Selective Corrosion Preparation and Sintering of Disperse α‐Al₂O₃ Nanoparticles

Abstract

Disperse fine equiaxed α‐Al2O3 nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe2O3 as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe2O3 acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al2O3 nanoparticles, resulting in disperse fine equiaxed α‐Al2O3 nanoparticles. These α‐Al2O3 nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al2O3 ceramics. The two‐step sintered bodies are nanocrystalline α‐Al2O3 with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al2O3 ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al2O3 nanoparticles have an excellent sintering activity.

Ultra‐Uniform Nanocrystalline Materials via Two‐Step Sintering

“…(In 8YSZ, it takes 500–1000 h at ≤1200 °C for deterioration to become evident. [ 42 ] ) Completely frozen microstructure was found in this work and in the literature for Al 2 O 3 , [ 15 ] BaTiO 3 , [ 17 ] Y 2 O 3 , [ 53 ] and Ni 0.2 Cu 0.2 Zn 0.6 Fe 2 O 4 [ 17 ] in T 2 ‐sintering for up to 40 h, which is enough to reach full density and is a comparable schedule used in industry, for example, for MLCC.…”

“…According to Hillert's solution that follows the mean‐field approach of Lifshitz, Slyozov, and Wagner (LSW), [ 6–9 ] the theoretical ratio σ, which is the ratio of the standard deviation Σ of the grain‐size distribution to the average grain size G avg , should be 0.354 in normal grain growth. Since real materials do not have grain boundaries of the same energy and mobility, they should have a larger σ during grain growth, which is true for a large number of dense ceramics sintered in various ways [ 4,10–45 ] as shown by the blue data points in Figure . Meanwhile, Figure 1 also reveals that smaller σ tends to come with a smaller G avg .…”

Mobility transition at grain boundaries in two‐step sintered 8 mol% yttria‐stabilized zirconia

“…Two‐step sintering has been practiced in a variety of ceramics, including Y 2 O 3, Al 2 O 3, doped ZrO 2, ZnO, TiO 2 , BaTiO 3 , and Ni–Cu–Zn ferrite . The idea is rooted in the hypothesis that grain growth can be suppressed in a sinterable system without necessarily suppressing grain boundary diffusion .…”

Response to Comment on “High-surface-area corundum by mechanochemically induced phase transformation of boehmite”

“…We do not agree with the points raised by Li et al (5) concerning our paper (1), although we should have included citations to their recent papers on the synthesis of α-Al 2 O 3 nanoparticles (NPs) as prior art (6)(7)(8). The omission was unintentional and has been corrected.…”