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Topological insulators (TI) are generating increasing interest as a new state of matter and due to the potential use of topologically- protected gapless surface states in spintronic devices and quantum computing. However, challenges such as high sensitivity to the atmosphere, the low surface-to-volume ratio, and the need for various material junctions currently limit their application. Here, a novel, natural and simple approach to the fabrication of volumetric TI heterostructures that can overcome these core challenges is presented, using the example of a Bi2Te3-Te eutectic composite. The proposed method based on directional solidification of eutectic composites, enables the formation of ensembles of parallel TI-other material heterojunctions through a self-organization process. It also offers control over the heterostructures’ dimensions/refinement. Electron microscopy techniques show that the heterostructure exhibits a lamellar/layered microstructure with atomically smooth Bi2Te3ǀǀTe interfaces. Angle-resolved photoelectron spectroscopy experiments confirm the existence of metallic surface states, while Kelvin probe force microscopy depicts the formed p-n junctions. The new degrees of freedom offered here, such as control of heterojunction chemical composition, packing density, and available fabrication techniques, may facilitate large-scale customized printing of topological devices.

Mechanically-alloyed CoCrFeNi high entropy alloys (MA-HEAs) show better mechanical properties than as-cast HEAs. However, the limited amount of powder milled in one-milling cycle creates a challenge in larger-scale applications. The low ball-to-powder ratio (BPR) milling used to increase powder yield resulted in the formation of Cr-rich second phases in the face-centered cubic matrix. To overcome the above issues, in this work, with a low BPR of 5:1, four different processes were analyzed: quicker milling intervals, higher milling speed, and the pre-alloying of Cr and Ni. All parameters mentioned before were used and tested together in the fourth sample. Samples were consolidated by spark plasma sintering technique. Microstructural characterization was made on a Scanning Electron Microscope equipped with EDS and EBSD detectors and a Transmission Electron Microscope. X-ray diffraction was employed to develop the structural evolution of powders sintered and annealed samples. The global hardness was measured using microhardness tests, while the nanohardness tests helped us correlate the evolution of secondary phases with their mechanical properties. The results show that pre-alloying elements with the lowest diffusion coefficient before the main milling process might be an attractive strategy to improve productivity and optimize microstructure without limiting the efficiency of low BPR MA-HEAs. The results described below demonstrate the promising perspective for the implementation of powder metallurgy techniques in a mass scale production as low BPR is essential for numerous applications like aerospace industry, car engines and medicine, where improved productivity of powder mixing is mandatory.

While gas nitriding of steel is currently used in industry, nitriding of aluminum alloys remains an open challenge. The main obstacle is aluminum’s high susceptibility to passivation. The oxide film provides an effective barrier to nitrogen diffusion. Attempts to overcome this problem have mainly focused on glow discharge nitriding using cathode sputtering of an oxide layer. The produced AlN layers exhibit no diffusion zone and show limited performance properties. In this work, the effect of hybrid treatment aimed at producing diffusion layers of nitrides other than AlN on aluminum alloys was investigated on the model system of iron nitride–aluminum substrate. Hybrid treatment combines an electrochemical process involving the removal of the aluminum oxide layer from the substrate, its subsequent iron plating, and a further gas nitriding in high-purity ammonia. The obtained results prove that the hybrid treatment allows the production, at 530 °C/10 h, of diffusion layers of Fe3N iron nitrides on aluminum substrates with a nitrogen diffusion zone range in aluminum of ca. 12 µm. In alloys containing magnesium, its unfavorable effect on the nitrogen diffusion and the functional properties of the layers was observed. An interesting direction for further research is hybrid treatment of precipitation-hardened alloys without magnesium.

Abstract Due to their broadband nonlinear optical properties, low-dimensional materials are widely used for pulse generation in fiber and solid-state lasers. Here we demonstrate novel materials, Bi2Te2Se (BTS) and Sn-doped Bi2Te2Se (BSTS), which can be used as a universal saturable absorbers for distinct spectral regimes. The material was mechanically exfoliated from a bulk single-crystal and deposited onto a side-polished fiber. We have performed characterization of the fabricated devices and employed them in polarization-maintaining ytterbium- and erbium-doped fiber lasers. This enabled us to obtain self-starting passively Q-switched regime at 1 µm and 1.56 µm. The oscillators emitted stable, linearly polarized radiation with the highest single pulse energy approaching 692 nJ. Both lasers are characterized by the best performance observed in all-polarization maintaining Q-switched fiber lasers with recently investigated new saturable absorbers, which was enabled by a very high damage threshold of the devices. This demonstrates the great potential of the investigated materials for the ultrafast photonics community.