No abstract

Metal halide perovskite nanocrystals (NCs) have emerged as new-generation light-emitting materials with narrow emissions and high photoluminescence quantum efficiencies (PLQEs). Various types of perovskite NCs, e.g., platelets, wires, and cubes, have been discovered to exhibit tunable emissions across the whole visible spectrum. Despite remarkable advances in the field of perovskite NCs, many nanostructures in inorganic NCs have not yet been realized in metal halide perovskites, and producing highly efficient blue-emitting perovskite NCs remains challenging and of great interest. Here, we report the discovery of highly efficient blue-emitting cesium lead bromide (CsPbBr3) perovskite hollow NCs. By facile solution processing of CsPbBr3 precursor solution containing ethylenediammonium bromide and sodium bromide, in situ formation of hollow CsPbBr3 NCs with controlled particle and pore sizes is realized. Synthetic control of hollow nanostructures with quantum confinement effect results in color tuning of CsPbBr3 NCs from green to blue, with high PLQEs of up to 81%.

Here we show that addition of Hf to Nb4Ta can significantly improve the high field performance of Nb3Sn, making it suitable for dipole magnets for a machine like the 100 TeV future circular collider (FCC). A big challenge of the FCC is that the desired non-Cu critical current density (Jc) target of 1500 A/mm 2 (16 T, 4.2 K) is substantially above the best present Nb3Sn conductors doped with Ti or Ta (~1300 A/mm 2 in the very best sample of the very best commercial wire). Recent success with internal oxidation of Nb-Zr precursor has shown significant improvement in the layer Jc of Nb3Sn wires, albeit with the complication of providing an internal oxygen diffusion pathway and avoiding degradation of the irreversibility field HIrr. We here extend the Nb1Zr oxidation approach by comparing Zr and Hf additions to the standard Nb4Ta alloy of maximum Hc2 and Hirr. Nb4Ta rods with 1Zr or 1Hf were made into monofilament wires with and without SnO2 and their properties measured over the entire superconducting range at fields up to 31 T. We found that group IV alloying of Nb4Ta does raise HIrr, though adding O2 still degrades this slightly. As noted in earlier Nb1Zr work with an O source, the pinning force density Fp is strongly enhanced and its peak value shifted to higher field by internal oxidation. A surprising result of this work is that we found better properties in Nb4Ta1Hf without SnO2, FpMax achieving 2.35 Times that of the standard Nb4Ta alloy, while the oxidized Nb4Ta1Zr alloy achieved 1.54 times that of the Nb4Ta alloy. The highest layer Jc(16 T, 4.2 K) of 3700 A/mm 2 was found in the SnO2free wire made with Nb4Ta1Hf alloy. Using a standard A15 cross-section fraction of 60% for modern PIT and RRP wires, we estimated that a non-Cu Jc of 2200 A/mm 2 is obtainable in modern conductors, well above the 1500 A/mm 2 FCC specification. Moreover, since the best properties were obtained without SnO2, the Nb4Ta1Hf alloy appears to open a straightforward route to enhanced properties in Nb3Sn wires manufactured by virtually all the presently used commercial routes employed today.

No abstract

REBa 2 Cu 3 O y coated conductors have recently become viable for high field superconducting magnets and this use may be the principal present driver of coated conductor development. Driving the transport critical current density (J c ) as high as possible has become one of the principal goals of CC manufacturers but this can only be done by developing highly engineered nanostructures that may not be easy to control in quantity production of long lengths. Protection of high field (B) magnets operating in the temperature (T) of 4-20 K range is challenging and one key data set needed for accurate quench modeling is a wide-ranging J c (B, T) data set. At the National High Magnetic Field Laboratory (NHMFL), 12 km of REBCO tapes were purchased for the allsuperconducting 32 T user magnet that successfully reached field recently. They were characterized at 4.2 K with field orientation B perpendicular to tape and at 18°off the tape-plane axis. Of the tapes selected for 32 T, three were chosen for additional J c (B, T) characterization from 4.2 to 75 K in the B^tape orientation in fields from 1 to 15 T. Although all tape lengths were bought to the same advanced pinning specification, in fact there was substantial variation of more than 2 in the low temperature, high field J c . Here we probe the reasons for this variability in the context of measurements of the transport J c (B, T) dependence of 3 representative samples from this distribution with Ginzburg-Landau models of vortex pinning using a power law for J c (B) and an exponential temperature dependence for T<45 K and 3 T<B<15 T. A fourth tape from the 32 T magnet procurement with J c outside this range was then selected to test the validity of our modelling. Using this extensive data set, the correlation between J c (B, 4.2 K) and J c (B, T) enabled us to predict J c (B, T) for tapes procured for the 32 T magnet with an expected accuracy of 10% or less for T<40 K and B up to 15 T. Transmission electron microscopy made clear that the BaZrO 3 (BZO) size, volume fraction and density varied significantly across the range of conductors studied, suggesting that nano-structural control is difficult during coated conductor manufacture and that the resulting J c variations may have to be accepted in procurement practice.

The poor reproducibility of intergrain critical current density J c in Fe-based superconductors is often believed to result from uncontrolled grain boundary (GB) connectivity degraded by extrinsic factors such as the local or global impurity concentration or GB porosity or cracks. Earlier we found that Ba and K can appear as oxide impurities at GBs, along with GB-wetting FeAs. In this study, we evaluated how the sample preparation environment and purity of the starting materials influence the polycrystalline J c in K-doped BaFe 2 As 2 (Ba122) bulks. Using a high-performance glovebox, the oxygen and water levels were significantly reduced, eliminating traces of FeAs. Oxide impurities and Ba (or K) segregation associated with oxygen in the starting materials were significantly reduced by using high purity starting materials. This combination essentially doubled the best J c (4.2 K) values to 2.3 × 10 5 at self-field and 1.6 × 10 4 A cm −2 at 10 T and analytical scanning transmission electron microscopy showed no GB or O segregation in the best samples, but did show dark Z-contrast and distinct nanoscale porosity. Our work shows that an inert synthesis environment and high purity K and Ba do reduce current-blocking oxygen impurity and GB impurity phases, allowing deeper exploration of the role of extrinsic and intrinsic GB blocking effects in controlling the J c of polycrystalline Ba122.