Liaison métallique | EPFL Graph Search

redresse=1.75|vignette| Diagramme représentant la distribution des électrons dans les bandes de différents types de matériaux à l'équilibre. De gauche à droite : métal ; semimétal ; semiconducteur (dopé p, intrinsèque, dopé n) ; isolant. L'énergie est représentée par l'axe vertical, tandis que l'épaisseur horizontale des bandes représente la densité d'états.La densité électronique par niveau d'énergie suit la statistique de Fermi-Dirac et est représentée par un dégradé de noir. Le niveau de Fermi E des métaux et des semimétaux se trouve dans au moins l'une des bandes, tandis qu'il se trouve loin de toute bande dans les isolants, et suffisamment proche d'une bande dans les semiconducteurs pour peupler la bande de conduction ou la bande de valence d'électrons ou de trous. Une liaison métallique est une liaison chimique résultant de l'action d'un fluide d'électrons délocalisés unissant des atomes ionisés positivement. Les matériaux métalliques purs ou alliés sont caractérisés par u

Inflight Microwave Drying Process of Micro-droplets for High Resolution 3D printing

Kwanghoon Choi

A new Additive-Manufacturing (AM) or 3D printing concept is proposed to improve the printing resolution for metal additive manufacturing in the frame of the SFA-AM project, Powder Focusing for Beam-Induced Laser 3D Printing. The project aims to transport small powder particles (largest diameter < 10 um) at a high throughput to a spot smaller than 20 µm. To realize the objective, a metal ion included liquid is suggested as a source medium similar to the inkjet process, and the droplet is employed as a container for the metal ion transport. The key challenges of achieving this goal are 1) droplet size control and 2) efficient microwave heating that can dry the liquid portion of the dynamic droplet at a high-speed (3 m/s). The most demanding part is the development of a miniaturized, very concentrated field density microwave resonator where the droplet is passing through. This follows from the extremely short interaction time between the microwave and the ejected droplets (research: 0.5 ms) which results from the requirement for a small system size for its integration. The primary medium examined for the experiment is water due to 1) a high dielectric loss proportional to the drying efficiency and 2) avoiding explosion risks at the laser sintering stage compared to organic solutions. Various droplet generation conditions were investigated to determine the optimum conditions for the custom droplet generator, in particular, two key parameters were focused on: frequency and flow rate. The droplets obtained were analyzed with respect to size, gap distance, and linearity affecting the uniformity of the AM process. Moreover, Navier-Stokes-based fluid dynamics simulations enabled understanding of droplets behaviors, especially break-up formation. Eventually, the optimal condition for producing the smallest droplets with a diameter of 100 µm is determined to be 15 kHz and 0.5 ml/min. Such droplets loaded with metallic particles could result in smaller than 20 um of remaining metal powders aggregate after the drying process.A new TEM (Transverse ElectroMagnetic) resonator has been developed for drying and sensing applications. A full-wave electromagnetic simulation software, CST Microwave Studio, supported the working mode of the device. Firstly, it is used for the dielectric characterizations of different water-ethanol mixtures contained in a micro capillary. The sensing relies on electric field perturbation due to the inserted sample, and is measured based on the change of both resonance frequency and Q-factor. These sample-dependent variations were analyzed to estimate the complex permittivity using different methods such as the Perturbation Method (PM), Least-Square Model (LSM), and Log-Linear Model (LLM). This work demonstrates that the proposed microwave module is capable of sensing nano-liter volume ranges even though the samples only partially influence the sensing area (error < 13 % in permittivity). In microwave heating, the developed resonator achieved a temperature increase of 28 K in the microwave exposal time of 0.5 ms when the microwave power of 45 W was applied. Finally, numerical models have been proposed to estimate the maximum temperature of the initial droplet before cooling starts attributable to heat and mass transfer. This model well corresponds to the microwave electromagnetic simulation resulting in similar temperature deviations. It helps to determine the net heating efficiency of the microwave device.

EPFL2022

Resonant optical trapping in hollow photonic crystal cavities and its potential use for bacterial characterisation

Rita Therisod

During the last decade, the development of optofluidic chips has become a large field of research. The integration of nano and microstructures with microfluidics layers allowed for the miniaturisation of a number of tools traditionally used in laboratories and optofluidic systems found their main applications in lab-on-a-chip and sensing platforms. At the same time, optical tweezers have become the tool of choice for the trapping and the manipulation of nano and micrometer-sized objects, that can be inert (dielectric or metallic particles) or biological (proteins, bacteria, viruses,...). In this context, nanostructures are the ideal candidate for tweezers miniaturisation, thanks to their ability to strongly confine light in small volumes and hence to generate intense gradient forces. This work reports on the fabrication of an optofluidic chip based on a two-dimensional photonic crystal cavity and on its use in particle and bacteria differentiation. The cavity has a hollow design that maximises the overlap between the confined field and the trapped objects and it supports Self-Induced Back-Action effects that allow for reducing the trapping power (down to few microwatts) and to simultaneously acquire information of the trapped specimen. The system is excited in an end-fire setup and the detection is carried out by recording the light intensity transmitted through the photonic crystal. The fabrication process, that is entirely carried out in the Institue of Physics and in the Center of MicroNanoTechnology cleanrooms, is first detailed. For the cavity design normally used, typical quality factors of 10000 were obtained. Moreover, SU8 mode adaptors were developed to increase the coupling with lensed fibers and a microfluidic membrane presenting two injection channels is proposed for rapid switch between liquids. The trapping and the differentiation of 250 and 500 nm polystyrene is then presented. The differentiation can be achieved qualitatively by direct observation of the transmitted intensity records and quantitatively by the use of histograms and of statistical moments. Finally, the trapping of seven species of living bacteria is shown and their Gram-type is determined by the analysis of the induced transmission increase. This ability to probe the cell wall of bacteria in a fast, label-free and non-destructive way paves the way for applications in biological and biomedical environments.

EPFL2019

Evidence of a coupled electron-phonon liquid in NbGe2

Philip Johannes Walter Moll, Mathieu François Padlewski, Matthias Carsten Putzke, Hao Yang

Whereas electron-phonon scattering relaxes the electron's momentum in metals, a perpetual exchange of momentum between phonons and electrons may conserve total momentum and lead to a coupled electron-phonon liquid. Such a phase of matter could be a platform for observing electron hydrodynamics. Here we present evidence of an electron-phonon liquid in the transition metal ditetrelide, NbGe2, from three different experiments. First, quantum oscillations reveal an enhanced quasiparticle mass, which is unexpected in NbGe2 with weak electron-electron correlations, hence pointing at electron-phonon interactions. Second, resistivity measurements exhibit a discrepancy between the experimental data and standard Fermi liquid calculations. Third, Raman scattering shows anomalous temperature dependences of the phonon linewidths that fit an empirical model based on phonon-electron coupling. We discuss structural factors, such as chiral symmetry, short metallic bonds, and a low-symmetry coordination environment as potential design principles for materials with coupled electron-phonon liquid. It was predicted that in the regime of strong electron-phonon interactions, electrons and phonons can form a coupled non-equilibrium state, characterized by the conservation of the total momentum and by hydrodynamic transport. Here, the authors report experimental evidence for such a coupled electron-phonon liquid in NbGe2.