Viscosity and structure of thoria—uranium sols

Viscosity and Structure of Thoria-Uranium Sols R. B. MATTHEWS, 1 P. H. TEWARI, AND T. P. COPPS Atomic Energy o f Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba, ROE 1LO Canada Received May 23, 1978; accepted September 6, 1978 Thoria sols transform from low viscosity, N e w t o n i a n fluids to pseudo-plastic, thixotropic gels with increasing p H , time, and u r a n i u m concentration. Shear stress v e r s u s s h e a r rate curves demonstrating this transformation have been generated. Electron microscopy of freeze-fractured samples with k n o w n viscosities has been used to show that the change in viscosity is a c c o m p a n i e d by a structured transformation, from a r a n d o m n e t w o r k of thoria particles to grains containing a textured rod-like lattice. It is suggested that the viscosity increases are d u e to increased mechanical interaction b e t w e e n the gelled rods.

are needed for high smear density spherepac fuel. We found, during preliminary investigations, that sphere formation was sensitive to the rheological properties of sols and we demonstrated that slow flowing, high density, viscous sols were desirable for consistent formation of large spheres (7). Concentrated thoria sols proved to be difficult to prepare and handle due to the seemingly unpredictable nature of flow behavior. During our preliminary attempts to prepare concentrated sols for coarse microspheres, we found that a loose sol transformed into a stiff gel after standing for several hours, but upon agitation the gel broke down and flowed readily. The sol to gel transformation could also be activated by varying pH, increasing uranium concentration, or removing water from the sol. As a consequence of these early observations, we began to investigate the variables affecting the rheological behavior of concentrated thoria sols.

INTRODUCTION

Sol-gel techniques have been developed for fabrication of spherepac, mixed oxide nuclear fuels (1, 2) and the surface chemistry and colloid stability of thoria sols have been thoroughly studied (3, 4). However, not much information has been published on the viscosity and structure of thoria sols. Anderson and Murray (5) have related viscosity-pH data ofthoria slips to ~-potential. McKorkle (3) and Ferguson (6) refer to some of the peculiar flow properties of thoria sols and suggest that the rheological behavior is due to a partially flocculated, long, stringy structure. This paper reports a group of experiments designed to measure some of the flow properties of concentrated thoria sols and to relate these properties to structure and ~-potential. The spherepac fabrication procedure starts with preparation of a stable mixedoxide sol containing thorium and uranium. Sol droplets are injected into columns containing an upward flow of 2 ethyl-hexanol where the drops gel into microspheres of various sizes. The spheres are then sintered and vibration compacted into fuel elements. Three sizes of spheres (10, 100, 1000/~m)

EXPERIMENTAL

The general experimental procedure consisted of preparing a nitrate-stabilized thoria sol by mixing at 80°C with the desired composition of uranyl nitrate. The pH was adjusted with incremental additions of nitric

1 N o w with: Battelle, Pacific N o r t h w e s t Laboratories, Richland, W a s h i n g t o n 99336. 260 0021-9797/79/020260- I 1$02.00/0 Copyright © 1979by Academic.Press,Inc. All rights of reproduction in any form reserved.

Journal of Colloidand Interface Science, Vol. 68, No. 2, February 1979

VISCOSITY AND STRUCTURE OF THORIA SOLS acid or ammonia. The sols were then cooled to room temperature and the viscosity was measured at some particular time after preparation. At the same time, samples of sol were quick-frozen for electron microscope observation. Sols were prepared by following the procedure used at the Oak Ridge National Laboratories (6). Thorium nitrate is first d e c o m p o s e d in steam at 475°C to thorium dioxide. The thoria is then mixed with uranyl nitrate (U/Th = 0.03) and sufficient nitric acid to suspend the particles. Peptization is complete after 30 min of mixing at 80°C, after which the p H is adjusted with ammonia to allow complete adsorption of uranium on the thoria crystallites. All p H values quoted were measured at the digestion temperature with a S a r g e n t - W e l c h combination electrode and a temperature compensated model N X p H meter. Apparent viscosity measurements were made with a Brookfield rotational viscometer at 12 rpm. Shear stress versus shear rate curves were generated with a H a a k e Rotovisco RV3 instrument using the NV and SV sensor systems. Specimens for microscopic examination were prepared by placing sol droplets on small specimen holders (1.0 mm in diameter) and plunging them into liquid Freon-22 (E. I. du Pont de Nemours) maintained near its melting point with liquid nitrogen. The frozen specimens were transferred under dry nitrogen to a previously cooled multiple specimen holder of a Denton DFE-3 FreezeEtch apparatus. The specimens were held at - 185°C under vacuum and freeze-fractured with a cooled scalpel. The specimens were then deep-etched by warming to -100°C to sublimate surface water. After shadowing with p l a t i n u m - c a r b o n at an angle of 45 ° the specimens were replicated with carbon at 90 ° . The replicas were floated off onto distilled water, washed, and picked up on specimen grids for examination in a Philips EM300 electron microscope operated at 80 kV.

261

This freeze-fracture technique was expected to show the structure of thoria sols and gels in an aqueous environment accurately because both the particulate structure and interparticle liquids are maintained in their dispersed state by the quick freezing technique. Electrophoretic mobility was measured by a zetameter to evaluate the ~-potential of the sols. Since the sols were very viscous and almost opaque in the zetameter cell, a part of the sol was centrifuged to separate the supernatant liquid. A few drops of the original sol were mixed with this supernatant solution to fill the zetameter cell and the mobility was measured at 22 _+ I°C. RESULTS

Effect of pH, Sol Density, Time, and Uranium Concentration on Viscosity The varying effect of pH on the apparent viscosity of a thoria sol is demonstrated in Fig. 1. The viscosity curve goes through two minimums; first, as the acid concentra-

pH 4

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0.01

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FIG. 1. Relationship between pH and apparent viscosity for a thoria-3 wt% uranium sol. Journal of Colloid and Inter[ace Science, Vol. 68, No. 2, February 1979

262

MATTHEWS, TEWARI, AND COPPS

tion is raised, the viscosity decreases then rapidly rises, and second, as the ammonia concentration is increased. A demonstration of the concentration and pH effects is shown in Fig. 2 where apparent viscosity is plotted against ammonia concentration in sols with three densities. Viscosity of the dilute sol is independent ofpH up to 3.7 after which the viscosity increases rapidly, for the more concentrated sols viscosity begins to rise at low pH and is extremely sensitive to ammonia concentration. This effect makes concentrated sols difficult to handle. We found that the viscosity of concentrated thoria sols increases with time, but the slope and shape of viscosity-time curves depend on the pH and density of the sols as shown in Fig. 3. High pH-high density sols exhibit a rapid rise in viscosity followed by a levelling off after standing for about 30 hr. The viscosity of low pH or dilute sols rises very slowly with time and very dilute sols (<1.4 Mg/m3) maintain a constant viscosity with time. Uranium ions readily adsorb on thoria (6) and the amount adsorbed is pH dependent. Figure 4 shows that the amount of uranium adsorbed on thoria particles increases when the pH of a thoria sol is raised.

Also the viscosity of sols increased markedly with increasing uranium concentration. We found that the sol-gel (Newtonian flow to thixotropic) transformation for thoria sols containing 2 and 3% uranium occurred at pH 3.6, but for a sol containing 4% uranium, the transformation occurred at pH 2.8.

Relationships between Viscosity and Structure The increase in viscosity with increasing pH, density, and time suggests that some type of structural transformation must be occurring in the sol. Figures 5-9 show the shear stress-shear rate curves and the associated sol structures developed with increasing pH for thorium3% uranium sol. At pH 3.0 (no ammonia addition) the sol behaves as a Newtonian fluid with a low viscosity (2 mPa. sec). The structure is a continuous random network of weakly bonded thoria particles surrounded by water. After the pH is adjusted to 3.3, the structure seems to be coagulating but the viscosity remains the same. The viscosity increases to 4 mPa. sec after the pH is adjusted to 3.5, the structure has coagulated further and textured rod-like shapes

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FIG. 2. Effect of ammonia concentration (pH) and density on the apparent viscosity of a t h o r i a 3 wt% uranium sol. Journal o f Colloid and Interface Science, Vol. 68, No. 2, February 1979

VISCOSITY AND STRUCTURE OF THORIA SOLS I

I

I

I

I

rod-like structure is maintained and some cross-linking begins to develop. The high pH sols have higher densities due to slower settling of coarse thoria particles through the viscous sols, and t h e greater volume fraction of solids also contributes to the high viscosity. Figures 10-12 demonstrate the effect of age on the flow behavior of the pH 3.5 sol. After sitting for 4 hr, the viscosity increases slightly and the structure is similar to the 2-hr sample. After 21 hr of aging, the apparent viscosity increases and the sol begins to show pseudo-plastic flow with a thixotropic loop, and a textured rod-like structure with some banding develops. Further aging for 45 hr leads to higher viscosity and flow behavior similar to the high pH sols. These high magnification micrographs are somewhat misleading because the orientation of the rods seems to imply an anisotropic structure throughout the sol. In fact, the sols are composed of randomly oriented grains containing the oriented rod structure. Figure 13a shows the intersection of two grains; unfortunately no low magnification micrographs were taken but Fig. 13b shows a schematic of a wide view of a typical high viscosity sol.

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FIG. 3. Effect of age on the apparent viscosity of thoria-3 wt% uranium sols.

are appearing. After the pH is adjusted to 3.7, the flow behavior changes from Newtonian to pseudo-plastic with a yield point. In addition the sol is thixotropic because the viscosity decreases with time at a constant shear rate. The structure consists of discrete, aligned rods of thoria flocs surrounded by water. Further increase in pH to 3.9 raises the viscosity and enhances the thixotropic hysteresis loop. The textured,

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Physical Properties of Sols The sols consist of thoria particles with uranium ions adsorbed on the surface as ob-

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FIG. 4. Effect of pH on uranium adsorption as a function of uranium concentration. Journal of Colloid and Interface Science, V o l . 68, N o . 2, F e b r u a r y 1979

264

MATTHEWS, TEWARI, AND COPPS I

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FIGS. 5-9. S h e a r stress-shear rate curves and associated electron micrographs of thoria-3 wt% uranium sols demonstrating structure and flow behavior as a function of pH. FIG. 5. pH 3.0 sol with low viscosity Newtonian flow and the random sol structure. The dark lines are thoria, the gray background is water.

served by X-ray photoelectron spectroscopy. Coulter Counter measurements show that the average size of dried sol particles is less than 1 /zm and the surface area obtained by BET nitrogen adsorption is about 70 m2/g. The measured ~-potentials for the sols between pH 3.0 and 3.9 were constantly high (-60 mV) for all the sols, and no correlation with viscosity was observed. Even though electrophoretic mobility was determined on sol samples that were extensively diluted, some trend in the values of ~potential should have been observed, if ~-potentials were related to viscosity. In fact, electrophoretic mobility did not change with progressive additions of the concentrated sol in the zetameter cell until its concentration had increased to the point where it was too opaque for mobility measurements. Journal of Colloid and Interface Science, Vol. 68, N o . 2, February 1979

DISCUSSION

The type of flow behavior we have observed in thoria sols has been seen in many colloid systems and explanations have evolved which can be applied to the thoriawater system. For example, viscosity-pH relationships similar to our results on thoria sols have been established for alumina slips (5, 8) and the behavior is explained in terms of changes in the electrical repulsive forces between dispersed particles. The viscosity minimums correspond to high ~-potential and strong repulsive forces. High viscosities are probably due to a partially flocculated gelled structure. Electrolyte additions affect the surface charge density by ion adsorption and/or dissociation of surface species. A decrease in viscosity corresponds to an increase in surface charge and the amphoteric nature of many oxides resuits in the ability to adsorb either H30 +

VISCOSITY A N D STRUCTURE OF THORIA SOLS I

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FIG. 7. Increasing the pH to 3.5 increases the viscosity slightly and a rod-like structure has developed. Journal of Colloid and Interface Science, Vol. 68, No. 2, February 1979

266

MATTHEWS, TEWARI, AND COPPS I

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FIG. 8. Increasing the pH to 3.7 causes a transformation to high viscosity, pseudo-plastic flow and a textured rod-like structure. I

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150

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FIG. 9. Increasing the pH to 3.7 further increases the viscosity and cross-linking between rods is developing. Journal of Colloid and Interface Science, Vol. 68, N o . 2, F e b r u a r y 1979

VISCOSITY AND STRUCTURE OF THORIA SOLS

I

267

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800

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1600

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FIGS. 10 to 12. Shear stress-shear rate curves and associated electron micrographs of a pH 3.5 thoria-3 wt% uranium sol demonstrating the effect of aging for 4 hr. The viscosity has increased slightly from the 2 hr sol shown in Fig. 7.

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I

160

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400

800

1200

1600

SHEAR RATE (s "1)

FIG. 11. After aging for 2 hr, the viscosity has increased, the sol shows pseudo-plastic flow, and the rod-like structure has set. Journal oJ' Colloid and Interface Science, Vol. 68, No. 2, February 1979

268

MATTHEWS, TEWARI, AND COPPS I

160

120 m o.

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x

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pH = 3.5

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800

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FIG. 12. After agingfor 45 hr, the sol shows a distincthigh viscosity,pseudo-plasticflow and the rod-like structure seems to be firmer. or OH- ions thereby explaining the deflocculation in either acid or base solutions as demonstrated in Fig. 1. An increase in viscosity results from flocculation promoted by the shielding effect of additional electrolyte and a diminishing surface charge. The pseudo-plastic flow and yield points observed in the high pH sols are related to the strength of the skeletal structure of flocs which breaks down with increasing rate of shear causing an apparent decrease in viscosity. The skeletal structure reforms with time probably due to Brownian diffusion of particles to form reversible bonds between flocs and the transformation from a sol to a linked gelled structure. This explains the aging effect shown in Fig. 3. The viscosity of dilute Newtonian suspensions increases linearly with increasing particle concentration. However, concentrated and non-Newtonian sols exhibit more complex behavior as shown in Fig. 2 due to Journal of Colloid and Interface Science, Vol. 68, No. 2, February 1979

charge interactions between particles and the breaking and reforming of bonded flocs. Our results suggest that the flow properties of thoria-uranium sols can be explained by combining parts of the colloid chemistry arguments with the structure-viscosity relationships established in this study. The overall picture that discrete, highly charged particles result in low viscosity and partially flocculated structures result in higher viscosity is clearly valid for thoria sols. However, our electrophoretic mobility resuits show that changes in viscosity are not accompanied by a changing g-potential. The potentials are fairly high throughout (=60 mV) hence the increase in the viscosity does not seem to be due to a lowering of ~-potential. It may be due to a flocculation in the secondary minimum, and chain-like structure formation has been proposed on the basis of theoretical calculations by van den Tempel (9) and Ruckenstein and

VISCOSITY AND STRUCTURE OF THORIA SOLS

269

a

D FIG. 13. (a) Electron micrograph showing a " g r a i n b o u n d a r y " in a gelled sol. (b) Schematic o f sol structure at low magnification ( - × 1000).

Mewis (10). The electron micrographs show that the change in flow behavior is accompanied by a change in structure from

a random network of thoria particles to grains containing a textured rod-shaped lattice. Further increase in viscosity coinJournal of Colloid and Interface Science, Vol. 68, No. 2, February 1979

270

MATTHEWS, TEWARI, AND COPPS

cides with the growth of branches from the rods and development of cross-links. The anisotropic structure of the thoria rods leads to oriented particle-to-particle bonds between zones of nonuniform surface charge density which results in a linked network of rods. The micrographs suggest that viscosity changes are a result of increased mechanical interference between rods and the oriented, cross-linked structure. Development of the rod-like structure is associated with attractive forces between thoria flocs and the formation of preferred, localized bonds resulting in the directional growth of rods and branches. The amount of hydrolyzed uranium species adsorbed on the thoria plays a significant role in the development of the gelled structures probably by affecting localized changes in surface charge to promote growth of branches and cross-links between flocs. In the low pH (3-3.5) sols uranium ions remain in solution, and the similarly charged surfaces result in a deflocculated, low viscosity sol with the random network structure. CONCLUSIONS

(i) The rheological properties of thoria sols containing uranium change from low viscosity, Newtonian flow to high viscosity pseudo-plastic flow with increasing pH and time. (ii) The structure of these sols transforms

Journal o f Colloid and Interface Science, Vol. 68, No. 2, February 1979

from a random network to a textured rodlike structure with increasing pH and time. (iii) The viscosity changes do not seem to be related to ~-potential. (iv) The adsorption of uranium ions enhances the high viscosity structure but the mechanism is not clear. ACKNOWLEDGMENTS We should like to thank Mr. D. M. Solberg for preparing the sols. REFERENCES 1. "Symposium on Sol-Gel Processes and Reactor Fuel Cycles," May 4-7, 1970, Gatlinburg, Tennessee. 2. " S o l - G e l Processes for Fuel Fabrication," Proceedings of a Panel on Sol-Gel Processes for Fuel Fabrication, May 21-24, Vienna, IAEA161 (1974). 3. McCorkle, K, H., "Surface Chemistry and Viscosity of Thoria Sols," Ph.D. Thesis, University of Tennessee, 1966. 4. Thomas, I. L., and McCorkle, K. H., J. Colloid Interface Sci. 36, 110 (1971). 5. Anderson, P. J., and Murray, P.,J. Amer. Ceram. Soc. 42, 70 (1959). 6. Ferguson, D. E., in "Progress in Nuclear Energy, Series III, Process Chemistry," Pergamon Press, p. 37, 1970. 7. Matthews, R. B., and Swanson, M. L., Amer. Ceram. Soc. Bull. 57(3), 361 (1978). 8. Hauth, W. E., J. Phys. Colloid Chem. 54, 142

(1950). 9. van den Tempel, M., "Rheology of Emulsions." Pergamon Press, London, 1963. 10. Ruckenstein, E., and Mewis, J., J. Colloid Interface Sci. 44, 532 (1973).