Magnetoelectric coupling effect of Ga0.8Fe1.2O3/Bi0.5(Na0.85K0.15)0.5TiO3 multiferroic composite films

Lead-free Ga_0.8Fe_1.2O₃/Bi_0.5(K_0.15Na_0.85)_0.5TiO₃ (GFO/BKNT) bilayer multiferroic composite films were fabricated on Pt(100)/Ti/SiO₂/Si substrates via sol-gel methods. The microstructure, domain structure, ferroelectric, piezoelectric, magnetic properties as well as magnetoelectric coupling effect were investigated for the composite films at room temperature. Well-defined interfaces between GFO and BKNT layers and clear electric domain structures are observed. A strong magentoelectric effect is obtained with magnetoelectric voltage coefficient of α_E=30.89 mV/cm Oe, which is attributed to excellent ferroelectric, piezoelectric, and magnetic properties, as well as the coupling interaction between ferromagnetic GFO and ferroelectric BKNT phases for lead-free bilayer composite films. Besides, GFO and BKNT demonstrate the similar perovskite structure with well lattice matching, which endows the outstanding coupling and fascinating magnetoelectric properties. The present work opens up the opportunity of lead-free magnetoelectric composite films for both further fundamental studies and practical device applications such as sensors, transducers and multistate memories.     

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.     

Growth orientation-related enhancement of electrical properties in (Na0.85K0.15)0.5Bi0.5TiO3-based composite films

TiO₂-coated (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) composite films with thicknesses of 250 nm and 650 nm are synthesized on Pt(111)/Ti/SiO₂/Si using an aqueous sol-gel method. X-ray diffraction (XRD) in ω-scan and ψ-scan modes is performed to study the crystal orientation of the films. The electrical properties of the TiO₂-coated NKBT composite films are investigated over a temperature range from -150 °C to150 °C. Compared to films without the TiO₂ layer, the TiO₂-coated NKBT films exhibit enhanced electrical properties, higher temperature stability, and better endurance performance, which can be ascribed to the combined effects of the highly (100)-preferred orientation and improved degree of crystallization. The composite film in which TiO₂ layers are attached on both sides of the 650 nm-thick NKBT film demonstrates the best ferroelectric performance with the highest remnant polarization (P_r) of 24.2(±1.2) μC/cm² under 750 kV/cm, the best piezoelectric performance with the highest effective piezoelectric constant (d₃₃^*) of 82(±4) pm/V, and the best electric performance with the lowest leakage current density of 4.5 × 10⁻⁶(±0.4 × 10⁻⁶) A/cm² at 20 V. Piezoresponse force microscopy (PFM) further confirms the enhanced ferroelectricity and domain switching of the TiO₂-coated composite films on a microscopic scale. In addition, the P_r values of the films increase gradually as the temperature decreases from 150 °C to-150 °C, while the d₃₃^* values exhibit the opposite trend, which is mainly attributed to the suppressed mobility of thedomain walls at low temperatures.     

Polarization behavior of` lead-free 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin films with enhanced ferroelectric properties

(Bi_0.5Na_0.5)TiO₃ based ferroelectric lead-free thin films have great potential for modern micro-devices. However, the multicomponent feature and volatile nature of Bi/Na makes the achievement of high quality films challenging. In this work, the morphotropic phase boundary composition, 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ thin films were successfully prepared by CSD method. Dense films with low dielectric loss and low leakage current density were obtained. A well-defined polarization hysteresis loop with a high remnant polarization was observed in the thin films. Moreover, the polarization behavior of the film at original state, under electric field and upon heating was investigated by PFM. A self-polarization and asymmetric domain switching behavior were observed. High temperature induced depolarization and the self-polarization recovered upon cooling. The thin films with good quality show a promising potential for the application in electrical devices, and the in-depth investigation of the polarization behavior improves the understanding of ferroelectric and piezoelectric properties of thin films.     

Microstructure,dielectric, piezoelectric,and ferroelectric properties of fine-grained 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

For preparing fine-grained 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free ferroelectric ceramics, the precursor powders were synthesized via sol-gel method and calcined at various temperatures. The precursor powders calcined at 520 °C, 550 °C, and 600 °C exhibit mean grain sizes of 30 ± 4 nm, 54 ± 3 nm, and 78 ± 5 nm, respectively. By optimizing the synthesis parameters, the fine-grained ceramics with high relative densities (>97%) and mean grain size around 100 nm were prepared. The ferroelectric, dielectric, and piezoelectric behavior were investigated. The ceramics prepared using the different precursor powders show different piezoelectric, ferroelectric, and dielectric behavior. The ceramic calcined at 550 °C and sintered at 900 °C exhibits the breakdown strength higher than 85 kV/cm, which exhibits the maximum polarization of 38.4 ± 0.3 μC/cm², remanent polarization of 20.6 ± 0.4 μC/cm².     

Ultra-high strain responses in lead-free (Bi0.5Na0.5)TiO3-BaTiO3-NaNbO3 ferroelectric thin films

Lead-free (Bi_0.5Na_0.5)TiO₃ (BNT)-based piezoelectric materials, have a great potential for high-precision actuators’ applications. In this work, the high-quality (0.94-x%)(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-x%NaNbO₃ (x = 2–10, BNT-6BT-xNN) thin films have been successfully deposited on Pt/TiO₂/SiO₂/Si substrates by sol-gel method. An ultra-high poling strain S_pol value of 1.7% with a unipolar strain S_uni value of 1.47% was reported in the BNT-6BT-6NN thin films. The coexistence of the ferroelectric phase and relaxor state was observed in the compositions of x = 2–8. Furthermore, the BNT-6BT-6NN thin films show more active domain switching compared to other compositions. It is demonstrated that the optimized strain responses in the BNT-6BT-6NN are attributed to a synergistic reaction of active domain switching and reversible electric-field-induced phase transition between the ferroelectric phase and relaxor state. Our systematic study demonstrates that the BNT-6BT-xNN thin films with an improved strain response are promising candidates for the applications of miniaturized actuators.     

Effects of annealing temperature on the microstructure,ferroelectric and dielectric properties of W-doped Na0.5Bi0.5TiO3 thin films

The effects of annealing temperature on the structure, morphology, ferroelectric and dielectric properties of Na_0.5Bi_0.5Ti_0.99W_0.01O_3+δ (NBTW) thin films are reported in detail. The films are deposited on indium tin oxide/glass substrates by a sol-gel method and the annealing temperature adopted is in the range of 560–620 °C. All the films can be well crystallized into phase-pure perovskite structures and show smooth surfaces without any cracks. Particularly, the NBTW thin film annealed at 600 °C exhibits a relatively large remanent polarization (P_r) of 20 μC/cm² measured at 750 kV/cm. Additionally, it shows a high dielectric constant of 608 and a low dielectric loss of 0.094 as well as a large dielectric tunability of 62%, making NBTW thin film ideal in the room-temperature tunable device 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.     

Dielectric,impedance and piezoelectric properties of (K0.5Nd0.5)TiO3-doped 0.67BiFeO3-0.33BaTiO3 ceramics

A novel (0.67-x)BiFeO₃-0.33BaTiO₃-x(K_0.5Nd_0.5)TiO₃ (KNT100x, x = 0.0, 0.02, 0.04, 0.06, 0.08 mol%) ceramics were fabricated and their microstructure and electrical properties were studied. All samples displayed a pseudo-cubic symmetry, and adding of KNT had little effect on grain size. The dielectric analysis displayed the dispersion increases with the addition of KNT compositions, showing strong relaxor properties. Besides, high dielectric constant (ε’) of 23000 and dielectric peak temperature (T_m) of 390 °C remain at 1 kHz in the x = 0.02 sample while the dielectric loss (tanδ) dropped below 0.5 in the range of 30–400 °C, showing excellent electrical insulation performance. In addition, doping of KNT had obvious influence on the strain, and a large strain (Smax) of 0.26% was obtained at x = 0.02 due to the increase of electrical insulation. More importantly, the strain at 50 kV cm^?1 enhanced significantly with temperature increasing, reaching a maximum strain of 0.75% with a small hysteresis coefficient of 30% at 110 °C. Particularly, KNT02 exhibited excellent fatigue resistance within 10⁵ fatigue cycles. Presumably these results are attributed to the coexistence of ferroelectric and non-ergodic relaxor domains and the thermally activated domain wall motion.     

Lead-free Na0.4K0.1Bi0.5TiO3 ceramic: Poling effect and enhancement in electromechanical and piezoelectric voltage coefficient

The effect of electric poling on the piezoelectric properties of the sintered Na_0.4K_0.1Bi_0.5TiO₃ is studied with varying poling field and temperature. An optimized poling condition (E_P =50 kVcm, T_P =110 °C) exhibited a high piezoelectric voltage (g₃₃∼ 85 mV m/N) and charge coefficients (d₃₃∼193pC/N). A combination of electric field induced irreversible transformation from polar nano regions embedded in a non-polar relaxor state to a long-range ordered ferroelectric state and increase in the structural ordering are responsible for the observed high piezoelectric properties. A mechanism is discussed to reveal the origin of high voltage coefficient due to poling, where the decrease of dielectric permittivity can facilitate high g₃₃. This investigation provides an approach for designing the high performance Na_0.4K_0.1Bi_0.5TiO₃based materials suitable for sensors and energy harvesting applications.     

Dielectric properties and relaxor behavior of 0.935(Na0.5Bi0.5)TiO3-0.065BaTiO3 lead free piezoelectric ceramic

Dielectric properties of 0.935(Na_0.5Bi_0.5)TiO₃-0.065BaTiO₃ lead free piezoelectric ceramic are studied. The study shows a diffuse phase transition (DPT) characterized by a frequency dispersion of permittivity which is related to cation disorder at A-site. Additionally to this dispersion, this solid solution exhibits relaxor behavior. The mechanism of relaxor behavior was discussed according to Debye, Vogel–Fulcher (V–F) and Power law models and the suitable model was predicted by means of goodness of parameter.     

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.     

BaTiO3 nanowires-induced phase transition and thermally stable strain in (Bi0.5Na0.5)TiO3 piezoelectric ceramics

Environment-friendly lead-free piezoceramics with high strain response and extremely excellent stability in a wide operating temperature range are critically important in practical actuator applications. Here, we develop a new strategy to tune the electrostrictive strain behavior in Bi_0.5Na_0.5TiO₃ (BNT)-based ceramics via using high aspect ratio BaTiO₃ nanowires (BT NWs) as a modifier. The addition of BT NWs generates a crossover from a typical ferroelectric (BT conventional spherical particles) to a complete ergodic relaxor (ER) phase at ambient temperature, accompanied by a large electrostrictive strain of ∼0.17% with d₃₃*(S_max/E_max) = 284 pm/V. Such a high electrostrictive strain is extremely thermally stable with only <7% fluctuation from 27 °C to 120 °C. In addition, the BT NWs-modified ceramics also exhibit acceptable fatigue endurance (<30% up to 10⁵ cycles) and frequency dependence (<20% at 10Hz–100Hz). These achieved exceptional performances can be ascribed to the BT NWs-driven complete ER phase at room temperature. The findings of this study can inspire enhanced interest in nanowires as a viable modifier to BNT-based materials due to promising potential for practical actuator applications in a wide temperature range.     

Phase transition and electrical properties of Bi0.5(Na0.8K0.2)0.5ZrO3 modified (K0.52Na0.48)(Nb0.95Sb0.05)O3 lead-free piezoelectric ceramics

In this work, (1−x)(K_0.52Na_0.48)Nb_0.95Sb_0.05O₃−xBi_0.5(Na_0.8K_0.2)_0.5ZrO₃ [abbreviated as (1−x)KNNS−xBNKZ, x=0–0.06] lead-free ceramics were fabricated using solid-state reaction method. The effects of BNKZ contents on the phase structure, piezoelectric and ferroelectric properties were investigated. The phase boundaries including orthorhombic-tetragonal (O-T) and rhombohedral-tetragonal (R-T) multiphase coexistence were identified by XRD patterns and temperature-dependent dielectric constant by adding different content of BNKZ. A giant field induced strain (~0.25%) along with converse piezoelectric coefficient d₃₃^* (~629.4 pm/V) and enhanced ferroelectricity P_r (~38 μC/cm²) were obtained when x=0.02, while the specimen with x=0.03 presented the optimal piezoelectric coefficient d₃₃ of 215 pC/N, due to the O-T or R-T phase coexistence near room temperature respectively. These results show that the introduction of Bi_0.5(Na_0.8K_0.2)_0.5ZrO₃ is a very effective way to improve the electrical properties of (K_0.52Na_0.48)(Nb_0.95Sb_0.05)O₃ lead-free piezoelectric ceramics.     

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.     

Nb-doped BaTiO3-(Bi0.5Na0.5)TiO3 ceramics with core-shell structure for high-temperature dielectric applications

Nb-doped 0.90BaTiO₃-0.10(Bi_0.5Na_0.5)TiO₃ temperature-insensitive ceramics with novel core-shell structure were sintered at low temperature by the conventional solid-state reaction method. The beneficial role of Nb in facilitating the formation of core-shell structure because of chemical inhomogeneity is verified, which is responsible for the weak temperature dependence of dielectric properties. Temperature dependence of permittivity measured at different frequencies shows high frequency dispersion at low temperature, while without relaxor characteristic at high temperature. The Vogel–Fulcher model was adopted to study the relaxor behaviour of Nb-doped 0.90BaTiO₃-0.10(Bi_0.5Na_0.5)TiO₃ ceramics at low temperature. The samples with an addition of 1.5?mol% Nb₂O₅ provide a temperature coefficient of capacitance meeting the requirements of the X9R characteristic, and result exhibits an optimum dielectric behaviour of ε_r ～1900, tanδ ～1.8% at room temperature, making the material a promising candidate for high temperature applications.     

Structure and ferroelectric behaviour of Na0.5Bi0.5TiO3-KNbO3 ceramics

Lead-free piezoelectric ceramics, (1?x)Na_0.5Bi_0.5TiO₃-xKNbO₃ (NBT-xKN), with x?=?0.02–0.08 were fabricated by solid-state reaction and sintering. The crystal structures and dielectric properties were measured for different KN contents. All compositions in the unpoled, as-sintered state were found to be single-phase pseudo-cubic. However, typical ferroelectric behaviour, with well-saturated polarisation-electric field hysteresis loops, was observed for certain compositions at high electric field levels. It is shown using high-energy synchrotron X-ray diffraction that the application of the electric field induced an irreversible structural transformation from the nano-polar pseudo-cubic phase to a ferroelectric rhombohedral phase. The changes in lattice elastic strain and crystallographic texture of a poled NBT-0.02KN specimen as a function of the grain orientation, ψ, conform well to those expected for a conventional rhombohedrally distorted perovskite ferroelectric ceramic. The dielectric permittivity-temperature relationships for all compositions exhibit two transition temperatures and a frequency-dependent behaviour that is typical of a relaxor ferroelectric. The transition temperatures and grain size decrease with the increasing KN content.     

(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.     

A位元素非化学计量Bi0.5(Na,K)0.5TiO3-BiFeO3压电陶瓷

采用传统陶瓷制备方法,制备了A位元素非化学计量无铅压电陶瓷0.79(Bi0.5Na0.5)tTiO3–0.18(Bi0.5K0.5)tTiO3–0.03BitFeO3(t=0.95～1.05)。研究了A位元素非化学计量对该体系陶瓷微观结构、压电性能的影响,同时通过测量不同外加应力下压电应变常数(d33),研究了影响d33和径向机电耦合系数(kp)的不同机理。结果表明:A位元素缺乏较多时,析出第二相。kp随A位元素过量与不足的增加而减少,d33随A位元素过量的增加基本不变,随A位元素不足的增加而减少。采用极化相位角(θmax)衡量陶瓷极化程度,发现kp随θmax增加而增加,d33随θmax增加变化不明显。d33在低于临界应力时基本不变,当应力高于临界应力后,随应力增加而下降。压电陶瓷中的应力场对畴壁运动与弹性偶极子的作用是影响d33的作用机理之一。     

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.