Preparation Of Magnesia And Magnesia-conatining Spinel Powders By Molten Salt Method

Molten salt method is recently developed as a novel synthetic method for inorganic materials. Based on one or more low melting point salts as a reaction medium, which can be dissolved by the appropriate solvent, the reaction is easily carried out via the media and pure product can be obtained after filtering and washing. Due the advantages of the simple process, low cost and synthetic temperature, short reaction time and high stability in chemical compositions, molten salt method has been widely applied in different fields, such as in the synthesis of high melting point oxide powders, electronic ceramic powders and other functional powder materials.Magnesium oxide powders were synthesized in LiCl molten salt media. The effects of different raw materials, such as reaction temperature, holding time and the relative content of molten salt on the performances of the products have been studied. The results indicated that Ca2+ ions were replaced by Mg2+ ions and produced magnesium carbonate and other intermediate products with MgCl2 and CaCO3 raw materials. Compared to natural magnesite, the as-prepared magnesium carbonate had a higher degree of crystallinity and its order of decomposition closed to 1, and the average apparent activation energy, Ea, is 221.23 kJ/mol. Using MgCl2 and CaCO3, and MgCl2 and CaO as raw materials respectively, the prepared magnesium oxides were characterized with a uniform size distribution and a near sphere-like morphology. Whereas using MgCl2·6H2O and CaCO3, and MgCl2 and MgCa(CO3)2 as raw materials, the both prepared products had the mixed morphologies and uneven size distributions, in which the former product has a relatively better reactive activity, a higher reaction temperature and longer holding time were favorable for the growth of magnesium oxide crystals, resulting in the decrease of crystal cell volume and reactive activity, and the increase of true density. The increase of LiCl addition content was favorable for the growth of magnesium oxide crystals, leading to the increase of crystalinity degree and mean particle size and the decrease of reactive activity.By adding different surfactants, the morphologies of the magnesium hydroxide are controlled to yield the corresponding morphologies of MgO powders. As for the MgCl2·6H2O, CaCO3 and LiCl reaction system, fibrous magnesium hydroxide was produced after the heat treatment and soaking of the PEG solution and it can be decomposed to form chain-like morphology of magnesium oxide. Accordingly, tetrahedral shape magnesium hydroxide by EDTA-PEG magnesium hydroxide can produce tetrahedron magnesium oxide. Seemly, as for MgCl2, CaCO3 and LiCl reaction system, fiber-like and flake-like magnesium hydroxide can be respectively obtained under the soaking of PEG and EDTA-PEG solutions, which can be transformed into porous rod and flake magnesium oxide, accordingly. The four different morphologic magnesium oxide powers were spayed on the surfaces of Si-steel billet respectively. The results of the microstructures and magnetic examinations showed that Si-steel billets were of high performance coated by porous rod-like Magnesium oxide powders, while they were of poor performance by tetrahedron magnesium oxide.Furthermore, MgCr2O4 and MgAl2O4 spinel powders were synthesized in molten salt media. The effects of different raw materials, salts, heating temperature and holding time on the performances of the final products have been studied respectively. It is found that both raw materials and salts played an important role in the formation of MgCr2O4 and its crystal growth. They were well-developed by using MgCl2, MgCl2·6H2O and CaCO3 as raw materials in NaCl-KCl mixture molten salt. Further research has found that a higher reaction temperature and longer holding time were favorable for the growth of MgCr2O4 spinel crystals, resulting in the increase of MgCr2O4 spinel crystallinity, true density and average particle size. The morphology of MgAl2O4 spinel powder was closely related to alumina materials. A higher reaction temperature and holding time favored the growth of MgAl2O4 spinel crystals, resulting in the increase of its crystallinity, true density and average particle size.Compared with single NaCl and single Na2CO3 salt, NaCl-KCl mixture salts were more favorable for the formation and growth of MgCr2O4 and MgAl2O4 spinel crystals. The two mechanisms, "dissolution-precipitation" and "template formation", were involved in MgCr2O4 or MgAl2O4 spinel formation. "dissolution-precipitation" mechanism acted a leading role in the course of MgCr2O4 synthesis while "template formation" was dominant in MgAl2O4 formation.