Preparation And Characterizations Of α-Al₂O₃ And MgAl₂O₄ Nanoparticles

Since Al2O3 and MgAl2O4 nanoceramic have extensively potential applications, it is of great signification to prepare nanosizedα-Al2O3 and MgAl2O4 powders. At first, using the cheap raw materials, theα-Al2O3 platelets and MgAl2O4 nanoparticles were prepared by a new method, namely the chemical precipitation-isolating phase assistant calcination technique. The influence of inorganic salt serving as isolating phase on the phase transformation of the alumina precursors, particle morphology, size and agglomeration degree was investigated. Then, the monodispersed alumina precursors were prepared by the homogenious precipitation method and hydrolysis of aluminum alkoxide, respectively. The influence of preparation conditions on the morphology of alumina precursors, particle size and agglomeration degree was studied. The phase transformation and morphology change of alumina precursors during the calcination were investigated.It is found that the MgAl2O4 nanoparticles can be prepared at a relatively low temperature of 800℃by calcining the mixture of MgSO4 andα-Al(OH)3. The inorganic salt MgSO4 acts not only as an isolating phase, but also a reactant in this process. The content of MgSO4 in the powder mixture has an influence on the particle size and agglomeration degree. As the Mg/Al atomic ratio is 20, the obtained MgAl2O4 nanoparticles have an average particle size of 12 nm, a narrow size distribution, and weak agglomeration. However, as the Mg/Al atomic ratio reduces to 5, the obtained MgAl2O4 nanoparticles have an average particle size of 22 nm, a broad size distribution, and severe agglomeration. Furthermore, these MgAl2O4 nanoparticles with particle size of 12 nm show excellent sinterability.Using the inorganic salt K2SO4 as an isolating phase, the dispersedα-Al2O3 platelets can be prepared at a relatively low temperature of 900℃by calcining the powder mixture of K2SO4 andα-Al(OH)3. The K2SO4 has a large influence on the phase transformation ofα-Al(OH)3. In calcination of K2SO4 andα-Al(OH)3 powder mixture, K3Al(SO4)3 forms in situ by solid reaction ofγ-Al2O3 and K2SO4. The decomposition of K3Al(SO4)3 can produceα-Al2O3 crystallite, which acts as a heterogeneous nucleation site and thus accelerates theγ-Al2O3 toα-Al2O3 transformation. The content of K2SO4 in the powder mixture has no influence on the particle morphology. As the K/Al atomic ratio is 12, the obtainedα-Al2O3 platelets have a diameter of 0.5-1.5μm and a thickness of 50-150 nm. The habit plane of obtainedα-Al2O3 platelets is {0001}. By addingα-Al2O3 seeds into the powder mixture, the diameter of obtainedα-Al2O3 platelets reduces to 300-600 nm, and thickness reduces to 30-60 nm.The monodispersed, spherical, and amorphous Al(OH)3 nanoparticles are prepared by the homogenious precipitation method. The particle size can be controlled by changing the concentration of Al3+. As the concentration of Al3+ reduces from 10 mmol/L to 0.5 mmol/L, the average particle size reduces from 350 nm to 60 nm. When the concentration of Al3+ increases to 0.21 mol/L, the average particle size reduces to 10 nm. However, the obtained Al(OH)3 nanoparticles have a severe agglomeration. The phase transformation sequence of Al(OH)3 nanoparticles obtained by homogenious precipitation is amorphous→γ-Al2O3→α-Al2O3. The near monodispersedα-Al2O3 nanoparticles can be prepared by calcining these Al(OH)3 nanoparticles at 1200℃for 2 h.The monodispersed, spherical, and amorphous Al2O3 particles are prepared by hydrolysis of Al(O-i-Pr)3 in acetonitrile-water system. The particle size can be controlled by changing the H2O/Al(O-i-Pr)3 molar ratio. As the H2O/Al(O-i-Pr)3 molar ratio reduces from 10 to 5, the average particle size of obtained amorphous Al2O3 particles reduces from 857 nm to 783 nm. However, as the H2O/Al(O-i-Pr)3 molar ratio reduces to 1, the obtained Al2O3 particles have no regular morphology. The phase transformation sequence of these Al2O3 particles is amorphous→γ-Al2O3→α-Al2O3. The monodispersedα-Al2O3 particles obtained by heating these amorphous Al2O3 particles at 1200℃for 2 h have porous structure.