Molecular dynamics study of nano-size silica melting by high heat flux

The melting and softening behavior of nano-size silica (SiO2) was studied using the molecular dynamics method. Energy supply was simulated by considering radiative heat transfer. The potential function of silica includes three-body interaction as well as two-body interaction. Initially, thermal treatment for making the amorphous state was performed for both thin film-type and sphere-type nano-size specimens. Then, pre-determined radiative energy, estimated by the spectroscopic concept of electromagnetic waves which are reflecting and refracting in the nano-size material, was applied to the specimen continuously. Depending on the temperature rise, each specimen exhibited softening or melting accompanied by dissociation of silicon oxide molecules. A change in the atomic structure was mainly recognized in the Si-O-Si bonding angle, which was effectively assessed using a novel parameter named the “B parameter”. By using this “B parameter”, both the strained state of SiO4 tetrahedra and their interconnection become obvious simultaneously. Regardless of the specimen shape, the coordination number and the “B parameter” have a certain relationship depending on the characteristic atomic-scale local structure of silica, such as SiO4, SiO3, or SiO2. It was found that a wide distribution of the “B parameter” for the Si-O-Si angle and increase in the distribution for the O-Si-O angle together indicate melting or softening of the nano-size silica.