Presintering Science

1. Presintering Science

2. Presintering Science Ceramic suspensions I 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 HardnessConcentration Soft 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) 

3. Presintering Ceramic suspensions II Organic 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:

4. 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 gas  liquid ∆S   ∆A solid Increasing the equilibrium surface between liquid and gas by  S will increase (decrease) the surface between solid and liquid (solid-gas) by: The resulting change in surface free energy is:

5. Presintering ° ° °   non-wetting wetting spreading Wetting and dihedral angles II At equilibrium G , and  tend to zero and the equilibrium wetting angleis given by: Three situations regarding the wetting angle can be distinguished: 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.

6. Presintering Surface charge I 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 H H H H  H 

7. Presintering Surface charge II Protonation or deprotonation will induce a surface charge. The pH for which the surface charge is zero is called point of zero charge pzc. Phase pH for pzc SiO2 1.9 Feldspar 3.3 Kaolinite 4.2 Calcite 8.5 Al2O3 9.5 MgO 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. 0.2 Al2O3 MgO 0.1 surface charge C/m2 Kaolinite 0 -0.1 Feldspar, Montm. SiO2 Calcite -0.2 0 1 2 3 4 5 6 7 8 9 10 11 12 pH

8. Presintering - + + - + - + + + + + + + + + + - - - - - - - - - - + - - - - - - - - - - - - - - - - - Electric double layer The 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. + + + + + + + + Potential (V) + Potential (V) + + slippage plane Zeta potential + + + + + + + + + + + + + distance distance Particle adsorbed diffusive bulk solution layers 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. Van der Waals forces Between very fine particles with a large specific surface attractive Van der Waals forces will act over short distances, counteracting the electrostatic repulsion.

9. Presintering Overall interaction energy I The overall potential energy acting between two particles is given by  permitivity of free space  dielectric constant e electron charge zi valence of the electrolyte species i ni number of electrolyte species i T temperature k Boltzmann constant  surface charge a grain radius H distance between the grains A12 Hamaker constant 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 . (Reed, 1995)

10. Presintering Overall interaction energy II 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.

11. Presintering Overall interaction energy III Attractive, repulsive and total potential between two like spherical particles as function of ionic strength and particle separation. (Ring, 1996)

12. Presintering Overall interaction energy IV Attractive, repulsive and total potential between two spherical particles as function of particle diameter and particle separation. (Ring, 1996)

13. Presintering Suspension stability The stability of a suspension depends on the free energy of floculation: Subscript f stays for flocculation. Schematic representation of the thermdynamic factors controlling steric stabilisation: Enthalpic stabilization H Combined stabilization G=0 stable Heat permanently stable unstable TS stable permanently unstable Heat Entropic stabilization unstable (Ring, 1996)

14. Presintering Steric stabilisation 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. hydrophobic tail Example of organic surfactants hydrophyllic head entanglement of the linear molecules = reduction of entropy 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. (Reed, 1995)

15. Presintering Sedimentation volume 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)

16. Presintering Binders 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. PVA Bridging of polymers Binders are used for the adjustment of the slurry viscosity, to provide green strength. They serve also as lubrication agent. C D  B Degree of adsorption as function of polymer conc. A equil. polymer conc.

17. Presintering Suspension additives Plasticizers Control 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 agents Increase or reduce the concentration of air bubbles Examples: foaming agents: sodum alkyl sulfate, polypropylene glycol ether anti-foaming agents: fluorocarbons, dimethylsilicones Lubricants Decrease die-wall and interparticle friction, must be compounds with high adhesion, but low shear strength Examples: paraffin wax, stearates, talc, graphite

18. Presintering Viscosity 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 dy dy A dx dx After time t high viscosity low viscosity (Reed, 1995)

19. Presintering Viscosity of different materials Typical Viscosities (Pa.s) Asphalt Binder --------------- Polymer Melt ----------------- Molasses ---------------------- Liquid Honey ----------------- Glycerol ----------------------- Olive Oil ----------------------- Water -------------------------- Acetic Acid -------------------- 100,000 1,000 100 10 1 0.01 0.001 0.00001

20. Presintering Non-Newtonian viscosity Different types of flow behaviour are possible: shear thickening with yield stress Bingham shear thinning with yield stress t shear thickening Newton shear thinning Shear rate 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). 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)

21. Presintering Shear thinning Sheared Unsheared Aggregatesbreak up Anisotropic Particles alignwith the Flow Streamlines Courtesy: TA Instruments 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.

22. Presintering Viscosity measurements Viscosimeter r 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)

23. Presintering Viscosity of suspensions 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: fP : volume fraction of particles fcr : critical volume fraction e.g. h r becomes infinite KH :shape factor (2.5 for spheres, > 2.5 for other shapes) (Reed, 1995)

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

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

26. 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 < 2m. 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)

27. Presintering Mixing dispersed random segregated Extreme particlearrangements in2- D mixtures 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 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. (Reed, 1995)

28. Presintering air water oil syrup putty Microstratifying Mixer Intensiv mixer Helical screws Planetary Paddles Anchors Flat Blade Turbines Propellers 10-2 100 102 104 106 108 viscosity Mixing instruments The instrument type used to mix suspensions depends on its viscosity. Instead of mixers different type of mills can be used for mixing. Impeller mixer Helical ribbon mixer Sigma blade intensive mixer (Reed, 1995)

29. Presintering Kenics mixer 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. http://www.mate.tue.nl/mate/movies/505.mpg

30. Presintering Spray dryer Drying of suspension For 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. 10 - 400mm slurry pressurized gas particle nozzle binder warm gas dense granule drying chamber cyclone collecting chamber hollow granule

31. Presintering Homogeneity of dried powder mixtures 50 mm 50 mm 2 mm 2 mm 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)