【作者】 张英才；

【导师】 向兰；

【作者基本信息】 清华大学 ， 化学工程与技术， 2006， 硕士

【摘要】 MgO晶须是一种高强度、低密度的耐高温增强材料,用途广泛。目前多采用高温气相沉积（CVD）法制备,存在条件苛刻、价格昂贵、工业放大困难等问题,限制了氧化镁晶须的应用。为此,本文以我国丰富的镁资源为原料,探索采用氨沉淀-水热-焙烧法制备MgO晶须的可行性,探讨了相关的过程规律及Mg（OH）₂定向生长机制,旨在为镁资源高度利用提供一条可供选择的技术途径,为相关技术的进一步工业应用提供基础数据。以MgCl₂为原料, NH₄HCO₃和NaOH为沉淀剂,在常温条件下首先合成MgCO₃.3H₂O晶须,再经陈化处理可得到Mg₅（CO₃）₄（OH）₄H₂O晶须。后者在热分解过程中由于脱除的组分较多,使最终形成的MgO晶须孔洞较多。以MgSO₄为原料,氨水为沉淀剂,通过常温沉淀-水热反应-焙烧转化途径可制备形貌较为规则的MgO晶须。小粒径且分散良好的Mg（OH）₂常温产物有利于Mg（OH）₂晶须的水热形成。在20℃及搅拌（400rpm）条件下,维持MgSO₄与NH₃摩尔比为1:2.6,陈化30-120分钟,即可制得小粒径、高分散Mg（OH）₂。维持较高初始过饱和度可促进成核和小颗粒的形成,后期低过饱和度则有利于减少团聚。水热过程参数（原料配比、温度、时间等）对Mg（OH）₂晶须性能影响显著。在140-160℃反应10-14小时,可制得直径0.1-0.2μm、长50-60μm的Mg（OH）₂晶须。水热处理时,产物经历Mg（OH）₂→5Mg（OH）₂MgSO₄₃H₂O→Mg（OH）₂相变过程,以下反应同时存在: Mg²⁺+2OH^-= Mg（OH）₂ 5Mg（OH）₂+Mg²⁺+SO^2-₄=5 Mg（OH）₂ MgSO₄3H₂O水热条件下前者过饱和度较高,反应较快,使Mg²⁺迅速消耗,从而促进5Mg（OH）₂MgSO₄3H₂O分解转化。水热过程中SO₄^2-进入Mg（OH）₂晶格并同周围离子相互作用,导致Mg（OH）₂生长方向发生改变,由垂直于c轴的二维生长变为沿[11 24]方向的一维生长。更多还原

【Abstract】 MgO whiskers are widely used as the refractory and reinforcing materials owing to their high intensity, perfect heat-resistant, low-density and the special one dimension morphology. MgO whiskers are usually synthesized by the Chemical Vapor Deposition (CVD) methods, the corresponding shortcomings such as the rigorous synthetical condition, the high cost due to the expensive raw materials and high energy cost limited the further scale-up of the CVD technology. The possibility to synthesize MgO whiskers via the ammonium precipitation–hydrothermal modification-calcination route was investigated in this paper, including the influence of process parameters on the properties of the Mg(OH)2 /MgO whiskers as well as the directional growth of Mg(OH)2 whiskers at hydrothermal conditions. The aim of this paper was to provide an alternative method for the utilization of the magnesium resource in the salt lake.The original MgCO3.3H2O whiskers were formed at room temperature, using MgCl2 as the Mg source, NH4HCO3 and NaOH as the precipitation agents. MgCO3.3H2O whiskers were converted to Mg5(CO3)4(OH)4H2O whiskers after aging for 30-120 minutes. Mg5(CO3)4(OH)4H2O whiskers were decomposed to porous MgO after heating at 600oC for 2 hours.MgO whiskers were synthesized via the coprecipitation-hydrothermal reaction–sintering route, using MgSO4 as the magnesium source and ammonia aqueous solution as the precipitation agent.Dispersive Mg(OH)2 particles, favoring the hydrothermal formation of Mg(OH)2 whiskers, were synthesized at the following condition: 20oC, 400 rpm, molar ratio of MgSO4 to NH3: 1:2.6, aging time 30-120 minutes. The high initial supersaturation was favorable for the formation of nuclei and fine particles, the later low supersaturation was favorable for the dispersion of particles.The morphology of Mg(OH)2 whiskers were influenced obviously by the hydrothermal process parameters such as the molar ratio of the raw materials, the hydrothermal temperature and reaction time, etc. Mg(OH)2 whiskers with a diameter of 0.1-0.2μm and a length of 50-60μm were synthesized after hydrothermal treatment of the Mg(OH)2 slurry formed at room temperature at 140-160oC for 10-14 hours.The following phase change occurred in the hydrothermal process: Mg(OH)2→5Mg(OH)2MgSO43H2O→Mg(OH)2The following precipitation reactions co-existed in the hydrothermal process: Mg2++2OH-= Mg(OH)2 5Mg(OH)2+Mg2++SO2-4=5 Mg(OH)2 MgSO43H2OThe precipitation of Mg(OH)2 was carried out quickly owing to its high supersaturation at the hydrothermal condition, leading to the quick consumption of the soluble Mg2+ and promoting the dissolution of 5 Mg(OH)2 MgSO43H2O.SO42- was inserted into the Mg(OH)2 crystal lattice after hydrothermal treatment, leading to the change of the growth direction of Mg(OH)2 from along (0001) planes to along [11 2? 4] direction.更多还原