C.H. Cheng

Improvement of critical current density in MgB2 superconductors by Zr doping at ambient pressure

We present the superconducting properties and phase compositions of Mg1−xZrxB2 bulk samples fabricated by a solid-state reaction at ambient pressure. It is found that a small amount of Zr atoms may be introduced into the lattice of MgB2, while the majority of them forms ZrB2 phase. The Mg0.9Zr0.1B2 sample shows the highest JC of 2.1×106 A/cm2 in 0.56 T at 5 K and 1.83×106 A/cm2 in self-field at 20 K, higher irreversibility field and larger upper critical field in MgB2 bulk samples. The combination of good grain connection, the reduction of grain size and small ZrB2 particles in the sample may be responsible for the significant enhancement of JC in Zr-doped samples. This technique has a great potential to prepare high performance MgB2 bulk samples and wires on an industrial scale.

Doping effect of nano-diamond on superconductivity and flux pinning in MgB2

The doping effect of diamond nanoparticles on the superconducting properties of MgB2 bulk material has been studied. It is found that the superconducting transition temperature Tc of MgB2 is suppressed by the diamond doping, however, the irreversibility field Hirr and the critical current density Jc are systematically enhanced. Microstructural analysis shows that the diamond-doped MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50–100 nm) with highly dispersed and uniformly distributed diamond nanoparticles (~10–20 nm) inside the grains. The high density of dislocations and diamond nanoparticles may be responsible for the enhanced flux pinning in the diamond-doped MgB2.

Enhancement of critical current density of MgB2 by doping Ho2O3

Mg1−x(Ho2O3)xB2 alloys were prepared by in situ solid state reaction to study the effect of magnetic Ho2O3 dopant on flux pinning behavior of MgB2. Crystal structure, Tc, and Hc2 were not affected by Ho2O3 doping; however, Jc and Hirr were significantly enhanced. In 5T field, the best sample (x=3%) reached Jc of 1.0×103, 2.0×104, and 1.2×105A∕cm2 at 20, 10, and 5K, respectively, much higher than those achieved by nonmagnetic impurity, such as Ti-, Zr-, and Y2O3-doped MgB2. The observed magnetic HoB4 nanoparticles were attributed to be the source for the enhanced flux pinning effects.

Comparison between nano-diamond and carbon nanotube doping effects on critical current density and flux pinning in MgB2

Doping effects of nano-diamond and carbon nanotubes (CNTs) on critical current density of bulk MgB2 have been studied. CNTs are found prone to be doped into the MgB2 lattice whereas nano-diamond tends to form second-phase inclusions in the MgB2 matrix, leading to a more significant improvement of Jc(H) by doping by nano-diamond than by CNTs in MgB2. TEM reveals tightly packed MgB2 nanograins (50–100 nm) with a dense distribution of diamond nanoparticles (10–20 nm) inside MgB2 grains in nano-diamond-doped samples. Such a unique microstructure leads to a flux pinning behaviour different from that in CNTs-doped MgB2.

Enhanced flux pinning in Zr-doped MgB2 bulk superconductors prepared at ambient pressure

We prepared a series of Mg1−xZrxB2 bulk samples with different x values by the solid-state reaction at ambient pressure and investigated the effects of the Zr doping on phase compositions, microstructure, and flux pinning behavior. The results indicate that the a axis increases with the increase of the Zr doping and the c axis remains unchanged. ZrB2 phases form and a small amount of Zr atoms may substitute Mg in Zr-doped samples. Critical temperature drops with increasing the Zr doping. It is found that critical current density Jc can be significantly enhanced by the Zr doping and the best data are achieved in Mg0.9Zr0.1B2. For this sample, Jc values are remarkably improved to 1.83×106 A/cm2 in the self field and 5.51×105 A/cm2 in 1 T at 20 K. Also, the irreversibility field is increased by the Zr doping. The high density, fine grain size of MgB2, and small ZrB2 particles in Zr-doped samples may be responsible for the enhancement of Jc. Also, the field dependence of Jc at different temperatures was brief...

Doping effects of carbon and titanium on the critical current density of MgB2

MgB2 bulks doped with Ti or/and C were prepared by an in?situ solid state reaction method to determine the combined effect of C and Ti doping and to probe the detailed mechanism. The magnetization measurement shows that Mg0.95Ti0.05B1.95C0.05 sample has significantly improved flux pinning compared to the MgB1.95C0.05 sample at 20?K, indicating that C and Ti are largely cooperative in improving the Jc(H) behaviour. No TiC phase was detected in the x-ray diffraction (XRD) patterns. Moreover, the overlap of the (100) peaks of MgB1.95C0.05 and Mg0.95Ti0.05B1.95C0.05 showed that Ti doping does not reduce the amount of C in MgB2. Microstructural analyses revealed that the addition of Ti eliminated the porosity present in the carbon-doped MgB2 pellet, resulting in an improved intergrain connectivity and an increase of effective current pass. Further, MgB2 doped with C and Ti, which mainly consists of spherical grains about 200?300?nm in size, shows an higher grain homogeneity than the C-doped sample, suggesting that the Ti doping in MgB1?xCx has played an important role in obtaining uniform grains.

Magnetic properties and critical current density of bulk MgB2 polycrystalline with Bi-2212 addition

Bulk samples of MgB2 were prepared with 0, 3, 5, and 10?wt% Bi2Sr2CaCu2O8?(Bi-2212) particles, added using a simple solid-state reaction route in order to investigate the effect of inclusions of a material with higher Tc than the superconducting matrix. The density, diamagnetic signal, and critical current density, Jc, of the samples change significantly with the doping level. It is found that Jc is significantly enhanced by the Bi-2212 addition. Microstructural analysis indicates that a small amount of Bi-2212 is decomposed into Cu2O and other impurity phases while a significant amount of unreacted Bi-2212 particles remains in MgB2 matrix, and these act as effective pinning centres for vortices. The enhanced pinning force is mainly attributable to these highly dispersed inclusions inserted in the MgB2 grains. Despite the effectiveness of the high-Tc inclusions in increasing superconducting critical currents in our experiment, our results seem to demonstrate the superiority of attractive centres over repulsive ones. A pinning mechanism is proposed to account for the contribution of this type of pinning centre in MgB2 superconductors.

Preparation, structure and superconductivity of Mg1-xAgxB2

Mg 1-x Ag x B 2 with 0 ≤ x ≤ 10% has been synthesized by the solid-state reaction. The solid solubility limit of Ag at Mg site is around x = 0.5%, and the dissolving process of Ag in MgB 2 depends on the starting material. The unit cell and Tc of the Mg 1-x Ag x B 2 steadily decrease with doping level. The underlying mechanism of superconductivity suppression by silver doping can be attributed to the chemical pressure effect.

A study of the Fe-based superconductor SmFeAsO1−xFx by x-ray photoelectron spectroscopy

The electronic structure of the Fe-based superconductor SmFeAsO1?xFx (x = 0 and 0.2) is studied with x-ray photoemission spectroscopy (XPS) through comparing the band structures of the nonsuperconducting parent material SmFeAsO and the superconducting SmFeAsO0.8F0.2 (Tc = 52.5?K). A small peak centered at 0.2?eV below the Fermi level, EF, in the valence band is observed in the parent material SmFeAsO, which is due to the low-spin state of the Fe?3d electrons. With fluorine doping, the peak at 0.2?eV disappears and a broad plateau forms near the Fermi level; in the meantime, the density of states at EF is slightly suppressed. The O?1s and Sm?3d core levels shift towards high energy by ~0.55?eV with fluorine doping, but in a sharp comparison, the Fe?2p and As?3d core levels do not shift significantly (the binding energy shift is less than 0.01?eV). It is deduced from the core-level shifts that the Fermi level of the system moves up by 0.55?eV by fluorine doping.

Enhancement of Jc by doping silver in grain boundaries of YBa2Cu3Oy polycrystals with solid-state diffusion method

Grain boundaries (GBs) of YBa2Cu3Oy (YBCO) are preferentially doped with Ag by the solid-state diffusion method. The distribution of Ag is highly localized in the GB. As a result of the Ag doping, the Jc-H behavior of YBCO textured polycrystalline samples is significantly improved, and at 60 K and 7 T, Jc is tripled. It is proposed that Ag could partially replace the Cu in CuO2 planes near GBs, therefore reducing the geometric distortion of Cu-O bonds and making the effective GBs thinner.