Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Microstructure (physique) --- Microstructure

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Élasticité --- Optimisation topologique --- Microstructure --- Materiau --- Coefficient de poisson negatif --- Poids minimum --- Homogeneisation --- Optimisation topologique --- Microstructure --- Materiau --- Coefficient de poisson negatif --- Poids minimum --- Homogeneisation

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

The technique of selective laser melting (SLM), developed in the late 20th century knows significant developments at present times. This technique, consisting of melting powder by successive layers, has the advantage of making extremely complex shapes, unachievable by traditional techniques. The use of different types of metal materials broadens its scope. The purpose of this work is the implementation of an Invar alloy by laser melting selective. Our main objective was to know the influence of each stage of heat treatment on microstructure and properties of Invar 36. Invar is an iron-nickel alloy with 36% Ni, which has the distinction of having a very low coefficient of thermal expansion in a wide temperature range around room temperature. This feature presents a major technological interest. After a brief explanation about the art on Invar, Invar effect and the SLM process, I will then present the pieces Invar manufactured by SLM for which a characterization of the microstructure was performed before and after heat treatment. The microstructural characterizations were performed with an optical microscope and scanning electron, and through Vickers hardness measurements and expansion. The results showed that the heat treatment does not significantly affect the microstructure and hardness of samples. The porosity rate decreases from the outside towards the inside of the building board and the thermal expansion coefficient of the alloys varies very little after heat treatment. In order to make a "preliminary validation and unoptimized" of the fabrication technique SLM Invar, a comparison is made between the parts produced by SLM and an extruded rod of Invar. An important discussion is then proposed on the influence of various parameters such as chemical composition, method of preparation and heat treatment on the properties and microstructure expansion.

Invar --- Selective laser melting --- Microstructure --- properties --- Ingénierie, informatique & technologie > Science des matériaux & ingénierie

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Ce mémoire de fin d’études a comme objectif principal d’optimiser des dépôts de pérovskite de type CH3NH3PbI3 (épaisseur de pérovskite, architecture de cellule, dopage aux chlorures) afin de préparer des cellules photovoltaïques avec des efficacités de conversion supérieures à 10%. La structuration 3D de films poreux de pérovskite est également étudiée afin de mettre en évidence l’effet de cette structuration sur l’efficacité des cellules correspondantes et sur les propriétés optiques des dépôts en termes de semi-transparence et/ou de coloration, en comparaison avec les films denses de pérovskite habituellement utilisés. Ce travail est donc divisé en 2 parties consacrées aux films denses de pérovskite et aux films poreux de pérovskite, respectivement. Les films de pérovskite sont réalisés via des dépôts en 1 étape (dépôt d’une solution contenant le précurseur de Pb et CH3NH3I) ou en 2 étapes (dépôt du précurseur de Pb puis conversion par CH3NH3I ou CH3NH3Cl). Pour la préparation des films poreux de pérovskite, des billes de polystyrène sont utilisées comme agent de structuration. 

La première partie du travail consacrée à l’optimisation des films denses montre que les films de pérovskite 1 étape non dopée CH3NH3PbI3 les plus homogènes sont obtenus avec le DMSO comme solvant, que la durée du traitement thermique à 100°C n’a pas d’influence sur la morphologie des films et que le lissage anti-solvant au sec-butanol permet d’améliorer encore l’homogénéité et le taux de couverture des dépôts. Cependant, les efficacités PV mesurées pour les cellules correspondantes sont faibles et une diminution est même observée avec le lissage au sec-butanol dû à un manque d’adhérence des films. Les films de pérovskite 1 étape dopée CH3NH3PbI3-xClx sont donc envisagés pour améliorer les transferts de charges et diminuer les recombinaisons. Les films obtenus, pourtant peu couvrants et peu uniformes, permettent d’augmenter les efficacités, passant de 2,1% à 4,0% en balayage direct, grâce au dopage. Les films de pérovskite 2 étapes non dopée CH3NH3PbI3 sont également étudiés. La conversion par immersion du film de PbI2 dans une solution de CH3NH3I donne les meilleurs résultats. Pour les films de pérovskite 2 étapes dopée CH3NH3PbI3-xClx, les meilleurs résultats sont obtenus par conversion d’un film de PbI2 par une solution mixte de CH3NH3I:CH3NH3Cl et une efficacité de 12,1% est atteinte pour notre cellule champion. L’utilisation de la pérovskite non dopée CH3NH3PbI3 en combinaison avec un film de TiO2 mésoporeux est également envisagée pour améliorer l’efficacité PV des cellules, avec 11,0% pour notre cellule champion. L’ajout d’un film de TiO2 mésoporeux ou le dopage de la pérovskite est donc indispensable pour atteindre une efficacité de conversion supérieure à 10%. 

Dans la deuxième partie du travail consacrée à la structuration 3D des films poreux de pérovskite, cinq tailles de billes de polystyrène sont utilisées : 300 nm, 540 nm, 810 nm, 1,0 m et 2,1 m, afin d’étudier l’effet de la tailles des billes sur les propriétés optiques des dépôts et sur l’efficacité PV des cellules correspondantes. Les dépôts 1 étape donnent les meilleurs résultats. Les solutions PbI2/CH3NH3I 0,7 M et PbCl2/CH3NH3I 1,0 M dans DMSO forment les films poreux de pérovskite 1 étape non dopée CH3NH3PbI3 et dopée CH3NH3PbI3-xClx, respectivement, les plus couvrants, homogènes et sans couche dense de pérovskite en surface. L’efficacité des cellules correspondantes augmente avec la taille des billes. Une nette amélioration de l’efficacité de conversion PV est observée suite à la structuration 3D pour les films de pérovskite non dopée CH3NH3PbI3. De plus, les films poreux CH3NH3PbI3-PS810, CH3NH3PbI3-PS1000, CH3NH3PbI3-PS2100 et CH3NH3PbI3-xClx-PS2100 possèdent une coloration nette, très intéressante pour l’intégration de ces cellules PV aux bâtiments. The main objective of this master thesis is to optimize CH3NH3PbI3-like perovskite coatings (thickness, cell architecture, chloride doping) in order to prepare photovoltaic cells with conversion efficiencies higher than 10%. The 3D structuring of porous perovskite films is also studied in order to highlight the effect of this structuration on the efficiency of the corresponding cells and on the optical properties of the films in terms of semi-transparency and/or coloration, in comparison with the dense perovskite films usually used. This work is therefore divided into two parts devoted to perovskite dense films and porous films, respectively. The perovskite films are prepared through 1-step process (deposition of a solution containing the Pb precursor and CH3NH3I) or 2-steps process (deposition of the Pb precursor followed by its conversion by CH3NH3I or CH3NH3Cl). For the preparation of the perovskite porous films, polystyrene beads are used as a structuring agent.

The first part of the work devoted to the optimization of dense films shows that the most homogeneous 1-step non-doped CH3NH3PbI3 perovskite films are obtained with DMSO as a solvent, the duration of the heat treatment at 100°C has no impact on the morphology of the films and the anti-solvent smoothing with sec-butanol allows for further improvement of the films homogeneity and coverage. However, PV efficiencies of the corresponding cells are low and a decrease is even observed with the sec-butanol smoothing due to a lack of adherence of the films. The 1-step doped CH3NH3PbI3-xClx perovskite films are therefore investigated to improve charge transfer and to reduce recombination. The obtained films, despite poor coverage and a lack of uniformity, lead to an increase of the efficiencies from 2.1% to 4.0% in direct scanning direction thanks to the doping. The 2-steps non-doped CH3NH3PbI3 perovskite films are also studied. The conversion of a PbI2 film by dipping in a CH3NH3I solution gives the best results. For the 2-steps doped CH3NH3PbI3-xClx perovskite films, the best results are obtained by converting a PbI2 film by a solution of both CH3NH3I:CH3NH3Cl and an efficiency of 12.1% is reached for our best cell. The use of the non-doped CH3NH3PbI3 perovskite in combination with a TiO2 mesoporous film is also considered to improve the PV efficiency of the cells, reaching 11.0% for our best cell. The addition of a TiO2 mesoporous film or the perovskite doping is therefore required to achieve conversion efficiencies higher than 10%.

In the second part of the work devoted to the 3D structuring of porous perovskite films, five polystyrene beads diameters are studied: 300 nm, 540 nm, 810 nm, 1.0 µm and 2.1 µm, to highlight the effect of the beads diameter on the optical properties of the films and on the PV efficiency of the corresponding cells. The 1-step process gives the best results. PbI2/CH3NH3I 0.7 M and PbCl2/CH3NH3I 1.0 M solutions in DMSO lead to the most covering, homogeneous and overlayer-free porous films of 1-step non-doped CH3NH3PbI3 and doped CH3NH3PbI3-xClx perovskite, respectively. The efficiency of the corresponding cells increases with the beads diameter. A significant improvement in the PV conversion efficiency is observed thanks to the 3D structuring for the non-doped CH3NH3PbI3 perovskite films. In addition, CH3NH3PbI3-PS810, CH3NH3PbI3-PS1000, CH3NH3PbI3-PS2100 and CH3NH3PbI3-xClx-PS2100 porous films are obviously colored, which is very interesting for building integration of these PV cells (BIPV).

pérovskite --- dopage --- structuration 3D --- propriétés optiques --- microstructure --- cellule photovoltaïque --- porosité organisée --- templating --- Physique, chimie, mathématiques & sciences de la terre > Chimie

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

This master’s thesis investigates the potential of liquidity indicators to help forecast the trade duration and their impact on the probability of flash crashes in financial markets. Our study uses autoregressive conditional duration (ACD) models for modelling the trade duration and compares the standard ACD with the Log-ACD model. Our analysis is based on high-frequency trading data over the period of the May 2010 flash crash. Our research incorporates the percentage effective
spread (PES), the volume-synchronized probability of informed trading (VPIN) and the average depth liquidity indicators into the ACD models. The results indicate that, although these indicators offer some understanding of trade duration, incorporating liquidity indicators into the models did not significantly improve model performances. Our study also reveals that longer trading durations are not systematically correlated with significant price variations, making trading duration alone an impractical predictor of flash crashes. The results suggest that the dynamics leading to extreme market events are probably influenced by a broader set of factors than liquidity and trading duration. This emphasizes the need for more comprehensive models that incorporate additional market variables.

Trade duration --- High-frequency data --- Autoregressive Conditional Duration --- Liquidity indicators --- Market microstructure --- Sciences économiques & de gestion > Finance --- Sciences économiques & de gestion > Méthodes quantitatives en économie & gestion