Laurence Courthéoux

Influence of the fuel on the thermal and catalytic decompositions of ionic liquid monopropellants

Binary aqueous mixtures of HAN, AN, ADN and HNF have been prepared. Two series of ternary mixtures have been synthesized with methanol and glycerol as fuels: one series with fuel excess and a second series with stoichiometric fuel content. Thermal decomposition of the solutions prepared has been analyzed. For binary HAN, HNF and ADN aqueous solutions, the thermal decomposition starts only once water has been fully vaporized and the oxidizer is in the liquid state. In the case of ammonium nitrate we do not observe any exothermic decomposition, but only the vaporization and dissociation of the liquid compound into ammonia and nitric acid. The influence of the fuel depends strongly on the oxidizer. Methanol is vaporized before the decomposition, leading to results close to those observed for binary mixtures. HAN-based solutions display the highest catalytic effect with a temperature decrease of about 100 °C. ADN-based propellants display the highest decomposition rates, but the catalytic effect remains weak. Catalytic decomposition of HNF leads to a two-step mechanism with slow decomposition rates. The ability order to oxidize completely glycerol in stoichiometric mixtures is: HAN < HNF < ADN

Catalytic decomposition of HAN-water binary mixtures

Binary aqueous HAN (hydroxylammonium nitrate NH3OHNO3) of different concentrations (20, 60, 83 wt-%) have been thermally and catalytically decomposed. The catalysts were prepared by impregnation of pure or doped supports (alumina, zirconia, barium, magnesium or zirconium aluminate) with monometallic or bimetallic active phases (noble and non-noble metals). The evaluation of the catalysts was carried out using a thermal analysis apparatus and a batch constant volume reactor. The thermal decomposition starts only when the water has been quantitatively removed and the onset temperature was 115 °C for the 83 % solution using the batch reactor. In the presence of a support alone, a catalytic effect is evidenced for the alumina and silica-doped alumina by a decrease of the decomposition temperature, due to the acidity of the surface; on the other hand, doping with barium leads to an inhibiting effect. The presence of an adequate active phase leads to a further drop of the onset temperature (< 40 °C), disclosing that the decomposition can be triggered at low temperature in the presence of water; the rate is one order of magnitude higher by comparison with the thermal decomposition. The evaluation of other active phases leads to various results; iridium supported on alumina displays a fair activity. INTRODUCTION In previous papers concerning the possible replacement of hydrazine by HAN (hydroxylammonium nitrate NH3OHNO3) based aqueous monopropellants, we have presented the design and use of a very versatile laboratory batch reactor, as well as the preliminary results on HAN-TEAN-water mixtures (TEAN = triethanolammonium nitrate NH(C2H4OH)3NO3). A comparison of this ternary mixture with the more simple HAN-water binary mixture (i.e. without a fuel) has revealed similar catalytic decomposition kinetics at low temperature (50-250 °C range), despite the very different energetic content of both mixtures, meaning that the first decomposition steps are the same for both samples as it was already stated by Klein et Leveritt: the early reactions for the HAN-TEAN-water blends involve only HAN decomposition and the reaction initiation takes place at 120°C. Figure 1 presents typical results obtained in batch reactor, after an injection of 50 μL of the ternary or binary mixtures onto 200 mg of the same catalyst at 130 °C. The HAN-water solution (83 wt-% HAN) displays a higher pressure slope than the ternary mixture (also called LGP 1846). Moreover, the reproducibility of the experiments is better for the binary solution and the pressure remains constant whereas it continues to increase slowly for the LGP sample. Therefore, we focused mainly on the potential reactivity of HAN-water binary mixtures in order to obtain more information regarding the first decomposition steps and the associate catalysts. The preliminary results obtained with this binary monopropellant on different catalytic formulations 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 7-10 July 2002, Indianapolis, Indiana AIAA 2002-4027 Copyright

IMPROVEMENT OF CATALYSTS FOR THE DECOMPOSITION OF HAN -BASED MONOPROPELLANT - COMPARISON BETWEEN AEROGELS AND XEROGELS

Different supports and catalysts have be en prepared and studied for the decomposition of aqueous HAN solutions (79 wt -%) (HAN = hydroxylammonium nitrate NH 3OHNO 3). The supports (silica doped alumina) were prepared by sol -gel synthesis using subcritical drying (leading to xerogels) or supercritic al drying with CO 2 (leading to aerogels). The active phase was introduced on the support surface through two ways: (i) by impregnation of thermally stabilized supports from aqueous metal precursor solutions or (ii) by one step addition of the precursor bef ore the sol formation. Characterization (X -ray diffraction, BET surface area, metallic dispersion, transmission electron microscopy) and evaluation of the catalysts (thermal analysis apparatus, constant volume reactor) were carried out. Aerogels present a much better thermal stability at high temperature (1200 °C) and are more homogeneously dispersed than xerogels. Nevertheless, aerogels are less efficient for the decomposition of HAN solutions at low temperatures. The one step procedure leads to an increas e of the thermal stability for the aerogel samples, but the catalytic activity is less efficient. The impregnated catalysts display always the best activity in relation with the lower crystallite size of the metallic particles. The most important results a re: (i) thermal stability of aerogels at high temperatures and (ii) possibility to decompose catalytically aqueous HAN solutions at low temperatures (less than 40 °C) with a short ignition delay (less than 1 s).