Formation and crystal-chemical studies of metastable and stable mullite-type aluminum borates

莫来石型硼酸铝晶须，特别是Al18B4O33 （A9B2）和Al4B2O9 （A2B）晶须具有良好的耐火、弹性和力学性能，可作为铝合金的增强材料。本文的主要目的是研究Al2O3 - B2O3二元体系中莫来石相的形成和晶体化学特征，重点研究了Al4B2O9和Al18B4O33 （Al5BO9）化合物。在这方面，在环境压力和有水或无水环境下，哪些因素（化学成分、反应路线、温度）在本质上控制莫来石型化合物的形成是特别令人感兴趣的问题。研究的另一个中心点是了解由溶胶-凝胶合成的亚稳前驱相形成的硼酸铝，以及这些相是否也具有莫来石型结构。主要的焦点将给予理解硼酸铝的结晶机制，有或没有形成中间亚稳相。考虑到可能的固溶体，这些研究应导致对二元体系相图的修订。根据不同的合成路线（硝酸盐分解、固相反应、水热合成），采用高温x射线和中子衍射方法、核磁共振、红外光谱和拉曼光谱以及热分析对粉末进行表征。化学成分将通过伽马激活和ICP-OES分析确定。

Whiskers of mullite-type aluminum borates, especially Al18B4O33 (A9B2) and Al4B2O9 (A2B) are used as reinforcement in aluminum alloys due to their highly refractory, elastic and mechanical properties. The main objective of this proposal is the investigation of the formation and the crystal-chemical characterization of mullite-like phases in the binary system Al2O3 – B2O3 with an emphasis on Al4B2O9 and Al18B4O33 (Al5BO9) compounds. In this context, the question which factors (chemical composition, reaction route, temperature) essentially control the formation of mullite-type compounds at ambient pressure and hydrous or anhydrous environments is of special interest. Another central point of the study is the understanding of aluminum borate formation from their metastable precursor phases obtained by sol-gel syntheses and whether these phases have also mullite-type structures. A main focus will be given to the understanding of the crystallization mechanisms of the aluminum borates with or without formation of intermediate metastable phases. The investigations should lead to a revised phase diagram of the binary system, considering possible solid solutions. Based on various synthesis routes (nitrate decomposition, solid-state reaction, hydrothermal synthesis) the powders will be characterized by HT X-ray and neutron diffraction methods, NMR, HT-IR and HT-Raman spectroscopy, and thermal analyses. The chemical compositions will be determined by prompt gamma activation and ICP-OES analyses.