Ceramic suspensions IPresintering Science

Deagglomeration and mixing of ceramic powders as well as most shaping techniques require the suspension of the powder in a liquid medium. The properties of the suspension are critical for the quality of the final ceramic part.

The principal liquid used in ceramic powder processing is water. Physical parameters of pure water: pH 7, specific conductivity 5 S/m (20°C)

CO2 and calcium carbonate are the most frequent dissolved species in water:

CO2 + H2O H2 CO3 H+ + HCO3+ H+ + CO3

2+

CaCO3 + CO2 + H2O Ca2+ + 2HCO3+

The hardness of water is defined as the concentration of precipitated carbonate grains, which is a function of the concentrations of calcium and magnesium ions, which can be calculated as shown in the equation: Hardness mg/l = 2,5 [conc. of Ca2+ (mg/l)] + 4,1 [conc. of Mg2+ (mg/l)]

Water supply classification Hardness ConcentrationSoft Water  0 to 17.1 mg/l (0 to 1 grain/gallon ) Slightly Hard Water    17.1 to 51.3 mg/l (1 to 3.5 grains/gallon) Moderately Hard Water  51.3 to 119.7 mg/l (3.5 to 7 grains/gallon) Hard Water  7 to 10.5 grains /gallon (119.7 to 179.55 mg/l) Very Hard Water    over 179.55 mg/l (over 10.5 grains/gallon) 

Ceramic suspensions IIOrganic suspension media

Certain ceramic powders, such as nitrides, are not stable in water. Organic liquids or mixtures thereof are used to suspend such powders. Physical parameters of organic suspension liquids:

Liquid Formula Dielectric constant

Surface tension (mN/m)

Viscosity(mPa· s)

Boiling Point (°C)

Flash Point (°C)

Water H2O 80 73 1.0 100Methanol CH3OH 33 23 0.6 65 18Ethanol C2H5OH 24 23 1.2 79 8n-Propanol C3H7OH 20 24 2.3Isopropanol C3H7OH 18 22 2.4 49 21n-Buthanol C4H9OH 18 25 2.9 100 38n-Octanol C8H17OH 10 28 10.6 171Ehylene glycol C2H6O2 37 48 20 >197 >116Glycerol C3H8O2 43 48 20 290Trichlorethylene C2HCl5 3 5.5 87Methlethyleneketone C4H8O 18 25 0.4 80 2

Presintering

Wetting and dihedral angles I The behavior of a liquid drop in contact with a solid at equilibrium is dictated by the minimum total energy for the boundaries present. The characteristic parameter is the so called wetting angle

solid

liquid

gas

∆A

∆S

The resulting change in surface free energy is:

Increasing the equilibrium surface between liquid and gas by S will increase (decrease) the surface between solid and liquid (solid-gas) by:

S = ΔAcos(θ − Δθ )

G = ΔAγ SL − ΔAγ SG + Aγ LG cos θ + Δθ( )

⇒ ΔGΔA

= γ SL − γ SG + γ LG cos θ + Δθ( )

Presintering

At equilibrium G , and tend to zero and the equilibrium wetting angle is given by:

Unfortunately, surface tension data are not available for all ceramic systems of interest. Furthermore, the surface tension ratio in the Young-Laplace equation is often > 1 and the angle cannot be calculated. In general the ceramic powders can be wetted by current suspension media, the wetting conditions are often improved by the addition of surfactants e.g. polymers adsorbed at the surface, which reduce SL and decrease, therefore, the wetting angle.

°

non-wetting

°

wetting

°

spreading

Wetting and dihedral angles II Presintering

Three situations regarding the wetting angle can be distinguished:

€

limΔG →0,Δθ →0 = γ SL − γ SG + γ LGcosθ ⇒ cosθ = γ SG − γ SL

γ LG

⎛ ⎝ ⎜

⎞ ⎠ ⎟

Surface charge I

The behavior of fine particles in a liquid e.g. in a suspension depend mainly on their surface characteristics. Surfaces may react with the suspension medium. For oxide powders in contact with water two possible situations are encountered:

vacuum H H H H H H H H

Water molecules may be adsorbed by the surface cations

The surface oxygens may be hydoxilated

In both cases, protonation and deprotonation is possible:

Low pH High pH

H H H HH+ H+ H H

Surface charge IPresintering

Phase pH for pzcSiO2 1.9Feldspar 3.3Kaolinite 4.2Calcite 8.5Al2O3 9.5MgO 12.2

Below the pzc the surfaces are charged negatively, above the pzc positively. A mixture of MgO- and SiO2 -powder in an aqueous slurry with pH 7will, therefore, immidiately coagulate. Like particles in suspensions with pH away from the pzc will repell each other through electrostatic repulsion.

0 1 2 3 4 5 6 7 8 9 10 11 12

0

0.1

0.2

-0.2

-0.1

pH

surfacecharge

C/m2

SiO2 Feldspar, Montm.

Kaolinite

Calcite

Al2O3MgO

Surface charge IIPresintering

Protonation or deprotonation will induce a surface charge. The pH for which the surface charge is zero is called point of zero charge pzc.

Electric double layerThe surface charge will be compensated by ions and polar molecules in the solution. A layer of adsorbed ions is followed by a diffuse layer with higher electrolyte concentration. The potential within this double layer decreases with increasing distance from the surface. The width of the double layer can be influenced by the concentration and the type of the electrolyte.

++++++++

--

-

-----

- ++ -

+ -+ -

+ -

+ -+ -

+ ---

-

-

-

- + -

+-

+ -

+ -

+ -

+ -

Particle adsorbed diffusive bulk solution layers

-

-

++++++++

Pote

ntia

l (V)

distance

++++++++

Pote

ntia

l (V)

distance

slippage planeZeta potential

Double layer model Potential for high and low electrolyte concentration0

For low electrolyte concentration the repulsive force between particles will extend farther into the bulk solution. Charged particles in a suspension will respont to an applied electrical field by moving to the oppositely charged poles. The particle drag with them part of the double layer, up to the slippage plane. The electrophoretic velocity of the particles depend on the potential at this plane, called the Zeta-potential.

Presintering

Van der Waals forcesBetween very fine particles with a large specific surface attractive Van der Waals forces will act over short distances, counteracting the electrostatic repulsion.

The overall potential energy acting between two particles is given by

Overall interaction energy I

permitivity of free space dielectric constante electron chargezi valence of the electrolyte species ini number of electrolyte species iT temperaturek Boltzmann constant surface chargea grain radiusH distance between the grainsA12 Hamaker constant

(Reed, 1995)

Presintering

€

VT = VR + VA = 4πε0εk 2T 2a2φ2

z2e2 2a + H( )exp −κH( ) + A12a

12H

κ =e2 nizi

2∑ε0εkT

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

12

If the repulsive potential (Coulomb potential) is small the attractive force will dominate and the particles will stick together. The suspension will coagulate or flocculate . In order to stabilize a suspension, one has to either increase particle size a, surface charge , or decrease the electrolyte concentration ni or choose a liquid with a high dielectric constant .

Overall interaction energy IIPresintering

The potential is attractive at a large and at a very close distance between the two particles. Inbetween the potential is repulsive. The stability of a suspension depends on the height of the energy barrier.

Attractive, repulsive and total potential between two like spherical particles as function of ionic strength and particle separation.(Ring, 1996)

Overall interaction energy IIIPresintering

Attractive, repulsive and total potential between two spherical particles as function of particle diameter and particle separation.

(Ring, 1996)

Overall interaction energy IVPresintering

(Ring, 1996)

Subscript f stays for flocculation.

Schematic representation of the thermdynamic factors controlling steric stabilisation:

G=0

TS

H

unstable

permanently unstable

unstable

Enthalpic stabilization

Entropicstabilization

Combined stabilization stable

permanently stable

stable

Heat

Heat

Suspension stability Presintering

The stability of a suspension depends on the free energy of floculation:

€

G f = ΔH f − TΔS f

hydrophobic tail

hydrophyllic head

When two particles meet the tails of the polymeric chains entangle limiting their mutual movement = decrease of the conformation entropy = increase of the free energy => system tries to lower the free energy which happens when the particles separate again = steric effect.A second process occurring the entanglement is the loss of the solvation shell of the polymer. When the entalpy change due to this process is larger than the entropy effect the polymer chains will remain entangled.

A way to enhance the stability of a colloidal suspension is the addition of organic surfactants, linear molecules with a hydrophobic and an hydrophylic tail. In a polar liquid, the hydrophobic end of a non-ionic surfactant will attach to the surface of the particles.

entanglement of thelinear molecules = reductionof entropy

Example of organic surfactants

(Reed, 1995)

Steric stabilisation Presintering

The sedimentation volume is an easy way to measure the stability of suspension. Well dispersed particles will lead to a denser packing than agglomerated particles.

Principle of the sedimentation volume Sedimentation volumes of rutile

powder as function of PVP concentration(Grobéty, 1986)

(Reed, 1995)

Sedimentation volumePresintering

Like organic surfactants, binders consist of polymeric molecules. The stabilizing vs. binding action depends on the polymer concentration. At low concentration they act as binders. The polymer chains attach on several grains. At higher concentration they may act as stabilizers. The most common binders ar polyvinyl alcohol (PVA), cellulose binders, polyethylenen glycol (PEG) and wax.

Binders are used for the adjustment of the slurry viscosity, to provide green strength. They serve also as lubrication agent.

PVA

Bridging of polymers

equil. polymer conc.

B

C D

Degree of adsorption as function of polymer conc.

A

BindersPresintering

PlasticizersControl rheology, allow granule plastic deformation. Liquid agents, which lower the glass transition temperature of binders.Examples: water, glycerol, ethylene glycol, dibutyl phtalate

Foaming and antifoaming agentsIncrease or reduce the concentration of air bubblesExamples: foaming agents: sodum alkyl sulfate, polypropylene glycol ether anti-foaming agents: fluorocarbons, dimethylsilicones

LubricantsDecrease die-wall and interparticle friction, must be compounds with high adhesion, but low shear strength Examples: paraffin wax, stearates, talc, graphite

Suspension additivesPresintering

To make a fluid flow a shear stress has to be applied. The proportionality constant relating the stress and the velocity gradient in the liquid is the viscosity of the liquid. The viscosity of a suspension is an important process parameter, which has to be adjusted for particular process steps.

F

Ady

dx After time thigh viscosity low viscosity

dy

dx

(Reed, 1995)

Viscosity Presintering

€

τ =−η dvdy ⎛ ⎝ ⎜

⎞ ⎠ ⎟= -η ˙ γ with dv = dx

dtτ : shear stress F/A˙ γ : shear rateη : prop. const., viscosity

Typical Viscosities (Pa.s)

Asphalt Binder ---------------Polymer Melt -----------------Molasses ----------------------Liquid Honey -----------------Glycerol -----------------------Olive Oil -----------------------Water --------------------------Acetic Acid --------------------

100,0001,0001001010.010.0010.00001

Viscosity of different materialsPresintering

t

Shear rate

Bingham

Newton

shear thinning

shear thickening

Different types of flow behaviour are possible:

The viscosity can be increased by - the addition of binders. Binders are usually non-ionic polymers like PVA or PVC.- the increase of the solid content- coagulation of the suspension (flocculation)

shear thinning with yield stress

shear thickeningwith yield stress

Suspensions containing chain-like molecules or anisotropic particles (clay) exhibit often shear thinning. Large shear rates orient the particles which leads to laminar flow in the suspension and a reduction of the viscosity. In such slurries the viscosity is often changing with process duration. This behavior is called thixotropy. The viscosity of suspensions showing shear thinning or shear thickening is not constant, but depends on the shear rate (power law dependency).

τ =τ 0 − η &γ

τ =−η & n−1 &γ

Non-Newtonian viscosityPresintering

Unsheared Sheared

Aggregatesbreak up

Anisotropic Particles alignwith the Flow Streamlines

Shear thinning behavior is often a result of:- Orientation of non-spherical particles in the direction of flow. An example of this phenomenon is the pumping of fiber slurries.- Breaking of particle aggregates in suspensions. An example would be stirring paint.

Courtesy: TA Instruments

Shear thinningPresintering

Viscosity measurements

Viscosimeter

The viscosity of a liquid/suspension is determined by measuring the force necessary to rotate an annulus of that medium. The shear stress is proportional to the torque produced.

(Reed, 1995)

˙ γ =−r dωdr

⎛ ⎝ ⎜ ⎞

⎠ ⎟

τ= F2πrL

= T2πr2L

T =F ⋅r

η=τ˙ γ ⇒ ηdω= T

2πLr3 dr

ηdω= T2πLr

dr⇒ ηa = Tωa

b2 −a2

a2b24πLb

a

∫0

ϖa

∫

r

Presintering

(Reed, 1995)

The relative viscosity of a suspension is the ratio between the viscosity of the suspension vs. the viscosity of the pure liquid medium. The main factor influencing the relative viscosity is the solid content of the suspension. The behavior of a suspension of equal sized and shaped particles is given by the Dougherty-Krieger equation:

ηr =ηSηL

= 1− fPV

fcrV

⎡ ⎣ ⎢ ⎢

⎤ ⎦ ⎥ ⎥

−KH fcrv

fP : volume fraction of particlesfcr : critical volume fraction e.g. h r becomes infiniteKH :shape factor (2.5 for spheres, > 2.5 for other shapes)

Viscosity of suspensions

Presintering

Flocculation of a suspension through changes in pH or the addition of a binder has dramatic consequences for the viscosity. (Reed, 1995)

pH dependence of the viscosity Presintering

A beneficiation procedure for powders is the reduction and homogenization of particle size through milling. The starting particle sizes are usually in the range where ball mills are best. In ball mills, the particle size is reduced through the impact of metallic or ceramic balls (alumina, zirconia) accelerated by the rotating movement of the milling container.

Milling

QuickTime™ and aGIF decompressor

are needed to see this picture.

andy.iamp.tohoku.ac.jp/~mio/anime-eng.html

Presintering

Milling efficiency

The milling efficiency measured as power consumption per unit size reduction is considerably better for vibration mills than for ball mills to obtain powders with grain sizes < 2m. Ball mills however a cheaper and easier to maintain.

The milling efficiency measured as increase of specific surface area is among others a function of the porosity and the shape of the starting powder. Porous calcined alumina is easier to grind than fused, monocrystalline alumina. Tabular dense alumina is easier to grind than the spherical fused alumina.

(Reed, 1995)

Presintering

segregated random dispersed

Extreme particlearrangements in2- D mixtures

s=Ci −C ( )2

0

N∑

N −1

⎡

⎣

⎢ ⎢ ⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ⎥ ⎥

12

The degree of mixedness of a two component powder mixture is given by the standard deviation between N samples

Standard deviation of N samples . Ci : conc.of first component in ith sample. C: bulk conc.

Mixing

(Reed, 1995)

Presintering

Mixing efficiency of two different mixers. To form gahnite powder, ZnO and alumina powders were mixed as suspensions with different solid fractions (prop. viscosity). The mixtures were annealed and the resulting spinel was determined by X-ray diffraction.

Mixing instruments

The instrument type used to mix suspensions depends on its viscosity. Instead of mixers differenttype of mills can be used for mixing.

viscosity10-2 100 102 104 106 108

PropellersFlat Blade Turbines

AnchorsPaddles

PlanetaryHelical screws

Intensiv mixerMicrostratifying Mixer

air water oil syrup putty

Impeller mixer Helical ribbon mixer Sigma blade intensive mixer

(Reed, 1995)

Presintering

Kenics mixer

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are needed to see this picture.

http://www.mate.tue.nl/mate/movies/505.mpg

The Kenics mixer is comprised of a series of mixing elements aligned at 90 degrees, each element consisting of a short helix of one and a half pipe diameters in length. Each helix has a twist of 180 degrees with right-hand and left-hand elements being arranged alternatively in the pipe.

Simulation of the passage of a two component powder, that is initially completely unmixed, through a 8-bladed Kenics mixer.

Presintering

Drying of suspensionFor certain shape giving processes, such as die pressing or hot isostatic pressing, the suspensions have to be dried again. Spray drying, the process of spaying a slurry into a warm drying medium, produces nearly spherical powder granules.

Spray dryer

slurry pressurized gas

warm gas

drying chamber

nozzle

cyclone

collecting chamber

particle

binder

dense granule

10 - 400mm

hollow granule

Presintering

Spray dried TiO2- AlN mixture with (left) and without (right) binder and the corresponding texture in the dry pressed disks. Granulation prevents segregation between the two powders (Grobéty, 1991)

2 m

50 m50 m

2 m

Homogeneity of dried powder mixturesPresintering