2.1 Materials

Thorium nitrate hexahydrate, Th(NO₃)₄·6H₂O, was obtained from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Concentrated ammonia and acetic acid, of analytical grade, were supplied by Sinopharm Chemical Reagent Company (Shanghai, China). Deionized water was obtained from laboratory.

2.2 Preparation and Characterization of Thorium Sols

The synthesis pathway used is based on the classical hydrolysis-polymerization reaction starting from thorium nitrate and ammonia. The solutions of thorium nitrate (0.6–1.6 M) were prepared by dissolving a certain amount of Th(NO₃)₄·6H₂O powder in deionized water at 30°C. The mixture was stirred (800–1000 r/min) in a thermostatic water bath at temperatures of 20°C–95°C. Concentrated ammonia was injected into the mixture at 30 mL/h first and 5 mL/h when pH of the solution was close to 3.0. The whole reaction process was monitored by a pH-meter (Metrohm pH-827). The viscosity was measured by a viscometer (Brookfield LVDV-II), and the turbidity by a desktop turbidimeter (Hach 2100AN). All the measurements were carried out at room temperature (~22°C). Sols, colorless and transparent, were obtained at pH 3.4–3.8. The density of the sols was measured by a densimeter (Rudolph DDM 2911). The particle size and size distribution of the sols were analyzed by a laser diffraction particle size analyzer (Brookhaven Zeta PALS/90 plus) at room temperature. The NH₄⁺/NO₃⁻ mole ratios were calculated according to the adding amount of NH₃·H₂O and Th(NO₃)_4. The reaction may be as follows^[17]:

(2)

3.1 Effect of NH₄⁺/NO₃⁻ mole ratio on thorium sol properties

The thorium sol was prepared by neutralizing 1.2 mol/L Th(NO₃)₄ solution with concentrated ammonia at 60°C. The change of pH, turbidity and viscosity of the mixed solution in the ammonia neutralization are shown in Fig. 1. The pH increased abruptly as the NH₄⁺/NO₃⁻ mole ratio increased from 0 to 30%, where it began to increase in a much lower rate. The pH was around 3.5 at the NH₄⁺/NO₃⁻ mole ratios of 75%–85%. The viscosity and turbidity of the solution showed almost no change before the NH₄⁺/NO₃⁻ mole ratio increased to 75%, where they began to increase dramatically. We found that sol could be prepared when the abrupt growth of viscosity and turbidity started and the pH was over 3.4, at the NH₄⁺/NO₃⁻ mole ratio of <75%. When the NH₄⁺/NO₃⁻ mole ratio was over 90%, the sol would become gel and lose mobility, but it could be reversed to sol again by adding of a small amount of acetic acid. The viscosity of the sol is a crucial parameter determining size distribution of the emulsion drops ^[18]. The present work revealed that for dispersion of gel particles, the NH₄⁺/NO₃⁻ mole ratio should be 75%–85% while the viscosity could be adjusted according to gel particle size requirement.

Fig. 1.Full-screen View

pH value (a) and viscosity/turbidity (b) of the mixed solution as a function of NH₄⁺/NO₃⁻ mole ratio.

3.2 Effect of reaction temperature on thorium sol properties

The effect of reaction temperature on properties of the sol was studied at 20°C–95°C and thorium concentration of 1.2 mol/L. Thorium sols could be obtained below 80°C while the milky suspension was obtained at <80°C. The reaction time required for preparing the sol deceased with increasing temperature, from 80 hours at 20°C to 8 hours at 60°C. This is due to that the peptizing process of precipitation is relatively slower at lower temperatures. Fig.2(a) shows the room temperature turbidity and viscosity of the final products prepared at different reaction temperatures and NH₄⁺/NO₃⁻ mole ratio of ~ 82%. The room temperature viscosity increased with the reaction temperature below 60°C, but greatly reduced at >80°C. Anyhow, the variation was relatively small with all the viscosities being in the range of 4–7 mPa·s. The slightly increasing viscosity at 20°C–60°C is probably due to the longer chain of polymers formed at higher reaction temperatures. The decreasing viscosity above 80°C is attributed to the formation of suspension. The turbidity increase slightly with temperature below 60°C, when it began to increase sharply. Above 80°C the increase trend slowed down to some extent. The increasing turbidity below 60°C should be due to the size increase of colloidal particles in the solution.

Fig. 2.Full-screen View

The temperature effects on viscosity/turbidity(a) and particle size distribution (b) of the thorium sols.

Fig. 2(b) shows the colloidal particle size distribution of the final sols prepared at 20°C, 40°C and 60°C. The average size of colloidal particle increased with the reaction temperature. As known, the sol formation is a thermodynamic process. The colloidal particles got more energy and became unstable with the rise of temperature. Colloidal particles grew without coagulation through collision as long as the temperature was not high enough. If the reaction temperature was sufficiently high, the violent movement of particles would cause coagulation of the solution, thereby forming a milky suspension. This is why the sol could not be obtained at >80°C. The present work revealed that the optimum reaction temperature for forming the thorium sol is about 60°C in consideration of the reaction time and the sol quality.

3.3 Effect of thorium concentration on thorium sol properties

The starting thorium concentrations should affect the sol properties of spherical degree and density. In the present study, the thorium sols were prepared by sol-gel method with the starting thorium concentrations of 0.6–1.6 mol/L at 60°C and ~84% of NH₄⁺/NO₃⁻ mole ratio. Fig.3(a) shows the density of the final sols as a function of starting thorium concentration. As expected, the sol density increased linearly with the thorium concentration,. As shown in Fig.3(b), both the viscosities of thorium nitrate solutions and the final sols increased with the thorium concentration. The viscosity of thorium sols is larger than that of the thorium nitrate solutions at the same thorium concentration. The sol turbidity did not change much as the thorium concentration increased.

Fig. 3.Full-screen View

The density(a) and viscosity/turbidity of thorium sol (b), vs the starting Th concentration, together with viscosity of Th(NO₃)₄ solutions.

The particle size distributions of final thorium sols prepared at different thorium concentrations are shown in Fig.4. The size of colloidal particles decreased with the increasing thorium concentrations, and the size distribution narrowed. These are consistent with the classical nucleation theory. According to the theory of Weimarn, the formation of colloidal particles can be divided into two stages. The first stage is nucleation and the second stage is the growth of the crystal nucleus^[19]. The nucleation and growth rates are all proportional to the supersaturation, but the nucleation rate is more strongly dependent on both supersaturation and suspension density. In the sol-forming process, as the precursor concentration increased, the nucleation rate was faster than growth, hence the formation of more crystal nucleuses, which led to smaller colloidal particle size. So, the sol particle size decreased with the increasing thorium concentrations.

Fig. 4.Full-screen View

Particlesize distribution of the final sols prepared at different thorium concentrations.

The sol particles can be well dispersed at lower thorium ions concentration, as the sol viscosity is smaller. But the spheres would become too soft and easily deformed during the heat treatment when thorium ions concentration is too low. When the thorium concentration is too high, it is hard to disperse the sol because of the large sol viscosity. To achieve good sphericity and high density of the microspheres, the initial thorium concentration of 1.2–1.4 mol/L is suggested.

3.4 Stability of the sol

The sol obtained at 60°C,NH₄⁺/NO₃⁻ mole ratio of 81.5% and the initial thorium concentration of 1.2 mol/L was tested after storing at room temperature for up to 744 hours. The results are given in Table 1. The storing time had a great impact on the thorium sol. The pH, viscosity, turbidity and particle size of the thorium sol increased with the storing hours. It seems that further condensation had occurred during the storing. If the storing time is over one month, the sol will easily turn into gel of three-dimensional network structure. It is confirmed that the sol can be stored at room temperature for 24 hours with only minor changes in the thorium sol properties.

3.5 Gelled Microsphere Formation

For preparing the thorium gelled spheres, sols were obtained at 60°C, NH₄⁺/NO₃^- mole ratio of 82.5% and initial thorium concentration of 1.2 mol/L. Urea and polyvinylalcholol (PVA) were used to adjust performance of the sol. After the mixed sol was prepared, it was slowly sprayed through a vibrating nozzle, during which the sol droplet was first formed in the air followed by pre-consolidation in NH₃ gas before eventually falling into ammonia solution^[20]. As shown in Fig.5, the gelled spheres are of good sphericity, without any cracking.

Fig. 5.Full-screen View

The sol droplet (a) and gelled spheres(b-c).