Structural and dielectric properties of Na1?xBaxNb1?x(Sn0.5Ti0.5)xO3 ceramics

Lead-free (1 − x)NaNbO₃/xBa(Ti_0.5Sn_0.5)O₃ (x = 0.1, 0.125, 0.15, 0.175, 0.2, and 0.3) ceramics were elaborated by the conventional ceramic technique. Sintering has been made at 1523 K for 2 h. The crystal structure was investigated by X-ray diffraction with CuKα radiation at room temperature. As a function of composition, these compounds crystallize with tetragonal or cubic symmetry. Dielectric measurements show that the materials have a classical ferroelectric behavior for compositions in the range 0.10 ≤ x ≤ 0.15 and relaxor one for compositions in the range 0.15 < x ≤ 0.30. Temperatures T _C or T _m decrease as x content increases. The ferroelectric behavior has been confirmed by hysteresis characterization. For x = 0.1, a piezoelectric coefficient d ₃₁ of 42.146 pC N⁻¹ was obtained at room temperature. The evolution of the Raman spectra was studied as a function of temperature for x = 0.1.     

Preparation and Magnetic Properties of?(La1?xSr1+x)(Mn0.5Co0.5)O4

A series of single phase (La_1−x Sr_1+x ) (Mn_0.5Co_0.5)O₄ (0≤x≤0.40) materials with a tetragonal K₂NiF₄ structure was prepared by a solid state reaction method at 1400 or 1450 °C with a short period of heating time. Powder X-ray diffraction (XRD) and the Rietveld refinement method were employed for the structural analysis. Unit cell a- and c-axes decrease with increasing amount of Sr substitution. A discrepancy between the zero-field-cooled and the field-cooled magnetization is observed in all samples investigated below a characteristic temperature, T ^*, which likely arises from the canted nature of spins or the random freezing of spins. It appears that T ^* decreases with increasing amount of Sr substitution, which is possibly due to the enhancement of chemical pressure induced by Sr and a corresponding increases of the valence of Mn and/or Co.     

Structural, spectroscopic and microwave characterizations of (Sm0.5Y0.5)Ti(Nb1?xTax)O6 ceramics

Microwave dielectric ceramics of the type (Sm_0.5Y_0.5)Ti(Nb_1−x Ta _x )O₆ were prepared for x = 0, 0.2, 0.4, 0.6, 0.8 and 1 through the conventional solid state ceramic route. The ball-milled compositions were calcined at 1,250 °C. The cylindrical pellets were sintered at temperatures between 1,385 and 1,450 °C. The densities were determined by Archimedes method. The structure was analyzed using X-ray diffraction method. The microwave dielectric properties of the polished pellets were measured using cavity resonator method. The morphological studies were done using Scanning Electron Microscopy and Transmission Electron Microscopy. The Raman spectra were recorded and analyzed to confirm the structural change. The photoluminescence spectra were also taken and the emission lines were identified. A correlation study was done among the measured properties and parameters. Most of the samples have high dielectric constant, high quality factor and low temperature coefficient of resonant frequency and hence suitable for microwave applications.     

Synthesis of New Pb-Based 1232 Cuprate Containing Boron in?the?(Pb0.5B0.5) Sr2(Er3?x?yCexSry)Cu2Oz System

New Pb-based layered cuprates containing boron with the 1232 structure have been synthesized in the (Pb_0.5B_0.5)Sr₂(Er_3−x−y Ce _x Sr _y )Cu₂O _z system. Nearly the single 1232 phase samples are obtained for the nominal composition of 1.6≤x≤1.9 and y=0.2. A sample with the composition of (Pb_0.5B_0.5)Sr₂(Er_1.2Ce_1.6Sr_0.2)Cu₂O _z shows resistivity-dropping phenomenon below 10 K and it shows a diamagnetic signal at about 9.0 K, though the superconducting volume fraction is very small. From these results, the sample may well be a new Pb-based superconductor with the 1232 structure.     

AC-field Frequency Response of?Cu0.5Tl0.5Ba2(Ca2?qMgq)Cu3O10?δ Bulk Superconductor

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ (q=0, 0.5, 1.0 1.5) superconductor samples were studied at two temperatures of 80 and 290 K by capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 KHz to 10 MHz. We have presented the measurements of the dielectric constants (ε′ and ε″), dielectric loss factor (tan δ) and ac-conductivity (σ _ac) as a function of frequency and temperature. A negative capacitance (NC) experience has been observed, which is most likely due to different contact electrodes and superconductor samples’ Fermi levels. Since metals have their Fermi levels higher than ceramics, there is a flow of the carriers from the ceramic samples towards the metal electrodes. The dielectric polarization phenomenon is observed, which is due to dislodgment of mobile charges from their equilibrium position relative to fixed charges of the reservoir layer. The improved inter plane coupling promoted by Mg substitution at Ca site would change the dielectric response of Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ superconductors. To observe such effects in Mg doped Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ superconductors, di- electric measurements were carried out both at room temperature (290 K) and in the superconducting state closer to the boiling point of liquid nitrogen (80 K). The excess conductivity arising due to superconducting state of material has been determined, and its role in the mechanism of superconductivity is suggested. The negative dielectric constant (ε′) and dielectric loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 80 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

Preparation of Y0.5[Yb1?xNdx]0.5Ba2Cu3Oz Superconductors Using Nitrate Routes

The chemical solution nitrate route was used to prepare Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase with x=0.1, 0.2 and 0.3. Different calcination temperatures from 870 to 900 °C were used. Both XRD and DSC techniques were utilized in monitoring the Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase and the non-superconducting impurity phases such as BaCuO₅, CuO and RE₂BaCuO₅. These two techniques were used to study the effect of impurity phases on the decomposition temperature of Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase. The value of x, the Nd amount, and the calcination temperature both influenced the type and number of impurity phases, which resulted in changing the eutectic and peritectic reaction temperatures.     

Structure of Sr1 ? xPbxFeO3 ? δ (0 < x < 0.5) perovskite-like materials

We have synthesized Sr_{1 − x} Pb _x FeO_{3 − δ} (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5) perovskite-like materials and studied their structure by X-ray diffraction, M?ssbauer spectroscopy, and electron microscopy. According to the X-ray diffraction data, the Pb solubility limit in the perovskite structure is x ≈ 0.15. The materials with x = 0.05 and 0.1 contained Pb_1.33Sr_0.67Fe₂O₅ inclusions 10–30 nm in size. Using chronopotentiometry and temperature-programmed desorption, we have estimated oxygen mobility in the materials with x = 0.05 and 0.1. The results demonstrate that Pb doping increases oxygen mobility in the strontium-ferrite-based materials.     

Characterization of ternary (Na0.5K0.5)1?xLixNbO3 lead-free piezoelectric ceramics prepared by molten salt synthesis method

(Na_0.5K_0.5)_1−x Li _x NbO₃ powders and ceramics were prepared by molten salt synthesis method. Pure perovskite-phase powder was obtained at a low temperature of 740 °C with a grain size of below 800 nm. The effects of the LiNbO₃ on phase transition, microstructure, electrical properties, and temperature stability were investigated. A morphotropic phase boundary was identified. The scanning electron microscopy indicated that the (Na_0.5K_0.5)_1−x Li _x NbO₃ powders and ceramics obtained by the molten salt synthesis method have a relatively uniform particle size and microstructure. The results indicate that these materials are promising candidates for lead-free piezoelectric ceramics for practical applications.     

Effect of Pb Doping on the Superconducting Properties of?(Cu0.5?xPbxTl0.5)Ba2Ca2Cu3O10?δ

The effect of Pb doping on the superconducting properties of (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) samples has been investigated. Lead is doped in Cu_0.5Tl_0.5Ba₂O_4−δ charge reservoir layer and at the CuO₂ planar sites. A multiphase material is achieved with the doping of Pb at the CuO₂ planar sites; however, a predominant single-phase (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) material is synthesized with the doping of Pb at the charge reservoir layers. Formation of multiphase material with the doping of lead at the planar sites showed that its substitution at the planar site is not possible and the formation of PbO₂ planes is less likely. In the samples doped at the charge reservoir layer, the zero critical temperature [T _c (R=0)] is systematically depressed with the increased concentration of lead. The T _c (R=0) and magnitude of the diamagnetism are enhanced after post-annealing the samples in oxygen atmosphere. An apical oxygen mode is observed around 438 cm⁻¹ in undoped samples, which is shifted to 457–461 cm⁻¹ in the Pb-doped samples. This shift in the peak position is most likely associated with the connectivity of apical oxygen atoms with Pb atoms of (Cu_0.5−x Pb _x Tl_0.5)Ba₂O_4−δ (x=0.0, 0.15, 0.25, 0.35) charge reservoir layers. The presence of Pb in the charge reservoir layer and its increased concentration, somehow, stops the flow of mobile carriers to the conducting CuO₂ planes. The decreased density of mobile carriers diminishes the critical temperature and magnitude of diamagnetism in the final compound. The increased oxygen diffusion in the unit cell achieved by post-annealing in oxygen replenishes the concentration of carriers in conducting CuO₂ planes, which increases the T _c (R=0) and the magnitude of diamagnetism. These experiments have shown that the density of mobile carriers plays a vital role in the mechanism of superconductivity and their depressed density suppresses the superconductivity parameters.     

AC Response of Cu0.5Tl0.5Ba2Ca2Cu3?xSnxO10?δ Superconductor

We have studied the ac response of Sn doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−x Sn _x O_10−δ superconductor samples from their ac-susceptibility measurements under different magnitudes of ac magnetic fields; H _ac=0.4, 4, 16 A/m. The samples with x=0.5 and 1.0 have shown strong flux pinning and intergrain coupling. However, the sample with Sn doping of x=1.5 has shown very poor flux pinning characteristics.     

Role of Mobile Charge Carriers and Fluctuation Induced Conductivity in (Cu0.5Tl0.5?xKx)Ba2Ca3Cu1Zn3O12?δ Superconductor

The main role played by mobile carriers and fluctuation induced conductivity (FIC) in (Cu_0.5Tl_0.5−x K _x )Ba₂Ca₃Cu₁Zn₃O_12−δ (x=0, 0.25) superconductor samples are analyzed in the light of the Aslamazov–Larken (AL) theory. The enhancement of three-dimensional (3D) conductivity and grain size is achieved with K-doping at Tl sites in the (Cu_0.5Tl_0.5−x K _x )Ba₂O_4−δ charge reservoir layer, which are further improved by post-annealing the samples in oxygen. The improvement of these factors causes the increase in cross-over temperature (T _o), zero resistivity critical temperature {T _c(R=0)} and T _c(onset) of diamagnetism to their higher values. We have also evaluated the exponents (λ _2D and λ _3D), zero temperature coherence lengths ξ _c(0), and interlayer coupling (J) for these samples and tried to correlate them to the superconductivity order parameters.     

Superconducting Properties of Carbon and Zn Double-doped Mg1?xZnx(B1?xCx)2

A series of polycrystalline samples of Mg_1−x Zn _x (B_1−x C _x )₂ (x=0.00, 0.02, 0.04, 0.06, 0.08, and 0.10) were synthesized by a conventional solid-state reaction method under a background pressure about 10⁻³ Pa. Phase identification, crystal structure and superconducting transition temperature (T _c) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicated that the lattice parameters a and c show no clear trend in their changes with increasing doping level, and it turned out that the dopants had a marked effect on the crystal-lattice parameters and changed the crystal structure of the samples. The T _c for Mg_1−x Zn _x (B_1−x C _x )₂ decreased with C and Zn doping, but the rate of decrease was slower than single C-doped. We propose that the suppression of T _c by doping originates largely from the structural change.     

Magnetization Processes of (La0.7Pb0.3MnO3)1?x(SiO2)x Composites

The ceramic composites, (La_0.7Pb_0.3MnO₃)_1−x (SiO₂) _x , with diluted magnetic properties are prepared using solid-sate sintering route. Magnetization processes of (La_0.7Pb_0.3MnO₃)_1−x (SiO₂) _x composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution induced by the increase of SiO₂ content. Clearly, irreversible behavior is observed in the zero-field cooling and the field cooling (ZFC–FC) curves at a low field of 100 Oe. Saturation magnetization decreases as x increases while ferromagnetic transition temperature remains around 346 K for all composites. All the composites exhibit ferromagnetic hysteresis behavior which can be modeled by the law of the approach to saturation in the form M=M _S(1−a/H). The term a/H expresses the deviation of magnetization from saturation. The smaller factor a for La_0.7Pb_0.3MnO₃-rich samples results in sharper square curve which should be associated with the long-range spin order of ferromagnetic coupling.     

Structure and piezoelectric properties of new (Bi0.5Na0.5)1?x?yBax(Yb0.5Na0.5)yTiO3 lead-free ceramics

New lead-free ceramics (Bi_0.5Na_0.5)_1−x−yBa_x(Yb_0.5Na_0.5)_yTiO₃ (x = 0.02–0.10 and y = 0–0.04) have been prepared by an ordinary sintering technique and their structure and piezoelectric properties have been studied. X-ray diffraction shows that Ba²⁺ and Yb³⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure and a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases is formed at 0.04 < x < 0.10. The partial substitutions of Ba²⁺ and Yb³⁺ for A-site ions of Bi_0.5Na_0.5TiO₃ decrease effectively the coercive field E _c and improve significantly the remanent polarization P _r. The ceramics with x = 0.06 and y = 0–0.02 situate within the MPB and possess the lower E _c and larger P _r, and thus exhibit optimum piezoelectric properties: d ₃₃ = 155–171 pC/N and k _p = 29.2–36.7%. The temperature dependences of the dielectric and ferroelectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above T _d.     

Preparation and properties of ceramics based on (Na0.5Bi0.5)1 ? xAxTiO3 (A = Sr, Cd) solid solutions

This paper describes the preparation and properties of (Na_0.5Bi_0.5)_{1 − x} A _x TiO₃ (A = Sr, Cd) lead-free ferroelectric ceramics. The (0.63–0.66)Na_0.5Bi_0.5TiO₃ · (0.37–0.34)Sr_0.7Bi_0.2TiO₃ ceramics are shown to have the best dielectric and piezoelectric properties: tanδ = 0.013–0.009, ɛ_RT = 1200–1500, d ₃₁ = (370–400) × 10⁻¹² C/N, and k _ρ = 0.4–0.58. The cadmium-containing ceramics offer low dielectric losses and high dielectric permittivity at room temperature.     

Electrical properties of Zn2(TiaSnb)1 ? xZrxO4 solid solutions

The electrical conductivity, band gap, dielectric permittivity, and molar polarizability of Zn₂(Ti _a Sn _b )_{1 − x} Zr _x O₄ solid solutions have been determined. All of the synthesized samples are dielectrics with semiconducting behavior of conductivity. The phase diagram of the Zn₂TiO₄-Zn₂SnO₄-Zn₂ZrO₄ system is presented.     

Improvement of Superconductivity with the Modification of Charge Reservoir Layer in (Cu0.5Tl0.5?xMx)Ba2Ca2Cu3O10?δ

The relatively higher electronegative elements (M = Pd, Nb, Bi, Hg) have been partially doped at Tl sites in Cu_0.5Tl_0.5−x M _x Ba₂O_4−δ (x=0,0.25) charge reservoir layer of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor. These elements may retain more oxygen in the charge reservoir layer due to their higher electronegativity as compared to Tl, and the higher population of oxygen in the charge reservoir layer can optimize the charge carriers’ density in the conducting CuO₂ planes. The optimum density of mobile charge carriers in the conducting CuO₂ increases Fermi wave-vector K _F and Fermi velocity v _F of the carriers, which results in the improvement of superconducting properties of the material.     

Superconducting Properties of Zn-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yZnyO10?δ Superconductors

A reproducible synthesis and characterization of Zn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Zn _y O_12−δ (y=0, 0.5, 1.0, 1.5) superconductors at a relatively lower synthesis temperature of 840°C are studied by using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. The X-ray diffraction (XRD) studies of these samples have shown a tetragonal structure in which the c-axis length has been found to decrease with increased Zn doping. The critical temperature and magnitude of diamagnetism have not been significantly affected with the doping of Zn at this synthesis temperature. The magnitude of diamagnetism in the as-prepared undoped samples is decreased, whereas it remains stable (unchanged) in oxygen post-annealed samples. The apical oxygen phonon’s modes of type Tl–O_A–M(2) and Cu(1)–O_A–M(2) {where M=Cu/Zn} and the planar oxygen phonon modes of type M(2)–O_P–M(2) are also softened with the increase of Zn doping. We interpreted the softening of these oxygen related phonon modes linked with the decreased c-axis length, reduced John–Teller distortions and increased mass of Zn (65.38 amu) as compared to that of Cu (63.54 amu) (Kaplan et al., Phys. Rev. B 65, 214509, 2002).     

Stabilization of the anatase phase of Ti1?xSnxO2 (x < 0.5) nanofibers

We experimentally investigate the stabilization of the anatase phase of Ti_1−x Sn _x O₂ (x < 0.5) nanofibers when synthesized by an electrospinning method. The as-spun nanofibers became nano-grained, polycrystalline nanofibers after calcination and the diameters of the nanofibers depend on Sn content. Stabilization of the anatase phase in Ti-rich compositions and incorporation of Sn ions were confirmed by X-ray diffraction, Raman, X-ray absorption near-edge structure, and photoluminescence (PL) spectroscopies. Results from the PL study also demonstrated the tunable nature of the optical properties, with the emission maximum shifting towards higher wavelength with increasing Sn concentration.      

Cd-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yCdyO10?δ (y=0,0.5,1.0,1.5,2.0) Superconductors

Superconducting properties of cadmium doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) samples have been studied using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. In X-ray diffraction studies these samples have shown to have tetragonal structure. The zero resistivity critical temperature and magnitude of diamagnetism are suppressed with the increased incorporation of Cd in the final compound. A change in the shape of FTIR absorption spectra, after doping, has shown the incorporation of Cd in the unit cell. A systematic hardening of the apical oxygen modes and softening of the CuO₂ planar modes of vibration with increased Cd doping have shown that it is incorporated in the unit cell of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) superconductors. The FTIR absorption measurements of these samples have shown that hardening of the apical oxygen modes of types Cu(1)–O(2)–Tl and Cu(1)–O(2)–Cu(2)/Cd _y (y=0,0.5,1.0,1.5,2.0) increases with the increase of Cd doping in the samples. A softening of the CuO₂ planar oxygen mode Cu(2)–O–Cu(2) is also observed with the increased Cd doping in the final compound. It is most likely that hardening of the apical oxygen modes and the softening of the planar modes of vibration are associated damped harmonic oscillations produced by heavier Cd atoms in the CuO₂ planes, which suppress the phonon population from a desired level, reducing the magnitude of superconductivity in the final compound.