722 research outputs found
Hydrogen storage in pillared Li-dispersed boron carbide nanotubes
Ab initio density-functional theory study suggests that pillared Li-dispersed
boron carbide nanotubes is capable of storing hydrogen with a mass density
higher than 6.0 weight% and a volumetric density higher than 45 g/L. The boron
substitution in carbon nanotube greatly enhances the binding energy of Li atom
to the nanotube, and this binding energy (~ 2.7 eV) is greater than the
cohesive energy of lithium metal (~1.7 eV), preventing lithium from aggregation
(or segregation) at high lithium doping concentration. The adsorption energy of
hydrogen on the Li-dispersed boron carbide nanotube is in the range of 10 ~24
kJ/mol, suitable for reversible H2 adsorption/desorption at room temperature
and near ambient pressure.Comment: 17 pages, 4 figure
Is Summary Useful or Not? An Extrinsic Human Evaluation of Text Summaries on Downstream Tasks
Research on automated text summarization relies heavily on human and
automatic evaluation. While recent work on human evaluation mainly adopted
intrinsic evaluation methods, judging the generic quality of text summaries,
e.g. informativeness and coherence, our work focuses on evaluating the
usefulness of text summaries with extrinsic methods. We carefully design three
different downstream tasks for extrinsic human evaluation of summaries, i.e.,
question answering, text classification and text similarity assessment. We
carry out experiments using system rankings and user behavior data to evaluate
the performance of different summarization models. We find summaries are
particularly useful in tasks that rely on an overall judgment of the text,
while being less effective for question answering tasks. The results show that
summaries generated by fine-tuned models lead to higher consistency in
usefulness across all three tasks, as rankings of fine-tuned summarization
systems are close across downstream tasks according to the proposed extrinsic
metrics. Summaries generated by models in the zero-shot setting, however, are
found to be biased towards the text classification and similarity assessment
tasks, due to its general and less detailed summary style. We further evaluate
the correlation of 14 intrinsic automatic metrics with human criteria and show
that intrinsic automatic metrics perform well in evaluating the usefulness of
summaries in the question-answering task, but are less effective in the other
two tasks. This highlights the limitations of relying solely on intrinsic
automatic metrics in evaluating the performance and usefulness of summaries
Adsorption of hydrogen molecules on the platinum-doped boron nitride nanotubes
Adsorption of hydrogen molecules on platinum-doped single-walled zigzag (8,0) boron nitride (BN) nanotube is investigated using the density-functional theory. The Pt atom tends to occupy the axial bridge site of the BN tube with the highest binding energy of −0.91 eV. Upon Pt doping, several occupied and unoccupied impurity states are induced, which reduces the band gap of the pristine BN nanotube. Upon hydrogen adsorption on Pt-doped BN nanotube, the first hydrogen molecule can be chemically adsorbed on the Pt-doped BN nanotube without crossing any energy barrier, whereas the second hydrogen molecule has to overcome a small energy barrier of 0.019 eV. At least up to two hydrogen molecules can be chemically adsorbed on a single Pt atom supported by the BN nanotube, with the average adsorption energy of −0.365 eV. Upon hydrogen adsorption on a Pt-dimer-doped BN nanotube, the formation of the Pt dimer not only weakens the interaction between the Pt cluster and the BN nanotube but also reduces the average adsorption energy of hydrogen molecules. These calculation results can be useful in the assessment of metal-doped BN nanotubes as potential hydrogen storage media
A New Two-Dimensional Functional Material with Desirable Bandgap and Ultrahigh Carrier Mobility
Two-dimensional (2D) semiconductors with direct and modest bandgap and
ultrahigh carrier mobility are highly desired functional materials for
nanoelectronic applications. Herein, we predict that monolayer CaP3 is a new 2D
functional material that possesses not only a direct bandgap of 1.15 eV (based
on HSE06 computation), and also a very high electron mobility up to 19930 cm2
V-1 s-1, comparable to that of monolayer phosphorene. More remarkably, contrary
to the bilayer phosphorene which possesses dramatically reduced carrier
mobility compared to its monolayer counterpart, CaP3 bilayer possesses even
higher electron mobility (22380 cm2 V-1 s-1) than its monolayer counterpart.
The bandgap of 2D CaP3 can be tuned over a wide range from 1.15 to 0.37 eV
(HSE06 values) through controlling the number of stacked CaP3 layers. Besides
novel electronic properties, 2D CaP3 also exhibits optical absorption over the
entire visible-light range. The combined novel electronic, charge mobility, and
optical properties render 2D CaP3 an exciting functional material for future
nanoelectronic and optoelectronic applications
Adsorption of hydrogen molecules on the platinum-doped boron nitride nanotubes
Adsorption of hydrogen molecules on platinum-doped single-walled zigzag (8,0) boron nitride (BN) nanotube is investigated using the density-functional theory. The Pt atom tends to occupy the axial bridge site of the BN tube with the highest binding energy of −0.91 eV. Upon Pt doping, several occupied and unoccupied impurity states are induced, which reduces the band gap of the pristine BN nanotube. Upon hydrogen adsorption on Pt-doped BN nanotube, the first hydrogen molecule can be chemically adsorbed on the Pt-doped BN nanotube without crossing any energy barrier, whereas the second hydrogen molecule has to overcome a small energy barrier of 0.019 eV. At least up to two hydrogen molecules can be chemically adsorbed on a single Pt atom supported by the BN nanotube, with the average adsorption energy of −0.365 eV. Upon hydrogen adsorption on a Pt-dimer-doped BN nanotube, the formation of the Pt dimer not only weakens the interaction between the Pt cluster and the BN nanotube but also reduces the average adsorption energy of hydrogen molecules. These calculation results can be useful in the assessment of metal-doped BN nanotubes as potential hydrogen storage media
Designs of fullerene-based frameworks for hydrogen storage
Two types of hybrid metallofullerene framework are theoretically designed, and their structural stabilities are examined using the density functional theory (DFT) computation. Both frameworks are constructed by connecting exohedral metallofullerene nodes with conjugated organic linkers, akin to the common metal–organic framework (MOF). The DFT calculations suggest that hydrogen molecules can be adsorbed in the frameworks with the hydrogen binding energies ranging from 0.15–0.50 eV, satisfying the optimal adsorption condition for hydrogen storage. Moreover, our computation suggests that the frameworks can entail molecular H2 binding in the range of 8.0–9.2 wt%, meeting the Department of Energy (DOE) target of 2010 or 2015
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