Silicon dioxide (SiO2) is an important compound in natural and industrial applications as a result of its thermal and chemical stability, mechanical strength and inertness, especially when compared to other silicates. Silicon dioxide has a simple crystal structure and can occur in multiple phases. Transitions between different phases can result in a number of important physical and chemical changes, including changes in density and hardness, increased porosity, and changes in electrical, optical, and mechanical properties. The different phases of SiO2 are end-member phases (amorphous and crystalline) and inter-phases, including glasses, zeolites, and gels.
At room temperature and pressure, SiO2 exists as quartz, one of its crystalline phases, but at high temperatures, crystalline SiO2 can melt and form silicate glasses. Glasses are disordered solids, with a structure and bonding between atoms that is similar to a liquid but retains its shape when cooled rapidly. The fragile and low-density nature of glasses has meant they have never been as useful as quartz or other SiO2 crystalline compounds; however, recent research is working to unlock the potential of SiO2 glasses enabled by advanced fabrication techniques.
One approach to unlocking the potential of SiO2 glasses involves crystallization mechanisms, or understanding the process by which amorphous or disordered materials spontaneously convert back to a crystalline form. Crystallization of SiO2 glasses can cause changes in their properties such as increased density, increased hardness, and altered electrical and optical behavior. By understanding how to control and activate the crystallization mechanisms, we can begin to look for ways to optimize the physical and chemical properties of SiO2 glasses, enabling new applications in a wide range of industries.
One way to control SiO2 crystallization is to use external stimuli, such as heat or light, to encourage crystallization, or even to transform existing crystals to other phases. The nature of the external stimulus is typically determined by the material and the end use application, with some applications requiring more conventional heating methods than others. For example, laser treatments at very high temperatures, such as ultrafast pulsed laser composites are often used to transform SiO2 glasses into higher-density crystalline structures.
In addition to external stimuli, SiO2 crystallization can also be influenced by a variety of internal pathways, such as dissolution-precipitation, grain growth, and heterogeneous nucleation, which work together to drive crystallization. For example, during dissolution-precipitation, SiO2 molecules in an aqueous solution break up into their constituent ions. These ions then condense and assemble into crystals, driven by the demands of thermodynamic equilibrium. Similarly, grain growth is a process by which SiO2 molecules or crystals slowly aggregate or grow together over time. This method increases crystallinity, but also increases grain size and therefore density. Finally, heterogeneous nucleation relies on surface-induced crystal growth, where particles adhering to the surface of a material catalyze the crystallization of the bulk material.
In addition to understanding crystallization mechanisms, researchers have also begun to explore ways to direct and optimize the structure of SiO2 crystals. This research combines strategies for tuning the structure of SiO2 crystals with the observation of crystallization mechanisms to make novel materials with properties tailored for specific applications. For example, researchers have demonstrated the fabrication of SiO2-based materials with improved bandgap, dielectric properties and conductivity by controlling the size and shape of the crystal structure.
Overall, the conversion of SiO2 between its amorphous and crystalline phases has emerged as a powerful tool to engineer materials with desired characteristics, which can enable new applications in various industries. While a detailed understanding of the crystallization mechanisms and structure of SiO2 crystals is still required, research has made tremendous strides in recent years in this area, with promising results prompting further exploration and development.
