Ignition of Propellants Through Nanostructured Materials

Open Access
Author:
Loomis, Matthew Paul
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Richard A Yetter, Thesis Advisor
  • Dr Jongguen Lee, Thesis Advisor
Keywords:
  • Carbon Nanotubes
  • graphene
  • fuel additives
  • photo-ignition
Abstract:
In the search for next generation fuels, research includes the use of additives in existing fuels as a method to tailor properties and enhance combustion characteristics. Carbon nanostructured materials, such as functionalized graphene sheets (FGS) and single-walled carbon nanotubes (SWCNTs), with high surface area and transport properties could facilitate selective enhancement of fuels through doping or catalytic action. Additionally, high optical adsorption provides the possibility of lower energy, higher reliability ignition methods over point ignition found in most combustion systems. FGS and SWCNTs are examined both in dry state and in solution. Dry material characterization is performed via scanning and transmission electron microscopy, energy dispersive spectroscopy, thermogravimetric analysis, surface area measure, and absorptance evaluation. Solutions are examined for dispersability, absorptance, droplet burning characteristics, and photo-sensitivity. Optical absorptance testing for both materials in solution revealed blackbody behavior throughout the visible regime with no particular wavelength sensitivity. This poses opportunity for selective heating of the nanostructured materials by choosing an incident wavelength outside of the absorptance capability of the base fuel. In solution, SWCNTs exhibit agglomeration and structure alteration with light exposure, making bulk characteristics difficult to determine in applications requiring moderately strong light. Droplet burning tests of nitromethane and Jet-A solutions at 500°C showed a two stage burning rate behavior in nitromethane based mixtures when ambient temperature was near the FGS sample’s onset of oxidation temperature (determined by TGA/DSC). Overall burning rate enhancement of nominally 30% was observed for nitromethane dispersed with small quantities of FGS that were highly functionalized (carbon to oxygen ratio of 19). This trend is consistent with the higher temperature oxidation onset of less functionalized (more reduced) FGS (carbon to oxygen ratio of 50). For example, at 600°C, a two stage burning profile was found for nitromethane dispersed with FGS (carbon to oxygen ratio of 50). The first stage burning rate was reduced from the pure nitromethane case by 49%, while the second stage was enhanced by 17%. As ambient temperature is increased the two stage burning appears to be no longer evident. SWCNTs exhibited little effect on solution burning rates, though there was a great degree of variance, particularly in the Jet-A sample (35% standard deviation). Laser induced photo-ignition attempts of solutions proved that inclusion of carbon nanostructured materials can enhance energy absorption, and offers validation of capability to selectively heat nanostructured materials in solution. This is the first such examination of FGS for photo-ignition behavior in solution. Repeatable internal nucleation was observed in both SWCNTs and FGS in nitromethane; the reaction decayed with each successive light pulse in SWCNTs. This is likely indicative of high energy absorption, and subsequent vaporization of surrounding fuel. The fact that no decay is evident for FGS samples indicates that nucleation is not likely a result of chemical reaction, suggesting decay in SWCNTs can be attributed to agglomeration with exposure as previously observed. This provides clues as to the source of nanostructure photo-ignition in air. Materials characterization provides thermal oxidation temperatures, relative structure, and material content of each sample to facilitate the analysis of the properties exhibited by bulk samples. FGS is found to possess many contaminants, which may influence bulk properties, and has varying agglomeration tendency between samples. SWCNTs vary in micron scale structure and composition based upon production method. Acid washes, used in production of FGS and purification of SWCNTs for later functionalization is proven to introduce significant quantities of contaminants (residue of oxidizers). Particularly in SWCNTs, differing ambient exposure conditions over the lifetime of the material is believed to greatly influence properties. This variability poses an inhibiting challenge to large scale use of carbon nanostructured materials, as material behavior cannot be considered reproducible from batch to batch.