Ashvin Kumar Vasudevan1,Prithwish Biswas1,Yujie Wang1,Michael Zachariah1
University of California, Riverside1
Ashvin Kumar Vasudevan1,Prithwish Biswas1,Yujie Wang1,Michael Zachariah1
University of California, Riverside1
Sodium borohydride is a potential fuel source for air-breathing propulsion applications. However, sodium borohydride by itself is stable in air and primarily releases hydrogen on heating. Literature work suggests that a strong Lewis acid such as boron trifluoride (BF<sub>3</sub>) is required to initiate decomposition into diborane and sodium tetrafluoroborate. Our current study shows that ionic liquids such as 1-Ethyl-3-methylimidazolium tetrafluoroborate, (EMIM BF<sub>4</sub>) can act as good sources of BF<sub>3</sub> and bring about complete decomposition forming sodium fluoride and diborane. Accordingly, a molar ratio of 3:1 (NaBH<sub>4</sub>: EMIM BF<sub>4</sub>) is stoichiometric for complete conversion to sodium fluoride and diborane. Ignition of the mixture shows rapid reaction of diborane with air. Successful combustion even at low amounts of ionic liquid indicates a unique decomposition mechanism initiated by BF<sub>3</sub>.<br/>Analysis of evolved products using T-jump time of flight mass spectrometry shows consistent amounts of diborane release at all molar ratios tested. When very low amounts of ionic liquid (molar ratio 16:1) are used, diborane is accompanied by hydrogen evolved from excess unreacted sodium borohydride. The solid products comprise mostly sodium fluoride with some traces of sodium tetrafluoroborate as seen by XRD. Mass losses observed in the TGA further suggest complete conversion to diborane and sodium fluoride at all molar ratios. Analysis of the gaseous and solid products alongside combustion results with 3d printed samples to describe the flame propagation will be presented and a mechanism specific to sodium borohydride decomposition in ionic liquids will be discussed.<br/>The use of thermally stable ionic liquids provides a unique avenue for generating diborane from borohydrides which can be incorporated to make air breathing propellants which have low ignition temperatures. Early results with potassium borohydride show that similar decomposition mechanisms are possible in other alkali borohydrides. Further work is needed to explore the potential of various hydrides and borohydrides for air breathing propulsion.