The need for wear- and scratch-resistant materials drives the quest for new superhard materials. In this work, we apply two design parameters to identify ultra-incompressible, superhard materials-high valence electron density and high bond covalency. Our first example of such a material is OsB2. The bulk modulus of OsB2 was measured using in situ high-pressure X-ray diffraction and was determined to be in the range of 365-395 GPa. While this value is slightly less than that of the bulk modulus of diamond, due to the anisotropic crystal structure of OsB2, the axis compressibility in the orthorhombic c-direction is less than the axis compressibility found in diamond. OsB2 also scratches the surface of a sapphire window, indicating that the hardness of OsB2 exceeds 2000 kg/mm2.

IntroductionAdditional materials characterization and data analysis were performed to supplement the results reported in "Zeolite-Templated Carbon Materials for High Pressure Hydrogen Storage" to assure reproducibility of the results, validate the templated structure of ZTCs, further analyze the micropore character of MSC-30 and ZTCs, and to probe differences in chemical bonding between MSC-30 and ZTCs that could clarify their significant difference in skeletal density.The correlation between BET surface area and excess hydrogen uptake across all temperatures and pressures in ZTCs and the other materials studied is central to the conclusions from this study. In addition to standard hydrogen adsorption/desorption measurements, we also include hydrogen cycling results to show full reversibility of hydrogen uptake and to verify that the experimental error in measurements is acceptable. Secondly, the BET method for characterizing the surface area of materials was supplemented by the Dubinin-Radushkevich method for determining microporous volume to determine the correlation between this value and excess hydrogen uptake at room temperature.Additional comparison of material properties of ZTC-3 and other ZTCs is important for validating the comparison of our high pressure results to those in the literature. Electron microscopy, both scanning (SEM) and transmission (TEM), was performed to show the similarity of ZTC-3 to "PFA-P7-H" synthesized by Ma et al. 38 , an approximately equivalent reference material to "P7(2)-H" of Nishihara et al. 16 Finally, some measurements were performed to determine if there were differences in sp 2 or sp 3 chemical bonding in ZTC-3 and MSC-30, including x-ray photolelectron spectroscopy (XPS), electron energy-loss spectroscopy (EELS), and solid-state 13 C nuclear magnetic resonance (NMR). The motivation for this additional work was elucidation of the nature of the different skeletal density between the two materials. However, no significant differences were detected that could account for the large difference in skeletal density. The results are consistent with those in the literature, for example by Yang et al. 33(XPS results) and Ma et al. 32 (NMR results). Hydrogen CyclingHydrogen uptake isotherms measured up to 30 MPa, using our newly constructed Sieverts apparatus specific to high pressure experiments, were cycled multiple times to ensure repeatability of the results. Hydrogen cycling in all materials studied was achieved without any loss of capacity on adsorption and desorption after many cycles, as expected for pure physisorbent materials. For example, three consecutive hydrogen adsorption/desorption cycles in ZTC-3 at 298 K are shown in Figure S1. The sample was degassed before cycling, as detailed in the Experimental Methods, but was not degassed in between cycles. Dubinin-Radushkevich Micropore VolumeNitrogen adsorption isotherms at 77 K were further analyzed to determine the Dubinin-Radushkevich (DR) micropore volume 21 of each sample. The DR method for treating the N 2 ...

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Interest in new ultraincompressible hard materials has prompted studies of transition metal diboride solid solutions. We have synthesized pure RuB 2 and solid solutions of Os 1-x Ru x B 2 . The mechanical properties of these materials are investigated using in situ high-pressure X-ray diffraction and Vickers hardness testing techniques. Both bulk moduli and hardness vary linearly with composition in accordance with Vegard's law, whereas the differing behavior among end-members can be explained by relativistic effects, core electron density, and differences in the cohesive energy of the parent metals. The results provide a refinement of the rules previously reported for the design of new superhard materials.