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首页> 外文期刊>Journal of Non-Crystalline Solids: A Journal Devoted to Oxide, Halide, Chalcogenide and Metallic Glasses, Amorphous Semiconductors, Non-Crystalline Films, Glass-Ceramics and Glassy Composites >Effect of urea on the mullite crystallization
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Effect of urea on the mullite crystallization

机译：尿素对莫来石结晶的影响

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Due to its chemical, physical and mechanical properties, mullite has been used in equipments subjected to extreme mechanical strain and high temperatures. This paper reports the study of the effect of urea on the mullite crystallization, synthesized through the colloidal and polymeric sol-gel processes, using XRD and FT-IR. The colloidal precursors were obtained from silicic acid, water, aluminum nitrate nonahydrate (ANN) and urea. The polymeric precursors were obtained from TEOS, ethanol, ANN and urea. For both methods, urea was used in the molar ratio urea/Al3+ equal to 0/1, 1/1 and 3/1. The urea addition in colloidal gels led to materials with higher concentrations of orthorhombic mullite, besides they crystallized mullite at lower temperatures (positive effect). However, a negative effect was observed when urea was added to polymeric gels. In addition, the amount of crystallized mullite increased with the urea content in the colloidal gels. The colloidal sample with the highest urea content proved to form mullite at a lower temperature. Moreover, this sample did not segregate the α-alumina phase. The positive effect of urea in the colloidal gels is related to its participation in the hydrolysis and condensation steps of aluminum and silicon, avoiding the extensive phase segregation of silica and alumina. The negative effect observed in the polymeric samples may have occurred due to a competition between the silanol, the ANN and urea by water molecules. The only water molecules that are present in the polymeric gel are those that are part of the ANN.
机译：由于其化学，物理和机械特性，莫来石已被用于承受极端机械应变和高温的设备中。本文利用XRD和FT-IR报道了通过胶体和聚合物溶胶-凝胶过程合成的尿素对莫来石结晶作用的研究。胶体前体得自硅酸，水，硝酸铝九水合物（ANN）和尿素。聚合物前体得自TEOS，乙醇，ANN和尿素。对于这两种方法，均使用尿素/ Al3 +的摩尔比等于0 / 1、1 / 1和3/1的尿素。在胶体凝胶中添加尿素导致材料中正交晶莫来石的浓度更高，除了它们在较低温度下结晶莫来石（正效应）。然而，当将脲添加到聚合物凝胶中时，观察到了负面影响。另外，结晶莫来石的量随胶体凝胶中尿素含量的增加而增加。尿素含量最高的胶体样品被证明在较低温度下会形成莫来石。而且，该样品没有偏析α-氧化铝相。脲在胶体凝胶中的积极作用与其参与铝和硅的水解和缩合步骤有关，避免了二氧​​化硅和氧化铝的广泛相分离。在聚合物样品中观察到的负面影响可能是由于硅烷醇，人工神经网络和尿素之间的水分子竞争所致。聚合物凝胶中存在的唯一水分子是ANN的一部分。

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