Intermetallic compounds formed from non-precious transition metals are promising cost-effective and robust catalysts for electrochemical hydrogen production. However, the development of monolithic nanoporous intermetallics, with ample active sites and sufficient electrocatalytic activity, remains a challenge. Here we report the fabrication of nanoporous Co7Mo6 and Fe7Mo6 intermetallic compounds via liquid metal dealloying. Along with the development of three-dimensional bicontinuous open porosity, high-temperature dealloying overcomes the kinetic energy barrier, enabling the direct formation of chemically ordered intermetallic phases. Unprecedented small characteristic lengths are observed for the nanoporous intermetallic compounds, resulting from an intermetallic effect whereby the chemical ordering during nanopore formation lowers surface diffusivity and significantly suppresses the thermal coarsening of dealloyed nanostructure. The resulting ultrafine nanoporous Co7Mo6 exhibits high catalytic activity and durability in electrochemical hydrogen evolution reactions. This study sheds light on the previously unexplored intermetallic effect in dealloying and facilitates the development of advanced intermetallic catalysts for energy applications.

We investigated the effect of low-flux electron irradiation with 125 keV to sputter-deposited amorphous germanium on the amorphous structure and electron-induced crystallization microstructure by TEM following our previous study on the effect of aging at room temperature. In samples aged for 3 days, coarse, spherical particles about 100 nm in diameter appear dominantly. By low-flux pre-irradiation to the samples, a reduction in the size and number of coarse particles, embedded in the matrix with fine nanograins of the diamond cubic structure, was noted with the increase in fluence. The crystal structure of these coarse particles was found to be not cubic but hexagonal. In samples aged for 4 months, a similar tendency was observed. In samples aged for 7 months, on the other hand, the homogeneous diamond cubic structured nanograins were unchanged by pre-irradiation. These results indicate that pre-irradiation as well as aging modifies the amorphous structure, preventing the appearance of a hexagonal phase. The elimination of a certain amount of medium-range ordered clusters by pre-irradiation, included in as-deposited samples and the samples aged for 4 months, apparently gives rise to a reduction in the size and number of coarse particles with a metastable hexagonal structure.

The structure of amorphous Ge (a-Ge) films prepared by sputter-deposition and the effects of aging at ambient temperature and pressure were studied by pair-distribution-function (PDF) analysis from electron scattering and molecular dynamics simulations. The PDFs of the as-deposited and aged samples for 3–13 months showed that the major peaks for Ge-Ge bonds decrease in intensity and broaden with aging for up to 7 months. In the PDFs of a-Ge of molecular dynamics simulation obtained by quenching liquid at different rates, the major peak intensities of a slowly cooled model are higher than those of a rapidly cooled model. Analyses on short- and medium-range configurations show that the slowly cooled model includes a certain amount of medium-range ordered (MRO) clusters, while the rapidly cooled model includes liquid-like configurations rather than MRO clusters. The similarity between experimental and computational PDFs implies that as-deposited films are similar in structure to the slowly cooled model, whereas the fully aged films are similar to the rapidly cooled model. It is assumed that as they undergo room-temperature aging, the MRO clusters disintegrate and transform into liquid-like regions in the same matrix. This transition in local configurations is discussed in terms of instability and the non-equilibrium of nanoclusters produced by a vapor-deposition process.

This paper addresses system identification and vibration control of a cantilever fabricated from piezoelectric materials, PZT, and shows how system identification and state estimation can be used to achieve self-maintenance of a self-sensing system. Currently, self-sensing systems that have concurrent actuation and sensing can be made by using a bridge circuit. However, hardware tuning is still needed due to the unstable nature of an imbalanced bridge circuit. This problem becomes serious in the space environment where human beings may not be available to perform the maintenance. A method of achieving self-sensing without a bridge circuit is proposed in this paper. Dynamics of this proposed system can be described as the state space expression with a direct transmission component. This means that the problem of balancing the bridge circuit is equivalent to the system identification and state estimation problem. By performing a simple experiment, a model of the system was identified using the 4SID, SubSpace State Space Identification method. Observer theory can be used to estimate state vectors which include information about the mechanical dynamics. Thus, system stability depends on the estimated values of the state vectors. The system can be stabilized using a state feedback controller such as a LQ controller. The proposed method was verified with experimental results, demonstrating that smart structures can achieve self-maintenance.

According to Hunt’s columnar-to-equiaxed transition (CET) criterion, which is generally accepted, a high-temperature gradient (G) in the solidification front is preferable to a low G for forming columnar grains. Here, we report the opposite tendency found in the solidification microstructure of stainless steels partially melted by scanning electron beam for powder bed fusion (PBF)-type additive manufacturing. Equiaxed grains were observed more frequently in the region of high G rather than in the region of low G, contrary to the trend of the CET criterion. Computational thermal-fluid dynamics (CtFD) simulation has revealed that the fluid velocity is significantly higher in the case of smaller melt regions. The G on the solidification front of a small melt pool tends to be high, but at the same, the temperature gradient along the melt pool surface also tends to be high. The high melt surface temperature gradient can enhance Marangoni flow, which can apparently reverse the trend of equiaxed grain formation.