Effect of two different dopants (Mg2+ and Ca2+) and processing parameters on γ-phase stabilization and conductivity of Bi4V2O11−δ

Mg²⁺ and Ca²⁺ doped Bi₄V₂O_11−δ systems are synthesized by a melt quench technique followed by heat treatment. The Ca²⁺ doped samples show higher density than Mg²⁺ doped samples. All the quenched samples show γ-phase stabilization irrespective of dopants and their concentration. The γ-phase stabilization takes place at lower dopant concentration than earlier reported systems. The conversion of γ-phase to ordered β-phase is observed with heat treatment for Bi₄V_2−xMg_xO_11−δ (x=0.05, 0.10 and 0.20) and Bi₄V_2−xCa_xO_11−δ (x=0.05 and 0.10). Ca²⁺ doped system, particularly high concentration (x=0.15 and 0.20) did not show γ→γ׳ phase transition. The lowest activation energy; E_a is observed for Bi₄Mg_0.15V_1.85O_11−δ sample ~0.74 eV in the temperature range 570–750 °C.     

Microwave dielectric properties of Ca1-xBaxMgSi2O6 ceramics

Ca_1-xBa_xMgSi₂O₆(x = 0–0.4) ceramics were prepared through a traditional solid-state reaction sintering route with various sintering temperatures. The effects of substituting Ba²⁺ for Ca²⁺, the relative density, phase composition, crystal morphology, and microwave dielectric properties of Ca_1-xBa_xMgSi₂O₆ (x = 0–0.4) ceramics were thoroughly studied. X-ray diffraction patterns indicate a single phase was formed in the samples when x ≤ 0.2, and the second phase BaMg₂Si₂O₇ appeared at x = 0.4. As the amount of Ba²⁺ substitution increases, the Q×f value first increases and then decreases due to the combined effects of FWHM of peak v₁₁ and atomic packing density, and the ${\epsilon }_r$ value was increased continuously which was closely corrected with the relative density and molecular polarization. The ${\tau }_f$ value improved slightly with the substituting Ba²⁺ for Ca²⁺. Typically, the Ca_0.88Ba_0.12MgSi₂O₆ ceramic can be well sintered at 1275 °C for 4 h with a maximum relative density of 99.3%, and possesses optimal microwave dielectric properties: ${\epsilon }_r=7.49$, $Q\times f=64310$ GHz, ${\tau }_f=-44.02$ ppm/°C.     

Enhanced thermoelectric properties of Na and Mg co−doped BiCuSeO

A series of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) was fabricated by a two?step solid?state reaction and spark plasma sintering (SPS), and the influence of Mg²⁺ doping on the thermoelectric properties of Bi_0.97Na_0.03CuSeO was systematically investigated. The SPS processed?Bi_0.97?xNa_0.03Mg_xCuSeO had a ZrSiCuAs?type tetragonal crystal structure (space group P4/nmm). The Mg²⁺ doping appreciably enhanced the electrical conductivity due to the increase in hole concentration. Furthermore, the Mg²⁺ doping increased the grain boundary areas and bulk porosity and induced the strain field and mass fluctuations, thereby reducing the phonon thermal conductivity. We significantly improved the thermoelectric performance of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) by enhancing the thermoelectric power factor and by reducing the thermal conductivity.     

Pb-free (Ba1−xCax) (Zr0·18Ti0.82)O3 dielectric tunable ceramics with giant figure-of-merit under low electric field

Dielectric tunable devices with improved overall tunability properties are in urgent demand for tunable applications. Hence, a series of Pb-free dielectric tunable ceramics based on (Ba_1?xCa_x) (Zr_0·18Ti_0.82)O₃ (abbreviated as BCZTx, x = 0.05–0.21, corresponding to BCZT05 to BCZT21) were carefully prepared using the traditional solid-state route in this work. The crystal structure, surface morphology, dielectric properties, and tunable performance of different BCZTx ceramics at room temperature (RT) were systematically studied. The temperature-dependent dielectric tunable performance was further investigated. The phase evolutions were co-confirmed by XRD and $\frac{d{\epsilon }_r}{dT}$-T curves. Interesting, high tunability (86.09%), together with a relatively low dielectric loss (~0.19% @7.42 kV/cm) were obtained in BCZT09 ceramics at 1 kHz, resulting in a giant figure-of-merit (FOM) of 448, which implied these ceramics are promising matrix for dielectric tunable applications that operated at RT. From the Tunability?T and FOM?T curves, it can be found that Ca²⁺ incorporation can improve the temperature stability of dielectric tunable performance to a certain extent. In addition, the FOM?T curve of BCZT09 showed advantages over other compositions in the temperature range of ?20 °C–85 °C, and its maximum FOM (~886) was reached at 40 °C. These observations suggest the BCZT09 ceramic as a promising matrix for application in dielectric tunable devices operating at RT. This work may guide the design of novel high-performance tunable ceramic materials.     

Effect of La substitution on the structural and electrical properties of BaBi4−xLaxTi4O15

Pure and lanthanum doped barium bismuth titanate BaBi_4−xLa_xTi₄O₁₅ (BBLT, x=0, 0.05, 0.15, 0.30) ceramics were prepared utilizing solid state method. The X-ray diffraction (XRD) data confirmed formation of single-phase Aurivillius compounds while SEM micrographs did not show evident grain size change of doped ceramics. Dielectric properties were investigated in 1.21 kHz to 1 MHz frequency range and in the temperature range of 20 to 727 °C. When Bi³⁺ is substituted with La³⁺, a significant disorder was induced and the material exhibited broadening of the phase transition. Impedance analysis confirmed the presence of two semicircular arcs in doped samples suggesting the existence of grain and grain-boundary conduction. The dc-conductivity and activation energies were evaluated for all compositions.     

Sol-gel synthesis,microstructure, and thermoelectric properties of Ca2.8-xBixDy0.2Co4O9+δ

Small-sized Ca_2.8-xBi_xDy_0.2Co₄O_9+δ (0 ≤ x ≤ 0.1) powders with a plate-like morphology were synthesized via the citric acid-assisted sol-gel method. The structural and thermoelectric properties of Ca_2.8-xBi_xDy_0.2Co₄O₉ samples were studied with an emphasis placed on the Bi content and the fabrication process. The as-sintered Ca_2.8-xBi_xDy_0.2Co₄O₉ samples exhibited a single Ca₃Co₄O_9+δ phase and a plate-like morphology. With increased Bi content, the grain size of the sintered Ca_2.8-xBi_xDy_0.2Co₄O₉ samples decreased, whereas the density of the sintered Ca_2.8-xBi_xDy_0.2Co₄O₉ samples increased. The incorporation of Bi up to x = 0.075 yielded high electrical conductivity. Meanwhile, the Seebeck coefficient decreased with increases in Bi content. The largest power factor (2.18 × 10⁻⁴ W m⁻¹ K⁻² at 800 °C) was obtained for the twice-sintered Ca_2.725Bi_0.075Dy_0.2Co₄O₉. The partial substitution of Bi for Ca and the twice sintering were a highly effective route for improving the thermoelectric properties of Ca_2.8Dy_0.2Co₄O₉.     

Electrical conduction behavior in nonstoichiometric BaBixNb5O15±δ tungsten bronze ceramics

The BaBi_xNb₅O_15±δ (BB_xN, 0.98 ≤ x ≤ 1.02) ceramics were synthesized via solid-state reaction to investigate the effect of Bi³⁺ nonstoichiometry on their microstructure and electrical conduction behavior. The study of the relationship between structure and conductive behavior revealed two main conclusions: (1) as the concentration of Bi³⁺ content increased, from deficiency to excess, the oxygen vacancies decreased, and the lattice unit volume V gradually increased; (2) there was a low-frequency Warburg electrode response besides the medium-frequency grain boundary response and high-frequency grain response, and the Bi³⁺ introduction could reduce conductivity. In addition, the dielectric anomalies indicated by the T₁ peak and the T₂ peak at 300 °C and 500 °C are related to the Warburg electrode response and to the Bi vacancy and oxygen vacancy defects, respectively.     

Electrical properties and light up conversion effects in Bi3.79Er0.03Yb0.18Ti3−xWxO12 ferroelectric ceramics

Modified ceramic compositions of Bi_3.79Er_0.03Yb_0.18Ti_3−xW_xO₁₂ with fixed Er and Yb content, and a varying W content (x=0.0, 0.01, 0.03, 0.06 and 0.10) are prepared. The site selectivity of Er³⁺, Yb³⁺, and W⁶⁺ cations is analyzed, and their influence on the electrical and light up conversion properties is studied. Formation of single phase orthorhombic structure is confirmed with enhanced grain growth up to x=0.03, and for (x≥0.04–0.10) the grain growth is inhibited, and the orthorhombic distortion is relaxed. Raman spectroscopy reveals W⁶⁺ cation substitutes preferentially at the B-site replacing Ti⁴⁺ ions in the Bi₄Ti₃O₁₂ lattice structure. Increasing W⁶⁺ donor concentration reduces the conductivity effects by lowering the oxygen vacancies. Reduced dielectric losses (tan δ=0.003) and dispersion with frequency in the range (10⁻²–10 Hz) are observed, and improvements in the remnant polarization (2P_r=28.86 μC/cm²) are seen up to an optimum content of x=0.03. At higher W content (x>0.03), the properties tend to degrade due to structural relaxation and microstructural changes. Up conversion photoluminescence (UC-PL) under 980 nm excitation shows strong emission in the green and red bands due to enhanced crystal field around the Er³⁺ ions for an optimum W content of x=0.06. A weak blue emission band around (~492 nm) is observed by cooperative emission (CE) due to radiative relaxation of an excited Yb–Yb pair from a virtual level. Variation of UC emission intensity with pump-power confirms a two-photon mechanism for the up conversion process.     

A simple Bi3+ self-doping strategy constructing pseudo-tetragonal phase boundary to enhance electrical properties in CaBi2Nb2O9 high-temperature piezoceramics

Calcium bismuth niobate (CaBi₂Nb₂O₉, CBN)-based ceramics are promising candidates for high temperature application, the electrical properties of which are commonly enhanced by complex ion substitution or texture processes. Here, we report that high piezoelectricity and high resistivity were achieved in Ca_1-xBi_2+xNb₂O₉ by constructing pseudo-tetragonal boundary through a simple strategy of Bi³⁺ self-doping. At the pseudo-tetragonal boundary, Ca_0.96Bi_2.04Nb₂O₉ ceramics maintain high Curie temperature T_c = 942 °C, and show high piezoelectric coefficient d₃₃ = 15.1 pC/N and high resistivity ρ_dc = 2 × 10⁶ Ω cm (@600 °C). It is proved that the good piezoelectric property mainly originates from the increase of domain density. In addition, Ca_0.96Bi_2.04Nb₂O₉ ceramics reveal good thermal depoling performance, remaining 90% of piezoelectricity after thermal depoling at 900 ℃, which is due to small thermal expansion and structural distortion. Our work provides a promising candidate for high temperature applications and an easy way to improve the performance of Aurivillius-type piezoelectric ceramics.     

Enhanced resistance in Bi(Fe1-xScx)O3-0.3BaTiO3 lead-free piezoelectric ceramics: Facile analysis and reduction of oxygen vacancy

We reported a facile analysis and reduction of oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}^{??}$) in 0.7Bi(Fe_1-xSc_x)O₃-0.3BaTiO₃ (0≤x≤0.08) ceramics. The leakage current mechanism was investigated intensively. Our results indicated that oxygen vacancies are the main cause for the high conductivity in BF-BT ceramics, and their concentration was quantitatively estimated from the Bi³⁺ content and the average oxidation state of iron. The ${\mathrm{V}}_{\mathrm{O}}^{??}$ concentration was effectively suppressed and the insulation resistance was enhanced by almost two orders of magnitude after doping 2%mol Sc³⁺. The enhanced insulation resistance contributed to excellent piezoelectric properties with d₃₃ = 165 pC/N, T_C = 505 °C, and k_p = 26%. The proposed analysis method used to quantify the ${\mathrm{V}}_{\mathrm{O}}^{??}$ concentration provides valuable indications to reduce the leakage current density and improve the piezoelectric properties of BF-BT based ceramic.     

Influence of WO3‐Doping on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics Sintered at 950°C

The phase evolution, microstructure, and electrical properties of WO₃‐doped ZnO–Bi₂O₃‐based varistors were investigated for different amounts x (0 ≤ x ≤ 1.60 mol%) of the dopant. When x was less than 0.40, the dissolved W⁶⁺ in the β‐Bi₂O₃ acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40 mol%, which meant an amount of WO₃ equal to that of Bi₂O₃, the electrical properties dramatically increased, which means the W⁶⁺ donor effect is removed at the grain boundaries because a new Bi₂WO₆ phase was formed in the grain‐boundary regions. The Bi₂WO₆ phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO₃ that is greater than the content of Bi₂O₃), the electrical properties were steadily reduced in comparison to the composition with x = 0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO₄ phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis.