Effect of silver content on the phase transition and electrical properties of 0.95[(K0.5Na0.5)NbO3]–0.05LiSbO3 ceramics

Lead free 0.95[(K_0.5Na_0.5)_1−xAg_xNbO₃]–0.05LiSbO₃ (KNAN–LS) ceramics with x = 0, 0.02, 0.04, 0.06 and 0.08 have been synthesized by conventional solid state reaction route (CSSR). X-ray diffraction (XRD) analysis confirmed the transformation of mixed structure to pure tetragonal structure with the increase in Ag content in KNN–LS ceramics. The Curie temperature (T_c) of the ceramics decreased from 385.5 °C to 331 °C with the increase in silver (Ag) content. The poling temperature was optimized for better piezoelectric properties. The KNAN–LS ceramics with x = 0.06 showed better piezoelectric and ferroelectric properties (d₃₃ = 227 pC/N, k_p = 42.5%, T_c = 368 °C and P_r = 21.9 μC/cm²).     

Microstructure,dielectric and piezoelectric properties of (Pb1−xSrx)Nb1.96Ti0.05O6 ceramics

(Pb_1−xSr_x)Nb_1.96Ti_0.05O₆ with 2 wt% excess PbO (x = 0, 0.02, 0.04, 0.06, 0.08) piezoelectric ceramics with high Curie temperature were fabricated via the conventional solid state reaction method. Effects of Sr²⁺ amount on crystallite structure, microstructure, dielectric and piezoelectric properties were studied. The substitution of Sr²⁺ ions for Pb²⁺ ions is effective to lower sintering temperatures. X-ray diffraction patterns indicate that all ceramics form the single orthorhombic ferroelectric phase. The doping of Sr²⁺ ions facilitates improving densification of the ceramics. Grain size and lattice parameters of the ceramics vary with the change of the Sr²⁺ contents. Both Curie temperature and maximum dielectric constant change with increasing the Sr²⁺ amounts. The dielectric constant data were also studied using the Curie–Weiss law and modified Curie–Weiss law. The ceramic with x = 0.04 possesses excellent piezoelectric and dielectric properties, presenting a high potential to be used in high-temperature applications as piezoelectric transducers.     

Effect of Niobium doping on structural, thermal, sintering and electrical properties of Bi4V1.8Cu0.2O10.7

Doping Bi₄V_1.8Cu_0.2O_10.7 with niobium has led to the formation of the Bi₄V_1.8Cu_0.2−xNb_xO_10.7+3x/2 solid solution. X-ray diffraction and thermal analysis have shown that only the compound with x=0.05 presents a tetragonal symmetry with a γ^′ polymorph while the other compositions are of β polymorph. The influence of sintering temperature on the microstructure of the samples was investigated by the scanning electron microscopy (SEM). The ceramics sintered at temperatures higher than 820 °C present micro-craks. The evolution of the electrical conductivity with temperature and the degree of substitution has been investigated by impedance spectroscopy. Among all compositions studied the sample with x=0.05 presents the highest value of the conductivity.     

Perovskite-type SrTi1−xNbx(O,N)3 compounds: Synthesis, crystal structure and optical properties

The synthesis, crystal structure, thermal stability and absorbance spectra of perovskite-type oxynitrides with the general formula SrTi_1−xNb_x(O,N)₃ (x=0.05, 0.10, 0.20, 0.50, 0.80, 0.90, 0.95) have been investigated. Oxide samples were prepared by a polymerized complex synthesis route and post-treated under ammonia at 850 °C for 24 h to substitute nitrogen for oxygen. Synchrotron X-ray powder diffraction (XRD) evidenced that the mixed oxide phases were all transformed into oxynitrides with perovskite-type structure during a thermal ammonolysis. SrTi_1−xNb_x(O,N)₃ with compositions x≤0.80 crystallized in a cubic and samples with x≥0.90 in a tetragonal structure. The Rietveld refinement indicated a continuous enlargement of the lattice parameters towards higher niobium content of the samples. Thermogravimetric analysis (TGA) and hotgas extraction revealed the dependence of the nitrogen incorporation upon the degree of niobium substitution. It showed that more nitrogen was detected in the samples with higher niobium content. Furthermore, TGA disclosed stability for all oxynitrides at T≤400 °C. Diffuse reflectance spectroscopy indicated a continuous decrease of the band gap’s width from 3.24 eV (SrTi_0.95Nb_0.05 (O,N)₃) to 1.82 eV (SrTi_0.05Nb_0.95(O,N)₃) caused by the increasing amount of nitrogen towards the latter composition.     

New ferroelectric and relaxor ceramics in the mixed oxide system NaNbO3–BaSnO3

New ferroelectric or relaxor ceramics were elaborated in the system Ba_1−xNa_xSn_1−xNb_xO₃ (BSNN) by solid-state reaction technique. The effect of cationic substitution of barium for sodium in the A sites and tin for niobium in the B sites in the NaNbO₃ perovskite lattice on symmetry and physical properties were investigated. Room temperature X-ray diffraction and dielectric permittivity in the temperature range from 80 to 600 K with frequencies from 0.1 to 200 kHz, respectively, were used. The material is relaxor with cubic symmetry when x<0.90 and becomes classical ferroelectric with a tetragonal cell when 0.90⩽x<1. Both T_C or T_m and the maximum of ε′_r decrease when BaSnO₃ is introduced in the lattice of NaNbO₃. The plot of T_m(f)/T_m (1 kHz) versus the logarithm of the frequency allows to determine the limit of composition between the ferroelectric and the relaxor state. Relaxor materials have been obtained at room temperature.     

Ferroelectric properties and ac conductivity studies of yttrium modified Pb(Fe0.5Nb0.5)O3 ceramics

Using a high-temperature standard solid-state reaction (i.e., mixed oxide) method several yttrium modified ferroelectric ceramics of type Pb_1−xY_x(Fe_0.5Nb_0.5)_1−x/4O₃ (PYFN)with different concentration (x = 0.00, 0.02, 0.04, 0.06, 0.08) were prepared. Preliminary X-ray structural analysis reveals that all the samples were crystallized in monoclinic crystal system. The crystal structure is almost invariant with small Y-concentration. Microstructure of the samples exhibits grain growth on increasing Y content. Curie temperature of Y-substituted samples decreases slightly as compared to that of PFN (Pb(Fe_1/2Nb_1/2)O₃). However, Y substitution results in a notable enlargement of remnant polarization (2P_r). The 2P_r of PYFN (x = 0.02) reaches to a large value (23 μC/cm²) which is nearly 5 times greater than that of x = 0.00 (4.6 μC/cm²). A phenomenological model (i.e., ferroelectric capacitor model) used to simulate hysteresis loops was found to be suitable to explain experimental results. Jonscher's single power law (σ_ac(ω) = σ(0) + Aωⁿ) was well fitted to the frequency dependence of conductivity for all the samples.     

Phase evolution,dielectric, ferroelectric,and piezoelectric properties of Bi(Mg0.5Hf0.5)O3–modified BiFeO3–BaTiO3

Bi(Mg_0.5Hf_0.5)O₃–modified BiFeO₃–BaTiO₃ ternary solid solutions of (0.725-x)BiFeO₃-0.275BaTiO₃-xBi(Mg_0.5Hf_0.5)O₃ (0 < x ≤ 0.05, abbreviated as BFO-BTO-xBMHO) were prepared for lead-free piezoelectrics. The addition of BMHO delivers a rhombohedral (R3c, denoted as R-phase) to tetragonal (P4mm, denoted as T-phase) phase transition at x = 0.05, giving the coexistence of R- and T-phase in intermediate compositions: R-phase dominated in x = 0.01–0.02 and T-phase dominated in x = 0.03–0.04. The increment of BMHO tunes the grain size, lowers the ferroelectric transition temperature (T_C) and dielectric loss (tanδ), and drives a gradually ferroelectric to relaxor transition. The morphotropic phase boundary between the R-and T-phases, together with the homogeneous morphology, results in the best performance for x = 0.04 case with piezoelectric d₃₃ of 130 pC/N, K_p of 0.286, Q_m of 58.993, electrostrain S_max of 0.18%, and T_C of 428 °C, showing potential applications for lead-free piezoelectric ceramics at considerably high temperature.     

Photoluminescent properties of white-light-emitting Li6Y(BO3)3:Dy3+ phosphor

In this study, lithium yttrium borate (LYBO) phosphor was doped with various concentrations of trivalent dysprosium ions. To produce these phosphors, the raw materials were sintered. The phase conformation, crystallinity, grain size, and overall morphology of the synthesized phosphors were studied with X-ray diffraction and scanning electron microscopy. The optimized LYBO phosphor, i.e., the LYBO phosphor that exhibited the highest X-ray- and ultraviolet (UV)-induced photoluminescent intensities, had a Dy³⁺ concentration of 4 mol%. Photoluminescence analysis showed that this phosphor could be easily excited with near-UV light (300–400 nm). The dominant photoluminescence bands were found in the blue (480 nm) and yellow (577 nm) regions of the visible spectrum. The light yield of the X-ray-induced luminescence of the optimized Li₆Y(BO₃)₃:Dy³⁺ was found to be 66% of that of the commercially available X-ray imaging material, Gd₂O₂S:Tb³⁺ (GOS). The chromaticity coordinates of the Li₆Y(BO₃)₃:Dy³⁺ phosphor were x = 0.34 and y = 0.32, which agree well with achromatic white (x = 0.33, y = 0.33). The results of this study show that the synthesized Li₆Y(BO₃)₃:Dy³⁺ phosphor could be used as X-ray imaging material.     

A role of BNLT compound addition on structure and properties of PZT ceramics

In this research, effects of lead-free bismuth sodium lanthanum titanate (BNLT) addition on structure and properties of lead zirconate titanate (PZT) ceramics were investigated. PZT ceramics with addition of 0.1–3.0 wt%BNLT were fabricated by a solid-state mixed oxide method and sintering at 1050–1200 °C for 2 h to obtain dense ceramics with at least 96% of theoretical density. X-ray diffraction indicated that complete solid solution occurred for all compositions. Phase identification showed both tetragonal and rhombohedral perovskite structure of PZT with no BNLT phase detected. Scanning electron micrographs of fractured PZT/BNLT ceramics showed equiaxed grain shape with both transgranular and intergranular fracture modes. Addition of BNLT was also found to reduce densification and effectively limited grain growth of PZT ceramic. Optimum Hv and K_IC values were found to be 4.85 GPa and 1.56 MPa.m^1/2 for PZT/0.5 wt%BNLT sample. Among PZT/BNLT samples, room temperature dielectric constant seemed to be improved with increasing BNLT content. The maximum piezoelectric coefficient values were observed in pure PZT ceramic and were slightly decreased in BNLT-added samples. Small reduction of remanent polarization and coercive field in hysteresis loops was observed in BNLT-added samples, indicating a slightly suppressed ferroelectric interaction in this material system.     

Tuning luminescent properties through solid-solution in (Ba1−xSrx)9Sc2Si6O24:Ce3+,Li+

A solid-solution of cerium-substituted alkaline earth scandium silicate phosphors, (Ba_1−xSr_x)₉Sc₂Si₆O₂₄:Ce³⁺,Li⁺ (x = 0, 0.25, 0.50, 0.75, 1), have been prepared by solid-state reaction. The structures, characterized using synchrotron X-ray powder diffraction, show the solid-solution closely follows Vegard's law. The substitution of Sr for Ba results in a decrease of the alkaline earth–oxygen bond distances by more than 0.1 Å at all three crystallographic sites, leading to changes in optical properties. The room temperature photoluminescent measurements show the structure has three excitation peaks corresponding to Ce³⁺ occupying the three independent alkaline earth sites. The emission of (Ba_1−xSr_x)₉Sc₂Si₆O₂₄:Ce³⁺,Li⁺ is red-shifted from the near-UV (λ_max = 384 nm) for x = 0 to blue (λ_max = 402 nm) for x = 1. The red-shifted photoluminescent quantum yield also increases when Sr is substituted for Ba in these compounds.     

The Crystal Chemistry of Lithium in the Alluaudite Structure: A Study of the (Na1−xLix)CdIn2(PO4)3 Solid Solution (x=0 to 1)

Several compounds of the (Na_1−xLi_x)CdIn₂(PO₄)₃ solid solution were synthesized by a solid-state reaction in air, and pure alluaudite-like compounds were obtained for x=0.00, 0.25, and 0.50. X-ray Rietveld refinements indicate the occurrence of Cd²⁺ in the M(1) site, and of In³⁺ in the M(2) site of the alluaudite structure. This non-disordered cationic distribution is confirmed by the sharpness of the infrared absorption bands. The distribution of Na⁺ and Li⁺ on the A(1) and A(2)′ crystallographic sites cannot be accurately assessed by the Rietvled method, probably because the electronic densities involved in the Na⁺→Li⁺ substitution are very small. A comparison with the synthetic alluaudite-like compounds, (Na_1−xLi_x)MnFe₂(PO₄)₃, indicates the influence of the cations occupying the M(1) and M(2) sites on the coordination polyhedra morphologies of the A(1) and A(2)′ crystallographic sites.     

Control of Co content and SOFC cathode performance in Y1−ySr2+yCu3−xCoxO7+δ

The electrochemical performance of the layered perovskite YSr₂Cu_3−xCo_xO_7+δ, a potential solid oxide fuel cell (SOFC) cathode, is improved by increasing the Co content from x = 1.00 to a maximum of x = 1.30. Single phase samples with x > 1.00 are obtained by tuning the Y/Sr ratio, yielding the composition Y_1−ySr_2+yCu_3−xCo_xO_7+δ (where y ≤ 0.05). The high temperature structure of Y_0.95Sr_2.05Cu_1.7Co_1.3O_7+δ at 740 °C is characterised by powder neutron diffraction and the potential of this Co-enriched material as a SOFC cathode is investigated by combining AC impedance spectroscopy, four-probe DC conductivity and powder XRD measurements to determine its electrochemical properties along with its thermal stability and compatibility with a range of commercially available electrolytes. The material is shown to be compatible with doped ceria electrolytes at 900 °C.     

Significantly Improved Colossal Dielectric Properties and Maxwell—Wagner Relaxation of TiO2—Rich Na1/2Y1/2Cu3Ti4+xO12 Ceramics

In this work, the colossal dielectric properties and Maxwell—Wagner relaxation of TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ (x = 0–0.2) ceramics prepared by a solid-state reaction method are investigated. A single phase of Na_1/2Y_1/2Cu₃Ti₄O₁₂ is achieved without the detection of any impurity phase. The highly dense microstructure is obtained, and the mean grain size is significantly reduced by a factor of 10 by increasing Ti molar ratio, resulting in an increased grain boundary density and hence grain boundary resistance (R_gb). The colossal permittivities of ε′ ~ 0.7–1.4 × 10⁴ with slightly dependent on frequency in the frequency range of 10²–10⁶ Hz are obtained in the TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics, while the dielectric loss tangent is reduced to tanδ ~ 0.016–0.020 at 1 kHz due to the increased R_gb. The semiconducting grain resistance (R_g) of the Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics increases with increasing x, corresponding to the decrease in Cu⁺/Cu²⁺ ratio. The nonlinear electrical properties of the TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics can also be improved. The colossal dielectric and nonlinear electrical properties of the TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics are explained by the Maxwell–Wagner relaxation model based on the formation of the Schottky barrier at the grain boundary.     

Pyroelectric and piezoelectric properties of new lead-free ceramics with composition Ba1-xNaxTi1-xO3

Ceramics with composition Ba_1-xNa_xTi_1-xNb_xO₃ are of either classical ferroelectric (0 ≤ × < 0.075) or relaxor ferroelectric types (0.075 ≤ x ≤0.55), and ferro- or antiferroelectric for compositions 0.55 < × ≤ 1. The dielectric study of ceramics with compositions close to NaNbO₃ showed a sharp peak of ε'_r without frequency dispersion. The value of T_c is decreasing as composition deviates from NaNbO₃. Ceramic samples are tetragonal at room temperature; they could be polarized and then show pyroelectric and piezoelectric properties up to 400K (p = 25nC.cm⁻².K⁻¹ and d₃₁30pC.N⁻¹ at 300K). This study aims at the preparation of environment-friendly lead-free relaxor ceramics which present a transition temperature close to room temperature.     

Thermal and electrical relaxation studies in Li(4+x)TixNb1−xP3O12 (0.0 ≤ x ≤ 1.0) phosphate glasses

A new superionic conducting transparent phosphate glasses with composition Li_(4+x)Ti_xNb_1?xP₃O₁₂ (0 ≤ x ≤ 1.0) were prepared by rapid melt quenching. As quenched samples were characterized by X-ray powder diffraction, differential scanning calorimetric and Fourier transform infrared spectroscopy studies. These glasses were found to have high thermal stability parameter and Li₄NbP₃O₁₂ has been found to have high glass forming ability. Electrical properties of the present glasses were studied by impedance and dielectric spectroscopy in the frequency range 10 Hz–3 MHz in the temperature range 323–523 K. Arrhenius behavior has been observed for all the glass in conductivity, dielectric loss and conductivity relaxation and their activation energies are explained and reported.     

Phase Diagram and Phase Transitions in the Relaxor Ferroelectric Pb(Fe2/3W1/3)O3-PbTiO3 System

A complete solid solution between relaxor ferroelectric Pb(Fe_2/3W_1/3)O₃ (PFW) and ferroelectric PbTiO₃ (PT), (1−x)PFW-xPT, was synthesized by a B-site precursor method and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric measurements. A phase diagram between PFW and PT has been established. The diffuse phase transition temperature (T_max≈180 K) of PFW was found to continuously increase with the increasing amount of Ti⁴⁺ ions on the B-site. At the same time, the relaxor ferroelectric behavior of PFW is gradually transformed toward a normal ferroelectric state, as evidenced by sharp and nondispersive peaks of dielectric permittivity around T_C for x≥0.25. At room temperature, a transition from a cubic to a tetragonal phase takes place with x increased up to 0.25. A morphotropic phase boundary is located within the composition interval 0.25≤x≤0.35, which separates a pseudocubic (rhombohedral) phase from a tetragonal phase.     

Dielectric properties of chemically co-precipitated tetragonal Pb(Mg1/3Nb2/3)0.65Ti0.35O3

Pyrochlore phase free Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ ceramics have been synthesized successfully by chemical co-precipitation method. It has been noted that formation of perovskite phase Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ without pyrochlore phase is tricky. The synthesized samples at optimized parameters were characterized using X-ray diffraction technique. Careful analysis of the XRD data predicts the tetragonal lattice structure. The morphological studies depict the presence of uniform grain size. Dielectric constant (ε′) and loss tangent (tan δ), at and well above room temperature, were studied. The variation of dielectric constant with temperature shows sharp peak at ferroelectric–paraelectric transition temperature. Further, the temperature dependent dielectric constant shows good fit with modified Curie–Weiss law, which suggests normal ferroelectric behavior of Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃.     

Structural variations and cation distributions in Mn3−xCoxO4 (0 ≤ x ≤ 3) dense ceramics using neutron diffraction data

Single phase ceramics of cobalt manganese oxide spinels Mn_3?xCo_xO₄ were structurally characterized by neutron powder diffraction over the whole solid solution range. For x < 1.75, ceramics obtained at room temperature by conventional sintering techniques are tetragonal, while for x ≥ 1.75 ceramics sintered by Spark Plasma Sintering are of cubic symmetry. The unit cells, metal–metal and metal–oxygen average bonds decrease regularly with increasing cobalt content. Rietveld refinements using neutron data show that cobalt is first preferentially substituted on the tetrahedral site for x < 1, then on the octahedral site for increasing x values. Structural methods (bond valence sum computations and calculations based on Poix's work in oxide spinels) applied to our ceramics using element repartitions and [M–O] distances determined after neutron data refinements allowed us to specify the cation distributions in all phases. Mn²⁺ and/or Co²⁺ occupy the tetrahedral site while Mn³⁺, Co²⁺, Co^III (cobalt in low-spin state) and Mn⁴⁺ occupy the octahedral site. The electronic conduction mechanisms in our highly densified ceramics of pure cobalt and manganese oxide spinels are explained by the hopping of polarons between adjacent Mn³⁺/Mn⁴⁺ and Co²⁺/Co^III on the octahedral sites.     

Synthesis, Crystal Structure and Physical Properties of LixMg0.857−xCu2.143O3−y

Solid solutions of Li-doped Mg_0.857Cu_2.143O₃ (Li_xMg_0.857−xCu_2.143O_3−y) were prepared at 950°C for 12 h in air by the solid-state method using Li₂CO₃, MgO and CuO powders. The solid solutions were obtained as the single α phase with the güggenite structure in 0≦x≦0.06 region. With the increasing of the Li content x, the lattice parameters a, b and unit cell volume V decreased, while c increased. On the basis of the charge neutrality, hole carrier estimated by the oxygen content increased with the Li substitution. The Seebeck coefficient at room temperature of x = 0.03 sample was +400 μV/K. The electrical resistivity ρ at room temperature drastically decreased with the increasing x. Temperature dependences of ρ for x = 0.01, 0.03 and 0.06 samples were semi-conductive behavior from room temperature to about 12 K. Interaction between Cu²⁺ and Cu²⁺ through O²⁻ seems to be somewhat large antiferromagnetic one. Sperconducting transition was not detected in the temperature range.     

Impedance of the Ag|Nd0.95Sr0.05F2.95 Crystal|Ag System

Electrochemical studies of the Ag|Nd_0.95Sr_0.05F_2.95 crystal|Ag system at 94 to 1072 K are carried out using impedance measurements over the 10⁻¹ to 10⁷ Hz frequency range. Contributions of conduction and bulk and electrode polarization are distinguished in the impedance spectra of the Ag| Nd_0.95Sr_0.05F_2.95|Ag cell. Anionic conductivity of the Nd_0.95Sr_0.05F_2.95 crystal varies from 3 × 10⁻¹³ to 0.4 S/cm. The mechanism of anionic conduction and models for the electrode and bulk polarization are discussed.__________Translated from Elektrokhimiya, Vol. 41, No. 8, 2005, pp. 1005–1009.Original Russian Text Copyright © 2005 by Sorokin.