Formation Of Nanovoids Research Articles

Deformation mechanisms at the tip of internal fatigue cracks in vacuum and in the presence of an air environment in a Ti alloy

Ultrasonic fully reversed tension fatigue tests have been performed in the Very High Cycle Fatigue (VHCF) regime (NR>107−108cycles) on Ti-6Al4V specimens containing a controlled internal notch. Two sets of samples have been used. The first one contains a central chimney along the specimen longitudinal axis which brings air to the internal notch; in the second series the notches are not connected to the surface. The microstructure present below the fracture surface of the broken specimens has been studied by electron microscopy (EBSD, TKD and TEM). The formation of nanograins and nanovoids was observed below the surface of the cracks growing in a vacuum environment but not below the surface of cracks connected with ambient air. In the latter case extensive striations were observed. Below each striation the formation of tensile {101̄2} twins was observed.

Nanofoamed Polyamide Membranes: Mechanisms, Developments, and Environmental Implications.

Thin film composite (TFC) polyamide membranes have been widely applied for environmental applications, such as desalination and water reuse. The separation performance of TFC polyamide membranes strongly depends on their nanovoid-containing roughness morphology. These nanovoids not only influence the effective filtration area of the polyamide film but also regulate the water transport pathways through the film. Although there have been ongoing debates on the formation mechanisms of nanovoids, a nanofoaming theory─stipulating the shaping of polyamide roughness morphology by nanobubbles of degassed CO2 and the vapor of volatile solvents─has gained much attention in recent years. In this review, we provide a comprehensive summary of the nanofoaming mechanism, including related fundamental principles and strategies to tailor nanovoid formation for improved membrane separation performance. The effects of nanovoids on the fouling behaviors of TFC membranes are also discussed. In addition, numerical models on the role of nanovoids in regulating the water transport pathways toward improved water permeance and antifouling ability are highlighted. The comprehensive summary on the nanofoaming mechanism in this review provides insightful guidelines for the future design and optimization of TFC polyamide membranes toward various environmental applications.

Enhancing separation performance of thin film nanocomposite membranes by self-etching of polydopamine-modified calcium carbonate during interfacial polymerization process

The acid-accepting and gas-generating properties, along with the formation of extensive nanovoids, make CaCO3 nanoparticles attractive as sacrificial nanofillers for fabricating thin-film composite (TFN) nanofiltration membranes. However, challenges such as severe agglomeration and limited acid etching efficiency of the nanoparticles hinder the pursuit of superior membrane performance. In this study, the dispersion and compatibility of CaCO3 nanoparticles within the membrane matrix were improved by modifying them with a polydopamine (PDA) coating. The abundant phenolic hydroxyl groups of PDA enhanced the hydrophilicity and negative charges of membrane surface. Additionally, the PDA capsule increased the etching extent of the CaCO3 nanoparticles by improving the nanoparticle dispersion and generating additional acid, which created sufficient water channels within the polyamide layer. It was demonstrated that incorporating 45 μg/cm2 of PDA-modified CaCO3 nanoparticles doubled the water permeance to 16.7 LMH/bar, while maintaining a low molecular weight cut-off of 249 Da and achieving rejections over 90 % for five types of per- and polyfluorinated substances. The PDA modification also overcame the membrane stability issue caused by the agglomerated CaCO3 nanoparticles. This study provides a novel strategy for applying properly modified self-etching nanofillers in TFN membrane development, showing great potential for water treatment applications.

Mechanical performance of amorphous diamond-like carbon nanowires

Amorphous carbon nanowires (NW) are a fundamental piece in the study of novel nanoarchitectures with unexpected mechanical properties. However, the failure mechanism of amorphous carbon NW is still missing. In this study, classical molecular dynamics was employed to conduct stress-strain tests to address the mechanical response of amorphous carbons NW with different radii. This research characterizes the mechanical properties of aC NW with different sp3 contents, including Youngs modulus and ultimate tensile stress. Our study reveals that plastic deformation is mediated by chemical modifications in carbon bonding, leading to transitions from sp2 to sp3 and vice versa. We observed that denser amorphous carbon materials undergo a transition from sp3 to sp2 bonding prior to failure, facilitated by the recombination of sp2 clusters. Additionally, plastic deformation in amorphous carbon NW is facilitated by the formation of shear transformation zones and nanovoids, with the deformation mechanism strongly dependent on the average coordination of carbon atoms. These insights provide valuable information for designing nanomaterials with tailored mechanical properties.

Revealing the degradation pathways of layered Li-rich oxide cathodes.

Layered lithium-rich transition metal oxides are promising cathode candidates for high-energy-density lithium batteries due to the redox contributions from transition metal cations and oxygen anions. However, their practical application is hindered by gradual capacity fading and voltage decay. Although oxygen loss and phase transformation are recognized as primary factors, the structural deterioration, chemical rearrangement, kinetic and thermodynamic effects remain unclear. Here we integrate analysis of morphological, structural and oxidation state evolution from individual atoms to secondary particles. By performing nanoscale to microscale characterizations, distinct structural change pathways associated with intraparticle heterogeneous reactions are identified. The high level of oxygen defects formed throughout the particle by slow electrochemical activation triggers progressive phase transformation and the formation of nanovoids. Ultrafast lithium (de)intercalation leads to oxygen-distortion-dominated lattice displacement, transition metal ion dissolution and lithium site variation. These inhomogeneous and irreversible structural changes are responsible for the low initial Coulombic efficiency, and ongoing particle cracking and expansion in the subsequent cycles.

Hydrogen trapping and embrittlement in a second-generation Ni-based single crystal superalloy

This study explores the hydrogen trapping capability and hydrogen embrittlement (HE) of a second-generation single crystal (SX) superalloy. Embrittlement susceptibility is greater as the duration of hydrogen charging is extended. The presence of hydrogen induces cleavage, micro-crack formation and denser slip traces. Moreover, hydrogen facilitates the formation of nano-voids and the activation of high-density dislocations, leading to denser slip bands which serve as initiation sites for micro-cracks. In addition, the desorption activation energy of hydrogen trapped at dislocations and soluble in the γ matrix is 33.7 kJ/mol, and hydrogen trapped at the γ/γ′ interface and vacancies is 42.4 kJ/mol. First-principles calculations have indicated that hydrogen reduces the binding strength at the γ/γ′ interface, which promotes the propagation of micro-cracks.

The beginning of iron corrosion - high-resolution visualization with 3D electron tomography

Understanding the genesis and early-phase reactions of iron corrosion is essential for the early detection, mitigation and prevention of metal degradation. In this work, high-resolution 3D tomography of metallic iron oxidation was acquired using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). In particular, dendritic capillaries (<0.5 nm) were observed during the initial oxidation of fresh nanoscale zero-valent iron due to the differential oxygen diffusion and iron atoms migration. This observation led to the proposal of a nanoscale “pothole” model for early-phase corrosion, wherein hollowing out of the metal nanoparticle and formation of nanovoids beneath the iron/oxide interface through Kirkendall effect. Coalescence of the nanocapillaries results in the ultimate collapse of metal structure and/or functional failure. Using nanoscale zero-valent iron as a research model, this work provides unprecedented insights into the nano- and atomic-scale mechanisms of iron oxidation, paving the way for advanced detection and prevention strategies for iron corrosion.

Subsurface damage evolution of β-Ga2O3 (010) substrates during lapping and chemical mechanical polishing

For semiconductor single crystal substrates, subsurface damage (SSD) is an important factor for device failure. For β-Ga2O3 with two cleavage planes, the SSD is the key issue that needs to be addressed. In this work, β-Ga2O3 (010) substrates were lapped and polished precisely. The formation and evolution mechanisms of SSD were analyzed by cross-section transmission electron microscopy and wet chemical etching techniques. The linear distribution of scratches was found on the substrates after lapping. After chemical mechanical polishing (CMP), the surface scratches were eliminated. However, the SSD layer may still exist under the surface. Chemical etching was developed to identify the "invisible damage" to the CMP-finished (010) substrate. The SSD including nanocrystalline, tilt boundary, twist boundary, and high-density dislocations in β-Ga2O3 was first reported. Distinctively, there was no slip occurrence in the SSD of the (010) plane different from the (100) and (001) planes. The Schmid factor of multiple slip systems was calculated to analyze the cause of no slip on the subsurface. The formation mechanism of SSD was believed that when the slip system could not be activated, the stress was released through rotation and distortion of the crystal plane. The accumulation of grain boundary dislocations at twist grain boundary junctions provided a driving force for the formation of nanovoids. The relationship of substrate damage evolution and crystal strain with different machining parameters was established by high-resolution X-ray diffraction and Raman spectra. A non-damaging β-Ga2O3 (010) substrate with a surface roughness of less than 0.2 nm was obtained by the optimized CMP process. The research results had guiding significance for damage control of β-Ga2O3 and other brittle crystals in ultra-precision machining.

Cosolvent-assisted construction of high-performance reverse osmosis membranes with multilayer nanovoid structures: Effect of amides in aqueous phase

The cosolvent-assisted interfacial polymerization (CAIP) has garnered significant attention due to its capacity to easily and effectively modify the performance of polyamide (PA) reverse osmosis (RO) membranes. To gain deeper understanding of the mechanism of a specific type of cosolvents on the IP reaction, namely N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N,N-dimethylpropionamide (DMPr) were used as aqueous-phase cosolvents to systematically investigate their effects on the membrane structures and properties under different conditions. Amide cosolvents in the aqueous phase facilitated the diffusion of the amine monomer and the formation of nanovoids within the PA layer as the C-chain length of amides increased. When 1,2-dichloroethane was also included in the organic phase, the extent of the two-phase miscible zone was further increased with its assistance, which contributed to generating the larger nanovoids within the layer, thicker PA layer and extensive exterior PA layers, as well as significantly enhancing the water permeability and selectivity of membranes. Importantly, this study successfully prepared high-performance RO membranes with multilayer nanovoid structures in situ by introducing cosolvents in the IP reaction with permeability up to 2.26 L·m−2·h−1·bar−1, providing theoretical and experimental bases for further enrichment of CAIP.

Prompted by surface-engineered honeycomb Turing nanostructures: Reflections on polyamide structure, growth and performance

The robust correlation between the performance of thin-film composite (TFC) reverse osmosis membranes and the polyamide (PA) active layer determines the necessity of precise design of the PA layer, and this regulation primarily involves the surface morphology and internal characteristics of the PA layer. In this study, we fabricated honeycomb Turing structure or nanovoid-rich TFC series membranes using polar/nonpolar organic phase additives. Characterization and simulation results confirmed that the enhancement of membrane performance relied more on the abundant nanovoids within the PA layer rather than the ordered honeycomb lattice surface. Notably, the TFC-nDCE membrane with multilayer nanovoids exhibited an approximately 89 % improvement in water permeability while maintaining satisfactory salt rejection (98.29 %). We emphasized the significant role of nanovoids in the diffusion process of amine monomers and investigated the potential relationship between nanovoids and the degree of surface cross-linking. Building upon this, a new insight into the formation of nanovoids induced by PA anisotropic growth was presented through a careful analysis of the amine monomer diffusion rate and the morphological evolution of nanovoids. The structure-property relationship between internal structures, surface morphologies, and performances was systematically and thoroughly investigated, pointing out the direction for the rational design of TFC membranes.

Tuning chemical short-range order for simultaneous strength and toughness enhancement in NiCoCr medium-entropy alloys

The pursuit of enhancing strength and toughness remains a critical endeavor in the field of structural materials. This study explores two distinct strategies to overcome the traditional strength-toughness trade-off. Specifically, we manipulate the chemical composition and short-range order (SRO) of the NiCoCr medium-entropy alloy, which has shown remarkable fracture toughness in recent experiments. Utilizing molecular dynamics simulations, we uncover nano-scale deformation mechanisms during crack propagation. Our findings highlight that optimizing the SRO degree leads to improvements in both atomic scale strength and toughness defined as the area underneath stress-strain curves from MD simulations. In contrast, a trade-off between strength and toughness persists when only manipulating the Ni content in the NiCoCr alloy. Based on the simulation results, we establish a strong correlation between toughness, strength, surface energies, and unstable stacking fault energies. These factors are influenced by the chemical composition and SROs in NiCoCr, with SROs acting as strong obstacles to dislocations, thereby contributing to additional strength. The exceptional toughness of NiCoCr with SRO arises from a synergy of intrinsic and extrinsic mechanisms, including dislocation glide, nanobridging during nanovoid coalescence and zigzag crack path. It is found that, in the presence of SRO, intrinsic toughening mechanisms usually associated with crack tip blunting and dissipation can also facilitate the onset of extrinsic toughening mechanisms of nanobridging and zig-zag crack path associated with nanovoid formation and coalescence. This study emphasizes the importance of tailoring SRO in designing materials with enhanced strength and toughness.

Beyond nothingness in the formation and functional relevance of voids in polymer films

Voids—the nothingness—broadly exist within nanomaterials and impact properties ranging from catalysis to mechanical response. However, understanding nanovoids is challenging due to lack of imaging methods with the needed penetration depth and spatial resolution. Here, we integrate electron tomography, morphometry, graph theory and coarse-grained molecular dynamics simulation to study the formation of interconnected nanovoids in polymer films and their impacts on permeance and nanomechanical behaviour. Using polyamide membranes for molecular separation as a representative system, three-dimensional electron tomography at nanometre resolution reveals nanovoid formation from coalescence of oligomers, supported by coarse-grained molecular dynamics simulations. Void analysis provides otherwise inaccessible inputs for accurate fittings of methanol permeance for polyamide membranes. Three-dimensional structural graphs accounting for the tortuous nanovoids within, measure higher apparent moduli with polyamide membranes of higher graph rigidity. Our study elucidates the significance of nanovoids beyond the nothingness, impacting the synthesis‒morphology‒function relationships of complex nanomaterials.

Nanovoid formation mechanism in nanotwinned Cu

Nanotwinned metals have been intensely investigated due to their unique microstructures and superior properties. This work aims to investigate the nanovoid formation mechanism in sputter-deposited nanotwinned Cu. Three different types of epitaxial or polycrystalline Cu films are fabricated by magnetron sputtering deposition technique. In the epitaxial Cu (111) films deposited on Si (110) substrates, high fractions of nanovoids and nanotwins are formed. The void size and density can be tailored by varying deposition parameters, including argon pressure, deposition rate, and film thickness. Interestingly, nanovoids become absent in the polycrystalline Cu film deposited on Si (111) substrate, but they can be regained in the epitaxial nanotwinned Cu (111) when deposited on Si (111) substrate with an Ag seed layer. The nanovoid formation seems to be closely associated with twin nucleation and film texture. Based on the comparative studies between void-free polycrystalline Cu films and epitaxial nanotwinned Cu films with nanovoids, the underlying mechanisms for the formation of nanovoids are discussed within the framework of island coalescence model.Graphical abstract

Novel polyaryl isatin polyelectrolytes with flexible monomers for anion exchange membrane fuel cells

The stability of the anion exchange polyelectrolytes is closely tied to the durability of anion exchange membrane fuel cells. Given their excellent alkali resistance, ether-free polymer backbones have attracted considerable attention for use in anion-exchange polyelectrolytes. Herein, a series of side-chain-type poly(terphenylene-co-dibenzyl N-(6-bromohexyl)-isatin) copolymers (QAPTDBHI-n) were synthesized via superacid-catalyzed polymerization and a subsequent quaternization reaction. The cyclic cardo groups in the backbones enabled the formation of nanovoids, which facilitated ionic clusters’ self-aggregation. The introduction of flexible bibenzyl monomers provided the polymers with a spatially foldable structure, promoting the aggregation of ion clusters and strengthening interchain interactions. Compared with the membrane without flexible monomers, QAPTDBHI-n membranes exhibited higher hydroxide conductivity (>115 mS cm−1 at 80 °C), outstanding mechanical stability (tensile strength: 50.91–60.76 MPa, elongation at break: 14.27%–26.06%), and superior alkali resistance (1 M NaOH at 80°C for >1500 h and <9.5% OH− conductivity loss). Regarding fuel cell performance, the AEMFC based on QAPTDBHI-40 ionomer achieved a peak power density of 1.14 W cm−2 (H2–O2) and 0.86 W cm−2 (H2-Air, CO2 free) at 80 °C due to its higher water uptake, OH− conductivity, and lower phenyl concentration.

Unraveling the nano-cracking mechanism in aged asphalt binder with consideration of rejuvenation effects

For achieving the sustainable utilization of reclaimed asphalt pavement, a popular approach is to incorporate rejuvenators to restore the cracking resistance of aged asphalt. However, research on the nanoscale rejuvenation mechanisms and the relationship with the cracking behavior is limited. To address this challenge, this study utilizes molecular dynamics simulations to explore the fundamental cracking mechanism in aged asphalt and elucidate how rejuvenators affect this process. A multiphysics analysis, including mechanical response, morphology relationship, energy evolution, deagglomeration and lubrication behavior is developed to comprehend the nano-cracking mechanism. Computational findings reveal that the cracking failure originates form cavitation, and the process primarily includes nanovoid formation, enlargement, filamentation, and separation. Oxidation enhances the aggregation of asphaltene molecules and reduces intermolecular rearrangement and mobility, which fundamentally accelerates the nano-cracking process. Owing to the diffusion of rejuvenators and their disturbances to asphaltene π-π stacking, rejuvenators can penetrate into aged asphalt molecules, lubricate and reduce intermolecular friction. This facilitates molecular rearrangement during the cracking process, thereby improving the cracking resistance and energy performance of aged asphalt binder. The finding from this study contributes to elucidating the fundamental cracking mechanism in rejuvenated asphalt binder and promoting the sustainable utilization of recycled pavement materials.

Thermal strain relaxation of GaAs overgrown on nanovoid based Ge/Si substrate

The integration of high quality III-V materials-based devices on the Si platform is considered as an enabling path towards achieving the long-standing goal of creating low-cost/high-performance devices. However, the heteroepitaxy of III-V compounds on a Si substrate generates high stress due to the difference in thermal expansion coefficient leading notably to crack generation. Here, we report on the reduction of the thermal strain of GaAs and Ge deposited on a Si substrate. The method is based on the formation of buried high density nanovoids by electrochemical etching of Ge/Si virtual substrate followed by direct epitaxial growth of Ge and GaAs. The thermal strain has been investigated for GaAs epilayer by High-Resolution X-Ray Diffraction and Raman spectroscopy showing a 30% decrease in strain. The strain reduction relies on the nanovoids-mediated thermal strain accommodation. Furthermore, photoluminescence measurements showed a 10-fold increase of the intensity, resulting in an improved optical and crystalline quality, making this virtual substrate promising for III-V based devices on Silicon.

Atomic insight into the nano-cracking behaviour of bitumen: considering oxidative aging effects

Experimental understanding of the nanoscale cracking mechanism in bitumen is challenging. This paper adopts molecular dynamics simulation to study the loading-induced cracking behaviour from four aspects: (1) stress-displacement states; (2) morphology evolution; (3) energy contribution; and (4) the relationship with chemical compositions. Simulation results indicated that the nano-cracking process can be divided into the following stages: increase of free volume (stage I), formation of nanovoids, (stage II), nucleation and propagation (stage III), filamentation (stage IV) and separation (stage V). The density drop was observed in the area where saturates agglomerated, which potentially caused the formation of nanovoids. Energy analysis indicated that the cracking behaviour was related to the non-bond interaction, especially van der Waals interaction. Oxidative aging increased the intermolecular bonding and reduced the molecular mobility, which resulted in larger tensile strength but lower ductility. In addition, the effects of temperature on the nano-cracking behaviour were also presented.

The Piezoresponse in WO3 Thin Films Due to N2-Filled Nanovoids Enrichment by Atom Probe Tomography.

Tungsten trioxide (WO3) is a versatile n-type semiconductor with outstanding chromogenic properties highly used to fabricate sensors and electrochromic devices. We present a comprehensive experimental study related to piezoresponse with piezoelectric coefficient d33 = 35 pmV-1 on WO3 thin films ~200 nm deposited using RF-sputtering onto alumina (Al2O3) substrate with post-deposit annealing treatment of 400 °C in a 3% H2/N2-forming gas environment. X-ray diffraction (XRD) confirms a mixture of orthorhombic and tetragonal phases of WO3 with domains with different polarization orientations and hysteresis behavior as observed by piezoresponse force microscopy (PFM). Furthermore, using atom probe tomography (APT), the microstructure reveals the formation of N2-filled nanovoids that acts as strain centers producing a local deformation of the WO3 lattice into a non-centrosymmetric structure, which is related to piezoresponse observations.

Demystifying the Role of Surfactant in Tailoring Polyamide Morphology for Enhanced Reverse Osmosis Performance: Mechanistic Insights and Environmental Implications.

Surfactant-assisted interfacial polymerization (IP) has shown strong potential to improve the separation performance of thin film composite polyamide membranes. A common belief is that the enhanced performance is attributed to accelerated amine diffusion induced by the surfactant, which can promote the IP reaction. However, we show enhanced membrane performance for Tween 80 (a common surfactant), even though it decreased the amine diffusion. Indeed, the membrane performance is closely related to its polyamide roughness features with numerous nanovoids. Inspired by the nanofoaming theory that relates the roughness features to nanobubbles degassed during the IP reaction, we hypothesize that the surfactant can stabilize the generated nanobubbles to tailor the formation of nanovoids. Accordingly, we obtained enlarged nanovoids when the surfactant was added below its critical micelle concentration (CMC). In addition, both the membrane permeance and selectivity were enhanced, thanks to the enlarged nanovoids and reduced defects in the polyamide layer. Increasing the concentration above CMC resulted in shrunken nanovoids and deteriorated performance, which can be ascribed to the decreased stabilization effect caused by micelle formation. Interestingly, better antifouling performance was also observed for the surfactant-assisted membranes. Our current study provides mechanistic insights into the critical role of surfactant during the IP reaction, which may have important implications for more efficient membrane-based desalination and water reuse.

(Invited) Computational Insights to Dielectric Hybrid Glasses Under Nanoscale Confinement for Next Generation Devices

Low and Ultra-k dielectric organosilicate glasses (OSG) are widely used in device interconnect applications such as shallow trench isolation as insulating units. However, the process of gap filling between conducting units has been challenging. One of the challenges is related to the undesired formation of nanovoids in the low-k dielectric material confined inside the trench geometry. Importantly, the effects of such nanoscale device constraints on the structure and mechanical reliability of low-k dielectric hybrid materials are not very well understood. To close this lack of understanding and guide the related experimental efforts we performed molecular dynamics simulations where we explore the role of the molecular structure and connectivity of different low-k dielectric OSG precursors under nanoscale confinement, and predict the resulting elastic and fracture properties. We demonstrated that hyperconnected OSG precursors with aromatic rings, such as the 1,3,5-benzene molecule, pack more homogenously under confinement leading to smaller nanovoids and better mechanical reliability compared to conventionally bridged OSG precursors, such as the ethylene bridged Et-OCS molecule.