Electrokinetic Properties of Advanced Powders in View of Their Colloidal Properties

Table of Contents

ABSTRACT

CHAPTER I INTRODUCTION
1.1. Task Description and Objectives
1.2. Work Load and Study Project Requirements Agreement

CHAPTER II COLLOIDAL PROCESSING OF CERAMICS- PARTICLE INTERACTIONS
2.1. van der Waal Forces
2.2. Electrostatic forces
2.3. Steric Forces
2.4. Electrosteric Forces
2.5. Depletion Forces

CHAPTER III MATERIALS AND METHODS
3.1. [Abbildung in dieser Leseprobe nicht enthalten] Properties
3.2. Dispersants Properties
3.3. Suspension Preparation
3.4. Investigation Techniques

CHAPTER IV RESULTS AND DISCUSSION
4.1. [Abbildung in dieser Leseprobe nicht enthalten] Suspension Characterization
4.2. Dispersant Dosage Optimization
4.3 Comparison of the dispersants effect on the zeta potential of the 5% [Abbildung in dieser Leseprobe nicht enthalten] suspension
4.4. Comparison of the dispersants effect on the ionic strength of the 5%- [Abbildung in dieser Leseprobe nicht enthalten] suspension
4.5. Assessment of the Dispersants with Reference to Point of Zero Charge (pzc)
4.6. Dispersant Behaviour Model
4.7. Experimental comparison of zeta potential and streaming potential of [Abbildung in dieser Leseprobe nicht enthalten] slurry

CHAPTER V CONCLUSION

REFERENCES

APPENDIX I

APPENDIX II

APPENDIX III

Abstract

Rapid development of technologies requires introduction of new materials as well as improvement of the existing one (Kudyba-Jansen et al. 2000).

The very fine fumed metal oxide [Abbildung in dieser Leseprobe nicht enthalten](d50 = 0.13nm, Desussa, Germany) has been investigated on the possibilities for development of green bodies though the innovative wet shaping process gel casting.

Thus, this study in particular is focused on characterisation of the suspension stabilities promoted by two commercially produced polyelectolytes Dolapix CE64 and Dolapix A88 (Zschimmer-Schwarz, Lahnstein, Germany). The optimal dispersant dosage has been found and the suspension stability has been further evaluated. Finally Dolapix CE64 has been found to be most favourable for enhancing the stability of aqueous [Abbildung in dieser Leseprobe nicht enthalten]

Acknowledgements

I would like to thank Prof. Ay and Dr. Gaydarzhiev (Chair of Mineral Processing, BTU Cottbus) for giving me the opportunity to carry out this interesting and exciting project. I am especially grateful to Stoyan for the scientific discussions with him which helped me to conduct and analyze my experiments for his valuable and constructive advice and for the numerous reviews of the manuscript.

I also appreciate the partial financial support of the chair of Mineral Processing for the time the measurements were made.

I would like to thank to Dr. Hitzen (Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, The Netherlands) who gave me access to some valuable literature sources that helped me in writing this paper.

List of Tables

TABLE 1. ILLUSTRATION OF THE INTERACTION POTENTIAL ENERGY AND RELEVANT LENGTH SCALE FOR DIFFERENT INTERPARTICLE INTERACTIONS, ADOPTED FROM LEWIS (2000)

TABLE 2. CHARACTERISTICS OF [Abbildung in dieser Leseprobe nicht enthalten] SAMPLE (SOURCE: WWW.DEGUSSA.DE)

TABLE 3. SOME IMPORTANT PROPERTIES OF THE DISPERSANTS IN USE

TABLE 4. ZETA POTENTIAL [MV] AS MEASURED WITH PCD AND ESA AND THE RESPECTIVE PH AT DIFFERENT DOSAGES OF DOLAPIX A88/ DOLAPIX CE

TABLE 5. STATISTICAL DATA FROM LINEAR REGRESSION ANALYSIS OF SUSPENSIONS WITH 5% SOLIDS LOADING AS EVALUATED FROM THE DATA AT FIG.13. ACCORDING TO EQ.(11.)

TABLE 6. HAMAKER CONSTANTS FOR SEVERAL CERAMIC MATERIALS INTERACTING UNDER VACUUM AND ACROSS WATER AT 289K

TABLE 7. ISOELECTRIC POINTS FOR SEVERAL CERAMIC MATERIALS

List of Figures

FIG. 1. STATE OF COLLOIDAL SYSTEMS AND PREDOMINANT INTERACTIONS (AFTER LEWIS 2000)

FIG. 2. [Abbildung in dieser Leseprobe nicht enthalten] SAMPLE

FIG. 3. SCHEMATIC REPRESENTATION OF SEDIMENT BED FORMED BY (LEFT) DISPERSED SUSPENSION AND (RIGHT) AGGREGATED PARTICLES (AFTER BESRA ET AL. 2005)

FIG. 4. SCHEMATIC ILLUSTRATION OF THE CST APPARATUS

FIG. 5. SCHEMATIC ILLUSTRATION OF DOUBLE LAYER FORMATION IN THE MEASURING GAP

FIG. 6. SCHEMATIC PRESENTATION OF CVI/ESA MEASUREMENT CELL SHOWING POLARIZATION OF THE ELECTRIC DOUBLE LAYER FOR A NEGATIVELY CHARGED PARTICLE (AFTER WÄSCHE ET AL. 2002)

FIG. 7. POTENTIOMETRIC TITRATION OF 1% AND 5% SUSPENSION 22 [Abbildung in dieser Leseprobe nicht enthalten]

FIG. 8. CST TEST RESULTS FOR DISPERSANT OPTIMIZATION

FIG. 9. VOLUMETRIC TITRATION OF 5% ALU-C AT DIFFERENT DISPERSANT DOSAGES WITH RESPECT CHANGE IN ZP

FIG. 10. VOLUMETRIC TITRATION OF 5% ALU-C AT DIFFERENT DISPERSANT DOSAGES WITH RESPECT TO CHANGE OF SUSPENSION CONDUCTIVITY

FIG. 11. COMPARATIVE POTENTIOMETRIC TITRATION AT DIFFERENT DISPERSANT DOSAGES

FIG. 12. SCHEMATIC ILLUSTRATION OF ADSORBED ANIONIC POLYELECTROLYTE SPECIES ON CERAMIC SURFACE AS A FUNCTION OF PH AND IONIC STRENGTH δ - IS THE ADLAYER THICKNESS

FIG. 13. ZETA POTENTIAL AS MEASURED BY THE CVI AS A FUNCTION OF THE STREAMING POTENTIAL AS MEASURED BY THE PCD METHOD FOR SUSPENSIONS AT A PARTICLE VOLUME FRACTION 5% W/V IN THE PRESENCE OF DISPERSANT

FIG. 14. GELCASTING FLOW CHART AS SUGGESTED BY THE OAK RIDGE NATIONAL LABORATORY

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Chapter I Introduction

1.1. Task Description and Objectives

The overall idea for this study project originates from the intention to investigate the possible use of the very fine fumed hydrophilic metal oxide, [Abbildung in dieser Leseprobe nicht enthalten] , produced by Degussa, Germany for sintering a green body with a homogenous, high density and optimal particle packing microstructure by gelcasting.

Gelcasting (GC) is a new shaping process for making high-quality complex-shaped ceramic parts developed by the scientists from Oak Ridge National Laboratory, USA in 1984 (Janney et al. 1998). An aqueous system using acrylamide as monomer was completed in 1988 (Omamete et al. 1991). However, concerns regarding health, safety and disposal of acrylamide, referred to as neurotoxin caused industrial rejection of the process. Development of a low toxicity process was initiated to deal with the lack of acceptance, and it was fully demonstrated in 1990 (Janney et al. 1998). In the gelcasting process, a small amount of organic monomer and cross linker is added to the ceramic aqueous slurry. The most successful systems are based on the monofunctional monomers methacrylamide (MAM), methoxy poly(ethylene glycol) monomethacrylate (MPEGMA), and n-vinyl pyrolidone (NVP), the difunctional monomers methylene bisacrylamid (MBAM) and poly(ethylene glycol) dimethacrylate (PEG(1000) DMA) (Janney et al. 1998, Rak 2000). None of the monomers interact adversely with standard ceramic processing aids such as dispersants and defoamers. Solids loading as high as 55-60 w/v were achieved in alumina slurries and 45-57 w/v in silicon nitride suspensions using these systems (Rak 2000). Upon heating, the monomer polymerises, and the resulting gel (which is ca. 90% water) stiffens the ceramic powder slurry into Electrokinetic Properties of Advanced Powders in View of Their Colloidal Processing the shape mould. The gelled powder is then easily dried to remove the water, and further pyrolysized to remove the gel. The gelcasting process is 50-80% faster than the slip casting technique, generating a more uniform powder packing, having a much higher strength and requiring simpler moulds.

Moreover, GC is a generic process applicable to any powder as the organic additives used do not have any cationic impurities after sintering and it can be quickly adapted for use with new materials and for new applications. Therefore, it will be interesting to know whether the very fine fumed aluminium oxide, [Abbildung in dieser Leseprobe nicht enthalten] (d50 = 13 nm), is compatible with the idea of GC. To perform an optimal GC process three inevitable stages should be considered (i) dispersant system optimization; (ii) solids loading optimization and (iii) production of a green body after gelation, its drying and sintering.

(i) Dispersant system optimization

GC requires high solids loading, so that the green body could shrink uniformly when dried and sintered. Therefore, an excellent dispersant system is one of the priorities here.

(ii) Solids loading optimization

GC requires high solids loading with reasonable viscosity so that the suspension can be poured into the respective mould. Therefore, finding the max volume with favourable rheological properties is the priority here.

(iii) Green body gelation

Following the methodology for low-toxicity gelcasting system developed by the Oak Ridge National Laboratory (Janney et al. 1998) this was the first attempt to produce green body at the Chair of Mineral Processing at BTU Cottbus.

Developing ceramic bodies for a new application by GC is a demanding task, which requires optimization of each process stage and furthermore detailed examination of the ceramic part with respect to its properties. For this reason, the author would like to stress that the scope within this time-limited study is elaborating in details the first stage of dispersant system optimizing and investigating the influence of the deflocculants on electrostatic stabilization of the powder and solids loading, as well as finding out which dispersant is more favourable and at which rate.

1.2. Work Load and Study Project Requirements Agreement

This study project is in agreement with the Study Project Requirements of the Environmental and Resource Management Study Course at the BTU Cottbus and it is a result of the following engagement and work load:

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All possible learning methods are combined in this study project. Moreover, the selforganized studies make up more than 80% of the total workload.

Chapter II Colloidal Processing of Ceramics - Particle Interactions

The term colloid is used to describe particles that have at least one dimension in the range 10-³ -1µm (Lewis 2000). A distinguishing feature of all colloid systems is that the contract area between the particles and the dispersing medium is large therefore surface (interparticle) forces strongly influence suspension behaviour (Lewis 2000; Bergström 2001).

It is critical to understand how one may manipulate suspension properties to achieve the desired rheological behaviour for the forming technique. The ability to control and influence the direction of the particle interactions is the first step towards optimized colloidal processing (Bergström 2001).

Through careful control of interparticle forces, colloidal suspensions can be prepared in dispersed, weakly flocculated or strongly flocculated state, as shown schematically on Fig. 1.

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Fig. 1. State of Colloidal Systems and Predominant Interactions (after Lewis 2000)

Colloidal stability is generally introduced by the total interparticle potential energy,

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where:

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The first two terms of the equation constitute the well-known DLVO theory developed independently by Derjaguin & Landau (1941), and Vervey & Oberbeek (1948) which predicts the stability of the colloidal particles suspended in polar liquids.

2.1. van der Waal Forces

All ceramic particles experience the long-range van der Waal forces. This force is electodynamic in origin as it arises from the interactions between the oscillating or rotating dipoles within the suspension (Bergström 2001, French 2000). The strength of vdW force depends on the dielectric properties of the interacting colloidal particles (Lewis 2000). For spherical particles of equal size, VvdW is given by the Hamaker expression:

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A is the Hamaker constant, which can be determined by spectral optical properties of the materials combined with knowledge of the configuration of the material. Experimentally determined values for several important ceramic materials are summarized in App. I - Table 7.

The long-range vdW forces are more complex interactions Eq.(4) that can be considered as resulting from the London dispersion forces together with the Debye and Keesom forces (French 2000).

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where

[Abbildung in dieser Leseprobe nicht enthalten] results from the orientation of the permanent dipoles and interaction of the dipole’s electric fields. It is an attractive force when the dipoles are antiparallel and a repulsive force when the dipoles are parallel. The Keesom force vanishes as the temperature increases. This permanent-dipole/ permanent-dipole interaction is the basis for the [Abbildung in dieser Leseprobe nicht enthalten](French 2000).

[Abbildung in dieser Leseprobe nicht enthalten] requires the presence of at least one permanent dipole, and therefore is not universally present for all atoms or molecules. The electromagnetic field of a single permanent dipole induces a dipole moment in the electron cloud of another atom or molecule. These induced high-frequency electronic dipole moments can couple with the lower frequency oscillations of the permanent dipolar molecule (French 2000).

[Abbildung in dieser Leseprobe nicht enthalten] is the universal contributor to the vdW forces. Just as the electromagnetic field of a permanent dipole induces dipole moment, so does the motion of any electron on the atom. This induced-dipole/ induced-dipole interaction results in an attractive force that is the basis for the [Abbildung in dieser Leseprobe nicht enthalten] (French 2000).

Long-range attractive vdW forces between particles need to be diminished or mitigated during colloidal processing to achieve the desired degree of the suspension stability. One approach is to make those attractive forces negligible by suspending the particle in an appropriate solvent (as suggested on Fig. 1.). However this approach has practically no importance for most ceramic powders. One must therefore rely on some type of interparticle forces, such as electrostatic (Fig. 1A), electrosteric (Fig. 1C) or depletion forces (Fig. 1D), to overcome vdW attraction, as illustrated below:

Table 1. Illustration of the interaction potential energy and relevant length scale for different interparticle interactions, adopted from Lewis (2000)

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2.2. Electrostatic forces

The stability of the aqueous colloidal system can be controlled by generating like charges of sufficient magnitude on the surfaces of the suspended ceramic particles. For example, immersing a ceramic powder in a polar liquid such as water, results in a built up of a charge at the solid-liquid interfaced. The interfacial (net) charge is a result of adsorption or desorption of ionic species in solution, e.g. by proton transfer reactions with the surface hydroxyl groups, or by adsorption of specifically adsorbed ions (Bergström 2001):

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In such a suspension the point of zero charge (pzc) is the pH where the surface concentration of MO- and MOH2+ are equal. The surface charge is negative at a pH>pHpzc and positive at pH<pHpzc. Ions of opposite charge are attracted to the charged interface and form a diffuse ion “cloud” adjacent to the particle surface. The thickness of this electrical double layer is a very important parameter, which determines the range of the double layer repulsion (Eq. 6.).

The resulting repulsive [Abbildung in dieser Leseprobe nicht enthalten] exhibits an exponential distance dependence whose strength depends on the surface potential induced on the interacting colloidal particles and the dielectric properties of the intervening medium. Exact analytical expression for the electrostatic potential energy cannot be given; therefore, analytical approximation or numerical solutions are used. For spherical particles of equal size that approach one another under conditions of constant potential, [Abbildung in dieser Leseprobe nicht enthalten] is given by:

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provided ka is sufficiently large (>10).

In contrast, when the double layer around each particle is extensive ( ka <5),[Abbildung in dieser Leseprobe nicht enthalten] is given by:

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and 1/ κ the Debye-Hückel is a measure for the thickness of the electrical double layer (Table 1A)

[illustration not visible in this excerpt]are the number density and valance of the counter-ions of type i and F is the Faraday constant.

Herewith, Ψ results from the dissociation of amphoteric hydroxyl groups present on oxide surfaces and dependent on pH. It can be estimated from the zeta potential ( ξ ), which measures the electrostatic potential at or near to the beginning of the diffuse double layer.

Thus, the DLVO theory predicts that dispersions can be rendered unstable by either increasing ionic strength or adjusting pH toward the isoelectric point (IEP) of a certain suspension. The isoelectric points of several oxides are summarized in App. I - Table 8.

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