Microstructure and properties of Ba1 − x La x TiO3 ceramics with submicron grain size

Submicron grain size Ba_{1 ? x} La _x TiO₃ (x = 0.025–0.05) barium titanate ceramics have been prepared by two-step heat treatment. The grain size of the ceramics was 200–400 nm at x = 0.025 and 300–700 nm at x = 0.05, and the ceramics had increased dielectric permittivity (?_max ?23 000 at x = 0.025 and ?_max ? 46 000 at x = 0.05) compared to large-grained (>1 μm) materials.     

Optical and electrical properties of Y3+ ion substituted orthorhombic mullite Y(x)Al(6−x) Si2O13 nanoparticles

Y³⁺ substituted mullite Y_(x)Al_(6?x)Si₂O₁₃ nanoparticles where x varied from 0.005 to 0.05 have been synthesized via co-precipitation technique. X-ray diffraction results revealed that orthorhombic mullite was the major phase in the samples of x = 0.0–0.025, whereas corundum α-Al₂O₃ was predominant at high Y³⁺-ion content of x = 0.05. Transmission electron microscope images showed orthorhombic-like structure for the pure sample. Meanwhile, the doped samples exhibited similar morphologies of larger particle sizes associated with small amount of glassy liquid phase. FT-IR spectrum evinced the formation of corundum particularly at high Y³⁺ ion content (5 %). The photoluminescence emission spectra were strongly affected by the Y³⁺ ion content. Moreover, mullite sample doped with 0.5 % Y³⁺ ion achieved the minimum electrical resistivity of 0.28 × 10⁹ Ω cm and the minimum dielectric loss value of 0.37 in the radiowave frequency region (10 MHz) as well as the minimum dielectric loss value of 0.41 in the microwave frequency region (1 GHz).     

Structure and dielectric properties of Nd x Sr1−x TiO3 ceramics for energy storage application

Polycrystalline Nd-doped SrTiO₃ ceramics with the formula Nd _x Sr_1?x TiO₃ (NSTO, x = 0, 0.024, 0.056, 0.104, 0.152, 0.200) were prepared by solid state reaction route. X-ray diffraction (XRD) analysis confirmed the formation of monophasic compounds and indicated the structure to be changed from cubic to tetragonal by increasing Nd doping concentrations. A remarkable decrease in grain size from ~30 μm for un-doped SrTiO₃ ceramics to ~1 μm for Nd-doped SrTiO₃ ceramics with x = 0.024 was observed by scanning electron microscopy. The grain size had a degree of increasing with further increasing Nd doping concentration and reached ~3 μm when the x value was 0.200. The dielectric properties of NSTO ceramics were measured at 1 kHz in ambient temperature. It revealed that the dielectric constant dramatically increases for the reason of Nd doping, leading to a maximum value of 19,800 for as-sintered sample with x = 0.104. The breakdown strength of all Nd-doped SrTiO₃ samples was found to be higher than 10 kV/mm. The relationship between dielectric properties and the microstructure feature, as well as the defect structures correlated with the charge compensation induced by trivalent Nd³⁺ doping, was discussed tentatively.     

Crystalline structure and dielectric properties of (Sr1−1.5x Bi x ) TiO3 ceramics

The crystalline structure, microstructure and dielectric properties of the (Sr_1–1.5x Bi _x )TiO₃ (0 x 0.267) ceramics were studied. Cubic solid solutions were determined for x 0.2 at room temperature. However, lattice distortion was detected by Raman spectra. A dense microstructure with the grain sizes of 2–4 m was obtained for (Sr_1–1.5x Bi _x )TiO₃ (0 x 0.2) ceramics. The Bi concentration was examined and found to be in agreement with the nominal composition and overall uniformly distributed in the sample. Different from the observations in the earlier literature for other doped quantum paraelectrics, where only an induced dielectric anomaly was reported, there are three Bi induced dielectric modes A, B, and C in the Bi doped SrTiO₃ samples. The occurrence of the impurity modes and the ferroelectric relaxor mode and their evolution are demonstrated as a function of Bi concentration.     

Dielectric properties of the Sr1−x La x Sn1−x Co x O3 system

The possibility of the formation of a solid solution in the Sr_1–x La _x Sn_1–x Co _x O₃ system has been explored. Single-phase solid solution forms in the compositions for x0.10. All single-phase solid solution compositions have a cubic structure similar to SrSnO₃. The dielectric behaviour of these solid solution compositions has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes significantly to their observed dielectric parameters. Microstructural studies show the presence of well-faceted grains. The average grain size in these samples is small.     

Structure and dielectric properties of Ba1−xSrxCuSi2O6-Ba0.55Sr0.45TiO3 ferroelectric-dielectric composite ceramics

yBa_1?xSr_xCuSi₂O₆-(1?y) Ba_0.55Sr_0.45TiO₃ (x?=?0, 0.2 and y?=?10 wt%, 30 wt%) ferroelectric-dielectric composite ceramics were prepared by the solid-state sintering method. The relationship among phase composition, microstructures and dielectric properties has been investigated. By adjusting the content of the Ba_1?xSr_xCuSi₂O₆ and the content of Sr²⁺ in the Ba_1?xSr_xCuSi₂O₆, the effect of the dielectric material on BST was studied. With the increase of Ba_1?xSr_xCuSi₂O₆ content and the increase of the Sr²⁺ content, the dielectric peak of the composite material is gradually suppressed and broadened, and the Curie temperature (Tc) moves to the low temperature. The introduction of Ba_1?xSr_xCuSi₂O₆ reduced the permittivity of the composite material to a certain extent, at the same time, the dielectric tunability and quality factor (Q) value are maintained at a high level. The dielectric tunability of the four composite ceramics obtained is still higher than 10% under the applied electric field (at 30 kV/cm). The 30 wt% BCSO–70 wt% BST55 system and the 30 wt% BSCSO–70 wt% BST55 system has lower dielectric permittivity (830, 577), higher dielectric tunability (17.3%, 10.2% at 30 kV/cm) and a decent Q value (149 at 1.25 GHz, 168 at 1.31 GHz).

     

Physical and electrical properties of screen-printed Zn x Cd1−x thick films

Screen-printed thick films of Zn _x Cd_1–x S have been prepared in the entire composition range from pure CdS to pure ZnS and sintered at 800° C on alumina substrates. Their structural and electrical properties have been studied as a function ofx. X-ray diffraction analysis of Zn _x Cd_1–x S established the presence of wurtzite structure for the range 0 x 0.8, whereas forx = 1.0 the presence of the sphalerite structure of ZnS is observed. The lattice parametersa andc vary withx in accordance with Vegard's law. Scanning electron micrographs reveal an enhancement in porosity with increasingx. The dark electrical resistivity of the film increases withx in the range 0 x 0.6, but beyond this range it starts decreasing. Photoconductivity is studied as a function ofx. An effect of H₂ annealing on the dark resistivity and photo-sensitivity is established.     

Dielectric properties of the system Ba1−xLaxTi1−xCo x O3

The dielectric behaviour of compositions withx=0.01, 0.05, 0.10 and 0.20 in the system Ba_1–x La _x Ti-_1-x Co _x O₃ was studied in the temperature range 300–473 K. The compositions withx = 0.01 and 0.05 show a diffuse ferroelectric-paraelectric phase transition, while other compositions do not show this transition in this temperature range. The frequency dependence of dielectric constant and dielectric loss in the samples withx0.05 indicates that spacecharge polarization contributes significantly to their observed dielectric parameters.     

Structural,electrical and magnetic properties of polycrystalline La0.67(Ca1−x Sr x )0.33MnO3 manganites

Polycrystalline La_0.67(Ca_1?x Sr _x )_0.33MnO₃ with different substitution level of strontium element, were synthesized via solid state reaction. Structure of samples was characterized by X-ray diffraction (XRD). XRD patterns reveal that La_0.67Ca_0.33MnO₃ exhibits orthorhombic structure with space group Pnma. Phase transitions from orthorhombic to rhombohedral take place as Ca ions were gradually substituted by Sr ions. The XRD data were further analyzed by Rietveld refinement technique. The data show that Mn–O–Mn bond angle increases as x increases. Microstructures obtained from SEM show that substitution of Sr ions has demoted the grain growth and densification process during sintering. The substitution of Sr ions has greatly influenced the hopping integral of electron via double exchange interaction, thus affecting the electrical properties and magnetic properties as well. The resistivity decreases and the metal–insulator transition temperature (T _p ) shifts to higher temperature as x increases. The magnetoresistance (MR) effect gradually decreases and MR peak shifts to higher temperature as x increases. The magnetization measured at room temperature is found to be increasing as x increases.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

Physical properties of YBa2Cu3O7−x superconductor sintered at different temperatures

YBa₂Cu₃O_7–x ceramics prepared by sintering at 920 and 1030° C were studied in some detail. Although samples prepared at 920° C had lower density, they had larger transport critical current at 77 K. A correlation between the transport critical current and the magnetic field dependence of the second maximum in the imaginary part of the initial susceptibility was found and discussed. According to our results samples prepared at higher temperature had larger but (in spite of almost ideal density) poorly connected grains.     

Physical and electrical properties of MnO2-doped Pb(Zr x Ti1−x )O3 ceramics

The effects of composition on the physical property change in the phase coexistence region between the tetragonal and rhombohedral phases have been investigated as a function of zirconium concentration, x, for the MnO₂-doped Pb(Zr _x Ti_1–x )O₃ (0.40x0.60) ceramics. The relative amount of phase coexisting between the tetragonal and rhombohedral phases affects greatly both dielectric and piezoelectric properties as a function of zirconium concentration. However, there are no detectable changes between the apparent density and microstructure. Also, in the coexistence region, the relative amount of coexistence of the rhombohedral phase increases with MnO₂ addition. The inflection points of the dielectric constant shift to lower zirconium concentration in proportion to the MnO₂ addition, owing to the substitution effect on the PZT lattice site.     

Structural refinement,optical and electrical properties of [Ba1−x Sm2x/3](Zr0.05Ti0.95)O3 ceramics

Samarium doped barium zirconate titanate ceramics with general formula [Ba_1?x Sm_2x/3](Zr_0.05Ti_0.95)O₃ [x = 0, 0.01, 0.02, and 0.03] were prepared by high energy ball milling method. X-ray diffraction patterns and micro-Raman spectroscopy confirmed that these ceramics have a single phase with a tetragonal structure. Rietveld refinement data were employed to model [BaO₁₂], [SmO₁₂], [ZrO₆], and [TiO₆] clusters in the lattice. Scanning electron microscopy shows a reduction in average grain size with the increase of Sm³⁺ ions into lattice. Temperature-dependent dielectric studies indicate a ferroelectric phase transition and the transition temperature decreases with an increase in Sm³⁺ ion content. The nature of the transition was investigated by the Curie–Weiss law and it is observed that the diffusivity increases with Sm³⁺ ion content. The ferroelectric hysteresis loop illustrates that the remnant polarization and coercive field increase with an increase in Sm³⁺ ions content. Optical properties of the ceramics were studied using ultraviolet–visible diffuse reflectance spectroscopy.     

Composition range and physicochemical properties of La1 − x Ba x Mn1 − y Fe y O3 solid solutions

We have determined the extent of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ solid solutions with orthorhombically and rhombohedrally distorted perovskite structures. A partial phase diagram of the LaMnO_{3 + δ}-BaMnO₃-BaFeO_2.5-LaFeO₃ system in air at a temperature of 1373 K has been proposed for the first time. We have measured the relative length change of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ samples and calculated their thermal expansion coefficients.     

Doping effects of structural transformation and soft mode in Ba x Pb1−x TiO3 nanoparticles

Ba _x Pb_1−x TiO₃ nanoparticles with various Ba concentrations were synthesized by stearic acid method. Structural transformation, ferroelectricity, and soft mode character under various Ba concentrations were examined by X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. When the Ba concentration in Ba _x Pb_1−x TiO₃ nanoparticle increases, tetragonality c/a reduces, transition temperature T _c decreases and the E(1TO) soft mode softens. A critical Ba doping concentration of x = 0.4 was found. Above the concentration tetragonality c/a reaches ∼1 and ferroelectricity disappears, and the E(1TO) soft mode also vanishes. Differential scanning calorimetric measurement on heating and cooling shows small thermal hysteresis ΔT = 4 °C in Ba _x Pb_1−x TiO₃ nanoparticle which manifests a weak first-order transition of the specimen. On the basis of the different electronic structures of Ba and Pb atoms, the mechanisms of the doping effects of ferroelectricity, structural transformation, and phonon properties are discussed and attributed to decrease in hybridization between Ti 3d and O 2p states via indirect interaction between the Ba 5p and O 2p states.     

Microstructures and photo-catalytic properties of B3+ and F− co-doped TiO2 films

The nano-crystalline B³⁺ and F^? co-doped titanium dioxide films were successfully prepared by the improved sol–gel process. The as-prepared specimens were characterised using X-ray diffraction (XRD), high-resolution field emission scanning electron microscopy (FE-SEM), the Brunauer–Emmett–Teller (BET) surface area, X-ray photo-electron spectroscopy, photoluminescence spectra and UV–Vis diffuse reflectance spectroscopy. The photo-catalytic activities of the films were evaluated by degradation of an organic dye in aqueous solution. The results of XRD, FE-SEM and BET analysis indicated that the TiO₂ films were composed of nano-particles. B³⁺ and F^? co-doping could obviously not only suppress the formation of brookite phase but also inhibit the transformation of anatase to rutile at high temperature. Diffuse reflectance measurements showed that co-doping could clearly extend the absorbance spectra of TiO₂ into visible region. Compared with pure TiO₂, B³⁺ doped or F^? doped TiO₂ film, the B³⁺ and F^? co-doped TiO₂ film exhibited excellent photo-catalytic activity. It is believed that the surface microstructure of the films and the doping methods of the two ions are responsible for improving the photo-catalytic activity.