Preparation And Characterization Of Magnesium Salt Whisker

Magnesium salt whiskers were prepared from salt-lake magnesium chloride (01 bittern) and ammonia (or sodium hydroxide) and sulfuric acid (or magnesium sulfate heptahydrate) by hydrothermal synthesis in autoclave. Chemical analysis, differentia thermal analysis-thermal gravity analysis (DTA-TG), X-ray diffraction (XRD) transmission electron microscopy (TEM) were employed to analyze the composition heat-stability and phase transformation, and morphology of the products and the preparation process.The results showed that magnesium salt whiskers with aspect ratios between 50 and 200 could be produced by hydrothermal synthesis from magnesium chloride ir autoclave. By controlling the reaction conditions, whiskers with different aspeci ratios could be obtained. It was determined, by using chemical analysis, DTA-TG anc XRD, that the products were of 152 type structure, which was denoted as MgSO4-5Mg(OH)2-2H2O.Whiskers could also be prepared from bittern without decolouring. This is 01 great interests for cost reduction, and is a newly-proposed way for bittern utilization.Kinetic parameters for the thermal decomposition of an as-prepared produd were measured by Kissinger method. Upon heating, the decomposition of magnesiurr hydroxide sulfate whiskers followed a three-step mechanism, finally resulting in the formation of porous oxide whiskers. Crystallized water was released in the first step, with the activation energy (Ea), the pre-exponential factor (A) and the reaction ordei (n) of 277.634kJ/mol, 1.9261013and 1.241 respectively. Hydroxide decomposed irthe second step with Ea, A and n being 492.273kJ/mol, 1.050xl026 and 0.721 respectively. Sulfur trioxide was released in the third step with Ea, A and n of 797.612kJ/mol, 1.954xl023 and 0.644 respectively.The optimized reaction condition for the production of magnesium hydroxide sulfate whiskers from magnesium chloride or bittern was determined as follows: reaction with ammonia solution and magnesium sulfate helptahydrate, pH 7~10,hydrothermal temperature of 170?0 with reaction time of 1~3 hours, stirring speed of 500?00rpm and [Cr]<0.20mol/L.Porous magnesium oxide whiskers could be obtained by heating magnesium hydroxide sulfate whiskers in muffle oven at temperatures higher than 1000. Theaspect ratio of the resulted oxide whiskers was about 40. At the same time, the morphology and structure change during the decomposition of the precursor whiskers was discussed based on the results of XRD and TEM. The precursor was perfectly crystallized 152 type magnesium hydroxide sulfate whisker. At temperatures about500, the products were amorphous fibres shorter than 400 nm, its structure couldbe MgSO4-(5-m)MgO-mMg(OH)2 (m<2) . At temperatures about 1050, the product was porous crystallized magnesium oxide whisker.