Skin-Inspired, Highly Sensitive, Broad-Range-Response and Ultra-Strong Gradient Ionogels Prepared by Electron Beam Irradiation

Skin, characterized by its distinctive gradient structure and interwoven fibers, possesses remarkable mechanical properties and highly sensitive attributes, enabling it to detect an extensive range of stimuli. Inspired by these inherent qualities, a pioneering approach involving the crosslinking of macromolecules through in situ electron beam irradiation (EBI) is proposed to fabricate gradient ionogels. Such a design offers remarkable mechanical properties, including excellent tensile properties (>1000%), exceptional toughness (100 MJ m-3), fatigue resistance, a broad temperature range (-65-200°C), and a distinctive gradient modulus change. Moreover, the ionogel sensor exhibits an ultra-fast response time (60 ms) comparable to skin, an incredibly low detection limit (1 kPa), and an exceptionally wide detection range (1 kPa-1 MPa). The exceptional gradient ionogel material holds tremendous promise for applications in the field of smart sensors, presenting a distinct strategy for fabricating flexible gradient materials.

Spontaneous Compositional-Interfacial Co-Modification Engineering via Ion Exchange Reaction Between Perovskite and Electron-Transporting Layer for Exceptionally Long-Term Stability of Photovoltaics

The long-term stability of perovskite solar cells (PSCs) is still challenging for commercialization and mainly linked to the life span of perovskite films. Herein, a spontaneous compositional-interfacial co-modification strategy is developed based on the ion exchange reaction by introducing ammonium hexafluorophosphate (NH4PF6) into antisolvent to form gradient structures through a simple one-step solvent engineering. With the assistance of the ion exchange reaction, NH4PF6 forms a multifunctional structure to protect perovskite films from both internal and external factors for the exceptionally long-term stability of photovoltaics. The reason for this is linked to the high hydrophobicity of NH4PF6 for preventing H2O invasion, suppressing ion migration by forming hydrogen bonding, and reducing perovskite defects. The resulting unencapsulated devices show exceptionally long-term stability under standardized the International Summit on Organic Photovoltaic Stability (ISOS) protocols, with over 94%, 81%, and 83% retained power conversion efficiencies after aging tests under N2 (ISOS-D-1I), ambient air (ISOS-D-1), and 85 °C (ISOS-D-2I) for 14016, 2500, and 1248 h, respectively. These performances compare well with the state-of-the-art stability of inverted PSCs. Further investigations are conducted to study the evolution of macroscopic morphology and microscopic crystal structure in aged perovskite films, aiming to provide evidence supporting the aforementioned improvements in stability.

Dual-Responsive Gradient Structured Actuator via Photopolymerization-Induced Diffusion

Stimuli-responsive materials have recently gained significant attention in the field of soft robotics, sensors, and biomimetic devices. The most facile way for the fabrication of such materials remains to endow bilayer structures which are fabricated with the combination of active and passive layers. Although, easily fabricated, these structures suffer from the generation of stress points between connection areas. In this work we develop a method to create a thin film with controlled cross-link variation across its thickness. The cross-link gradient is achieved through polymerization induced diffusion of dithiol molecules in thiol-ene network. As a result, the film exhibits bending deformation upon illumination with light or exposure to a chemical solvent, thereby demonstrating dual responsiveness. Light actuation of the film is achieved via photothermal effects due to the incorporation of dye into the system which can absorb UV light and heat the network. While solvent induced actuation is due to anisotropic swelling. Furthermore, the straightforward fabrication procedure allows for the creation of more complex deformations by patterning the film using a photomask during photopolymerization.

In Situ Construction of Morphologically Different Hydroxyapatite-Mineralized Structures on a Three-Dimensional Bionic Chitin Scaffold

Slow healing at the tendon-bone interface is a prominent factor in the failure of tendon repair surgeries. The development of functional biomaterials with 3D gradient structures is urgently needed to improve tendon-bone integration. The crystalline form of hydroxyapatite (HAP) has a crucial impact on cell behavior, which directly influences protein adsorption, such as bone morphogenetic protein 2, the adhesion, proliferation, and osteogenic differentiation with cells. This work aimed to generate gradient mineral structures in situ by stabilizing calcium and phosphate ions using a polymer-induced liquid precursor process. To regulate the crystalline growth of HAP at the interface of β-chitin, this work made use of the surface properties of the organic matrix found in cuttlefish bone. These techniques allowed us to prepare an organic-inorganic composite gradient scaffold comprising plate-like HAP mineralized in situ on the surface of the scaffold and fibrous HAP in the scaffold's interior. Organic-inorganic composite gradient materials are anticipated for use in tendon-bone healing produced via the in situ construction of gradient-distributed HAP mineralization layers having varying crystalline morphologies on chitin scaffolds that possess a three-dimensional bionic structure.

Scaled Production of Functionally Gradient Thin Films Using Slot Die Coating on a Roll-to-Roll System

Polymer thin films with a cross-web gradient structure is a burgeoning area of research, having received more attention in the last two decades, for improvements in the performance and material properties. Such patterned films have been fabricated using several techniques, but in practice these techniques are non-scalable, material-dependent, wasteful, and not highly efficient. Slot die coating, a well-known scalable manufacturing process, is used to fabricate gradient polymer thin films which will be investigated herein. By incorporating slot die with the custom roll-to-roll imaging system, gradient thin films are successfully fabricated by forcing two fluidic materials into the slot die simultaneously and by manipulating the viscous, diffusive, and inertial forces. The materials will be allowed to intermix, with the aim of having approximately a 50% mix along the centerline of any two contiguous stripes. Moreover, several characterizations such as FTIR, UV-vis spectroscopy, and SEM are performed to assess the quality of the gradient polymer thin films. The gradient structure fabricated using functional and nonfunctional materials has successfully improved the functional properties compared to fully blended two materials. This work will provide an understanding of the mechanisms to obtain gradient polymer thin-film structures that exhibit the desired geometric structure and performance.

Dual-Targeted Metal Ion Network Hydrogel Scaffold for Promoting the Integrated Repair of Tendon-Bone Interfaces

The tendon-bone interface has a complex gradient structure vital for stress transmission and pressure buffering during movement. However, injury to the gradient tissue, especially the tendon and cartilage components, often hinders the complete restoration of the original structure. Here, a metal ion network hydrogel scaffold, with the capability of targeting multitissue, was constructed through the photopolymerization of the LHERHLNNN peptide-modified zeolitic imidazolate framework-8 (LZIF-8) and the WYRGRL peptide-modified magnesium metal-organic framework (WMg-MOF) within the hydrogel scaffold, which could facilitate the directional migration of metal ions to form a dynamic gradient, thereby achieving integrated regeneration of gradient tissues. LZIF-8 selectively migrated to the tendon, releasing zinc ions to enhance collagen secretion and promoting tendon repair. Simultaneously, WMg-MOF migrated to cartilage, releasing magnesium ions to induce cell differentiation and facilitating cartilage regeneration. Infrared spectroscopy confirmed successful peptide modification of nano ZIF-8 and Mg-MOF. Fluorescence imaging validated that LZIF-8/WMg-MOF had a longer retention, indirectly confirming their successful targeting of the tendon-bone interface. In summary, this dual-targeted metal ion network hydrogel scaffold has the potential to facilitate synchronized multitissue regeneration at the compromised tendon-bone interface, offering favorable prospects for its application in the integrated reconstruction characterized by the gradient structure.

Dynamic Covalent Bonded Gradient Structured Actuators with Mechanical Robustness and Self-Healing Ability

Flexible actuators with excellent adaptability and interaction safety have a wide range of application prospects in many fields. However, current flexible actuators have problems such as fragility and poor actuating ability. Here, inspired by the features of nacre structure, a gradient structured flexible actuator is proposed with mechanical robustness and self-healing ability. By introducing dynamic boronic ester bonds at the interface between MXene nanosheets and epoxy natural rubber matrix, the resulting nanocomposites with ordered micro-nano structures exhibit excellent tensile strength (25.03 MPa) and satisfactory repair efficiency (81.2%). In addition, the gradient distribution structure of MXene nanosheets endows the actuator with stable photothermal conversion capability, which can quickly respond to near-infrared light stimulation. The interlayer dynamic covalent bond crosslinking enables good response speed after multiple bending and is capable of functional self-healing after damage. This work introduces gradient structure and dynamic covalent bonding into flexible actuators, which provides a reference for the fabrication of self-healing soft robots, wearable, and other healable functional materials.

Yolk-Shell Gradient-Structured SiOx Anodes Derived from Periodic Mesoporous Organosilicas Enable High-Performance Lithium-Ion Batteries

Gradient-structured materials hold great promise in the areas of batteries and electrocatalysis. Here, yolk-shell gradient-structured SiOx -based anode (YSG-SiOx /C@C) derived from periodic mesoporous organosilica spheres (PMOs) through a selective etching method is reported. Capitalizing on the poor hydrothermal stability of inorganic silica in organic-inorganic hybrid silica spheres, the inorganic silica component in the hybrid spheres is selectively etched to obtain yolk-shell-structured PMOs. Subsequently, the yolk-shell PMOs are coated with carbon to fabricate YSG-SiOx /C@C. YSG-SiOx /C@C is comprised of a core with uniform distribution of SiOx and carbon at the atomic scale, a middle void layer, and outer layers of SiOx and amorphous carbon. This unique gradient structure and composition from inside to outside not only enhances the electrical conductivity of the SiOx anode and reduces the side reactions, but also reserves void space for the expansion of SiOx , thereby effectively mitigating the stress caused by volumetric effect. As a result, YSG-SiOx /C@C exhibits exceptional cycling stability and rate capability. Specifically, YSG-SiOx /C@C maintains a specific capacity of 627 mAh g-1 after 400 cycles at 0.5 A g-1 , and remains stable even after 550 cycles at current density of 2 A g-1 , achieving a specific capacity of 519 mAh g-1 .

MOF Derivatives with Gradient Structure Anchored on Carbon Foam for High-Performance Electromagnetic Wave Absorption

The impedance matching and high loss capabilities of composites with homogeneous distribution are limited owing to high addition and lack of structural design. Developing composites with heterogeneous distribution can achieve strong and wide electromagnetic (EM) wave absorption. However, challenges such as complex design and unclear absorption mechanisms still exist. Herein, a novel composite with a heterogeneous distribution gradient is successfully constructed via MOF derivatives Co@ nitrogen-doped carbon (Co@NC) anchored on carbon foam (CF) matrix (MDCF). Notably, the concentration of MOF can easily control the gradient structure. In particular, the morphologies of MOF derivatives on the surface of CF undergo a transition from the collapse of the inner layer to the integrity of the outer layer, accompanied by a continuous reduction in the size of Co nanoparticles. Correspondingly, enhanced interface polarization from the core-shell of Co@NC and good impedance matching of MDCF can be obtained. The optimized MDCF exhibits the minimum reflection loss of -68.18 dB at 2.01 mm and effective absorption bandwidth covering the entire X-band. Moreover, MDCF exhibits lightweight characteristics, excellent compressive strength, and low radar cross-section reduction. This work highlights the immense potential of composites with heterogeneous distribution for achieving high-performance EM wave absorption.

Photo-Induced Geometry and Polarity Gradients in Covalent Organic Frameworks Enabling Fast and Durable Molecular Separations

Azobenzene, which activates its geometric and chemical structure under light stimulation enables noninvasive control of mass transport in many processes including membrane separations. However, producing azobenzene-decorated channels that have precise size tunability and favorable pore wall chemistry allowing fast and durable permeation to solvent molecules, remains a great challenge. Herein, an advanced membrane that comprises geometry and polarity gradients within covalent organic framework (COF) nanochannels utilizing photoisomerization of azobenzene groups is reported. Such functional variations afford reduced interfacial transfer resistance and enhanced solvent-philic pore channels, thus creating a fast solvent transport pathway without compromising selectivity. Moreover, the membrane sets up a densely covered defense layer to prevent foulant adhesion and the accumulation of cake layer, contributing to enhanced antifouling resistance to organic foulants, and a high recovery rate of solvent permeance. More importantly, the solvent permeance displays a negligible decline throughout the long-term filtration for over 40 days. This work reports the geometry and polarity gradients in COF channels induced by the conformation change of branched azobenzene groups and demonstrates the strong capability of this conformation change in realizing fast and durable molecular separations.

Study on Microstructure Evolution Mechanism of Gradient Structure Surface of AA7075 Aluminum Alloy by Ultrasonic Surface Rolling Treatment

The materials with grain size gradient variation on the surface, which were prepared with mechanical-induced severe plastic deformation, always show high resistance to high and low cycle fatigue and frictional wear because of their good strength-ductility synergy. The ultrasonic surface rolling treatment (USRT) has the advantages of high processing efficiency, good surface quality, and large residual compressive stress introduced to the surface after treatment. The USRT was used to prepare aluminum alloy (AA7075) samples with a surface gradient structure; meanwhile, the microstructural evolution mechanism of the deformation layers on the gradient structure was studied with XRD, SEM, and TEM. The microstructure with gradient distribution of grain size and dislocation density formed on the surface of AA7075 aluminum alloy after USRT. The surface layer consists of nanocrystals with random orientation distribution, and high-density dislocation cells and subgrains formed in some grains in the subsurface layer, while the center of the material is an undeformed coarse-grained matrix. The results show that the dislocation slip dominates the grain refinement process, following the continuous cutting and refinement of dislocation cells, subgrains, and fragmentation of the second precipitates. This study systematically clarified the mechanism of grain refinement and nanocrystallization on the surface of high-strength aluminum alloys and laid a theoretical foundation for further research on mechanical behavior and surface friction and wear properties of high-strength non-ferrous materials with gradient structure.

PTFE/PVA-PVDF Conjugated Electrospun Nanofiber Membrane with Triboelectric Effect Used in Face Mask

COVID-19 broke out all over the world, and the medical protective mask is an important epidemic prevention equipment. Traditional medical protective masks use electret polypropylene melt-blown cloth as the core filter material. However, it relies heavily on electrostatic filtration and has high filtration resistance. The one-time electret makes the static charge decay rapidly with the water vapor generated by breathing, which affects the service life of the mask. In this paper, PTFE/PVA fiber and PVDF fiber were fabricated by conjugate electrospinning method, and the PTFE/PVA-PVDF layer blending fluffy fiber membrane was obtained by on-line mixing. Under the action of air slip effect and triboelectric secondary electret, the fiber membrane has higher filtration efficiency, lower filtration resistance and longer service life. The initial filtration efficiency of the fiber membrane is above 95%, the filtration efficiency is near to 100% after 24 times of cyclic filtration, the filtration resistance is about 110 Pa, the air permeability of the fiber membrane is 262.88–370.70 mm/s, and the moisture permeability is as high as 7721–8471 g/ (m2·24 h).

Fabrication of an Organic-Inorganic Hybrid Hard Coat with a Gradient Structure Controlled by Photoirradiation

Organic-inorganic materials have attracted attention because of the advantages of both organic and inorganic resins. Among their disadvantages, hard coating films made of organic-inorganic mixtures of resins have opacity and interface peeling problems because of organic-inorganic phase separation and surface segregation of inorganic resins. Although an organic-inorganic gradient-structured material comprising an inorganic-rich domain at the air interface and an organic-rich domain at the organic substrate has the potential to solve these problems, the fabrication of a gradient-structured material has not yet been achieved. Here, we describe the fabrication of an organic-inorganic gradient film by impeding the movement of organic and inorganic resins through radical photopolymerization of organic and inorganic oligomers. Moreover, we successfully enhanced gouge hardness by cross-linking with photobase-catalyzed sol-gel reactions of inorganic resins at the air interface. As a result, the organic-inorganic gradient coating contributed excellent gouge hardness (pencil hardness >9H), adhesion to an organic substrate such as polycarbonate, and transparency (visible light transmittance >99%T). In addition, we demonstrated that the formation of organic-inorganic gradient structures is dominated by the surface free energy and viscosity of each resin. Achieving a gradient structure required a significant difference in surface free energy (>20 mJ/m²) and high mixture viscosity (>65 mPa·s).

The Effect of Radial-Shear Rolling Deformation Processing on the Structure and Properties of Zr-2.5Nb Alloy

The rheological properties of the Zr-2.5Nb alloy by the strain rate range of 0.5-15 s^-1 and by the temperature range of 20-770 °C was studied. The dilatometric method for phase states temperature ranges was experimentally determined. A material properties database for computer FEM simulation regards the indicated temperature-velocity ranges were created. Using this database and DEFORM-3D FEM-softpack, the radial shear rolling complex process numerical simulation was carried out. The contributed conditions for the ultrafine-grained state alloy structure refinement were determined. Based on the simulation results, a full-scale experiment of Zr-2.5Nb rod rolling a on a radial-shear rolling mill RSP-14/40 was carried out. It takes in seven passes from a diameter of 37-20 mm with a total diameter reduction ε = 85%. According to this case simulation data, the total equivalent strain in the most processed peripheral zone 27.5 mm/mm was reached. Due to the complex vortex metal flow, the equivalent strain over the section distribution was uneven with a gradient reducing towards the axial zone. This fact should have a deep effect on the structure change. Changes and structure gradient by sample section EBSD mapping with 2 mm resolution were studied. The microhardness section gradient by the HV 0.5 method was also studied. The axial and central zones of the sample by the TEM method were studied. The rod section structure has an expressed gradient from the formed equiaxed ultrafine-grained (UFG) structure on a few outer millimeters of the peripheral section to the elongated rolling texture in the center of the bar. The work shows the possibility of processing with the gradient structure obtaining and enhanced properties for the Zr-2.5Nb alloy, and a database for this alloy FEM numerical simulations are also presents.

Biomimetic Gradient Bouligand Structure Enhances Impact Resistance of Ceramic-Polymer Composites

Biological materials relied on multiple synergistic structural design elements typically exhibit excellent comprehensive mechanical properties. Hierarchical incorporation of different biostructural elements into a single artificial material is a promising approach to enhance mechanical properties, but remains challenging. Herein, a biomimetic structural design strategy is proposed by coupling gradient structure with twisted plywood Bouligand structure, attempting to improve the impact resistance of ceramic-polymer composites. Via robocasting and sintering, kaolin ceramic filaments reinforced by coaxially aligned alumina nanoplatelets are arranged into Bouligand structure with a gradual transition in filament spacing along the thickness direction. After the following polymer infiltration, biomimetic ceramic-polymer composites with a gradient Bouligand (GB) structure are eventually fabricated. Experimental investigations reveal that the incorporation of gradient structure into Bouligand structure improves both the peak force and total energy absorption of the obtained ceramic-polymer composites. Computational modeling further suggests the substantial improvement in impact resistance by adopting GB structure, and clarifies the underlying deformation behavior of the biomimetic GB structured composites under impact. This biomimetic design strategy may provide valuable insights for developing lightweight and impact-resistant structural materials in the future.

Multichannel Gradient Piezoelectric Transducer Assisted with Deep Learning for Broadband Acoustic Sensing

As an important part of human-machine interfaces, piezoelectric voice recognition has received extensive attention due to its unique self-powered nature. However, conventional voice recognition devices exhibit a limited response frequency band due to the intrinsic hardness and brittleness of piezoelectric ceramics or the flexibility of piezoelectric fibers. Here, we propose a cochlear-inspired multichannel piezoelectric acoustic sensor (MAS) based on gradient PVDF piezoelectric nanofibers for broadband voice recognition by a programmable electrospinning technique. Compared with the common electrospun PVDF membrane-based acoustic sensor, the developed MAS demonstrates the greatly 300%-broadened frequency band and the substantially 334.6%-enhanced piezoelectric output. More importantly, this MAS can serve as a high-fidelity auditory platform for music recording and human voice recognition, in which the classification accuracy rate can reach up to 100% in coordination with deep learning. The programmable bionic gradient piezoelectric nanofiber may provide a universal strategy for the development of intelligent bioelectronics.

Strengthening a Medium-Carbon Low-Alloy Steel by Nanosized Grains: The Role of Asymmetrical Rolling

A medium-carbon low-alloy steel was prepared via the asymmetric rolling process with different ratios of upper and down roll velocities. Subsequently, the microstructure and mechanical properties were explored by using SEM, EBSD, TEM, tensile tests and nanoindentation. The results show that asymmetrical rolling (ASR) can significantly improve strength while retaining good ductility compared with conventional symmetrical rolling. The yield strength and tensile strength of the ASR-steel are 1292 ± 10 MPa and 1357 ± 10 MPa, respectively, which are higher than the values of 1113 ± 10 MPa and 1185 ± 10 MPa for the SR-steel. The ASR-steel retains good ductility of 16.5 ± 0.5%. The significant increase in strength is related to the joint actions of the ultrafine grains, dense dislocations and a large number of nanosized precipitates. This is mainly because of the introduction of extra shear stress on the edge under asymmetric rolling, which induces gradient structural changes hence increasing the density of geometrically necessary dislocations.

Crashworthiness Study of 3D Printed Lattice Reinforced Thin-Walled Tube Hybrid Structures

Based on the advantages of thin-walled tubes and lattice structures in energy absorption and improved crashworthiness, a hybrid structure of lattice-reinforced thin-walled tubes with different cross-sectional cell numbers and gradient densities was constructed, and a high crashworthiness absorber with adjustable energy absorption was proposed. The experimental and finite element characterization of the impact resistance of uniform density and gradient density hybrid tubes with different lattice arrangements to withstand axial compression was carried out to investigate the interaction mechanism between the lattice packing and the metal shell, and the energy absorption of the hybrid structure was increased by 43.40% relative to the sum of its individual components. The effect of transverse cell number configuration and gradient configuration on the impact resistance of the hybrid structure was investigated, and the results showed that the hybrid structure showed higher energy absorption than the empty tube, and the best specific energy absorption was increased by 83.02%; the transverse cell number configuration had a greater effect on the specific energy absorption of the hybrid structure with uniform density, and the maximum specific energy absorption of the hybrid structure with different configurations was increased by 48.21%. The gradient density configuration had a significant effect on the peak crushing force of the gradient structure. In addition, the effects of wall thickness, density and gradient configuration on energy absorption were quantitatively analyzed. This study provides a new idea to optimize the impact resistance of lattice-structure-filled thin-walled square tube hybrid structures under compressive loading through a combination of experiments and numerical simulations.

Obtaining an Equiaxed Ultrafine-Grained State of the Longlength Bulk Zirconium Alloy Bars by Extralarge Shear Deformations with a Vortex Metal Flow

The method of radial shear rolling makes it possible to achieve comparable to high pressure torsion (HPT) method ultrahigh degrees of total strain level in combination with the vortex metal flow character for long-length large bulk bars unable by HPT and many other processes of sever plastic deformation (SPD). Sequential rolling of the Zr-1%Nb alloy was carried out under extreme conditions on two radial shear rolling mills with a total diameter reduction ε = 185% and a maximum total strain level = 46 mm/mm. The strain level and its cross-section distribution assessment by finite element method (FEM) simulation was studied. The final bar cross-section structure type distribution detailed study 1 mm resolution by electron back scatter diffraction (EBSD) mapping was performed. A gradient structure with a predominance of the equiaxed ultrafine-grained (UFG) state was found. The deformation level rising did not allow to refine it in the periphery zone more than that obtained nearly middle of the processing, but it allows for significant change in the axial zone structure. The additional large warm deformations by radial shear rolling have no additional grain refinement effect for already 300-600 nm refined zone. An equiaxed UFG structure was obtained in a relatively large volume of the sample with a reduced gradient towards the non-UFG center zone in regard to known works.

Development of Strong and Tough β-TCP/PCL Composite Scaffolds with Interconnected Porosity by Digital Light Processing and Partial Infiltration

Strong and tough β-TCP/PCL composite scaffolds with interconnected porosity were developed by combining digital light processing and vacuum infiltration. The composite scaffolds were comprised of pure β-TCP, β-TCP matrix composite and PCL matrix composite. The porous β-TCP/PCL composite scaffolds showed remarkable mechanical advantages compared with ceramic scaffolds with the same macroscopic pore structure (dense scaffolds). The composite scaffolds exhibited a significant increase in strain energy density and fracture energy density, though with similar compressive and flexural strengths. Moreover, the composite scaffolds had a much higher Weibull modulus and longer fatigue life than the dense scaffolds. It was revealed that the composite scaffolds with interconnected porosity possess comprehensive mechanical properties (high strength, excellent toughness, significant reliability and fatigue resistance), which suggests that they could replace the pure ceramic scaffolds for degradable bone substitutes, especially in complex stress environments.

Improvement of Aerosol Filtering Performance of PLLA/PAN Composite Fiber with Gradient Structure

Since commercial non-woven air filtering materials have unstable filtering efficiency and poor moisture permeability for the abundant condensed aerosol particles in the highly humid atmospheric environment, the PLLA/PAN composite fiber material with a hydrophobic and hydrophilic gradient structure is designed and prepared by using electrode sputtering electro spinning technology. By characterizing and testing the filtrating effect of SEM, XRD, FTIR, wettability, mechanical property, N₂ adsorption isotherm, and BET surface area, NaCl aerosol of PLLA fiber, PAN fiber, and PLLA/PAN composite fiber membranes, the study found that the electrode sputtering electrospinning is fine, the fiber mesh is dense, and fiber distribution is uniform when the diameter of the PAN fiber is 140-300 nm, and the PLLA fiber is 700-850 nm. In this case, PLLA/PAN composite fiber materials gather the hydrophobicity of PLLA fiber and the hydrophilicity of PAN fiber; its electrostatic effect is stable, its physical capturing performance is excellent, it can realize the step filtration of gas-solid liquid multiphase flow to avoid the rapid increase of air resistance in a high-humidity environment, and the filtrating efficiency η of NaCl aerosol particles with 0.3 μm reaches 99.98%, and the quality factor Q_F 0.0968 Pa^-1. The manufacturing of PLLA/PAN composite fiber material provides a new method for designing and developing high-performance air filtration materials and a new technical means for the large-scale production of high-performance, high-stability, and low-cost polylactic acid nanofiber composites.

Preparing Thick Gradient Surface Layer in Cu-Zn Alloy via Ultrasonic Severe Surface Rolling for Strength-Ductility Balance

A gradient structure (GS) design is a prominent strategy for strength-ductility balance in metallic materials, including Cu alloys. However, producing a thick GS surface layer without surface damage is still a challenging task limited by the available processing technology. In this work, a gradient structure (GS) surface layer with a thickness at the millimeter scale is produced in the Cu-38 wt.% Zn alloy using ultrasonic severe surface rolling technology at room temperature. The GS surface layer is as thick as 1.1 mm and involves the gradient distribution of grain size and dislocation density. The grain size is refined to 153.5 nm in the topmost surface layer and gradually increases with increasing depth. Tensile tests indicate that the single-sided USSR processed alloy exhibits balanced strength (467.5 MPa in yield strength) and ductility (10.7% in uniform elongation). Tailoring the volume fraction of the GS surface layer can tune the combination of strength and ductility in a certain range. The high strength of GS surface layer mainly stems from the high density of grain boundaries, dislocations and dislocation structures, deformation twins, and GS-induced synergistic strengthening effect. Our study elucidates the effect of the thick GS surface layer on strength and ductility, and provides a novel pathway for optimizing the strength-ductility combination of Cu alloys.

A review on control of droplet motion based on wettability modulation: principles, design strategies, recent progress, and applications

The transport of liquid droplets plays an essential role in various applications. Modulating the wettability of the material surface is crucial in transporting droplets without external energy, adhesion loss, or intense controllability requirements. Although several studies have investigated droplet manipulation, its design principles have not been categorized considering the mechanical perspective. This review categorizes liquid droplet transport strategies based on wettability modulation into those involving (i) application of driving force to a droplet on non-sticking surfaces, (ii) formation of gradient surface chemistry/structure, and (iii) formation of anisotropic surface chemistry/structure. Accordingly, reported biological and artificial examples, cutting-edge applications, and future perspectives are summarized.

A Multifunctional Gradient Solid Electrolyte Remarkably Improving Interface Compatibility and Ion Transport in Solid-State Lithium Battery

Solid electrolytes with both interface compatibility and efficient ion transport have been an urgent technical requirement for the practical application of solid-state lithium batteries. Herein, a multifuctional poly(1,3-dioxolane) (PDOL) electrolyte combining the gradient structure from the solid state to the gel state with the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) interfacial modification layer was designed, in which the "solid-to-gel" gradient structure greatly improved the electrode/electrolyte interface compatibility and ion transport, while the solid PDOL and LLZTO layers effectively improved the interface stability of the electrolyte/lithium anode and the inhibition of the lithium dendrites via their high mechanical strength and forming a stable interfacial SEI composite film. This gradient PDOL/LLZTO composite electrolyte possesses a high ionic conductivity of 2.9 × 10^-4 S/cm with a wide electrochemical window up to 4.9 V vs Li/Li⁺. Compared with the pristine PDOL electrolyte and PDOL solid electrolyte membrane coated with a layer of LLZTO, the gradient PDOL/LLZTO composite electrolyte shows better electrode/electrolyte interfacial compatibility, lower interface impedance, and smaller polarization, resulting in enhanced rate and cycle performances. The NCM622/PDOL-LLZTO/Li battery can be stably cycled 200 times at 0.3C and 25 °C. This multifunctional gradient structure design will promote the development of high-performance solid electrolytes and is expected to be widely used in solid-state lithium batteries.

Slippery Passive Radiative Cooling Supramolecular Siloxane Coatings

Polymer coatings with comprehensive properties including passive radiative cooling, anti-fouling, and self-healing constitute a promising energy-saving strategy but have not been well documented yet. Herein, we reported a class of novel multifunctional supramolecular polysiloxane composite coatings showing the combination of these features. The coatings have a hybrid structure with a slippery liquid-infused porous surface and a gradient polymer-Al₂O₃ composite matrix constructed by reversible hydrogen bonding. The gradient matrix consists of a polymer-rich top and a particle-rich bottom favoring coating attachment on rigid substrates. Such a complex structure can be obtained by simply casting the suspending solutions of the polydimethylsiloxane (PDMS)-urea copolymer and Al₂O₃ on substrates followed by swelling silicone oil. Obtained coatings display good passive daytime radiative cooling (a temperature drop of ∼2 °C), self-healing ability, and anti-fouling properties. Since the comprehensive performances and the facile fabrication, the coatings should have application potential for various thermal management purposes.

Simultaneous Improvement of Yield Strength and Ductility at Cryogenic Temperature by Gradient Structure in 304 Stainless Steel

The tensile properties and the corresponding deformation mechanism of the graded 304 stainless steel (ss) at both room and cryogenic temperatures were investigated and compared with those of the coarse-grained (CGed) 304 ss. Gradient structures were found to have excellent synergy of strength and ductility at room temperature, and both the yield strength and the uniform elongation were found to be simultaneously improved at cryogenic temperature in the gradient structures, as compared to those for the CG sample. The hetero-deformation-induced (HDI) hardening was found to play a more important role in the gradient structures as compared to the CG sample and be more obvious at cryogenic temperature as compared to that at room temperature. The central layer in the gradient structures provides stronger strain hardening during tensile deformation at both temperatures, due to more volume fraction of martensitic transformation. The volume fraction of martensitic transformation in the gradient structures was found to be much higher at cryogenic temperature, resulting in a much stronger strain hardening at cryogenic temperature. The amount of martensitic transformation at the central layer of the gradient structures is observed to be even higher than that for the CG sample at cryogenic temperature, which is one of the origins for the simultaneous improvement of strength and ductility by the gradient structures at cryogenic temperature.

Light-Responsive Nanofibrous Motor with Simultaneously Precise Locomotion and Reversible Deformation

Light-powered micromotors have drawn enormous attention because of their potential applications in cargo delivery, environmental monitoring, and noninvasive surgery. However, the existing micromotors still suffer from some challenges, including slow speed, poor controllability, single locomotion mode, and no deformation during movement. Herein, we employ a combined electrospinning with brushing of Chinese ink to simply fabricate a light-responsive gradient-structured poly(vinyl alcohol)/carbon (PVA/carbon) composite motor. Because of the surface deposition and ultrahigh loading amount of carbon nanoparticles (ca. 43%), the motor exhibits rapid (39 mm/s), direction-controlled, and multimodal locomotion (vertical movement, horizontal motion, rotation) under light irradiation. Simultaneously, gradient alignment structure of the PVA nanofibrous matrix endows the motor with controllable and reversible deformation during locomotion. We finally demonstrate the potential applications of the motors in leakage monitoring, object salvage, smart access, and intelligent assembly. The present work will inspire the design of novel photosensitive motors for applications in various fields, such as microrobots, environmental monitoring, and biomedicine.

Intermetallic Phase Evolution of Cold-Sprayed Ni-Ti Composite Coatings: Influence of As-Sprayed Chemical Composition

Owing to low-temperature deposition conditions and high deposition rate, cold spray offers unique advantages in manufacturing a wide variety of metallic and composite coatings including metal matrix composites produced from physically blended powders. One of the challenges of producing composite coatings using cold spray is the deviation of coatings composition from the blended feedstock powder composition. This is of utmost importance as it affects the composition and phase evolution of intermetallic forming coatings during post spray heat treatment. In this work, cold spray of composite Ni-Ti coatings and formation of intermetallics from post spray heat treatment were investigated as a first step to examine the potential of producing equiatomic bulk Ni-Ti by cold spray. Three different physically blended Ni and Ti powders mixtures were sprayed on titanium substrates to address the coating composition variation from the blended feedstock powder and study its influence on phase evolution during post spray heat treatment. High-density and well-dispersed composite coatings were achieved for each case. EDS analysis revealed as-sprayed coatings with 10.5, 35.9 and 56.9 at.% Ni (and with balanced Ti ratios) from the three powder mixtures. Annealing treatments were conducted at 400, 500 and 900 °C for 1 and 2 h and comparative studies of the intermetallic compound formations were carried out. Microstructural investigation showed that all three equilibrium intermetallics phases of binary Ni-Ti phase diagram (Ni3Ti, Ti2Ni and NiTi) formed in the two Ni-rich composite coatings with NiTi phase being maximum in the coating with the closest composition to equiatomic ratio while only Ti2Ni phase formed in the Ti-rich coating after annealing. Thermal etching analysis of coatings showed that NiTi phase forms with a gradient microstructure from Ti splats boundary toward the center of splats, which is attributed to the grain refinement of CS samples at splat boundary and intermetallic nucleation mechanism.

The Strongest Size in Gradient Nanograined Metals

Conventional polycrystalline metals become stronger with decreasing grain size, yet softening starts to take over at the nanometer regime, giving rise to the strongest size at which the predominate strengthening mechanism switches to softening. We show that this critical size for the onset of softening can be tuned by tailoring grain size gradient, and raising in the gradient shifts the size toward the smaller value. The decrease in the strongest size is prompted by mitigation of grain boundary-mediated softening processes accompanying by enhanced intragranular plastic deformations. We found that the nanograins smaller than 6 nm, mainly involving intergranular sliding in homogeneous structures, reveal anomalous plastic deformation in gradient systems, which is mediated by partial dislocation nucleation, faulting and twinning activated in a gradient stress field. The results on extended dislocation slip and gradient plasticity, stemming from the structure heterogeneity, shed light on an emerging class of heterogeneous nanostructured materials of improved strength-ductility synergy.

Biomimetic Color-Changing Hierarchical and Gradient Hydrogel Actuators Based on Salt-Induced Microphase Separation

There have been more challenges for hydrogel actuators to meet the combined requirement of discoloration, complex deformation, and simple preparation. Structural coloration is widely used to fabricate discolored hydrogel via microrearrangement of photonic crystals in the hydrogel framework. However, precise regulation is usually required. Besides, the macro-optical properties are unstable. Herein, we develop a hierarchical and gradient hydrogel actuator with complex deformation and color-changing functions using an electrophoresis method. A simple but effective strategy is presented for fabrication of hierarchical hydrogel composed of homopolymers and copolymers via salt-induced microphase separation during the polymerization of the N-isopropylacrylamide (NIPAm) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (DMC). Meanwhile a gradient distribution of DMC is also formed during the polymerization due to migration of DMC under electric field. The hierarchical and gradient structures are characterized by confocal laser scanning microscope (CLSM), small-angle X-ray scattering measurement (SAXS), temperature-variable Fourier transform infrared (FTIR), etc. The discoloration mechanism is proposed. The as-prepared hydrogel can undergo fast and complex thermo-triggered deformation and discoloration. Bionic movements of discoloration flowering and information encryption are successfully demonstrated, promising great potential in the application of biomimetic materials.

Effect of Shear Strain Rate on Microstructure and Properties of Austenitic Steel Processed by Cyclic Forward/Reverse Torsion

In this work, commercial AISI 304 stainless steel rods were subjected to cyclic forward/reverse torsion (CFRT) treatments at low-speed and high-speed torsion at room temperature. Microstructures in the core and surface layers of the CFRT-treated samples were systematically characterized. Results show that the CFRT treatment can introduce martensite phase on the surface of the rods via strain-induced martensitic transformation. High-speed twisting is more effective in inducing martensite in the surface layer compared to low-speed twisting. During the stretching process, the overall strain-hardening behavior of the gradient material is related to the content of its gradient defects. Higher gradient martensite content results in a higher surface hardness of the material, but less overall tensile properties. The effect of twisting speed on torsion behavior and the strain-hardening mechanisms in tensile of the gradient structured steels was also addressed.

Gradient Structure Design of Flexible Waterborne Polyurethane Conductive Films for Ultraefficient Electromagnetic Shielding with Low Reflection Characteristic

Highly efficient electromagnetic shielding materials entailing strong electromagnetic wave absorption and low reflection have become an increasing requirement for next-generation communication technologies and high-power electronic instruments. In this study, a new strategy is employed to provide flexible waterborne polyurethane composite films with an ultra-efficient electromagnetic shielding effectiveness (EMI SE) and low reflection by constructing gradient shielding layers with a magnetic ferro/ferric oxide deposited on reduced graphene oxide (rGO@Fe₃O₄) and silver-coated tetraneedle-like ZnO whisker (T-ZnO/Ag) functional nanoparticles. Because of the differences in density between rGO@Fe₃O₄ and T-ZnO/Ag, a gradient structure is automatically formed during the film formation process. The gradient distribution of rGO@Fe₃O₄ over the whole thickness range forms an efficient electromagnetic wave absorption network that endows the film with a strong absorption ability on the top side, while a thin layer of high-density T-ZnO/Ag at the bottom constructs a highly conductive network that provides an excellent electromagnetic reflection ability for the film. This specific structure results in an "absorb-reflect-reabsorb" process when electromagnetic waves penetrate into the composite film, leading to an excellent EMI shielding performance with an extremely low reflection characteristic at a very low nanofiller content (0.8 vol % Fe₃O₄@rGO and 5.7 vol % T-ZnO/Ag): the EMI SE reaches 87.2 dB against the X band with a thickness of only 0.5 mm, while the shielding effectiveness of reflection (SE_R) is only 2.4 dB and the power coefficient of reflectivity ( R) is as low as 0.39. This result means that only 39% of the microwaves are reflected in the propagation process when 99.9999998% are attenuated, which is the lowest value among the reported references. This composite film with remarkable performance is suitable for application in portable and wearable smart electronics, and this method offers an effective strategy for absorption-dominated EMI shielding.

Microstructures and Mechanical Properties of Commercially Pure Ti Processed by Rotationally Accelerated Shot Peening

Gradient structured materials possess good combinations of strength and ductility, rendering the materials attractive in industrial applications. In this research, a surface nanocrystallization (SNC) technique, rotationally accelerated shot peening (RASP), was employed to produce a gradient nanostructured pure Ti with a deformation layer that had a thickness of 2000 μm, which is thicker than those processed by conventional SNC techniques. It is possible to fabricate a gradient structured Ti workpiece without delamination. Moreover, based on the microstructural features, the microstructure of the processed sample can be classified into three regions, from the center to the surface of the RASP-processed sample: (1) a twinning-dominated core region; (2) a “twin intersection”-dominated twin transition region; and (3) the nanostructured region, featuring nanograins. A microhardness gradient was detected from the RASP-processed Ti. The surface hardness was more than twice that of the annealed Ti sample. The RASP-processed Ti sample exhibited a good combination of yield strength and uniform elongation, which may be attributed to the high density of deformation twins and a strong back stress effect.

Incorporating 4-tert-Butylpyridine in an Antisolvent: A Facile Approach to Obtain Highly Efficient and Stable Perovskite Solar Cells

The synthesis and growth of CH₃NH₃PbI₃ films with controlled nucleation is a key issue for the high efficiency and stability of solar cells. Here, 4-tert-butylpyridine (tBP) was introduced into a CH₃NH₃PbI₃ antisolvent to obtain high quality perovskite layers. In situ optical microscopy and X-ray diffraction patterns were used to prove that tBP significantly suppressed perovskite nucleation by forming an intermediate phase. In addition, a gradient perovskite structure was obtained by this method, which greatly improved the efficiency and stability of perovskites. An effective power conversion efficiency (PCE) of 17.41% was achieved via the tBP treatment, and the high-efficiency device could maintain over 89% of the initial PCE after 30 days at room temperature.