Catastrophic Recombination/Reaction of Aluminum Atoms in Cryogenic Parahydrogen Solids.

Abstract

We report the results from experiments testing the limits of chemical energy storage in Al-atom-doped cryogenic parahydrogen (pH2) solids produced by codeposition of Al vapor and pH2 gas. These results map out three regimes of behavior. As described in the immediately preceding companion manuscript, for target Al atom concentrations, [Al]target ≲ 200 parts-per-million (ppm), the Al/pH2 solids are optically transparent and primarily contain isolated Al atoms, with a small admixture of AlxHy molecules formed by Al atom recombination/reaction. For [Al]target ≳ 500 ppm, the depositions fail immediately, producing highly optically scattering solids with evidence of extensive Al atom recombination/reaction. For intermediate [Al]target concentrations, near-threshold metastable transparent solids are formed which suffer catastrophic recombination/reaction as they grow past some critical thickness. Analysis of these catastrophic events using a minimal two parameter model [Jackson. J. Chem. Phys. 1959, 31, 722-729] yields a critical Al atom concentration near [Al] ≈ 280 ppm and an Al atom diffusion length ≈ 100 μm. While this diffusion length appears unphysically large, it is roughly compatible with even larger 300-900 μm wavelengths of spatial inhomogeneities visible in images of a remnant postrecombination sample. This spatial pattern is evocative of Turing structures or perhaps the structures sometimes formed upon phase separation; however, the actual mechanism for morphogenesis remains undetermined. The largest achieved Al atom concentrations of [Al] ≈ 300 ppm are roughly two orders-of-magnitude lower than those required for advanced chemical propulsion applications.