Electrokinetic Properties of Advanced Powders in View of Their Colloidal Properties

Acknowledgements

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.

iv

Table of Contents

  ABSTRACT  

CHAPTER I  NA  

  INTRODUCTION  1

1.1.  Task Description and Objectives  1

1.2.  Work Load and Study Project Requirements Agreement  3

CHAPTER II  NA  

  COLLOIDAL PROCESSING OF CERAMICS PARTICLE INTERACTIONS  4

2.1.  van der Waal Forces  5

2.2.  Electrostatic forces  8

2.3.  Steric Forces  10

2.4.  Electrosteric Forces  10

2.5.  Depletion Forces  11

CHAPTER III  NA  

  MATERIALS AND METHODS  12

  γ  

  −  

2  O Al  

3.1.  Properties  12

  3  

3.2.  Dispersants Properties  14

3.3.  Suspension Preparation  15

3.4.  Investigation Techniques  16

CHAPTER IV  NA  

  RESULTS AND DISCUSSION  22

  γ  

  −  

2  O Al  

4.1.  Suspension Characterization  22

  3  

4.2.  Dispersant Dosage Optimization  23

v

Table of Contents

4.3  Comparison of the dispersants effect on the zeta potential of the  5%

  γ  

  −  

2  O Al  

  suspension  25

  3  

4.4.  Comparison of the dispersants effect on the ionic strength of the  5%-

  γ  

  −  

2  O Al  

  suspension  26

  3  

4.5.  Assessment of the Dispersants with Reference to Point of Zero Charge (pzc)  27

4.6.  Dispersant Behaviour Model  30

4.7.  Experimental comparison of zeta potential and streaming potential of  

  γ  

  −  

2  O Al  

  slurry  32

  3  

CHAPTER V  NA  

  CONCLUSION  36

  REFERENCES  38

APPENDIX I  42  42

APPENDIX II  43  43

APPENDIX III  44  44

vi

List of Tables

TABLE 1.  ILLUSTRATION OF THE INTERACTION POTENTIAL ENERGY AND RELEVANT LENGTH SCALE FOR  

  DIFFERENT INTERPARTICLE INTERACTIONS ADOPTED FROM LEWIS (2000)  7

  γ  

  −  

2  O Al  

TABLE 2.  CHARACTERISTICS OF SAMPLE (SOURCE: WWW DEGUSSA DE)  12

  3  

TABLE 3.  SOME IMPORTANT PROPERTIES OF THE DISPERSANTS IN USE  14

TABLE 4.  ZETA POTENTIAL MV AS MEASURED WITH PCD AND ESA AND THE RESPECTIVE PH AT  

  DIFFERENT DOSAGES OF DOLAPIX A88 DOLAPIX CE64  32

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 )  33

TABLE 6.  HAMAKER CONSTANTS FOR SEVERAL CERAMIC MATERIALS INTERACTING UNDER  VACUUM

  AND ACROSS WATER AT 289K  42

TABLE 7.  ISOELECTRIC POINTS FOR SEVERAL CERAMIC MATERIALS  42

vii

List of Figures

  FIG 1 STATE OF COLLOIDAL SYSTEMS AND PREDOMINANT INTERACTIONS (AFTER LEWIS 2000)  4

  γ  

  −  

2  O Al  

  FIG 2  13

  SAMPLE  

  3  

  FIG 3 SCHEMATIC REPRESENTATION OF SEDIMENT BED FORMED BY (LEFT) DISPERSED SUSPENSION  

  AND (RIGHT) AGGREGATED PARTICLES (AFTER BESRA ET AL 2005)  17

  FIG 4 SCHEMATIC ILLUSTRATION OF THE CST APPARATUS  18

  FIG 5 SCHEMATIC ILLUSTRATION OF DOUBLE LAYER FORMATION IN THE MEASURING GAP  19

  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)  21

  FIG 7 POTENTIOMETRIC TITRATION OF 1 AND 5  22

  SUSPENSION  

  γ  

  −  

2  O Al  

  3  

  FIG 8 CST TEST RESULTS FOR DISPERSANT OPTIMIZATION  23

  FIG 9 VOLUMETRIC TITRATION OF 5 ALU-C AT DIFFERENT DISPERSANT DOSAGES WITH RESPECT  

  25  

  CHANGE IN ZP  

  FIG 10 VOLUMETRIC TITRATION OF 5 ALU-C AT DIFFERENT DISPERSANT DOSAGES WITH RESPECT  

  26  

  TO CHANGE OF SUSPENSION CONDUCTIVITY  

  FIG 11 COMPARATIVE POTENTIOMETRIC TITRATION AT DIFFERENT DISPERSANT DOSAGES  29

  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  

  30  

  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%

  34  

  W V IN THE PRESENCE OF DISPERSANT  

  FIG 14 GELCASTING FLOW CHART AS SUGGESTED BY THE OAK RIDGE NATIONAL LABORATORY  43

viii

Annotation and Abbreviations

a

minimum separation distance between particle

h

surfaces

electrostatic potential energy between charged

V

elect

particles

steric potential energy between particles resulting

V

steric

from adsorbed species

structural potential energy between particles

V

structural

resulting from non-adsorbed species

total interparticle potential energy

V

tot

total interparticle potential energy

V

vdW

Keesom forces

F

Keesom

Debye forces

F

Debye

London forces

F

LD

number density of ions of type i in solution

N

i

valance of ions of type i in solution

r

i

χ

CVI Colloid Vibration Current

CST Capillary Suction Test

ESA Electrokinetic Sonic Amplitude

GC Gelcasting

IEP isoelectric point

PCD Particle Charge Detector

PZC point of zero charge

ix

1. Introduction

1.1. Task Description and Objectives

The overall idea for this study project originates from the intention to investigate the

γ

possible use of the very fine fumed hydrophilic metal oxide,

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

1. Introduction

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

γ

know whether the very fine fumed aluminium oxide,

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.

Electrokinetic Properties of Advanced Powders in View of Their Colloidal Processing

1. Introduction

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:

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

Electrokinetic Properties of Advanced Powders in View of Their Colloidal Processing