A new method for preparing ultra-fine titanium dioxide powder

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　New method for preparing ultra-fine titanium dioxide powder. Ultra-fine powders generally refer to calcined powder materials with particle sizes ranging from 1 to 100 μm. It differs somewhat from the general concept of nanomaterials. Typical nanomaterials often do not take into account the synthesis process of the material, focusing only on its particle size. Ultra-fine powders generally refer to dry powder materials that have undergone a calcination process. During the calcination process, nanoparticles tend to reduce their surface free energy; as a result, the calcination process easily leads to particle agglomeration and sintering. In this process, determining how to take measures to maintain the dispersion properties of the material is key to preparing ultra-fine powders. The common methods used to prepare ultra-fine titanium dioxide powder include the solid-phase method, the gas-phase method, and the liquid-phase method. The solid-phase method makes it difficult to obtain particles with a particle size of less than 1 μm; the gas-phase method requires high levels of technical expertise and specialized equipment; whereas the liquid-phase method allows for uniform temperature of the reaction medium, facilitates control over particle size, and makes industrial implementation easier. The liquid-phase methods for preparing titanium dioxide include hydrothermal method, sol-gel method, and TiCl4 hydrolysis method. This experiment introduces an innovation to the previous TiCl4 hydrolysis method; by controlling key factors in the preparation process, ultra-fine titanium dioxide powder with good dispersibility was produced. 1 Test Methods 1.1 Test Materials Titanium tetrachloride (GR), hexamethylenetetramine (AR), monoethanolamine (CP), sodium hexametaphosphate (AR), oleic acid (AR), toluene (AR), propane (AR), ammonia or ammonium hydroxide (AR), sodium dodecylbenzenesulfonate (AR), cetyltrimethylammonium bromide (AR), etc. SRJX-4-13 type tubular electric furnace, CQ50 type ultrasonic oscillator, infrared dryer, 2X4AS type rotary vane vacuum pump, 2K-82B type vacuum drying oven, HITACHIH-800 type transmission electron microscope, etc. 1. 2 　Powder preparation: A certain amount of TiCl4 is measured using a very dry pipette, mixed into the organic phase in a specific ratio, and gently stirred with a mixer. After dissolving the dispersant in a certain amount of pure water, the organic phase and surfactant are added, and the mixture is placed in a cold water bath with high-speed stirring to produce a stable W/O emulsion. Slowly add the organic diluted TiCl4 solution to the emulsion. Stir at high speed while introducing ammonia gas, and continuously measure the pH value until a large amount of gel forms. Stirring continued for 30 min; the resulting gel was washed with anhydrous alcohols to remove excess water, and then filtered under vacuum. The product obtained by vacuum drying was rapidly heated to 500 °C in a tubular furnace under the protection of flowing N2, yielding rutile-type titanium dioxide powder. Transmission electron microscopy testing was performed in the same propyl solution. 2 Results and Discussion 2.1 Nucleation process During the liquid-phase reaction, the concentration of the solute continues to increase; once it exceeds a certain value C_max, rapid nucleation occurs, causing the solute concentration to drop sharply to C_min. After that, the crystal nuclei enter the growth phase, which continues until the solute concentration approaches its solubility limit. See Figure 1. Figure 1 Lamer model describing particle formation. In order to obtain particles of uniform size from the liquid phase, it is necessary to separate the nucleation process from the growth process of the nuclei, so that the crystal nuclei can grow synchronously. If the nucleation rate is insufficient, the solute concentration will remain fluctuating between C max and C min for a long time. The nucleation process is inevitably accompanied by the growth of crystal nuclei, resulting in non-uniform and poorly dispersed particle sizes. 2.2 Liquid-phase reactions: In the liquid-phase reaction stage, the main cause of particle agglomeration is the van der Waals forces between particles; common measures to suppress agglomeration include the use of double layers and the addition of protective agents. Ultrafine particles acquire a charge on their surface due to the adsorption of H+ or OH- ions from aqueous solutions. The surface of titanium dioxide carries different charges in aqueous solutions depending on the pH value. If the pH value of the reaction solution is too low, the gelation reaction does not occur completely. The reaction should take place under weakly alkaline conditions. At this time, the surface charge of titanium dioxide is negative, and the effective equilibrium ions of the double layer are Na+, NH4+, etc. The types of protective agents include hydrophilic polymers, surfactants, and chelating agents, such as various polyamine salts, polyethylene glycol, gelatin, etc. They adsorb on the particle surfaces, and through Coulombic forces and steric effects, they generate repulsive forces between particles, thereby preventing agglomeration from forming. 2.3 Drying and calcination stages: Theoretical analysis and experimental studies show that water between ultra-fine particles during synthesis by the liquid-phase method is the main factor causing particle agglomeration. Water will pull the particles together through capillary force. In the final stage of dehydration, salt impurities in the reaction mixture form crystalline solid bridges between the particles, thereby connecting them together. Ultra-fine powders possess a very large specific surface area and high reactivity; compared to ordinary particles, they can sinter at lower temperatures. Under conditions that meet the product requirements, the calcination temperature should be as low as possible. Furthermore, after the water removal steps and drying phase, the resulting dried gel needs to be calcined in a nitrogen atmosphere to produce rutile-type titanium dioxide ultra-fine powder. Although this preparation method still belongs to the liquid-phase gelation approach, it differs from traditional methods in that the amount of water in the reaction system is fixed, and an organic liquid phase replaces the conventional aqueous solution phase, thereby eliminating most of the moisture that could cause agglomeration. The water removal measures during the drying stage further eliminated particle agglomeration caused by water. Therefore, in fact, during the calcination stage, its function is to remove the residual organic liquid phase between the particles and to convert the titanium dioxide particles from anatase form to rutile form. Refer to Figures 2 and 3. By replacing the aqueous phase with an organic phase and using a controlled amount of water in that organic phase, the ultra-fine titanium dioxide particles synthesized through hydrolysis have a smaller particle size compared to those synthesized via traditional aqueous-phase hydrolysis, and they exhibit better dispersion properties. Figure 2: Hydrolysis synthesis in the aqueous phase; Figure 3: Hydrolysis synthesis in the organic phase. 2.4 Water removal measures: As mentioned earlier, water is the main cause of agglomeration in ultra-fine titanium dioxide powders prepared by the liquid-phase method. However, water is also one of the essential reactants. This experiment innovatively adopted a series of measures, including quantifying the reactant water, carrying out the reaction in the organic phase, and removing water after the reaction, to produce ultra-fine powders with small particle sizes and good dispersion properties. Since TiCl4 can undergo a strong hydrolysis reaction with the water in the air, even though the amount of water added as a reactant is controlled quantitatively, water present in the air still participates in the hydrolysis of TiCl4, resulting in an excess of water as a reactant. To minimize the amount of water in the reaction system, this experiment adopted a method of quantitative reaction using water as one of the reactants. According to the molar ratio of reaction between water and TiCl4, a certain amount of water is mixed with the organic phase; after an emulsion is formed under the action of a surfactant, it is then reacted with TiCl4. In this way, on the one hand, the formation of an emulsion causes the water in the reaction system to be dispersed by the organic phase into extremely small water-phase particles; within these particles, the gel formed as a result of the reaction is also dispersed by the organic phase, which helps to prevent the aggregation of these particles to a certain extent. On the other hand, after quantitative water reacts completely with TiCl4, not much water remains, which can help to reduce the agglomeration of particles caused by water. After the reaction is complete, washing the product with alcohol is also a water-removal measure. The resulting wet gel contains a large amount of organic solvent and water. During the gel drying process, the decrease in gel volume is equal to the volume of liquid that has evaporated; since there is no gas-liquid interface, capillary forces do not occur. As the liquid continues to evaporate, voids form within the gel, and numerous meniscus-shaped liquid surfaces appear in these voids. As a result, capillary forces push the particles together, leading to their agglomeration, as shown in Figure 4. The greater the surface tension of the liquid in the gel, the stronger the capillary force. During drying, agglomeration between particles becomes more severe. Based on the data regarding the surface tension of different liquids (as shown in Table 1), a liquid with lower surface tension can be chosen to replace one with higher surface tension, in order to prevent particle agglomeration. Figure 4: Schematic diagram of the capillary forces that cause particle agglomeration. Table 1: Surface tension values of certain liquids (at a temperature of 293 K, at the gas-liquid interface). Relevant liquids: water, ethanol, n-butanol, xylene. Surface tension ×10^-3 N/m: 72.88, 22.39, 22.25, 28.52. As can be seen from Table 1, ethanol has a much lower surface tension than water; washing the wet gel several times with anhydrous ethanol replaces the water in the gel, thereby reducing the surface tension of the liquid and consequently decreasing the degree of particle agglomeration. Figure 5: Before ethanol is used for water removal. Figure 6: After ethanol is used for water removal. Refer to Figures 5 and 6. During the gel drying stage prior to calcination, ethanol is used to replace the water in the gel; the particle size obtained in this way is approximately 50% of that of the particles produced without using ethanol for replacement. 3 　Conclusion (1) Experiments have shown that by incorporating a quantitative amount of water into the hydrolysis reaction of titanium salts and carrying out this hydrolysis reaction in an organic phase, it is possible to effectively eliminate the effect of water on the agglomeration of ultra-fine particles during drying and calcination processes. (2) The degree of particle agglomeration can also be reduced by replacing water with alcohols having lower surface tension. This post was last edited by zxh6267 on 2009-3-7 09:49.]