Puneesh Puri

Flame Propagation of Nanoaluminum-Water Mixtures

A theoretical investigation on the combustion behavior of nano-aluminum (nAl) and liquid water is conducted. Linear burning rates of nAl and liquid water as a function of pressure and equivalence ratio are reported. A 38-nm diameter aluminum particle with a 3.1-nm thick oxide layer has been chosen for the present study. The simulation is carried out for a pressure range of 0.1 - 3.65 MPa. The equivalence ratio considered in the present study varies from 0.5 to 1.25. The model predicts an increase in linear burning rate from 0.7 to 8.1 cm/s at a pressure of 3.65 MPa, as the equivalence ratio is increased from 0.5 to 1.25. The predicted burning rates are in good agreement with the experimentally measured values. The flame thickness predicted by the model is 108 microns.

Thermo-Mechanical Behavior of Nano Aluminum Particles with Oxide Layers

Molecular dynamics simulations are performed using micro canonical (NVE) ensembles to analyze the thermo-mechanical behavior of nano-sized aluminum particles with oxide layers. The Streitz-Mintmire potential is selected for the present study because of its ability to capture the size dependence of thermodynamic properties and to simulate the interactions between aluminum and oxygen atoms. The simulation commences with a spherical aluminum particle in an FCC crystal structure covered with a shell of oxide layer. The whole system is first equilibrated and then heated continuously. A combination of structural and thermodynamic parameters, including the potential energy, Lindemann index, translationalorder parameter, and radial-distribution functions, are used to characterize the melting process. Molecular dynamics simulations provide exclusive insight into the atomistic mechanisms involved for smaller nanoparticles which can’t be explained by continuum laws. Nano-sized aluminum particles, along with the diffusion of aluminum cations through the oxide layer, exhibit thermo-mechanical characteristics distinct from their micron-sized counterparts. The melting temperature of the oxide layer is considerably lower than its counterpart for bulk alumina. The oxide thickness exerts a weak influence on the particle melting temperature for the size range (up to 8 nm) considered in the present work. The ensuing phase changes in the core and shell, as well as diffusion of aluminum cations in the oxide layer, are explored in depth. The study is an important milestone towards the development of a multi-scale theory for the ignition and combustion of particulate aluminum.

Molecular Dynamics Study of Melting of Nano Aluminum Particles

Molecular dynamics simulations are performed using isobaric-isoenthalpic (NPH) ensembles to predict the melting of nano-sized aluminum particles in the range of 2-9 nm and to investigate the effect of charge development on the melting. Five different potential functions (i.e., the Lennard-Jones, Glue, Embedded Atom, Streitz-Mintmire, and SuttonChen potentials are employed, and the results are compared using the size dependence of melting phenomenon as a benchmark. A combination of structural and thermodynamic parameters such as the potential energy, Lindemann index, translational-order parameter, and radial-distribution functions are employed to characterize the melting process. Both bulk and particle melting are considered. The former is characterized by a sharp increase in structural and thermodynamic properties, whereas the latter involves surface pre-melting. The effect of surface charges on the melting point is found to be insignificant for nano-sized aluminum particles. The melting point of a nano particle increases monotonically with increasing size and approaches the bulk melting point at approximately 8 nm. Two-body potentials like the Lennard-Jones potential fail to capture the thermodynamic melting phenomenon. The Sutton-Chen potential, fitted to match structural properties, also fails to capture the size dependence of the particle melting point. Many-body potentials like the Glue and Streitz-Mintmire potentials result in accurate melting temperature as a function of particle size.

Thermo-Mechanical Behavior of Nickel-Coated Nano- Aluminum Particles

Thermo-mechanical behavior of nickel coated nano-aluminum particles, in the size range of 4-16 nm, is studied using molecular dynamics simulations. The analysis is carried out in isothermal-isobaric and isochoric-isoenergetic ensembles using an embedded atom method. Emphasis is laid on analyzing the melting of Al core, diffusion of Al and Ni atoms, and intermetallic reactions for different core sizes and shell thicknesses. The melting point of the Al core is found to exceed the heterogeneous melting point of pure nAl particle and approach the homogenous melting point of Al, irrespective of the shell thickness. The diffusion of Al atoms, after melting, is accompanied by self-sustaining inter-metallic reactions between Al and Ni atoms. The advent of these reactions is, to some extent, delayed for a thicker shell and expedited for a larger core. The amount of heat release due to the reactions increases as the Al or Ni atomic fraction increases to 0.5. Adiabatic reaction temperatures close to 2300 K are predicted for near-equiatomic particles with a 1 nm thick Ni shell. The simulation results indicate the possibility of ignition of these particles in an inert environment and also help in explaining their reduced ignition temperatures.