COMBUSTION OF NANOENERGETIC MATERIALS:

A HEAT CONDUCTION PERSPECTIVE

Doctoral Defense of

Murali Gopal Muraleedharan

Tuesday, October 2nd 2018, 2:30 – 4:30 pm

Venue: Montgomery Knight 317

Abstract

     Metal-based composite energetic materials have substantially high volumetric energy density when compared with monomolecular compounds such as trinitrotoluene (TNT). Micron-sized metal particles have been routinely used for energetic applications since the 1950’s. They, however, suffer from several drawbacks such as high ignition temperatures, agglomeration, and low reaction rates, resulting in low energy release rates. Nanoparticles exhibit beneficial physicochemical properties compared to their micron-scale counterparts for combustion applications. Due to the large SSA, they also offer tailorable surface properties that have the potential to allow precision control of thermal transport and chemical kinetics. Hence, during the mid-1990’s, widespread replacement of microparticles with nanoparticles created a new class of energetic materials called nanoenergetic materials.

Among the different candidate metals, aluminum is desired because of its abundance, high oxidation enthalpy, low cost of extraction, and for its relatively safe combustion products. This study provides a heat conduction perspective to combustion wave propagation in nano-energetic materials by using nano-aluminum – water system as an example system. A fundamental treatment of heat transport in nanoparticles and interfaces is carried out. Firstly, ab initio and atomistic scale simulations were performed to investigate the nanoscopic nature of heat transport in bulk and nanosized aluminum and aluminum oxide, as well as at the interface of these materials. Atomistically informed macroscale modeling techniques were then employed to treat heat transport in mixtures of nanoparticles in liquid oxidizer to study combustion wave propagation. The key findings of this research are summarized herein.

As the first step, a detailed analysis of phonon transport properties in aluminum (Al) and aluminum oxide (Al2O3) has been performed via lattice dynamics (LD) using input from density functional theory (DFT) calculations. DFT-LD methods reproduce the transverse and longitudinal phonon branches in Al and Al2O3 along the edges of Brillouin zone accurately. Furthermore, temperature dependent phonon thermal conductivity (TC) of Al and Al2O3 are also evaluated by solving the Boltzmann transport equation (BTE) under the relaxation time approximation (RTA), and the thermal conductivity accumulation functions were also evaluated. Spectral distribution of TC was also analyzed to assess the possibility of engineering phonon transport properties. These studies provide a fundamental understanding of phonon frequencies and their contribution in pristine bulk Al and Al2O3 crystals.

Building on this understanding of thermal transport in bulk Al and Al2O3, the study was extended to understand heat transport across Al/Al2O3 interfaces. The thermal interfacial conductance (G) of the aluminum (Al)-aluminum oxide (α-Al2O3) interface along the crystal directions (111) Al || (0001) Al2O3 was accurately predicted. Two fundamentally different formalisms were used to make these predictions in the temperature range 50-1800 K: interfacial conductance modal analysis (ICMA) and the atomistic green function (AGF) method. ICMA formalism is based on phonon correlation theory whereas AGF is based on phonon gas model (PGM). The study reveals the fundamental flaws in the PGM when describing the interfacial heat flux, and its exclusion of full anharmoncity of vibrational modes.

Subsequently, to assess the role of dynamic heat transport mechanisms in nano-suspensions, a rigorous study of the effect of dynamic mechanisms of thermal conductivity enhancement was conducted. An alternative explanation to the unusually high thermal conductivity of nano-suspensions obtained using Green-Kubo relations is provided. This study questions the traditional nanofluid theory that substantiates the presence of nanolayers and Brownian motion in enhancing thermal conductivity of nanoparticle suspensions.

Finally, building on the knowledge of nanoscale heat transport properties, a heat conduction perspective to flame propagation in nanoenergetic materials is developed. A detailed numerical analysis of flame propagation in nano-aluminum (nAl) - water (H2O) mixtures is performed. Considering a multi-zone framework, the nonlinear energy equation is solved iteratively using the Gauss-Seidel method. Thermal conductivity of nanoparticles is modeled using thermal conductivities of aluminum and oxide layer, as well as interfacial conductance. The effective thermal conductivity of the mixture is modeled using the Maxwell-Eucken-Bruggeman model as a function of temperature, spatial coordinate, and local mixture composition. Sensitivity of rb to changes in thermal conductivities of aluminum (kAl) and aluminum oxide (kAl2O3), and interface conductance (G) is also studied for various particle sizes in the nanometer range. This study gave a heat conduction perspective to combustion wave propagation in nanoenergetic materials, providing a solid foundation to their bottom-up rational design.

Committee members:

Prof. Vigor Yang, Professor, AE, Georgia Tech (adviser)

Prof. Asegun Henry, Associate Professor, ME, MIT (co-adviser)

Prof. G. P. Peterson, President of Georgia Tech and Professor, ME, Georgia Tech

Prof. Jerry Seitzman, Professor, AE, Georgia Tech

Prof. Julian Rimoli, Associate Professor, AE, Georgia Tech