Novel NSTEX System Based on Ti/CuO/NC Nanothermite Doped with NTO

Abstract

Modern energetic materials (EMs), e.g., nanothermite and NSTEX (Nanostructured Thermites and Explosive) compositions are attracting increasing research interest. In this work, we present the results of our investigation on the properties of a novel Ti/CuO nanothermite system doped with cellulose nitrate (NC) and 5-Nitro-1,2-dihydro-3H-1,2,4-triazin-3-one (NTO). In terms of safety parameters, the friction 40–>360 N), impact (40–>50 J) and laser irradiation sensitivity were determined for tested systems, which indicated tunable properties. The combustion velocity (up to 735 m/s), pressure parameters for combustion in closed vessel, thrust parameters and open-air combustion behaviour were measured. Moreover, in order to deeply study the impact of NTO on the combustion mechanism, the SEM, DSC/TG and XRD tests were performed. The obtained results indicate that the Ti/CuO/NC/NTO system is extremely promising for future applications.

1. Introduction

The last two decades have seen a stable and continuous development of new energetic materials (EMs), which are used in many civilian and military areas [1,2,3]. A groundbreaking development occurred in the field of nanotechnology, and it has been utilised just as thoroughly in medicine, environmental protection and ceramic materials, as in defence applications [4,5,6,7]. Novel nanoenergetic materials, containing nanosize particles or bearing nanoscale structural features have aroused significant research interest. The application of material nanoengineering principles to the preparation of EMs enables detailed control over the composition and density of the EMs, as well as their morphology and particle size distributions at the nanometer scale. Nanothermites, also referred to as metastable intersititial composites (MICs) are good examples of the above trend [8,9,10,11,12].

Nanothermites are highly reactive and solid-phase EMs composed of nanometric metallic fuels and oxidising agents that are intimately mixed with each other [13,14,15,16]. Compared to micrometric thermites, nanothermites are characterized by a high surface area–to–volume ratio, high energy density, the rapid release of energy and high sensitivity to stimuli. These features increase the reaction rate, heat release, pressure and thrust output of nanothermites [17,18,19]. On the other hand, the existing limitations in the applications of nanothermites result from the agglomeration of nano-reactants before ignition, incomplete combustion and relatively long ignition delays [2,20,21].

The electronegativity of metals used as fuels is one of the key parameters considered in the design of nanothermites, with metals, which have electronegativity in the range of 1.5–2.5 on the Pauling scale, being a favourable choice [11,22]. This stems from the fact that a viable fuel for nanothermite compositions must exhibit significant reducing properties, high density, high heat of formation, a moderate amount of gases released, low melting point, stability, compatibility with other components and low toxicity. Consequently, aluminium (Al), magnesium (Mg), silicon (Si), titanium (Ti), tantalum (Ta) and even boron (B) are commonly used as fuels [23], though the latter two less so, due to their significant toxicity.

Taking into account the thermochemical properties of the fuels, nanometric titanium (Ti) is a promising fuel for producing nanothermites, as it exhibits high reactivity and the presence of a protective oxide layer, which is responsible for the passivation of its surface. The temperature, at which the oxidation of nanometric Ti begins, is 100–200 °C, while complete oxidation occurs only at 1000 °C. Ti is also known to readily react with copper(II) oxide, lead oxides and permanganates, providing a clue as to potential oxidising agents to be used in tandem with it [23,24]. The most frequently used oxidising agents are copper(II) oxide (CuO), iron(III) oxide (Fe₂O₃), molybdenum(VI) oxide (MoO₃), bismuth(III) oxide (Bi₂O₃), tungsten(VI) oxide (WO₃), sulfates, iodates and periodates [25,26].

Nanothermites, as modern, interdisciplinary materials, play a significant role, both academic (e.g., in materials engineering) and technological, increasing the effectiveness in many industry sectors (both civil and military) [18]. Nanothermites can be used in micro- and nanowelding, materials synthesis, ejection seats and safety airbags. Propellant fuels and pyrotechnic materials based on nanothermites increase energy densities and lower sensitivity to external stimuli of propulsion systems used in the aerospace industry [8,27]. In the military industry, nanothermites are used as energetic materials for advanced munitions and demolition systems. They are used in microigniters, microthrusters, primers, bullets, correction engines, delay systems and gas generators [28]. The NSTEX-type (NanoStructured Thermites and Explosives) composition can also be used as a “green” substitute for lead salts in the design of initiating agents [29]. Moreover, nanothermites are increasingly used in materials science and materials engineering, through the development of modern composites, coatings and even energy storage devices [30,31,32].

The aim of this work was to develop an NSTEX composition that would exhibit high safety parameters, while having high performance parameters. A secondary aim was to elucidate the effect of changes in the EM content, in our case, the EM being NTO, on the parameters and mechanism of combustion of this group of NSTEX formulations.

2. Materials and Methods

2.1. Materials and Preparation

Copper(II) oxide (>99.99%, d ≈ 40 nm, Iolitec GmBH, Heilbronn, Germany), titanium (>99.99%, d ≈ 50 nm, Iolitec GmBH, Heilbronn, Germany), propan-2-ol (>99.9%, Electro Chem, Gliwice, Poland) and NTO (>99.5%, d ≈ 200 μm, Nitrochem S.A., Bydgoszcz, Poland) were used as received. Nitrocellulose was obtained through the drying of 4 wt.% solution (Sigma-Aldrich, Darmstadt, Germany).

The equivalence ratio of basic nanothermite was calculated according to Equation (1), without taking into account the presence of titanium oxides:

$$\varphi ={\displaystyle \frac{n_{\mathrm{fuel}}/n_{\mathrm{oxidising} \mathrm{agent}}}{n_{\mathrm{stechiometric} \mathrm{fuel}}/n_{\mathrm{stechiometric} \mathrm{oxidising} \mathrm{agent}}}}$$

(1)

where

• $\varphi$—equivalence ratio of the nanothermite;
• n_fuel/n_{oxidising agent}—the molar ratio between actual fuel (i.e., Ti)/oxidising agent (i.e., CuO) dispersed in the electrolyte;
• n_{stoichiometric fuel}/n_{stoichiometric oxidising agent}—the fuel/oxidising agent molar ratio in a stoichiometric reaction.

The stoichiometric molar ratio of fuel to the oxidising agent was 1:2.

The NSTEX composition samples were prepared for 4 different NTO contents, on the base of the Ti/CuO composition with an equivalence ratio of 1.4, as presented in

Table 1. Content of tested compositions.

Composition Name	NTO wt.%	Ti wt.%	CuO wt.%	NC wt.%
1NTO	1	29	69	1
3NTO	3	28	68	1
5NTO	5	28	66	1
10NTO	10	26	63	1

. All compositions contained 1 wt.% of NC additive.

In order to prepare compositions, the wet-mixing method was employed, followed by electrospraying.

All components were weighed and dried for 12 h at 60 °C in air. In the next step, components were transferred into glass vessels, which were filled with propan-2-ol to achieve a solid loading equal to 125 mg/mL. The suspensions were stirred for 30 min and sonicated for 30 min (0.7/0.3 sonication/break cycle), followed by stirring for 48 h. After this process, the compositions were electrosprayed in 1 cm³ batches through stainless steel nozzles (0.5 mm internal diameter, 10 mm length) at a 19 kV voltage difference between the nozzle and collector. The flow rate was equal to 3 cm³/h, and the distance between collector and nozzle was fixed at 10 cm. After the deposition process, the collector plates were dried for 6 h at 60 °C in air. In the final step, the NSTEX compositions were gently removed from the collector plates and stored in sealed containers.

2.2. Friction and Impact Measurement

All tests were performed in accordance with international standards [33,34], using a Peters’ apparatus and a BAM Fallhammer apparatus.

2.3. Laser Sensitivity Tests

Laser irradiation sensitivity ($\lambda$ = 520 nm) tests were carried out for optical power values in the range of 100–300 mW (Laser Green 520-1000-SMA pulse generator, SpectraLaser, Opole, Poland), which were validated using a MIER-4W-USB thermal radiation power detector (SpectraLaser, Opole, Poland). Ignition time was calculated as a time between the UV-VIS (350–850 nm) photodetector reaction and signal from the pulse generator. Both the photodetector and pulse generator were connected with a Rigol MS05104 digital (produced by Rigol Technologies, Beijing, China) oscilloscope, with 10 MSa/s sampling. Tests were performed for 15 mg samples with the set-up presented in Figure 1.

2.4. SEM-EDS Tests

The morphology of the compositions and post-combustion residues were investigated with an FEI Inspect S50 (FEI, Hillsboro, OR, USA) scanning electron microscope (SEM). The chemical compositions of the post-combustion residues were researched with an X-ray energy dispersive spectrometer with a Detector EDS Octane Elect Plus and Analyzer EDAX Z2-i7 (Bruker, Billerica, MA, USA), enabling the simultaneous acquisition of micrographs and Energy Dispersive X-ray (EDS) maps of the investigated samples. A working distance of 10 mm, an acceleration voltage of the incident electrons in a range from 5 kV to 30 kV, a current intensity of about 95 μA and the spot size of the electronic beam size equal to 5 nm² were assumed as working parameters.

2.5. XRD Tests

X-ray diffraction (XRD) with Cu Kα radiation ($\lambda$ = 1.54183 $\dot{\mathrm{A}}$) was performed to identify the crystalline phases using Rigaku MiniFlex 600 (Rigaku Co., Tokyo, Japan) at RT using a one-dimensional detector (Rigaku D/teX Ultra 250) and zero-background sample holder (monocrystalline Si). The X-ray tube was operated at 40 kV and 15 mA. Additional measurement parameters were 2$\theta$ range of 22.5°–62.5°, IHS slit = 5 mm, Soller slits = 2.5°, DS slit = 1.25°, scanning step size 0.01° and exposure time at each point of 1.67 s without sample rotation.

2.6. DSC/TG Tests

DSC/TG tests were conducted using a TA Instruments SDT Q600 apparatus, employing a heating rate of 20 °C/min and purging with argon gas (5.0. purity grade, 200 cm³/min). Tested samples were poured into ceramic crucibles and enclosed with a gold disc with a 0.3 mm hole in the central part. The construction of closing discs allows the escape of gases, while maintaining limited contact of samples with the environment.

2.7. Closed Vessel Combustion

All tests were performed for 50 mg samples of compositions. Pre-weighed samples were poured into a closed vessel (volume of 15 cm³), equipped with a pressure sensor (PCB Piezotronics 102B, PCB Piezotronics, New York, NY, USA). The sensor was connected with a signal conditioner (PCB Piezotronics 480E09, PCB Piezotronics, New York, NY, USA) and digital oscilloscope (Tektronix TBS2401B, Tektronix, Beaverton, OR, USA), with 25 MS/s sampling. In each case, the samples were ignited with a laser pulse (700 mW, 100 ms).

2.8. Thrust Force Measurements

A custom experimental set-up (Figure 2) was utilised to conduct measurements, as described in detail in the Supplementary Information.

The specific impulse was calculated according to Equation (2):

$$I_{sp}={\displaystyle \frac{F\Delta t}{g\Delta m}}$$

(2)

where

• I_sp —specific impulse [s];
• F—thrust force [N];
• Δt—combustion time [s];
• g—gravitational constant = 9.81 [m/s²];
• Δm—mass of combusted composition [kg].

2.9. Open-Air Combustion

In order to study the combustion of NSTEX compositions, a set-up (Figure 3) analogous to ones used in the literature [35,36,37] was utilised. Tested NSTEX samples (15 mg) were poured onto a ceramic plate and ignited with a laser pulse (700 mW, 100 ms).

The combustion process was recorded with a Phantom 9.1. high-speed camera, placed at a distance of 500 mm from the sample, with 6400 frames/second. Between the camera and the sample, 2 mm thick PMMA screens were placed in order to protect the optical elements. The ignition time and time of combustion were calculated based on image analyses.

2.10. Combustion Velocity Measurements

The tested charges made from PMMA tubes (70 mm long, 4 × 2 mm in diameter) were closed at one end with a PMMA plate (2 mm thick) and elaborated with tested NSTEX samples (Figure 4) in order to achieve a density of 9 % TMD for each composition.

Tests for the 3NTO composition also included the measurement of combustion velocity for charges with densities of 10.5 % TMD and 12 % TMD.

After elaboration, the charges were closed with 3D-printed plugs (PETG, printed with Prusa Mk3, Prusa Research, Holesovice, Czech Republic). Prepared charges were placed on a testing stand, between two steel jaws to ensure the reproducible mounting of samples. The Phantom 9.1 high-speed camera was placed 1.5 m away from the sample at 90 degrees to the charge axis. Each sample was ignited with a laser pulse (700 mW, 100 ms). The time of combustion was calculated based on image analyses. The average combustion velocities were calculated based on the integral mean value theorem [38].

3. Results and Discussion

3.1. Friction and Impact Sensitivity Measurements

The tested samples are insensitive to impact (

Table 2. Friction and impact sensitivity of tested samples.

Sample	Friction Sensitivity [N]	Impact Sensitivity [J]
1NTO	128	>50
3NTO	96	>50
5NTO	84	>50
10NTO	40	>50
NTO	>360	40

), which is primarily related to the presence of the usually impact-insensitive nanothermite and secondary EM (NTO) in the tested formulation.

The friction sensitivity of the studied samples increases with an increasing amount of NTO in the sample, contrary to what is reported in the literature for nanothermite/secondary EM systems [39]. Most likely, this is due to the texturising effect of NTO, as SEM analyses have confirmed the presence of larger grains for compositions containing higher NTO amounts.

3.2. Laser Sensitivity Tests

Regardless of the radiation energy, an increase in the ignition time as a function of the NTO content of the composition can be observed in almost every case studied (Figure 5).

The increase in ignition time can be attributed to one or more of several possible factors. The explanation could be a lengthening of the reaction time scale, where the addition of NTO slows down the ignition and full, stable combustion development of the composition. Moreover, it has been shown that the irradiation of porous surfaces of secondary EMs can result in the formation of cracks and free spaces and, therefore, also faults in heat and mass diffusion, which could delay the ignition process [40]. Moreover, NTO has a significantly lower heat transfer coefficient (0.14 W/K·m [41]), much lower than those of Ti and CuO (21.9 W/K·m and 18 W/K·m, respectively [42,43]), which limits heat transfer in a system where the ignition process of compositions of this type is dominated by heat transfer via conduction and convection. Changes in the morphology of the composition should be taken into account as the NTO content increases, and as the grain size rises, there is a decrease in their sensitivity to thermal stimuli.

From the point of view of thermal explosion theory [44], the ignition is possible when the amount of heat generated during the reaction exceeds the heat losses to subsequent layers of the composition and to the environment. According to this, the increase in NTO consent should facilitate easier combustion, due to lower effective heat exchange factors. Nevertheless, the observed increase in ignition times could be explained through the local overheating of NTO, causing it to either volatilize or undergo decomposition, producing gases, which promote heat loss from the sample.

3.3. SEM-EDS Tests

SEM-EDS analyses (Figure 6 and Figure A1–Figure A4) reveal that the tested samples have very well-developed surfaces, which significantly improves the bilateral adhesion of the particles. The direct adhesion of the oxidising agent to the fuel was confirmed, which is expected to improve the combustion properties of nanothermites.

Spherical titanium covered with well-developed copper oxide agglomerates was observed. Some of the particles have a smooth surface, others have a developed surface and a regular spherical shape. In each case, highly porous, multi-walled compositional agglomerates, formed from submicron particles are present. With increasing NTO content, it is possible to observe not only an increase in the size of these agglomerates but also the presence of more spherical and spindle-shaped structures of the submicron crystals that form part of the larger clusters. This apparent change in morphology is due to the fact that with increasing NTO content, the crystallisation of solid particles occurs more rapidly during the electrospraying process.

SEM-EDS analyses of the tested combustion products show quite good homogeneity of Cu, O, N and C in 1NTO combustion products (Figure A5). The Ti atoms are agglomerated in a few areas, visible as multi-walled, porous sinters or spherical particles. In both cases, these are most likely titanium oxides, trapped inside a remoulded copper and copper oxide matrix.

In the case of 3NTO combustion products (Figure A6), the morphology is quite different. One can spot the uneven distribution of Cu and Ti, which occur alternately. Similar to the previous sample, the multi-walled agglomerates are composed mostly of Ti and O. The C, O and N atom distributions are equal across the sample surface.

The 5NTO sample (Figure A7) presents a morphology and atom contribution similar to those of the 1NTO sample, with titanium oxides trapped inside a Cu-based matrix. Nevertheless, few areas with lower Ti and Cu concentrations are visible.

The 10NTO sample (Figure A8) shows morphology in line with the typical morphology. The Cu and Ti are distributed unequally, and the main part of the surface is formed from massive, multi-walled sintered structures composed of titanium oxides. The C, O and N atom distributions are uniform. The EDS tests suggest changes in the reaction mechanism with an increase in NTO content (Figure 7), which leads to the stronger disproportion of Cu and Ti in compositions and increase in the presence of multi-walled structures.

3.4. X-ray Diffractometry

The XRD patterns of four decomposition products are presented in Figure 8. Three main phases, Cu (PDF Card No. 00-004-0836), rutile TiO₂ (PDF Card No. 00-021-1276) and Cu₂O (PDF Card No. 01-071-3645), were identified in all four samples. The TiO₂ anatase phase (PDF Card No. 01-075-2547) was also found in the 10NTO sample. The occurrence of anatase may indicate that unreacted Ti was present in the combustion products, as anatase transforms into rutile in temperatures above 1267 K [45]. All the NTO intensity peaks were identified by the single-phase $\alpha$-NTO crystal structure (triclinic symmetry) previously published in [46].

3.5. DSC/TG Tests

In Figure 9, the DSC/TG curves for pure NTO and 1NTO composition are presented. In case of NTO, we may notice a single exothermic peak with a maximum temperature equal to 283.06 °C and heat of reaction equal to 946 J/g, whereas the mass loss process starts at approximately 190 °C.

In each run for NSTEX composition, a decrease in mass can be observed with an analogous course for all tested samples (Figure 10). The initial slight decrease in mass (up to about 150 °C) is related to the removal of adsorbed water and residual solvent from the electrospraying process. The subsequent rapid decrease in mass in the temperature range of 15–400 °C (with a sharp drop of about 330 °C) is related both to the decomposition of nitrocellulose and nitrotriazolone, as well as to reactions occurring in the Ti-CuO system in the 300–400 °C to, due not only to the decomposition of CuO to Cu₂O, Cu and O₂, but also to the reaction between the fuel and oxidising agent.

Subsequently, in the temperature range of 400–750 °C, the uniform mass decrease is related to the oxidising agent decomposition and reactions between the metallic fuel and oxidising agent (Figure 11). The uniform mass decrease is noteworthy, independent of the thermal effects occurring. Above this temperature, a stabilisation of the mass of all the samples can be observed, or a slight increase in the case of the 10NTO composition, which is due to the reaction of titanium with the atmospheric air from the back-pressure air flows generated in the apparatus chamber.

The overall decrease in mass is consistent with the NTO content of the compositions. The mass decrease in the first stage shows an analogous trend, which indicates the correctness of the assumptions made above. Additionally, the value of the mass decrease is greater than the NTO and NC content of the compositions, indicating the presence of the CuO decomposition and reaction in the thermite system. The discrepancy between the mass decrease and the theoretical NTO and NC content increases with increases in their content in the system. The further decrease in mass is quite similar, with, however, a noticeable downward trend, with increasing NTO content indicating that the CuO decomposition reaction and the thermite reaction are facilitated with increasing NTO content. This difference is particularly evident for the 10 NTO composition (Figure 10).

For all compositions, the observed thermally induced transformations are relatively similar, with compositions 1NTO and 3NTO showing three distinct exothermic peaks and compositions 5NTO and 10NTO showing a small exothermic fourth peak.

The first reaction occurs in the temperature range of about 250–320 °C. The heats of these reactions increase rapidly with increasing NTO content (36; 81; 121; and 217 J/g). The curves themselves indicate a two-stage reaction, where the breakdown is more pronounced with increasing NTO content.

In this stage, the decomposition of NTO (268.2 °C [47], 274–288 °C [48]) takes place. However, given the mass drop values, there is also an initial reaction of the nanothermite itself, or at least the decomposition of the oxidising agent.

The subsequent reaction, occurring in the temperature range 400–500 °C is characterised by the highest heat and is the main reaction of the nanothermite system. Note the double peak; the lower the NTO content in the system, the more pronounced it is, which may indicate that the titanium–copper oxide reaction is limited by the intermediate products of combustion (titanium oxides) and that the amount of gases evolving upon the combustion of the composition has a strong influence on the combustion mechanism itself, with a large amount of gaseous products facilitating permeation through the titanium oxide layer into the reactive core of the Ti grains. The heat of the reaction reaches a maximum for the 3NTO composition and decreases rapidly with further increases in NTO content.

Another process, occurring in the temperature range of 600–700 °C, shows analogous trends, with a maximum heat release for the 3NTO composition. This process is most likely related to consecutive reactions of the Ti/CuO system to more stable products.

The percentage contributions of the individual reactions to the total decomposition heat of the composition are presented in Figure 12.

With increasing NTO content, the contribution of reaction 1 increases at the expense of reactions 2 and 3. Considering the total reaction heats (525; 625; 439; and 372 J/g in function of NTO content), this indicates not only the influence of NTO, which increasingly dominates the reaction mechanism, but also a reduction in the reactivity of the composition. As a limiting composition, the 5NTO composition should be distinguished, where the contribution of reaction II is still dominant; at the same time, the other decomposition stages are characterised by similar values.

As mentioned previously, the mass loss observed below 400 °C is higher than stemming straight from the NC and NTO content in the compositions, where this difference rises with NTO content. Based on the contribution of the first process on the net reaction heat, an initial reaction between Ti and CuO takes place.

The minor addition of gas-generating compounds was expected to enhance the nanothermite reaction, due to the formation of reaction “micro-clusters”. In this case, the evolving gaseous decomposition products of NTO and NC constitute a pressure wave that prevents sintering. The hindered sintering process allows maintaining the nanometric size of reacting particles, leading to a more efficient combustion. Simultaneously, the presence of gaseous species influences the heat transfer in the combustion zone, due to their facile migration through the bulk of the nanothermite composition.

In the case of samples containing larger shares of NTO, the unit volume of evolving gases may be sufficient to disperse the solid components of the formulations, simultaneously obstructing solid–solid reactions and facilitating the escape of oxygen from the reaction zone. In such an event, combustion via the reactive sintering mechanism is no longer viable, leading to combustion via a typical diffusion mechanism, limited by gaseous products. This will disrupt the equivalence ratio of the components in subsequent reactions and result in significantly lowering the net heat of combustion.

3.6. Closed Vessel Combustion

The pressurization rate observed for the nanothermite samples (Figure 13) appears to be directly correlated with the system [49]. The pressurization rate (dP/dt) of the entire system reaches the maximum for 3 wt.% NTO.

This is due to the fact that the reactions occurring during combustion provide the greatest amount of heat, as shown in the section above. Through increasing the NTO content, the composition is shifted to a state of oxidising agent deficiency, resulting in a reduced oxidation of Ti by CuO. Interestingly, the P_max for the tested compositions does not correspond to the net heat of combustion.

Nevertheless, the reaction of NTO occurs with the generation of a large amount of gas, resulting in an increasing maximum pressure value as the amount of NTO in the composite increases. The total combustion time of the compositions tested is fairly constant, with the exception of 3NTO, for which it is equal to approximately 64 ms.

Based on the obtained results, it can be concluded that NSTEX composition with 3 wt.% NTO is an optimal formulation in terms of energetic performance. In ranking the compositions in terms of decreasing pressurization rate, the sequence of 3NTO > 5NTO > 10NTO > 1NTO is obtained.

3.7. Thrust Force

The results of the thrust force measurements are presented in Figure 14.

Initially, the addition of NTO results in increasing the maximum thrust force, but exceeding 3 wt.% NTO in the composition results in a rapid decrease in the thrust force. Although the increasing NTO content contributes to the amount of gases evolving from a unit mass of the composition, it also prolongs the combustion of the sample, resulting in a significantly lower thrust force. This is due to the fact that thrust is significantly affected by the mass flow rate and velocity of the exhaust gases [50]. Conversely, the total impulse decreases to a much lesser extent, due to the higher combustion times and high gas emission for samples containing larger amounts of NTO contents.

The increase in combustion time points to the decelerating effect of NTO addition on the ignition process. Nevertheless, the rise in gas production allows the maximum thrust to reach higher values for compositions with 1–3 wt.% of NTO.

3.8. Open-Air Combustion

Open-air combustion tests were conducted for loosely poured samples (Figure 15) that were not confined in any way. Consequently, higher experimental variance was encountered than in the case of other described experiments, making the in-depth analysis of the results unreliable. Nevertheless, the obtained top-level information provides a significant insight into the behaviour of the compositions.

Firstly, increasing the NTO content up to 3 wt.% results in prolonging the time required for igniting the composition, but further NTO content increases do not significantly increase ignition times. This is in line with the aforementioned postulate that NTO decelerates the ignition and combustion development of the investigated compositions. Secondly, several combustion stages can be identified for all compositions. Initially, point ignition develops into rapid combustion with high brightness and narrow flame. This stage transitions into a deformed hot-gas zone, surrounded by spallating solid particles, that continue to combust during this process. The duration of the main combustion stage of the investigated NSTEX samples indicates the stabilization of the combustion process. The full combustion develops more rapidly for compositions containing lower amounts of NTO (Figure 16). Conversely, the time of the entire combustion process is relatively similar for all samples except 10NTO, for which it is noticeably longer. Usually, however, the afterburning of scattered solid particles lasts longer than the combustion of gases.

3.9. Combustion Velocity

Contrary to the results of open-air and thrust force tests, combustion velocity measurements (Figure 17A) revealed the lack of any significant dependence on NTO content (i.e., a minor combustion velocity increase upon the transition from 3NTO to 5NTO) on the measured values. A significant dependence on the density, in relation to TMD, was, however, observed, with increasing density sharply decreasing combustion velocity, as expected for nanothermites.

High-speed camera image analysis allowed the development of ignition in the compositions to be followed, along with the formation of a gaseous piston that thickens the composition on the head of the combustion front (Figure 18), initially resulting in a slower ignition and combustion development. At a certain point, the initiation of the combustion front occurs behind the thickened area and the rapid development of the combustion process, combined with the simultaneous combustion of the aforementioned area, become apparent.

A significant curvature of the combustion front is also apparent, resulting from the area in which the composition has re-ignited. Most likely, the observed thickening effect is connected with a pressure wave propagating before the flame front [51]. A similar phenomenon was observed for all tested compositions, indicating the occurrence of a two-step combustion development process: the main ignition of the composition, associated with the initial initiating stimuli, leading to the formation of a gas piston, and a second ignition behind the initial flame front that rapidly develops into stable combustion.

Contrary to the effect of the NTO content in the compositions, increasing their density (expressed in relation to the theoretical maximum density, TMD) brings about a significant decrease in the measured combustion velocity. This inversely proportional relationship between combustion velocity and density is typical of nanothermites and compositions based on them [52]. This effect is related to the limitations of mass and heat transfer through a system, where the porosity decreases with increasing density. The studied compositions produce relatively high amounts of gaseous products upon combustion; hence, their contribution to the combustion process will become increasingly strong with increasing NTO content. Simultaneously, hindering gas permeation is expected to have a more significant effect on the achieved combustion parameters.

4. Conclusions

The tested NSTEX compositions show high safety parameters, i.e., are completely insensitive to impact, and show minor sensitivity to friction; their radiation sensitivity is moderate and inversely proportional to the NTO content. The compositions were readily prepared via electrospraying and were highly uniform in both component distribution and morphology, even though the latter is dependent on the presence and amount of NTO. The increasing NTO content results in the NSTEX becoming composed of increasingly larger and progressively less porous particles.

In terms of performance, the change in NTO content significantly influences the combustion mechanism. This is manifested as both changes in the contribution of the individual reactions taking place during thermally induced decomposition and changes in the combustion parameters observed in closed and open systems.

The total reaction heat was the highest for samples containing 3 wt.% NTO. Analogous conclusions were derived from the closed vessel combustion and thrust force tests—the pressurization rate, thrust force and specific impulse achieved maxima for a NTO content of 3 wt.%. Open-air combustion confirmed the radiation sensitivity tests results and proved the occurrence of multi-stage combustion for the studied compositions.

Conversely, combustion velocity measurements indicated that increasing NTO content also increases the linear combustion velocity of the tested compositions. This points to the crucial role of hot gases in the combustion of these NSTEXs, as evolving gases drive the reaction via heat exchange through convection.

It should be noted that the performed investigations were conducted with the limitation of at most 10 wt. % NTO in the NSTEX formulations, as the secondary EM should not constitute a large share of the NSTEX. Further increasing the NTO content may rapidly change the combustion mechanism, up to a detonation regime.

Overall, the investigated NSTEXs, based on the Ti/CuO nanothermite system supplemented by NTO, provided favourable performances (e.g., a combustion velocity of up to 735 m/s), while maintaining high safety parameters. This makes them extremely promising materials for application in, e.g., igniters and pyrotechnic relays.

Due to the fact that this report is devoted to the novel NSTEX system, we limited our investigation of the microstructure of the produced NSTEX samples. This is the next logical step in describing the Ti/CuO/NC system, as fine-tuning this microstructure will likely lead to maximising the performance of these NSTEXs. This, however, requires an in-depth study and optimisation of the electrospraying process used for the preparation of samples and will necessitate its own dedicated report.

Future investigations of the tested compositions should be dedicated to deepening our understanding of the initially proposed combustion mechanism, including kinetic factors and actual application of these compositions in blasting devices, such as igniters or fuses.