Porous thick-film silver metallisation for thermally mismatched brazing operations in tokamak magnetic sensor

A novel sensor based on thick-film + LTCC (low-temperature cofired ceramic) technology has been recently developed for sensing high-frequency 3D magnetic fields in tokamak fusion devices. For integration within the walls of the tokamak, the sensor has to be connected to the mineral-insulated cabling, which is carried out by brazing to ensure sufficient thermal stability. However, thermal mismatch stresses between the braze and the cable vs. the alumina substrate may cause local cracking of the latter during cooling, as the basic dense silver metallisation of the alumina does not provide a sufficient degree of stress decoupling. To address this issue, a series of porous metallisations have been formulated by incorporation of a mix of silver and fugitive graphite powder into a thick-film paste. To allow co-firing of thick, multi-layered prints. Such porous metallisations have allowed successful brazing operations, without cracking of the alumina substrate. Metallisations were assessed by measuring their electrical resistivity and shear stress have been realised as preliminary results to measure the influence of the porosity on the maximal stress before cracking.