Powder technology and sintering-Shaping-2 ... · Green body formation Powder conditioning...
Transcript of Powder technology and sintering-Shaping-2 ... · Green body formation Powder conditioning...
Typical course of the pressure during pressing and ejection
Upper punch
Lower punch
Time
Pre
ssur
e
(Single action)
Green body formation
Powder conditioning
Granulation Slip casting Ceramic powder/polymerMixing
Agglomerates
Wet forming methods Plastic forming methods
Green body
Slips Plastic mass(Feedstock)
Dry forming methods
Wet forming methods
Powder
Suspension (Slip)
Solid-liquid separation
Slip casting Tape casting of slips
Mechanical separation of
liquid and powder(filtration)
Thermal separation of liquid and powder
(evaporation)
Surface charges of metal oxides in water
MeO
MeOH2O MeO
MeOH
OH
MeO
MeOH
OH
(1)
(2)
MeO
MeO+
OHMeO
MeO-
OH
H+ OH-
H
H
Low pH High pH
Wet forming
Potential
Distance
0
Powder particles in water
(Gouy-Chapman model)
Positive counter ions
Diffuse layer = “Cloud” made of counter ions
Negative surface potential(= Nernst potential)
Layer model for powder particles in electrolyte-containing water
(Stern model)
Diffuselayer
Electrolyte-containing system
Stern layerInner
OuterHelmholtz layer
Negativ, dehydratedNegativ, hydratedPositive, hydrated
Not fixed hydrated positive and negative ions
Fixed hydrated positive ions
Fixed dehydrated negative ions
Potential
Layer model for powder particles in electrolyte-containing water
Potential of the inner Helmholtz layer (i)
Potential of the outer Helmholtz layer= Stern potential (s, a)
Nernst potential0
Exponential decay in the diffuse layer
Decay to zerois simplified
Diffuse Layer
Zeta potential
Diffusing particleStern potential
Nernstpotential
Partial separation of the diffuse layer Potential difference
Potential at the shear plane S = Zeta potential (ZP)(= Potential difference)
Diffuse layer
Measurement of the zeta potential by electrophoresis
-- --
-
-
---
-
-+-
+
+
+Cathode Anode
+v
: Zeta potentialv: Particle velocity: Viscosity of the dispersion medium0: Dielectric constant in vacuum r: Relative dielectric constantE: Electrical field strengthv/E: Electrophoretic mobility
Er 0 v
Zeta potential
Estimation of the physical stability by means of the zeta potential
Features of the stability Zeta potential (mV)
Maximum agglomeration and sedimentation +3 to 0
Distinct agglomeration and sedimentation -1 to -4
Barrier to agglomeration -5 to -10
Slight agglomeration -21 to -30
No agglomeration -31 to -40
Good stability -41 to -50
Very good stability -51 to -60
Excellent stability -61 to -80
Maximum stability -81 to -100
DLVO theory
Description of the interaction energy VT between two particles:
VT = VA + VR
VA: Contribution of the van der Waals attraction
VR: Contribution of the electrostatic repulsion
Van der Waals attraction
Repulsion
Attraction
xVA(x) = – A·r /(12x)
A: Hamaker constantr: Particle radiusx: Distance of the particle surfaces
Particles floccuate
VA
Electrostatic repulsion VR
VR(x)=2rr02 exp(–x)0: Dielectric constant in vacuumr: Relative dielectric constant: Surface potentialr: Particle radiusx: Distance of the particle surfaces: Debye-Hückel parameter
Particles dispersed
Contribution of the double layer to the interaction energy:
Attraction
Repulsion
x
VR
Thickness of the diffuse layer
21
2201
i
ii
r
zNekT
: Debye-Hückel parameter0: Dielectric constant in vacuumr: Relative dielectric constantk: Boltzmann constantT: Absolute temperaturee: Elementary chargeNi: Number of ions per volume unitzi: Charge number of ion species i
xx e
: Potential at the particle surface(Stern potential)
x: Potential at distance x from the surface
Stern potential
Nernstpotential Decay to 1/e
Decay to zero
Stern layer Thickness of the diffuse layer 1/
Thickness of the diffuse layer
Concentration dependence
1:1 electrolyte (e. g. NaCl)
Dependence on the ionic charge
2:1 electrolyte (e. g. CaCl2)3:1 electrolyte (e. g. AlCl3)
HO C C CO
H
H
H
C C C H
7
H
H
7
H
HO C C CO
H
H
H
C C C H
7
H
H
7
H
HO C C CO
H
H
H
C C C H
7
H
H
7
H
Adsorption of oleic acid at oxide particles
Oxide particle
Combined particle interaction
V
Van der Waals contribution
Steric contribution
Total potential
Steric stabilization by long-chained molecules (long range)
V Steric contribution
Van der Waals contribution
Steric stabilization by short-chained molecules (short range)
Wet forming methods
Powder
Suspension
Solid-liquid separation
Slip casting Tape casting
Mechanical separation of
liquid and powder(filtration)
Thermal separation of liquid and powder
(evaporation)
Slip casting
Porous, water absorbing mold (gysum, CaSO42H2O
Cast copies the inner contour of the mold
Realization of small wall thicknesses:Increase of the cast thickness dx:
dtxcdx
c: constant (depending on the permeability and drain volume of the cast; solid yield and viscosity
of the suspensiont: time
Integration (x =0 at t=0):
ctx 2
x
t
ctx
2
Processing additives
Additive
Solvent
Wetting agent
Deflocculant
Binder
Plasticizers (softener)
Antifoaming agents
Preservatives
Separating agents
Lubricants
Function
Suspending agent
Dispersing agent, reduction of the surface tension of the solvent
Inluence on particle interaction, control of surface charges and pH,
steric stabilizationGreen strength/machinability
FlexibilityModification of binder properties
Avoidance of bubbles
Attack by bacteria, fungi, algae
Mold release
Friction, punching properties
Examples
Water, alcohols, trichloroethylene, toluene
Stearates, dodecyl ammonium acetate
Citrates, polyacrylic acid
Polyvinylbutyral, polyacrylic acid
Phthalates, glycols, water
Octanol
Bactericides, fungicides, Cu salts
Fatty acids, fatty acid esters, wax
Glycerin
Slip
Doctor bladewith micro screw
Exhaust airDrying chamber
Preheated air
flexible green tape
Tape casting