Dielectric properties Ca-substituted barium strontium titanate ferroelectric ceramics

The dielectric and ferroelectric properties of Ba_1−xSr_xTiO₃ (BST) (x=0.10,0.20,0.30,0.40 and 0.60) ceramics and Ba_1−2xSr_xCa_xTiO₃ (BSCT) (x=0.10,0.20,0.30) ceramics have been investigated. The low temperature phase transitions of BST ceramics vanish after Ca²⁺ substitution while the high temperature transition is diffused and relaxed, which becomes more obvious with increasing x. Ca²⁺ substitution obviously decreases the dielectric constant maximum, K_m, of BST ceramics and changes the temperature of dielectric constant maximum, T_m, of BST ceramics. The shift of T_m in BST is attributed mainly to the Sr²⁺ and Ba²⁺ concentration. BST ceramics exhibit almost normal ferroelectric characteristics, while a typical relaxor behavior was observed in BSCT ceramics. The relaxor behavior observations may be understood by a random electric field induced domain state.     

Photoluminescence at room temperature in polycrystalline BaxSr1−xTiO3 ceramics obtained by solid-state reaction method

Stoichiometric Ba_xSr_1−xTiO₃ ceramics (x=0.7, 0.8, and 0.9) have been prepared by solid-state reaction method. It has been observed in our experiments that the photoluminescence (PL) spectra in such samples at room temperature were centered around 800 nm; the results have been found to differ from observations in current literature. An explanation of this phenomenon was given, and the origin of such photoluminescence is ascribed to certain defects existing in these compounds, such as oxygen vacancies.     

Ferroelectric-relaxor behavior of (Na0.5K0.5)NbO3-based ceramics

We report that ferroelectric-relaxor behavior is induced by doping of SrO and TiO₂, or BaO and TiO₂ into classic ferroelectric (Na_0.5K_0.5)NbO₃. It is found that [(Na_0.5K_0.5)_0.9Sr_0.1](Nb_0.9Ti_0.1)O₃ ceramics exhibit a pronounced ferroelectric-relaxor behavior, comparable to that of [(Na_0.5K_0.5)_0.9Ba_0.1](Nb_0.9Ti_0.1)O₃ ceramics. Our results indicate that the relaxor behavior is closely related to the appearance of micropolar regions in these systems. The relaxor behavior should arise from the dynamic response of micropolar clusters. Raman spectra of [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics measured in the wavenumber range from 100 to 1200 cm⁻¹ confirm that the first order scattering is dominant in phonon bands should result from both short-range ordered region (micropolar regions) and disordered matrix. The frequency dependence of dielectric permittivity measurements show that the relaxor behavior of SrO and TiO₂, or BaO and TiO₂ doped (Na_0.5K_0.5)NbO₃ ceramics is not a Debye type in the radio frequency range.     

Dielectric and piezoelectric properties of lead-free (Na0.5K0.5)NbO3-SrTiO3 ceramics

In this article, we report successful preparation of dense [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ (x=0.005-0.100) ceramics by ordinary sintering in air. The dependence of phase structure on doping content of SrO and TiO₂ has been determined by the X-ray diffraction technique. It was found that the crystal structure changed from orthorhombic to tetragonal at x≈0.040. Dielectric study revealed that the dielectric relaxor behavior was induced by doping of SrO and TiO₂ into (Na_0.5K_0.5)NbO₃. The samples in the composition range from x=0.005 to 0.020 exhibited excellent electrical properties, piezoelectric constant of electromechanical planar and thickness coupling coefficients of k_p=26.6-32.5% and k_t=39.8-43.8%. The results show that the [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics are one of the promising lead-free materials for electromechanical transducer applications.     

Electromechanical properties and dielectric behavior of (Bi1/2Na1/2)(1−1.5x)BixTiO3 lead-free piezoelectric ceramics

Bismuth doped bismuth sodium titanate ceramics [(Bi_1/2Na_1/2)_(1−1.5x)Bi_xTiO₃, x=0 to 0.06] were prepared, and the resulting effects on the microstructure and dielectric properties were examined. All of the Bi-doped ceramics exhibited a single phase of perovskite structure with rhombohedral symmetry. The poling leakage current was significantly reduced by the doping of Bi, facilitating the poling process of the ceramics. The doping with Bi enhances the piezoelectric properties and increases the dielectric constant and the dielectric loss of the ceramics. At 2 mol% Bi-doping level, the ceramics exhibit a large remanent polarization of 47 μC/cm² and a relatively low coercive field of 71 kV/cm, while their d₃₃ and k_p reach a maximum value of 95 pC/N and 21%, respectively.     

Effect of Sr concentration on microstructure and magnetic properties of (Ba1−xSrx)(Ti0.3Fe0.7)O3 ceramics

Fe-doped (Ba_1−xSr_x)TiO₃ ceramics were prepared by solid-state reaction, and ferromagnetism was realized at room temperature. The microstructure and magnetism were modified by the Sr concentration control (0≤x≤75 at%) at a fixed Fe concentration, and the relevant magnetic exchange mechanism was discussed. All the samples are shown to have a single perovskite structure. When increasing the Sr concentration, the phase structure is transformed from a hexagonal perovskite into a cubic perovskite, with a monotonic decrease in lattice parameters induced by ionic size effect. The room-temperature ferromagnetism is expected to originate from the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti sites mediated by the O²⁻ ions. The increase in Sr addition modifies two main influencing factors in magnetic properties: the ratio of pentahedral to octahedral Fe³⁺ and the concentration of oxygen vacancies, leading to a gradually enhanced saturation magnetization. The highest value, obtained for Fe-doped (Ba_0.25Sr_0.75)TiO₃, is an order of magnitude higher than that of the Fe-doped BaTiO₃ system with similar Fe concentration and preparation conditions, which may indicate (Ba_1−xSr_x)TiO₃ as a more suitable matrix material for multiferroic research.     

Structural distortion and phase transition studies of Aurivillius type Sr1−xPbxBi2Nb2O9 ferroelectric ceramics

In this paper, effects of lead doping on the lattice response and phase transitions of Sr_1−xPb_xBi₂Nb₂O₉ (x=0.0-0.5 in steps of 0.1) ferroelectric ceramics are reported. It is observed that structure attains more tetragonality with doping of lead up to 40%. Increased orthorhombic distortion is observed for undoped SBN and 50 at.% lead substituted SBN. Phase transitions for all samples were studied using Curie temperature measurements and are explained in terms of lattice response of these ceramics. Sample with x=0.5 shows decreased tetragonal strain and Curie temperature. Relationship of polarization with lattice response is discussed.     

Effect of fabrication parameters on the electrical conductivity of YxSr1−xTiO3 for anode materials

Yttrium-doped strontium titanate (Y_xSr_1−xTiO₃), as probable anode material for SOFC, was prepared by solid-state reaction. The solubility of yttrium in SrTiO₃ at different temperatures was examined and the electrical conductivities of Y_xSr_1−xTiO₃ were measured from 500 to 1000 °C. The effects of doping amount, fabrication atmosphere, and sintering temperature on the electrical conductivity of Y_xSr_1−xTiO₃ were investigated. Y_xSr_1−xTiO₃ with x=0.08 was found to give the maximum electrical conductivity, 71 S/cm at 800 °C in pure hydrogen. Reducing atmospheres and appropriate sintering temperatures play a positive role in improving the electrical conductivity.     

Effect of elevated concentrations of strontium and iron on the structural and dielectric characteristics of Ba(1−x−y)Sr(x)Ti Fe(y)O3 prepared through sol–gel technique

Nano-composite Ba_1−xSr_(x)TiO₃ (BST), where x=0.01–0.50 and doped with different concentrations of iron Ba_(1−x−y)Sr_(x)TiFe _(y)O₃ (BSTF), where x=0.01 and y=0.01–0.05 powders were prepared by sol–gel method.     

Microstructure and electrical properties of Mn-doped barium strontium titanate thin films prepared on copper foils

Ba_0.7−xSr_0.3Mn_xTiO₃ (x = 0, 0.025, 0.05) thin films have been prepared on copper foils using sol-gel method. The films were processed in an atmosphere with low oxygen pressure so that the substrate oxidation is avoided and the formation of the perovskite phase is allowed. XRD and SEM results showed that Mn doping enhanced the crystallization of the perovskite phase in the films. The Mn substitution prevents the reduction of Ti⁴⁺ to Ti³⁺, which is supported by XPS analysis. The Ba_0.7−xSr_0.3Mn_xTiO₃ film with x = 0.025 (BSMT25) exhibits preferred dielectric behavior and a lower leakage current density among the three thin films. The dielectric constant and loss of the BSMT25 film are 1213.5 and 0.065 at 1 MHz and around zero field, which are mostly desired for embedded capacitor applications. The mechanism of Mn doping on improving the electrical properties of barium strontium titanate (BST) thin films was investigated.     

Atomistic computer simulation studies of La1−xSrxVO3

Static computer simulation techniques have been employed for structural investigation of the La_1−xSr_xVO₃ series. Potential parameters for V³⁺-O²⁻ and V⁴⁺-O²⁻ have been derived which reproduces the crystal structures of end members with sufficient accuracy. Variations of lattice parameters and bond distances with Sr concentration have been studied. The calculated lattice parameters decrease with increase in the Sr concentration. A structural phase transition from orthorhombic to cubic is observed at 50% Sr doping level.     

Structure, ferroelectric and piezoelectric properties of?(Bi0.98?xLa0.02Na1?x)0.5BaxTiO3 lead-free ceramics

Lead-free (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics have been prepared by an ordinary sintering technique and their structure, ferroelectric and piezoelectric properties have been studied. The results of X-ray diffraction show that La²⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) exists at 0.04<x<0.10. Compared with pure Bi_0.5Na_0.5TiO₃ ceramics, the (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics possess much smaller coercive field E _c and larger remanent polarization P _r. Because of the low E _c (3.38 kV/mm), large P _r (46.2 μC/cm²) and the formation of the MPB of rhombohedral and tetragonal phases, the piezoelectric properties of the ceramics are significantly enhanced at x=0.06: d ₃₃=181 pC/N and k _p=36.3%. The depolarization temperature T _d reaches a minimum value near the MPB. The ceramics exhibit relaxor characteristic, which is probably a result from the cation disordering in the 12-fold coordination sites. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both polar and non-polar regions at the temperatures above T _d.     

Dielectric and piezoelectric properties of (1−x)Ba0.7Sr0.3TiO3−xBa0.7Ca0.3TiO3 perovskites

Dielectric and piezoelectric properties of (1?x)Ba_0.7Sr_0.3TiO₃?xBa_0.7Ca_0.3TiO₃ (BST?xBCT) (x=0.2–0.9) perovskite ceramics have been investigated. BCT has fully incorporated into BST lattice, forming a complete perovskite solid solution, whose lattice constant χ decreases almost linearly with increase in x from 0.2 to 0.4, while showing an anomalous expansion at 0.4<x≤0.6. This, together with the deviation of tetragonal–orthorhombic phase transition temperature (T_O–T) from the linear relation T_O–T (K)=?103.7x+239.3 at x=0.5, suggests that a small amount of Ca²⁺ has substituted for Ti⁴⁺. Curie temperature T_C increases linearly with increase in x from 0.2 to 0.9, which is mainly contributed to the increase of the Ba/Sr ratio. The calculated degree of relaxation (γ) is in the range of 1.41–1.53, indicating that the BST–xBCT ceramics are ferroelectric materials with diffuse phase transition. Strain and piezoelectric constant (d₃₃) decrease with increasing x, whereas planar electromechanical coefficient (k_p) reaches a maximum (17.0%) at x=0.6.     

Critical concentrations in Ba-doped incipient ferroelectric SrTiO3

Temperature-induced variations of light refraction and dielectric permittivity in single-crystal Sr_1−x Ba_xTiO₃ (x=0.02, 0.05, 0.07, and 0.14), Sr_1−x Ca_xTiO₃ (x=0.014), and in nominally pure strontium titanate have been studied within the 17–300 K temperature range. The spontaneous polar contribution to the refractive index has been isolated. It was used to calculate the temperature and concentration dependences of the polarization autocorrelation function 〈P _s ^2〉 in the Sr_1−x Ba_xTiO₃ system. For x⩽0.07, the polarization P _s=〈P _s ² 〉^1/2 varies proportional to (x−x _g)^1/2, where x _g=0.0027 is the new critical concentration in Sr_1−x Ba_xTiO₃, below which short-range polar order vanishes. Fiz. Tverd. Tela (St. Petersburg) 39, 704–710 (April 1997)     

Luminescence characteristics of Ca1−xSrxS:Ce nanophosphors

Luminescence characteristics of Ca_1−xSr_xS:Ce (x = 0, 0.25, 0.50, 1) nanophosphors have been investigated. XRD of all the samples show a single cubic phase of Ca_1−xSr_xS:Ce. TEM micrographs exhibit the rod like structure of the samples with a decrease in diameter with decreasing amount of Ca. The results of TEM were found to be in good agreement with the XRD results. The photoluminescence spectrum comprises of a main peak in the range 480-510 nm with a shoulder in the range 530-565 nm, which may be ascribed to transitions from 5d-4f levels of cerium in the mixed host lattice. The red shift in the emission wavelength with increasing Ca content may be correlated with the change in crystal field of mixed host lattice for different Ca and Sr concentrations. We have also investigated TL response of Ca_1−xSr_x:Ce to ⁶⁰Co-γ rays. All the samples with different Sr and Ca contents show different TL response. TL response for the sample with x = 0.75 shows the simplest TL glow curve with the maximum TL intensity, for which we have calculated the activation energy using glow curve deconvolution functions.     

Luminescence and chromaticity of alkaline earth aluminate MxSr1−xAl2O4:Eu (M: Ca,Ba)

In order to prepare fluorescent material for UV-LED used as illumination light source, two series of Eu²⁺ doped (1 mol%) alkaline earth aluminate phosphors Ca_xSr_1−xAl₂O₄ and Ba_xSr_1−xAl₂O₄ were prepared. The crystal structure, relative quantum efficiency(Qr), peak wavelength(λ_p), color tuning and chromaticity were investigated by XRD patterns and photoluminescence (PL) on samples prepared by solid solution system (s series) and powder mixing system (m series) respectively. For the s series, the synthesized Ca_xSr_1−xAl₂O₄:Eu²⁺ powders show that the structure transforms from monoclinic to hexagonal at x?0.5, and λ_p increases from 442.3 to 529.7 nm with decreasing x. For the Ba_xSr_1−xAl₂O₄:Eu²⁺ system, the structure transforms from monoclinic to hexagonal at x?0.3, and λ_p decreases from 520.5 to 502.2 nm continuously from x=0 to 1. The shift in λ_p could be explained by the crystal field effect, which is affected by different coulomb attractive forces due to the various fraction of alkaline earth cation in the host lattice. Different phosphor properties prepared by either solid solution or powder mixing methods were characterized by chromaticity measurements for both reflective and transmissive modes.     

Preparation of LixCa1−xTiO3 solid electrolytes by the sol-gel method

Solid electrolytes based on lithium doped CaTiO_3,Li_xCa_1−xTiO₃ (x=0-0.5) were prepared by the sol-gel method in an ethanol and water mixture medium. Phase identification and morphology observation of the products were carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the Li_xCa_1−xTiO₃ powders sintered above 700 ^°C are of cubic perovskite structure and the mean size of Li_xCa_1−xTiO₃ powders is about 80 nm. A study of ionic conductivity by AC impedance implies that the conductivity of Li_xCa_1−xTiO₃ increases with the increase of substituted Li⁺ ions and reaches a maximum value of 4.53×10⁻⁴ S cm⁻¹ at x=0.1, and then decreases for x>0.1.     

Superconductivity in Sr and Co co-doped PrFeAsO

Polycrystalline Sr and Co co-doped superconducting oxypnictides Pr_1−ySr_yFe_1−xCo_xAsO were synthesized by solid state reaction method, and superconductivity was investigated by measuring resistivity, magnetic susceptibility and thermopower. In the PrFe_1−xCo_xAsO samples with only Co doping, T_c reaches a maximum of 16 K at x=0.075-0.1, and the variation of T_c with Co content (x) is dome-shaped, consistent with the other Co-doped 1 1 1 1 phase systems such as LaFe_1−xCo_xAsO and SmFe_1−xCo_xAsO. In the Co and Sr co-doped Pr_0.8Sr_0.2Fe_1−xCo_xAsO system, T_c reaches a maximum of about 16 K at x=0.15, and the T_c‐x dome shifts to higher Co content in the phase diagram of T_c versus Co content (x). Our result indicates that Sr doping effectively induces hole-type charge carriers in the PrFeAsO system and the disorder caused by Sr and Co dopants has little effect on T_c. The thermopower data confirms that Sr dopant is hole-type and Co dopant is electron-type. The shift of T_c‐x dome to higher Co content caused by Sr doping implies that the Sr dopant can compensate for the Co doping effect. The results suggest that there could be a universal dependence of T_c on charge carrier density in both hole-type and electron-type oxypnictide superconductors.     

Microstructure and piezoelectric properties of Bi0.5(Na0.82K0.18)0.5TiO3−NaSbO3 ceramics

Lead-free piezoelectric ceramics (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ have been prepared by a conventional ceramics technique, and their microstructure and electrical properties have been investigated. The addition of NaSbO₃ has no remarkable effect on the crystal structure within the studied doping content; however, an obvious change in microstructure took place. With increase in NaSbO₃ content, the temperature from a ferroelectric to antiferroelectric phase transition increases, and the temperature for a transition from antiferroelectric phases to paraelectric phases changes insignificantly. Simultaneously, the temperature range between the rhombohedral phase transition point and the Curie temperature point becomes smaller. The piezoelectric properties significantly increase with increase in NaSbO₃ content and the piezoelectric constant and electromechanical coupling factor attain maximum values of d₃₃=160 pC/N and k_p=0.333 at x=0.01. The results indicate that (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ ceramic is a promising lead-free piezoelectric candidate material.     

Properties of vanadium-doped SrBi4Ti4O15 ferroelectric ceramics

Using the standard solid-state reaction method, several vanadium-doped ferroelectric ceramics of type SrBi_4−x/3Ti_4−xV_xO₁₅ (SBTV−x) were synthesized. The vanadium doping content, x, rangs from 0.00 to 0.06. The crystal structure of SrBi₄Ti₄O₁₅ is not affected by V-doping. The electric breakdown voltage of the samples increases with V content. Meanwhile, V-doping results in a notable enlargement of remnant polarization (2P_r). The 2P_r of STBV−0.03 reaches a very large value, which is over 50 μC/cm² and is nearly twice greater than that at zero doping. The Curie temperatures of V-doped samples decrease slightly in comparison with that of SrBi₄Ti₄O₁₅. V-doping can improve the electric properties of SrBi₄Ti₄O₁₅ without sacrificing its thermal stableness.