Preparation of Nb<Subscript>3</Subscript>Al Superconducting Tapes by a Powder-in-Tube Method Combined with Hot-Pressed Sintering

The precursors of Nb-Al tape were fabricated by a powder-in-tube (PIT) technique. Supersaturated solid solutions of Nb-Al powder were obtained by high-energy ball milling, followed by a powder-in-tube process to prepare series of Nb₃Al precursor tapes. Compared with sintering under normal pressure, the hot-pressing sintering greatly increased the critical current density of the tapes. The results showed that the Nb₃Al tapes prepared via the powder-in-tube method and hot-pressed sintering could significantly improve the J _c performance, and the value of J _c (8 K, 0 T) was higher than 6 × 10⁵ A/cm². The Nb₃Al tapes after pressing under the pressure of 20 MPa and sintering at 950 ^°C for 3 h had the best magnetic flux pinning performance, and the sample reached the maximum flux pinning force (F _p,max) of 3.28109 N/m³ in the magnetic field of 2.2 T.     

Superconducting microwave cavities based on Nb<Subscript>3</Subscript>Sn electrolytic coatings

We demonstrate that Nb₃Sn coatings suitable for use in superconducting microwave devices can be produced by electrocodeposition of Nb and Sn. Superconducting cavities have been fabricated by electrodeposition in a galvanostatic mode using such coatings, and their performance has been evaluated.     

Nb<Subscript>3</Subscript>Sn-based superconducting helicoids

A procedure is proposed for the fabrication of superconducting helicoids which includes consecutive electrodeposition of Nb and Nb₃Sn layers from molten salts onto the surface of a plane-turn copper tape helix.     

Properties of fluorine-doped PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> ceramics

We report the first fluorine doping of lead magnesium niobate in the PbMg _{(1 + x)/3}Nb_{(2 ? x)/3}O_{3 ? x} F _x system in a wide composition range, x = 0.025 to 0.625. The fluorine content of the samples is shown to be substantially lower than the intended one because of the fluorine volatilization in the form of HF during synthesis and sintering in air. The ceramics consist of magnesium and lead oxides undetectable by x-ray diffraction, and a perovskite phase whose composition can be represented by the formula PbMg_{(1 + m)/3}Nb_{(2 ? m)/3}O_{3 ? m} F _m , where the fluorine content after sintering is m ≤ 0.12. The PbO and MgO contents of the ceramics depend on the starting mixture composition (x) and heat-treatment conditions (hydrogen fluoride and lead oxide volatilization). As a result of the low fluorine content, the diffraction patterns of the samples show no superlattice reflections, and their lattice parameter varies insignificantly with x. Data are presented on the temperaturedependent dielectric permittivity of ceramic samples sintered and annealed under different conditions.     

Nanocrystalline PbIn<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> ceramics and its properties

A ferroelectric relaxor PbIn_0.5Nb_0.5O₃ (PIN) ceramics has been obtained using a modified ceramic technology, with the sintering stage preceded by compression and shear straining of the synthesized charge in Bridgman anvils. The dimensions of ceramic grains after this pretreatment are spread over a range from 100 to 1250 nm. A comparative investigation of the properties of PIN ceramics obtained using the standard and modified technology showed that the proposed mechanical action at the charge preparation stage can be used for controlled modification of the properties of ferroelectric ceramics.     

The Ideal Strengths of Superconducting MgCNi<Subscript>3</Subscript> and CdCNi<Subscript>3</Subscript>

The stress-strain curves under tensile deformation in the 〈100〉, 〈110〉, and 〈111〉 directions and under shear deformation in the (001)〈110〉, \((110)\langle \overline {1}10\rangle \), \((111)\langle 1\overline {1}0\rangle \), and \((111)\langle 11\overline {2}\rangle \) slip systems have been systematically calculated by first-principles method to study the ideal strengths of superconducting MgCNi₃ and CdCNi₃. The ideal strengths in the three tensile directions are found to be reduced in the order of 〈100〉 → 〈110〉 → 〈111〉 and those for the four shear slip systems in the order of \((110)\langle \overline {1}10\rangle \rightarrow (111)\langle 11\overline {2}\rangle \rightarrow (111)\langle 1\overline {1}0\rangle \rightarrow (001)\langle 110\rangle \) for both superconductors. Their lowest ideal tensile strengths are found to be larger than the corresponding highest ideal shear strengths, which could explain why both superconductors have the ductility. The obtained lattice constants and elastic properties coincide well with the the available experimental and theoretical values.     

PTCR effect in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>-doped BaTiO<Subscript>3</Subscript> ceramics

The influences of BaBi₂Nb₂O₉ content on the electrical property and the microstructure of BaTiO₃-based materials have been studied. With an increase in BaBi₂Nb₂O₉ content the grain size decreases. All the prepared BaBi₂Nb₂O₉ doping BaTiO₃-based thermistors show typical PTC effect. As the amount of BaBi₂Nb₂O₉ added in BaTiO₃-based ceramics increases, resistivity appears to exhibit a minimum value. At high BaBi₂Nb₂O₉ content (≥0.0875), the resistivity increased again with increasing BaBi₂Nb₂O₉ content. At a given content of BaBi₂Nb₂O₉, the influence of sintering temperature on the electrical properties of samples has been investigated. A minimum of room temperature resistivity is obtained at the sintering temperature equal to 1,290 °C at a given content of BaBi₂Nb₂O₉.     

Structure and physical properties of tetragonal PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> solid solutions

The principal bond distances in the structure of tetragonal PbMg_1/3Nb_2/3O₃-PbTiO₃ (PMN-PT) solid solutions have been determined by the Rietveld method. The ferroelectric state of (PbMg_1/3Nb_2/3O₃)_0.62(PbTiO₃)_0.38 samples is shown to be governed by the displacement of the Pb ions from their ideal crystallographic site. In the frequency range 4000–5000 cm^?1, the refractive index of the samples is 2.70–2.75, as determined by the Kramers-Kronig method.     

Preparation and electrical properties of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> solid solutions

Ceramic samples of lead magnesium niobate (PMN) and (1 ? x)Pb(Mg_1/3Nb_2/3)O₃?xPbTiO₃ (PMN-PT) solid solutions with x = 0, 0.05, 0.10, and 0.30 have been prepared by solid-state reactions, and their structural, electrical, and piezoelectric properties have been studied using x-ray diffraction, Rietveld profile analysis, impedance spectroscopy, and the resonance/antiresonance method. The results indicate that the use of nonstoichiometric columbite niobates enables the synthesis of phase-pure PMN and PMN-PT.     

Crystal structure and electrical properties of CaNaBi<Subscript>2</Subscript>Nb<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics

Monophasic CaNaBi₂Nb₃O₁₂ powders were synthesized via the conventional solid-state reaction route. Rietveld refinement of the X-ray powder diffraction (XRD) data and selected area electron diffraction (SAED) studies confirmed the phase to be a three-layer Aurivillius oxide associated with an orthorhombic B2cb space group. The dielectric properties of the ceramics have been studied in the 300–800 K temperature range at various frequencies (1 kHz to 1 MHz). A dielectric anomaly was observed at 676 K for all the frequencies corresponding to the ferroelectric to paraelectric phase transition as it was also corroborated by the high temperature X-ray diffraction studies. The incidence of the polarization–electric field (P vs. E) hysteresis loop demonstrated CaNaBi₂Nb₃O₁₂ to be ferroelectric.     

Residual stress in Nb<Subscript>3</Subscript>Sn-based composite electrolytic coatings

A combined nondestructive technique utilizing X-ray diffraction strain measurements has been developed for evaluating the residual stress and Poisson’s ratio in Nb₃Sn-based composite electrolytic coatings. The magnitude and nature of the residual stress in niobium stannide on molybdenum, nickel, and copper substrates have been determined.     

Structural and electrical properties of Ba(Fe<Subscript>1/3</Subscript>Nb<Subscript>1/3</Subscript>Ta<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> perovskite

Ba(Fe_1/3Nb_1/3Ta_1/3)O₃ (BFNT) perovskite compound (phase purity>99%) was synthesized by conventional ceramic preparation method. XRD, microstructure, impedance spectroscopy and ac conductivity properties were analyzed in this study. BFNT compound has a cubic crystal structure, having grain size of 0.31 μm. This compound has shown normal ferroelectric behaviour but not obeying Curie-Weiss law. The impedance and electrical studies have been performed as a function of frequency and temperature. Impedance as a function of frequency revealed single relaxation process. The impedance spectroscopic plots exhibit the major response due to grains with partial contribution from the grain boundary and negligible electrode effect. Complex impedance plot showed data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the bulk grains. Also, the centre of semicircle lies below the real axis indicating non-Debye type relaxation. Relaxation time was calculated from Z″_max of Cole–Cole plots. It is observed that conduction is due to hopping of charge carriers. Activation energies were computed from the Arrhenius plots of the sample.     

Structural and dielectric properties of PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions synthesized at high pressures and temperatures

(1 ? x)PbMg_1/3Nb_2/3O₃ · xPbZrO₃ (1 ? x)PMN · xPZ) solid solutions have been synthesized at a pressure of 5 GPa and temperatures from 1300 to 1700 K, and their structural and dielectric properties have been studied. The composition dependences of the average unit-cell parameter and dielectric permittivity for the solid solutions indicate that the PMN-PZ system has a morphotropic phase boundary near x = 0.65. The solid solutions have a cubic structure for x < 0.65, a rhombohedral structure in the range 0.65 < x < 0.9, and an orthorhombic structure (similar to that of PbZrO₃) for x > 0.9. The temperature and frequency dependences of dielectric permittivity suggest that the (1 ? x)PMN · xPZ samples with x < 0.65 consist of two ferroelectric phases: a relaxor with antipolar dipole order and a normal ferroelectric with a diffuse phase transition. The effect of annealing temperature on the ferroelectric state of the samples with x < 0.65 is examined. In the composition range 0.65 < x < 0.9, the samples have normal ferroelectric properties, independent of annealing temperature.     

Synthesis of anisometric KSr<Subscript>2</Subscript>Nb<Subscript>5</Subscript>O<Subscript>15</Subscript> particles in the SrNb<Subscript>2</Subscript>O<Subscript>6</Subscript>–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–KCl system

Anisometric and agglomerate-free template particles are important for fabrication of grain-oriented ceramics. In the present work, preparation of acicular KSr₂Nb₅O₁₅ (KSN) particles was firstly explored in the SrNb₂O₆–Nb₂O₅–KCl system by molten salt synthesis (MSS) method. It was found that the molar ratio of SrNb₂O₆ to Nb₂O₅, the amount of KCl salt and synthesis time could significantly affect the phase structure and morphology of KSN particles. When calcined at 1,150 °C for 6 h with the molar ratio of SrNb₂O₆ to Nb₂O₅ was 1 and the weight ratio of salt to oxide source was 1.50, pure KSN particles with well-developed acicular morphology were successfully obtained in this system. They were agglomerate-free and with proper scale in the size range of 5–30 μm, which made them the ideal templates for fabricating textured ceramics. In addition, some new reaction and growth mechanisms were proposed in this work.     

Dielectric responses in Mg<Subscript>1/3</Subscript>Ta<Subscript>2/3</Subscript>-replaced Pb[(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>),Ti]O<Subscript>3</Subscript> ceramics

Octahedral lattice sites of Pb[(Zn_1/3Nb_2/3),Ti]O₃ were replaced by 20 at.% Mg_1/3Ta_2/3 complex to enhance perovskite development, especially at Pb(Zn_1/3Nb_2/3)O₃-rich compositions. Resultant changes in the perovskite formation and associated dielectric responses were investigated. A perovskite structure was identified at Pb(Zn_1/3Nb_2/3)O₃-rich compositions by X-ray diffraction, although the development was rather incomplete. Phase transition modes in the dielectric constant spectra changed from diffuse to sharp ones, regardless of the introduction of Mg_1/3Ta_2/3. Dielectric maximum temperatures of the ceramics shifted linearly with the compositional change.     

Structural,dielectric and electrical properties of CaBa<Subscript>4</Subscript>SmTi<Subscript>3</Subscript>Nb<Subscript>7</Subscript>O<Subscript>30</Subscript> ferroelectric ceramic

The polycrystalline sample of CaBa₄SmTi₃Nb₇O₃₀, a member of tungsten bronze family, was prepared by solid-state reaction method. X-ray diffraction analysis shows the formation of single-phase compound with an orthorhombic structure at room temperature. Scanning electron micrograph of the material shows uniform distribution of grains. Detailed studies of dielectric properties of the compound as a function of temperature at different frequencies suggest that the compound has a dielectric anomaly of ferroelectric to paraelectric type at 198°C, and exhibits non-relaxor kind of diffuse phase transition. The ferroelectric nature of the compound has been confirmed by recording polarization-electric field hysteresis loop. Piezoelectric and pyroelectric studies of the compound have been discussed in this paper. Electrical properties of the material have been analyzed using complex impedance technique. The Nyquist plots manifest the contribution of grain boundaries (at higher temperature), in addition to granular contribution (at all temperatures) to the overall impedance. The temperature dependence of dc conductivity suggests that the compound has negative temperature coefficient of resistance (NTCR) behaviour. The frequency dependence of ac conductivity is found to obey Jonscher’s universal power law. The observed properties have been compared with calcium free Ba₅SmTi₃Nb₇O₃₀ compound.     

Criteria for Selecting Substrates for Nb<Subscript>3</Subscript>Sn Electrodeposition

We studied the effect of substrate material on the microstructure and properties of Nb₃Sn coatings produced by electrochemical coreduction of Nb and Sn ions in the cathodic zone in molten salts. The results demonstrate that continuous superconducting Nb₃Sn coatings can be produced on molybdenum, tantalum, niobium, copper, nickel, Invar and the 60% Ni + 22% Mo + 12% Fe alloy. The coatings grown on molybdenum, tantalum, and niobium have the highest purity. To ensure good superconducting properties of electrodeposited coatings, preference should be given, all other factors being the same, to substrate materials with thermal expansion coefficients close to or smaller than that of Nb₃Sn.     

Polycrystalline materials in MoO<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript>–V<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Melt quenching technique was applied to study tendency for phase formation and amorphization in the MoO₃–ZrO₂–V₂O₅ system. By X-ray diffraction were detected the main crystalline phases separated during the quenching: Zr(MoO₄)₂, V₂MoO₈, (Mo_0.3V_0.7)₂O₅, V_0.95Mo_0.97O₅ but in a wide concentration range the dominant crystalline phase was monoclinic ZrO₂. The average particle sizes of the obtained crystal phases were in the range 30–50 nm. A narrow glass formation area was situated, near MoO₃–V₂O₅ side. The glass-crystalline samples were obtained in the MoO₃- and V₂O₅-rich compositions. The phase formation was proven by IR analysis also. IR data showed that the main structural units built up the glass network are corner shared VO₅ and MoO₆ groups while in the corresponding crystal V₂MoO₈ phase MeO₆ (Me = V, Mo) octahedra are corner and edge shared (band at 580 cm⁻¹).     

Effect of Blocking and Superconducting Layer Doping on the Superconductivity and Magnetic Properties of Polycrystalline Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>6</Subscript>

The possible difference in the properties upon doping the Sr₂CaCu₂O₆ superconducting or blocking layers with Fe and Eu respectively was investigated in this work. The homogeneous Sr_2?yEu_yCaCu₂O_6+δ and Sr₂CaCu_2?xFe_xO_6+δ (y = 0, 0.1, 0.5, x = 0, 0.05) compounds were produced by a high-pressure synthesis route. Judging by the magnetic susceptibility measurements, all samples exhibit a superconductivity transition and the Eu/Fe concentration dependencies on the diamagnetic moment and average T_c have been constructed using the experimental data. As a result, an unusual behavior of the T_c value was observed for the samples with doped Eu: a fivefold reduction in the europium concentration in the sample does not give a noticeable effect on the transition temperature value while the diamagnetic signal becomes more stronger. Complex superconducting dome was found for Eu-doped material: 0.1 ≤ y ≤ 0.5 region T_c vs. concentration data were approximated by inverted parabola-like curve with a maximum at y = 0.3. Difference in properties of the Eu and Fe-doped samples was also found in the behavior of the hysteresis loops showed the opposite orientations.     

Dielectric and magnetic properties of Ca(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>/NiZn ferrite composites

Dense Ca(Zn_1/3Nb_2/3)O₃/NiZn ferrite composites with homogeneously fine microstructures were prepared through conventional solid-state method. The powder XRD patterns confirm the coexistence of the two phases. The dielectric properties in the low frequency range (100 Hz–1 MHz) follow the rule of Maxwell–Wagner interfacial polarization. The dielectric and magnetic properties in the high frequency range (10 MHz–1 GHz) are also reported. The results show that this kind of magnetic–dielectric composites could be used in high-frequency communications for the capacitor-inductor integrating devices such as electromagnetic interference filters and antennas.