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. Metal Foam Precursor to Achieve Hollow Ligaments
Foam thickness in
(Try for thicker material
Develop larger block capability)
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