Morphological Instability Research Articles

Effect of low-frequency AC forcing on the morphological instability arising in electrodeposition

Effect of low-frequency AC forcing on the morphological instability arising in electrodeposition

Patterning coexisted micro-/nanostructures for consequential camouflage via mechanical constraint harnessed surface instability

Coexisting micro-/nanostructures on a stretchable substrate offer localized functionality with programmability and dynamic regulation. Yet, the combination of different fabrication techniques is challenging. In this paper, a one-step methodology for such a surface is proposed by harnessing wrinkle instability at the targeted area with selected mechanical constraint during soft lithography. Partly covered by patterned constraints, a surface with nanostructures that replicate a template is obtained while the unconstraint part wrinkles as a result of classical morphology instability. The effect of constraints is investigated experimentally to guide the generation of two optical performances, chemical color and structural color, at the coexisting surface structures. A camouflage demonstration is illustrated, utilizing the strict consequence of wrinkle-flattening and structural color redshift upon stretching.

Morphological instability of the G-phase induced a different contribution to hardening of ferrite phase in a duplex stainless steel

G-phase plays an indispensable role in the hardening of ferrite during thermal aging process of the duplex stainless steel. Although there are many studies on quantifying the contribution of spinodal decomposition and G-phase precipitation to the hardening of ferrite, details regarding the influence of G-phase morphology transformation on hardening are lacking. These knowledge gaps hinder a deep understanding of the correlation between G-phase and ferrite hardening, and hence hinder the exploration of new techniques for repairing performance degradation. In this study, after aging at 400 °C and 475 °C, driven by the minimization of interfacial energy, the morphology of G-phase changes from sphere to cube, and consequently the contribution of G-phase to hardening increases significantly. After annealing at 550 °C, the G-phase does not dissolve, but coarsens, resulting in a further increase in the contribution to hardening. The difference from annealing is that after pulsed electric field treatment at 460 °C, the unstable spherical G-phase precipitates are almost completely dissolved, while the stable cubic G-phase is spheroidized. The behaviors of dissolution and spheroidization are driven by the intervention of the electric free energy, and as a result, the hardening caused by G-phase is eliminated or relieved due to the dissolution and refinement of G-phase. It is conceivable that this work is a decisive step to completely repair the deteriorated performance of duplex stainless steel after long-term operation.

Asymptotic stability for a free boundary tumor model with a periodic supply of external nutrients

For tumor growth, the morphological instability provides a mechanism for invasion via tumor fingering and fragmentation. This work considers the asymptotic stability of a free boundary tumor model with a periodic supply of external nutrients. The model consists of two elliptic equations describing the concentration of nutrients and the distribution of the internal pressure in the tumor tissue, respectively. The effect of the parameter μ representing a measure of mitosis on the morphological stability are taken into account. It was recently established in Huang et al. (2019) that there exists a critical value μ∗ such that the unique spherical periodic positive solution is linearly stable for 0μ∗. In this paper, we further prove that the spherical periodic positive solution is asymptotically stable for 0<μ<μ∗ for the fully nonlinear problem.

Post-synthesis treatment of graphene oxide/silica particles nanocomposite with piranha acid for functionalization

The discovery of 2D materials has led researchers to a broad material platform. Their excellent physical, chemical and electrical properties along with the layered structure have found applications in various fields. However, these materials also have limitations and functionalisation is one of the mechanisms that improves their properties. In our previous work, we observed surface-enhanced Raman spectroscopy (SERS) after covalent attachment of protein to the graphene nanocomposite where piranha acid was used to generate the functional groups. The current work describes the synthesis and characterisation of a graphene oxide-silica particle nanocomposite after piranha acid treatment at different time intervals. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy were performed to indicate structural changes which facilitated the protein attachment. The SEM and TEM results indicated that the sample which was piranha acid activated for 3 min displayed better arrangement of silica particles on the graphene sheets with exposition of the highest net surface area in the graphene sheet, compared to the other samples and determined to be the best functionalised nanocomposite for further applications. Morphological instability of the graphene sheets and clustering of silica particles were observed in the samples treated for more than 3 min. Interestingly, the same degree of graphitisation was observed in all the samples when I D /I G ratios {(≤0.99) ≠ 0} were determined by Raman spectroscopy.

Onset of fuzz formation in plasma-facing tungsten as a surface morphological instability

Based on simulation results and theoretical analysis according to an atomistically informed continuum-scale model for the surface morphological response of tungsten plasma-facing component (PFC) in nuclear fusion devices, we demonstrate that the onset of the well-known surface nanostructure (known as ``fuzz'') formation in PFC tungsten is the outcome of a stress-induced surface morphological instability. Specifically, we show that formation and growth of nanotendrils emanating from the exposed surface of PFC tungsten, a precursor to fuzz formation, is caused by a long-wavelength surface morphological instability triggered by compressive stress in the PFC near-surface layer due to over-pressurized helium (He) bubbles forming in this near-surface region through implantation of low-energy He ions from the plasma. Using linear stability theory (LST), we predict the onset of surface growth in response to low-amplitude perturbations from a planar PFC surface morphology and calculate the average spacing between nanotendrils growing from the PFC surface, in excellent agreement with self-consistent dynamical simulations of surface morphological response according to the fully nonlinear surface evolution model starting with random fluctuations from the planar surface morphology that result in nanoscale surface roughness. In addition, we examine the morphological response of the PFC surface to low-amplitude perturbations of very long wavelengths from its planar morphology and interpret fully the simulation results on the basis of a weakly nonlinear tip-splitting instability theory, which predicts a post-instability nanotendril pattern formation with nanotendril separation consistent with the LST predictions regardless of the initial surface perturbation. Finally, we compare our simulation predictions for surface nanotendril growth with experimental measurements of fuzz layer growth on the exposed surface of PFC tungsten. Our simulation results are in very good agreement with the surface growth measurements at the early stages of fuzz growth, further establishing the onset of fuzz formation in PFC tungsten as the outcome of a stress-driven PFC surface morphological instability.

Electroconvection near an ion-selective surface with Butler–Volmer kinetics

We study the effects of interfacial kinetics on the electro-hydrodynamics of ion transport near an ion-selective surface using a combination of linear stability analysis and numerical simulation. The finite kinetics of the electrolyte–electrode interface affects the ion transfer and electroconvection in many ways. On a surface of fixed topography, such as a metal surface of slow and stable ion deposition or covered by a polymer membrane, the finite kinetics reduces the current in one-dimensional ion diffusion/migration, increases the critical voltage for the onset of the electroconvective instability, changes the dynamics of the electroconvection and the overlimiting current, and enhances the lateral ion diffusion within the interfacial layer. The first three effects are indirectly caused by the reaction kinetics and can be characterized by an effective voltage difference across the liquid electrolyte. In comparison, the last effect is controlled by a direct interplay between kinetics and nonlinear electroconvection. Scaling laws for ion transport and features of electroconvection are proposed. We also analyse the linear stability of a surface which evolves under ion deposition and find that the finite kinetics decreases the growth rate of both electroconvective and morphological instabilities and therefore modifies the wavenumber of the most unstable mode.

Alkaline-carbonate-templated carbon: Effect of template nature on morphology, oxygen species and supercapacitor performances

Nanocasting techniques are widely adopted for the fabrication of nanocarbons. However, there is little research on the effect of template nature on the physicochemical properties and performances of the nanocarbons. Herein, we investigated how the morphology, oxygen species as well as supercapacitor performance of the derived nanocarbons were governed by the morphology, surface basicity and thermal stability of MgCO3, CaCO3 and BaCO3 when the alkaline metal carbonates were used as templates. Using MgCO3 as catalytic template, the MgC973 produced after carbonization at 973 K has ultrathin nanosheet morphology (∼20 nm), large specific surface area (952 m2/g), and superior supercapacitor performances, showing high capacitance of 421F/g@1.0 A/g (maintaining 306F/g@20 A/g), and large energy density of 31.1 Wh/kg@1000 W/kg. The superiority can be ascribed to the ultrathin morphology and thermal instability of the MgCO3 template, which upon decomposition generates CO2 and in situ activates the carbon skeleton. Moreover, the surface alkalinity of templates is conducive to the retention of electrochemically active oxygen, and BaC973 is with oxygen content (9.1 atom%) higher than MgC973 (7.9 atom%) and CaC973 (8.8 atom%). The present work provides insightful ideas for the synthesis of unique carbon materials with specific functionalities through the selection of suitable templates.

Silver Nanowire Networks: Ways to Enhance Their Physical Properties and Stability.

Silver nanowire (AgNW) networks have been intensively investigated in recent years. Thanks to their attractive physical properties in terms of optical transparency and electrical conductivity, as well as their mechanical performance, AgNW networks are promising transparent electrodes (TE) for several devices, such as solar cells, transparent heaters, touch screens or light-emitting devices. However, morphological instabilities, low adhesion to the substrate, surface roughness and ageing issues may limit their broader use and need to be tackled for a successful performance and long working lifetime. The aim of the present work is to highlight efficient strategies to optimize the physical properties of AgNW networks. In order to situate our work in relation to existing literature, we briefly reported recent studies which investigated physical properties of AgNW networks. First, we investigated the optimization of optical transparency and electrical conductivity by comparing two types of AgNWs with different morphologies, including PVP layer and AgNW dimensions. In addition, their response to thermal treatment was deeply investigated. Then, zinc oxide (ZnO) and tin oxide (SnO2) protective films deposited by Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD) were compared for one type of AgNW. We clearly demonstrated that coating AgNW networks with these thin oxide layers is an efficient approach to enhance the morphological stability of AgNWs when subjected to thermal stress. Finally, we discussed the main future challenges linked with AgNW networks optimization processes.

(Keynote) Metal Electrodeposition across a Nanometre-Scale Gap

Growth on a quasi 1-dimensional substrate is of fundamental interest for what it can reveal about dendrite formation and morphological instabilities. It also provides a means to fabricate nano-gaps for molecular electronics and sensor applications. Here we present a novel experimental configuration consisting of a metal-insulator-metal sandwich structure, of which the sides are exposed to electrolyte. When an appropriate potential is applied to the top and bottom metal layers, electrodeposition can take place at their edges, which form quasi 1-dimensional electrodes separated by the thickness of the insulator layer (typically 50 nm). As electrodeposition proceeds, metal grows across the gap between the edges of the top and bottom electrodes, and this process may be studied by monitoring the current between them resulting from a known applied bias potential. If the gap between the two electrodes closes smoothly, this current increases exponentially due to tunnelling between them. If, on the other hand, the gap is closed by rough or dendritic growth, the conductance is observed to increase in larger steps corresponding to conductance quantization and smaller steps that we attribute to surface relaxation. Interestingly, we can control how the gap closes, either by employing an additive (benzotriazole for Cu electrodeposition) or by varying the bias applied between the electrodes: increasing the bias appears to favour rough growth due to electric field effects.

Mesoscale Analysis of Interfacial Stability in Solid-State Batteries

The utilization of lithium metal anodes along with inorganic solid electrolytes holds the potential to enhance the energy density, power density and safety of conventional lithium-ion batteries. Despite the theoretical promise of solid electrolytes, the morphological instability of lithium metal anodes during electrochemical operation continues to pose major challenges for the advancement of solid-state batteries. Electrodeposition stability at the solid-solid interface is governed by a wide range of factors including the surface roughness, microstructural arrangement, transport and mechanical properties, relevant to the solid electrolyte and lithium metal. In addition, the competing kinetics of vacancy diffusion and electrochemical reaction alters the metal morphology during discharge and governs the interfacial contact. In this work, a mechanistic understanding of the electrochemical-mechanical interactions underlying the electrodeposition and dissolution stability of lithium metal anodes during fast charge and discharge operation of solid-state batteries will be presented.

Long-Term Cycling Stability of Ni-Rich Li-Ion Batteries Cells Using the Encapsulation of Multifunctional Materials

Ni-rich cathode materials have been considered as a next-generation of the lithium-ion batteries, used for advanced applications such as electric vehicles. Although they have high specific capacities, the major drawbacks of these materials result in the obstacle of commercialization. In this work, the composite materials of high ionic-conductivity metal oxide and conductive carbon materials were coated on the surface of the Ni-rich cathode materials by mechano-fusion technique, forming the core@shell structure to protect the morphological instability, as well as the electrolyte decomposition. Also, the multi-functional shell elevates the Li-ion diffusion and ionic conductivity. Moreover, we firstly reported the novel formation protocol of cylindrical cell type lithium-ion battery (18650). As a result, the optimized 18650 Li-ion batteries show higher overall charge-storage performances.

Irregular growth of the γ′ phase in a Ni-based superalloys under slow cooling rate

The effects of cooling rate on the evolution of γ′ phase were investigated by homogenization treatments performed at the γ′ super-solvus temperature (1160 ℃) with the soaking time of 30 h, following cooled by furnace cooling. Irregular growth and morphological instability of the γ′ phase under slow cooling condition was identified. The shape of γ′ phase was mixture of cubic, octets and dendrites in this process. The growth of the γ′ phases during cooling was consistent with a diffusion-controlled mechanism. Therefore, chemical composition of γ′ phase and γ matrix was analyses, it is found that the γ′ phase was enriched in Al, Ti, Ni and poor in Co, Cr, Fe. Based on the experimental results, the irregular growth process of γ′ phases was discussed, and supersaturation and sufficient diffusivity are the dominated mechanisms of this process.

Surface morphology and mechanical properties changes induced in Ti3InC2 (M3AX2) thin nanocrystalline films by irradiation of 100 keV Ne+ ions

We report on the structure of the Ti3InC2 (M3AX2) thin nanocrystalline films (TNCFs) produced by ion beam sputtering of the Ti, In, and C targets with subsequent thermal annealing, and irradiation by 100 keV Ne+ ions with two different fluences of 1 · 1014 cm−2 and 1 · 1016 cm−2. The Ti3InC2 TNCFs were characterized by a set of microscopic techniques (AFM, STEM/EDS, and HRTEM) as well ion beam analytical methods RBS and NRA. The STEM/EDS analysis revealed that the structure and chemical composition of the Ti3InC2 TNCF remained intact after irradiation with the lower fluence of 1 · 1014 cm−2 Ne+. However, a significant morphological instability was observed after irradiation with the higher fluence of 1 · 1016 cm−2 Ne+. The simulation of the nanobeam electron diffraction (NBED) patterns by using CrystalMaker code has pointed at the existence of new ε-Ti2C0.06 (MXene) structures and separation of Indium (In) from the Ti3InC2 matrix. The analysis by nanoindentation showed that the irradiated Ti3InC2 TNCFs exhibited promising mechanical properties considering Young's modulus and hardness even for the locally disordered structure. M3AX2 TNCFs are a new type of 2D materials designed for applications in extreme environments, such as nuclear facilities with intensive radiation fields. However, the research is still in its infancy and requires further study on the synthesis of high-quality, dense, and homogeneous M3AX2 TNCFs. This work is an initial study related to the radiation resistance of Ti3InC2 TNCFs.

Improving the Morphology Stability of Spiro-OMeTAD Films for Enhanced Thermal Stability of Perovskite Solar Cells.

To guarantee a long lifetime of perovskite-based photovoltaics, the selected materials need to survive relatively high-temperature stress during the solar cell operation. Highly efficient n-i-p perovskite solar cells (PSCs) often degrade at high operational temperatures due to morphological instability of the hole transport material 2,2',7,7'-tetrakis (N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene (Spiro-OMeTAD). We discovered that the detrimental large-domain spiro-OMeTAD crystallization is caused by the simultaneous presence of tert-butylpyridine (tBP) additive and gold (Au) as a capping layer. Based on this discovery and our understanding, we demonstrated facile strategies that successfully stabilize the amorphous phase of spiro-OMeTAD film. As a result, the thermal stability of n-i-p PSCs is largely improved. After the spiro-OMeTAD films in the PSCs were stressed for 1032 h at 85 °C in the dark in nitrogen environment, reference PSCs retained only 22% of their initial average power conversion efficiency (PCE), while the best target PSCs retained 85% relative average PCE. Our work suggests facile ways to realize efficient and thermally stable spiro-OMeTAD containing n-i-p PSCs.

In situ observation of faceted growth and morphological instability of a complex-regular eutectic in Zn−Mg−Al system

The growth dynamics of a three-phase eutectic during solidification of a Zn−3Mg−4Al (wt.%) alloy was investigated using synchrotron X-radiography. It was observed in situ that the three-phase eutectic grows with a macrofaceted morphology after nucleating on primary Mg2Zn11. The facets of the three-phase eutectic are due to the leading growth of Mg2Zn11. Density functional theory based calculations confirm that this leading phase has a highly anisotropic equilibrium shape. It was also observed that the eutectic−liquid interface destabilizes in solidification, generating macrosteps into the constitutionally undercooled liquid. The onset of the interfacial instability may have an orientation dependence due to the crystalline anisotropy of Mg2Zn11.

Confocal microscopy observations of electrical pre-breakdown of bi-layer elastomer dielectrics

At high electric fields, the electrical energy stored in a soft elastomer dielectric can be comparable to the mechanical deformation energy it produces. This has led to the development of a class of electrically controlled, large strain dielectric elastomer actuators for soft robotics and energy harvesting devices. At large electric fields, the electro-mechanically induced deformation can lead to pseudo-periodic surface morphological instabilities which then grow with increasing field into stable pre-breakdown defects prior to final, irreversible electrical breakdown. Under these extremes of combined large electrical and mechanical deformations, the morphological evolution of the pre-breakdown defects has not hitherto been reported. In contrast to the filamentary breakdown of much stiffer dielectrics, fluorescence confocal microscopy reveals an array of defects that evolve through a complex, reversible series of morphologies, transitioning from axi-symmetric “pits” to “crack-like” shapes that can “twist” and deflect, and finally open to form an array of holes. The observations suggest that the transitions, from axi-symmetric pits to flat, slit-like defects and then to an array of holes, are geometric instabilities. The implications for using a soft elastomer layer to increase the dielectric breakdown of a stiffer dielectric are discussed.

Formation and 3D morphology of interconnected α microstructures in additively manufactured Ti-6Al-4V

Three-dimensional characterization methods, such as 3D electron backscatter diffraction (3D-EBSD), have been used to reveal phase transformation and microstructural evolution mechanisms in multi-phase materials such as steel or titanium alloys. While 3D techniques have enabled many findings in steels, fine dual phase microstructures in titanium alloys such as the basketweave structure have been challenging to resolve. Now, advances in 3D-EBSD methods using sectioning with a plasma focused ion beam have allowed in-depth analyses of fine α microstructures. We apply 3D-EBSD to investigate the microstructures formed in Ti-6Al-4V by electron powder bed fusion (E-PBF) using different scanning strategies. Basketweave, acicular, and colony microstructures are produced from linear, Dehoff, and random scanning strategies, respectively. Different types of 3D interconnectivity were revealed in each microstructure including within clusters of platelets in the basketweave microstructure, within a grain boundary allotriomorph in the acicular microstructure, and between platelets in colonies. These observations are discussed in terms of the formation mechanisms of interconnectivity, including sympathetic nucleation, impingement, and morphological instability. Morphological instability was found to potentially play a role in both the basketweave and colony structures while the interconnectivity in the acicular structure likely forms via sympathetic nucleation or impingement. This information allows for a more complete description of the phase evolution of Ti-6Al-4V during thermal cycling in E-PBF than previously available and represents new insights into the complex branching reported in different titanium microstructures.

Spinal alignment shift between supine and prone CT imaging occurs frequently and regardless of the anatomic region, risk factors, or pathology

Computer-assisted spine surgery based on preoperative CT imaging may be hampered by sagittal alignment shifts due to an intraoperative switch from supine to prone. In the present study, we systematically analyzed the occurrence and pattern of sagittal spinal alignment shift between corresponding preoperative (supine) and intraoperative (prone) CT imaging in patients that underwent navigated posterior instrumentation between 2014 and 2017. Sagittal alignment across the levels of instrumentation was determined according to the C2 fracture gap (C2-F) and C2 translation (C2-T) in odontoid type 2 fractures, next to the modified Cobb angle (CA), plumbline (PL), and translation (T) in subaxial pathologies. One-hundred and twenty-one patients (C1/C2: n = 17; C3-S1: n = 104) with degenerative (39/121; 32%), oncologic (35/121; 29%), traumatic (34/121; 28%), or infectious (13/121; 11%) pathologies were identified. In the subaxial spine, significant shift occurred in 104/104 (100%) cases (CA: *p = .044; T: *p = .021) compared to only 10/17 (59%) cases that exhibited shift at the C1/C2 level (C2-F: **p = .002; C2-T: *p < .016). The degree of shift was not affected by the anatomic region or pathology but significantly greater in cases with an instrumentation length > 5 segments (“∆PL > 5 segments”: 4.5 ± 1.8 mm; “∆PL ≤ 5 segments”: 2 ± 0.6 mm; *p = .013) or in revision surgery with pre-existing instrumentation (“∆PL presence”: 5 ± 2.6 mm; “∆PL absence”: 2.4 ± 0.7 mm; **p = .007). Interestingly, typical morphological instability risk factors did not influence the degree of shift. In conclusion, intraoperative spinal alignment shift due to a change in patient position should be considered as a cause for inaccuracy during computer-assisted spine surgery and when correcting spinal alignment according to parameters that were planned in other patient positions.

Numerical Analysis of Solidification Behavior during Laser Welding Nickel-based Single-crystal Superalloy Part IV: Optimization of Thermo-metallurgical Factors

Important metallurgical factors, such as alloying aluminum redistribution, supersaturation and undercooling of dendrite tip around solid/liquid interface, are separately optimized to alleviate stray grain formation and columnar/equiaxed transition (CET) with series of welding conditions and provide a very efficient method for microstructure control through modification of growth kinetics of dendrite tip under nonequilibrium solidification conditions of ternary Ni-Cr-Al molten pool. Asymmetrical (001)/[110] welding configuration is inferior to symmetrical (001)/[100] welding configuration, because overall area-weighted alloying redistribution, supersaturation and undercooling of dendrite tip throughout the solid/liquid interface of weld pool are consistently severer to exacerbate solidification behavior and microstructure development and incur morphology instability of columnar/equiaxed transition. High heat input, such as combination of higher laser power and slower welding speed, monotonically increases aluminum enrichment, supersaturation and undercooling of dendrite tip near solidification interface to simultaneously deteriorate nucleation and growth of stray grain formation and weaken columnar dendrite morphology, while low heat input, such as combination of lower laser power and faster welding speed, decreases solute buildup, relieves supersaturation and beneficially suppresses dendrite tip undercooling to minimize equiaxed dendrite morphology in the crack-susceptible region, and thereby facilitate single-crystal epitaxial growth with decrease of thermo-metallurgical factors for columnar/equiaxed transition in order to provide prerequisite for optimization of welding conditions. Favorable solidification conditions are obtainable with preferential crystallographic orientation to eliminate columnar/equiaxed transition under which the epitaxy of single-crystal metallurgical properties across fusion boundary of substrate is predominantly promoted to essentially reduce stray grain formation in (001)/[100] welding configuration, and is kinetically capable of significant reduction of microstructure anomalies and nonuniform solidification behavior. The useful relationship among welding conditions, alloying aluminum redistribution, supersaturation and undercooling of dendrite tip is properly established within dendrite stability range through thorough analysis. In addition, the validation of theoretical predictions is fairly reasonable by the experiment results. It is worth that the contributions of kinetics-related solidification phenomena with advancement of solid/liquid interface are imposed altogether to understand why stray grain formation occurs on the basis of controlling mechanism of minimum undercooling or minimum velocity by the reproducible methodology procedure.