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Ouyang et al. Microstructures 2023;3:2023027 https://dx.doi.org/10.20517/microstructures.2023.22 Page 7 of 10

“polarizability”, i.e., early saturation of the electric polarization in comparison to that of the low-
temperature deposited BaTiO films.
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In Figure 5A, we present typical polarization-electric field (P-E) hysteresis loops of the LaNiO buffered
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BaTiO film at successively increasing voltages of 100 V, 120 V, and 160 V. The P-E loops did not show
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signs of saturation until under the largest applied voltage of 160 V, at which the applied electric field
approached 314 MV/m and the electric polarization reached ~75 C/cm . The recyclable energy densities
2
W as functions of the applied electric field were computed via the integration of the P-E loops
rec
( ) . The relative dielectric permittivity , where EdP is the total
[13]
stored electric energy under field E, was obtained from monopolar P-E loops (not shown here). Figure 5B
[21]
presents the W and ε values of the LaNiO-buffed BaTiO along with those of a BaTiO ceramic . Under
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r
3
rec
an applied maximum field of ~314 MV/m, the electric energy density W of the buffered BaTiO film
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rec
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reached ~81 J/cm , and the relative dielectric permittivity ε ranged between 470 (@low field) and ~215
r
(@low field). In contrast, the maximum electric energy density W of the BaTiO ceramic, which has the
rec
3
largest reported bulk dielectric strength , is only ~1.8 J/cm . The W -E curves of the two materials overlap
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[21]
rec
fairly well at low electric fields (< ~50 MV/m), but the corresponding ε -E curves are quite different. In the
r
BaTiO ceramic, ε quickly drops to ~200 at E = 30 MV/m and remains constant as the field increases until it
r
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reaches the breakdown limit of ~90 MV/m. In contrast, in the BaTiO film, ε gradually decreases from ~300
3
r
at E = 30 MV/m to ~215 at E ~314 MV/m, indicating a much slower saturation of the electric polarization
compared to the bulk BaTiO . The grain size of BaTiO thin films is only several tens of nanometers, which
3
3
is much smaller than that of its ceramic counterpart (ranging from a few micrometers to tens of
micrometers). Due to this extremely small grain size, a full polarization alignment in BaTiO thin films
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becomes very difficult under an intermediate electric field (~10 MV/m). In bulk ceramics, this level of field
is usually sufficient to saturate the electric polarization. However, in the nanoscale grains of the BaTiO thin
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films, a field level of about one order of magnitude higher (~100 MV/cm) is required to align all accessible
polarization vectors, resulting in a slow electric polarization saturation. The delayed saturation of the
electric polarization enables the absorption of additional electrical energy and greatly extends the electric
breakdown limit. Consequently, our BaTiO films exhibit outstanding energy storage capacities under
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electric fields of several hundred MV/m.
Furthermore, the unbuffered, randomly oriented BaTiO films (on Pt/Ti/Si) showed a distinct energy
3
storage feature compared to the (001)-textured, LaNiO -buffered BaTiO films. Unlike the latter, which have
3
3
[13]
shown a thickness-scalable relative dielectric permittivity ε /energy density by maintaining an aspect-ratio
r
of in-plane diameter/grain length, the randomly oriented films demonstrate an increasing energy density
W and ε at a given low field with the film thickness [Figure 6A and B, Table 2]. These behaviors, as
rec
r
predicted by the microstructural analyses, can be attributed to the discontinuous columnar nanograin
morphology with mixed crystalline orientations and the growth of these nanograins with the film thickness.
It is worth noting that at a high voltage of 460 V, the 2,610 nm thick film showed a low dielectric loss (< 3%)
and a high electric polarization (> 75 C/cm ). This can be attributed to its large thickness (460 V
2
corresponding to a low field of ~117 MV/m) and good film quality/robust insulation.
CONCLUSIONS
In summary, the study investigates the energy storage capabilities of BaTiO thick films (~0.5 μm to
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~2.6 μm) deposited at 500 °C, with two types of engineered grain structures. The films with a (001)-textured,
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continuous columnar nanograin structure showed a high recyclable energy density W of ~81 J/cm at
rec
E = 314 MV/m. This represents a substantial improvement compared to films of the same type of films