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Xu et al. Microstructures 2023;3:2023034 https://dx.doi.org/10.20517/microstructures.2023.19 Page 9 of 13

To understand further the intrinsic reason for the high E of the doped SMZ sample, impedance tests were
b
conducted on the NN-SMZ sample. Because the impedance test pattern curve was semicircular and there
was almost no difference between the grain boundary and the grain response, a set of RQC structures was
used to fit the resistance of the NN-SMZ sample [33,34] . The test and fitting results are shown in Figure 9A-D.
In Supplementary Figure 5, the curve of the imaginary part changing with frequency has only a single peak,
which also proves this point. Supplementary Figure 6 shows the relationship between the AC conductivity
of the NN-SMZ sample and the frequency change. The incorporation of SMZ reduced the conductivity of
the sample, which was beneficial for obtaining a higher breakdown field strength.

Figure 9E shows that under the test condition of 560 °C, the resistance of the sample increased gradually
with the incorporation of SMZ. When x = 0.15, the resistance of the sample reached a maximum value of
70,515 Ω. According to the activation energy calculation formula, the activation energy of each component
is shown in Figure 9G. The activation energy of the sample increases with increasing SMZ doping. In
general, the higher the activation energy of the sample, the larger its energy barrier, the more energy it needs
to break down, and the higher its breakdown field strength [34,35] . To study the bandgap of the NN-SMZ
ceramics further, ultraviolet test results of the samples are shown in Figure 9F. The Tauc equation was used
to estimate the E value . The E values of the NN-SMZ samples were 3.31, 3.37, 3.42, and 3.45 eV, which
[36]
g
g
also indicated that the bandgap of the samples gradually increased . This may have been caused by the
[37]
incorporation of Sm O and ZrO , which had large bandgaps. In summary, due to its significantly smaller
2
2
3
grain size, the NN-SMZ sample obtained a higher breakdown field strength than the pure NN sample.
Although the average grain sizes of each component increased with the increase of SMZ doping amount, the
resistance and conductivity of each component increased. In terms of intrinsic factors, the band gap of the
sample was increased, which helped the sample to obtain a high E .
b
The charge-discharge test of the sample was helpful for exploring its actual energy storage capacity. The
data for the NN-SMZ samples under different electric fields at room temperature are shown in
Figure 10A and B. In the environment of 2-18 kV, the I of the sample gradually increased to 54.9 A, the
max
current density of the sample increased from 172.7 A/cm to 777.1 A/cm , and the power density of the
2
2
sample increased to 69.93 MW/cm 3[35,38] . The charge-discharge test of samples was performed in the range of
30 °C-100 °C under a field strength of 16 kV/mm. The results are shown in Supplementary Figure 7. The I
max
value of the sample was 48.93 ± 3 A, the current power was 692 ± 40 A/cm , and the change rate of the test
2
results was within 5%, indicating that NN-SMZ can maintain stable performance under working
environments with different temperatures [38,39] .

To judge the feasibility of the practical application of the NN-SMZ ceramics, a 210-Ω resistor was connected
to perform an overdamped charge-discharge test on the sample. As shown in Figure 10C and D and
Supplementary Figure 8A, when the applied electric field of the 0.08 SMZ sample gradually increased to
18 kV/cm at 30 °C, the current increased to 11.5 A, the discharge energy density W increased to 0.33 J/cm ,
3
d
and an ultrafast stable discharge time (t ) of 57 ± 5 ns was obtained. As shown in
0.9
Supplementary Figure 8B-D, when the temperature of the 0.08 SMZ sample increased from 30 °C to 100 °C
under the condition of 16 kV/cm, the current remained at a stable value of 10.185 A ± 1%, and the value of
t was stable at 55 ± 3 ns. This shows that 0.08 SMZ can maintain stable performance under working
0.9
conditions with different temperatures and meet the needs of actual pulse capacitors [40-42] .

CONCLUSION
In this study, NN-SMZ was prepared by solid-phase sintering. The crystal phase of NN-SMZ gradually
changed from the orthorhombic phase (Pnma) to the pseudocubic phase (Pm3m) with increasing SMZ