Date of Award

2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Committee Chair

Robert A. Frederick

Committee Member

James K. Baird

Committee Member

Jason T. Cassibry

Committee Member

George Nelson

Committee Member

James J. Swain

Subject(s)

Solid propellants., Electrolysis.

Abstract

Some electric solid propellants repeatedly ignite and extinguish through application and removal of electrical energy. Responses to electrical factors and electrode configurations were evaluated at atmospheric conditions using the Design of Experiment methodology for development of valid correlations. Experiment results were compared with an existing electrolytic theory describing electrochemical reactions occurring proportional to current. The high performance electrical propellant 501a formulation containing a hydroxylammonium nitrate ionic liquid and polyvinyl alcohol polymer binder was used in all experiments. External flame impingement produced charring without energetic ignition, no self-sustained burning, and extinguishment upon flame removal. Application and removal of electrical energy through stainless steel electrodes resulted in propellant ignition and extinguishment. Burning rate as a function of current density was determined using a power fitted regression equation having a current density exponent of 0.958. The nearly direct proportionality supports the electrolytic theory. The measured mass loss was 5-50 times the mass loss predicted by the electrolytic theory suggesting a significant thermochemical component exists. For experiments having equal electrode surface areas, preferential anodic burning occurred as more chemically reactive oxidative species are predicted by the electrolytic theory. For most experiments having unequal surface areas, burning occurred at the smaller electrode surface area regardless of polarity due to greater current density and ohmic heating predicted by the electrolytic theory. Hydroxylammonium and nitrate diffusion coefficients determined for platinum electrodes were from 3.11x10-7 to 3.62x10-7 cm2/s and 2.67x10-7 to 3.45x10-7 cm2/s, respectively. Ion mobility and drift velocity relate to diffusion coefficient thereby describing ionic transport characteristics affecting current and ultimately burning rate. The approximated electrical conductivity range for platinum electrodes was 3.8-22.0 S/m for frequencies of 0.1-10 kHz. Conductivity increased with frequency suggesting potential burning rate control through frequency modulation. Electrical response was a highly localized effect limited to the propellant/electrode interface. The electrode where burning occurred exhibited electrostatic discharge behavior as evidenced in the video, current, and voltage data suggesting the polymer breakdown voltage was achieved. Dielectric breakdown was proposed as an additional theoretical component connecting the experiment results with the electrolytic theory and augmenting the electrochemical and thermochemical processes for the overall combustion mechanism.

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