Extensivité et intensivité (physique)

Résumé

Les grandeurs extensives et intensives sont des catégories de grandeurs physiques d'un système physique :

une propriété est « intensive » si sa valeur ne dépend pas de la taille du système (en particulier, si sa valeur est la même en tout point d'un système homogène) : par exemple, la température ou la pression ;
une propriété est « extensive » si elle est proportionnelle à une quantité caractéristique du système : par exemple, la masse ou le volume.

Si deux chevaux courent côte à côte et chacun à , à eux deux, ils font un ensemble allant aussi à (la vitesse est intensive) ; par contre, à eux deux, ils font un passage deux fois plus important qu'un cheval seul (débit, puissance et masse sont doublés : ce sont des grandeurs extensives). Le rapport entre deux propriétés extensives d'un même objet est une grandeur physique intensive. Ainsi, par exemple, le rapport entre la masse et le volume d'un objet est sa masse volumique moyenne, ce qui permet de mesurer la masse volumique in

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Magnetic structure of the topological semimetal Co3Sn2S2

Henrik Moodysson Rønnow, Jian Rui Soh, Ivica Zivkovic

Co3Sn2S2 has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here, we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co3Sn2S2 below TC = 177 K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal c axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged (k = 0) in-plane 120?? antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between -90 K and TC. A discontinuous change in bulk properties and neutron polarization observed at T = 125 K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co3Sn2S2 is a magnetic Weyl semimetal.

AMER PHYSICAL SOC2022

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Solvent-induced surface hydroxylation of a layered perovskite Sr3FeCoO7−δ for enhanced oxygen evolution catalysis

Jun Gu, Xile Hu, Fang Song, Xiang Xu, Kongliang Xu

Efficient oxygen evolution reaction (OER) catalysts made of earth abundant elements are essential for the cost-effective generation of solar fuels. Surface modification is a novel strategy to enhance the activity of OER catalysts, however, it is commonly done under harsh and energy intensive conditions. Herein, we present a facile approach of solvent treatment to hydroxylate the surface of a layered perovskite, Sr3FeCoO7−δ. The catalytic activity correlates with the degree of surface hydroxylation, which influences the absorption energy of intermediates on the surface. The optimized catalyst exhibits higher activity than the current benchmark perovskite OER catalysts. This work introduces a mild and convenient method for surface modification, which may be applicable for the improvement of other nanoscale electrocatalysts.

2018

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Seismic swarms are often interpreted to be driven by natural fluid pressurization in the Earth’s crust, when seismicity is observed to spread away from a common origin and follows approximately a square-root-of-time pattern of growth. On the other hand, a growing body of literature suggests that aseismic fault slip seems to be a frequent result of fluid injections and may trigger seismicity due to the stress transfer of quasi-statically propagating ruptures in critically stressed regions. Although in some conditions a nominal pore pressure perturbation front may evolve proportionally to the square root of time, much less is known about the temporal patterns of fluid-driven aseismic slip fronts. The latter hinders efforts to distinguish whether some seismic swarms are driven by aseismic slip episodes or not. In this contribution, we provide an extensive set of physics-based solutions that describes the evolution of fluid-driven aseismic slip fronts for a wide range of conditions in terms of in-situ stress state and fluid flow. Our solutions show that fluid-driven aseismic slip fronts may result in many different patterns of propagation, depending on the characteristics of the fluid source (e.g., constant-pressure source, constant-rate source, among others) and also if simplified 2-D or fully 3-D elasticity is considered. Other parameters such as the initial stress state and fault hydraulic properties are also relevant in the propagation of the slip fronts. Our family of solutions includes cases in which aseismic slip fronts propagate following a square-root-of-time dependence, a linear expansion with time, power laws of time with exponents lower than ½, and some other more complex evolutions. These results are based on the model of a fluid-driven frictional shear crack that propagates on a planar fault interface characterized by a constant friction coefficient and a constant permeability, embedded in an infinite linearly elastic medium with an initially uniform state of stress. Although the basic assumptions of the model are simple, it results in a significant amount of complexity in terms of possible spatio-temporal patterns of rupture propagation. Since a constant friction coefficient corresponds to a fault interface with zero fracture energy, we show by analyzing the rupture-front energy balance of fluid-driven aseismic slip transients with non-zero fracture energy, that an asymptotic regime in which the fracture energy is negligible is always ultimately reached. This regime is approached asymptotically when the rupture has propagated over a distance larger than a characteristic length-scale depending on the frictional fracture energy and the in-situ stress state. We expect our results to provide a simple means to interpret observations of seismic swarms for which fluid-driven aseismic slip transients are thought to be a relevant mechanism in the triggering of seismicity.

EGU2022

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