Âge du fer

Résumé

vignette|redresse=1.5|Reconstitution d'une maison de l'âge du fer au , Royaume-Uni. L’âge du fer est une période archéologique caractérisée par la métallurgie du fer et faisant généralement suite à l'âge du bronze. Toutefois, les limites chronologiques de l'âge du fer varient considérablement selon l'aire culturelle et géographique considérée. L'âge du fer peut être considéré comme appartenant à la Préhistoire, à la Protohistoire ou à l'Histoire selon les populations et régions concernées. L'âge du fer débute vers en Anatolie, vers 1020 à en Grèce, vers 800 à en Europe de l'Ouest, et peut-être dès le en Afrique centrale. La métallurgie du fer nécessite une température plus élevée que celle du bronze, atteignable grâce à l'évolution technologique des fours. Historiographie L'existence d'un âge du fer est déjà évoquée dans le De rerum natura de Lucrèce, mais comme simple hypothèse philosophique. L'invention de l'expression « âge du fer » est attribuée au chercheur dano

Source officielle

https://fr.wikipedia.org/wiki/%C3%82ge_du_fer

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées (19)

Chargement

Afficher plus

Publications associées

Chargement

Personnes associées (1)

Personnes associées

Chargement

Unités associées (1)

Unités associées

Chargement

Concepts associés (74)

Concepts associés

Chargement

Cours associés (7)

Cours associés

Chargement

Séances de cours associées (15)

Séances de cours associées

Chargement

Publications associées (19)

Seasonal phytoplankton and geochemical shifts in the subsurface chlorophyll maximum layer of a dimictic ferruginous lake

Hannah Elisa Chmiel

Subsurface chlorophyll maxima layers (SCML) are ubiquitous features of stratified aquatic systems. Availability of the micronutrient iron is known to influence marine SCML, but iron has not been explored in detail as a factor in the development of freshwater SCML. This study investigates the relationship between dissolved iron and the SCML within the dimictic, ferruginous lake Grosses Heiliges Meer in northern Germany. The occurrence of the SCML under nonferruginous conditions in the spring and ferruginous conditions in the fall are context to explore temporal changes in the phytoplankton community and indicators of primary productivity. Results indicate that despite more abundant chlorophyll in the spring, the SCML sits below a likely primary productivity maximum within the epilimnion, inferred based on colocated dissolved oxygen, delta C-13(DIC), and pH maxima. The peak amount of chlorophyll in the SCML is lower in the fall than in the spring, but in the fall the SCML is colocated with elevated dissolved iron concentrations and a local delta C-13(DIC) maximum. Cyanobacteria and Chlorophyta have elevated abundances within the SCML in the fall. Further investigation of the relationship of iron to primary productivity within ferruginous SCML may help to understand the environmental controls on primary productivity in past ferruginous oceans.

2022

Chargement

Carbides constitute a class of materials used for many applications, where excellent mechanical properties, such as stiffness, hardness and fracture toughness, are required. The machining industry is a typical example where carbide are crucial materials. Whether it be in tool steels or in cermets, the most performant machining alloys are actually composed of ternary carbides, i.e. contain two different metallic elements in addition to carbon, present as a reinforcement in a metallic matrix. Ternary carbides constitute better reinforcements providing enhanced mechanical properties in comparison with binary carbides; however, our knowledge of ternary carbides per se is insufficient, and only the properties of the alloys have been characterized. The main reason explaining such a lack of data is the relative small size of these particles, in the micro-range, which complicate the measurement of their mechanical properties. This thesis aims to contribute towards filling this gap by means of micromechanical methods able to measure locally the main properties of individual microscopic carbide particles. This is now possible thanks to the recent developments of experimental techniques such as nanoindentation and FIB. The challenges of this thesis are then to be able to 1) produce carbides particles embedded in iron with tailored compositions, 2) test mechanically individual particles and 3) explain the evolution of the properties with changes in composition, in order to find an optimal composition characterized by enhanced mechanical properties. The main challenges come from the small dimension of the particles (< 40 µm) and the fact that such particles are embedded in a steel matrix, more compliant than the particles, which can then bias the measurement of elastic properties. Standard nanoindentation techniques cannot measure accurately the mechanical properties of such matrix/particle combinations and new methods have been developed in this thesis in order to overcome this problem. Carbide particles have been grown in situ by arc-melting cast iron with transition metal high purity chips, in order to properly tune the composition. Three mechanical properties have been measured: elastic modulus, hardness and fracture toughness, by indenting a polished surface, or by bending up to fracture notched micro-cantilever beams. This trio of properties is essential in order to characterize properly a material in view of an application in machining industry. By using binary carbide compositions as references, we have characterized five ternary carbide systems, namely (Ti,W)C, (Ti,V)C, (Ti,Ta)C, (Ti,Nb)C and (Ta,V)C, as well as the quaternary (Ti,Ta,V)C system. The ratio of the metallic elements has been varied in order to cover the entire range of compositions. For the ternary systems, we found compositions that exhibited both a higher hardness and modulus than the two corresponding binary carbides, enhancing the properties by ~15-20%. By investigating the filling of atomic bonds involving d-orbitals, we can provide a partial explanation to the elastic modulus and hardness evolution with composition, the maxima in hardness corresponding to a valence electron concentration between 8.4 and 8.6. The quaternary system aims to further improve mechanical response of the ternary carbides: it allows to highlight a very promising quaternary composition, characterized by the combination of both a very high modulus and hardness of respectively 636 and 41 GPa.

EPFL2018

Chargement

Light absorption by brown carbon over the South-East Atlantic Ocean

Athanasios Nenes, Lu Zhang

Biomass burning emissions often contain brown carbon (BrC), which represents a large family of light-absorbing organics that are chemically complex, thus making it difficult to estimate their absorption of incoming solar radiation, resulting in large uncertainties in the estimation of the global direct radiative effect of aerosols. Here we investigate the contribution of BrC to the total light absorption of biomass burning aerosols over the South-East Atlantic Ocean with different optical models, utilizing a suite of airborne measurements from the ORACLES 2018 campaign. An effective refractive index of black carbon (BC), m(eBC) = 1.95 + ik(eBC), that characterizes the absorptivity of all absorbing components at 660 nm wavelength was introduced to facilitate the attribution of absorption at shorter wavelengths, i.e. 470 nm. Most values of the imaginary part of the effective refractive index, k(eBC), were larger than those commonly used for BC from biomass burning emissions, suggesting contributions from absorbers besides BC at 660 nm. The TEM-EDX single-particle analysis further suggests that these long-wavelength absorbers might include iron oxides, as iron is found to be present only when large values of k(eBC) are derived. Using this effective BC refractive index, we find that the contribution of BrC to the total absorption at 470 nm (R-BrC,(470)) ranges from similar to 8 %-22 %, with the organic aerosol mass absorption coefficient (MAC(OA,470)) at this wavelength ranging from 0.30 +/- 0.27 to 0.68 +/- 0.08 m(2) g(-1). The core-shell model yielded much higher estimates of MAC(OA,470) and R(BrC,470)( )than homogeneous mixing models, underscoring the importance of model treatment. Absorption attribution using the Bruggeman mixing Mie model suggests a minor BrC contribution of 4 % at 530 nm, while its removal would triple the BrC contribution to the total absorption at 470 nm obtained using the AAE (absorption Angstrom exponent) attribution method. Thus, it is recommended that the application of any optical properties-based attribution method use absorption coefficients at the longest possible wavelength to minimize the influence of BrC and to account for potential contributions from other absorbing materials.

COPERNICUS GESELLSCHAFT MBH2022

Chargement

Afficher plus

Light absorption by brown carbon over the South-East Atlantic Ocean

Athanasios Nenes, Lu Zhang

COPERNICUS GESELLSCHAFT MBH2022

EPFL2018