P. E. Kazin

On the origin of near-IR luminescence in Bi-doped materials (II). Subvalent monocation Bi+ and cluster Bi5(3+) luminescence in AlCl3/ZnCl2/BiCl3 chloride glass

Broadband NIR photoluminescence (from 1000 to 2500 nm) was observed from partially reduced AlCl₃/ZnCl₂/BiCl₃ glass, containing subvalent bismuth species. The luminescence consists of three bands, assigned to Bi⁺ , Bi₂⁴⁺, and Bi₅³⁺ ions. The physical and optical characteristics of these centers and possible contribution to NIR luminescence from bismuth-doped oxide glasses are discussed.

Flux pinning improvement in Bi-2212 silver sheathed tapes with submicron SrZrO3 inclusions

Abstract Silver sheathed tapes of Bi-2212 undoped and doped with SrZrO 3 have been prepared, using co-precipitated oxalates as precursors. Thermal treatment conditions have been optimised, varying the maximum processing temperature T max . A comparative study on thermal treatment conditions, microstructure, and flux pinning properties for doped and undoped samples has been performed in order to evaluate the effect of SrZrO 3 addition. The doped samples contain SrZrO 3 submicron particles, appearing as aggregates included between Bi-2212 lamellae with a Cu-free phase and the rest of the liquid, and also distributed inside Bi-2212 grains. The presence of SrZrO 3 surplus leads to higher critical current densities in samples prepared in a wide T max range; in addition, the lowest T max , at which high J c can be obtained, becomes 5 deg lower (875°C). At T = 5 K critical currents for all the samples are of the order of 10 5 A/cm 2 and are independent of doping, while at 60 K doped samples exhibit roughly two times higher J c in zero field and more than an order of magnitude higher J c at H = 40 mT. A magnetisation decay study reveals that samples with SrZrO 3 surplus have a larger fraction of pinning centres with high energy. The enhanced pinning properties can be related to the finely dispersed zirconate inclusions found in the superconductor matrix.

Study on the superconducting composite material formation in the system Bi2Sr2CaCu2O8+x/Al-containing phases

Abstract Phase evolution in the BiSrCaCuAlO system was studied. Two Al-containing phases BiSr 1.5 Ca 0.5 Al 2 O z and (Sr 1− x Ca x ) 3 Al 2 O 6 ( x = 0.4 − 0.45) were determined to be chemically compatible with Bi 2.18 Sr 2 CaCu 2 O 8+ x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr 1.5 Ca 0.5 Al 2 O z was found to be in equilibrium only with the melt and the (Sr 1− x Ca x ) 3 Al 2 O 6 phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr 1.5 Ca 0.5 Al 2 O z ↔ (Sr 1− x Ca x ) 3 Al 2 O 6 + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + n BiSr 1.5 Ca 0.5 Al 2 O z and Bi-2212 + m (Sr 1− x Ca x ) 3 Al 2 O 6 were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr 1.5 Ca 0.5 Al 2 O z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr 1− x Ca x ) 3 Al 2 O 6 imbedded in Bi-2212 lamellas. The Bi-2212 + m (Sr 1− x Ca x ) 3 Al 2 O 6 materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content.

Synthesis and Magnetic Properties of SrO–Fe2O3–B2O3 Glass-Ceramics

Glasses with nominal compositions of SrFe12O19 + 8SrB2O4 (I) and SrFe12O19 + 12Sr2B2O5 (II) are prepared by rapid quenching from the liquid state and are converted to glass-ceramics containing fine magnetic particles of SrFe12O19 by heat treatment between 600 and 950°C. The materials are characterized by x-ray diffraction, differential thermal analysis, electron microscopy, and magnetic measurements. The phase transformations accompanying glass crystallization are identified. The glass composition and heat-treatment conditions are shown to influence the aspect ratio of the forming submicron-sized strontium hexaferrite particles. The strongest coercive fields reached in glass-ceramics I and II are 504 and 456 kA/m, respectively.

Synthesis and Properties of Colored Copper-Containing Apatites of Composition Ca5(PO4)3CuyOy + δ(OH)0.5 − y − δX0.5 (X = OH, F, Cl)

Ca5(PO4)3CuyOy + δ(OH)0.5 − y − δX0.5 compounds (for X = OH, y = 0.01–0.3; for X = F, y = 0.01–0.1; for X = Cl, y = 0.1) have been synthesized by heat treatment of oxide-carbonate mixtures at 1150°C in air and have been characterized by X-ray diffraction, electronic spectroscopy, and magnetic measurements. The compounds have an apatite structure in which copper atoms substitute for part of the hydroxyl hydrogen atoms in hexagonal channels. The electronic spectrum shows two main absorption bands due to d-d transitions in copper(II) linearly coordinated to two oxygen atoms, as well as extra, weaker bands, whose contribution to the overall spectrum decreases with decreasing y. The latter are assignable to copper atoms occupying other sites in the crystal lattice. The temperature dependence of the magnetic susceptibility of the compounds obeys the Curie-Weiss law with a Curie constant close to zero. The Weiss constant characterizes the copper(II) content of the compounds and correlates qualitatively with the intensity of the main absorption bands in the visible spectrum. The fraction of copper(II) in the total amount of copper in the apatites increases in the substituent order X: Cl, OH, F, as well as upon the low-temperature annealing of the compounds in air. At the same time, copper(II) fraction depends only slightly on the total copper content. As the copper(II) content increases, the color of apatite changes from pink to dark claret.

AC SUSCEPTIBILITY WEAK-LINK CHARACTERIZATION IN THE BI-PB-CA-SR-CU-O THICK-FILMS ON (IN) AG TAPE

Abstract The thick films of the Bi-Pb-Ca-Sr-Cu-O superconductor with thickness 20 μm on (in) Ag tape have been prepared by cold rolling, pressing and annealing at 820–830°C cycles with overal thermal treatment time of 90 h. AC susceptibility data show that intragrain T c is about 108 K, while transport supercurrents start to flow at lower temperatures (70–102 K). The temperature dependences of the intergrain critical current density J c (T) have been calculated on the base of the χ″(T) data using the model worked out. On the J c (T) curves sometimes one can see a bend near 65 K, which may be explained by the existence of the second superconducting phase (2122), switching on new percolation paths for the supercurrents below that temperature. It has been found that low temperature distortion of χ″(T) and J c (T) curves is connected with cracks in the superconducting layer, the cracks restricting critical current flow through the film. The best films obtained have J c (77 K) 830 and 440 A/cm 2 for the films in and on Ag tape accordingly. Additional annealing of these samples at 830°C for 50 h leads to the J c rising for the “on Ag” film up to 1200 A/cm 2 , but J c for the “in Ag” film remains the same. The J c rise for “on Ag” film is in accordance with decreasing of J c suppression by cracks due to sintering, while for the “in Ag” film this process is accompanied with partial destruction of 2223 phase.

Synthesis and properties of colored copper-containing alkaline-earth phosphates with an apatite structure

Colored compounds A5(PO4)3Cu0.1OHz, where A = Ba1-xSrx (x=0,0.5,0.8), Sr1-y Cay (y=0,0.2, 0.5, 1), and Ba1/3Sr1/3Ca1/3 with an apatite structure containing copper ions in the hexagonal channels have been synthesized by the thermal treatment of the corresponding carbonates, oxides, and ammonium hydrophosphate at 1150°C. The use of the composition with A = Ba0.5Ca0.5 gives a multiphase product containing apatite (Ba1-xCax)5(PO4)3CuyOHz with x ∼ 0.2 as the major phase. The hexagonal lattice parameters of apatite depend linearly on the content of solid solution in the alkaline-earth metals. In the series of A = Ba, Ba-Sr, Sr, Sr-Ca, and Ca, the color of the samples changes smoothly from blue through violet to pink, whereas the samples with A = Ba0.5Ca0.5 and Ba1-3Sr1-3Ca1-3 are light gray. The diffuse reflectance electronic spectra exhibit two absorption bands characteristic of the copper ions in the hexagonal channels. Their positions depend on the alkaline-earth metal content of the compound. The third characteristic band appears in the spectrum of calcium apatite. Its relative intensity increases when calcium is gradually replaced by strontium and especially by barium, which results in worsening and even loss of the sample color. These spectral changes are presumably due to the transition of some copper ions from the hexagonal channels to other positions of the crystal lattice.

Formation of Bi-2212-based composites with submicrometre-grained (Sr, Ca)SnO3

Phase assemblage was investigated in the Bi-Sr-Ca-Cu-Sn-O system at temperatures of Bi-2212 phase crystallization. It is found that the Sr1-x Cax SnO3 phase (x = 0.1-0.2) is compatible with Bi-2212 as well as with Bi-2201, CuO, alkaline-earth metal cuprates and bismuthates. The solubility of Sn in Bi-2212 is limited to low values. The superconducting composites Bi-2212-(Sr, Ca)SnO3 were prepared by melt crystallization using single precursor and mixed precursor routes. The materials consisted of Bi-2212 matrix with submicron (Sr, Ca)SnO3 grains included in Bi-2212 lamellas and agglomerated in between. The composites exhibited enhanced flux pinning at T = 60 K, while at 5 K the effect of (Sr, Ca)SnO3 addition was not revealed.

Formation of Bi(Pb)-2223 with chemically compatible V-rich phase

Abstract Phase assemblage was studied in the samples obtained by thermal treatment of fine nitrate mixtures containing cations of Bi, Pb, Sr, Ca, Cu, and V. Large Bi(Pb)-2223 phase fraction was observed for the initial composition Bi(Pb)-2223+Sr 3 V 2 O 8 . It was found that vanadium-containing surplus resulted in the formation of Sr 3− x Ca x Cu y V 2 O z phase which proved to be chemically compatible with Bi(Pb)-2223, while vanadium solubility in the Bi(Pb)-2223 did not exceed 5 mol%. The new vanadate phase had diffraction pattern different from rhombohedric Sr 3 V 2 O 8 . Composites of Bi(Pb)-2223+ n Sr 2.8 Ca 0.2 Cu 0.3 V 2 O z ( n =0, 0.1, 0.5, 1) were prepared by thermal treatment of the homogeneous precursors at 860°C. In the samples with large vanadium content the formation rate of Bi(Pb)-2223 phase was higher, although the presence of vanadium caused a few degrees T c drop. Low fraction of the vanadate phase in the composite did not influence markedly the sample magnetization at 5, 30 and 77 K, while high fraction of this phase resulted in strong decrease of the hysteresis magnetization at T ≥30 K.

Synthesis of colloidal solutions of SrFe12O19 plate-like nanoparticles featuring extraordinary magnetic-field-dependent optical transmission

Hard-magnetic plate-like nanoparticles of SrFe12O19 were synthesised hydrothermally. The particles, fixed in a polymer, exhibited a coercivity in the range 0.9–1.8 kOe, and the saturation magnetization values reached 64 emu g−1. Electrostatically stabilized aqueous colloidal solutions of the plate-like particles with average thickness of 5–8 nm and average diameter of 30–80 nm were prepared. The nanoparticles readily oriented when applying a weak magnetic field as low as a few Oe, and due to the linear dichroism phenomena a strong change in the optical transmittance of the solution took place. The extinction coefficient ratio for the perpendicular and the parallel lights electric field vector relative to the magnetic field direction was found to be up to 4.5. The experimental results were compared with a theoretical model, in which the optical absorption of the nanoparticles was calculated from the polarizability of oblate ellipsoids.