On the importance of the electrokinetic properties of the starting nano powders for processing of ul

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On the importance of the electrokinetic
properties of the starting nano powders for
processing of ultra pure silica crucibles via
pressure casting followed by laser
sinteringJürgen G. Heinrich, Janka Seeger,
Birgit Palm
Rusnanotech09, Moscow

• Motivation
• - Processing of ultra pure
  silicon single crystals (semiconductor
  applications) and polycrastaline silicon (solar
  applications)
• - Processing of ultra pure SiO2
  crucibles from nano powders without organic
  additives
• Stabilisation of slurries made out of nano
  powders
• -Theoretical considerations
• - Slurry stabilisation with nano powders
• - Proping effect and demolding
• The pressure casting process
• - The demolding problem at
  pressure casting of micro powders has to be
  solved
• Outlook and conclusion

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Motivation / Background

• Manufacturing of high purity silicon
• High purity SiO2 crucibles
• Czochralsky- Process
• Several days of process time
• Process temperature gt1420 C
• Silicon single crystals Dm. gt 300 mm
• High corrosion of crucible

R Wacker Chemie AG
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The Czochralsky Process
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Ultra Pure Wacker Wasic Crucible Production
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Diffuse electric double layer at a solid particle
surface in aqueous suspension and potential
gradient between particle surface and suspension
(schematic)
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Slip stability and potential energy as a result
of the surface condition of particles in an
aqueous suspension
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Zeta potential and viscosity of Al2O3
suspensions (from Graule et al. and Heinrich et
al.)
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Parameters with influence on the zeta potential
fH Henry constant ? Viscosity of the
electrolyte ?e electrophoretic velocity for an
imposed electric field E ?r dielectric
constant ?0 dielectric constant in vacuum E
imposed electric field Henry fH 1 d ?
? gt 100 fH 1,5 d ? ? lt 1 d
particle size ?-1 thickness of the diffuse
electric double layer
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Grain size influence on the zeta potential
Henry fH 1 d ? ? gt 100 fH 1,5 d
? ? lt 1 d particle size ?-1 thickness of
the diffuse electric double layer
?-1 9,6nm )
particle in nm range d lt 9,6nm gt fH 1,5
particle in µm range d gtgt 9,6nm gt fH 1
) according to R.G. Horn Particle Interactions
in Suspensions. In R.A. Terpstra, P.P.A.C. Pex,
A.H. de Vries Ceramic Processing. Chapman
Hall, London (1995)
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Theoretical grain size dependence of the zeta
potential
D.C. Henry Proc. Roy. Soc. London, 133, 106-140
(1931)
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Forces in suspensions between ions and
nanoparticles
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Theoretical grain size dependence of the zeta
potential

• At a certain grain size in the nanometer range
• With just a few ions on the surface the forces
  are no longer continuous
• Mutual repulsion of ions occurs
• ? ? should decrease again

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Experimental

• Determination of grain size distribution
  (Lasergranulometer Coulter LS230) and specific
  surface area (Areameter)
• Slurry solid yield 2 Vol.-
• Measuring of the zeta potential vs pH by ESA 8000
• Conversion of the ESA signal
• into the zeta potential

? Viscosity of the liquid e Dielectric
constant of the liquid F volume fraction of the
particles ?? Density difference between
particles and liquid c sound velocity in the
suspension G(a) correction factor for the mass
inertia of the particles
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Experimental

• Grain Size and Specific Surface Area of the SiO2-
  Powders (Lasergranulometer Coulter LS230,
  Areameter)

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Results and Discussion

• Zeta-Potential vs Grain Size

?
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Results and Discussion

• Possible reasons for deviation from theory
• OBriens formula for the conversion of the ESA
  signal into the zeta
• potential is valid just for the following
  assumtions
• Diluted slurry (solid content lt 10 Vol.-)
• d/?-1 gt 50 some of the investigated particles
  possibly didnt
• fullfill this assumption
• Narrow grain size distribution of the powder,
• otherwise the correction in the calculation of
  the effectife radius (reff) and therefore G(a)
  might be faulty
• - Nano powders are partially soluble in the
  liquid

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Zeta potential of SiO2 slurry without and with
nano particles for stabilization
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Viscosity vs. Shear Rate of High Purity
SiO2-Slurries
Ph.D. Thesis Daniel Raschke
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Pressure Casting Theoretical View

• Casting process - Process of filtration is
  diffusion controlled
• Parameters - Soaking time
• - Casting pressure
• - Slip viscosity

x cast thickness km factor of permeability p
pressure difference t soaking time a
factor of slip h slip viscosity
cast thickness x ?
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Demolding model for slurries with and without
nano powder fraction
Demolding That's the problem !
Demolding No problem !
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Double Layer Crucible

• Inner Crucible

A 1st Step Casting
B Mould change
Pressure casting of Double Layer Crucibles
(schematic)
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Laser Treatment of SiO2 Preforms
coefficient of expansion 5.5 x 10-7 unusually
high thermal shock resistance
Ph.D. Thesis Sven Engler
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Outlook

• Assessments
• There should be a maximum of the zeta potental
  between zero and 1 micrometer grain size
• The maximum could not be confirmed with SiO2 -
  nano powders so far
• Nano powders can act as deflocculants
• The demolding problem after pressure casting can
  be solved with nano powders
• Open questions
• Is measuring of the zeta potential vs pH by ESA
  8000 the right method (The influence ot G(a)
  should be reduced at lower measuring
  frequencies)?
• How can the grain size of nano powders be
  measured exactly?
• Future activities
• Manufacturing of nano powders in the range of 5nm
  through 1000 nm

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Start with nano, end up huge!
28crucible, ultra pure SiO2 (wall thickness 10
mm)