Ralph Schefflan

Formation of Aluminum Nanoparticles upon Condensation from Vapor Phase for Energetic Applications

A mathematical model of the nanoparticles formation process from deposition from the vapor phase process was developed and applied to the manufacture of alumina-coated aluminum nanoparticles. This process involves conversion of gaseous aluminum in the presence of helium carrier gas to solid aluminum nanoparticles. These activities effectively prepare the aluminum for reaction with oxygen gas to create an alumina coating in the remainder of the process. The basis of the calculations is the General Dynamic Equation for aerosols, which was formulated as an explicit numerical equation. The equation is solved over a user specified surface with particle volume (equivalent to particle diameter) and reactor holding time as the independent variables. The solution produces the number distribution function of the nanoparticles over the solution space. After all of the gaseous aluminum has solidified, a moment equation is employed to calculate the number of particles in each of the size distribution ranges. The mathematical model is useful to study the trends on the dependence of the nanoparticle size distribution on the operating parameters such as pressure and temperature profile in the reactor. A number of case studies are included to demonstrate the utility of the mathematical model.

Mathematical Model for a Fed-Batch Crystallization Process for Energetic Crystals to Achieve Targeted Size Distributions

In the manufacture of energetic materials including RDX, HMX, CL-20, it is a challenge to obtain the targeted size distribution. Generally blending is costly and regrinding of the crystals increases the defect densities to give rise to increased sensitivity. The ability to predict apriori the size distribution of various energetic crystalline materials upon recrystallization as a function of the operating conditions, allows the optimization of the process parameters to achieve the desired size distribution without having to regrind or blend different size populations. Here a comprehensive mathematical model of the fed-batch crystallization process consisting of two groups of equations is presented. These include first the dynamic material and energy equations, and second, a population balance model for the prediction of the number density of crystals as a function of time and size as functions of the nucleation and growth kinetics for the particles. A numerical solution to the general problem, which involves the alternate solution of the equations at each time step, was developed considering that the reactor volume changes with each time step. Typical results are presented to demonstrate the utility of the mathematical model of the recrystallization process.