Outgassing Analysis of different Copper Materials for Ultra-High Vacuum by Thermal Desorption Spectroscopy

An extensive outgassing study has been performed for oxygen free electronic copper (Cu-OFE) with different surface treatments (electropolishing, passivation and SUBU) and for electroformed copper. Other materials like stainless steel or Molybdenum Carbide - Graphite composites have also been studied to complement the copper study. All these materials are used or will be used in different parts of the CERN accelerators complex for the vacuum chambers or in components that operate in ultra-high vacuum conditions. Thermal desorption spectroscopy (TDS) is the method used to analyse the outgassing of these materials. The study was mainly focused on the outgassing of hydrogen and water. To complement the outgassing results obtained from the TDS, other analysis methods have been applied: X-ray photoelectron spectroscopy to analyse the surface and its oxidation states and optical microscope and scanning electron microscope to analyse the microstructure of the samples. For the Cu-OFE it has been observed how the outgassing does not change between the electropolishing and the passivation treatments, but for the SUBU treatment around 10 times more outgassing of water is detected. This water is coming from the dissociation of the components that form the SUBU bath that are trapped in pinholes present at the surface. In general, for the Cu-OFE, it has been found that the hydrogen dissolved in the bulk of the copper can produce the reduction of the oxide layer when it diffuses through the surface oxide. The diffusion of hydrogen in copper happens at lower temperatures than the dissociation of the copper oxide. For the electroformed copper it has been possilbe to stablish a relations between the electroforming parameters, the amount of outgassing, the outgassing temperature and the microstructure of the copper. One of these relations is that the concentration of trapped hydrogen increases with the thickness. Also a correlation has been observed between the transient curves and the TDS outgassing results in terms of how the hydrogen is trapped in the copper.