Accelerated microwave synthesis of magnesium sulfide with the pro-heating medium of graphite
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Xiaoyue Xiaoa) Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China (Received 11 April 1994; accepted 12 October 1994)
Two reaction routes have been tried for the synthesis of magnesium sulfide by applying microwave irradiation. In the first trial, finely ground Mg and S were mixed thoroughly and heated in a microwave oven for various lengths of time (5 + 5 + 8 + 10 and 1 2 + 1 2 + 35 + 45 min) in a sealed quartz tube. In the second trial, the pro-heating medium (PHM) of graphite was introduced into the mixture of Mg and S and microwaved for only 1 min. Results of x-ray diffraction analyses of the reaction products indicated that MgS polycrystallites (cubic, a0 = 5.201 ± 0.001) had formed in the second trial, and that the MgS yield was greater than 98%. Data of EDAX and EPMA gave a formula of MgS with atomic ratio of Mg:S = 1.0:1.0. In contrast, MgS could not be identified in the reaction mixtures in the first trial. Obviously, graphite, as a PHM, played a key role in the dramatic enhancement of the rate of the reaction between Mg and S powders. Furthermore, the effect of different molar ratios of graphite to Mg on the rate of microwave synthesis was investigated.
I. INTRODUCTION Magnesium sulfide, as one of the alkaline-earth sulfides, is technologically important as a host material of phosphors with high quantum efficiency.1"7 In recent years, it has been considered as a material with high potential for use in thin film electroluminescent (TFEL) displays.8'9 Other applications of MgS include sensors,10 optical windows,11 erasable optical memory,12 additives into cathode materials of transition metal sulfides,13 etc. Generally, there are two methods for the synthesis of poly crystalline MgS: (a) direct sintering of ground elemental mixtures,14 and (b) deoxidization of purified MgSO 4 in a CS 2 -N 2 (or Ar) stream.1-3-5-6 These processes require either high energy consumption (high sintering temperature and long reaction duration) or complicated apparatus. Recent efforts have been made to increase the rate of selected chemical reactions and to simplify synthetic processes by means of microwave irradiation. Microwave irradiation may provide a heating rate as fast as 100 °C/s for solid reactions, so that reactions could be completed in minutes.15'16 Several investigators indicated that microwave irradiation treatment may be a powerful method for synthesis of metal sulfides from metal powders and sulfur.1516 Metal powa)
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J. Mater. Res., Vol. 10, No. 2, Feb 1995 Downloaded: 25 Aug 2015
ders, unlike bulk metals, have large surface areas so they could avoid electric discharge during the microwave irradiation. The rate of temperature rise by microwave irradiation would be quite different for different targets.17 Those targets with low dielectric loss may be unsuitable for microwave heating, thus limiting applications of microwave synthesis. The reaction rate for materials that couple poorly wit
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