Ba Substitution Research Articles

Enhancement of Tc by Sr substitution for Ba in Hg-2212 superconductor

The Ba substitution by Sr has been studied in two Hg-2212 series: Hg 2(Ba 1− y Sr y ) 2YCu 2O 8− δ and Hg 2(Ba 1− y Sr y ) 2(Y 0.80Ca 0.20)Cu 2O 8− δ . In both series a T c enhancement of about 40 K is observed when Sr substitutes Ba from y=0 to 1.0. The y=0 compound of the first series is the non-superconducting Hg 2Ba 2YCu 2O 8− δ prototype. In the second series, this y=0 compound is already superconducting at 21 K. Indeed the members of this series present a higher charge carrier density in their CuO 2 superconducting planes than their homologues of the first series due to the doping introduced by the substitution of 20% of Y by Ca. The compounds of both series were synthesized in high pressure (3.5 GPa)–high temperature (950–1050 °C) conditions. In both cases Sr substitution was successful up to the full replacement of Ba ( y=1.0). The Hg-2212 phases were characterized by XRD, SEM, EDX and a.c. susceptibility.

Spatial variation of superconducting properties and compositional fluctuation in large melt-processed (Sm–Eu)123 single domains

Abstract Large size (Sm 0.5 Eu 0.5 )Ba 2 Cu 3 O 7− δ single domains were grown in air via a hot-seeding melt process. Superconducting properties were found to gradually enhance as the grain growth proceeds, in that the grown grain displayed low T c and J c values at the initial crystallization in the top center of single domain while exhibiting high T c and J c in the edge section. The experiment unveiled that this was mainly induced by different RE–Ba substitution levels (RE=Sm and Eu). Such a spatial modulation of composition could be effectively suppressed by employing a fast cooling in the initial growth. The modified melt growth significantly improved the field trapping ability of single domain bulks.

Local property measurement in PTC thermistors

The local electrical properties of a positive temperature coefficient (PTC) thermistor based on donor doped BaTiO 3 with additions of Sr, Pb and Ca, have been characterised and correlated with the results of compositional, microstructural and crystallographic analyses. Wavelength dispersive spectroscopy showed a significant depletion in lead near the pellet surface, caused by volatilisation during sintering. Compensation for the lead loss was principally by Ba substitution, with only a small increase in the levels of Ca and Sr. Local resistance–temperature ( R– T) measurements showed a slight reduction in switching temperature, together with an increase in the pre-switching resistance, within the lead depleted zone. Electron backscatter pattern (EBSP) analysis revealed a significant proportion of Σ=3 and 9 grain boundaries, which are reported to be PTC inactive, in the microstructure. The proportion of Σ=3 grain boundaries was significantly higher in the lead-depleted zone than in the bulk of the pellet.

The effect of processing conditions on the properties of spray dried Nd 1Ba 2Cu 3O y/Ag composite superconductors

The aim of this study is to process Ag-doped Nd 1Ba 2Cu 3O y (NBCO) powders using spray drying and consequent calcinations and sintering. The influence of sintering conditions on the microstructure and superconductivity is investigated using thermal analysis (TG-DTA), X-ray diffractometry (XRD), Scanning electron microscopy (SEM), susceptibility and resistivity measurements. During sintering, a low oxygen partial pressure (1% O 2/Ar) was necessary to avoid Nd for Ba substitution. Sintering was performed at temperatures ⩾990 °C. After annealing in pure oxygen, superconducting material exhibiting Tc=95 K was obtained. The addition of Ag whiskers led to improved grain growth and decreased intergrain contact in the superconducting phase. Negligible structural and morphological interaction between NBCO and Ag was observed.

Surface morphology of Sm 1+ xBa 2− xCu 3O 6+ δ films deposited by metal organic chemical vapour deposition

We have prepared the Sm 1+ x Ba 2− x Cu 3O 6+ δ thin films with various values of x on MgO and SrTiO 3 by metal organic chemical vapour deposition using a liquid-state precursor. In this work we investigate the influence of the Sm–Ba substitution on superconducting properties ( T c, J c) and surface morphology. As the value of x approaching to 0 (stoichiometry), T c and J c showed the maximum values on MgO, about 91 K and 1.4 MA/cm 2, respectively. By the atomic force microscopy analysis, it was found that the grain size was large and the surface was flat for x>0. On the other hand, for x<0 the surface was rough due to many outgrowths of irregular shapes.

Reactivity between Nd1+xBa2−xCu3O7−δ+x/2 and Nd4−2zBa2+2zCu2−zO10−2z phases in superconducting NdBaCuO powders and melt textured bulk samples

The high temperature reaction between the superconducting Nd123(x) (Nd1+xBa2−xCu3O7−δ+x/2) phase and Nd422(z) (Nd4−2zBa2+2zCu2−zO10−2z), leading to substitutions of Nd in the Ba sites of Nd123, was analysed. For this purpose, differential thermal analysis (DTA) was performed on powders with different Nd422 addition. Measurement results were compared with the values, reported in the literature, of the peritectic temperatures in function of Nd123 substitution. The superconducting transition critical temperature analysis of melt-textured samples shows that the use of Barium rich Nd422 allows significant improvements of the superconducting parameters of produced samples, avoiding the occurrence of the Nd–Ba substitutions.

The influence of BaCuO2−δ addition in Gd–Ba–Cu–O single-grain superconductors fabricated in air

High quality Gd–Ba–Cu–O single-grain bulks with different BaCuO2−δ adding contents were grown in air via a hot-seeding melt process. At 77 K, all samples show a well-developed secondary peak effect with high Jc values, which are comparable to that of OCMG-processed Ga–Ba–Cu–O samples. The BaCuO2−δ addition can effectively suppress the Gd–Ba substitution and therefore enhance the superconducting properties. For the sample with a composition of Gd123:BaCuO2−δ = 1:0.3 in a molar ratio, the Jc(B = 0) and Jc,peak(B = 1.3 T) reach 6.45 × 104 A cm−2 and 6.04 × 104 A cm−2, respectively, which are the record high Jc values at 77 K for Gd–Ba–Cu–O samples melt processed in air.

The reduction of Eu3+ to Eu2+ in BaMgSiO4∶Eu prepared in air and the luminescence of BaMgSiO4∶Eu2+ phosphor

The reduction of Eu3+ to Eu2+ in air has been observed in a silicate matrix for the first time in BaMgSiO4∶Eu prepared by high-temperature solid-state reaction. Emission and excitation spectra were employed to detect the presence of Eu2+ ions in the compound and this reduction was explained by a charge compensation model proposed previously. In BaMgSiO4∶Eu2+, Eu2+ ions occupy three different lattice sites by substitution for Ba2+ ions. Eu2+ ions on Ba(1) and Ba(2) sites gave emissions at about 500 nm while that on Ba(3) site showed an emission band at 398 nm. All the emissions of Eu2+ ions in BaMgSiO4∶Eu2+ were not quenched at room temperature.

Piezoelectric Properties of Ba and Ca Doped SrBi2Nb2O9 Based Ceramic Materials

The crystalline structure and dielectric and piezoelectric properties of SrBi2Nb2O9 (SBN)-based materials were investigated with the substitution of Ca and Ba ions into the Sr site in this study. X-ray diffraction analysis revealed that these substitutes formed solid solutions throughout the compositions. The Ca substitution increases the ferroelectric transition temperature and maintains the phase transition as that of the normal ferroelectrics, while the Ba substitution lowers the ferroelectric transition temperatures and changes the transition from the normal to the diffuse ferroelectric one. The Ba-substituted SBN shows a smooth elastic anomaly for the Ba substitution of 0.2. A dielectric measurement revealed that the smooth elastic anomaly might be related to small dielectric dispersion characteristics observed around 300°C.

Large grain growth and superconducting properties of GdBa 2Cu 3O 7− δ fabricated in air with BaCuO 2− x addition

The melt-textured growth of Gd123 single domain was studied in air via a hot-seeding melt process. Gd–Ba substitution can be effectively suppressed by adding BaCuO 2− x . At 77 K, both J c value at self-field and irreversibility field B irr showed a peak with increasing the amount of BaCuO 2− x . B irr(77 K) was higher than 7 T with a nominal composition of Gd123 + 0.33BaCuO 2− x in a molar ratio. A further addition of BaCuO 2− x deteriorated superconducting properties. A surface field of 0.7 T was trapped in a 2.5 cm single domain pellet at 77 K.

Single-grain growth, microstructure and superconducting properties of (Sm, Eu)123 superconductors with BaCuO2−x addition

High quality (Sm0.5, Eu0.5)Ba2Cu3O7−δ single domains were grown in air with BaCuO2−x addition. It was found that a RE–Ba substitution (RE: Sm and Eu) could be effectively suppressed by a small addition of BaCuO2−x. The 94 K superconductivity with sharp transition was obtained in the composition range of 0.16 ∼ 0.33 BaCuO2−x and (Sm0.5, Eu0.5)Ba2Cu3O7−δ in a molar ratio. At 77 K, a surface field of 0.8 T was trapped in a single-domain pellet of 3.2 cm diameter. A further addition of BaCuO2−x deteriorated the microstructure and degraded the superconducting performance.

Massive barite deposits and carbonate mineralization in the Derugin Basin, Sea of Okhotsk: precipitation processes at cold seep sites

An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite build-ups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples show a Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals (∼1%) in the barite fabric imply the presence of H2S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative δ13C values of these carbonates (>−43.5‰ PDB) indicate methane as major carbon source; δ18O values between 4.04 and 5.88‰ PDB correspond to formation temperatures, which are certainly below 5°C. One core also contained shells of Calyptogena sp. at different core depths with 14C-ages ranging from 20 680 to >49 080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO42- concentrations and a parallel increase of H2S with depth. Additionally, S and O isotope data of barite sulfate (δ34S: 21.0–38.6‰ CDT; δ18O: 9.0–17.6‰ SMOW) strongly point to biological sulfate reduction processes. The isotope ranges of both S and O can be exclusively explained as the result of a mixture of residual sulfate after a biological sulfate reduction and isotopic fractionation with ‘normal’ seawater sulfate. While massive barite deposits are commonly assumed to be of hydrothermal origin, the assemblage of cheomautotrophic clams, methane-derived carbonates, and non-thermally equilibrated barite sulfate strongly implies that these barites have formed at ambient bottom water temperatures and form the features of a Giant Cold Seep setting that has been active for at least 49 000 yr.

Comparison of surface morphology in pulsed laser deposited Sm 1+ xBa 2− xCu 3O 6+ δ (0⩽ x⩽0.12) thin films

We have fabricated Sm 1+ x Ba 2− x Cu 3O 6+ δ (SmBCO) thin films by means of the pulsed laser deposition technique, using the stoichiometric and the Sm-rich SmBCO ( x=0.08 and 0.12) composition laser targets. In this work, the influence of the Sm–Ba substitution on the film surface morphology in the SmBCO system was investigated using the atomic force microscopy. With the increase of the Sm–Ba substitution, it was found that the surface of SmBCO thin films was very smooth and highly stable in the air. It is observed that Sm-rich SmBCO thin films using laser targets of x=0.08 and 0.12 have a root mean-square surface roughness in a 2×2 μm 2 area of about 2.1 and 1.7 nm, respectively. It was concluded that the Sm–Ba substitution influences the smoothness and the stability on the surface of the SmBCO thin films. We speculate that the surface morphology of the SmBCO thin films is influenced by the Sm–Ba substitution and the migration potential.

Structural Investigations on BaxPb1-x(Mg1/3Nb2/3)O3 Solid Solutions Using the X-Ray Rietveld Method

Solid solutions in the series (BaxPb1-x)(Mg1/3Nb2/3)O3 which belong to ABO3 type compounds were studied using the X-ray Rietveld technique. The compositions corresponding to Ba=0.2 and 0.4 have the same structure as that for Ba=0 (pm3m). The composition corresponding to Ba=0.6, 0.8 and 1.0 show ordering at the B-site. The structure of these compounds is solved in the space group P3m1. The temperature factor of Pb atom in Pb(Mg1/3Nb2/3)O3 is very high. The temperature factor of Pb/Ba reduces with Ba substitution. The octahedral distortions are small for the compositions in this series. There is an increase in Nb–O bond length with the substitutions of Pb.

Piezoelectric and Ferroelectric Properties of the Modified SrBi 2 Nb 2 O 9 Ceramics

Piezoelectric properties of Bismuth layer compound ferroelectrics SrBi 2 Nb 2 O 9 (SBN) were investigated. Their piezoelectric properties were evaluated from m 150°C to 450°C. Good resonant characteristics were obtained throughout the whole temperature range. Temperature coefficients of the resonance frequency (TCf's) are small, around 20ppm/°C. These good resonant characteristics, especially the good TCf's, are strongly related to the elastic anomaly observed in the temperature range approximately between 250°C and 350°C. When Ba ion substitutes Sr in SBN, the elastic anomaly becomes smoother, and the TCf decreases. SBN also shows a relaxor characteristics with increasing the Ba substitution.

Piezoelectric and Ferroelectric Properties of the Modified SrBi 2 Nb 2 O 9 Ceramics

Piezoelectric properties of Bismuth layer compound ferroelectrics SrBi 2 Nb 2 O 9 (SBN) were investigated. Their piezoelectric properties were evaluated from m 150°C to 450°C. Good resonant characteristics were obtained throughout the whole temperature range. Temperature coefficients of the resonance frequency (TCf's) are small, around 20ppm/°C. These good resonant characteristics, especially the good TCf's, are strongly related to the elastic anomaly observed in the temperature range approximately between 250°C and 350°C. When Ba ion substitutes Sr in SBN, the elastic anomaly becomes smoother, and the TCf decreases. SBN also shows a relaxor characteristics with increasing the Ba substitution.

Role of B Ion (B = Nb, Ta) on perovskite development, lattice parameters and dielectric properties of (Ba, Pb)(Zn1/3B 2/3)O3 ceramics

Ceramic powders of Pb(Zn1/3B″2/3)O3 (where B″ is Nb or Ta ions) complex-perovskite compositions, with gradual replacements of Pb by Ba, were prepared by using a B-site precursor method. Phases developed in both systems were identified by X-ray diffraction and perovskite contents were evaluated as a function of the amount of Ba substitution. Variations of lattice parameters of the pyrochlore and perovskite structures with composition modifications are interpreted in terms of the 6- and 12-fold cation sizes. Weak-field dielectric properties of the two systems in the low-frequency range are compared.

Synthesis and Dielectric Characterization of Pyrochlore-Free Pb1-xBax(Zn1/3Nb2/3)O3Ceramics

The dielectric properties and synthesis of pyrochlore-free lead zinc niobate ceramics with Ba substituting for Pb were investigated. Ba partial substitution for Pb was effective in stabilizing the perovskite structure in PZN ceramics, where the minimum amount of Ba substitution needed was about 20 mol%. The dielectric loss and the temperature coefficient of dielectric constant of PZN were reduced markedly with Ba substitution, while the dielectric constant was greater than 110. Good dielectric properties were obtained for the composition of Pb0.3Ba0.7(Zn1/3Nb2/3)O3:e = 133.5, tan δ = 0.0009, τe = −811 ppm/°C.

Solubility limit and crystallization of (Sm 0.5Eu 0.5) 1+ xBa 2− xCu 3O 7− δ

The phase relation and crystallization behavior were investigated in a series of (Sm 0.5Eu 0.5) 1+ x Ba 2− x Cu 3O 7− δ (−0.1⩽ x⩽0.3). The single phase rare earth (RE)–Ba substitution window (RE: Sm and Eu) was found to range from 0 to 0.3 when quenched from 1010 °C for nominal substitution parameter, x⩽0.05, from 990 °C for x=0.1 and 0.2, and from 970 °C for x=0.3 under ambient atmosphere. Onset T c values were 94 K for x⩽0 and fell sharply with increasing x. After Ar post-annealing (ArPA), the countersubstitution of RE by Ba was observed associated with a slight improvement of onset T c (0.5 K). A dramatic enhancement of onset T c for the sample with RE rich composition ( x>0) was observed with ArPA. It was also found that crystallization rate was large for the compositions with x⩽0 while small for x>0.

HRTEM and EELS Study for Superconducting and Non-Superconducting (Cu,Cr)-1212 Cuprates

The crystal structures of high temperature superconducting cuprates consist of alternative stacking of charge-reservoir (CR) block and conduction block. in case of YBa2Cu3Oy, (YBCO), CR block contains one-dimensional Cu(I)-O chain and BaO layers, and conduction block contains two-dimensional Cu(II)O2 planes and Y layers. YSr2Cu3Oy (YSCO), where Ba-sites of the YBCO are replaced by Sr completely can be obtained only by high-pressure synthesis. However, by replacing Cu(I) partly by Cr, YSr2Cu3-xCrxOy can be obtained even under ambientpressure conditions. The compound YSr2Cu2.8Cr0.2Oy (x=0.2) is non-superconductive, but, superconductivity is induced by partial replacement of Sr-sites with Ba; the compounds with YSr2-xBaxCu2.8Cr0.2Oy (x≤0.5) is non-superconductive, but the compound with x≥1.0 is superconductive. in order to obtain the structural information about the transition to superconducting phases, the effect of the Ba substitution is investigated by transmission electron microscopy (TEM).In the present study, the compounds with YSr2-xBaxCu2.8Cr0.2Oy (x=0-1.5) are examined by high resolution TEM (HRTEM) and electron energy-loss spectroscopy (EELS) to elucidate structural and electrical modification induced by Ba-substitution.