Speaker
Description
For the high-luminosity LHC (HL-LHC), CMS will install a completely new silicon tracker. The future Outer Tracker will consist of two barrel parts and two endcaps, one at each end. Two types of silicon detector modules are being utilized, the Strip-Strip (2S) modules for the outer radii and the Pixel-Strip (PS) modules for the inner radii. For the PS modules, the entire bottom of around 5 x 13 cm^2 must be thermally coupled to the mechanics to allow efficient cooling, for which a thermal interface material is needed. The current candidate materials for the use in the endcaps are room temperature curing two component thermal gap fillers.The contribution will outline the measurements and highlight the results to qualify gap filler materials to the radiation dose expected for the lifetime of the CMS Outer Tracker. Three different types have been tested thermally and mechanically in this campaign. A thermal test setup determines the thermal conductivity of a test sample by measuring the temperature gradient with a controlled amount of heat flow through a sample. Mechanical tests are needed to ensure structural integrity of the thermal interface even when under some extent of thermal stress. Resembling the style of an ISO 4587 lap shear test, and an ISO 25217 mode-1 fracture test, test samples were made with a large 5 x 5 cm^2 adhesion overlap using plasma cleaned carbon fibre plates to have a surface comparable to its intended use case. The testing method developed for this study will be presented and motivated. After testing of unirradiated samples, they have been irradiated to 600 kGy. The measured mechanical and thermal properties will be presented and the results before and after irradiation will be compared. We found that the gap filler material hardens significantly, however its thermal and adhesive properties are maintained. The hardening reduces the cohesion failure, leading to an increased mechanical strength of the bonds. A specific product has been chosen as suitable material. A follow-up campaign is being prepared to further quantify the properties before and after irradiation.Applying the thermal interface material in a thin layer reliably on the entire bottom of the module is a challenge. Particularly as many modules are mounted on a large structure in close proximity, the available space around the module being installed is very limited. The intended method of application will be explained. The reliability and quality of the interface are evaluated with thermal tests using a thermal dummy module and a cooling structure resembling the mechanics of the final detector.
