Thus, lowering the phase transformation temperatures is the
target for preparing the nanoscale a-Al2O3. Currently, there are
many reports about lowering the phase transformation temper-
atures of a-Al2O3. Li et al. [19] proposed that citrate polymeric
precursor derived from aluminum nitrate and citric acid mixed
solution promoted the g-Al2O3 transformed to a-Al2O3 directly,
which greatly reduced the transformation of a-Al2O3. Some studies
[20–22] indicated that phase transformation temperature of
u-Al2O3!
can be influenced by some metal cations such as Fe2+,Cr3+and Ti4+. However, the addition of inorganic ion inevitably
introduced impurities, which reduced the purity of a-Al2O3,
therefor limits its application in some areas. The previous studies
[23–25] have indicated that the a-Al2O3 seeding can provide the
nucleation sites in the process of the phase transition to reduce the
activation energy and phase transformation temperature, besides,
the a-Al2O3 seeding can prevent the formation of vermicular
structure during sintering.