Thermally Conductive Carbon Foam Thermally Conductive Carbon Foam PropertiesProperties

M. Gilchriese, M. Cepeda - LBNLM. Gilchriese, M. Cepeda - LBNL

W. Miller, W. Shih, R. Ramirez-AllcompW. Miller, W. Shih, R. Ramirez-Allcomp

VG VG 22

Foam ProgressFoam Progress

• GeneralGeneral– Processed and tested two basic foams, 12in by 12in by 1in blocksProcessed and tested two basic foams, 12in by 12in by 1in blocks

• All from Allcomp, IncAll from Allcomp, Inc

• 100ppi and 130ppi (ppi=pores per inch)100ppi and 130ppi (ppi=pores per inch)

• Several foam densitiesSeveral foam densities– TestingTesting

• Thermal conductivity by laser diffusivity and direct methodThermal conductivity by laser diffusivity and direct method

• Tensile and Compressive strengthsTensile and Compressive strengths– Modulus of elasticityModulus of elasticity– Recorded nonlinear compressive stress-strain over wide strainRecorded nonlinear compressive stress-strain over wide strain

• LBNL Measured CTE of foamLBNL Measured CTE of foam– Virgin and resin filledVirgin and resin filled

Foam (limit to CTE)Foam (limit to CTE) Foam with HYSOL 9396 Foam with HYSOL 9396 Foam with HYSOL 9396 and BN filled Foam with HYSOL 9396 and BN filled

• Very recently, CTE measurements by company(Delsen)Very recently, CTE measurements by company(Delsen)

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

VG VG 44

Foam Strength Test ResultsFoam Strength Test Results

• Table compares recent K7 (100ppi) and K9 (130ppi) with Table compares recent K7 (100ppi) and K9 (130ppi) with prior dataprior data– K9 exhibits higher tensile strength (K9 exhibits higher tensile strength (σσ)) and higher modulus (E) than K7 and higher modulus (E) than K7

and prior 100pi foamsand prior 100pi foams

• Foam density is value following foam “K” designator, e.g. K3/0.09Foam density is value following foam “K” designator, e.g. K3/0.09– Density in this case is 0.09gm/cc Density in this case is 0.09gm/cc

– Part of the increased strength and stiffness is due to the higher density 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 0.23gm/cc versus 0.19gm/cc, but clearly K9 stands out from a tensile propertyproperty

K3/0.09 K3+/0.19 K4/0.19 K4+/0.27 K7/0.22 K9/0.23 σ-psi E-psi σ-psi E-psi σ-psi E-psi σ-psi E-psi σ-psi E-psi σ-psi E-psi 104 12600 124 16613 333 29026 183 12732 481 39481 104 9918 143 14702 311 26885 326 31306 212 9917 553 45541 112 13041 158 13936 296 25541 328 30221 208 9698 108 10869 132 14518 344 23068 355 33059 107 11607 139 14942 317 25164 336 30903 201 10782 517 42511

Tensile properties

VG VG 55

Foam Strength Test ResultsFoam Strength Test Results

• Compressive properties favor the 130ppi K9 foamCompressive properties favor the 130ppi K9 foam– K9 compressive strength only slightly higher than K7K9 compressive strength only slightly higher than K7

• However, its modulus is noticeably higherHowever, its modulus is noticeably higher

• Foam compressive behavior decidedly different than tensileFoam compressive behavior decidedly different than tensile– Tensile failure has a clear fracture pointTensile failure has a clear fracture point– Compression failure is not so clearCompression failure is not so clear

• Strain in a stave is comparatively low in relation to strain to failure in Strain in a stave is comparatively low in relation to strain to failure in Allcomp testsAllcomp tests

– Compressive stress peak is followed by discontinuities in the Compressive stress peak is followed by discontinuities in the stress-strain curve----plastic buckling of the foam stress-strain curve----plastic buckling of the foam

K3/0.09 K3+/0.19 K4/0.19 K4+/0.27 K7/0.22 K9/0.23 σ-psi E-psi σ-psi E-psi σ-psi E-psi σ-psi E-psi σ-psi E-psi σ-psi E-psi

40 1278 157 14152 167 30185 325 46100 204 19612 285 40863 33 7914 176 15438 195 32693 305 37244 206 15646 237 44562 43 8449 184 17552 188 28485 337 47715 192 14990 40 7467 173 14854 141 24489 311 51430 179 14135 39 6277 172 15499 173 28963 320 45622 195 16096 261 42712

Compressive properties

VG VG 66

Foam Compressive BehaviorFoam Compressive Behavior

• Notice compressive peak Notice compressive peak followed by rapid unloading of followed by rapid unloading of Instron machineInstron machine– Instron loading recovers and then Instron loading recovers and then

an oscillatory behavior followsan oscillatory behavior follows– Thermal solutions of the stave Thermal solutions of the stave

prototypes (25C down to -35C) prototypes (25C down to -35C) results in plastic strain nominally results in plastic strain nominally 0.02 0.02

• Tests are being conducted to Tests are being conducted to see what damage if any is see what damage if any is encountered at strain less than encountered at strain less than 0.050.05– Initial dramatic unloading a testing Initial dramatic unloading a testing

issue?issue?

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0 0.05 0.1 0.15

StrainC

om

pre

ssio

n S

tres

s-p

si

K9-4-center

K7-SN2-4-center

1inch high specimen compressed to 0.75in, 25% strain

VG VG 77

Foam Tensile CharacteristicFoam Tensile Characteristic

• Tensile curveTensile curve– Fairly linear, repeatableFairly linear, repeatable

• Fracture strength of Fracture strength of K9 higher than K7K9 higher than K7

• Tensile strength well Tensile strength well above stave stressesabove stave stresses

0

100

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500

600

0 0.005 0.01 0.015 0.02 0.025 0.03

Strain

Str

ess-

psi

K7 100-SN2-3-edgeK7-2-centerK7-3-centerK9-1-edgeK9-3-center

VG VG 88

Foam Thermal ConductivityFoam Thermal Conductivity

• Foam Laser Diffusivity MeasurementsFoam Laser Diffusivity Measurements– K9 @ 0.23gm/cc K~38W/K, K7 @ 0.22g//cc K=22W/mKK9 @ 0.23gm/cc K~38W/K, K7 @ 0.22g//cc K=22W/mK

• Variance in diffusivity measurements led to direct K Variance in diffusivity measurements led to direct K measurementsmeasurements

Foam (Block) Density g/cc

Test Facility

Thermal Conductivity W/mK

Kx Ky Kz 100 ppi

K3 ( ) 0.093 VPI 2.5 3.3 2.6 K3+ 0.200 VPI - - -

K4 ( ) 0.188 VPI 13.3 15.0 13.0 K4+ 0.280 VPI - - -

K5 ( ) 0.172 VPI 17.6 18.9 18.2 K7 ( 2) 0.220 VPI 21.3 27.1 22.2

K7 coated (3) 0.238 OU - - 11.3* K8 ( 1) 0.441 OU - - 73.2 K8 (1) 0.441 OU - - 84.2 K7 ( ) 0.225 OU - - 28.9

K7 coated (3) 0.238 OU 22.9 27.0 9.8* K7 (2) 0.205 OU 15.2 17.9 10.2 K7 (2) 0.219 OU 26.4 15.1 31.0 K7 (2) 0.205 OU - - 25.0

130 ppi K9 (1) 0.208 OU - - 29.7 K9 (1) 0.226 OU - - 33.9 K9( (1) 0.229 OU 34.3 31.1 41.1

K8 used in stripstave prototypes

VG VG 99

Conductivity by Direct MethodConductivity by Direct Method

• Direct ConductivityDirect Conductivity– Complication from the contact faces being foamComplication from the contact faces being foam

• Used thermal grease to reduce thermal Used thermal grease to reduce thermal resistanceresistance

• Present method uses two different foam Present method uses two different foam heights to back out effect of interface heights to back out effect of interface thermal conductancethermal conductance

– Data contained in Table based on initial test Data contained in Table based on initial test methodmethod

• Published SS data is 16 w/mKPublished SS data is 16 w/mK

• Confirms K7 Confirms K7 20 and K9 20 and K9 40 40

Cooler

Heater

h represents interface conductance

VG VG 1010

HYSOL 9396/Foam Thermal CTEHYSOL 9396/Foam Thermal CTE

• LBNL CTE Measurements(around 20C)LBNL CTE Measurements(around 20C)– Virgin HYSOL 9396, literature value is 70ppm/CVirgin HYSOL 9396, literature value is 70ppm/C– The basic setup is shown below. It consists of an aluminum block that 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 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 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 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 granite table. An IR camera (out of the picture) is used to measure the temperature as well as thermocouple as shown in the picture. the temperature as well as thermocouple as shown in the picture.

– Next setup at operating T, we hopeNext setup at operating T, we hope

Material CTE in ppm Hysol 9396 77±6 Hysol 9396/K9 foam 66±7 Hysol 9396/BN(30% of total weight) 60±6 Hysol 9396/BN/K9 foam 42±4 K9 foam <1

LBL

Delsen

Hysol 9396 72Hysol 9396/20% BN 67

VG VG 1111

Foam Properties UniformityFoam Properties Uniformity

• Testing objective, and future Testing objective, and future focus, is on block material focus, is on block material uniformityuniformity– Core samples are extracted Core samples are extracted

throughout a 12in by 12in by 1in throughout a 12in by 12in by 1in thick foam block for measuring:thick foam block for measuring:

• TensileTensile

• CompressiveCompressive

• Thermal conductivityThermal conductivity

• Density Density

– Focus of work at Allcomp, IncFocus of work at Allcomp, Inc density 0.23 g/cc

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0 0.05 0.1 0.15 0.2 0.25

Strain

Co

mp

ress

ion

Str

ess-

psi

K9-4-center

K9-2-edge

VG VG 1212

Foam Summary: Tests in Foam Summary: Tests in ProcessProcess

• Filled resin thermal properties under evaluation, K (W/mK) Filled resin thermal properties under evaluation, K (W/mK) obtained thus far indicated below obtained thus far indicated below

• Property of Foam and Resin CombinationProperty of Foam and Resin Combination– HYSOL 9396 filled BN infiltrates into the foam at the bonded interfacesHYSOL 9396 filled BN infiltrates into the foam at the bonded interfaces

• Stress area under study is around the cooling tubeStress area under study is around the cooling tube– Predicted compressive stress is high but no damage is notedPredicted compressive stress is high but no damage is noted– Foam strength is enhanced by resinFoam strength is enhanced by resin

To what extent remains to be determinedTo what extent remains to be determined• Tensile and compression tests of foam/resin combinations are Tensile and compression tests of foam/resin combinations are

underwayunderway– 20 to 25 %BN additive20 to 25 %BN additive

– Thermal properties to be used in the Stave Prototype FE modelThermal properties to be used in the Stave Prototype FE model

Sample Description Result-(W/mK) 01 HYSOL 9396-(press) 0.23 05 HYSOL 9396+25% wt BN (press) 1.18 06 HYSOL 9396+25% wt BN (10#) 0.84 07 HYSOL 9396+20% wt BN (10#) 0.57 08 HYSOL 9396+20% wt BN (10#) 0.56 09 HYSOL 9396+35% wt BN (10#) 1.77 10 HYSOL 9396+35% wt BN (10#) 1.76

VG VG 1313

12 cm Long Stave Prototype 12 cm Long Stave Prototype

• Typical PhotographsTypical Photographs– Illustrate:Illustrate:

• BN filled adhesive migrates BN filled adhesive migrates into foam in regions of the into foam in regions of the bond-linebond-line

• Extent of penetration variesExtent of penetration varies– 0.5mm to 1mm(to facing)0.5mm to 1mm(to facing)– Appears that 0.1mm bond Appears that 0.1mm bond

line exists only in small line exists only in small regionsregions

• Adhesive/Foam RegionAdhesive/Foam Region– Material property unknown, but Material property unknown, but

estimate made for FEAestimate made for FEA

• Tests underway to measure Tests underway to measure tensile and compressive tensile and compressive propertiesproperties

– Tensile and compressive Tensile and compressive moduli/strength important moduli/strength important to understanding the FEAto understanding the FEA

– Adhesive stress running Adhesive stress running high in FEAhigh in FEA

Facing Tube exits foamHysol+BN in foam

Tube cut open

Facing Tube exits foamHysol+BN in foam

Tube cut open

Facing Tube exits foamHysol+BN in foam

Tube cut open

Facing Tube exits foamHysol+BN in foam

Tube cut open

VG VG 1414

FE Model – 12cm PrototypesFE Model – 12cm Prototypes

• Model based on cutting on Model based on cutting on planes of symmetryplanes of symmetry– Heater by implication of Heater by implication of

symmetry is mounted on both symmetry is mounted on both opposite sidesopposite sides

– 12 cm mode (4cm wide) 12 cm mode (4cm wide) becomes 2cm by 6cmbecomes 2cm by 6cm

• Length OK, since stress field Length OK, since stress field localized at tube exit, localized at tube exit, becoming constant toward Z becoming constant toward Z mid-planemid-plane

• Material properties are linear Material properties are linear except for foamexcept for foam– Non-linear stress-strain Non-linear stress-strain

patterned after K9 patterned after K9 measurementsmeasurements

• Stress –strain based on Stress –strain based on compressive strain curvecompressive strain curve

– SS tube, 3D CC end capSS tube, 3D CC end cap– Adhesive/foam region, Adhesive/foam region,

E=5.3GPa, CTE 41ppm/CE=5.3GPa, CTE 41ppm/C

Adhesive foam region

Split planes of symmetry

z

x

y

Pipe

VG VG 1515

Foam Stress-What Output is Foam Stress-What Output is Critical?Critical?

• FEA stress field FEA stress field – Three normal stresses, three shear stresses, plus:Three normal stresses, three shear stresses, plus:

• Minimum Principal stress, calculated from 3 normal and 3 shear Minimum Principal stress, calculated from 3 normal and 3 shear stresses but it stresses but it physicallyphysically represents a plane in the element where the represents a plane in the element where the normal stress is a maximum and the shear stress vanishesnormal stress is a maximum and the shear stress vanishes

• Maximum Principal stress, same basis as minimum principal, Maximum Principal stress, same basis as minimum principal, but in but in this case it is less than minimum principalthis case it is less than minimum principal (compressive) and is tensile (compressive) and is tensile

• Intermediate Principal stress, less than both min and maxIntermediate Principal stress, less than both min and max

• Von Mises stress- also calculated from 3 normal and 3 shear stresses, Von Mises stress- also calculated from 3 normal and 3 shear stresses, but has no physical meaning, used as criteria in assessing ductile but has no physical meaning, used as criteria in assessing ductile material failure stressmaterial failure stress

– CriteriaCriteria

• Use normal element stress?.- this compressive stress is less than the Use normal element stress?.- this compressive stress is less than the minimum principal. May not be conservative to use as failure criteriaminimum principal. May not be conservative to use as failure criteria

• Use minimum principal stress?- assumed to be more conservative in Use minimum principal stress?- assumed to be more conservative in comparing to uni-axial compressive test. This value represents comparing to uni-axial compressive test. This value represents physically a stress in the element, which reflects internal normal and physically a stress in the element, which reflects internal normal and shearing stressesshearing stresses

VG VG 1616

FE Model Foam PropertiesFE Model Foam Properties

• Foam stress-strainFoam stress-strain– Describes a non-linear behavior for Describes a non-linear behavior for

an “elastic-plastic” analysisan “elastic-plastic” analysis– FE computes element strain and FE computes element strain and

then then Von MisesVon Mises element stress element stress

• 2 successive solution steps 2 successive solution steps used to update the “apparent” used to update the “apparent” modulusmodulus

– Process simplified by using the Process simplified by using the compressive stress-strain curve for compressive stress-strain curve for tensile (upper quadrant)tensile (upper quadrant)

• Possible to use different tensile Possible to use different tensile and compressive curves, not and compressive curves, not warranted at this time since warranted at this time since tensile stress in foam not tensile stress in foam not issue.issue.

• Data points upper quadrant for Data points upper quadrant for element 18167 show the Von element 18167 show the Von Mises stress is tracking the Mises stress is tracking the input curveinput curve

– Points in lower quadrant are the Points in lower quadrant are the solution steps for “minimum solution steps for “minimum principal” stress in the foam at the principal” stress in the foam at the adhesive interfaceadhesive interface

-2500000

-2000000

-1500000

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-500000

0

500000

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2500000

-0.06 -0.04 -0.02 0 0.02 0.04 0.06

Element strain

Str

ess-

Pa

tensile input

compressive input

181687 Min Principal

181687-Von Mises Stress

VG VG 1717

Nastran solution for 60Nastran solution for 60ºC DeltaºC Delta

• Element 181687 in foam under state of compression Element 181687 in foam under state of compression from tube shrinking and composite facing expandingfrom tube shrinking and composite facing expanding– Color bar represents range in nodal stresses, shading is Color bar represents range in nodal stresses, shading is

smoother than plotting with average element stresssmoother than plotting with average element stress– 181687 is nominally a cube, with 0.3mm (12mils) on a side181687 is nominally a cube, with 0.3mm (12mils) on a side

• Minimum Principal stress decays rapidly away from Minimum Principal stress decays rapidly away from the tube exit regionthe tube exit region– Peak minimum principal stress is 1.99MPa (289psi), right at Peak minimum principal stress is 1.99MPa (289psi), right at

the K9 foam limitthe K9 foam limit

Element 181687

Element 181656 Element 181662 Shaded image is foam core

only, high stress region (blue) is interface to adhesive/foam region, which is 0.5mm thick

VG VG 1818

Nastran Solution for 60Nastran Solution for 60ºC DeltaºC Delta

• Figure illustrates how rapidly the minimum principal Figure illustrates how rapidly the minimum principal stress drops away from the end of the stavestress drops away from the end of the stave– Stress within the foam is less than the compressive K9 test resultStress within the foam is less than the compressive K9 test result– 0.45mm into the foam the “minimum principal” stress is <230psi0.45mm into the foam the “minimum principal” stress is <230psi

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element number

Stre

ss-

(Pa)

end element-181687 min principal

end element-181656-min principal

End of foam

VG VG 1919

FEA Result as Function of Temp FEA Result as Function of Temp ChangeChange

• Figure descriptionFigure description– Displays minimum principal Displays minimum principal

stress in axial direction stress in axial direction along the foam and tube along the foam and tube adhesive interfaceadhesive interface

• Foam surface stress at Foam surface stress at tube exit increases with tube exit increases with increasing temperature increasing temperature change e.g., starting at change e.g., starting at room and decreasing to room and decreasing to -125-125ºC (150C)ºC (150C)– However, sub-surface However, sub-surface

stress remains largely stress remains largely unchangedunchanged

– The plastic deformation The plastic deformation built into the stress-strain built into the stress-strain curve moderates the stress curve moderates the stress magnitudemagnitude

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Min

Pri

ncip

al S

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s (P

a)

Delta T 60C

Delta T 95C

Delta T 150C

End of foamTube VM stress=37.6ksi 60C deltaTube VM stress=47ksi 150C delta

VG VG 2020

How Realistic is Sandwich Model How Realistic is Sandwich Model FEA?FEA?

• Model based on marginal material informationModel based on marginal material information– Material data for composite facing and foam is more complete than for:Material data for composite facing and foam is more complete than for:

• Modulus of elasticity for foam/adhesive/BN regionModulus of elasticity for foam/adhesive/BN region– Will have a measure of this soonWill have a measure of this soon– Possibly model is using too high of a valuePossibly model is using too high of a value

• Strength of foam/adhesive/BN region is unknownStrength of foam/adhesive/BN region is unknown– If model predicts tensile stress beyond material strength this If model predicts tensile stress beyond material strength this

will be an indicator something is amisswill be an indicator something is amiss– Strength test underwayStrength test underway

• CTE of foam/adhesive/BN was determined by LBNLCTE of foam/adhesive/BN was determined by LBNL– Allcomp to determine CTE of adhesive/BN as check at Delsen Allcomp to determine CTE of adhesive/BN as check at Delsen

LaboratoryLaboratory

• Physically no damage has been recorded to the foam Physically no damage has been recorded to the foam surface as result of temperature cycling – see next talk.surface as result of temperature cycling – see next talk.– Need to continue modeling until test results converge with model Need to continue modeling until test results converge with model

predictionspredictions