Microwave dielectric properties of the (BaxSr1−x)TiO3 thin films on alumina substrate

Barium strontium titanate, (Ba_xSr_1?x)TiO₃ (BST) thin films have been prepared on alumina substrate by sol–gel technique. The X-ray patterns analysis indicated that the thin films are perovskite and polycrystalline structure. The interdigital electrode with 140 nm thickness Au/Ti was fabricated on the film with the finger length of 80 μm, width of 10 μm and gaps of 5 μm. The temperature dependence of dielectric constant of the BST thin films in the range from ?50 °C to 50 °C was measured at 1 MHz. The dielectric properties of the BST thin films were measured by HP 8510C vector network analyzer from 50 MHz to 20 GHz.     

Reduced leakage current,enhanced energy storage and dielectric properties in (Ce,Mn)-codoped Ba0.6Sr0.4TiO3 thin film

Ba_0.6Sr_0.4TiO₃, Ce-doped Ba_0.6Sr_0.4TiO₃, Mn-doped Ba_0.6Sr_0.4TiO₃, (Ce,Mn) co-doped Ba_0.6Sr_0.4TiO₃ (abbreviated as BST, BSTCe, BSTMn, BSTCeMn) thin films were deposited on LaNiO₃(LNO)/Si substrates. The effects of ion doping on the microstructure and electrical properties of BST-based thin film have been researched and discussed. The X-ray diffraction pattern shows that each sample has pure perovskite phase structure with high (l00) peaks. The microstructure of each film is quite dense with uniform size. Compared with pure BST, improved insulating properties can be found in ion-doped BST thin films. For all the films, Ohmic conduction, space charge limited conduction and interface-limited Fowler-Nordheim tunneling should be the main conduction mechanisms within different electric field regions. For the case of BSTCeMn thin film, it possesses enhanced energy storage performance with a recoverable energy storage density (18.01?J/cm³) and a energy storage efficiency (75.1%) under 2000?kV/cm. This can be closely related to the small remanent polarization value (P_r=?1.89 μC/cm²), large maximum polarization value (P_max=?28.08?μC/cm²) as well as big maximum electric field (2000?kV/cm). Also, it exhibits a large dielectric constant of 405 and a small dissipation factor of 0.075 at 500?kHz.     

A new family of sodium niobate-based dielectrics for electrical energy storage applications

Although tremendous achievements have been made in enhancing recoverable energy storage density (W_rec) of lead-free dielectric ceramics for electrical energy storage applications in recent years, these ceramics with high W_rec still have two disadvantages: complex chemical composition and difficult preparation process. In this work, we selected NaNbO₃ (NN)-based ceramics as base materials and used Bi₂O₃ as a sintering aid to reduce porosity and enhance dielectric breakdown strengths. Encouragingly, high W_rec, simple chemical composition and facile preparation process were simultaneously realized in 0.77NaNbO₃-0.23BaTiO₃ (0.77NN-0.23BT) ceramics. A large W_rec of 1.5 J cm^?3 at 175 kV cm^?1 and excellent thermal stability (variation of W_rec < 15% over the temperature range of 20?140 °C) were simultaneously achieved in 0.77NN-0.23BT ceramics. More importantly, this work could bring out the development of a series of NN-based ceramics for electrical energy storage in the future such as NN-ABO₃ (A = Ca and Sr; B = Ti and Zr).     

Improved energy storage performance of Bi(Mg0.5Ti0.5)O3 modified Ba0.55Sr0.45TiO3 lead-free ceramics for pulsed power capacitors

(1–x)Ba_0.55Sr_0.45TiO₃–xBi(Mg_0.5Ti_0.5)O₃ (x = 0, 0.08, 0.1, 0.12, 0.15, 0.2) ceramics were fabricated via a solid-state reaction route. The ultrahigh recoverable energy density (W_rec = 4.05 J cm^?3), efficiency (η = 78%), maximum polarization (P_max = 51.40 μC cm^?2), and high dielectric breakdown strength (BDS = 230 kV cm^?1) were achieved for the 0.9BST?0.1BMT ceramic. The fast discharge rate (t_0.9～0.14 μs), current density (C_D～637.02 A cm^?2), high power density (P_D～38.70 MW cm^?3), good temperature stability (20?180 °C), frequency stability (10?500 Hz), and fatigue endurance for cycling (10⁵) of 0.9BST?0.1BMT ceramic make it suitable for the development of energy-storage devices. The relaxor behavior with a high W_rec (3.06 J cm^?3) and η (93%) at BDS (220 kV cm^?1) was also achieved for the 0.8BST?0.2BMT ceramic. This study systematically investigates the correlation among the structural, dielectric, impedance, and energy storage properties of BMT-doped BST ceramics.     

Stronger B-site ionic disorder boosting enhanced dielectric energy-storage performance in BNT-based relaxor ferroelectric ceramics

Because of their possible applications in dielectric energy-storage capacitor devices, (Bi_0.5Na_0.5)TiO₃-based (BNT) relaxor ferroelectric (RFE) ceramics are feasible alternatives to lead-containing electroceramics. Good energy-storage performance (ESP), including high recoverable energy density (W_rec) and good energy discharge efficiency (η), is required to achieve device miniaturization and long device lifetimes. An advanced method was used to overcome the challenges of A-site ionic disordered RFE in achieving high inducible polarization and low hysteresis, with the former dictating a large W_rec and the latter dictating a high η. In this study, an ABO₃ perovskite-structured complex end-member Bi(Mg_2/3Nb_1/3)O₃ (BMN) was added to a 0.7Bi_0.5Na_0.4K_0.1TiO₃–0.3Ba_0.5Sr_0.5TiO₃ (0.7BNKT–0.3BST) matrix. The differences in the valence states and ionic radii of Mg²⁺, Ti⁴⁺, and Nb⁵⁺ increased the local electric field fluctuation, which contributed to the expanded dielectric relaxation properties. The combined substantial prevention of hysteresis and remanent polarization suggests high potential applicability for ESP. Finally, an enhancement in W_rec to 4.98 J/cm³ was achieved in 0.595BNKT–0.255BST–0.15BMN with an ultrahigh η of 97.3% in a medium-strength electric field of 300 kV/cm. The ESP also demonstrated good thermostability between 30 and 120 °C. Furthermore, the strategy used in this study to generate RFEs can serve as a guide for future research.     

Excellent energy storage performance of paraelectric Ba0.4Sr0.6TiO3 based ceramics through induction of polar nano-regions

Dielectric energy storage materials with congenitally high power densities and ultrafast discharge rates have been extensively studied for emergent applications. As a typical and traditional dielectric material, paraelectric Ba_0.4Sr_0.6TiO₃ (BST) ceramic exhibits a moderate dielectric constant (ε_r), low dielectric loss and slightly nonlinear P–E hysteresis. However, its energy storage density (W) is extremely low because of its low maximum polarisation (P_max) and weak breakdown strength (BDS). In this study, ferroelectric Na_0.5Bi_0.5TiO₃ (NBT) was introduced into paraelectric BST to enhance energy storage performance. The results show that the introduction of NBT induced polar nano-regions (PNRs) in the paraelectric matrix, resulting in a slim hysteresis loop with low remnant polarisation (P_r) and high P_max simultaneously. Furthermore, owing to a decrease in the oxygen vacancy concentration and an increase in the band gap energy, the BDS of the BST ceramic also significantly increased. As a consequence, a remarkable energy storage density (W_rec = 3.89 J/cm³) and a high energy storage efficiency (η = 83.8%) were realised in the 0.75Ba_0.4Sr_0.6TiO₃-0.25Bi_0.5Na_0.5TiO₃ (0.75BST–0.25NBT) ceramic under a practical electric field of 360 kV/cm. Moreover, the ceramic also exhibited an excellent current density (～1029.7 A/cm²) and ultrahigh power density (～128.7 MW/cm²). The attained energy storage performances indicate that the NBT-modified BST ceramics are promising materials for high energy storage capacitor applications field.     

Energy storage performance of (K,Na)NbO3 ferroelectric thin films with Mn-Ta and Mn-Ti co-doping

The lead-free KNN (K_0.5Na_0.5NbO₃) films were co-doped with Mn-Ta and Mn-Ti via the Sol-Gel method. With Mn doping, the leakage behavior in the KNN films is significantly weakened. Annealed at 700 °C, the KNNM film shows good crystallinity, dense morphology and better energy storage performance. With Mn-Ta and Mn-Ti co-doping, the KNNM4R (K_0.5Na_0.5Nb_0.96-xMn_0.04R_xO₃, R = Ta, Ti) films still show tetragonal phase and the smoothest surface morphology at 6mol% Ta or Ti content, with about 200 nm in thickness. Under the breakdown field strength of 2700 kV/cm and 2800 kV/cm, the (P_m-P_r) of Ta6 and Ti6 films are 20.7 μC/cm² and 22.4 μC/cm², the W_rec and η are up to 20.7 J/cm³, 23.9 J/cm³ and 57%, respectively.     

High-performance (Na0.5Bi0.5)(Ti0.97Fe0.03)O3-based heterostructure thin films for energy storage capacitors

This work presents a simple process to fabricate Na_0.5Bi_0.5(Ti_0.97Fe_0.03)O₃/Ba_(1–x)Sr_xTiO₃–based heterostructure thin films with a compositional graded sequence, and their energy storage properties are investigated systematically. A simulation technique is used to predict the experimental results. Interestingly, improved energy storage properties are obtained in the “up–graded” film, which is associated with the stress/strain. Furthermore, the “up–graded” film after aging with treating exhibits a higher breakdown strength (E_BD = 3176.4 kV/cm), recoverable energy storage density (W_rec = 67.18 J/cm³) and efficiency (η = 75.65%). This can be attributed to the ordered defect dipoles induced by aging with treating driving domain switching, which is favorable for high (P_m–P_r) and large W_rec. These results provide an approach and guidance to design thin film-based devices with optimal energy storage performance.     

Thickness effects on microwave properties of (Ba,Sr)TiO3 films for frequency agile technologies

We fabricated (Ba_0.6Sr_0.4)TiO₃ (BST) thin films of various thicknesses on sapphire (−1 1 2 0) substrates using metal-organic decomposition method. These films showed grain growth from 160 to 650 nm with an increase in the thickness from 90 to 1050 nm. At microwave frequencies, the measured capacitances of the planar capacitors decreased with the film thickness because the electro-magnetic field propagates across high permittivity BST films to the low permittivity sapphire substrate. However, we found that the BST-thin film permittivity remained large up to 90 nm thick, based on electro-magnetic field analysis using the finite element method. On the other hand, the BST thin film tunability decreased with the film thickness.     

Thickness dependence of microstructure,dielectric and leakage properties of BaSn0.15Ti0.85O3 thin films

The BaSn_0.15Ti_0.85O₃ (BTS) thin films are prepared on Pt-Si substrates with thickness ranging from ~ 60?nm to ~ 380?nm by radio frequency magnetron sputtering. The effects of thickness on microstructure, surface morphologies and dielectric properties of thin films are investigated. The thickness dependence of dielectric constant is explained based on the series capacitor model that the BTS thin film is consisted by a BTS bulk layer and an interfacial layer (dead layer) between the BTS and bottom electrode. The thin films with thickness of 260?nm give the largest figure of merit of 76.9@100?kHz, while the tunability and leakage current density are 64.6% and 7.46?×?10^?7 A/cm² at 400?kV/cm, respectively.     

Relaxor antiferroelectric-like characteristic boosting enhanced energy storage performance in eco-friendly (Bi0.5Na0.5)TiO3-based ceramics

Ideal relaxor antiferroelectrics (RAFEs) have high field-induced polarization, low remnant polarization and very slim hysteresis, which can generate high recoverable energy storage W_rec and high energy storage efficiency η, thus attracting much attention for energy storage applications. True RAFEs, on the other hand, are extremely rare, and the majority of them contain environmentally hazardous lead. In this work, we use a viscous polymer rolling process to synthesize a novel and eco-friendly 0.65Bi_0.5Na_0.4K_0.1TiO₃-0.35[2/3SrTiO₃-1/3Bi(Mg_2/3Nb_1/3)O₃] (BNKT-ST-BMN) dielectric material, which possesses a very typical RAFE-like characteristic. As a result, this material has a high W_rec of 4.43 J/cm³ and a η of 86% at an electric felid of 290 kV/cm, as well as a high thermal stability of W_rec (>3 J/cm³) over a wide range of 30–140 °C at 250 kV/cm. Our findings suggest that the BNKT-ST-BMN material could be a potential candidate for use in energy storage pulse capacitors.     

Enhanced piezoelectricity and energy storage performances of Fe-doped BNT–BKT–ST thin films

Fe-doping is an effective way to improve physical performances of piezoelectric and ferroelectric materials. Under such circumstances, x mol% (x?=?0.0, 1.0, 1.5, 2.5) Fe-doped 0.72Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃–0.10SrTiO₃ (BNT–BKT–ST–xFe) thin films were prepared by sol-gel method and the relationships between the content of Fe and electromechanical properties of the films were studied. The BNT–BKT–ST–1.0Fe thin films exhibit the best electromechanical properties, whose S_max/E_max, W_rec, η, P_max, P_rem and ε_r of are 68.00?pm/V, 20.34?J/cm³, 65.17%, 71.5?μC/cm², 14.8?μC/cm², 868 respectively. These results indicate that BNT–BKT–ST–1.0Fe thin films are promising for applications for advanced piezoelectric materials and capacitors with high energy-storage density.     

Fabrication and Dielectric Properties of Ba0.63Sr0.37TiO3 Thin Films on SiC Substrates

Ba_0.63Sr_0.37TiO₃ (BST) films were first deposited on SiC substrates with LNO bottom electrodes by magnetron sputtering. The BST/LNO/SiC thin films exhibit high dielectric tunability and low dielectric loss while maintaining excellent temperature coefficient of dielectric constant in the temperature range between 250 and 350 K. We also investigated the effect of film thickness on the dielectric properties. BST(430 nm)/LNO/SiC film has higher tunability (68.09% @700 kV/cm), lower loss tangent (tanδ = 0.00987) and quite a bit higher figure of merit (FOM = 68.99) as compared with that of BST(300 nm)/LNO/SiC film. Our results demonstrate that combining ferroelectric BST films with SiC substrates is very promising for the development of tunable devices over a large temperature range.     

Simultaneously achieving high energy storage density and efficiency under low electric field in BiFeO3-based lead-free relaxor ferroelectric ceramics

BiFeO₃-BaTiO₃-based relaxor ferroelectric ceramic has attracted increasing attention for energy storage applications. However, simultaneously achieving high recoverable energy storage density (W_rec) and efficiency (η) under low electric field has been a longstanding drawback for their practical applications. Herein, a novel relaxor ferroelectric material was designed by introducing (Sr_0.7Bi_0.2)TiO₃ (SBT) into the composition 0.67BiFeO₃-0.33BaTiO₃ (BF-BT-xSBT). A large W_rec of ∼2.40 J/cm³ and a high η of ∼90.4 % were simultaneously realized under a low electric field of 180 kV/cm, which is superior to that of most previously reported lead-free ceramics. Moreover, moderate temperature endurance and excellent frequency stability were also obtained. More importantly, this ceramic has a large discharge current density (∼289.18 A/cm²), a discharge power density (∼14.46 MW/cm³) and short discharge time (<0.25 μs). These results not only demonstrate superior potential in BF-BT-SBT ceramics, but also offer a new design to tune the energy storage performance of lead-free relaxor ferroelectric ceramics.     

Enhanced ferroelectric properties of La-substituted BiFeO3 thin films on LaSrCoO3/Pt/TiO2/SiO2/Si (1 0 0) substrates prepared by the soft chemical method

Bi_0.85La_0.15FeO₃ (BLFO015) thin films were deposited by the polymeric precursor solution on La_0.5Sr_0.5CoO₃ substrates. For comparison, the films were also deposited on Pt bottom electrode. X-ray diffraction data confirmed the substitutions of La into the Bi site with the elimination of all secondary phases under a substitution ratio x = 15% at a temperature of 500 °C for 2 h. A substantial increase in the remnant polarization (P_r) with La_0.5Sr_0.5CoO₃ bottom electrode (P_r ≈ 34 μC/cm²) after a drive voltage of 9 V was observed when compared with the same film deposited on Pt substrate. The leakage current behavior at room temperature decreased from 10^?8 (Pt) to 10^?10 A/cm² on (La_0.5Sr_0.5CoO₃) electrode under a voltage of 5 V. The fatigue resistance of the Au/BLFO015/LSCO/Pt/TiO₂/SiO₂/Si (1 0 0) capacitors with a thickness of 280 nm exhibited no degradation after 1 × 10⁸ switching cycles at a frequency of 1 MHz.     

Preparation of La1?x Sr x CoO3 Oxide Electrode Material and its Influence on the Structure and Dielectric Performance of the Supported Ba1?x Sr x TiO3 Thin Films

Using the sol–gel method, La_1−x Sr _x CoO₃ (LSCO) electrode films were first fabricated on the Si (100) substrates, followed by the growth of Ba_1−x Sr _x TiO₃ (BST) thin films on the LSCO electrode film. The crystal structure and surface morphology of these films were characterized by XRD and SEM. The effects of Sr-doping and annealing temperature on the structure and electric resistivity of the LSCO films and the dielectric properties of the BST films were studied. Results show that the La_0.5Sr_0.5CoO₃ electrode annealed at 750 °C has the lowest electric resistivity, 1.1 × 10⁻³Ω cm. The relative permittivity of the La_0.5Sr_0.5CoO₃-supported BST films first increases and then decreases with Sr-doping. The relative permittivity of the BST film decreases while the dielectric loss increases with frequency. Among the studied BST films, Ba_0.5Sr_0.5TiO₃ has the largest relative permittivity and the smallest dielectric loss (95 and 0.1, respectively) when the frequency is 1 kHz.     

Ultraviolet pulsed laser crystallization of Ba0.8Sr0.2TiO3 films on LaNiO3-coated silicon substrates

In this work, Ba_0.8Sr_0.2TiO₃ (BST) films on LaNiO₃-buffered SiO₂/Si (LNO/SiO₂/Si) substrates were crystallized by pulsed laser irradiation. Solution-derived amorphous barium–strontium–titanate precursor layers were crystallized with a KrF excimer laser in oxygen ambient at fluences ranging from 50 to 75 mJ cm⁻². With the substrate temperature set to 500 °C, the number of pulses and film thickness were varied until high-quality crystallinity could be achieved. It was found that films with a thickness of 40 nm are fully crystallized with a uniaxial {00l} orientation which is predetermined by the LaNiO₃ orientation. On the other hand, for 160 nm thick films, crystallization was observed after 12,000 pulses in the 70 nm close to the surface, while the rest of the film remained amorphous. The large temperature difference between the film surface and interface due to the low thermal conductivity of the amorphous BST is suggested as the origin of this behavior. Films thicker than 80 nm cracked on crystallization due to the stress caused by the different thermal expansion coefficients of film and substrate, as well as the large temperature variations within the BST film.     

High energy storage performance in BaTiO3-based lead-free relaxors via multi-dimensional collaborative design

The application of advanced pulse power capacitors strongly depends on the fabrication of high-performance energy storage ceramics. However, the low recoverable energy storage density (W_rec) and energy efficiency (η) become the key links limiting the development of energy storage capacitors. In this work, a high W_rec of ～5.57 J cm^?3 and a large η of ～85.6% are simultaneously realized in BaTiO₃-based relaxor ceramics via multi-dimensional collaborative design, which are mainly attributed to the ferroelectric-relaxor transition, enhanced polarization, improved breakdown electric field, and delayed polarization saturation. Furthermore, the excellent temperature stability (ΔW_rec < ± 5%, 25–140 °C), frequency stability (ΔW_rec < ± 5%, 1–200 Hz), and outstanding charge/discharge performance (current density ～1583.3 A cm^?2, power density ～190.0 MW cm^?3) with good thermal stability are also achieved. It is encouraging that this work demonstrates that multi-dimensional collaborative design is a good strategy to develop new high-performance lead-free materials used in advanced dielectric capacitors.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Optimized energy storage performances in morphotropic phase boundary (Na0.8K0.2)0.5Bi0.5TiO3-based lead-free ferroelectric thin films

As microelectronic devices move toward integration and miniaturization, the thin film capacitors with high energy density and charge/discharge efficiency have attracted immense interests in modern electrical energy storage systems. Despite morphotropic phase boundary (Na_0.8K_0.2)_0.5Bi_0.5TiO₃-based lead-free materials with outstanding ferroelectric and piezoelectric properties, while large ferroelectric hysteresis with high remanent polarization (P_r) hinder to improve energy storage capability. Here, novel lead-free relaxor-ferroelectric (RFE) thin film capacitors with high energy density are successfully prepared in (1-x) (Na_0.8K_0.2)_0.5Bi_0.5TiO₃-xBa_0.3Sr_0.7TiO₃ [(1-x)NKBT-xBST] systems. Introducing BST into the NKBT systems is expected to reduce remanent polarization (P_r) on account of coupling reestablishment of the polar nano-regions (PNRs) and improving the relaxation behavior. As a result, 0.6NKBT-0.4BST thin film exhibits high energy density (W_rec ～ 54.79 J/cm³) together with satisfactory efficiency (η ～ 76.42%) at 3846 kV/cm. The stable energy storage performances are achieved within the scope of operating temperatures (20–200 °C) and fatigue cycles (1-10⁷ cycles). This work furnishes a new technological way for the design of high energy-density thin film capacitors.