Combustion Properties of Metal Particles as Components of Modified Double-Base Propellants

Xiaofei Qi, Hongyan Li Ning Yan2, Ying Wang2 and Xueli Chen2(.Xi’an Modern Chemistry Research Institute, Xi’an 70065, China; 2.Science and Technology on Combustion and Explosion Laboratory, Xi’an 70065, China)

Many practical applications support continuing interest in metal combustion processes[1-2]. Metals like magnesium (Mg) has been studied by many researchers for a long time, and it was proposed in the early research, based on the low boiling point of Mg, that the reaction occurs in the vapor phase[3-4]. Aluminum (Al) particles are added to propellants to boost their combustion, as a prime candidate for high enthalpy fuel and propellant formulations due to its high energy release[5-6]. Although boron (B) is traditionally not considered as a true metal, it seems like a metallic element with high melting point and energy content, and has attracted a considerable research interest in the past 30 years[7-8]. Moreover, nickel (Ni) is widely used in propellants as a catalytic agent, and it might be used to tailor the combustion performance of propellants[9-10].

Although metals are generally not considered to be flammable materials because of their high ignition temperatures, they burn extremely vigorously once ignition is achieved. In propulsion systems, the combustion of metals typically occurs via small diameter particles, which can be either of micro- or nano-size. One of the most complex aspects of understanding combustion of these propellants is describing the physics of burning metal particles. Burning metal particles is different from hydrocarbons because of the presence of condensed species. The highest temperatures in the flame are associated with localized particle combustion, and a significant amount of the heat release to objects in the flame is due to a particle deposition. The oxidation of the metal particles tends to occur over much longer time scales than the conversion of the primary propellant (oxidizer/binder/catalysts/ plasticizer), during which time the particles are convected away from the burning surface. When burning at elevated pressures, as rocket motors are designed to operate, the propellant burns rapidly and metal particles are probably convected away from the surface as they are exposed. When burning under ambient conditions, as in an accident scenario, the metal particles tend to stay on the surface of the propellant for a period of time without burning. These particles cluster together to form larger agglomerates and are then lofted into the plume, where burning takes place.

1 Experimental

1.1 Materials and specimen

Cyclotrimethylenetrinitramine (RDX, ≥99.6%), nitrocotton (NC, ≥99.5%), nitroglycerine (NG, ≥99.2%), lead phthalate (φ-Pb, >99.5%), cupric 2,4-dihydroxybenzoate (β-Cu, >99.8%), 1,3-dimethyl-1,3-diphenyl urea (C2, ≥99.0%), carbon black (C.B., >99.8%), aluminum (Al, ≥99.8%), nickel (Ni, ≥99.5%), magnesium (Mg, ≥99.5%), boron (B, ≥99.5%) and Mg3Al4alloy (Mg/Al, ≥99.6%) were used as components of modified double-based propellants. The propellants involved in this study differ only in the type of reactive metals, where as other components and their ratios were identical. The composition and physical properties of the samples involved in this research are listed in Tab.1.

Tab.1 Composition of propellants

All the samples involved in this investigation were prepared by mould process at the temperature of 35 ℃, solidified for 96 h at 70 ℃, and machined to fixed dimensions (shape: cylinder; length: 200-250 mm; diameter: 5-8 mm).

1.2 Instruments and experimentation

1.2.1Surface morphology and particle size distribution

The surface morphology of metal particles were measured by scanning electron microscopy (SEM) (Model JSM-5800,Japan).

1.2.2Particle size distribution

The particle size distribution of the metals was investigated and quantified by Malvern laser granulometer (Britain).

1.2.3Flame structure analysis

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The Thermal Video Systems(China) was used to take photos of the flame at different pressure. A coated sample (dimension 150 mm×5 mm×5 mm) was placed vertically on an ignition rack, and the rack was fixed in a combustion chamber with a vitreous window. The chamber was filled with dynamic nitrogen atmosphere from bottom to top achieving a definite pressure, which ensured the flame transparency in the chamber. A nickel-chrome wire was used to ignite the propellant samples. In this way, the flame pictures were obtained.

1.2.4Non-contact wavelet-based measurement of flame temperature distribution

Thermographs make use of the infrared (IR) spectral band of the electromagnetic spectrum. IR involves four bands: near infrared (0.75-3 μm), middle infrared (3-6 μm), far infrared (6-15 μm) and extreme infrared (15-100 μm). Infrared video cameras are passive, i.e. emit no energy, but merely collect thermal radiation emitted from the flame zone. The IR camera used in this paper operates in the far infrared range. When infrared images of the flame were obtained, their source data were input into a software that can calculate and judge the temperature profiles of the flame by different RGB color value.

2 Results and Discussion

2.1 Metal particle shape and size

Fig.1 shows SEM micrographs of five different metal particles related to different preparation routes. It is shown that Al, Ni, B are of spherical shapes, while Mg, Mg/Al are of irregular shape and larger particle size.

Fig.1 SEM micrographs of the surfaces of different metal particles

The particle size distribution is one of the important parameters for characterization of quality of metal powders. Tab.2 summarizes the particle size distribution of the metal particles obtained by laser granulometer and reports calculated average diameters. Obviously, the dimension of Al and B coincides with the average particle diameter, which means that all particles are almost monocrystalline. However, in case of Al/Mg alloy and Mg, the particle size is larger (d50>80 μm) and of wider distribution range, while the particle size of B is the smallest, being even at the submicron level (around 0.7 μm).

Tab.2 Particle size distribution of the metals

2.2 Flame structure

Fig.2 Flame structure comparison for propellants at pressure of 1 MPa and 4 MPa

Fig.2 shows a typical flame structure for the propellants at a pressure of 1 MPa and 4 MPa.p1indicates a low temperature zone, where there is an inert atmosphere instead of a fuel gas;p2shows a spray combustion zone of metal particles, where the flame temperature is extremely high;p3shows the burnout of the metal particles;p4presents incombustible components sprayed from the condensed phase;p5shows severe agglomeration of the metal particles with other components, andp6is the pressure sensor of the testing device. A thin dark zone exists above the burning surface, and a luminous reddish flame appears above the dark zone. For the CMDB, the luminous flame is blown away from the burning surface with a pressure of less than 1 MPa, and the luminous flame gradually approaches the burning surface as the pressure increases. With the incorporation of metal species, the burning process of the propellants becomes quite unstable, showing a wave-shaped flame zone, while the propellant containing nickel exhibits a stable one-dimensional luminous flame. It should be noted that the flame structure of the propellants containing Al and Ni shows nearly a disappearance of dark zones at the pressure of 4 MPa. Specifically, it was found that the flame region of Al-CMDB propellant approaches the liquid surface, leading to an increase in the concentration gradient of Al in the gas phase, which is beneficial for matching the increase of vaporization rate of Al at a higher pressure. On the other hand, the increase in the temperature of the burning surface can accelerate the vaporization rate of Al, which is responsible for accelerating the burning rate of the propellants. Thus far, the combustion behavior of Al at a micrometer scale has been well studied, and the results show large differences with regard to those of B and Mg metal particles, which demonstrate a detached flame structure at a lower pressure.

Moreover, it is deduced from Fig.2 that different metal particles show considerable differences in both the combustion efficiency and flame structure. As for Al-CMDB and Mg/Al-CMDB propellants, the luminous flame is unstable and heterogeneous, and the distinctive metallic flame (Fig.2-p2) is embedded in the flame matrix zone where the burning rates are relatively higher. However, both B-CMDB and Mg-CMDB show a low combustion efficiency, which is attributed to the unburned metal agglomeration (Fig.2-p3) suspended in the flame zone. In addition, a smoke can be clearly observed in the flame photos of these two kinds of propellants. More details about the flame structure will be theoretically analyzed in the following passages.

2.3 Flame temperature distributions

Flame temperature distributions of solid propellants with and without reactive metals are shown in Fig.3 and Fig.4, respectively. “D” refers to the dark zone of the flame composed of a gaseous mixture of thermolysis products, and “F” denotes the uniform flame zone featuring stabilized combustion as well as a homogeneous temperature distribution. “B” in Fig.4 denotes inert gas bubbles in the flame zone where the temperature is extraordinary low, and “Fm” refers to the localized metallic flame. It was found that all the propellants show the combustion waves with multi-flame characteristics, and the unburnt binder layer can be observed in the surface reaction zones. As for the propellants containing Al, Mg and Mg/Al, the hot spots (Fig.2) starting from the ejected metal particles lead to much higher temperature of the flame zone compared with that of CMDB. It is worth noting that the ejected particles in the flame of Al-CMDB and Mg/Al-CMDB propellants show more straight tracks, which is attributed to a higher rate of the generation of gaseous products, leading to generation of a large amount of energy for the ejection of the metal particles. In fact, a large amount of data is available with regard to the combustion process of aluminum based propellants[11-13]. The melting point and boiling point of Al are 933 K and 2 600 K, respectively, while for the oxide, the melting and boiling points are 2 323 K and 3 273 K. In this case, with the increase of temperature, the Al melts first followed by the melting and coalescence of the oxides, which allows the diffusion of Al vapor into the gas phase of the flame, with consequence of the ignition of the metal particles. Consequently, Al particles burn in a gas phase reaction, and the flame front is at a detached distance of 1.5-4.0 times the droplet radius from the droplet surface. The hot oxide products dissociate outward, and Al2O3condenses and forms a smoke cloud that emits thermal radiation at a temperature of 3 036 K, which is shown in Fig.4.

Fig.3 Temperature distribution for the flames of propellants containing nickel and boron

Fig.4 Temperature distribution comparison between the flames of propellants containing different metals and a blank propellant at 4 MPa

The flame temperature of the propellant containing magnesium reaches as high as 3 832 K. This is because the low melting point of magnesium leads to a sufficient release of the latent heat of metals. It should be noted that the propellants containing B and Ni exhibit a homogeneous distribution of the flame temperature, which is much lower than that of the propellants containing Al and Mg (Fig.3). Also, the flame propagation is steady and homogeneous due to the slow process of the energy release. In particular, the Ni-CMDB propellant represents a steady flame with a uniform temperature distribution in the burning process, which makes nickel a promising combustion stabilizer regardless of the slight decrease in the energy.

As for B-CMDB and Ni-CMDB propellants, both B and Ni exhibit a slower oxidation process compared with Al, Mg and Mg/Al alloy, which is due to the difficulty in exploiting the energy potential of B and Ni. In fact, the oxide coating generated around the B particle would be liquefied at a very low temperature (723 K at 0.1 MPa), which can prevent the B particle from being attacked by the oxidizer, resulting in the ignition delay of the B particle[14]. In addition to the effect of the protective oxide coating, combustion of B and Ni is difficult to achieve due to their high vaporization temperature (4 139 K for B and 3 860 K for Ni at 0.1 MPa), which substantially hinders the vapor phase burning and restricts oxidation, which in turn slows down the heterogeneous surface reactions. During combustion, the condensation of products such as B2O3and NiO is thermodynamically prohibited, especially in the nozzle expansion where the temperature is below the boiling point of the oxide, which facilitates the dissociation of the products. In the presence of oxidizing species such as HCN, CH2O and H2O, originating from the thermolysis of the RDX and NC/NG binder[15], the products change in favor of gas phase species (HBO2, BN), resulting in a lower net energy release, though this may be offset to some extent by the formation of a much lower energy cyclic trimer of HBO2[14].

3 Hypothetical Physical Model for Metal Particle Combustion

In the hypothetical combustion model, the metal particles are assumed to be spherical, and coated with a negligibly thin oxide layer. The ignition and combustion modes for single micro-sized particles and agglomeration of nano-sized particles are shown in Fig.5.

Mode A-a spherical metal particle; Mode B-an agglomeration of metals with less granularity in the modified double-base propellantFig.5 Schematic illustration of two combustion modes

The combustion modes for a spherical metal particle and a metal agglomeration are shown in Fig.5. It was found that each mode consists of four steps, step 1: liquid RDX and binders enwrap the metal particle; step 2: metal particles absorb heat and start to melt; step 3: the temperature increases fast and the whole metal particle melts and the outer layer starts to vaporize and decompose; step 4: a part of the liquid RDX and binders mix with the liquid metal, and then a high temperature oxidation reaction starts between the metal particle and the oxidants produced by the thermolysis of the RDX and binders. Mode A presents a single spherical metal particle coated with the RDX powder and NC/NG components. An increase in temperature leads to the melting of the RDX and produces an NC/NG foam layer, which is beneficial for the uniform distribution of coatings, as well as the enhancement of the interfacial interaction between the coating layer and the metal particles (step 1). Then, the temperature of the metal particles increases greatly and an acute gas flow sprays into the flame zone as a result of the thermolysis of the liquid RDX and NC/NG (step 2); as the temperature reaches at the melting point of the metal, the metal begins to react with the gaseous oxidant (step 3). The temperature increases up to the boiling points of the metals like Mg, Al and Mg/Al, then the liquid metal particles begin to vaporize, the vaporized metal fuel then diffuses outward and reacts with the internal diffusing oxidizer gas such as CH2O, NO2, N2O and HCN in the diffusion flame (step 4). Mode B represents a similar process, where an agglomerate can be considered as a larger spheric metal particle.

The oxidation of the metal particles shows a strong dependence on the outer oxide layer, which serves as a cap on the particle surface. The exposed liquid metal particle evaporates and is oxidized in a gaseous cloud around the particle, and the resultant oxides such as Al2O3, B2O3, NiO, MgO undergo a random diffusion to join the cap. The combustion mechanism is closely related to the oxidizer type and pressure, as the phase transition temperature varies a lot with respect to the chemical composition in the atmosphere and pressure. Moreover, the transport phenomenon through the condensed and gas phases further complicates the combustion mechanism. Due to the formation of high temperature condensed-phase products, radiation effects also play a crucial role in the energy conservation of burning particles. Moreover, compatibility of the metal and its products can also affect the combustion behavior and can lead to a disruption or break up of solid metal.

Hence, the combustion mechanism of a particle is strongly dependent on ambient temperature. When the temperature is higher than the vaporization temperature of the metal, the particle burns through a vapor phase mechanism, although a detached flame cannot be observed because of the fast transport rates in the surrounding environment. When temperature is higher than the melting temperature of the oxide, but lower than the vaporization temperature of the metal, then the oxide can form a cap on the surface due to the surface tension difference between the molten oxide and the metal, which leads to a heterogeneous oxidation at the molten metal surface. At still lower environmental temperatures, the transport of metal and oxygen through the solid oxide shell with a subsequent reaction will dominate.

4 Conclusions

The characteristics of metal particles, combustion properties and burning models under different experimental conditions have been studied and discussed in detail. The conclusions could be made as follows.

①Observations of the propellant burning process with Ni and Mg show a nearly uniform bright region immediately above the propellant surface, in contrast to large agglomerates of B and Al that leave the surface as discrete isolated particles burning individually at some distance from the surface.

②Mg, Al and Mg/Al are excellent energetic components because of their large heat of combustion on either a mass or volume basis. In combustion of Mg, Al, and Mg/Al in propellants, there are both gas phase reactions and surface oxidation resulting in volatile and non volatile products which include oxide and suboxide species. However, for metalloid B and transition metal Ni, there are only gas phase reactions due to a higher melting point.

③Combustion of metal particles may cause a condensation of the oxide vapor at the particle surface, releasing the condensation heat that can be directly used by the metal particles for gasification. Spherical metals with higher activity (such as Al and Mg) could burn in an oxidizing environment with an infinitely fast surface reaction.

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