Collaborative Research: Bulk synthesis of stishovite near ambient pressure and temperature

非技术描述：该项目的目标是在热液条件下合成辉石，这是一种SiO2（二氧化硅）的形式，也是人类已知的最坚硬的物质之一。目前，这种珍贵形式的二氧化硅只能在极高的压力下（大约是大气压力的10万倍）以技术上非常困难和昂贵的方法获得，而且产量很小。这个项目的新方法模拟地质条件，从大量的水溶液中生长晶体。该过程在接近环境压力和温度条件下进行，因此成本低廉且可扩展。这促进了硅辉石作为磨料，切削工具和其他应用中的硬陶瓷材料的新应用。技术细节：硅辉石是二氧化硅（SiO2）的一种多晶型，与立方氮化硼一起，硬度仅次于金刚石。此外，在已知的最硬的材料中，它是唯一的氧化物，因此自然地抗氧化降解。尽管具有这些优异的性能，但硅辉石并没有被用作工程材料，因为它只能在高于8gpa的压力下合成。虽然技术上可行，但高压路线不可扩展且相当昂贵。在本项目中，开发了一种动力学控制的水热合成辉石的方法。该过程发生在环境压力和温度条件下，类似于石英晶体的工业生产。与工业石英生长的不同之处在于，使用的不是石英种子晶体，而是针长石种子晶体。高压灭菌系统中的温度梯度允许生长室中的硅酸盐溶液过饱和，并从过饱和溶液中动态控制辉石种子晶体的生长。实验以计算为指导，探索和预测最佳合成条件。该研究是在近环境条件下从溶液中合成高压相的开创性方法，对高压相的合成具有变革性的影响。该项目旨在培养高压和水热合成技术以及计算材料化学方面的研究生。研究结果是研究生水平课程“固态化学”的一部分。

NON-TECHNICAL DESCRIPTION: The project targets synthesis of stishovite, a form of SiO2 (silica) and one of the hardest materials known to mankind at hydrothermal conditions. Presently, this precious form of silica can only be attained at extremely high pressure (ca. 100,000 times atmospheric pressure) in a technically very difficult and expensive process that yields only small quantities. The new approach of this project mimics geological conditions, and grows the crystals from aqueous solutions in bulk quantities. The process occurs close to ambient pressure and temperature conditions, and consequently, will be inexpensive and scalable. This facilitates new applications of stishovite as an abrasive, in cutting tools, and as a hard ceramic material in other applications. TECHNICAL DETAILS: Stishovite, a polymorph of silica (SiO2), is - together with cubic boron nitride - only second in hardness to diamond. Moreover, among the hardest materials known it is the only oxide, thus naturally resistant against degradation through oxidation. Despite these outstanding properties, stishovite is not used as an engineering material, because it can be synthesized only at pressures above 8 GPa. Although technically feasible, the high-pressure route is not scalable and rather expensive. In this project, a kinetically controlled hydrothermal synthesis approach for stishovite is being developed. The process occurs near ambient pressure and temperature conditions and is similar to the industrial production of quartz crystals. The difference to industrial quartz growth is that instead of quartz seed crystals, stishovite seed crystals are employed. The temperature gradient in the autoclave system allows for the supersaturation of the silicate solution in the growth chamber and the kinetically controlled growth of the stishovite seed crystals from the supersaturated solution. The experiments are guided by computations that explore and predict optimal synthesis conditions. The research is a seminal approach to synthesize high pressure phases near ambient conditions from solution which suggests a transformational impact on the synthesis of high-pressure phases. The project is training graduate students in high-pressure and hydrothermal synthesis techniques and computational materials chemistry. The results from the research are part of the graduate level course "Solid State Chemistry".