Hélium liquide

Résumé

vignette|Hélium liquide superfluide dans un récipient. L’hélium 4 peut être liquéfié à pression ambiante sous une température d'environ , soit . Son isotope, l'hélium 3, se liquéfie à pression ambiante sous une température d'environ . L'hélium est le seul élément qui ne peut être solidifié à pression ambiante. L'hélium solide peut être obtenu seulement lorsqu'une très grande pression y est appliquée. Historique de la liquéfaction L’hélium 4 est liquéfié pour la première fois le par le physicien hollandais Heike Kamerlingh Onnes à Leyde aux Pays-Bas. Cette prouesse majeure, résultat de dix années de travail intensif, permet d'atteindre les températures les plus basses de l'époque (le record de froid avant la liquéfaction de l'hélium était détenu depuis 1898 par le physicien anglais James Dewar avec la liquéfaction de l'hydrogène à , soit environ ). Heike Kamerlingh Onnes obtient le prix Nobel de physique en 1913 "pour sa recherche des propriétés de la matière à basse tem

Source officielle

https://fr.wikipedia.org/wiki/H%C3%A9lium_liquide

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Embrittlement induced by the synergistic effects of radiation damage and helium in structural steels

Kun Wang

Ferritic/martensitic steels are candidate materials for fusion reactor structural components, liquid metal containers of spallation neutron sources, and accelerator driven systems, with good radiation resistance and thermo-mechanical properties. However, embrittlement resulting from the combined effects of radiation induced displacement damage (measured in dpa = displacement per atom) and transmutation products, especially helium gas, is one of the key issues. Four different steels were selected for mechanical and microstructural studies to understand the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after irradiation in SINQ, the Swiss spallation source. The irradiations conditions were in the range: 10.7 ¿ 20.4 dpa with 850-1750 appm He at 160-300 °C. The evolution of the mechanical properties after irradiation was investigated by tensile and hardness tests. Radiation-induced defect clusters and helium bubbles were quantified by transmission electron microscopy (TEM). Emphasis was put on the deformation mechanisms under the different observed fracture mode, (ductile, quasi-cleavage and intergranular), whose occurrence depends on the irradiation conditions (dpa, He content and irradiation temperature). The tensile stress-strain curves and the scanning electron microscopy images of fracture surfaces showed distinct fracture mechanisms under different irradiation and test conditions. The tensile tests showed a yielding stress increase and loss of ductility of irradiated specimens. Hardness was measured on the specimens before tensile testing. The hardness results demonstrated an increasing trend with irradiation dose and helium content. TEM observations were done for all irradiated fractured specimens. Small defect clusters were observed in the 12.3 dpa specimen, but large defect clusters with loop-shape were very few. In the specimens of 17.2, 17.7 and 20.4 dpa, many large dislocation loops were detected besides small clusters. In addition, helium bubbles were observed in all specimens. The average size of defect clusters increased from 4.2 nm to 11.8 nm with dose increasing from 12.3 dpa to 20.4 dpa, whereas the number density did not change significantly. Meanwhile, the average size of visible helium bubbles increased from 1.03 nm to 1.93 nm. The microstructures in deformed area of irradiated specimens were observed and defect free channels with {110} and {112} slip planes were found in some specimens, indicating plastic flow localization. The average width of the channels is about 100 nm. Regarding the brittle samples, the TEM-lamella were extracted directly below intergranular fracture surfaces or cleavage surfaces by focused ion beam. Strikingly, deformation twinning was observed as the main feature in three irradiated specimens at high dose. Only twins with {112} planes were observed in all of these samples. The average thickness of twins is about 34 nm. Twins started from a fracture surface became gradually thinner with distance away from the fracture surface and stopped in the matrix finally. Features such as twin-precipitates interaction, twin-grain boundary and/or lath boundary interaction were observed. Twinning bands were seen to be arrested by grain boundaries or large precipitates, but could penetrate martensitic lath boundaries. Unlike the case of defect free channels, inside twins small defect-clusters, dislocation loops and dense small helium bubbles were observed.

EPFL2015

Chargement

Several aspects in the field of molecular self assembly are addressed in this thesis. Thanks to the scanning tunnelling microscope (STM), structural, electronic and magnetic properties on surfaces can be studied down to the size of a single atom. In this thesis, I designed and built a STM operating in ultra high vacuum at liquid helium temperature. In Chapter 2 I highlight the design and key features of this system. I describe the development of UHF cabling under severe design constraints, intended to be used for a state of the art STM time resolved technique with nanosecond time resolution. The performance of the instrument is demonstrated by a few benchmark measurements. The arrangement of molecules on surfaces is sensitive to intermolecular interactions, but also to molecule-surface interactions. The surfaces used as support for molecules must therefore be well characterized from the structural, chemical and electronic points of view. Using STM, work function (WF) variations at the nanoscale on hexagonal boron nitride on rhodium (h-BN/Rh(111)) are investigated. These variations are the key to understanding the arrangement of molecules on this surface, yet their magnitude is disputed. Local WF is measured in two ways and compared with KPFM maps. The results provide insight about hot electronic states near surfaces, called image potential state, and their lifetimes. A new phase of h-BN/Rh(111) was discovered and a preliminary characterization of its origin is presented. STM can provide valuable information for understanding the arrangement and the electronic properties of molecules on surfaces. These capabilities are harnessed in Chapter 4 to study large molecules on metallic surfaces, and on thin dielectric surfaces. Angiotensin-II is a peptide, comprising 8 amino acids, small enough to fit in the nanometric cavities of h-BN/Rh(111). This peptide can be brought to the surface intact using electrospray ion-beam deposition. On a Cu surface, we observe that the peptides interact strongly with the substrate, and this interaction determines their arrangement and electronic properties. However, on a single layered sheet of h-BN/Rh(111), the peptides are sufficiently decoupled from the metal, and their electronic properties conserved. In a series of preliminary measurements, these electronic properties are studied in order to discriminate between the amino acids the peptide is made of. Additional strategies that may enable peptide sequencing by STM are discussed. Turning to still larger molecules, cytochrome-C is a protein made of 104 amino acids. On plain metal surfaces, its size and flexibility make the number of conformations available to it enormous. In Chapter 5 this protein is deposited on a nanopatterned surface, forcing it to adhere to a square grid. This makes the number of possible conformations manageable. A detailed look at these conformations reveals many details about how the proteins move on the surface and find their place on the grid. For example, it can inferred that short segments of the protein can diffuse on the surface at room temperature, while longer segments are immobilized. The critical length where diffusion begins is used to estimate the diffusion barrier energy. Furthermore, a simplified simulation of the landing process yields a good fit to the experimentally observed conformation distribution. This fit confirms the flexibility of the proteins and the negligible role of the amino acid side chains.

EPFL2016

Chargement

An apparatus for pulsed ESR and DNP experiments using optically excited triplet states down to liquid helium temperatures

Arnaud Comment, Tim Rolf Eichhorn, Martin Haag, Sami Jannin, Jacques Van Der Klink

In standard Dynamic Nuclear Polarization (DNP) electron spins are polarized at low temperatures in a strong magnetic field and this polarization is transferred to the nuclear spins by means of a microwave field. To obtain high nuclear polarizations cryogenic equipment reaching temperatures of 1 K or below and superconducting magnets delivering several Tesla are required. This equipment strongly limits applications in nuclear and particle physics where beams of particles interact with the polarized nuclei, as well as in neutron scattering science. The problem can be solved using short-lived optically excited triplet states delivering the electron spin. The spin is polarized in the optical excitation process and both the cryogenic equipment and magnet can be simplified significantly. A versatile apparatus is described that allows to perform pulsed dynamic nuclear polarization experiments at X-band using short-lived optically excited triplet sates. The efficient He-4 flow cryostat that cools the sample to temperatures between 4 K and 300 K has an optical access with a coupling stage for a fiber transporting the light from a dedicated laser system. It is further designed to be operated on a neutron beam. A combined pulse ESR/DNP spectrometer has been developed to observe and characterize the triplet states and to perform pulse DNP experiments. The ESR probe is based on a dielectric ring resonator of 7 mm inner diameter that can accommodate cubic samples of 5 mm length needed for neutron experiments. NMR measurements can be performed during DNP with a coil integrated in the cavity. With the presented apparatus a proton polarization of 0.5 has been achieved at 0.3 T. (C) 2013 Elsevier Inc. All rights reserved.

Academic Press Inc Elsevier Science2013

Chargement

Embrittlement induced by the synergistic effects of radiation damage and helium in structural steels

Kun Wang

EPFL2015

EPFL2016