Increasing Ba Content Research Articles

Phase relations and hydrogenation behavior of Sr1−xBaxAl2 (0≤x≤0.5)

The phase relations and hydrogenation behavior of Sr1−xBaxAl2 (0≤x≤0.5) alloys have been studied. It was found that the maximum solubility of Ba in the Zintl phase SrAl2 was about x=0.082. When x=0.082∼0.156, the alloys consisted of the Zintl phase, (Ba,Sr)7Al13 and (Sr,Ba)Al4. When x≥0.156, the Zintl phase disappeared and (Ba,Sr)7Al13 and (Sr,Ba)Al4 remained. Under a hydrogen pressure of 7 MPa, the Zintl phase (Sr,Ba)Al2 was first hydrogenated to (Sr,Ba)Al2H2 and then it disproportionated into an unknown hydride and Al. In the disproportionation course, an amorphous hydride was formed as an intermediate phase. (Ba,Sr)7Al13 was first hydrogenated into an amorphous hydride and then (Sr,Ba)Al4 crystallized in the amorphous matrix. The amorphous hydride also transformed into the crystalline unknown hydride and Al when the hydrogenation temperature further increased. Increasing Ba content led to a rise of the formation temperature of the unknown hydride.

The Influence of Composition and Temperature on the Phases in Sr1−xBaxZrO3 Perovskites: A High-Resolution Powder Diffraction Study

High-resolution synchrotron X-ray diffraction measurements on a series of polycrystalline samples of Sr1−xBaxZrO3 at room temperature show that the symmetry increases with increasing Ba content, the sequence of phases being orthorhombic Pnma, orthorhombic Imma, tetragonal I4/mcm, and cubic Pm3m. The effect of temperature has also been studied for a number of compositions and indicates that the same series of phases is present. The transition temperatures decrease with increasing Ba content.

Internal Chemical Pressure Effect and Magnetic Properties of La0.6(Sr0.4−xBax)MnO3

The effect of isovalent chemical substitution of Ba2+ into the Sr2+ sites on the structural and magnetic transition in La0.6(Sr0.4−xBax)MnO3 (x=0–0.4) has been investigated. The rhombohedral structure with space group R−3c is observed in this series of materials. Substitution of Sr and Ba in the La site changes the structural parameters such as the Mn–O bond lengths, thus allowing the study of the effect of the structure on magnetic properties. An increase in the Mn–O bond distance with increasing Ba content leads to a decrease in TC (the temperature of the paramagnetic to ferromagnetic transition).

Phase Formations and Electrical Properties of (SrxBa1-x)Bi2Ta2O9 Thin Films

The phase formation and electrical properties of (SrxBa1-x)Bi2Ta2O9 (SBBT) thin films prepared by the metal-organic decomposition method on Pt/Ti/SiO2/Si substrates have been investigated. It was observed that the microstructure and electrical properties of SBBT thin films dramatically varied with the Ba content. While the remanent polarization and dielectric constant decreased with increasing Ba content in SBBT thin films, the insulating properties significantly improved. The SBBT films also exhibited good fatigue characteristics under bipolar stressing to 1010 cycles.

Low-temperature synthesis of the (Ba, Sr, Ca)–Cu–O system from hydroxometallate precursors

Alkaline-earth cuprates of Sr 1− x Ca x CuO 2 and (Sr 1− x Ba x ) 2CuO 3+ y have been synthesized at temperatures as low as 400–500°C by thermal decomposition of hydroxometallate precursors of Sr 1− x Ca x Cu(OH) 4·H 2O and (Sr 1− x Ba x ) 2Cu(OH) 6·H 2O, respectively. In the first step of the synthesis, we succeeded in obtaining new hydroxometallate precursors Sr 1− x Ca x Cu(OH) 4·H 2O and (Sr 1− x Ba x ) 2Cu(OH) 6·H 2O through precipitation in a highly concentrated NaOH solution. In the second step, the hydroxometallate precursors were heated at various temperatures. In the case of Sr 1− x Ca x Cu(OH) 4·H 2O, the ambient-pressure phase of SrCuO 2 was obtained at 500°C in flowing N 2 gas. In the case of (Sr 1− x Ba x ) 2Cu(OH) 6·H 2O, three kinds of phases of (Sr 1− x Ba x ) 2CuO 3+ y with different excess oxygen content y were obtained at 400°C: y∼0, ∼0.1 and ∼0.3–0.5. It has been found that the excess oxygen content y tends to increase with increasing Ba content x, with increasing oxygen partial pressure during the heat treatment and with decreasing heat-treatment temperature.

Van der Waals epitaxy with C 60 molecules and change of the growth mechanism by Ba-doping

Hot-Wall Epitaxy (HWE) allows to grow epitaxial layers very close to thermodynamic equilibrium, which is very essential in the case of Van der Waals epitaxy of fullerenes. The semiclosed nature of the HWE reactor provides a growth regime at high vapour pressures without loss of source material, and offers the possibility to dope the epilayers with Ba or to form Ba-compounds during growth. As soon as Ba was used in the growth reactor, the growth rate of the C 60 layers was mainly controlled by the Ba source temperature. This behaviour can be interpreted by the assumption that the incorporated Ba causes a charge transfer to C 60 and controls, in that way, the sticking coefficient for the C 60 molecules that are always present in a surplus in the vapour. In that way the growth mechanism changes from Van der Waals epitaxy, which is typical of pure fullerenes, to a different mechanism resulting from a stronger bonding type evoked by the charge transfer. A clear decrease of the resistivity with increasing Ba content in the C 60 layers was observed with a saturation value of 4 × 10 −4 Ω cm.

Synthesis and rheological properties of soluble poly(hydroxyethyl methacrylate) and some copolymers

AbstractMethods to synthesise soluble poly(2‐hydroxyethyl methacrylate) (PHEMA) with varying molar mass were developed. Conversions over 90% were achieved without losing solubility. Steady state tests (range 0.01–590 Pa) and oscillatory tests (range 0.001–40 Hz) were carried out on 40% solutions in dimethyl formamide using a Carri‐Med rheometer. Only PHEMA having molar mass over 500000 showed significant shear thinning and viscoelastic properties. The copolymers of 2‐hydroxyethyl methacrylate (HEMA) with n‐butyl acrylate (BA) and 2‐hydroxypropyl acrylate (HPA) showed higher viscosities and viscoelastic properties compared with PHEMA homopolymer prepared under identical conditions. Also, viscosity and viscoelasticity increased with increase in acrylate content in the initial mixture. This was attributed to the higher molar mass of copolymers, which resulted from faster polymerisation rates owing to the inclusion of relatively highly reactive acrylates. However, in the case of HEMA : BA copolymers, first, the viscosity and viscoelasticity increased with increasing BA content and then dropped again, giving a maximum around HEMA : BA 75 : 25. This anomaly was explained by taking the effects of changes in inter‐and intra‐molecular hydrogen bonding as well as conformational differences caused by inclusion of BA in the PHEMA chain into account.

Preparation of a Chain-Type Composite Crystal, BaxTiS3 (x = 1.00 - 1.05)

Barium titanium sulfide, BaxTiS3 (x = 1.00 - 1.05), has been prepared by the reaction of CS2 with a mixture of BaCO3 and TiO2. The X-ray powder diffraction and electron diffraction patterns show that the structure is a chain-type composite crystal similar to SrxTiS3 and BaxFe2S4. The structure consists of two interpenetrating cells, probably TiS3 and Ba cells, which have the parameter a in common, whereas the parameter c is different (c1 and c2 in a hexagonal setting). c1 shrinks with increasing Ba content, while c2 expands. The electron diffraction spots and the Xray powder diffraction peaks cannot be indexed because of an incommensurate structure; they were, therefore, indexed based on a four-dimensional formalism using the parameters a, c1, and c2. In the expression of the four-dimensional formalism, this sulfide is represented by a single structure with a homogeneity range of x = 1.00 - 1.05.

A STUDY OF RAMAN SCATTERING IN Ba-DOPED Bi-Sr-Cu-O SINGLE CRYSTALS

The phonon vibration characteristics of the Bi2Sr2-xBaxCuOy (x=0, 0.2，0.4) single crystal samples was studied by using Raman scattering measurement. The exp-erimental results show that in the Raman spectra of the Bi2Sr2-xBaxCuOy system the typical phonon vibrational modes mainly appear in the frequency range of 196, 282-310, 460, 625 and 660cm-1, and the intensities of the 196, 282-310, 625, and 660 cm-1 lines enhance with increasing Ba content x. In previous work, we have studied the influence of Ba-doping on the incommensurate modulation structure of the 2201 phase. On the basis of those results, in this article we analyze the relation of the incommensurate modulation structure and phonon vibration characteristics emphatically, thus point out that the intensifying of the superstructure modulation (i.e. the decr-ease of modulation period) changes the symmetry of the unit cell seriously so that the phonon Raman activity of the unit cell varies obviously.

BARIUM-DOPING EFFECTS ON THE SUPERCONDUCTIVITY OF Bi2Sr2CaCu2Oy

Ba-doping effects on the superconducting properties in the Bi2Sr2-xBaxCaCu2Oy single crystals (0≤x≤0.15) and polycrystalline samples (0≤x≤0.3) have been studied. The results show that Ba２＋ enters the superconducting phase with a solubi-lity limit. For 2212 phase single crystals, the length of c-axis and the supercondu-cting transition temperature Tc both increase with increasing Ba content. For slowly cooled sintered samples, Tc is significantly increased by Ba doping, whereas for quenched samples Tc is little affected. Changes in Tc with Ba doping have been explained by changes in the hole concentration.

Preparation, electric conductivity and crystal structure of (Sr 2− xBa x)CuO 2+ y(CO 3) z

(Sr 2− x Ba x )CuO 2+ y (CO 3) z solid solutions with 0≤×≤1.75 were synthesized at 1173 K under atmospheres of O 2–CO 2 gases (partial pressures of CO 2( P(CO 2)) 10 1−10 4 Pa). The lattice parameters of the solid solutions increased with increasing Ba content x from a′ = 5.5186 (1) and c′ = 7.497 (1) Å ( x = 0; a′ = a √2 and c′ = c 2 ) to a = 5.6760(1) and c = 7.9276(2) A ̊ ( x = 1.75). The average Cu valence determined from iodometric titration changed from 2.010 ( x = 0) to 2.043 ( x = 1.75). The electric conductivity of the samples increased from 2.0 × 10 −1 to 5.0 S cm −1 with increasing x at 290 K. All samples showed semiconducting behavior and no superconducting transition at temperatures down to 12 K The crystal structures of the solid solutions with x = 1.00 and x = 1.75 were refined by means of Rietveld analysis for neutron powder diffraction data. There was a deficiency of about 12% of the C atoms in the solid solution with x = 1.75. Excess oxygen atoms which may dope holes into the CuO 2 sheets were found at the carbonate deficient sites.

Mechanical and thermodynamic properties of butyl acrylate-N-vinylpyrrolidone hydrogels

Copolymerization to almost 100% conversion has been effected by the γ-irradiation of mixtures of n-butyl acrylate (BA) and N-vinyl-2-pyrrolidone (VP) in the presence of a crosslinking agent, 1,1,1-trimethylolpropane trimethacrylate (TPT). Swelling in water at 294 K yielded hydrogels having equilibrium water contents in the range 1–89%, the corresponding extension ratios and volume fractions of water being obtained from dimensional changes on swelling. Stress(compression)-strain measurements yielded the compression and Young's moduli, the effective crosslinking density ( v e) and the copolymer-water interaction parameters (χ). From the linear dependence of v e on the theoretical value based on the concentration of TPT, the efficiency of crosslinking was found to increase with increasing BA content; a finite v e was also interpolated to exist even in the absence of added TPT. Increasing the VP content of xerogels produced a corresponding increase and decrease in the fractional contents of water and BA respectively within the hydrogels, whereas the fractional content of VP displayed an increase followed by a decrease. This latter behaviour mirrors that exhibited by v e and the elastic moduli. Possible explanations are proposed.