2,371 research outputs found
Improved H_2 Storage in Zeolitic Imidazolate Frameworks Using Li^+, Na^+, and K^+ Dopants, with an Emphasis on Delivery H_2 Uptake
We use grand canonical Monte Carlo simulations with first principles based force fields to show that alkali metal (Li^+, Na^+, and K^+)-doped zeolitic imidazolate frameworks (ZIFs) lead to significant improvement of H_2 uptake at room temperature. For example, at 298 K and 100 bar, Li-ZIF-70 totally binds to 3.08 wt % H_2, Na-ZIF-70 to 2.19 wt % H_2, and K-ZIF-70 to 1.62 wt % H_2, much higher than 0.74 wt % H_2 for pristine ZIF-70. Thus, the dopant effect follows the order of Li-ZIF > Na-ZIF > K-ZIF, which correlates with the H_2 binding energies to the dopants. Moreover, the total H_2 uptake is higher at lower temperatures: 243 K > 273 K > 298 K. On the other hand, delivery H_2 uptake, which is the difference between the total adsorption at the charging pressure (say 100 bar) and the discharging pressure (say 5 bar), is the important factor for practical on-board hydrogen storage in vehicles. We show that delivery H_2 uptake leads to Na-ZIF-70 (1.37 wt %) > K-ZIF-70 (1.25 wt %) > Li-ZIF-70 (1.07 wt %) > ZIF-70 (0.68 wt %), which is different from the trend from the total and excess uptake. Moreover, the delivery uptake increases with increasing temperatures (i.e., 298 K > 273 K > 243 K)! To achieve high delivery H_2 uptake at room temperature, the large free volume of ZIFs is required. We find that higher H_2 binding energy needs not always lead to higher delivery H_2 uptake
Zeolitic Imidazolate Frameworks as H_2 Adsorbents: Ab Initio Based Grand Canonical Monte Carlo Simulation
We report the H_2 uptake behavior of 10 zeolitic−imidazolate frameworks (ZIFs), based on grand canonical Monte Carlo (GCMC) simulations. The force fields (FFs) describing the interactions between H_2 and ZIF in the GCMC were based on ab initio quantum mechanical (QM) calculations (MP2) aimed at correctly describing London dispersion (van der Waals attraction). Thus these predictions of H_2 uptake are based on first principles (non empirical) and hence applicable to new framework materials for which there is no empirical data. For each of these 10 ZIFs we report the total and excess H_2 adsorption isotherms up to 100 bar at both 77 and 300 K. We report the hydrogen adsorption sites in the ZIFs and the relationships between H_2 uptake amount, isosteric heat of adsorption (Q_(st)), surface area, and free volume. Our simulation shows that various ZIFs lead to a variety of H_2 adsorption behaviors in contrast to the metal−organic frameworks (MOFs). This is because ZIFs leads to greater diversity in the adsorption sites (depending on both organic linkers and zeolite topologies) than in MOFs. In particular, the ZIFs uptake larger amounts of H_2 at low pressure because of the high H_2 adsorption energy, and ZIFs have a variety of H_2 adsorption sites. For example, ZIF-11 has an initial Q_(st) value of ~15 kJ/mol, which is higher than observed for MOFs. Moreover, the preferential H_2 adsorption site in ZIFs is onto the organic linker, not nearby the metallic joint as is the case for MOFs
Planting and harvesting innovation - an analysis of Samsung Electronics
This study explores how firms manage the entire life cycle of innovation projects based on the framework of harvesting and planting innovation. While harvesting innovation seeks new products in the expectation of financial performance in the short term, planting innovation pursues creating value over a long time period. Without proper management of the process of planting and harvesting innovation, firms with limited resources may not be successful in launching innovative new products to seize a momentum in high tech industries. To examine this issue, the case of Samsung Electronics (SE), now an electronics giant originated from a former developing country, is analyzed. SE has shown to effectively utilize co-innovation to maintain numerous planting and harvesting innovation projects. Both researchers and practitioners would be interested in learning about how SE shared risks of innovation investment with external partners at the early stage of innovation cycles
Revisiting small-world network models: Exploring technical realizations and the equivalence of the Newman-Watts and Harary models
We address the relatively less known facts on the equivalence and technical
realizations surrounding two network models showing the "small-world" property,
namely the Newman-Watts and the Harary models. We provide the most accurate (in
terms of faithfulness to the original literature) versions of these models to
clarify the deviation from them existing in their variants adopted in one of
the most popular network analysis packages. The difference in technical
realizations of those models could be conceived as minor details, but we
discover significantly notable changes caused by the possibly inadvertent
modification. For the Harary model, the stochasticity in the original
formulation allows a much wider range of the clustering coefficient and the
average shortest path length. For the Newman-Watts model, due to the
drastically different degree distributions, the clustering coefficient can also
be affected, which is verified by our higher-order analytic derivation. During
the process, we discover the equivalence of the Newman-Watts (better known in
the network science or physics community) and the Harary (better known in the
graph theory or mathematics community) models under a specific condition of
restricted parity in variables, which would bridge the two relatively
independently developed models in different fields. Our result highlights the
importance of each detailed step in constructing network models and the
possibility of deeply related models, even if they might initially appear
distinct in terms of the time period or the academic disciplines from which
they emerged.Comment: 11 pages, 5 figures, 1 table, to appear in J. Korean Phys. So
Circuit Structure and Control Method to Reduce Size and Harmonic Distortion of Interleaved Dual Buck Inverter
A new circuit structure and control method for a high power interleaved dual-buck inverter are proposed. The proposed inverter consists of six switches, four diodes and two inductors, uses a dual-buck structure to eliminate zero-cross distortion, and operates in an interleaved mode to reduce the current stress of switch. To reduce the total harmonic distortion at low output power, the inverter is controlled using discontinuous-current-mode control combined with continuous-current-mode control. The experimental inverter had a power-conversion efficiency of 98.5% at output power = 1300 W and 98.3% at output power = 2 kW, when the inverter was operated at an input voltage of 400 V-DC, output voltage of 220 V-AC/60 Hz, and switching frequency of 20 kHz. The total harmonic distortion was < 0.66%, which demonstrates that the inverter is suitable for high-power dc-ac power conversion.11Ysciescopu
Dynamical mean-field theory of Hubbard-Holstein model at half-filling: Zero temperature metal-insulator and insulator-insulator transitions
We study the Hubbard-Holstein model, which includes both the
electron-electron and electron-phonon interactions characterized by and
, respectively, employing the dynamical mean-field theory combined with
Wilson's numerical renormalization group technique. A zero temperature phase
diagram of metal-insulator and insulator-insulator transitions at half-filling
is mapped out which exhibits the interplay between and . As () is
increased, a metal to Mott-Hubbard insulator (bipolaron insulator) transition
occurs, and the two insulating states are distinct and can not be adiabatically
connected. The nature of and transitions between the three states are
discussed.Comment: 5 pages, 4 figures. Submitted to Physical Review Letter
HierSpeech++: Bridging the Gap between Semantic and Acoustic Representation of Speech by Hierarchical Variational Inference for Zero-shot Speech Synthesis
Large language models (LLM)-based speech synthesis has been widely adopted in
zero-shot speech synthesis. However, they require a large-scale data and
possess the same limitations as previous autoregressive speech models,
including slow inference speed and lack of robustness. This paper proposes
HierSpeech++, a fast and strong zero-shot speech synthesizer for text-to-speech
(TTS) and voice conversion (VC). We verified that hierarchical speech synthesis
frameworks could significantly improve the robustness and expressiveness of the
synthetic speech. Furthermore, we significantly improve the naturalness and
speaker similarity of synthetic speech even in zero-shot speech synthesis
scenarios. For text-to-speech, we adopt the text-to-vec framework, which
generates a self-supervised speech representation and an F0 representation
based on text representations and prosody prompts. Then, HierSpeech++ generates
speech from the generated vector, F0, and voice prompt. We further introduce a
high-efficient speech super-resolution framework from 16 kHz to 48 kHz. The
experimental results demonstrated that the hierarchical variational autoencoder
could be a strong zero-shot speech synthesizer given that it outperforms
LLM-based and diffusion-based models. Moreover, we achieved the first
human-level quality zero-shot speech synthesis. Audio samples and source code
are available at https://github.com/sh-lee-prml/HierSpeechpp.Comment: 16 pages, 9 figures, 12 table
IMPACT OF D-DIMER LEVEL ON MYOCARDIAL INJURY IN PATIENTS WITH ST-SEGMENT ELEVATION MYOCARDIAL INFARCTION: A MAGNETIC RESONANCE IMAGING STUDY
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