Metal additives have been utilized in solid propellants and fuels for some time and have been shown to dramatically increase combustion enthalpies and quality. In addition, these metalized propellants offer increases in the overall energy density of the fuel and increase specific impulse, and they effectively reduce the tank storage volume. In the current state-of-the-art implementation, energetic additives offer a high volumetric enthalpy of combustion, facilitating transportation of more payload per given fuel volume. However, given that the energetic additive sizes are in the micron range and sometimes even in the millimeter range, there are numerous side effects to the combustion process, including ignition delays, slow burn rates, and incomplete combustion of large (micron-sized) metal particles. Furthermore, the stability of liquid-based fuels is also a major concern; conventional liquid fuels may need to be remixed or processed before use, because of rapid settling of the energetic additive particles. New approaches and advances in nanotechnology are being developed to mitigate several of the disadvantages of metal particle additions, which will enable their large-scale implementation as viable secondary energy carriers .
Nanoparticle-laden fuels are known to exhibit significantly different thermophysical properties when compared to the base fuel. When metallic particles approach length scales on the order of nanometers, significant changes in thermophysical properties often occur. At these dimensions, the surface-area-to-volume ratio of the particle increases considerably, and this enables providing a larger contact surface area during the rapid oxidation process . For instance, due to size-dependent properties, energetic materials containing nanoparticles can release more than twice the energy of even the best molecular explosives . Several studies have reported lower melting points and lower heats of fusion for decreasing sizes of metal particles [4–6]. In particular, there are numerous combustion enhancements that result from the addition of ultrafine or nano-aluminum (n-Al) particles to gelled and solid-based propellants. Several investigators [7, 8] have reported enhanced burning rates and reduced ignition delay in solid-based ammonium perchlorate propellants, in a wide array of formations. Based on these developments, research in the relatively new area of nano-energetics has become a topic of significant interest.
While there are a number of combustion enhancements resulting from the addition of nanoparticles to gelled and solid-based propellants, little investigative study has been done on the combustion properties of biofuel nanofluids. Nanoscale structures (<100 nm) stably suspended in biofuel nanofluids give rise to exciting new properties and phenomena. Previous studies have shown that the addition of nanoparticles to liquids, such as water, may improve the heat and mass transfer inside the liquid [9, 10], even at low concentrations (<1 vol.%). Tyagi et al.  determined that adding n-Al to diesel fuel resulted in an enhancement of ignition probability when compared to the base fuel alone. With aluminum volume fractions of 0, 0.1, and 0.5%, hot plate droplets were found to have much higher ignition probability regardless of the aluminum size or form. Experimental studies with aluminum hydroxide and graphene sheets in nitromethane (NM) monopropellant resulted in significantly greater burning rates (×1.75 for graphene sheets) . Likewise, nano-aluminum (n-Al)-gelling agent additives in NM resulted in increased linear and mass burning rates . Suspended metallic colloids also have the ability to be optically ignited, resulting in a multipoint or "distributed ignition" within a combustion engine . Experimental studies with cerium oxide fuels are known to display increased catalytic activity, causing oxidation of hydrocarbons and functioning as an oxygen buffer against NO
formation. Cerium oxide additives to biodiesel resulted in reductions of NO
by approximately 30% and reductions of hydrocarbon emissions by 25-40% . Therefore, nanoparticles can function as a catalyst and an energy carrier, as well. In addition, due to the small scale of nanoparticles, the stability of the fuel suspensions should be markedly improved.
Aluminum is used due its numerous applications as an energetic material; however, current theoretical models cannot fully explain n-Al ignition in certain environmental conditions and size ranges. The phenomena of the growth of the oxide layer, effect of mechanical stresses or strains, and solid-solid phase changes or solid-liquid presence in the core are not completely understood . A number of experimental investigations on aluminum additive combustion have reported a wide range of ignition temperatures even within the same particle distribution. Furthermore, the n-Al burning rate is increased with decreased particle size and is strongly dependent on temperature and pressure .
Previous studies have suggested that the change in oxidation temperature is triggered by metal/metalloid impurities [16
], or an increasing fraction of lattice defects, or surface irregularities with decreasing particle size [4
]. Trunov et al. [18
] suggested that this is a result of the sequence of four polymorphic phase transformations (amorphous, γ, and α-alumina) [19
], leading to a step-wise particle mass increase. In the first stage, as the metal is heated, the natural amorphous alumina layer grows until it reaches a critical thickness (approximately 5 nm), and then the oxide layer fractures and transforms into a crystalline γ-alumina phase. In the second stage, the γ-alumina oxide layer increases in density, and molten aluminum leaks through the γ-alumina faults, growing into the third stage as one of the similar intermediary transitions, such as δ or θ. In the final polymorph stage, the oxidation rate increases, and the crystalline structure becomes significantly dense as α-alumina. A qualitative analysis [18
] suggested that, within the multistage oxidation, different particle self-heating rates were responsible for the range of ignition temperatures. Smaller particle ranges triggered transition to the second oxidation stage (γ-alumina) at lower temperatures; however, the transition to the second stage was delayed under higher heating rates. Rai et al. [20
] proposed that aluminum nanoparticle oxidation occurs in two distinct regimes. At temperatures below the melting point of aluminum, a slow oxidation occurs with oxygen-limited diffusion through the aluminum oxide shell. At temperatures above the melting point of aluminum, a fast oxidation occurs with both aluminum and oxygen diffusing through the oxide shell, followed by a hollowing of the aluminum core at temperatures in excess of 1000°C. Recently, a new fast oxidation mechanism, referred to as the melting-dispersion mechanism, was discovered for n-Al particles under heating rates on the order of 107
]. These rates are not well understood and cannot be explained by current diffusion-oxidation models. The change in volume due to fast melting of the n-Al core induces pressures in the range of 0.1-4 GPa and causes spallation of the oxide shell. As a result, further experimental studies are needed to fully characterize the n-Als as a nanoenergetic material. In this study, the combustion properties and performance of n-Al and n-Al2
additions to liquid ethanol (C2
OH) are qualitatively and quantitatively investigated. Previous studies have shown a 20% increase in the thermal conductivity of ethanol with the addition of 4% volume fraction of AlN (20 nm) [22
]. The primary objective of this experimental study is to characterize the combustion and gain a better understanding of n-Al oxidation in a multicomponent heterogeneous system. In order to reduce greenhouse gases from fossil-fuel use, ethanol is widely used as a biofuel and/or a fossil-fuel additive, and its complete combustion products in pure oxygen are CO2
O, both of which are possible oxidizers for aluminum [23
], under certain environmental conditions:
Ethanol is also biodegradable and has a relatively low bio-toxicity; any spillage of pure ethanol may be simply diluted with water and disposed of down the drain .
Aluminum is used because of its numerous applications as an energetic material, high volumetric heat of combustion (HoC), high thermal conductivity, excellent surface absorption, and low melting/ignition temperatures. If oxygen is assumed as the primary oxidizer for aluminum combustion, then the global reaction mechanism is as follows:
The main combustion product of aluminum, Al2O3, is environmentally stable and may be recycled back to pure aluminum with an electrolytic reduction [1, 25]. Therefore, aluminum combustion with ethanol could potentially be regarded as a more environmentally sustainable fuel than conventional petrol if its energetic value is practical. Aluminum oxide was regarded as a heavily passivated metal and used for comparison with the ignition of pure aluminum; hence, it was hypothesized that aluminum oxide would not participate reactively in the experiments.
The nomenclature for the aluminum suspension samples will be as follows: for an aluminum nanoparticle suspension volume fraction of 5% in ethanol, it will be indicated by Eth + 5% Al, or Eth + 5% Al2O3 for alumina. The basic combustion studies here may be extended to more complex nanoenergetic systems, such as bimodal aluminum compositions, mechanically alloyed metals, or metastable intermolecular composite materials.