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Effect of Silane Capping on the Dispersion and Combustion Characteristics of Sub-Micron Boron Particles Loaded in Jet A-1 Pawan Kumar Ojha, and Srinibas Karmakar Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02490 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018
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Energy & Fuels
Effect of Silane Capping on the Dispersion and Combustion
Characteristics
of
Sub-Micron
Boron Particles Loaded in Jet A-1 Pawan Kumar Ojha and Srinibas Karmakar* Department of Aerospace Engineering, Indian Institute of Technology Kharagpur, West Bengal, 721302, India *
corresponding author, Email:
[email protected], Phone: +91-3222-283012
ABSTRACT: The present investigation deals with the surface modification of sub-micron boron particles with octadecyltrimethoxysilane (a silane compound) in order to improve the dispersion stability of boron particles in liquid fuel. Characterizations of as-received and silane-coated boron particles in terms of particle size, morphology, surface chemistry, ignition temperature, and oxidation profile have been conducted using typical material characterization methods such as SEM, STEM, XPS and TGA. The results show that the surfaces of as-received boron particles have been successfully functionalized via condensation reaction of hydroxyl function groups (OH) with octadecyltrimethoxysilane (OTMS) molecules. The capping of OTMS on boron surface makes the particle stable against air oxidation. The dispersion stability of OTMS-capped boron in Jet A-1 at particle loading of 1%, 5% and 10% are found out to be 20 hours, 18 hours and 2 hours respectively. Ignition and combustion characteristics of as-received and silanecoated boron particles loaded in Jet A-1 at desired concentrations have been analyzed to understand the effect of silane coating. TGA, true colour flame images and spectroscopic results show that the burning process of OTMS-capped boron is slightly delayed compared to asreceived boron. The droplet diameter regression profiles show smooth regression up to 70-80% of droplet lifetime with some intermittent puffing with disruptions at later stage in both the particle cases. However, the intensity of disruption is stronger in case of OTMS-capped boron because of the formation of more compact shell inside the droplet due to the melting of OTMS layer (particularly towards the end of droplet lifetime). The micrographs of combustion residue
1 ACS Paragon Plus Environment
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reveal that some tiny holes are present on the residue surface in case of as-received boron whereas multiple blow holes are there in case of OTMS-capped boron. A blanket of silicon seems to cover the particle surface which makes them to stick together.
Keywords Boron, Silane compounds, Surface functionalization, Dispersion, Droplet combustion
1. INTRODUCTION Hydrocarbon fuels are in use in air-breathing propulsion systems since many decades 1. However, the volumetric heating value of these fuels is limited. Synthesis of hydrocarbon fuel with higher density and higher energy density than that of currently available petroleum-based fuels has certain limitations. For example, JP-10 (density of around 0.94 g/cc and volumetric heating value of around 39.6 MJ/L) is almost the limit of the readily synthesized liquid hydrocarbon fuels at present 2. Further improvement on density and energetic potential is difficult. Therefore, one of the ways to enhance the volumetric energy content as well as burning behaviour of currently available liquid hydrocarbon fuels is by adding energetic solid metal and metalloid particles
3–9
. Of many available energetic solid additives, elemental boron has drawn
significant attention because of its very high gravimetric and volumetric energy content. This makes it a prime choice as the potential additive for liquid fuels. On gravimetric basis, the energy density of boron is approximately 40% higher than that of hydrocarbon fuels (58 vs. 44 KJ/g), and it is 200% higher on volumetric basis (136 vs. 42 KJ/cm3)
10
. Additionally, the higher
density of boron (around three times) compared to hydrocarbon fuels makes it an obvious choice for volume limited propulsion systems. Furthermore, the rate of heat release from boron combustion increases when the boron particles are in nanometric size range
11,12
. Theoretically,
addition of boron nanoparticles in liquid fuel has many advantages, particularly the enhanced energy release upon combustion; however, one of the major issues lies in achieving long time suspension of boron particles in liquid fuel. The poor dispersion stability of boron particles puts a serious limitation on the practical viability of boron laden liquid fuels. To counter this problem, surface modification of boron particles is required in order to mitigate the particle agglomeration and sedimentation in liquid fuel. 2 ACS Paragon Plus Environment
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Energy & Fuels
The capping of boron surface by organic materials has been tried by some researchers in recent times 13–18. Bellott et al. 13 tried to functionalize boron with bromine (Br) and fluorine (F). Treatment of boron nanoparticles with Br2 and XeF2 in benzene were done to get brominated and fluorinated boron nanoparticles respectively. Pickering et al.
18
presented a simple convenient
route to produce octyloxy-capped boron nanoparticles at room temperature. First, the boron particles were treated with boron tri-bromide (BBr3) to get a bromide-capped intermediate which then reacted with octanol to obtain octyloxy-capped boron. Ziu et al.
16
tried to modify the
surface of boron particle with many amphiphilic ligands. They tested seven ligands with identical chemical structures. However, trioctylphosphine oxide (TOPO) was found out to be the most effective and the boron particles (at loading concentration of 12.7%) remained stable in JP-10 up to 6 weeks. Using high-energy ball milling, Anderson et al. 14,15 tried to modify boron surface by oleic acid and cerium oxide. In the process of ball milling, the boron particles were crushed which enable fresh un-oxidized boron surface to bind with the oleic acid and cerium oxide. Recently, few studies
19–22
were conducted to assess the stability of ionic liquids (ILS) capped
boron. The ILs-capped boron are dispersible in ionic liquids but not in hydrocarbon fuels 21. In a recent study, Du and Li
17
produced silane-capped boron nanoparticles by functionalizing the
surface of boron with silane (hexadecyltrimethoxysilane). They reported that the suspension of silane-capped boron and decalin was stable for 2 months for 1% particle loading. The reported studies on surface modification of boron through different organic materials enable the boron to suspend in liquid fuels for quite long time. It is important to note that the boron particles used in these studies are smaller in size (~ 40 to 50 nm). Inherent advantage of small particles is that they are capable of floating in liquid for longer period than their bigger size counter parts (sub-micron or higher). Most of the boron nanoparticles available commercially are mixture of nano-sized and sub-micron size particles. In fact, a significant portion of the particles are in the sub-micron range. Getting controlled size nanometric (
