Dennis H. Mayo

Combustion Behavior of Solid Fuels Based on PTFE/Boron Mixtures

An experimental study was conducted to understand the combustion behavior of polytetrafluoroethylene (PTFE)/boron–based solid fuels for future hybrid rocket motor applications. Fuels were loaded with 10–40% boron powder (w/w). Two different types of PTFE were examined in this study, while a single type of boron powder was considered. No significant differences in the decomposition mechanisms for PTFE and a candidate solid fuel mixture were observed by differential scanning calorimetry (DSC) and temperature-jump (T-jump)/Fourier transform infrared (FTIR) experiments. Diffusion flame studies between solid fuels and gaseous oxygen were carried out to measure regression rates and to develop a fundamental understanding of the combustion behavior. The fuels with the lowest boron content readily extinguished upon removal of the supplemental oxygen flow. The fuels with the highest loadings of boron self-propagated after ignition. X-ray diffraction on postcombustion residue of the self-propagating material revealed graphite and boron carbide as the remaining products, while particles captured leaving the surface of the fuel under normal burning conditions were found to be mostly boric acid. Boron oxidation and magnesium fluorination were observed in the flame zone of the diffusion flame by UV-Vis emission spectroscopy (magnesium is the major impurity in the elemental boron powder used). The results of this study suggest that solid fuels comprising PTFE and boron show promise for improving the energy density of hybrid rockets.

Synthesis, Structure, and Properties of Al((R)bpy)3 Complexes (R = t-Bu, Me): Homoleptic Main-Group Tris-bipyridyl Compounds.

The neutral homoleptic tris-bpy aluminum complexes Al((R)bpy)3, where R = tBu (1) or Me (2), have been synthesized from reactions between AlX precursors (X = Cl, Br) and neutral (R)bpy ligands through an aluminum disproportion process. The crystalline compounds have been characterized by single-crystal X-ray diffraction, electrochemical experiments, EPR, magnetic susceptibility, and density functional theory (DFT) studies. The collective data show that 1 and 2 contain Al(3+) metal centers coordinated by three bipyridine (bpy(•))(1-) monoanion radicals. Electrochemical studies show that six redox states are accessible from the neutral complexes, three oxidative and three reductive, that involve oxidation or reduction of the coordinated bpy ligands to give neutral (R)bpy or (R)bpy(2-) dianions, respectively. Magnetic susceptibility measurements (4-300 K) coupled with DFT studies show strong antiferromagnetic coupling of the three unpaired electrons located on the (R)bpy ligands to give S = (1)/2 ground states with low lying S = (3)/2 excited states that are populated above 110 K (1) and 80 K (2) in the solid-state. Complex 2 shows weak 3D magnetic interactions at 19 K, which is not observed in 1 or the related [Al(bpy)3] complex.

The Role of Ligand Steric Bulk in New Monovalent Aluminum Compounds

The tetrameric Al(I) cyclopentadienyl compound Al4Cp*4 (Cp* = C5Me5) is a prototypical low-valence Al compound, with delocalized bonding between four Al(I) atoms and η5 ligands bound to the cluster exterior. The synthesis of new [AlR]4 (R = C5Me4Pr, C5Me4iPr) tetramers is presented. Though these systems failed to crystallize, comparison of variable-temperature 27Al NMR data with density functional theory (DFT) calculations indicate that these are Al4R4 tetramers analogous to Al4Cp*4 but with increased ligand steric bulk. NMR, DFT, and Atoms in Molecules analyses show that these clusters are enthalpically more stable as tetramers than the Cp* variant, due in part to noncovalent interactions across the bulkier ligand groups. Thermochemistry calculations for the low-valence metal interactions were found to be extremely sensitive to the DFT methodology used; the M06-2X functional with a cc-pVTZ basis set is shown to provide very accurate values for the enthalpy of tetramerization and 27Al NMR shifts. This computational method is then used to predict geometrical structures, noncovalent ligand interactions, and monomer/tetramer equilibrium in solution for a series of Al(I) cyclopentadienyl compounds of varying steric bulk.

Low oxidation state aluminum-containing cluster anions: Cp∗AlnH−, n = 1–3

Three new, low oxidation state, aluminum-containing cluster anions, Cp*AlnH(-), n = 1-3, were prepared via reactions between aluminum hydride cluster anions, AlnHm (-), and Cp*H ligands. These were characterized by mass spectrometry, anion photoelectron spectroscopy, and density functional theory based calculations. Agreement between the experimentally and theoretically determined vertical detachment energies and adiabatic detachment energies validated the computed geometrical structures. Reactions between aluminum hydride cluster anions and ligands provide a new avenue for discovering low oxidation state, ligated aluminum clusters.

Growth of metalloid aluminum clusters on graphene vacancies

Ab initio simulations are used to show that graphene vacancy sites may offer a means of templated growth of metalloid aluminum clusters from their monohalide precursors. We present density functional theory and ab initio molecular dynamics simulations of the aluminum halide AlCl interacting with a graphene surface. Unlike a bare Al adatom, AlCl physisorbs weakly on vacancy-free graphene with little charge transfer and no hybridization with carbon orbitals. The barrier for diffusion of AlCl along the surface is negligible. Covalent bonding is seen only with vacancies and results in strong chemisorption and considerable distortion of the nearby lattice. Car-Parrinello molecular dynamics simulations of AlCl liquid around a graphene single vacancy show spontaneous metalloid cluster growth via a process of repeated insertion reactions. This suggests a means of templated cluster nucleation and growth on a carbon substrate and provides some confirmation for the role of a trivalent aluminum species in nucleating a ligated metalloid cluster from AlCl and AlBr solutions.

Tetra­bromidobis(dicyclo­hexyl­phosphane-κP)digallium(Ga—Ga)

The title compound, a GaII dimer, [Ga2Br4(C12H23P)2], was synthesized by reaction of GaBr(THF)n (THF is tetrahydrofuran) with dicyclohexylphosphine in toluene. At 150 K the crystallographically centrosymmetric molecule exhibits disorder in which one of the two independent cyclohexyl groups is modelled over two sites in a 62 (1):38 (1) ratio. In d 6-benzene solution, the compound exhibits virtual C 2h symmetry as determined by 1H NMR. The coordination environment of the GaII atom is distorted tetrahedral.

Mechanistic Studies of [AlCp*]4 Combustion

The combustion mechanism of [AlCp*]4 (Cp* = pentamethylcyclopentadienyl), a ligated aluminum(I) cluster, was studied by a combination of experimental and theoretical methods. Two complementary experimental methods, temperature-programmed reaction and T-jump time-of-flight mass spectrometry, were used to investigate the decomposition behaviors of [AlCp*]4 in both anaerobic and oxidative environments, revealing AlCp* and Al2OCp* to be the major decomposition products. The observed product distribution and reaction pathways are consistent with the prediction from molecular dynamics simulations and static density functional theory calculations. These studies demonstrated that experiment and theory can indeed serve as complementary and predictive means to study the combustion behaviors of ligated aluminum clusters and may help in engineering stable compounds as candidates for rocket propellants.