Hydrogen-enhanced combustion of a composite propellant with ZrH2 as the fuel

Combustion and Flame 187 (2018) 67–76

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Hydrogen-enhanced combustion of a composite propellant with ZrH2 as the fuel Yanjing Yang a, Fengqi Zhao a,∗, Huixiang Xu a, Qing Pei a, Hanyu Jiang a, Jianhua Yi a, Chunlei Xuan a, Sanping Chen b a

Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research Institute, Xi’an 710065, Shaanxi, China Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Material Science, Northwest University, Xi’an 710065, Shaanxi, China

b

a r t i c l e

i n f o

Article history: Received 20 February 2017 Revised 1 September 2017 Accepted 5 September 2017 Available online 16 October 2017 Keywords: Solid propellant Combustion Reaction mechanism Zirconium hydride Hydrogen Ammonium perchlorate

a b s t r a c t The reaction mechanisms of metal hydrides in the combustion of solid propellants are closely related to their effects on the energetic and combustion properties of propellants. Here, we report a first attempt to investigate the reaction mechanisms of ZrH2 , a promising candidate for fuels, in the combustion of HTPB propellants using ammonium perchlorate (NH4 ClO4 , AP) as the oxidizer. ZrH2 is determined to possess a good resistance to the direct oxidation by AP. Thus, it would dehydrogenate independently to generate H2 and metallic Zr, which is believed to be very favorable for decreasing the gaseous molecular masses. On the other hand, due to the attachment of gaseous reactants to it, ZrH2 was found to tune the decomposition behaviors of AP by enhancing the generation of NO in the high-temperature decomposition stage. More interestingly, the hydrogen released from ZrH2 is evaluated to promote the combustion reactions in gaseous phases and be responsible for the two-stage combustion behaviors of the corresponding propellant, which are distinct from those when metallic Al is used as the fuel. The findings in this work validate the potential of ZrH2 as a fuel for high-energy solid propellants. © 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction In solid rocket propulsion for both civil and military applications, the pursuit for longer range necessitates the development of novel materials to improve the energetic and combustion performances of solid propellants [1,2]. Metal hydrides, which are also very useful for a range of applications including hydrogen storage [3–6], hydrogen generation [7,8] and reducing agents [9–11], have attracted sustainable attentions as a series of promising metal fuels for solid rocket propellants. It is supposed that, the metal hydrides are capable of providing simultaneously high energy from metal oxidation and low gaseous molecular mass due to hydrogen (H2 ) released during the combustion processes [12–14], thus resulting in enhanced specific impulses of solid propellants. Considerable efforts have been devoted investigating the applications of promising metal hydrides in propellants, such as aluminum hydride (alane, AlH3 ) [15–19], beryllium hydride (BeH2 ) [20], magnesium hydride (MgH2 ) [17,21], titanium hydride (TiH2 ) [22,23], lithium hydride (LiH) [24] and ammonia borane (NH3 BH3 ) [25]. Especially, aluminum hydride (AlH3 ), which is proposed to be a promising al-

∗

Corresponding author. E-mail address: [email protected] (F. Zhao).

ternative to the most widely used metal fuel aluminum (Al), was found to be capable of increasing the specific impulse and decreasing the two-phase flow losses [12,15]. However, AlH3 is a fuel with relatively low density (1.477 g/cm3 ), which is significantly lower than those of aluminum (2.70 g/cm3 ) and conventional propellants (about 1.60–1.70 g/cm3 ). Thus, if AlH3 is adopted to replace Al in the aluminized propellants, the volumetric loading of the resulting propellants would be evidently reduced. Obviously, this is unfavorable for their practical applications. Zirconium hydride (ZrH2 ), which is now under intense investigations in several fields including hydrogen storage [26,27], neutron shielding [28], blowing agents for foaming metals [29] and moderator in nuclear reactors [30], on the other hand, possesses a much higher density of 5.61 g/cm3 . Therefore, the introduction of ZrH2 to propellants is proposed to be capable of providing both high energy and low gaseous molecular mass without reducing the densities [31]. As stated above, the high energy and low gaseous molecular mass offered by metal hydrides originate from the hydrogen and metal generated upon their dehydrogenation. However, it is wellknown that, metal hydrides are reductive due to the existence of Hδ − in them, thus resulting in a high possibility of the occurrence of interactions between metal hydrides and the strong oxidizers in solid propellants, such as ammonium perchlorate (AP), cyclotetramethylene-tetranitramine (HMX) and cyclotrimethylene-

http://dx.doi.org/10.1016/j.combustflame.2017.09.004 0010-2180/© 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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Y. Yang et al. / Combustion and Flame 187 (2018) 67–76 Table 1 Main ingredients of the HTPB propellants. Compositions HTPB Ammonium perchlorate Aluminium ZrH2 DOS TDI

Binder Oxidizer Metal fuel Metal fuel Plasticizer Cross-linking agent

trinitramine (RDX). Furthermore, the above interactions are believed to result in the direct oxidation of metal hydrides, which would give rise to the generation of metal oxides and water instead of the expected H2 and metals. Obviously, occurrence of the redox reactions between hydrides and oxidizers is very unfavorable for lowering the gaseous molecular mass and increasing the specific impulse of propellants. On the contrary, the independent dehydrogenation of metal hydrides in the existence of oxidizers to give H2 and metals would benefit the energetic performances of solid propellants. As a consequence, for metal hydrides used as the fuels for solid propellants, it is of great importance to determine their interaction characteristics with the oxidizers. In addition, the role of hydrogen in hydride, which is believed to be very critical for propellant combustion, should be also evaluated. Previously, Lempert et al. investigated theoretically the specific impulses, densities and combustion temperatures of several ZrH2 -based composite propellants with different oxidizers and binder [31,32]. However, little is known about the interaction characteristics of ZrH2 with the oxidizers and especially, the role of hydrogen in ZrH2 . For several decades, ammonium perchlorate (AP) has been widely utilized as an oxidizer for solid rocket propellants [33–35]. As a major ingredient of the formulations, its interactions with metal fuels are closely related to the performances of APcontaining propellants. Herein, we investigate the interaction characteristics between ZrH2 and AP, and especially, the role of hydrogen in the combustion of HTPB propellants containing ZrH2 as the metal fuel. It is evaluated that, ZrH2 would not be oxidized directly by AP or its decomposition products. Instead, it dehydrogenates to produce hydrogen and metallic zirconium which would promote the combustion of propellants. The new insights into the reaction mechanisms of ZrH2 in propellant combustion are very useful for the applications of metal hydrides in solid rocket propellants.

Contents (mass%)

Contents (mass%)

Contents (mass%)

10.22 71 15

10.22 71 10 5 3 0.68

10.22 71 5 10 3 0.68

3 0.68

Contents (mass%) 10.22 71 15 3 0.68

01-073-2076). It was found that, both AP and ZrH2 showed good crystallinity. In addition, no impurities were detected by XRD and FTIR, suggesting their high purities. Aluminum (Al, 5 μm), which is now the most widely adopted metal fuel in solid propellants, was used for comparison with ZrH2 . Zirconium (Zr) was also used in evaluating the role of hydrogen in ZrH2 . The binder, HTPB (hydroxyl–terminated polybutadiene), was obtained from Liming Research Institute of Chemical Industry. Moreover, in this work, dioctyl sebacate (DOS) and 2,4-tolylene diisocyanate (TDI) were utilized as the plasticizer and cross-linking agent, respectively. The compositions of propellant formulations are listed in Table 1 and no combustion catalysts were used in the formulations. The propellant samples were prepared in 500 g batches using a vertical planetary mixer of 2 L capacity. All batches were mixed and cast under vacuum by a slurry process. After HTPB, DOS and TDI were mixed evenly, AP was carefully introduced into the mixture followed by additional mixing. Then, the fuel (Al, ZrH2 ) was added to the sample and mixed evenly. It is worth mentioning that the addition of ZrH2 should be conducted under the protection of nitrogen. The propellant samples were cured at 50 °C for 72 h in a water jacketed oven. After that, the propellant densities were measured and densities were determined to be (1.75 ± 0.01) × 103 , (1.78 ± 0.01) × 103 , (1.81 ± 0.01) × 103 and (1.84 ± 0.01) × 103 kg/m3 for the propellants containing 15 mass% of Al, 10 mass% of Al + 5 mass% of ZrH2 , 5 mass% of Al + 10 mass% of ZrH2 and 15 mass% of ZrH2 . These densities are almost identical to the corresponding theoretical ones of 1.75 × 103 , 1.77 × 103 , 1.81 × 103 and 1.83 × 103 kg/m3 , suggesting that the propellant samples possess good quality. The AP/ZrH2 combination system used for thermal analyses was prepared by physically mixing AP and ZrH2 evenly using pestle and mortar.

2. Experimental section Ammonium perchlorate is an energetic compound sensitive towards impact and friction. When handling AP, propellant samples containing AP and its mixtures with metal fuels, proper protective measures including ear protection, Kevlar gloves, face shield, body armor, and earthed equipment should be used. Handling of the hydride was performed in a glove box (MBRAUN 200B) equipped with a circulation purifier to keep the concentration of O2 and H2 O bellow 0.1 ppm. In addition, extra attentions should be paid to the manufacturing of sample propellant containing ZrH2 since ZrH2 possesses higher reducing activity than Al. 2.1. Materials and sample preparation The oxidizer ammonium perchlorate (AP, NH4 ClO4 , 120 μm), which is obtained from Dalian Gaojia Chemical Co., Ltd, was dried in vacuum at 70 °C for 24 h before use. Zirconium hydride (ZrH2 , 10 μm) was obtained from Aladdin and used as received. Figure S1 (Supplementary material) exhibits the XRD patterns and FTIR spectra of AP (PDF number: 00-043-0648) and ZrH2 (PDF number:

2.2. Structural characterization Phase identification was conducted using powder X-ray diffraction (XRD) on a PANalytical X’Pert Pro X-ray Diffractometer with ˚ at 40 kV and 40 mA. The data were Cu Kα radiation (λ = 1.5418 A) collected from 5° to 90° (2θ ) in steps of 0.05 ° at ambient temperature. The vibrational characteristics of chemical bonds were determined using a Bruker Tensor 27 Fourier Transform Infrared (FTIR) spectrometer. The spectra of the samples (as KBr pellets with a KBr to sample mass ratio of approximately 30:1) were acquired in the range of 40 0 0–40 0 cm−1 in the transmission mode with a resolution of 4 cm−1 . X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCALAB 250 Xi XPS microprobe (Thermo Scientific, U.K.). The sample was prepared by sprinkling powder on a carbon tape which was attached to the sample holder in the glove box. The sample was transferred into the vacuum prep chamber by a commercial transfer vessel with overnight pumping before the measurement. XPS spectra were recorded using monochro-

Differential scanning calorimetry (DSC) examination, thermal gravimetric analysis (TGA), mass spectroscopy (MS) and FTIR spectroscopy measurements were conducted on a Netzsch STA449C thermal analyzer attached with a QMS 403C mass spectrometer and a Nicolet 5700 FTIR spectroscopy to determine the heat flow and gases released during the thermal decomposition of AP and AP/ZrH2 system. The experiments were conducted under a gas flow of 50 ml Ar/min and a heating rate of 10 °C/min was adopted. About 1.0 mg of sample was loaded into an Al2 O3 crucible and heated from room temperature to 500 °C. For all the combustion property evaluations, a nickel–chromium alloy wire ( = 0.15 mm) was utilized for ignition in the experiments. In order to investigate the combustion flame structure of the HTPB propellants, their flame profiles at different pressures were recorded by a camera using the single frame amplification photography method. Strand burning rates of the propellants were determined at pressures of 4–15 MPa by utilizing the acoustic emission technique. This method involves the combustion of propellant samples with dimensions of 150 mm × 5 mm × 5 mm in a stellar bomb filled with pressurized nitrogen and water. The combustion of propellant samples occurs under water. The combustion wave distributions of propellant samples were obtained using the  type double tungsten-rhenium thermocouple [36]. The thermocouple with wire size of 25 μm was embedded in the propellant sample (diam. = 7 mm, length = 120 mm). In addition, the side of the propellant samples was coated by polyvinyl alcohol for flame retardance. During the combustion process, the burning surface moves gradually to the thermocouple, and finally, the thermocouple gets into the flame zone. Thus, the whole combustion process was recorded and the combustion wave structure from the condensed phase to gas phase was obtained. 3. Results and discussions 3.1. Interaction characteristics between AP and ZrH2 In order to evaluate the interactions between AP and ZrH2 , the pristine ZrH2 was mixed evenly with AP at a mass ratio of 1:1 to get the AP/ZrH2 system. The AP/ZrH2 combination system was then subjected to DSC and TG measurements as demonstrated in Fig. 1. And the results of the pristine AP were also presented for comparison. Upon heating, the pristine AP undergoes three thermal events. The first sharp endothermic event peaked at 244 °C corresponds to the phase transition of AP from the low-temperature orthorhombic modification to the high-temperature cubic modification. After that, AP decomposes via a two-step reaction with an onset temperature of about 277 °C. An exothermic peak at 298 °C, which originates from the low-temperature decomposition of AP, is followed by another exothermic event peaked at 344 °C due to the hightemperature decomposition. These thermal-decomposition characteristics fit well with the results of Liu et al. [37]. However, it also should be noted that, the peak temperatures are lower than the data in some reports [33,35,38–41], possibly due to the supplier of the chemical (Dalian Gaojia Chemical Co., Ltd). The DTG result of the pristine AP shown in Fig. 1(c) confirms this two-step decomposition behavior. As for the AP/ZrH2 system, a sharp endothermic peak at 243 °C and two exothermic peaks at 295 and 361 °C were detected. Based on the DSC and DTG results, this endother-

Weight Loss (%)

69

AP AP-ZrH2 o

100 80 60

o

361 C

o

295 C 243 C

exo→

2.3. Thermal analysis and combustion property evaluation

(a) DSC

o

344 C

o

298 C

o

244 C

(b) TG

AP AP-ZrH2

50.3 wt%

40 20

100 wt%

0

(c) DTG

DTG (a.u.)

matic Al-Kα (1486.6 eV) X-ray sources under an ultimate pressure of 5 × 10−10 mbar. All data were calibrated by using the adventitious C 1s signal at 284.8 eV as a reference. The binding energy spectra were fitted by the XPSPEAK software.

Intensity (a.u.)

Y. Yang et al. / Combustion and Flame 187 (2018) 67–76

AP AP-ZrH2 o

o

363 C

o

345 C

295 C

o

299 C

100

200

300

o

Temperature ( C)

400

500

Fig. 1. DSC (a), TG (b) and DTG (c) curves of the AP-ZrH2 system and pristine AP.

mic event should be attributed also to the phase transition of AP, and the exothermic events are believed to originate from AP decomposition. Specifically, the exothermic events peaked at 295 and 361 °C are due to low- and high-temperature decomposition of AP, respectively. It is noticed that, thermal decomposition behaviors of the AP/ZrH2 mixture are very similar to those of the pristine AP, suggesting that no chemical reactions between AP and ZrH2 occur in the mixture. However, an upshift of the peak temperature from 344 °C to 361 °C was observed for the high-temperature decomposition step of AP. As reported previously, the low-temperature decomposition of AP occurs mainly in the solid phase, whereas during the high-temperature step, the exothermic reactions occur on the surface of AP and in the gaseous phase above the surface [35]. Therefore, it is supposed that, in the combination system, the gaseous products released from AP, which would further react to release energy, are adsorbed to the surface of ZrH2 particles, resulting in an increased reaction temperature of the hightemperature step. Figure 1(b) presents the weight losses of the AP/ZrH2 system and the pristine AP upon heating. The weight loss of the pristine AP initiates approximately at 277 °C, and no solid residues remain after the temperature is increased to 400 °C. An almost identical onset temperature for weight loss is determined (275 °C) for the AP/ZrH2 mixture and a weight loss of 50.3 mass% is achieved at 400 °C. Further dehydrogenation test on ZrH2 (Fig. S3, Supplementary material) show its onset temperature for endothermic decomposition to be 350 °C. This means that hydrogen release from ZrH2 also contributes to weight loss of the mixture at 400 °C (50.3 mass%). Considering that the AP content is 50 mass% in the mixture, it is proposed that dehydrogenation instead of oxidation happen to ZrH2 since oxidation of ZrH2 would make the weight loss lower than 50 mass%. The decomposition product of this combination system at 400 °C was also collected and characterized structurally by using XRD to get insights into its reaction mechanisms (Fig. S4, Supplementary material). It was found that, AP disappeared completely after decomposition at 400 °C, whereas no discernible changes happened to ZrH2 , thus confirming the above proposition. According to the previous reports, the exothermic oxidation of ZrH2 in air occurs at temperatures higher than 430 °C [42,43], suggesting that it possesses good resistance to oxidation. It is thus believed that, the good resistance to oxidation and high thermal stability of ZrH2 makes AP hard to oxidize it to generate H2 O and ZrO2 . Therefore, it is believed that, for the solid propel-

70

Y. Yang et al. / Combustion and Flame 187 (2018) 67–76

(a) AP-ZrH2

H2 m/e=2 o

296 C

o

366 C

AP

HCl m/e=35

600

Gram Schmidt temperature

400

H2O m/e=18

NO m/e=30 (b) AP

o

348 C

o

302 C

200

Intensity (a.u.)

Intensity (a.u.)

NH3 m/e=17 N2O m/e=44

HCl m/e=35 H2O m/e=18

20 min

27 min 29 min 32 min

0 40 min

600

AP-ZrH2 400

O2 m/e=32

200

NH3 m/e=17

28.5 min

N2O m/e=44 NO m/e=30

100

200

300

400

Temperature (oC)

O2 m/e=32

20 min

500

o

Temperature ( C) Fig. 2. MS curves of the AP-ZrH2 combination system (a) and the pristine AP (b).

lants using AP as the oxidizer, ZrH2 could act as a fuel to provide hydrogen and metallic Zr instead of reacting directly with the oxidizer. Gaseous decomposition products of the AP/ZrH2 combination system and the pristine AP were evaluated by means of mass spectroscopy (MS) as shown in Fig. 2. Gas release from the pristine AP starts at about 275 °C. NO, N2 O, NH3 , O2 and H2 O are detected in both the low- and high-temperature stages with peak temperatures of 302 and 348 °C, respectively. But hydrogen chloride (HCl), another major product of AP decomposition, is only observed in the high-temperature stage. For AP in the mixture, NO, N2 O, NH3 , O2 and H2 O are also released in the whole process via two stages, and HCl is produced only in the high-temperature stage, either. With respect to HClO and Cl2 , which are reported to be decomposition intermediates [37], detection on them was also failed (Fig. S2, Supplemental material), possibly due to the small amount of sample for test and their fast consumption in the exothermic reactions. It is worth noting that, the gas-release behaviors observed for the pristine AP and the AP/ZrH2 mixture are similar. This phenomenon further suggests the absence of redox reactions between ZrH2 and AP during heating. Nevertheless, an upshift from 348 to 366 °C is detected for the second peaks of NO, N2 O, NH3 , O2 and H2 O, which coincides with the above DSC results. As for H2 , the gaseous decomposition product of ZrH2 , it was not found during decomposition of the mixture. Fail to detect hydrogen in the decomposition products is proposed to be due to the high thermal stability, low hydrogen content (2.16 mass%) and poor dehydrogenation kinetics of ZrH2 . This proposition is reasonable, since till 500 °C, only 0.13 mass% of hydrogen could be desorbed very slowly (Fig. S3, Supplementary material). It is noticed that, the MS peaks of NO, N2 O, NH3 , O2 and H2 O released from AP in the mixture are broadened in comparison with those of the pristine AP. Furthermore, their relative intensities are also changed after the introduction of ZrH2 . This alteration of gasrelease characteristics suggests that ZrH2 influence the decomposition reactions of AP in some way, although no chemical reactions are believed to occur between them. In order to achieve a better understanding on the interactions between ZrH2 and AP, the insitu FTIR measurements were conducted on the mixture since the primary gaseous decomposition products of AP (H2 O, NO and N2 O) are IR-active. Figure 3 demonstrates the Gram Schmidt plots of the pristine AP and the mixture, providing information related to the total IR absorbance of gaseous products in the whole spectral range. For the pristine AP, IR-active gaseous products are generated at temperatures of 282–387 °C (24.5–35.0 min) via two reaction

0

10

20

26.7 min

33 min

30

0

40 min

40

50

Time (min) Fig. 3. Gram Schmidt plots of the AP-ZrH2 system and the pristine AP.

stages, fitting well with the thermal-analysis and MS results. The intensity of the second Gram–Schmidt peak is noticed to be higher, indicating the formation of higher amounts of IR-active gases including NO, N2 O, H2 O and HCl. Similarly, the release of IR-active gases from the AP/ZrH2 mixture occurs in two steps at the temperature range of 273 °C–402 °C (23.5–36.5 min). However, the GramSchmidt peaks of this mixture are broadened when compared with the pristine AP, probably due to the presence of ZrH2 . Figure 4 shows the in-situ FTIR spectra (as stack plot) collected during the pyrolysis processes and Fig. S5 (Supplementary material) exhibits the enlarged view of the spectra. As shown in Figs. 4(a) and S5(a), for the pristine AP, absorption bands in the spectral ranges of 20 0 0–230 0 cm−1 , 150 0–170 0 cm−1 and 120 0– 1350 cm−1 , which should be attributed to nitrogen oxides, were observed. The absence of the absorption bands of H2 O at 1630 and 3440 cm−1 is believed to due to the water compensation performed automatically by the software [44]. The intensity of the absorption band in 20 0 0–230 0 cm−1 varies with temperature and two peaks belonging to the low- and high-temperature decomposition stages are identified. Moreover, synchronistical changes are found to occur to the band in 1200–1350 cm−1 . As for the band in 150 0–170 0 cm−1 , only one intensity peak with a shoulder was observed. The in-situ FTIR spectra of the AP/ZrH2 system, which possess worse signal-to-noise ratio, are shown in Fig. 4(b). It is found that, the variation in intensity with temperature is also discernible for the two bands in 20 0 0–230 0 cm−1 and 150 0–170 0 cm−1 . However, their relative intensities are different in comparison with those of the pristine AP. More specifically, for the combination system, the band in 150 0–170 0 cm−1 possesses higher intensity than the one in 20 0 0–230 0 cm−1 , whereas the higher intensity was found for the band in 20 0 0–230 0 cm−1 during the decomposition of the pristine AP. The FTIR spectra of gaseous products obtained at different decomposition stages, i.e. before the initiation of decomposition, at the peak of each decomposition steps, between the two decomposition peaks and after the completion of decomposition (as displayed in Fig. 3), are demonstrated in Fig. 5. For the pristine AP, FTIR spectra collected at 20, 27, 29, 32 and 40 min are exhibited, whereas spectra obtained at 20, 26.7, 28.5, 33 and 40 min are utilized for analyses of the AP/ZrH2 combination system. As shown in Fig. 5(a), before the initiation of decomposition, except for CO2 which should come from the air, no IR-active gases are detected at 237 °C (20 min) for the pristine AP. Upon further heating, absorption bands of N2 O at 1270, 1303, 2204 and 2238 cm−1 are observed at the peak of the low-temperature decomposition stage

Y. Yang et al. / Combustion and Flame 187 (2018) 67–76

71

Fig. 4. In-situ FTIR spectra (as stack plot) of the gases released during the decomposition of (a) the pristine AP and (b) AP-ZrH2 system.

a 2238 cm-1

436 oC (40 min)

Absorbance (a.u.)

2204 cm-1 1630 cm-1 1593 cm-1 1303 cm-1 -1 1270 cm

357 oC (32 min) 327 oC (29 min) 307 oC (27 min) 237 oC (20 min)

4000

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1)

b 436 oC (40 min)

Absorbance (a.u.)

1630 cm-1

1595 cm-1 1301 cm-1

2238 cm-1 2207 cm-1

1270 cm-1

367 oC (33 min) 322 oC (28.5 min) 304 oC (26.7 min) 237 oC (20 min)

4000

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1) Fig. 5. FTIR spectra of the gaseous decomposition products of (a) AP and (b) APZrH2 combination system at different temperatures.

(307 °C, 27 min). In addition, two bands at 1593 and 1630 cm−1 of NO with lower intensities are also discernible at this temperature. The above results suggest that, both N2 O and NO are generated in the first decomposition stage and the content of N2 O is higher in the products. When the temperature is further increased to 327 °C (29 min), at which the high-temperature decomposition stage has already been initiated but the low-temperature one has not com-

pleted, the relative intensities of N2 O bands decrease, whereas those of NO increase, thus suggesting the enhanced NO release in the high-temperature stage. As shown in the spectrum collected at 357 °C (32 min), peak temperature of the high-temperature stage, both NO and N2 O are released. Nevertheless, comparing with the relative intensities of NO bands in the low-temperature decomposition, the ones in the high-temperature step are noticed to be higher, confirming the promoted NO generation. In addition, this phenomenon is believed to be closely related to the differences in reaction mechanisms between the low- and high-temperature decomposition stages of AP [35]. Figure 5(b) shows the spectra of gases released from the AP/ZrH2 combination system. Similar to the pristine AP, N2 O and NO are released both in the first and second decomposition stages. Especially, for the low-temperature stage, very similar gas-release behaviors to those of the pristine AP are found. The spectrum collected at 304 °C (26.7 min) indicates the primary nitrogen oxide generated in this stage to be N2 O. And the absorption bands of NO are also discernible with lower intensities. On the other hand, although both NO and N2 O are detected in the high-temperature stage, their gas-release behaviors are evidently different from those of the pristine AP. As exhibited in the spectra obtained at 322 °C (28.5 min) and 367 °C (33 min), it is interesting to find that the relative intensities of NO bands to N2 O bands are significantly higher than those of the pristine AP, suggesting that the presence of ZrH2 could influence the high-temperature decomposition reaction of AP and promote the formation of NO. According to the composition of AP (NH4 ClO4 ), there is enough oxygen for the complete oxidation of NH4 + (NH3 ) to NO. Therefore, it is proposed that, in the fast exothermic reaction that occurs in gaseous phase (high-temperature stage), insufficient contact of NH3 (N-Ⅲ) with the oxidizing agents would lead to formation of N2 O (NⅠ) instead of NO (NⅡ). As reported previously, NH3 is apt to coordinate to Zr in ZrH2 to form complexes [27,45,46–47]. Moreover, the gaseous oxidizing agents like ClO2 could also be adsorbed to the surfaces of solid ZrH2 particles, which possess good thermal stability. The attachment of reactants for the high-temperature decomposition of AP is believed to favor the sufficient contact of NH3 with the oxidizing agents both on the surfaces of ZrH2 and in the gaseous phase, thus promoting the formation of nitrogen with a higher valence (N2+ in NO instead of N+ in N2 O). Therefore, we believe that, when ZrH2 coexists with AP, the attachment of gaseous reactants to ZrH2 accounts for the change in decomposition behaviors of AP.

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Y. Yang et al. / Combustion and Flame 187 (2018) 67–76 Table 2 Energetic properties of HTPB propellants containing different metal fuels. Calculated theoretical specific Al ZrH2 (mass%) (mass%) impulse (N s/kg)

1 2 3 4

15 10 5 0

0 5 10 15

2.59 2.55 2.47 2.38

× × × ×

103 103 103 103

Based on the above results, it is proposed that, no chemical reactions between AP and ZrH2 occur in their combination system. Instead, AP and ZrH2 are believed to decompose separately, although the high-temperature decomposition of AP is influenced by ZrH2 due to attachment of reactants to its surfaces. As a consequence, the dehydrogenation of ZrH2 gives H2 and metallic Zr, validating its potential as a metal fuel in high-energy solid propellants. 3.2. Effects of ZrH2 on the energetic properties of HTPB propellants In order to further examine its applications in solid propellants, ZrH2 was introduced into HTPB formulations as the fuel. It is well-known that, addition of metal fuels such as Al is a classical approach to increase the theoretical specific impulse of composite propellants, due to the high energy provided by the fuel combustion. Therefore, before investigating the combustion properties of HTPB propellants containing ZrH2 , the effects of ZrH2 on energetic performances of propellants were evaluated by calculating the theoretical specific impulses based on the minimum free energy method. The calculation was performed by using the REAL for windows program [48,49]. For now, metallic aluminum (Al) is the most widely adopted metal fuel for solid rocket propellants, thus a comparison was made between the performances of propellants containing Al and ZrH2 as the fuel. Table 2 demonstrates the theoretically determined results of HTPB propellants utilizing different metal fuels. In addition, the density specific impulses of the four formulations were also obtained based on the theoretical specific impulses and densities. The density of HTPB propellant containing 15 mass% Al was calculated to be 1.74 × 103 kg/m3 . As expected, replacement of Al with ZrH2 leads to an increase in densities due to the higher density of ZrH2 (5.67 × 103 kg/m3 ) than Al (2.7 × 103 kg/m3 ), and the density of formulation containing 15 mass% of ZrH2 was measured to be 1.84 × 103 kg/m3 . On the other hand, however, replacement of Al with ZrH2 is found to be unfavorable for the theoretical specific impulse of propellant. The basic formulation with 15 mass% of Al possesses a specific impulse of 2.59 × 103 N s/kg. Nevertheless, the replacement of 5, 10 and 15 mass% of Al with ZrH2 result in the evidently decreased theoretical specific impulses of 2.55 × 103 , 2.47 × 103 and 2.38 × 103 N s/kg, respectively. It is well known that, both the high heat of combustion and low gaseous molecular mass are critical for increasing the theoretical specific impulse. Although ZrH2 could act as an hydrogen supplier to decrease the gaseous molecular mass as stated above, its hydrogen content is relatively low (2.16 mass%), suggesting the limited decrease in molecular mass. Furthermore, the combustion heat of ZrH2 (12 MJ/kg) is evidently lower than aluminum (31 MJ/kg), which means that the decrease in gaseous molecular mass may not compensate the decrease in combustion heat. As a consequence, it is reasonable that replacing Al with ZrH2 would lead to a decrease in the theoretical specific impulses of the formulations in this work. The density specific impulse, on the other hand, shows a different behavior with the increase in ZrH2 contents. Due to the higher densities of ZrH2 -containing propellants, they

Experimentally determined density (kg/m3 ) (1.75 (1.78 (1.81 (1.84

± ± ± ±

0.01) 0.01) 0.01) 0.01)

× × × ×

Density specific impulse (N s/m3 )

103 103 103 103

(4.53 (4.54 (4.47 (4.38

16

± ± ± ±

0.02) 0.02) 0.02) 0.02)

× × × ×

106 106 106 106

15 wt% Al 10 wt% Al + 5 wt% ZrH2

15

Burning Rate (mm/s)

No.

5 wt% Al + 10 wt% ZrH2 15 wt% ZrH2

14 13 12 11 10 9 8 7 4

5

6

7

8

9

10

11

12

13

14

15

Pressure (MPa) Fig. 6. Burning rates of HTPB propellants with different metal fuels.

do not change monotonically with the variation in ZrH2 contents and a maximum impulse could be obtained at a certain composition. The formulation containing 5 mass% Al and 10 mass% ZrH2 possess the highest density specific impulse of (4.53 ± 0.01) × 106 N s/m3 , slightly higher than the one containing only Al ((4.51 ± 0.01) × 106 N s/m3 ). The above results show that, the high-density hydrides like ZrH2 are very favorable for increasing the densities and volumetric loading of the propellants. It is well-known that, high propellant density is favorable for the density specific impulse. Moreover, it is worth noting that, in comparison with the high-density metals like Zr and Ti, their hydrides (ZrH2 , TiH2 ) have higher heat of combustion and could provide H2 to reduce the gaseous molecular mass during combustion. On the other hand, the high hydrogen contents, which are often related to light metal hydrides and complex hydrides (metal alanate, metal borohydrides and their derivatives) with low densities, could increase the theoretical specific impulses effectively. Therefore, it is proposed that, utilizing a hydride composite composed of highdensity hydrides (ZrH2 TiH2 ) and high hydrogen-content hydrides as the fuel is an effective and promising approach to improve the energetic properties of propellants. 3.3. Effects of ZrH2 on the combustion performances of HTPB propellants It is well-known that, the compositions, chemical properties and contents of metal fuels are closely related to the combustion performances of solid propellants. Figure 6 shows the burning rates determined at 4–15 MPa for the as-prepared HTPB propellants containing different metal fuels. As expected, the burning rates of all the four sample propellants increase with the rise in pressure. The basic formulation, i.e. the one containing 15 mass% of Al as the metal fuel, exhibits burning rates from 7.87 to 14.16 mm/s in the testing pressure range. Partial replacement of Al with 5 mass% or 10 mass% of ZrH2 leads to a slight increase in burning rates. In addition, these two sample propellants are noticed to possess almost identical burning rates at different pressures. This phenomenon

Y. Yang et al. / Combustion and Flame 187 (2018) 67–76

73

Table 3 Pressure exponents of HTPB propellants with different metal fuels. No.

Zr Al ZrH2 (mass%) (mass%)

Pressure exponent 4–7 MPa

1 2 3 4 5

15 10 5 0 0

0 5 10 15 0

0 0 0 0 15

0.37 0.34 0.30 0.31 0.36

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

indicates that, for the two formulations containing both Al and ZrH2 , the contents of ZrH2 seems to have little effects on the burning rates. The reason behind this phenomenon is proposed to lie in the interactions between the in-situ formed Zr from ZrH2 dehydrogenation and Al occurred during the combustion process, which may lead to the formation of Zr–Al alloys as the metal fuel [50,51]. As for the propellant containing 15 mass% of ZrH2 as the metal fuel, it possesses the highest burning rates at 4–15 MPa among the four sample propellants under investigation. The burning rate at 4 MPa was determined to be 8.47 mm/s and it reached 15.61 mm/s when the pressure was elevated to 15 MPa. Obviously, ZrH2 is more effective in enhancing the burning rates of propellants than Al. More interestingly, for the sample propellant containing only ZrH2 , it is noticed that the increase in burning rates with pressure is accelerated at pressures higher than 10 MPa. This is evidently different from the combustion behaviors of propellant with metallic Al, indicative of the higher sensitivity of combustion to pressure when a hydride (ZrH2 ) instead of a metal (Al) exists in the formulation. The pressure exponents of burning rate were calculated and shown in Table 3, further confirming this. The higher sensitivity to pressure of the propellant containing only ZrH2 means that the combustion reactions occurred in gaseous phases become the ratelimiting step [52]. This alteration in combustion behaviors when a metal (Al) is replaced by a hydride (ZrH2 ) may suggest the critical role of hydrogen in propellant combustion. In order to get a further understanding on the role of hydrogen in ZrH2 , a propellant containing 15 mass% of metallic zirconium (Zr) as the metal fuel was prepared and its burning rates were measured under the same conditions for comparison. As demonstrated in Fig. S6 (Supplementary material), there is an apparent discrepancy between the combustion behaviors of propellants with Zr and ZrH2 . The burning rates of the formulation containing Zr were determined to be 8.35, 10.03, 11.65, 13.32 and 14.11 mm/s at 4, 7, 10, 13 and 15 MPa, respectively. At 4 and 7 MPa, the propellant with ZrH2 exhibits almost identical burning rates to those of the one containing Zr. When the pressure was increased to higher than 10 MPa, the adoption of ZrH2 instead of Zr results in significant higher burning rates. Especially, the higher is the pressure, the bigger the difference between the burning rates of the two samples would be. The pressure exponents of the burning rates were also determined and listed in Table 3. It is found that, in the pressure range of 4–7 MPa, the exponent is lower for ZrH2 -containing propellant than the Zr-containing one. On the contrary, however, when the pressure is higher than 7 MPa, the propellant containing ZrH2 exhibits higher pressure exponents than the one containing metallic Zr. As stated above, this also means that the combustion of propellant with the hydride ZrH2 is more sensitive to pressure than the one with the metal Zr. In other words, when ZrH2 instead of Zr is adopted as the metal fuel for propellants, an increase in pressure would enhance the combustion of propellant more effectively, which suggests that the diffusion and chemical reactions in gaseous phases are more critical for the combustion of propellant with ZrH2 [52]. As proved above, ZrH2 would dehydrogenate to generate H2 and Zr to participate in the combustion when AP

7–10 MPa 0.43 0.50 0.49 0.45 0.33

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

10–13 MPa 0.53 0.53 0.61 0.73 0.47

± ± ± ± ±

0.02 0.02 0.02 0.02 0.01

13–15 MPa 0.57 0.57 0.50 0.62 0.40

± ± ± ± ±

0.02 0.02 0.02 0.02 0.01

is used as the oxidizer. Therefore, it is reasonable to believe that the hydrogen released from ZrH2 , whose combustion should occur in the gaseous phases and is closely related to diffusion, plays an important role in the combustion of propellants. Furthermore, the combustion product of the sample propellant utilizing 15 mass% of ZrH2 as the fuel was collected and characterized via XPS to determine the chemical state of Zr after combustion. Figure S7 (Supplementary material) presents the high-resolution spectrum of Zr 3d. It is seen that the spectrum could be resolved into two signals peaked at 181.88 and 184.28 eV, respectively, by peak fitting. These two peaks are attributed to ZrO2 , suggesting that all the ZrH2 transforms to ZrO2 after combustion. 3.4. Reaction mechanisms of ZrH2 in propellant combustion The combustion flames of the HTPB propellants with different fuels were also recorded and exhibited in Fig. 7. The photos were obtained at two different pressures, i.e. 2 MPa and 4 MPa. For all the four sample propellants, the combustion at 4 MPa is more violent than combustion at 2 MPa, suggesting that high pressures are favorable for the combustion of HTPB propellants. In addition, light tracks are observed in the flames of four propellants, due to the movement of burning fuel particles. However, it is noticed that, the combustion-flame characteristics of the propellant utilizing the classical metal fuel of Al are evidently different from those of the one containing ZrH2 . At both 2 and 4 MPa, the flame of propellant is darker when ZrH2 instead of Al is used as the metal fuel. This phenomenon indicates its combustion temperature to be lower, fitting well with the previous report, in which the theoretical calculations revealed that the combustion temperature of propellant became lower while metal was replaced by metal hydride [53]. Furthermore, this also confirms the critical role of hydrogen in the combustion of HTPB propellants containing ZrH2 . More interestingly, at the pressure of 2 MPa, the propellants containing both Al and ZrH2 burn with more homogeneous flames than the one containing only Al or ZrH2 , further suggesting that there are some interactions between ZrH2 and Al during the combustion. As stated above, the thermal-analysis results imply that, ZrH2 would dehydrogenate independently to generate Zr and H2 without direct oxidation by AP. Moreover, it has been proven that, hydrogen plays a very important role in the combustion of propellant containing ZrH2 . In order to obtain further insights into the reaction mechanisms of ZrH2 in combustion of HTPB propellants, the combustion wave structures were determined for HTPB-15 mass% Al and HTPB-15 mass% ZrH2 sample propellants and shown in Fig. 8. Figure 8a exhibits the combustion wave of the HTPB15 mass% Al propellant burning at 2 and 4 MPa. At both pressures, the temperature increases quickly after ignition due to the intense energy release from propellant combustion and then reaches a plateau, which indicates the luminous flame zone. The temperature-rise rate at 4 MPa is calculated to be about 2.3 × 105 °C/s, evidently higher than the rate of 9.9 × 104 °C/s at 2 MPa, confirming that the increase in pressure promotes the combustion reactions of propellants. In addition, for this Al-containing

74

Y. Yang et al. / Combustion and Flame 187 (2018) 67–76

Fig. 7. Combustion flames of the HTPB propellants with different metal fuels burning at 2 and 4 MPa ((a): 15 mass% Al, 2 MPa; (b): 15 mass% Al, 4 MPa; (c): 10 mass% Al + 5 mass% ZrH2 , 2 MPa; (d) 10 mass% Al + 5 mass% ZrH2 , 4 MPa; (e) 5 mass% Al + 10 mass% ZrH2 , 2 MPa; (f) 5 mass% Al + 10 mass% ZrH2 , 4 MPa; (g) 15 mass% ZrH2 , 2 MPa; (h) 15 mass% ZrH2 , 4 MPa).

propellant, the temperature of luminous flame at 4 MPa was determined to be 2378 °C, higher than that at 2 MPa of 2203 °C as expected. As for the HTPB propellants with ZrH2 as the metal fuel, evidently lower flame temperatures of 2033 and 2170 °C were determined for combustion at 2 and 4 MPa, respectively, which are favorable for the low signature propellants. The lower flame temperature of propellant containing ZrH2 than that of the one with Al is believed to due to its evidently lower combustion heat. More interestingly, a completely different structure of combustion wave is found as shown in Fig. 8b. At both 2 and 4 MPa, the temperature first increases gradually after ignition followed by an evident acceleration in temperature rise, showing a two-stage behavior. As demonstrated in Fig. 8b, the temperature-rise rates of the first stage are 1.5 × 104 and 1.6 × 104 °C/s for the combustion at 2 and 4 MPa, respectively. But in the second stage, the rates at 2 and 4 MPa were determined to be 1.6 × 105 °C/s and 2.2 × 105 °C/s, respectively, significantly higher than those in the first stage. The transition points from the first to second stage were determined to be about 360 °C and 335 °C for combustion at 2 and 4 MPa, respectively. Obviously, the change in combustion wave structures originates from the change in combustion mechanisms of propellants, which should be lie in the differences between reaction mechanisms of metallic Al and hydride ZrH2 in propellants. Considering that the dehydrogenation temperature of ZrH2 is about 350 °C (Fig. S3, Supplementary material), very close to the transition points, it is believed that the Zr and hydrogen (H2 ) from ZrH2 decomposi-

tion interacts with the oxidative species generated during AP decomposition to release a large amount of energy and thus result in the acceleration of temperature-rise rate. XPS measurement on the decomposition product of AP-ZrH2 at 400 °C suggests the formation of ZrO2 layer on ZrH2 particles, further confirming the above proposition. The relative low temperature-rise rates of the propellants using ZrH2 in the first stage, on the other hand, is proposed to be due to the lower thermal conductivity of ZrH2 than Al. Therefore, it is confirmed that, in contrast to metallic Al which is oxidized directly to Al2 O3 during the combustion of propellant, the hydride ZrH2 would first dehydrogenate to generate H2 and Zr as the products. Then, the decomposition products are involved in the combustion reactions to release energy and H2 plays a critical role in the combustion of propellant. Moreover, it is the differences in reaction mechanisms between ZrH2 and Al that are responsible for the different combustion wave structures between the corresponding propellants. 4. Conclusion In this work, the application of zirconium hydride ZrH2 in solid rocket propellants and especially, its reaction mechanisms in propellant combustion was investigated systematically by means of thermal analyses, structural characterizations and combustion evaluations. It is demonstrated that, during the combustion of HTPB propellants utilizing ZrH2 as the metal fuel and AP as the oxi-

Y. Yang et al. / Combustion and Flame 187 (2018) 67–76

a

Supplementary materials 2500

2378 oC

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2017.09. 004.

2203 oC

Temperature (oC)

2000 2.3 105 oC/s

HTPB-15 wt% Al 2 MPa 4 MPa

1500

References

9.9 104 oC/s

1000 500 0 0.00

0.01

0.02

0.03

0.04

Time (s)

b

2500 2170 oC

2000

Temperature (oC)

75

2033 oC

2.2 105 oC/s HTPB-15 wt% ZrH2 2 Mpa 4 MPa

1500 1.6 105 oC/s

1000 500

335 oC 1.6 104 oC/s 360 oC

0 4 o

1.5 10 C/s

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Time (s) Fig. 8. Temperature profiles in the combustion waves of the HTPB propellants utilizing Al and ZrH2 as the metal fuels.

dizer, ZrH2 would not be oxidized directly by AP. Instead, ZrH2 dehydrogenates to release H2 and generate metallic Zr, which is supposed to be very favorable for increasing the theoretical specific impulses of propellants, validating its potential as a fuel for highenergy solid propellants. On the other hand, due to the attachment of gaseous reactants to the surfaces of ZrH2 , the formation of NO in the high-temperature decomposition stage of AP is enhanced. Moreover, the hydrogen released from ZrH2 is determined to promote the combustion effectively by enhancing the diffusion and reactions in gaseous phases, thus playing a critical role in the combustion of HTPB propellants. The burning rate of HTPB propellant containing 15 mass% of ZrH2 at 15 MPa is 15.61 mm/s, higher than those of propellants containing 15 mass% of Zr (14.11 mm/s) and Al (14.16 mm/s). The findings in this work provide important insights into the reaction mechanisms of metal hydrides in the combustion of solid propellants, which will guide the applications of metal hydrides as metal fuels for propellants.

Acknowledgment We gratefully acknowledge financial support received from the National Natural Science Foundation of China (21503163), the China Postdoctoral Science Foundation (2015M582720) and the Postdoctoral Science Foundation of Shaanxi Province (2016BSHEDZZ19).

[1] E.L. Dreizin, Metal-based reactive nanomaterials, Prog. Energy Combust. Sci. 35 (2009) 141–167. [2] Q. Zhang, J.N.M. Shreeve, Energetic ionic liquids as explosives and propellant fuels: a new journey of ionic liquid chemistry, Chem. Rev. 114 (2014) 10527–10574. [3] J. Yang, A. Sudik, C. Wolverton, D.J. Siegel, High capacity hydrogen storage materials: attributes for automotive applications and techniques for materials discovery, Chem. Soc. Rev. 39 (2010) 656–675. [4] S.-I. Orimo, Y. Nakamori, J.R. Eliseo, A. Zuttle, C.M. Jensen, Complex hydrides for hydrogen storage, Chem. Rev. 107 (2007) 4111–4132. [5] B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Metal hydride materials for solid hydrogen storage: a review, Int. J. Hydrog. Energy 32 (2007) 1121–1140. [6] L. George, S.K. Saxena, Structural stability of metal hydrides, alanates and borohydrides of alkali and alkali-earth elements: a review, Int. J. Hydrog. Energy 35 (2010) 5454–5470. [7] E.Y. Marrero-Alfonso, A.M. Beaird, T.A. Davis, M.A. Matthews, Hydrogen generation from chemical hydrides, Ind. Eng. Chem. Res. 48 (2009) 3703–3712. [8] D.A. Rodriguez, E.L. Dreizin, E. Shafirovich, Hydrogen generation from ammonia borane and water through combustion reactions with mechanically alloyed Al•Mg powder, Combust. Flame 162 (2015) 1498–1506. [9] M. Hayward, M. Green, M. Rosseinsky, J. Sloan, Sodium hydride as a powerful reducing agent for topotactic oxide deintercalation: synthesis and characterization of the nickel (I) oxide LaNiO2 , J. Am. Chem. Soc. 121 (1999) 8843–8854. [10] W. Xu, R. Wang, G. Wu, P. Chen, Calcium amidoborane, a new reagent for chemoselective reduction of [small alpha],[small beta]-unsaturated aldehydes and ketones to allylic alcohols, RSC Adv. 2 (2012) 6005–6010. [11] Z. Huang, T. Autrey, Boron–nitrogen–hydrogen (BNH) compounds: recent developments in hydrogen storage, applications in hydrogenation and catalysis, and new syntheses, Energy Environ. Sci. 5 (2012) 9257–9268. [12] L. DeLuca, L. Galfetti, F. Severini, L. Rossettini, L. Meda, G. Marra, B. D’Andrea, V. Weiser, M. Calabro, A. Vorozhtsov, Physical and ballistic characterization of AlH3 -based space propellants, Aerosp. Sci. Technol. 11 (2007) 18–25. [13] S.C. Shark, T.R. Sippel, S.F. Son, S.D. Seister, T.L. Pourpoint, Theoretical performance analysis of metal hydride fuel additives for rocket propellant applications, 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, San Diego, California (2011), p. 5556. [14] T. Bazyn, H. Krier, N. Glumac, N. Shankar, X. Wang, T.L. Jackson, Decomposition of aluminum hydride under solid rocket motor conditions, J. Propul. Power 23 (2007) 457–464. [15] J.P. Flynn, G.A. Lane, J.J. Plomer, Plasticized nitrocellulose propellant composition containing aluminum hydride and nitronium perchlorate, US Patents US3865656, Adamas Carbide Corporation, US, 1975. [16] J.P. Flynn, Particulate aluminum hydride with nitrocellulose coating suitable for use in solid propellants, US Patents US3855022, The Dow Chemical Company, US, 1974. [17] D.R. Carley, J.H. Dunn, Encapsulation of light metal hydrides as rocket propellants, US Patents US3376173, Ethyl Corporation, US, 1968. [18] D. Schmidt, Non-solvated particulate aluminum hydride coated with a cyanocontaining compound useful in solid propellants, US Patents US3850709, The Dow Chemical Company, US, 1974. [19] L.T. DeLuca, L. Rossettini, C. Kappenstein, V. Weiser, Ballistic characterization of AlH3 -based propellants for solid and hybrid rocket propulsion, AIAA Paper 2009, 4874. [20] W.E. Baumgartner, P.G. Butts, Propellant composition containing beryllium hydride, nitrocellulose and nitrate co-plasticizers, US Patents US3861970, The United States of America as represented by the Secretary of the United States Air Force, US, 1975. [21] L. Jin, P. Du, M. Yao, The influence of magnesium hydride on the thermal decomposition properties of nitrocellulose, J. Energy Mater. 32 (2014) S13–S21. [22] W. Pang, X. Fan, F. Zhao, H. Xu, W. Zhang, H. Yu, Y. Li, F. Liu, W. Xie, N. Yan, Effects of different metal fuels on the characteristics for HTPB-based fuel rich solid propellants, Propellants Explos. Pyrotech. 38 (2013) 852–859. [23] M.A. Cooper, M.S. Oliver, The burning regimes and conductive burn rates of titanium subhydride potassium perchlorate (TiH1.65 /KClO4 ) in hybrid closed bomb-strand burner experiments, Combust. Flame 160 (2013) 2619–2630. [24] X.Y. Ding, Y.J. Shu, N. Liu, M.J. Wu, J.G. Zhang, B.W. Gou, H.M. Wang, C.L. Wang, S.N. Dong, W. Wang, Energetic characteristics of HMX-based explosives containing LiH, Propellants Explos. Pyrotech. 41 (2016) 1079–1084. [25] X. Bi, J. Liu, Detonation properties of high explosives containing ammonia borane, J. Inorg. Gen. Chem. 642 (2016) 773–777. [26] W.E. Wang, D.R. Olander, Thermodynamics of the Zr–H system, J. Am. Ceram. Soc. 78 (1995) 3323–3328. ¨ [27] Y.J. Choi, Y. Xu, W.J. Shaw, E.C. Ronnebro , Hydrogen storage properties of new hydrogen-rich BH3 NH3 -metal hydride (TiH2 , ZrH2 , MgH2 , and/or CaH2 ) composite systems, J. Phys. Chem. C 116 (2012) 8349–8358.

76

Y. Yang et al. / Combustion and Flame 187 (2018) 67–76

[28] T. Hayashi, K. Tobita, Y. Nakamori, S. Orimo, Advanced neutron shielding material using zirconium borohydride and zirconium hydride, J. Nucl. Mater. 386 (2009) 119–121. [29] F. Von Zeppelin, M. Hirscher, H. Stanzick, J. Banhart, Desorption of hydrogen from blowing agents used for foaming metals, Compos. Sci. Technol. 63 (2003) 2293–2300. [30] W. Zhu, R. Wang, G. Shu, P. Wu, H. Xiao, First-principles study of different polymorphs of crystalline zirconium hydride, J. Phys. Chem. C 114 (2010) 22361–22368. [31] D. Lempert, G. Nechiporenko, G. Manelis, Energetic performances of solid composite propellants, Cen. Eur. J. Energ. Mater. 8 (2011) 25–38. [32] D. Lempert, G. Nechiporenko, G. Manelis, Energetic capabilities of high-density composite solid propellants containing zirconium or its hydride, Combust. Explos. Shock Waves 47 (2011) 45–54. [33] A.G. Keenan, R.F. Siegmund, Thermal decomposition of ammonium perchlorate, Q. Rev. Chem. Soc. 23 (1969) 430–452. [34] P.W.M. Jacobs, H.M. Whitehead, Decomposition and combustion of ammonium perchlorate, Chem. Rev. 69 (1969) 551–590. [35] V.V. Boldyrev, Thermal decomposition of ammonium perchlorate, Thermochim. Acta 443 (2006) 1–36. [36] A.J. Sabadell, M. Summerfield, J. Wenograd, Measurement of temperature profiles through solid-propellant flames using fine thermocouples, AIAA J. 3 (1964) 1580–1584. [37] Z. Liu, Thermal analysis for energetic materials, National Defense Industry Press, Beijing, 2008. [38] K. Kishore, K. Sridhara, Solid propellant chemistry: condensed phase behaviour of ammonium perchlorate-based solid propellants, Defence Research & Development Organisation, Ministry of Defence, 1999. [39] W.A. Rosser, S.H. Inami, H. Wise, Thermal decomposition of ammonium perchlorate, Combust. Flame 12 (1968) 427–435. [40] L. Bircumshaw, B. Newman, The thermal decomposition of ammonium perchlorate. II. The kinetics of the decomposition, the effect of particle size, and discussion of results, Proc. R. Soc. Lond. A: Math. Phys. Eng. Sci. 227 (1955) 228–241. [41] L. Bircumshaw, B.H. Newman, The thermal decomposition of ammonium perchlorate. I. Introduction, experimental, analysis of gaseous products, and thermal decomposition experiments, Proc. R. Soc. Lond. A: Math. Phys. Eng. Sci. 227 (1954) 115–132.

[42] V.Z. Shemet, A. Pomytkin, V. Lavrenko, V.Z. Ratushnaya, Decomposition of metal hydrides in low temperatures and in high-temperature oxidation, Int. J. Hydrog. Energy 18 (1993) 511–516. [43] V.Z. Shemet, V. Lavrenko, O. Teplov, V.Z. Ratushnaya, High-temperature oxidation of zirconium-hydride powders, Oxid. Met. 38 (1992) 89–98. [44] V. Malhotra, S. Jasty, R. Mu, FT-IR spectra of water in microporous KBr pellets and water’s desorption kinetics, Appl. Spectrosc. 43 (1989) 638–645. [45] X. Liu, Z. Wu, Z. Peng, Y.-D. Wu, Z. Xue, Synthesis and structure of an unusual zirconium hydride amide complex. Mechanistic studies of the reactions of transition-metal amides with silanes, J. Am. Chem. Soc. 121 (1999) 5350–5351. [46] M. Zhou, M. Chen, L. Zhang, H. Lu, Reactions of zirconium and hafnium atoms with ammonia. Matrix infrared spectra and density functional calculations of the MNH3 and H2MNH (M= Zr and Hf) molecules, J. Phys. Chem. A 106 (2002) 9017–9023. [47] X. Wang, L. Andrews, Infrared spectra, structure, and bonding of the group 6 and ammonia M:NH3 , H2 N−MH, and NMH3 reaction products in solid argon, Organometallics 27 (2008) 4885–4891. [48] A. Denisyuk, Y.G. Shepelev, S. Yudaev, I. Kalashnikov, Combustion of systems containing linear nitramines, Combust. Explo. Shock Waves 41 (2005) 206–214. [49] G.V. Belov, REAL for Windows v3. 0, Moscow, 2004. [50] T. Shoji, A. Inoue, Hydrogen absorption and desorption behavior of Zr-based amorphous alloys with a large structurally relaxed amorphous region, J. Alloys. Compd. 292 (1999) 275–280. [51] L. Yu, Y. Feng, J. Yang, T. Qiu, L. Pan, Mechanical and thermal physical properties, and thermal shock behavior of (ZrB2 +SiC) reinforced Zr3[Al(Si)]4C6 composite prepared by in situ hot-pressing, J. Alloys Compd. 619 (2015) 338–344. [52] D.L. Reid, A.E. Russo, R.V. Carro, M.A. Stephens, A.R. LePage, T.C. Spalding, E.L. Petersen, S. Seal, Nanoscale additives tailor energetic materials, Nano Lett. 7 (2007) 2157–2161. [53] M. Li, F. Zhao, S. Xu, E. Yao, H. Hao, T. An, L. Xiao, Y. Tan, X. Li, Energetic characteristics of composite propellant containing different metal hydride, J. Solid Rocket Technol. 37 (2014) 86–90.