Mechanical, electrical, and thermal properties of boron-rich manganese diboride (MnB2) synthesized using reactive plasma sintering

Boron-rich manganese diboride MnB 2 with the AlB 2 -type hexagonal crystal structure was synthesized by combining high-energy ball-milling (HEBM) with reactive spark-plasma-sintering (RSPS) directly from elements. To investigate the influence of excess boron on sintering, densification, hardness, and transport properties, the B/Mn molar ratio was varied (2.5, 3.0, 3.5, and 4.0). While powder X-ray diffraction (PXRD) analysis revealed that the MnB 2 main phase was obtained regardless of the starting nominal composition, excess boron strongly affected the sintering behavior, hardness, and thermal transport properties. Synthesized boron-rich MnB 2 exhibited superior Vickers hardness compared to reported high-pressure high-temperature (HPHT) MnB 2 , with composition B/Mn = 3.5 particularly exhibiting 32.8 GPa and 23.7 GPa at low and high loads, respectively. The enhanced hardness is attributed to the combined effects of improved densification, dispersion strengthening by uniformly distributed secondary phases, and grain refinement of the MnB 2 matrix. The boron-rich MnB 2 displayed metallic electrical conductivity with a room-temperature resistivity of 75 μΩ cm for composition B/Mn = 2.5. The boron-rich composition B/Mn = 4.0 showed the highest thermal conductivity among all compositions, reaching ≈ 48 W m −1 K −1 at room temperature, likely due to improved densification and the presence of boron-rich β-rhombohedral boron and B 6 O secondary phases. Interestingly, the thermal conductivity of non-stoichiometric MnB 2 is dominated by phonon transport, in contrast to stoichiometric AlB 2 -type hexagonal transition-metal diborides where electronic contribution prevails. This shift in heat-transport mechanism may arise from interface modifications induced by excess boron.