Liquid-Like, Self-Healing Aluminum Oxide during Deformation at Room Temperature

Predicted Fracture Tendency of Naturally Occurring Aluminum Surface Coatings under Tensile Loading

Naturally occurring coatings on aluminum metal, such as its oxide or hydroxide, serve to protect the material from corrosion. Understanding the conditions under which these coatings mechanically fail is therefore expected to be an important aspect of predictive models for aluminum component lifetimes. To this end, we develop and apply a molecular dynamics (MD) modeling framework for conducting tension tests that is capable of isolating factors governing the mechanical strength as a function of coating chemistry, defect morphology, and variables associated with the loading path. We consider two representative materials, including γ-Al2O3 and γ-Al(OH)3 (i.e., oxide and hydroxide), both of which form readily as aluminum surface coatings. Our results indicate that defects have a significant bearing on the strength of aluminum oxide, with grain boundaries serving to reduce the strain at failure from εzz = 0.300 to 0.219, relative to perfect single crystal. Our simulations also predict that porosity lowers the elastic stiffness and yield strength of the oxide. Relative to perfect crystal, we find porosity factors of 5%, 10% and 20% decrease the yield stress by 26%, 36% and 53%, respectively. MD predicts that perfect hydroxide and oxide single crystal have respective strains at failure of 0.08 and 0.31 under tensile uniaxial strain loading, and that the corresponding yield stresses are respectively 1.6 and 11.1 GPa. These data indicate that the hydroxide is substantially more susceptible to mechanical failure than the oxide. Our results, coupled with literature findings that indicate hot and humid conditions favor formation of hydroxide and defective oxide coatings, indicate the potential for a complicated dependence of aluminum corrosion susceptibility and stress corrosion cracking on aging history.

Atomistic mechanisms of water vapor-induced surface passivation

The microscopic mechanisms underpinning the spontaneous surface passivation of metals from ubiquitous water have remained largely elusive. Here, using in situ environmental electron microscopy to atomically monitor the reaction dynamics between aluminum surfaces and water vapor, we provide direct experimental evidence that the surface passivation results in a bilayer oxide film consisting of a crystalline-like Al(OH)3 top layer and an inner layer of amorphous Al2O3. The Al(OH)3 layer maintains a constant thickness of ~5.0 Å, while the inner Al2O3 layer grows at the Al2O3/Al interface to a limiting thickness. On the basis of experimental data and atomistic modeling, we show the tunability of the dissociation pathways of H2O molecules with the Al, Al2O3, and Al(OH)3 surface terminations. The fundamental insights may have practical significance for the design of materials and reactions for two seemingly disparate but fundamentally related disciplines of surface passivation and catalytic H2 production from water.

Unraveling the Reaction Mystery of Li and Na with Dry Air

Li and Na metals with high energy density are promising in application in rechargeable batteries but suffer from degradation in the ambient atmosphere. The phenomenon that in terms of kinetics, Li is stable but Na is unstable in dry air has not been fully understood. Here, we use in situ environmental transmission electron microscopy combined with theoretical simulations and reveal that the different stabilities in dry air for Li and Na are reflected by the formation of compact Li₂O layers on Li metal, while porous and rough Na₂O/Na₂O₂ layers on Na metal are a consequence of the different thermodynamic and kinetics in O₂. It is shown that a preformed carbonate layer can change the kinetics of Na toward an anticorrosive behavior. Our study provides a deeper understanding of the often-overlooked chemical reactions with environmental gases and enhances the electrochemical performance of Li and Na by controlling interfacial stability.

A Super Anticorrosive Ultrathin Film by Restarting the Native Passive Film on 316L Stainless Steel

The corrosion resistance of stainless steel is attributed to the extraordinary protectiveness of the ultrathin native passive film (~3 nanometers) on alloy surface. This protectiveness, independent of alloying, can possibly be further increased by modifying the native film to resist corrosion in harsh conditions. However, the modification based on the film itself is extremely difficult due to its rapid, self-limiting growth. Here we present a strategy by using low-temperature plasma processing so as to follow the growth kinetics of the native film. The native oxide film is restarted and can uniformly grow up to ~15 nanometers in a self-limiting manner. High-resolution TEM found that the film exhibited a well-defined, chemical-ordering layered structure. The following corrosion tests revealed that the anodic current density of the alloy decreased by two orders of magnitude in 0.6 M NaCl solution with a remarkable increase of pitting potential. This enhancement is also observed in Fe-Cr alloys with Cr contents above ~10.5 wt.%. The superior protectiveness of the alloy is thus attributed to the continuous and thickened high-quality ultrathin Cr₂O₃ layer in the restarted film.

Flexible-in-rigid polycrystalline titanium nanofibers: a toughening strategy from a macro-scale to a molecular-scale

TiO₂ nanomaterials, especially one-dimensional TiO₂ nanofibers fabricated by electrospinning, have received considerable attention in the past two decades, for a variety of basic applications. However, their safe use and easy recycling are still hampered by the inherently subpar mechanical performance. Here, we toughened polycrystalline TiO₂ nanofibers by introducing Al³⁺-species at the very beginning of electrospinning. The resultant long-and-continuous TiO₂ nanofibers achieved a Young's modulus of 653.8 MPa, which is ca. 25-fold higher than that of conventional TiO₂ nanofibers. Within each nanofiber, amorphous Al₂O₃-based oxide effectively hindered the coalescence of TiO₂ nanocrystals and potentially repaired the surface groves. The solid-state ¹⁷O-NMR spectra further revealed the toughening strategy on a molecular scale, where relatively flexible Ti-O-Al bonds replaced rigid O-Ti-O bonds at the interfaces of TiO₂ and Al₂O₃. Moreover, the modified TiO₂ nanofibers exhibited superb sinter-resistance, without cracking over 900 °C, which was dynamically monitored by TEM. Therefore, flexible-in-rigid TiO₂ fibrous mats can be facilely folded into 3D sponges through origami art. As a potential showcase, the TiO₂ sponges were demonstrated as a duarable and renewable filtrator with a high filtration efficiency of 99.97% toward PM_2.5 and 99.99% toward PM₁₀ after working for 300 min. This work provides a rational strategy to produce flexible oxide nanofibers and gives an in-depth understanding of the toughening mechanism from the macro-scale to the molecular-scale.

Molybdenum Oxide Functional Passivation of Aluminum Dimers for Enhancing Optical-Field and Environmental Stability

In this contribution, we present an experimental and numerical study on the coating of Al plasmonic nanostructures through a conformal layer of high-refractive-index molybdenum oxide. The investigated structures are closely coupled nanodisks where we observe that the effect of the thin coating is to help gap narrowing down to the sub-5-nm range, where a large electromagnetic field enhancement and confinement can be achieved. The solution represents an alternative to more complex and challenging lithographic approaches, and results are also advantageous for enhancing the long-term stability of aluminum nanostructures.

Surface-Condition-Dependent Deformation Mechanisms in Lead Nanocrystals

Serving as nanoelectrodes or frame units, small-volume metals may critically affect the performance and reliability of nanodevices, especially with feature sizes down to the nanometer scale. Small-volume metals usually behave extraordinarily in comparison with their bulk counterparts, but the knowledge of how their sizes and surfaces give rise to their extraordinary properties is currently insufficient. In this study, we investigate the influence of surface conditions on mechanical behaviors in nanometer-sized Pb crystals by performing in situ mechanical deformation tests inside an aberration-corrected transmission electron microscope (TEM). Pseudoelastic deformation and plastic deformation processes were observed at atomic precision during deformation of pristine and surface-oxidized Pb particles, respectively. It is found that in most of the pristine Pb particles, surface atom diffusion dominates and leads to a pseudoelastic deformation behavior. In stark contrast, in surface-passivated Pb particles where surface atom diffusion is largely inhibited, deformation proceeds via displacive plasticity including dislocations, stacking faults, and twinning, leading to dominant plastic deformation without any pseudoelasticity. This research directly reveals the dramatic impact of surface conditions on the deformation mechanisms and mechanical behaviors of metallic nanocrystals, which provides significant implications for property tuning of the critical components in advanced nanodevices.

Preparation of Ti-Al-Si Gradient Coating Based on Silicon Concentration Gradient and Added-Ce

Titanium and titanium alloys have excellent physical properties and process properties and are widely used in the aviation industry, but their high-temperature oxidation resistance is poor, and there is a thermal barrier temperature of 600 °C, which limits their application as high-temperature components. The Self-generated Gradient Hot-dipping Infiltration (SGHDI) method is used to prepare the Ti-Al-Si gradient coating based on the silicon concentration with a compact Ti(Al,Si)3 phase layer, which can effectively improve the high-temperature oxidation resistance of the titanium alloy. Adding cerium can effectively inhibit the generation of the τ2: Ti(AlxSi1−x)2 phase within a certain hot infiltration time so as to form a continuous dense Al2O3 layer to further improve the oxidation resistance of the coating. Studies have found that multiple Ti-Al binary alloy phase layers are formed during the high-temperature oxidation process, which has the effect of isolating oxygen and crack growth, and effectively improving the high-temperature resistance of the coating oxidation performance.

Solution-Processed Organic Photodetectors with Renewable Materials

An organic photodetector prepared by a simple solution method based on renewable citrus pectin with an optimized concentration of aluminum nitrate (AlC05) is introduced herein. The effects of different concentrations of aluminum nitrate on the morphology and optical properties were investigated through various characterization methods. An AlC concentration of 0.5 mg/mL was found to provide the highest on/off ratio and acceptable rise and decay times. Also, the optimized device (Al/AlC0.5/ITO) exhibited good stability and repeatability at a 0.1 V bias under 440 nm visible light. Based on these results, citrus pectin materials were successfully used to fabricate an organic photodetector with a simple and cost-efficient fabrication process, while taking into account environmental commitments.

Stress-Affected Oxygen Reduction Reaction Rates on UNS S13800 Stainless Steel

This work investigates the previously unexplored impact of tensile stress on oxygen reduction reaction (ORR) kinetics of a precipitation-hardened, stainless-steel fastener material, UNS S13800. ORR is known to drive localized and galvanic corrosion in aircraft assemblies and greater understanding of this reaction on structural alloys is important in forecasting component lifetime and service requirements. The mechano-electrochemical behavior of UNSS13800 was examined using amperometry to measure the reduction current response to tensile stress. Mechanical load cycles within the elastic regime demonstrated reversible electrochemical current shifts under chloride electrolyte droplets that exhibited a clear potential dependence. Strain ramping produced current peaks with a strain rate dependence, which was distinct from the chronoamperometric shifts during static tensile load conditions. Finally, mechanistic insight into the dynamic and static responses was obtained by deoxygenation, which demonstrated ORR contributions that were distinct from other reductive processes.

Predicting Fracture Propensity in Amorphous Alumina from Its Static Structure Using Machine Learning

Thin films of amorphous alumina (a-Al₂O₃) have recently been found to deform permanently up to 100% elongation without fracture at room temperature. If the underlying ductile deformation mechanism can be understood at the nanoscale and exploited in bulk samples, it could help to facilitate the design of damage-tolerant glassy materials, the holy grail within glass science. Here, based on atomistic simulations and classification-based machine learning, we reveal that the propensity of a-Al₂O₃ to exhibit nanoscale ductility is encoded in its static (nonstrained) structure. By considering the fracture response of a series of a-Al₂O₃ systems quenched under varying pressure, we demonstrate that the degree of nanoductility is correlated with the number of bond switching events, specifically the fraction of 5- and 6-fold coordinated Al atoms, which are able to decrease their coordination numbers under stress. In turn, we find that the tendency for bond switching can be predicted based on a nonintuitive structural descriptor calculated based on the static structure, namely, the recently developed "softness" metric as determined from machine learning. Importantly, the softness metric is here trained from the spontaneous dynamics of the system (i.e., under zero strain) but, interestingly, is able to readily predict the fracture behavior of the glass (i.e., under strain). That is, lower softness facilitates Al bond switching and the local accumulation of high-softness regions leads to rapid crack propagation. These results are helpful for designing glass formulations with improved resistance to fracture.

Light, strong, and stable nanoporous aluminum with native oxide shell

Aluminum (Al) metal is highly reactive but has excellent corrosion resistance because of the formation of a self-healing passive oxide layer on the surface. Here, we report that this native aluminum oxide shell can also stabilize and strengthen porous Al when the ligament (strut) size is decreased to the submicron or nanometer scale. The nanoporous Al with native oxide shell, which is a nanoporous Al-Al₂O₃ core-shell composite self-organized in a galvanic replacement reaction, is nonflammable under ambient conditions and stable against coarsening near melting temperatures. This material is stronger than conventional foams of similar density consisting of pure Al or Al-based composites, and also lighter and stronger than most nanoporous metals reported previously. Its light weight, high strength, and excellent stability warrant the explorations of functional and structural applications of this material, if more efficient and scalable synthesis processes are developed in the future.

Hydrophobic AlOx Surfaces by Adsorption of a SAM on Large Areas for Application in Solar Cell Metallization Patterning

A resist-free metallization of copper-plated contacts is attractive to replace screen-printed silver contacts and is demonstrated on large-area silicon heterojunction (SHJ) solar cells. In our approach, a self-passivated Al layer is used as a mask during the plating process. In this study, Al/AlO_x or Al₂O₃ plating masks are further functionalized by a self-assembled monolayer (SAM) of octadecyl phosphonic acid (ODPA). The ODPA adsorption is characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy in attenuated total reflectance (FTIR-ATR) (in situ), and contact angle measurements to link the surface chemical composition to wetting properties. The SAM leads to homogeneous hydrophobic surfaces on large-area textured solar cells and planar flexible printed circuit boards (PCBs), which allows reproducible patterning of narrow lines by inkjet printing of an etchant. Selective copper plating is then performed to complete the metallization process and produce Cu contacts in the patterned areas. Silicon heterojunction (SHJ) solar cells metallized by the complete sequence reached up to 22.4% efficiency on a large area.

Self-healing on mismatched fractured composite surfaces of SiC with a diameter of 180 nm

Self-healing on fractured surfaces of silicon carbide (SiC) is highly desirable, to avoid the catastrophic failure of high-performance devices working at extreme environments. Nevertheless, self-healing on a fractured surface of an amorphous and crystalline (AAC) composite structure of a brittle nanowire (NW) has not been demonstrated. In this study, self-healing is demonstrated on mismatched fractured surfaces of the AAC composite structure of a brittle solid for a SiC NW with a diameter of 187 nm. Fracture strength is 10.18 GPa for the AAC structure, recovering 11.7% after self-healing on its mismatched fractured surfaces. To the best of our knowledge, we firstly report the self-healing on mismatched fractured surfaces of the AAC structure for a brittle NW. This is a breakthrough of the previous prediction that self-healing could not be realized on a brittle NW with a diameter over 150 nm. A growth of 3 nm was found after self-healing on the gap induced by mismatched fractured surfaces, which is different from previous reports for pure amorphous and monocrystalline brittle NWs. To reduce the potential energy, coherent rebonding and debonding were performed to realize the atomic migration to fill the gap, resulting in the growth of gap of 3 nm to perform self-healing. Our findings shed light on the potential of self-healing for design and fabrication of next-generation high-performance SiC devices used in the vacuum and aerospace industries.

Surface- and Strain-Mediated Reversible Phase Transformation in Quantum-Confined ZnO Nanowires

The phase stability of ZnO in a quantum-confinement size regime (sub-2-nm) remains fiercely debated. Applying in situ (scanning) transmission electron microscopy, we present the atomistic view of the phase transitions from the original wurtzite structure to an intermediate body-centered tetragonal and h-MgO structure under tensile strain in quantum-confined ZnO nanowires. Strikingly, such structural transitions are reversible after releasing the stress. Further theoretical calculations mirror the transition pathway and provide basic insight into the overall landscape regarding surface- and strain-dependent phase transition behavior. Our results provide the critical piece to solve the puzzle in phase stability of ZnO, which may prove essential for advancing a variety of nanotechnologies, e.g., quantum-dot light-emitting devices.

Breakdown of Native Oxide Enables Multifunctional, Free-Form Carbon Nanotube-Metal Hierarchical Architectures

Passive oxide layers on metal substrates impose remarkable interfacial resistance for electron and phonon transport. Here, a scalable surface activation process is presented for the breakdown of the passive oxide layer and the formation of nanowire/nanopyramid structured surfaces on metal substrates, which enables high-efficiency catalysis of high-crystallinity carbon nanotubes (CNTs) and the direct integration of the CNT-metal hierarchical architectures with flexible free-form configurations. The CNT-metal hierarchical architecture facilitates a dielectric free-energy-carrier transport pathway and blocks the reformation of passive oxide layer, and thus demonstrates a 5-fold decrease in interfacial electrical resistance with 66% increase in specific surface area compared with those without surface activation. Moreover, the CNT-metal hierarchical architectures demonstrate omnidirectional blackbody photoabsorption with the reflectance of 1 × 10^-5 over the range from ultraviolet to terahertz region, which is 1 order of magnitude lower than that of any previously reported broadband absorber material. The synergistically incorporated CNT-metal hierarchical architectures offer record-high broadband optical absorption with excellent electrical and structural properties as well as industrial-scale producibility.

The Aluminum-Ion Battery: A Sustainable and Seminal Concept?

The expansion of renewable energy and the growing number of electric vehicles and mobile devices are demanding improved and low-cost electrochemical energy storage. In order to meet the future needs for energy storage, novel material systems with high energy densities, readily available raw materials, and safety are required. Currently, lithium and lead mainly dominate the battery market, but apart from cobalt and phosphorous, lithium may show substantial supply challenges prospectively, as well. Therefore, the search for new chemistries will become increasingly important in the future, to diversify battery technologies. But which materials seem promising? Using a selection algorithm for the evaluation of suitable materials, the concept of a rechargeable, high-valent all-solid-state aluminum-ion battery appears promising, in which metallic aluminum is used as the negative electrode. On the one hand, this offers the advantage of a volumetric capacity four times higher (theoretically) compared to lithium analog. On the other hand, aluminum is the most abundant metal in the earth's crust. There is a mature industry and recycling infrastructure, making aluminum very cost efficient. This would make the aluminum-ion battery an important contribution to the energy transition process, which has already started globally. So far, it has not been possible to exploit this technological potential, as suitable positive electrodes and electrolyte materials are still lacking. The discovery of inorganic materials with high aluminum-ion mobility—usable as solid electrolytes or intercalation electrodes—is an innovative and required leap forward in the field of rechargeable high-valent ion batteries. In this review article, the constraints for a sustainable and seminal battery chemistry are described, and we present an assessment of the chemical elements in terms of negative electrodes, comprehensively motivate utilizing aluminum, categorize the aluminum battery field, critically review the existing positive electrodes and solid electrolytes, present a promising path for the accelerated development of novel materials and address problems of scientific communication in this field.

Construction of Al-ZnO/CdS photoanodes modified with distinctive alumina passivation layer for improvement of photoelectrochemical efficiency and stability

ZnO/CdS-based nanorod arrays (NRs) are an excellent class of photoanode materials, which possess high photoelectric response for solar-driven water splitting. A highly efficient photoanode system consisting of Al-doped ZnO NRs as effective electron-transfer layers and CdS as a light harvesting layer was rationally designed. Al doping increased the conductivity of ZnO NRs and simultaneously coarsened the surface of ZnO due to expansion of ZnO lattice. The rough surface favoured the growth of a CdS coating layer on it through a successive ionic layer adsorption reaction. The integrated ZnO/CdS photoanode exhibited photocurrent of 10.4 mA cm^-2 at 1.23 V versus RHE (reversible hydrogen potential) and conversion efficiency of 5.75% at 0.38 V versus RHE for 60 SILAR CdS cycles. The coating of a protective Al₂O₃ passivation layer through the direct current magnetron sputtering technique significantly improved the stability of the electrode, and it was better than that of the conventional atomic layer deposition method.

Renewable Cr2O3 Nanolayer on Cr(W)N Surface for Seizure Prevention at Elevated Temperatures

Chromium nitride coating is now the norm for improving the wear resistance of high-performance mechanical components. Even so, to prevent the seizure issue between the contacting interfaces, the prerequisites are oil or solid lubricants which would however lose the lubricating functionality at elevated temperatures due to breakdown or degradation. In this research, we utilize a Cr₂O₃ nanolayer formed on modified Cr(W)N coating to prevent the adhesive seizure for steel-based components. X-ray photoelectron spectroscopy (XPS) analyses show that the chromium oxide can be generated at 200-400 °C. At 400 °C, the Cr₂O₃ nanolayer is in situ formed and maintains a consistent thickness of 2.2 nm due to the oxide renewal during the heating-sliding operation. The in situ, renewable oxide nanolayer provides a novel approach to the technically unsolved seizure problem occurring in high-performance machines operated at elevated temperatures.