Synthesis and Characterization of reinforced Ni-Al Dual Functional Energetic Structural Materials, Transition State

Abstract

Introduction INARY energetic materials are of significant interest to the energetic materials community, because of 1) their capability to release substantial amount of heat during a chemical reaction and 2) their relative insensitivity. Intermetallic or metal-metal oxide composites are an example of binary energetic materials that has received substantial attention in the recent years. Even more interesting are mixtures consisting of binary energetic materials as one component, which in addition exhibit strength characteristics, and appropriate reinforcements, as another component. Mixtures of this type provide dual functionality of structural strength along with energetic properties. Among these, micron, submicron, and nanostructured composites have been the object of thorough investigation both in the fields of theoretical modeling, first principles simulation and experimentation. While single-phase materials are sufficiently straightforward to be described by continuum and molecular dynamics models, multi-phase composites pose a problem due to their inherent complexity and due to the lack of sufficient experimental evidence necessary for calibrating the material constants in the models. With these considerations in mind, this paper is part of a systematic approach to study multi-phased multi-functional (MF) structural energetic (SE) composites, with a specific system selected for investigation. The material considered is a mixture of nickel, aluminum, binders and reinforcements. This is not an alloy or a compound, but a powder mixture with porosity present. In the present work we first discuss a procedure for synthesizing bulk nanosolids starting from Ni and Al submicron and nanosized powders, epoxy and carbon nanotubes. The fabrication is consolidation from nanocrystaline powders achieved by cold isostatic pressing after initial mixing into an epoxy matrix. Among the parameters investigated are porosity, grain size, and choice of binder, whose effect on strength and energetic characteristics is analyzed. The guidelines for obtaining a successful multifunctional structural energetic material are based on the failure criteria of the fabricated mixture. There are two essential requirements for such materials. First, when strength is desired, reaction should not initiate. Second, the material should not fail (or crack) due to dynamic loading. While both criteria are important, each of them represents a considerable research on its own. Thus, this paper concentrates on the first, and the second is presented in a separate work. A successful synthesis technique benefits to a great extent by a thorough characterization of the material. This is achieved by looking into the constitutive relationship of the individual components and the mixture itself, and by considering the chemical reaction of nickel, aluminum, binders and carbon nanotubes, and more specifically, its transition state (TS). In pursuit of formulating the constitutive relationships, a first step is the determination of the thermodynamically complete equation of state P=P(ρ,T) for the selected intermetallic mixture of nickel and aluminum, without the binder, for pressures up to 300GPa and temperatures up to 1000K. In another paper, Lu and Hanagud carried out calculations for the static-lattice EOS in the framework of the density functional theory (DFT), and for the phonon modes using the density functional perturbation theory (DFPT). First, they obtained the EOS for each species based on ab initio prediction of the electron ground state and thermal excitations. Then, they obtained the EOS for the mixture by first continuum mixture theories and second by a super cell approach with first principle methods. Lu and Hanagud observed good comparison of the theoretically derived