SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
ReB2 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Re6+ is bonded in a 8-coordinate geometry to eight equivalent B3- atoms. There are two shorter (2.23 Å) and six longer (2.27 Å) Re–B bond lengths. B3- is bonded in a 7-coordinate geometry to four equivalent Re6+ and three equivalent B3- atoms. All B–B bond lengths are 1.83 Å.
Sm2(ReB2)3 crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. Sm3+ is bonded in a 10-coordinate geometry to ten B3- atoms. There are a spread of Sm–B bond distances ranging from 2.73–3.18 Å. There are two inequivalent Re4+ sites. In the first Re4+ site, Re4+ is bonded in a 6-coordinate geometry to six B3- atoms. There are a spread of Re–B bond distances ranging from 2.13–2.30 Å. In the second Re4+ site, Re4+ is bonded in a 8-coordinate geometry to eight B3- atoms. There are a spread of Re–B bond distances ranging from 2.25–2.38 Å. There are three inequivalent B3- sites. In the first B3- site, B3- is bonded in a 8-coordinate geometry to three equivalent Sm3+, four Re4+, and one B3- atom. The B–B bond length is 1.82 Å. In the second B3- site, B3- is bonded in a 9-coordinate geometry to four equivalent Sm3+, three Re4+, and two B3- atoms. There is one shorter (1.82 Å) and one longer (1.88 Å) B–B bond length. In the third B3- site, B3- is bonded in a 9-coordinate geometry to three equivalent Sm3+, three Re4+, and three B3- atoms. The B–B bond length is 1.80 Å.
The high-entropy boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 material was synthesized under high-pressures and high-temperatures in a large-volume Paris-Edinburgh (PE) press from a ball-milled powder mix of HfO2, MoO3, Nb2O5, Ta2O5, ZrO2, carbon black, and boron carbide. The transformation process was monitored in situ by energy-dispersive x-ray diffraction with conversion starting at 1100 °C and completed by 2000 °C with the formation of a single hexagonal AlB2-type phase. The synthesized sample was recovered, powdered, and mixed with platinum pressure marker and studied under high pressure by angle-dispersive x-ray diffraction in a diamond anvil cell. The hexagonal AlB2-type phase of (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 was found to be stable up to the highest pressure of 220 GPa reached in this study (volume compression V/V0 = 0.70). The third order Birch-Murnaghan equation of state fit to the high-pressure data up to 220 GPa results in an ambient pressure unit cell volume V0=28.16±0.04 Å3, bulk modulusKo = 407 ± 6 GPa, pressure derivative of bulk-modulus K0′ = 2.73 ± 0.045 GPa. Our study indicates that this high-entropy boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 material is stable to ultrahigh pressures and temperatures and exhibit high bulk modulus similar to other incompressible transition metal borides like ReB2 and Os2B3.