Ammonia is an essential chemical that underpins modern agriculture and industry, but the direct conversion of dinitrogen to ammonia under mild conditions remains highly challenging. To this end, molybdenum nitride complexes bearing 1,3-bis(di-tert-butylphosphinomethyl)benzimidazole-2-ylidene pincer ligands have been shown to catalyse ammonia formation from dinitrogen using samarium diiodide and water. However, the catalytic mechanism remains poorly understood owing to limited information on the key intermediates and the solution-state speciation of the samarium reductant. Here we show a full catalytic mechanism by combining mechanistic experiments and theoretical analyses. We isolate molybdenum methylimide and molybdenum methylamide complexes as stable analogues of catalytic intermediates, providing direct experimental evidence for the proposed catalytic pathway. Theoretical studies further suggest a plausible structure of the tetrahydrofuran-solvated samarium diiodide–water complex, and subsequent mechanistic analyses reveal that ammonia formation proceeds through proton-coupled electron transfer. Building on these experimental and theoretical findings, we identify the molybdenum–imide formation reaction as the most energy-demanding step. These findings provide a framework for understanding catalytic ammonia formation and inform future efforts to improve dinitrogen reduction catalysts. Ammonia can be synthesised from dinitrogen using a mononuclear molybdenum–nitride complex with samarium diiodide as the electron source and water as the proton source, but the mechanism is not fully known. Here, the authors isolate intermediate analogues and provide experimental and theoretical evidence for the proposed catalytic pathway.
This study applies a topology optimization approach to the design of a Junction Termination Extension (JTE), which is one of the edge-termination structures for vertical GaN power devices. Conventional parameter optimization requires independent tuning of the width, depth, and impurity concentration of the JTE region to achieve the desired breakdown voltage. As the number of target regions increases, the combinations of design parameters grow explosively. Consequently, severe constraints such as enforcing identical impurity concentrations across regions are often imposed, which substantially limit the design freedom. Focusing on the fact that a JTE structure can be represented as a dose distribution, we perform optimization with high design freedom using topology optimization. Since breakdown voltage correlates with the maximum electric field strength under reverse bias, we optimize the dose distribution to reduce the maximum electric field strength of the device. For a vertical GaN device biased at 900 V in reverse, the proposed method reduces the maximum electric field strength by 12.5% compared with a structure obtained by parameter optimization.
This study proposes an elastic-plastic incremental simulation method based on the continuum theory of dislocations. First, the elastic stress field for an infinite strip containing a pair of edge dislocations is derived. Next, for the external force boundary condition, a simulation method is proposed wherein the yield condition of finite-difference elements is satisfied by solving a system of linear equations for which the number of unknowns equals the number of yielding finite-difference elements. Similarly, for the displacement boundary condition, a simulation method is proposed wherein the boundary condition on the material-tool contact surface is satisfied by solving a system of linear equations for which the number of unknowns equals the number of contacting finite-difference elements. Finally, the simulation results agree with those obtained using the conventional elastic-plastic finite-element method but differ slightly from those obtained using the crystal-plasticity finite-element method.
