Strong and ductile titanium-oxygen-iron alloys by additive manufacturing.

Titanium alloys are advanced lightweight materials, indispensable for many critical applications1,2. The mainstay of the titanium industry is the α-ß titanium alloys, which are formulated through alloying additions that stabilize the α and ß phases3-5. Our work focuses on harnessing two of the most powerful stabilizing elements and strengtheners for α-ß titanium alloys, oxygen and iron1-5, which are readily abundant. However, the embrittling effect of oxygen6,7, described colloquially as 'the kryptonite to titanium'8, and the microsegregation of iron9 have hindered their combination for the development of strong and ductile α-ß titanium-oxygen-iron alloys. Here we integrate alloy design with additive manufacturing (AM) process design to demonstrate a series of titanium-oxygen-iron compositions that exhibit outstanding tensile properties. We explain the atomic-scale origins of these properties using various characterization techniques. The abundance of oxygen and iron and the process simplicity for net-shape or near-net-shape manufacturing by AM make these α-ß titanium-oxygen-iron alloys attractive for a diverse range of applications. Furthermore, they offer promise for industrial-scale use of off-grade sponge titanium or sponge titanium-oxygen-iron10,11, an industrial waste product at present. The economic and environmental potential to reduce the carbon footprint of the energy-intensive sponge titanium production12 is substantial.

Uniting tensile ductility with ultrahigh strength via composition undulation.

Metals with nanocrystalline grains have ultrahigh strengths approaching two gigapascals. However, such extreme grain-boundary strengthening results in the loss of almost all tensile ductility, even when the metal has a face-centred-cubic structure-the most ductile of all crystal structures1-3. Here we demonstrate that nanocrystalline nickel-cobalt solid solutions, although still a face-centred-cubic single phase, show tensile strengths of about 2.3 gigapascals with a respectable ductility of about 16 per cent elongation to failure. This unusual combination of tensile strength and ductility is achieved by compositional undulation in a highly concentrated solid solution. The undulation renders the stacking fault energy and the lattice strains spatially varying over length scales in the range of one to ten nanometres, such that the motion of dislocations is thus significantly affected. The motion of dislocations becomes sluggish, promoting their interaction, interlocking and accumulation, despite the severely limited space inside the nanocrystalline grains. As a result, the flow stress is increased, and the dislocation storage is promoted at the same time, which increases the strain hardening and hence the ductility. Meanwhile, the segment detrapping along the dislocation line entails a small activation volume and hence an increased strain-rate sensitivity, which also stabilizes the tensile flow. As such, an undulating landscape resisting dislocation propagation provides a strengthening mechanism that preserves tensile ductility at high flow stresses.

Electronic Modulation of Nickel Disulfide toward Efficient Water Electrolysis.

Developing highly efficient earth-abundant nickel-based compounds is an important step to realize hydrogen generation from water. Herein, the electronic modulation of the semiconducting NiS2 by cation doping for advanced water electrolysis is reported. Both theoretical calculations and temperature-dependent resistivity measurements indicate the semiconductor-to-conductor transition of NiS2 after Cu incorporation. Further calculations also suggest the advantages of Cu dopant to cathodic water electrolysis by bringing Gibbs free energy of H adsorption at both Ni sites and S sites much closer to zero. It is noteworthy that water dissociation on Cu-doped NiS2 (Cu-NiS2 ) surface is even more favorable than those on NiS2 and Pt(111). Thus, the prepared Cu-NiS2 shows noticeably improved performance toward alkaline hydrogen and oxygen evolution reactions (HER and OER). Specifically, it requires merely 232 mV OER overpotential to drive 10 mA cm-2 ; in parallel with Tafel slopes of 46 mV dec-1 . Regarding HER, an onset overpotential of only 68 mV is achieved. When integrated as both electrodes for water electrolysis, Cu-NiS2 needs only 1.64 V to drive 10 mA cm-2 , surpassing the state-of-the-art Ir/C-Pt/C couple (1.71 V). This work opens up an avenue to engineer low-cost and earth-abundant catalysts performing on par with the noble-metal-based one for water splitting.

Big to Small: Ultrafine Mo2 C Particles Derived from Giant Polyoxomolybdate Clusters for Hydrogen Evolution Reaction.

Due to its electronic structure, similar to platinum, molybdenum carbides (Mo2 C) hold great promise as a cost-effective catalyst platform. However, the realization of high-performance Mo2 C catalysts is still limited because controlling their particle size and catalytic activity is challenging with current synthesis methods. Here, the synthesis of ultrafine ß-Mo2 C nanoparticles with narrow size distribution (2.5 ± 0.7 nm) and high mass loading (up to 27.5 wt%) on graphene substrate using a giant Mo-based polyoxomolybdate cluster, Mo132 ((NH4 )42 [Mo132 O372 (CH3 COO)30 (H2 O)72 ]·10CH3 COONH4 ·300H2 O) is demonstrated. Moreover, a nitrogen-containing polymeric binder (polyethyleneimine) is used to create MoN bonds between Mo2 C nanoparticles and nitrogen-doped graphene layers, which significantly enhance the catalytic activity of Mo2 C for the hydrogen evolution reaction, as is revealed by X-ray photoelectron spectroscopy and density functional theory calculations. The optimal Mo2 C catalyst shows a large exchange current density of 1.19 mA cm-2 , a high turnover frequency of 0.70 s-1 as well as excellent durability. The demonstrated new strategy opens up the possibility of developing practical platinum substitutes based on Mo2 C for various catalytic applications.

Ultrahigh piezoelectricity in ferroelectric ceramics by design.

Piezoelectric materials, which respond mechanically to applied electric field and vice versa, are essential for electromechanical transducers. Previous theoretical analyses have shown that high piezoelectricity in perovskite oxides is associated with a flat thermodynamic energy landscape connecting two or more ferroelectric phases. Here, guided by phenomenological theories and phase-field simulations, we propose an alternative design strategy to commonly used morphotropic phase boundaries to further flatten the energy landscape, by judiciously introducing local structural heterogeneity to manipulate interfacial energies (that is, extra interaction energies, such as electrostatic and elastic energies associated with the interfaces). To validate this, we synthesize rare-earth-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), as rare-earth dopants tend to change the local structure of Pb-based perovskite ferroelectrics. We achieve ultrahigh piezoelectric coefficients d33 of up to 1,500 pC N-1 and dielectric permittivity ε33/ε0 above 13,000 in a Sm-doped PMN-PT ceramic with a Curie temperature of 89 °C. Our research provides a new paradigm for designing material properties through engineering local structural heterogeneity, expected to benefit a wide range of functional materials.

Confinement Impact for the Dynamics of Supported Metal Nanocatalyst.

Supported metal nanoparticles play key roles in nanoelectronics, sensors, energy storage/conversion, and catalysts for the sustainable production of fuels and chemicals. Direct observation of the dynamic processes of nanocatalysts at high temperatures and the confinement of supports is of great significance to investigate nanoparticle structure and functions for practical utilization. Here, in situ high-resolution transmission electron microscopy photos and videos are combined with dynamics simulations to reveal the real-time dynamic behavior of Pt nanocatalysts at operation temperatures. Amorphous Pt surface on moving and deforming particles is the working structure during the high operation temperature rather than a static crystal surface and immobilization on supports as proposed before. The free rearrangement of the shape of Pt nanoparticles allows them to pass through narrow windows, which is generally considered to immobilize the particles. The Pt particles, no matter what their sizes, prefer to stay inside nanopores even when they are fast moving near an opening at temperatures up to 900 °C. The porous confinement also blocks the sintering of the particles under the confinement size of pores. These contribute to the continuous high activity and stability of Pt nanocatalysts inside nanoporous supports during a long-term evaluation of catalytic reforming reaction.

Nano-RuO2 -Decorated Holey Graphene Composite Fibers for Micro-Supercapacitors with Ultrahigh Energy Density.

Compactness and versatility of fiber-based micro-supercapacitors (FMSCs) make them promising for emerging wearable electronic devices as energy storage solutions. But, increasing the energy storage capacity of microscale fiber electrodes, while retaining their high power density, remains a significant challenge. Here, this issue is addressed by incorporating ultrahigh mass loading of ruthenium oxide (RuO2 ) nanoparticles (up to 42.5 wt%) uniformly on nanocarbon-based microfibers composed largely of holey reduced graphene oxide (HrGO) with a lower amount of single-walled carbon nanotubes as nanospacers. This facile approach involes (1) space-confined hydrothermal assembly of highly porous but 3D interconnected carbon structure, (2) impregnating wet carbon structures with aqueous Ru3+ ions, and (3) anchoring RuO2 nanoparticles on HrGO surfaces. Solid-state FMSCs assembled using those fibers demonstrate a specific volumetric capacitance of 199 F cm-3 at 2 mV s-1 . Fabricated FMSCs also deliver an ultrahigh energy density of 27.3 mWh cm-3 , the highest among those reported for FMSCs to date. Furthermore, integrating 20 pieces of FMSCs with two commercial flexible solar cells as a self-powering energy system, a light-emitting diode panel can be lit up stably. The current work highlights the excellent potential of nano-RuO2 -decorated HrGO composite fibers for constructing micro-supercapacitors with high energy density for wearable electronic devices.

Facilitation of Ferroelectric Switching via Mechanical Manipulation of Hierarchical Nanoscale Domain Structures.

Heterogeneous ferroelastic transition that produces hierarchical 90° tetragonal nanodomains via mechanical loading and its effect on facilitating ferroelectric domain switching in relaxor-based ferroelectrics were explored. Combining in situ electron microscopy characterization and phase-field modeling, we reveal the nature of the transition process and discover that the transition lowers by 40% the electrical loading threshold needed for ferroelectric domain switching. Our results advance the fundamental understanding of ferroelectric domain switching behavior.

On the wurtzite to tetragonal phase transformation in ZnO nanowires.

There is a long standing contradiction on the tensile response of zinc oxide nanowires between theoretical prediction and experimental observations. Although it is proposed that there is a ductile behavior dominated by phase transformation, only an elastic deformation and brittle fracture was witnessed in experiments. Using molecular dynamics simulations, we clarified that, as the lateral dimension of zinc oxide nanowires increases to a critical value, an unambiguous ductile-to-brittle transition occurs. The critical value increases with decreasing the strain rate. Factors including planar defects and surface contamination induce brittle fracture prior to the initiation of phase transformation. These findings are consistent with previous atomistic standpoints and experimental results.

Effect of a High Density of Stacking Faults on the Young's Modulus of GaAs Nanowires.

Stacking faults (SFs) are commonly observed crystalline defects in III-V semiconductor nanowires (NWs) that affect a variety of physical properties. Understanding the effect of SFs on NW mechanical properties is critical to NW applications in nanodevices. In this study, the Young's moduli of GaAs NWs with two distinct structures, defect-free single crystalline wurtzite (WZ) and highly defective wurtzite containing a high density of SFs (WZ-SF), are investigated using combined in situ compression transmission electron microscopy and finite element analysis. The Young's moduli of both WZ and WZ-SF GaAs NWs were found to increase with decreasing diameter due to the increasing volume fraction of the native oxide shell. The presence of a high density of SFs was further found to increase the Young's modulus by 13%. This stiffening effect of SFs is attributed to the change in the interatomic bonding configuration at the SFs.

Manipulation of Nanoscale Domain Switching Using an Electron Beam with Omnidirectional Electric Field Distribution.

Reversible ferroelectric domain (FD) manipulation with a high spatial resolution is critical for memory storage devices based on thin film ferroelectric materials. FD can be manipulated using techniques that apply heat, mechanical stress, or electric bias. However, these techniques have some drawbacks. Here we propose to use an electron beam with an omnidirectional electric field as a tool for erasable stable ferroelectric nanodomain manipulation. Our results suggest that local accumulation of charges contributes to the local electric field that determines domain configurations.

Determination of Young's Modulus of Ultrathin Nanomaterials.

Determination of the elastic modulus of nanostructures with sizes at several nm range is a challenge. In this study, we designed an experiment to measure the elastic modulus of amorphous Al2O3 films with thicknesses varying between 2 and 25 nm. The amorphous Al2O3 was in the form of a shell, wrapped around GaAs nanowires, thereby forming an effective core/shell structure. The GaAs core comprised a single crystal structure with a diameter of 100 nm. Combined in situ compression transmission electron microscopy and finite element analysis were used to evaluate the elastic modulus of the overall core/shell nanowires. A core/shell model was applied to deconvolute the elastic modulus of the Al2O3 shell from the core. The results indicate that the elastic modulus of amorphous Al2O3 increases significantly when the thickness of the layer is smaller than 5 nm. This novel nanoscale material can be attributed to the reconstruction of the bonding at the surface of the material, coupled with the increase of the surface-to-volume ratio with nanoscale dimensions. Moreover, the experimental technique and analysis methods presented in this study may be extended to measure the elastic modulus of other materials with dimensions of just several nanometers.

Effects of loading misalignment and tapering angle on the measured mechanical properties of nanowires.

Loading misalignment and tapering of nanowires are usually unavoidable factors in compression and tensile mechanical property testing of nanowires. Herein, we report quantitative finite element analyses and experimental measurements on how these two factors affect the measured compression and tensile mechanical properties if they are not included in the data analysis. The results obtained show that ignoring these two factors leads to different degrees of underestimation of the critical load, Young's modulus and tensile fracture strength.

Anelastic behavior in GaAs semiconductor nanowires.

The mechanical behavior of vertically aligned single-crystal GaAs nanowires grown on GaAs(111)B surface was investigated using in situ deformation transmission electron microscopy. Anelasticity was observed in nanowires with small diameters and the anelastic behavior was affected by the crystalline defects in the nanowires. The underlying mechanism for the observed anelasticity is discussed. The finding opens up the prospect of using nanowire materials for nanoscale damping applications.

Strengthening brittle semiconductor nanowires through stacking faults: insights from in situ mechanical testing.

Quantitative mechanical testing of single-crystal GaAs nanowires was conducted using in situ deformation transmission electron microscopy. Both zinc-blende and wurtzite structured GaAs nanowires showed essentially elastic deformation until bending failure associated with buckling occurred. These nanowires fail at compressive stresses of ~5.4 GPa and 6.2 GPa, respectively, which are close to those values calculated by molecular dynamics simulations. Interestingly, wurtzite nanowires with a high density of stacking faults fail at a very high compressive stress of ~9.0 GPa, demonstrating that the nanowires can be strengthened through defect engineering. The reasons for the observed phenomenon are discussed.

Room-temperature super-elongation in high-entropy alloy nanopillars.

Nanoscale small-volume metallic materials typically exhibit high strengths but often suffer from a lack of tensile ductility due to undesirable premature failure. Here, we report unusual room-temperature uniform elongation up to ~110% at a high flow stress of 0.6-1.0 GPa in single-crystalline <110>-oriented CoCrFeNi high-entropy alloy nanopillars with well-defined geometries. By combining high-resolution microscopy and large-scale atomistic simulations, we reveal that this ultrahigh uniform tensile ductility is attributed to spatial and synergistic coordination of deformation twinning and dislocation slip, which effectively promote deformation delocalization and delay necking failure. These joint and/or sequential activations of the underlying displacive deformation mechanisms originate from chemical compositional heterogeneities at the atomic level and resulting wide variations in generalized stacking fault energy and associated dislocation activities. Our work provides mechanistic insights into superplastic deformations of multiple-principal element alloys at the nanoscale and opens routes for designing nanodevices with high mechanical reliability.

Formation of Head/Tail-to-Body Charged Domain Walls by Mechanical Stress.

Domain walls (DWs) in ferroelectric materials are interfaces that separate domains with different polarizations. Charged domain walls (CDWs) and neutral domain walls are commonly classified depending on the charge state at the DWs. CDWs are particularly attractive as they are configurable elements, which can enhance field susceptibility and enable functionalities such as conductance control. However, it is difficult to achieve CDWs in practice. Here, we demonstrate that applying mechanical stress is a robust and reproducible approach to generate CDWs. By mechanical compression, CDWs with a head/tail-to-body configuration were introduced in ultrathin BaTiO3, which was revealed by in-situ transmission electron microscopy. Finite element analysis shows strong strain fluctuation in ultrathin BaTiO3 under compressive mechanical stress. Molecular dynamics simulations suggest that the strain fluctuation is a critical factor in forming CDWs. This study provides insight into ferroelectric DWs and opens a pathway to creating CDWs in ferroelectric materials.

Electrosynthesis of chlorine from seawater-like solution through single-atom catalysts.

The chlor-alkali process plays an essential and irreplaceable role in the modern chemical industry due to the wide-ranging applications of chlorine gas. However, the large overpotential and low selectivity of current chlorine evolution reaction (CER) electrocatalysts result in significant energy consumption during chlorine production. Herein, we report a highly active oxygen-coordinated ruthenium single-atom catalyst for the electrosynthesis of chlorine in seawater-like solutions. As a result, the as-prepared single-atom catalyst with Ru-O4 moiety (Ru-O4 SAM) exhibits an overpotential of only ~30 mV to achieve a current density of 10 mA cm-2 in an acidic medium (pH = 1) containing 1 M NaCl. Impressively, the flow cell equipped with Ru-O4 SAM electrode displays excellent stability and Cl2 selectivity over 1000 h continuous electrocatalysis at a high current density of 1000 mA cm-2. Operando characterizations and computational analysis reveal that compared with the benchmark RuO2 electrode, chloride ions preferentially adsorb directly onto the surface of Ru atoms on Ru-O4 SAM, thereby leading to a reduction in Gibbs free-energy barrier and an improvement in Cl2 selectivity during CER. This finding not only offers fundamental insights into the mechanisms of electrocatalysis but also provides a promising avenue for the electrochemical synthesis of chlorine from seawater electrocatalysis.

Influence of microstructures on mechanical behaviours of SiC nanowires: a molecular dynamics study.

The tensile behaviours of [111]-oriented SiC nanowires with various microstructures are investigated by using molecular dynamics simulations. The results revealed the influence of microstructures on the brittleness and plasticity of SiC nanowires. Plastic deformation is mainly induced by the anti-parallel sliding of 3C grains along an intergranular amorphous film parallel to the plane and inclined at an angle of 19.47° with respect to the nanowire axis. Our study suggests that the wide dispersion of mechanical properties of SiC nanowires observed in experiments might be attributed to their diverse microstructures.

Self-healing of fractured GaAs nanowires.

In-situ deformation experiments were carried out in a transmission electron microscope to investigate the structural response of single crystal GaAs nanowires (NWs) under compression. A repeatable self-healing process was discovered in which a partially fractured GaAs NW restored its original single crystal structure immediately after an external compressive force was removed. Possible mechanisms of the self-healing process are discussed.