Subscriber access provided by Kaohsiung Medical University
C: Energy Conversion and Storage; Energy and Charge Transport
Ignition of Nano-Scale Titanium/Potassium Perchlorate Pyrotechnic Powder: Reaction Mechanism Study Miles C. Rehwoldt, Yong Yang, Haiyang Wang, Scott Holdren, and Michael R. Zachariah J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03164 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Ignition of Nano-scale Titanium/Potassium Perchlorate Pyrotechnic Powder: Reaction Mechanism Study Miles C. Rehwoldt1, Yong Yang1, Haiyang Wang1, Scott Holdren1, Michael R. Zachariah 1,* 1
Department of Chemistry and Biochemistry and Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States * Corresponding Author Email:
[email protected] Abstract The reaction mechanism and ignition characteristics of the pyrotechnic composite of titanium nanoparticles and micron sized potassium perchlorate was investigated under rapid heating conditions (~5× 10 K/s) by Temperature Jump (T-Jump) Time of Flight Mass Spectrometry (TOFMS). X-ray Photoelectron Spectroscopy (XPS) surface analysis and Transmission Electron Microscopy (TEM) characterization of titanium nanoparticles show a reactive oxide layer (~6 nm) composed of amorphous TiO2 and roughly 20% crystalline TiN and TiON. T-Jump and thermogravimetric analysis reveals the oxide layer to be responsible for catalysis of oxygen release from KClO4 resulting in ignition temperatures as low as 720 K in atmospheric pressure argon. Fast and slow in situ heating TEM corroborate the findings of oxygen atmosphere ignition characteristics which illustrate KClO4 melting and coating of titanium nanoparticles immediately before oxidizer decomposition and titanium oxidation. Unlike aluminum which has been shown to have a rapid loss of surface area prior to combustion as a result of sintering, this was not observed for Ti which retained its high surface area. A combination of a reactive shell and the preservation of titanium nanostructure under rapid heating may lead to enhanced oxygen diffusion and increased potential for transient energy release.
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 26
1. Introduction Nano-scale metal particles such as aluminum, boron, zirconium, magnesium, and titanium have been the focus of recent research regarding their potential as fuels in energetic composites. The combination of relatively high combustion enthalpies and reduced reaction length scales make nano-scale metalized energetic systems particularly interesting for their potential for transient, high yield energy release.1-7 While the favorable thermodynamics of such systems are well known, their kinetics are relatively slow, and specific reaction mechanics largely unknown. The nature of such reaction mechanics depends heavily on the individual and collective characteristics of the fuel and oxidizer under the rapid heating conditions indicative of combustion.1 Although there have been several mechanistic studies of this type involving aluminum nanoparticles due to its relatively low cost and high energy density, there have been few such studies which have investigated titanium nanoparticles as a viable alternative fuel source.8 As a fuel with strong oxidizers, titanium can boast a higher theoretically normalized combustion enthalpy per unit volume compared
to
aluminum
and
typical
organic
monopropellants
cyclotrimethylenetrinitramine(RDX) and 2,4,6-trinitrotoluene (TNT).1,
2
such
as
Significant differences
in physical and thermal characteristics between aluminum, titanium, and their respective metal oxides make such an investigation enticing when considering their possible consequences on ignition mechanics (Table S1). This paper investigates the reaction mechanism of titanium nanoparticles (nTi) with the commonly used, strong pyrotechnic oxidizer potassium perchlorate (KClO4) under rapid heating conditions.3,
4, 9-11
This class of energetic mixture is commonly used in energetic components
such as thermal igniters and/or mechanical actuators, taking advantage of a superior combustion 2 ACS Paragon Plus Environment
Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
enthalpy compared to more traditional thermites which use metal oxide oxidizers (Figure 1). Titanium/potassium perchlorate mixtures (TKP) were previously studied in a mechanistic manner at slow heating conditions by Sandia National Laboratories as a part of their mission to gain intuition and the “ability to predict and model pyrotechnic ignition thresholds”.12 Our investigation expands upon the findings from the Sandia work by subjecting the nano-scale energetic composite to controlled high heating rates while simultaneously monitoring gas phase species using Time of Flight Mass Spectrometry.
Figure 1: Maximum Combustion Enthalpies (φ=1)
2. Experimental Methods 2.1. Materials/ Sample Preparation Titanium nanoparticles (50-80 nm) were purchased from US Research Nanomaterials, Inc. and anatase TiO2 nanoparticles (
