282 research outputs found
Aluminum Oxide Layers as Possible Components for Layered Tunnel Barriers
We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with
ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma
oxidation, before and after rapid thermal post-annealing of the completed
structures at temperatures up to 550 deg C. Post-annealing at temperatures
above 300 deg C results in a significant decrease of the tunneling conductance
of thermally-grown barriers, while plasma-grown barriers start to change only
at annealing temperatures above 450 deg C. Fitting the experimental I-V curves
of the junctions using the results of the microscopic theory of direct
tunneling shows that the annealing of thermally-grown oxides at temperatures
above 300 deg C results in a substantial increase of their average tunnel
barriers height, from ~1.8 eV to ~2.45 eV, versus the practically unchanged
height of ~2.0 eV for plasma-grown layers. This difference, together with high
endurance of annealed barriers under electric stress (breakdown field above 10
MV/cm) may enable all-AlOx and SiO2/AlOx layered "crested" barriers for
advanced floating-gate memory applications.Comment: 7 pages, 6 figure
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
Direct observation of nm-scale Mg- and B-oxide phases at grain boundaries in MgB2
Here we describe the results of an atomic resolution study of the structure
and composition of both the interior of the grains, and the grain boundaries in
polycrystalline MgB2. We find that there is no oxygen within the bulk of the
grains but significant oxygen enrichment at the grain boundaries. The majority
of grain boundaries contain BOx phases smaller than the coherence length, while
others contain larger areas of MgO sandwiched between BOx layers. Such results
naturally explain the differences in connectivity between the grains observed
by other techniques
Polymorphism in Ruddlesden-Popper : Discovery of a Hidden Phase with Distinctive Layer Stacking
We report the discovery of a novel form of Ruddlesden-Popper (RP) oxide,
which stands as the first example of long-range, coherent polymorphism in this
class of inorganic solids. Rather than the well-known, uniform stacking of
perovskite blocks ubiquitously found in RP phases, this newly discovered
polymorph of the bilayer RP phase adopts a novel stacking
sequence in which single and trilayer blocks of octahedra alternate
in a 1313 sequence. Crystals of this new polymorph are described in space group
Cmmm, although we note evidence for a competing Imcm variant. Transport
measurements at ambient pressure reveal metallic character with evidence of a
charge density wave transition with onset at T = 134 K, which lies intermediate
between that of the standard 2222 polymorph of (space group
Amam) and the trilayer RP phase, . The discovery of such
polymorphism could reverberate to the expansive range of science and
applications that rely on RP materials, particularly the recently reported
signatures of superconductivity with as high as 80 K above 14 GPa in
bilayer .Comment: 37 pages, 10 figure
Direct synthesis and chemical vapor deposition of 2D carbide and nitride MXenes
Two-dimensional (2D) transition metal carbides and nitrides (MXenes) are a
large family of materials actively studied for various applications, especially
in the field of energy storage. MXenes are commonly synthesized by etching the
layered ternary compounds, MAX phases. We demonstrate a direct synthetic route
for scalable and atom-economic synthesis of MXenes, including phases that have
not been synthesized from MAX phases, by the reactions of metals and metal
halides with graphite, methane, or nitrogen. The direct synthesis enables
chemical vapor deposition (CVD) growth of MXene carpets and complex
spherulite-like morphologies that form through buckling and release of MXene
carpet to expose fresh surface for further reaction. The directly synthesized
MXenes showed excellent energy storage capacity for Li-ion intercalation.Comment: 9 pages, 4 figure
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
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
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