Recent trends in nanothermites: Fabrication, characteristics and applications

Energetic materials (EMs) are a group of distinctive materials that release an enormous amount of amassed chemical energy in a short time when incited by external mechanical or thermal factors. They comprise of propellants, explosives, and pyrotechnics. Unlike conventional micro-energetic materials, nano energetic materials (nEMs), due to their smaller particle size ranging from 1–100 nm, exhibit higher specific surface area (~10–50 m2 g−1), reduced ignition temperatures from 2350 K to approx.1000 K for particle size from 100 μm to 100 nm respectively, higher energy densities (up to 50 MJ kg−1), burning rates ~30.48 mm s−1 at 6.894 kPa with specific impulses up to 542 s (5320 m s−1), low impact sensitivity (<4–35 J). Such exceptional properties of nano energetic composites, i.e., thermites (a combination of metal-fuel/metal oxide particles), find applications, namely in, munitions, pyrotechnics, energetic micro-electromechanical system (MEMS) chips. This review provides valuable insight into the synthesis methods of nano energetic composite systems (e.g., Al/CuO, Al/KMnO4, Al/Fe2O3, Al/SnO2, Silicon-based systems), their characteristic properties, behavior under certain conditions and applications. Furthermore, the review converses about the advancements made in the last few decades by many researchers, along with the technological gaps that need to be addressed for futuristic applications.


Introduction
Energetic Materials cataloged as Propellants, Pyrotechnics, and Explosives [1,2] are materials that are capable of providing a large quantity of energy by releasing their stored chemical energy when instigated by external factors like impact, friction or shock.
Historically, humans got familiarized with the energetic materials in 220 BC when few Chinese alchemists faced an accidental explosion due to the unintentional production of black powder, also known as Gun powder. Using Roger Bacon's experimental details from 13th century as a reference, Berthold Schwartz further validated the composition of black powder and also scrutinized its properties [3]. By the beginning of the 17th century, the Gun powder was used in detonating mines but had the risk of mine explosions. The need to develop more benign substitute explosive materials led to the discovery of nitroglycerine (NG) and dynamite in the 19th century followed by the development of nitrocellulose (NC), trinitrotoluene (TNT) in the 20th century and various energetic organic nitrate/nitramine compounds like cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), triaminotrinitrobenzene (TATB), in the later years [4][5][6]. Though energetic materials have been around for a while, the continuous need for enhanced, efficient, safe, and secure energetic materials drives researchers to keep on exploring the world of energetic materials and advance from the conventional energetic compounds to advanced EMs.
Many researchers contributed to understanding the molecular structure [7], the chemical reactions [8], the physicomechanical [9], and optical properties [10], the hazards associated with EMs [11], and the development of modern energetic materials. Owing to the limited energy densities (ex. 2094 J g −1 for TNT) of conventional EMs [12], researchers moved towards the usage of high energy density materials (approx. 30 kJ g −1 ), i.e., metals as fuels in the synthesis of EMs. With the advent of nanotechnology, nanoparticles of the order of 100 nm or less, because of their exceptional property of high specific surface area, replaced the micron-sized metal particles, thus leading to the fabrication of nanothermites (a mixture of fuel and oxidizers with particle size ranging in nanometric scale), a new realm of nano energetic materials [13]. Building upon the foundation led by the researchers several decades ago, the transition from conventional energetic compounds to advanced nanoenergetic materials took place.
Last two decades have witnessed a remarkable development in the domain of energetic materials. There has been a gradual shift from the usage of nitrocarbon energetic materials such as TNT, RDX, CL-20 to microstructured composites to nanothermites. This paper starts by giving a holistic view of the classification of EMs and then introducing the nanothermites and elaborates the trends in the fabrication methods involved in processing of nanothermites as well as it emphasizes mainly on the most commonly used Al-based nanothermites and their exceptional properties that render them suitable for miscellaneous applications in military, aerospace and civilian sector. Furthermore, it addresses the challenges that need to be resolved in pursuit of the enhanced NEMs for future development.

Al-based energetic materials
Of all the metals, Aluminum ordinarily serves to be a suitable candidate as metal fuel. Earth's crust is rich with aluminum, thus making it the third most copiously found element. It has high reactivity and characterized by high heat and efficient combustion, a high specific energy density of approximately 30 kJ g −1 , high enthalpy, high calorific value, and so forth [16,43,44]. Al augments the material's reactive power by escalating the combustion velocity by virtue of its high thermal conductivity [45]. Since it is non-toxic and shows excellent catalytic behavior, Aluminum is perpetually employed as a competent metallic fuel in composite EMs [1,46,47].
The aluminum powder acts as a crucial element in pyrotechnics, rocket propellants, and explosives. The factors like particle size and aluminum content should be considered judiciously as it affects the burning time, ignition delay time, viscosity of a rocket propellant mixture, specific impulse (I sp ), and ignition temperature.

Micron-sized particles
Several researchers have fabricated energetic composite materials by incorporating micron-sized particles [48,49]. Gibot et al [50] assessed the pyrotechnic performance for Al/SnO 2 energetic composite system comprised of SnO 2 with particle size <10 μm and Al particles with size~50 nm with the equivalence ratio (f) ranging between from 0.8 to 1.8. The combustion velocities ranged from 480 m s −1 to optimum value of approximately 580 m s −1 for equivalence ratio between 1 to 1.4. Equivalence ratio [48,50,51] is computed as: where F=fuel, A=oxidizer, act and st in the subscript indicate the real and stoichiometric ratios, respectively. In another study, Kang et al [28] successfully synthesized micron-sized Potassium Perchlorate and Zirconium (KClO 4 /Zr) composite by employing a chemical solution -deposition method. As the amount of KClO 4 varied (38 wt%, 42 wt %,71 wt %), different structures were observed, as shown in figure 3. (∼6 μm) KClO 4 /Zr (3-6 μm) composites displayed an extended light-radiation period and greater light-radiation energy/power. Whereas, Brown et al has enlisted experimental burning rates for a variety of binary pyrotechnic combinations with different compositions having different fuel particles size (in microns). For instance, reduction in the fuel particles radius from 14 μm to 2 μm while keeping the constant oxidizer particle radius in the Sb/KMnO 4 (13 μm) system [30], led to a rise in the burning rate ranging from 2 mm s −1 to 8 mm s −1 . Similarly, the decrease in the fuel particle radius of Mo from ∼18 μm to 7 μm in the Mo/peroxide systems like Mo/SrO 2 (∼2 μm), Mo/BaO 2 (∼5 μm) showed a substantial boost in the experimental burning rate values. Thus, it is ascertained that diminution in the particle size leads to a significant improvement in the desirable properties of the composite systems. Unlike the inadequacies like high ignition temperature, particle agglomeration, low energy release rate of micron-sized particles, researchers moved on towards using nanoparticles (figure 4) [52, 53].
Standard equation of a thermite reaction [57] is as follows: Some of the thermite reactions along with their adiabatic reaction temperature, heat of reaction and generation of gas have been given in table 1 [12]. The parameters, namely, ignition temperature, reaction rate, combustion velocities, ignition time and decomposition temperature in pyrotechnic mixtures get influenced by the reduced size of the fuel and oxidizer particles and show significant improved properties. Nanothermites/MICs are also called as Superthermites depending upon the extent of reactivity. Nanoparticles, unlike microparticles, increase the intimate interaction between oxidizer and fuel, that lessens the diffusion distance for mass transport, thus accelerating the reaction or burning rate [42] and reducing the mechanical sensitivity and ignition time [12]. They have increased ignition sensitivity due to its high specific surface area [58], the surplus energy of surface atoms, and strong surface activity. Pantoya et al performed a study on the effect of nano and micron fuel particles on the combustion velocities as shown in figure 5 which shows the dependence of combustion velocity on the Al particle size [48]. However,the synthesis of the nanoparticles can be done using various processing techniques. Aluminum nanoparticles were synthesized by the vapor phase condensation method by Schefflan et al [59]. The vapor depostion method produces nano-sized metal particles by cooling the gaesous form of the metal that is carried away by an inert gas. Whereas, Electrical Explosion of Wires (EEW) [41,60,61] tecnhique was one of the other methods used to obtain ultra fine metal powders. This method is characterized by high voltage source to generate high current pulses of about thousands of amperes, high plasma temperatures(∼10000 K), pulse duration ranging from μs to nanoseconds. Accordingly, it proves to be more advantageous than other evaporation methods as the electrical energy gets directly transmitted into heat. Furthermore, Elbasuney et al employed the hydrothermal synthesis method for fabricating colloidal CuO and Fe 2 O 3 nanoparticles for their integration in the energetic systems [12], as shown in figure 6. CuO and Fe 2 O 3 are the most commonly used metal oxides for energetic applications.
Similarly, the hydrothermal method [62] was also used to prepare nano-bismuth oxide particles having size of around 47 nm by Wang et al. This method involved preparing a mixture by adding 2.425 g of Bi(NO 3 ) 3 ·5H 2 O to 10 ml of (CH 2 OH) 2 , which is further stirred with C 2 H 5 OH for 30 min. This solution is then transferred to autoclaves and heated for 10 h at ∼160°C. Upon cooling, they were washed with deionized water and alcohol, respectively. Drying for 6 h at 60°C and calcination for 2 h at 325°C eventually produces nano Bi 2 O 3 particles.
On the other hand, Chowdhury et al synthesized nano Fe 2 O 3 particles by polymer matrix encapsulation technique for their use as an oxidizer for energetic applications [63]. Depending on the Fe 3+ concentration, the   . Some of the different composite systems reinforced with nanoparticles of metal fuel or oxidizers, their comparison and influence on the properties and have been discussed. Sanders et al analyzed the optimization of composites like Al/MoO 3 , Al/CuO, Al/WO 3 , and Al/Bi 2 O 3 as regards to the propagation speed (or burn rate) pressure output. It was concluded that the propagation depends on the state of the products and produced gas. Higher gas generation and liquid or gas products leads to higher propagation speed [65]. Besides, Glavier et al [66] performed a comparative analysis of burning rates, pressure peaks, and pressurization rate of the nanocomposites. The burning rate varied from 65 m s −1 (Al/CuO foils) to 420 m s −1 (Al/Bi 2 O 3 ). The pressurization rate was maximum for Al/Bi 2 O 3 nanocomposite with a value of approximately 5760 kPa.μs −1 at 30% TMD. Whereas the pressure peak of 41.7 MPa were reported to be highest at 50% TMD for Al/CuO. Figure 8. displays the SEM images for all the four energetic composite system.
Bockmon et al showed the impact of particle size on the pressure and combustion velocities of Al/MoO 3 metastable interstitial composites. Upon reducing the particle size from 121 nm to 44 nm, the combustion velocities showed a rise in the combustion velocity values from 600 m s −1 to 1000 m s −1 , respectively, as shown  in table 2 [40]. Additionally, Granier and Pantoya [67] performed a comparative analysis of the ignition times for the nanoAl/MoO 3 and micro Al/Mo4 3 energetic systems. The results reveal that the ignition times reduce significantly from 6 s for micron-sized (∼20 μm) Al particle composites up to 12 ms for nanothermites (∼17 nm). As depicted in table 3 [67], the burn rates showed an increase from 4 m s −1 to 12 m s −1 when the size of aluminum particles was reduced from 200 nm to 50 nm.

Si-based energetic systems
Aluminum nanoparticles having average size of 80 nm show high electrostatic discharge sensitivity and at a very low discharge energy level of 0.98 mJ, it tends to ignite. Thus, Thiruvengadathan et al [51] developed Silicon-  based nanoenergetic composite that displays decreased sensitivity and because of its property of surface passivation are capable of replacing aluminum nanoparticles in specific applications requiring lower combustion performance. The highest pressurization rate of ∼2.7 MPa μ/s and combustion wave speed from 1200 to 1500 m s −1 were reported as a function of equivalence ratio (f=0.9).
Brown et al [30] did a comprehensive study on Si-based and other fuel/oxidant systems. It was assumed that particles of fuel and oxidant are spherical in order to calculate the total number of point of contact (N R ) between them and compare it with the experimental burn rates of Si/Pb 3 O 4 , Sb/KMnO 4 , Si/Fe 2 O 3 , Si/Sb 2 O 3 , Si/KNO 3 pyrotechnic mixtures. The results of those fuel/oxidant systems have been represented in tables 4-6 [30]. It was concluded from the results that a minute variation in size of particle dramatically influences the contact points of fuel-oxidizer, and accordingly shows a significant influence on combustion velocity.

Hybrid/other composite systems
Recently, Wang et al [68] succeeded in incorporating ammonium perchlorate (AP) to a composite of fluoropolymer named polytetrafluoroethylene (PTFE) and Aluminum. This 9 wt% addition of AP to PTFE/Al energetic composite displayed substantial escalation in the output of energy (8863 J g −1 ), burn rate (1626 m s −1 ) and pressurization rate (340 MPa ms −1 ) as compared to pure PTFE/Al which has output of energy of 2019 J g −1 , burn rate of 260 m s −1 and pressurization rate 29.3 MPa ms −1 (figures 9-11).
A far greater burn rate and totally different flame structure are seen for PTFE/Al with AP, which shows that introduction of AP possibly enhances energy output [68] and combustion kinetics. Zamkov et al [69] Table 2. Combustion velocity and pressure values for Sample A (particle diameter=44 nm), Sample B (particle diameter=80 nm), Sample C (particle diameter=121 nm) [40].  investigated the chemistry involved in Al/Teflon AF nanoenergetic material using Ultrafast mid-infrared (IR) spectroscopy, where Teflon AF is a copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5difluoro-1,3-dioxole (dioxole). As compared to the theoretical value of heat of combustion of 8 kJ cm −3 for TNT, Al/Teflon mix shows a significantly high value of 21 kJ cm −3 .    Zhu et al did the latest study on hybrid nanothermite composites fabricated on a substrate (silicon) by lodging CL-20 with arrays of CuO/Al nanothermite. The properties of heat release were upgraded with 18.2% decrease in activation energy of integrated CL20 and increase in the total heat of reaction. The synthesized nanoenergetic composite shows an appropriate behavior of burning with an intense yet stable combustion flame [70].

Synthesis and properties of energetic materials
3.1. Evaporation-assisted (Vapor deposition) Layered vapor deposition usually is an adaptable process as nearly all the recurrently used metalloids, metal oxides and metals are able to be made by selecting appropriate deposition factors with easy control over the layer thicknesses. In the interim, the dense and distinct reactive multilayer nano foil (RMF) geometry makes the theoretical modeling more simple and boosts accuracy but this technology has some constraints. This method is expensive and challenging to scale up. Moreover, the problem of premixing becomes severe with minimal bilayer spacing. The interfacial free energies, chemical, and elastic strain can obliterate the structure of the layers. Conclusively, when total RMF is exceptionally thick, the unpredictability is a matter of concern, when reactants with diverse properties or with substrates are deposited. Manesh et al employed a magnetron sputter deposition method to prepare a layered Al/CuO films of thickness upto 3 μm [71]. Likewise, Petrantoni et al [72] deposited micro/nanostructured thermite of Al/CuO on SiO 2 wafers as displayed in figure 12 [72].  Ferguson et al made use of the atomic layer deposition (ALD) process to sequentially deposit thin and uniform layers of SnO 2 onto Al nanoparticles (diameter ∼50 nm) to prepare the Al/SnO 2 nanothermite with the help of H 2 O 2 and SnCl 4 reactants in a fluidized bed reactor [73]. ALD is considered to be an ideal process due to its self-terminating surface chemical reaction mechanism that involves purging in of inert gases after every pulse of the precursors which lead to high conformality and uniformity in the thickness of the thin deposited films. Upon ignition, it was observed that the SnO 2 coated Al particles reacted intensely as compared to uncoated Al particles thus demonstrating that ALD method can be utilized in fabrication of improved thermite systems.

Sol-gel technique
The sol-gel process includes chemical reactions taking place in the solution for initial production of nanoparticles called as 'sols' that are connected in a 3D solid system, called as 'gel', and the remaining solution is filled in the open pores. Figure 13 represents the sol-gel methodology [74,75]. Primarily there are three steps in sol-gel procedure; hydrolysis, condensation and drying. The sol-gel process is comparatively an easy method executed at low temperatures, relatively economical and it has the potential of producing utterly novel energetic materials with sought after properties [74].
Comparative study of sol−gel Ta/WO 3 nanocomposites with conventional powder mixtures was done. This study revealed that the heat released by sol−gel composite was approximately 30% to 35% greater as compared to the powder mixture due to the carbon presence in the sol−gel composite. In addition, Ta/WO 3 nanocomposite produced by the sol−gel were found to be unresponsive to spark, friction and impact ignition [76,77].

Electrophoretic deposition
Electrophoretic deposition [31,81,82] is an effective method used to make films on different surfaces of the conductive materials. Figure 16 shows the electrophoretic deposition setup arrangement [83]. The composition and quality of deposited coatings can be quantitatively altered by regulation of concentration of charged particles, time of deposition and field strength. It has extensive use for deposition of charged particles over irregular substrate surfaces the and as anticorrosive coatings. Lately, Wang et al explored the exothermic and      combustion properties of the Si/CuO based energetic systems produced by Electrophoretic deposition method [31] . The heat release value i.e., 244.3 kJ kg −1 was highest when the equivalence ratio was 1.0.

Solvent-non-solvent crystallization method
Comparing with the other few techniques available (figure 17) [84], this method has many advantages, like low cost, easy control over the process, simple crystallization principle, low operating temperature and wet state. Many NEMs prepared by this technique find application in slapper detonator and various other explosive devices. The liquid phase crystallization theory is the theoretical base for solvent/non-solvent process for the preparation of NEM. The solution has three different regions and three different states due to dissimilar concentrations during certain temperature and pressure conditions: unstable region, metastable region, stable region and supersaturated state, saturated state, unsaturated state. Making the solution attain state of supersaturation and nucleate swiftly in a very little time by combining non-solvent or weak solvent(commonly called non-solvent) and energetic material solution (liquid) strongly and quickly under precise conditions, is the key in making of NEM with solvent/non-solvent method. Nano scale energetic particles are prepared [84] by keeping under control the crystal nucleation and growth process and restraining the growth speed of crystal nuclei. Using this technique, a novel nanoenergetic composite was synthesized by coating Al/CuO with nano potassium perchlorate (KClO 4 ) and was found to have three times the burning rate of conventional compounds [85].

Powder mixing or mechanical/ultrasonic mixing
The ease of availability of n-Al powders enables them to be used widely for most of the metal-based reactive nanomaterials by utilizing simple powder mixing method. By means of commercially accessible ultrasonic cell disruptors or high intensity ultrasonic actuators the mixing is stereotypically carried out. In the lab assessments it is comparatively effective whereas the processing of large batches by ultrasonic mixing of nanopowders is very hard to scale up and may cause poor quality of mixing. Lately, nanothermites were synthesized by mixing the Fe 2 O 3 and Al nanopowders through the rapid expansion of a supercritical dispersion (RESD) technique. Relative to the conventional ultrasonic mixing a much better mixing was accomplished. The RESD method is wellmatched for continuous operation than ultrasonic mixing [41] , in addition to giving a better mixing quality.

Self-assembly
For preparing reactive nanocomposite compositions with functionalized nanosized oxide particles and starting with nanosized aluminum powder, the self-assembly [62] methodology was taken into consideration. The metal particles were set over the outside surface area of oxide nanorods or in the ordered pore structure of the mesoporous oxidants in composites to create ordered assemblies. For instance, in an Al-CuO system, the selfassembly was attained with initial functionalization of CuO nanorods by using poly(4)-vinyl pyridine (P4VP), a monofunctional polymer. The nanorods gets ordered within the material since Al nanoparticles hold on to the functionalized nanorods. Better hold on the material properties along with the better reaction rates in practical applications render the ordered nanocomposites more appealing compared to others. The inadequacies of this technique comprises of the high price of customized oxides, the existence of functionalizing agents that are usually responsible for reducing the energy density in energetic formulation, and the intrinsically high porosity of the materials [41,62] produced.

Additive manufacturing: 3D printing
A printable reactive ink consisting of micron (75 micron sieve) and nanoscale aluminum (80 nm) solid inclusions based on the fluoro-polymer (THV-a polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride) was created with its rheology suitably altered for direct-write assembly. Using usual pen type technique [86] the reactive inks were printed. Shen et al formulated a colloidal ink comprising of 90 wt% Al-CuO and the remaining 10 wt % of polymers like polystyrene, nitrocellulose and hydroxy propyl methyl cellulose (HPMC) to ease the fabrication process of high density reactive composites without hampering the combustion behavior and mechanical strength of the nanothermite [87]. Similarly, Wang et al used the direct ink writing(DIW) methodology and developed ink containing 10 wt% hybrid polymer of HPMC and polyvinylidene fluoride (PVDF) and 90 wt % nanothermite Al/CuO (Al=∼85 nm, CuO=∼40 nm). The ability of DIW technique to build structures with such high particle loading without the loss of mechanical strength along with the ease of adjustment of equivalence ratio, burn rate and rate of energy release has been drawing a lot of attention [88]. Mao et al [89] introduced a formula for new Al/CuO nanothermite ink that surpasses the existing limits and shortcomings of Al/CuO thermite and Direct-Ink-Writing (DIW). The presence of binder, F2311, helps in formulating hydrothermal inks with sound rheology shear-thinning properties, which proves to be useful in achieving the 3D printing of Al/CuO nanothermite. The printed patterns with loading as high as 75-90 wt% nanothermite are capable of attaining adjustable burn rates varying from 32 mm s −1 to 352 mm s −1 . It is reported that the heat released increases with the loading of nanothermite (table 7) [89]. The competence of this method to achieve high resolution and precise patterns of nanothermite, makes it appropriate for different applications in micro energetic device with boosted combustion performance (figure 18) [89]. For 3D printing, this new strategy can very well be used with other nanothermite inks. Westphal et al have effectively deposited nanothermites like Al/CuO and Al/Bi 2 O 3 on silicon substrate by inkjet printing and studied the effects of confinement and controllable fracturing performance that is generally employed in the electromechanical system security [90].
In addition, Murray et al proved that nAl (80 nm)/CuO (50 nm) nanothermite [91] can be successfully fabricated by reactive inkjet printing with improved safety, security and shelf stability. According to the high speed thermal imaging results, a difference of ∼200 K prevailed between the maximum reaction temperature of samples printed with the dual nozzle and a single nozzle technique. Though wide range of energetic materials can be synthesized using this method, the lack of bulk characterization techniques opens up opportunities for researchers to work on providing solutions to this problem in the future developments.

Heat of reaction
Occurring at a constant pressure, the variation in the enthalpy of a chemical reaction or the difference in the heat of formation of the reactants and products is called as the heat of reaction or enthalpy of reaction. It is used for calculating the energy per mole either produced or released in a reaction. Table 8 shows the values of heat of reaction for some of the Al-based nanothermites [39, 45, 92, 93].

Ignition time
The ignition time is the time taken by the hot gas to come in contact with the propellant and cause the emission of light [94]. It is reported that the reduction in the fuel and oxidizer particles size reduces the diffusion distance between them thus increasing their intermixing and reaction rates and leading to less ignition times. The effect of particle diameter variation on the ignition time has been depicted in figure 19 [52, 67].

Ignition temperature
The ignition temperature is the temperature at which the energetic material combusts. It is another important facet of nano-thermite characterization in determining the suitability of the thermite for a particular application. This property is influenced by the particles size of the constituents of the thermite [95]. The reduction of size of Al particles in Al/MoO 3 thermite from 10-14 μm to 40 nm displays decrease in the ignition temperature from 955°C to 458°C respectively [48,54].

Burning rate and pressure
It has been demonstrated by several researchers that faster burning rates can be achieved by using nano-sized particles instead of the micron-sized particles (figure 20) [96][97][98][99]. The burn rate-pressure relationship plays a vital role in determining the practicality of the nano thermite for a specific application. Mostly, as the pressure is  elevated or reduced the burn rate increases or decreases. Also, temperature of the propellant has great influence on burn rate and as temperature increases it increases and as temperature decreases it decreases. All other parameters being same, the calorific value of the propellant influences the burn rate. The Vielle's Law [21,100] i.e. the exponential form of burn -rate law, is given as:

Specific impulse (I sp )
The foremost vital single ballistic property for rocket propellants [21] is the Specific impulse (I sp ). This property is very crucial in determining the mass of propellant needed to suffice the ballistic requirements and is expressed as the thrust per unit weight flow rate of propellant (w). I sp of a propellant can be determined using equation (4).
where F=thrust; w=weight flow rate.  [42]. Moreover the pressure impulses (∼11 MPa) and higher values of energy densities render their application in microthrusters [100,102] and micropropulsion systems. When compared with the hexane milling, the self-assembled nanoenergetic materials Al/Bi 2 O 3 and Al/I 2 O 5 , displays elevation in the pressure discharge. Minimal quantities as low as 2 to 10 mg of manufactured nanoenergetic composites utilized in microthrusters are capable of producing a force ranging between 0.002 N to 0.6 N. For instance, 6000 N force can be generated by a group of 10,000 microthrusters on a 0.5×0.5 m surface. Nanoenergetic composites also find applications in Microelectromechanical Systems (MEMS) [43,45]. Table 10 shows the trend in the synthesis and development of novel energetic materials.  Teflon: a copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3dioxole (dioxole), the mass ratio of Teflon AF /Al=4.6

Applications
Al+CF 2 or CF 3 and the faster involving Al+CFO.
The reactions with CFO were >10 times faster than reactions with CF 2 or CF 3 .

Gun primers and electric matches
By varying size of particles or by usage of different nanoscale materials and varying the ratio of fuel to oxidizer of the composite, the super-thermite materials are made to be more sensitive towards thermal stimuli like resistive heating thus rendering them capable of replacing the lead-containing compounds in the electric matches/ igniters as well as non-toxic gun or electric primers ( figure 21) [54, 66, 105, 106].

Biocidal agent
Sullivan et al examined the reaction and ignition of Al/AgIO 3 thermites for plausible usage in various biocidal applications. Its ignition temperature of the Al/AgIO 3 was found to be between 1175 K to 1255 K whereas the pressurization rate outperforms that of Al/CuO [92]. Ivan Davila [107] composed a ternary-thermite system, which acted as NGG, having composition 10/75/15 wt % of I 2 O 5 /Ag 2 O/Al. The highly pathogenic microorganisms or bacteria are destroyed by the mixtures ability imparted to it by the production of biocidal gases from this composition. In this study, due to the production of a strong biocidal environment by the gaseous silver and iodine produced from NGG combustion, the living strain cells of Escherichia coli (E. coli) K-12 were destroyed. Likewise, the potent biocidal properties of Iodine-containing gases have been employed by Wang et al and successfully prepared a reactive ternary composite system of Mg-B-I 2 by mechanical milling of powders in two steps. Such reactive composites have potential to be used in munitions that can be used against biological weapons [108].

Molecular delivery applications
Patel et al ignited Co 3 O 4 /nAl and Co 3 O 4 (calcined at 400°C)/nAl nanoenergetic composites and reported the peak pressure/pressurization rates [109] and combustion front-wave speed. The heat of reaction was recorded to be 1.02 kJ g −1 for calcined Co 3 O 4 -400/nAl and 0.96 kJ g −1 for Co 3 O 4/ nAl nanoenergetic systems. The Co 3 O 4 -400/nAl and Co 3 O 4 /nAl nanoenergetic composite propagated at a maximum flame-front speed of 830±75 m s −1 and 781±50 m s −1 respectively. The maximum peak pressure, nearly similar to the CuObased nanoenergetic system, was obtained at an equivalence ratio of 1.6, but due to low gas generation during the combustion process the maximum pressurization rate with Co 3 O 4/ nAl (0.47±0.1 MPa μs −1 ) was much lower to that of CuO/nAl nanoenergetics. The Co 3 O 4 /nAl nanoenergetic system can create pressurization rate from ∼0.03 to 0.19 MPa μs −1 whereas mild peak pressure ranging from 12.6±1 MPa to 20±2 MPa that attributes to the low gas generation characteristics, and thus find application in low intensity pressure-pulse based microporation of soft matters such as bacterial cells without analysis [109].

Conclusion
Comprehensive analysis of various fabrication approaches and research accomplishments so far has been presented in the review paper. The reduction in size of particles to nanometric scale and its further incorporation to prepare nanoenergetic composites have elevated the potential of conventional energetic compounds and led to development of novel thermite systems. Nanothermites are a promising candidate for developing advanced energetic systems with desirable high energy densities, faster energy release rates, lower impact sensitivity, high burning rates, high specific impulse and so on for applications in Nanoenergetic Gas Generators (NGG), microthrusters and micropropulsion system, microelectromechanical systems (MEMS). Al-based metastable interstitial composites have been extensively explored by researchers by employing the traditional synthesis techniques. However, nowadays, other metallic fuels like Si, Sb, Mn, Zr, and advanced synthesis processes like Direct Ink Writing (DIW) 3D printing have grabbed the attention of the researchers. Despite the advantages of the nanothermite systems, the safety and security issues hinders the usage of nanoenergetic materials to a certain extent. Thus, a significant work can be done to broaden the scope of nanoenergetic materials for diverse future applications.