Key parameters such as optimal viscosity, minimal amount of dispersant and high solids loading are beneficial for both mixing and casting in high concentrated slurry processing [51,52,56]. Some researchers have used zeta-potential studies, sedimentation, Capillary Suction Time (CST) measurements and viscosity measurements at fixed shear rates for low concentrated (e.g., < 60 mass% of solids loading) suspensions in assessing stability [1,46]. But for high concentrated systems, there exist insufficient reports and or studies.
It is, therefore, important to find out maximum solids loading and evaluate its effects on dispersion stability and rheological properties of ZrO2
nanosuspensions [1,26,31]. Thus, to prepare highly stable slurry with optimal viscosity the maximum solid loading in suspensions was investigated by rheological measurements of ZrO2
nanoparticle suspension batches. The solid loading concentration was varied from 75 to 78 mass% by measuring the suspension viscosity (η) and shear stress (τ), where the dispersant concentration in all suspensions was held at constant 0.9 mass%.
For rheological measurements, the first series of analyses were performed on suespensions containing of 78 mass% solids loading of ZrO2
nanoparticles with a constant amount of 0.9 mass % dispersant. But this batch of nanosuspension exhibited a very strong shear-thinning behavior, paste structure, non-fluid and with high thixotropy which was not possible to apply for casting process. From this point of view, the experimental study centered on ZrO2
nanoparticle suspensions of 75 mass % up to 77 mass % solids loading as a maximum possible solids loading. To evaluate rheological properties of each suspension composition, a rheological flow curve can be used. Flow curve can provide information that relates to the interactions among the nanoparticles, the polymer, and the aqueous media. Furthermore, the strength of the interactions can be estimated with the information at various shear-rate conditions . In particular, if the data can be represented by an appropriate rheological flow model, the evaluation may become more convenient and effective .Therefore, the results from rheological measurements are shown in figures 2A and 2B.Figure 2:
Rheological behavior of ZrO2
nanoparticle suspensions with different solid loadings of 75 mass%, 76 mass% and 77 mass% with a constant amount 0.9 mass% of dispersant (Dolapix CE64); A) shows shear stress-versus-shear rate (flow curves), and B) shows dependence of viscosity on solid loading against shear rate.
Figures 2A and 2B, shows the variations in the rheological flow curves of shear stress τ (Pa) and viscosity η (Pa.s) as a function of shear rate γ < 0.03-1000 s-1
>) of concentrated ZrO2
nanoparticle suspensions by different solid loadings with addition of a constant 0.9 mass% dispersant. Figures 2A and 2B, shows the loop lines of the shear stress and viscosity of the three suspensions with different solid loadings 75-77 mass% versus applied shear rates, γ ? < 0.03-1000 (s-1
) >. It is evident that all the three suspensions are characterised by “shear thinning” behavior [52-55].
Both the degree of shear thinning and the viscosity at high shear rates increase with increasing of solid loadings. This behaviour is common for all of concentrated, colloidally stable powder suspensions which are investigated in recent studies (also Al2
and ATZ suspensions) [3,36,41,56] and can be explained as a perturbation of the suspension structure by applied shear [57,58]. The thixotropy hysteresis was found and appeared more apparent when the solid loading of the suspension was over 75 mass%. On the other hand, figures 3A and 3B, shows the variations of shear stress and viscosity with respect to three different solid loadings for dispersion of ZrO2
nanoparticle suspensions with constant addition (0.9 mass%) of Dolapix CE64 at three different shear rates of 50, 100 and 1000 s-1
at equivalent pH 9.0.
From figures 3A and 3B, it can be seen that,both shear stress and viscosity increase with solid loading which mainly attribute to the increase of hydrodynamic interaction between nanoparticles. That is to say, the viscosity of concentrated suspensions at a constant amount 0.9 mass% of dispersant depends strongly on solid loading, in this study.The viscosity at certain shear rate, γ 50 s-1
, with solids loading below 75 mass% shows minor changes and increases with the solids loading up to maximum 77 mass%. Keeping at shear rate as 50 s-1
, viscosity values of 0.05, 0.11 and 0.28 Pa.s were obtained with increasing solid concentration from 75, 76 to 77 mass%, respectively, see table 1.Figure 3 (A, B):
The effect of solid loading on incremental increasing of shear stress τ (Pa) and viscosity η (Pa.s) of suspensions with constant amount (0.9 mass%) of dispersant (Dolapix CE64) at three different shear rates of 50, 100 and 1000 s-1
. (Equivalent pH); a) shear stress τ (Pa), and b) viscosity η (Pa.s) vs. solid concentration (mass%).
Sample Viscosity Power law Herschel-Bulkley Bingham Thixotropy
(mass%) η (Pa.s) τ = k γ n τ = τ0 + k γ n τ = τ0 + ργ
|(L= ?) at γ = 50 (s-1) k n r τ0 kn r τ0ρ r Pa/s
750.05 0.44 0.42 0.999 0.06 0.45 0.43 0.999 2.56 0.006 0.99 215
760.11 1.6 0.39 0.999 0.10 2.73 0.42 0.998 4.97 0.005 0.80 474
770.28 2.6 0.37 0.999 0.75 2.79 0.17 0.998 13.52 0.02 0.92 567
78 impossible - - - - - - - - - - Paste
The effect of solid loading on rheological properties (viscosity, yield stress and relative variations) of concentrated ZrO2
nanosuspensions with constant amount 0.9 mass % of dispersant estimated by different flow models of: Power law, Herschel-Bulkley and Bingham models.
Table 1 shows the relationship between solid loading (?) and rheological properties such as viscosity, yield stress (τ0
) and relative variations in ZrO2
nanoparticle suspensions by different flow models. A pronounced increase in τ0
is apparently seen as maximum solids loading (?max
) exceeds 77 mass% at a constant amount 0.9 mass% of dispersant. Table 1 also shows the experimental values of the shear-thinning (pseudoplastic) index (n), consistency coefficient (k) and yield stress (τ0
) of the above selected three suspensions with different solid loadings of 75, 76 and 77 mass%, which calculated automatically by fitting of their relative flow curves to the three different flow models of: power law , Herschel-Bulkley  and Bingham [60,61] from an extrapolation of the τ - γ (linear dependence) to γ = 0 s-1
. The value of (n) clearly demonstrates that all of the suspensions have shear thinning behaviour (n <1, n decreases) and exhibit a finite yield stress.
It is obvious that by increasing the solid loading of suspension, the viscosity, yield stress and thioxtropy of suspensions increases and also subsequenlty the fluid factor (k) drastically increases. This phonemenon is because of the larger number of interactions that any given particle has with its neighbors as ? is raised in the suspension system [62,63]. Finally, fitting the experimental data of table 1 by Herschel-Bulkley model, correlation coefficients of at least 0.998 are obtained indicating that the viscosity behavior of the examined suspensions can be described very well by the Herschel-Bulkley model.
As a supplementary step, to visualize the results obtained from rheological measurements, we test a hypothesis followed by a nanoscale schematic colloidal model. Our hypothesis is that at a constant amount (in this study: 0.9 mass% Dolapix CE64) of dispersant increasing of solid loadings from 75 mass% to 77 mass% not only influences on the rheological properties of concentrated ZrO2
nanosuspensions, but also improves the packing structure of green bodies after casting. Accordingly, as solid loading increases in concentrated suspension it causes viscosity increasing. In addition, it helps green microstructures having better packing density that minimizes microstructure porosity. To visualize this hypothesis, a schematic nanoscale illustration of three types of colloidal models, using ZrO2
nanoparticles as host particles and polyelectrolyte chains of dispersant Dolapix CE64 as the coating polymer, and mobile fluid as a background aqueous water, is shown in figures 4A-4C.
Figure 4: Three schematic nanoscale colloidal models with related scanning electron microscopy images illustrating the effect of three different solid loadings of: A) 75 mass%, B) 76 mass%, and C) 77 mass% on colloidal packing and microstructure quality of samples prepared by concentrated ZrO2 nanoparticle suspensions with a constant amount 0.9 mass% of anionic dispersant Dolapix CE64.
A: Sample with 75 mass% ZrO2 NPs and open packing microstructure and extra mobile liquid area
B: Sample with 76 mass% ZrO2 NPs and open packing microstructure and available mobile liquid area
C: Sample with 77 mass%ZrO2 NPs and highly dense packing microstructure and immobile (less available) liquid area
In addition, for further verification of our hypothesis in relation to each proposed colloidal model a scanning electron microscopy image of drop-cast sample after drying is included, see figure 4 (A, B, C). Our hypothesis focuseson the effect of solid loadings in the formation of high concentrated nanosuspensions with the relative lower viscosity (η < 0.2 Pa.s for shear rates above 100 s-1
, suitable for casting applications) at relatively constant pH value of 9.0, see figures 4A-4C. Figure 4A, shows a schematic illustration of our proposed model of colloidally dispersed ZrO2
nanosuspension with constant 0.9 mass% dispersant and lower solid loading of 75 mass% ZrO2
nanoparticles. As it can be seen, in this model dispersed nanoparticles have free colloidal space next to each other and there exists extra mobile aqueous liquid that can move among nanoparticles which helps lowering the viscosity. However, because of having lower solid loading and extra mobile liquid in this type of nanosuspension system it will cause aggregation among nanoparticles that increases larger pores and heterogeneity in green microstructure after casting. As it is evident from figure 4A, the SEM image shows large aggregation with pores among nanoparticles and hetegenuous microstructure.
However, figure 4B, shows a schematic illustration of our proposed second model for colloidally dispersed ZrO2
nanosuspension with constant 0.9 mass% dispersant and higher solid loading of 76 mass% ZrO2
nanoparticles. As it can be seen, in this model dispersed nanoparticles have less free colloidal space next to eachother and there exists available mobile aqueous liquid. Comparing to first model illustrated in figure 4A, in this model as its SEM image shows concentrated nanosuspension system has smaller aggregation among nanoparticles. But its mictrostructure still shows existing of heterogenuous packing formation which will lead to increasing viscosity and microstructure porosity that will negatively effect on the quality of final cast products.
In contrast to figures 4A and 4B, figure 4C shows a schematic illustration of third model of dispersed ZrO2
nanosuspension with constant 0.9 mass% dispersant and maxmimum solid loading of 77 mass% ZrO2
nanoparticles. As it can be seen, in this model dispersed nanoparticles have denser position next to eachother and there is not sufficient mobile liquid among them. In addition, the SEM image in figure 4C shows concentrated nanosuspension system which is denser and has much smaller aggregation and pores among nanoparticles with improved homogeneity in microstructure.
As a result, figure 4C demonstratesthe concentrated ZrO2
nanosuspension that is dispsersed with constant 0.9 mass% dispersant and maxmimum solid loading of 77 mass% ZrO2
nanoparticles has better rheological properties, green microstructure and packing quality, and therefore suitable for casting applications.