Search

Artificial promotion of self-sintering was studied to clarify the factors associated with higher intergranular connectivity in ex situ MgB2. Starting MgB2 powders with different magnesium precursor powder size and carbon additive in the form of B4C powder were prepared and used to synthesize sintered bulks to investigate the influences of interface impurities and intragrain lattice defects on self-sintering of MgB2. Laboratory-made high purity MgB2 powder with less oxidized surface and carbon-substituted MgB2 powder were effective to stimulate self-sintering at 900 °C by increasing the specific surface area and enhancing the driving force of mass transfer. Microstructural analyses showed that ex situ MgB2 bulks synthesized from such pretreated powders demonstrated increased sintered grain boundary area with the evolution of intergrain necks. Record high connectivity (35%) was achieved by a sintering period of 24 h, which is one-fourth of the conventional sintering time. The results suggest that carbon substitution stimulates sintering of grain boundaries and increases the upper critical field of the grains; this leads to quantitative and intensive enhancement of flux pinning strength in a high connectivity matrix. [ABSTRACT FROM AUTHOR]

A new iron-based superconductor, (Ca,Pr)- FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P21/ m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a Tc of ∼20 K in both magnetization and resistivity measurements. [ABSTRACT FROM AUTHOR]

In this work, scintillation properties of Ce1%, Pr1%, Eu1% and non-doped La3-xLu2+xGa3O12 (LLGG) single crystals were evaluated. At first, in order to investigate single phase region of La3-xLu2+xGa3O12, crystal growth was carried out for undoped LLGG crystal. After that, Ce1%, Pr1% and Eu1% doped La2.5Lu2.5Ga3O12 crystals were grown by the μ-PD method. Radioluminescence spectra and photoluminescence decay curve of these crystals ware measured. Energy spectra were collected under 5.5 MeV α-ray excitation (241Am source) and detected by an PMT R7600U (Hamamatsu) in order to determine light yield. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: We have discovered three new layered nickel pnictides with perovskite-type layer, (Ni2As2)(Ba3Sc2O5), (Ni2As2)(Ba4Sc2O6) and (Ni2As2)(Ba4Sc3O8). They have tetragonal crystal structures with space groups of I4/mmm for (Ni2As2)(Ba3Sc2O5) and (Ni2As2)(Ba4Sc3O8), and P4/nmm for (Ni2As2)(Ba4Sc2O6). Their lattice constants were a = 4.158 Å and c = 27.51 Å for (Ni2As2)(Ba3Sc2O5), a = 4.150 Å and c = 16.42 Å for (Ni2As2)(Ba4Sc2O6), and a = 4.15 Å and c = 37.0 Å for (Ni2As2)(Ba4Sc3O8). (Ni2As2)(Ba3Sc2O5) and (Ni2As2)(Ba4Sc2O6) exhibited bulk superconductivity with Tc(onset)’s of 4.4K and 3.5K, respectively. Tc of (Ni2As2)(Ba3Sc2O5) is the highest among the layered nickel pnictide superconductors. Their relatively high Tc may originate from their long a-axis. [Copyright &y& Elsevier]

We have previously reported that dilute impurity doping technique universally enhances the critical current density of RE123 superconductors. These doping effects on the Jc properties depend on substituted cation sites. In this paper, we report the flux pinning properties of the dilute Sr- or Co-doped Dy123 melt-solidified bulks in a wide temperature range. It was found that the dilute Sr-doping effectively enhance the Jc for Dy123 melt-solidified bulks with low level Dy/Ba substitution. For the dilute Sr-doped Dy123 samples, the temperature coefficient of Jc in 30 kOe was found to be approximately two times larger than for undoped materials above 40 K, meaning that it is an effective method for developing high-Jc RE123 materials operating at low temperatures. [ABSTRACT FROM AUTHOR]

Easy axes of magnetization at room temperature in REBa2Cu3Oy (RE: rare earth) and Bi2Sr2Ca1-xRExCu2Oy were clarified using their powders oriented in a static magnetic field. The easy axes clearly depended on the type of RE which could be mainly explained in terms of second-order Stevens factors and magnetic anisotropy of the CuO2 plane. Furthermore, we found the difference in magnetic effects of Er3+for orientation through systematic evaluations of Er-doped Y123 and Bi2212 powders. This result means that magnetic anisotropies of RE3+ ions are sensitive to not only the type of RE but also local structure of oxygen ions surrounding RE3+ ions. [ABSTRACT FROM AUTHOR]

Tri-axial orientation of twin-free powders of RE2Ba4Cu7O15-δ (RE247, RE = Y, Dy, Ho, Er) was attempted by modulated rotating magnetic fields. The c-axis orientation was almost completely achieved for all the four RE247 compounds, whereas the degrees of in-plane orientation depended on RE element. Especially for Y247, the degrees of in-plane and c-axis orientation were determined to be ∼ 2.5° and < 2°, respectively, by the Φ-scanning and rocking curve measurements. This excellent in-plane orientation is comparable or superior to those for coated conductors ( 4°) and melt-solidified bulks (∼ 8°) of REBa2Cu3O7-δ. Magnetic field dependence of Lotgering factors revealed that the degrees of in-plane orientation were sensitive to the applied magnetic field. The required magnetic field for the tri-axial orientation is determined by the in-plane magnetic anisotropy rather than the magnetic anisotropy between in-plane and out-of-plane directions. [ABSTRACT FROM AUTHOR]

XRD peak profile analysis was performed on polycrystalline MgB2 bulks and tapes manufactured by various heat treatment conditions in order to investigate statistical value of crystallite size. Volume-weighted and area-weighted mean diameter of a bulk sample heated at 600°C for 6 h were estimated to be approximately 90 nm and 20 nm, respectively. These suggest that crystallite size distribution of MgB2 is very wide, although a lot of very small crystallites exist. Similar tendencies were observed for other samples. On the other hand Jc values were found to be rather low compared to the expected values considering its small Da. This indicates that a small portion of crystallite interfaces act as effective pinning centers. [ABSTRACT FROM AUTHOR]

Pr:LuAG crystals possess interesting properties, such as high density, high light yield, and very fast 5d-4f emission decay time. Recently, we developed a Pr:LuAG single crystal with a diameter of 2 inches for scintillator applications, such as medical imaging. In this study, Pr-doped Lu3 Al5 O12 (Pr:LuAG) single crystals with a diameter of 2 inches were grown by means of the Czochralski method; a body length of up to 90 mm was achieved. The Pr:LuAG crystals produced a homogeneous light yield with approximately 2.5 times less intensity compared to that produced by a conventional Ce:LSO crystal. A sample with dimensions of 5 mm × 5 mm × 1 mm exhibited energy resolutions of 4.8% at 662 keV, as measured by using a photomultiplier (PMT: Hamamatsu H6531), and 5.8% at 662 keV, as measured by using an Avalanche photodiode (APD: Hamamatsu S8664-55). The Pr:LuAG crystals exhibited a very fast rise time of approximately 0.4 ns, excited by a pulsed X-ray at room temperature. The decay time was approximately 18 ns, along with a noticeable presence of slower decay components (with a decay time of 55.0 ns). Furthermore, the Pr:LuAG crystals showed good linearity between the energy and pulse height within approximately 4% of the standard deviation in the range from 122 keV (152Eu) to 1.4 MeV (241Am). [ABSTRACT FROM AUTHOR]

Abstract: Effect of Ga substitution in a (Ce,Lu)3Al5O12 scintillator was examined at the crystals grown by the micro-pulling down(μ-PD) method. Strong suppression of unwanted host luminescence due to an exciton localized around Lu–Al antisite defect was observed even at the Ga concentration of 10mol%. Less-intense slower components in scintillation decay were obtained upon increasing the amount of Ga. While the radioluminescence intensities of the 5d–4f luminescence of Ce3+ were not strongly changed, light yield was increased by Ga substitution. The 20% Ga-substituted sample showed even higher light yield than typical Czochralski-grown Ga-free LuAG:Ce. [Copyright &y& Elsevier]

In this study, high-density magnesium diboride (MgB2) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (Jc-B) in MgB2 bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and Jc-B using two methods, ex situ (sintering MgB2 synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB4 and MgO were found to be effective in improving the Jc-B characteristics. In the ex situ method, the degradation of MgB2 due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the Jc-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and Jc-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (fP) analysis revealed the possibility of refined MgO inclusions and MgB4 phase as new pinning centers, which greatly contributed to the Jc-B properties. The contributions of the sintering conditions on fP for both synthesis methods were analyzed. [ABSTRACT FROM AUTHOR]

In this work we tried to grow 2-inch-diameter and high quality Pr:LuAG single crystals by the Czochralski (Cz) method. To grow the large diameter Pr:LuAG single crystal, we have optimized growth conditions and the furnace design. As a result, we have succeeded in the growth of 2-inch-diameter Pr:LuAG single crystal with a length of 110 mm. To determine light yield and energy resolution the energy spectra were collected under 662 keV γ-ray excitation (137Cs source) and detection by a photomultiplier (Hamamatsu H7826). The light yield was around three times higher than that of BGO and almost homogeneous with increasing solidification fraction up to 0.32. The energy resolution was around 8%. Furthermore, scintillation decay time was around 23 ns and almost homogeneous all over the crystal. [ABSTRACT FROM AUTHOR]

Abstract: Pr-doped Lu3Al5O12 (Pr:LuAG) single crystals were grown by the micro-pulling-down (μ-PD) method and Czochralski (Cz) method. The crystals were seeded-grown in the 〈111〉 direction. Dimensions up to 30mm in length and 5mm in diameter by μ-PD and up to 100mm in length and 22mm in diameter by Cz method were achieved. Concentration of Pr3+ in Cz-grown Pr:LuAG was measured. Radioluminescence spectra and photoluminescence decay of Pr:LuAG were investigated and compared to Pr-doped Y3Al5O12 (Pr:YAG) single crystals. Pr:LuAG crystals show better temperature stability of radioluminescence intensity and decay time compared to Pr:YAG, and luminescence was not quenched even at room temperature (RT). Scintillation light yield of Cz-grown Pr:LuAG was measured and compared to the standard BGO sample. Light yield of Pr:LuAG was more than two times higher than BGO and the relationship between Pr3+ concentration and light yield was clarified. [Copyright &y& Elsevier]

Several concentration of Yb-doped Lu3Al5O12 (Yb:LuAG) and Lu3Ga5O12 (Yb:LGG) single crystals were grown by the micro-pulling-down method. The crystals were seeded-grown in the 〈1 1 1〉 direction and transparent and crack free crystals were obtained. Photoluminescence spectra and decay kinetics of these crystals were studied. Charge transfer luminescence of Yb3+ was observed in both crystals. Mean decay time of about 25 ns at 90 K and strong thermal quenching at room temperature was measured for Yb 5%:LuAG. Radioluminescence intensity was compared to the standard BGO sample. [Copyright &y& Elsevier]