Pixel detector local supports: carbon monolithic stave

1. Pixel detector local supports:carbon monolithic stave ATLAS Inner Detector Engineering Review CERN, October 22nd 1998 M.Olcese INFN-Genova

2. Basic Barrel Local Support Concept • The barrel local support (stave) is a basic unit designed to support 13 modules (total length 800 mm ca.) with a z overlap to ensure hermeticity • The stave design have to fit within a three layer barrel layout providing the required F coverage with the minimum amount of material • the stave must be attached to the global support structure (half shells) with the minimum number of supports to reduce material three points support = minimum stability requirement

3. Summary of Stave Requirements • High alignment precision: Better than 5 mm (F), 10 mm (z, r) • High stability: Max allowable distortion: 25 mm (F), 50 mm (z, r) • Max temperature on sensor: -6°C • Average radiation length design goal: less than 0.7%

4. Material Choice: Major Design Issue • Very aggressive specifications: many constraints • few candidates combining all critical properties: • High stiffness vs.. low mass and Z • Low CTE and CME • Good thermal properties in all directions • Radiation hardness • Coolant inert

5. The carbon monolitic stave C-C TMT • cooling tube made of an W-shaped Carbon Fibre Reinforced Polymer (CFRP) material glued on a Carbon-Carbon (C-C) composite Thermal Management Tile (TMT) • TMT machined from a C-C plate with a simple stepping sequence easy to fabricate and to align Omega piece Top view Bottom view

6. Carbon monolithic stave Pro’s • very stiff (carbon fibre based) and light • simple geometry (only two parts per stave) • low radiation length % (nearly 90% of Carbon) • good thermal efficiency and reliability (fluid in direct contact with TMT) Con’s • glue joint reliability

7. The Carbon-Carbon TMT: a proper choice to address the thermal issue • C-C is a full carbon material (CFRP where the matrix is carbonised through a high temperature process) • combine excellent mechanical properties (high stiffness, low CTE) with good thermal properties also in through thickness direction (K//=200 W/mK, Kt=32 W/mK - XN50-2D) Max DT = 2.5°C TMT must collect heat along transverse direction and transport it along the in plane direction Q = 0.6 W/cm2 400 mm • but it is porous (10% void fraction)……….

8. How to address the C-C sealing issue • Two processes have been explored: • surface coating (introduce an additional thermal impedance and it does not guarantee long term reliability) • impregnation: • does not affect the TMT thermal properties • it is reliable (sealing is not affected by surface effects, like fluid chemical attach) • we have adopted impregnation

9. The impregnation process • A vacuum impregnation process followed by a pressure curing has been developed • different impregnation resins have been tested • CIBA LY5052 resin gives the best results: • the max He leak rate over the entire stave is less than 0.05 cc/year • the leak rate is still the same even after 33 MRads (6.1*1013 55 MeV protons/cm2)dose C-C piece Impregnation chamber

10. The omega piece • Provides the required stiffness to the stave • the material (CFRP: Toray M60J fibres + Fiberite 954/2A cyanate ester resin) and the lay-up have been optimised to be as close as possible to the longitudinal CTE of the TMT(minimise cool-down distortions) • three CFRP layers have been used (300 mm total thickness) with two different lay-ups: • 0°-90°-0° • 0°-15°-0° • a graphite mould has been adopted to minimise the distortions after curing

11. The stave manufacturing process • Omega forming using a graphite mould • C-C pre-machining to 20x1,2 mm cross section plates (with no step) • C-C impregnation • C-C re-machining to the final step sequence and thickness • gluing of the 4 (now, 2 in the next prototype) C-C piece together • C-C + omega gluing (curing at room temperature + a post-curing at 60 °C to enhance the glue joint strength) • stave post milling (side thinning of the upper steps)

12. Performances assessment • Detailed FEAs have been made simulating the stave performances as function of: weight, cool-down, fluid pressure • Two prototypes (with the two different omega lay-ups) meeting all design parameters have been built • both prototypes have been extensively tested and the results have been compared with the calculation finding always a good agreement

13. Finite element analysis • A detailed FE model of the stave has been made: each CFRP layer of the omega piece has been modelled, allowing to simulate not only the stave bending but also twisting due to cool-down

14. Thermal performances • tests made with equivalent water-ethanol binary ice mixture • fluid inlet temperature = -16.5 °C • ice = 5 % ~ 8 °C ~ 5 °C

15. Prototypes: “as built” geometry • The prototype “as built” geometry at room temperature has been investigated using a CMM tool, attaching the stave on three supports aligned at the same r position • Both prototypes show: • individual stepplanarity: better than 30 mm • average step side tilting: < 30 mm • The max r deviation from nominal profile (measured along the stave z axis) • 0-90-0 stave: 150 mm • 0-15-0 stave: 60 mm Dr (step side tilting)

16. Prototype “as built geometry”:stave longitudinal shape deviations 0-15-0 stave centerline 0-90-0 stave centerline +60 mm +150 mm -50 mm -60 mm Stave longitude Stave longitude Support location Hunchbacks due to the non complete accommodation of the free omega bowing (2mm sag) The stave longitudinal r fluctuations are linked to the C-C TMT manufacturing process (four parts glued together due to supplier tooling limits: this problem will be solved with the next prototypes)

17. Cool-down stability • Stave distortions during cool-down are induced by different material CTEs • while the DCTE TMT-omega can be made small enough optimising the omega lay-up (current value is ~0,24 ppm °C-1), the DCTE TMT-module (~5 ppm °C-1) cannot be cut down: the only way to reduce the distortions is to decouple the module from the TMT by using a flexible glue (a few possible candidate have been identified) • Two independent cool-down tests have been performed on staves (without modules) using different technologies to validate FE calculation, FE analysis has been then used to evaluate the effect of the glue joint (TMT-module) elasticity Max displacement (mm) DCTE TMT-module DCTE TMT-omega

18. Cool down stability tests CMM tooling 0-15-0 stave longitudinal r displacements Measuring head -20 mm A Stave 0-90-0 stave longitudinal r displacements A +20 mm +13 mm Cold box B ESPI setup Stave longitude A curve: FEA surface impregnatedTMT B curve: FEA deep impregnatedTMT Support location

19. Cool-down tests: 0-15-0 stave twisting CMM-touch probe measurements Dy=60 mm • Beside to a longitudinal deflection the 0-15-0 omega stave also shows an undesirable twisting due the 15° CFRP layer • preventing the rotation of the ends supports, the max side displacement due to tilting is within the specifications (20 mm), but the torque applied by the two end support is not negligible (13 Nmm) Free end support rotation ESPI cross check Stave mid support location Dy=260 mm Rotation fully constrained 8th step 9th step Trans. Tilt angle Dy=20 mm Dy (side displacement)

20. Combined cool-down and weight deflections: 0-90-0 stave • gravity sag (total (stave+modules) design weight 75 g (*)) = 42 mm • cool-down deflection (modules glued with soft adhesive): 12 mm (*) very conservative! The current ladder weight should be of the order of 50 g (the stave weights 23 g) Worst case (top location) 52 mm Best case (bottom location) Cool-down deflection Gravity deflection Mid support End support 33 mm

21. Moisture effects • Moisture release (dry operating environment) + coolant absorption induce swelling in resin matrix based materials • likely CTE, differential swelling behaviour of impregnated C-C and omega piece produce r deflections of the stave. • A proper resin choice (standard for impregnated C-C and low moisture absorption for the omega piece) counterbalancing the different resin content of impregnated C-C (8%) and omega piece (33%) minimisethe effect (very close moisture strains of the two parts) • if C4F10 absorption swelling is, as expected, just scaled by the same factor for the C-C and omega parts, the corresponding r stave distortions should be negligible too (long term test have to be done)

22. Stave dynamic behaviour FEA dynamic analysis assuming as stave weight 75 g (design weight) 2nd mode: 273 Hz Fundamental mode: 89 Hz Mid support 108 Hz assuming 50 g weight (current estimation) End support

23. Effect of coolant pressure Inside pressure assumed to be conservatively - 1 bar (relative) stave lower step top view FEA Max TMT out-of-plane = 18 mm • FEA calculated max module out-of-plane: • upper steps: 2 mm • lower steps: 12 mm ESPI

24. How to address the glue joint reliability issue • Fluid operating pressure will be below the outside pressure: the glue joint will be normally in compression (safe loading condition) • anyway, in order to evaluate the glue joint strength and since the maximum allowable inside pressure, peeling tests have been performed and are still in progress • peeling tests have been made as function of: temperature, radiation dose, coolant action (immersion in C6F14 liquid fluid) • stave samples (15mm long) have been prepared using two different glue candidates (CIBA AY103 and 420=A/B)

25. Peeling tests Glue: CIBA Araldite 420 A/B (a) Glue: CIBA AY103 (b) = 33 bar fluid pressure = 26 bar fluid pressure (1) Fluence = 9.4*1014 neq/cm2 (1) Fluence = 1.88*1015 neq/cm2 (3) 1800 houres @ 20 C • glue peeling strengths better than supplier data-sheet values • 420 A/B shows a higher peeling strength than AY103 in agreement with supplier data

26. Stave supports: installation/removal End support assembly Support ring insert Stave end support Fastening clip Stave cooling channel termination Individual stave removal sequence alignment pin

27. Conclusions • All major design issues have been addressed (thermal performances, cool-down stability, fluid pressure, fabrication). We did not find any show stopper • stave weight = 23 g, average X0 = 0.53 % per layer including overlap • tests on the most critical issue (glue joint reliability) indicate that the safety margin against glue joint break is quite large even after irradiation and coolant exposure • medium long term test program (stability against coolant action/absorption) will be started as soon as a baseline decision will be taken