Nanoengineering and Synthesis of Metal-Based Materials for Enhanced Energetic Performance
Restricted (Penn State Only)
- Author:
- Agarwal, Prawal
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 27, 2023
- Committee Members:
- Robert Rioux, Professor in Charge/Director of Graduate Studies
Robert Rioux, Chair & Dissertation Advisor
Ali Borhan, Major Field Member
Stephanie Velegol, Major Field Member
Richard A Yetter, Special Member
Raymond Schaak, Outside Unit & Field Member - Keywords:
- nanoenergetics
nonthermal plasma
PECVD
self-propagating high-temperature synthesis reactions
thermal oxidation
hydrogen plasma
solid-state reactions
surface chemistry
material synthesis
material characterization
thermite reactions
intermetallic reactions
metal nanoparticles
boron
magnesium
aluminum - Abstract:
- Metal-based energetic materials are a pathway for clean and sustainable energy applications because of their high energy densities and ability to oxidize readily and release large amounts of heat. They are sustainable sources of green combustion and can easily be stored and transported because they are chemically stable solids compared to hydrocarbon fuels. These merits lead to their applications in volume-limited propulsion, solid fuels, explosives, space exploration, self-destructing energetic chips, electrochemical energy storage, and hydrogen generation/storage. To be used effectively in these applications, nanometer-sized particles are beneficial because of the fast ignition, more complete combustion, and enhanced heat transfer and reaction rates due to the larger specific surface area available for the reaction. However, some roadblocks exist in harnessing the benefits of metal nanoparticles (NPs). The surfaces of the metal NPs are highly reactive. Hence, there is a formation of a native oxide layer on their surface. This native oxide occupies a significant fraction of mass in the sample that does not contribute to the oxidative heat release of the sample and also acts as a diffusion barrier on the metal NP surface that delays the contact of oxidizer with metal in the core and thus restricts the combustion process. Various methods are available in the literature to minimize the inhibiting effects of the native oxide layer on oxidative heat release. These methods are based on surface functionalization using solution-based approaches, reactive milling, coating reactive metals on other metals, and high-temperature sintering to synthesize metal borides. These methods helped us to determine how to approach solving the problem of the native oxide layer and investigating possible routes to improve the oxidative energy release from metal-based nanomaterials. We can either reduce native oxide or convert them into a reactive component such that they contribute to the oxidative heat release. In our work, we used nonthermal plasma processing and intermetallic chemistry based on self-propagating high-temperature synthesis (SHS) reactions. Nonthermal plasma is a low-temperature operation that triggers selective and rapid reactions on the surface. Due to low-temperature operation, this process uses energy efficiently. We used hydrogen plasma to generate reactive hydrogen species that can reduce native oxides of metal at room temperature. We also used plasma-enhanced chemical vapor deposition (PECVD) through argon plasma to deposit reactive nanofilms on the metal NPs surfaces to enhance the energy performance during oxidation and to passivate their surfaces to inhibit oxide growth in extreme temperature and humid conditions during storage. Using SHS, we synthesize solid solutions of metals with long storage life because of their thermal stability and with enhanced oxidative heat release due to the reduction of less reactive metal oxide with a more reactive metal. The process temperature is selected so that there is no sintering and agglomeration of NPs during the process. Both the above processes are dry-phase process and reduces the contamination of metals. Using nonthermal plasma processing, we enhanced the oxidative heat release from boron (B) NPs by developing an in-situ process in which hydrogen plasma reduces B oxide and PECVD coats the surface with a thin fluorocarbon film to stop reoxidation when NPs are exposed to the environment. PECVD is used to deposit reactive nanofilms of perfluorodecalin and oleic acid on the surface of aluminum (Al) NPs, which lead to superior energy performance of Al NPs. The plasma-based oleic acid nanofilms performed better than graphene oxide. Hydrogen plasma doped the Al NPs with hydrides such that during oxidation, channels are formed on the surface due to gas transport, leading to better oxidation of metal in the core. Boron is a desirable candidate for energetic applications with the highest gravimetric and volumetric energy density of 58 kJ/g and 140 kJ/mL. The energy from B can be exploited by the addition of reactive metals with reasonable gravimetric energy density, such as Al and magnesium (Mg), in the form of a mechanical mixture or solid solutions, which can undergo an exothermic redox reaction to reduce native oxide and enrich metallic B. We used SHS and mechanical mixing to form Mg/B solid solutions and energy-optimized Al/B systems to synthesize energetic materials. We also combined plasma chemistry and intermetallic chemistry to investigate the integrated effects of these processes on B energetics. Hence, we reduced native oxides and/or converted them into energetic components via nanoengineering by fabricating core-shell architectures and synthesizing energetic nanomaterials with enhanced energy performance and extended storage stability.