Open Access Research Article Just Accepted
Dielectric ultracapacitors based on columnar nano-grained ferroelectric oxide films with gradient phases along the growth direction
Journal of Advanced Ceramics
Available online: 04 June 2024
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In this work, dielectric ultracapacitors were designed and fabricated using the combination of the phase boundary and nanograin strategies. Such ultracapacitors are based on submicron-thick Ba(Zr0.2Ti0.8)O3 ferroelectric films sputter-deposited on Si at 500 ℃. With a composition near a PPB, a compressive strain and a high nucleation rate due to the lowered deposition temperature, these films exhibit a columnar nanograined microstructure with gradient phases along the growth direction. Such a micro-structure presents three-dimensional polarization inhomogeneities on a nanoscale, thereby significantly delays the saturation of the overall electric polarization. Consequently, a pseudo-linear, ultra-slim polarization-electric field hysteresis loop was obtained, featuring a high maximum applicable electric field (~5.7 MV/cm), a low remnant polarization (~5.2 μC/cm2) and a high maximum polarization (~92.1 μC/cm2). Such a P-E loop corresponds to a high recyclable energy density (Wrec~208 J/cm3) and charge-discharge efficiency (~88%). An in-depth electron microscopy study revealed that the gradient ferroelectric phases consist of tetragonal (T) and rhombohedral (R) polymorphs along the growth direction of the film. The T-rich phase is abundant near the bottom of the film and gradually transformed into the R-rich one near the surface. These films also exhibited a high Curie temperature of ~460 ℃ and a stable capacitive energy storage up to 200 ℃. These results suggest a feasible pathway for design and fabrication of high-performance dielectric film capacitors.

Open Access Research Article Issue
Pushing the high-k scalability limit with a superparaelectric gate layer
Journal of Advanced Ceramics 2024, 13 (4): 539-547
Published: 30 April 2024
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To meet the expectation set by Moore’s law on transistors, the search for thickness-scalable high dielectric constant (k) gate layers has become an emergent research frontier. Previous investigations have failed to solve the “polarizability–scalability–insulation robustness” trilemma. In this work, we show that this trilemma can be solved by using a gate layer of a high k ferroelectric oxide in its superparaelectric (SPE) state. In the SPE, its polar order becomes local and is dispersed in an amorphous matrix with a crystalline size down to a few nanometers, leading to an excellent dimensional scalability and a good field-stability of the k value. As an example, a stable high k value (37±3) is shown in ultrathin SPE films of (Ba0.95,Sr0.05)(Zr0.2,Ti0.8)O3 deposited on LaNiO3-buffered Pt/Ti/SiO2/(100)Si down to a 4 nm thickness, leading to a small equivalent oxide thickness of ~0.46 nm. The aforementioned characteristic microstructure endows the SPE film a high breakdown strength (~10.5 MV·cm−1 for the 4 nm film), and hence ensures a low leakage current for the operation of the complementary metal oxide semiconductor (CMOS) gate. Lastly, a high electrical fatigue resistance is displayed by the SPE films. These results reveal a great potential of superparaelectric materials as gate dielectrics in the next-generation microelectronics.

Open Access Research Article Issue
Simultaneously achieving high energy density and responsivity in submicron BaTiO3 film capacitors integrated on Si
Journal of Advanced Ceramics 2024, 13 (2): 198-206
Published: 08 March 2024
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In the research field of energy storage dielectrics, the “responsivity” parameter, defined as the recyclable/recoverable energy density per unit electric field, has become critically important for a comprehensive evaluation of the energy storage capability of a dielectric. In this work, high recyclable energy density and responsivity, i.e., Wrec = 161.1 J·cm–3 and ξ = 373.8 J·(kV·m2)–1, have been simultaneously achieved in a prototype perovskite dielectric, BaTiO3, which is integrated on Si at 500 ℃ in the form of a submicron thick film. This ferroelectric film features a multi-scale polar structure consisting of ferroelectric grains with different orientations and inner-grain ferroelastic domains. A LaNiO3 buffer layer is used to induce a {001} textured, columnar nanograin microstructure, while an elevated deposition temperature promotes lateral growth of the nanograins (in-plane diameter increases from ~10–20 nm at lower temperatures to ~30 nm). These preferably oriented and periodically regulated nanograins have resulted in a small remnant polarization and a delayed polarization saturation in the film’s PE behavior, leading to a high recyclable energy density. Meanwhile, an improved polarizability/dielectric constant of the BaTiO3 film has produced a much larger maximum polarization than those deposited at lower temperatures at the same electric field, leading to a record-breaking responsivity for this simple perovskite.

Open Access Research Article Issue
Synergically improved energy storage performance and stability in sol–gel processed BaTiO3/(Pb,La,Ca)TiO3/BaTiO3 tri-layer films with a crystalline engineered sandwich structure
Journal of Advanced Ceramics 2023, 12 (12): 2300-2314
Published: 02 January 2024
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Achieving an excellent energy storage performance, together with high cycling reliability, is desirable for expanding technological applications of ferroelectric dielectrics. However, in well-crystallized ferroelectric materials, the concomitant high polarizability and low polarization-saturation field have led to a square-shaped polarization–electric field loop, fatally impairing both recoverable energy density (Wrec) and efficiency (η). Nanocrystalline ferroelectric films with a macroscopically amorphous structure have shown an improved Wrec and η, but their much lower polarizability demands an extremely high electric field to achieve such performances, which is undesirable from an economic viewpoint. Here, we propose a strategy to boost the energy storage performances and stability of ferroelectric capacitors simultaneously by constructing a tri-layer film in which a well-crystallized ferroelectric layer was sandwiched by two pseudo-linear dielectric layers with a dominant amorphous structure. In sol–gel-derived BaTiO3/(Pb,La,Ca)TiO3/BaTiO3 (BTO/PLCT/BTO) tri-layer films, we show that the above design is realized via rapid thermal annealing which fully crystallized the middle PLCT layer while left the top/bottom BTO cap layers in a poor crystallization status. This sandwiched structure is endowed with an enhanced maximum polarization while a small remnant one and a much-delayed polarization saturation, which corresponds to large Wrec ≈ 80 J/cm3 and high η ≈ 86%. Furthermore, the film showed an outstanding cycling stability: its Wrec and η remain essentially unchanged after 109 electric cycles (ΔW/W < 4%, Δη/η < 2%). These good energy storage characteristics have proved the effectiveness of our proposed strategy, paving a way for the utilization of sandwiched films in applications of electric power systems and advanced pulsed-discharge devices.

Open Access Research Article Issue
Achieving a high energy storage density in Ag(Nb,Ta)O3 antiferroelectric films via nanograin engineering
Journal of Advanced Ceramics 2023, 12 (1): 196-206
Published: 19 December 2022
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Due to its lead-free composition and a unique double polarization hysteresis loop with a large maximum polarization (Pmax) and a small remnant polarization (Pr), AgNbO3-based antiferroelectrics (AFEs) have attracted extensive research interest for electric energy storage applications. However, a low dielectric breakdown field (Eb) limits an energy density and its further development. In this work, a highly efficient method was proposed to fabricate high-energy-density Ag(Nb,Ta)O3 capacitor films on Si substrates, using a two-step process combining radio frequency (RF)-magnetron sputtering at 450 ℃ and post-deposition rapid thermal annealing (RTA). The RTA process at 700 ℃ led to sufficient crystallization of nanograins in the film, hindering their lateral growth by employing short annealing time of 5 min. The obtained Ag(Nb,Ta)O3 films showed an average grain size (D) of ~14 nm (obtained by Debye–Scherrer formula) and a slender room temperature (RT) polarization–electric field (P–E) loop (Pr ≈ 3.8 μC·cm−2 and Pmax ≈ 38 μC·cm−2 under an electric field of ~3.3 MV·cm−1), the P–E loop corresponding to a high recoverable energy density (Wrec) of ~46.4 J·cm−3 and an energy efficiency (η) of ~80.3%. Additionally, by analyzing temperature-dependent dielectric property of the film, a significant downshift of the diffused phase transition temperature (TM2–M3) was revealed, which indicated the existence of a stable relaxor-like AFE phase near the RT. The downshift of the TM2–M3 could be attributed to a nanograin size and residual tensile strain of the film, and it led to excellent temperature stability (20–240 ℃) of the energy storage performance of the film. Our results indicate that the Ag(Nb,Ta)O3 film is a promising candidate for electrical energy storage applications.

Open Access Research paper Issue
Achieving excellent energy storage reliability and endurance via mechanical performance optimization strategy in engineered ceramics with core-shell grain structure
Journal of Materiomics 2022, 8 (3): 601-610
Published: 30 November 2021
Abstract Collect

Although dielectric ceramic capacitors possess attractive properties for high-power energy storage, their pronounced electrostriction effect and high brittleness are conducive to easy initiation and propagation of cracks that significantly deteriorate electrical reliability and lifetime of capacitors in practical applications. Herein, a new strategy for designing relaxor ferroelectric ceramics with K0.5Na0.5NbO3-core/SiO2-shell structured grains was proposed to simultaneously reduce the electric-field-induced strain and enhance the mechanical strength of the ceramics. The simulation and experiment declared that the bending strength and compression strength of the core-shell structured ceramic were shown to increase by more than 50% over those of the uncoated sample. Meanwhile, the electric-field-induced strain was reduced by almost half after adding the SiO2 coating. The suppressed electrical deformation and enhanced mechanical strength could alleviate the probability of generation of cracks and prevent their propagation, thus remarkably improving breakdown strength and fatigue endurance of the ceramics. As a result, an ultra-high breakdown strength of 425 kV cm−1 and excellent recoverable energy storage density (Wrec ~ 4.64 J cm-3) were achieved in the core-shell structured sample. More importantly, the unique structure could enhance the cycling stability of the ceramic (Wrec variation < ±2% after 105 cycles). Thus, mechanical performance optimization via grain structure engineering offers a new paradigm for improving electrical breakdown strength and fatigue endurance of dielectric ceramic capacitors.

Open Access Research Article Issue
Boosting energy storage performance of low-temperature sputtered CaBi2Nb2O9 thin film capacitors via rapid thermal annealing
Journal of Advanced Ceramics 2021, 10 (3): 627-635
Published: 05 March 2021
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CaBi2Nb2O9 thin film capacitors were fabricated on SrRuO3-buffered Pt(111)/Ti/Si(100) substrates by adopting a two-step fabrication process. This process combines a low-temperature sputtering deposition with a rapid thermal annealing (RTA) to inhibit the grain growth, for the purposes of delaying the polarization saturation and reducing the ferroelectric hysteresis. By using this method, CaBi2Nb2O9 thin films with uniformly distributed nanograins were obtained, which display a large recyclable energy density Wrec ≈ 69 J/cm3 and a high energy efficiency η ≈ 82.4%. A superior fatigue-resistance (negligible energy performance degradation after 109 charge–discharge cycles) and a good thermal stability (from –170 to 150 ℃) have also been achieved. This two-step method can be used to prepare other bismuth layer-structured ferroelectric film capacitors with enhanced energy storage performances.

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