Synthesis of Anisotropic Na0.5Bi0.5TiO3 Crystals Using Different Topochemical Microcrystal Conversion Methods

Na_0.5Bi_0.5TiO₃ (NBT) platelets with high aspect ratio were synthesized from Na_0.5Bi_4.5Ti₄O₁₅ (NBIT) precursors via a topochemical microcrystal conversion in molten salt conditions. The effect of the synthesis parameters, such as the molten salt system, synthesis temperature, and the molar ratio of Na₂CO₃ and NBIT, was investigated. The results showed that NaCl–KCl molten salt environment and excess Na₂CO₃ played a positive role in the synthesis, square‐shaped NBT was obtained at 950°C in NaCl–KCl molten salt and a TiO₂‐free environment, and it was a suitable template candidate to achieve NBT‐based textured ceramics using the reactive template grain growth (RTGG) method.     

Large Piezoresponse and Ferroelectric Properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi(Mg0.5Ti0.5)O3 Thin Films Prepared by Chemical Solution Deposition

Bulk ceramic 72.5 mol%(Bi_0.5Na_0.5)TiO₃–22.5 mol%(Bi_0.5K_0.5)TiO₃–5 mol%Bi(Mg_0.5Ti_0.5)O₃ (BNT–BKT–BMgT) has previously been reported to show a large high‐field piezoelectric coefficient (d₃₃* = 570 pm/V). In this work, the same composition was synthesized in thin film embodiments on platinized silicon substrates via chemical solution deposition. Overdoping of volatile cations in the precursor solutions was necessary to achieve phase‐pure perovskite. An annealing temperature of 700°C resulted in good ferroelectric properties (P_max = 52 μC/cm² and P_r = 12 μC/cm²). Quantitative compositional analysis of films annealed at 650°C and 700°C indicated that near ideal atomic ratios were achieved. Compositional fluctuations observed through the film thickness were in good agreement with the existence of voids formed between successive spin‐cast layers, as observed with electron microscopy. Bipolar and unipolar strain measurements were performed via double laser beam interferometry and a high effective piezoelectric coefficient (d_33,f) of approximately 75 pm/V was obtained.     

Mechanism of Bi0.5Na0.5TiO3 and Bi4.5Na0.5Ti4O15 template synthesis during topochemical microcrystal conversion and texturing of Bi0.5(Na0.8K0.2)0.5TiO3 piezoelectric ceramics

The mechanism by which Bi_0.5Na_0.5TiO₃ and Bi_4.5Na_0.5Ti₄O₁₅ templates are synthesized via a topochemical microcrystal conversion method using Bi₄Ti₃O₁₂ precursor and TiO₂ particles was investigated based on their crystal structures. The Bi_0.5Na_0.5TiO₃ template consisted of a mixture of plate-like and equiaxed particles, whereas the Bi_4.5Na_0.5Ti₄O₁₅ template consisted only of plate-like particles. The size of the plate-like and equiaxed particles was dependent on the size of the Bi₄Ti₃O₁₂ precursor and TiO₂ particles, respectively. The Lotgering factor and piezoelectric constant of textured Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ ceramics prepared using the Bi_0.5Na_0.5TiO₃ template were lower than those of the textured Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ ceramics prepared from the Bi_4.5Na_0.5Ti₄O₁₅ template. This can be attributed to the small amount of plate-like particles in the Bi_0.5Na_0.5TiO₃ template caused by the inevitable co-existence of equiaxed particles.     

Dielectric temperature stability of Nb-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

In this study, the phase structure, microstructure and dielectric properties of Bi_0.5(Na_0.78K_0.22)_0.5(Ti_1-xNb_x)O₃ lead-free ceramics prepared by traditional solid phase sintering method were studied. The second phase pyrochlore bismuth titanate (Bi₂Ti₂O₇) was produced in the system after introduction of Nb⁵⁺. The dielectric constant of the sample (x = 0.03) sintered at 1130 °C at room temperature reached a maximum of 1841, and the dielectric loss was 0.045 minimum. It had been found that the K⁺ and Nb⁵⁺ co-doped Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics exhibited outstanding dielectric-temperature stability within 100–400 °C with T_cc ≤±15%. Result of this research provides a valuable reference for application of BNT based capacitors in high temperature field.     

Fabrication of [1 0 0]-oriented bismuth sodium titanate ceramics with small grain size and high density for piezoelectric materials

A layered titanate H_1.07Ti_1.73O₄·nH₂O (HTO) with a plate-like particle morphology was used as a template for the fabrication of [1 0 0]-oriented bismuth sodium titanate (Na_0.5Bi_0.5TiO₃, or BNT) ceramics by a reactive-templated grain growth (RTGG) method. The oriented BNT ceramic with a high degree of orientation (95%), high density (98%), and small grain size (2 μm) was fabricated for the first time by the RTGG method using a HTO-TiO₂-Bi₂O₃-Na₂CO₃ reaction system. The oriented BNT ceramic is formed by a topotactic transformation reaction of plate-like HTO template particles to plate-like BNT mesocrystal particles, and then epitaxial crystal growth of BNT on the BNT mesocrystal particles. The epitaxial crystal growth reaction is affected by TiO₂/HTO mole ratio, chemical component of the starting material, and calcination temperature program. The fabricated oriented BNT ceramic shows a higher d₃₃* value than the non-oriented BNT ceramic, suggesting the promising application to high performance Pb-free piezoelectric materials.     

Growth of NBT template particles through topochemical microcrystal conversion and their structural characterization

In this study, uniform rectangular plate‐like perovskite Na_0.5Bi_0.5TiO₃ templates were obtained by molten salt synthesis method in three steps. Perovskite plate‐like Na_0.5Bi_0.5TiO₃ templates were synthesized from bismuth‐layered Na_0.5Bi_4.5Ti₄O₁₅ by topochemical microcrystal conversion method at the third and final step. The most important point was that the plate‐like morphology remained as desired, but templates were obtained in cubic perovskite structure. The Na₂CO₃ excess in the third step was the main controlling parameter. The crystalline orientation of the particles was investigated by electron backscatter diffraction analysis. The results indicated that the particles are single crystalline in nature. In the case of the 90 mol% excess of the stoichiometric Na₂CO₃ ratio, the NBT platelets had highly regular, rectangular prismatic morphology and single‐crystal nature. The structure was also confirmed through high‐resolution transmission electron microscope and selected area electron diffraction analysis.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Mg0.5Ti0.5)O3 Lead‐Free Piezoceramics

Lead‐free BNT‐based piezoceramics, (1?x)Bi_0.5Na_0.5TiO₃–xBi(Mg_0.5Ti_0.5)O₃ [(1?x)BNT–xBMT] (0.00 ≤ x ≤ 0.06) binary system, were synthesized using a conventional ceramic fabrication method. Effect of Bi(Mg_0.5Ti_0.5)O₃ (BMT) substitution on room temperature (RT) crystal structure, and temperature dependence of electric properties were investigated. The XRD indicates that a pure perovskite phase is formed. The introduction of BMT decreases E_C of BNT from 7.3 to 4.0 kV/mm, and increases d₃₃ from 58 pC/N to 110 pC/N for the x = 0.05. The system shows a typical ferroelectric (FE) polarization loop P(E) and butterfly bipolar strain‐electric S(E) curve at RT. For the composition of 0.95BNT–0.05BMT antiferroelectric (AFE) phase appears near 80°C, characterized by a constricted P(E) loop and altered bipolar S(E) butterfly, and gradually prevails with increasing temperature. Temperature dependence of dielectric constant shows that T_C increases from 310°C for pure BNT to 352°C for the x = 0.05. The results indicate that the piezoelectric properties of BNT have been improved by means of Bi(Mg_0.5Ti_0.5)O₃ substitution.     

Development of texture in Bi0.5Na0.5TiO3 prepared by reactive-templated grain growth process

The effect of sodium concentration on texture development in Bi_0.5Na_0.5TiO₃ [BNT] bulk ceramics was examined. The 〈1 0 0〉-textured specimens were prepared by the reactive-template grain growth process using platelike Bi₄Ti₃O₁₂ particles. Texture did not extensively develop in stoichiometric and Na-deficient BNT, but excess Na promoted extensive texture development together with densification. The role of excess Na was discussed based on the formation of a liquid phase.     

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.     

Approaches for preparing 〈1 1 1〉-textured Bi0.5Na0.5TiO3-based ceramics by hetero-templated grain growth

The preparation of 〈1 1 1〉-textured Bi_0.5Na_0.5TiO₃ (BNT) ceramics was attempted by the templated grain growth method. This method uses anisometric grains as the template for texture development. In this work, plate-like BaTiO₃ grains with the 〈1 1 1〉 direction perpendicular to the plate faces were used. A mixture of the plate-like BaTiO₃ grains and equiaxed BNT grains was consolidated by tape casting to prepare green compacts in which the plate-like BaTiO₃ grains were dispersed in the matrix of the BNT grains and were aligned with their plate faces parallel to the casting direction. The textured BNT ceramics could not be obtained by the sintering of the green compacts, because of the disappearance of the plate-like BaTiO₃ grains by dissolution in the BNT grains. Several approaches were examined to obtain 〈1 1 1〉-textured ceramics. They included the use of plate-like (Sr, Ba)TiO₃ grains as the template, the use of equiaxed grains of BNT–BaTiO₃ and BNT–Bi_0.5K_0.5TiO₃ solid solutions as the matrix, and the addition of excess Na₂CO₃ and Bi₂O₃ to form liquid phases. The only approach successful in obtaining the textured ceramics was the one involving the addition of excess Na₂CO₃. The excess Na₂CO₃ formed a liquid phase and promoted the formation of a shell of BNT grains around a plate-like BaTiO₃ grain, prior to the dissolution of the BaTiO₃ grain. The BNT shell had the same crystallographic orientation as the BaTiO₃ grain and acted as the template for texture development.     

Luminescence and electrical properties of Eu-modified Bi0.5Na0.5TiO3 multifunctional ceramics

The Eu³⁺-modified Bi_0.5Na_0.5TiO₃ (BNT) ceramics have been fabricated by the solid-state reaction method. The impact of Eu³⁺ doping on the structure, photoluminescence, and electrical properties has been studied by XRD, SEM, PL spectra, and LCR meter. X-ray diffraction analysis reveals that the crystal structure of the samples is well matched with the trigonal perovskite, and the optimal temperature of presintering is 880°C. The Eu³⁺-doped BNT ceramics show excellent red fluorescence at 614 nm corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺ under 466 nm excitation and relatively long fluorescence lifetime. The BNT-0.02Eu ceramic density is up to 5.68 g/cm³ and the relative density is up to 94.6% with sintering temperature 1075°C. The piezoelectric constant (d₃₃) of samples has been significantly improved up to 110 pC/N by Eu³⁺ doping. The BNT-0.03Eu ceramic presintered at 880°C and sintered at 1050°C has good dielectric properties and excellent luminescence properties. Eu³⁺-doped BNT ceramics make it potential applications for novel integrated electro-optical and multifunctional devices.     

Dielectric and ferroelectric properties of (1 − x)BaTiO3–xBi0.5Na0.5TiO3 ceramics

Lead-free (1 − x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (x = 0.01, 0.02, 0.05, 0.1, 0.2, 0.3) ferroelectric ceramics were fabricated by the conventional ceramic technique. Sintering was made at 1200 °C for 2–4 h in air atmosphere. The crystal structure was investigated by X-ray diffraction. The dielectric and ferroelectric properties were also studied. Room temperature permittivity was found to decrease as Bi_0.5Na_0.5TiO₃ (BNT) content increases. Only the sample with 0.3 mol BNT was found to have relaxor behaviour. The T_c shifted slightly only for BNT addition lower than 0.1 mol. The highest T_c (about 150 °C) was obtained for 0.2 mol BNT addition. The remanent polarization, P_r, decreases whereas the coercive field, E_c, increases monotonously as the BNT content increases.     

Structure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of BiFeO3‐BaTiO3‐Bi0.5Na0.5TiO3 Lead‐Free Multiferroic Ceramics

Lead‐free multiferroic ceramics of BiFeO₃‐BaTiO₃‐Bi_0.5Na_0.5TiO₃ have been prepared by a conventional ceramic technique. The microstructure, multiferroic, and piezoelectric properties of the ceramics have been studied. The ceramics sintered at 1000°C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and tetragonal phases is formed at x = 0.02. After the addition of Bi_0.5Na_0.5TiO₃, two dielectric anomalies are observed at high temperatures (T_m ~ 510°C–570°C and T₂ ~ 720°C). The phase transition around T_m becomes wider gradually with increasing x. The ferroelectricity, piezoelectricity, and ferromagnetism of the ceramics are significantly improved after the addition of Bi_0.5Na_0.5TiO₃. High resistivity (~1.3 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 27.4 μC/cm²), good piezoelectricity (d₃₃ =140 pC/N, k_p = 31.4%), and weak magnetic properties (M_r =0.19 emu/g) are observed.     

High-temperature dielectric and relaxation behavior of Yb-doped Bi0.5Na0.5TiO3 ceramics

Microstructure, phase transition and dielectric properties of Yb-doped Bi_0.5Na_0.5TiO₃ (BNT) ceramics were investigated. It is found that ytterbium promotes the grain growth and densification of the ceramics while Ti-rich impurity appears due to the compensation of Ti-vacancy. The dielectric operational temperature range of the ceramics with a±15% tolerance was greatly broaden until 500 °C by ytterbium doping. Meanwhile, the diffuseness of the diffuse phase transition increases with the increase of doping Yb. BNT ceramics with 3 mol% Yb doping shows a near-plateau dielectric behavior in a broad temperature range from 147 to 528 °C and a low dielectric loss (<0.025) from 154 to 356 °C, indicating that it is a promising material for applications in high-temperature capacitor.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Al0.5Ga0.5)O3 Lead‐Free Piezoceramics

A new lead‐free BNT‐based piezoelectric ceramics of (1 ? x)Bi_0.5Na_0.5TiO₃–xBi(Al_0.5Ga_0.5)O₃ (x = 0, 0.02, 0.03, 0.04, and 0.05) were synthesized using a conventional ceramic fabrication method. Their structures and electrical properties were investigated. All the samples show a typical ferroelectric P(E) loops and S(E) curves at room temperature. The optimal properties are obtained at the composition of the x = 0.03. The substitution of Bi(Al_0.5Ga_0.5)O₃ enhances piezoelectric constant and increases Curie temperature from 58 pC/N and 310°C of pure BNT to 93 pC/N and 325°C of the x = 0.03. The temperature‐dependent P(E) loops and S(E) curves of 0.97BNT–0.03BAG indicate that phase transition from ferroelectric to antiferroelectric takes place over a very wide temperature region from 80°C to 180°C. The results show that the introduction of BAG improves the electrical properties of BNT.     

Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics

Na_0.5Bi_0.5TiO₃ (NBT) based oxide-ion conductor ceramics have great potential applications in intermediate-temperature solid oxide fuel cells (SOFCs) and oxygen sensors. Na_0.5Bi_0.49Ti_1−xMg_xO_3−δ ceramics with x=0, 0.01, 0.02, 0.03, 0.05 and 0.08 were prepared by conventional solid-state reaction. XRD measurement and SEM analysis revealed the formation of pure perovskite structures without secondary phase. MgO doping greatly decreased the sintering temperature and inhibited grain growth. AC impedance spectroscopy measurement was adopted to measure the total conductivity, which was found to increase with MgO doping content ranging from 0 to 3 mol% and subsequently to decrease. High oxygen ionic conductivity σ_t=0.00629 S/cm was achieved for sample doped with 3 mol% MgO at 600 °C in air atmosphere.     

Structural,dielectric and electrical properties of BaSnO3 and BaSeO3 modified Bi0.5Na0.5TiO3 ceramics

In this communication, preliminary structural and detailed electrical (dielectric, polarization, impedence, and conductivity) characteristics of BaSnO₃ and BaSeO₃ modified complex Bi_0.5Na_0.5TiO₃ ceramics with a general chemical formula and composition, (1–2x)[(Bi_0.5Na_0.5) TiO₃]+x(BaSnO₃)+x(BaSeO₃) (with x = 0, 0.05, 0.1, 0.15) (BNT–BSn–BSe) ceramics, synthesized by a high-temperature mixed-oxide technique (calcinations temperature = 925 °C and sintering temperature = 950 °C, for time = 4 h each) have been reported. Analysis of room temperature X-ray diffraction data has confirmed the formation of single phase perovskite of BNT–BSn–BSe in rhombohedral phase and provided crystal data. Studies of impedance spectroscopy and electrical conductivity of the materials in a frequency range of (1 kHz-1MHz) at different temperatures (25–500 °C) have shown the composition (x) effect on electrical characteristics of BSn and BSe modified BNT.The electric field dependent polarization study at room temperature exhibits the existence of ferroelectricity in the materials. Study of the impedance spectroscopy through Nyquist plots shows the contributions of the grains and grain boundary in the resistive and capacitive properties of the materials. This study also determines the existence of NTCR (negative temperature coefficient of resistance) behavior the prepared materials. The J–E characteristics demonstrate the Ohomic behavior with slope closer to 1.     

Effect of Texture on Temperature‐Dependent Properties of K0.5Na0.5NbO3 Modified Bi1/2Na1/2TiO3–xBaTiO3

Textured (1?x?y)Bi_1/2Na_1/2TiO₃–xBaTiO₃–yK_0.5Na_0.5NbO₃ (BNT–100xBT–100yKNN) ceramics with a {001} pseudocubic (pc) orientation were fabricated by templated grain growth using Bi_1/2Na_1/2TiO₃ templates. Temperature‐dependent electromechanical results demonstrate that the strain response of templated BNT–xBT–yKNN ceramics is stable from room temperature (RT) to 125°C. The temperature‐dependent strain and polarization response are compared to randomly oriented ceramics, for BNT–100xBT–2KNN (0.05 ≤ x ≤ 0.07). Textured BNT–7BT–2KNN reached a maximum 0.47% strain response at 5 kV/mm, an almost 50% increase compared to randomly oriented BNT–7BT–2KNN. Over the temperature range RT–125°C, the strain response of templated BNT–6BT–2KNN degraded from 0.38% to 0.22% (?42.1%) compared to 0.37% to 0.18% (?51.4%) for randomly oriented ceramics. The temperature‐dependent strain response suggests that templated BNT–100xBT–100yKNN ceramics are well suited for elevated temperature applications.     

Syntheses of bismuth titanate templates obtained by the molten salt method

Bismuth titanate templates (Bi₄Ti₃O₁₂) were synthesized by the molten salt method in Na₂SO₄ and K₂SO₄ fluxes, using an amorphous Bi₄Ti₃O₁₂ precursor and a mechanically mixed Bi₂O₃+TiO₂ mixture as the starting materials. The templates were characterized by means of X-Ray Diffraction, FT-IR, FT-Raman, FEG-SEM and TEM. The templates are free of secondary phases and present orthorhombic structure with orientation in the c-plane. FT-IR suggests no traces of sulfate groups revealing that the molten salt synthesis was beneficial for elimination of inorganic species and for the arrangement of individual nanocrystals into ordered lattices. FEG-SEM analyses of BIT templates revealed that most of the grains were homogeneous with a length of 3.1 µm and a width of 0.3 µm and had plate-like morphology. TEM investigations show that the c-axis of the perovskite units is parallel to the thickness direction of the grains and no liquid-phase was formed during BIT phase formation.     

Enhanced energy density and thermal stability in relaxor ferroelectric Bi0.5Na0.5TiO3-Sr0.7Bi0.2TiO3 ceramics

Sr_0.7Bi_0.2TiO₃ (SBT) was introduced into Bi_0.5Na_0.5TiO₃ (BNT) via a standard solid-state route to modulate its relaxation behaviour and energy storage performance. With increasing SBT content, the perovskite structure of BNT transforms from a rhombohedral phase to a weakly polarized pseudo-cubic phase, and the relaxation behaviour is enhanced. In particular, the E_DBS is improved from 120 kV/cm of BNT to 160 kV/cm of 0.6BNT-0.4SBT, which displays a large recoverable energy storage density (W_rec = 2.20 J/cm³), implying a large potential ability of energy storage for the 0.6BNT-0.4SBT ceramic. Moreover, both dielectric properties (28–326 °C) and energy storage properties (20–140 °C) exhibit a good thermal stability for the same 0.6BNT-0.4SBT composition. These characteristics suggest 0.6BNT-0.4SBT ceramic could be a promising candidate to be applied in a pulse power system over a broad temperature range.