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Aerogels: Structure, properties and applications

  • October 2011
  • Conference: 13th Meeting on Supercritical Fluids, ISASF 2011
Authors:
Lorenz Ratke at German Aerospace Center (DLR)
Lorenz Ratke
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Abstract

Aerogels prepared via a sol-gel route and supercritical drying posses a unique microstructure with specifics like a 3D open porous network of nanosized particles, huge specific surface area up to a few thousand square meters per gram, extremely low thermal conductivity, extremely low density, such that applications in a variety of industrial sectors seemed obvious 30 years ago. The structure and properties are essentially independent of the chemical nature of the aerogels, which can be organic, inorganic, a combination of both or composites made from them. Although the large potential initiated intensive research it took almost twenty years before the still costly production process allows making aerogels finding their way into more and more industrial sectors. Aerogels are now utilized as super insolating materials in shoe soles or apparel, daylight illumination systems, pipeline isolation mats, medium temperature isolating materials, tennis rackets, drug delivery systems, foundry core and mould materials, building construction materials and many more are being developed in the last decade. The paper describes briefly typical aerogel structures, properties and then concentrates on a comprehensive presentation of industrial applications today and potential for the future.
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Aerogels -
Structure, properties and applications
Lorenz Ratke
Institut für Materialphysik im Weltraum
51147 Köln
Email: lorenz.ratke@dlr.de
What are aerogels?
Aerogels are
nanostructured, open porous
solids made via sol-gel technology
Types of aerogels
!   Oxides (Quarz, Titania, Zirkonia. Mixed oxides)
!   Polymers (Resorcin-, Melamin-Formaldehyd)
!   Carbon (pyrolysed Polymere)
!   cellulose, starch, alginates,...
!   And almost everything that can be gelled
Synthesis of aerogels
!   General technique is “sol-gel processing” meaning
!   Mixing of chemicals (alcoxides, water, alcohol, catalyst)
!   Gelation (catalyst, temperature, fluid flow)
!   Drying
!   Final product
DLCA-Simulation von B.Halperin , NWU Evanston
Synthesis of aerogels
!   General technique is “sol-gel processing” meaning
!   Mixing of chemicals (alcoxides, water, alcohol, catalyst)
!   Gelation (catalyst, temperature, fluid flow)
!   Drying
!   Final product
Preparation of cellulose aerogels
Dissolution of cellulose in e.g. salt-hydrate melts
• dissolution
• gelation
• regeneration
• aging
Mixing Dissolution
Gelation on cooling (80°C)
and regeneration
The wet gel is ready and now?
Drying – determines everything*!
Drying under ambient condition
leads to Xerogels
!   huge shrinkage
!   Destruction of the nanosized
pore system
!   Fragmentation of the gel body
!   Porosity between 5 and 50 %
!   but:
proper drying routine can give
perfectly shaped xerogels!
methods: air freeze drying supercritical
!
wetting stresses
lead to a collapse
of the gel network
p ! 1/R
2R
capillary forces
* not really, but it is the essential step
• Shock or rapid freezing (after solvent
exchange) to minimize freezing
effects on the solid gel structure
• Sublimation along the
sublimation line by
– pressure changes and
– temperature increase
Drying – determines everything!
methods: air freeze drying supercritical
• Solvent exchange gel liquid
against CO2
• Temperature increase above
critical point
• Tc=31.1°C, 73.8 bar.
• Real process ≈ 40 °C with
pressure of ≈ 90 bar
• Slow depressurization
Drying – determines everything!
methods: air freeze drying supercritical
And the result?
RF polymeric aerogel
Carbon aerogel
Answer: almost always a nanostructure
Cellulose aerogel
RF-Aerogel (HCl catalysed)
Carbon aerogel Silica aerogel
Appearance of some aerogels
RF-Aerogel
C-Aerogel
TiO2
V2O5
(G.Reichenauer)
Appearance and structure of Cellulose aerogels
Cellulose-Aerogels are generally
opaque and milky with densities
of around 5 - 60 kg/m3
Structure: nanofelt of microfibrils
Aerogels with
0,5 % Cellulose 1,0 % Cellulose 2,0 % Cellulose
Cai et al, ChemSusChem 2008
Properties
Properties of aerogels
UV Infrared
! Low density - high porosity
" 90 - 99,9 % porosity
! Large sound absorption - low sound velocity
" damping > 50 dB, vl ≈ 100 m/s
! Low thermal conductivity
" 0.005 - 0.1 W/mK
! Transparent or opaque or can be coloured
" variable refractive index (1,001 to 2,1)
! No reaction with metallic melts up to 950°C (Silica and
some other oxide aerogels)
! Huge internal surface
" 10 - 2000 m2/g
! Surface can be functionalized
" Hydrophobic by polymeric residues (-CH3)
" CVD deposition of magnetic layers
! Functionalisation of the gel body
" Embedding of particles (e.g. dyes, ferroelectrics)
" Interpenetrating hybrid aerogel networks
Mechanical properties of aerogels
E(MPa)
ρ (g/cm3)
Young’s modulus
pressure"
σ
ΔL
L
after Gross et al. JNCS 1992
EE
L
L!=!
"
=
#$
0
Fracture strength:
Silica-Aerogels: 20 kPa bis 2 MPa
RF-Aerogels: 100 kPa bis 10 MPa
!
"
B#
$
1.5 E#
$
3.6
Are there applications?
Industrial areas of application
!   thermal insulation
" home building, vehicles, pipelines, packaging
!   electronic components
" capacitors, humidity sensors, batteries, fuel cell,
soft magnets, Geiger counter, IR detector
!   foundry technology
" aerogelic binders, aerogel granules for foundry sands
!   architecture
" illumination, superlight concrete, wall isolation
!   superlight composites
!   non-linear optics (SHG)
!   Cosmetics
!   Pharmacy (drug delivery)
!   Apparel & sportsware
" Tennis rackets
" Security shoes
" Sport shoes
!   Space
" Space suits
" Anti-sloshing in tanks
" Cryogenic tank insulation
Industrial areas of application
!   thermal insulation
" home building, vehicles, pipelines, packaging
!   electronic components
" capacitors, humidity sensors, batteries, fuel cell,
soft magnets, Geiger counter, IR detector
!   foundry technology
" aerogelic binders, aerogel granules for foundry sands
!   architecture
" illumination, superlight concrete, wall isolation
!   superlight composites
!   non-linear optics (SHG)
!   Cosmetics
!   Pharmacy (drug delivery)
!   Apparel & sportsware
" Tennis rackets
" Security shoes
" Sport shoes
!   Space
" Space suits
" Anti-sloshing in tanks
" Cryogenic tank insulation
Cryogenic tank
insulation
Suppliers? Vendors?
Who delivers what?
Company: Cabot (Frankfurt)
!   Material: water glass
!   Shape: polyhedric 0.5 - 4 mm
!   transparent
!   super hydrophobic
!   price ≈ 2000 € /cbm
!   dried at ambient conditions
Aerogel granulates
TL - Translucent
Company: Dow Corning
!   material: pure quartz
!   super hydrophobic
!Application in cosmetic products
density 40 - 100 kg/cbm
Particle size 5 - 15 µm
Surface area 600 - 800 m2/g
porosity > 90%
Aerogel tiles
company: Airglass (Schweden)
!   material: Silica (precursor TMOS:Methanol)
!   Hydrophilic
!   Drying is made supercritically (methanol)
price:
•  tiles 10 x 10 x 1 inch ≈ 500 €
•  granulate 1 L ≈ 150 €
Airglass AB
BOX 150
245 22 Staffanstorp
SWEDEN
www.airglass.se
TEL: +46 46 255200
FAX: +46 46 25692
Aerogel pieces & papers
company: MarkeTech (USA)
!   material:
!   Silica (TEOS:Methanol)
!   RF aerogel pieces and paper
!   Carbon aerogel (pieces and paper
!   hydrophilic
!   shape: small tiles and pieces
!   price:
!   Random pieces: 100 g ≈ 880 US$
!   Tiles (4x8x0,5“) = 1228 US$
Marketech International Inc.
107B Louisa Street
Port Townsend, WA 98368
Tel: 360-379-6707
Toll Free: 877-452-4910
Fax: 360-379-6907
info@mkt-intl.com
http://www.mkt-intl.com/index.shtml
Aerogel blankets
company: Aspensystems (USA)
!   material: fibre composite
!   Silica-Aerogel on polymeric fiber felt
!   Silica-Aerogel on glass fibers
!   superhydrophobic
ASPEN AEROGELS, INC.!
30 Forbes Road, Building B!
Northborough, MA 01532!
USA!
Tel: 1.508.691.1111!
Fax: 1.508.691.1200 !
http://www.aerogel.com/
Aerogel blankets
Composite of mineral wool and silica aerogel
Brand name: Aerowoλλe®
Thermal conductivity 0.019 W/mK
Koebel 2007
And some other companies
!  Aerogel Composite
!   Product: Polymeric (RF) and Carbon aerogels, crushed
and compressed into arbitrary shapes
!   http://www.aerogelcomposite.com
!  American Aerogel
!   Product: VIPs
!   http://www.americanaerogel.com
!  Nano High-tech Co, China
!   Products: everything all others fabricate
Granular materials, tiles, blankets, hydrophobic, fibre
reinforced...
Examples of industrial applications
Illuminating systems - diffusers
Basic idea: transparent to translucent aerogel granules scatter light into all directions
Technology: fill hollows plates with aerogel grains.
Company: Cabot (brand name: Nanogel) in co-operation with window companies
Effects:
a) Diffusive illumination of rooms
b) Thermal insulation
before after
Isolation of subsea pipelines
Basic idea:
Utilize low thermal conductivity of aerogels
Challenges:
- Huge mechanical loads in subsea double walled
pipelines (dynamic and static)
- High temperature of the raw oil -> cooling leads to
clogging
- High service life
Insulation of Pipelines - solutions
Vendors:
!   Cabot
!   Aspenssystems
Different technical solutions
Cabot: Compression/Expansion-Pack
Aspen: aerogel blankets
source: Cabot.com
Electronic components - capacitors
Basic idea: use the large internal surface of aerogels
capacity is proportional to area!
Material: Carbon aerogels as thin films
Specific values
100 - 300 F/g
Fibre reinforced and CNT
additions of aerogels lead
to ≈ 750 F/g
Home building & insulation - Challenges
!   Energy savings by thermal isolation
measures
!   Regulation of indoor climate
!   Light weight material (1-person
structural panel)
!   High fire protection class (> F120)
!   Utilization of natural construction
materials
!   No colonization by fungi
!   Low cost material
A solution
aerogel concrete
Aerogel concrete is a
!  composite made from
" hydrophobic Aerogel granulate
and
" inorganic Binder/Matrix
!  binder / matrix materials
" gypsum or plaster
" cement or rendering
!  technical challenges
" Extreme density difference
aerogel / matrix
" Bonding strength aerogel-
matrix interface
Aerogel granules + Goldband TM
Aerogel granules + cement
Density
Lowest density with hydrophobic Aerogel
granules:
Values between 600 and 1100 kg/m3
Lowest density material made by us:
Pure building plaster with ≈ 80% aerogel
granules at 250 kg/m3
Fire protection
!  requirement: better than fire class F90
!  Laboratory tests close to DIN-Norm
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Zeit [min]
Temperatur [°C]
Rückseite Flammseite
Thermal conductivity and U-value
Aerogel content
[vol%]
Heat
conduction
[W/mK]
U-value
(25 cm wall
thickness)
65% (+10% Sand) 0,16 0,64
70% (+10% Sand) 0,11 0,44
70% (without Sand) 0,13 0,52
construction material wall thickness
common brick (17,5 cm) plus PUR
damping
30 cm 0,32
The future of aerogel concrete in the
Netherlands?
Canoe made form high strength
concrete and aerogel to save
weight.
Built by students of the
University Duisburg-Essen
Other solutions
!  Structural panels:
• Composite of PU-foams or
styrofoams plates with
aerogel-infiltarted felts
U-value=0.34
!  Aerogel wrap for storefront
Stadur-Süd
Foundry applications
Casting of metals in a mould needs a mould material
Mould materials: sands = sand casting
metals = permanent mould casting
ceramics = investment casting
Sand grains must be bonded!
Binders used:
oils, polymers (phenol based, polyurethane, epoxy resins,...)
water glass,...
new: binder = RF Aerogel = AeroSand
Cerabeads
Al2O3
Techn ology:
Preparation of RF-sol, mixing with sand, core shooting,
ambient drying, ready!
Brück-Ratke Giessereiforschung 56 (2004) 55 - 65
Voss-Ratke Giessereiforschung 57 (2005) 18-25
Reuß-Ratke Giessereiforschung 61 (2009) 24 - 33
AeroSands - real structure
RF-Aerogel with corundum sand
Carbon aerogel with
Corundum sand
AeroSands - properties
!  Easy core removal
!  Thermal conductivity 0,3 - 2 W/Km
!  High strength 100 to 600 N/cm2
!  Smooth surfaces
!  No shrinkage
!  No gas release during casting
!  Insensitive to humidity
Aerogel based additives
!   Granulate from
!   Silica-Aerogel (sc and ambient
dried)
!   RF Aerogel
!   Carbon aerogel
!   Particle size:
!   Fits to sand grain size
!   Volume fraction: ≤ 5 Vol.%
!   Binder:
!   Whatever is used in a foundry
shop
!   RF-aerogel binder
idea:
Mix aerogel granules with foundry sands to improve the cast piece and work
without a facing
utilizing:
•  Large surface area
•  High adsorption capability
•  Non-wetting with metals
Aero-Additives
Effects of aerogels in brass, aluminium
and bronze castings
Use of Aero-Additive in a brass cast shop
with 1.5 vol.-% RF-Aerogel
w/o Nano-Additives
aluminium brass
„The global market for aerogels is expected to grow up from
$62 Million in 2006 (700t) to $951 Million in 2011 (12,679t).
JOM 58 (August 2006) 10-11
Summary
Oxide aerogels, polymeric aerogels, mixed aerogels, hybrids,
composites with fibers and felts offer a wide variety of applications
going far beyond what is explored today and is on the market.
Acid/Base-Mediated Uptake and Release of Halide Anions with a Preorganized Aryl-Triazole Foldamer
Article
A new approach for the construction of artificial receptors capable of selectively uptake and release of halides to mimic the biological halide ions pumps is developed, in which the preorganized aryl-triazole foldamer was designed to bear a resorcinolic group in the central strand as a switch regulator. By using 1,8-diazabicyclo[5.4.0]undec-7-ene/picric acid as the trigger, the foldamer can be switched between "w"-shape and helical conformation. Due to the large, half-open cavity as well as the additional electrostatic repulsion between oxyanions and guest halide, the foldamer in "w"-shape possesses a much weaker affinity for chloride, bromide, and iodide anions than those in the helical conformation in 6:94 (v/v) [D6 ]DMSO/CDCl3 . When the foldamer and chloride ions have the same initial concentrations of 1 mM, 70 % chloride ions in the solution could be reversibly bound or released upon switching. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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