powder technology - epfl · granules 8. particle packing • empirical models • theoretical and...
TRANSCRIPT
LTPÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE
Powder Technology
From landslides to concrete and from avalanches to
chocolate….
Prof. P. Bowen (EPFL), Dr. P. Derlet (PSI)
1
SUMMARY
Most materials e.g. ceramics, metals, polymers or concrete pass during their processing one
or more steps in the form of powders. This course discusses and presents the science &
technology of important powder processing steps like compaction, dispersion, sintering and
novel densification technologies. EXERCISES – new this year ..use models…
CONTENT
• Theoretical and empirical models for powder packing and compaction including discrete
element modelling (DEM) (examples for ceramics and metals)
• Particle- particle interactions (colloidal chemistry, DLVO theory, non-DLVO forces ,
polymer adsorption, colloidal stability assessment). Examples from cement and concrete,
landslides, ceramic powder granulation, paper coating.
• Introduction to atomistic modelling - with examples from grain boundary segregation of
dopants in ceramics, polymer adsorption and crystal growth
• Sintering mechanisms (metal, ceramics, influence of the microstructure, simulation)
• Novel technologies (includes rapid prototyping, spark plasma sintering, laser sintering)
• The support material for the course are copies of the slides used to present the course along
with a few key text books and review articles - which the students are encouraged to use to
supplement the documents provided.
Applied Powder Technology – Fundamental approach – modelling – predict
behaviour from measurable experimental parameters or access something not
possible by experiment …lots of equations but oral exam…typical questions…
Learning Prerequisites - Recommended courses
Ceramics, Ceramic processing, material science
Important concepts to start the course
microstructure property relationships
Applications of powders…..Ceramics
Hip jointsMedicineConcrete
Food
Nanotechnology
Magnetic nanoparticles
Photography
- Paper coating
Composites
Self –healing carbon fibre
reinforced polymers
Cosmetics
Metals
P. Bowen, EPFL. 4
Course Contents - Plan1. Introduction – general introduction to course
– example transparent ceramics
2. Particle Packing and Powder Compaction - Theoretical and empirical models (PB)- Powder compaction (PD)
3 Particle-Particle Interactions (PB)- Colloidal Dispersions- DLVO –theory and limitations- non-DLVO and steric forces
Exercises – particle particle intercations and rheology – HAMAKER & YODEL
4. Introduction to Atomistic Scale Simulations (PD)- introduction to modeling of surfaces and interfaces at the atomic scale - defects in metals – towards sintering
5. Sintering mechanisms (PD)- metals, ceramics- influence of microstructure- simulation
Exercises – Atomistic modeling and DEM - LAMMPS
6. New Powder Processing Technologies (PB)- rapid prototyping- laser sintering, Spark Plasma Sintering
SiC - abrasive
« La neige » Snow …
• The Colloidal Domain – D. F. Evans & H. Wennerström, Wiley, 1999,
• Principles of Ceramic Processing – J.S.Reed , Wiley, 1995. English
• Les Céramiques, J. Barton, P. Bowen, C. Carry & J.M. Haussonne, Les Traité des Matériaux, Volume 16, PPUR, 2005
P. Bowen, EPFL. 5
Teaching plan 2018
• Files of lectures and notes to be found on LTP website : http://ltp.epfl.ch/Teaching
Week-
DATE
File.
no.
Powder Technology – Wednesday 10.15-13.00 – MXG 110
1- sept 19 1&2 PB Introduction – example rheology – Yodel - Powder packing and compaction – 1 (i) – (3hrs)
2 – sept 26 2&3 PB
MS
Powder packing and compaction – 1(ii), 2- Examples and DEM guest lecturer – (3hrs)
3 – oct 3 4 PD Powder packing and compaction -3 & 4(i) – (3hrs)
4 – oct 10 4&5 PD PB Powder packing and compaction - 4 (ii) – (1hr)
Particle – Particle Interactions 1 - 2hrs
5 – oct 17 6&7 PB Particle – Particle Interactions 2 & 3(i) – (3hrs) – Download Hamaker
6 – oct 24 7 PB Particle – Particle Interactions – 3(ii) YODEL-PB (1hr)
Exercises – Intro to Hamaker & YODEL software & groups project (2hrs)
7 – Oct 30 AKM Exercises - Hamaker and Yodel Modelling – group projects
8 – nov 7 8 PB PD Exercises –presentation of interparticle project results (1 hr)
Introduction to atomistic scale simulations – (2hrs)
9 – nov -14 9& 11 PD Compaction, Sintering & Defects in metals at atomistic scale (2hrs)
Sintering Mechanisms – 1(i) (1 hr)
10 – nov 21 11 PD Sintering Mechanisms - 1 (ii) & 2 (3hrs)
11 -nov-28 PD Excercises -Introduction to Molecular Dynamics Modelling using LAMMPS (3hrs) .
12 - dec 5 PD Excercises - MD- DEM modelling exercise using LAMMPS –particle packing - Effect of parameters
(3 hrs)
13 – dec 12 10 PB New Technologies -1 Processing – Forming – Shaping (2hrs) & Exercises or invited lecture or
visit
14 – dec 19 10 PB New Technologies-2 – Sintering Methods & Exercises or invited lecture or visit & Exam method
PB – Prof. Paul Bowen (EPFL), PD – Dr. Peter Derlet (PSI)
MS- Dr. Mark Sawley (EPFL), AKM - Aslam Kuhni Mohamed(EPFL)
This week (1) and next week (2)
• Introduction – Brief overview of course contents
• Revision of rheology of suspensions (course Céramiques procédés – Vol 16
Les Traité des Matériaux,Volume 16 "Les Céramiques« , J. Barton, P.
Bowen, C. Carry & J.M. Haussonne)
• Practical example of importance of particle packing and colloidal stability –
particle-particle interactions
– Transparent polycrstalline ceramics – rheological model …Yodel…
• Next week - Particle packing
– Spheres and regularly shaped particles (cylinder…)
– Irregularly Shaped Particles
– Effect of size on packing
– Effect of size distribution (log-normal)…..
– Models - Numerical and Analytical (empirical)
– Bi-modal distributions – multimodal distributions
6
Responsable – Echanges Section Matériaux – Prof. Paul Bowen
Mountain Ash Comprehensive School,
WALES 1969-1976 ( I was 19 in 1976)
•From Wales via
•Imperial College London 1976-79
•University of Cambridge PhD 1979-82
•British Petroleum Research Centre 1983-86
•Since 1987 at EPFL
Passions
•Rugby
•Beer
•Music
•Physics
Passions
•Mountains
•Wine
•Dance
•Powder Technology
Introduction
•What is Powder Technology
•What is a powder?
• A particulate material from nm to m
•Characterization and control behavior – in various systems
•Synthesis of ceramic powder synthetic
•Crystallization of sugar or salt
Visco-elasticity
•Dry- flour, instant coffee, snow ...
•Wet - suspensions in the treatment of ceramics, concrete placement
•Landslides -rocks up to 3 m and clay fractions of 20 nm in
thickness
•Chocolate - size distribution of the phases - rheology and taste ....
Alumina platelets
Calcium Phosphate
Granules
8
Particle packing
• Empirical models
• Theoretical and numerical models
• ( e.g. DEM, Discrete Element Modelling)
• Particle Packing; Log normal size distribution
• Particle Shape.....
• Neural network
Atomised steel powder used in car industry
9
DEM – perfect elastic vs friction & adhesion
Z - coordination number
10
Particle Packing – Log Normal distribution
•RCP - Good correlation with experimental results
•RLP – only for s > 2.5
s < 2.5 bridging – stable numerically– but not in reality – gravity etc… –need to know forces used in models
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0 1 2 3 4 5 6
RCP-ModelRLP-ModelRCP-ExperimentRLP-Experiment
Pa
ckin
g F
ractio
n
Standard Deviation (s)
Numerical
•Model - Nolan & Kavanagh - Spheres, RCP & RLP
s > 2.5
s < 2.5
Réseau neurone – Neural Network
• IW – weighted
hidden layer
- non-linear
function
• LW – weighted
output layer
- linear function
• b- bias
• Adjust until target
value achieved
Surface and colloidal forces &
Introduction to atomistic modelling
A User Friendly Programme for Interparticle Interaction
Energy Calculations - Hamaker - the First Step in
Successful Nanosized Powder Dispersion.
• Uli Aschauer, Paul Bowen
WP8 – Modelling & Simulation
LTP website (ltp.epfl.ch) – Research – Powder Processing– Colloidal Stability
- http://hamaker.epfl.ch
13
Colloidal Stability Calculations and Rheology
♦ Program written similar to Bergström approach
♦ Studied Gamma alumina with C1-C4 carboxlic acids
♦ Compared calculations with rheological behaviour
♦ Yield stress of gel to estimate the energy to separate particles
L. Bergström, C.H. Schilling, I.A. Aksay, .Am.Ceram.Soc., 75(12) 3305-14 (1992).
Acetic AcidFormic Acid
Propionic AcidButyric Acid
Real relative size
14
Colloidal Stability
• Electrostatic
interactions (zeta
potential)
• Steric interactions
• Polyelectrolytes
(Polyethylenimide
PEI, etc)
• Magnetically
induced interactions
h
(a)
(b)
++
+
+
++
+
+
+
++
++
+
+
++
+
+
+
++
(distance h between particles)
15
Magnetic interaction between particles
•Other interparticle interactions…salt
concentration…zeta potential…
•In-Situ GOLD self assembly TEM movie
•(30-50nm) – Liu et al 2013 (CTA+coated)
• Effect of NaCl concentration and modification
•of zeta potential under electron beam
• movie : http://pubs.acs.org/doi/suppl/10.1021/ja312620e
attraction
b
repulsion
magn. field lines
Important for transport behaviour – e.g. iron oxide
16
Changes in polymer and protein adsorption and
/or conformation – biomedical applications
0
20
40
60
80
0 2 4 6 8 10 12
pH
Nu
mb
er
we
igh
ted
PC
S s
ize
[n
m]
R = 18.4
R = 13.8
Conformation changes of the PVA at the
particles surface (swelling, hyrogel)
Dawson
17
Atomistic modelling – Dr. Peter Derlet (PSI)
• Introduction to atomistic modeling
of surfaces and
interfaces…ubiquitous in powders
• Energy minimization
• Molecular dynamics
• Metadynamics
• Examples in course
– Adsorption of polymers onto
inorganic surfaces in water
– Defects in metals……
18Water not visualised for clarity
Compaction – densification – sintering - metals
19
Analysing the mechanical behaviour under
cold and/or hot compaction:
• Phenomenological description based on
experimental observation.
• Macroscopic constitutive equation for a porous
solid, often based on phenomenological
assumptions.
• Micromechanical approach to analyse the
deformation of individual particles in detail.
20
Macroscopic constitutive model
• Equilibrium equations (balance of forces transmitted through the material)
• Continuity equation (conservation of mass)
• Geometry of the problem
• Constitutive behavior of the powder (stress– strain behavior)
• Boundary conditions including loading (e.g.,displacement and velocity) and friction between the tooling and the powder
• Initial conditions (e.g., initial relative density of powder)
21
DCP Model
Application of the Drucker-Prager-Cap Model (DPC) on the compaction
of a real ceramic part
The DPC Model at low hydrostatic pressures is a shear failure model, similar to those
used in granular flow like the Cam-Clay Model
22
Sintering
Irreversible Thermodynamics
23
24
Solid State Sintering of real
powder compacts
Literature:
Sintervorgänge , Grundlagen
Wernerr Schatt
VDI Verlag1992
25
Solid State Sintering- real powder compacts
26
Experiment – 2 orders of magnitude
greater than predicted… .defects…!
Dislocation movement…!
New (Forming &) Sintering
methods
27
2003/06/17, Lausanne
Spark Plasma Sintering - Images
28
CS1250oC - 2hr, 98.5%, 7mm SPS 900oC - 3min, 99.8%, 0.2mm
Examples of SPS – nanostructured
ceramics - PLZT
29
Future – Transparent polycrystalline alumina
– Sintered by SPS !
30
RIT of 7.8% RIT of 57%
directly on the sheet
• PCA samples (Ø 12 mm, thickness 1mm) – Spark Plasma Sintering*
• SPS - Stockholm Univ. Prof. Zhao Zhe
*M. Stuer et al . J.Eur. Ceram
Soc. 30 (2010) 1335-1343
Key factors
1. Eliminate pores
2. Small Grain size or
Orient grains
HOW?
1. Better Processing
2. Better Powders
Selective Laser Sintering (SLS) –
additive manufacturing- 3D or Ink-jet printing
31
Simple quick video….http://www.youtube.com/watch?v=SVkUwqzjGJY
32
Advantages of the SLS - process
motivation
speed of fabrication – rapid prototyping
complexity of piece geometry
recycling /re-use of primary materials
nature of materials that can be used
32
Revision - Rheology of Suspensions - liquids
• Viscosity of a liquid resistance of a liquid to flow .
• Ratio of shear stress t, to shear rate (rate of shear strain) dg/dt or
• Shear stress – stress that causes successive parallel layers of a material body to move in their own planes
• Shear strain g – relative in plane displacement, x, of two parallel layers in a material body divided by their separation distance y,
• Sheare rate - rate of change of shear strain
d dv
dt dy
g Shear rate
A
FtShear stress
g
t viscosity
g
moving plate
stationary plate
Flow curves and viscosity
1’000’00012’0001004-152-51.51.00.65
Bitumin-tarHoneyOlive oilBloodOrange
juice
MercuryWaterPetrol
Tableau 3.4.6. Typical values of viscosity of well know liquids 20˚C in mPa·s.
Liquid
Shaft
Torsion wire
Stand
Hollow cylindre
Solid cylindre
Couette viscometer.
(a) Newtonian (b) shear thining (c) shear thickening
Flow curves
Shear
str
ess
Shear rate
Rheology of suspensions
• Principal factors that control the behaviour are:
– volume fraction of particles (),
– type of polymeric additive in the dispersing liquid
– interparticle forces (colloidal stability),
– Particle size distribution and morphology
– Maximum particle packing fraction
• For dilute suspensions, ignoring interparticle forces, Einstein derived a
simple expression for the viscosity of a suspension
s - suspension viscosity, - volume fraction of particles ,
l - viscosity of the liquid
is a constant which depends on the particul shape - 2.5 for spheres > 2.5,
for anisotropic particles – effective volume fraction
1lsEq. 3.4.22
35
Volume Fractions >2%
• Einstein relation valid for up to 2% (further if ideal monodispersed system
• > 2% - need to take into account higher order terms using a Taylor series
• Accounts for interparticle interactions
.......1 32 b ls Eq. 3.4.23
For fine particles - significant changes in the effective volume can become
apparent due to the thickness of the electrical double layer or an adsorbed layer
of polymer.
For agglomerated particles, liquid is lost inside the agglomerate and cannot
contribute to the movement of the particles - i.e the effective volume of the
particles is increased (effective volume of liquid decreased) with respect to the
real volume of the solid in the suspension – next slide
Important for suspensions with low ionic concentrations eg 10-5 M
10 nm particles can have an effective diameter of 100 nm (Tadros, 1995). 36
Wetting of powder – to produce flowing
suspension - solids volume fraction…..
SAMPLES……… 37
Effect of VolumeFraction
• Particles treated as hard spheres (eg. Silica in cyclohexane) – viscosityincreases as increases
• A shear thinning behaviour is observedbetween the low 0 high shear limits ∞.
• For the low shear limit, volume fraction at which the viscosity diverges is near to 0.58 which corresponds to a randomloose packed limit.
• For higher shear rates this limit is higherand closer to the close packed limit of 0.64 suggests a certain degree of ordering (eg : hexagonal close packed = 0.74) this allows the particles to move despite the high volume fraction (Russel 1991).
Figure 3.4.23. Effect of volume fraction
on the relative viscosity of a suspension -
for hard spheres (a) low shear limit 0 (b)
high shear limit ∞.(Russel, 1991). 38
Particle Packing – Size Distribution & Shape
• Particle Size Distribution (PSD)
• log-normal distribution* - spheres
• maximum packing increases vs width
*G.T. Nolan and P.E. Kavanagh, Powder Technology, 231-238, 78 (1994).
64
68
72
76
1 1.5 2 2.5
Packing density (%)
Packin
g d
en
sity (
%)
Geometric Standard Deviation
0.0 0.5 1.0
0.4
0.5
0.6
0.7
Fra
ctio
nal
den
sity
Relative roundness
Particle Shape
maximum packing decreases
As sphericity$ decreases
$ R.M. German "Particle Packing Characteristics", Metal Powder Industries Federation, Princeton, NJ, 1989.
Rheology of Clay Pastes
• For traditional ceramics rheology is more complicated by the anisotropic shape andd inhomogeneous charge distribution on clay surfaces.
• At pHs below 6 the kaolinite platelets carry negative charges on their basal planes and positive charges on their edges.
• This leads to a house of cards type structure which at high volume fractions gives us a continuous attractive particle network with a yield stress.
• At higher pH all the surfaces become negative and a minimum in viscosity is observed (Figure 3.4.25, next slide).
• An Example of importance in practice is the geolocial phenomena of landslides –Alpine Debris Flow – later in course
pH< 6
pH>8
40
Figure 3.4.25. Change in viscosity Kaolin and "ball clay " suspensions as a fonction of pH
(Reed, 1995) and a schematic representation of the suspension microstructure.
pH< 6
pH>8
Rheology of Clay Pastes
41
Rheological Models Different types of fluid behaviour – Newtonian and non-Newtonian
Various models used to extract information e.g. yield stress – all give very similar results
M. Palacios. -Química del Cemento – Enero 2010
(Ferraris, C. F. , 1999)
• Yeild Stress – t0 Minimum stress to make liquid or suspension flow
• Typical of systems with attractive network
• Yoghurt – Ciment - Ceramic slurries
42
Paul Bowen, Michael Stuer
Laboratoire de Technologie des Poudres, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
Fundamental Issues in the Processing of
Transparent Aluminas : From Interparticle Forces to
Dense Transparent Ceramics
Example
44
Plan of Talk
• Introduction – background...
• Ceramic properties – forming methods...why quality powders
• Dispersion & Colloidal stability – Hamaker Programme
• From interparticle forces and particles size distributions to rheological properties
• YODEL – a Yield stress mODEL for concentrated suspensions
• Transparent Polycrystalline Alumina
• Conclusions
Ceramics come in all shapes and sizes
Variety of applications/properties – dictates powder & forming method
Hip joints
Tableware
Automobile – spark
plugsBuilding materials
Electronic circuits – the mobile phone – cars…
45
Properties depend on - microstructures
Controlled by powder surfaces - grain boundaries after
sintering
Design microstructures – better powders – better
processing…?
Why quality powders - Alpha alumina–effect of agglomerates
Particle size distribution shows small tail of agglomerates – leads to
defects in microstructure and low sintered densities (94%)
99.99
0.1
1
.01 .1 1 5 10 2030 50 7080 9095 99 99.9
AKP50-non-broyéeAKP50-broyée
ES
Dia
mè
tre
(µ
m)
% volume cumulés
46
Powder Influences Microstructure - Properties
Transport properties -
• Electrical, Mechanical, Optical
Influenced by - Grain size and
Grain boundary composition
Processing – high quality powder*
• High compact density
• Narrow pore size distribution
• Close pores as late as possible
• Better microstructure
• Understand surfaces and interfaces
• Control microstructure
• Colloidal processing – even dry
pressing – need good dispersion for
spray drying
As received – slip cast – 94 %
Attrition milled 1hr – slip cast – 99%
*F-S. Shiau, T-T. Fang, T-H Leu, Materials Chemistry and Physics, 57, 33-40 (1998).
47
A User Friendly Programme for Interparticle
Interaction Energy Calculations – Hamaker*.
WP8 – Modelling & Simulation
• Uli Aschauer – Easy to use program - http://hamaker.epfl.ch
or
• http://ltp.epfl.ch – Research – Powder Processing – Colloidal
Stability
*U. Aschauer, et al. J. Dispersion Science Technology. 32(4), 470 – 479 (2011)
Steric -polymer adsorption – layer thickness
Interparticle Forces - simple to use freeware – Hamaker*
Repulsive
Electrostatic, ion adsorption, dissociation, polyelectrolyte
h
(a)
(b)
++
+
+
++
+
+
+
++
++
+
+
++
+
+
+
++
(distance h between particles)
hak
al
r = ( h + 2a )
*U. Aschauer, et al J. Dispersion Science Technology, 32(4), 470 – 479 (2011).
( ), , 212k lha a h
aF A
h 2 k l
k l
a aa
a a
Harmonic average radius
2
2
0 22
1
h L
ES h L
eF a
e
Electrostatic potential
From zeta potential)
1/ Electrical double
layer thickness
5
3
2
3 2, 2 1
5
Bster k l
k T LF a a a
s h
L - Adsorbed layer thickness, s - Spacing of adsorbed moleculesIn mushroom configuration – geometry important
Attractive - dispersion or Van der Waals forces – A(h) – Hamaker constant
49
Overall Interaction Energy – DLVO*
♦ Net force is algebraic sum of
repulsive and attractive forces
0
Inte
rac
tio
n E
ne
rgy
charge
polymer
Attraction - VdW
h
(-)
(+)
1-4 nm
Repulsion total
♦ Bergström$- good qualitative
results with alumina & fatty acids
♦ Not quantitative - used identical
spheres - need to use PSD
♦ Yield stress mODEL (YODEL)#
Uses PSD
♦ Predicts yield point
♦ Used for cement £ – complex
mixture of 4 or more minerals –
certain degree of success ,h Disp ES Steha rG F F F
$Bergström, et al J.Am.Ceram.Soc., 75(12) 3305-14 (1992). *Derjaguin & Landau - Vervey & Overbeck #Flatt&Bowen, J. Am. Ceram. Soc., 89 [4] 1244–1256 (2006), £Houst et al 38 1197–1209 (2008), Perrot et al
Cem.&Conc.Res. 42 (2012) 937–944, Palacios et al Mater. de Construcción, 489-513, 62(308), 2012
Total Interaction
VT = VA + VR
Maximum Energy Barrier,
50
Taking into account Particle Size Distributions (PSD)
• Suspension may form an attractive
network - yield stress
• To flow have to break ”pairs”
• Reduces the effective volume
fraction
• To predict - need all the possible
pair interactions as a function of
zeta potential, adsorbed layer
thickness, PSD etc....
• Suzuki & Oshima* statistical model
,h Disp ES Steha rG F F F
Total Interaction Force
1 2
1 2
2
a a
a aa
All forces – function of harmonic radius
(*M. Suzuki, T. Oshima, Estimation of the coordination number in a multicomponent mixture of
spheres, Powder technology, 1983, 35, pp. 159-166) 51
YODEL - Effective volume - aggregates
No. of “bonds” – coordination number from packing models
Strength of bond from interparticle force calculations
Certain no. of “bonds” break under a certain shear
How does effective volume of solids change?
Robert J. Flatt, Paul Bowen, J. Am. Ceram. Soc., 89 [4] 1244–1256 (2006)
Yodel: A Yield Stress Model for Suspensions
52
YODEL - Effective volume - aggregates
No. of “bonds” – coordination number from packing models
Strength of bond from interparticle force calculations
Certain no. of “bonds” break under a certain shear
How does effective volume of solids change?
Robert J. Flatt, Paul Bowen, J. Am. Ceram. Soc., 89 [4] 1244–1256 (2006)
Yodel: A Yield Stress Model for Suspensions
53
YODEL - Increased effective volume - aggregates
Truncated coneEnclosing sphere
Several geometries looked at – some minor differences but all
give same general trends
Best fit to alumina slurries – Enclosing sphere model
Robert J. Flatt, Paul Bowen, J. Am. Ceram. Soc., 89 [4] 1244–1256 (2006)
Yodel: A Yield Stress Model for Suspensions
54
YODEL - Volume fraction functionality
t
ss
**
01m
Factor, m1 includes:
- particle size (a)
- particle size distribution
- interparticle force, G (a,h)
- distance of closest approach, H
- radius of curvature of contact, a*
0
1000
2000
3000
4000
5000
6000
7000
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
AKP-50AKP-30AKP-20AKP-10
Yie
ld s
tress
[Pa]
Volume fraction [-]
Model validated with data from attractive network - careful study on alumina
slurries near the isoelectric point$
Yield stress, t, as a function of volume fraction () and maximum packing
fraction (s) , percolation threshold 0
$Zhou, Z., Solomon, M. J., Scales, P., Boger,
D. V. - J. Rheol. 43(3) 651-671(1999)
Pores / precipitates
Grains themselves
Transparent Polycrystalline Alumina - General Context
Scattering sources
Sample surfaces
Incident light
Reflected light
Transmitted light
Reflected light
Grain boundaries
a b
c
Hexagonal lattice
• na = nb=1.760
• nc = 1.768
Birefringent
n = [ 0.0, nmax = |na - nc| ]
nmax = 0.008
PCA
56
Sapphire
Real In line Transmittance
RIT = 86%
Transparent Polycrystalline Alumina – applications
• Apetz & van Bruggen *
• Real–In-line Transmission (RIT)
• PCA - 50-60% (70%)
• Krell * “careful” colloidal Processing
• sinter 95% density close porosity – post HIP
*R. Apetz & M.P.B. van Bruggen, J.Am Ceram.Soc. 2003, *A. Krell & J.Klimke J. Am. Ceram. Soc.(2006)
• Swiss Watch industry needs >80%
• Why polycrystalline
• Easier to shape than sapphire - “hard” work!!
• How can we provide the required microstructures?
• Reduce grain growth
• Avoid second phases (dopants) and porosity
• Can we do it using SPS – can we do it by dry pressing?
57
0
10
20
30
40
50
60
70
80
90
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
RIT
[%
]
grain size [mm] Diameter
Δn=0.005 porosity: 0.00% vol.
Δn=0.004 porosity: 0.00% vol.
Δn=0.005 porosity: 0.01% vol.
Δn=0.005 porosity: 0.05% vol.
Transparent PCA - Light Scattering Theory
= 640 nm; thickness = 1 mm; pore size = 50 nm
• <∆n>
(alignment) and
porosity affect
shape of curve
• Porosity reduces
maximum RIT
•But very
difficult to verify
density > 99.8%
To improve the real in-line transmittance (RIT), one needs:
FULL DENSIFICATION + GRAIN ALIGNMENT
AND/OR SMALLER GRAINS 58
59
RIT and Porosity - difficult to measure
Pore analysis by 3D-FIB tomography:*
Sample name: 350A 350B 350C
RIT [%]: 48.9 9.5 42.2
Porosity [vol%]: 0.036 0.191 0.048
Dv50,pores [nm]: 51.8 81.9 61.5
350A
350B
350C
4.11µm x 4.11µm x
4.11µm
*M.Stuer, C. Pecharroman, Z. Zhao, M. Cantoni, P. Bowen " Adv. Funct. Mat.. 22(11) 2303 (2012).
60*R. Apetz , M. P. B. van Bruggen , J. Am. Ceram. Soc. 2003 , 86 , 480 .
J. G. J. Peelen , R. Metselaar , J. Appl. Phys. 1974 , 45 , 216 .
0
10
20
30
40
50
60
70
80
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
RIT
[%
]
grain diameter [ mm]
350A; porosity: 0.036%vol.
350B; porosity: 0.191%vol.
350C; porosity: 0.048%vol.
other experimental data points
n=0.005; porosity: 0.00% vol.
n=0.005; porosity: 0.01% vol., Dv50 50nm
n=0.005; porosity: 0.05% vol., Dv50 50nm
@ 640 nm and 0.8mm
Pore scattering overestimated by order of magnitude
Need better optical model… not just pore size
distribution
Optical model with scattering from pores*
Modified model from Pecharromán et al* - fits data well....
Modified characteristic pore and grain sizes with absorption (C2, C3)
Absorption term measured and required for samples C2 and C3
C2: Rayleigh approximation no longer valid
61
The optical model: new description*
*C. Pecharroman , et al, Opt. Express 2009 , 17 , 6899*M. Stuer, et al " Adv. Funct. Materials. 22(11) 2303 (2012).
62
Spark Plasma Sintering – Processing
• Freeze drying & doping – dry pressing in SPS dye (Z. Zhe, Stockholm)
– not granulated - systematic study of dopant effects:
62
15 doping strategies
4 sintering parameters
M. Stuer et al.-Transparent polycrystalline alumina using spark plasma sintering: Effect of Mg, Y
and La doping JECS 30 (2010) 1335-1343
SPS
63
Spark Plasma Sintering
Best results for each dopant and sintering strategy
100 MPa
225 ppm 450 ppm
DopantsRIT
[%]
Soak temp.
[°C]
RIT
[%]
Soak temp.
[°C]
M00 32.17 1400 52.27 1250
0Y0 55.19 1310 54.71 1350
00L 52.56 1350 50.10 1370
MY0 48.34 1350 54.76 1350
M0L 51.31 1330 54.63 1350
0YL 56.89 1350 49.37 1350
MYL 55.77 1330 56.95 1350
Regardless of the doping strategy RIT > 50% (0.8 mm @ 640 nm )
Better than literature for standard SPS (39% @ 640 nm (0.8 mm))
RIT mainly defect controlled (sintering parameters)
Improved processing required to get intrinsic dopant
contribution?
SPS - Processing – Limitation – Freeze Drying
Inhomogeneous microstructure from inhomogeneous powder packing –
aggregates observed after freeze drying:
Increased grain size distribution RIT ↓ decreases
64
100 MPa
0
10
20
30
40
50
60
70
80
90
100
0
2
4
6
8
10
12
0.01 0.1 1 10 100 1000
Cu
mm
ula
tive F
req
uen
cy [%
]
Fre
qu
en
cy [%
]
Particle size [μm]
as received
freeze dried
freeze dried + UH
64
65
100 MPaSyringe pump
VUp to 2kV
Piezo
Power
Generator
Up to 2000 Hz
100 μm tip
Vibrating
membrane
Charged ring
Liquid N2 bath
Better green bodies – Freeze granulation – dry pressing
Requirements for freeze granulation with
“Encapsulator”:
• Low viscosity suspension <0.25 Pa.s @
used flow rate
• Laminar flow (best possible flow speed,
just below turbulent flow)
• Homogeneous and stable suspension
• Particle size at least 8x lower than tip size
Final granule size >2x size of tip
66
Freeze granulation – suspension rheology
Effect of dopant additions 450 ppm @ pH5.5 and 35%vol. solid load
After dopant addition suspension changes behavior:Newtonian Shear thinning with yield stress
0
2
4
6
8
10
12
14
16
18
20
0 50 100 150 200
She
arst
ress
[P
a]
Shear rate [s-1]
undoped pH5.5
450MYL pH5.5
Hamaker Program*- Interparticle potentials - Dopants
Alumina – effect of dopants 450 ppm – ionic concentration...
• Hamaker constant: 3.6710-20J
• PSD –
• Dv10 =200 nm
• Dv50=500 nm
• Dv90 =1600nm,
• pH=4, zeta potential 60 mV
• ionic strength (IS-0.006M)
• Dopants Mg2+ ,Y3+ 450 ppm
• (IS - 0.022-0.025M)
*U. Aschauer, O. Burgos-Montes, R. Moreno, P. Bowen,
J Dispersion Science Technology. Accepted - In Press (2011)67
Interparticle potentials - Alumina doping – Hamaker 2.1
68
-20
-10
0
10
20
30
40
50
0.00E+00 1.00E-08 2.00E-08 3.00E-08 4.00E-08 5.00E-08 6.00E-08 7.00E-08 8.00E-08
HNO3 - 60 mV (0.006M)
Mg-60mV (0.022M)
Y-60mV (0.025M)
• Always secondary
miniumum –
• Small yield even with
HNO3 –
• Minimum deeper and
closer as with dopants
• Stronger for Y3+ cf Mg2+
• Expected Shultz-Hardy
rule…
h (m) interparticle distance
inte
rpar
ticl
epo
tenti
al/k
T
Yield Stress Model – Yodel
Alumina Slurries for freeze granulation
YODEL – predicts yield stress for volumes fractions 36% and PSD
Alumina AA04 - pH=4, zeta 60 mV, zeta plane 2 nm, (no polymer)
Powder
Measured
yield stress
(pa)
Predicted
yield stress
(pa)
Undoped 0.2 ±0.2 0.7
Mg2+ 4.6±0.3 5.8
Y3+ 5.0±0.3 6.5
La3+ 5.0±0.3 6.5
H
a a
r
r = ( h + 2a )
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6
Yie
ld S
tre
ss [
Pa]
Volume fraction [-]
Experimental
YODEL Model
YODEL - Volume fraction functionality
- Example Mg doped – vg ....
69
YODEL - Volume fraction functionality*
Maximum packing:
needs perfectly dispersed
suspensions-filter pressing with
HNO3
Parameters difficult to define
Percolation threshold: function of
particle shape - floccs/agglomerates
– network structure – from
sedimentation density – confirmed
Y lower percolation threshold
Contact curvature:
smallest curvature for each particle,
average or distribution ? used 37
nm but…..
INPUT PARAMETERS: Mg-doped Y-doped
Hamaker constant: 3.67E-20 3.67E-20
Minimum separation [nm]: 23.4 16.8
Contact curvature [nm]: 37 37
Percolation threshold [-]: 0.16 0.11
Maximum packing [-]: 0.64 0.64
Yield at =0.45 [Pa] 15.3 40.7
Yttrium –doped – needed lower percolation threshold
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6
Yie
ld S
tre
ss [
Pa]
Volume fraction [-]
Experimental
YODEL Model
*M. Stuer and P. Bowen-
Advances in Applied Ceramics,
111(5/6) 254-261 (2012)
3. Curvature of contact point– Alumina?
EUROMAT 2007
Nürnberg, 10 – 14 September
Particle
-
-
-
--
-
--
-
-
--
-
Used 37 nm but....
71
Better green bodies – Freeze granulation*• pH 5.5 not possible needed - electro-steric barrier – PAA - pH9
• with PAA mol.wt 2000 and 5000 - if brush - 12 and 30 nm steric
barrier...rheology improved and granules produced (PVA and PEG for pressing)
• OK but still not good enough – perhaps mushroom or pancake configuration
because of complexation with Mg2+ and Y 3+
72
0
2
4
6
8
10
12
14
16
18
20
0 50 100 150 200
She
arst
ress
[P
a]
Shear rate [s-1]
450MYL pH5.5
undoped with PAA, PVA and PEG pH9
450MYL with PAA, PVA and PEG pH9*M. Stuer , Z. Zhe and P.
Bowen, J.Eur.Ceram.Soc
32(11) 2899-2908 (2012)
Pancake
Brush
Mushroom
PAA-complexation – Adsorbed Layer Thickness ?
• Complexation with Mg2+ and Y 3+ - two effects*
1 - Reduces the effective ionic strength and thus in secondary minimum distance
and depth
2- modifies the adsorption conformation of PAA on the surface
• >50% of dopants
complexed/adsorbed with
PAA
• Used YODEL and
Hamaker to compute
closest approach, H, by
matching with
experimental yield stress
• Reduction of ionic
concentration more
important than steric
contribution....
73*M. Stuer and P. Bowen , Advanced Engineering materials ,16(6),774-784 (2014)
– Green body densities 56%
– SPS > 99.9% dense……
– RITs - 53%* - slightly lower than freeze dried
– Improvements still needed...suspension need higher solids load
– But successful use of “standard” processing for SPS – simpler
than slip-casting ....easier and cheaper for industrial application
– Next step apply same process to finer powders, Dv50- 130nm –
dispersion still challenging.....
– Best result so far 65% RIT at 150 MPa ......>70% soon.
– ....in fact ..........
Results – Freeze Granulation - dry pressing*
74*M. Stuer , Z. Zhe and P. Bowen, J.Eur.Ceram.Soc 32(11) 2899 (2012).
– Baikoswki powder Dv50 – 130nm, Dn50 – 80 nm
– Single doped La (480 ppm) using nitrate but……washed out excess
dopant ions– reduced ionic concentration – re-dipsered with HNO3
– Slip casting – green bodies (Vincent Garnier - Lyon)
– RIT 71%...best SPS results so far (complex sintering cycle)
Roussel et al* …reduced ionic concentration – RIT 71%!!
75*Roussel et al., J.Am.Ceram.Soc., 1-4, 2013
2.8 cm above text
Conclusions
76
• Dispersion – not easy to predict or always understand – without calculations
• Agglomerates - poor microstructures or no flow!!
• Hamaker programme - estimate charge and/or steric barrier needed overcome
van der Waals attractive forces
YODEL -Yield stress can be predicted - at least semi quantitatively using
• Particles Size Distribution, Maximum Packing Fraction, Percolation
Threshold, Hamaker Constant, Distance of closest approach of particles, H,
• Curvature (radius) of contact point between particles a*
Limitations
• Last parameter can be seen as “fitting” parameter - takes into account shape
and perhaps other factors not perfectly captured by YODEL
• but yield stress predictions are very coherent once fixed for a given system
Transparent Alumina
• Future work - Try Baikowski Dn50 80nm suspension but by granulation…..
• Collaboration with Yves Jorand and Vincent Garnier (Lyon) – master project…
• Soon perhaps dry pressed >70% RIT …application….becoming possible…
Course Contents - Plan
4 semaines
3 semaines
2 semaines
2 semaines
1 semaine1. Introduction – general introduction to course– example transparent ceramics
2. Particle Packing and Powder Compaction - Theoretical and empirical models (PB)- Powder compaction (PD)
3 Particle-Particle Interactions (PB)- Colloidal Dispersions- DLVO –theory and limitations- non-DLVO and steric forces
4. Introduction to Atomistic Scale Simulations (PD)- introduction to modeling of surfaces and interfaces at the atomic scale - defects in metals – towards sintering
5. Sintering mechanisms (PD)- metals, ceramics- influence of microstructure- simulation
6. New Powder Processing Technologies (PB)- rapid prototyping- laser sintering, Spark Plasma Sintering
2 semaines
SiC - abrasive
« La neige » Snow …
• The Colloidal Domain – D. F. Evans & H. Wennerström, Wiley, 1999,
• Principles of Ceramic Processing – J.S.Reed , Wiley, 1995. English
• Les Céramiques, J. Barton, P. Bowen, C. Carry & J.M. Haussonne, Les Traité des Matériaux, Volume 16, PPUR, 2005
This week (1) and next week (2)
• Introduction – Brief overview of course contents
• Revision of rheology of suspensions (course Céramiques procédés – Vol 16
Les Traité des Matériaux,Volume 16 "Les Céramiques« , J. Barton, P.
Bowen, C. Carry & J.M. Haussonne)
• Practical example of importance of particle packing and colloidal stability –
particle-particle interactions
– Transparent polycrstalline ceramics– rheological model …Yodel…
• This week start - Next week finish - Particle packing
– Spheres and regularly shaped particles (cylinder…)
– Irregularly Shaped Particles
– Effect of size on packing
– Effect of size distribution (log-normal)…..
– Models - Numerical and Analytical (empirical)
– Bi-modal distributions – multimodal distributions
78