Study On Bonds Restructuring Of MgAl₂O₄ Precursors During Mechanical Activation

With magnesium hydroxide, magnesium oxide, gibbsite and alumina as raw materials, two different components of alumina spinel precursor, mixture of magnesium hydroxide-gibbsite and mixture of magnesium oxide-alumina, had been prepared respectively. Based on the above mentioned mixture system of different time mechanical activation, the effect of mechanical activation time along with components and crystal structure on the change of chemical bond of alumina spinel precursor during the process of mechanical activation, and then further study that influence of change of chemical bond on initial generating temperature and complete generating temperature of alumina spinel was caused by mechanical activation had been carried on.Obvious change of chemical bond of mixture of magnesium hydroxide-gibbsite existed in process of mechanical activation. XRD results showed that crystallinity dropped down with extension of time for mechanical activation, presenting the trend of amorphism; SEM, BET and EDS analysis had proved that grain size of precursor gradually refined by mechanical force, magnesium, aluminum component in precursor mixed uniformity, and embed each other in sub-micron after mechanical activation 20 h; TG-DTA study indicated that, structure of hydroxide had been damaged under the mechanical activation, which caused dehydration mechanism of raw materials to change. Initial decomposition temperature of magnesium hydroxide decreased from 426℃before mechanical activation to 270.9℃after mechanical activation, while initial decomposition temperature of gibbsite reduced from 331℃before mechanical activation to 132.5℃; obvious damage of chemical bond in mixture of mixture of magnesium hydroxide-gibbsite caused by mechanical activation was shown by FTIR research. Partial O-H, Al-O and Mg-O of precursor strengthened to a certain degree and reduction, torsion, transformation, or fracture of chemical bond occurred. Companding and deformation vibration of O-H in the precursor and translational peak of Al-OH-Al bond of gibbsite after mechanical activation 20h gradually weakened to disappear, characteristic peak of Mg-O and Al-O bond reduced obviously, broadened and moved toward blue high wavenumber. Precursors prepared in different mechanical activation time had been calcined in the conditions of different temperature. XRD results indicated that mixture of magnesium hydroxide-gibbsite was after mechanical activation 20h, Initial generating temperature of alumina spinel decreased from 1000℃before mechanical activation to 700℃, complete generating temperature of that decreased from 1300℃before mechanical activation to 800℃.Comparatively speaking, analogous law of bond change appeared in the mixture of magnesium oxide-alumina during mechanical activation process. XRD, SEM, BET and EDS results showed that change of chemical bond in the mixture of magnesium oxide-alumina was lower than that in mixture of magnesium hydroxide-gibbsite. FTIR analysis proved that with extension of time and enhancement of mechanical force during mechanical activation, Al-O and Mg-O bond in mixture of magnesium oxide-alumina had been damaged badly, chemical reaction generally weakened between the atoms, and further chemical bond appeared to twist, deformate or even partial chemical bond fractured. But compared with mixture of magnesium hydroxide-gibbsite, change tendency of Infrared characteristic peaks in system of oxide obviously reduced. Precursors prepared in different mechanical activation time had been calcined in the conditions of different temperature. The results indicated that mixture of magnesium oxide-alumina was after mechanical activation 20h, Initial generating temperature of alumina spinel decreased from 800℃before mechanical activation to 700℃, complete generating temperature of that decreased from 1300℃before mechanical activation to 1000℃.In the process of mechanical activation, the nature reason for change of bond is that gain refining of raw materials caused by mechanical activation, and damaged crystal lattice, increase of degree of disorder, increase of flaws inside crystal, embedment between components, reduction of diffusion length in solid-phase reaction, and improvement of reactants, which could reduce initial generating temperature and complete generating temperature of alumina spinel effectively. Elastic modulus and hardness of magnesium hydroxide-gibbsite are smaller that of magnesium oxide-alumina, and so it is easy to cause chemical bond to change. Under same activation situation, reduction of initial generating temperature of alumina spinel is more obvious with mixture of magnesium hydroxide-gibbsite as precursor...