Structural Complexity and Phonon Physics in 2D Arsenenes

In the quest for stable 2D arsenic phases, four different structures have been recently claimed to be stable. We show that, due to phonon contributions, the relative stability of those structures differs from previous reports and depends crucially on temperature. We also show that one of those four phases is in fact mechanically unstable. Furthermore, our results challenge the common assumption of an inverse correlation between structural complexity and thermal conductivity. Instead, a richer picture emerges from our results, showing how harmonic interactions, anharmonicity and symmetries all play a role in modulating thermal conduction in arsenenes. More generally, our conclusions highlight how vibrational properties are an essential element to be carefully taken into account in theoretical searches for new 2D materials

In Situ Dry Matter Ruminal Degradation Kinetics of Prairie Grass and Annual Ryegrass Forage

The objective of this study was to evaluate dry matter ruminal degradation kinetics, of prairie grass (Bromus catharticus Vahl.) and annual ryegrass (Lolium multiflorum), by using in sacco technique, on seven different harvesting dates. The forage was incubated in nylon bags, suspended in the rumen of four Hereford steers with a rumen fistula. The dry matter ruminal degradation was calculated through the exponential model P=a+b(1-exp(-ct)). Potential degradabilities (PD) were high and reduced across a temporal trend, ranging between 92,31 and 81,79g 100g -1 of dry matter (DM) for prairie grass (PG) and 95,06 and 77,44g 100g -1 DM for annual ryegrass (AR). Effective degradabilities (ED) had the same tendency as PD, ranging between 75,85 and 53,80g 100g -1 DM for PG and 79,68 and 53,60g 100g -1 DM for AR. For both parameters and most of the harvesting dates, AR was superior to PG. Except on two harvesting dates, both species presented similar degradation rates, ranging between 16,30% h-1 and 5,35% h-1. It was concluded that, during its main period of utilization, AR showed more ED and PD than PG, though, except on October, differences were of scarce magnitude. However, PD and ED decay across a temporal trend were more pronounced in RG. On the last harvest dates PG tended to stabilize its degradabilities

Physically founded phonon dispersions of few-layer materials, and the case of borophene

An increasing number of theoretical calculations on few-layer materials have been reporting a non-zero sound velocity for all three acoustic phonon modes. In contrast with these reports, here we show that the lowest phonon dispersion branch of atomistically described few-layer materials should be quadratic, and this can have dramatic consequencies on calculated properties, such as the thermal conductivity. By reformulating the interatomic force constants (IFC) in terms of internal coordinates, we find that a delicate balance between the IFCs is responsible for this quadraticity. This balance is hard to obtain in ab-initio calculations even if all the symmetries are numerically enforced a posteriori, but it arises naturally in our approach. We demonstrate the phenomenon in the case of borophene, where a very subtle correction to the ab-initio IFCs yields the physically correct quadratic dispersion, while leaving the rest of the spectrum virtually unmodified. Such quadraticity nevertheless has a major effect on the computed lattice thermal conductivity, which in the case of borophene changes by more than a factor 2, and reverses its anisotropy, when the subtle IFC correction is put in place

Reentrant phase behaviour for systems with competition between phase separation and self-assembly

In patchy particle systems where there is competition between the self-assembly of finite clusters and liquid-vapour phase separation, reentrant phase behaviour is observed, with the system passing from a monomeric vapour phase to a region of liquid-vapour phase coexistence and then to a vapour phase of clusters as the temperature is decreased at constant density. Here, we present a classical statistical mechanical approach to the determination of the complete phase diagram of such a system. We model the system as a van der Waals fluid, but one where the monomers can assemble into monodisperse clusters that have no attractive interactions with any of the other species. The resulting phase diagrams show a clear region of reentrance. However, for the most physically reasonable parameter values of the model, this behaviour is restricted to a certain range of density, with phase separation still persisting at high densities.Comment: 13 pages, 10 figure

Sobre la estructura del clínker de cemento portland trabajo presentado en la Ill reunión internacional sobre reactividad de sólidos, con este mismo título

Not availableEstudiada por el autor, en dos trabajos, la distribución heterogénea de la cal libre, así como la de otros componentes de carácter ácido y fundente, en los gránulos de clínker de cemento portland, al considerar en ellos zonas externas, intermedias e internas, se confirman y amplían en la presente comunicación los resultados anteriormente obtenidos. Para ello se ha utilizado un método de trabajo distinto, que permite comprobar dos de las conclusiones emitidas previamente como eventuales. Los resultados hallados permiten apreciar un desigual reparto de componentes químicos entre las distintas zonas de los gránulos de clínker, en el sentido de que en las capas externas de éstos abundan más los fundentes y, por tanto, los aluminatos y aluminoferritos cálcicos, así como la magnesia, mientras en el interior son más abundantes la sílice, el silicato bicálcico y la cal libre. Los módulos hidráulico, silicito y de fundentes, así como los porcentajes de fase líquida dados por las fórmulas de Lea y Parker, varían, de acuerdo con lo anterior, en la forma previsible

Surface roughness and thermal conductivity of semiconductor nanowires: going below the Casimir limit

By explicitly considering surface roughness at the atomic level, we quantitatively show that the thermal conductivity of Si nanowires can be lower than Casimir's classical limit. However, this violation only occurs for deep surface degradation. For shallow surface roughness, the Casimir formula is shown to yield a good approximation to the phonon mean free paths and conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact treatment of roughness scattering is in stark contrast with a previously proposed perturbative approach, which is found to overpredict scattering rates by an order of magnitude. The obtained results suggest that a complete theoretical understanding of some previously published experimental results is still lacking.Comment: 11 pages, 4 figure