Effect of strontium substitution on the structure and dielectric properties of unfilled tungsten bronze Ba4-xSrxSmFe0.5Nb9.5O30 ceramics

The effects of Sr²⁺ substitution for Ba²⁺ on phase structure, microstructure, dielectric and electric properties for Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2, 3 and 4) ceramics were systematically researched. X-ray diffraction patterns show that Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2 and 3) ceramics are tetragonal tungsten bronze compound with a space group of P4bm, while the sample for x = 4 is an orthorhombic structure compound. The result can be corroborated by the analysis of Raman spectroscopy. As the Sr²⁺ contents increase from 0 to 3, the full width at half maximum of Raman lines of all samples increase gradually, indicating that the degree of lattice distortion increase. All tetragonal tungsten bronze ceramics exhibited a broad permittivity peaks, accompanied by frequency dispersion, indicating all samples are relaxor. The electrical properties of BSSFN ceramics were further studied by complex impedance spectroscopy. XPS spectrum shows that Fe²⁺ and Fe³⁺ coexist in Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ ceramics, and their proportion varies with the concentration of Sr²⁺.     

Correlation between crystal structure and dielectric response for orthorhombic tungsten bronze ceramics Ba4(Nd1-xSmx)28/3(Ti0.95Zr0.05)18O54

Ba₄(Nd_1-xSm_x)_28/3(Ti_0,95Zr_0,05)₁₈O₅₄ ceramics with (x = 0; 0.2; 0.4; 0.8 and 1) were synthesized by solid method at 1250 °C for 10 h. The effects of Nd/Sm ratio on the structure and dielectric behavior were studied by changing the value of x. The study of Ba/RE order with (RE = Nd and Sm) in the solid solution Ba₄(Nd_1-xSm_x)_28/3(Ti_0,95Zr_0,05)₁₈O₅₄ was realized by X-ray diffraction. The crystal structure of these phases belongs to the tungsten bronze type, which is constructed on the basis of the (3x3) Ti/ZrO₆ octahedron more than (2x2) (orthorhombic symmetry, space group Pnma, a ≈22.3 Å, b ≈ 7.67 Å, c ≈ 12.1 Å). A structural model has been established, corresponding to an order within the structure. The model of formula [Ba₄]_A2[Ba_2-a(Nd_1-xSm_x)_1.33+b□_c]_A1'[(Nd_1-xSm_x)_8-d□_d]_A1(Ti_0.95Zr_0.05)₁₈O₅₄, corresponds to a model associated to infinite perovskite rows parallels to the Oy axis, constructed on the basis of the octahedron (3x3) Ti/ZrO₆. Scanning electron microscopy (SEM) has showed that Ba₄(Nd_1-xSm_x)_28/3(Ti_0,95Zr_0,05)₁₈O₅₄ ceramics have a typical columnar grain, which indicates that the phase structure of tungsten bronze exists in the x range and all samples show a dense microstructure. The average grain size ranges from 1.179 to 0.912 μm. The dielectric properties were studied by complex impedance spectroscopy in the temperature range from 30 °C to 800 °C where an anomaly was observed in a few compositions characterized by a maximum of the dielectric permittivity that shifts with increasing frequency at higher temperatures. The presence of a strong dispersion over a wide range of temperatures, is probably related to cationic disorder within the Ba₄(Nd_1-xSm_x)_28/3(Ti_0,95Zr_0,05)₁₈O₅₄ structure.     

Phase-structure evolution and electrocaloric effect in Sr2NaNb4.3Ta0.7O15-based filled tungsten bronze ceramics

To investigate the effect of A-site substitution on phase-structure evolution and electrocaloric effect (ECE) in filled tungsten bronze ceramics, Sr_2–3x/2La_xNaNb_4.3Ta_0.7O₁₅ (x = 0, 0.02, 0.04, and 0.06) ceramics are prepared through a two-step solid-state reaction method. Rietveld refinement shows that La₂O₃ addition achieves a partial transition from orthorhombic (Im2a) to tetragonal phase (P4bm). A decrease in phase transition temperature, enhancement in distortion degree, and relaxation behavior are observed with increasing La₂O₃ content, as demonstrated by dielectric, in situ Raman spectra, and ferroelectric properties. The indirectly measured ECE shows a non-monotonic variation, with room-temperature ECE ranging from 0.19, 0.15, 0.23 to finally 0.19 K for x = 0, 0.02, 0.04, and 0.06 compositions, respectively. This evolution trend is explained by an interplay of relaxation and multiphase coexistence. Directly measured ECE is also performed and a positive ECE character with a good temperature stability is observed. This work not only establishes relationships between phase-structure evolution and ECE in filled Sr₂NaNb_4.3Ta_0.7O₁₅ tungsten bronze ceramics, but also presents its possibility in applications of thermally stable EC devices.     

Strong tribocatalytic dye degradation by tungsten bronze Ba4Nd2Fe2Nb8O30

The triboelectric effect has recently demonstrated its great potential in environmental remediation and even new energy applications for triggering a number of catalytic reactions by utilizing trivial mechanical energy. In this study, Ba₄Nd₂Fe₂Nb₈O₃₀ (BNFN) submicron powders were used to degrade organic dyes via the tribocatalytic effect. Under the frictional excitation of three PTFE stirring rods in a 5 mg/L RhB dye solution, BNFN demonstrates a high tribocatalytic degradation efficiency of 97% in 2 h. Hydroxyl radicals (?OH) and superoxide radicals (?O₂－) were also detected during the catalysis process, which proves that triboelectric energy stimulates BNFN to generate electron-hole pairs. The tribocatalysis of tungsten bronze BNFN submicron powders provides a novel and efficient method for the degradation of wastewater dye by utilizing trivial mechanical energy.     

Grain-size-insensitive dielectric properties of Sr0.6Ba0.4Nb2O6 relaxor ferroelectric ceramics with tetragonal tungsten bronze structure

The influence of grain size on the dielectric properties of Sr_0.6Ba_0.4Nb₂O₆ (SBN60) ceramics with a tetragonal tungsten bronze structure was examined. The conventional and two-step sintering procedures were used to fabricate dense and phase-pure SBN60 ceramics with grain sizes ranging from 2.2 μm to 17.3 μm. In the temperature dependence of the dielectric permittivity, all samples showed a relaxor-like broad dielectric peak with a maximum at around 60 °C. The maximum dielectric permittivity and dielectric maximum temperature of the SBN ceramics were found to be less dependent on the grain size. The reason for the grain-size-insensitive dielectric properties of the SBN60 ceramics is discussed based on the analysis of the dielectric properties and Raman spectra.     

Temperature‐stable unfilled tungsten bronze dielectric ceramics: Ba3.5Sm1.5Fe0.75Nb9.25O30

Unfilled tungsten bronze ceramic Ba_3.5Sm_1.5Fe_0.75Nb_9.25O₃₀ was prepared by a conventional solid‐state reaction method. Its structure, microstructure, and dielectric properties were investigated. Dielectric behaviors indicate that the ceramics are ferroelectric at room temperature with a remnant polarization ~1.61 μC/cm² and a coercive field ~13.91 kV/cm. Ba_3.5Sm_1.5Fe_0.75Nb_9.25O₃₀ ceramics have temperature stability with a high dielectric constant (ε_r) varying from 172 to 202 at the temperature range ?55 to 200°C (at 1 MHz). Capacitance change rate of the sample is less than 15%. These results indicate that Ba_3.5Sm_1.5Fe_0.75Nb_9.25O₃₀ ceramics might be promising in temperature‐stable multi‐layer ceramic capacitors.     

Relaxor behavior and ferroelectric properties of a new Ba4SmFe0.5Nb9.5O30 tungsten bronze ceramic

Unfilled tungsten bronze ceramics with a composition of Ba₄SmFe_0.5Nb_9.5O₃₀ were prepared by the conventional solid-state sintering method. The phase, microstructure, dielectric and ferroelectric properties were studied. Room temperature XRD results indicated that the ceramic occurs in the tetragonal space group P4bm phase with cell parameters of a=b=12.4712(2) Å and c=3.9430(2) Å. The temperature-dependent dielectric properties, XRD data and Raman spectra data indicated that BSFN ceramics exhibit no phase changes from 35 °C to 450 °C. Fitting of a Vogel-Fulcher relationship with an activated energy E_a of 0.11 eV indicates an unambiguous dielectric relaxor state near room temperature. Furthermore, the BSFN ceramics exhibited residual polarization and coercive field of 3.45 µC/cm² and 24.65 kV/cm, respectively.     

Crystal structure,relaxor behaviors and energy storage performance of (Sr0.7Ba0.3)5LaNb7Ti3O30 tungsten bronze ceramics

In this work, a new (Sr_0.7Ba_0.3)₅LaNb₇Ti₃O₃₀ system ceramic with a filled tetragonal tungsten bronze structure was proposed and fabricated by the traditional solid phase method. Crystal structure, relaxor behaviors and energy storage capabilities were studied. Large relaxor activation energy indicates a “weakly coupled relaxor” mechanism, which is advantageous for obtaining better energy storage performance, and lower conductance activation energy can further prove the formation of the filled tungsten bronze structure. More importantly, a dielectric breakdown strength of up to 35.5 kV/mm was obtained. The releasable energy density and efficiency at 24 kV/mm are 1.36J/cm³ and 91.9%, respectively. In addition, due to the high current density and high dielectric breakdown strength, a current density of 477.5 A/cm² and a power density of 42.9 MW/cm³ are achieved, meaning that SBLNT ceramic is a potential candidate dielectric ceramic for energy storage.     

Enhanced dielectric,ferroelectric and magnetic properties of Ba4Sm2Fe2Nb8O30 RT multiferroics prepared by microwave sintering

High density Ba₄Sm₂Fe₂Nb₈O₃₀ (BSFN) multiferroics ceramics with tetragonal tungsten bronze structure had been prepared by microwave sintering (MS) for 30min and conventional sintering (CS) methods for 4 h at 1275 °C. Single tungsten bronze phase and equiaxial grains are obtained for the MS BSFN ceramics, while a small amount secondary phase of SmNbO₄ is observed in the CS BSFN ceramics with columnar grains. Compared to Ba₄Sm₂Fe₂Nb₈O₃₀ ceramics prepared by CS method, enhanced dielectric, ferroelectric and magnetic properties are achieved for the MS BSFN ceramics. The values of electric polarization Pr and coercive electric field Ec are 2.11 μC/cm² and 7.14 kV/cm for the MS BSFN ceramics, respectively. Meantime, the magnetic polarization Mr of 0.410emu/g and coercive magnetic field Hc of 2930Oe are also obtained for the MS BSFN ceramics. Based on the density, crystal structure, point defect and grain, the reasons of enhanced dielectric, ferroelectric and magnetic properties are discussed for the MS BSFN ceramics It is indicated that Ba₄Sm₂Fe₂Nb₈O₃₀ is an intrinsic room temperature multiferroic materials.     

Effect of annealing atmosphere on the structure and dielectric properties of unfilled tungsten bronze ceramics Ba4PrFe0.5Nb9.5O30

Unfilled tungsten bronze ceramics with the nominal formula Ba₄PrFe_0.5Nb_9.5O₃₀ were synthesized via the standard solid-state sintering route, and the effects of oxygen vacancies on the dielectric and electrical properties were investigated in addition to the structure. Room-temperature X-ray diffraction showed that the N₂-annealed sample had the largest cell volume. Low-temperature spectrum showed that N₂ annealing rendered the dielectric constant and dielectric loss more frequency dispersive, whereas O₂ annealing inhibited the frequency dispersion. The dc conductivity of all the samples originated from the electrons produced in the second ionization of oxygen vacancies and was most likely controlled by a mixed conduction mechanism of the electron and oxygen-vacancy ions. The N₂-annealed sample has the highest dc conductivity owing to its high concentration of oxygen vacancies. The broadening of the Raman lines and the decrease of Raman intensity for the N₂-annealed sample originated from a significant structural disorder. X-ray photoelectron spectra demonstrated that the increased oxygen vacancies caused by the change of valences of Fe and Pr ions contributed to the structural disorder.     

Chemical interpretation,enhanced ferroelectric property and dielectric breakdown strength of CuO-added Sr2Nb2O7 ceramics

The structural interpretation and electrical properties of perovskite layer structured (PLS) Sr₂Nb₂O₇-xwt%CuO ceramics prepared by solid-state reaction method are investigated. The chemical interpretation of enhanced piezoelectricity is confirmed to be attributed to the rotation and/or distortion of oxygen octahedron caused by possible Cu²⁺ substitution at the A-site of Sr₂Nb₂O₇ by XRD refinement and variable-temperature Raman spectra. Sr₂Nb₂O₇-xwt%CuO (x?=?0.3, 0.5 and 0.7) ceramics shows enhanced ferroelectric properties with a larger P_r of ～4.1?μC/cm² and a smaller E_c of ～63.1?kV/cm. This study further explains the cations in A-site play a major structural role in the polarization process for PLS system. It was found that dielectric breakdown strength increases up to 258.8?kV/cm and then decreases gradually with the increase of CuO content. Impedance spectroscopy indicated that CuO addition could be helpful in increasing the grain boundary resistance then dielectric breakdown strength.     

Relaxor behavior and energy storage density induced by B-sites substitutions in (Ca0.28Ba0.72)2.1Na0.8Nb5O15 Tungsten bronze ceramics

Lead-free relaxor ferroelectrics (Ca_0.28Ba_0.72)_2.1Na_0.8Nb_5-xSb_xO₁₅ (CBNNS) and (Ca_0.28Ba_0.72)_2.1Na_0.8Nb_5-yTa_yO₁₅ (CBNNT) with tungsten bronze structure were fabricated via solid-state reactions. The obtained CBNNS and CBNNT ceramics showed different dielectric behaviors. Only the CBNNS ceramics revealed an intensified diffusion and relaxor-like characteristics, which could be verified by the modified Curie–Weiss law. The relaxor behaviors in CBNNS were attributed to the radii difference between Sb⁵⁺ and Nb⁵⁺ ions co-occupying in B-sites. For the substitution of Nb⁵⁺ by Sb⁵⁺ in CBNNS ceramics, the change from macroscopic polarization to local polarization could also give rise to the obvious relaxor behavior. The Raman spectra verified a larger off-centering of the cation and a higher distortion degree for BO₆ octahedron in the ab plane for CBNNS ceramics when compared with those of CBNNT. In addition, the ferroelectric properties of CBNNS ceramics further indicated the relaxor ferroelectric nature, and also confirmed that the relaxor behavior helped to improve the energy-storage performance.     

Correlation between the radius of acceptor ion and the dielectric properties of co-doped TiO2 ceramics

There is an urgent need to seek colossal dielectric materials with better dielectric properties because of the rapid development of the electronics industry. In this work, (M_0.5Nb_0.5)_0.01Ti_0.99O₂ (M = Al, In, Eu) ceramics are synthesized by traditional solid-state reaction, and recorded as ANTO, INTO and ENTO ceramics, respectively. The effects of different radius acceptor ions on the phase structure, microstructure and dielectric properties of TiO₂ based ceramics are investigated. It is found that all samples are rutile phases without the generation of the second phase, but the difference of ion radius resulted in lattice distortion of different degrees, which affects the defect structure in the material. The dielectric properties of the materials are related to the defect structure in the material, and the ability to localize electrons in different defect structures is different. The dielectric properties of ANTO ceramics are similar to those of un-doped TiO₂; INTO ceramics have the lowest dielectric loss (0.039), the best temperature and frequency stability, and there are no dielectric loss peak near the room temperature; The ENTO ceramics has the maximum permittivity (2.01 × 10⁵).     

Investigations of nano-crystalline Sr0.5Ba0.5Nb2O6 and bulk ceramics synthesized by a polymerization method using PEG400

Nano-crystalline Sr_0.5Ba_0.5Nb₂O₆ was synthesized by a one-pot method using PEG400 and citric acid. Calcination of the (Sr,Ba,Nb)-gel at 600 °C leads to Sr_0.5Ba_0.5Nb₂O₆ with a crystallite size of 24(2) nm and a specific surface area of 38.5(10) m² g^?1. Sintering up to 1325 °C leads to ceramics with globular or irregular-shaped grains and average grain sizes between 1.3 and 2.4 μm, whereas higher temperatures lead to a rod-like microstructure. The indirect allowed optical band gap varies between 3.70(5) and 3.29(5) eV. Dielectric measurements show a diffuse phase transition and weak relaxor properties. The maximum of the permittivity occurs between 116 and 147 °C. The frequency dependence of the impedance can be well described by one or two RC-circuits depending on sintering temperature. The melting temperature is determined as 1506(7) °C with ΔH_f = 140(20) kJ mol^?1. The average linear thermal expansion coefficient is found to be 10.5(5)?10^?6 K^?1.     

Significant increase in comprehensive energy storage performances of Ca0.5(Sr0.5Ba0.5)2Nb5O15-based tungsten bronze relaxor ceramics

A series of unfilled tungsten bronze Ca_0.5(Sr_0.5Ba_0.5)₂Nb_5-xSb_xO₁₅ (CSBNS_x) ceramics were synthesized by traditional solid-phase method to investigate the relationship between energy storage performances (ESPs) and structural changes including the lattice distortion and structural modulation. The Sb⁵⁺ substitution prompted the crystal structure to evolve from tetragonal P4bm to orthorhombic Ama2 and then to tetragonal P4/mbm symmetry. The appearance of incommensurate Ama2 structure and the polar nanoregions were the main factors for the stronger relaxation behavior. Moreover, the effectively enhanced bulk and grain boundary resistivity were conducive to the increase of breakdown field E_b. The P-E loops become more elongated and the T_c peak moved towards lower temperatures. Ultimately, an excellent recoverable energy density (W_rec, 2.38 J/cm³), energy storage efficiency (η, 85%), power density (P_D, 200.9 MW/cm³) and current density (C_D, 1545.6 A/cm²) were achieved simultaneously for CSBNS_0.2 ceramics. The excellent ESPs make the CSBNS_0.2 show considerable potential in high-power capacitor applications.     

The effect of co-substitution on the microwave dielectric properties of Ba6-3xNd8+2xTi18O54(x=2/3) ceramics

In this work, the microwave dielectric properties of Ba₄(Nd_1-yBi_y)_28/3Ti_18-x(Al_1/2Ta_1/2)_xO₅₄(0≤x≤2, 0.05≤y≤0.2) ceramics co-substituted by A/B-site were studied. Firstly, (Al_1/2Ta_1/2)⁴⁺ was used for substitution at B-site. At 0≤x≤1.5, the above mentioned ceramic was found to exist in single-phase tungsten bronze structure, but at x = 2.0, the secondary phase appeared. Although the dielectric constant decreased by doping the (Al_1/2Ta_1/2)⁴⁺, but the quality factor was observed to improve by 40% and the temperature coefficient of resonant frequency decreased by 75%. Based on the above results, Bi³⁺ was introduced to Ba₄Nd_28/3Ti₁₇(Al_1/2Ta_1/2)O₅₄. The introduction of Bi³⁺ reduced the sintering temperature, greatly improved the dielectric constant, and ultimately decreased the temperature coefficient of resonant frequency, but it led to deterioration of quality factor. At last, with appropriate site-substitution content control (x = 1.0,y = 0.15), excellent comprehensive properties (ε_r = 89.0, Q × f = 5844 GHz @ 5.89 GHz,TCF = +8.7 ppm/°C) were obtained for the samples sintered at 1325 °C for 4 h.     

Effects of preparation method on the microstructure and electrical properties of tungsten bronze structure Sr2NaNb5O15 ceramics

Herein, Sr₂NaNb₅O₁₅ (SNN) ceramics with a filled tungsten bronze structure were synthesized by the various methods of conventional mixed-oxide (CMO), two-step sintering (TSS), reactive sintering (RS) and molten salt synthesis (MSS). It was found that varying the preparation method could result in significant differences in the ceramic morphology, relative density, and electrical properties. The TSS, RS and MSS methods all produced a pure tungsten bronze phase SNN ceramic, though the CMO method could not. Further characterization revealed a sharp drop in the Curie temperature and significant deterioration of the dielectric properties in the ceramics prepared by MSS compared to the other methods, likely owing to residual K⁺ from the molten salt. Systematic comparison of the electrical properties of the ceramics produced by the RS and TSS method found that the RS method was the most suitable preparation method for SNN ceramics. The superior ferroelectric and piezoelectric properties in the sample produced by the RS method were attributed to the highly distorted NbO₆ octahedron, as suggested by Raman spectroscopy.     

Structural,dielectric and ferroelectric properties of lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramics prepared by Plasma Activated Sintering

Lead-free Ba_0.85Ca_0.15Zr_0.10Ti_0.90O₃ (BCZT) ceramics were prepared by Plasma Activated Sintering (PAS). The influence of PAS sintering temperature on the crystalline phase, microstructure, and, dielectric and ferroelectric properties of BCZT ceramics were studied. The phase structure of BCZT ceramics first changed from rhombohedral phase to the coexistence of rhombohedral and tetragonal phases and then to tetragonal phase as the sintering temperature increased. Microstructural characterization of BCZT ceramics indicated that PAS can obtain a compact microstructure at lower temperatures of 1150–1300 °C compared with that from common pressureless sintering. The BCZT ceramics showed different degrees of diffuseness with increased temperature, and the diffuseness exponents C are all approximately on the order of 10⁵ °C. The dielectric and ferroelectric properties of BCZT ceramics were enhanced with increased sintering temperature. BCZT ceramics sintered at 1250 °C exhibited optimum properties of room-temperature ε_r=2863, ε_m=6650, and 2P_r=25.24 μC/cm², resulting from the relatively higher tetragonal phase content of the MPB between tetragonal and rhombohedral phases together with a compact microstructure.     

Relationship between distortion of crystal structure and dielectric properties of complex perovskite oxide Ba(Co,Zn)1/3Nb2/3O3 thin films

The oxygen octahedral can be distorted by epitaxial strain due to the lattice mismatch. The epitaxial strain (ε_yy) linearly decreases from ? 0.244% to ? 0.445% with the growth temperature. Thin films grow along c axis on the SrTiO₃ substrate and exhibit the epitaxial relationship of Ba(Co,Zn)_1/3Nb_2/3O₃ (001) // SrTiO₃ (001). The superlattice reflections arising from in-phase tilting of the oxygen octahedra are clearly visible along [010] and [111] zone axis. The IR modes at 330 cm^?1 and 390 cm^?1 related to in-phase tilting are observed in far-infrared reflectivity spectroscopy. The calculated Q×f values from far-infrared reflectivity spectra of films grown at 550 °C to 700 °C increases from 51,000 GHz to 91,000 GHz mainly due to the enhancement of crystalline quality. The intrinsic quality factor (Q) is mainly contributed by O-(Co, Nb)-O and O-(Zn, Nb)-O bonding modes, while in-phase tilting in BCZN films may result in enhanced dielectric constants.     

Effects of in situ BiScO3 nanoparticles on the microstructure and dielectric properties of Ba0.7Sr0.3Ti0.9925Tm0.0103@BiScO3 ceramics

Ba_0.7Sr_0.3Ti_0.9925Tm_0.01O₃ (BST) and core-shell structured BST@BiScO₃ ceramics were prepared via a sol-precipitation and conventional sintering method. The single perovskite structure of the BST@BiScO₃ ceramic powder and bulk materials were confirmed via X-ray diffraction. Additionally, transmission electron microscopy analysis revealed that the surface of the BST powder was successfully coated in situ with amorphous BiScO₃ nanoparticles. Scanning electron microscopy analysis indicated that the grain sizes of the BST@x mol.% BiScO₃ (x = 1, 2, 3) ceramics decreased with increasing coating amounts. For x = 4 mol.%, the grain size of the BST@BiScO₃ ceramic significantly increased relative to that of BST@x mol.% BiScO₃ (x = 1, 2, 3), and some smaller particles were found to have precipitated. The results revealed that the dielectric constants of the BST@BiScO₃ ceramics decreased at room temperature, but remained above 10000, and that the dielectric loss was significantly reduced. In addition, the Curie peak of BST@x mol.% BiScO₃ (x = 1, 2, 3, 4) broadened and shifted to a higher temperature. The Curie peak of the BST@4 mol.% BiScO₃ ceramic material was nearly a flat line. In this work, dielectric materials with high dielectric constants and low dielectric losses were successfully prepared; thus, they met the requirements of miniaturization and temperature stability for dielectric ceramics.