Hydrogen | EPFL Graph Search

Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, and highly combustible. Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter. Stars such as the Sun are mainly composed of hydrogen in the plasma state. Most of the hydrogen on Earth exists in molecular forms such as water and organic compounds. For the most common isotope of hydrogen (symbol 1H) each atom has one proton, one electron, and no neutrons. In the early universe, the formation of protons, the nuclei of hydrogen, occurred during the first second after the Big Bang. The emergence of neutral hydrogen atoms throughout the universe occurred about 370,000 years later during the recombination epoch, when the plasma had cooled enough for electrons to remain bound to protons. Hydrogen is

Synthesis of carbon nanotubes and characterization of nanotubes as atomic force microscopy tips

Yanki Keles

This thesis describes the synthesis of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) and the evaluation of them as tips of atomic force microscope (AFM) probes. Both arc-discharge and CVD grown nanotubes were investigated as AFM tips. Two methods were used to produce nanotube tipped AFM probes: (i) nanotubes were directly grown on the apex of AFM probes by CVD and (ii) pre-grown nanotubes were mechanically attached onto the apex of AFM probes. The first part of the thesis describes experiments that sought to control the growth of carbon nanotubes by chemical vapor deposition. The effects of various parameters e.g. catalyst, the carbon source, the addition of hydrogen and the growth temperature, on carbon nanotube growth were investigated. Ethylene was found to lead to better quality nanotubes than the acetylene. Grown nanotubes' quality was found to improve upon flow of hydrogen during the synthesis. Nanotubes synthesized on 3 nm iron film at 840°C using ethylene and hydrogen led to utilizable carbon nanotube tipped AFM probes. The second part of the thesis focuses on the mechanical production of nanotube tipped AFM probes in high resolution electron microscopes using nanomanipulators. Scanning electron microscopy (SEM) was found to be more convenient than transmission electron microscopy (TEM) to carry out the nanomanipulation process. Both arc-discharge and CVD grown nanotubes were successfully attached onto the AFM probes using a custom made nanomanipulator. Nanotubes were glued onto the probes by focused electron beam (FEB) induced amorphous carbon deposits. FEB irradiation deposited amorphous carbon performed more reliable attachment on CVD nanotubes than the arc-discharge nanotubes. The last part of the thesis describes tapping mode AFM measurements done using nanotube tipped AFM probes. Different nanotube tipped probes were investigated as AFM tips for scanning colloidal gold particles with different diameters and gratings from Digital Instruments™ AFMs. High resolution AFM images were obtained by arc-discharge nanotube tips. CVD grown nanotubes were shown to be more resilient to tip crashes than the arc-discharge grown nanotubes. However, all the AFM images acquired from the scans by CVD nanotubes had the same kind of ringing artifact, in good agreement with a previously reported data about the artifacts of the CVD nanotube tipped AFM probes. Open ends of the CVD nanotubes, lack of crystallinity and the defects in their structure are suggested to be the reason for these artifacts.

EPFL2006

Delayed Hydride Cracking in Irradiated and Unirradiated Zircaloy-2 Cladding

Aaron William Colldeweih

Zirconium alloys used in the nuclear industry are exposed to extreme conditions undergoing high levels of irradiation damage and corrosion. Zircaloy-2 is used as nuclear fuel cladding in boiling water reactors and for the encapsulation of the spallation target in the neutron source SINQ at the Paul Scherrer Institute. As a result of oxidation, hydrogen is produced and partially taken up into the zirconium matrix. As the hydrogen reaches the solubility limit, it precipitates in the form of a zirconium hydride. A number of deleterious effects of hydrides have been extensively studied, including the embrittlement of the material and a unique cracking phenomenon, namely delayed hydride cracking (DHC). DHC occurs as hydrogen in solid solution diffuses towards locations of higher tensile stress, where it subsequently precipitates as the local solvus limit is reached. Once the hydride grows to a critical size, it fractures, which can continue to propagate in repeated steps by the same mechanism.Several studies have been performed in order to understand properties of DHC based on parameters like, alloy type, cracking temperature, hydrogen content, and cracking direction. The most important mechanical properties include the fracture toughness and cracking velocity, which governs the material failure conditions. Crystallographic properties have been investigated in order to understand how complex hydride precipitation develops within the context of DHC conditions. In this project, radial DHC is explored in irradiated and unirradiated thin-walled Zircaloy-2 nuclear fuel cladding with and without an inner liner, in addition to spallation target rod material. A unique three-point bending test, applicable also for irradiated samples in the hotlab, was developed to induce a consistent radially propagating outside-in crack. Results have provided insight into the effects of the inner liner on cracking velocity, showing correlations with the hydrogen depletion. The combined DHC and creep mechanisms, which cannot be decoupled, is coined as âcreep-delayed hydride crackingâ. As the initiating conditions for DHC are of great technical importance, the threshold stress intensity factor has been investigated, under ideal DHC conditions, where a minimum value occurs around 6 MPaâm. Quantitative analysis of the hydrogen distribution around the tip of an arrested DHC crack tip has been performed with neutron radiography at the SINQ. Trends show that the hydrogen concentration around the crack tip increases with temperature resulting likely from the larger hydrogen diffusion, and that the liner reduces the hydrogen available for DHC. Crystallographic analysis has provided insight on the hydride phases responsible for DHC with synchrotron micro X-ray diffraction, performed at the Swiss Light Source at PSI. The results directly suggest that the Î³-hydride is a stable phase precipitated at low temperatures. Testing of irradiated material provided insight into the crack initiation of DHC as a result of the strongly irregular outer surface generated from the oxidation. The propagation pattern is highly irregular compared to the unirradiated material due to the embrittled nature of the cladding from irradiation damage.

EPFL2022

Performance evolution and modeling of PEM fuel cell systems

Nicolas Roggo

In the context of climate change mitigation, different alternatives to reduce the atmospheric CO2 emissions are; increasing the efficiency, switching to renewable resource and reduce emissions from fossil resources by CO2 capture and storage (CCS) . Hydrogen is considered as potential alternative to cover the energy needs by using it as a fuel in internal combustion engines or fuel cells, or as chemical source. In proton exchange membrane (PEM) fuel cells, also known as polymer exchange membrane fuel cells, pure hydrogen fuelis combined with the oxygen from the atmosphere to produce water, heat and electricity. Many research has been done in recent years on fuel cell systems leading to higher efficiencies. The performance is influenced by the membrane properties, the catalysts, the number of cells and the operating conditions. For studying the performance of fuel cell systems process modeling is a well-known approach. Here the evolution of fuel cell efficiency, especially of PEMFC is studied within the aim of updating previously developed PEMFC process models to integrate them with different hydrogen production processes using fossil and renewable resources in order to assess the competitiveness on the energy market.

2012