steric stabilization of colloidal aluminium particles … · 2019. 10. 4. · for advanced...

Sherif Elbasuney, Mostafa A. Radwan, Hany A. Elazab

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STERIC STABILIZATION OF COLLOIDAL ALUMINIUM PARTICLES FOR ADVANCED METALIZED-LIQUID ROCKET PROPULSION SYSTEMS

Sherif Elbasuney1, Mostafa A. Radwan2, Hany A. Elazab2,3

ABSTRACT

Reactive metal particles can act as high energy density material, therefore the achievement of stable colloidal suspension of reactive metal particles in liquid propellant is crucial for enhanced thrust per unit mass. Aluminium is of interest due to its availability, stability, and high combustion enthalpy (32000 J/g). In this manuscript, ultra-fine aluminium particles of a spherical shape and a particle size of 5 µm were developed by wet milling. Aluminium particles were effectively surface modified with polymeric surfactant and sterically stabilized into organic solvent (toluene). The surface modification process of aluminium reveals a unique change in surface properties from hydro-philic to hydrophobic with an efficient and remarkable transfer from aqueous phase to organic phase. The stabilized particles were effectively dispersed in liquid rocket propellant (hydrazine). The theoretical impact of colloidal alu-minium particles on hydrazine combustion characteristics was evaluated using ICT Thermodynamic Code (Institute of Chemical Technology in Germany, 2008). Aluminium particles offered an increase in combustion temperature, oxygen balance, characteristic exhaust velocity (C*), and specific impulse (Is). The optimum solid loading level of colloidal aluminium in hydrazine fuel was found to be 6 wt. %.

Keywords: liquid rocket propulsion, colloids, surface modification, steric stabilization, metal-based fuels.

Received 08 January 2019Accepted 18 June 2019

Journal of Chemical Technology and Metallurgy, 54, 6, 2019, 1281-1290

1 Nanotechnology research center, School of Chemical Engineering Military Technical College, Kobry El-Kobba, Cairo, Egypt2 Department of Chemical Engineering, Faculty of Engineering The British University in Egypt, El-Shorouk City, Cairo, Egypt3 Nanotechnology Research Centre (NTRC), the British University in Egypt (BUE), El-Sherouk City, Suez Desert Road, Cairo, 11837, Egypt E-mail: [email protected], [email protected]

INTRODUCTION

The maximum heat of combustion of hydrocarbon liquid propellants is generally limited by the enthalpy of formation of their reaction products, CO2 and H2O [1 - 2]. Consequently, the energy density of liquid propellant is relatively low. Higher combustion energies and energy density can be enhanced by combusting metal fuels, such as Mg, Al, B, and Ti [3]. The considerable strength and energetic formation of the metal-oxygen, account for the excellent fuel properties of many metallic elements [4]. Metallic fuels can offer massive combustion heat, up to 70 KJ/g for beryllium (Fig. 1), [3].

Recent developments in the area of metallic fuels have largely focused on ultra-fine powders [5 - 6]. The

effective dispersion and stabilization of reactive metal particles into liquid propellants could have a great impact on rocket propulsion technologies [7 - 8]. It has been reported that the suspension of beryllium particles in liquid propellant increased the specific impulse by 12 - 18 %, but the propellant plume was toxic [1, 9]. Boron is reasonably stable element that can be oxidized to yield high heat output. Its high cost limits its extended applications [4, 10].

Aluminium offers many advantages in terms of per-formance, cost, availability, safety, and chemical stabil-ity [11 - 12]. Aluminium, with combustion heat of 32000 J/g, is the most extensively used fuel in rocket propulsion technologies [13 - 14]. Aluminium has unique ability to react with inert gasses (Equations 1 - 5), [12, 15].

Journal of Chemical Technology and Metallurgy, 54, 6, 2019

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2Al(s)+3/2O2→ Al2O3 (s)+1700 KJ/mol (1)2Al (s)+3CO (g) →Al2O3+3C+ 1251 kJ/mol (2)2Al (s)+3H2O (g)→Al2O3 (s)+3H2 (g)+ 866 kJ/mol (3)2Al (s)+3CO2 (g) →Al2O3 + 3CO + 741 kJ/mol (4)2Al (s) + N2 (g) → 2AlN + 346 kJ/mol (5)

This series of exothermic reactions represents the secondary combustion pathway according to reaction spontaneity and reaction rate [4, 16 - 17]. Therefore, a large amount of heat would be liberated with minimum consumption of oxygen [2, 16]. Ultra-fine aluminium particles have the potential to initiate this series of secondary combustion process due to the increased interfacial surface area, and reactivity [4, 18 - 20]. Yet, benefits expected from aluminium are not fully exploited [21]. On the other hand, hydrazine is the most common fuel in use for liquid propellant rocket engines (LPREs) [1]. Hydrazine is characterized by a positive heat of formation, and it offers a good performance when compared with many common liquid fuels [22]. It is spontaneously ignitable with nitric acids [1, 23]. The proper dispersion (suspension) of ultra-fine aluminium particles in hydrazine has the potential to increase its heat of combustion and subsequently can positively impact its specific impulse [24 - 25].

The achievement of stable colloidal suspension of reactive metal particles in liquid propellant is crucial

issue for enhanced thrust per unit mass of liquid propel-lant [26 - 27]. Consequently mono-dispersed particles are to be developed, isolated, and stabilized into liquid propellant [28]. It is possible to stabilize reactive metal particles in order to prevent the coagulation by introduc-ing a thick adsorbed organic layer which constitutes a steric barrier [29 - 30].

The surface coating layer must be thick enough (typically > 3 nm) in order to keep the points of closest approach outside the van der Waals forces range [31 - 33]. Further details about nanoparticle stabilization via surface modification with different surfactants can be found in the following references [21, 34 - 36, 50].

This paper reports on the development of ultra-fine aluminium particles by wet milling. Organic solvent was employed to avoid metal oxidation, to dissipate the generated heat, as well as to act as a medium for subsequent surface modification. Aluminium particles of 5 µm average particle size were reported from SEM micrographs. Aluminium particles were effectively surface modified with poly (ethylene-co-acrylic acid) as polymeric surfactant. Organic modified aluminium demonstrated complete change in surface properties from hydrophilic to hydrophobic. Organic modified particles exhibited effective phase transfer from the aqueous phase to the organic phase. Organic modified colloidal aluminium particles were re-dispersed in hy-drazine fuel. The impact of colloidal aluminium particles

Fig. 1. Gravimetric and volumetric heats of oxidation for common metal fuels.


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on hydrazine ballistic performance was evaluated using chemical equilibrium code named ICT Thermodynamic Code (Institute of Chemical Technology in Germany, version 2008). This thermodynamic code is based on the chemical equilibrium and steady-state burning model, which encompasses two methods of calculation: frozen equilibrium and shifting equilibrium. The employed frozen equilibrium model is based on the assumption that the composition is invariant (the product composition at the nozzle exit is identical to that of its chamber condi-tion). Liquid propellant rocket motor (LPRM) based on metalized hydrazine as a fuel and red fuming nitric acid (REFNA) as oxidizer was employed as a case study. Aluminium particles offered an increase in combustion temperature and oxygen balance. The optimum solid loading level of aluminium in hydrazine fuel was found to be 6 wt. %; this ratio offered the balanced specific impulse (Is) and characteristic exhaust velocity of gase-ous products (C*). The ballistic performance could be further enhanced by decreasing the oxidizer feeding rate. Therefore, high performance can be achieved with lower consumption rate of fuel and/or oxidizer. Consequently, extended range can be achieved with minimal change in aerodynamic designs.

EXPERIMENTAL Materials

Commercial aluminium powder of 50 µm (Alpha chemika, India) was employed for the preparation of ultra-fine aluminium. Ethanol (97 %, Fisher) was em-ployed as a solvent for milling process. Poly(ethylene-co-acrylic acid) (Aldrich) was employed as polymeric surfactant for aluminium surface modification. Urea (Aldrich) was used as a source for OH- ions which were employed for ionizing the carboxylic acid (anchoring group) of the employed surfactant. Toluene (97 %, Fisher) was employed as solvent for harvesting the organic modified aluminium nanoparticles. Hydrazine (Aldrich) was used as the hosting liquid propellant for organic modified aluminium particles.

Development of ultra-fine aluminium particlesCommercial aluminium particles were separated ac-

cording to particle size by using Retsch VS1000 sieving machine. The finest aluminium particles (particle size ≤ 32 μm) were separated. Aluminum particles (< 32 µm) were dispersed in 250 ml ethanol. The colloidal

particles were ground using ball mill Retch 100 for 12 hours. Ethanol was employed as the grinding medium in order to dissipate any generated heat (during grinding process) and to protect ultra fine aluminium from further oxidation. The particle size of developed aluminium particles was visualized using scanning electron micro-scope SEM, Zeiss EVO-10 by Carl Zeiss Corporation. SEM was equipped with three types of detectors sec-ondary electrons (SE), back scattered electron (BSE), and energy dispersive X-ray spectrometer (EDX). The crystalline phase was investigated with X-ray diffrac-tion (XRD) D8 advance by Bruker Corporation over the angle range 2θ from 5 to 65 degrees to evaluate the stability of nano-aluminium against oxidation during milling process.

Surface modification of aluminium particlesSurface modification of aluminium particles was

conducted using poly(ethylene-co-acrylic acid). This copolymer is able to bind to numerous surface sites at the same time, forming a durable adsorption surfactant layer [37 - 39], that can produce a barrier of sufficient thickness. The conformation of the adsorbed polymer is a major controlling factor in determining the steric barrier stability [40]. The adsorbed polymer has three possible segments: (a) segments at the solid-liquid in-terface, called trains; (b) segments bound at both ends, called loops; (c) segments bound at one end, called tails [41]. Non-interacting groups are responsible for the oc-currence of tails and loops [40]. Variation in train, loop, and tail length controls the adsorbed layer thickness [42]. Fig. 2 is a schematic drawing of polymer conformation at the solid-liquid interface.

The adsorbed barrier can overcome van der Waals forces of attraction between particles [44 - 45]. Poly(ethylene-co-acrylic acid) was dissolved in 200 ml toluene. Con-

Fig. 2. Schematic of adsorbed polymer molecule at the solid-liquid interface [43].


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sequently, 1 g of aluminium particles was dispersed in poly(ethylene-co-acrylic acid) solution using ultrasonic prop homogenizer (Heschler) for 20 minutes. This pro-cess was imperative in an attempt to break down any aggregates as well as to enhance the dispersion of col-loidal particles before conducting surface modification process. Polar groups of employed surface coating agent have to be ionized in order to anchor the surface of metal particles [33, 41]. Anchoring groups (carboxylic groups) of poly(ethylene-co-acrylic acid) were ionized using urea. Aqueous solution of 0.003 M urea was added to the previously prepared toluene colloid. The colloidal mixture (toluene-water) was heated in a hydrothermal cell at 90oC, under sonication in an ultrasonic bath (Retsch UR1) for 40 minutes. Urea was hydrolyzed in water with the generation of hydroxyl ions according to equation 6 [46].

(NH2)2 CO + 3H2O→ CO2 + 2NH4 + 2OH (6) This hydrolysis reaction took place slowly and oc-

curred near the boiling point of water. The generated hydroxyl ions have the potential to ionize the weak carboxylic groups of poly(ethylene-co-acrylic acid), and to improve the surface coating efficiency. The final colloid was allowed to be separated into two phases, organic phase at the top and aqueous phase at the bottom. Organic modified aluminium particles demonstrated complete phase transfer from the aqueous phase to the organic phase. The attachment of the surfactant to the surface of aluminium particles was investigated with FTIR (8400, Shimadzu).

Theoretical performance evaluation of metalized hydrazine

The impact of aluminium particles on hydrazine combustion characteristics particularly the oxygen bal-ance, combustion temperature, characteristic exhaust velocity of gaseous products, and specific impulse were evaluated using ICT Thermodynamic Code (Institute of Chemical Technology in Germany, 2008). This code is based on the chemical equilibrium and steady-state burning model, which is based on two methods of cal-culation: frozen equilibrium and shifting equilibrium. The employed frozen equilibrium model is based on the assumption that the composition is invariant (the product composition at the nozzle exit is identical to that of its

chamber condition). Different percentages (wt. %) of Al in hydrazine were investigated. The impact of aluminium content on combustion characteristics and ballistic per-formance was evaluated using liquid propellant rocket engine encompasses hydrazine as a fuel (1.05 Kg/s) and red fuming nitric acid (REFNA) (75 % HNO3 and 25 % N2O4) as an oxidizer (3.4 Kg/s).

RESULTS AND DISCUSSIONCharacterization of aluminium particles

The morphology of developed aluminium particles was investigated with SEM. Al particles in the shape of spheres with 5 µm average particle size were reported from SEM micrograph (Fig. 3).

The crystalline phase of developed aluminium par-ticles was investigated with XRD. XRD diffractogram included five clearly sharp peaks all of them agreed with the Joint Committee on Powder Diffraction Standards (JCPDS) from the International Centre for Diffraction Data (ICDD) (Fig. 4).

XRD diffractogram confirmed that no hydrolysis or oxidation reactions could take place during milling process. This milling approach secured aluminium par-ticles with high crystalline structure free from defects and any interfering substances. The organic modified particles demonstrated complete change in surface prop-erties from hydrophilic to hydrophobic; they exhibited effective transfer from the aqueous layer to the organic layer (Fig. 5).

The adsorbed organic polymeric layer could steri-cally stabilize the aluminium particles through:

Fig. 3. SEM micrograph of aluminium particles developed by wet milling.


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Volume restriction which prevents nanoparticle ap-proaching one another (Figure 6-a).

Osmotic effect where the solvent molecules sur-rounding the particles are squeezed out upon close approach. The generated osmotic pressure tends to suck the liquid into the space between the particles, thus increasing the energy required for the particles to

coagulate [27 - 28], (Fig. 6b).Steric stabilization offered uniform nanoparticle

dispersion into the liquid fuel; no flocculation could take place over time. FTIR spectra of organic modified alu-minium indicated the attachment of carboxylic surfactant to aluminium surface. Organic modified aluminium exhibited enhanced absorption in IR region compared to uncoated-aluminium (Fig. 7).

The difference in IR absorption of poly(ethylene-co-acrylic acid)-aluminium to uncoated aluminium was correlated to the carboxylic group (C=O and O-H stretch) of the attached surfactant [50]. One of the main features of aluminium is that aluminium surfaces are readily oxidized by the oxygen in the air, and a tight surface coating of aluminium oxide (Al2O3) is formed. This passive layer protects the inner metal from further oxidation (Fig. 8), [12].

Even though aluminium particles are more reactive with decrease in particle size, they can be stored for extended periods with little loss of reactivity [4, 21].

Fig. 4. XRD diffractogram of developed aluminium particles.

Fig. 5. Extraction of sterically stabilized nano-aluminium to the organic layer.

Fig. 6. Schematic of steric stabilization through: volume restriction (A), and the osmotic effect (B) [29].


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Theoretical ballistic performance evaluationThe integration of aluminium in hydrazine offered

an enhanced oxygen balance with dramatic increase in combustion temperature (Fig. 9).

This enhanced combustion characteristics are atrib-uted to the high enthalpy of combustion of aluminium

fuel (32000 J/g). Aluminium could consume less oxygen and in the mean time produce much heat compared with hydrocarbon fuels. This could withstand the enhanced oxygen balance with aluminium content.

The main combustion criteria which need to be pre-cisely evaluated and measured include specific impulse

Fig. 7. FTIR spectra of poly(ethylene-co-acrylic acid)-aluminium to uncoated aluminium.

Fig. 8. Aluminium nanoparticles with passivated layer of Al2O3 [12].

Fig. 9. Impact of aluminium content on oxygen balance and combustion temperature.


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and characteristic exhaust velocity of gaseous products [22]. Specific impulse is the thrust imparted to a vehicle per combustion of unit weight of effective propellant. Aluminium not only offered enhanced combustion tem-perature and oxygen balance, but also it offered metal-ized gaseous products (Al2O3). It has been reported that metalized gaseous product could also offer enhanced specific impulse [1]. This is why aluminium positively impacts both the characteristic exhaust velocity of gase-ous products and the specific impulse (Fig. 10).

Aluminium loading level of 6 wt. % offered the high-est characteristic exhaust velocity. Aluminium content

higher than 6 wt. % demonstrated further increase in combustion temperature which could be detrimental to engine structure materials. However the enhanced oxy-gen balance at 6 wt. % solid loading level could offer a reduction in the feeding rate of the oxidizer. The impact of oxidizer feeding rate on combustion characteristics of metalized hydrazine (6 wt. % Al) was investigated. There was a further increase in characteristic exhaust velocity and specific impulse with the decrease in oxi-dizer feeding rate (Fig. 11).

Oxidizer is usually used for cooling the combustion chamber, this would be a vital factor to be considered

Fig. 10. Impact of aluminium fuel on combustion characteristics of hydrazine.

Fig. 11. Impact of oxidizer feeding rate on combustion characteristics of metalized hydrazine at 6 wt. % Al.


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to maintain acceptable limit of cooling efficiency with enhanced ballistic performance. This manuscript dem-onstrated the state of the art for the real development of metalized liquid rocket propellant. Furthermore, extended range can be achieved with minimal change in the feeding systems and aerodynamic shape of the existing missile systems.

CONCLUSIONS

Aluminium particles, in the shape of spheres with 5 µm average particle size, were developed by wet mill-ing. The developed particles were effectively surface modified with polymeric surfactant, organic-modified aluminium particles demonstrated complete change in surface properties from hydrophilic to hydrophobic. They exhibited complete phase transfer from aqueous phase to organic phase. The dispersion and stabiliza-tion of aluminium particles in hydrazine was achieved via steric stabilization. Aluminium particles offered enhanced combustion characteristics in terms of specific impulse and characteristic exhaust velocity of gaseous products. In the mean time it was possible to decrease the feeding rate of oxidizer with a further increase in specific impulse. This manuscript shaded the light on the state of the art for the real development of metalized liquid rocket propellant.

AcknowledgementsMilitary Technical College is acknowledged for

funding the research project entitled “Novel metalized liquid rocket propellants”

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steric stabilization of colloidal aluminium particles … · 2019. 10. 4. · for advanced...

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