Synthesis And Properties Characterization Of Micro/Nanostructured Alumina Materials

This study focused on the controlled synthesis of γ-AlOOH and α-Al2O3Micro/Nanostructured materials through liquid chemical methods, including hydrothermal and sol-gel methods. The preparation, the formation mechanism, as well as characterization and properties of the materials were discussed, including the sol-gel process and formation of the ceramic α-Al2O3using γ-AlOOH nanoparticles as the precursor; the hydrothermal preparation, formation mechanism and adsorption properties of y-AlOOH nanocrystals with different morphologies; the preparation of α-Al2O3microcrystallite ceramic grits, as well as their microstructure and mechanical properties. Through the study of the control synthesis, characterization and performance of the y-AlOOH and α-Al2O3micro/nanostructured materials, the formation mechanisms in the liquid phase synthesis were investigated. The relationship between the microstructure and properties of the materials are established, which might provide a theoretical basis for the preparation of γ-AlOOH and α-Al2O3with good performance.1. The Sol-Gel Process and the Formation of α-Al2O3Ceramic Using γ-AlOOH Nanoparticles as PrecursorThe stable sol was obtained using y-AlOOH nanoparticles as precursor and nitric acid as the peptizing agent. The gelation was achieved by the introducing electrolytes into the sol or volatilizing some of the free water. The α-Al2O3microstructured ceramic was synthesized by calcined the xerogel with additive. The products with different microstructures would be obtained when the additives were different, and the corresponding hardness was also different.The products and in-process products were characterized by XRD, TEM, IR, TG, DSC, SEM, particle size distribution, conductivity measurement in detail. The results showed that peptization process is a decentralized process of the nanoparticles aggregation in the presence of electrolyte. In this process, the adsorption of electrolyte ions at the particle surface leads to gradually increased electrostatic repulsion between the particles, then the particles dispersed to form a relatively stable sol. In the sol, the electrostatic repulsion and the van der Waals gravity between the colloid particles reached equilibrium. In the absence of any additives, the calcined product is a porous structure with worm-like hole. The addition of additives makes the changes of product's crystallinity and microstructure. And the hardness varies as the change of microstructure.2. γ-AlOOH Nanomaterials with Regular Shapes:Hydrothermal Fabrication and the Adsorption for CrO72-Well-defined boehmite (γ-AlOOH) nanomaterials with regular shapes including nanorods, nanobelts and nanoplates were prepared by hydrothermally treating the γ-AlOOH nanoparticles, in which H2SO4, CH3COOH and HC1were applied to regulate the shape of the products. The products and in-process products were characterized by XRD, SEM, TEM, HRTEM, TG, FT-IR, N2adsorption-desorption techniques in detail. The formation of the nanomaterials with different shapes could be attributed to adsorption of SO42-, CH3COO-and Cl-anions on the crystal planes, which affected the oriented-aggregation of the raw y-AlOOH nanoparticles and the growth of the aggregates. The difference in the adsorption abilities of the acid anions resulted in the different shapes of the products. The corresponding shape-retained γ-Al2O3nanomaterials could be obtained after calcination. Furthermore, the nanomaterials with different shapes exhibited different adsorption capabilities for CrO72-anions in aqueous solution, and the adsorption percentage by the nanobelts is up to95.5%. The exposed crystal facet of the γ-AlOOH nanomaterials is the important factor affecting the adsorption ability.3. Preparation, Microstructure and Properties of α-Al2O3Microcrystalline Ceramic GritsDense α-Al2O3microcrystalline ceramic particles were synthesized through a sol-gel process using the commercial γ-AlOOH nanoparticles as raw material. The products and in-process products were characterized by XRD, SEM, EDS, TEM, SAED, IR, TG-DSC techniques in detail. The hardness and fracture toughness of the products were recorded on a microhardness tester. The relationship between the grain size and hardness was studied. The α-Al2O3ceramic particles are constituted by the polyhedron microcrystalline with average grain size of140nm. The microcrystalline grains were sintered and linked closely together, almost no pores were observed among the grains. The products show excellent mechanical properties, hardness of23.6GPa, fracture toughness of5.1MPa·m1/2, which are all higher than the fused corundum. The excellent mechanical properties are due to its unique microcrystalline structure. The relation between the hardness and grain size follows the hybrid Hall-Petch relation.