Study and Characterization of Magnetic Materials for Beam Intensity Monitors at CERN

The number of particles in the beams used at CERN is measured by a family of devices called the Beam Current Transformers (BCTs). One of such devices is the DC Current Transformer (DCCT) measuring the total number of particles in an accelerator. The DCCT is built around a magnetic core made of a soft magnetic material. Currently, the DCCT cores are purchased from one of a few industrial partners. This situation might result in some signiï¬cant issues with development of the DCCTs. Firstly, the choice of the core can be made only among the already developed and manufactured cores. Secondly, the industrial partners do not normally guarantee long-term availability of their products which could manifest itself as future maintenance problems. Thirdly, the cores available on the market have not been developed with CERNâs intended application in mind and, hence, might be non-optimal for the DCCTs. These three factors, among others, led to a decision to investigate a possibility of in-house core manufacturing. BCT cores are made up of a soft magnetic material in ribbon-form between 20 and 30 microns thick, wound into a toroidal shape. They are amorphous or nanocrystalline cobalt and iron-based alloys that show high permeabilities and low coercivities. Several commercial alloys in their amorphous as-cast state were purchased after identifying the key parameters of the materials for the application, contacting suppliers and studying the available materials to ï¬nd the most suitable ones. This choice was made by confronting the as-cast properties of the alloys with the needs of the instruments which would house the magnetic cores. The materials were then characterised to determine how to thermally treat them in order to obtain a range of diï¬erent ï¬nal magnetic properties. This study also resulted in establishing the best candidates to fabricate the sample cores. After this, three alloys were selected (iron-based alloy Finemet FT-3 from Metglas and two-cobalt based materials, 6025 G40 from Vacuumschmelze and 2705 M from Metglas) based on their as-cast properties. The cores and ribbons of the selected materials were then annealed at diï¬erent temperatures and durations to study the eï¬ect of the treatment in both. Three treatments were done on the selected samples, below the Curie temperature, between the Curie and the crystallisation temperature and above the crystallisation temperature. Measurements of the samples were taken after the thermal treatment and compared to the results of the as-cast materials to see the eï¬ects of the annealing. This gave a range of diï¬erent ï¬nal materials with various permeability vs frequency behaviours, BH-curves and Barkhausen Noise. These new materials, in addition to the untreated as-cast ones, provide a good range of samples to choose from for applications with diï¬erent needs. Based on the results presented in this thesis, one can suggest that the best material to build DCCT cores would be the iron-based nanocrystallised Metglas Finemet FT-3 material. It shows a high permeability up to 10 kHz, low coercivity and a rounded BH-curve. Regarding FBCTs, the cores need a very ï¬at BH-curve with high permeability and minimum coercivity. From all the materials presented, Vacuumschmelzeâs cobalt-based Vitrovac 6025 G40 is the one that would be the most suitable. It has the lowest coercivity. However, the perfect material would need magnetic thermal annealing to create a high magnetic material anisotropy for a flat curve.

Optimisation of low pressure processing for honeycomb sandwich structures

Sandra Sequeira Lopes Tavares

Vacuum-bag processing of honeycomb sandwich structures faces particular challenges compared to autoclave sandwich fabrication for aeronautics, a consequence of using, at the most, atmospheric pressure for their manufacturing. In particular, the adhesion of the skin to the honeycomb core is reduced and the skins tend to have higher values of porosity. On the other hand, autoclave processing is expensive, in particular for large structures, and a growing interest has been observed for vacuum-bag only processing of sandwich structures. During honeycomb sandwich cure, the skin represents a barrier to air flow between the honeycomb and the vacuum-bag so the air transport through the prepreg, adhesive and consumables or other additional materials may be a crucial factor to increase skin debulking, reduce void content and extract air from the honeycomb cells. The pressure inside the honeycomb is, therefore, a result of the through thickness permeability to air of the skins and vacuum pump work. The goal of this thesis is to develop methods for improving the low pressure processing of honeycomb sandwich structures by quantifying the through thickness air permeability of fibre reinforced composites during cure and analysing related processing issues and governing phenomena. An experimental method based on a falling pressure measurement was proposed to evaluate the through thickness air permeability of prepregs during cure, with two set-up variants. One concerns the measurement of the inherent air permeability and the other applies to processing conditions, implying the use of both consumables and vacuum-bag. The methods were successfully tested and fulfil the criterions used in soil science for the evaluation of the air permeability. Prepreg and adhesive permeabilities were determined separately and in skin combination. Prepreg permeability to air during cure varies between 10-19 m2 and 10-17 m2 and was found to depend on the evolution of the resin viscosity, which shifts from an initially immobile phase to a mobile one and then back to an immobile phase. The adhesive film was found to have very low initial permeability, at least two orders of magnitude lower than that of the prepreg. A range of initial skin through thickness air permeability could be achieved by perforating the prepreg plies, the adhesive layer, or a combination of both. An adhesive deposition method which results in the centre of honeycomb cells being free of adhesive was also tested. A corresponding range of achievable pressure was measured inside the honeycomb. The evolution of the through thickness permeability with the curing cycle was determined for each case. The adhesive layer was identified as the element that reduces the initial through thickness air permeability the most in skin manufacturing. This suggests that one of the ways of increasing the initial through thickness air permeability of the skin is to modify the permeability of the adhesive layer. The role of the resulting pressure inside the honeycomb on skin-core adhesion and skin quality was then evaluated. Skin permeability was found to control skin-core adhesion through the pressure drop in the honeycomb cells and potential outgassing of the adhesive layer. An optimal range of pressure inside the honeycomb was found to be between 40 kPa and 70 kPa. A process window is proposed for achieving the optimal pressure inside the honeycomb, as a function of the skin permeability and time of vacuum application. A semipreg alternating dry and resin impregnated areas along the fibre bed surface was also investigated as an alternative to prepreg in skin manufacturing. The semipreg through thickness air permeability before cure is approximately three orders of magnitude higher than that of a unidirectional prepreg impregnated with the same resin, thus allowing the reduction of air pressure in the honeycomb. A model was proposed for the air permeability change during cure, as dry areas get infiltrated. Due to resin pouring inside the honeycomb cells, this type of semipreg is viable as a skin only if combined with a material that has low permeability to resin, e.g., a prepreg. The resulting initial pressure in the honeycomb cells might then be tailored to specific needs. Finally, it was possible to produce sandwich samples combining a consolidation system, such as a prepreg, with an infiltration system, as wet lay-up or with a semipreg. The combination of a prepreg with a semipreg revealed to be a very practical and clean system, where it is possible to have the same resin in both materials. Moreover, it allows to tailor the initial honeycomb pressure as well as during cure, and to have the initial permeability assessed.

EPFL2009

Optimisation of low pressure processing for honeycomb sandwich structures

Sandra Sequeira Lopes Tavares

EPFL2009