Tungsten diboride, WB2, usually has a hexagonal structure with the space group P63/mmc (number 194); and molybdenum diboride, MoB2, has a trigonal structure with R3̅m (number 166). Other than these phases, both diborides are reported to have a phase with an AlB2-type structure (P6/mmm, number 191). AlB2-type MoB2 is easy to synthesize and has been extensively studied, whereas AlB2-type WB2 is very difficult to synthesize and has appeared only once in a report by Woods et al. in 1966 (Woods, H. P.; Wawner, Jr., F. E.; Fox, B. G. Science1966, 151, 75.) We have investigated these diborides by means of first-principles calculations and found that boron defects are responsible for the difference in their synthesizability. AlB2-type MoB2 became stable enough with some boron defects added, while AlB2-type WB2 became minimally stable, suggesting it may not actually exist. Following our calculations, we attempted to synthesize AlB2-type WB2 with the optimum quantity of boron defects but observed no trace of it. We conclude, from both calculations and experiments, that AlB2-type WB2 does not exist stably in the W-B phase diagram and that the compound produced in Woods et al.'s report might have contained some impurities.
A preliminary feasibility study of a laser ramjet SSTO has been conducted using engine cycle analysis. Although a large amount of laser energy is lost due to chemically frozen flow at high altitudes, the laser ramjet SSTO was found to be feasible with 100 MW laser power for 100 kg vehicle mass and 1 m 2 vehicle cross-section area. Obtained momentum coupling coefficient, C m , was validated by means of CFD. As a result, the engine cycle analysis was underestimating C m. This would be because of the strong unsteady energy input in the actual heating process and the spatially localized pressure on the afterbody.
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