| Περίληψη: | This thesis aims to determine optimal parameters for the diffusion bonding process which is used as a main step in the fabrication work-flow of the accelerating structures.
The focus is on investigating the minimum necessary pressure during the heating cycle of the diffusion bonding, in order to acquire a good bonded joint but at the same time avoid any excessive deformations and preserve the micro-precision tolerances.
As an additional parameter, the flatness and shape of the bonding samples have been examined in order to determine the optimum tolerances and bring the fabrication cost down.
The RF tuning method \cite{shi2010tuning} allows for internal volume variations after the bonding procedure and therefore can compensate for these errors. Nevertheless, the final design of CLIC RF structures does not foresee this tuning step and, consequently, these assembly errors must be predicted and reduced already in the design phase.
In this respect, diffusion bonding tests were conducted with disks made of OFE Oxygen-Free Electronic Copper (Cu-OFE)
with variable geometries, values of applied pressure and interface flatness. A series of experimental tests was performed in order to
qualify the bonded joints; including ultrasonic examination, crystallographic analysis of the interfaces as well as tensile tests. The results of the tensile tests are validated with finite element simulations by using fracture mechanics theory. The results of the deformations are compared with an analytical approach which uses creep mechanisms
so as to predict the permanent deformations after the heating cycle.
In order to address several questions that are related to the bonded joint, a further analysis on the atomic level using Molecular Dynamics (MD) simulations was required. Several simulations where conducted with Cu atoms and the influence of temperature and pressure had been investigated on the time needed for nano-scale voids to be filled. Those voids represent the roughness (Ra) of two surfaces in contact before the heating process. The main aim of this study was to extrapolate the findings of the simulations in voids of nano-meter range to micro-meter voids size. Therefore, a formula which calculates the closing time in bigger void sizes and different temperatures had been invented. In parallel, calculations of the diffusion coefficient in the group of atoms which belong to the GBs, verify a well known creep law and adds even more value in the simulated results. Going a step further the aspect of applied pressure together with the high temperature is examined for crystals with the same crystallographic orientation.
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