Optimisation of Graphite-Matrix Composites for Collimators in the LHC Upgrade
Creators: name: Guardia-Valenzuela, Jorge
Publisher: name: Zenodo
Year: 2019
Subjects: Graphite
Composite
Sintering
Liquid Phase Sintering
Ceramic
Thermal Management
Materials
Catalytic Graphitisation
Dissolution-Precipitation Mechanism
Carbon
Refractory
Carbide
X-Ray Diffraction (XRD)
Thermo-physical
Characterisation
Powder Metallurgy
Pyrolytic Graphite
Carbon Fibres
Microstructure
Microscopy
LHC
Particle Accelerator
Collimator

description: The present PhD Thesis has been carried out at the European Organization for Nuclear Research (CERN), located in Geneva (Switzerland).<br> At CERN the structure and physics of matter are studied with the aid of high energy particle beams; “Large Hadron Collider” (LHC) produces the world highest energy particle beam. The beam total energy is equivalent to the energy of a full Airbus A320 (66 t) plane travelling at 380 km/h, but concentrated in the diameter of a pencil lead. Beam intercepting devices, in particular collimators, are required as safety protection devices for using these high energy particle beams. Therefore, collimator absorber materials have to withstand one of the hardest man-made environments. On top of that, they must not disturb the normal particle accelerator operation, introducing additional requirements such as high electrical conductivity and ultra-high vacuum compatibility. Finally, the materials have to be produced industrially, therefore reliable and cost-efficient processes are required.<br> In the coming years, an important upgrade, known as High Luminosity LHC (HL-LHC), is foreseen, requiring more performing collimator materials. This calls for the development of novel advanced materials.<br> The uncommon combination of properties required can be met by certain graphite-matrix composites, particularly those including the advantages of liquid phase sintering. Apart from high densification through void infiltration, an adequate liquid phase induces catalytic graphitisation by dissolution of individual carbon atoms and precipitation of “fresh” graphite.<br> The research in this thesis aimed at significantly extending the knowledge of the physical mechanisms --- such as catalytic graphitisation --- that govern the behaviour of these materials. One of the main goals of this research was to find new production routes reducing the material cost and facilitating the sintering control, therefore increasing the market dissemination potential. A broad spectrum of candidate elements to replace molybdenum has been explored and chromium was selected for the study.<br> Four chromium carbide - graphite (CrGr) grades were produced and extensively characterised. Thermo-physical characterisation results have been presented for all the grades produced. Microstructural and phase analysis have been performed. The thermal expansion behaviour and stability of all relevant phases have been characterised by in-situ X-ray diffraction (XRD) up to 600 °C. The results have been compared with two high quality commercial graphites: pyrolytic graphite (PG) and fine-grained nuclear isotropic graphite.<br> The investigated carbide-graphite composites are mainly composed of oriented graphite, therefore they exhibit a transversely isotropic behaviour with in-plane properties comparable to those of pure ordered graphites. However, unlike the latter, which are extremely anisotropic, the composites keep reasonable thermo-physical and mechanical properties also in the direction perpendicular to the basal planes.<br> Besides high energy physics, the combination of properties of these carbide - graphite composites is appealing for a broad range of thermal management applications, particularly in aerospace and electronics, where reduced thermal expansion and low density are required along with high thermal conductivity.
description: Thesis Defence on 20 Sept. 2019