6,167 research outputs found
Defective hierarchical porous copper-based metal-organic frameworks synthesised via facile acid etching strategy
Introducing hierarchical pore structure to microporous materials such as
metal-organic frameworks (MOFs) can be beneficial for reactions where the rate
of reaction is limited by low rates of diffusion or high pressure drop. This
advantageous pore structure can be obtained by defect formation, mostly via
post-synthetic acid etching, which has been studied extensively on water-stable
MOFs. Here we show that a water-unstable HKUST-1 MOF can also be modified in a
corresponding manner by using phosphoric acid as a size-selective etching agent
and a mixture of dimethyl sulfoxide and methanol as a dilute solvent.
Interestingly, we demonstrate that the etching process which is time- and
acidity- dependent, can result in formation of defective HKUST-1 with extra
interconnected hexagonal macropores without compromising on the bulk
crystallinity. These findings suggest an intelligent scalable synthetic method
for formation of hierarchical porosity in MOFs that are prone to hydrolysis,
for improved molecular accessibility and diffusion for catalysis.Comment: 14 pages, 8 figure
The physical and structural effects of 1-MCP on four different apple cultivars during storage
The impact of the ethylene inhibitor, 1-methylcyclopropene (1-MCP), on four apple cultivars (Braeburn, Fuji, Jazz and Golden Delicious) over 150 days of storage at 2 °C was assessed. Proton transfer reaction quadrupole mass spectrometry (PTR-QUAD-MS) was used to monitor changes in VOC composition, while texture analysis and X-ray microcomputer tomography (µ-CT) scanning were used to study microstructural changes. The application of 1-MCP on apples reduced VOC emissions, concurrently maintaining a firmer texture compared to the untreated apples at each time point. The µ-CT scanning revealed how changes in specific morphological characteristics such as anisotropy, connectivity and porosity, size and shape, as well as the interconnectivity of intracellular spaces (IS) influenced texture even when porosity was similar. Additionally, this study showed that the porosity and connectivity of IS were associated with VOC emission and increased simultaneously. This study highlights how the morphological parameters of an apple can help explain their ripening process during long-term storage and how their microstructure can influence the release of VOC
Dynamical invariants in non-Markovian quantum state diffusion equation
We find dynamical invariants for open quantum systems described by the
non-Markovian quantum state diffusion (QSD) equation. In stark contrast to
closed systems where the dynamical invariant can be identical to the system
density operator, these dynamical invariants no longer share the equation of
motion for the density operator. Moreover, the invariants obtained with from
bi-orthonormal basis can be used to render an exact solution to the QSD
equation and the corresponding non-Markovian dynamics without using master
equations or numerical simulations. Significantly we show that we can apply
these dynamic invariants to reverse-engineering a Hamiltonian that is capable
of driving the system to the target state, providing a novel way to design
control strategy for open quantum systems.Comment: 6 pages, 2 figure
The Structure on Invariant Measures of generic diffeomorphisms
Let be an isolated non-trival transitive set of a generic
diffeomorphism f\in\Diff(M). We show that the space of invariant measures
supported on coincides with the space of accumulation measures of
time averages on one orbit. Moreover, the set of points having this property is
residual in (which implies the set of irregular points is also
residual in ). As an application, we show that the non-uniform
hyperbolicity of irregular points in with totally 0 measure
(resp., the non-uniform hyperbolicity of a generic subset in )
determines the uniform hyperbolicity of
Solutions of the Ginsparg-Wilson Relation
We analyze general solutions of the Ginsparg-Wilson relation for lattice
Dirac operators and formulate a necessary condition for such operators to have
non-zero index in the topologically nontrivial background gauge fields.Comment: 6 pages, latex, no figures, set T to 1 in eqs. (10)--(13
Effect of mono- and divalent extra-framework cations on the structure and accessibility of porosity in chabazite zeolites
Chabazite (CHA), one of the most common zeolite framework types, has a remarkable capacity to accommodate a wide range of different cations within the unique CHA framework. This has led to CHA being applied extensively in ion exchange, and studied for highly selective gas sorption, most notably through a trapdoor mechanism. Here, we report the systematic study of a series of six chabazite zeolites (i.e.K-CHA, Cs-CHA, Ca-CHA, Ba-CHA, Sr-CHA and Zn-CHA) obtained by subjecting the parent chabazite (KNa-CHA) to exchange operations with cations of different valences and atomic radii. These samples were examined using numerous techniques and it was found that the differences in valence and size between extra-framework cations exert a significant effect on the abundance of these cations positioned in the framework, resulting in differing nitrogen sorption ability measured in the synthesised chabazite zeolites. These findings will help to understand how the zeolite counter-cation affects the ability of the CHA material to selectively sequester and separate gases through the use of the trapdoor mechanism.</p
Effect of mono- and divalent extra-framework cations on the structure and accessibility of porosity in chabazite zeolites
Sound absorption in Hilbert Fractal and Coiled Acoustic Metamaterials
We describe here a class of acoustic metamaterials with fractal Hilbert
space-filling and coiled geometry with equal tortuosity for noise mitigation.
Experiments are performed using a four-microphone impedance tube and
benchmarked against non-viscous and viscothermal Finite Element models related
to configurations spanning up to five fractal/geometry orders. We show that the
acoustic absorption can be predicted by the resonance of the cavities
associated to the tortuous paths. For a given fractal/geometry order, the
acoustic absorption at specific frequencies is also enhanced by maximising the
difference between the minimum and maximum fluid particle velocity of the air
inside the patterns. These principles can be used to design high-performance
acoustic metamaterials for sound absorption over broad frequency ranges.Comment: 4 pages, 3 figures, Submitted to Applied Physics Letter
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