterns largely affected the resistance. Regardless
of the sintering temperature, the sheet resistance values were
consistently the lowest, <1 Ω, when d=20 μm, D=70 μm,
and 90 μm. Even though the sintering temperature was only
60 °C, the resistance was as low as 0.274 Ω. By contrast, the
3
00 °C. The surface of the paper was ablated with a laser in
values of the resistance were highest at any sintering temp-
erature when d=10 μm, D=50 μm, and 70 μm. When
T=60 °C, the highest initial resistance of the electrodes—
that is, 6.745 Ω—was achieved. For the electrode with an
unmodified substrate, the Ag particles could only be sintered
at 120 °C, and the electrical resistance was higher than that of
all electrodes with the modified substrates. These results
indicated that the electrode could be sintered at a low temp-
erature with the best conductivity when d=20 μm and
D=70 μm or 90 μm.
A small number of patterns and unmodified regions were
not joined with the Ag NPs and thus could not contribute to
the conductivity. Therefore, the percentage of the A region in
all patterns and the percentage of the B-type region in all
unmodified regions were valuable parameters for measuring
the electrical conductivity of the electrodes. The resistance of
the electrodes is shown in figure 1(q) when d=10 μm and
D=70 μm. As the sintering temperature decreased, the
percentages of the A and B regions decreased monotonically.
At the same temperature, the percentage of the A region was
always higher than that of the B region, which proved that
laser modification significantly improved the sintering per-
formance. The resistance of the electrodes when T=120 °C
accordance with the preset points array, as shown in
figure 1(c).
The SEM image of the modified paper are given in
figure 1(d), and these dark patterns were the ablative area. The
heat generated by the laser causes the temperature at the array
position to rise at an extremely high rate, resulting in the
oxidation of surface components. The conductive ink was
then coated on the modified substrates, as shown in
figure 1(e). The SEM image of the Ag NPs in the conductive
ink is presented in figure 1(f). The diameter of Ag NPs was
∼50 nm, and many of the gaps between these particles were
smaller than 10 nm. After coating, the sample was stored in a
drying oven (figure 1(g)), and the residual ethylene glycol
completely evaporated. These Ag NPs were sintered together,
and a flexible electrode was obtained.
The optical image of the electrode is shown in
figure 1(h), with the sintered Ag layer as the central square
area. Notably, the method used during flexible property
testing was folding, indicating that the deformation angle of
the electrode was 180°, as shown in figure 1(i). High-relia-
bility joining between the Ag layer and the substrate ensured
that the electrode could withstand the rigorous folding test.
Sintering and joining Ag NPs showed different states in is shown in figure 1(r). For all pattern sizes, the percentage of
the modified region (dark patterns) and the unmodified the A region was always higher than that of B. The increase in
region. Not all dark patterns resulted in reliable joining with pattern diameter contributed more to Ag NP sintering.
the Ag layer. The same occurrence was true for the unmo- Therefore, the percentage of the A region with a diameter of
dified region. The patterns that joined reliably with the well- 20 μm was always higher than that with a diameter of 10 μm.
sintered Ag layer were defined as the A-type region, and the
unmodified regions that joined reliably with the well-sintered
Ag NPs were defined as the B-type region, as shown in
figure 1(j).
3
.2. Folding test
To evaluate their flexibility, electrodes with patterns of
Figure 1(l) presents the SEM images of the entire sintered varying sizes and sintering temperatures were subjected to
Ag layer with the diameter of the dark patterns (d) equal to extreme folding cycle test, as shown in figure 2. Figure 2(a)
2
0 μm, the central distance of two round patterns (D) equal to shows the ratio of the sheet resistance (R ) of the electrodes to
s
9
0 μm, and the sintering temperature (T) set to 60 °C. For the R0 during the folding cycles when d=10 μm and
A-type region (figure 1(k)), Ag NPs were sintered together D=30 μm. The resistance of the electrode sintered at 60 °C
and joined with the particles in the modified substrates, increased rapidly during the initial 100 folding cycles, and
thereby achieving the desired conductivity and reliable join- R /R reached 7.092. In the subsequent 1900 cycles, the ratio
s
0
ing. For the B-type region (figure 1(m)), the Ag particles were fluctuated within the 2.093–3.925 range and reached 3.062
sintered into a weak network. Such a slightly continuous Ag until failure occurred. For the electrode sintered at 90 °C, Rs
layer damaged the electrical conductivity.
increased rapidly during the initial 50 folding cycles, and
4