Synthesis,structural and electrical properties of novel pyrochlores in the Bi2O3–CuO–Ta2O5 ternary system

A series of non-stoichiometric cubic pyrochlores with general formula, Bi_3?xCu_1.8Ta_3+xO_13.8+x (BCT) was successfully prepared by solid state reaction at the firing temperature of 950 °C over 2 days. The solid solution mechanism is proposed as one-to-one replacement of Bi³⁺ for Ta⁵⁺, together with a variation in oxygen content in order to achieve electroneutrality. The solid solution limit is confirmed by X-ray diffraction technique (XRD) for which linear variation of lattice constants is observed at 0 ≤ x ≤ 0.6. The refined lattice constants are found to be in the range of 10.4838 (8) Å–10.5184 (4) Å and the grain sizes of these samples determined by scanning electron microscopy (SEM) fall between 1 and 40 μm. Meanwhile, thermal analyses show no physical or chemical change for the prepared pyrochlores. The relative densities of the densified pellets for AC impedance measurements are above 85% and the measured relative permittivity, ?′ and dielectric loss, tan δ for composition, x = 0.2 at ambient temperature are ～60 and 0.07 at 1 MHz, respectively. The calculated activation energies are 0.32–0.40 eV and the conductivity values, Y′ are in the order of 10^?3 at 400 °C. The conduction mechanisms of BCT pyrochlores are probably attributed to the oxygen non-stoichiometry and mixed valency of copper within the structure.     

High-temperature conductivity,stability and redox properties of Fe3−xAlxO4 spinel-type materials

Iron-based oxides are considered as promising consumable anode materials for high temperature pyroelectrolysis. Phase relationships, redox stability and electrical conductivity of Fe_3?xAl_xO₄ spinels were studied at 300–1773 K and p(O₂) from 10^?5 to 0.21 atm. Thermogravimetry/XRD analysis revealed metastability of the sintered ceramics at 300–1300 K. Low tolerance against oxidation leads to dimensional changes of ceramics upon thermal cycling. Activation energies of the total conductivity corresponded to the range of 16–26 kJ/mol at 1450–1773 K in Ar atmosphere. At 1573–1773 K and p(O₂) ranging from 10^?5 to 0.03 atm, the total conductivity of Fe_3?xAl_xO₄ is nearly independent of the oxygen partial pressure. The conductivity values of Fe_3?xAl_xO₄ (0.1 ≤ x ≤ 0.4) at 1773 K and p(O₂) ～10^?5 to 10^?4 atm were found to be only 1.1–1.5 times lower than for Fe₃O₄, showing high potential of moderate aluminium additions as a strategy for improvement of refractoriness for magnetite without significant deterioration of electronic transport.     

Effect of CaO addition on the structure and electrical conductivity of the pyrochlore-type GdSmZr2O7

Polycrystalline GdSm_1?xCa_xZr₂O_7?x/2 (0 ≤ x ≤ 0.20) ceramics have been prepared by the solid-state reaction method. The effects of CaO addition on the microstructure and electrical properties of the pyrochlore-type GdSmZr₂O₇ ceramic were investigated. GdSm_1?xCa_xZr₂O_7?x/2 (x ≤ 0.05) ceramics exhibit a pyrochlore-type structure; however, GdSm_1?xCa_xZr₂O_7?x/2 (0.10 ≤ x ≤ 0.20) ceramics consist of the pyrochlore-type structure and a small amount of CaZrO₃. The total conductivity of GdSm_1?xCa_xZr₂O_7?x/2 ceramics follows the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. GdSm_1?xCa_xZr₂O_7?x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10^?4–1.0 atm at each test temperature. The highest total conductivity is about 1.20 × 10^?2 S cm^?1 at 1173 K for the GdSm_0.9Ca_0.1Zr₂O_6.95 ceramic.     

Microwave dielectric properties of Bi-substituted La(Mg1/2Ti1/2)O3

The system La(Mg_1/2Ti_1/2)O₃–Bi(Mg_1/2Ti_1/2)O₃ (LMT–BMT) was investigated in respect to formation of perovskite solid solutions based on lanthanum magnesium titanate. Single-phase perovskite (1 − x)LMT–xBMT ceramics (0 ≤ x ≤ 0.3) were prepared and their crystal structure and dielectric properties were studied. It has been found that within the solubility range the crystal structure of Bi-substituted LMT remains monoclinic, P2₁/n. Unit cell volume was evaluated to be almost independent on x, varying within the experimental error. Relative permittivity of the ceramics increases by almost a factor of 3 in the range 0 ≤ x ≤ 0.3 and its value is 40–45 at the compositional region where temperature coefficient of the resonant frequency passes a zero-value. Compositional and temperature variations of the dielectric parameters for LMT–BMT estimated at different frequency ranges are considered in comparison with those observed in other Bi-substituted ceramics based on LMT.     

Raman spectra and extended X-ray absorption fine structure characterization of La(2−x)/3NaxTiO3 and Nd(2−x)/3LixTiO3 microwave ceramics

A-site deficient perovskite compounds, La_(2?x)/3Na_xTiO₃ (0.02 ≤ x ≤ 0.5) and Nd_(2?x)/3Li_xTiO₃ (0.1 ≤ x ≤ 0.5) microwave ceramics, were investigated by Raman scattering. Nd_(2?x)/3Li_xTiO₃ (0.1 ≤ x ≤ 0.5) was also investigated by extended X-ray absorption fine structure (EXAFS) measurement. The Raman shifts of the E (239 cm^?1) and A₁ (322 cm^?1) modes of La_(2?x)/3Na_xTiO₃ were found to decrease with x. However, the E (254 cm^?1) and A₁ (338 cm^?1) of Nd_(2?x)/3Li_xTiO₃ were found to blueshift with x, which was caused by Li substitution. The redshift of the A₁ (471 cm^?1) phonon of Nd_(2?x)/3Li_xTiO₃ (0.1 ≤ x ≤ 0.3) indicates that O–Ti–O bonding forces lessen with Li concentration, which is consistent with the EXAFS result that Ti–O bond lengths increase for 0.1 ≤ x ≤ 0.3. For x > 0.3, the EXAFS result shows that Ti–O bond lengths decrease. Moreover, Ti–O bond lengths show strong correlation with the microwave dielectric constants of Nd_(2?x)/3Li_xTiO₃.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Phase transition behavior and electrical properties of (1 − x)Bi0.5Na0.5TiO3–x(Na0.53K0.44Li0.04)(Nb0.88Sb0.08Ta0.04)O3 lead-free ceramics

(1 ? x)Bi_0.5Na_0.5TiO₃–x(Na_0.53K_0.44Li_0.04)(Nb_0.88Sb_0.08Ta_0.04)O₃ (BNT–xNKLNST) with x = 0–0.10 lead-free piezoelectric ceramics were prepared by a solid state method, and the structure and electrical properties were investigated in this study. It is found that a morphotropic phase boundary (MPB) of rhombohedral (R) and tetragonal (T) phase exists in the range of 0.03 ≤ x ≤ 0.05 and the structure changes to paraelectric phase when x > 0.07. The samples with x = 0.05 exhibit improved electrical properties owing to the formation of MPB, which are as follows: piezoelectric constant d₃₃ = 120 pC/N, remnant polarization P_r = 39.4 μC/cm² and coercive field E_c = 3.6 kV/mm. These results indicate that the enhanced piezoelectric properties for BNT can be achieved by forming the coexistence of R and T phase.     

Abnormal variation of microwave dielectric properties in A/B site co-substituted (Ca1−0.3xLa0.2x)[(Mg1/3Ta2/3)1−xTix]O3 complex perovskite ceramics

A/B site co-substituted (Ca_1?0.3xLa_0.2x)[(Mg_1/3Ta_2/3)_1?xTi_x]O₃ ceramics (0.1 ≤ x ≤ 0.5) were prepared by solid state reaction and the structures, microstructures and dielectric properties were investigated. B site 1:2 cation ordering and oxygen octahedra tilting lead to monoclinic symmetry with space group P2₁/c for x = 0.1. For x above 0.1, the ordering was destroyed and the crystal structure became orthorhombic with space group Pbnm. The B site 1:2 cation ordering tended to be destroyed to form 1:1 ordering by the A site La³⁺ substitution. The dielectric constant increased linearly with increasing content of Ti⁴⁺ as the increasing second Jahn–Teller distortion enhanced the B site cation rattling. The temperature coefficient of resonant frequency and Qf values showed abnormal variations, which were refined to be caused by the increasing A site cation vacancy and diffused distribution of small size ordering domains respectively. Good combination of microwave dielectric properties was obtained at x = 0.5, where ?_r = 48, Qf = 21,000 GHz and τ_f = 2.2 ppm/°C.     

Relationships between crystal structure and electrical properties of Li0.055[Agx(K0.5Na0.5)1−x]0.945(Nb1−yTay)O3 ceramics

Electrical properties of perovskite Li_0.055[Ag_x(K_0.5Na_0.5)_1?x]_0.945(Nb_1?yTa_y)O₃ (0.00 ≤ x ≤ 0.04, 0.01 ≤ y ≤ 0.09) ceramics were investigated based on the structural characteristics. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phase was detected through the entire range of compositions. With increasing of Ta⁵⁺ content, the dielectric constant (?_r), piezoelectric coefficient (d₃₃) and electromechanical coupling factor (k_p) of Li_0.055(K_0.5Na_0.5)_0.945(Nb_1?yTa_y)O₃ ceramics were increased up to y = 0.07 and then decreased, while mechanical quality factor (Q_m) was increased. However, the ?_r, d₃₃, k_p and Q_m of Li_0.055[Ag_x(K_0.5Na_0.5)_1?x]_0.945(Nb_0.07Ta_0.03)O₃ ceramics were not changed remarkably with Ag⁺ content. The dependence of temperature coefficient of k_p (TCk_p) on the oxygen octahedral distortion was also discussed by Raman-active vibrations modes.     

Relation between piezoelectric properties of ceramics and output power density of energy harvester

(1?x)Pb(Zr_0.47Ti_0.53)O₃–xPb[(Zn_0.4Ni_0.6)_1/3Nb_2/3]O₃ [(1?x)PZT–xP(ZN)N] ceramics with 0.26 ≤ x ≤ 0.31 were sintered at 1100 °C, and their energy harvesters were fabricated. All specimens exhibit a similar energy convergence efficiency. However, the transduction coefficient (d₃₃ × g₃₃) increased with x, reaching 21.5 × 10^?15 m²/N for the x = 0.31specimen; the figure-of-merit of the specimens shows a similar variation. The output energy density of the energy harvester also increased with x, and a high output energy density of 231 mW/cm³ was obtained for the harvester fabricated using the x = 0.31 specimen, indicating that the d₃₃ × g₃₃ value significantly affects the output energy density of the energy harvester.     

Effect of B2O3 addition on the sintering temperature and microwave dielectric properties of Zn2SiO4 ceramics

B₂O₃ (25.0 mol%) was added to Zn_2?xSiO_4?x ceramics (0.0 ≤ x ≤ 0.5) to decrease the sintering temperature. Specimens with 0.0 ≤ x ≤ 0.3 sintered at 900 °C were well sintered with a high density due to the formation of a B₂O₃ or B₂O₃–SiO₂ liquid phase. The Q × f value of the Zn₂SiO₄ ceramic was relatively low, 32,000 GHz, most likely due to the presence of a ZnO second phase. A maximum Q × f value of 70,000 GHz was obtained for the specimens with x = 0.2–0.3, and their ?_r and τ_f values were approximately 6.0 and ?21.9 ppm/°C, respectively. Ag metal did not interact with the 25.0 mol% B₂O₃-added Zn_1.8SiO_3.8 ceramic, indicating that Zn_2?xSiO_4?x ceramics containing B₂O₃ are a good candidate materials for low temperature co-fired ceramic devices.     

Crystal structure,microstructure and electrical properties of (1−x−y)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBiFeO3 ceramics near MPB prepared via the combustion technique

(1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yBiFeO₃ (BNKFT-x/y with 0.12≤x≤0.24, 0≤y≤0.07) lead-free piezoelectric ceramics have been prepared by the combustion technique. The effects of amounts of x and y on structures and electrical properties were examined. Powders and ceramics can be well calcined and sintered at 750 °C for 2 h and 1025–1050 °C, respectively. The results indicated that the crystalline structure and microstructure changed with the increase of x and y concentrations. XRD results of BNKFT-x/0.03 and BNKFT-0.18/y ceramics with 0.12≤x≤0.24 and 0≤y≤0.07 showed the rhombohedral–tetragonal morphotropic phase boundary (MPB). The addition of y caused a promoted grain growth while the addition of x suppressed the grain growth. The highest density (ρ=5.85 g/cm³), superior dielectric properties at T_c (ε_r=7846 and tan δ=0.02), remnant polarization measured at 40 kV/cm (P_r = 20.1 μC/cm²) and piezoelectric coefficient (d₃₃=213 pC/N) were obtained for x=0.18 and y=0.03.     

The effects of sintering temperature and content of x on phase formation,microstructure and dielectric properties of (1−x)(Bi0.4871Na0.4871La0.0172TiO3)−x(BaZr0.05Ti0.95O3) ceramics prepared via the combustion technique

(1?x)(Bi_0.4871Na_0.4871La_0.0172TiO₃)?x(BaZr_0.05Ti_0.95O₃) ceramics (abbreviated (1?x)BNLT?xBZT) where 0.1≤x≤0.3 were fabricated by the combustion technique using glycine as fuel. BNLT and BZT powders were calcined at temperatures of 825 °C for 4 h and 925 °C for 6 h, respectively. After that they were mixed with the different compositions. It was found that the optimum sintering temperature of (1?x)BNLT?xBZT ceramic was obtained at 1125 °C for 2 h. This ceramic had the highest density. The structure of the (1?x)BNLT?xBZT ceramics exhibited the co-existence of tetragonal and rhombohedral phases with x≤0.1. The tetragonality increases with the increase of x content. The average grain size, the density and the Curie temperatures decrease with increasing x content. The maximum dielectric constant and the highest P_r were at about 4850 and 12.7 μC/cm², respectively, and were obtained by the 0.85BNLT?0.15BZT sample.     

Thermal expansion and thermal conductivity of SmxZr1−xO2−x/2 (0.1 ≤ x ≤ 0.5) ceramics

A study was conducted of the effect of additions of samarium oxide on the thermal expansion and thermal conductivity of zirconium oxide for thermal barrier coatings. Sm_xZr_1?xO_2?x/2 (0.1 ≤ x ≤ 0.5) ceramic powders synthesized with a chemical-coprecipitation and calcination method were sintered at 1873 K for 15 h. Structures of the synthesized powders and sintered ceramics were identified by X-ray diffractometer. The morphologies of ceramic powders were observed by transmission electron microscope. The thermal expansion coefficients and thermal diffusion coefficients of Sm_xZr_1?xO_2?x/2 ceramics were studied with a high-temperature dilatometer and a laser flash diffusivity technique from room temperature to 1673 K. The thermal conductivity was calculated from thermal diffusivity, density and specific heat of bulk ceramics. Sm_0.1Zr_0.9O_1.95 ceramics consists of both monoclinic and tetragonal structures. However, Sm_0.2Zr_0.8O_1.9 and Sm_0.3Zr_0.7O_1.85 ceramics only exhibit a defect fluorite structure. Sm_0.4Zr_0.6O_1.8 and Sm_0.5Zr_0.5O_1.75 ceramics have a pyrochlore-type lattice. With the increase of Sm₂O₃ content, the linear thermal expansion of Sm_xZr_1?xO_2?x/2 ceramics increases except for Sm_0.1Zr_0.9O_1.95. The thermal conductivities of Sm_xZr_1?xO_2?x/2 ceramics ranged from 1.41 at 873 K to 1.86 W m^?1 K^?1 at room temperature in a test temperature range of room temperature to 1673 K, and the results can be explained by phonon scattering mechanism.     

Dependence of dielectric properties on structural characteristics of (Zn1/3A2/3)0.5 (Ti1−xBx)0.5O2 (A = Nb5+, Ta5+, B = Ge4+, Sn4+) ceramics at microwave frequencies

The effects of structural characteristics on the dielectric properties of (Zn_1/3A_2/3)_0.5(Ti_1?xB_x)_0.5O₂ (A = Nb⁵⁺, Ta⁵⁺, B = Ge⁴⁺, Sn⁴⁺) (0.1 ≤ x ≤ 0.3) ceramics were investigated at microwave frequency. The sintered specimens showed solid solutions with a tetragonal rutile structure within the solid solution range of compositions. With an increase of BO₂, the temperature coefficient of resonant frequency (TCF) and dielectric constant (K) decreased with a decrease of oxygen octahedral distortion and dielectric polarizabilites, respectively. However, the quality factor (Qf) of the sintered specimens was increased with BO₂ due to the reduction of Ti⁴⁺ ions. The Qf value of the specimens with A = Ta was higher than that of the specimens with A = Nb.     

Ionic conductivity across the disorder–order phase transition in the SmO1.5–CeO2 system

Samples of Sm_xCe_1 ? xO_2 ? δ (0.05 ≤ x ≤ 0.55) were prepared by solid-state reactions and the disorder–order phase transition and grain ionic conductivity were investigated using XRD and ac impedance spectroscopy technique, respectively. For 0 ≤ x ≤ 0.35 the material has a fluorite structure and gradually stabilizes into a C-type rare-earth structure at 0.40 ≤ x ≤ 0.55 because of oxygen-vacancy ordering. The highest grain ionic conductivity observed is 0.0565(37) S cm^?1 at 700 °C for Sm_0.20Ce_0.80O_2 ? δ with an associated activation energy (E_A) of 0.791(7) eV. The slopes for E_A and pre-exponential factor change during phase transition and the conductivity decreases monotonically. Upon comparison of the E_A between the SmO_1.5–CeO₂ and NdO_1.5–CeO₂ systems, it is seen E_A for the SmO_1.5–CeO₂ system is lower than NdO_1.5–CeO₂ system at compositions with less than 25% trivalent rare earth element while higher E_A is observed for the SmO_1.5–CeO₂ system at Nd/Sm concentrations above 25%.     

Uptake of chloride and carbonate ions by calcium monosulfoaluminate hydrate

Decommissioning of old nuclear reactors may produce waste streams containing chlorides and carbonates, including radioactive ³⁶Cl^? and ¹⁴CO₃^2?. Their insolubilization by calcium monosulfoaluminate hydrate was investigated. Carbonates were readily depleted from the solution, giving at thermodynamic equilibrium monocarboaluminate, monocarboaluminate + calcite, or calcite only, depending on the initial ratio between the anion and calcium monosulfoaluminate hydrate. Chloride ions reacted more slowly and were precipitated as Kuzel's salt, Kuzel's and Friedel's salts, or Friedel's salt only. Rietveld refinement of X-Ray powder diffraction patterns was successfully used to quantify the phase distributions, which were compared to thermodynamic calculations. Moreover, analysing the lattice parameters of Kuzel's salt as a function of its chloride content showed the occurrence of a restricted solid solution towards the sulfate side with general formula 3CaO·Al₂O₃·xCaCl₂·(1 ? x)CaSO₄·(12 ? 2x)·H₂O (0.36 ≤ x ≤ 0.50).     

Extended X-ray absorption fine structure,X-ray diffraction and Raman analysis of nickel-doped Ba(Mg1/3Ta2/3)O3

Raman, X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements of xBa(Ni_1/3Ta_2/3)O₃ + (1 − x)Ba(Mg_1/3Ta_2/3)O₃ samples with x = 0–0.03 were performed to reveal the nickel doping effect on the microwave properties. EXAFS result clearly shows that the nickel is located on the Mg lattice site. We also found that, as the nickel concentration increases, microwave dielectric constant decreases with the TaO and NiO bond distances. X-ray diffraction shows that the 1:2 ordered structure is degraded with the increasing of nickel concentration. The stretching phonon of the TaO₆ octahedra, that is A_1g(O) phonon near 800 cm⁻¹, are strongly correlated to the microwave properties of xBa(Ni_1/3Mg_2/3)O₃ + (1 − x)Ba(Mg_1/3Ta_2/3)O₃ samples. The large Raman shift and the large width of the A_1g(O) imply rigid but distorted oxygen octahedral structure, therefore, the effect of nickel doping lowers the dielectric constant and the Q × f value of Ba(Mg_1/3Ta_2/3)O₃ ceramic.     

The PZT system (PbTixZr1−xO3, 0≤x≤1.0): Dielectric response of solid solutions in broad temperature (10≤T≤1000 K) and frequency (10−2≤f≤107 Hz) ranges (Part 4)

Solid solution (SS) ceramics of the PZT (PbTi_xZr_1?xO₃, 0≤x≤1.0) system were studied in broad temperature (10≤T≤1000 K) and electric field frequency (10^?2≤f≤10⁷ Hz) ranges. Several groups of SS were distinguished, which differ by nonmonotonic behavior of dielectric parameters in the cryogenic temperature range and at Т>300 K, which results both from the defective state and from the polymorphism of SS. A conclusion is made on the expediency of use of the obtained data during the application of materials based on the PZT system in the broad range of external actions.     

White light emitting from YVO4/Y2O3:Eu3+,Bi3+ composite phosphors for UV light-emitting diodes

A series of (1−x)YVO₄/xY₂O₃:Eu³⁺_0.006,Bi³⁺_0.006 (0≤x≤0.54) composite phosphors was synthesized in one step by high temperature solid state reaction and the photoluminescence properties were investigated. By means of co-doping Eu³⁺ and Bi³⁺ ions into the composite matrices composed of YVO₄ and Y₂O₃ crystals, the YVO₄/Y₂O₃:Eu³⁺,Bi³⁺ phosphor exhibits simultaneously the blue (418 nm), green (540 nm) and orange-red (595, 620 nm) emissions. The broad blue and green emissions are attributed to the ³P₁–¹S₀ transitions of Bi³⁺ ion both in Y₂O₃ and in YVO₄ matrices. Moreover, the sharp orange-red emissions are attributed to the ⁵D₀–⁷F_1,2 transitions of Eu³⁺ ion in YVO₄ matrix. By tuning the mole ratio of YVO₄/Y₂O₃ matrices the white light-emitting could be obtained. The results indicated that when the mole ratio of Y₂O₃ (x) is at 0.11–0.54 mol, the (1−x)YVO₄/xY₂O₃:Eu³⁺_0.006,Bi³⁺_0.006 phosphors emit white light by combining the blue, green and orange-red emissions under the excitation of 360–370 nm wavelength which matches the emission of the commercial UV-LED diode. This implies that the phosphors may be the promising white light materials with broad absorption band for white light-emitting diodes.