NACRE II: an update of the NACRE compilation of charged-particle-induced thermonuclear reaction rates for nuclei with mass number A<16A < 16

An update of the NACRE compilation [Angulo et al., Nucl. Phys. A 656 (1999) 3] is presented. This new compilation, referred to as NACRE II, reports thermonuclear reaction rates for 34 charged-particle induced, two-body exoergic reactions on nuclides with mass number $A<16$, of which fifteen are particle-transfer reactions and the rest radiative capture reactions. When compared with NACRE, NACRE II features in particular (1) the addition to the experimental data collected in NACRE of those reported later, preferentially in the major journals of the field by early 2013, and (2) the adoption of potential models as the primary tool for extrapolation to very low energies of astrophysical $S$-factors, with a systematic evaluation of uncertainties. As in NACRE, the rates are presented in tabular form for temperatures in the $10^{6}$ $\simeq\leq$ T $\leq$ $10^{10}$ K range. Along with the 'adopted' rates, their low and high limits are provided. The new rates are available in electronic form as part of the Brussels Library (BRUSLIB) of nuclear data. The NACRE II rates also supersede the previous NACRE rates in the Nuclear Network Generator (NETGEN) for astrophysics. [http://www.astro.ulb.ac.be/databases.html.]Comment: 86 figure

Nanoscale assembly processes revealed in the nacroprismatic transition zone of Pinna nobilis mollusc shells

Intricate biomineralization processes in molluscs engineer hierarchical structures with meso-, nano-, and atomic architectures that give the final composite material exceptional mechanical strength and optical iridescence on the macroscale. This multiscale biological assembly inspires new synthetic routes to complex materials. Our investigation of the prism-nacre interface reveals nanoscale details governing the onset of nacre formation using high-resolution scanning transmission electron microscopy. A wedge polishing technique provides unprecedented, large-area specimens required to span the entire interface. Within this region, we find a transition from nanofibrillar aggregation to irregular early-nacre layers, to well-ordered mature nacre suggesting the assembly process is driven by aggregation of nanoparticles (~50-80 nm) within an organic matrix that arrange in fiber-like polycrystalline configurations. The particle number increases successively and, when critical packing is reached, they merge into early-nacre platelets. These results give new insights into nacre formation and particle-accretion mechanisms that may be common to many calcareous biominerals.Comment: 5 Figure

Assessment of crystallographic influence on material properties of calcite brachiopods

Calcium carbonate biominerals are frequently analysed in materials science due to their abundance, diversity and unique material properties. Aragonite nacre is intensively studied, but less information is available about the material properties of biogenic calcite, despite its occurrence in a wide range of structures in different organisms. In particular, there is insufficient knowledge about how preferential crystallographic orientations influence these material properties. Here, we study the influence of crystallography on material properties in calcite semi-nacre and fibres of brachiopod shells using nano-indentation and electron backscatter diffraction (EBSD). The nano-indentation results show that calcite semi-nacre is a harder and stiffer (H {approx} 3–5 GPa; E = 50–85 GPa) biomineral structure than calcite fibres (H = 0.4–3 GPa; E = 30–60 GPa). The integration of EBSD to these studies has revealed a relationship between the crystallography and material properties at high spatial resolution for calcite semi-nacre. The presence of crystals with the c-axis perpendicular to the plane-of-view in longitudinal section increases hardness and stiffness. The present study determines how nano-indentation and EBSD can be combined to provide a detailed understanding of biomineral structures and their analysis for application in materials science

Big Bang Nucleosynthesis updated with the NACRE Compilation

We update the Big Bang Nucleosynthesis calculations on the basis of the recent NACRE compilation. The average values of the calculated abundances of light nuclei do not differ significantly from that obtained using the previous Fowler's compilation. ${^7}Li$ is slightly depressed at high baryon to photon ratio $\eta$. The main uncertainty concerns the $D(p,\gamma){^3}He$ reaction rate affecting the synthesis of ${^7}Li$ (via the ${^3}He(\alpha,\gamma){^7}Be(e\nu){^7}Li$) at rather high baryonic density. On the left part of the lithium valley the uncertainty is strongly reduced due to the improvement of the measurement of the $T(\alpha,\gamma)^{7}Li$ reaction rate. We use lithium-7 as the main baryometer, since, though much efforts have been devoted to the determination of Deuterium in absorbing clouds in the line of sight of remote quasars, the statistics is very poor compared to the long series of lithium measurements. Taking into account the lithium constraints, two possible baryonic density ranges emerge, $\eta_{10}= 1.4 - 1.9$ and $\eta_{10} = 3.3 - 5.1$. The Be and B abundances produced in the big bang are orders of magnitudes lower, and spallation of fast carbon and oxygen is probably their unique source, in the early Galaxy.Comment: 8 pages, 5 figures, accepted in Astronomy and Astrophysic

A multiscale tribological study of nacre : Evidence of wear nanomechanisms controlled by the frictional dissipated power

Sheet nacre is a hybrid biocomposite with a multiscale structure, including nanograins of CaCO3 (97% wt.% – 40 nm in size) and two organic matrices: (i) the “interlamellar” mainly composed of β-chitin and proteins, and (ii) the “intracrystalline” mainly composed by silk-fibroin-like proteins. This material is currently studied as small prostheses with its tribological behaviour. In this work, the latter is studied by varying the frictional dissipated power from few nW to several hundreds mW, in order to study the various responses of the different nacre’s components, independently. Results reveal various dissipative mechanisms vs. dissipated frictional power: organic thin film lubrication, tablet’s elastoplastic deformations, stick-slip phenomenon and/or multiscale wear processes, including various thermo-mechanical processes (i.e., mineral phase transformation, organics melting and friction-induced nanoshocks process on a large range). All these mechanisms are controlled by the multiscale structure of nacre – and especially by its both matrices and respective orientation vs. the sliding direction