In this research, for studying the influence of size and heat treatment temperature of initial Al(OH) 3 on the physical properties of porous mullite ceramics, porous mullite ceramics were prepared by in situ reaction sintering of amorphous silica and treated Al(OH) 3 . The transition phases χ-Al 2 O 3 , к-Al 2 O 3 , and stable phase α-Al 2 O 3 can be obtained in turn when the treatment temperature of raw Al(OH) 3 is 500, 1000, and 1500 • C, respectively. The coarser the raw Al(OH) 3 , the higher the strength of porous mullite ceramics. When the sintering temperature is 1500 • C, the bending strengths of PS500-C, PS1000-C, and PS1500-C (PSx-C represents that the specimen was prepared by the coarse grade Al(OH) 3, which was previously treated at x • C) are 40.3 ± 2.1, 54.9 ± 5.2, and 64.8 ± 4.8 MPa, respectively. In addition, although the activated Al 2 O 3 can decrease the formation temperature (∼100 • C) of porous mullite ceramics, the strength and density of porous mullite ceramics prepared by activated Al 2 O 3 will decrease at the same sintering temperature. It is believed that the increase of defects and pores during the phase transformation should be responsible for this phenomenon.
To lower the sintering temperature and improve the mechanical properties of aluminum titanate (AT, Al2TiO5) flexible ceramics, γ‐Al2O3 is employed to partially replace the α‐Al2O3 as the starting powder. The results show that the addition of γ‐Al2O3 is sufficient to promote the reaction, improve the flexibility, raise the strength, and accelerate the grain growth of AT flexible ceramics when sintered at 1300°C. With that, the combination of optimum fracture strength (∼33.52 MPa) and high flexibility (∼1.05%) is obtained when the addition of γ‐Al2O3 is 20 wt.%. As compared to the virgin, the optimization of 21.67% in fracture strength and 176% in strain can be obtained. It is believed that the increase of crack deflection and branching should take the responsibility. However, the mechanical strength degrades with the increase of sintering temperature, which is caused by the grain defects that reduce the bearing capacity of AT grains and lead to the grain fracture model transition. In addition, the effect of γ‐Al2O3 content on AT grain growth transfers from promotion to inhibition with the increase of sintering temperature. The main reason is that the growth of the nanocrystals at AT grain boundary causes the uneven distribution of the intermediate phase, leading to the roughening transition of the boundary and hence reducing the grain growth rate. On these bases, the grain growth mechanism that dominates by grain boundary is proposed. It is believed that the discovery of this study will provide a new grain growth model and the reference for solving the abnormal grain growth or grain growth stagnation.
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