Combustion and Flame 187 (2018) 67–76 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame Hydrogen-enhanced combustion of a composite propellant with ZrH 2 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 b 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 a r t i c l e i n f o a b s t r a c t Article history: The reaction mechanisms of metal hydrides in the combustion of solid propellants are closely related to Received 20 February 2017 their effects on the energetic and combustion properties of propellants. Here, we report a first attempt to Revised 1 September 2017 investigate the reaction mechanisms of ZrH 2 , a promising candidate for fuels, in the combustion of HTPB Accepted 5 September 2017 propellants using ammonium perchlorate (NH ClO , AP) as the oxidizer. ZrH is determined to possess a Available online 16 October 2017 4 4 2 good resistance to the direct oxidation by AP. Thus, it would dehydrogenate independently to generate Keywords: H 2 and metallic Zr, which is believed to be very favorable for decreasing the gaseous molecular masses. Solid propellant On the other hand, due to the attachment of gaseous reactants to it, ZrH 2 was found to tune the de- Combustion composition behaviors of AP by enhancing the generation of NO in the high-temperature decomposition Reaction mechanism stage. More interestingly, the hydrogen released from ZrH 2 is evaluated to promote the combustion reac- Zirconium hydride tions in gaseous phases and be responsible for the two-stage combustion behaviors of the corresponding Hydrogen propellant, which are distinct from those when metallic Al is used as the fuel. The findings in this work Ammonium perchlorate validate the potential of ZrH 2 as a fuel for high-energy solid propellants. ©2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction ternative to the most widely used metal fuel aluminum (Al), was found to be capable of increasing the specific impulse and de- In solid rocket propulsion for both civil and military applica- creasing the two-phase flow losses [12,15]. However, AlH3 is a fuel tions, the pursuit for longer range necessitates the development of with relatively low density (1.477 g/cm 3 ), which is significantly novel materials to improve the energetic and combustion perfor- lower than those of aluminum (2.70 g/cm 3 ) and conventional pro- 3 mances of solid propellants [1,2]. Metal hydrides, which are also pellants (about 1.60–1.70 g/cm ). Thus, if AlH3 is adopted to re- very useful for a range of applications including hydrogen storage place Al in the aluminized propellants, the volumetric loading of [3–6] , hydrogen generation [7,8] and reducing agents [9–11] , have the resulting propellants would be evidently reduced. Obviously, attracted sustainable attentions as a series of promising metal fuels this is unfavorable for their practical applications. Zirconium hy- for solid rocket propellants. It is supposed that, the metal hydrides dride (ZrH2 ), which is now under intense investigations in several are capable of providing simultaneously high energy from metal fields including hydrogen storage [26,27] , neutron shielding [28] , oxidation and low gaseous molecular mass due to hydrogen (H2 ) blowing agents for foaming metals [29] and moderator in nuclear released during the combustion processes [12–14] , thus resulting reactors [30] , on the other hand, possesses a much higher density 3 in enhanced specific impulses of solid propellants. Considerable ef- of 5.61 g/cm . Therefore, the introduction of ZrH2 to propellants forts have been devoted investigating the applications of promising is proposed to be capable of providing both high energy and low metal hydrides in propellants, such as aluminum hydride (alane, gaseous molecular mass without reducing the densities [31] . AlH3 ) [15–19], beryllium hydride (BeH2 ) [20], magnesium hydride As stated above, the high energy and low gaseous molecular (MgH2 ) [17,21], titanium hydride (TiH2 ) [22,23], lithium hydride mass offered by metal hydrides originate from the hydrogen and (LiH) [24] and ammonia borane (NH3 BH3 ) [25]. Especially, alu- metal generated upon their dehydrogenation. However, it is well- minum hydride (AlH3 ), which is proposed to be a promising al- known that, metal hydrides are reductive due to the existence of δ− H in them, thus resulting in a high possibility of the occur- rence of interactions between metal hydrides and the strong ox- ∗ Corresponding author. idizers in solid propellants, such as ammonium perchlorate (AP), E-mail address: [email protected] (F. Zhao). 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. 68 Y. Yang et al. / Combustion and Flame 187 (2018) 67–76 Table 1 Main ingredients of the HTPB propellants. Compositions Contents Contents Contents Contents (mass%) (mass%) (mass%) (mass%) HTPB Binder 10.22 10.22 10.22 10.22 Ammonium perchlorate Oxidizer 71 71 71 71 Aluminium Metal fuel 15 10 5 ZrH 2 Metal fuel 5 10 15 DOS Plasticizer 3 3 3 3 TDI Cross-linking agent 0.68 0.68 0.68 0.68 trinitramine (RDX). Furthermore, the above interactions are be- 01-073-2076). It was found that, both AP and ZrH2 showed good lieved to result in the direct oxidation of metal hydrides, which crystallinity. In addition, no impurities were detected by XRD and would give rise to the generation of metal oxides and water in- FTIR, suggesting their high purities. stead of the expected H2 and metals. Obviously, occurrence of the Aluminum (Al, 5 μm), which is now the most widely adopted redox reactions between hydrides and oxidizers is very unfavor- metal fuel in solid propellants, was used for comparison with ZrH2 . able for lowering the gaseous molecular mass and increasing the Zirconium (Zr) was also used in evaluating the role of hydrogen specific impulse of propellants. On the contrary, the independent in ZrH2 . The binder, HTPB (hydroxyl–terminated polybutadiene), dehydrogenation of metal hydrides in the existence of oxidizers to was obtained from Liming Research Institute of Chemical Industry. give H2 and metals would benefit the energetic performances of Moreover, in this work, dioctyl sebacate (DOS) and 2,4-tolylene di- solid propellants. As a consequence, for metal hydrides used as the isocyanate (TDI) were utilized as the plasticizer and cross-linking fuels for solid propellants, it is of great importance to determine agent, respectively. The compositions of propellant formulations their interaction characteristics with the oxidizers. In addition, the are listed in Table 1 and no combustion catalysts were used in the role of hydrogen in hydride, which is believed to be very critical for formulations. propellant combustion, should be also evaluated. Previously, Lem- The propellant samples were prepared in 500 g batches us- pert et al. investigated theoretically the specific impulses, densi- ing a vertical planetary mixer of 2 L capacity. All batches were ties and combustion temperatures of several ZrH2 -based composite mixed and cast under vacuum by a slurry process. After HTPB, propellants with different oxidizers and binder [31,32] . However, DOS and TDI were mixed evenly, AP was carefully introduced into little is known about the interaction characteristics of ZrH2 with the mixture followed by additional mixing. Then, the fuel (Al, the oxidizers and especially, the role of hydrogen in ZrH2 . ZrH2 ) was added to the sample and mixed evenly. It is worth For several decades, ammonium perchlorate (AP) has been mentioning that the addition of ZrH2 should be conducted under widely utilized as an oxidizer for solid rocket propellants [33–35] . the protection of nitrogen. The propellant samples were cured at As a major ingredient of the formulations, its interactions with 50 °C for 72 h in a water jacketed oven. After that, the propel- metal fuels are closely related to the performances of AP- lant densities were measured and densities were determined to be containing propellants. Herein, we investigate the interaction char- (1.75 ± 0.01) × 10 3 , (1.78 ± 0.01) × 10 3 , (1.81 ± 0.01) × 10 3 and ± × 3 3 acteristics between ZrH2 and AP, and especially, the role of hydro- (1.84 0.01) 10 kg/m for the propellants containing 15 mass% + + gen in the combustion of HTPB propellants containing ZrH2 as the of Al, 10 mass% of Al 5 mass% of ZrH2 , 5 mass% of Al 10 mass% metal fuel. It is evaluated that, ZrH2 would not be oxidized directly of ZrH2 and 15 mass% of ZrH2 . These densities are almost identi- by AP or its decomposition products. Instead, it dehydrogenates to cal to the corresponding theoretical ones of 1.75 × 10 3 , 1.77 × 10 3 , produce hydrogen and metallic zirconium which would promote 1.81 × 10 3 and 1.83 × 10 3 kg/m 3 , suggesting that the propellant the combustion of propellants. The new insights into the reaction samples possess good quality. mechanisms of ZrH2 in propellant combustion are very useful for The AP/ZrH2 combination system used for thermal analyses was the applications of metal hydrides in solid rocket propellants. prepared by physically mixing AP and ZrH2 evenly using pestle and mortar. 2. Experimental section Ammonium perchlorate is an energetic compound sensitive to- 2.2. Structural characterization wards impact and friction. When handling AP, propellant samples containing AP and its mixtures with metal fuels, proper protective Phase identification was conducted using powder X-ray diffrac- measures including ear protection, Kevlar gloves, face shield, body tion (XRD) on a PANalytical X’Pert Pro X-ray Diffractometer with armor, and earthed equipment should be used.