| One important application of α-Al2O3 nanoparticles is to sintering of nanocrystalline ceramics. Compared with conventional ceramics, nanocrystalline ceramics have extremely rich grain boundaries, and the diffusional flow of atoms along the grain boundaries contribute to the plastic deformation of nanocrystalline ceramics and can lead to ductility of nanocrystalline ceramics. Therefore, the nanocrystalline ceramics may be fundamental strategy to overcome the brittleness of the ceramic materials. Disperse fine uniform equiaxed α-Al2O3 nanoparticles are essential for sintering Al2O3 nanocrystalline ceramics. Fine α-Al2O3 nanoparticles have large specific surface areas and high surface energy, which can provide sufficient driving force in the sintering process to lower sintering temperature and reduce grain size. Uniform size is another important factor for sintering dense nanocrystalline ceramics. A narrow particle size distribution is desired because large particles will grow at the expense of small particles during sintering, resulting in coarse-grained ceramics rather than expected nanocrystalline ceramics. Good dispersity ensures good fluidity of nanoparticles. Large agglomerates will lead to large pores in the green compacts, which can be hardly exhausted during sintering. Fabrication of Al2O3 nanocrystalline ceramics is extremely difficult so far due to the difficulty in synthesis of disperse fine uniform equiaxed α-Al2O3 nanoparticles. Al2O3 has several polymorphs. α-Al2O3 is the thermodynamically stable phase of bulk Al2O3 at standard pressure and temperature conditions. However, the surface energy of α-Al2O3 is higher than that of γ-Al2O3. When the particle size is reduced to be smaller than 15 nm for spherical particles, the free energy of α-Al2O3 will be higher than that of γ-Al2O3. α-Al2O3 is not the thermodynamically stable phase any more. Besides, the phase transition temperature of transition Al2O3 to α-Al2O3 is usually higher than 1000?C. Fine α-Al2O3 nanoparticles tend to sinter to large agglomerates at such high temperatures. For these reasons, synthesis of disperse fine uniform equiaxed α-Al2O3 nanoparticles is extremely difficult.In this work, the α-Al2O3 nanoparticles with an average size of 13.3 nm and a size distribution of 2-250 nm were synthesized by mechanochemical method and HCl selective corrosion. The α-Al2O3 nanoparticles exhibit good dispersity but fail in narrow size distribution width. Broad size distribution width limits the performance of the nanoparticles. Especially, to sintering of nanocrystalline ceramics, large particles will grow at the expense of small particles during sintering, resulting in excessive growth of grains. Here, fractionated coagulation was applied to separate α-Al2O3 nanoparticles of different sizes by using HCl as coagulating agent to obtain the α-Al2O3 nanoparticles with narrow size distribution. The synthetic α-Al2O3 nanoparticles with an average size of 13.3 nm and a size distribution of 2-250 nm were suspended in order at HCl concentrations of 1.4, 1.2, 1.0, 0.8 mol/L HCl, α-Al2O3 nanoparticles with average particle sizes of 5.2, 6.5, 7.9, and 9.2 nm and size distributions of 2-9, 3-11, 4-14, and 5-15 nm were separated.When the α-Al2O3 nanoparticles are suspended in the deionized water, the surfaces of will adsorb a layer of H+, and a small amount of OH? will be adsorbed by H+. Here, H+ is called potential ion, and OH? is called counter ion. The adsorbed potential ions and counter ions form the adsorption layer. The excess OH? is distributed in the liquid near the surfaces of the particles, the concentration of OH? decreases as the distance increases, this is called diffusion layer. The adsorption layer and diffusion layer form a double electric layer. In the process of thermal motion of colloidal particles, the plane between colloidal particle surface and liquid is called shear plane. The potential difference between shear plane and liquid is called zeta potential. The colloid with the higher zeta potential is more stable duo to the higher electrostatic repulsion between the colloidal particles. By adding HCl into the Al2O3 colloid, Cl? ionized by HCl enters into the double electric layers of α-Al2O3 nanoparticles, resulting in the decreases of zeta potential and electrostatic repulsion. Finally, α-Al2O3 nanoparticles coagulate by the van der waals forces. The smaller α-Al2O3 nanoparticles have a higher surface energy, stronger adsorption capacity, thicker double electric layer and higher zeta potential. So the colloid is more stable, the electrolyte concentration which coagulate the colloid will be higher. Small α-Al2O3 nanoparticles can suspend in an HCl solution with a high HCl concentration whereas large α-Al2O3 nanoparticles coagulate.By suspending α-Al2O3 nanoparticles to be separated with an average size of 10.5 nm and a size distribution of 3-100 nm in order in HCl silutions of 1.6, 1.4, 1.2, 1.0, 0.8, 0.6, 0.4, 0.2, and 0 mol/L, separated α-Al2O3 nanoparticles have average sizes 6.6, 7.7, 8.9, 10.0, 11.3, 14.1, 16.9, 20.0 and 40.5 nm and size distributions of 3-10, 4-12, 5-13, 5-15, 6-17, 7-25, 9-33, 10-46 and 15-100 nm. Large α-Al2O3 nanoparticles with an average size of 40.5 nm and a size distribution of 15?100 nm were added into α-Al2O3 nanoparticles to be separated and mixed at a large-to-small nanoparticle mass ratio of 10:1. The mixed α-Al2O3 nanoparticles were separated in order at HCl concentrations of 1.6, 1.4, 1.2, 1.0, 0.8, 0.6, 0.4, 0.2, and 0 mol/L. Separated α-Al2O3 nanoparticles have average sizes of 4.7, 5.8, 6.5, 7.6, 9.2, 10.9, 14.2, 17.4, and 36.5 nm and size distributions of 3-7, 4-8, 4-9, 5-11, 6-13, 6-16, 7-31, 8-35, and 10-100 nm. Large α-Al2O3 nanoparticles(40.5 nm) and α-Al2O3 nanoparticles to be separated were mixed at a large-to-small NP mass ratio of 5:1. α-Al2O3 nanoparticles separated at HCl concentrations of 1.6, 1.4, 1.2, 1.0, and 0.8 mol/L have average sizes of 5.2, 6.0, 6.8, 8.1, and 9.7 nm and size distributions of 3-8, 4-9, 4-10, 5-12, and 6-14 nm. When the large-to-small nanoparticle mass ratio was adjusted to 20:1, α-Al2O3 nanoparticles separated at HCl concentrations of 1.6, 1.4, 1.2, 1.0, and 0.8 mol/L have average sizes of 4.4, 5.4, 6.0, 6.9, and 8.0 nm and size distributions of 3-6, 3-8, 4-8, 4-9, and 5-11 nm.Compared with the separation results of α-Al2O3 nanoparticles without addition of large ?-Al2O3 nanoparticles(40.5 nm) separated at the same HCl coincentrations, the average sizes of separated α-Al2O3 nanoparticles can be reduced and their size distribution widths can be narrowed with addition of large α-Al2O3 nanoparticles. With increasing amount of large α-Al2O3 nanoparticles(40.5 nm) added, separated α-Al2O3 nanoparticles show smaller average sizes and narrower size distribution widths at the same separation concentrations.To sinter Al2O3 nanocrystalline ceramics, α-Al2O3 nanoparticles with an average particle size of 7.9 nm and a size distribution of 4-14 nm were pressed into a green compact at a pressure of 600 MPa and then sintered by a two-step pressureless sintering method to suppress the final-stage grain growth(heating to 1230°C without hold and decreasing to 1080 °C with a 40 h hold). The sintered bodies have a relative density of 99.5%, an average grain size of 60 nm, and a grain size distribution width of 20–130 nm. The sintering result reveals that the separated disperse fine equiaxed α-Al2O3 nanoparticles with a narrow size distribution exhibit a good sintering activity. |
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