| AVS 64th International Symposium & Exhibition | |
| Surface Science Division | Wednesday Sessions |
| Session SS-WeM |
| Session: | Deposition and Growth at Surfaces |
| Presenter: | Koroush Shirvan, MIT |
| Authors: | K. Shirvan, MIT E. Forrest, Sandia National Laboratories |
| Correspondent: | Click to Email |
The morphology and microstructure of organic explosive films formed using physical vapor deposition (PVD) processes strongly depend on local deposition conditions, especially the homologous temperature. Given the low melting points of many organic explosives, changes in surface temperature by only a few degrees can result in drastically different explosive morphology and performance. Given the difficulty of accurately measuring surface temperature during energetic material deposition, modeling may provide some insight into local conditions. This work focuses on using a multi-physics computational fluid dynamics tool, STARCCM+, to simulate PVD of pentaerythritol tetranitrate (PETN). The PETN vapor and solid phase are simulated using the volume of fluid method. The deposition fixture, copper cooling block, and assembly rotation are incorporated into the model. Implicit time-dependent simulations in 2D and 3D are performed to derive insight into the governing physics for growth of thin PETN films via PVD.
PETN is deposited at a rate of 0.8 µm/min at 142.9 oC on a 500 µm thick silicon wafer with an initial temperature of 22oC. The sublimation of PETN on the wafers is calculated to occur at a heat transfer coefficient (HTC) of 400 W/m2-K. The HTC for sublimation proved to be the most sensitive parameter in determining the local surface temperature during deposition. Previous experimental studies found noticeable microstructural changes with the use of fused silica wafers of equivalent thickness in place of silicon during the PETN deposition. Effects were likely due to the change in local deposition temperature from the reduction in substrate thermal conductivity. This work confirms the effect, showing that use of fused silica substrates reduces initial wafer cool down and results in ~ 10 oC difference in the surface temperature when depositing a 500 µm thick PETN film. It was also found that the deposition surface temperature is insensitive to the cooling power of the copper block due to the very large heat capacity and thermal conductivity relative to the heat input from the PVD process. Future work will involve incorporation of local stress fields during PETN deposition and cooling.