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1.
Nat Commun ; 12(1): 7170, 2021 Dec 09.
Artículo en Inglés | MEDLINE | ID: mdl-34887395

RESUMEN

Two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates. No such pores have been demonstrated experimentally. Here we study gas transport through individual graphene pores created by low intensity exposure to low kV electrons. Helium and hydrogen permeate easily through these pores whereas larger species such as xenon and methane are practically blocked. Permeating gases experience activation barriers that increase quadratically with molecules' kinetic diameter, and the effective diameter of the created pores is estimated as ∼2 angstroms, about one missing carbon ring. Our work reveals stringent conditions for achieving the long sought-after exponential selectivity using porous two-dimensional membranes and suggests limits on their possible performance.

2.
Nature ; 588(7837): 250-253, 2020 12.
Artículo en Inglés | MEDLINE | ID: mdl-33299189

RESUMEN

Capillary condensation of water is ubiquitous in nature and technology. It routinely occurs in granular and porous media, can strongly alter such properties as adhesion, lubrication, friction and corrosion, and is important in many processes used by microelectronics, pharmaceutical, food and other industries1-4. The century-old Kelvin equation5 is frequently used to describe condensation phenomena and has been shown to hold well for liquid menisci with diameters as small as several nanometres1-4,6-14. For even smaller capillaries that are involved in condensation under ambient humidity and so of particular practical interest, the Kelvin equation is expected to break down because the required confinement becomes comparable to the size of water molecules1-22. Here we use van der Waals assembly of two-dimensional crystals to create atomic-scale capillaries and study condensation within them. Our smallest capillaries are less than four ångströms in height and can accommodate just a monolayer of water. Surprisingly, even at this scale, we find that the macroscopic Kelvin equation using the characteristics of bulk water describes the condensation transition accurately in strongly hydrophilic (mica) capillaries and remains qualitatively valid for weakly hydrophilic (graphite) ones. We show that this agreement is fortuitous and can be attributed to elastic deformation of capillary walls23-25, which suppresses the giant oscillatory behaviour expected from the commensurability between the atomic-scale capillaries and water molecules20,21. Our work provides a basis for an improved understanding of capillary effects at the smallest scale possible, which is important in many realistic situations.

3.
Nature ; 579(7798): 229-232, 2020 03.
Artículo en Inglés | MEDLINE | ID: mdl-32161387

RESUMEN

Despite being only one-atom thick, defect-free graphene is considered to be completely impermeable to all gases and liquids1-10. This conclusion is based on theory3-8 and supported by experiments1,9,10 that could not detect gas permeation through micrometre-size membranes within a detection limit of 105 to 106 atoms per second. Here, using small monocrystalline containers tightly sealed with graphene, we show that defect-free graphene is impermeable with an accuracy of eight to nine orders of magnitude higher than in the previous experiments. We are capable of discerning (but did not observe) permeation of just a few helium atoms per hour, and this detection limit is also valid for all other gases tested (neon, nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen shows noticeable permeation, even though its molecule is larger than helium and should experience a higher energy barrier. This puzzling observation is attributed to a two-stage process that involves dissociation of molecular hydrogen at catalytically active graphene ripples, followed by adsorbed atoms flipping to the other side of the graphene sheet with a relatively low activation energy of about 1.0 electronvolt, a value close to that previously reported for proton transport11,12. Our work provides a key reference for the impermeability of two-dimensional materials and is important from a fundamental perspective and for their potential applications.

4.
Zhonghua Nei Ke Za Zhi ; 33(11): 770-2, 1994 Nov.
Artículo en Chino | MEDLINE | ID: mdl-7600869

RESUMEN

Right ventricular ejection fraction (RVEF), right ventricular peak filling rate (RVPFR) and right atrial early diastolic emptying rate (RAER) were measured with radionuclide gated blood pool scintigraphy in 19 healthy subjects and 15 cases of COPD with cor pulmonale and right heart catheterization was performed in the latter group. It was shown that RVEF in the group of cor pulmonale patients with pulmonary arterial hypertension (PAH) was significantly lower than that of the group of healthy subjects and cor pulmonale patients without PAH (P < 0.001, P < 0.001), no difference in RVEF was found between cor pulmonale patients without PAH and healthy subjects. As pulmonary arterial pressure increased, RAER and RVPFR decreased gradually, and the reduction of RAER and RVPFR occurred earlier than that of RVEF. It is suggested that right ventricular diastolic function may be impaired before right ventricular systolic function.


Asunto(s)
Enfermedades Pulmonares Obstructivas/fisiopatología , Enfermedad Cardiopulmonar/fisiopatología , Función Ventricular Derecha , Ventriculografía de Primer Paso , Adulto , Anciano , Femenino , Humanos , Hipertensión Pulmonar/complicaciones , Enfermedades Pulmonares Obstructivas/complicaciones , Enfermedades Pulmonares Obstructivas/diagnóstico por imagen , Masculino , Persona de Mediana Edad , Enfermedad Cardiopulmonar/complicaciones , Enfermedad Cardiopulmonar/diagnóstico por imagen , Volumen Sistólico
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