Y.P. Sun

Preparation and photoluminescence properties of amorphous silica nanowires

Abstract Bulk-quantity amorphous silica nanowires (SiONWs) have been synthesized by carbothermal reduction reaction between silicon dioxide and active carbons. Transmission electron microscopy (TEM) image shows the formation of the nanowires at a diameter of 60–110 nm and a length up to hundreds micrometers. Besides most smooth-surface polyp-shaped nanowires, two other forms of nanowires, named amoeba-shaped and frog-egg-shaped nanowires, have also been observed. The nanowires can emit stable and high brightness blue light at 435 nm (2.85 eV) under excitation at 260 nm (4.77 eV). The formation of the nanowires into different shapes may be explained by the vapor–liquid–solid (VLS) mechanism.

Preparation and photoluminescence properties of crystalline GeO2 nanowires

Abstract Bulk-quantity GeO 2 nanowires (GeONWs) have been synthesized by carbothermal reduction reaction between germanium dioxide and active carbons. Transmission electron microscopy (TEM) image shows the formation of the nanowires at a diameter about 50–120 nm and a length up to hundreds of micrometers. The nanowires can emit stable and high brightness blue light at 485 nm (2.56 eV) under excitation at 221 nm (5.61 eV). The intensity of the emission is one order of magnitude higher than that of GeO 2 powders. The photoluminescence (PL) may originate from radiative recombination between an electron on V O × and a hole on ( V Ge , V O ) × in the GeONWs. The nanowires may have potential applications in one-dimensional optoelectronic nanodevices.

Enhanced intergrain tunneling magnetoresistance in double perovskite Sr2FeMoO6 polycrystals with nanometer-scale particles

Polycrystalline ordered double perovskite Sr2FeMoO6 bulk samples with grain size in the range of 29–45 nm have been synthesized at temperatures from 900 to 1000u200a°C, using a sol-gel method. We find that the intergrain magnetoresistance is closely correlated with the grain size. The sample with the grain size of 29 nm shows large magnetoresistance Δρ/ρ0, 30%–20% at a low magnetic field of 4 kG over a wide temperature range from 20 to 300 K. The results can be explained in terms of spin-dependent intergrain tunneling model.

Large room-temperature magnetoresistance and phase separation in La1−xNaxMnO3 with 0.1⩽x⩽0.3

The structural, magnetic, and electronic properties of the polycrystalline La1−xNaxMnO3 (x=0.10, 0.15, 0.20, and 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in a rhombohedrally distorted structure with space group R3C. The magnetic measurement shows that Curie temperature TC of the studied samples is near or above room temperature. The temperature dependence of resistivity shows that all samples undergo a sharp transition accompanying a paramagnetic to ferromagnetic with the decrease of temperature, however, for x⩾0.15 samples, double transition peaks with a single ferromagnetic transition is observed. In the meanwhile, a large room-temperature magnetoresistance with low applied magnetic field is observed. The co-existing ferromagnetic metallic phases and ferromagnetic insulating (FMI) phases induced by the electronic inhomogeneity as well as the additional FMI phases caused by the presence of vacancies at the A sites, a...

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.

Structural, magnetic, and transport properties in a Cu-doped La0.7Ca0.3MnO3 system

The structural, magnetic, and transport properties of La0.7Ca0.3Mn1−xCuxO3 (0⩽x⩽0.2) have been investigated. The maximum magnetoresistance (MR) is about 4×106% for the sample with x=0.15, which is about ∼104 times the maximum MR for samples with x⩽0.1. A change in the structural parameters and magnetic and resistivity properties of the samples shows that Cu exists in Cu3+ ionic form in the sample with x⩽0.05. However, Cu2+ ions begin to appear and then they increase with further Cu doping.

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.

Enhanced flux pinning of silver-sheathed tapes with nano-MgO particle addition

Nano-MgO particles have been introduced into silver-sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes fabricated by the rod-in-tube process. It is found that the transport properties of the nano-MgO particle doped sample are noticeably improved compared with those of the undoped sample. The irreversibility line of the MgO-doped sample shifts to higher temperatures and magnetic fields, as compared with the undoped sample. Therefore the introduction of nano-MgO particles can effectively enhance the flux pinning.

Dependence of internal friction and young's modulus on the oxygen deficiency in the superconductor YBa2Cu3O7−δ

Abstract The dependence of the internal friction and Youngs modulus on the oxygen deficiency in the superconductor YBa 2 Cu 3 O 7−δ was studied at frequencies near 2.7 kHz. Peaks observed near 110 and 230 K decreased with decreasing oxygen stoichiometry in a sample with the orthorhombic structure, and were absent in a sample with the tetragonal structure. Peaks at about 88 and 260 K did not change with the oxygen stoichiometry, and were observed both in the sample with orthorhombic structure and in the sample with tetragonal structure. The peaks at about 88 and 110 K are characteristics of relaxation processes. It is suggested that the activated event for the 110 K peak is the jump of the hole or electron in the Cu(1)-O plane and that of the 88 K peak is that in the Cu(2)-O plane. The peak at about 260 K is indicative of a phase transition. The change in the microscopic structure leading to this peak may be associated with the oxygen deficiency ordering.

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%).