High recoverable energy storage density in (1-x)Bi0.5(Na0.8K0.2)0.5TiO3-xSrZrO3 thin films prepared by a sol-gel method

Lead-free (1-x)Bi_0.5(Na_0.8K_0.2)_0.5TiO₃-xSrZrO₃ (abbreviated as BNKT-100xSZ) thin films were deposited on Pt(111)/Ti/SiO₂/Si using sol-gel/spin coating method. With the addition of SZ, the long-range ferroelectric order dominant in BNKT is disrupted, which boosts the ferroelectric relaxor behavior. Consequently, a high recoverable energy density of 34.69?J/cm³ combined with an efficiency of 59.32% was achieved at the optimal composition of BNKT-15SZ under a high electric field of 2100?kV/cm, which can be ascribed to the slim P-E loops induced by strong relaxor behavior (γ?=?1.93) and the enhanced breakdown strength. Moreover, BNKT-15SZ thin film capacitor presents a good thermal stability with minimal variations of the energy density (<10%) and the energy storage efficiency (<5%) over a wide temperature range of 30–100?°C. The results indicated that the BNKT-100xSZ thin films may be a promising environmental-friendly material for energy storage applications.     

Enhanced dielectric and energy-storage properties in BiFeO3-modified Bi0.5(Na0.8K0.2)0.5TiO3 thin films

Lead free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ thin films doped with BiFeO₃ (abbreviated as BNKT-xBFO) (x = 0, 0.02, 0.04, 0.08, 0.10) were deposited on Pt(111)/Ti/SiO₂/Si substrates by sol-gel/spin coating technique and the effects of BiFeO₃ content on the crystal structure and electrical properties were investigated in detail. The results showed that all the BNKT-xBFO thin films exhibited a single perovskite phase structure and high-dense surface. Reduced leakage current density, enhanced dielectric and ferroelectric properties were achieved at the optimal composition of BNKT-0.10BFO thin films, with a leakage current density, dielectric constant, dielectric loss and maximum polarization of < 2 × 10⁻⁴ A/cm³, ~ 978^, ~ 0.028 and ~ 74.13 μC/cm² at room temperature, respectively. Moreover, the BNKT-0.10BFO thin films possessed superior energy storage properties due to their slim P-E loops and large maximum polarization, with an energy storage density of 22.12 J/cm³ and an energy conversion efficiency of 60.85% under a relatively low electric field of 1200 kV/cm. Furthermore, the first half period of the BNKT-0.10BFO thin film capacitor was about 0.15 μs, during which most charges and energy were released. The large recoverable energy density and the fast discharge process indicated the potential application of the BNKT-0.10BFO thin films in electrostatic capacitors and embedded devices.     

Bi(Mg0.5Ti0.5)O3 addition induced high recoverable energy-storage density and excellent electrical properties in lead-free Na0.5Bi0.5TiO3-based thick films

(1-x)Na_0.5Bi_0.5TiO₃-xBi(Mg_0.5Ti_0.5)O₃ (NBT-BMT) thick films were designed for achieving large recoverable energy-storage density (W_rec). A large W_rec of 40.4 J/cm³ was detected in the thick film for x = 0.4, which was more than 4 times larger than that of the pure NBT film. The addition of BMT induced slim polarization hysteresis (P-E) loops at room temperature. The slim P-E loops improved the difference between the maximum polarization (P_max) and the remnant polarization (P_r). Besides, a breakdown strength field (BDS) of 2440 kV/cm was also detected in the thick film for x = 0.4. The high BDS was caused by the reduced leakage current density. Furthermore, the thick film for x = 0.4 possessed superior energy-storage stability under different temperature, frequency and electric-field cycling. In addition, 90% of the pulsed discharge energy density could be released in less than 1100 ns by using a pulsed discharge measurement.     

Pyroelectric,ferroelectric, piezoelectric and dielectric properties of Na0.5Bi0.5TiO3 ceramic prepared by sol-gel method

Sodium bismuth titanate (BNT) nanopowder of molar composition 50/50 (Na_0.5Bi_0.5TiO₃) was prepared by a sol-gel processing method. The structure and microstructure of the precursor gel as well as the ferroelectric, pyroelectric, dielectric and piezoelectric properties of the BNT were studied. BNT crystallized in the rhombohedra perovskites structure Na_0.5Bi_0.5TiO₃ was obtained from the precursor gel by heating at 700 °C for 2 h in air. The BNT ceramic at 1100 °C sintering temperature present high crystallinity, good dielectric properties at 1 kHz (ε′=885, tan δ=0.03, T_c=370 °C), piezoelectric properties (k₃₃=0.39, c₃₃=105 GPa, e₃₃=12.6 C/m², d₃₃=120 pC/N), high remnant polarization (P_r=47 μC/cm²) and pyroelectric coefficient ($p$=707 μC/m² K) and low coercive field (E_c=55 kV/cm). Hence, the BNT prepared by sol-gel method could be used for silicon based memory device application where a low synthesis temperature is a key requirement.     

Thickness-dependent switching behavior of 0.8(Bi0.5Na0.5)TiO3-0.2SrTiO3 lead-free piezoelectric thin films

0.8(Bi_0.5,Na_0.5)TiO₃-0.2SrTiO₃ (BNT-0.2ST) thin films, with thicknesses ranging from 90 to 364 nm, were fabricated on platinized silicon substrates by sol-gel method. These films were investigated by switching spectroscopy piezoresponse force microscope (SS-PFM) as a function of frequency at room temperature, revealing the enhanced ferroelectric response in ∼ 210 nm film at all frequencies (0.1 Hz - 1.5 Hz). This enhancement was ascribed to the largest thermally-activated stress at such thicknesses generated during film fabrications. As the temperature of the investigated films increases from room temperature to 200 ^oC, the piezoelectric parameters were obtained from SS-PFM, such as switching polarization (R_s), coercive bias (V₀), work of switching (A_s), maximum strain (S_max), and negative strain (S_neg), indicating an occurrence of phase transition from ferroelectrics to relaxors. This work revealed that thickness plays a crucial role for ferroelectric response and temperature-dependent phase transition in BNT-0.2ST films, since it affects the stress state and switching behavior.     

Construction of multi-domain coexistence enhanced piezoelectric properties of Bi0.5Na0.5TiO3-based thin films

Although the multi-phase coexistence makes Bi_0.5Na_0.5TiO₃-based piezoelectric thin films possess stronger piezoelectric properties and more spacious application prospects in electronic devices, the domain reversal mechanism of Bi_0.5Na_0.5TiO₃-based thin films cannot be accurately understood due to the size effect. In this study, the relationship between domain structure and piezoelectric properties of the (0.94-x)Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-xBi(Fe_0.95Mn_0.03Ti_0.02)O₃ thin films are studied by using visualization technology PFM, structure and electrical properties characterizations. The results show that the addition of Bi(Fe_0.95Mn_0.03Ti_0.02)O₃ creates a long-range ordered/short-range disordered nanodomain coexisting structure. This kind of coexisting domain structure can realize the long-range reversal driven by disordered nanodomains under the external electric field, reduce the potential barrier and the hysteresis, and significantly enhance the piezoelectric properties of the thin films. Under the same conditions, the piezoelectric properties of the 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ thin films are enhanced nearly 2.3 times. This provides a reference for exploring the physical mechanism of high performance lead-free piezoelectric thin films.     

Piezoelectric enhancement and vacancy defect reduction of lead-free Bi0.5Na0.5TiO3-based thin films

To explore new lead-free piezoelectric materials that is both environmentally friendly and healthy to provide the possibility for material selection for microelectromechanical systems. Lead-free piezoelectric (1-x)(0.8Bi_0.5Na_0.5TiO₃-0.2Bi_0.5K_0.5TiO₃)-xBi(Ni_0.5Zr_0.5)O₃ thin films (abbreviated as BNT-BKT-xBNZ) (x=0.00, 0.01, 0.02, 0.03, 0.04) were prepared on Pt(111)/Ti/SiO₂/Si substrates by a sol-gel method. Impacts of Bi(Ni_0.5Zr_0.5)O₃ content on the microstructure, dielectric, ferroelectric, and piezoelectric properties were also investigated detailedly. It found that the Bi(Ni_0.5Zr_0.5)O₃ composition had a great influence on the increase of relaxor and the decrease of the oxygen vacancies, which is influential to the promotion of thin-film properties. Thin-film of BNT-BKT-0.02BNZ showed the optimum electrical properties with the polarization of 40.27 μC/cm², dielectric constants of 477 and effective inverse piezoelectric coefficient reach up to 125.9 p.m./V. Results revealed that the BNT-BKT thin films with 0.02 mol% Bi(Ni_0.5Zr_0.5)O₃-doped are a kind of lead-free piezoelectric materials with superior manifestations with a great development prospect for applications.     

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.     

Effects of Mn doping on dielectric properties and energy-storage performance of Na0.5Bi0.5TiO3 thick films

Lead-free Na_0.5Bi_0.5Ti_1−xMn_xO₃ (NBTMn_x, x=0, 0.01, 0.03 and 0.05) ferroelectric thick films have been fabricated on LaNiO₃/Si(100) substrate by using a polyvinylpyrrolidone-modified sol-gel method and the effects of Mn content on their microstructure, dielectric properties and energy-storage performance were investigated. Compared with the pure Na_0.5Bi_0.5TiO₃ (NBT) thick films, NBTMn_x thick films exhibited a large enhancement in dielectric properties and energy-storage performance. Particularly, a giant recoverable energy-storage density (W) of 30.2 J/cm³ and the corresponding efficiency (η) of 47.7% were obtained in NBTMn_0.01 thick film at 2310 kV/cm. Moreover, the NBTMn_0.01 thick film displayed good energy-storage stability over a large temperature range at different frequency.     

Synthesis of Na0.5Bi0.5TiO3 whiskers and their nanoscale piezoelectricity

The perovskite structured lead-free system Na_0.5Bi_0.5TiO₃ (NBT) whiskers were synthesized from whiskers of layered tunnel structured Na₂Ti₆O₁₃ (NT), using a topochemical route. Both NT and NBT whiskers show high aspect ratios with an average length of 15 µm and diameter of 1 µm. By prolonging the reaction time from 2 h to 6 h at 900 °C, NT whiskers with monoclinic phase completely transformed to NBT whiskers with pseudocubic phase. Typical strip-like nanodomains are observed in a NBT whisker, which are parallel to each other. The piezoelectric response amplitude for a NBT whisker indicates a large electric field induced strain, corresponding to a S_max/E_max value of as high as 300 pm/V. This work provides an in-depth instruction to prepare pure NBT whiskers, and gives the detailed piezoelectricity of NBT whiskers to promote their applications in energy harvesting and micro-electromechanical systems.     

Enhanced temperature stable dielectric properties and energy-storage density of BaSnO3-modified (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

A series of (1-x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-xBaSnO₃ (BNBT-100xBSN, x = 0–20) lead-free ceramics were synthesized using a conventional high-temperature solid-state reaction route. The effects of BaSnO₃ on the dielectric, ferroelectric and energy-storage performance of BNBT-BSN were systematically investigated. Temperature dependent permittivity curves indicated the obviously enhanced relaxor ferroelectric property. The introduction of BaSnO₃ reduced the temperature corresponding to the first dielectric anomaly, which facilitated the dielectric temperature stability. △ε'/ε'_150°C varied no more than 15% within the temperature range of up to 338 °C (45–383 °C) for BNBT-15BSN. A slimed P-E loop was obtained with the remnant polarization of 0.4 μC/cm² for BNBT-15BSN. Moreover, the breakdown field intensity of BNBT-BSN increased effectively from 80 kV/cm to 115 kV/cm. Therefore, an optimum energy-storage performance was obtained in BNBT-15BSN with the energy-storage density of 1.2 J/cm³ whose energy-storage efficiency reached 86.7%. Furthermore, the possible contributions of defect and vacancy to relaxation and conductance mechanism were discussed by studying the impedance and electric modulus. The results above indicated the BNBT-100xBSN be a promising lead-free candidate for energy-storage capacitors.     

(Bi0.5Na0.5)TiO3 ferroelectric ceramics: Achieving high depolarization temperature and improved piezoelectric properties

(Bi_1/2Na_1/2)TiO₃-based materials have received much attention due to large electro-strain and high piezoelectric constant (d₃₃), but the tough issue is that the existence of inherent depolarization temperature (T_d) limits the temperature stability and application temperature range. Previously, reports about the formation of BNT/oxide (i.e., ZnO, Al₂O₃) composites thought that T_d can be deferred to a higher temperature and then thermal depolarization improves. However, the deferred T_d of BNT/oxide composites is limited, accompanied by a low d₃₃. Here, we design the {[Bi_0.5(Na_0.8K_0.2)_0.5]_1-xPb_x}TiO₃ ceramics, leading to a big shift of T_d from 77 ℃ to 390 ℃. Large d₃₃ (140 pC/N) and high T_d (∼263 ℃) can be simultaneously achieved for the sample with Pb=0.05, and T_d could be further deferred higher (390 ℃) for Pb=0.20. The off-centre displacement of Pb induced by Pb-O hybridization in the PbO₁₂ polyhedron and ferroelectric order stabilized by the addition of Pb can provide the driving force to strengthen the ferroelectric order, and then promote the thermal stability.     

[Bi0.5(Na0.4-xLixK0.1)]0.96Sr0.04Ti0.975Ta0.025O3 lead-free RELAXOR ceramics with the enhanced recoverable energy density

The high recoverable energy density (W_rec) of 1.83 J/cm³ was achieved at 120 kV/cm by simply introducing 0.050 mol. % Li⁺ ions into the A-site of Bi_0.5(Na_0.4K_0.1)]_0.96Sr_0.04Ti_0.975Ta_0.025O₃ lead-free relaxor ceramics. Bi_0.5(Na_0.4-xLi_xK_0.1)]_0.96Sr_0.04Ti_0.975Ta_0.025O₃ (BNKSTT-100xLi) ceramic samples were prepared by solid-state reaction method. The influence of Li⁺ ions on structure and electrical properties was analyzed in detail. XRD patterns and Raman spectra of these samples were revealed a pure perovskite-type structure. BNKSTT-100xLi ceramics exhibited an obvious relaxor characteristic with the downshift of T_F-R to ambient temperature. J - E loops confirmed the relaxor ferroelectric like nature. Leakage current density was proved to decrease with the increasing Li⁺ ions concentration. At the composition x = 0.050, remnant polarization (P_r) kept steady while maximum polarization (P_max) and breakdown strength (DBS) increased obviously, which was beneficial to the enhancement of recoverable energy density.     

Enhanced energy-storage performance of 0.94NBT-0.06BT thin films induced by a Pb0.8La0.1Ca0.1Ti0.975O3 seed layer

0.94(Na_0.5Bi_0.5TiO₃)-0.06BaTiO₃ ferroelectric thin films with and without the Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer were deposited on platinum-buffered silicon substrates by using Sol–Gel process. The influence of Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer and annealing temperatures on the microstructures, ferroelectric properties and energy-storage performances of the as-prepared films were investigated in details. The low annealing temperature and Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer could improve the values of electric break-down field strength and P_max-P_r, which play a vital role for high recoverable energy-storage density. Owing to the high electric break-down field strength value of 3310 kV/cm, a large recoverable energy density of W=17.2 J/cm³ and a high energy efficiency of η=74.3% were obtained for the 0.94(Na_0.5Bi_0.5TiO₃)-0.06BaTiO₃ thin film with the Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ seed layer, which was annealed at 450 °C.     

High thermal stability of energy storage density and large strain improvement of lead-free Bi0.5(Na0.40K0.10)TiO3 piezoelectric ceramics doped with La and Zr

Properties of lead-free Bi_0.5-xLa_xNa_0.40K_0.10Ti_0.98Zr_0.02O₃ (x?=?0.000–0.040) ceramics were investigated. All ceramics have a pure perovskite structure. A high energy storage density (～1.00?J/cm³) at room temperature (RT) is noted for the x?=?0.030 sample, while x?=?0.020 and 0.040 samples have very high thermal stability of energy storage density of ～3% (at 75–150?°C). Furthermore, the x?=?0.030 and 0.040 samples have the highest energy storage efficiency (η) value of 94% at 125?°C with high thermal stability (η?=?84–95% at 25–150?°C). The x?=?0.005 sample has high electric field-induced strain (S_max?=?0.42%) and high normalized strain coefficient (d^*₃₃?=?S_max/E_max?=?700?pm/V) with large improvements (～200% and 163% for S_max and d^*₃₃, respectively), as compared to the based composition. This ceramic system has potentials for piezoelectric and/or energy storage density applications.     

Giant electro-strain and enhanced energy storage performance of (Y0.5Ta0.5)4+ co-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 lead-free ceramics

0.94(Bi_0.5Na_0.5)(Y_0.5Ta_0.5)_xTi_1-xO₃-0.06BaTiO₃ lead-free piezoelectric ceramics were prepared by a conventional solid-state reaction method to study their excellent electro-strain properties and energy storage characteristics systematically. All ceramics exhibited a dense surface morphology. The introduction of (Y_0.5Ta_0.5)⁴⁺ complex ions destroyed the long-range ferroelectric order, which reduced the T_F-R to the operating temperature, resulting in an easier field-induced transition between relaxor and ferroelectric phase. Therefore, for x = 0.01 component attained unipolar strain of 0.37% under 52 kV/cm, and the corresponding normalized strain d₃₃* was 708 pm/V. Besides, the destruction of the ferroelectric phase also induced a pinched hysteresis loop and the maximum storage density of 1.215 J/cm³ with the efficiency of 68.7% obtained at 98 kV/cm for BNYT30 ceramics. These all demonstrated that the doping of complex ions (Y_0.5Ta_0.5)⁴⁺ made the BNT-BT an outstanding candidate for actuators and energy storage devices.     

Microstructure and electric properties of (Na0.85K0.15)0.5Bi0.5TiO3 composited films with alternative TiO2 layers

In this work, in order to investigate the effect of TiO₂ layer on the microstructure and piezoelectric properties of (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) thin films, TiO₂ layer was inserted at the interface between the NKBT thin film and substrate and on both sides of the NKBT, i.e., at the interface and on the top of the NKBT thin film. NKBT composited films with alternative TiO₂ layer were deposited on Pt/Ti/SiO₂/Si substrate by aqueous sol‐gel method. X‐ray diffraction observation found that the degree of (100) preferred orientation strengthened with TiO₂ layers added, especially on both sides of NKBT thin film. The TiO₂/NKBT/TiO₂ composited film with both TiO₂ layer of 40 nm thickness exhibited a remnant polarization value P_r of 22.6 μc/cm³ and effective piezoelectric coefficient of approximate 77 pm/V, which are much larger than that of the single‐layered NKBT thin film with P_r value of 13.7 μc/cm³ and of 56 pm/V, respectively. According to the investigation of the temperature‐dependent ferroelectric property, it was found that the P_r gradually increased, and in the meantime the coercive voltage gradually moved to higher voltage with testing temperature varied from 20 to ?150°C. Besides, applied voltage dependence of leakage current density measurement indicated that the TiO₂ layer would effectively lower the leakage current of the films, and the TiO₂/NKBT/TiO₂ composited film both TiO₂ layer of 40 nm exhibited the lowest leakage current.     

Large electrostrain and high energy-storage of (1-x)[0.94(Bi0.5Na0.5) TiO3-0.06BaTiO3]-xBa(Sn0.70Nb0.24)O3 lead-free ceramics

(1-x)[0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃]-xBa(Sn_0.70Nb_0.24)O₃ (abbreviated as BNTBT-100xBSN) lead-free ceramics were fabricated with a relative density greater than 96 %, and the structure as well as performance were tested. BNTBT-100xBSN ceramics are pseudo-cubic perovskite structure, with dense surface morphology. Doping BSN can effectively reduce the dielectric loss of ceramics and increase the relaxation properties to a certain extent. The randomly distributed ferroelectric phase was replaced by polar nano regions, thereby improving the electro-strain and energy storage performance of the system. The largest electro-strain and the corresponding normalized strain (d₃₃*) reach ~ 0.43 % and 633 pm/V respectively in the BNTBT-1BSN ceramic. The largest effective energy storage density reaching ~ 1.28 J/cm³ was tested in BNTBT-2BSN. BNTBT-100xBSN ceramics provide a feasible idea for the systematic research of lead-free ferroelectrics and improvements in electro-strain and energy storage applications.     

High energy storage density achieved in Bi3+-Li+ co-doped SrTi0.99Mn0.01O3 thin film via ionic pair doping-engineering

The demand for lead-free dielectric capacitors with rapid charge-discharge ability and high energy storage density is increasing owing to the rapid development of electronic equipment. Lead-free Sr_1-x(Bi_0.5Li_0.5)_xTi_0.99Mn_0.01O₃ (x = 0.02, 0.025, 0.03, 0.035) thin films grown on Pt/Ti/SiO₂/Si substrates were prepared by sol-gel method. A huge enhancement in polarization was found in Bi³⁺-Li⁺ co-doped SrTiO₃ thin films. The large lattice distortion and local broken-symmetry due to formation of Bi³⁺-Li⁺ ionic pairs are responsible for ferroelectricity and high polarization. The maximum polarization (42.1 μC/cm²) and largest energy storage density of 47.7 J/cm³ at 3307 kV/cm were both achieved in Sr_0.975(Bi_0.5Li_0.5)_0.025Ti_0.99Mn_0.01O₃ thin film. Moreover, an excellent temperature-dependent stability was also obtained in Sr_0.975(Bi_0.5Li_0.5)_0.025Ti_0.99Mn_0.01O₃ thin film from 30 to 110 °C.     

钛酸铋钠钾锂无铅陶瓷的柠檬酸制备技术

以钠、钾、锂的碳酸盐为原料，采用柠檬酸盐-凝胶法制备了Biol5（Na1-x-y，LixKy）0.5TiO3无铅压电陶瓷，研究了影响凝胶合成的各种工艺条件，利用TG-DTA，SEM，XRD，纳米粒度分析等技术分析了凝胶预烧温度及预烧粉体的粒度分布。研究结果表明：柠檬酸与金属离子的物质的量比、溶液浓度、pH是影响前驱液与凝胶形成的主要因素；用柠檬酸盐一凝胶法合成的Bi0.5（Na1-x-y，LixKy）TiO3粉体粒度细小均匀，合成温度低；陶瓷的压电常数d33可达138pC／N，其平面机电耦合系数kp为0．30。