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Freeze Casting of Porous Ceramics

Dr. xxxxxxxxxx

31.01.2012

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Colloidal Near-Net-Shaping Techniques

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Colloidal Near-Net-Shaping Techniques

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Prozessschritte von Freeze CastingÜberblick

J.Ceram.Soc.Jpn. 2011, 119, 147-160

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Kristallwachstum

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Einfluss der Einfriergeschwindigkeit

Phil. Trans. R. Soc. A 2010 368, 2099-2121

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Einfluss der Gefriereinrichtung

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Einfluss des Lösungsmittels (LöMi)

Flüssiges Wasser Wasser als Eis

Temperaturausdehnung (TAD) beim Einfrieren

+9% !!! 

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Bildung von Eiskristallen

Adv.Eng.Mater. 2008, 10, 3, 155-169

H2O:Water (m.p.= 0°C)

TAD= +9%

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Anderes LöMi  – Andere Struktur

Camphene

(m.p.= 45-46°C)TAD= -3,1%

Solidified camphene dendrites 

tert-Butanol

(m.p.= 26°C)

TAD= +2%

Prismatic Channels 

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Einfluss der Partikelgröße anPorenmorphologie

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Freeze Casting: Vorteile

• Low shrinkage• Independent on material• Easy control through sintering step• Porosity design• Relatively high mechanical properties in the green state• Environmentally friendly

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Freeze Casting: BestimmendeParameter

•

Solid content of the suspension (10÷50 vol% = 20÷85wt%)• Particle size (20nm ÷6.5 μm)

• Solvent (pure solvents and eutectic mixtures; aqueous and non-aqueous

techniques: viscosity, thermal conductivity, surface tension, melting point,

volume change, vapor pressure, crystal structure)

•Freezing direction, rate and temperature (velocity of freezing

 front, temperature gradient)

• Freezing device and cryoprotector additions(isotropic/anisotropic cooling = homogeneous/direction solidification)

• Pressure and temperature of freeze-dryer (sublimation step)

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Freeze Casting: Probleme und Grenzen

• During the whole process the slurry must be absolutely stable againstsedimentation

• Solid load is limited (breakthrough concentration)• Most investigations reported  – 1÷3 cm (lab scale). Homogeneity of the temperature

filed and the freezing kinetics will become problematic for larger bodies

(wärmeleitfähigkeit und viscosität des LöMi, temperaturgradient)  – möglicheLosung -> Einfrieren unterm Druck

• Temperaturausdehnung may become critical during freezing and lead to cracks

and flaws

• Flaws during sublimation of the solvent – has been never optimized – terra

incognita

• Larger particles will lower the critical velocity (cv  – the particles become

encapsulated by solid)

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Beispiele aus Literatur

Materials Powder

Particle Size Solvent   Cooling Conditions  Pore Size 

Sintering

Temperature 

 Alumina 700 nm Silica Sol Cold Plate, -30°C 80 μm No sintering

 Alumina 700 nm Silica Sol Metallic Mould, -40°C 50 μm 1100-1500

 Alumina 400 nm CampheneMould, from 55 to

22°C5-20 μm 1600°C

 Alumina 400 nm Water From 22 to -100°C 2-200 μm 1500°C

 Alumina 200 nm t-BuOH From 0 to -15 50 μm 1500°C

Glass 6.5 μm Water Mould, -55°C 100-150 μm 620°C

Bioglass - Camphene Mould, from 60 to20°C

20-40 μm 700-1100°C

SiC 16 μm Water Mould, -40°C 300 μm No sintering

TiO2 150 nm Water Cold support, -18°C 15 μm 1000°C

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