Preparation And Characterizations Of Amorphous Alumina Nanoparticles

Due to the large elastic modulus, good thermal stability, high hardness, abrasion resistance, corrosion resistance, good insulation, light weight, low cost, abundant source of raw materials, etc., alumina is widely used in many applications such as electronics, automotive, chemical industry, cutting tools, aerospace, and other fields. The market demand for alumina production will continue to grow with the rapid development of world’s electrolytic aluminum, ceramics, pharmaceuticals, electronics, machinery, and other industries. Different from crystalline alumina, amorphous alumina(a-Al2O3) lacks long-range order and is metastable. Therefore, compared with crystalline alumina, amorphous alumina have different characteristics and applications. Amorphous alumina may have important applications in the field of insulating materials, catalyst supports, nano-glasses, and so on. So the preparation and characterizations of amorphous alumina are of important significance.This work has two main purposes. First, we hope to develop new methods for preparation of amorphous alumina nanoparticles with low-cost, simple process, and high yield. Second, we hope to achieve the amorphous alumina nanoparticles with good dispersity, spherical shape, narrow size distribution, and controlled particle sizes. In this work, cheap aluminum nitrate, aluminum sulfate, and formamide were used as starting materials to prepare the precursors of amorphous alumina using a facile homogeneous precipitation method. The effect of the type and amount of precipitating agent, aluminum sulfate/nitrate molar ratio, and precipitation method on the average particle size, size distribution and particle morphology of amorphous alumina nanoparticles were studied. Finally, well-dispersed spherical amorphous alumina nanoparticles with a narrow size distributions and controlled particle sizes were successfully prepared.The results show that the pH of the solution in the environment has a major effect on the structure of precursor when the aluminum nitrate and ammonia were used as raw materials and the direct precipitation method was applied. When the pH value of the solution is not more than 7, the precursor prepared is amorphous. If the solution is an alkaline environment, the precursor prepared contain crystalline γ-AlOOH and α-Al(OH)3. Calcination temperature also has an important effect on the final structure of alumina. Amorphous alumina was obtained when the calcination temperature is below 700°C, and γ-Al2O3 forms when the calcination temperature is 700°C. The amorphous alumina particles prepared is irregular shape, serious agglomeration and the aggregates has broad size distribution.If urea was used as precipitant, the mixtures of aluminum sulfate and aluminum nitrate with different molar ratios were used as aluminum sources for preparing alumina precursor particles. The amorphous alumina with severe agglomeration and larger particle size were obtained when the calcination temperature is below 700°C. Under the optimal conditions, the amorphous alumina with particle size of 100 nm and severe agglomeration were obtained at an aluminum sulfate/nitrate molar ratio of 1:9 and a total concentration of Al3+ of 2 mmol/L.If formamide was used as the precipitant, the mixtures of aluminum sulfate and aluminum nitrate with different molar ratios were used as aluminum sources for preparing alumina precursor particles. The amount of formamide added clearly strongly affects the particle size distribution and average particle size of the final amorphous alumina nanoparticles. This means that the particle size of the final product can be controlled by adjusting the amount of formamide added in the reaction. The average particle size of amorphous alumina particles decreases with increasing amount of formamide. In a typical experiment, the average particle sizes are 37 and 23 nm, respectively, when 10 and 100 mL formamide was added and the aluminum sulfate/nitrate molar ratio was 1:9. The aluminum sulfate/nitrate molar ratio also affects the particle size distribution and average particle size of the final amorphous alumina nanoparticles. The average particle size of amorphous alumina nanoparticles increases with increasing aluminum sulfate/nitrate molar ratio. The average particle sizes are 23 and 70 nm, respectively, at aluminum sulfate/nitrate molar ratios of 1:9 and 5:5 if 100 mL formamide is added.Well-dispersed spherical amorphous alumina particles with a narrow size distribution and the average particle size of 13, 23, 34, 37, 45, 56, 70, 78, 111, 141, and 163 nm were successfully obtained by adjusting the amount of formamide and aluminum sulfate/nitrate molar ratio.