20 research outputs found
Multiwavelength Raman spectroscopy analysis of a large sampling of disordered carbons extracted from the Tore Supra tokamak
Disordered carbon often exhibit a complex Raman spectrum, with four to six
components. Here, a large variety of disordered carbons, forming a collection
of samples with a great variety of structures, are analysed using
multi-wavelength Raman microscopy (325.0, 514.5, 785.0 nm). They allow us to
extend Raman behaviour known for nano-crystalline graphite to amorphous
carbons, (dependence with the excitation wavelength) and other known for
amorphous carbons to nano-crystalline graphite, (differentiation of the
smallest cluster size probed using different excitation wavelengths).
Experimental spectra were compared to simulated spectra, built using known
laws, to evidence a new source of broadening
Raman Microscopy: A Suitable Tool for Characterizing Surfaces in Interaction with Plasmas in the Field of Nuclear Fusion
Raman microscopy, which is sensitive to chemical bonds, defects, structure, is a suitable tool that can give information on how a material can be modified by interacting with ions. We will first give concrete examples on how it can be used to characterize with a micrometric resolution samples extracted from tokamaks. We will then give concrete examples on what information can be obtained by doing a study on laboratory synthesized materials, benchmarking Raman microscopy with quantitative techniques. The first part of the chapter is focused on carbonâbased material analysis. We will show how Raman spectra are sensitive to the presence of hydrogen, a major safety issue in the field. The second part of the chapter will be focused on berylliumâ and tungstenâbased material analysis. We will show that hydrogen can be stored as an hydride after ion implantation and that it can be released easily in tungsten oxide
Raman spectroscopy investigation of the H content of heated hard amorphous carbon layers
We revisit here how Raman spectroscopy can be used to estimate the H content
in hard hydrogenated amorphous carbon layers. The H content was varied from 2
at.% to 30 at.%, using heat treatments of a a-C:H, from room temperature to
1300 K and was determined independently using ion beam analysis. We examine the
correlation of various Raman parameters and the consistency of their thermal
evolution with thermo-desorption results. We identify a weak band at 860 cm-1
attributed to H bonded to C(sp2). We show that the HD/HG parameter (Height
ratio between the D and G bands) is quasi-linear in the full range of H content
and can thus be used to estimate the H content. Conversely, we show that the
m/HG parameter (ratio between the photoluminescence background, m, and the
height of the G band), often used to estimate the H content, should be used
with care, first because it is sensitive to various photoluminescence quenching
processes and second because it is not sensitive to H bonded to C(sp2)
Raman micro-spectroscopy as a tool to measure the absorption coefficient and the erosion rate of hydrogenated amorphous carbon films heat-treated under hydrogen bombardment
We present a fast and simple way to determine the erosion rate and absorption
coefficient of hydrogenated amorphous carbon films exposed to a hydrogen atomic
source based on ex-situ Raman micro-spectroscopy. Results are compared to
ellipsometry measurement. The method is applied to films eroded at different
temperatures. A maximum of the erosion rate is found at ~ 450 {\degree}C in
agreement with previous results. This technique is suitable for future
quantitative studies on the erosion of thin carbonaceous films, especially of
interest for plasma wall interactions occurring in thermonuclear fusion
devices
Long Term H-Release from Amorphous Carbon Evidenced by in Situ Raman Microscopy under Isothermal Heating
We study the kinetics of the H release from plasma-deposited hydrogenated
amorphous carbon films under isothermal heating at 450, 500 and 600 {\degree}C
for long times up to several days using in situ Raman microscopy. Four Raman
parameters are analyzed. They allow the identification of different processes
such as the carbon network reorganization and the H release from sp3 or sp2
carbon atoms and the corresponding timescales. Carbon reorganization with
aromatization and loss of sp3 hybridization occurs first in 100 minutes at 500
{\degree}C. The final organization is similar at all investigated temperatures.
Full H release from sp3 carbon occurs on a longer timescale of about 10 hours
while H release from sp2 carbon atoms is only partial, even after several days.
All these processes occur more rapidly with higher initial H content, in
agreement with what is known about the stability of these types of films. A
quantitative analysis of these kinetics studies gives valuable information
about the microscopic processes at the origin of the H release through the
determination of activation energies
Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on Hâ yield
International audienceHighly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56MHz plasma reactor. Relative yields of surface-produced Hâ ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (âŒ3-5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing Hâ production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions
Raman study of CFC tiles extracted from the toroidal pump limiter of Tore Supra
International audienceThe structure of six tiles extracted from the erosion and deposition zones (thin and thick deposition) of the Tore Supra toroidal pump limiter (TPL) have been analysed in the framework of the DITS campaign using micro-Raman spectroscopy. This post-mortem analysis gives information on both carbon structure and D content. We have found that the carbon structure is most often similar to that of plasma-deposited hard amorphous carbon layers. The role of the surface temperature during the discharge in the D content is investigated: in all locations where the temperature does not reach more than 500°C the D content seems to be roughly uniform with D/D+C â 20%
Microscopie Raman en sciences des matériaux : Des carbones désordonnés aux parois de tokamaks
Multiwavelength Raman spectroscopy analysis of a large sampling of disordered carbons extracted from the Tore Supra tokamak
International audienceDisordered carbon often exhibit a complex Raman spectrum, with four to six components. Here, a large variety of disordered carbons, forming a collection of samples with a great variety of structures, are analysed using multi-wavelength Raman microscopy (325.0, 514.5, 785.0 nm). They allow us to extend Raman behaviour known for nano-crystalline graphite to amorphous carbons, (dependence with the excitation wavelength) and other known for amorphous carbons to nano-crystalline graphite, (differentiation of the smallest cluster size probed using different excitation wavelengths). Experimental spectra were compared to simulated spectra, built using known laws, to evidence a new source of broadening