# Al/SnO 2 Nanothermite ESD Desensitization by Means of the Elaboration of Tailored SnO 2 –Polypyrrole Composites

Virginie Goetz, Pierre Gibot

► To cite this version:

Virginie Goetz, Pierre Gibot. Al/SnO 2 Nanothermite ESD Desensitization by Means of the Elaboration of Tailored SnO 2 –Polypyrrole Composites. ACS Applied Materials & Interfaces, 2023, 15 (7), pp.9830-9840. 10.1021/acsami.2c20700. hal-04245721

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# Al/SnO₂ nanothermites ESD desensitization by means of the elaboration of tailored SnO₂-polypyrrole composites

Virginie Goetz*, Pierre Gibot

Laboratoire des Nanomatériaux pour les Systèmes Sous Sollicitations Extrêmes (NS3E)

ISL-CNRS-UNISTRA UMR 3208

Institut Franco-Allemand de Recherches de Saint Louis (ISL)

5 rue du Général Cassagnou 68301 Saint-Louis – France

Type of article: Research paper.

Declarations of interest: none.

* Corresponding author: E-mail: goetz.virginie.m@gmail.com (Virginie Goetz)

Co-author : pierre.gibot@cnrs.fr
**Abstract:**

Incidents can occur when handling energetic materials (EMs) due to their sensitivity to external solicitations. Mechanical (friction, impact) and electrical (electrostatic discharge (ESD)) stimuli can trigger unvolunteered ignition of EMs. Nanothermites, defined as highly reactive mixtures of metal and metal oxide, show interesting reactive properties (high temperature/amount of heat...). However, their extreme sensitivity to ESD is a significant drawback in their development. With a sensitivity threshold lower than the millijoule, and considering the human body capacity discharge at a few tens of millijoule, the handling of such energetic mixtures is extremely hazardous. ESD desensitization is therefore of crucial importance for the use of these materials. To achieve this goal, polypyrrole (PPy) conductive polymer was used as an additive within an Al/SnO₂ energetic formulation. This polymer is able to easily conduct the electrostatic charges which is useful to avoid an ignition after a spark solicitation. PPy was added in the nanothermite through the elaboration of SnO₂-PPy composites. Delaying the SnO₂ introduction time during pyrrole's polymerization (t = 0, 1, 24 h) changed the microstructure of composites. The various SnO₂-PPy composites highlighted that the additive's distribution within the composite is a key component for the modulation of ESD sensitivity. For example, this threshold is higher for a nanothermite formulated with the composite elaborated at t = 0 h (96.5 mJ with 7.5 vol.% of polymer) than a composition based on the composite at t = 24 h (16.9 mJ with 8.2 vol.% of polymer). Meanwhile, the reactive properties are decreased. The loss of reactivity (e.g. combustion speed) was explained by the thermal insulating property of the additive. Overall, the aim of this study is to help in the understanding of ESD desensitization of nanothermites in order to make them safer for operators and adapted to the required applications.

**Key-words:**

Spark sensitivity; polypyrrole; microstructuring; desensitization; energetic formulations; conducting polymer
# 1. Introduction

Electrostatic discharge, often referred as ESD, is a phenomenon known since decades and is defined as a sudden electric current between two electrically charged objects for these an electrical potential difference occurs ¹. This phenomenon explains many cases of explosions of dust, powders, flammable substances and/or fires ²⁻³. ESD is also a high constraint in the development of energetic materials considering their sensitivity to this solicitation. Any electrostatic solicitation could trigger an ignition of these specific materials due to an excessive charge accumulation on the particle surface ⁴⁻⁵.

Among energetic materials, nanothermites – mixtures of metal (fuel) and metal oxide (oxidizer) nanoparticles ⁶⁻⁷ – could not develop their potential because of their extreme sensitivity to electrostatic discharge ⁸. Even if there are many stated benefits (release of high amount of heat, modulation of components, easy processing, combustion velocity up to 2500 m/s ⁷⁻⁹), nanothermites react very easily to a spark stimulus (< 1 mJ ⁵, ⁸, ¹⁰). Given a human body capacity discharge value between 7 and 20 mJ ¹, ¹¹⁻¹³, the handling of nanothermites is hazardous. To prevent this risk and enable the development of these promising energetic materials, the scientific community has addressed the concept of desensitization for several years. Authors emphasized the modulation of sensitivity by adding a third component of various nature (carbon ¹⁴⁻¹⁷, polymers ¹⁸⁻²¹, oxides ²²⁻²³ and metals ²⁴⁻²⁵...). For example, adding carbon or polymer additive as nanotubes (in each case) lead to the raise of ESD threshold from 0,14 mJ (without additive) to 41 mJ and 120 mJ with respectively 3 wt.% of carbon ²⁶ and 5 wt.% of polymer ²⁰. However, the additive introduction has a negative impact on the reactivity of compositions especially with a decrease of the combustion velocity ¹⁴⁻¹⁵, ²⁰. Consequently, the addition of a third component within nanothermites should be performed with care in order to desensitize nanothermites without reducing their reactive behaviour.

Recent works showed the influence of the additive structuring to meet this challenge. Through elaboration of composites between a well-chosen additive and one component of the nanothermite (the oxide), it seems possible to reach a compromise by reducing the sensitivity properties and maintaining an acceptable reactive behaviour ²⁷⁻²⁸. Conductive polymers were selected as additive for their
remarkable electrical conduction properties<sup>29-30</sup>. They can help to prevent electrostatic charges accumulation on the reactant's particles surface which enable a raise of the electrostatic discharge sensitivity threshold. For instance, polyaniline (PAni) presents encouraging results for the ESD desensitization of nanothermite<sup>20, 27-28</sup>. Indeed, the elaboration of a SnO<sub>2</sub>-Polyaniline composite led to ESD desensitization (47,7 mJ) with only 2 wt. % of polymer within the Al/SnO<sub>2</sub> energetic composition<sup>28</sup>. In the same time, the combustion velocity was measured closed to 480 m/s. This property was kept at an interesting level, i.e. about 60 % of Al/SnO<sub>2</sub> binary energetic system (800 m/s). The structuring of the additive with the oxide leads to desensitization but also maintains the reactivity of the energetic mixture due to the small amount of additive introduced.

This study focuses on the investigation of the polypyrrole (PPy) conducting polymer and specifically on its microstructure in the SnO₂-PPy composite. The polymer was already used as a desensitizer thanks to a ternary addition within the energetic medium and showed promising results for the ESD desensitization <sup>31</sup>. The Al/SnO₂ composition, chosen for this work, is extremely sensitive to the ESD solicitation with a threshold below 0,14 mJ (device limits). This extreme value represents a significant risk for the operators during handling (human body discharge capacity estimated between 7 and 20 mJ) <sup>1, 11-13</sup>. To avoid any risk, the goal is to conduct efficiently and quickly the charges through the medium to avoid hot spots formation by using the conducting polymer. Based on the encouraging results obtained by using SnO₂-PAni composites, the elaboration of SnO₂-PPy composites was performed using in-situ polymerization <sup>32</sup>. Compared to the previous works, the aim of this study is to go further by focusing an additive's arrangement in the SnO₂-PPy composite. By a late addition of the oxide during the polymerization step, the polymerization occurs differently and different organisations were obtained between the oxide and the polymer. Thus, from an experimental point of view, two main parameters were modulated during the study: the introduction time of SnO₂ particles during the polymerisation and the polymer rate within the composites. All oxide-polymer composites were mixed with aluminium nanoparticles to formulate nanothermites. Sensitivity properties and reactive performance were evaluated and discussed herein.
## 2. Experimental section

### 2.1 Chemicals

Nanopowders of tin (IV) oxide (SnO₂, ≤ 100 nm powder) aluminium (pure Al wt.% of 69.3) were purchased from Alfa Aesar and Intrinsiq Materials, respectively. Pyrrole (C₄H₅N, ≥ 97.0 %), methyl orange (C₁₄H₁₄N₃NaO₃S, dye content 85 %) and acetonitrile (CH₃CN, > 99.8%) were obtained from Sigma Aldrich. Iron (III) chloride hexahydrate (FeCl₃·6H₂O) and hydrochloric acid (HCl, 37%) were from Carl Roth.

### 2.2 Synthesis of polypyrrole and SnO₂-Polypyrrole composites

The work of Sapurina et al. ³² was followed for the elaboration of the SnO₂-PPy composites. The different SnO₂-PPy composites were made by the oxide introduction at different times during the pyrrole polymerization process (t = 0 h, 1 h and 24 h).

For instance, in the experiment with an early SnO₂ addition (t = 0 h), 0.563 g of iron (III) chloride hexahydrate were dissolved in 125 mL of water. In a separate way, pyrrole (0.14 mL) was dissolved in water (125 mL), then 0.03 g of methyl orange were added. The molar ratio of iron (III) chloride hexahydrate to pyrrole monomer was equal to 1. After 30 seconds of ultrasounds, 0.5 g of SnO₂ particles were added under stirring. The solution of pyrrole-SnO₂ was added to the oxidant solution to start the polymerization. The mixture was stirred during 30 minutes and left at rest for 24 hours at cold temperature (5°C). Then, the solid was retrieved by centrifugation (4500 rpm, 4 min) and washed with 1 M hydrochloric acid and ethanol. Finally, the composite was dried under vacuum overnight at 80 °C. Concerning the preparation of the other SnO₂-PPy composites, the same process was followed but with a delayed SnO₂ introduction, 1 hour (t = 1 h) and 24 hours (t = 24 h) after the beginning of the polymerization. In particular, the SnO₂ addition was followed by 30 minutes of stirring to boost the mix of oxide within the medium. The concentration of polypyrrole in SnO₂-PPy was modulated from 1 to 20 vol %.
## 2.3 Formulation of ternary PPy-doped Al/SnO₂ nanothermites

The ternary PPy-doped Al/SnO₂ nanothermites were prepared by physical mixing of the SnO₂-PPy composites with aluminium. The combustion reaction between fuel and oxidizer can be written as (1):

$$4 \text{Al}_{(\text{s})} + 3 \text{SnO}_{2(\text{s})} \rightarrow 2\text{Al}_{2}\text{O}_{3(\text{s})} + 3 \text{Sn}_{(\text{s})} + \Delta\text{H} \quad (1)$$

where ΔH is the amount of energy released during the reaction.

To enhance reactive performance, an excess of aluminium is frequently preferred ³³. The excess of metal fuel is determined on the basis of the equivalence ratio (φ) in Equation (2). For the Al/SnO₂ energetic system, this ratio is set to 1.2 ³⁴:

$$\Phi = \frac{\binom{M_F}{M_O}}{\binom{M_F}{M_O}_{\text{st.}}} (2)$$

where M<sub>F</sub>/M<sub>O</sub> corresponds to the mass ratio of fuel (Al) to oxidizer (SnO<sub>2</sub>) and the subscripts “exp.” and “st.” mean experimental and stoichiometric conditions, respectively.

Experimentally, aluminium (Al), tin oxide (SnO<sub>2</sub>), polypyrrole (PPy) or tin oxide-polypyrrole composites (SnO<sub>2</sub>-PPy) nanopowders were mixed in a round bottom flask with 100 mL of acetonitrile. The dispersion was stirred during one hour with ultra-sonication interruptions (5 min each 30 min) to from a homogeneous mixture. The solvent was evaporated under 200 mbar of pressure at 80 °C until dry greyish powder was obtained. Before complete recovery, the resulting energetic powder was dried at 80 °C for 4 hours. An Al/SnO<sub>2</sub> composition without PPy additive was formulated for comparison purposes.

For the sake of clarity, the PPy-enriched Al/SnO₂ nanothermites were named according to SnO₂ introduction time - during the polymerization of pyrrole - and the polymer concentration. The ternary nanothermites with the composites elaborated with an early SnO₂ particles introduction time (t = 0 h) were identified with the upper-case alphabet namely Al/SnO₂-PPy-A (B C and D for the different PPy
concentrations). In the same way, the nanothermites formulated thanks to SnO₂-PPy composites with a late oxide introduction were labelled with the lower-case alphabet Al/SnO₂-PPy-a (b c and d) for 1-hour delay (t = 1 h) and with the Greek alphabet Al/SnO₂-PPy-α (β γ and δ) for 24-hour delay (t = 24 h).

## 2.4 Characterization techniques

The chemical composition of oxide-polymer composites was identified by Fourier transform infrared (FTIR) spectroscopy. The analyses were conducted using a Bruker Tensor 27 spectrometer in transmission mode (KBr disks). Spectra were recorded in the 4000 - 400 cm⁻¹ range with a resolution limit of 4 cm⁻¹. The concentrations of polymer within oxide-polymer composites were determined using thermogravimetric analyses (TGA). The analyses, performed on a SDT Q600 device from TA Instrument, were conducted between ambient temperature to 900 °C under air (100 mL/min) and following a 5 °C/min heating rate. The morphological characterizations of polymer-based components were performed by a scanning electron microscope (SEM FEI NovaSEM 450, FEI) working at 10 kV. The electrical conductivities of energetic compounds were measured with a two-point probing set up. The measurements were carried out with a low noise multimeter (Model 2010, Keithley). The powdered sample (± 50 mg) was placed in an insulating hollow cylindrical mold (inner diameter = 4 mm). A copper electrode was inserted on either side of the mold. The sample was pressed at 2.5 MPa (1 min), pressure allowing to have a stable electrical conductivity measurement for all investigated samples. The electrical conductivity was determined by using the Equation (3):

$$ \sigma = \frac{1}{R} * \frac{e}{S} \quad (3) $$

with R the electrical resistance (Ω, average value on five measurements taken all minute), e the thickness of the sample (cm) and S the section surface (cm²) of the pelletized materials.

The performance of nanothermites, especially the sensitivity and the reactivity properties, were characterized by using specific devices. These properties were determined on loose powders.
The sensitivities of the energetic compositions towards the mechanical (impact, friction) and electrostatic discharge solicitations were determined according to standard procedures from NATO organization ³⁵⁻³⁷. Impact and friction tests were performed, respectively, on a BAM Fall Hammer device (BFH-12) and on a BAM Friction apparatus (FSKM-10) from OZM Research. For the electrostatic discharge, the sensitivity tests were performed on an ESD 2008 testing device (OZM Research). These tests were described in details in the literature ²⁸, ³⁸.

The reactive properties were determined in an open burn configuration. The samples were ignited by means of an optical flash igniter equipped with a Xenon lamp and providing an energy density of about 0,11 J/cm² ³⁹. In details, a small amount of energetic powder (10 mg) was placed on glass substrate over the igniter device. To delimit the optical flash to the surface covered by the investigated sample, a metal washer (inner diameter = 4 mm) was inserted between the igniter and glass slide. The combustion process of nanothermites, prepared with and without polymer additive, was recorded using a Photron FASTCAM high-speed camera (10 000 frames per second).

## 3. Results and discussion

### 3.1 Characterization of SnO₂-Polypyrrole composites

The presence of the PPy conducting polymer in the organic/inorganic composites was verified by Fourier transformed infrared spectroscopy. For the sake of clarity, one SnO₂-PPy composite at each SnO₂ introduction time (t = 0 h, t = 1 h and t = 24 h) was represented in **Figure 1** (SnO₂-PPy composites with the polymer concentration modulation are available in *Supporting Information S1*). FTIR spectra show absorption bands of tin oxide and polypyrrole for the different composites. The vibration bands in the range of 500 – 700 cm⁻¹ correspond to the SnO₂ material ⁴⁰ (the black dashed rectangle in **Figure 1**). The two bands at 669 and 620 cm⁻¹ could be assigned to stretching modes of O-Sn-O and Sn-O bonds inside the tin oxide (IV) oxide phase, respectively. Concerning the polypyrrole, several characteristics absorption bands of the pyrrole ring were distinguished: at 1545, 1470 and 1170 cm⁻¹ corresponding to
C-C stretching vibration, C-N stretching vibration, and C-H/C-N breathing vibration, respectively. The peaks located at 1263 and 1315 cm⁻¹ were assigned to C-H or C-N in-plane deformation vibrations. The N-H⁺ deformation vibration was observed at 1092 cm⁻¹ and the peak at 1050 could be attributed to C-H or N-H in-plane deformation vibration. Below 1000 cm⁻¹, peaks were assigned to C-H and C-C deformation vibrations in the ring ⁴¹⁻⁴². The polypyrrole polymer was characterized as conductive salt form, according to the bands at 1545, 1170, 1092 cm⁻¹, even if a band at 1470 cm⁻¹ could be attributed to the deprotonation of polypyrrole ⁴¹⁻⁴². Other infrared bands (1620, 1600, 1385, 1115, 1006, 838 and 819 cm⁻¹) were assigned to the methyl orange used as a structuring agent in the synthesis (despite multiples washes) ³², ⁴³.

<figure><img src="image_2.png" /><figcaption>**Figure 1** : FTIR spectra of the SnO₂-PPy composites. PPy bands are identified from vertical lines and the SnO₂ material with the black dashed rectangle. The sign "+" correspond to IR bands of methyl orange.</figcaption></figure>

Then, the SnO₂-PPy composites were characterized by thermogravimetric analyses to determine the weight percentage of polymer within the inorganic/organic composites. **Figure 2** represents the thermogravimetric analyses of one SnO₂-PPy composite at each SnO₂ particles introduction time (t = 0 h, t = 1 h and t = 24 h. The *Supporting Information S2* gathers the TG analyses of the other SnO₂-PPy composites with an increasing polymer concentration. The different thermogravimetric curves were similar with two weight losses in the selected temperature domain. The first loss, in the temperature range between 25 and 200 °C, corresponds to the removal of water and solvent (ethanol) embedded in
the polymer matrix. The second weight loss, recorded between 200 and 600 °C, was attributed to the degradation of the polypyrrole organic phase ⁴¹. Above 600 °C, no mass loss was observed, confirming complete polymer degradation and thermal stability of the tin oxide (IV) material ⁴⁴.

<figure><img src="image_3.png" /><figcaption>**Figure 2** : Thermogravimetric curves (under air) of the SnO₂-PPy composites at three different SnO₂ particle introduction time (SnO₂-PPy-A for t = 0 h, SnO₂-PPy-α for t = 1 h and SnO₂-PPy-α for t = 24 h).</figcaption></figure>

The polymer concentrations within the SnO₂-PPy composites were reported in **Table 1** with volume percentage for a better understanding on the polymer distribution within the composite. The volume percentages of the species were calculated from the ratio of their own volume to the volume of a sample pellet (considering porosity). The volume of the species considered was determined from its mass (determined by TGA) and density.
<table><caption>**Table 1** : Summary table of the different SnO₂-PPy composites. All weight losses were determined thanks to TGA analyses shown in Supporting Information S2. From these weight losses, attributed to the removal of the polymer, the volume percentages of the polymer in the different composites (in the form of pellets used for the electrical conductivity measurement) were calculated.</caption><thead><tr><th></th><th></th><th>Weight loss in % (200 – 600 °C) from TGA analyses</th><th>Volume percentage calculated of PPy</th></tr></thead><tbody><tr><td rowspan="4">t = 0 h</td><td>SnO₂-PPy-A</td><td>12.9</td><td>19.5</td></tr><tr><td>SnO₂-PPy-B</td><td>6.9</td><td>11.2</td></tr><tr><td>SnO₂-PPy-C</td><td>4.1</td><td>7.7</td></tr><tr><td>SnO₂-PPy-D</td><td>2.2</td><td>4.5</td></tr><tr><td rowspan="4">t = 1 h</td><td>SnO₂-PPy-a</td><td>13.7</td><td>19.8</td></tr><tr><td>SnO₂-PPy-b</td><td>8.2</td><td>13.4</td></tr><tr><td>SnO₂-PPy-c</td><td>6.2</td><td>10.8</td></tr><tr><td>SnO₂-PPy-d</td><td>2.5</td><td>4.6</td></tr><tr><td rowspan="4">t = 24 h</td><td>SnO₂-PPy-α</td><td>14.9</td><td>19.8</td></tr><tr><td>SnO₂-PPy-β</td><td>8.4</td><td>13.0</td></tr><tr><td>SnO₂-PPy-γ</td><td>5.0</td><td>9.4</td></tr><tr><td>SnO₂-PPy-δ</td><td>1.5</td><td>2.7</td></tr></tbody></table>

The microstructures of the SnO₂-PPy composites were characterized by SEM (Figure 3). Two kinds of populations were distinguished in each sample (regardless the SnO₂ particle's introduction time): spherical particles and fibrous elements. The spherical particles could be attributed to the tin (IV) oxide particles and the fibrous elements to polypyrrole material. Indeed, according to the work of Sapurina et al. 32, the polymerization of pyrrole leads to the preparation of long nanotubes (several micrometres in length), as characterised by TEM analysis (Supporting Information S3). Figure 3 highlights that the oxide particles introduction time has an influence on the SnO₂-PPy composite microstructures. With a tin oxide introduction at the beginning of the polymerization of pyrrole (SnO₂-PPy-A, Figure 3A and B) or only one hour after (SnO₂-PPy-a, Figure 3C and D), the composites present a majority of SnO₂ spherical particles and a low amount of fibrous elements. By contrast, with a lately SnO₂ particles
introduction time (SnO₂-PPy-α, **Figure 3E** and **F**) more fibrous elements can be seen. The quantity of fibrous elements seems to increase with the delay time of oxide addition. The introduction of SnO₂ at an earlier stage of polymerization (t = 0 and 1 h) seems to prevent the formation of nanotubes but instead promote the formation of a polymer coating on SnO₂ particles (in an inhomogeneous way). On the other

<figure><img src="image_4.png" /><figcaption>**Figure 3** : SEM images of the SnO₂-PPy composites. A and B corresponded to SnO₂-PPy-A (t = 0 h), C and D to SnO₂-PPy-α (t = 1 h), E and F to SnO₂-PPy-α (t = 24 h).</figcaption></figure>

hand, when the oxide was added at the end of the polymerization, the nanotubes grow (independently) as nanotubes of several microns in length.

## 3.2 Characterization of ternary Al/SnO₂-Polypyrrole nanothermites
The different composites SnO₂-PPy were mixed with an aluminium nanopowder to formulate Al/SnO₂-PPy nanothermites with a polymer content in the range of 1.5 and 12.4 vol.% (Table 2).

<table><caption>**Table 2** : Summary table of the concentration of PPy in the SnO₂-PPy composites and the equivalence for the Al/SnO₂-PPy nanothermites. Composites and nanothermites in pellet form.</caption><thead><tr><th></th><th></th><th>PPy rate in SnO<sub>2</sub>-PPy (vol. %)</th><th>PPy rate in Al/SnO<sub>2</sub>-PPy (vol. %)</th></tr></thead><tbody><tr><td rowspan="4">t = 0 h</td><td>SnO<sub>2</sub>-PPy-A</td><td>19.5</td><td>12.0</td></tr><tr><td>SnO<sub>2</sub>-PPy-B</td><td>11.2</td><td>7.5</td></tr><tr><td>SnO<sub>2</sub>-PPy-C</td><td>7.7</td><td>3.9</td></tr><tr><td>SnO<sub>2</sub>-PPy-D</td><td>4.5</td><td>2.1</td></tr><tr><td rowspan="4">t = 1 h</td><td>SnO<sub>2</sub>-PPy-a</td><td>19.8</td><td>11.4</td></tr><tr><td>SnO<sub>2</sub>-PPy-b</td><td>13.4</td><td>8.5</td></tr><tr><td>SnO<sub>2</sub>-PPy-c</td><td>10.8</td><td>6.4</td></tr><tr><td>SnO<sub>2</sub>-PPy-d</td><td>4.6</td><td>2.5</td></tr><tr><td rowspan="4">t = 24 h</td><td>SnO<sub>2</sub>-PPy-α</td><td>19.8</td><td>11.5</td></tr><tr><td>SnO<sub>2</sub>-PPy-β</td><td>13.0</td><td>8.2</td></tr><tr><td>SnO<sub>2</sub>-PPy-γ</td><td>9.4</td><td>4.9</td></tr><tr><td>SnO<sub>2</sub>-PPy-δ</td><td>2.7</td><td>1.5</td></tr></tbody></table>

### 3.2.1. Electrical conductivity of PPy-enriched Al/SnO₂ energetic composition and sensitivity to electrostatic discharge (ESD)

Before processing to the determination of ESD sensitivity thresholds, electrical conductivity measurements were performed on the different Al/SnO₂-PPy nanothermites (Figure 4).
<figure><img src="image_5.png" /><figcaption>**Figure 4** : Electrical conductivity of Al/SnO₂-PPy composites as a function of the PPy concentration (vol.%).</figcaption></figure>

According to **Figure 4**, the conductivity changed when the PPy concentration increased within nanothermites, regardless of the SnO₂ particle's introduction time (t = 0 h, t = 1 h, t = 24 h). This increase comes from the electrical conduction of the pure PPy value, measured at 0.85 S/cm. In details, in the Figure 4, two trends were observed:

- For less than 5 vol. %, low values of electrical conductivities were reported. By comparison to the conductivity value of the nanothermite without polymer (0 vol. %, σ = 6.16 x 10⁻⁷ S/cm), it seems that the introduction of SnO₂ particles during polymerization (t = 0, 1, 24 h) had no significant impact on the electrical conductivity.

- On the contrary, for more than 5 vol. %, the electrical conductivity raises for all type of composites and as a function of the PPy concentration. Here, the microstructure of the conducting polymer in the SnO₂-PPy composite has an influence on the electrical conductivity values. A delayed introduction time of SnO₂ (t > 1 h) led to higher conductivity values in contrast to nanothermites with SnO₂-PPy composites synthesized at the beginning of the polymerization (t = 0 h). For example, the conductivity value of the nanothermite elaborated from a SnO₂-PPy composite at t = 0 h and with 7.5 vol. % of polypyrrole (σ = 4.14 x 10⁻⁷ S/cm) was two orders of magnitude lower
than that obtained for a nanothermite consisting of SnO₂-PPy composites created at t = 1 and 24 h, measured at 1.58 x 10⁻⁵ S/cm (8.5 vol. %) and 6.76 x 10⁻⁵ S/cm (8.2 vol. %), respectively. This difference argues that the organization of polymer within the nanothermite have an influence on the ability to conduct the charges. As suggested in study about oxide-polymer composite microstructure (section 3.1, **Figure 3**), the SnO₂-PPy composite present more nanotubes of polymer as the delay of SnO₂ particle introduction is important (t = 1 and 24 h). With a large amount of nanotubes, it might be easier to develop a continuous three-dimensional conductive network within the nanothermite and thus obtain significant electrical conductivity values. SEM analysis seems to validate this assumption of a continuous network with the presence of an important amount of polypyrrole nanotubes within the energetic sample (**Figure 5C D E and F**). For the sample where SnO₂ particles have been added early (t = 0 h), the electrical conductivity values were lower. In this case, the conductive phase is less significant due to the lack of polypyrrole nanotubes (which enables the conductive phase to be the three-dimensional) (**Figure 5A and B**).
<figure><img src="image_6.png" /><figcaption>**Figure 5:** SEM images of the Al/SnO₂-PPy nanothermites. A and B correspond to SnO₂-PPy-A (t = 0 h, PPy rate = 12 vol. %), C and D to SnO₂-PPy-α (t = 1 h, PPy rate = 12.4 vol. %), E and F to SnO₂-PPy-α (t = 24 h, PPy rate = 11.5 vol. %).</figcaption></figure>

Then, PPy-enriched Al/SnO₂ nanothermites were characterized by means of ESD sensitivity tests (Figure 6). All values were compared to the sensitivity threshold of the Al/SnO₂ binary nanothermite (< 0.14 mJ) to quantify the impact of the introduction of a conductive compound on the ESD sensitivity. Moreover, to guarantee the operators safety and by extent to be sure that the nanothermite is effectively desensitized, the human body discharge capacity is taken as reference at 20 mJ (values in literature are between 7 and 20 mJ ¹, ¹¹⁻¹³).
<figure><img src="image_7.png" /><figcaption>**Figure 6:** ESD sensitivity as a function of the PPy concentration (vol.%).</figcaption></figure>

**Figure 6** shows different evolution of the sensitivity thresholds according to the oxide-polymer composite structuring. Electrostatic discharge sensitivity thresholds were raised with the addition of PPy regardless the type of SnO₂-PPy oxide-polymer composites prepared. All thresholds exceed the value of the polymer free Al/SnO₂ nanothermite (0.14 mJ). For nanothermites with an earlier SnO₂ introduction time within composites (t = 0 and 1 h), sensitivity thresholds exceeded the reference value (20 mJ) with a PPy rate around 5 vol. %. On the contrary, for a delayed SnO₂ particles introduction, a higher amount of PPy (> 8 vol. %) is required to overcome this value. Moreover, when the polymer rate increased (11 – 12 vol. %), the nanothermite with an earlier SnO₂ introduction (t = 0 and 1 h) showed a higher ESD sensitivity threshold than nanothermite with a delayed oxide introduction (t = 24 h): 246.40 mJ for t = 0 and 1 h against 120.74 mJ for t = 24 h. This difference in ESD sensitivity threshold can be surprising. Indeed, similar electrical conductivities were measured for these compositions (10⁻² – 10⁻³ S/cm, **Figure 4**). It seems that the electrical conductivity study of nanothermite is not the only factor to explain the trend of the ESD sensitivity threshold of nanothermites. As mentioned before, the evolution of ESD threshold for Al/SnO₂-PPy at t = 0 h and 1 h were similar as their organisation: they have some fibrous element in low amount and as well as a polymer layer on the oxide particles (especially for nanothermites at t = 0 h) (**Figure 5A B C D**). With this kind of structuring, electrostatic charges could be dissipated easier thanks to the polymer which is present in a homogenous way inside of the energetic
matter. Polymer and oxide materials were closely linked together and there were many contact areas between the two elements. This was advantageous to prevent the charges accumulation during the spark solicitation. Due to the presence of the polymer, the aluminothermy reaction needs more energy to ignite. As a result, the ESD sensitivity threshold gets higher. By contrast, for the Al/SnO₂-PPy t = 24 h nanothermites, a lot of PPy nanotubes were identified (Figure 5E F) and a heterogeneous distribution of PPy and SnO₂ were observed in all the volume of nanothermite. In this case, the nanothermite ignition required a lower energy. The ESD threshold remained at a risky level: the polymer did not play the role of charges dissipater. These results highlight the importance of the homogeneous distribution of conductive polymer for desensitization of nanothermite.

### 3.2.2. Sensitivity tests to mechanical solicitations (friction, impact)

Mechanical stimuli sensitivities of the different PPy-rich nanothermites were studied by means of friction and impact tests. The values measured are gathered in Table 3. The Al/SnO₂ binary nanothermite, taken as a reference, has an insensitivity to friction and impact tests, with thresholds over devices limits: i.e. > 360 N and > 100 J, respectively.

<table><caption>**Table 3** : Mechanical sensitivity thresholds of the Al/SnO₂-PPy energetic composites developed from SnO₂-PPy composites at t = 0, 1 and 24 h.</caption><thead><tr><th></th><th></th><th>PPy rate in Al/SnO<sub>2</sub>-PPy<br>(vol. %)</th><th>Friction (N)</th><th>Impact (J)</th></tr></thead><tbody><tr><td rowspan="4">t = 0 h</td><td>Al/SnO<sub>2</sub>-PPy-A</td><td>12.0</td><td>&gt; 360</td><td>&gt; 100</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-B</td><td>7.5</td><td>&gt; 360</td><td>75</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-C</td><td>3.9</td><td>324</td><td>90</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-D</td><td>2.1</td><td>288</td><td>&gt; 100</td></tr><tr><td rowspan="4">t = 1 h</td><td>Al/SnO<sub>2</sub>-PPy-a</td><td>12.4</td><td>&gt; 360</td><td>&gt; 100</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-b</td><td>8.5</td><td>&gt; 360</td><td>&gt; 100</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-c</td><td>6.4</td><td>&gt; 360</td><td>90</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-d</td><td>2.5</td><td>192</td><td>60</td></tr></tbody></table>
<table><tr><td rowspan="4">t = 24 h</td><td>Al/SnO<sub>2</sub>-PPy-α</td><td>11.5</td><td>> 360</td><td>> 100</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-β</td><td>8.2</td><td>> 360</td><td>> 100</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-γ</td><td>4.9</td><td>252</td><td>> 100</td></tr><tr><td>Al/SnO<sub>2</sub>-PPy-δ</td><td>1.5</td><td>324</td><td>> 100</td></tr></table>

At first, all nanothermites studied have sensitivity thresholds above international standards defined for the transport of dangerous goods and established at 80 N for the friction and 2 J for the impact tests <sup>45</sup>. Therefore, Al/SnO<sub>2</sub>-PPy nanothermite could be transported on public roads without any risks regarding mechanical solicitations.

On the one hand, a slight phenomenon of sensitivity to friction was noted for nanothermites with less than 5 vol. % of PPy (Table 3). To explain this trend, a hypothesis could be formulated in relation with the thermal decomposition of polypyrrole, which occurs at lower temperature (200 - 500 °C, Figure 1) than the nanothermite ignition (> 500 °C <sup>46</sup>), could trigger this phenomenon. The gaseous residues from the decomposition of the polymer can heat the Al and SnO<sub>2</sub> reactants, with the formation of hot spots on their surface leading to an earlier thermite reaction. For nanothermites with a higher PPy content, the amount of polymer is enough to act as a physical barrier between the reactive species <sup>28</sup>, thus requiring more energy for ignition, resulting in higher friction sensitivity threshold values.

On the other hand, the sensitivity to impact was quite different depending on the kind of Al/SnO<sub>2</sub>-PPy nanothermites. For the Al/SnO<sub>2</sub>-PPy t = 24 h nanothermites, no mitigation on impact sensitivity was noted. A sensitization was observed at low PPy content (< 7 vol. %) within Al/SnO<sub>2</sub>-PPy at t = 0 and 1 h. The difference of sensitivity threshold could be explained by a more homogenous PPy distribution for these latter compositions, leading to a possible thinner polymer thickness between aluminium and oxide nanoparticles. With a thinner polymer thickness, the polymer can decompose more easily because of the profitable thermal energy available (coming from the transformation of mechanical to thermal energy during the impact phenomena). As a result, the compression of PPy decomposition gases can lead to the formation of hot spots at the Al/SnO<sub>2</sub> interface and allow the ignition of the nanothermite. For energetic composition with more than 7 vol. %, the polymer content is enough to delay the ignition
by the formation of a physical barrier between reactive with low thermal conductivity. More energy is needed to obtain the ignition, meaning a raise of sensitivity threshold.

### 3.2.3. Reactivity test in open burn configuration

The reactive properties of Al/SnO₂-PPy nanothermites were studied in an open mode using a flash technique described in section 2.4. Taking into consideration the ESD sensitivity results presented in the previous section, the reactive properties of nanothermites were focused on compositions in accordance with safety reference (20 mJ), meaning that only nanothermites with more than 5 vol. % of polypyrrole for the series at t = 0 and 1 h, and more than 8 vol. % for t = 24 h, are described here. The data about combustions events of the different Al/SnO₂-PPy nanothermites is available in *Supporting Information S4*.

The combustion of nanothermite without any polymer was quick and complete (no residual matter was found on the glass slide). An intense luminous flash was noted and the combustion time was determined equal to 4.6 ± 0.1 ms (Figure 7).

The reactive behaviour of Al/SnO₂-PPy nanothermites depend on the polypyrrole content within nanothermite: the higher the amount of polymer, the slower the combustion rate (Figure 7). This could be explained by the low thermal properties of the polypyrrole and especially the thermal conductivity with a value between 0.02 – 0.12 W/m.K⁴⁷. This value is lower than the other reactants within the nanothermite: i.e 27.3 W/m.K for Al₂O₃⁴⁸⁻⁴⁹ (at the surface of aluminium particles), 30 W/m.K for SnO₂⁵⁰ and 237 W/m.K for aluminium⁵¹. In this way, the polymer acts as a thermal barrier. It consequently restricts the heat transfer through the energetic material. That is why combustion time of PPy-rich nanothermites were increased, meaning a decreased of the combustion rate. In Figure 7, it appears that the combustion time varies slightly with the nanostructuring of polymer with the oxide. For example, a comparison was made between the nanothermites with high PPy rate meaning: Al/SnO₂-PPy-A (PPy rate = 12 vol. %), Al/SnO₂-PPy-a (PPy rate = 12.4 vol. %), Al/SnO₂-PPy-α (PPy rate = 11.5 vol. %). Among these three energetic compositions, the Al/SnO₂-PPy-α nanothermite (t = 24h) showed a faster combustion time (66.0 ± 1.0 ms) than the two other compositions (83.0 ± 1.0 ms for Al/SnO₂-PPy-a and
69.0 ± 1.0 ms for Al/SnO₂-PPy-A). However, considering the gap between the above-mentioned values, it seems to be quite difficult to argue that the reactive behaviour really depends on the polymer microstructure within nanothermites.

<figure><img src="image_8.png" /><figcaption>**Figure 7: Combustion time (ms) as a function of the PPy concentration (vol.%).**</figcaption></figure>

### 3.3 Discussion

According to the literature, the desensitization of nanothermites was studied by the introducing different kind of additives such as carbon<sup>14, 16, 17, 26</sup>, oxides<sup>22-23</sup>, metals<sup>24-25</sup> and polymers<sup>18-21</sup>. Encouraging results were obtained after adding conductive polymers as a third component<sup>20, 27, 28, 31</sup>. The introduction of conductive polymer as an additive was done by two main ways: as an individual third component and as via the elaboration of a polymer/oxide composite before adding aluminium fuel. These two ways showed a possible desensitization to ESD but at different level.

Indeed, according to previous works<sup>20, 28</sup>, the addition of polyaniline (PAni) as a third component required a significant amount of polymer to increase the ESD threshold. The morphology of polymer was investigated and the authors showed that fibres of polymer with a high L/Ø shape factor (length (L) = 1 - 2 µm, diameter (Ø) = 50 nm) were more advantageous to conduct easily the charges within the medium. The addition of long fibres of polymer increased the ESD threshold with 5 wt.% of PAni (ESD
= 120 mJ) ²⁰. Nevertheless, by the structuring between the oxide and the polymer additive, the desensitization to spark solicitation (ESD = 47 mJ) was obtained with the addition of only 2 wt. % of PAni ²⁸. The authors highlighted the important effect of the additive dispersion to prepare desensitized nanothermites. The elaboration of oxide-additive composites leads to a homogenous polymer dispersion within the energetic material, which is very efficient and suitable for rapidly conducting the charges.
Therefore, a better raise of ESD threshold was achieved in this case.

Thanks to this current study, it seems interesting to consider the additive microstructure as a parameter for the ESD desensitization of nanothermites. The close link between the oxide and the polymer is a significant parameter in order to rapidly dissipate the charges after the electrostatic solicitation. Based on this work, it is better to formulate the composite at the beginning of the polymerization in order to prepare a composite with a polymer covering around the oxide particle surface. Compared to the literature, the sensitivity threshold reached here (ESD = 24 mJ - 6.4 vol.% of PPy or 4.2 wt.% of PPy), which allows to consider the composition as desensitised, is slightly higher than that reported in 28.
These authors highlighted that 2 wt.% of additive is enough to desensitize the nanothermite (ESD = 47 mJ). Nevertheless, the comparison is a bit tricky because different polymers were used in these two studies: polypyrrole (in this work) versus polyaniline ²⁸.

A complete study of desensitization should take the reactive aspect into account. By the introduction of an inert component with low thermal properties as those of the conducting polymer, the reactive properties were slowed down compared to the composition without additive (Figure 7). But combustions events were maintained for the different energetic compositions (*Supporting Information S4*), which is encouraging for the preparation of safer and reactive nanothermites.

## 4. Conclusion

In this study, the oxide-conductive polymer structuring was investigated for the desensitization of nanothermites.
Different kind of structuring between the oxide and the conductive polymer additive were proposed especially by delaying the oxide particle introduction time during the polymerization process; i.e at t = 0 h, 1 h and 24 h (end of polymerization). By delaying the SnO₂ particles' introduction time, the microstructure of the inorganic/organic composite was clearly modified. This had consequences on ESD sensitivity of derived energetic composites. When the oxide was added earlier during the polymerization, an ESD desensitization was achieved at low polymer content (5 vol. %) than structuring obtained after a long time of polymerization. The closer structuring between the polymer and the oxide is a key factor for the desensitization because it leads to a better conduction capacity and thus easier charges dissipation making energetic composites safer for operators. Tests in open burn configuration were done on the different energetic compositions. Unfortunately, the low thermal conduction property of polypyrrole coupled to its role of physical barrier between reactive components slowed the reactivity down. Shapes of incandescent particles confirmed the maintaining of combustion, even if it is less violent than in the case of binary nanothermite combustion.

The next step could be the elaboration of a composite with all the components of the nanothermite - the aluminium (Al), the oxide (SnO₂) and the additive. A one-step synthesis could be imagined to combine the elaboration of the nanothermite as well as the polymerization of the additive. This could lead to a complete structuring between the Al/SnO₂ nanothermite and the polymer while limiting the handling by the operator. This specific structuring would lead to the formation of a continuous conducting phase between the reactants, which could promote new performances of the as-formulated Al/SnO₂/additive energy formulation.

## Supporting Information

Additional graphs of FITR and TG analyses are available on Supporting Information S1 and S2. SEM images as well as images of combustion phenomena are available on Supporting Information S3 and S4.

## Acknowledgments
The authors gratefully acknowledge the French Defence Innovation Agency (DGA/AID, Paris, France)

French-German Institute of Saint-Louis (ISL, Saint-Louis, France) for the operational sponsorship and

the financial support of the Ph.D. project.

The authors thank MM. Boehrer (ISL, Saint-Louis, France) and Oudot (NS3E, Saint-Louis, France) for
the combustion video recordings and mechanical sensitivity tests respectively and MM. Schnell and
Lallemand (NS3E, Saint-Louis, France) for the scanning electron microscopy analyses. They also thank
M. Morel-Fourier and Mrs Schmelz (ISL, Saint-Louis, France) for the detailed proofreading of the
manuscript.

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