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EUROPEAN COMMISSION

Deliverable no: D5.3 FOI designation no. FOI-2010-1487

Dissemination level: PU Date: 31/08/2012

HISP has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 262099.

Preliminary combustion evaluation

Filippo Maggi (POLIMI)

Stefano Dossi (POLIMI)

Marco Fassina (POLIMI)

Giovanni Colombo (POLIMI)

Luigi T. DeLuca (POLIMI)

John Zevenbergen (TNO)

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Deliverable no: D5.3 Due date of deliverable: 31/05/2012 Actual submission date: 31/05/2012 Updated: 31/08/2012 Version: 2 FOI designation no. FOI-2010-1487 Responsible: Filippo Maggi (POLIMI) Author(s): Filippo Maggi (POLIMI), Stefano Dossi (POLIMI), Marco Fassina

(POLIMI), Giovanni Colombo (POLIMI), Luigi T. DeLuca (POLIMI), John Zevenbergen (TNO)

Number of pages: 26 Dissemination level: Public Start date of project: 01/03/2011 Duration: 3 years Webpage: www.hisp-fp7.eu On the front page: SEM of the candidate ingredients tested in this deliverable

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Summary

HISP project aims to develop solid propellants with high specific impulse, leveraging on the

proper choice of binder, oxidizer and fuel. Work package 5 focuses on the use of new high-

energy density fuels. In the past reports, a full chemical and physical characterization was

performed on a set of powders proposed by partners and grouped into three main families:

nanometric aluminum, activated micrometric aluminum and aluminum hydride. After the

initial screening, a downselection was also performed, limiting the focus on one type of

nanometric aluminum (n-Al03) and one type of activated aluminum (A-Al02).

The present report focuses on the behavior of such ingredients in a standard propellant

formulation of the kind HTPB/AP/Fuel. Density, porosity, ballistic properties, ignition delay

and agglomeration attitude are characterized. Burning rate analysis was performed in a strand

burner up to 40 bar pressure, depending on the type of characterization. Ignition delay

investigation was performed at ambient pressure, in nitrogen atmosphere, using a CO2 laser

as heating source. Agglomerate measurements were conducted through a consolidated optical

technique. High-energy density fuels were used pure or in blend with standard micrometric

aluminum particles. Formulation details are given in the body of the report. Ignition

properties of the aluminum ingredients in air, already dealt in the deliverable D5.2, were

integrated with results relevant to multimodal mixtures.

Finally, one part of this report includes an integration relevant to the activities at TNO for

alane production, showing the status for the assessment of a process that leads to a stable

product over time. Compatibility with ADN is assessed as well.

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Contents

1 Introduction .................................................................................................................................... 5

2 Characterization of propellants ..................................................................................................... 7 2.1 Density analysis ................................................................................................................. 8 2.2 Ballistic characterization .................................................................................................... 8 2.2.1 Standard aluminum propellants ......................................................................................... 9 2.2.2 Monomodal-fuel propellants ............................................................................................ 10 2.2.3 Multimodal-fuel propellants .............................................................................................. 11 2.3 Radiant ignition delay ...................................................................................................... 12 2.4 Agglomeration .................................................................................................................. 16

3 Ignition temperature of fuel mixtures ........................................................................................... 19

4 Discussion on metal fuel application ........................................................................................... 21

5 Alane production at TNO ............................................................................................................. 23

6 Conclusions ................................................................................................................................. 25

7 References .................................................................................................................................. 26

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1 Introduction HISP project aims to develop high specific impulse propellants, taking advantage of proper

modifications in propellant composition. From one side, the objective is accomplished by

addressing an ADN/GAP baseline formulation which grants itself a higher specific impulse,

when compared to standard AP/HTPB propellants, as widely reported in the literature [1,2]

and in deliverable D1.1 of HISP project. On the other side, the use of high-energy density

ingredients should contribute to specific impulse increment by increasing the theoretical value

or reducing the losses. For this reason, three families of ingredients were investigated by the

HISP project in the body of the work package 5 which addressed three types of

nanoaluminum, three of micrometric activated aluminum and one batch of aluminum hydride.

Details are given in Table 1.

Table 1. High energy density fuels tested during HISP project.

Label Nom.

Size

Description

n-Al01 50 nm Industrial ALEX

TM nanoaluminum naturally coated

by aluminum oxide.

n-Al02 100 nm Industrial ALEX

TM nanoaluminum naturally coated

by aluminum oxide.

n-Al03 100 nm Industrial ALEX

TM nanoaluminum coated by stearic

acid before air exposure.

A-Al01 3 µm Activated aluminum powder 1 from Valimet.

A-Al02 3 µm Activated aluminum powder 2 from Valimet.

A-Al03 3 µm Activated aluminum powder 3 from Valimet.

Valimet 3 µm Spherical aluminum produced by Valimet, H3 cut.

Al-01 15 µm Space-grade spherical aluminum.

Al-02 70 µm Industrial-grade aluminum with shape of flakes

produced by Metalpolveri S.p.A.

Al-03 30 µm Space-grade spherical aluminum (added after the first

plenary meeting).

AlH3 Aluminum hydride

Powders were tested for basic physical, chemical and ignition properties as well as SEM

(scanning electron microscopy) and XRD (X-ray diffraction). Deliverable D5.2 collected the

whole set of results and, on this basis, from the initial list of fuels, only A-Al02 and n-Al03

powders were considered for further characterization. The selection resulted in a compromise

between high active metal content and powder reactivity.

Moreover, after the first plenary session hold in Milan at the beginning of March 2012, a

change in the baseline group was agreed, introducing the powder labeled Al-03, nominally 30

µm spherical which features a BET of 0.1 m2/g, surface-weighted mean diameter 31.15 µm

and mass weighted mean diameter 52.10 µm. A SEM performed on this powder is reported in

Figure 1. Powder distribution obtained through laser granulometry is reported in Figure 2.

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Figure 1. SEM micrograph of Al-03 powder.

Figure 2. Laser granulometry on Al-03 new baseline powder.

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2 Characterization of propellants Powders selected after the deliverable D5.2 are now used in standard HTPB/AP/Fuel

propellants. A general formulation reproducing an industrial standard is selected as reference

(see Table 2). The binder matrix is based on Sartomer HTPB R-45 oligomer, cured with an

isocyanate according to SPLab in-house standard procedures. Propellants are produced in

batches of about 95 g mass.

Table 2. Propellant general composition.

Id. Mass fraction, % Description

AP 200 58 Oxidizer, nominal size: 200 μm

AP 10 10 Oxidizer, nominal size: 10 μm

HTPB 14 HTPB-based elastomer

Fuel 18 ---

A total number of eleven propellants were produced and analyzed. Seven propellants used a

monomodal metal fuel, that is only one type and size of aluminum powder. This group

consisted in one composition containing a nanometric aluminum (n-Al03), two formulations

with activated aluminum (A-Al01 and A-Al02), and four standard aluminum-based

compositions containing Al-01, Al-02, Al-03, and Valimet. Two propellants were formulated

using a bimodal fuel where a minor fraction of nanoaluminum or activated aluminum was

compounded along with the baseline Al-03 batch. Also a propellant with a three-modal fuel

composition was compounded using a major quantity of baseline Al-03 and a minor amount

of n-Al03 and A-Al02, equally allocated. In multimodal compositions the mass ratio between

the high-energy density fuel and the standard aluminum was fixed to 3/15. In fact, on the

basis of previous investigations on nanoaluminum, performed both at the SPLab and by other

research groups [3,4,5,6,7], a stronger effect on ballistic performance can be obtained for

about 20% replacement ratio. Above this level, the benefit decreases progressively. Missing

specific investigations, this principle was here extended also to formulations containing

activated aluminum. Finally, a propellant with multimodal A-Al02 and n-Al03 was

considered. Details of the formulations are reported in Table 3.

Table 3. Propellant labels and relevant details of the fuel. Quantities are expressed on a mass-basis and referred to the whole propellant mass.

Id. Details of fuel

P-Al-01 18% of Al-01

P-Al-02 18% of Al-02

P-Al-03 18% of Al-03 (baseline)

P-Valimet 18% of Valimet

P-A-Al01 18% of A-Al01 (for comparison)

P-A-Al02 18% of A-Al02

P-n-Al03 18% of n-Al03

P-MIX-Al-03-A-Al02 15% Al03 and 3% A-Al02

P-MIX-Al-03-n-Al03 15% Al03 and 3% n-Al03

P-MIX-Al-03-n-Al03-A-Al02 15% Al03, 1.5% n-Al03 and 1.5% A-Al02

P-MIX-A-Al02-n-Al03 15% A-Al02, 3% n-Al03

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2.1 Density analysis

Measurement of propellant density is a quality indication of the formulation process. It is also

an indicator for the presence of bubbles or gas inclusions, left inside the bulk during the

production process or generated during the curing process by undesired chemical reactivity.

Measurement is performed by means of hydrostatic weighing technique, using ethyl alcohol

as working fluid. Three tests were performed for each propellant. Results and relevant

uncertainty are reported in Table 4. The theoretical propellant density, assuming 100% of

aluminum in the fuel, is 1.761 g/cm3. Porosity is computed as the relative difference between

experimental and ideal density.

Table 4. Density characterization of propellants. Uncertainty is computed assuming a t-student

distribution and a 95% of confidence.

Id. Density, g/cm3 Porosity, %

P-Al-01 1.739 ± 0.007 1.24

P-Al-02 1.744 ± 0.004 0.99

P-Al-03 1.751 ± 0.010 0.56

P-Valimet 1.749 ± 0.007 0.70

P-A-Al01 1.681 ± 0.004 4.45

P-A-Al02 1.727 ± 0.004 1.94

P-n-Al03 1.729 ± 0.005 1.81

P-MIX-Al-03-A-Al02 1.704 ± 0.022 3.22

P-MIX-Al-03-n-Al03 1.745 ± 0.006 0.89

P-MIX-Al-03-A-Al02-n-Al03 1.744 ± 0.004 0.94

P-MIX-A-Al02-n-Al03 1.743 ± 0.005 1.03

2.2 Ballistic characterization

The experimental ballistic characterization was performed in a strand burner of about 1 liter

volume where a set of servovalves, operated by a controller and a transducer, keep the

pressure constant. Burning rate evaluation is performed with an optical technique. A scheme

of the experimental rig is reported in Figure 3. Finally, data are correlated through a standard

Vieille law rb = a pn, where rb is the burning rate, p is the pressure in bar, a and n are fitting

constants experimentally determined.

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Figure 3. Scheme of the experimental rig for ballistic characterization. The same assembly is used for visualization of agglomerate formation during combustion.

2.2.1 Standard aluminum propellants

The first group of burning rate data reports about the ballistic performance of propellants

containing standard micrometric ingredients (see Figure 4). In this set of data it is possible to

find the baseline for future comparison (P-Al-03), the former baseline (P-Al-01), an industrial

flaky micrometric aluminum (P-Al-02) and the initial raw aluminum used for the activation

process (Valimet).

Valimet powder is the finest one and has the highest burning rate. The propellant featuring the

slowest burning rate contains the baseline powder Al-03. Nominally, the powder Al-02 is the

coarsest but the burning rate of its propellant is in the middle of the others. However, this

behavior becomes consistent if the result of BET analysis is considered. This flaky powder

also yields a rather low pressure exponent (n = 0.41). However, the maximum rb increase with

respect to the baseline composition obtained by this class of ingredients (standard powders,

micrometric range, no chemical activation) does not exceed 20%.

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Figure 4. Steady burning rate characterization: propellants containing standard aluminum fuels.

2.2.2 Monomodal-fuel propellants

The second group of combustion data are related to propellants fueled with advanced high-

energy density ingredients, both micrometric and nanometric (see Figure 5). The P-Al-03

baseline is plotted for comparison. With respect to the standard micrometric ingredients, the

propellant with n-Al03 powder (P-n-Al03) features a burning rate gain of about +85%. The

pressure exponent tends to increase slightly, being around 0.5. The propellant with A-Al02

powder (P-A-Al02) performs halfway between the baseline and the n-Al, with an increment

of about +40-50% of burning rate with respect to P-Al-03. In both cases, the pressure

exponent tends to slightly increase as well.

Figure 5. Steady burning rate characterization: propellants with monomodal high-energy density fuels.

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As a matter of comparison, propellants containing two different activated fuels (P-A-Al01 and

P-A-Al02) are characterized and compared in Figure 6 to the Valimet-based propellant (P-

Valimet) and P-Al-03 baselines. Such comparison should shed light on the effect of the

activation process and on the consequences of different activation strengths on the ballistic

performance. A-Al powders do not have a strong effect on burning rate increment, limiting

the gain to less than +20% with respect to Valimet raw aluminum. However, an effect on the

ballistic exponent of the Vieille law can be observed. Whereas the original Valimet powder

and the baseline aluminum have about the same value (n = 0.46-0.47), the activation process

progressively increases the sensitivity to pressure, in correlation to the strength of the

chemical modification. In fact, A-Al02 has a milder activation with respect to A-Al01, as

stated by the producer and confirmed by chemical analysis in deliverable D5.2.

Figure 6. Comparing steady burning rates of propellants with A-Al powders and Valimet.

2.2.3 Multimodal-fuel propellants

Finally, the results for propellants containing multimodal fuel blends are reported in Figure 7.

A 3% replacement of the standard fuel by nanoaluminum (P-MIX-Al-03-n-Al03) yielded a

gain of the rb of about +40-60% which represents a good compromise when compared to the

increment of +60-85% obtained for a propellant containing only n-Al fuel. This bimodal-fuel

composition, though limiting the presence of nanometric fuel to an extent of 3% over the total

propellant mass, shows the same increase of the ballistic exponent obtained by P-n-Al03.

Regarding activated aluminum, the increment obtained for the mixed composition (P-MIX-

Al-03-A-Al02) was about +15%, whereas the full substitution yielded an increase of +35-

45%. An interesting observation is the trend of the pressure exponents. This bimodal-fuel

propellant features a remarkable reduction of the pressure exponent in the Vieille law (n =

0.44), when compared to the corresponding monomodal-fuel propellant (n = 0.49 for P-A-

Al02). The value is even lower than in the baseline (n = 0.46 for P-Al-03). Moreover, when

the three-modal blend P-MIX-Al-03-A-Al02-n-Al03 is considered, even a more consistent

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decrement of the pressure exponent in the Vieille law is observed (n = 0.42). For such

propellant, it is interesting to note that the absolute increment of the burning rate in the range

30-40 bar is less than 0.5 mm/s. This fact may open the possibility of plateau burning in this

pressure range for this specific combination of ingredients but further investigation is

required, involving higher pressure combustion tests. Finally, results for the propellant

containing a blend of A-Al02 and n-Al03, without any other baseline metal powder, suggest

that this combination of fuels yields the highest burning rates within this group. The gain with

respect to the baseline P-Al-03 is about +65-70%. However, also the ballistic exponent is

relatively high (n = 0.52), comparable to the value obtained for n-Al propellants.

Figure 7. Steady burning rate characterization: propellants containing multimodal high-energy density fuels.

2.3 Radiant ignition delay

The evaluation of the radiant ignition delay was performed using a CO2 laser as heating

source. A scheme of the experimental rig is reported in Figure 8. A 10x10 mm propellant

sample is placed in a vessel with optical access while operating at an initial ambient pressure.

The vessel is purged and filled with nitrogen gas. A minor fraction of the laser beam is

diverted towards an infrared (IR) detector. A photodiode detects the ignition event. The

signals coming from both detectors are recorded by an oscilloscope. Ignition delay stems from

the direct comparison of the signals. Different radiant fluxes in the range of 30 to 100 W/cm2

were considered, performing at least three tests per condition.

The experimental results are summarized in the following figures. Details of interpolation are

reported for each propellant in Figure 9 a) through h). Finally, for a matter of global

interpretation, fitting curves are grouped into three different plots.

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Figure 8. Scheme of the experimental rig for evaluation of propellant radiant ignition delay.

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-Al03

y = (1946 ± 632.7) * x(-0.731 ± 0.079)

R2 = 0.892

a)

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-A-Al02

y = (1415 ± 413.9) * x(-0.821 ± 0.071)

R2 = 0.891

b)

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-n-Al03

y = (2373 ± 1272) * x(-1.007 ± 0.130)

R2 = 0.878

c)

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-MIX-Al03-n-Al03

y = (4473 ± 2008) * x(-1.057 ± 0.110)

R2 = 0.912

d)

Figure 9 continues on the next page

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Continued from the former page

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-MIX-Al03-A-Al02

y = (7300 ± 2856) * x(-1.100 ± 0.095)

R2 = 0.920

e)

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-MIX-Al03-A-Al02-n-Al03

y = (4826 ± 1614) * x(-1.071 ± 0.081)

R2 = 0.898

f)

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-Valimet

y = (8264 ± 2477) * x(-1.211 ± 0.074)

R2 = 0.902

g)

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-MIX-A-Al02-n-Al03

y = (9540 ± 3105) * x(-1.299 ± 0.079)

R2 = 0.960

h)

Figure 9 a-h. Ignition delay. Detailed results and fitting parameters for tested propellants. Standard deviations of fitting parameters as well as coefficient of determination are reported on the plots.

Fitting of experimental data was performed with the simplified thermal model ti=A/qm

[8,9,10]. Least-squares fitting procedure over the set of valid experimental data relevant for

each propellant is performed by means of the AxumTM

5.0 software. A rejection criterion

based on a 90% confidence interval is applied to raw data. Each fitting parameter is reported

in Figure 9 along with standard deviation and coefficient of determination.

Fitting laws show that the exponent m settles in the range between 0.7-1.3, suggesting that the

energetic materials under examination are subjected to volumetric absorption and/or reaction

under the effect of laser radiation. A totally opaque and inert condition would have resulted in

m = 2.0.

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101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-Al03 (baseline)

P-Valimet

P-A-Al02

P-n-Al03

a) monomodal fuel

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-Al03 (baseline)

P-MIX-Al03-n-Al03

P-MIX-Al03-A-Al02

P-MIX-Al03-A-Al02-n-Al03

P-MIX-A-Al02-n-Al03

b) multimodal fuel

101 1022 3 4 5 6 7 8 9

Radiant Flux, W/cm2

101

102

103

Ign

itio

n D

ela

y,

ms

P-Al03 (baseline)

P-A-Al02

P-n-Al03

P-MIX-A-Al02-n-Al03

c) A-Al and n-Al comparison

Figure 10 a-c. Comparison of ignition delay for different formulations: a) monomodal fuel formulations, b)multimodal fuel formulations, c) comparison between A-Al and n-Al propellants.

Comparison of the ignition performance among monomodal propellants is reported in Figure

10a). As expected, the most responsive formulation is based on n-Al, followed by A-Al and

then by Valimet while the baseline aluminum is the least reactive among this group. It is

interesting to note that Valimet and n-Al feature a similar steepness of the curve (m ≈ 1.0-1.2)

while A-Al and 30 μm Al have a reduced exponent (m ≈ 0.7-0.8). In this respect, Valimet and

its derived A-Al do not produce parallel curves. Thanks to the activation process, the ignition

delay of A-Al is close to the values obtained for n-Al for radiant fluxes in the lower range of

this investigation. On the contrary, the ignition delay at 100 W/cm2 overlaps the one

performed by Valimet.

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The results for propellants fueled with multimodal mixes are reported in Figure 10b). Ignition

delay decreases progressively as a blend with higher reactivity is embedded in the propellant.

Whereas the baseline has the longest ignition delay, the mixture P-MIX-A-Al02-n-Al03 (15%

A-Al and 3% n-Al) is the most responsive within this group with a reduction of up to 50%

with respect to the baseline, depending on the radiant flux. The P-MIX-Al-03-A-Al02 (15%

30 μ Al and 3% A-Al) does not demonstrate interesting differences for low fluxes, while

about 20-30% reduction is obtained at higher fluxes. The addition of only 3% A-Al may not

suffice to improve the overall performance. Also in the case of P-MIX-Al-03-n-Al03 and P-

MIX-Al-03-A-Al02-n-Al03 the presence of a small fraction (1.5%) of A-Al in the latter

composition does not provide an improvement and it is likely that n-Al is driving the

performance with a 30 to 45% reduction of the ignition delay, depending on the radiant flux.

Finally, Figure 10c) reports the ignition delay of n-Al, A-Al and its mix. The n-Al

composition has the lowest ignition delay but the mixture P-MIX-A-Al02-n-Al03 containing

only 3% of n-Al has quite similar performance and replicates the behavior of 100% n-Al at

higher radiant fluxes. Considering the high viscosity and the compounding problems

associated with a fully n-Al propellant, a formulation containing 15% A-Al and only 3% n-Al

results in a good performance compromise while enabling proper processing of the propellant.

2.4 Agglomeration

Metal fuels in solid propellants create a higher gravimetric specific impulse and density but

also generate condensed combustion products (CCP). A detailed description of the

agglomeration process was already given in Deliverable 5.1. In this section, visualization and

measurement of incipient agglomerates are presented. Measurements are performed by means

of an optical technique, post-processing high-speed and high-resolution video recordings. The

experimental rig is similar to the scheme reported in Figure 3, using a different video camera

and an appropriate optical apparatus that obtain a resolution of 6.8 μm/pixel. Combustion

experiments were performed at 10 and 20 bar. About 200 measurements of single

agglomerates per propellant are taken by means of a well-established hand-made

measurement process. Mass-mean diameter D43 of the agglomerates was measured at the

detachment from the combustion surface and is summarized for tested propellants in Figure

11.

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P-Al03

P-A-Al02

P-Valimet

P-MIX-Al-03-A-Al02

P-MIX-Al-03-n-Al03

P-MIX-A-Al02-n-Al03

P-MIX-Al-03-A-Al02-n-Al03

50 60 70 80 90 100 110 120 130

10 bar

20 bar

Mass-mean Diameter, D43, um

Figure 11. Mass-mean agglomerate diameters for tested propellants. No measurements were possible for P-n-Al03 and for P-MIX-A-Al02-n-Al03 at 20 bar.

Larger agglomerates are produced by the baseline propellant P-Al-03 while the finest result

from the multimodal propellant formulations containing n-Al. Sample pictures of propellants

burning at 10 bar are reported in Figure 12 and Figure 13. Considering the results obtained by

compositions containing A-Al, it is possible to say that the activation process does not deliver

benefits in terms of agglomeration. In fact, the P-A-Al02 and the P-Valimet feature very

similar results both at 10 and 20 bar. Also the P-MIX-Al-03-A-Al02 has the same

agglomeration behavior. Sample images are reported in Figure 12 b), d) and Figure 13 a).

It was not possible to perform measurements from videos of propellant P-n-Al03. Distributed

combustion of very fine CCP is observed at the burning surface (Figure 12 c). Agglomeration

tendency is very low and the field of view appears extremely confused. The same problem

occurred for P-MIX-A-Al02-n-Al03 at 20 bar. In propellants fueled with multimodal

powders, a small addition of n-Al contributes to almost 50% reduction of the mass-mean

agglomerate size. In the qualitative images reported in Figure 13 b), c) and d), smaller

agglomerates are present along with a number of very fine bright CCP, presumably produced

by the combustion of n-Al.

a) P-Al-03

b) P-A-Al02

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c) P-n-Al03

d) P-Valimet

Figure 12 a-d. Burning surface visualization of propellants containing monomodal fuels burning at 10 bar. Field of view is 5.5 mm wide and 4.1 mm high.

a) P-MIX-Al-03-A-Al02

b) P-MIX-Al-03-n-Al03

c) P-MIX-A-Al02-n-Al03

d) P-MIX-Al-03-n-Al03-A-Al02

Figure 13 a-d. Burning surface visualization of propellants containing mixed multimodal fuels burning at 10 bar. Field of view is 5.5 mm wide and 4.1 mm high.

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3 Ignition temperature of fuel mixtures Ignition of metal powders is correlated to the reactivity of the powder itself and, as a

consequence, to the agglomeration process in solid propellants. An extensive screening of

HISP advanced metal powder ingredients was already performed on this topic and results

were presented in Deliverable D5.2. In this document, multimodal fuel powder mixtures were

investigated thoroughly and, as support, ignition temperature in air is presented in this section.

The analysis was conducted by means of the experimental rig already described in

Deliverable D5.2, Section 3.2 and integrates the results already made available in the former

report.

Powder blends were produced by means of mechanical shaking using a ResodynTM

resonant

mixer. Powder mix identification labels and compositions (by mass) are reported in Table 5

while relevant results are given in Table 6. Figure 14 represents a global comparison among

ignition data collected in the HISP framework.

Table 5. Powder mix identification, respective propellant label and composition by mass.

Mix id. Propellant id. Details of powder blend

Valimet P-Valimet Valimet aluminum

Mix 1 P-MIX-Al03-A-Al02 15 parts Al-03 and 3 parts A-Al02

Mix 2 P- MIX -Al03-n-Al03 15 parts Al-03 and 3 parts n-Al03

Mix 3 P- MIX -A-Al02-n-Al03 15 parts A-Al02 and 3 parts n-Al03

Mix 4 P- MIX -Al03-nAl03-A-Al02 15 parts Al-03, 1.5 parts n-Al03 and 1.5

parts A-Al02

Table 6. Ignition temperature of mixtures in air at ambient pressure. Uncertainty is computed assuming a t-student distribution and a 95% of confidence. (*) Mix 4 has only one valid test.

Test No. Valid Tests T Ignition, K Uncertainty, %

Valimet 11 5 1155.2 ± 18.0 1.56

Mix 1 21 6 1104.0 ± 27.8 2.52

Mix 2 38 5 800.2 ± 13.3 1.66

Mix 3 30 4 776.9 ± 8.8 1.13

Mix 4* 17 1 883.1 N.A.

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Figure 14. Global comparison of HISP ignition temperatures in air at ambient pressure. Uncertainty is computed assuming a t-student distribution and a 95% of confidence. Mix 4 is not reported due to excessive measurement uncertainty.

Ignition temperature of mixtures do not follow peculiar trends but are influenced by the

presence of the most reactive (or inert) ingredient in the blend. Mix 1 is a mechanical mix of

standard 30 μm Al and A-Al aluminum (15/3 by mass). The ignition temperature settles above

1100 K, close to the value found for Valimet powder but more than 200 K higher than pure

A-Al. It is worth remembering that pure Al-03 powder did not ignite under the testing

conditions. Recalling the literature review already addressed in Deliverable D5.1, nanometric

ingredients ignite at temperatures even below the melting point of aluminum (933K) while, in

the micrometric range, it is necessary to wait for melting of the Al2O3 passivating layer

(above 2300K). Indeed, a minor fraction of A-Al02 confers ignitability to the mixture and

triggers the combustion of coarser powders but the thermal inertia of Al-03 delays the event.

Mix 2 and Mix 3 feature almost the same ignition temperature. They both have a minor

fraction of n-Al03 (3 %) with, respectively, standard 30 μm Al and A-Al (in both cases 15 %).

Mix 2 ignites only 25 K above Mix 3 due to the presence of activated aluminum. However, in

both cases their ignition temperature falls in the region of the n-Al, at 800K or below. In both

cases, the most reactive powder is represented by a minor fraction of n-Al and its ignition

drives the combustion of the whole heap of material. Thermal inertia of the other powder in

the mixture does not have an influence since it almost replicates the behavior of n-Al03 and

even when A-Al is used, the contribution of the activation process is rather limited.

Mix 4 was tested with the same procedure but only one valid test was obtained during the

campaign. The ignition temperature equaled 883K, which is coherent with the content of the

Mix (half of n-Al with respect to Mix 2 and 3). However, statistical validity of this result is

not assessed.

Generally speaking, temperature measurements during tests on all mixtures presented some

anomalies such as multiple ignition events, probably due to the heterogeneous solid-solid

blend. For this reason the rate of success in these experiments (ratio between the number of

valid tests and of performed runs) was very low, when compared to the monomodal test

series. Under these circumstances, a photodiode was also used to monitor the combustion and

to obtain a double-check of the ignition event. Despite several attempts, Mix 4 did not reach a

sufficient number of valid tests during the campaign.

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4 Discussion on metal fuel application It is now possible to come to a combined interpretation of combustion results and

characterization of HISP advanced metal fuels. From the dataset collected up to now it

appears that the reactivity grading obtained by the characterization of monomodal metal fuels

was only partially confirmed by propellant burning features. Nanoaluminum n-Al03 was the

most reactive material according to both pre-combustion and ignition temperature

characterization. An 10 m2/g specific surface was found for the reduced geometric size even

though the presence of an organic coating prevented from higher BET values. As a

consequence of high reactivity, metal ignition in air occurs at 771 K, in accordance with

results obtained for other uncoated nanometric fuels. When added to propellant P-n-Al03, the

reactive nature of the metal powder gave the best combustion characteristics among the group

of powders tested in this stage of the investigation. Comparing to baseline P-Al-03, an

enhancement of up to 85% was observed in ballistic properties while a reduction of 30 to 45%

in ignition delay was obtained. The same ignition tests yielded that the composition was more

opaque than the baseline. Regarding the agglomeration process, the reduction of CCP size

was so remarkable that a reliable measurement with the consolidated optical technique was

not possible even at 10 bar. The combustion of n-Al propellant generated a distributed

combustion of very fine combustion products. Large agglomerates in the measurable range

(above 30 μm in the current experimental rig) were very uncommon.

The pre-combustion and ignition investigation on A-Al02 and, in general, in the A-Al family

suggested that the chemical activation process improved the reactivity of the Valimet raw

powder and, in comparison to the baseline Al-03, a remarkable increment was observed.

Whereas standard 30 μm Al did not ignite in air with the SPLab in-house testing protocol and

Valimet ignited at 1155 K, combustion of the A-Al family occurred in the range 850 to 900K,

about 100 K higher than n-Al. However, this result was anticipated by BET which resulted in

a range of 2.2 through 3.3 m2/g, two-to-three times higher than Valimet. Despite some minor

differences in ignition temperatures within the A-Al family, ballistic properties of A-Al01 and

A-Al02 did not have any visible difference, meaning that the details of the activation process

did not influence the propellant behavior. Regarding ignition delay, P-A-Al02 showed a

reduction of the ignition time up to 30% with respect to the baseline, settling just above the P-

n-Al03 composition. Finally, a negligible effect was obtained in terms of agglomeration

process since its results were similar to the Valimet powder. In conclusion, despite an

enhanced reactivity of the powder itself, this type of fuel acts almost as a standard aluminum

when embedded in a propellant, as of steady combustion properties (namely, agglomeration

and burning rate).

Interesting results were obtained with the multimodal mixed fuel formulations. Ignition tests

on powder mixes demonstrated that the most reactive powder trails the combustion of the

blend. However, not all the powders showed adequate reactivity in the mixture. In fact,

whereas only minor improvements were obtained when a small amount of A-Al powder was

added to standard Al-03 (in ratio 3 to 15), the same addition of n-Al resulted in the same

reactivity of nanometric fuel to the whole mixture. It appears that ignition of nanometric

particles triggered the reaction of the whole group. Interesting improvements with partial n-Al

substitution were obtained also for burning rate (+60%) and for agglomeration (almost 50%

mass-mean diameter of agglomerates). Intermediate results were obtained for ignition delay.

Despite the reduced effect obtained with a composition when A-Al is used in minor quantity

or as pure fuel, very interesting improvements were obtained when A-Al02 was compounded

in a propellant with a minor fraction of n-Al03 (15/3 ratio). In this case, a 70% improvement

of the ballistic properties was obtained and ignition delay was reduced almost to the level of a

pure n-Al formulation. Also regarding agglomeration, a reduction of almost 50% in CCP

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mean size was obtained at 10 bar while it was not possible to measure agglomerate sizes at 20

bar since they were too fine. Intermediate ballistic results were obtained by the addition of a

small amount of n-Al and A-Al to Al-03, being correlated to the amount of n-Al. However,

both ignition delay and agglomeration features at higher pressure behaved like other

compositions containing Al-03 and n-Al03.

The mixed formulations actually represent an interesting solution when a compromise

between performance and processability is requested. Low amounts of nanometric fuel limits

the increment of viscosity as well as the interaction with the curing process. In this sense, the

best performing composition is represented by P-A-Al02-n-Al03, which includes both the n-

Al03 and the A-Al02. However, in terms of mission profile requirements, the burning rate of

this propellant is excessive. In this sense, formulation P-Al-03-n-Al03-A-Al02 yields a

similar ignition delay and interesting improvements in terms of agglomerate size. However,

ballistic performance increment is limited because only a reduced amount of n-Al is used.

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5 Alane production at TNO The synthesis process of and procedure for aluminum hydride was changed in light of the

purity and stability problems encountered in the original samples and highlighted in

deliverable D5.2. This has resulted in the production of only aluminum hydride without

aluminum as a side product and hence an improved impurity. The aluminum hydride phase

present is the mixture contains two or more crystal structures of aluminum hydride, but this is

merely a matter of adjusting the heating process to obtain the required crystal structure, being

alpha aluminum hydride. The other crystal structures all thermodynamically favor the

transformation to the alpha structure, as this is the most stable state.

Point of concern remains the fact that the nonstabilized aluminum hydride, as originally

offered by TNO in this project, seems to be incompatible with ADN, the major ingredient in

the formulation. To this end a small sample of stabilized aluminum hydride was used to check

the compatibility with ADN. The XRD spectrum of the stabilized aluminum hydride is shown

in Figure 15. From this figure it can be seen that there are two (crystalline) compounds

present, being alpha aluminum hydride and aluminum.

This sample was also subjected to thermal analyses, using TGA/DSC, with the results shown

in Figure 16. Using the data from this figure it can be calculated that the sample contains

roughly 95% alpha aluminum hydride, with the rest being aluminum. Knowing that the

sample was almost pure, this sample was mixed according to the STANAG standard in a

50:50 ratio with ADN and TGA/DSC measurements in triplo were performed. The result is

shown in Figure 17 and from the given curves and using the compatibility definitions of

STANAG, it can be concluded that ADN is fully compatible with stabilized alpha aluminum

hydride. As such, this potential show-stopper is off the table.

Curently research is focusing on stabilizing the aluminium hydride of TNO and subsequent

scale-up of this process.

Figure 15. XRD spectrum of stabilized alpha aluminum hydride.

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Figure 16. TGA measurement of stabilized alpha aluminum hydride.

Figure 17. TGA measurement of the 50:50% mixture of alpha aluminum hydride and ADN.

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6 Conclusions On the basis of these results, some recommendations can be issued for the prosecution of the

project.

The n-Al03 represents the best choice, when improvement of both burning rate and

agglomeration performance are considered. Tested HTPB/AP/Fuel propellants with

nanoaluminum outperform in terms of ballistic properties (+85% with respect to the baseline)

but they also feature a higher ballistic exponent. Two concerns may prevent the use of

nanoaluminum in HISP project: the limited aluminum content due to low active metal content

(from deliverable D5.2) and the high combustion rate for the specific mission profile (from

deliverable D1.1).

The first issue (low active metal content) does not represent a real problem. In fact, from

sensitivity analysis of the ideal specific impulse on active metal content in a GAP/ADN

propellant, the computed loss is about -0.15% per each percent decrement of metal content.

That is, mixed nano-micro propellant formulations have the advantage of maintaining high

metal content, which is granted by the micrometric fraction, while acting with the reactivity of

a nanometric fuel.

The second issue can represent a problem and must be thoroughly considered, especially

when ADN/GAP chemical systems are used in the second part of the HISP project. This class

of propellants tends to have a burning rate which is higher than AP/HTPB formulations and

the addition of a highly reactive metal is likely to further increment of the ballistic properties,

presumably increasing the pressure sensitivity as well.

The mixed micro-nano-activated aluminum represents an interesting option that should be

considered for its high metal content (granted by the micrometric fraction), for the limited

pressure exponent, and for its intermediate regression rate at higher operating pressure.

Among the tested compositions, two recommendations are issued. Composition P-A-Al02-n-

Al03 (fuel fraction made by 15 parts of A-Al02 and 3 parts of n-Al03) yields high regression

rate improvements, while the ignition time and agglomeration level are comparable to n-Al

propellants. Processability is easier, most of the ingredients being micrometric. Despite the

fact that the active aluminum content is reduced by the activation process of A-Al02 and the

oxide shell of n-Al03, a limited level of agglomeration more than compensates for the other

losses. If a slower propellant is required, the composition P-Al03-n-Al03-A-Al02 gives a

lower burning rate with respect to the composition mentioned above, but still superior to the

baseline. Higher metal content and processability are obtained as well as interesting

improvements in terms of agglomerate size and ignition delay. However, it is not possible to

assess right now if these properties can be reproduced in a composition with GAP and ADN.

In perspective of the HISP program, we should consider that both ADN/GAP propellants

(without additives) and ADN/GAP/Fuel propellants can represent a valuable choice, the

former granting specific impulse improvements itself when compared to current standard

formulations. Moreover, the effect of an active binder on the combustion of these innovative

metal fuels is quite unknown. As a general strategy, it would be important to have a parallel

experimental campaign involving on the one side the study of the ADN/GAP combination

and on the other the effect of a fuel, avoiding the use of complex additive combinations as

starting point. Progressive extension may be done later on.

Finally, the updated state for the development of a stable form of alane is encouraging, though

a production status has not yet been reached. However, from TNO’s activity one important

answer came out relevant to the compatibility of the alpha-stable alane with ADN. This

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uncertainty came after the first investigations performed on an unstable form of alane

suggesting the possibility of its non-compatibility (deliverable D5.2). Thanks to these

assessments, it is possible to consider alane as a candidate for HISP program, if a proper

production status will be reached.

7 References 1. K. Menke et al. Formulation and Properties of ADN/GAP Propellants. Propellants,

explosives and pyrotechnics, 34(3):218-30, 2009

2. Anders Larsson and Niklas Wingborg. Green Propellants Based on Ammonium

Dinitramide (ADN). Advances in Spacecraft Technologies, Chapter 7, 2011.

3. A. Dokhan et al. Combustion Mechanisms of Bimodal and Ultra-Fine Al in AP Solid

Propellants, AIAA Paper, No. 2002-4173, 2002.

4. L.T. DeLuca et al. Microstructure Effects in Aluminized Solid Rocket Propellants.

Journal of Propulsion and Power, 26(4):724-33, 2010.

5. L. Meda et al. Nano-Aluminum as Energetic Material for Rocket Propellants.

Materials Science and Engineering C, 27(5–8): 1393–96, 2007.

6. A. Olivani et al. Aluminum Particle Size Influence on Ignition and Combustion of

AP/HTPB/Al Solid Rocket Propellants, in: Advances in Rocket Propellant

Performance, Life and Disposal, MP-091, Paper 31, 2002.

7. K. Jayaraman et al. Effect of Nano-aluminum in Plateau-burning and Catalyzed

Composite Solid Propellant Combustion. Combustion and Flame, 156(8):1662–73,

2009.

8. L. Strakovskiy et al. Laser Ignition of Propellants and Explosives. Army Research

Laboratory, ARL-TR-1699, 1998.

9. L.T. DeLuca et al. Radiative Ignition of Double-Base Propellants: I. Some

Formulation Effects. AIAA Journal, 14(7):940-946, 1976

10. L.T. DeLuca et al. Radiative Ignition of Double Base Propellants: II.Pre-ignition

Events and Source Effects. AIAA Journal, 14(8):1111-1117, 1976.