Pierre Gibot
email: pierre.gibot@isl.eu

Virginie Goetz
email: virginie.goetz@isl.eu

SnO2-polyaniline composites for the desensitization of Al/SnO2 thermite composites

Keywords: Nanothermites, Tin (IV) oxide, Electrostatic discharge, Conducting polymer, Reactive properties

Nanothermites consisting of a reducing metal and a metal oxide nanopowder represent a new generation of energetic materials in pyrotechnics due to their impressive reactive properties.

However, their extreme sensitivity regarding electrostatic discharge appears to be detrimental to their future practical applications. Herein, the mitigation of the sensitivity thresholds of the aluminium/tin (IV) oxide energetic nanocomposite is successfully achieved by using a conducting polymer, polyaniline (PAni). PAni was introduced within the thermite by the chemical polymerization of an oxidizer/PAni hybrid matrix. The SnO2-PAni composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, electrical conductivity measurements and transmission electron microscopy. The derived Al/SnO2-PAni thermites were investigated in term of sensitivities and reactivity. Results revealed gradual desensitization of the Al/SnO2 thermite as a function of the concentration of PAni for both the electrostatic discharge (0.14 mJ to 1212 mJ) and friction (216 N to 360 N) tests while maintaining reactive energetic composites. This work presents a way for the preparation of insensitive and reactive energetic formulations.

Introduction

Energetic composites or thermites are physical mixtures made of fuel and oxidizer powders having an exothermic redox reaction characterized by impressive heat release and high temperature. Fischer et al. have established a list of possible energetic mixtures, by varying the chemical nature of the reactants, accompanied by their thermochemical data. [START_REF] Fischer | Theoretical energy release of thermites, intermetallics, and combustible metals[END_REF] When downsized, thermites called nanothermites or metastable intermolecular composites (MICs) have attracted a growing interest for both civil and military applications due to their many advantages compared to their micron-scale counterparts. For example, nanothermites show higher combustion velocities, lower ignition delay times and reduced ignition temperatures. [START_REF] Pantoya | [END_REF]3 However, nanothermites can present some drawbacks such as sensitivity toward diverse forms of stresses leading to unwanted burning during the preparation, handling and transportation steps.

The stresses, which can cause such unexpected effects/reactions, are essentially of mechanical and electrostatic nature. For example, most of the thermite compositions are extremely sensitive to electrostatic discharge (< 1.5 mJ) 4 widely below what that the human body can generate (8.33-20 mJ) 5,6 , and in more isolated cases, they are sensitive to the friction phenomenon which can develop between moving particles. The high mechanical and electrostatic discharge sensitivities may limit the use of such energetic nanocomposites in numerous potential applications. The desensitization of thermite energetic nanocomposites may enable important applications.

In the literature, the mitigation of the sensitivity of the nanothermites is addressed either by acting on the fuel (especially when aluminium is used), modifying the particle size and changing the alumina/aluminium ratio constituting the fuel particles 7,8 , or inserting a third compound (desensitizer) within the conventional binary fuel/oxidizer energetic mixture. [9][10][11][12][13][14][15][16][17][18][19] With regards to the use of an additive, different kinds of carbonaceous compounds (carbon black, graphite, nanotubes, nanofibers, graphene and nanodiamonds), polymers (polytetrafluoroethylene and polyaniline) and inorganic species (molybdenum sulphide) have been tested, and many of them have a positive effect on the electrostatic discharge (spark) desensitization as well as a lesser effect toward mechanical stress such as friction. While the conducting properties of desensitizers are exploited when the energetic composite is submitted to the spark test, their lubricant properties are solicited during the friction phenomena. The different compounds were added either as "isolated/individual" nanoparticles [9][10][11][12][13][14][16][17][18] within the fuel/oxidizer energetic composite or through the synthesis of a desensitizer/oxidizer or a desensitizer/fuel composite 15,19,20 before it was mixed with the fuel or the metal oxide to form the final energetic mixture. The use of polyaniline conducting polymer (PAni) as an additive is particularly interesting and promising, as reported by Gibot et al. 13 For example, when added to an Al/WO3 nanothermite, the PAni additive allowed the considerable mitigation of the electrostatic sensitivity (with the friction threshold increasing) while preserving an acceptable reactivity for the energetic mixture considered. 13 In this work, the conducting polyaniline nanofibers were separately synthesized before they were added to the fuel/oxidizer nanocomposite.

Herein, the addition of the intrinsic conducting polyaniline polymer (PAni) in an aluminium/tin (IV) oxide energetic mixture was examined. This time, the polymer was introduced within the thermite from the preparation of an inorganic/organic hybrid material such as tin (IV) oxide/polyaniline material (SnO2-PAni). The hybrid material was prepared by the chemical polymerization of aniline in an aqueous dispersion of SnO2 nanoparticles. The as-prepared SnO2-PAni composites have been characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, electrical conductivity measurements and transmission electron microscopy. The sensitivity properties and combustion ability of the Al/SnO2-PAni thermites were investigated as a function of the concentration of PAni additive.

Experimental section

Tin (IV) oxide material (SnO2, < 10 µm, 99%) and aniline (C6H5NH2, 99.5%) were purchased from Sigma Aldrich. Aluminium nanopowder Al-50P (Al, 50 nm, 99%) and potassium dichromate (K2Cr2O7) were obtained from Nanotechnologies Inc. and Merck, respectively. Hydrochloric acid (HCl, 37%) and acetonitrile (CH3CN, 99%) were obtained from Prolabo. All chemicals were used without any further purification.

Elaboration of the SnO2-polyaniline composites

The SnO2-PAni composites were synthesized according to a method reported by Chen et al. 21 Typically, 300 mg of SnO2 powder was vigorously dispersed in 50 mL of demineralized water before acidification of the solution to 1 mol/L by means of hydrochloric acid. While stirring, 0.1 g of K2Cr2O7 was added, followed by 0.2 mL of aniline monomer. The resulting solution was stirred for 10 min., ultrasonicated for 30 min. and stirred again for 6 hours. The solution stood for at least 24 hours at room temperature for chemical polymerization of aniline. The suspension was centrifuged, and the collected solid phase was washed (x 3) with demineralized water and ethanol and then it was dried at 60 °C for 6 hours. Finally, the as-obtained green material was ground in a mortar. Different SnO2-PAni composites were thus prepared by fixing the concentration of PAni at 3.7, 7.5, 11.2 and 14.7 wt. %. It should be noted that the values will correspond to concentrations of PAni of 2.5, 5, 7.5 and 10 wt. %, respectively in the Al/SnO2-PAni thermites.

The aniline/potassium dichromate weight ratio was kept constant for all of the syntheses. The different samples were denoted SnO2-PAni-X, where X is the theoretical weight percent of polyaniline polymer within the composite considered.

Preparation of the Al/SnO2-polyaniline thermites

The SnO2-PAni composites were mixed with the aluminium nanopowder Al-50P to form the energetic mixtures whose combustion reaction is given in (1):

4Al + 3SnO2 → 2Al2O3 + 3Sn (1)
According to the stoichiometry, the fuel/oxidizer weight ratio or equivalence ratio (Φ) is equal to 0.239 for the energetic formulation investigated. The equivalence ratio is calculated from the formula reported in (2):

Φ = (F/O)exp./(F/O)st. ( 2
)
where F and O are the concentrations of fuel (reactive Al) and oxidizer (SnO2), respectively, and the subscripts exp. and st. are the experimental and stoichiometric values, respectively. It should be noted that aluminium nanoparticles exhibit a core/shell structure with a native alumina layer (Al2O3) surrounding a pure aluminium core representing 52.2 wt. % of the sample (determined by means of thermogravimetric analysis).

The Al/SnO2-PAni energetic mixtures were prepared as follows: the passivated-Al powder and the SnO2-PAni-X composite were mixed in a round-bottomed flask with 100 mL of acetonitrile according to an excess of aluminium fuel of 1.2 (Φ). The dispersion was stirred (1 hour) and sonicated (3 x 5 min.) to break up agglomerates and obtain a homogeneous solid phase mixture.

The removal of the solvent was achieved by means of a rotary evaporator (100 rpm, 80 °C and 200 mbars). The resulting mixture was placed in an oven at 80 °C for 4 hours. The process was repeated with each SnO2-PAni-X composite while keeping the same experimental parameters.

The as-prepared energetic formulations were denoted as Al/SnO2-PAni-X. An Al/SnO2 nanothermite without PAni additive was formulated as a reference.

Characterization techniques

Characterization of the SnO2-PAni composites

The structures of the SnO2-PAni nanocomposites were examined by X-ray diffraction analysis performed on a D8 Advance diffractometer (XRD, Cu K radiation,  = 0.154026 nm, Bruker) operating at 40 kV and 40 mA. The concentration of polyaniline (PAni) in the different SnO2-PAni composites was determined by thermogravimetric analysis (TGA) performed on a SDT Q600 device (TA Instrument). The composites were heating between 25-1000 °C under air (100 mL/min.) with a 5 °C/min heating rate. The Fourier transform infrared (FTIR) spectroscopy was used to investigate the chemical compositions of the SnO2-PAni composites. The analyses were performed on a Tensor 27 spectrophotometer (Bruker) using the transmission mode (KBr pellets).

The samples were dried in a desiccator overnight. Spectra were recorded between wavenumbers ranging from 450 to 4000 cm -1 and with a resolution limit of 4 cm -1 . The microstructures of the composites were determined by transmission electron microscopy (TEM) using an ARM-200F microscope (JEOL, Japan) operating at 200 kV. Samples were dispersed in ethanol, and one drop was supported on a carbon-coated copper grid. For the electrical conductivity measurements a two point probing set up similar to was used. 10 A high resistance low conductance meter (HRLC 2, Alpha Lab Inc., USA) was used to measure the electrical resistance (in Ohms) with an accuracy of 2%. The sample (± 50 mg) was located in an insulating hollow cylindrical matrix (inner diameter: 4 mm) to prevent interference from the external sources. Two copper electrodes, connected to the HRLC device, were inserted to each either end of the matrix till being in contact with the sample. Prior recording the electrical resistance, the sample was pressed at 2.5 MPa for 60 s. The electrical conductivity (in Siemens per meter, S/m) was determined by using the equation given below:

𝜎 = 1 𝑅 × 𝑒 𝑆 (3)
with R the electrical resistance (Ω), e the thickness (m) and S the section surface (m 2 ), respectively, of the sample through which the current passes.

Characterization of the Al/SnO2-PAni energetic composites

The sensitivities of the thermites toward the mechanical (impact and friction) and electrostatic discharge stresses were determined according to the standard characterization techniques defined by the NATO organization. [START_REF]STANAG 4489. In: Standardization Agreement (STANAG) on Explosives, Impact Sensitivity Tests[END_REF][START_REF]STANAG 4487. In: Standardization Agreement (STANAG) on Explosives, Friction Sensitivity Tests[END_REF][START_REF]STANAG 4490. In: Standardization Agreement (STANAG) on Explosives, Electrostatic discharge Sensitivity Tests[END_REF] Photographs of the different devices are given in the Supporting Information (Figure S1). Regarding the impact and friction sensitivities, a fall-hammer apparatus and a Julius Peter BAM device (Suceska 1992) were used, respectively. The impact sensitivity threshold (expressed in joules) is determined by submitting 40 mm Combustion tests of the thermites in an unconfined configuration were performed by means of an optical flash igniter equipped with a Xenon lamp. [START_REF] Ohkura | [END_REF] The powdered mixtures were placed on a glass substrate placed on the optical flash igniter. A metal washer (thickness of 1 mm, inner diameter of 4 mm) interlaced between the igniter and the glass holder was used to delimit the optical flash and avoid luminous disturbance during the recording. Under these conditions the powders were submitted to an energy density of 0.11 J/cm 2 . The combustion phenomena were recorded by using a Photron FASTCAM high-speed camera (1×10 4 frames/s) placed in front of the igniter.

Results and Discussion

Characterizations of the SnO2-PAni composites

The different as-prepared SnO2-PAni-X composites were characterized by X-ray diffraction (XRD). The corresponding X-ray patterns, which are all similar, are shown in Figure 1 (B to E).

The diffraction peaks can be attributed to a tetragonal structure with a P42/mnm space group, which describes a rutile structure as adopted by the SnO2 Cassiterite phase described in the JCPDS card No. 041-1445 (Figure 1A). The diffraction peaks of the composites were not subjected to any significant position modifications compared to the international card, suggesting that the experimental protocol involved for the preparation of the SnO2-PAni composites does not modify the structure of the SnO2 oxide ceramic. The polyaniline component probably exhibits an amorphous or a poorly crystallized structure since no other peaks were observed in the diffractograms. 21 To verify the presence of polyaniline conducting polymer in the inorganic/organic composites, Fourier transform infrared analyses (FTIR) were performed on the different SnO2-PAni-X samples. The corresponding FTIR spectra are shown in Figure 2 and compared to the pure PAni and SnO2 phases spectra. Two main domains were observed for wave numbers near 800 cm -1 .

Below this limit, the wide vibration band located between 500-700 cm -1 is characteristic of tinoxygen bonds. 26 The three peaks observed at 535 cm -1 , 604 cm -1 and 673 cm -1 can be assigned to the stretching modes of Sn-O bonds (terminal and bridged) in the SnO2 tin (IV) oxide phase.

Regarding the peaks above 800 cm -1 , they are in good agreement with the vibration bands characteristic of the pure PAni phase (Figure 2E). Moreover, a small shift to higher wavenumbers was observed that can be originated to SnO2-PAni chemical interactions. Based on the literature 27 , the vibration bands at 1560 cm -1 and at 1488 cm -1 can be assigned to the C=C stretching and bending vibrations of the quinoïd (Q) and the benzenoïd (B) rings, respectively, in PAni polymer exhibiting the emeraldine salt phase. The peak at 823 cm -1 describes the out-of-plane bending vibration of the C-H bond in a meta-substituted aromatic cycle, while the band at approximately 1300 cm -1 is characteristic of the C-N stretching vibration of the amine group with an aromatic ring. The IR bands observed at 1245 cm -1 and 1146 cm -1 correspond to the C-N + stretching vibration in the polaron lattice and the vibration mode of the -NH + = in the Q=NH + -B or B-NH + -B groups. Their presence translates to a high degree of electron delocalization. Finally, the two vibration bands observed at 1640 cm -1 and around 3200 -3400 cm -1 on the SnO2 spectrum (Figure 2F) were attributed to the vibration modes of physisorbed water molecules at the surface of the material. Thermogravimetric analyses (TGA) were performed to determine the mass percentages of organic and inorganic matter in the different as-synthesized SnO2-PAni hybrid materials (Figure 3). All of the thermogravimetric curves were similar and presented three mass losses in the temperature domain investigated. From room temperature to 200 °C, the mass loss was attributed to desorption of water molecules on the composite surface. The second phenomenon, recorded between 200-350 °C, may correspond to the removal of the by-products formed during the polymerization of aniline (visible on the TGA curve of pure PAni, not shown here). Finally from 350 to 550 °C, the degradation of the organic phase takes place. The different concentrations of organic phase within the SnO2-PAni-X nanocomposites, whose theoretical X values were 3.8, 7.5, 11.2 and 14.7 wt.

%, were thus experimentally determined (from the TGA analysis) to be equal to 2.8, 6.4, 11.6 and 16 wt. %, respectively. The electrical conductivity values of the different SnO2/PAni-X composites are presented in Table 1. The conductivity of the pure PAni phase, synthesized according to the same experimental procedure except without the addition of SnO2 particles, is given for comparison. %, presented in the Supporting Information (Figure S3), a significant increase of the conductivity is recorded for a PAni concentration higher than 6% followed by a pseudo-plateaus below 12 wt.

%. This trend suggests that the percolation threshold of the conducting phase is probably around 8 -10 wt. %. To remember, the percolation threshold is the value for which a minority phase in a composite forms a homogeneous and continue network in all the considered composite. In the current case, the network is conducting and allows the flow of current through all the matter without interference. Further characterization was carried out by TEM to observe the microstructures of the SnO2-PAni-X nanocomposites. Figure 4 shows the SnO2-PAni composite prepared with a PAni concentration of 11.6 wt. % (value determined by TGA). As seen in the image at low magnification, the composite is constituted of agglomerates of SnO2 nanoparticles (dark areas) embedded in a PAni matrix (light areas). The insert in Figure 4 (higher magnification) shows obvious evidence supporting this result. No isolated SnO2 primary particles covered by the polymer were visible, but in contrast, in some places of the view, elongated alone PAni particles were observed. From meticulous observation, uncoated SnO2 agglomerates were also observed.

Accordingly, based on these microstructural observations, it may be concluded that the SnO2-PAni-11.6 composite does not seem homogeneous even if the SnO2-PAni multimaterials represent the majority of the composite. Finally, based on these analyses, the synthesis of SnO2-PAni nanocomposites with the oxide component embedded in a conducting polymer matrix was demonstrated.

Pyrotechnic properties of the Al/SnO2-PAni thermites

The SnO2-PAni-X nanocomposites were mixed with the aluminium nanopowder according to an equivalence ratio () of 1.2 in order to prepare the corresponding Al/SnO2-PAni-X energetic composites (thermites). Taking into account the aluminium quantity added, the concentrations of PAni (X) within the Al/SnO2-PAni-X energetic composites were calculated again to be equal to 1.8, 4.3, 7.7 and 10.8 wt. %, respectively. These data will be referenced in the text below. The performances of the various thermite mixtures, in term of sensitivity and reactive behaviour, were assessed and are discussed below.

Sensitivities of the Al/SnO2-PAni thermites

The interest in adding polyaniline within the Al/SnO2 thermite is to raise its electrostatic discharge sensitivity threshold. Since the polymer is an electrical conducting material, it should be able to channel the electrical energy through the energetic formulation forcing it to bypass the reactant powder (fuel/oxidizer). 10 Thus, the energy density (energy provided by the electrostatic discharge), which could appear at the surface of the aluminium particles, will not be enough to lead to ignition of the mixture. Accordingly, no joule heating at the surface of the aluminium component will occur, and no hot spots will be formed. Hot spots are generally considered to be the origin of the ignition phenomena of energetic materials. 28 The electrostatic discharge sensitivity thresholds of the Al/SnO2 energetic composites doped or not doped with PAni are gathered in Table 2. A value of 0.14 mJ was determined for the Al/SnO2 reference thermite (without the addition of PAni), which is far below the energy dissipated by the human body (8.33-20 mJ). 5,6 At this extremely low level, the energetic composite shows a considerable hazard since an unwished or out of control decomposition reaction can occur during operator handling or transport causing injuries and damage. As expected, the use of the conducting polyaniline allows the electrostatic discharge (ESD) sensitivity of the binary Al/SnO2 thermite to be mitigated ( The literature data relating to the ESD desensitization of the thermites are gathered in Table 3.

Basically, the present results are relatively equivalent to the other works with the advantage that the PAni polymer is low cost, easy to synthesize, chemically stable and its conductivity can be tailored (doping process). Attempting a direct comparison between the different approaches is very complex because numerous parameters are different such as the nature and equivalence ratio (ϕ) of the fuel/oxidizer couple, the implemented fuel particle size (micro-and nanosized Al), the intrinsic conductivity of the additive considered and the fact that the percolation threshold depends on the microstructure (the shape and the dimensions) of the additive. 30 Further characterizations were then performed on the promising Al/SnO2-PAni-7.7 energetic formulation such as the impact and friction tests, and the results are reported in Table 2. Regarding the binary Al/SnO2 thermite, taken as a reference in the context of this investigation, insensitivity and a moderate sensitivity to the impact and friction tests were established, respectively. 29 For example, the sensitivity thresholds were determined to be higher than 49.6 mJ and equal to 216 N for the impact and friction stresses, respectively. Additionally, these values are higher than the international regulations, set at 2 J and 80 N by the United Nations for the transport of dangerous goods on public roads. [START_REF]Recommendations on the transport of dangerous goods: Manual of tests and criteria, 4 th revised edition[END_REF] By adding 7 wt. % of PAni to the Al/SnO2 thermite, the impact insensitivity is preserved with a sensitivity value also beyond 49.6 mJ, for which no positive reactions (combustion, coloration, spark, sound, etc.) were observed. However, as this energy level is the highest one that the apparatus can determine, it is not possible to know if the sensitivity threshold really varied between the Al/SnO2 energetic composite prepared with or without PAni polymer. With regards to the friction test, the addition of PAni (7.7 wt. %) within the Al/SnO2 thermite led to an increase in the insensitivity level to a value superior to 360 N, thereby ranking the Al/SnO2-PAni-7.7 energetic composite as totally insensitive. 29 To explain such a trend (desensitization with PAni additive) two main reasons can be suggested without being necessary independent phenomena. First, as the tin (IV) oxide (SnO2) material is embedded in a PAni polymer matrix, it is physically moved away from the Al fuel. The contact zones between the fuel and the oxidant are then considerably reduced, and the formation of hot spots generated by the mechanical stress at the Al/SnO2 interface is a little more difficult. It is then necessary to provide a higher energy to crush the polymeric layer, thus bringing the fuel and the oxidizing species closer together and allowing the combustion reaction. Another explanation could be the one given in 13,[START_REF] Song | [END_REF] . The authors, referring to the fact that polyaniline material is made of the stacking of polymeric planes, suggest a sliding of these planes with respect to each other when the polyaniline material is mechanically stressed. This can result in the absorption of a portion of the energy initially provided by the friction phenomenon, leading to a residual energy that is potentially not enough for triggering the redox reaction between the Al and the SnO2 components. Generally speaking, regardless of the possible purpose of the PAni additive, a physical barrier between the SnO2 and Al reactants or the effect of its lubricating properties, the higher the concentration of PAni within the Al/SnO2 thermite is, the higher the energy absorption is and the higher the mechanical force intensity must be for the combustion reaction to take place. Regarding, the NATO criteria for the transport of hazardous goods, the Al/SnO2-PAni-7.7 is in good compliance with the required standards for the impact and friction since its sensitivity thresholds are widely superior to the standards (2 J and 80 N, respectively). 29

Combustion of the Al/SnO2-PAni thermites

The combustion test of the Al/SnO2 and Al/SnO2-PAni-7.7 energetic composites were evaluated according to the protocol described in section 2.3.2 contrary to the one of the conventional binary Al/SnO2 energetic composite. Some selected video frames of the combustion of these two energetic composites are displayed in Figure 5A and 5B, respectively. The Al/SnO2 thermite, selected as a reference, quickly burnt, and no residual solid matter was found on the glass substrate at the end of the experiment. A clapping sound was also heard during the combustion, supporting the hypothesis of a gaseous release previously mentioned in the dedicated literature. [START_REF] Fischer | Theoretical energy release of thermites, intermetallics, and combustible metals[END_REF]33,34 When observing the combustion frames of this energetic composite (Figure 5A, excluding the optical flash frame), an intense luminous signal is observed in the three first images of the sequence, followed by huge blue/purple fumes moving above the optical igniter device. Taking into account the time scale, the duration of the combustion reaction seems to not exceed 3-4 ms (Figure 5A, images 4 to 5). In contrast, the combustion reaction of the Al/SnO2 thermite enriched with the PAni polymer, according to a weight ratio of 7.7 wt. %, took in much prolonged time, and a very small concentration of unreactive substance was collected on the glass support at the end of the test. Moreover, the combustion reaction was quieter, which is likely due to the less powerful energetic character of this ternary composite. 13 This lower combustion energy may be due to the fact that the PAni polymer additive absorbs a portion of the combustion energy of the Al/SnO2 composite for its own decomposition/combustion, thereby leading to a mitigation of the global energy released by the energetic system considered. This result can be especially pronounced, as the additive is not an energetic species but rather a chemically inert substance from a pyrotechnics point of view, as is the case in the present investigation. Similar observations are mentioned in the literature, in particular with carbon particles and polymers selected as desensitizers. 12,13 When scanning the corresponding sequence of the combustion images of the Al/SnO2-PAni-7.7 energetic composite (Figure 5B), the combustion phenomenon is totally different from that of the binary Al/SnO2 (Figure 5A). A sheaf of incandescent particles evolving at the surface of the energetic mixture is observed. The orange luminous sheaf grows up in a regular way, without the formation of fumes, until it reaches the maximum flame temperature symbolized by the appearance of orange/yellow incandescent particles (Figure 5B, beyond image 9). Nevertheless, it can be speculated that the reaction temperature for the Al/SnO2-PAni-7.7 energetic composite is not as high as that for the Al/SnO2 system since the colour of the incandescent particles is warmer in the first case (not a white luminous signal). The energy consumption by the polymer combustion, decreasing the quantity of released energy, is likely the source of such a thermal evolution. Based on the time scale, the duration of the combustion phenomenon may be estimated to be 15-20 ms (Figure 5B, images 16 to 20). Since for both energetic composites equivalent concentrations of matter were tested (see the Experimental section) and assuming that they exhibit equivalent apparent densities, close to 10 % of the theoretical maximum density (TMD) as generally attributed for powdered samples 35 , it can be determined that the combustion velocity for the PAni-doped Al/SnO2 thermite is lower than that of the undoped Al/SnO2 composite. This fact may result in at least two main parameters. The first one is the one cited above relating to the fact that the polyaniline, for combustion, draws a part of the energy released by the Al/SnO2 reactive couple during its own exothermic reaction. The second one is based on the fact that the SnO2 oxidizer is embedded in a PAni-polymer matrix. Accordingly, the direct interfacial contacts between the SnO2 oxidizer and the Al fuel, which facilitate the ignition and front flame propagation, are considerably reduced. Based on these attempts to explain the phenomena, the combustion speed should be dependent of the concentration of polyaniline polymer within the Al/SnO2 energetic composite. In other words, the larger the concentration of PAni is, the lower the combustion velocity should be. To verify that, a set of combustion tests on the different Al/SnO2 samples enriched with polyaniline (Al/SnO2-PAni-X with X = 1.8, 4.3, 7.7 and 10.8 wt. %, respectively) were performed.

The different sequences of combustion pictures are gathered in the Supporting Information (Figure S4). In all cases, reactive formulations were observed, and some trends have been identified validating the previous observations made between the undoped and Al/SnO2-PAni-7.7 In summary, even if the addition of polyaniline has a prejudicial effect on the combustion behaviour of the Al/SnO2 thermite with a significant decrease of the combustion velocity, it is important to note that for all PAni-doped Al/SnO2 energetic composites, the self-propagation of the combustion reaction has been demonstrated regardless of the concentration of polymer used.

This work provides additional evidence for the use of polyaniline polymer in the ESD desensitization of nanothermites.

Conclusion

This investigation showed that the polymerization synthesis of SnO2-polyaniline (PAni) was checked, a significant decrease of the combustion velocity was observed. The next step of this approach could be the elaboration of SnO2-polyaniline core/shell nanostructures, which should lead to a better optimization of the addition of PAni (better coating, lower concentration) within the thermite energetic composites and thereby establishing a better ratio/compromise between the sensitivities and reactivity. (2-column fitting image)

Figure captions

Table 3: Literature data on to the desensitization of the thermite energetic composites. * L, l, t, Ø, Øo and Øi for the length, the width, the thickness, the diameter, the outer diameter and the inner diameter of the different particles investigated, respectively.

(Single-column fitting image)

  3 of energetic mixture to the drop of a 5 kg weight from a variable height, thus adjusting the stress intensity. The friction sensitivity threshold (expressed in newtons) is defined by scrubbing 10 mm 3 of energetic formulation between a ceramic stick against a ceramic plate. The friction intensity of the stick is a function of the position of weights hung on a lever. Regarding the electrostatic discharge test, an ESD 2008 electrostatic spark sensitivity device from OZM Research s.r.o. was used. A low concentration of matter placed between two electrodes 1 mm away is submitted to a spark whose energy (in joules) is calculated from the voltage and capacitance implemented. The sensitivity threshold values for the different Al/SnO2-PAni-X thermites correspond to the value of six consecutive "no-go", i.e., without a combustion reaction.

Figure S1 :

 S1 Figure S1: Photographs of the (A) impact, (B) friction and (C) electrostatic discharge apparatus, respectively, used for the determination of the sensitivities of the energetic thermite composites.

Figure 1 :

 1 Figure 1: XRD patterns of the (A) SnO2 Cassiterite phase (JCPDS card No. 041-1445) and the SnO2-PAni-X hybrid nanocomposites with the PAni concentration X equal to (B) 3.8, (C) 7.5, (D) 11.2 and (E) 14.7 wt. %, respectively. For clarity's sake, the XRD pattern of the commercial SnO2 material used in this work is reported in the Supporting Information (S2).

Figure S2 :

 S2 Figure S2: XRD patterns of the commercial SnO2 material used in this work and the corresponding JCPDS card.

Figure 2 :

 2 Figure 2: FTIR spectra of the SnO2-PAni-X nanocomposites with the PAni concentration X equal to (A) 3.8, (B) 7.5, (C) 11.2 and (D) 14.7 wt. %, respectively. The spectra ogf the pure PAni and SnO2 phases are reported as (E) and (F), respectively. For clarity's sake, the vibrational assignments were only reported in the main text.

Figure 3 :

 3 Figure 3: Thermogravimetric curves (under air) of the SnO2-PAni-X nanocomposites with the PAni concentration X equal to (A) 3.8, (B) 7.5, (C) 11.2 and (D) 14.7 wt. %, respectively.

First, compare to

  the pure conducting PAni phase, the electrical conductivities of the SnO2-PAni-X composites are lower that can be explain itself by the low electrical conductivity of the SnO2 material (3×10 -7 S/m) major component of the composites. Then, taken into account only the different SnO2-PAni-X materials, the electrical conductivity increases approximately from 0.2 to 2.4 S/m with the PAni conducting polymer concentration (X) ranging from 2.8 to 16 weight percent within the composite. Based on the plot of the conductivity as a function of the PAni wt.

Figure S3 :

 S3 Figure S3: Electrical conductivity of the SnO2/PAni-X composites as a function of the PAni weight % (X).

Figure 4 :

 4 Figure 4: Transmission electron microscopy views of the SnO2-PAni-X hybrid nanocomposite (X = 11.6 wt. % of PAni).

Figure 5 :

 5 Figure 5: Sequences of the combustion images of the (A) Al/SnO2 and (B) Al/SnO2-PAni-7.7 thermites. The elapsed time between images is 1 ms.

  composites. For example, the smaller the concentration of PAni is, the shorter the combustion reaction is (ascending image numbers to record the combustion phenomena for Al/SnO2 < Al/SnO2-PAni-1.8 < Al/SnO2-PAni-4.3 < Al/SnO2-PAni-7.7 < Al/SnO2-PAni-10.8, respectively), the louder the sound is (presence of purple fumes for Al/SnO2 and Al/SnO2-PAni-1.8, a luminous yellow/white torch for the Al/SnO2-PAni-4.3 and an orange luminous sheaf for the Al/SnO2-PAni-7.7 and Al/SnO2-PAni-10.8, respectively) and finally less residual matter remains (much more unreactive matter for the Al/SnO2-PAni-10.8).

13

 13

Figure S4 :

 S4 Figure S4: Combustion image sequences of the Al/SnO2-PAni-X thermites with X being the PAni weight %. (A) X = 0 (i.e., the Al/SnO2 thermite reference), (B) X = 1.8 wt. %, (C) X = 4.3 wt. %, (D) X = 7.7 wt. % and (E) X = 10.8 wt. %, respectively. The elapsed time between images is 1 ms.

  composites can be efficient to mitigate both the electrostatic discharge (ESD) and friction sensitivities of an aluminium-based energetic composite (Al/SnO2). The addition of 7-8 wt. % of the conducting PAni within an Al/SnO2 nanothermite allowed an ESD sensitivity threshold of ~ 100 mJ to be reached compared to < 1 mJ for the binary Al/SnO2 energetic system. The formation of a homogenous and continuous electrostatic charge-conducting polyaniline-based pathway in the entire volume of the energetic formulation was thus suggested. As a result, no joule heating of fuel nanoparticles by the electrostatic discharges occurs, thus avoiding ignition of the energetic composition at a level comparable to what a human body can deliver. At the same time, the friction sensitivity was also mitigated to obtain a value of > 360 N (vs. 216 N for the undoped Al/SnO2 nanothermite). The role of a physical barrier played by the polymer placed between the fuel and the oxidizer and its lubricant properties, made possible by the sliding of the polymeric planes between each other when stressed, were advanced to justify this trend. Nevertheless, this concentration of conducting polyaniline has a negative impact on the combustion behaviour of the Al/SnO2 energetic composite because even if the self-propagation of the combustion reaction

Figure 1 :

 1 Figure1: XRD patterns of the (A) SnO2 Cassiterite phase (JCPDS card No. 041-1445) and the SnO2-PAni-X hybrid nanocomposites with the PAni concentration X equal to (B) 3.8, (C) 7.5, (D) 11.2 and (E) 14.7 wt. %, respectively. For clarity's sake, the XRD pattern of the commercial SnO2 material used in this work is reported in the Supporting Information (S2).

Figure 2 :

 2 Figure2: FTIR spectra of the SnO2-PAni-X nanocomposites with the PAni concentration X equal to (A) 3.8, (B) 7.5, (C) 11.2 and (D) 14.7 wt. %, respectively. The spectra of the pure PAni and SnO2 phases are reported as (E) and (F), respectively. For clarity's sake, the vibrational assignments were only reported in the main text.

Figure 1 :

 1 Figure 1: Thermogravimetric curves (under air) of the SnO2-PAni-X nanocomposites with X the PAni concentration equals to (A) 3.8, (B) 7.5, (C) 11.2 and (D) 14.7 wt. %, respectively.

Figure 4 :

 4 Figure 4: Transmission electron microscopy views of the SnO2-PAni-X hybrid nanocomposites (X = 11.6 wt. % of PAni).

Figure 5 :(Figure 5 :

 55 Figure 5: Sequences of the combustion images of (A) the Al/SnO2-PAni-7.7 and (B) the Al/SnO2 thermites. The elapsed time between images is 1 ms. (Figure 5: Color online only)

Figure captions

 captions Figure captionsClick here to access/download;Figure;Figures captions_Revised version.docx

  Figure captionsClick here to access/download;Figure;Figures captions_Revised version.docx

Figure 1

 1 Figure 1

Figure 2

 2 Figure 2

Figure 3

 3 Figure 3

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure S1 Click

 S1 Figure S1

Figure S2 Click

 S2 Figure S2

Figure S3 Click

 S3 Figure S3

Figure S4A Click

 S4A Figure S4A

Figure S4B Click

 S4B Figure S4B

Figure S4C Click

 S4C Figure S4C

Figure S4D Click

 S4D Figure S4D

Figure S4E Click

 S4E Figure S4E

Table 1 : Electrical conductivity values of the different SnO2/PAni-X composites as a function of the PAni concentration (X).

 1

Table 2 : Electrostatic discharge sensitivity thresholds of the Al/SnO2-PAni thermite depending on the PAni wt. %.

 2

Table 2

 2

	surges by a factor 134 (96.5 mJ vs. 0.72 mJ), while the concentration of conducting phase only
	grows by a factor 1.8 (7.7% vs. 4.3%). In contrast, when the PAni concentration varies from 1.8
	to 4.3 wt. % (x 2.4), the ESD sensitivity is only raised by a factor 1.6 (0.72 mJ against 0.46 mJ).
	Accordingly, the percolation threshold, which characterizes the minimal concentration of a
	chemical species to assure a continuous network, is likely in the range 4.8-7.7 wt. % of PAni
	added within the Al/SnO2 thermite (15-22 vol. %). The electrical conductivity determination
	performed on the two Al/SnO2-PAni-7.7 and Al/SnO2 energetic composites confirmed a more

). For example, the ESD threshold value increases from 0.46 to 1212 mJ with a concentration of PAni ranging from 1.8 to 10.8 wt. %, respectively, which can be interpreted by the fact that with PAni, it is necessary to deliver a higher electrostatic discharge to ignite the combustion reaction of the Al/SnO2 thermite. The efficiency of the polyaniline as a desensitizing agent to electrostatic discharge in thermites is obvious. By precisely examining the variation of the ESD sensitivity values, a drastic increase is recorded between 4.3 and 7.7 wt. % of PAni. Indeed, the ESD value

29

Accordingly, based on this result, if a nanothermite (as examined in this work) does not ignite close to an ESD of 100 mJ (as defined for the microscale systems), it can be ranked as potentially insensitive. However, caution should still be applied with this kind of energetic composite.

Table 3 : Literature data on the desensitization of thermite energetic composites. * L, l, t, Ø, Øo and Øi stand for the length, width, thickness, diameter, outer diameter and inner diameter of the different particles investigated, respectively.

 3

Table captions Table 1 : Electrical conductivity values of the different SnO2/PAni-X composites as a function of the PAni concentration (X).

 captions1

	(2-column fitting image)

Table 2 : Electrostatic discharge sensitivity thresholds of the Al/SnO2-PAni thermite depending on the PAni wt. %.

 2

Table captions

 captions Click here to access/download;Table;Tables captions_Revised version.docx 1

	PAni wt. % (X)	2.8	6.4	11.6	16	100
	𝜎 (S/m)	0.198	0.500	1.867	2.379	5.67

Table 1

 1 Click here to access/download;Table;Table 1_Revised version.docx

	PAni wt. %	ESD (mJ)	Impact (J)	Friction (N)
	0	0.14	> 49.6	216
	1.8	0.46	n.d.	n.d.
	4.3	0.72	n.d.	n.d.
	7.7	96.5	> 49.6	> 360
	10.8	1212	n.d.	n.d.

Table 2

 2 Click here to access/download;Table;

Table 2 .docx 1 Energetic formulations (fuel/oxidizer) Microstructure of the energetic components fuel/oxidizer (size)* Desensitizer (dimensions)* Desensitizer amount (%) ESD threshold (mJ) Ref.

 2

	Vol.	Wt.

Acknowledgements

The authors gratefully acknowledge the French National Centre for Scientific Research (CNRS), the French-German Research Institute of Saint-Louis (ISL, Saint-Louis, France) and the University of Strasbourg (UNISTRA, Strasbourg, France) for funding as well as D. Spitzer

(director of NS3E, Saint-Louis, France) for his support and help in obtaining the funding of the second-year Master's Degree internship of V. Goetz. The authors thank Y. Boehrer (ISL, Saint-Louis, France) and L. Vidal (IS2M, University of Mulhouse, France) for the combustion video recordings and the scanning electron microscopy analysis, respectively.


G A Bocan

H Breiss

S Szilasi

A Momeni

E M Staicu Casagrande

M S Gravielle

E A Sánchez

H Khemliche

Anomalous KCl(001) surface corrugation from fast He diffraction at very grazing incidence

Keywords: numbers: 34.35.+a, 34.50.-s, 79.20.Rf, 79.60.Bm

We present theoretical and experimental evidence of an anomalous surface corrugation behavior in He-KCl(001) for incidence along 110 . When the He normal energy decreases below 100 meV, i.e. He-surface distances Z > 2 Å, the corrugation unexpectedly increases up to an impressive > ∼ 85%. This is not due to van der Waals interactions but to the combination of soft potential effects and the evolution of He-cation and He-anion interactions with Z. This feature, not previously analyzed on alkali-halide surfaces, may favor the alignement properties of weakly-interacting overlayers.

The design of promising new materials in the form of thin film stacks brings along the challenge of producing such heterostructures to the highest quality and thus requires an accurate knowledge of the relevant features that rule quasi-epitaxial growth. One such feature is the substrate surface corrugation, which has been shown to drive the overlayer-substrate relative orientation even for weakly-interacting interfaces, such as organic/inorganic or transition metal dichalcogenides systems [1,2].

Grazing incidence fast atom diffraction (GIFAD) [3,4] is a highly sensitive and non-damaging probe of the projectile-surface interaction potential. Particularly well-suited for a thorough exploration of the surface electronic corrugation, it has already provided valuable structural information on a variety of systems [5][6][7][8][9][10][11], and its grazing geometry makes it compatible with in situ monitoring of thin film growth [12,13].

GIFAD occurs when keV-energy projectiles impinge on a surface along a low-index crystallographic direction, under axial surface channeling conditions [14]. The GI-FAD pattern arises from the combination of: a) interchannel interference, originated from the periodic array of channels, giving the Bragg peaks and b) intrachannel interference, originated from the corrugation of the interaction potential within a given channel, giving the rainbow peak as well as the supernumerary rainbows [15]. The result on the detection plane is a sequence of Bragg maxima modulated by the underlying intrachannel interference [15,16].

In this work we employ GIFAD to investigate the corrugation of the KCl(001) surface when probed with low normal energy 4 He 0 projectiles along the 110 channel. We present experimental data, information provided by the potential energy surface (PES) and simulations of the scattering dynamics, all of which put forward a counterintuitive and notorious increase of the surface corrugation with decreasing normal energy E ⊥ in the low E ⊥ < ∼ 60 meV range, i.e. for He-surface distances Z > 2.2 Å. Although long-distance surface corrugation increases have been experimentally observed by GIFAD for He/LiF(001) (∼ 12%) [17] and He/KBr(001) (∼ 45%) [18], this is, to our knowledge, the first time such an overwhelmingly large ( > ∼ 85%) increase is reported. Furthermore, we prove that the ruling factors of this puzzling feature are the evolution of the He-anion and He-cation interactions with Z and the continuance of the projectile-surface interaction during the whole scattering process (i.e. soft potential effects, SPE).

The experiments were performed at room temperature in a GIFAD setup similar to that of Ref. [13]. The KCl(001) sample was cleaved in air and immediately transferred to the ultra-high-vacuum chamber. Once mounted on a five-axis manipulator it was prepared insitu by annealing at about 650 K to ensure its cleanness and crystallographic order (corroborated from the well defined GIFAD patterns obtained [19]). The 4 He 0 beam was formed by neutralizing a He + beam with primary energies in the 300-600 eV range. Following the neutralization cell, a set of apertures of 0.4 × 0.09 mm 2 placed 0.36 m apart were used to reduce the beam divergence to less than 1 mrad. The two-dimensional angular distributions of scattered projectiles were collected by placing a detector formed by a Microchannel plate, a phosphor-coated screen and a CCD camera in the forward direction at a distance of 1.27 m. In Figure 1 we depict a GIFAD pattern for 600 eV 4 He atoms with E ⊥ =21 meV. The simulated pattern, that shows good accord with the experiment, was obtained with the Surface Initial Value Representation (SIVR) [20,21], a wellestablished semi-quantum approach to describe the scattering dynamics [22,23]. Note that collimation [21], surface defects and thermal vibrations may contribute to the elongation of the Bragg spots along the polar angle, though the latter two are not included in the simulations. Such polar spread does not affect our GIFAD-derived results, based on the elastic contribution to the intensity (θ f = θ i ± 0.035 • for the central Bragg order) [19]. The experimental corrugation, presented in Fig. 2(a), was obtained by processing the GIFAD patterns to get the effective interaction potential [7,19,24]. Since low-E ⊥ scattering takes place far from the surface, it should be ruled by a mild potential landscape, yielding a decreasing surface corrugation with decreasing E ⊥ but, intriguingly, we find that the corrugation increases with decreasing E ⊥ and this increase gets particularly sharp below 40 meV.

In order to gain more insight into this unexpected behavior we built a high-precision potential energy surface (PES) for the He/KCl(001) system based on Density Functional Theory (DFT), using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [25] (henceforth the PBE-PES, further details in Ref. [22]). When the PES is averaged along the channeling direction (x), the axial potential V 2D (Y, Z) that actually rules the scattering process [26] is obtained, where Y is the position across the channelling direction. In Fig. 2(b) we show equipotential curves of the axial PBE-PES, from which the intrinsic corrugation for a given E ⊥ is obtained from the corresponding amplitude ∆Z = Z max -Z min .

Energy regions for which atoms scatter relatively far from the surface (Z > 2 Å) may bring along nonnegligible van der Waals (vdW) interactions, left aside in the PBE-PES. Hence, we built two additional PESs [27,28], where we replaced the PBE exchange-correlation functional by the rVV10 [29] and the DF2b86r [30] ones (henceforth respectively labeled vdW1 and vdW2), which include vdW interactions in a self consistent fashion though with different approaches. The intrinsic corrugations obtained with the three PESs give very similar curves (see Fig. 2(a)), in good accord with the trend of the experimental values, though about 25% below them. The intrinsic corrugation increase is thus not due to vdW interactions but should rather be explained through a thorough analysis of the axial PBE-PES [31].

The equipotential profiles across the 110 channel (see inset in Fig. 2(b)) have maxima over the Cl -rows. At mid-channel, over the K + row, there is a central minimum for E ⊥ < ∼ 60 meV and a local maximum for E ⊥ > ∼ 60 meV. The latter creates two off-center minima which migrate away from mid-channel with increasing E ⊥ . The equipotential curves show that the He-Cl - interaction (as obtained from DFT calculations) is always more repulsive than the He-K + one. The evolution of these interactions upon changing E ⊥ , particularly the slow sinking of the central minimum due to the vanishing He-K + interaction, makes Z min vary slower than Z max , explaining the intrinsic corrugation increase.

The residual discrepancy between the intrinsic and the experimental corrugations remains nevertheless intriguing, leading us to address the rainbow angle [32], a bright peak in the intrachannel contribution at maximal angular deflection [6,14,15,32]. The rainbow angle shows a wellknown high sensitivity to the projectile-surface interaction. While the corrugation affects mainly the intensity of the specular region to which only trajectories reflecting in the flattest regions of the axial PES contribute, the rainbow probes the steepest regions of the axial PES. For a given E ⊥ , and within a hard-corrugated wall (HCW) approach [33], we determined the PES-rainbow from the steepest slope of the corresponding equipotential curve, while the experimental rainbow was obtained from the processing of the GIFAD pattern [7,19,24].

In Fig. 3(a) we show the experimental rainbow angle together with those obtained from the three PESs considered. The increase observed in the experimental rainbow angle for decreasing low E ⊥ is analogous to the one obtained for the corrugation. The PESs rainbows show a mild increase, associated to the sinking central peak in the potential profiles, but they cannot reproduce the experimental rainbow trend for E ⊥ < 100 meV. The magnitude of the disagreement, once again unrelated to vdW interactions, strikingly contrasts with the fairly good accord discussed for the corrugation. Extracting information directly from the E ⊥ equipotential curves takes into account the projectile-surface interaction only at the turning point (HCW approach), thus neglecting the continuance of the projectile-surface interaction during the rest of the scattering process (i.e. soft potential effects (SPE)). We hence performed simula-tions of the scattering dynamics for each PES considered, by means of the semi-quantum SIVR approach [20,21] (henceforth labeled PBE-SIVR, vdW1-SIVR and vdW2-SIVR). The simulations were performed considering coherent illumination of a single channel (no interchannel interference) so that the rainbow angle can be directly obtained from the diffracted distribution. Strikingly, the PBE-SIVR rainbow reproduces the experiments in the energy range considered with an almost quantitative agreement. Including vdW contributions results in no significant differences, with vdW1-SIVR and vdW2-SIVR running slightly above PBE-SIVR.

The notorious difference observed between the PES and the SIVR rainbow angles, combined with the very good SIVR-Experiment accord, reveal in a quantitative manner the importance of SPE in central aspects of GI-FAD. These dynamical effects are naturally included in both the SIVR and the experimental rainbows (obtained by processing the GIFAD patterns) while the PES ones assume unrealistic broken-line trajectories.

Soft potential effects for the PBE-SIVR case can be visualized in Fig. 3(b) where equipotential curves of the axial PES are shown in 1:1 scale. For 3 < ∼ Z < ∼ 5 Å this plot shows a noteworthy shallow (< 7 meV) attractive region, due to polarization and lower-order attractive interactions within the DFT PES, which markedly alters the shape and corrugation of the equipotential curves. For high enough E ⊥ this well is inconsequencial, but low-E ⊥ projectiles traversing this region are strongly affected. This is illustrated in Fig. 3(b) for E ⊥ = 15 meV. Each equipotential curve deviates the trajectory according to the slope at their contact point so that a steeper slope favors E z → E y energy conversion. SIVR trajectories deviate from the HCW one upon entering the well region and also during their way out. In the asymptotic condition the projectile-surface interaction fades out and trajectories become linear but not parallel to the HCW trajectory. In the light of this analysis, the difference between the intrinsic and the experimental corrugations could also be due to SPE. Hence, from our SIVR simulations we selected trajectories contributing to the specular intensity. These trajectories are reflected either around the maximum or the minimum of the equipotential, with near-normal incidence and scattering directions. We can compute their SIVR accumulated phase difference (Eq. (10) in Ref. [20]) and obtain an effective corrugation from ∆Φ SIV R = 2 k ⊥ (∆Z) SIV R (depicted in Fig. 2(a)). The agreement with experiments is remarkable and shows that the PBE-SIVR calculation captures the factors determining the corrugation increase. From Fig. 2(a), the differences between the SIVR and PES curves, together with the SIVR-Experiment accord, provide a quantitative measure of SPE on the effective experimental corrugation obtained by means of the GIFAD technique.

In summary, we report a counter-intuitive increase of the corrugation and the rainbow angle for decreasing low-E ⊥ , by means of the GIFAD technique, in 4 He-KCl(001) along the 110 direction. This increase reaches an overwhelmingly large > ∼ 85% and is independently obtained from experiments and simulations. Its origin, unrelated to vdW effects, lies in the different evolution of He-cation and He-anion interactions, combined with SPE. As the latter do not strongly affect the corrugation dependence on E ⊥ , the experimental curve preserves the intrinsic corrugation trend, meaning that GIFAD provides reliable information on this surface feature. In contrast, the rainbow increase is almost fully explained in terms of SPE, strongly enhanced by the shallow well, whose steepest regions markedly deviate low-E ⊥ rainbow trajectories.

The same two factors are likely responsible for the corrugation increases at low E ⊥ reported for He/KBr(001) [18] and He/LiF(001) [17] (and expected for other alkalihalide surfaces) along this same channel. Particularly, for He/LiF, the much less spectacular corrugation increase is probably only due to SPE as the He-Li interaction is far vanished in the energy range considered. Also, we expect SPE to rule a marked rainbow increase feature, given the reported presence of shallow wells [34].

Thin alkali-halide layers (mainly NaCl and LiF) are often used to improve charge injection in organic electronic devices. The enhanced corrugation of KCl(001) at typical physisorption distances may favor the alignment properties of weakly interacting overlayers [1,2], improving their crystalline quality and thus the device performance. The measured corrugation may also be enhanced for probes either more reactive or with larger polarizability such as H or Ne [35][36][37]. Further investigations should address He GIFAD on other alkali-halide surfaces, aiming to maximize the intrinsic corrugation increase.

FIG. 1 :

 1 FIG. 1: GIFAD patterns for 4 He→ 110 KCl(001) with incidence energy E = 600 eV and E ⊥ = 0.021 eV. ϕ f (θ f ): azimuthal(polar) angular position of the scattered atoms.

FIG. 2 :

 2 FIG. 2: (a) Surface corrugation for 4 He→ 110 KCl(001) obtained from experiments, the three PESs considered and the respective SIVR simulations. (b) PBE-PES equipotential curves for 0 ≤ E ⊥ ≤ 200 meV. Inset: Surface geometry.

FIG. 3 :

 3 FIG. 3: (a) Rainbow angle for 4 He→ 110 KCl(001). (b) PBE equipotential curves (Only energies (meV) not shown in Fig. 2(b) are labeled) and trajectories contributing to the rainbow intensity (Dashed black line: HCW, Full black line: SIVR).

The authors acknowledge funding from ANPCYT (PICT-2017-1201; PICT-2017-2945) CONICET (PIP 112 201301 00386 CO; PIP 112 201501 00274 CO) and U.N. Cuyo (06/C590).