Introduction

Dinitramide salts were first discovered by American researchers in 1988 [1]. Since their initial report of the synthesis of dinitramide acid and dinitramide salts, the dinitramide salts have attracted intense attention [2–10]. A potential use of dinitramide salts is as an environment-friendly replacement for ammonium perchlorate. The application of dinitramide salts in composite solid propellants eliminates the emission of chlorinated exhaust products from rocket [5]. These salts have high oxygen density, high heat of formation and high densities [11]. They are promising candidates for highly energetic materials with potential industrial or military application. The main advantages of such salts are the high specific impulse, the high burning rates, less production of smoke and a cheaper price [12, 13]. Ammonium dinitramide (ADN) is the most well-known salts of dinitramide [11, 14] because of its superior performance in composite solid propellants. However, the application of ADN is restricted by a big problem of high moisture [15]. And many other metal ion salts, such as K+, Cu(NH3) +4 and Pd(NH3) +4 as the cations, also have the same problem [16]. Numerous studies have exploited some organic ion salts of dinitramide to solve the problem [11, 14, 17]. The preparation of many new organic energetic salts of dinitramide needs appropriate dinitramide transfer reagents [18]. Free dinitramine (HN(NO2)2) is one of dinitramide transfer reagents. HN(NO2)2 did not appear to be promising because of its instability, decomposing at 273.15 K [19]. Generally, silver dinitramide (AgN(NO2)2) was used as the dinitramide transfer reagents [2, 16, 20, 21].

AgN(NO2)2 was first prepared through the metathesis reaction of potassium dinitramide (KDN) and silver nitrate in 1994 [19]. However, this compound was just characterized by melting point, elemental analysis [19], IR, Raman, 109Ag and 14N NMR [18]. The crystal structure of AgN(NO2)2 was reported in 2008 by Klapötke [22]. It is well known that AgN(NO2)2 is an energetic compound and an excellent dinitramide transfer reagent [23]. It is essential to have as much information as possible about the mechanism of the thermal decomposition, especially the products and energetics of each decomposition step. There are few reports about the thermal decomposition of AgN(NO2)2 up to date.

The main purpose of this work is to investigate the mechanism of the thermal decomposition of AgN(NO2)2 comprehensively. The thermal decomposition of AgN(NO2)2 was monitored by coupled DSC and TG/EGA-FTIR techniques under nitrogen atmosphere. The decomposition products were analyzed by UV–Vis, FT-IR, Raman and XRD techniques, and the thermodynamics analysis of its degradation was analyzed using Kissinger method and Ozawa method.

Experimental

Materials and instruments

Materials

Potassium hydroxide (AR), acetone (AR), amino acid (AR), isopropanol (AR), sulfuric acid (AR), silver nitrate (AR), ethanol (AR) and 95 % fuming nitric acid (industrial grade) were purchased from Cheng Du Kelong Chemical Co., Ltd. (China). Liquid nitrogen (99.99 %) was purchased from Mianyang Chang Jun Gas Co., Ltd. (China). All solvents were used without further purification.

Instruments

UV–Vis spectra were measured with a UV-2102 PCS spectrometer using demonized water as solvent. IR spectra were recorded with a Nicolet 5700 spectrometer (4000–400 cm−1 with accuracy of recording 1 cm−1) using KBr pellets. Raman spectra were recorded with an InVia spectrometer. TG measurements, coupled with the evolved gas analysis by FTIR (EGA-FTIR), were carried out with a TG209F1 instrument connected to a Nicolet 5700 FTIR spectrometer. AgN(NO2)2 sample (11.32 mg) was placed in a platinum crucible in the microbalance and was heated from room temperature to 1200 K. The analysis was performed under 90 mL min−1 supplied nitrogen flow and at a heating rate of 10 K min−1. The gases released were introduced through a heated tube into a FTIR cell. The absorption spectra were recorded in the region 4000–400 cm−1 with a resolution of 1 cm−1 by a Nicolet 5700 spectrometer to follow the progress of decomposition. Differential scanning calorimetric (DSC) curves of sample were recorded on a TA Q200 differential scanning calorimeter at heating of 5, 10, 15 and 20 K min−1 under nitrogen atmosphere. In the DSC, about 1.5 mg of sample was placed in an aluminum sample pan. The solid products of the thermal decomposition of AgN(NO2)2 were analyzed by XRD, Raman and FTIR techniques. The pure AgNO3 (purchased from Cheng Du Kelong Chemical Co., Ltd. China) was analyzed by Powder X-ray diffraction (XRD) as reference. XRD patterns were recorded on an X’PERT PRO diffractometer, using Cu-Kα-radiation. Room temperature diffract grams were recorded at scanning from 30° to 140°.

Synthesis

The synthesis route of AgN(NO2)2 is shown in Eqs. 1 and 2. Potassium dinitramide [KN(NO2)2, KDN] synthesis was carried out according to the literature [6] as the following procedure (reactions 1 and 2). AgN(NO2)2 was prepared by a modified procedure (reactions 1 and 2) according to the literature [18].

$${\text{HSO}}_{3} {\text{NH}}_{2} \mathop{\longrightarrow}\limits^{\text{KOH}}{\text{KSO}}_{3} {\text{NH}}_{2} \mathop{\longrightarrow}\limits_{ - 35\,{^\circ }{\text{C}}}^{{{\text{H}}_{2} {\text{SO}}_{4} ,{\text{HNO}}_{3} }}{\text{HN}}({\text{NO}}_{2} )_{2}$$
(1)
$${\text{HN}}({\text{NO}}_{2} )_{2} \mathop{\longrightarrow}\limits^{\text{KOH}}\mathop {{\text{KN}}({\text{NO}}_{2} )_{2} }\limits_{\text{KDN}} \mathop{\longrightarrow}\limits_{ - {\text{KNO}}_{3} }^{{{\text{AgNO}}_{3} }}{\text{AgN}}({\text{NO}}_{2} )_{2}$$
(2)

The preparation of potassium sulfamate

Sulfamic acid (71.00 g) was suspended in 50.0 mL deionized water, and 50.0 mL 15.4 mol L−1 potassium hydroxide solution was added into the suspended sulfamic acid under stirring at room temperature. After adding potassium hydroxide solution, the mixture was kept stirring for 25 min until it became clear. Then the neutral solution was poured into 200.0 mL ethanol, and white solid precipitated. The precipitate was filtered on a filter paper and was washed three times with ethanol. Then solid product was dried in vacuum drying oven at 343 K, to obtain 83.50 g potassium sulfamate.

Under vigorous stirring, 17.00 g potassium sulfamate was gradually added into a 60 mL mixed acid system sulfuric acid–nitric acid (3:1, vol/vol). The nitric–sulfuric acid was kept cool from 238.15 to 228.15 K, with a mixture of liquid nitrogen and ethanol. The viscosity increased significantly as the reaction proceeded, and KHSO4 precipitated. After reacting 25 min, the reaction mixture was poured into a bath including 300.0 g mixture of crushed ice and deionized water, and the cooled mixture was neutralized immediately with 200 mL 5.0 mol L−1 cold potassium hydroxide solution; during this process, the temperature of system must be kept below 273 K. The solution became green-yellow when the neutralization approached its terminal point. The potassium hydroxide solution was kept adding into reaction mixture until the pH of solution equaled 8.

The reaction mixture was concentrated by a rotary evaporator until the mixture became dry powder. And the latter was extracted with 150 mL acetone, and the unsolvable solid was filtered. The acetone solution was concentrated with an evaporator to about one-tenth of volume and was poured into 120 mL 2-propanol. The precipitated salt was filtered and dried in vacuum oven at 343 K to obtain 9.3 g KDN, with a yield rate of 53 %.

The preparation of AgN(NO2)2

KDN (1.45 g) was dissolved in 100 mL of boiling ethanol, and 1.70 g AgNO3 was dissolved in 100 mL of boiling ethanol. The above two boiling solutions were mixed, and KNO3 was precipitated. When the system was cooled to 293 K, KNO3 was filtered from the solution. The filtrate was concentrated in a rotatory evaporator to about one-fifth of the initial volume, and some impurity precipitated. The impurity was filtrated while hot. The filtrate was evaporated to dryness to obtain yellow crystals 1.75 g AgN(NO2)2, with a yield of 82 %.

Results and discussion

Sample characterization

The UV–Vis spectra of AgN(NO2)2 in water are shown in Fig. 1. It was shown that the aqueous of AgN(NO2)2 exhibited intense bands at 212 and 284 nm and a band of low intensity (a shoulder) at 335 nm, which corresponded to the dinitramide anion [24].

Fig. 1
figure 1

UV–Vis spectra of silver dinitramide

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The Raman spectra of AgN(NO2)2 are shown in Fig. 2. Characteristic absorption peaks at 1506 and 1423 cm−1 in the Raman spectra were attributed to –NO2 asymmetric stretching vibrations. The peaks at 1317, 1191, 1172 and 1121 cm−1 were ascribed to –NO2 symmetric stretching vibrations. Bands at 1039 and 970 were related to group N3 asymmetric and symmetric stretching vibrations, respectively. The peaks at 831, 754 and 493 were identified as –NO2 flexural vibrations. The Raman spectra of AgN(NO2)2 were in accordance with the literature [25].

Fig. 2
figure 2

Raman spectra of AgN(NO2)2

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The IR spectra of AgN(NO2)2 are shown in Fig. 3. Characteristic absorption peaks at 1536 and 1431 cm−1 in the IR spectra were attributed to –NO2 anti-symmetric stretching vibrations. The peaks at 1384, 1344, 1205 and 1178 cm−1 were ascribed to –NO2 symmetric stretching vibrations. Bands at 1032 and 953 cm−1 were related to group N3 anti-symmetric and symmetric stretching vibrations, respectively. The peaks at 827, 732, 762 and 471 cm−1 were identified as –NO2 flexural vibrations. Raman and IR data for AgN(NO2)2 assignment are presented in Table 1. The vibration data (Raman, IR) were in good agreement with the previously reported literature [6, 25]. Above discussions revealed that the sample contained N(NO2) −12 . The melting point of AgN(NO2)2 was 405 K (Fig. 5), which was consistent with the Ref. [24]. Combining the analysis of FTIR, Raman with TG, the structure of AgN(NO2)2 can be confirmed.

Fig. 3
figure 3

FT-IR spectra of AgN(NO2)2

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Table 1 IR and Raman data of AgN(NO2)2
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Thermal decomposition of silver dinitramide

Thermal decomposition behavior is very important for energetic compounds. In order to get better understanding about thermal decomposition of AgN(NO2)2, TG and DSC methods were adopted to study the decomposition behaviors of AgN(NO2)2.

Figures 4 and 5 showed the TG and DSC curves for AgN(NO2)2 at a heating rate of 10 K min−1 in nitrogen atmosphere, respectively. As shown in Fig. 4, the decomposition process of AgN(NO2)2 could be divided to two stages. The first stage took place in 408–483 K with a mass loss of 21.86 %. It could be visualized in DSC curve (Fig. 5) that there was an endothermic peak at 405 K, which was attributed to the melt of AgN(NO2)2 [24]. It started to decompose at 407.5 K. A big exothermic peak at 437.7 K could be found, which indicated that a tremendous amount of heat was released quickly during the compound decomposing. The second stage with a mass loss of 28.24 % took place in 483–800 K. It started to lose mass at 523 K. It also could be observed in DSC curve that there was an endothermic peak appearing at about 625 K. It explained that some substance decomposed or sublimated, observing the mass losing from TG curve. A big endothermic peak appeared at 714 K, which may be ascribed to the decomposition of silver nitrate according to the references [26, 27].

Fig. 4
figure 4

TG curves for AgN(NO2)2 at heating rate of 10 K min−1 under nitrogen

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Fig. 5
figure 5

DSC curve of AgN(NO2)2 at heating rate of 10 K min−1 under nitrogen

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To investigate the proposed mechanism of decomposition of AgN(NO2)2, TG, EGA-IR, Raman and XRD technology were employed. The gases released from AgN(NO2)2 thermal decomposition were detected simultaneously by FT-IR spectroscopy (EGA-IR). The EGA-IR analysis results are shown in Fig. 6. It was observed that N2O (1985 cm−1) appeared in the first stage. The maximum evolution of N2O (1985 cm−1) was peaked at around 462 K. NO2 (1380 cm−1) forms in the second stage. The amount of released NO2 reached the maximum at ~760 K. These results manifested that NO2 (1380 cm−1) and N2O were the main evolved gases.

Fig. 6
figure 6

IR spectra of gas products formed in the thermal decomposition of AgN(NO2)2

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In order to study the solid product of the AgN(NO2)2 decomposition, FT-IR, Raman and XRD techniques were used. The AgN(NO2)2 was heated up to 490 K and cooled down to room temperature in nitrogen atmosphere, and the solid residue was analyzed with FT-IR and Raman detector. The FT-IR and Raman results are shown in Figs. 7 and 8a, respectively. In Fig. 7, characteristic bands of NO −3 were observed at 1762, 1384, 1274 and 823 cm−1. The strong peaks at 1384 and 1274 cm−1 were related to asymmetric NO −3 stretching vibrations. The peak at 823 was ascribed to NO −3 symmetric bending vibrations. The peak observed at 1762 cm−1 was attributed to the combination bands (as a result of ν 1 + ν 4, the symmetric stretching and in-plane bending modes) of the nitrate ion [28]. It suggested that the solid residue was nitrate. However, the weak absorption peak at ~2369 cm−1 was attributed to the symmetric stretching vibrations of CO2, which was adsorbed by sample.

Fig. 7
figure 7

FT-IR spectra of residues of AgN(NO2)2 after 490 K

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Fig. 8
figure 8

Raman spectra of residues of AgN(NO2)2 after 490 K (a) and pure AgNO3 (b)

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The Raman spectra of pure AgNO3 solid (purchased from Cheng Du Kelong Chemical Co., Ltd. China) are shown in Fig. 8b. The peaks at ~1045, ~1370 and ~723 cm−1 could be observed in Fig. 8a, b. The NO −3 symmetric stretching band peak appeared at ~1045 cm−1 and the asymmetric stretching appeared at ~1370 cm−1 [29]. The above analysis indicated that AgNO3 was the solid product of AgN(NO2)2 in first stage decomposition.

The AgN(NO2)2 was heated up to 873 K and cooled down to room temperature in nitrogen atmosphere, and the solid residue was analyzed by XRD. The result is shown in Fig. 9b. As shown in Fig. 9b, all detectable peaks were indexed as the SiO2 and Ag with structure comparable to the standard data PDF no. 80-2157 (SiO2) and PDF no. 00-004-0783(Ag), respectively. Therefore, the above analysis indicated that the Ag was the solid products of decomposition in second stage.

Fig. 9
figure 9

The XRD diffraction patterns of Ag (a) and residues of AgN(NO2)2 after 873 K (b)

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Based on the above analysis of TG (Fig. 4) and DSC curves (Fig. 5), the mass loss 21.86 % of the first decomposition was close to the theoretical mass loss calculated for the decomposition AgN(NO2)2 to AgNO3 and N2O (20.56 %). The mass loss 28.24 % of the second decomposition was close to the total mass loss expected for the conversion of AgNO3 to NO2, Ag and O2 (29.91 %). The EGA-IR analysis (Fig. 6), IR (Fig. 7) and Raman (Fig. 8) analysis revealed that the main products of AgN(NO2)2 decomposition were AgNO3 and N2O in first stage (408–483 K). The thermal decomposition of AgN(NO2)2 in the first decomposition could be described by reaction 3. The EGA-IR analysis (Fig. 6) and XRD analysis proved that Ag and NO2 were the main products in the second decomposition stage. Taking the EGA, XRD analysis and thermal analysis into account, the intermediate decomposition product AgNO3 decomposed into Ag, O2 and NO2 in 483–800 K in nitrogen atmosphere. In the literature, the decomposition of AgNO3 was studied in argon and artificial atmosphere. It was proved that Ag, O2 and NO2 were the decomposition product of AgNO3 [28]. Therefore, the degradation of intermediate decomposition product AgNO3 could be described by reaction 4. Therefore, the mechanism of decomposition of AgN(NO2)2 could be proposed (Eqs. 3 and 4):

$${\text{AgN}}({\text{NO}}_{2} )_{2} \mathop{\longrightarrow}\limits^{{408 - 483\,{\text{K}}}}{\text{AgNO}}_{3} + {\text{N}}_{2} {\text{O}}$$
(3)
$${\text{AgNO}}_{3} \mathop{\longrightarrow}\limits^{{483 - 800\,{\text{K}}}}{\text{Ag}} + {\text{NO}}_{2} + {\text{O}}_{2} .$$
(4)

Thermal decomposition kinetics of silver dinitramide

To further understand the thermal decomposition behavior of silver dinitramide, Kissinger [30, 31] method and Ozawa method [32, 33] were employed to calculate the exothermal decomposition reaction kinetic parameters [apparent activation energy E a (kJ mol−1) and the pre-exponential constant A (min−1)] of AgN(NO2)2. In the temperature range 298–873 K, the DSC curves of silver dinitramide at heating rates of 5, 10, 15 and 20 K min−1 are shown in Fig. 10. In Fig. 10, it could be easily observed that the decomposition temperature moved toward high temperature as the heating rate increased. The basic data for exothermic decomposition of AgN(NO2)2 are listed in Table 2.

Fig. 10
figure 10

DSC curves of AgN(NO2)2 at heating rates of 5, 10, 15 and 20 K min−1

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Table 2 Decomposition temperature T p, 1/T p, ln (β/T 2p ) for thermal behavior of AgN(NO2)2 at different heating rates
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Kissinger method was used to calculate activation energy:

$$\ln \left[ {\beta /T_{\text{p}}^{2} } \right] = \ln \left[ {AR/E_{\text{a}} } \right] - \left[ {E_{\text{a}} /(RT_{\text{p}} )} \right].$$
(5)

β is the heating rate (K min−1), T p is the decomposition temperature (K), E a is the apparent activation energy (kJ mol−1), A is the pre-exponential factors, and R is the ideal gas constant.

The relationships between ln [β/T 2p ] and 1/T p were evaluated with linear regression analysis, which is shown in Fig. 11a. The activation energy was calculated from the relation between ln [β/T 2p ] and 1/T p, E a = 66.59 kJ mol−1, pre-exponential factors, A = 16260816, lg A = 7.21 min−1, correlation coefficient, linear correlation coefficient, r = 0.9913.

Fig. 11
figure 11

Linear relationships of ln (β/T 2p ) and 1/T p (a) and linear relationships of lg β and 1/T (b)

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Ozawa method was used to calculate activation energy:

$$\lg \beta = \lg \left[ {AE_{\text{a}} /RG(\alpha )} \right] - 2.315 - 0.4567E/RT.$$
(6)

The relationships between lg β and 1/T were evaluated with linear regression analysis, which is shown in Fig. 11b. The activation energy was calculated from the relation between lg β and 1/T, E a = 70.53 kJ mol−1, correlation coefficient, linear correlation coefficient, r = 0.9987.

The kinetics parameters of thermal decomposition of AgN(NO2)2 calculated by Kissinger and Ozawa methods are listed in Table 3. It could be easily concluded from Table 3 that values of E a and lgA calculated by Kissinger’s method were in approximate agreement with the values calculated using Ozawa method. This meant that one could use either of Eqs. (4) and (5) to calculate the active energy of thermal decomposition.

Table 3 Kinetics parameters of thermal decomposition of AgN(NO2)2 calculated by Kissinger and Ozawa method
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Conclusions

The thermal decomposition behavior of AgN(NO2)2 was studied by DSC, TG, EGA, Raman, FT-IR and XRD. Its thermal decomposition could be divided into two stages: the first stage (408–483 K) with a mass loss of 21.86 % and the second stage (483–800 K) with a mass loss of 28.24 %. The first stage gaseous decomposition product is N2O, and solid decomposition product is AgNO3. The second stage gaseous decomposition products are NO2 and O2, and solid decomposition product is Ag. The mechanism of decomposition of AgN(NO2)2 was proposed.

Apparent activation energy E a = 66.59 kJ mol−1, the pre-exponential constant lg A = 7.21 min−1 and linear correlation coefficient, r = 0.9913, were calculated by Kissinger method. E a = 70.53 kJ mol−1 and r = 0.9987 were calculated by Ozawa method.