Category: Thesis

Author: Daniel Adrien Franco Lespinasse; Type of thesis: PhD Thesis
Abstract: In the framework of Eucard project, it has been carried out, in collaboration with CERN, the R&D on magnetron sputtering deposition on the HIE-ISOLDE cavity geometry, as an alternative method to deposit niobium thin films. In this research a new magnetron configuration source was tested at the National Institute of Nuclear Physics (INFN-LNL), in order to deposit a uniform niobium thin film onto copper superconducting Quarter Wave Resonator cavities. The methodology was divided in three. A first part, in which a test dummy cavity and a test cathode were used in order to deposit stainless steel onto copper quartz. The purpose of the use of steel has been finding the right parameters of sputtering and also to analyze the uniformity of the film. In this first phase it have been tested several magnetic confinements, which allowed the optimization of the deposition. In parallel it was performed the deposition of stainless steel onto copper strips, to realize the stripping test as a method to analyze the uniformity of the film. The second part was focused on the deposition of niobium thin film onto quartz samples placed along the resonator to improve the superconducting properties, specifically Residual Resistivity Ratio (RRR) and Critical Temperature (Tc); nevertheless other magnetic confinements were tested to maintain the uniformity of the coating. It was studied the influence on superconducting properties of two principal parameters of the sputtering process: the power and the substrate temperature. After setting the deposition parameters, a definitive magnetron confinement was used to deposit the real copper QWR. The RF performance was also measured after the design, construction and installation of a test cryostat. Finally, it was found the magnetic source to deposit a niobium thin film uniformly over QWR cavities. Increasing the substrate temperature and the sputtering power, the transition temperature of the niobium thin film was around 9,3K and it was obtained a maximum RRR of 61. Only 30 min were necessary to deposit the film with a uniformity of 2±1 μm along the cavity. SEM results allowed to analyze the microstructure of the niobium film. Bigger grains were founds on the inner conductor closer to the magnetron source. In addition a test cryostat was successfully built in order to measure the RF performance; the system can be useful to perform measurements at 4.2 and 1.8 K. Respect to the RF performance the first Nb/Cu cavity is under the specifications of CERN with a maximum Q value of 2e8 and an accelerating field of 2MV/m; however this first result is extremely important to start with the optimization phase. Some parameters will be changed in order to improve the performance and push the SRF community to use the magnetron sputtering technique as an economical method to deposit superconducting cavities in short times.

Author: Vlada Pastushenko; Type of thesis: PhD Thesis
Abstract: This work deals with an industrial research on ecological innovative descaling treatments for stainless steels, in substitution of the acid etching process: from the study and the research on samples, the most efficient techniques and their application to industrial purpose are explained. The research has basically covered the study of two pre-finishing treatments (high pressure water blasting and “dry ice”-blasting) and two etching treatments (electropolishing with ionic liquids and in aqueous media). In the field of stainless steels, the surface oxide of iron Fe (III) is combined with the multiple elements added to alloys in order to increase their characteristics (carbon, chromium, nickel and other). Moreover, the surface oxide is presented as a layer very tenacious and compact. In addition, heat treatment leads to the formation of a layer without chromium more readily attacked by oxygen from the atmosphere. Surface treatments are required to remove the surface oxide and recover the
chromium layer. The chemical pickling, that is one of the most common etching processes, depends on many factors, such as the size of the pieces, the type of plant, the type of alloy et al. In general, the traditional solutions contain from 10% to 20% by weight of nitric acid, and 1% to 5% by weight of hydrofluoric acid. The oxidizing environment is provided by nitric acid, which effectively removes the oxide surface, and is subsequently used without the hydrofluoric acid to restore the passive layer (passivation). From the environmental point of view, however, the use of this reagent is very costly:
• Air pollution: the formation of nitrogen oxides (NOx) during the process causes fumes and vapours. These gases are harmful to health, highly polluting (production of acid rain) and extremely aggressive towards metals.
• Water pollution: the high concentration of nitrates and nitrites is one of the causes of eutrophication. In particular, nitrites may form carcinogenic compounds such as nitrosamines, which can enter the food chain through fish.
• Health and safety: hydrofluoric acid is highly corrosive and a poison. It should be handled with extreme attention, using protective equipment and safety precautions. Once absorbed into the blood through the skin, it reacts with blood calcium and may cause cardiac arrest. In addition, it combines with calcium and magnesium of the bones. Since its action can be delayed for many hours, it can distribute throughout the body, causing the erosion of bones. These features have shown how the study of alternative “green” treatments is crucial.