Study of SEY degradation of amorphous carbon coatings

Deposition of low secondary electron yield (SEY) carbon coatings by magnetron sputtering onto the inner walls of the accelerator seems to be the most promising solution for suppressing the electron cloud problem. However, these coatings change their electron emission properties during long term exposure to air. The ageing process of carbon coated samples with initial SEY of about 0.9 received from CERN is studied as a function of exposure to different environments. It is shown that samples having the same initial SEY may age with different rates. The SEY increase can be correlated with the surface concentration of oxygen. Annealing of samples in air at 100-200 {\deg}C reduces the ageing rate and even recovers previously degraded samples. The result of annealing is reduction of the hydrogen content in the coatings by triggering its surface segregation followed by desorption.Comment: 4 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba, Italy; CERN Yellow Report CERN-2013-002, pp.149-15

Modelling of gas permeation based on the morphology of a natural polymer material

Natural polymers have been studied during the last years for the transport and separation of liquid and gas mixtures, in terms of solubility and permeability data, and their structure and mechanical properties have been characterised. However, no transport models have been reported, relating transport with the material morphology. Cork is a natural cellular material containing three structural polymers (suberin, lignin and polysaccharides). Cork is considered a natural polymer, with economic relevance due to its sealing, non-toxic, stable and low-density properties. Cork was characterised in this work in terms of its solubility and permeability data in relation to various gases with different molecular mass: He, O-2, N-2, CO2 and 1,1,1,2-tetrafluoroethane (R134a). A morphological analysis of the structure of the cork sample chosen in this work was also performed using SEM (scanning electron microscopy) and TEM (transmission electron microscope) image analysis, which took into account the variation of each relevant structural parameter. A transport model was developed supported on the morphology of cork characterised in this work. The transport model developed considers that gas permeation occurs through the plasmodesmata, which are channels with approximately 100 nm of diameter that cross the cell walls of the cork cells. It was found that gas transport follows a Knudsen mechanism, as proved by the gas permeability behaviour with increasing gas molecular mass, with a negligible contribution of viscous transport to the total flux.publishersversionpublishe