High-purity,homogeneous and ultra fine LaB6 powders were prepared by combustion synthesis.The effects of reactant ratio and molding pressure on the phase and morphology of the combustion products were studied.The combustion products and leached products were analyzed by XRD,SEM and EDS.The results indicate that the combustion product consists of LaB6,MgO and a little Mg3B2O6.The combustion product becomes denser and harder when the molding pressure increases.The purity of LaB6 is higher than 99.0%.The LaB6 particle size is in range of 1.92-3.00 μm and the lattice constant of LaB6 is a=0.414 8 nm.
High temperature self-propagating synthesis （SHS） process is very rapid, the reaction process becomes un-controlled after the SHS reaction is ignited. So the initial reaction conditions will have great effects on phase compositions and microstructures of reaction products. In this paper, the effects of the proportioning amount of Mg on the yield ratio and particle sizes of CeB6 were studied. The SHS reaction products and leached products were characterized by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results indicated that the SHS products consisted of MgO, CeB6 and little Mg3B206. The single CeB6 phase was contained after the SHS reaction products were leached. The purity of CeB6 was higher than 99.0 mass%, and the minimum particle sizes of CeB6 were within 30-70 nm. When the propor- tioning amount of Mg was 25% more than the theoretic amount, the yield of CeB6 was 68.68%. The antioxidant ability of CeB6 was rather stronger, which was oxidized step by step, and the initial oxidation temperature was 750 ℃ which indicated that it had good high temperature stability. The apparent activation energies of oxidation reactions of CeB6 were 200.09 and 312.10 kJ/mol, respectively, and reaction orders were 0.69 and 0.40, respectively.
The amorphous boron powders with high activity were prepared by the high-energy ball milling-combustion synthesis method. The effects of the milling rate and milling time on the crystallinity, microscopic morphology and reactivity of amorphous boron powder were studied. The results show that the crystallinity of amorphous nano-boron powder is only 22.5%, and its purity reaches 92.86%. The high-energy ball milling can significantly refine boron powder particle sizes, whose average particle sizes are smaller than 50 nm, and specific surface areas are of up to 70.03 m2/g. When the transmission electron beam irradiates the samples, they rapidly melt. It can be seen that the monomer amorphous boron size is less than 30 nm from the specimen melting traces, which indicates that the samples have high reactivity.
CeB6 powders were prepared by high-temperature self-propagating synthesis （SHS） in which CeO2, B203 and Mg were taken as reactants. The adiabatic temperature and dynamics of SHS reactions were investigated. The SHS reaction products and leached products were characterized by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results indicated that the adiabatic temperature of Mg-B2Oa-CeO2 reaction system was rather higher than 1800 K to make the reaction propagate by itself, and the apparent activation energy （Ea） and reaction order （n） of exothermic peak on Mg-BzO3-CeO2 differential scanning calorimetry （DSC） curve were 23.03 kJ/mol and 1.31, respectively. The apparent activation energy was lower, so the reaction occurred easily. The SHS products consisted of MgO, CeB6 and little MgaB2O6. The leached products consisted of single CeB6 phase and its purity was higher than 99.0%, and the average particle sizes of CeB6 were smaller than 150 nm.
The preparation process of amorphous nanometer boron powders through combustion synthesis was investigated, and the effects of the reactant ratio, the heating agent and the milling rate on the activity and particle size of amorphous boron powders were studied. The results show that the boron powders exist in the form of an amorphous phase which has the crystallinity lower than 30.4%, and the panicle size of boron powder decreases with an increase of the high-energy ball milling rate. The purity of amorphous boron powder is 94.8% and panicle sizes are much smaller than 100 nm when the mass ratio of B2O3/Mg/KClO3 is 100：105：17 and the ball milling time is 20 min with the milling rate of 300 r/min. At the same time, the amorphous boron nano-fibers appear in the boron powders.
