Ballistic Properties of Nano-Aluminized Solid Rocket Propellants a L.T. DeLuca*1, L. Galfetti1, F. Severini1 B.N. Kondrikov2, A. Vorozhtsov3, V. Sedoi4
1
Politecnico di Milano, SP Lab, 20158 Milan, Mi, Italy,
[email protected] 2
Mendeleev University of Chemical Technology, 125047 Moscow, Russia 3 4
Tomsk State University, 634034 Tomsk, Russia
Institute of High Current Electronics, 634055 Tomsk, Russia.
ABSTRACT Experimental data concerning the ballistic characterization of several nano-aluminum powders of Russian production, their influence on physical and chemical properties of condensed combustion products, and their flame structure near burning surface are investigated. All studies were performed at SP Lab, Milan, Italy with model composite solid rocket propellants based on ammonium perchlorate as oxidizer and HTPB as binder (AP/HTPB/Al formulation with respectively 68/17/15 % mass fractions).
The ballistic
properties of the tested formulations are presented in this paper; the physical characterization of the used nano-Al powders and chemical analyses of solid combustion residues are discussed in a companion paper.
Results obtained under a wide variety of operating
conditions typical of rocket propulsion indicate, for increasing nano-Al mass fraction or decreasing nano-Al size, larger steady burning rates with essentially the same (commercial grade AP) or less (propulsive grade AP) pressure sensitivity. A variety of other effects connected with nano-Al powders appears, depending on the details of the burning formulations. Different flame structures near the burning surface are revealed by high-speed, high-resolution color digital video recordings. Visual inspections also disclose a different structure of the particle-laden flame zone.
a
Proceedings of International Conference on Combustion and Detonation Zel’dovich Memorial II (Moscow, 2004) 1
1. BACKGROUND
Nanotechnology, as a method to produce and implement solid materials with sub-micron particles dimension, is nowadays an effective technology in many branches of modern industry. While metal mirrors of 0.01 µm thickness have been known for centuries, the first investigation on sub-micron aluminum powder was carried1 out in Russia by Gen, Frolov, and Storozhev in 1978.
They used nano-Al produced by vaporization and consequent
condensation of the metal vapors in argon, a method developed2 by Gen et al. already in 1959. A remarkable decrease of dimensions of the aluminum burning condensed products, very valuable in reducing the severe two-phase losses of rocket motors, was observed. The idea of applying nano-sized particulate metals as ingredients to radically improve performance of high-energy materials was introduced in the 80-ies, mainly by Ivanov in Russia and Barbee in USA.
Ivanov used preeminently the electrical wire explosion (EEW) process, whereas
Barbee worked with multi-layer packs of nano-scale metal mirrors.
One of the main ideas of Ivanov was that the nano-powders comprise the source of great surface energy. In particular Ivanov and Tepper, in 1996, suggested3 that Alex, a well-known commercial form of nano-aluminum, contains an additional amount of stored internal energy, on the order of 400 cal/g, that would be released at a relatively low threshold temperature and increase the propellant steady burning rate.
This idea was repeatedly verified but no
experimental or theoretical corroboration could be found. Barbee et al. proposed4 to use an initiator device consisting of energetic metallic nanolaminate foil coated with a sol-gel derived energetic nano-composite [4-6]. The device structure consists of a precision sputter deposition synthesized nano-laminate energetic foil of non-toxic and non-hazardous metals along with a ceramic-based energetic sol-gel produced coating made up of non-toxic and non-hazardous components such as ferric oxide and aluminum metal. The nano-laminate serves as the mechanically sensitive precision igniter and the energetic sol-gel functions as a low-cost, non-toxic, non-hazardous booster in the ignition train.
The current high interest in nano-particles in general is motivated from both applied and fundamental reasons.
As a matter of fact, nano-ingredients are useful for a range of
applications spanning from energetic materials (propellants, explosives, pyrotechnics…) to 2
structural materials (especially carbon tubes). Of course, chemical rocket propulsion covers both areas of interest, but applications are also seen for airbreathing propulsion (scramjets, pulsed detonation engines …), underwater propulsion, electric propulsion, and so on. From a fundamental viewpoint, nano-ingredients are of interest for understanding of deflagration and detonation waves structure, flame structure of nano-based mixtures, burning of particles (single vs. groups), and basic physical and chemical laws for nano-sized objects.
Potential applications include improvement of current ballistic performance of solid, liquid, hybrid chemical rocket propulsion, monopropellant and gels systems, airbreathing; in particular advantages can be seen for space launcher boosters, upper stages for planetary missions, airbreathing systems in general (thanks to the decreased ignition delay of fuels). Use of nano-particles could also favor the development of new technologies otherwise difficult if not impossible to achieve, such as cryogenic solids and water slurries in chemical rocket propulsion, colloid thrusters in electrical propulsion, increased thermal conductivity of suspensions, etc. Propulsive nano technology might be useful for propulsive missions to Mars. A peculiar short-term interest lies in setting up and refining a prospective dual oxidizer (AN+AP) metallized propellant as a low-cost alternative to the current space launcher boosters.
Aim of this research program is to experimentally assess the influence of Al particle size (see Table 1) on the ballistic properties of solid rocket propellants (steady burning rates, radiant ignition delays, ageing effects, burning surface characteristics, flame structure visualization, solid combustion residues, etc.). Burning rate and ignition delay measurements, high speed video-camera recordings, and residual combustion products analyses have been obtained for many aluminized propellant formulations (see Tables 2-5), designed and manufactured at the SP Lab (Solid Propulsion Laboratory) of Politecnico di Milano.
In this paper the steady ballistic properties of the tested formulations are presented; the physical characterization of nano-Al powders and chemical analyses of solid combustion residues were discussed in recent meetings5-6-7 and in a companion paper offered at this same conference8. In the first part of this paper, the tested solid propellant formulations and their general properties are discussed. In the second part, the observed ballistic properties are presented. Some concluding remarks and hints of future work complete the paper.
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2. SOLID PROPELLANT PROPERTIES Combustion of metallized solid rocket propellants leads to the formation of condensed combustion products (CCP), which have a critical influence on rocket motor performance. Following the Russian literature nomenclature9-10-11, CCP consist of two fractions: smokeoxide particles (SOP) and agglomerates. The SOP’s, formed by condensation of the gas phase reaction products with a typical size around 1 µm, account for the largest part of the produced oxide; they are effective in damping high frequency oscillations. Agglomerates, consisting of aluminum and aluminum oxide, are formed from the oxide skin surrounding the original Al particle or by condensed phase surface reactions during the particle combustion. Being their size of the order of hundreds and even thousands of micrometers, these particles are useful to damp low to mid frequency oscillations. Enlargement of the metal particle size originally contained in the propellant can occur via agglomeration (inert atmosphere) or sintering (oxidative atmosphere) of the molten metal. This phenomenon unfortunately limits the actual performance of metallized propellants in rocket motors, but at the same time is a powerful means to control combustion instabilities. Details of the concentration-sintering-detachmentagglomeration process affect12-13 the site and extent of the overall combustion process as well as the size of the product oxide droplets. These properties (impacting burning rate, combustion efficiency, combustion stability, and slag formation) do in turn depend on the propellant formulation and operating conditions.
Tested Formulations To investigate ballistic effects connected with nano-Al, emphasizing in particular differences with respect to the usual micrometric Al, a reference composite formulation consisting of 68% ammonium perchlorate (AP), 17% HTPB binder, and 15% aluminum (Al) was selected.
As oxidizer only AP has been used in different grain size distribution: for bimodal formulations the grain size is 70-80 µm (fine) and 140-160 µm (coarse), for monomodal formulations 80-140 µm. For practical reasons, AP provided from two different suppliers had to be used: AP-1 from a propulsive supplier and AP-2 from a commercial supplier. Al powders were used produced with different techniques and covering both nanometric and micrometric (spheres and flakes) size ranges; a wide range of products from international sources is under examination; see Table 1 for a comprehensive summary. In this paper, for the largest part attention is focused on nanometric or ultra-fine uncoated aluminum particles of Russian production. The binder typically employed is the common Hydroxyl-Terminated 4
PolyButadiene – DiOctilAdipate - IsoPhorone-DIsocyanate (HTPB – DOA- IPDI). In general HTPB R-20 has been used, only in one case HTPB R-45 has been used provided by a different supplier; occasionally, other hydrocarbon binders have also been used.
Nomenclature Source, Production Technique, and Size Type 01 Type 02 02a 02b 02c Type 03 03a 03b 03c 03d Type 04 04a 04b Type 05 Type 06 Type 07 Type 08 Type 09
Source: Russia EEW; (similar to Alex); uncoated; nominal size: 0.10 µm Source: Russia (Institute of High Current Electronic, RAS) EEW; uncoated; nominal size: 0.17 µm; production: 2003 EEW; uncoated; nominal size: 0.17 µm; production: 2002 EEW; uncoated; nominal size: 0.17 µm; production: 1999 Source: Russia (Tomsk State University) Mechanical; uncoated; nominal size: 0.2 µm Mechanical; uncoated; nominal size: 0.4 µm Mechanical; uncoated; nominal size: 0.8 µm Mechanical; uncoated; nominal size: 2.5 µm Source: Russia Plasma condensation; coated; nominal size: 0.20 µm Plasma condensation; coated; nominal size: 0.28 µm Source: Italy (from space industry) spherical; uncoated; nominal size: 30 µm Source: Italy (commercial supplier) flakes; uncoated; nominal size: 50 µm Source: Russia EEW; (similar to Alex); coated; nominal size: 0.10 µm Source: Russia (Institute of High Voltage Research, TPU) EEW, coated; nominal size: < 0.10 µm Source: USA Plasma condensation; uncoated; nominal size: 0.04 µm
Tab. 1. Summary of Al powders currently under investigation and nomenclature.
Thus, all tested compositions contain 83% mass fraction of solids and may differ for the supplier and size distribution of AP particles (monomodal vs. bimodal but always for a total mass fraction of 68%); nature and size distribution of Al particles (monomodal vs. bimodal but always for a total mass fraction of 15%); nature of the binder (but always for a total mass fraction of 17%). Bimodal AP size distributions are based on a coarse/fine ratio of 4 (c/f = 4): coarse particles (80%) in the range 140-160 µm and fine particles (20%) in the range 70-80 µm. Monomodal AP size distributions only include particles in the range 70-80 µm. Bimodal Al size distributions are based on a mixture on micrometric (coarse) and nanometric (fine) particles with parametrically variable c/f ratio: coarse particles are typically in the range 30-50 µm and fine particles in the range 0.1-0.2 µm. All propellants were manufactured, handled, and burned at SP Lab of Politecnico di Milano.
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Out of the several dozens of compositions tested, four different AP/HTPB/Al propellant series are discussed in this paper (Tables 2 – 5). Series I is meant to contrast the effects of micro-Al type 06 replaced by the same amount of nano-Al type 02b (see Table 2). This series includes seven Al compositions between the two bracketing formulations P_06 and P_02b respectively consisting of 100% micro-Al flakes and 100% nano-Al particles.
The remaining five
propellants of Series I (P_07_Bx, P_08, P_09) feature a bimodal Al distribution, with 50 or 30 µm (coarse) and 0.17 µm (ultra-fine) particle sizes, according to the indicated c/f ratio. Propellant Short Notation
AP supplier and granulometry (µm)
type
nominal size
Binder (17%)
P_06
AP-2; 70-80, 140-160
100% type 06
50 µm
HTPB R-20
80% type 06 20% type 01 80% type 06 20% type 02b 80% type 06 20% type 03d 50% type 06 50% type 02b 50% type 05 50% type 02b
50 µm 0.17 µm 50 µm 0.17 µm 50 µm 0.17 µm 50 µm 0.17 µm 30 µm 0.17 µm
100% type 02b
0.17 µm
P_07_B01 P_07_B02b P_07_B03d P_08 P_09 P_02b
AP-2; 70-80, 140-160 AP-2; 70-80, 140-160 AP-2; 70-80, 140-160 AP-2; 70-80, 140-160 AP-2; 70-80, 140-160 AP-2; 70-80, 140-160
Al (15%)
HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20
Table 2 – First series of propellant formulations: bimodal AP-2, monomodal or bimodal Al, HTPB R-20 binder. Series II is meant to assess how burning rate scales with Al particle size and thus includes only monomodal Al distribution (see Table 3). Propellant P_05 includes 100% of the microAl type 05 used in propulsive applications (spheres of 30 µm diameter). Propellant P_03d includes Al particles of 2.5 µm nominal size; propellant P_04a includes Al particles of 0.2 µm nominal size and protected by a hydrocarbon coating; propellant P_01 includes Al particles of 0.1 µm nominal size and very similar to the commercial Alex product. To allow a meaningful comparison of both effects, a common baseline propellant (called P_06) has been enforced for both series I and II.
Propellant P_06 include 100% of micro-Al type 06, a low-cost
commercial product (flakes of 50 µm characteristic dimension). All propellants of series I and II employ a bimodal size distribution (c/f = 4) of AP-2 as oxidizer (from a commercial supplier) and HTPB R-20 as binder.
6
Propellant Short Notation
AP supplier and granulometry (µm)
type
nominal size
Binder (17%)
P_06
AP-2; 70-80, 140-160
100% type 06
50 µm
HTPB R-20
P_05
AP-2; 70-80, 140-160
100% type 05
30 µm
HTPB R-20
P_03d
AP-2; 70-80, 140-160
100% type 03d
2.5 µm
HTPB R-20
P_04a
AP-2; 70-80, 140-160
100% type 04a
0.2 µm
HTPB R-20
P_01
AP-2; 70-80, 140-160
100% type 01
0.1 µm
HTPB R-20
Al (15%)
Table 3 – Second series of propellant formulations: bimodal AP-2, monomodal Al, HTPB R-20 binder.
Details of the formulation ingredients concerning the oxidizer and binder were investigated in propellant Series III (Table 4). In this series, comparative burning tests were conducted with several AP-1 (instead of AP-2) grain size distributions and a different HTPB binder (R-45 instead of R-20).
Propellant AP supplier and Short granulometry (µm) Notation P_BL_05
AP-1; 70-80, 140-160
P_BL_01
AP-1; 70-80, 140-160
P_BL_02
AP-1; 70-80, 140-160
P_BL_01a
AP-1; 80-140
P_BL_02a
AP-1; 80-140
P_BL_02b
AP-1; 80-140
Al (15%) nominal size Type
Binder (17%)
100% type 05
30 µm
HTPB R-20
100% type 01 50% type 01 50% type 05 100% type 01 50% type 01 50% type 05 50% type 01 50% type 05
0.1 µm
HTPB R-20
0.1 µm 30 µm 0.1 µm 0.1 30 0.1 30
µm µm µm µm
HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-45
Table 4 – Third series of propellant formulations: monomodal or bimodal AP-1, monomodal or bimodal Al, HTPB R-20 or HTPB R-45 binder.
In Series IV (Table 5), specific effects regarding the nano-Al types under test were systematically investigated using a bimodal size distribution (c/f = 4) of AP-1 as oxidizer (from a propulsive industry supplier) and HTPB R-20 as binder .
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Propellant AP supplier and Short granulometry (µm) Notation P_Al_01
AP-1; 70-80, 140-160
P_Al_02a
AP-1; 70-80, 140-160
P_Al_02b
AP-1; 70-80, 140-160
P_ Al_02c
AP-1; 70-80, 140-160
P_ Al_03a
AP-1; 70-80, 140-160
P_ Al_03b
AP-1; 70-80, 140-160
P_ Al_03c
AP-1; 70-80, 140-160
P_ Al_03d
AP-1; 70-80, 140-160
P_Al_04a
AP-1; 70-80, 140-160
P_Al_04b
AP-1; 70-80, 140-160
Al (15%) Type
nominal size
80% type 05 20% type 01 80% type 05 20% type 02a 80% type 05 20% type 02b 80% type 05 20% type 02c 80% type 05 20% type 03a 80% type 05 20% type 03b 80% type 05 20% type 03c 80% type 05 20% type 03d 80% type 05 20% type 04a 80% type 05 20% type 04b
30 µm 0.1 µm 30 µm 0.17 µm 30 µm 0.17 µm 30 µm 0.17 µm 30 µm 0.2 µm 30 µm 0.4 µm 30 µm 0.8 µm 30 µm 2.5 µm 30 µm 0.20 µm 30 µm 0.28 µm
Binder (17%) HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20 HTPB R-20
Table 5 – Fourth series of propellant formulations: bimodal AP-1, bimodal Al, HTPB R-20 binder.
Safety Remarks Friction, impact, moderate temperature increase, etc. do not hinder laboratory handling of nano-Al. Electrostatic discharges may be risky and thus require proper attention. Mix of 100 g are routinely manufactured and handled at the SP Lab with the precautions usually taken when highly energetic materials are employed. The powder was stored in a well closed container at a temperature of 4-10 °C. Handling of aluminum powder was made in the presence of a stream of nitrogen. Never the powder was put in contact with pure ammonium perchlorate and a well defined sequence of the reagents in the propellant preparation was established to avoid this event. Each powder sample was tested before the preparation in very small weight to investigate the reactivity and the safety degree. The operator was equipped with gloves, goggles and an antistatic device. Small quantities of the metal which are not recyclable or recovered are destroyed by treatment with a dilute solution of hydrochloric acid.
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Fig. 1 – Visualization of the propellant interparticle matrix: nanometric Al (upper pictures) and micrometric Al (lower pictures). Optical images on the left, fluorine visualizations on the right. Propellant of first series (see Table 2).
Propellant Texture Filling of the propellant interparticle matrix (the fine material distributed among the coarse oxidizer particles) appears of much better quality (uniform and well-distributed) in the case of nanometric Al, see Fig. 1 upper pictures, rather than in the case of micrometric Al, see Fig. 1 lower pictures. The interparticle matrix material includes Al particles, binder (HTPB R-20), and also the smallest AP particles making the oxidizer mass fraction. In both cases, the coarse oxidizer grains (bimodal AP-2) of irregular shape are well visible under ordinary light illumination; each picture includes in its upper strip a mark for the 200 µm scale. Special fluorine visualization techniques were used to reveal the Al distribution.
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3. STEADY BURNING RATE MEASUREMENTS Experimental Technique Propellant samples (4.5 x 4.5 x 30 mm) of the tested formulations were burned in a nitrogenflushed window bomb in order to measure the steady burning rate. Samples were ignited by a hot Nichrome wire. Pressure was kept constant during the whole combustion process with a feedback pressure control system. Steady burning rates were measured in the range 1-70 bar, using an automated image processing technique from high-speed video recordings. Several samples (at least 3) were used for each experimental point and for each sample several burning rate readings were performed, according to well-established SP Lab procedures.
Micrometric vs. Nanometric Al powder: Partial or Full Replacement (AP-2) As shown in Fig. 2 propellant P_06, assumed as baseline, has the lowest burning rate. All remaining formulations containing nano-particles exhibit significantly higher burning rates, thanks to the strong reactivity of the nano-sized Al particles. By replacing 50 µm Al flakes with the same amount of 0.1 µm nano-Al, burning rate approximately increases by 40% (P_07), 60% (P_08), 100% (P_02b) respectively corresponding to 20%, 50%, 100% nanometric replacement of the micrometric Al powder. Thus, steady burning rates increase by increasing the fraction of ultra-fine Al. For both monomodal Al distribution propellant (P_05) and bimodal Al distribution propellant (P-09), no significant burning rate difference is observed by replacing 50 µm Al flakes with 30 µm Al spheres.
Fig. 2 – Increasing steady burning rates with decreasing c/f fraction of Al powder (Series I propellants, bimodal AP-2).
10
Micrometric vs. Nanometric Al powder: Size Effect (AP-2) The second propellant series shows a similar trend, as illustrated in Fig. 3. Formulations including different nano-Al powders (type 04a and 01 of Tab. 1) feature significantly higher burning rates, while Al particles in the range 2.5 to 50 µm makes no appreciable difference.
Fig. 3 – Increasing steady burning rates for decreasing Al powder size (Series II propellants, bimodal AP-2). In AP-based composite propellants AP grains, after a crystalline phase transition (from orthorhombic to cubic at 513 K), undergo14 an exothermic degradation generating hot gases. Approximately, 70% of AP degrades through this exothermic reaction, while the remaining 30% sublime into NH3 and HClO4, which react in a premixed flame, very close to the burning surface, over a distance of about 0.5 µm (against 20 µm of the main diffusive flame) under normal rocket pressures. Combustion gases, characterized by a rich oxygen content, mix with hydrocarbon gases generated by the pyrolyzing binder and react in a main flame responsible for the conductive heat flux to the burning surface, which provides the degradation of both AP and binder.
Metal powders in the micrometric range (or above) burn according to a distributed mechanism extending much beyond the gas-phase flame thickness and thus not affecting the essentially diffusive combustion process just described. However, experimental results in the nanometric range show a remarkable steady burning rate augmentation with either increasing replacement of micrometric by nanometric Al (Fig. 2) or decreasing size of the nanometric Al
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(Fig. 3). The fact that ultrafine metal particles neatly increase steady burning rates testifies that the gas-phase flame structure is now affected by the combined effects of earlier ignition and quick premixed burning15-16-17. The fact that the burning rate increase is further enhanced by decreasing size of nanometric Al points out a primary effect of specific surface, typically increasing by at least an order of magnitude from micrometric to nanometric Al.
Within the explored operating conditions of Figs. 2-3, the basic flame structure appears only slightly affected, as revealed by the minor changes of the burning rate pressure sensitivity. In agreement with other studies18, the findings can be summarized as follows:
For bimodal Al formulations, replacing micro with nano Al yields a neat increase of burning rates with growing fraction of nano Al;
For monomodal Al formulations, decreasing nano Al size yields prominent increase of burning rates in the range 0.1 - 0.2 µm;
After elucidating these general trends, specific effects of the formulations were explored.
Effect of AP Type (AP-1 vs. AP-2) An unexpected consequence of the oxidizer powder is revealed in Fig. 4, illustrating the burning rates produced by a propulsive grade of ammonium perchlorate (short hand notation, AP-1) under a variety of formulations. Over the investigated pressure range and for the same
4
P_05 reference P_BL_05
3
burning rate, rb, mm/s
2
101 8 7 6 5 4 3
2
100
100
2
3
4
5
6
7 8
101
2
3
4
5
6
7 8
102
pressure, p, bar
Fig. 4 – Increasing steady burning rates for bimodal AP-1 vs. bimodal AP-2.
12
grain size distribution, AP-1 shows burning rates systematically higher than the commercial grade AP-2. Contrast the burning rates of P_05 with P_BL_05 obtained by using the same bimodal AP, provided by two different suppliers, with monomodal micro-Al type 05. The better processability of AP1 is confirmed by the higher propellant density. The faint trend toward reduced pressure sensitivity (from 0.392 for P_05 with bimodal AP-2 to 0.342 for P_BL_05 with bimodal AP-1) is more convincing in further results obtained with AP-1.
Pressure sensitivity is reduced by the tested bimodal nano-Al and/or monomodal AP-1 (see Fig. 5). The introduction of 50% nano-Al type 01 (P_BL_02), replacing the same amount of micro-Al type 05 (P_BL_05), increases the burning rate of bimodal AP-1 and further decreases its pressure sensitivity from 0.342 to 0.242. The complete replacement of micro-Al type 05 (P_BL_02a) with nano-Al type 01 (P_BL_01a), again increases the burning rate of monomodal AP-1 and further decreases its pressure sensitivity from 0.237 to 0.196.
Also the consequence of the tested monomodal AP-1 size distribution on burning rates was unforeseen. For practical needs the selected distributions of the oxidizer particle size had to be rather similar: 70-80 µm fine and 140-160 µm coarse for the bimodal distribution
4
P_BL_05 P_BL_02 P_BL_01a P_BL_02a
3
burning rate, rb, mm/s
2
101 8 7 6 5 4 3
2
100
100
2
3
4
5
6
7 8
101
2
3
4
5
6
7 8
102
pressure, p, bar
Fig. 5 – Increasing steady burning rates and decreasing pressure exponent are distinct for monomodal nano-Al with monomodal AP-1.
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against 80-140 µm for the monomodal distribution.
However, the beneficial effect of
monomodal AP-1 revealed striking for the tested monomodal nano-Al type 01 (see P_BL-01a with n = 0.196), but just fading for the tested bimodal Al formulations (see P_BL_02 and P_BL_02a in Fig. 5). Thus, AP-1 helps to increase burning rate, decrease pressure sensitivity and magnify the importance of the oxidizer grain size distribution. Nanometric Al Powder Type 01 The most remarkable ballistic effects among all nano-Al so far tested were given by nano-Al type 01 (nominal size 100 nm, actual size was somewhat larger).
A detailed physical
characterization8 revealed a bimodal distribution of the powders with bigger particles in the range 200-300 nm scattered in a significant fraction of nano-particles of spherical shape in the range 10-50 nm.
Nanometric Al Powder Type 02: Effects of Age on Burning Rate The steady burning rates of three identical formulations differing only for the time production of the used nano-Al type 02 are reported in Fig. 6. The formulations using nano-Al of recent production (2a and 2b) show very close results, while the formulation using nano-Al of old production (2c) features a moderate but systematic increase of burning rate.
4
P_05 reference P_02a 2003 P_02b 2002 P_02c 1999
3
burning rate, rb, mm/s
2
101 8 7 6 5 4 3
2
100
100
2
3
4
5
6
7 8
101
2
3
4
5
6
7 8
102
pressure, p, bar
Fig. 6 – Increasing steady burning rates of bimodal Al type 02 (bimodal AP-1).
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The physical characterization8 of the powders so far attempted failed to reveal any significant differences: the crystalline metallic Al decreases from 95.5% to 92.2% while the crystalline AlN increased from 4.3% to 7.8%. However, a literature survey shows that ageing of Al particles may be important19. In fact:
No significant effects take place for 17 µm (or more) Al particles, protected by a natural oxide layer of 0.5 µm thickness;
On the contrary, important effects take place for nano-Al in the range 0.1- 0.2 µm, due to surface deterioration leading to Al(OH)3 formation rather than Al2O3 and adsorption of gas (2-7% against 2-5% of oxidation products and 86-92% of Al metal for Alex20);
In general, anti-ageing coatings need to be implemented for passivation: for example, the commercial L-Alex uses an organic coating of palmitic acid.
Nanometric Al Powders Type 03: a Peculiar Behavior An effort was made to discriminate burning rates in the range 0.2 – 2.5 µm using nano-Al type 03. For propellants P_03-a and P_03-b, containing the same kind of Al type 03, the only difference is the nominal particle size, respectively 0.2 µm and 0.4 µm.
4
P_03a 0.2 micron P_03b 0.4 micron P_03c 0.8 micron P_05 reference
3
burning rate, rb, mm/s
2
101 8 7 6 5 4 3
2
100
100
2
3
4
5
6
7 8
101
2
3
4
5
6
7 8
102
pressure, p, bar
Fig. 7 – Similar steady burning rates for bimodal Al type 03a and 03b (bimodal AP-1).
Although showing an increased burning rate and decreased pressure exponent with respect to micro-Al (see P_06 and P_05), the two obtained rate curves are very close (Fig. 7). In fact,
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SEM analyses indicate that the real dimension of both Al powders is centered around 1.7 µm, with some minor differences in their size distributions around the average value, explaining the overlapping of the burning rate curves. The actual distribution of the Al powder type 03b and type 03c shows8 respectively a characteristic size of 1.73 µm and 1.86 µm, revealing a micrometric dimension of the corresponding Al grain sizes. Similar results are found for the Al powder type 03a. For all nano-Al type 03, the only recognized crystalline phase is metallic Al with no AlN. Also, the typical properties of nano-structures well manifest in nano-Al type 01 are missing in type 03. This surprising result stresses the need of a full characterization of the used nano-Al independently on their nominal specifications.
Nanometric Al Powders Type 04 A coated kind of nano-Al obtained by plasma condensation was also tested. The only difference for propellants P_04a and P_04b, containing the same kind of coated Al type 04, is the nominal particle size, respectively 0.20 µm and 0.28 µm. Although showing an increased burning rate and decreased pressure exponent with respect to micro-Al (see P_06 and P_05), the two obtained rate curves are close (Fig. 8).
4
P_04a-AP1 0.20 micron P_04b-AP1 0.28 micron P_05 reference
3
burning rate, rb, mm/s
2
101 8 7 6 5 4 3
2
100
100
2
3
4
5
6
7 8
101
2
3
4
5
6
7 8
102
pressure, p, bar
Fig. 8 – Similar steady burning rates for bimodal Al type 04a and 04b (bimodal AP-1).
The physical characterization8 of the powders revealed, as for nano-Al type 01 and type 02, the presence of two crystalline phases: metallic Al (98% in type 04a and 81.6% in type 04b)
16
and AlN (2% in type 04a and 18.4% in type 04b). Notwithstanding the large Al fraction of type 04a and the large AlN fraction of type 04b, steady burning rates were about the same (see Fig. 8). The protective oxide layer thickness of nano-Al type 04 was estimated8 to be about 20 nm against 10 nm of type 01 and type 02.
Effect of Binder Type (AP-1) The influence of two different kinds of HTPB binder is illustrated in Fig. 9: P_BL_02a is bonded with HTPB R-20, while P_BL_02b is bonded with HTPB R-45. Both propellants feature a monomodal AP-1 distribution with bimodal Al (50% micro type 05 + 50% nano type 01). While burning rates overlap near atmospheric pressure, pressure sensitivity for the two formulations is quite different: HTPB R-45 (n=0.408) is faster and neatly more pressure sensitive (almost twice !) than HTPB R-20 (n=0.237). Thus, the decreased pressure exponent due to monomodal AP-1 vanishes with HTPB R-45. At 70 bar, steady burning rate for HTPB R-45 is just twice than the value for HTPB R-20. However, curing is longer and yields a less dense composition; several properties of Alex and L-Alex concerning compatibility with and incorporation in HTPB and other binders were already elucidated19.
4
P_05 reference P_BL_05 P_BL_02a P_BL_02b
3
burning rate, rb, mm/s
2
101 8 7 6 5 4 3
2
100
100
2
3
4
5
6
7 8
101
2
3
4
5
6
7 8
102
pressure, p, bar
Fig. 9 – Comparing two different HTPB formulations (R-45 and R-20) using AP-1.
17
Summary of Experimental Steady Burning Rate Laws The standard Vieille ballistic laws and densities measured for the indicated tested compositions are summarized in Table 6. Practical reasons forced the authors to switch to a different supplier for AP; although the nominal specifications of the oxidizer are the same, ballistic properties were modified. Likewise, the binder had to be changed. A common remark is that monomodal nano-Al formulations (P_04a, for example) may yield sensibly higher propellant densities than micro-Al formulations (P_06, P_05, and P_BL_05). Vieille burning rate law, rb = a pn a n
Propellant Short Notation
Density (g/cm3)
P_01
1.67
2.42 ± 0.07
0.38 ± 0.01
P_02b
1.65
2.00 ± 0.04
0.44 ± 0.01
P_03d
1.56
1.46 ± 0.06
0.37 ± 0.01
P_04a
1.70
1.86 ± 0.04
0.36 ± 0.01
P_05
1.52
1.32 ± 0.03
0.39 ± 0.01
P_06
1.59
1.26 ± 0.03
0.41 ± 0.01
P_07-B01
1.63
2.04 ± 0.07
0.35 ± 0.01
P_07-B02b
1.60
1.47 ± 0.03
0.42 ± 0.01
P_07-B03d
1.57
1.27 ± 0.10
0.44 ± 0.03
P_08
1.63
1.58 ± 0.03
0.48 ± 0.01
P_09
1.67
1.57 ± 0.07
0.47 ± 0.02
P_BL_05
1.56
1.89 ± 0.07
0.34 ± 0.01
P_BL_02
1.63
3.25 ± 0.11
0.24 ± 0.01
P_BL_01a
1.65
5.17 ± 0.16
0.20 ± 0.01
P_BL_02a
1.61
3.53 ± 0.13
0.24 ± 0.01
P_BL_02b
1.54
3.59 ± 0.15
0.41 ± 0.01
P_Al_01
1.61
2.608
0.28
P_Al_02a
1.61
1.987
0.35
P_Al_02b
1.61
2.035
0.35
P_Al_02c
1.53
2.593
0.33
P_Al_03a
1.52
2.82 ± 0.25
0.26 ± 0.03
P_Al_03b
1.56
2.45 ± 0.14
0.30 ± 0.02
P_Al_03c
1.58
1.82
0.32
P-Al_04a
1.69
2.11 ± 0.09
0.30 ± 0.01
P_Al_04b
1.64
2.93 ± 0.02
0.23 ± 0.02
P_BL_01
P_Al_03d
Tab. 6 - Best fitting of Vieille steady burning rate law for all tested propellants.
18
In Series I propellants, bimodal Al compositions (P_07_B, P_08, P_09) with bimodal AP-2 and HTPB R-20 binder feature, with respect to the baselines P_05 and P_06, a burning rate sensibly increased by nano-Al replacement while the pressure sensitivity trend is slightly increased (manual mixing).
In Series II propellants, monomodal Al compositions (P_05, P_04a, P_03d, P_02b, P_01) with bimodal AP-2 and HTPB R-20 binder feature, with respect to the baselines P_05 and P_06, a burning rate sensibly increased by decreasing monomodal nano-Al (P_04a, P_01) while the pressure sensitivity trend is slightly affected.
In Series III propellants, with respect to the baselines P_05 and P _06, burning rates increase with decreasing pressure exponent for AP-1 replacing AP-2; on the opposite burning rates increase with increasing pressure exponent with HTPB R-45 replacing HTPB R-20. Combining a monomodal Al type 01 and monomodal AP-1 distribution in HTPB R-20 (see P_BL_01a) produced a propellant of high density with the fastest burning rate and smallest
pressure exponent of the group.
In Series IV propellants, all with bimodal AP-1 and HTPB R-20 binder, with respect to the baselines P_05 and P _06, bimodal nano-Al type 02 (compositions P_Al _02a, P_Al _02b, P_Al _02c) yields increased burning rates while pressure sensitivity is slightly decreased. Likewise, bimodal nano-Al type 03 yields burning rates sensibly increased while pressure sensitivity is decreased; this effect is more marked for nano-Al type 03a and 03b (compositions P_Al _03a, P_Al _03b) and less for Al type 03c (composition P_Al _03c). About the same effect is observed for bimodal nano-Al type 04a and 04b (compositions P_Al_04a, P_Al_04b). The reason being the fact that all of these propellants have the same formulations except for a 20% replacement of 30 µm Al with the same amount of the corresponding nano-Al, notwithstanding the different production technique (EEW for nano-Al type 02 and type 03 but plasma condensation for nano-Al type 04).
19
4. IGNITION DELAY While ignition of solid propellants is a largely investigated subject
21-22-23
. the behavior of
compositions including ultra-fine powders is scarcely investigated. The ignition delay vs. radiant flux, from a continuous wave CO2 laser source, was measured for several of the propellants investigated in this research project. The baseline propellant (P_06) showed the highest ignition delay. The reason can be attributed to the Al large thermal conductivity which slows down the surface temperature rise. By progressively replacing micro-Al with nano-Al, while keeping constant the total Al mass fraction (15 % for all the propellants considered), the ignition delay decreases. During laser heating, the enhanced reactivity of nano-Al powders manifests as an earlier build-up of the transient flame (leading to self-sustained combustion regime) with respect to not only micrometric Al but even unmetallized compositions. This is in qualitative agreement with the reported possible lower melting temperature of nano-Al24 and quicker25-26 ignition of nano-Al based reactive mixtures. For a matter of space, detailed results are not reported.
5. VISUALIZATION OF THE BURNING SURFACE A high-speed, up to 2000 fps at full resolution, digital color video-camera allows a convenient visualization of combustion surface phenomena, probably the controlling zone for burning rate augmentation due to ultrafine Al. Figures 10a - 10b show some frames taken from tests performed at 2 bar for propellants P_06 and P_02, respectively containing 100% 50 µm and 100% 0.1 µm Al particles. The brightness of the region immediately above the burning surface, detected during the combustion process of propellants with 0.1 µm Al particles, can be associated to the rapid combustion of Al in that region, which enhances near surface heat release, thus increasing surface temperature and burning rate. This observation, in qualitative agreement with the spectroscopic data by Weiser et al.27, explains the burning rate augmentation shown in Figs. 2-9.
The results of Fig. 10c show the details of the near burning surface region for P_01 propellant, revealing an unusual coralline formation reminiscent of the interparticle matrix material visualized in Fig. 1 (top pictures). Although the variety and complexities of the observed phenomena require further studies, the enhanced reactivity of nano-Al is well manifest. Ignition occurs when the protective oxide coating surrounding Al particles is cracked by fractures due to thermal effects. Near-surface hot AP/binder flamelets, called “Leading Edge Flames”, are supposed28 to be responsible for this phenomenon. 20
6. COLD AND HOT AGGLOMERATION Cold agglomeration, a characteristic trait of the fluffy material making nano-Al, takes place at room temperature (or close to) during storage, handling, and incorporation in the propellant batch. This kind of agglomeration is responsible of micro-sized clusters reducing the specific surface of nano-sized ingredients and thus works against the major, although not unique, rationale of their enhanced reactivity. All precautions should be taken, including coating of the particles, to lessen the propension of nano-Al to cold agglomeration (i.e., cluster of particles).
t = 0ms
t = 6ms
t = 12ms
t = 18ms
Fig. 10a – Burning surface of 50 µm Al flakes propellant at 2 bar (P_06, exposure time 1/500 s).
t = 0ms
t = 2ms
t = 4ms
t = 6ms
Fig. 10b – Burning surface of 0.1 µm nano-Al propellant at 2 bar (P_02, exposure time 1/4000 s).
Fig. 10c – Burning surface of 0.1 µm nano-Al propellant at 10 bar (P_01, exposure time 1/4000 s).
21
Hot agglomeration is a well-known trait of micro-Al occurring under high temperatures at the burning surface or in the gas phase of the combustion wave. Although possible also for nanoAl as well, size and combustion times15 of isolated nano-Al particles are so small that its occurrence is actually sensibly reduced; in addition, resealing after oxide layer crack is less likely to occur13. As a matter of fact, this is the main reason for the successful application of nano metals in advanced energetic materials for rocket propulsion. Babuk et al.9 show the influence of the oxidizer particle size on the mass of the hot agglomerates: a decrease in oxidizer particle size first decreases and then increases the agglomerate mass. Babuk11 also comments on the effect of binder properties on agglomeration concluding that the binder influence is determined by the content of carbon and easily gasifying elements in the binder itself.
Coating Al particles with high-melting metal films (Ni, Cu, Fe) decreases hot
agglomeration29-30; a similar effect can also be reached by using organic substances for coating31.
7. CONCLUSIONS AND FUTURE WORK AP/HTPB-based aluminized composite solid rocket propellants, containing nanometric Al particles, show faster steady burning rates compared to the corresponding propellants containing micrometric Al particles. This increase is mainly determined by the intense energy released by ultra-fine particle oxidation closer to the burning surface and may be further supported by the higher propellant density. Nano-Al particles manifest a strong reactivity (equivalent to an increase of the premixed heat release contribution) mainly due to an increased specific surface (from 1.27 m2/g of 17 µm Al to around 12 m2/g of Alex19) notwithstanding the simultaneous decrease of active Al content (from 99.6% of 17 µm Al to 88.2% of Alex19). But probably this effect is accompanied by others as well. Burning rate is not affected by particles in the micrometric range (say, ≥ 1 µm) but strongly increased by decreasing size particles in the nanometric range (say, 0.1-0.2 µm ); the intermediate interval 0.2-1 µm being currently little explored. Within the limit of the carried out experimentation, pressure exponent, while subject to minor changes for commercial grade AP, is reduced for monomodal AP of propulsive grade. This is reassuring in view of the pressure exponent increase observed in other investigations.
AP/HTPB-based aluminized composite solid rocket propellants, containing nanometric Al particles, show a earlier build-up of the transient flame with respect to not only micrometric 22
Al but even unmetallized compositions. Again, an appreciable decrease was found for nanoAl in the range 0.1 - 0.2 µm, while little effect was observed for micro-Al.
Experimental evidence indicates that ultra-fine Al enhances the near burning surface heat release. The intense luminosity, detected by the high speed video-camera recording, characterizing combustion processes in the region immediately above the burning surface when nano-sized Al is used, confirms the very fast Al combustion and improved flame structure in such region pointed out by other experiments and investigations32. The number and speed of particles / agglomerates gradually increase with decreasing micro-Al size in the range 90 - 15 µm, while a remarkable decrease of flame heterogeneity is observed for nano-Al in the range 0.1 - 0.2 µm.
In conclusion, clear support has been collected, in this investigation or from the competent literature, for increased burning rate, decreased ignition delay, increased heat feedback, vigorous subatmospheric burning, reduced hot agglomeration, more efficient metal burning, reduced specific impulse losses, and reduced particulate damping especially at low pressures33-34. However, it should be emphasized that the exact results may depend on the many details of the tested propellant formulation: in particular, the pressure exponent of steady burning rates has been observed to follow different trends according to the specifics of the composition. This fact suggests that, in addition to the sheer increase of the specific surface, other subtle chemical and physical factors (intermetallic compounds, crystalline structure, etc.) may be important in nano-Al enhanced reactivity.
Using nano-Al, the most common among nano-size ingredients, density and specific impulse are improved, while cost and safety aggravate. Cold (typical of nano-Al) and hot (typical of Al in general) agglomeration phenomena control the size of the metal particle before and during combustion. This in turn impacts many aspects of propulsive systems. Most of the nano-Al particles used in this research program had only a natural coating of oxide layer to protect against inadvertent ignition and slow down ageing. Next generation of nano-Al will focus on coated particles for a range of goals: passivation against natural oxidation and ageing (AlN is easily oxidized and hydrolyzed35), cutback of cold agglomeration propension, and reduction of hot agglomeration phenomena; coating can also be used to increase the powder energy contents35. On the other hand, an intentional passivation coating meant to prolong storage lifetime not only decreases active Al (from 88.2% of Alex to 86.1% of L-Alex19) but 23
was also shown to favor19 cold agglomeration; likewise, the natural passivation of oxide layer is known to decrease36 performance.
Questions still open and to be clarified in future work are as follows:
How does steady burn rate exactly scale with specific surface?
What are the achievable maximum values of specific surface?
What additional chemical or physical factors contribute to the enhanced reactivity of nano-Al powders?
What are the best coatings for nano-Al particles?
Will small-scale motor testing confirm the above strand burner indications?
From a practical viewpoint, the actual properties of the available nominal Al have to be carefully checked before any use. With due precautions, nano-Al can be handled with safety in lab scale experiments, but extension to motor scale is at this time an unknown.
ACKNOWLEDGEMENTS Financial support by AMRDEC (Contract No. N62558-04-M-0303), ASI (Contract No. I/R/37-00), and MURST (Contract No. MM09163342_001) is greatly appreciated.
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