H.K. Liu

High-transport critical current density above 30 K in pure Fe-clad MgB2 tape

Abstract Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg+2B mixture was used as the central conductor core and reacted in situ to form MgB 2 . The tapes were sintered in pure Ar at 800°C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 μm. The Fe-clad tape shows a sharp transition with transition width of ΔT c of 0.2 K and a T c0 of 37.5 K. We have achieved the highest transport critical current reported so far at 1.7×10 4 A/cm 2 for both 29.5 K in 1 T and 33 K in zero applied field. Resistivity temperature dependence and transport critical current were also measured in magnetic fields applied perpendicular and parallel to the tape plane. Not only is the use of an Fe sheath necessary for the successful processing of in situ reacted powder-in-tube MgB 2 , it confers on the finished wire the additional benefit of magnetic screening.

Innovative nanosize lithium storage alloys with silica as active centre

Abstract Two types of nanosize intermetallic alloy powders, NiSi and FeSi, are prepared by high-energy ball-milling. The alloys are used as electrode materials in lithium test cells. During lithium insertion into the alloy electrodes, Si acts as active centres, which react with Li to form Li x Si alloys. A high lithium storage capacity of 1180 mA h g −1 is observed for the NiSi electrode, with some reversibility. A mechanism for the reaction of NiSi and FeSi with Li + is proposed.

Large low-field magnetoresistance over a wide temperature range induced by weak-link grain boundaries in La0.7Ca0.3MnO3

Polycrystalline bulk porous samples with a large number of weak-link grain boundaries and high density polycrystalline bulk samples with strong-link boundaries were synthesized by conventional solid-state reaction and a partial melting technique. The weak-link samples showed large magnetoresistance (MR,Δρ/ρH=[ρ0−ρH]/ρ0), 20–30%, at a low magnetic field of 300 mT and over a wide temperature range from the magnetic transition at 235 to 77 K. The partially melted samples exhibited the same magnetoresistance behavior as that of single crystals with a maximum peak MR of 15% at a narrow temperature range around the ferromagnetic transition. It is suggested that grain boundaries are necessary but not sufficient to account for the MR at low field over a large temperature range. Weak-link grain boundaries rather than strong-link boundaries are responsible for the MR at low field over a large temperature range.

Spinel Li[Li1/3Ti5/3]O4 as an anode material for lithium ion batteries

Abstract The spinel Li[Li1/3Ti5/3]O4 compound was synthesised via a solid-state method and its electrochemical performance in lithium ion cells was examined. Lithium ions intercalate into and deintercalate from Li[Li1/3Ti5/3]O4 with high reversibility. The spinel Li[Li1/3Ti5/3]O4 demonstrated a very stable structural characteristic for lithium ion insertion and extraction without passivation on its surface. The spinel Li[Li1/3Ti5/3]O4 as an anode material was coupled with LiCoO2 and LiMn2O4 as cathodes to construct lithium ion cells. These cells provide 2.4–2.5 V operating voltage and without the safety concerns associated with using lithium metal or carbon anodes.

Very fast formation of superconducting MgB2/Fe wires with high Jc

Abstract In this paper, we have investigated the effects of sintering time and temperature on the formation and critical current densities of Fe-clad MgB 2 wires. MgB 2 wires were fabricated using the powder-in-tube process and sintered for different periods of time at predetermined temperatures. All the samples were examined using XRD, SEM, and magnetisation measurements. In contrast to the common practice of sintering for several hours, the present results show that there is no need for prolonged heat treatment in the fabrication of Fe-clad MgB 2 wires. A total time in the furnace of several minutes is more than enough to form nearly pure MgB 2 with high performance characteristics. The results from T c , J c and H irr show convincingly that the samples which were sintered for 3 min above 800°C are as good as those sintered for longer times. In fact, the J c field performance for the most rapidly sintered sample is slightly better than for all other samples. J c of 4.5×10 5 A/cm 2 in zero field and above 10 5 A/cm 2 in 2 T at 15 K has been achieved for the best Fe-clad MgB 2 wires. As a result of such a short sintering there is no need for using high purity argon protection and it is possible to carry out the heat treatment in a much less protective atmosphere or in air. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB 2 wires.

Substitution-induced pinning in MgB2 superconductor doped with SiC nano-particles

By doping MgB2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB2(SiC)x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.

Tungsten Disulfide Nanotubes for Lithium Storage

WS 2 nanotubes were synthesized by sintering amorphous WS 3 at high temperature under flowing hydrogen. High-resolution transmission electron microscopy observation revealed that the as-prepared WS 2 nanotubes have an open end with an inner hollow core of about 4.6 nm. We studied the lithium intercalation behavior of WS 2 nanotubes. The WS 2 nanotubes demonstrated a stable cyclability in a wide voltage range (0.1-3.1 V vs. Li/Li + ). The nanotubes could provide a new class of electrode materials for lithium-ion batteries.

Improvement of electrochemical properties of the spinel LiMn2O4 using a Cr dopant effect

Abstract A series of LiCr x Mn 2− x O 4 spinels were synthesised by the Pechini method which enables dopant Cr ions to distribute at Mn sites homogeneously. Neutron diffraction and EDS analysis confirmed that Cr ions do occupy 16d sites (octahedral intestial) evenly in the spinel structure. The Cr dopant effect improves the cyclability of spinel LiMn 2 O 4 electrodes and decreases the self-discharge rate substantially. Cyclic voltammetry and AC impedance spectroscopy were employed to characterise the reactions of lithium insertion into and extraction from LiCr x Mn 2− x O 4 electrodes. It was found that a thicker surface layer was formed on the surface of the pure LiMn 2 O 4 electrode than on the LiMn 2 O 4 electrode.

Formation mechanism of high-Tc and critical current in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tape

Abstract X-ray diffraction method was used to characterise the phase composition and investigate the formation mechanism of the (Bi,Pb)2Sr2Ca2Cu3O10 phase from the precursor with (Bi,Pb)2Sr2CaCu2O8 as the main phase. The reaction is found to be a two-dimensional nucleation (random)-growth type, −[ ln (1 − F)] 1 2 = kt , where F is the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 and t is the sintering time. The size of the critical current of the tape is quantitatively related to the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 to (Bi,Pb)2Sr2Ca2Cu3O10 phase. At low fraction regime of (Bi,Pb)2Sr2CaCu2O8, the critical current of a tape shows no clear dependence on the remaining Bi-2212 phase, other factors such as grain alignment, colony size, contact between colonies, and fine nonsuperconducting particles become important in controlling the Jc. Also, in this regime, the predominant weak links to be the colony boundaries rather than the Bi-2212 phase.

Secondary aqueous lithium-ion batteries with spinel anodes and cathodes

Abstract Secondary aqueous lithium-ion batteries with spinel Li 2 Mn 4 O 9 or Li 4 Mn 5 O 12 as the anode and LiMn 2 O 4 as the cathode are investigated. The aqueous electrolyte contains 6 M LiNO 3 and 0.0015 M OH − . The Li 2 Mn 4 O 9 /LiNO 3 /LiMn 2 O 4 and Li 4 Mn 5 O 12 /LiNO 3 /LiMn 2 O 4 aqueous cells deliver approximately 100 m Ah g −1 capacity at an average voltage of 1–1.1 V. This aqueous lithium-ion system eliminates safety concerns and offers considerably cost-effective technology for manufacturing.