Electrical conductivity and stability of concentrated aqueous alumina suspensions
Introduction
Ongoing interest in the properties of concentrated suspensions and their stability calls for a broader range of methods to assess the variables that might control it and establish a hierarchy on the importance they have in the wet processing of ceramic powders.
Major efforts have been made to relate the properties and variables at the intermediate processing stages with the mechanical properties of the suspensions. Those have been focused not only on rheology stress–strain measurements (stress-controlled) and mechanical impedance analysis (oscillation sweep), carried out with the suspensions, but also on the consolidation methods (namely, pressure filtration [1], [2], [3], slip-casting [4], centrifugation [5], [6]), and on the properties of the resulting consolidated bodies, before sintering (green body porosity [7], green body plasticity [8], [9], [10], dried body properties [11]).
The experimental results so obtained led to the development of models relating the changes in suspension viscosity, yield stress and shear modulus with the characteristics of the dispersed phase, such as concentration (solids content) and particle shape and size distribution, and with those of the suspending medium, such as its viscosity, temperature, and ionic strength [12], [13], [14].
The primary objectives of such research works still are to find out how the individual powder particles are spatially arranged within the suspension [15], [16], for how long that arrangement is kept (i.e., colloidal stability), and how it propagates through the consolidation methods into the green body, after the removal of the suspending liquid. The structure of the final body is irrevocably established during the consolidation stage, leaving a rather limited manoeuvering range for the correction of possible particle packing defects during the sintering stage. Even when such defects can be corrected during sintering and the desired densification reached, this stage is more concerned with other unwanted microstructural changes that can occur, like excessive grain growth and/or shrinkage [17], [18], [19].
Also, the novel techniques of wet consolidation, e.g., freeze and gel casting, temperature induced forming (TIF), direct coagulation casting (DCC), solid freeform fabrication (SFF), used to produce high performance ceramics, show a high degree of complexity and their success and optimisation demand an even better understanding of the controlling parameters, like pH, electrolyte type and content, solids load, and physicochemical nature [20].
Such novel processes also require special characterisation techniques, as they induce new or more important defects, caused by the manipulation of the suspension (e.g., air bubbles due to energetic stirring, or gas bubbles from the decomposition of unusual processing additives) [21].
Although conventional methods have produced good relationships between micro and macro properties, such methodologies remain laborious, show great interpretation complexity and demand sophisticated experimental setups of precision mechanical construction and a high degree of automation, hence high cost (e.g., modern rheometers with dedicated software packages) which often precludes their use outside the ideal laboratory conditions.
However, there is an alternative technique to study and control the stability of concentrated suspensions that can complement the conventional ones and that has not been thoroughly explored, which is the use of the suspensions electrical properties, namely their conductivity. Such procedure is very common in the characterisation of diluted suspensions (e.g., particle size determination [22] and electrophoretic mobility [23]). The study of the electrical properties displayed by suspended particles have contributed to further knowledge of the relationship between particles and suspending medium and to confirm, or dismiss, theories that try to explain the interparticle relationships and their stability, like the DLVO theory [23], [24].
This work investigates how the changes in the volume fraction of solids affect the static low frequency electrical conductivity of concentrated alumina suspensions, with various contents of added NH4Cl. Maxwell's model [25] is used to explain the behaviour of submicron particles subjected to an external alternating electric field.
Section snippets
Maxwell's model for the conductivity of suspensions
Maxwell's work on conductivity in heterogeneous media [25] proposes a model that can be applied to the conductivity of suspensions of ceramic particles. The model predicts that the conductivity of a mixture of two phases, a continuous phase (the suspending liquid in our case), of conductivity , and a disperse phase of spherical particles, of conductivity , is a function of , , and the volume fraction of the disperse phase, given by By
Materials
The ceramic powder used throughout this work was the commercial α-alumina CT3000SG (Alcoa, Ludwigshafen, Germany), 99.7% pure, with 6.5–8.5 m2/g specific surface area and a measured average particle size of 520 nm (LS 230 Particle Size Analyzer, Beckman–Coulter GmbH, Germany). The suspending liquid was distilled water (of measured conductivity 1.0 μS/cm). Mixed cation/anion ion-exchanger beads (Merck, Darmstadt, Germany) were used to reduce the amount of ions in solution. The beads were
Conductivity of the electrolyte (calibration curve)
Salt solutions show much higher electrical conductivities than distilled water, which can be correlated with the ionic concentration once the ionic mobility is known. Therefore, a NH4Cl solution was used as reference electrolyte to obtain a calibration curve for all conductivity measurements. Fig. 1 shows that, indeed, the solution conductivity, , increases almost linearly with the salt concentration, as it is expected for monovalent electrolytes [32]. Slight deviations from linearity in the
Combining DLVO theory with Maxwell's model
The estimates of the colloidal stability produced with DLVO theory combined with the experimental measures of the conductivity can be used as a basis for the discussion of the applicability of Maxwell's model to predict the particle behaviour in a given electrolyte of known ionic strength. Fig. 5 shows the theoretical effect, as predicted by Maxwell's model, of the solids content on the relative conductivity () for a set of constant values of the conductivity ratio, . This figure
Conclusions
This work is part of a broader investigation of the relationships among the stability of concentrated alumina suspensions, their electrical conductivity and volume fraction of solids, as well as the ionic strength of the suspending liquid. A particularly careful suspension preparation methodology (combined dialysis) led to the precise knowledge of the type and content of the ions in solution. Hence the ionic strength effect could be isolated from the solids content effect. Maxwell's model and
Acknowledgments
The authors appreciate the financial support received from the Brazilian Research Agency CAPES (R.C.D. Cruz, Ph.D. Grant) and from Deutsche Forschungsgemeinschaft (DFG Research Unit FOR 371).
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