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InfluenceofAerogelMorphologyandReinforcementArchitectureonGasConvectioninAerogelComposites

FrancesI.Hurwitz1,MatthewMeyer2,Haiquan Guo3,RichardB.Rogers1, JeffreyDeMange4,HayleyRichardson2

1NASAGlennResearchCenter,Cleveland,OH441352UniversitiesSpaceResearchAssociation(USRA),Cleveland,OH441353OhioAerospaceInstitute,Cleveland,OH441424UniversityofToledo,Toledo,OH43606

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Background:• Aerogels are very effective insulators, primarily in

suppressing heat transfer by gas convection, but also in providing a tortuous path for solid conduction.

• Alumina and aluminosilicate aerogels can maintain mesoporous structure to temperatures of up to 1200˚C.

• These aerogels are fragile, but an be reinforced with fabrics, papers or foams to form composites in which the aerogel serves as the matrix.

Objective:Characterize gas permeability of aluminosilicate aerogels with various ceramic fabric, paper or felt reinforcements.

AlO6 octahedra S.Brühne,Cryst.GrowthDes.,2008,8(2),pp 489–493n

Boehmite [AlO(OH)]

Synthesis Approach: Boehmite alone to form alumina aerogel; Boehmite + TEOS co-condense to form aluminosilicate.

+

TEOS

hydrogel aerogelsupercriticalCO2

P.R.Aravind,etalMicroporous andMesoporous Materials96 14–20(2006).

Introduction of silica into alumina lattice inhibits transformation to α-alumina(Hurwitz, et al., manuscript in preparation)

Si OOO

O

3

1100˚C, 24h 1100˚C, 48h 1100˚C, 96h

1200˚C, 24h

As supercritically dried

Micrographs showing persistence of mesoporous structure within aluminosilicate aerogel heated at 1100 and 1200˚C.

600˚C, 24h

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CeramicReinforcement

Thickness(mm)

Density(g/cm3)Asreceived

CompositeDensity(g/cm3)

UpperUseTemperature Composition(%)

Astroquartz (503plainweave)Unsized

0.11 0.95 0.610-0.997 1070° C 99.99SiO2

Nextel440 0.3 1.0 1.09 1100-1200˚C γ-alumina +mullite +amorphousSiO2

ZYW30A

Saffil Paper 0.5,1.0 0.15(wbinder)0.125(1000C) 0.144-0.194 1600° C95-97Al2O3,3.0-5.0SiO2,

Aerogel impregnation into woven fabrics fills crossover points between tows as well as inter-fiber spacing within tows, decreasing gas permeability.

Example shown in Astroquartz503 (plain weave)

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2-D Fabric composites

APA-2/ aerogel composite

Density of 0.14 g/cm3, lighter than Microtherm HT (0.3428g/cm3)

Aerogel bonds to fibers; unlike commercial materials, particles do not spall. Aerogel/fiber bond achieved by heat treatment of alumina paper to remove all binders prior to sol impregnation.

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APA-1, binder removed

APA-2, no organic binder (inorganic may include Si, S, Ba, K,Ca)

Saffil paper, binder removed

Saffil paper, 1000˚C

inorganic

silica

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Fiberfrax 972AH

47-52 Al2O3, 48-53 SiO2,

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Reinforcement Massfractionaerogel

Astroquartz 0.10

Nextel440 0.05

ZYW30A 0.06

APA-2 0.37

Saffil 0.30

Fiberfrax 0.28

Quartzpapers 0.52-0.62

Quartzfelt 0.6-0.9

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11

11

Astroquartz

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8

Airflow(C

FM/SF)

DifferentialPressure,in.H2O

Astroquartz Comparison

AQ1fabric

AQ2fabric

AQ4composite

AQ38composite

43.4

37.4

0.557 0.1530

5

10

15

20

25

30

35

40

45

50

Airflow(C

FM/SF)

Astroquartz503permeabilityat0.50in.H2O

AQ1fabric

AQ2fabric

AQ4composite

AQ38composite

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AF10withsizing (left)andwithoutsizing(right).

BF10withoutsizing BF20withoutsizing

Nextel fabrics

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13

0

50

100

150

0 0.2 0.4 0.6 0.8

Airflow(C

FM/SF)

DifferentialPressure(in.H2O)

Nextelfabrics

N440BF10w/sizingN312AF10w/sizingN312AF-10w/osizingN440BF-10w/osizingN440BF-20

52.62

1.15

31.75

0.35

11.14

0.340

10

20

30

40

50

60

70

Airflow(C

FM/SF)

ComparisonofNextelweavesw/osizingandtheircompositesat0.50in.H2O

N312nocompositeN312CompositeN440BF10nocompositeN440BF10compositeN440BF20nocompositeN440BF20composite

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14

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8

Airflow(C

FM/SF)

DifferentialPressure(in.H2O)

Saffil papercomparison

.5mmpaperwithsizing

1mmpaperwithsizing

SP1151mmwithoutsizing1000C

SP1181mmwithoutsizing1000C

SP1710.5mmwithoutsizing600C

SP1621mmwithoutsizing600C

SP1611mmwithoutsizing600C

0100200300400500600700800

0 5 10 15

Airflow(C

FM/SF)

DifferentialPressure(in.H2O)

Saffil0.5mmpaperandcomposites

.5mmpaperwithsizing

SP171.5mmwithoutsizing600C

SP143.5mmcomposite1000C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 5 10 15

Airflow(C

FM/SF)

DifferentialPressure(in.H2O)

Saffil 0.5mmcomposites

SP173.5mmcompositenopreheattreatment,600Cposttreatment1600-3.5mmcompositenopreheattreatment,noposttreatmentSP143.5mmcomposite1000C,600Cposttreatment

Note variations in scale!

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10

Airflow(C

FM/SF)

Sensor1DifferentialPressure(inH2O)

APA2Comparison(0.5mmnozzle)

APA2-658paper

APA2-648Composite

APA2-653Composite

APA2-654Composite

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024681012141618

0 2 4 6 8 10

Airflow(C

FM/SF)

Sensor1DifferentialPressure,inH2O

Fiberfraxcomparison

972AHasreceived

972AH-41composite

Incorporation of aerogel provides marked reduction in permeability for both papers, but APA2 paper has much lower permeability than Fiberfraxpaper alone.

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0

10

20

30

40

50

60

70

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Airflow(C

FM/SF)

DifferentialPressure(in.H2O)

Wovenfabriccomparison

N312AF-10

Astroquartz503

N440BF-10

ZYW30A

N440BF-20

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0

20

40

60

80

100

120

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Airflow(C

FM/SF)

DifferentialPressure(in.H2O)

PapercomparisonAPA2

Saffil0.5mmw/osizing

Saffil0.5mmw/sizing

Saffil1mmw/osizing

Saffil1mmw/sizing

Fiberfrax972AHasreceived

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17

40.4

52.62

31.75

11.1421.9

72.48

13.9

95.45

0

20

40

60

80

100

120

Airflow(C

FM/SF)

Comparisonofnon-compositematerialsw/osizingat0.5inwater

Astroquartz

Nextel312AF10

Nextel440BF10

Nextel440BF20

ZYW30A

Saffil1mmpaper

Fiberfrax972AH(wbinder)

APA2

Saffil 0.5mmpaperw/obinderwastoofragiletobetestedat0.50in.H2O.

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0.355

0.574

0.354 0.342

0.098

0.292

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8Airflow,CFM

/SF

Comparisonofwovenfabriccompositesat0.50in.H2O

AstroquartzComposite

N312AF10composite

N440BF10composite

N440BF20composite,noposttreatmentN440BF20composite,600C

ZYW30Acomposite

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0.066

0.15

0.260.24

0.08

0.19

0.11

0

0.05

0.1

0.15

0.2

0.25

0.3

Airflow,CFM

/SF

Comparisonofpaperandfelt/aerogelcompositesat0.50in.H2O(extrapolated)

APA2

Saffil0.5mmcomposite,1000C/600CSaffil0.5mmcomposite,none/600CSaffil0.5mmcomposite,none/noneSaffil1mmcomposite

Fiberfrax972AHcomposite

Quartzelcomposite

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Hydrophobic treatments availableUse of PTMS shows some reduction of surface area at high temperature.Some additional silica incorporation occurs; PTMS does not produce cristobalite seen with other silanes.

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050100150200250300350400450

0 400 800 1200 1600

BETsurfaceaream

2/g

Time,min

Untreated,asdriedaerogelUntreated,600C

Untreated,1000C

Untreated,1100C

Untreated,1200C

PTMS,600C

PTMS,1000C

PTMS,1100C

PTMS,1200C

cristobaliteδ,θ

21-22˚-amorphous hump

mullitemullite

mullite

X-ray Diffraction of Hydrophobic Samples Heated at 1200C, 24h

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Trades:

• Woven fabrics incorporate low fraction of aerogel, but can reduce gas permeability, reducing convective heat transfer.

• Alumina fiber provides opacification; alumina papers offer best thermal performance.

• Saffil paper contains some small silica fibers which bridge fiber cross-over points on heating to 1000˚C, stiffening paper. Aerogel composite thermal properties and permeability are comparable to APA-2. Saffilavailable as 0.5 mm paper, thinner than 1 mm APA-2, and therefore slightly more flexible.

• Fiberfrax 972AH offers highest flexibility, but slightly higher thermal conductivity and permeability and contains respirable fiber. (APA-1 and Saffil also contain small diameter fibers). Most flexible, particularly with “slip” layer.

• Quartz felts offer highest aerogel incorporation on a volume fraction basis (fiber volume fraction is low), and offer lowest density and more robust mechanical strength than many of the papers.

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Conclusions:

• In 2-D fabrics, weave architecture strongly influences permeability. Removal of organic sizing permits spreading of tows to reduce gas flow.

• Impregnation of 2-D fabrics with aerogel provides more than an order of magnitude reduction in permeability. N440 (BF20) provided lowest permeability of options tested, but is a heavy option (1.1 g/cm3).

• Ceramic papers generally offer further reduction in permeability and lighter weight than fabrics, but are more fragile mechanically, and composites of most papers are less flexible than the 2-D weaves.

• Waterproofing techniques have been developed; however, influence on permeability has not as yet been evaluated.

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