M H Pu

Crystalline gallium oxide nanowires: intensive blue light emitters

Abstract Gallium oxide nanowires (GaONWs; diameter, ca. 60 nm; length, hundreds of micrometers) have been synthesized by a carbothermal reduction reaction. The nanowires have been confirmed as crystalline β -Ga 2 O 3 by powder X-ray diffraction and selected area electronic diffraction. The GaONWs can emit stable and high brightness blue light at 446 nm (2.78 eV) under excitation at 378 nm (3.28 eV), which may have potential applications in one-dimensional optoelectronic nanodevices.

Simultaneous growth of α-Si3N4 and β-SiC nanorods

Abstract Well-separated α-Si 3 N 4 and β-SiC nanorods have simultaneously been synthesized by the reaction of SiO 2 nanoparticles and active carbon at 1450°C in nitrogen atmosphere. TEM (transmission electron microscopy) photographs confirm α-Si 3 N 4 nanorods with the diameter of 10–100 nm and length up to 15μm, and β-SiC nanorods with the diameter around 40 nm and length up to about 1 μm. The chemical reaction conditions and mechanisms have been discussed.

Enhanced flux pinning in (Bi, Pb)-2223/Ag tapes by slight Pr substitution

Bi1.8Pb0.4Sr2Ca2.2-xPrxCu3Oy silver sheathed ((Bi, Pb)-2223/Ag) tapes with x = 0.0 (un-doped), 0.001 and 0.002 (Pr doped) have been investigated. X-ray diffraction analyses and transmission electron microscopy observation show that in the Pr-doped tapes there are many defective regions induced by the partial Pr substitution for Ca with a size of about 10 nm. As compared with the un-doped tape, the magnetic field dependence of critical current densities of the Pr-doped tapes are noticeably improved at 77 K. Their irreversibility lines are also shifted to higher temperatures and magnetic fields, and their activation energies of flux motion are increased drastically. The pinning force densities of the tapes are fitted with a improved law from the Dew-Hughes model; the results of this fitting show that the flux pinning enhanced in Pr-doped tapes mainly originates from the fine normal-like defects.

Colossal Magnetoresistance in Fine Particle of Layered‐Perovskite La2—2xCa1+2xMn2O7 (x = 0.3) Synthesized at Low Temperatures

By means of a sol-gel method, we prepare layered-perovskite samples of La 2-2x Ca 1+2x Mn 2 O 7 (x = 0.3) at synthesis temperatures from 1000 to 1200 °C. The grain size changes from 20 to 45 nm. A magnetoresistance above 25% at low temperatures is acquired in a field of 0.8 T. We also find that decreasing grain size is determining for getting a larger magnetoresistance. These results may be beneficial to future application.

Improved Jc behavior under magnetic field in Bi1.8Pb0.4Sr2Ca2.2Cu3-xCrxOy/Ag tapes

Bi(2223)/Ag tapes with CrO 3 doping have been fabricated by Powder-in-Tube (PIT) method. It is found that a small amount of CrO 3 doping improves the flux pinning, with the effective pinning center due to Cr substitution for Cu, while excessive Cr-ion doping degrades the superconductivity of Bi(2223). XRD analysis shows that a small amount of CrO 3 addition has no effect on the phase composition of Bi(2223)/Ag tape. SEM micrographs of Bi(2223) tapes do not show a distinct difference between the pure sample and CrO 3 doped samples (x ≤ 0.4%).

Enhanced flux pinning in (Bi, Pb)-2223/Ag tapes by slightly over-doped Pb

Bi2.2-xPbxSr2Ca2.2Cu3Oy silver sheathed ((Bi, Pb)-2223/Ag) tapes with different Pb doping amounts, x = 0.4, 0.5, 0.6, are prepared by the powder-in-tube method. X-ray diffraction analyses manifest that the tape with x = 0.5 has more Pb-rich phase than that of a tape with x = 0.4. For the tape with x = 0.6, there is a large amount of Ca2PbO4 phase and a small amount of Pb-rich phase. Scanning electron microscopy and transmission electron microscopy observations show that the Pb-rich phase with nanometre order of magnitude is embedded in the (Bi, Pb)-2223 grain matrix of samples. The magnetic field dependence of the critical current density of the tape with x = 0.5 is noticeably improved at 77 K, and its irreversibility line is also shifted to higher temperatures and magnetic fields in comparison with the tape with x = 0.4, which is usually considered as the standard Pb doping level in preparing (Bi, Pb)-2223/Ag tapes. The resistivity measurement of the normal state reveals that the anisotropy of the tape with x = 0.5 is also decreased compared with the tape with x = 0.4. The enhanced flux pinning seems to mainly originate from the fine Pb-rich phase particles themselves and the extra defects induced by the larger Pb-rich phase particles.

Enhancement of flux pinning in (Bi, Pb)-2223/Ag tapes doped with MgO nanorods

MgO nanorods with an average diameter of 30?nm and lengths of 2-3??m have been incorporated into (Bi,?Pb)-2223/Ag tapes. X-ray diffraction analysis shows that a small addition of MgO nanorods does not obstruct the formation of (Bi,?Pb)-2223 phase. Scanning electron microscopy images show that the size of the (Bi,?Pb)-2223 grains and the degree of texture in the samples doped with 1?wt% of MgO nanorods are improved. Transmission electron microscopy observation also shows that the MgO nanorods do not agglomerate or grow during the sintering process. The transport critical current densities of the samples doped with 1?wt% of MgO nanorods are enhanced noticeably at high temperatures and magnetic fields and the irreversibility line of the samples doped with 1?wt% of MgO nanorods shifts to higher temperatures and magnetic fields as compared with the undoped samples. The possible pinning mechanism of the samples doped with MgO nanorods is discussed. The results reveal that the introduction of MgO nanorods as columnar defects can effectively enhance the flux pinning.

Enhanced flux pinning in Bi2212 single crystal embedded with MgO particles

Bi2Sr2CaCu2O8+ single crystals embedded with MgO particles were successfully synthesized. Preliminary results indicate that MgO particles introduced during the single crystal growth process have little effect on Tc of the Bi2212 superconductor. The critical current density of the MgO-added single crystal has been increased by over a factor of three at low temperatures below 20 K with high magnetic fields parallel to the c -axis as compared with samples without MgO added. The underlying flux pinning mechanism is discussed in detail.

Effect of Liquid Nitrogen Quenching on Phase Formation in (Bi, Pb)-/2223Ag Tapes

The effect of liquid-nitrogen (LN2) quenching on the phase formation and critical current density (J c ) of (Bi, Pb)-2223 silver-sheathed tapes has been studied. The results show that the LN2 quenching can effectively accelerate the increasement of the (Bi, Pb)-2223 phase and so shorten the production period of the tapes. The texture of the superconducting cores in the tapes is improved and J c of LN2-quenched samples is enhanced. The possible reasons are presented.