On the Feasibility of Perpetual Growth in a Decentralized Economy Subject to Environmental Constraints

We propose an endogenous growth model of a decentralized economy subject to environmental constraints. In a basic version, we consider an economy where final production requires some material input and where research activities allow simultaneously productive firms to reduce the dependency of their production process on this input and to improve the quality of their output. We adopt a material balance approach and, in spite of the optimistic assumption that the material input is perfectly recyclable (and thus never exhausted), we show that material output growth is always a transitory phenomenon. When it exists, a balanced growth path is necessarily characterized by constant values of the material variables, long term economic growth taking exclusively the form of perpetual improvements in the quality of consumption goods. The material resource constraint is not solely a long term issue since it is also shown to affect the whole transitory dynamics of the (material) growth process. Renewable energy is introduced in an extension of our basic model. This extension does not affect qualitatively the features of a feasible balanced growth path but make its conditions of existence more restrictive.material balance, endogenous growth, recycling

Comprehension of chip formation in laser assisted machining

Laser Assisted Machining (LAM) improves the machinability of materials by locally heating the workpiece just prior to cutting. Experimental investigations have confirmed that the cutting force can be decreased, by as much as 40%, for various materials. In order to understand the effect of the laser on chip formation and on the temperature fields in the different deformation zones, thermo-mechanical simulations were undertaken. A thermo-mechanical model for chip formation was also undertaken. Experimental tests for the orthogonal cutting of 42CrMo4 steel were used to validate the simulation. The temperature fields allow us to explain the reduction in the cutting force and the resulting residual stress fields in the workpiece.Contrat Plan Etat Région (CPER) Pays de la Loir

Modelling, identification and application of phenomenological constitutive laws over a large strain rate and temperature range

A review of the different phenomenological thermo-viscoplastic constitutive models often applied to forging and machining processes is presented. Several of the most common models have been identified using a large experimental database (Hor et al., 2013). The latter consists of the tests were done in compression on cylindrical shaped specimens and in shear using hat-shaped specimens. The comparison between these different models is shown that the group of decoupled empirical constitutive models (e.g. the Johnson and Cook (1983) model), despite their simple identification procedures, are relatively limited, especially over a large range of strain rates and temperatures. Recent studies have led to the proposal of coupled empirical models. Three models in this class have also been studied. The Lurdos (2008) model shows the best accuracy but requires a large experimental database to identify its high number of parameters. After this comparison, a constitutive equation is proposed by modifying the TANH model (Calamaz et al., 2010). Coupling between the effects of strain rate and temperature is introduced. This model is easier to identify and does not require knowledge of the saturation stress. Compared to other models, it better reproduces the experimental results especially in the semi-hot and hot domains. In order to study real machining conditions, an orthogonal cutting tests is considered. The comparison between experimental test results and numerical simulations conducted using the previously identified constitutive models shows that the decoupled empirical models are not capable of reproducing the experimental observations. However, the coupled constitutive models, that take into account softening, improve the accuracy of these simulations

Effet de l'oxydation et du dopage sur les modes Raman de couches minces de phosphore noir

Le phosphore noir est un semi-conducteur 2D très prometteur pour les prochaines générations de dispositifs électroniques. Il possède une bande interdite direct modulée par son épaisseur en plus d’avoir des propriétés électriques, mécaniques et optiques hautement anisotropes grâce à sa structure ondulée. La valeur de sa bande interdite se situe entre 1.8 eV pour la mono-couche et 0.33 eV pour le matériau massif. Tout comme les autres isotopes du phosphore, le phosphore noir est instable lorsque exposé aux conditions ambiantes d’air, d’eau et de lumière. Le processus d’oxydation ajoute des défauts à sa structure et crée de l’acide phosphorique à sa surface. Lorsque exposé à de grands champs électriques induits par l’adsorption d’atomes de potassium, la bande interdite de couches minces de phosphore noir diminue dûe à l’effet Stark. La fermeture du gap, soit la transition de semi-conducteur à semi-métal, a été observée pour des concentrations surfaciques d’électrons au delà de 8 ×10 13 cm −2. Ce mémoire porte sur la caractérisation optique de couches minces de phosphore noir à l’aide de la spectroscopie Raman. L’objectif est d’identifier des marqueurs du spectre Raman pour l’oxydation et la transition de semi-conducteur à semi-métal. Pour y arriver, les modes actifs en Raman A1g et A2g sont étudiés en fonction du temps d’oxydation, du dopage au potassium et de l’énergie d’excitation. L’expérience d’oxydation a permis d’établir la présence des modes phonon-défauts, soit des modes Raman de second-ordre activés par les défauts dans le matériau. L’intensité des modes phonon-défauts dépend de la densité de défauts ainsi que de l’énergie d’excitation. Les expériences de dopages ont permis de caractériser la modulation de la bande interdite grâce au ratio des intensités D/Ag. Les résultats de cette recherche permettront de mieux caractériser les couches minces de phosphore noir. Les résultats de l’expérience d’oxydation permettront de quantifier la qualité et la pureté des échantillons. Les résultats de l’expérience de dopage permettront de quantifier la modulation du gap à partir de la spectroscopie Raman. Ces conclusions assisteront la recherche et le développement de technologies à base de couches minces de phosphore noir.----------Abstract Black phosphorus is a 2D semiconductor material exhibiting a thickness-tunable direct bandgap and pronounced electrical, mechanical, and optical in-plane anisotropies resulting from its puckered structure. Its energy gap range from 1.8 eV for the monolayer to 0.33 for the bulk material. Black phosphorus is unstable when exposed to ambient conditions of air, water and light. The oxidation process adds defects in the structure and creates phosphorus acid onto its surface. Exposed to the high electric field induced by adsorbed potassium atoms, the band gap of few-layer black-phosphorus shrinks. At doping levels above 8 ×10 13 cm −2, a semiconductor to semimetal transition is observed. We aim to explain these mechanisms with Raman spectroscopy by identifying markers of the oxidation process and semiconductor to semimetal transition. To achieve this, we analyse the evolution of Raman-allowed A1g and A2g modes as a function of oxidation time, potassium doping, and excitation wavelength. The oxidation experiments reveal the presence of phonondefect modes, second-order Raman modes activated by defects in the structure, and its excitation wavelength dependence. The doping experiments allow us to identify changes of the band-gap energy with the D/Ag ratio. These results will help the community with a better understanding of the Raman spectra of black phosphorus and a way to measure the defect density with the phonon-defect modes. These modes also provide evidence of the gap modulation due to an electric field