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A novel structurally stable 2D-boron nitride material, namely di-BN, is predicted by means of the first-principles simulations. This monolayer BN system is composed of the azo (N N) and diboron (B B) groups. Its in-plane stiffness is close to the monolayer h-BN. Usually, the boron nitride materials are semiconductors with large band gaps. However, the monolayer di-BN possesses a moderate direct band gap of 1.622 eV obtained from our HSE06 calculation. Although the GW correction enlarges the band gap to 2.446 eV, this value is still in the range of the visible light. The detailed investigation of its band arrangement reveals that this material is able to product hydrogen molecules in a photocatalytic water splitting reaction. Furthermore, its charge carrier mobilities are significantly higher than the other popular 2D semiconductors, e.g., MoS 2 and phosphorene. Therefore, this 2D-BN material could have huge application potentials in the electronics and solar energy conversion fields. • A new boron nitride monolayer system, di-BN composed of B B and N N pairs, is proposed. • Unlike the other boron nitride systems with large band gaps, di-BN possesses an optimal direct band gap about 1.622 eV at HSE06 level. • The di-BN is a promising photocatalytic material for hydrogen evolution by water splitting. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coble's densification model has been adopted to investigate the densification behavior of a freeze-casted Al 2 O 3 sample. • A shift of the densification mechanism from grain boundary to lattice diffusion has been elucidated at 1500 °C. • Glassy phases at grain boundary would facilitate densification when grain boundary diffusion dominates below 1500 °C. • Densification also would be facilitated by oxygen vacancies formed at grain boundary due to segregation of impurities. • Proper selection of dopants is important for densification to obtain beneficial grain boundary characteristics. Coble's densification theory has been adopted to investigate the densification behavior of a freeze-casted Al 2 O 3 sample. The successful adoption of bulk theory determined the dominant densification mechanisms of the porous structure. We noted a shift of the densification mechanism from grain boundary to lattice diffusion at 1500 °C. Interestingly, a very low activation energy value of 72.17 kJ/mol was measured for grain boundary diffusion while a general value of 455.83 kJ/mol for lattice diffusion. This outcome was accompanied by the presence of glassy phases at the grain boundary that would facilitate densification when grain boundary diffusion dominates the overall densification kinetics. Otherwise, the concentration of oxygen vacancies was high at the grain boundaries without the glassy phases. Both resulted from the segregation of intrinsic impurities, emphasizing the need for the proper selection of dopants to determine the grain boundary characteristics and hence potential densification strategies based on sintering science. [ABSTRACT FROM AUTHOR]