344 research outputs found
Generalised analytical method unravels framework-dependent kinetics of adsorption-induced structural transition in flexible metal–organic frameworks
ゲート型吸着剤はガス分子をどう取り込む? --サブ秒でのX線回折測定が動的過程を紐解く--. 京都大学プレスリリース. 2023-11-08.Flexible metal–organic frameworks (MOFs) exhibiting adsorption-induced structural transition can revolutionise adsorption separation processes, including CO₂ separation, which has become increasingly important in recent years. However, the kinetics of this structural transition remains poorly understood despite being crucial to process design. Here, the CO₂-induced gate opening of ELM-11 ([Cu(BF₄)₂(4, 4’-bipyridine)₂]n) is investigated by time-resolved in situ X-ray powder diffraction, and a theoretical kinetic model of this process is developed to gain atomistic insight into the transition dynamics. The thus-developed model consists of the differential pressure from the gate opening (indicating the ease of structural transition) and reaction model terms (indicating the transition propagation within the crystal). The reaction model of ELM-11 is an autocatalytic reaction with two pathways for CO₂ penetration of the framework. Moreover, gas adsorption analyses of two other flexible MOFs with different flexibilities indicate that the kinetics of the adsorption-induced structural transition is highly dependent on framework structure
Fast Gas‐Adsorption Kinetics in Supraparticle‐Based MOF Packings with Hierarchical Porosity
Metal–organic frameworks (MOFs) are microporous adsorbents for high-throughput gas separation. Such materials exhibit distinct adsorption characteristics owing to the flexibility of the crystal framework in a nanoparticle, which can be different from its bulk crystal. However, for practical applications, such particles need to be compacted into macroscopic pellets, creating mass-transport limitations. In this work, this problem is addressed by forming materials with structural hierarchy, using a supraparticle-based approach. Spherical supraparticles composed of nanosized MOF particles are fabricated by emulsion templating and they are used as the structural component forming a macroscopic material. Zeolitic imidazolate framework-8 (ZIF-8) particles are used as a model system and the gas-adsorption kinetics of the hierarchical material are compared with conventional pellets without structural hierarchy. It is demonstrated that a pellet packed with supraparticles exhibits a 30 times faster adsorption rate compared to an unstructured ZIF-8 powder pellet. These results underline the importance of controlling structural hierarchy to maximize the performance of existing materials. In the hierarchical MOFs, large macropores between the supraparticles, smaller macropores between individual ZIF-8 primary particles, and micropores inherent to the ZIF-8 framework collude to combine large surface area, defined adsorption sites, and efficient mass transport to enhance performance
The K2K SciBar Detector
A new near detector, SciBar, for the K2K long-baseline neutrino oscillation
expe riment was installed to improve the measurement of neutrino energy
spectrum and to study neutrino interactions in the energy region around 1 GeV.
SciBar is a 'fully active' tracking detector with fine segmentation consisting
of plastic scintillator bars. The detector was constructed in summer 2003 and
is taking data since October 2003. The basic design and initial performance is
presented.Comment: 7 pages, 4figures, Contributed to Proceedings of the 10th Vienna
Conference on Instrumentation, Vienna, February 16-21, 200
Search for two-neutrino double electron capture on Xe with the XMASS-I detector
Double electron capture is a rare nuclear decay process in which two orbital
electrons are captured simultaneously in the same nucleus. Measurement of its
two-neutrino mode would provide a new reference for the calculation of nuclear
matrix elements whereas observation of its neutrinoless mode would demonstrate
lepton number violation. A search for two-neutrino double electron capture on
Xe is performed using 165.9 days of data collected with the XMASS-I
liquid xenon detector. No significant excess above background was observed and
we set a lower limit on the half-life as years at 90%
confidence level. The obtained limit has ruled out parts of some theoretical
expectations. We obtain a lower limit on the Xe two-neutrino double
electron capture half-life of years at 90% confidence
level as well.Comment: 6 pages, 3 figures, accepted for publication in Physics Letters
Search for exotic neutrino-electron interactions using solar neutrinos in XMASS-I
We have searched for exotic neutrino-electron interactions that could be
produced by a neutrino millicharge, by a neutrino magnetic moment, or by dark
photons using solar neutrinos in the XMASS-I liquid xenon detector. We observed
no significant signals in 711 days of data. We obtain an upper limit for
neutrino millicharge of 5.410 at 90\% confidence level
assuming all three species of neutrino have common millicharge. We also set
flavor dependent limits assuming the respective neutrino flavor is the only one
carrying a millicharge, for , for , and for .
These limits are the most stringent yet obtained from direct measurements. We
also obtain an upper limit for the neutrino magnetic moment of
1.810 Bohr magnetons. In addition, we obtain upper limits for
the coupling constant of dark photons in the model of
1.310 if the dark photon mass is 1 MeV,
and 8.810 if it is 10 MeV
Direct dark matter search by annual modulation in XMASS-I
A search for dark matter was conducted by looking for an annual modulation
signal due to the Earth's rotation around the Sun using XMASS, a single phase
liquid xenon detector. The data used for this analysis was 359.2 live days
times 832 kg of exposure accumulated between November 2013 and March 2015. When
we assume Weakly Interacting Massive Particle (WIMP) dark matter elastically
scattering on the target nuclei, the exclusion upper limit of the WIMP-nucleon
cross section 4.310cm at 8 GeV/c was obtained and we
exclude almost all the DAMA/LIBRA allowed region in the 6 to 16 GeV/c range
at 10cm. The result of a simple modulation analysis, without
assuming any specific dark matter model but including electron/ events,
showed a slight negative amplitude. The -values obtained with two
independent analyses are 0.014 and 0.068 for null hypothesis, respectively. we
obtained 90\% C.L. upper bounds that can be used to test various models. This
is the first extensive annual modulation search probing this region with an
exposure comparable to DAMA/LIBRA.Comment: 5 pages, 4 figure
Resolving theta_{23} Degeneracy by Accelerator and Reactor Neutrino Oscillation Experiments
If the lepton mixing angle theta_{23} is not maximal, there arises a problem
of ambiguity in determining theta_{23} due to the existence of two degenerate
solutions, one in the first and the other in the second octant. We discuss an
experimental strategy for resolving the theta_{23} octant degeneracy by
combining reactor measurement of theta_{13} with accelerator nu_{mu}
disappearance and nu_{e} appearance experiments. The robustness of the
theta_{23} degeneracy and the difficulty in lifting it only by accelerator
experiments with conventional nu_{mu} (and nu_{mu}-bar) beam are demonstrated
by analytical and numerical treatments. Our method offers a way to overcome the
difficulty and can resolve the degeneracy between solutions sin^2 theta_{23} =
0.4 and sin^2 theta_{23} = 0.6 if sin^2 (2 theta_{13}) \gsim 0.05 at 95% CL by
assuming the T2K phase II experiment and a reactor measurement with an exposure
of 10 GW.kt.yr. The dependence of the resolving power of the octant degeneracy
on the systematic errors of reactor experiments is also examined.Comment: 23 pages, 9 figures, version to appear in PR
- …