Defect profile and electrical properties of Nb2O5-doped BaTiO3 materials

This work deals with the changes in properties induced by the incorporation of donor ions into the BaTiO₃ lattice. We focused on the relationship between microstructural development and the defect profile of Nb-based compositions around the semiconducting–insulating transition at 0.3% of donor oxide. The results indicated that the defect structure that developed controlled both microstructural and electrical properties. On sintering slightly doped BaTiO₃, large semiconducting grains and low barium vacancy concentration were detected. This behavior was explained by considering an electronic compensation mechanism based on dopant incorporation. Further dopant addition led to a dramatic grain-growth inhibition and a clear transition to a material with higher resistivity. In addition, an extraordinary generation of barium vacancies was observed. Heavily doped materials consisted of core-shell structures with a Ti-rich phase surrounding BaTiO₃ grains. This phenomenon originated a significant grain-boundary barrier layer response.     

Superconductivity in compound Y1-2x Ce x Ba2+x Cu3O6·5+δ system

Partial substitution of Ce in place of Y has been achieved by taking care of charge compensation and compounds with the general formula Y_1-2 xCe_xBa₂₊ xCu₃O_6·5+δ have been successfully prepared. X-ray powder diffraction analysis reveals that this substitution gives orthorhombic single phase for 0⩽x⩽0·3. TheT _c remains nearly unchanged between 94-90 K.     

Effect of Ni2+ substitution on structural,magnetic and electrical traits of Ba1-xNixFe2O4

Nickel substituted Barium ferrites is synthesized by citrate gel method. Nickel which is transition metal used to tailor the properties of the Ba spinel ferrites. The characterization techniques such as thermogravimetry–differential thermal analysis (TG-DTA), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), Fourier transform infrared spectroscopy (FTIR), ultra-violet diffuse-reflectance spectroscopy (UV-DRS), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM), Raman analysis and direct current (DC) electrical conductivity are used to investigate various properties. XRD and rietveld refinement confirmed the existence of orthorhombic as well as cubic phase with the decrease in lattice parameters in orthorhombic phase. FTIR spectra confirmed the presence of absorption bands for tetrahedral and octahedral sites. Tauc’s plots accredited the increase in band gap with increase in Ni content. The magnetic hysterisis loop exhibited large variation in coercivity and remanence. Increased Yafet-Kittel (Y-K) angle indicated redistribution of ions amongst the sites strenthening the A-B exchange interactions. DC electrical conductivity observed the conductive switching from n-type to p-type. Thermally activated charge carriers and hopping of electrons boost drift mobility. The activation energy of conduction is observed in the range of 0.40 to 0.85 eV. Raman peaks confirmed formation of orthorhombic as well as cubic phase.     

Effect of thermal treatment on various characteristics of undoped and V2O5-doped Co3O4/TiO2 catalysts

The thermal treatment of undoped and V₂O₅-doped Co₃O₄/TiO₂ catalysts was studied in the temperature range, 330–600° C both in vacuum and in air. The wide difference in the catalytic behaviour of the two catalysts could be attributed to surface as well as bulk diffusion of the active cobalt oxide particles. Although in both cases the total Co³⁺ ions of various energy states were considered to be the active species for the given reaction, the distribution of various cobalt species, namely Co-t and Co-o, occupying tetrahedral and octahedral sites in the support-defective structure, seemed to be seriously affected by doping with V₂O₅. This dopant was supposed to have two-fold effect: part is incorporated into the surface Co₃O₄ crystallites leading to smaller more mobile particles, easily reducible and more dispersed, and another part diffuses a few atomic layers deeper in the support causing the redistribution of cobalt species. Upon heating, the increased mobility and the increased availability of the support tetrahedral sites may be responsible for the deactivation behaviour. The bulk diffusion enhanced by doping might cause some modification in the porosity characteristics of the titania support.     

Preparation and blue–white luminescence properties of Bi3+-doped Ba5SiO4Cl6

The Ba₅SiO₄Cl₆:Bi³⁺ phosphor was synthesized by high-temperature solid-state reaction and its luminescence property was investigated. The results showed that the Bi³⁺-doped Ba₅SiO₄Cl₆ phosphors exhibited an intense blue–white light emission located at 480 nm and a broad excitation band from 230 to 340 nm. The Bi³⁺-doped Ba₅SiO₄Cl₆ phosphors can be efficiently excited by the incident light of 220–340 nm, and the emission properties of the Bi³⁺-doped Ba₅SiO₄Cl₆ samples are strongly dependent on the excitation wavelength. The emission color tunability can be obtained by changing the excitation wavelength. The visible region emission characteristics of Ba₅SiO₄Cl₆:Bi³⁺ indicates that it can potentially be used as a new efficient blue–white luminescent material.     

Microstructure, electrical properties, and dc aging characteristics of Tb4O7-doped ZnO-based varistors

The microstructure, electrical properties, and dc-accelerated aging characteristics of Tb₄O₇-doped ZnO-based varistors were investigated for different Tb₄O₇ amounts and sintering temperatures. The sintered density increased with increasing Tb₄O₇ amount and sintering temperature. The average grain size decreased with increasing Tb₄O₇ amount and increased with increasing sintering temperature. The varistor voltage and nonlinear coefficient increased with increasing Tb₄O₇ amount and decreased with increasing sintering temperature. The stability was worse with increasing Tb₄O₇ amount for the varistors sintered at 1,300 °C. The 0.5 mol% Tb₄O₇-doped varistors sintered at 1,350 °C exhibited a good stability for dc-accelerated aging stress of 0.95 V _{1 mA}/150 °C/24 h.     

A novel green-emitting phosphor of Eu2+-doped Cs2MgSi5O12

A new phosphor of Eu²⁺-doped Cs₂MgSi₅O₁₂ was synthesized by conventional solid-state reaction. The phase formation was confirmed by X-ray powder diffraction measurements. The photoluminescence excitation and emission spectra were measured. The phosphor presents bright yellowish-green-emitting luminescence. The site-occupancy of Eu²⁺ ions doped in Cs₂MgSi₅O₁₂ was discussed according to the crystal structure and the luminescence characteristics of Eu²⁺ ions. Two different Eu²⁺ centers were assigned in Cs₂MgSi₅O₁₂ lattices. The dependence of luminescence intensity on the heating temperatures was measured and discussed. The chromaticity coordinates and activation energy (ΔE) for the thermal quenching of Cs₂MgSi₅O₁₂:Eu²⁺ were reported.     

Cu2+对掺Pb2+的β-BaB2O4纳米材料发光特性的影响

采用共沉淀法合成了掺杂Cu2 ,Pb2 的β-BaB2O4纳米材料,通过激发光谱和发射光谱研究了其发光性质,并探讨了Cu2 对Pb2 发光性能的影响.在β-BaB2O4:Pb2 的发射光谱中观察到位于630nm的强而稳定的发射峰.由于Cu2 和Pb2 在β-BaB2O4中形成复合发光中心,Cu2 和Pb2 共掺样品的发射光谱与Pb2 单独样品的发射光谱有很大的不同,Pb2 在β-BaB2O4:Cu2 ,Pb2 中的发光强度显著增强.     

Water vapor absorption and proton conductivity of (Ba1−x La x )2In2O5+x

The proton conductivity of (Ba_1−x La _x )₂In₂O_5+x system has been investigated as a function of the La content, temperature and amount of absorbed water. The proton conductivity increased with La content up to x = 0.10, to reach a maximum of 1.12 × 10⁻⁵ (S/cm) at 400 °C. From that point on, it decreased. From the results of thermogravimetry and mass spectra, we confirmed that the water was absorbed in the (Ba_1−x La _x )₂In₂O_5+x system, in a maximum quantity of 0.14 mol/mol (sample). The proton conductivity increased monotonically with the quantity of water uptake, suggesting that this variable is one of the dominant parameter of proton conductivity in this system.     

掺铒TeO2-Nb2O5-ZnO系统玻璃的上转换发光性能

稀土离子掺杂碲酸盐系统玻璃是一类应用前景良好的上转换发光材料.研究了含铒TeO2-Nb2O5-ZnO系统玻璃在980nm抽运下的上转换光谱,结果发现存在3个上转换荧光谱带,分别对应于2H11/2→4I15/2、4S3/2→4I15/2和4F9/2→4I15/2,而且Er3 含量的增加可明显提高材料在530、550和660nm附近的发光性能.少量的ZnO引入既可以大幅提升材料的上转换发光强度,又能保持铌碲酸盐玻璃良好的化学稳定性.材料的发光机制主要是激发态吸收(ESA)和能量转移(ET),最大声子能量的降低是上转换发光增强的主要原因.     

Crystal structures of Ba4CaCu3O8+δ and Ba6CaCu3O10+δ

The crystal structures of the two ternary compounds, Ba₄CaCu₃O_8+δ and Ba₆CaCu₃O_10+δ, have been investigated by means of X-ray diffraction combined with Rietveld analysis. We found that the Ba₄CaCu₃O_8+δ phase has a cubic structure (Im–3m, a=8.1515(1) Å, δ=+0.8) for high oxygen content as reported in the literature but undergoes a transformation into a tetragonal structure (I4/mmm, a=8.1888(1) Å, c=8.0634(1) Å for δ=−0.3) when the oxygen content is lowered. The crystal structure of Ba₆CaCu₃O_10+δ (I4/mmm, a=4.0463(1) Å, c=21.7322(4) Å for δ=+0.2) is confirmed. Changes in the c value with sintering conditions suggest a variable oxygen content with no structure transformation.     

Phenomenological investigation of inhomogeneities in Nd3+-doped Ba2NaNb5O15 single-crystal fibres grown by the laser-heated pedestal growth technique

Inhomogeneities in Ba2NaNb5O15 (BNN) single-crystal fibres undoped or doped with 1–5 at% Nd3+ and grown by the laser-heated pedestal growth technique were investigated by energy-dispersive X-ray microprobe analysis (EDXA), second-harmonic generation (SHG) measurements and interference contrast microscopy after chemical etching. The observed internal morphology of BNN fibres was correlated to SHG and EDXA measurements and involves convective flow instabilities in the molten zone and possibly unsteady interface kinetics. From this study, it appears that for a doping concentration equal to 3 at% Nd3+, no growth striations nor any kind of inhomogeneity could be detected, indicating, as also shown by differential thermal analysis, that this mixture congruently melts, ensuring very-high-optical-quality crystals are obtained as no cracks nor microtwins are also present for this amount of neodymium. © 1998 Chapman & Hall     

Investigation of the crystal structure and electrical properties of La3+-doped SrBi2Ta2O9 ceramics

The layered-perovskite ferroelectric ceramics of La³⁺-doped SrBi₂Ta₂O₂ (SBT), with the chemical formula of SrBi_{(2 - x)}La _x Ta₂O₉ (SBLT), have been prepared by the conventional mixed-oxide method. The effect of substitution of La³⁺ for Bi³⁺ in the crystal structure and electrical properties of SBT ceramics was explored with the aid of X-ray diffraction, (T) curve and ferroelectric hysteresis loop measurements. The electrical properties such as dielectric constant () and remanent polarization (P _r) showed a dependence on the crystal structure, and both reached maxima of 243 and 25 C cm^–2, respectively, with 6 at % La³⁺ substitution, accompanying the greatest structure change. Theoretical considerations were presented to suggest that the atomic displacements and the crystal deformation implied by the crystal structure change are responsible for the improvement of electrical properties. On the other hand, degradation of fatigue resistance was observed in SBLT ceramics, which is believed to be caused by the chemical environment change of the perovskite layers arising from La³⁺ substitution on Bi³⁺ sites.     

Synthesis and photoluminescence properties of Eu2+-doped Ca2AlSi3O2N5 green phosphors

Novel Eu²⁺-doped Ca₂AlSi₃O₂N₅ phosphors with a general formula of Eu_xCa_2?xAlSi₃O₂N₅ were successfully prepared via a solid-state reaction method under a nitrogen atmosphere. The produced phosphors were effectively excited by UV–vis light in the wavelength range between 250 and 400 nm, and featured an intense green emission band which peaked at about 500 nm. The emission spectra featured a red-shift over increasing Eu²⁺ content and the temperature of heat treatment. The maximum intensity of emission was obtained for x = 0.014 and heat treatment at 1450 °C. The photoluminescence properties of the produced Ca₂AlSi₃O₂N₅:Eu²⁺ phosphors qualify them for consideration in potential use as green phosphors in UVLED-based white LED.     

Ce3+-doped Lu2Si2O7 luminescent fibers derived from electrospinning: facile preparation and flexible fiber molding

One-dimensional Lu₂Si₂O₇:Ce³⁺ (LPS:Ce) luminescent fibers were prepared by the sol–gel process combined with electrospinning with polyvinyl butyral as polymer in this study. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, thermogravimetric and differential scanning calorimetry, Fourier transform infrared spectroscopy, photoluminescence, and kinetic decays were used to characterize the resulting samples. After calcinated at 1200 °C, pure crystalline phase of Lu₂Si₂O₇ was obtained with well maintenance of the fiber morphology. The excitation and emission curves of LPS:Ce samples are influenced by cerium-doping concentrations. LPS:0.5%Ce fibers show the most intense emission among the samples. A fast decay time of tens of nanosecond was also observed in LPS:Ce fibers. Furthermore, LPS:0.5%Ce powders and Y co-doped LPS:0.5%Ce fibers were also prepared in a similar way for comparison. As a result, LPS:0.5%Ce fibers present a much stronger emission and higher quantum efficiency than that of LPS:0.5%Ce powders, and a close efficiency compared with Y_0.8Lu_1.2Si₂O₇:0.5%Ce fibers. In addition, a facile and efficient fiber molding process can be realized to assemble one-dimensional LPS:Ce fibers into three-dimensional (3D) fibers structures with different shapes like bracelet and spool. Such 3D fiber structures are dense enough and well-shaped even if after high temperature calcination. The high performance LPS:Ce fibers and LPS:Ce 3D fibers may have great potential applications for luminescent clad sensors, optoelectronic devices, or scintillating detectors     

Comparison of Y-Ba-Cu-O compounds prepared from BaCuO2 and Ba2Cu3O5+δ

Series of Y-Ba-Cu-O compositions were prepared from barium cuprates and Y₂O₃, using a two-step synthesis route. It has been shown that Ba₂Cu₃O_5+δ is essential in formation of the YBa₂Cu₃O_7?δ superconducting phase while BaCuO₂ is not an appropriate starting material. A wide composition range fromx=2 to 17 was prepared from Ba₂Cu₃O_5+δ in the YBa _x Cu_1.5x O _z series without disappearance of macroscopic superconductivity atT _c>77 K. Resistivity measurements hint at the existence of two superconducting phases. ESR investigations revealed a baseline hysteresis, depending on the actual value ofx.     

Magnetic and electrical properties of Al3+-substituted MgFe2O4

The structural, magnetic and electrical properties of the Al³⁺-substituted disordered spinel system Mg(Fe_2–x Al _x )O₄ have been investigated by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained through Mössbauer spectroscopy. The variation of saturation magnetization per formula unit as a function of aluminium context, x, has been satisfactorily explained on the basis of Neel's collinear spin model and the slight discrepancy between the observed and calculated n _B values can be explained in terms of a random canting model. The Néel temperatures calculated theoretically by applying molecular field theory agreed well with the experimentally determined values from thermal variation of susceptibility and electrical resistivity. An unusual metal-like thermo-electric behaviour was found for the compositions with x 0.3 which was attributed to the decrease in the Fe-Fe separation distance arising from aluminium substitution.     

Polyol-mediated synthesis and photoluminescent properties of Ce3+ and/or Tb3+-doped LaPO4 nanoparticles

Rare-earth ion (Ce3+, Tb3+) doped LaPO4 nanoparticles were prepared by the polyol method and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV-vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and lifetimes. The results of XRD indicate that the as-prepared nanoparticles are well-crystallized at 160 degrees C and assigned to the monoclinic monazite structure of the LaPO4 phase. The obtained LaPO4:Ce3+, Tb3+ nanoparticles are spherical with narrow size distribution and average size of 20 nm. The doped rare-earth ions show their characteristic emission in LaPO4 nanoparticles, i.e., Ce3+ 5d-4f and Tb3+ 5D4-7FJ (J = 6-3) transitions, respectively. The optimum doping concentration for Tb3+ in La(0.8-x)Ce0.2TbxPO4 nanoparticles is determined to be 15 mol% (x = 0.15). The luminescence decay curves of Ce3+ in LaPO4:Ce3+ and LaPO4:Ce3+, Tb3+ nanoparticles present a single-exponential behavior, and the lifetimes (tau) of Ce3+ decrease with increasing Tb3+ concentrations (at the constant Ce3+ concentration) in LaPO4:Ce3+, Tb3+ nanoparticles due to the energy transfer from Ce3+ to Tb3+. The energy-transfer efficiency from Ce3+ to Tb3+ was calculated, which depends on the doping concentrations of Tb3+ if the concentration of Ce3+ is fixed.