Preperation, Sintering Characteristics And Catalytic Property Of Magnesia Nano-powders

Nanometer magnesia, as a new inorganic functional material, was applied assuperconducting materials, rubber filler, acidic gas adsorbent, catalyst support, etc. It wasalso applied as radar adsorbing materials and antibacterial materials, which showedpromising application and huge economic potential. In this paper, magnesia nanopowderswere prepared by hydrothermal method with cheap inorganic magnesium salt as rawmaterials, the effect of reaction media on the morphologies of nanometer magnesiapowders was discussed, the sintering characteristics of magnesia nanopowders weresystematically studied by spark plasma sintering (SPS) method. the mechanical propertiesof solidified magnesia ceramics were tested and the catalytic properties of nanometermagnesia powders on thermal decomposition of ammonium perchlorate（AP）wereinvestigated systematically.The effect of the processing parameters such as the molar ratio of precipitant andMg2+, hydrothermal reaction time and temperature on the yield, grain size andmorphologies of nanometer magnesia powders was studied by orthogonal experimentsmethod. The optimal process conditions of nanometer magnesia powders were determinedat2:1ratio of precipitant to Mg2+, hydrothermal reaction time of3h and hydrothermalreaction temperature of160℃. The precipitant and calcined temperatures of precursorswere determined by single-factor experiment method. The morphology and structure ofnanometer magnesia powders were discussed when ethylenediaminetetraacetic acid(EDTA) and potassium chloride (KCl) were added in the reaction system as the media.When EDTA or KCl was added to the reaction system, the morphology was found totransform from hexagonal to quasi-circular nanoflakes, however, the phase composition atdifferent conditions was uniform. The mechanism of grain growth and crystal morphologychange was discussed by anion coordination polyhedron theory of growth.The phase composition and grain morphologies of MgO bulk ceramics sintered atdifferent temperatures (900℃，1050℃，1200℃，1300℃，1420℃) were studied byspark plasma sintering (SPS). Sintering temperatures had great influence on grain growth and ceramic structure. XRD showed that the bulk ceramics had face-centered phase, singlephase composition and high purity. The sintered ceramics at different temperaturesexhibited more sharp and narrow peaks than those of the magnesia powders, reflectinggrain growth during the sintering processes. SEM showed that the particle shape graduallyvaried from hexagonal to polygonal and continuous pores gradually transformed intoisolated pores with increasing SPS temperature. The kinetic grain growth exponent n andgrain growth activation energy Q were determined by the classical phenomenologicalkinetic equation. The analysis showed that sintering and grain growth mechanism wascontrolled by surface diffusion between900℃and1050℃, surface diffusion aided byplastic deformation at1200℃, and grain boundary diffusion aidedby plastic deformationwithin the range of1300-1420℃.The microhardness of MgO ceramics sintered at the different temperatures and holdtime was tested by Vichey microhardness method. The bending strength was also tested bythree-point bending experiment method. The effect factor on hardness was discussed，showing that porosity and grain size were the main factors. From the microstructure ofceramic materials, the strength was mainly controlled by grain size, porosity anddislocation density.The catalytic properties of nanometer magnesia powders were evaluated based onthree aspects including exothermic peak at high temperatures, apparent decompositionheat and weight loss at low temperatures. Nanometer magnesia powders had goodcatalytic properties on AP, which lowered the exothermic peak at high temperatures,increased the apparent decomposition heat and raised weight loss of AP at lowtemperatures. The dynamic parameters of AP decomposition were studied by Kissingermethod. The catalytic mechanism of nanometer magnesia powders on AP was investigatedaccording to the electron transfer theory. Various catalytic activity centers such as a largenumber of hydroxyl radicals on the crystal surface, the formation of hydroxyl groups at alow coordinate that became traps for capturing electrons promoted the electron transfer;the smaller crystal size of the nanomaterials increased the number of surface atoms withunsaturated coordination. Correspondingly, various factors such as large numbers ofhighly reactive edges, corner defect sites, and unusual lattice planes greatly promoted the catalytic properties.