Nickel hydrazine nitrate/TNT hybrid: toward novel castable energetic nanocomposite with customized performance and decomposition kinetics via novel synergistic effect

Advanced energetic metal–organic frameworks (MOFs) can expose novel characteristics. Highly crystalline nickel hydrazine nitrate (NHN) MOF of 50 nm size was developed via wet coordination. NHN experienced superior decomposition enthalpy of 3100 J g−1 at 222 °C using differential scanning calorimetry. NHN could act as a sensitizer for trinitrotoluene (TNT) and could boost its decomposition enthalpy, detonation velocity, and decomposition kinetics. NHN nanoparticles were effectively integrated into TNT energetic matrix; proper particle dispersion was verified via energy-dispersive X-ray analysis, whereas TNT experienced decomposition enthalpy of 340 J g−1; NHN boosted TNT decomposition enthalpy by 282%, with decrease in main decomposition temperature by 31 °C. NHN/TNT nanocomposite revealed a decrease in TNT activation energy by − 23.5% and − 22% via Kissinger and KAS models, respectively. The synergistic effect of NHN on TNT was ascribed its high decomposition enthalpy, gaseous products. NHN could secure novel catalytic effect on TNT decomposition with increase in detonation velocity by 11%; this was ascribed to the release of active hydrogen that could support CH-α attack with the exclusive formation of nickel nanocatalyst.


Introduction
Trinitrotoluene (TNT), the universal explosive in use, exposes low performance.TNT suffers from lack of heat output [1][2][3][4][5].On the other hand, energetic metal organic frameworks (MOFs) can secure novel characteristics in terms of high interfacial surface area, decomposition enthalpy, and stability [6,7].Hydrazine nitrate has found wide applications as liquid explosive with explosion heat of 3829 J g −1 .
Hydrazine-based energetic MOFs can find superior applications as green initiators, percussion compositions, and catalysts [8].Nickel hydrazine nitrate (NHN) is one of the most prominent energetic MOFs [9][10][11][12].NHN secured low production cost and high safety compared to other traditional explosive as RDX and HMX [13,14].NHN has the potential to decompose with the release of large amount of gases and nickel metal.
NHN revealed high thermal stability up to 200 °C; NHN experienced violent decomposition at higher temperature, followed by explosion reaction [15].The evolved nickel can act as a catalyst for secondary decomposition reactions.Integration of NHN into energetic materials (i.e., TNT) can improve the decomposition temperature, explosion heat, and the detonation velocity.
In this study, NHN nanoparticles (NPs) of 50 nm were developed via hydrothermal coordination.Thermal behavior and decomposition enthalpy of NHN was assessed via DSC and TGA.A melt cast composition based on NHN/ TNT (5/95 mass%) was developed.NHN/TNT nanocomposite experienced an increase in TNT decomposition enthalpy by 282%, with decrease in TNT main decomposition temperature by 31 °C.NHN/TNT nanocomposite revealed a decrease in TNT activation energy by − 23.5% and − 22% via Kissinger and KAS models, respectively.Furthermore, 1 3 NHN demonstrated an increase in TNT detonation velocity by 11% using EXPLOMET equipment.It can be concluded that NHN could act as a sensitizer and high energy dense material for TNT.[16,17].This superior catalytic effect was ascribed to the high interfacial surface area as well as exclusive formation of Zn nanocatalyst during NHN decomposition.

Synthesis of nickel hydrazine nitrate (NHN)
Nickel nitrate (Aldrich, 98%) was employed as the precursor of nickel ion.Hydrazine hydrate (Aldrich, 99%) was adopted as an energetic legend.Nickel nitrate solution 8% was heated to 95 °C with mechanical stirring in a steel autoclave.Hydrazine (7 mL) was added drop wise to 50 mL of nickel nitrate under mechanical mixing for 40 min.The suspension was stirred for 15 min at 95 °C.The purple colored product was filtered and washed three times with distilled water.The product was washed with ethanol and dried for 10 h at 65 °C.

Formulation of NHN/TNT nanocomposite
TNT (9.5 g) was weighed in 100 mL beaker; subsequently, TNT was heated in water bath at 83 °C with mechanical stirring for 45 min.NHN (0.5 g) was integrated into the molten TNT, as three equal small portions.The whole mixture was stirred for 30 min to ensure good homogeneity.The integration of NHN into TNT secured good particle dispersion.While pristine TNT demonstrated light yellow color, NHN/TNT nanocomposite demonstrated dark orange color (Fig. 1).

Morphology of developed NHN was investigated via transmission electron microscope (TEM) (JEM-2010F
by Joel Corporation).NHN crystalline structure was investigated via Hiltonbrooks X-ray diffractometer (XRD), over the angle range 2θ from 5 to 65 degrees.Chemical structure of NHN was assessed via Fourier-transform infrared spectroscopy (FTIR) over the range 400-4000 cm −1 with 4 cm −1 resolution via JASCO spectrometer Model 4100 (Japan).Scanning electron microscope (SEM), ZEISS SEM EVO 10 MA, was employed to investigate the morphology of NHN and NHN/TNT nanocomposite.Elemental mapping was investigated via EDAX detector.

Thermal behavior of NHN/TNT nanocomposite
The thermal behavior and decomposition enthalpy of developed NHN was investigated using DSC Q200 by TA; the tested sample (0.5 mg) was heated to 500 °C at 10 °C min −1 , under 50 mL min −1 N 2 flow.The impact of NHN on TNT decomposition was further assessed using TGA.Thermal behavior of developed nanocomposite was further investigated using TGA55 by TA; the tested sample was heated to 500 °C at 10 °C min −1 , under 50 mL min −1 N 2 flow.

Characterization of NHN
TEM micrographs demonstrated NHN particles of 50 nm average particle size (Fig. 2).
The relative crystal size of NHN was estimated to be 48 nm from the XRD line using Scherrer equation; this value was found to be in good agreement with TEM results [23].The degree of NHN crystallinity (X c ) was evaluated via Eq. 3.
A c is the area of the crystalline peak, and A a is the area of the amorphous peak.Degree of crystallinity was found to be 62.5%; this extent of crystalline structure was suitable for XRD without assisted polymers like polyvinylpyrrolidone (PVP).Elemental mapping using EDAX analysis confirmed the uniform dispersion of main NHN elements including (N, O, and Ni) (Fig. 4).
NHN was effectively integrated into TNT energetic matrix.Nickel was adopted as the key element to assess uniform dispersion of NHN within TNT matrix.Elemental (3) XC = AC AC + Aa mapping of nickel into TNT nanocomposite was investigated via EDAX detector (Fig. 6).
It can be concluded that uniform dispersion of NHN with TNT energetic matrix was accomplished.Uniform particle dispersion is fundamental requirement for enhanced performance and decomposition kinetics.

Thermal behavior of NHN
Thermal behavior of NHN was investigated using DSC.The sample size of 0.5 mg was employed in closed aluminum pan.The tested sample was heated at constant heating rate of 10 °C min −1 , under N 2 flow of 50 mL min −1 .NHN demonstrated main exothermic decomposition peak at 222 °C with the evolution of 3100 J g −1 calculated (Fig. 7).
This high level of decomposition enthalpy could inherit NHN novel characteristics as high stoichiometric balanced high energy-dense material.Thermal behavior of NHN was further investigated using TGA.Rapid exothermic reaction took place at 222 °C with mass gain reach to of 250% due to the rapid release of gas exerting a force on the balance (Fig. 8).

Thermal behavior of NHN/TNT nanocomposite.
Thermal behavior NHN/TNT composite was investigated using DSC to virgin TNT.The tested sample (0.5 mg) was heated at 10 °C min −1 .Virgin TNT demonstrated an endothermic melting of 110 J g −1 at 80 °C, with subsequent exothermic decomposition of 340 J g −1 at 291 °C.Castable NHN/TNT nanocomposite demonstrated decrease in endothermic melting by 45%.In the meantime, the main exothermic decomposition was enhanced by 282% compared to virgin TNT (Fig. 9).

Synergistic effect of NHN on TNT thermolysis
TNT aromatic ring fission could take place after the removal of all substituent [26,27].It is widely accepted that TNT decomposition includes two mechanisms: • C-NO 2 homolysis is the more favorable mechanism, with the rearrangement of C-NO 2 to C-ONO [28].Cleavage of the O-NO bond requires energy of 6.7 kcal mol −1 with the formation of thermodynamically favorable O and NO(g) [29].• C-H α attack depends on the hydrogen atom transfer from CH 3 group to the O-nitro with the elimination of water molecule and NO gas [30][31][32].
NHN decomposition, with the release of active hydrogen, could support the catalytic decomposition of TNT via the attack of O-nitro group (Fig. 11).Additionally, NHN decomposition could be accompanied with the release of nickel nanocatalyst.Nickel nanocatalyst could secure facile decomposition at lower temperature and could absorb the evolved on the surface.The oxidation of nickel to nickel oxide could contribute to decomposition enthalpy.In an attempt to assess the evolution of nickel nanocatalyst during NHN decomposition, certain amount of NHN was burned in ceramic crucible.The decomposition products were investigated with EDAX detector to assess the evolution of nickel nanocatalyst upon NHN decomposition (Fig. 12).
Formation of nickel particles upon NHN decomposition is obvious the evolved particles would act as a catalyst for TNT decomposition.Quantification of NHN decomposition products confirmed the evolution of nickel particles.This confirmed the synergistic effect of NHN on TNT decomposition.Integration of NHN into TNT secured enhanced detonation velocity, detonation velocity was assessed via precise measurement of the required time for the detonation wave to travel a measured distance longitudinally.Whereas TNT demonstrated detonation velocity of 5650 ± 75 m s −1 , NHN/TNT demonstrated 6270 ± 11.5 m s −1 using EXPLOMET micro-counter.The exclusive formation of nickel nanocatalyst could support the detonation velocity.

Kinetic parameters via Kissinger model
The thermal decomposition kinetics of TNT and NHN/ TNT nanocomposite was investigated using nonisothermal technique of TGA with four heating rate 1, 3, 5, 7, and 10 °C min −1 with different mass losses as shown in (Fig. 13).The residual related to carbon and nickel, respectively.
Activation energy was determined using the Kissinger model (Eq.1).The activation energy was retrieved from the slop of the straight line from plotting ln (βT −2 ) versus Transmittance/% Wavenumber/cm -1 Fig. 5 Infrared absorption spectra of NHN T −1 where T is the decomposition peak temperature, which obtained from the DTG curve (Fig. 14).
Whereas TNT demonstrated an activation energy of 120 kJ mol −1 , NHN/TNT nanocomposite demonstrated an activation energy of 92 kJ mol −1 [33].This finding confirmed the catalytic effect of NHN on TNT decomposition.

Kinetic parameters via Kissinger-Akahira-Sunose (KAS) model
The activation energy at the different fractional conversion was determined via modified Kissinger-Akahira-Sunose (KAS) method (Eq.2).The kinetics parameters of TNT and TNT/NHN nanocomposite are tabulated in Table 1.
The mean value of the activation energies of TNT and NHN/TNT nanocomposite was 122 kJ mol −1 and 95 kJ mol −1 .This result was found to be in good accordance with outcomes from Kissinger model.Furthermore, it confirmed the superior catalytic effect of NHN on TNT decomposition.

Conclusions
Energetic NHN MOF was synthesized via hydrothermal coordination.NHN experienced decomposition enthalpy of 3100 J g −1 at 222 °C.Castable NHN/TNT nanocomposite was developed.NHN offered an increase in TNT decomposition enthalpy from 345 to 1300 J g −1 with a decrease in main decomposition temperature by 31 °C.NHN experienced decrease in TNT activation energy by − 23.5% and − 22% using the Kissinger and KAS models, respectively.The detonation velocity increased from 5650 to 6270 m s −1 .NHN can act as novel catalyst and high energy-dense material for TNT decomposition.NHN could catalyze TNT decomposition via releases hydrogen that could support CH-α attack with the evolution of nickel catalyst.

Fig. 6 Fig. 9 Fig. 10 Fig. 11
Fig. 6 Elemental mapping of nickel within TNT nanocomposite International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material.If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.