Fig. 1 Electron microscopy characterization of starting boron powder. (a) SEM image of boron powder, showing the shape and size of two kinds of grains in equiaxial and rectangular morphology. (b) High-angle annular dark-field STEM image showing the size of particles consistent with (a). (c) BF-STEM image of boron powder particles. (d-f) Corresponding EDS maps highlighting the (d) B, (e) O, (f) Mg elements in red, blue and orange color, respectively.

Fig. 2. X-ray diffraction pattern of starting boron powder showing β-B106 and τ-B106 phases.

Fig. 3. TEM characterization of starting boron powder. (a) TEM image and corresponding SAED pattern confirm the β-boron. (b) Zig-Zag crystalline lattice and corresponding SAED pattern confirm the τ-boron. (c, d) Images reveal the amorphous structure obtained from such typical particles analyzed in (e). (e) TEM-EDS confirms such particles composed of boron and oxygen elements.

Fig. 4. High-resolution TEM images of rhombohedral β-boron displayed from different orientations: (a) [001], (b)$\left[ 1\bar{1}0 \right]$, (c) [111] and (d) [211]. The insets showing the SAED patterns (upper right corner) and simulated HRTEM images (lower right corner) from the same direction, indicating the rhombohedral boron unit cell contains 106 boron atoms.

Fig. 5. X-ray diffraction patterns of (a) boron powder and samples sintered at different temperatures of (b) 1650 °C, (c) 1750 °C and (d) 1800 °C using SPS. Yellow, blue and red dashed lines match well with characteristic peaks of β-B, τ-B and boron suboxide phases, respectively.

Fig. 6. Experimental XRD profile of SPSed β-B compact is compared with the calculated (simulated) diffraction pattern of B96O4. The intensity peaks of calculated B96O4 match well with the unidentified peaks in experimental profile (shown by green dotted lines). The solid lines (orange) below correspond to boron suboxide (B6O).

Fig. 7. (a) A representative image of SPSed boron compact sample. SEM micrographs of samples sintered at temperatures of (b) 1650 °C, (c) 1750 °C and (d) 1800 °C, showing the porosity and the precipitates formed during SPS process.

Fig. 8 STEM-EDS mapping of the precipitates on a sintered sample surface at 1650 °C. (a) HAADF-STEM of an area with precipitates in the sample. Corresponding EDS maps highlighted with the (b) B (red color), (c) O (blue color), (d) Mg (orange color).

Fig. 9. (a) Bright field STEM and (b) corresponding line intensity EDS scanning of boron (B), oxygen (O) and magnesium (Mg).

Fig. 10. Characterization of boron bulk sample sintered at 1650 °C. (a) TEM image showing the β-B and boron-rich oxide grains. (b) The HRTEM image displays the interface between the precipitate and the matrix, highlighted by the red dotted line revealing semi coherent interface. The image shows the crystal lattice of β-B $\left( 1\bar{2}0 \right)$ (on left) and the crystal lattice of boron rich oxide of $\left( 1\bar{1}00 \right)$ plane (on right). (c) SAED pattern obtained from the boron-rich oxide grain along [0001] direction. (d) Atomic resolution image of boron rich oxide along [0001] direction.

Fig. 11. Characterization of boron compact sample sintered at 1800 °C. (a) HAADF-STEM of an area with boron-rich oxide precipitates in sample sintered at 1800 °C. (b, c) SAED pattern and HRTEM image taken along $\left[ 1\bar{1}00 \right]$ zone axis from a precipitate marked with white box. (d) Enlarged HRTEM image of the area in (c) marked by a square.

Fig. 12. Experimental TEM diffraction patterns acquired from the newly formed boron suboxide crystals along the directions of (a)${{\left[ 111 \right]}_{r}}={{\left[ 0001 \right]}_{h}}$, (b) ${{\left[ 110 \right]}_{r}}={{\left[ 1100 \right]}_{h}}$(c) ${{\left[ \bar{1}10 \right]}_{r}}$=$[1\bar{2}1{{\overline{3}]}_{h}}$ and (d) ${{\left[ 0\bar{1}0 \right]}_{r}}=[01\bar{1}{{\overline{1}]}_{h}}$match well (in both reciprocal space distances and angles) with the simulated diffraction patterns of B96O4. The symbol ‘r’ is denoted for rhombohedral and ‘h’ is denoted for hexagonal direction.

Fig. 13. (a) HAADF-STEM image of a boron-rich oxide particle on the β-B sample sintered at 1800 °C, as marked by a square. (b, c) Elemental 2-D distribution maps of boron and oxygen in the area marked by a square in (a), respectively. (d) EELS spectra extracted from β-B and B-O marked region in Fig. 10(a).

Fig. 14. (a) Representative load-depth curves and (b) hardness-depth curves from the nanoindentation on the dense boron sample sintered at 1650 °C, 1750 °C and 1800 °C.

Fig. 15. Nanoindentation characterization of 1800 °C sintered sample. (a) Topology image after nanoindentation tests at applied loads of 10 mN and 5 mN. (b) Representative load-depth curves taken from the β-B and boron-rich oxide at 5 mN load. A pop-in can be detected on the curve of β-boron, which may correspond to the phase transformation. The obtained hardness (c) and reduced modulus (d) of boron and boron-rich oxide.

Fig. 16. Proposed structure model of B96O4 showing icosahedral boron (B12) clusters in blue and oxygen atoms in red color.