Nanoscale-SiC doping for enhancing Jc and Hc2 in the Superconducting MgB2

The effect of nanoscale-SiC doping of MgB2 was investigated using transport and magnetic measurements. It was found that there is a clear correlation between the critical temperature Tc, the resistivity r, the residual resistivity ratio, RRR = R(300K)/R(40K), the irreversibility field H* and the alloying state in the samples. SiC-doping introduced many nano-scale precipitates, provoking an increase of r(40K) from 1 mW-cm (RRR = 15) for the clean limit sample to 300 mW-cm (RRR = 1.75) for the SiC-doped sample, leading to significant enhancement of Hc2 and H* with only minor effect on Tc. EELS analysis revealed a number of nano-scale impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, BC and unreacted SiC in the doped sample. TEM study showed an extensive domain structure of 2-4nm domains induced by SiC doping. The Jc for the 10% nano-SiC doped sample increased substantially at all fields and temperatures compared to the undoped samples, due to the strong increase in Hc2 and H* produced by SiC doping

Inversion of two-band superconductivity at the critical electron doping of (Mg,Al)B-2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)AlxB2. A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximate to 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T-c< 7 K up to about x approximate to 55. Only the pi band had detectable density of states for 0.33 less than or similar to x less than or similar to 0.55, implying an inversion of the two-band hierarchy of MgB2 in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T-c are discussed

Control of crystal size tailors the electrochemical performance of alpha-V2O5 as a Mg2+ intercalation host

α-V2O5 has been extensively explored as a Mg2+ intercalation host with potential as a battery cathode, offering high theoretical capacities and potentials vs. Mg2+/Mg. However, large voltage hysteresis is observed with Mg insertion and extraction, introducing significant and unacceptable round-trip energy losses with cycling. Conventional interpretations suggest that bulk ion transport of Mg2+ within the cathode particles is the major source of this hysteresis. Herein, we demonstrate that nanosizing α-V2O5 gives a measurable reduction to voltage hysteresis on the first cycle that substantially raises energy efficiency, indicating that mechanical formatting of the α-V2O5 particles contributes to hysteresis. However, no measurable improvement in hysteresis is found in the nanosized α-V2O5 in latter cycles despite the much shorter diffusion lengths, suggesting that other factors aside from Mg transport, such as Mg transfer between the electrolyte and electrode, contribute to this hysteresis. This observation is in sharp contrast to the conventional interpretation of Mg electrochemistry. Therefore, this study uncovers critical fundamental underpinning limiting factors in Mg battery electrochemistry, and constitutes a pivotal step towards a high-voltage, high-capacity electrode material suitable for Mg batteries with high energy density

Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb

High purity niobium (Nb), subjected to the processing methods used in the fabrication of superconducting RF cavities, displays micron-sized surface patches containing excess carbon. High-resolution transmission electron microscopy and electron energy-loss spectroscopy measurements are presented which reveal the presence of nanoscale NbC coherent precipitates in such regions. Raman backscatter spectroscopy on similar surface regions exhibit spectra consistent with the literature results on bulk NbC but with significantly enhanced two-phonon scattering. The unprecedented strength and sharpness of the two-phonon signal has prompted a theoretical analysis, using density functional theory (DFT), of phonon modes in NbC for two different interface models of the coherent precipitate. One model leads to overall compressive strain and a comparison to ab-initio calculations of phonon dispersion curves under uniform compression of the NbC shows that the measured two-phonon peaks are linked directly to phonon anomalies arising from strong electron-phonon interaction. Another model of the extended interface between Nb and NbC, studied by DFT, gives insight into the frequency shifts of the acoustic and optical mode density of states measured by first order Raman. The exact origin of the stronger two-phonon response is not known at present but it suggests the possibility of enhanced electron-phonon coupling in transition metal carbides under strain found either in the bulk NbC inclusions or at their interfaces with Nb metal. Preliminary tunneling studies using a point contact method show some energy gaps larger than expected for bulk NbC.Comment: Phys. Rev. B, accepte

Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V₄Nb₁₈O₅₅

While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities in comparison to Li-ion batteries if a high-voltage electrode material, such as a transition-metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg^{2+} insertion and extraction at high voltages; this is thought to be due to poor Mg^{2+} diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg^{2+} into the tetragonal tungsten bronze V_{4}Nb_{18}O_{55}, with a maximum reversible electrochemical capacity of 75 mA h g^{–1}, which corresponds to a magnesiated composition of Mg_{4}V_{4}Nb_{18}O_{55}. Experimental electrochemical magnesiation/demagnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg^{2+}); when magnesiation is limited to a composition of Mg_{2}V_{4}Nb_{18}O_{55}, this hysteresis can be reduced to only 0.5 V. Hybrid-exchange density functional theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg^{2+} intercalation involved concurrent V and Nb redox activity and changes in crystal structure, as confirmed by an array of complementary methods, including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, it can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries

Enhanced charge storage of nanometric ζ-V₂O₅ in Mg electrolytes

V2O5 is of interest as a Mg intercalation electrode material for Mg batteries, both in its thermodynamically stable layered polymorph (α-V2O5) and in its metastable tunnel structure (ζ-V2O5). However, such oxide cathodes typically display poor Mg insertion/removal kinetics, with large voltage hysteresis. Herein, we report the synthesis and evaluation of nanosized (ca. 100 nm) ζ-V2O5 in Mg-ion cells, which displays significantly enhanced electrochemical kinetics compared to microsized ζ-V2O5. This effect results in a significant boost in stable discharge capacity (130 mA h g−1) compared to bulk ζ-V2O5 (70 mA h g−1), with reduced voltage hysteresis (1.0 V compared to 1.4 V). This study reveals significant advancements in the use of ζ-V2O5 for Mg-based energy storage and yields a better understanding of the kinetic limiting factors for reversible magnesiation reactions into such phases

Electron doping in MgB2 studied by electron energy-loss spectroscopy

The electronic structure of electron-doped polycrystalline Mg1-xAlx(B1-yCy)(2) was examined by electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) and first-principle electronic structure calculations. We found significant changes in the boron K edge fine structure, suggesting the two bands of the B K edge, the sigma and the pi band, are being simultaneously filled as the electron doping concentration of Mg1-xAlx(B1-yCy)(2) was increased. Our density-functional theory calculations confirm the filling of the sigma band states close to the Fermi level, which is believed to cause the loss of superconductivity in highly doped MgB2, since the electron-phonon coupling of these states is thought to be responsible for the high superconducting transition temperature. Our results do not show significant differences in the electronic structure for electron doping on either the Mg or the B site, although many superconducting properties, such as T-c or H-c2 differ considerably for C- and Al-doped MgB2. This behavior cannot be satisfactorily explained by band filling alone, and effects such as interband scattering are considered