Epitaxial Ni?Mn?Ga Films for Magnetic Shape Memory Alloy Microactuators

Active materials such as piezoelectrics are established in the field of microsystems application despite their low achievable strains which often require the integration of additional gear mechanisms. The ongoing search for new active materials has focused on magnetic shape memory (MSM) alloys such as Ni? Mn? Ga since they combine macroscopic strains of up to 10% with a cycling frequency well above the frequencies of conventional thermal shape memory alloys. The present review focuses on preparation and analysis of Ni? Mn? Ga films that can eventually be integrated in microsystems. Single crystal like films are prepared by epitaxial growth on suitable substrate materials. Since the magnetically induced reorientation of variants is blocked by a rigid substrate, we present different methods for releasing films from the substrates. We show that the sacrificial layer technology is the most promising approach. Further processing of the freestanding film requires a microtechnology which is adjusted to the film laminate structure. The properties of the freestanding films are compared with films on a rigid substrate. Although we observe stress‐induced twin boundary motion, the twinning stress is too high to be overcome by an external magnetic field. Therefore, it is necessary to develop suitable training methods to reduce the twinning stress below 2 MPa to enable the activation of the material by means of an external magnetic field.     

Key Properties of Ni?Mn?Ga Based Single Crystals Grown with the SLARE Technique

Magnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of Ni? Mn? Ga based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of Ni? Mn? Ga, Ni? Mn? Ga? Fe, and Ni? Mn? Ga? Co of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. Ni? Mn? Ga alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as Ni? Mn? Ga with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.     

Periodically Laser Patterned Fe?B?Si Amorphous Ribbons: Phase Evolution and Mechanical Behavior

The properties of amorphous alloys are significantly influenced by structural relaxation and partial/full crystallization induced by thermal annealing of the alloy. In this paper, the phase evolution and mechanical behavior of laser‐patterned Fe? B? Si amorphous alloys are reported. The laser patterning was employed to cause localized thermal effects on the surface of amorphous ribbons. The laser irradiation with a lower fluence (12 J · cm^?2) caused significant embrittlement of the alloy due to the structural relaxation. The partial crystallization of an amorphous alloy into α‐Fe(Si) was also observed with laser irradiation using higher laser fluences (15 and 17 J · cm^?2). The embrittlement effect due to laser‐irradiation‐induced crystallization was more severe than that due to structural relaxation.     

Controlling Hot Charge Carrier Transfer in Monolithic Al?Si?Al Heterostructures for Plasmonic On-Chip Energy Harvesting

The generation of hot carriers by Landau damping or chemical interface damping of plasmons is of particular interest to the fundamental aspects of extreme light-matter interactions. Hot charge carriers can be transferred to an attached acceptor for photochemical or photovoltaic energy conversion. However, these lose their excess energy and relax to thermal equilibrium within picoseconds and it is difficult to extract useful work thereof with thermodynamic efficiencies that are of interest for practical devices. Without a detailed understanding of the underlying plasmon decay processes and transfer mechanisms, proper material matching and design considerations for novel plasmonic devices are extremely challenging. Here, a multifunctional Al Si Al heterostructure device with tunable Schottky barriers is presented to control plasmon-induced hot carrier injection at an abrupt metal-semiconductor interface. Light absorption, surface plasmon generation, and separation of hot carriers arising from the non-radiative decay of surface plasmons are realized in a monolithic Schottky barrier field effect transistor. Aside from barrier modulation, a virtual p–n junction can be emulated in the semiconductor channel with the distinct merit that carrier concentration and polarity are tunable by electrostatic gating. The investigations are carried out with a view to possible use for CMOS-compatible plasmonic photovoltaics, with versatile implementations for autonomous nanosystems.     

Unfolding B?O?B Bonds for an Enhanced ORR Performance in ABO3‐Type Perovskites

Identifying the relationship between catalytic performance and material structure is crucial to establish the design principle for highly active catalysts. Deficiency in B? O bond covalency induced by lattice distortion severely restricts the oxygen reduction reaction (ORR) performance for ABO₃‐type perovskite oxides. Herein, a rearrangement of hybridization mode for B? O bond is used to tune the overlap of the electron cloud between B 3d and O 2p through A‐stie doping with larger radius ions. The B? O bond covalency is strengthened with a B? O? B bond angle recovered from intrinsic structural distortion. As a result, the adsorption and the reduction process for O₂ on the oxide surface can be promoted via shifting the O‐2p band center toward the Fermi Level. Simultaneously, the spin electrons in the Mn 3d orbit become more parallel. It will lead to a high electrical conductivity by the enhanced double exchange process and thereof mitigate the ORR efficiency loss. Further density functional theory calculation reveals that a flat [BO₂] plane will make contribution to the charge transfer process from lattice oxygen to adsorbed oxygen (mediated with B ions). Through such exploration of the effect of crystal structure on the electronic state of perovskite oxides, a novel insight into design of highly active ORR catalysts is offered.     

Biomorphic Co?N?C/CoOx Composite Derived from Natural Chloroplasts as Efficient Electrocatalyst for Oxygen Reduction Reaction

Natural chloroplasts containing big amounts of chlorophylls (magnesium porphyrin, Mg‐Chl) are employed both as template and porphyrin source to synthesize biomorphic Co? N? C/CoO_x composite as electrocatalyst for the oxygen reduction reaction (ORR). Cobalt‐substituted chlorophyll derivative (Co‐Chl) in chloroplasts is first obtained by successively rinsing in hydrochloric acid and cobalt acetate solutions. After calcining in nitrogen to 800 °C, Co‐Chl is transferred to Co? N? C; while other parts of chloroplasts adsorbed with Co ions are transferred to CoO_x retaining the microarchitecture of chloroplasts. The abundant active Co? N? C sites are protected by circumjacent biocarbon and CoO_x to avoid leakage and agglomeration, and at the same time can overcome the poor conductivity weakness of CoO_x by directly transporting electrons to the carbonaceous skeleton. This unique synergistic effect, together with efficient bioarchitecture, leads to good electrocatalytical performance for the ORR. The onset and half‐wave potentials are 0.89 and 0.82 V versus reversible hydrogen electrode, respectively, with better durability and methanol tolerance than that of commercial Pt/C. Different from the traditional concept of biomorphic materials which simply utilize bioarchitectures, this work provides a new example of coupling bioderivative components with bioarchitectures into one integrated system to achieve good comprehensive performance for electrocatalysts.     

Thin Film Synthesis of Ti3SiC2 by Rapid Thermal Processing of Magnetron‐Sputtered Ti?C?Si Multilayer Systems

MAX phase coating could have interesting technical applications in many fields. This paper describes the synthesis of the M_n+1AX_n phase Ti₃SiC₂ by a rapid thermal annealing process of physical vapor deposited Ti? C? Si multilayer thin films on Si (100) and SiO₂ substrates. Annealing temperatures of 800–1000 °C affected the solid state reaction of titanium, carbon and silicon creating titanium‐carbides, ‐silicides, and Ti₃SiC₂. The film structures and chemical compositions were observed by grazing incidence X‐ray diffraction, transmission electron microscopy, and glow discharge optical emission spectroscopy. Analysis after the rapid thermal processing revealed the formation of the polycrystalline M_n+1AX_n phase Ti₃SiC₂ in coexistence with TiSi₂, TiC and Ti₅Si₃, even within a 0 s annealing process. This synthesis method has a high potential for the formation of MAX phases as a high temperature electrical contact material.     

Engineering C?S?Fe Bond Confinement Effect to Stabilize Metallic-Phase Sulfide for High Power Density Sodium-Ion Batteries (Small 37/2023)

Metallic-phase iron sulfide (e.g., Fe₇S₈) is a promising candidate for high power density sodium storage anode due to the inherent metal electronic conductivity and unhindered sodium-ion diffusion kinetics. Nevertheless, long-cycle stability can not be achieved simultaneously while designing a fast-charging Fe₇S₈-based anode. Herein, Fe₇S₈ encapsulated in carbon-sulfur bonds doped hollow carbon fibers (NHCFs-S-Fe₇S₈) is designed and synthesized for sodium-ion storage. The NHCFs-S-Fe₇S₈ including metallic-phase Fe₇S₈ embrace higher electron specific conductivity, electrochemical reversibility, and fast sodium-ion diffusion. Moreover, the carbonaceous fibers with polar C S Fe bonds of NHCFs-S-Fe₇S₈ exhibit a fixed confinement effect for electrochemical conversion intermediates contributing to long cycle life. In conclusion, combined with theoretical study and experimental analysis, the multinomial optimized NHCFs-S-Fe₇S₈ is demonstrated to integrate a suitable structure for higher capacity, fast charging, and longer cycle life. The full cell shows a power density of 1639.6 W kg⁻¹ and an energy density of 204.5 Wh kg⁻¹, respectively, over 120 long cycles of stability at 1.1 A g⁻¹. The underlying mechanism of metal sulfide structure engineering is revealed by in-depth analysis, which provides constructive guidance for designing the next generation of durable high-power density sodium storage anodes.     

Formation of B?N?C Coordination to Stabilize the Exposed Active Nitrogen Atoms in g‐C3N4 for Dramatically Enhanced Photocatalytic Ammonia Synthesis Performance

It is an important issue that exposed active nitrogen atoms (e.g., edge or amino N atoms) in graphitic carbon nitride (g‐C₃N₄) could participate in ammonia (NH₃) synthesis during the photocatalytic nitrogen reduction reaction (NRR). Herein, the experimental results in this work demonstrate that the exposed active N atoms in g‐C₃N₄ nanosheets can indeed be hydrogenated and contribute to NH₃ synthesis during the visible‐light photocatalytic NRR. However, these exposed N atoms can be firmly stabilized through forming B? N? C coordination by means of B‐doping in g‐C₃N₄ nanosheets (BCN) with a B‐doping content of 13.8 wt%. Moreover, the formed B? N? C coordination in g‐C₃N₄ not only effectively enhances the visible‐light harvesting and suppresses the recombination of photogenerated carriers in g‐C₃N₄, but also acts as the catalytic active site for N₂ adsorption, activation, and hydrogenation. Consequently, the as‐synthesized BCN exhibits high visible‐light‐driven photocatalytic NRR activity, affording an NH₃ yield rate of 313.9 µmol g^?1 h^?1, nearly 10 times of that for pristine g‐C₃N₄. This work would be helpful for designing and developing high‐efficiency metal‐free NRR catalysts for visible‐light‐driven photocatalytic NH₃ synthesis.     

B?N Pairs Enriched Defective Carbon Nanosheets for Ammonia Synthesis with High Efficiency

Electrochemical synthesis has garnered attention as a promising alternative to the traditional Haber–Bosch process to enable the generation of ammonia (NH₃) under ambient conditions. Current electrocatalysts for the nitrogen reduction reaction (NRR) to produce NH₃ are comprised of noble metals or transitional metals. Here, an efficient metal‐free catalyst (BCN) is demonstrated without precious component and can be easily fabricated by pyrolysis of organic precursor. Both theoretical calculations and experiments confirm that the doped B? N pairs are the active triggers and the edge carbon atoms near to B? N pairs are the active sites toward the NRR. This doping strategy can provide sufficient active sites while retarding the competing hydrogen evolution reaction (HER) process; thus, NRR with high NH₃ formation rate (7.75 µg h^?1 mg_cat. ^?1) and excellent Faradaic efficiency (13.79%) are achieved at ?0.3 V versus reversible hydrogen electrode (RHE), exceeding the performance of most of the metallic catalysts.     

Saturated Coordination Lu?N6 Defect Sites for Highly Efficient Electroreduction of CO2

Metal single-atom and internal structural defects typically coexist in M–N–C materials obtained through the existing basic pyrolysis processes. Identifying a correlation between them to understand the structure–activity relationship and achieve efficient catalytic performance is important, particularly for the rare-earth (RE) elements with rich electron orbitals and strong coordination capabilities. Herein, a novel single-atom catalyst based on the RE element lutetium is successfully synthesized on a N–C support. Structural and simulation analyses demonstrate that the formation of a Lu N₆ structural site with an individual defect because of pyrolysis is thermodynamically favorable in Lu–N–C. Using KHCO₃-based electrolytes facilitates the fall of the K⁺ cations into the defective sites of Lu–N–C, thus enabling improved CO₂ capture and activation, which increases the catalyst conductivity for Lu–N–C. In this study, the catalyst exhibits a Faradaic efficiency of 95.1% for CO at a current density of 18.2 mA cm⁻² during carbon dioxide reduction reaction. This study thus provides new insights into understanding RE–N–C materials for energy utilization.     

Ultrafast Sodium Full Batteries Derived from X?Fe (X = Co,Ni, Mn) Prussian Blue Analogs

Exploring high‐rate electrode materials with excellent kinetic properties is imperative for advanced sodium‐storage systems. Herein, novel cubic‐like X? Fe (X = Co, Ni, Mn) Prussian blue analogs (PBAs), as cathodes materials, are obtained through as‐tuned ionic bonding, delivering improved crystallinity and homogeneous particles size. As expected, Ni‐Fe PBAs show a capacity of 81 mAh g^?1 at 1.0 A g^?1, mainly resulting from their physical–chemical stability, fast kinetics, and “zero‐strain” insertion characteristics. Considering that the combination of elements incorporated with carbon may increase the rate of ion transfer and improve the lifetime of cycling stability, they are expected to derive binary metal‐selenide/nitrogen‐doped carbon as anodes. Among them, binary Ni_0.67Fe_0.33Se₂ coming from Ni‐Fe PBAs shows obvious core–shell structure in a dual‐carbon matrix, leading to enhanced electron interactions, electrochemical activity, and “metal‐like” conductivity, which could retain an ultralong‐term stability of 375 mAh g^?1 after 10 000 loops even at 10.0 A g^?1. The corresponding full‐cell Ni‐Fe PBAs versus Ni_0.67Fe_0.33Se₂ deliver a remarkable Na‐storage capacity of 302.2 mAh g^?1 at 1.0 A g^?1. The rational strategy is anticipated to offer more possibilities for designing advanced electrode materials used in high‐performance sodium‐ion batteries.     

In Situ Reconstruction Ni?O Octahedral Active Sites for Promoting Electrocatalytic Oxygen Evolution of Nickel Phosphate

Electrochemical activation strategy is very effective to improve the intrinsic catalytic activity of metal phosphate toward the sluggish oxygen evolution reaction (OER) for water electrolysis. However, it is still challenging to operando trace the activated reconstruction and corresponding electrocatalytic dynamic mechanisms. Herein, a constant voltage activation strategy is adopted to in situ activate Ni₂P₄O₁₂, in which the break of Ni O Ni bond and dissolution of PO₄³⁻ groups could optimize the lattice oxygen, thus reconstructing an irreversible amorphous Ni(OH)₂ layer with a thickness of 1.5–3.5 nm on the surface of Ni₂P₄O₁₂. The heterostructure electrocatalyst can afford an excellent OER activity in alkaline media with an overpotential of 216.5 mV at 27.0 mA cm⁻². Operando X-ray absorption fine structure spectroscopy analysis and density functional theory simulations indicate that the heterostructure follows a nonconcerted proton–electron transfer mechanism for OER. This activation strategy demonstrates universality and can be used to the surface reconstruction of other metal phosphates.     

Fe?N Coordination Induced Ultralong Lifetime of Sodium-Ion Battery with the Cycle Number Exceeding 65 000

Sodium-ion batteries (SIBs) have received increasing attention because of their appealing cell voltages and cost-effective features. However, the atom aggregation and electrode volume variation inevitably deteriorate the sodium storage kinetics. Here a new strategy is proposed to boost the lifetime of SIB by synthesizing sea urchin-like FeSe₂/nitrogen-doped carbon (FeSe₂/NC) composites. The robust Fe N coordination hinders the Fe atom aggregation and accommodates the volume expansion, while the unique biomorphic morphology and high conductivity of FeSe₂/NC enhance the intercalation/deintercalation kinetics and shorten the ion/electron diffusion length. As expected, FeSe₂/NC electrodes deliver excellent half (387.6 mAh g⁻¹ at 20.0 A g⁻¹ after 56 000 cycles) and full (203.5 mAh g⁻¹ at 1.0 A g⁻¹ after 1200 cycles) cell performances. Impressively, an ultralong lifetime of SIB composed of FeSe₂/Fe₃Se₄/NC anode is uncovered with the cycle number exceeding 65 000. The sodium storage mechanism is clarified with the aid of density function theory calculations and in situ characterizations. This work hereby provides a new paradigm for enhancing the lifetime of SIB by constructing a unique coordination environment between active material and framework.     

Liberating N‐CNTs Confined Highly Dispersed Co?Nx Sites for Selective Hydrogenation of Quinolines

Selective hydrogenation of quinoline and its derivatives is an important means to produce corresponding 1,2,3,4‐tetrahydroquinolines for a wide spectrum of applications. A facile and efficient “laser irradiation in liquid” technique to liberate the inaccessible highly dispersed Co? N_x active sites confined inside N‐doped carbon nanotubes is demonstrated. The liberated Co? N_x sites possess generic catalytic activities toward selective hydrogenation of quinoline and its hydroxyl, methyl, and halogen substituted derivatives into corresponding 1,2,3,4‐tetrahydroquinolines with almost 100% conversion efficiency and selectivity. This laser irradiation treatment approach should be widely applicable to unlock the catalytic powers of inaccessible catalytic active sites confined by other materials.     

Enlarged Co?O Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt‐containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen‐deficient perovskites. Here, a systematic study of spinel ZnFe_xCo_2?xO₄ oxides (x = 0–2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal–oxygen covalency and an enlarged Co? O covalency by 10–30 at% Fe substitution is responsible for the activity enhancement. While the pH‐dependent OER activity of ZnFe_0.4Co_1.6O₄ (the optimal one) indicates decoupled proton–electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p‐band center relative to Fermi level governed by the spinel's cation deficient nature.     

A Transmetalation Synthetic Strategy to Engineer Atomically Dispersed MnN2O2 Electrocatalytic Centers Driving High-Performance Li?S Battery

Sluggish sulfur redox reaction (SROR) kinetics accompanying lithium polysulfides (LiPSs) shuttle effect becomes a stumbling block for commercial application of Li S battery. High-efficient single atom catalysts (SACs) are desired to improve the SROR conversion capability; however, the sparse active sites as well as partial sites encapsulated in bulk-phase are fatal to the catalytic performance. Herein, high loading (5.02 wt.%) atomically dispersed manganese sites (MnSA) on hollow nitrogen-doped carbonaceous support (HNC) are realized for the MnSA@HNC SAC by a facile transmetalation synthetic strategy. The thin-walled hollow structure (≈12 nm) anchoring the unique trans-MnN₂O₂ sites of MnSA@HNC provides a shuttle buffer zone and catalytic conversion site for LiPSs. Both electrochemical measurement and theoretical calculation indicate that the MnSA@HNC with abundant trans-MnN₂O₂ sites have extremely high bidirectional SROR catalytic activity. The assembled Li S battery based on the MnSA@HNC modified separator can deliver a large specific capacity of 1422 mAh g⁻¹ at 0.1 C and stable cycling over 1400 cycles with an ultralow decay rate of 0.033% per cycle at 1 C. More impressively, a flexible pouch cell on account of the MnSA@HNC modified separator may release a high initial specific capacity of 1192 mAh g⁻¹ at 0.1 C and uninterruptedly work after the bending-unbending processes.