New Proposed Foam Developments

1. New Proposed Foam Developments M. Gilchriese, LBNL W. Miller, W. Shih, Allcomp

2. Carbon Foam Topics • Allcomp Carbon Foam Current Technology 0.2g/cc density • Focused on “core” application in long stave-like sandwich structures • Both strength and cooling design issues • Low radiation length penalty • Allcomp’s answer was open carbon cell structure processed for optimum K • Foam blocks produced 12in by 12in by 1in thickness • Highly thermally conductive, ligament K~1700 W/mK • Thermal evaluation by laser diffusivity for bulk thermal conductivity • Strongly dependent on final density • Predictive methods for correlating results have been proposed and studied • Future Development Opportunities • Applications: • Short structures, vertical and horizontal orientation, long suspension spans not primary focus • Ultra-low density >50% reduction in mass? • Detectors bonded directly to foam? • Re-think foam processing methods

3. Present Foam Appearance and Size • Foam blocks • Notice ligament structure • Solid cross-section • Foam density is a variable for the user • Strength and thermal properties increase with increasing density • Foam surface • Un-sealed • Co-curing composite facing has been demonstrated • Machined to fit sandwich structures with embedded cooling tube • Proven to work well 12in by 12in by 1in foam blocks

4. Allcomp Foam Photomicrograph • Photos for both 100ppi and 130ppi foam • First distinction to keep in mind for later is the cross-section of RVC core member • RVC precursor has low thermal conductivity • For future foam development, idea is to eliminate this portion of the ligament, which lowers mass • At some expense to strength, question how much? CVD graphite RVC precursor

5. Approach to Eliminate RVC Precursor • Objective is to lower density and have solely highly conductive graphite • Approach to eliminating RVC precursor • Precursor initially needed for skeleton • CVD carbon over sacrificial precursor • Remove precursor, leaving hollow ligaments • Heat treat CVD carbon to obtain highly conductive graphite • Present precursor of choice • Nickel foam, commercially available • Need to try different ppi foam to settle on approach • Nickel is removed via acid digestion • Alternate is high temperature exposure to chlorine gas • Result is hollow ligaments, all graphite • Sounds easy, but?

6. Where Does Hollow Core Foam Fit? Target area? Hollow ligament foam would be lower density and lower K

7. To What Extent Do We Lose Strength? Data normalize in fashion described by Ashby, ratio varies as square of density ratio Structural model ~500,000 elements Characteristics? FE cell models used to predict foam modulus, using modulus of elasticity for carbon 3.5Msi and graphite 5Msi

8. Foam Surface Sealing • Foam surface specimen is sealed by CVD carbon • Objective to develop micro sealing of foam surface • Progress in limbo because of funding • Sealing option for future work • Micro-Seal with polycarbosilane • High temperature pyrolysis • Test for effect on foam thermal conductivity • 1 mil 1000W/mK grapfoil carbonized to surface • Advantage • Direct bonding of pixel detectors to conductive interface without laminates sealed

9. Where We Are Now • Allcomp high conductivity comes from: • Special thermal processing of CVD material, converting material conductivity to graphite, 4 times that of copper • Conductivity variations appear connected to final CVD material structure • Strongly proportional to increased foam density • Foam conductivity is weak function of cell size, as presently processed • More dependent on density • Noticed in our data and in foam literature • In density range of interest 130ppi has a conductivity improvement over 100ppi • At very low densities 100ppi should be higher, since the CVD coating over the 130ppi foam is thinner • But that was not the general case • 130ppi foam processing at 0.2g/cc and above produces higher conductivity than 100ppi foam

10. What Would Our Approach Be in The Future? • Develop foam processing for hollow ligament graphite structure foam • Start with 100 and 130ppi foam cell size • Good machinability • Does it make sense to try 30ppi to 45 ppi foam? • Column strength of ligament possibly greater, since ligament shell cross-section would have a greater perimeter • Ultra-lightweight foam • Micro-seal foam surface with Si • CVD silicon onto overcoat, micro polish? • Appropriate for hollow ligament foam? • First order evaluations for new foam • Measurement • Compression and tensile testing, extend analytical model to cover ultra-low density foam • Fabricate sample sandwich structure • Simple bend tests

11. Foam thickness 0.125 in (Try for thicker material Develop larger block capability) Metal Foam Precursor to Achieve Hollow Ligaments • Metal Base material • Deposit carbon onto precursor • Etch metal away • Grind up foam • Perform crystallographic analysis to demonstrate graphite purity Metal foam precursor

12. Hollow Foam Ligaments • Foam Structure Presents New Challenges • Processing of course is an issue • Removal of precursor, that poses less of a problem, but carbon purity could be an issue • However to what extent are the primary parameters, thermal conductivity and strength affected? • More testing, with and without surface sealing • Surface sealing may prove useful to stabilizing the hollow ligaments • With foil and with polycarbosilane • What is the best approach to modeling? • Combination of empirical solutions and FEA? • FEA of CAD generated cell geometry? • Some success • FEA of scanned foam specimens? • Before ligaments were represented as solids, now need shell elements • Model requires gazillion elements!

13. Summary on Allcomp Foam Material • Allcomp will focus on 130ppi foam for applications using 0.2g/cc • We have answered our satisfaction what caused several blocks to have low thermal conductivity • This was not a trivial aspect to chase down—had to do with samples taken from edges • Progress to date • Achieving high conductivity low density carbon foam is a success in large batch processing • Low density, solely carbon based material equates to very long radiation length • Solid density of the ligaments is ~1.98g/cc • Makes the solid radiation length 212mm or for the foam 1909mm at 0.22g/cc • Pixel stave <5mm thickness or 0.26% of a radiation length • Goal for radiation length for foam is 8500mm or ~0.05% versus current 1909mm