CuO as sintering additive to Sr0.4Ca0.6La4Ti5O17 ceramics

The effects of CuO addition on the sintering behavior and microwave dielectric properties of Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics were investigated. CuO was selected as a liquid phase sintering aid to lower the sintering temperature of Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics. With CuO addition, the sintering temperature of Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics was effectively reduced from 1550 °C to 1475 °C. The crystalline phase exhibited no phase difference and no second phase was detected at all addition levels. The electric permittivity was not significantly affected by various amounts of CuO addition and ranged from 52 to 54. Small values (<+7 ppm/K) of the temperature coefficient of resonant frequency were obtained for Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics. However, the unloaded quality factor Q _u was strongly dependent upon the CuO concentration. Q _u f _o ～ 10500 GHz was obtained for Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics with 0.5 wt.% of CuO addition, sintered at 1475 °C.     

Effect of Zr+4 ion substitution on the structural, dielectric and electrical properties of Sr5LaTi3Nb7O30 ceramics

Polycrystalline Zr-modified Sr₅LaTi₃Nb₇O₃₀ compound was prepared by a high-temperature solid-state reaction technique. X-ray structural analysis confirmed the formation of compound in an orthorhombic system at room temperature. Detailed studies of the dielectric parameters (dielectric constant and tangent loss) as a function of frequency (1–100 kHz) at different temperature (150 to 650 K) were carried out. It was found that as Zr⁺⁴ concentration increases in Sr₅LaTi_{3 – x} ZrxNb₇O₃₀, the value of dielectric constant decreases. These compounds show ferroelectric-paraelectric phase transition of diffuse type at 283, 305 and 320 K for x = 0, 1 and 2 respectively. The transition temperature (T _c) shifts towards higher temperature and maximum dielectric constant value (_max) decreases with increasing Zr⁺⁴ concentration for x = 0 to x = 2. When Ti⁺⁴ cations were completely replaced by Zr⁺⁴ cations (i.e., Sr₅LaZr₃Nb₇O₃₀), the compound does not show any phase transition. Impedance-spectroscopic studies provided an insight into the electrical properties and understanding of relaxation behavior of the material. Measurement of electrical conductivity as a function of temperature suggests that the compounds have a negative temperature coefficient of resistivity (NTCR) with a typical semiconductor behavior.     

Phase formation and microwave dielectric properties of Pb2+ and Sr2+ doped La4Ti9O24 ceramics

Phase formation and microwave dielectric properties of the Pb²⁺ and Sr²⁺ doped La₄Ti₉O₂₄ ceramics were investigated. Using electron diffraction and Rietveld analysis of the X-ray powder diffraction patterns, we show that the increase in the concentration of Pb²⁺ and Sr²⁺ doping results in the structural transition from La₄Ti₉O₂₄ to a La_2/3TiO₃-type phase (Ibmm, No. 74). A change in the crystalline phase considerably affects the microwave dielectric properties, increasing the ɛ_r from 37 to 130, reducing Q × f from 25,000 to 5500, and increasing temperature coefficient of the resonant frequency (TCF) from 15 to 300 ppm/°C.     

Microstructural and dielectric properties of donor doped (La3+) CaCu3Ti4O12 ceramics

The effect of La³⁺ doping on Ca²⁺ sites in CaCu₃Ti₄O₁₂ (CCTO) was examined. Polycrystalline samples in the chemical formula Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ with x = 0, 0.5, 1 were synthesized via the conventional solid state reaction route. X-ray powder diffraction analysis confirmed the formation of the monophasic compounds and indicated the structure to be remaining cubic with a small increase in lattice parameter with increase in La³⁺ doping. The dielectric and impedance characteristics of Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ were studied in the 100 Hz–10 MHz frequency range at various temperatures (100–475 K). A remarkable decrease in grain size from 50 μm to 3–5 μm was observed on La³⁺ substitution. The dielectric constant of CaCu₃Ti₄O₁₂ decreased drastically on La³⁺ doping. The frequency and temperature responses of dielectric constant of La³⁺ doped samples were found to be similar to that of CaCu₃Ti₄O₁₂. The effects of La³⁺ doping on the electrical properties of CaCu₃Ti₄O₁₂ were probed using impedance spectroscopy. The conducting properties of grain decreased while that of the grain boundary increased on La³⁺ doping, resulting in a decrease of the internal barrier layer effect. A decrease in grain boundary capacitance and stable grain response in La³⁺ doped CCTO ceramics were unambiguously established by modulus spectra studies.     

Preparation,characterization and dielectric properties of Ba3−xSrxM4Ti4O21 (M = Nb,Ta and 0 ≤ x ≤ 3) ceramics

Ba_3−xSr_xM₄Ti₄O₂₁ (M = Nb, Ta and 0 ≤ x ≤ 3) ceramics have been prepared through solid state ceramic route. The structure and microstructure of the compositions have been studied using powder X-ray diffraction and Scanning Electron Microscopic studies. In the present study, both Ba₃Nb₄Ti₄O₂₁ and Ba₃Ta₄Ti₄O₂₁ are obtained as single phase compositions whereas strontium rich compositions exhibit multiphase in nature. Energy dispersive X-ray analysis has been performed to identify the nature of additional phases present in the strontium rich compositions. The dielectric constant, dielectric loss and temperature variation of dielectric constant of the well sintered ceramic specimens have been studied in the low frequency region (< 13 MHz) using an impedance analyzer. The present study reveals that high dielectric compositions can be realized by Sr-substitution in the BaO–TiO₂–Nb₂O₅/Ta₂O₅ ternary systems.     

Dielectric properties and electrical response of grain boundary of Na1/2La1/2Cu3Ti4O12 ceramics

The dielectric properties and electrical response of grain boundaries of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics were investigated as a function of temperature. High dielectric permittivity (6.1–8.7 × 10³) and low loss tangent (0.032–0.038 at 10 kHz) were observed in Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics. Through analyses using complex impedance and electric modulus, it was found that the dielectric properties of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics are closely related to the electrical response of grain boundaries. The investigation of non-Ohmic characteristics at various temperatures suggests that the potential barrier at the grain boundaries of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics is due to the Schottky effect. The giant low frequency dielectric response in Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics is attributed to Maxwell–Wagner polarization at the grain boundaries.     

Ca<Subscript>4</Subscript>La<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript>: a novel low loss dielectric ceramics in the CaO–La<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2 </Subscript>system

A novel low loss dielectric material Ca₄La₂Ti₅O₁₇ was prepared by solid state ceramic route. The structure and microstructure of the Ca₄La₂Ti₅O₁₇ ceramics was investigated using XRD, SEM and EDX techniques. XRD result showed cubic perovskite like structure for Ca₄La₂Ti₅O₁₇ ceramics. The ceramics were sintered in the temperature range of 1,450–1575 °C. The microwave dielectric properties of the material were investigated using a network analyzer in the frequency range of 3–6 GHz. The variation in microwave dielectric properties of the Ca₄La₂Ti₅O₁₇ ceramics with sintering temperature was correlated with bulk density of the material. Ca₄La₂Ti₅O₁₇ has ε_r of 73, Q_uxf of 16,000 at 3.3 GHz, and τ_f of 127 ppm/°C at the optimized sintering temperature of 1,550 °C/4h.     

Modification of Ba4Sm2Ti4Ta6O30 dielectric ceramics

Modification of promising tungsten bronze-type dielectric ceramic Ba₄Sm₂Ti₄Ta₆O₃₀ has been investigated by adjusting the composition in Ba_4-3xSm_2+2xTi₄Ta₆O₃₀. The temperature coefficient of dielectric constant has been significantly improved by such modification together with the suppressed dielectric loss, but the dielectric constant decreased significantly. © 2000 Kluwer Academic Publishers     

Dielectric relaxation and polaronic hopping in the single-layered perovskite La<Subscript>1.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>4</Subscript> ceramics

Single tetragonal La_1.5Sr_0.5CoO₄ ceramics with the space group of I 4/mmm (139) were prepared by a solid-state reaction process, and dielectric characteristics were investigated on a broad frequency and temperature range. There was one obvious dielectric relaxation around room temperature plus a low temperature upturn on the curve of temperature dependence of dielectric properties for La_1.5Sr_0.5CoO₄ ceramics. This dielectric relaxation was a thermal-activated process. It should be attributed to the mixed-valence structure (Co²⁺/Co³⁺) since its activation energy was similar to that of small polaronic hopping process. After annealing the sample in O₂ atmosphere, dielectric constants and ac conductivities of La_1.5Sr_0.5CoO₄ ceramics increased and decreased after annealing the sample in N₂ atmosphere. This abnormal phenomenon should be attributed to the variation of concentration for holes (Co³⁺).     

Structure,chemical stability and electrical conductivity of perovskite La0.6Sr0.4M0.3Fe0.7O3−δ (M = Co,Ti) oxides

The perovskite La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) powders have been synthesized by the citrate gel method. The structural and chemical stability of the La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) oxides were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. The electrical conductivities of the sintered La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) ceramics were measured. The results demonstrate the chemical stability in H₂/helium (He) atmosphere of the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3?δ oxide is improved significantly compared to that of the La_0.6Sr_0.4Co_0.3Fe_0.7O_3?δ oxide. The incorporation of Ti^3+/4+ ions in the perovskite structure can significantly stabilize the neighboring oxygen octahedral due to the stronger bonding strength, leading to the enhanced structural and chemical stability of the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3?δ. In addition, the perovskite La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) oxides possess much higher chemical stability in CO₂/He atmosphere than that of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ oxide, in which the perovskite structure is destroyed completely in a flowing CO₂-containing atmosphere.     

Effects of La3+ doping ions on dielectric properties and formation of Schottky barriers at internal interfaces in a Ca2Cu2Ti4O12 composite system

Influences of La³⁺ substitution on the dielectric properties and formation of Schottky barriers at internal interfaces of a Ca₂Cu₂Ti₄O₁₂ (CaTiO₃/CaCu₃Ti₄O₁₂) composite system were investigated. It was found that electrostatic potential barrier height was greatly reduced by doping with La³⁺, leading to a large decrease in the total resistance of internal interfaces between grains. This observation was attributed to the creation of conduction electrons, which were possibly induced by electrical charge compensation of La³⁺ substitution into Ca²⁺ sites. Variations in the dielectric properties of La³⁺-doped CaTiO₃/CaCu₃Ti₄O₁₂ composite ceramics and nonlinear properties can be described based on the electrical responses at the internal interfaces between CaCu₃Ti₄O₁₂–CaCu₃Ti₄O₁₂ grains and CaTiO₃–CaCu₃Ti₄O₁₂ grains. Influence of possible charge compensation due to different levels of La³⁺ dopant on the formation of potential barriers was discussed.     

Tunable and microwave dielectric properties of Ba0.5Sr0.5TiO3-BaWO4 composite ceramics doped with Co2O3

Co₂O₃ doped BaWO₄-Ba_0.5Sr_0.5TiO₃ composite ceramics, prepared by solid-state route, were characterized systematically, in terms of their phase compositions, microstructure and microwave dielectric properties. Doping of Co₂O₃ promoted grain growth, reduced Curie temperature and broadened phase-transition temperature range of BaWO₄-Ba_0.5Sr_0.5TiO₃, which were attributed mainly to the substitution of Co³⁺ for Ti⁴⁺ at B site in the perovskite lattice. Dielectric diffusion behaviors of the composite ceramics were discussed. The composite ceramics all had dielectric tunability of higher than 10% at 30 kV/cm and 10 kHz, with promising microwave dielectric properties. Specifically, the sample doped with 0.2 wt.% Co₂O₃ exhibited a tunability of 20%, permittivity of 225 and Q of 292 (at 1.986 GHz), making it a suitable candidate for applications in electrically tunable microwave devices.     

Structural and electrical properties of Na2Pb2Eu2W2Ti4X4O30 (X = Nb, Ta) ferroelectric ceramics

A new group of tungsten bronze family Na₂Pb₂Eu₂W₂Ti₄X₄O₃₀ (X = Nb, Ta) having all the valence elements (I-VI) were prepared by a high-temperature solid-state reaction technique. The formations of the compounds were confirmed by X-ray diffraction technique with an orthorhombic structure. Surface morphology of the compounds was studied by scanning electron microscope (SEM). Studies of dielectric properties (?_r and tan δ) of the above compounds at different frequencies in a wide temperature range (300-700 K) with an impedance analyzer exhibit a ferroelectric phase transition at 580 and 394 K for Na₂Pb₂Eu₂W₂Ti₄Nb₄O₃₀ and Na₂Pb₂Eu₂W₂Ti₄Ta₄O₃₀, respectively. Ferroelectric properties of these compounds were confirmed with polarization (hysteresis) study.     

Etude du comportement de l'antimoine v et du strontium dans les oxydes a structure en cages du type A3M8O21

New compounds Ba₃(Sb₄Ti₄)O₂₁ and Ba_3?xSr_x(M₄Ti₄)O₂₁ (M = Nb, Ta, Sb) isostructural with the niobates A₃(Nb_8?xM_x)O₂₁ previously described, have been synthesized. The influence of the antimony and strontium ions on the stability of the A₃M₈O₂₁ structure has been discussed in terms of electrostatic repulsion forces, polarization and covalency. Atomic positions, determined from powder diffraction patterns, are given for Ba₃(Sb₄Ti₄)O₂ and Sr₃(M₄Ti₄)O₂₁ (M = Nb, Ta).     

Ba4Nd2Ti4Ta6O30 dielectric ceramics modified by Bi substitution for Nd

Modification of dielectric properties for Ba₄Nd₂Ti₄Ta₆O₃₀ ceramic was investigated through Bi partial substitution for Nd. The dielectric constant increased and the dielectric loss decreased with increasing concentration of Bi, and the dielectric constant reached 142, combined with a low dielectric loss of 10^–4 (at 1 MHz) for the composition Ba₄(Nd_0.975Bi_0.025)₂Ti₄Ta₆O₃₀. The temperature coefficient () can be slightly improved.     

Microwave dielectric properties of Sr2Ce2Ti5O16 ceramics

Sr₂Ce₂Ti₅O₁₆ dielectric ceramics were prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics were investigated by X-ray diffraction and scanning electron microscopic methods. The Sr₂Ce₂Ti₅O₁₆ has a psuedocubic structure. It has ɛ_r of 113, unloaded quality factor (Q_u × f) of 8000 GHz and temperature coefficient of resonant frequency of 306 ppm/°C. The effects of various dopants on the structure, microstructure and microwave dielectric properties of the material have been investigated. It is found that addition of small amount of dopants such as PbO, Al₂O₃, Nd₂O₃, MoO₃, CeO₂, La₂O₃, Fe₂O₃ and NiO improve the microwave dielectric properties of Sr₂Ce₂Ti₅O₁₆.     

Crystallization, microstructure development and dielectric behaviour of glass ceramics in the system [SrO · TiO2]-[2SiO2 · B2O3]-La2O3

Various glasses in the system (65 – x)[SrO · TiO₂]-(35)[2SiO₂ · B₂2O₃]-(x)La₂O₃, where x = 1,5,10 (wt%) were prepared by melting in alumina crucible (1375–1575 K). Heat treatment schedules were selected from DTA plots of respective glasses. X-ray diffraction studies of glass ceramic samples containing different concentrations of La₂O₃ revealed the formation of Sr₂B₂O₅, Sr₃Ti₂O₇ and TiO₂ (rutile) phases. The addition of La₂O₃ results in the development of well formed, elongated crystallites of different phases. Results of the dielectric behaviour demonstrate higher values of dielectric constant for some of the glass ceramic samples. This can be ascribed to the relaxation polarization at the crystal-glass interface due to conductivity differences between crystalline and glassy phases.     

Ferroelectric Bi3.25La0.75Ti3O12 thin films on a conductive Sr4Ru2O9 electrode obtained by pulsed laser deposition

Strontium ruthenate and Bi_3.25La_0.75Ti₃O₁₂ (BLT) layers were grown on Si(100) substrate using pulsed laser deposition technique. Starting from a Sr₂RuO₄ target, we obtained single phase films composed of Sr₄Ru₂O₉; on these strontium ruthenate electrodes, textured and non-textured BLT were grown at 700 °C. Structural characterizations of these double layers were done by X-ray diffraction, scanning electron microscopy, normal and high-resolution transmission electron microscopy. The Van der Pauw's resistivity measurements indicate that Sr₄Ru₂O₉ can be used as a back electrode. The temperature dependence of the resistivity at low temperatures is , which corresponds to a variable-range hopping mechanism.     

Microwave dielectric properties of Ba2 − xSrxLa3Ti3NbO15 ceramics

High dielectric constant and low loss ceramics in the system Ba_2 − xSr_xLa₃Ti₃NbO₁₅ (x = 0-1) have been prepared by conventional solid-state ceramic route. Ba_2 − xSr_xLa₃Ti₃NbO₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 48.34 to 43.03, quality factor Q_u × f from 20,291 to 39,088 GHz and temperature variation of resonant frequency from 8 to 1.39 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

Magnetocaloric and Colossal Magnetoresistance Effect in Layered Perovskite La1:4Sr 1:6Mn2O7

Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La_1:4Sr_1:6Mn₂O₇ compound have been studied. A broad peak of magnetic entropy change (-ΔS_M) is found above the Curie temperature (T_C=120 K), which can be associated with the existence of two-dimensional short range ferromagnetic order. Additionally, the curvilinear shape of -ΔS_M for layered perovskite is quite different from that of the Ln_1-xA_xMnO₃ probably arising from magnetocrystalline anisotropy. At the same time, a wide peak of colossal magnetoresitance effect near T_C is found in the layered provskite La_1:4Sr_1:6Mn₂O₇.