1. Thermally Conductive Carbon Foam Properties M. Gilchriese, M. Cepeda - LBNL W. Miller, W. Shih, R. Ramirez-Allcomp

2. VG 2 Foam Progress General General –Processed and tested two basic foams, 12in by 12in by 1in blocks All from Allcomp, Inc All from Allcomp, Inc 100ppi and 130ppi (ppi=pores per inch) 100ppi and 130ppi (ppi=pores per inch) Several foam densities Several foam densities –Testing Thermal conductivity by laser diffusivity and direct method Thermal conductivity by laser diffusivity and direct method Tensile and Compressive strengths Tensile and Compressive strengths –Modulus of elasticity –Recorded nonlinear compressive stress-strain over wide strain LBNL Measured CTE of foam LBNL Measured CTE of foam –Virgin and resin filled  Foam (limit to CTE)  Foam with HYSOL 9396  Foam with HYSOL 9396 and BN filled Very recently, CTE measurements by company(Delsen) Very recently, CTE measurements by company(Delsen)

3. Foam Appearance Foam blocks – –Thermally conductive lightweight carbon foam are produced in block form Material readily machinable to into precise thin sections – –Foam density is a variable for the user Strength and thermal properties increase with increasing density 12in by 12in by 1in foam blocks

4. VG 4 Foam Strength Test Results Table compares recent K7 (100ppi) and K9 (130ppi) with prior data Table compares recent K7 (100ppi) and K9 (130ppi) with prior data –K9 exhibits higher tensile strength (σ) and higher modulus (E) than K7 and prior 100pi foams Foam density is value following foam “K” designator, e.g. K3/0.09 Foam density is value following foam “K” designator, e.g. K3/0.09 –Density in this case is 0.09gm/cc –Part of the increased strength and stiffness is due to the higher density 0.23gm/cc versus 0.19gm/cc, but clearly K9 stands out from a tensile property Tensile properties

5. VG 5 Foam Strength Test Results Compressive properties favor the 130ppi K9 foam Compressive properties favor the 130ppi K9 foam –K9 compressive strength only slightly higher than K7 However, its modulus is noticeably higher However, its modulus is noticeably higher Foam compressive behavior decidedly different than tensile Foam compressive behavior decidedly different than tensile –Tensile failure has a clear fracture point –Compression failure is not so clear Strain in a stave is comparatively low in relation to strain to failure in Allcomp tests Strain in a stave is comparatively low in relation to strain to failure in Allcomp tests –Compressive stress peak is followed by discontinuities in the stress- strain curve----plastic buckling of the foam Compressive properties

6. VG 6 Foam Compressive Behavior Notice compressive peak followed by rapid unloading of Instron machine Notice compressive peak followed by rapid unloading of Instron machine –Instron loading recovers and then an oscillatory behavior follows –Thermal solutions of the stave prototypes (25C down to -35C) results in plastic strain nominally 0.02 Tests are being conducted to see what damage if any is encountered at strain less than 0.05 Tests are being conducted to see what damage if any is encountered at strain less than 0.05 –Initial dramatic unloading a testing issue? 1inch high specimen compressed to 0.75in, 25% strain

7. VG 7 Foam Tensile Characteristic Tensile curve Tensile curve –Fairly linear, repeatable Fracture strength of K9 higher than K7 Fracture strength of K9 higher than K7 Tensile strength well above stave stresses Tensile strength well above stave stresses

8. VG 8 Foam Thermal Conductivity Foam Laser Diffusivity Measurements Foam Laser Diffusivity Measurements –K9 @ 0.23gm/cc K~38W/K, K7 @ 0.22g//cc K=22W/mK Variance in diffusivity measurements led to direct K measurements Variance in diffusivity measurements led to direct K measurements K8 used in strip stave prototypes

9. VG 9 Conductivity by Direct Method Direct Conductivity Direct Conductivity –Complication from the contact faces being foam Used thermal grease to reduce thermal resistance Used thermal grease to reduce thermal resistance Present method uses two different foam heights to back out effect of interface thermal conductance Present method uses two different foam heights to back out effect of interface thermal conductance –Data contained in Table based on initial test method Published SS data is 16 w/mK Published SS data is 16 w/mK Confirms K7  20 and K9  40 Confirms K7  20 and K9  40 h represents interface conductance

10. VG 10 HYSOL 9396/Foam Thermal CTE LBNL CTE Measurements(around 20C) LBNL CTE Measurements(around 20C) –Virgin HYSOL 9396, literature value is 70ppm/C –The basic setup is shown below. It consists of an aluminum block that is water cooled. A channel is cut in the block for the specimen. The specimens are about 20cm (196.5mm typically) long and 3.18 mm x 3.18 mm. The dial indicators have 0.0001” markings and are fixed to the granite table. An IR camera (out of the picture) is used to measure the temperature as well as thermocouple as shown in the picture. –Next setup at operating T, we hope LBL Delsen Hysol 939672 Hysol 9396/20% BN67

11. VG 11 Foam Properties Uniformity Testing objective, and future focus, is on block material uniformity Testing objective, and future focus, is on block material uniformity –Core samples are extracted throughout a 12in by 12in by 1in thick foam block for measuring: Tensile Tensile Compressive Compressive Thermal conductivity Thermal conductivity Density Density –Focus of work at Allcomp, Inc

12. VG 12 Foam Summary: Tests in Process Filled resin thermal properties under evaluation, K (W/mK) obtained thus far indicated below Filled resin thermal properties under evaluation, K (W/mK) obtained thus far indicated below Property of Foam and Resin Combination Property of Foam and Resin Combination –HYSOL 9396 filled BN infiltrates into the foam at the bonded interfaces Stress area under study is around the cooling tube Stress area under study is around the cooling tube –Predicted compressive stress is high but no damage is noted –Foam strength is enhanced by resin  To what extent remains to be determined Tensile and compression tests of foam/resin combinations are underway Tensile and compression tests of foam/resin combinations are underway –20 to 25 %BN additive –Thermal properties to be used in the Stave Prototype FE model

13. VG 13 12 cm Long Stave Prototype Typical Photographs Typical Photographs –Illustrate: BN filled adhesive migrates into foam in regions of the bond-line BN filled adhesive migrates into foam in regions of the bond-line Extent of penetration varies Extent of penetration varies –0.5mm to 1mm(to facing) –Appears that 0.1mm bond line exists only in small regions Adhesive/Foam Region Adhesive/Foam Region –Material property unknown, but estimate made for FEA Tests underway to measure tensile and compressive properties Tests underway to measure tensile and compressive properties –Tensile and compressive moduli/strength important to understanding the FEA –Adhesive stress running high in FEA FacingTube exits foamHysol+BN in foam Tube cut open FacingTube exits foamHysol+BN in foam Tube cut open FacingTube exits foamHysol+BN in foam Tube cut open FacingTube exits foamHysol+BN in foam Tube cut open

14. VG 14 FE Model – 12cm Prototypes Model based on cutting on planes of symmetry Model based on cutting on planes of symmetry –Heater by implication of symmetry is mounted on both opposite sides –12 cm mode (4cm wide) becomes 2cm by 6cm Length OK, since stress field localized at tube exit, becoming constant toward Z mid-plane Length OK, since stress field localized at tube exit, becoming constant toward Z mid-plane Material properties are linear except for foam Material properties are linear except for foam –Non-linear stress-strain patterned after K9 measurements Stress –strain based on compressive strain curve Stress –strain based on compressive strain curve –SS tube, 3D CC end cap –Adhesive/foam region, E=5.3GPa, CTE 41ppm/C Split planes of symmetry z x y Pipe

15. VG 15 Foam Stress-What Output is Critical? FEA stress field FEA stress field –Three normal stresses, three shear stresses, plus: Minimum Principal stress, calculated from 3 normal and 3 shear stresses but it physically represents a plane in the element where the normal stress is a maximum and the shear stress vanishes Minimum Principal stress, calculated from 3 normal and 3 shear stresses but it physically represents a plane in the element where the normal stress is a maximum and the shear stress vanishes Maximum Principal stress, same basis as minimum principal, but in this case it is less than minimum principal (compressive) and is tensile Maximum Principal stress, same basis as minimum principal, but in this case it is less than minimum principal (compressive) and is tensile Intermediate Principal stress, less than both min and max Intermediate Principal stress, less than both min and max Von Mises stress- also calculated from 3 normal and 3 shear stresses, but has no physical meaning, used as criteria in assessing ductile material failure stress Von Mises stress- also calculated from 3 normal and 3 shear stresses, but has no physical meaning, used as criteria in assessing ductile material failure stress –Criteria Use normal element stress?.- this compressive stress is less than the minimum principal. May not be conservative to use as failure criteria Use normal element stress?.- this compressive stress is less than the minimum principal. May not be conservative to use as failure criteria Use minimum principal stress?- assumed to be more conservative in comparing to uni-axial compressive test. This value represents physically a stress in the element, which reflects internal normal and shearing stresses Use minimum principal stress?- assumed to be more conservative in comparing to uni-axial compressive test. This value represents physically a stress in the element, which reflects internal normal and shearing stresses

16. VG 16 FE Model Foam Properties Foam stress-strain Foam stress-strain –Describes a non-linear behavior for an “elastic-plastic” analysis –FE computes element strain and then Von Mises element stress 2 successive solution steps used to update the “apparent” modulus 2 successive solution steps used to update the “apparent” modulus –Process simplified by using the compressive stress-strain curve for tensile (upper quadrant) Possible to use different tensile and compressive curves, not warranted at this time since tensile stress in foam not issue. Possible to use different tensile and compressive curves, not warranted at this time since tensile stress in foam not issue. Data points upper quadrant for element 18167 show the Von Mises stress is tracking the input curve Data points upper quadrant for element 18167 show the Von Mises stress is tracking the input curve –Points in lower quadrant are the solution steps for “minimum principal” stress in the foam at the adhesive interface

17. VG 17 Nastran solution for 60ºC Delta Element 181687 in foam under state of compression from tube shrinking and composite facing expanding Element 181687 in foam under state of compression from tube shrinking and composite facing expanding –Color bar represents range in nodal stresses, shading is smoother than plotting with average element stress – 181687 is nominally a cube, with 0.3mm (12mils) on a side Minimum Principal stress decays rapidly away from the tube exit region Minimum Principal stress decays rapidly away from the tube exit region –Peak minimum principal stress is 1.99MPa (289psi), right at the K9 foam limit Shaded image is foam core only, high stress region (blue) is interface to adhesive/foam region, which is 0.5mm thick

18. VG 18 Nastran Solution for 60ºC Delta Figure illustrates how rapidly the minimum principal stress drops away from the end of the stave Figure illustrates how rapidly the minimum principal stress drops away from the end of the stave –Stress within the foam is less than the compressive K9 test result –0.45mm into the foam the “minimum principal” stress is <230psi End of foam

19. VG 19 FEA Result as Function of Temp Change Figure description Figure description –Displays minimum principal stress in axial direction along the foam and tube adhesive interface Foam surface stress at tube exit increases with increasing temperature change e.g., starting at room and decreasing to - 125ºC (150C) Foam surface stress at tube exit increases with increasing temperature change e.g., starting at room and decreasing to - 125ºC (150C) –However, sub-surface stress remains largely unchanged –The plastic deformation built into the stress-strain curve moderates the stress magnitude End of foam Tube VM stress=37.6ksi 60C delta Tube VM stress=47ksi 150C delta

20. VG 20 How Realistic is Sandwich Model FEA? Model based on marginal material information Model based on marginal material information –Material data for composite facing and foam is more complete than for: Modulus of elasticity for foam/adhesive/BN region Modulus of elasticity for foam/adhesive/BN region –Will have a measure of this soon –Possibly model is using too high of a value Strength of foam/adhesive/BN region is unknown Strength of foam/adhesive/BN region is unknown –If model predicts tensile stress beyond material strength this will be an indicator something is amiss –Strength test underway CTE of foam/adhesive/BN was determined by LBNL CTE of foam/adhesive/BN was determined by LBNL –Allcomp to determine CTE of adhesive/BN as check at Delsen Laboratory Physically no damage has been recorded to the foam surface as result of temperature cycling – see next talk. Physically no damage has been recorded to the foam surface as result of temperature cycling – see next talk. –Need to continue modeling until test results converge with model predictions