J.D. Guo

Synthesis of nanoscale superconducting YBCO by a novel technique

Abstract A novel technique using citrate pyrolysis was developed to prepare nanoscale superconducting oxide materials. This paper describes the details on synthesizing nanocrystalline YBCO with a T c of ∼80 K using this method. The morphology and structure of the nanoscale products were characterized by powder X-ray diffraction and scanning electron microscopy. The obtained YBCO grains have a mean particle size of 40–60 nm (for unannealed samples) and 100–150 nm (for the annealed products). The crystalline size was only ∼20 nm. Currently there are attempts at using such fine powder to fabricate longer superconducting tapes, which should induce a technical revolution in the production of superconducting tapes.

A novel technique by the citrate pyrolysis for preparation of iron oxide nanoparticles

A novel technique by the citrate pyrolysis is developed to prepare metal oxide nanocrystals. In this paper details on synthesizing iron oxide nanoparticles through this method is described. The structure and morphology of nanoscale products have been characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The obtained iron oxide particles have two lattice types and a mean particle size distribution of 20 and 50 nm, respectively.

Au-doping effects in the Mg1−xAuxB2 series

Abstract A series of Au-doped MgB 2 superconductors with different doping levels were prepared by means of solid-state reaction. X-ray diffraction analysis indicates that Mg 1− x Au x B 2 samples with x ≦0.236 can still be identified as having a hexagonal structure, with a different space group from the pure MgB 2 . The resistive measurements show that the addition of Au to MgB 2 results in a Tc decrease. The dependence of the crystal structure and the superconducting transition temperature on the doping concentration is discussed.

Superconducting HgBa2CaCu2Oy thin films growth on NdGaO3, SrTiO3, LaAlO3 and Y–ZrO2 substrates

Abstract Superconducting HgBa 2 CaCu 2 O y (Hg-1212) films have been fabricated on (110) NdGaO 3 , (100) SrTiO 3 , (100) LaAlO 3 and (100) Y–ZrO 2 substrates by a two-step process. Good reproducible superconducting properties, zero resistance temperatures ( T c )>115 K and critical current density ( J c )>4000 MA/m 2 at 77 K in zero field, are obtained. X-ray diffraction patterns indicate that the films have an epitaxial structure with the c -axis perpendicular to the surface. Scanning electron microscopy measurements show a layered growth structure with square and octagonal grains on the Hg-1212 film surfaces. (110) NdGaO 3 is demonstrated to be a good substrate for growing the Hg-cuprate thin films. A lower superconducting transition temperature is observed in films deposited on Y–ZrO 2 substrates, which can be attributed to variations of the microstructure in the films.

Preparation of superconducting HgBa2CaCu2Ox films with a zero-resistance transition temperature of 121 K

Abstract Hg-1212 films were prepared by means of annealing of mercury-free precursor films with bulk Hg-1212 pellets in an evacuated quartz tube. The mercury free precursor films were deposited by laser ablation on (100) SrTiO 3 substrates. The annealed films exhibited the (00 n ) oriented X-ray diffraction pattern of Hg-1212. The superconducting transition temperature, T c ( ϱ = 0), was 121 K.

MgB2 thick film grown on stainless steel substrate with ductility

MgB2 thick film was synthesized on the stainless steel substrate by the technique of hybrid physical-chemical vapor deposition (HPCVD), using Mg ingot and B2H6 as the raw materials. The film thickness is about 10 μm. The scanning electron microscope (SEM) images reveal that it is consisting of highly dense MgB2 crystals with the size ranging from 0.2 to 3 μm. The superconducting transition occurs at 38 K (Tc, onset) and ends at 27 K (Tc, zero), giving the transition width of 11 K. The fabricated film exhibits high ductility and remains attached to the substrate after it was bent to a curvature of about 200 μm.

Preparation of superconducting HgBa2CaCu2Ox and Hg0.8Bi0.2Ba2CaCu2Ox films by means of annealing of mercury-free precursor films

Abstract HgBa2CaCu2Ox films and Hg0.8Bi0.2Ba2CaCu2Ox films were prepared by means of annealing of mercury-free precursor films with the Hg-1212 bulks in an evacuated quartz tube. The mercury-free precursor films were deposited by laser ablation on (100) SrTiO3 substrates. The annealed films exhibited the (00n) oriented X-ray diffraction pattern of the Hg-1212. The superconducting transition temperatures, Tc(ϱ=0), are 121 K and 100 K respectively.

Synthesis of Au-doped Mg1−xAuxB2 superconductors

The recent discovery of the MgB2 superconductor has renewed the great interest in the search for superconductivity in the inter-metallic diboride compounds. Here we report the synthesis of a series of Au-doped MgB2 by solid-state reactions. The obtained Mg1−xAuxB2 samples could be indexed by a hexagonal unit cell with a=4.612 A and c=8.525 A. The resistive measurements showed that the addition of Au to MgB2 results in a slightly lower superconducting transition temperature at Tc∼37 K than the pure binary MgB2, which is expected.

Growth of HgBa2CaCu2Ox thin films using seed-buffer layers of YBa2Cu3Oy

Abstract High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been fabricated on SrTiO3 (STO) substrates using YBa2Cu3Oy (YBCO) as a seed-buffer layer (SBL). X-Ray diffraction indicates that the Hg-1212 thin films have a c-axis epitaxial structure with (100)Hg-1212//(100)STO. The φ-scans and rocking curves demonstrate that the SBL results in a better orientation of the Hg-1212 films. Scanning electron microscopy (SEM) also shows that the film surfaces are more regular. The Hg-1212 thin films on YBCO SBL have a superconducting transition temperature Tc=115–124 K and a critical current density Jc>1×106 A/cm2 at 77 K in zero field.

Growth of HgBa2CaCu2Oy films with a buffer layer

Using YBa2 Cu3 O7 (YBCO) as a buffer layer high Tc superconducting HgBa2 CaCu2 Oy (Hg-1212) thin films with zero resistance temperature (Tc ) of 115-124 K and critical current density (Jc ) of 1 MA cm-2 at 77 K in zero magnetic field have been grown on (100)Y-ZrO2 substrates. The high Tc , Jc and good morphology indicate that YBCO is a good buffer for growing the Hg cuprate films. This novel buffer makes it possible to grow Hg cuprate films on many substrates, including some metallic tapes.