Hierarchical Composite Structures Research Articles

Free-standing TiO2 nanowire-embedded graphene hybrid membrane for advanced Li/dissolved polysulfide batteries

The increasing demand for electric vehicles and large-scale smart grids has aroused great interest in developing high energy density storage devices. Lithium–sulfur (Li–S) battery has attracted much attention owing to its high theoretical energy density and abundance, but many challenges such as rapid capacity fade and low sulfur loading and utilization have impeded its practical use. Here, we present a free-standing TiO2 nanowire/graphene hybrid membrane for Li/dissolved polysulfide batteries with high capacity and long cycling life. Graphene membrane with high electrical conductivity is used as a current collector to effectively reduce the internal resistance in the sulfur cathode and physically immobilize the dissolved lithium polysulfides. The TiO2 nanowires introduced into the graphene membrane offer a hierarchical composite structure, in which the TiO2 nanowires not only have strong chemical binding with the lithium polysulfides, but also show a strong catalytic effect for polysulfide reduction and oxidation, promoting a fast redox reaction kinetics with high capacity and low voltage polarization. This hybrid electrode delivers a high specific capacity of 1327mAhg−1 at 0.2C rate, a Coulombic efficiency approaching 100%, high-rate performance of 850mAhg−1 at 2C rate, and long cyclic stability with a capacity of 1053mAhg−1 at 0.2C rate over 200 cycles, demonstrating great prospect for application in high energy Li–S batteries.

Superior high-rate capability of hierarchically structured flower-like magnetite–carbon–graphene composite for Li-ion anode

A nano/micro hierarchical composite structure is prepared by solvothermal synthesis from iron alkoxide in an inert atmosphere. It consists of graphene sheet and flower-like Fe3O4 particle covered by carbon coating (designated as Fe3O4@C). The synthesized composite is an excellent anode for Li-ion batteries. This is attributed to its double structure of current collector. High reversible lithium storage capacity (1439 mA h/g at 0.2 C after 47 cycles) and superior high-rate capability (619 mA h/g at 5 C) are examples of this property. The carbon coating of the Fe3O4@C is in direct contact with the Fe3O4 particles to form local conductive connections, Fe3O4@C tightly covering graphene sheets can be employed as the current collector as well as bridge connection of the Fe3O4@C particles. Therefore, the double structure of current collector can form superior-conductivity networks, providing an efficient electron transport between active materials and Cu current collectors. This double structure prevents the break down of electric connection between the active material and the current collector during protracted cycles of charge and discharge.

Recent advances in hierarchical macroporous composite structures for photoelectric conversion

Macroporous structures have attracted great interest for a wide range of applications due to their unique structure-determined physical and chemical properties. In regard to the inherent low surface area, decoration of macroporous structures with small building blocks (including nanoparticles, nanorods, nanowires, nanosheets, etc.) to form macroporous composite materials therefore have been highly pursued very recently. In this review, we summarize the recent advances in synthesis of hierarchical macroporous composite materials and their potential applications as photoelectrode materials for dye-sensitized solar cells (DSSCs), quantum dot-sensitized solar cells (QDSSCs) and photoelectrochemical (PEC) cells.

Straightforward Oxidation of a Copper Substrate Produces an Underwater Superoleophobic Mesh for Oil/Water Separation

A superhydrophilic and underwater superoleophobic Cu(OH)2-covered mesh with micro- and nanoscale hierarchical composite structures is successfully fabricated through a one-step chemical oxidation of a smooth-copper mesh. Such mesh, without any further modification, can selectively separate water from oil/water mixtures with high separation efficiency, and possess excellent stability even after 60 uses. This method provides a simple, low-cost, and scalable strategy for the purification of oily wastewater.

A Correlated Parts Model for Object Detection in Large 3D Scans

AbstractThis paper addresses the problem of detecting objects in 3D scans according to object classes learned from sparse user annotation. We model objects belonging to a class by a set of fully correlated parts, encoding dependencies between local shapes of different parts as well as their relative spatial arrangement. For an efficient and comprehensive retrieval of instances belonging to a class of interest, we introduce a new approximate inference scheme and a corresponding planning procedure. We extend our technique to hierarchical composite structures, reducing training effort and modeling spatial relations between detected instances. We evaluate our method on a number of real‐world 3D scans and demonstrate its benefits as well as the performance of the new inference algorithm.

Laser Printing of Nanoparticle Toner Enables Digital Control of Micropatterned Carbon Nanotube Growth

Commercialization of materials utilizing patterned carbon nanotube (CNT) forests, such as hierarchical composite structures, dry adhesives, and contact probe arrays, will require catalyst patterning techniques that do not rely on cleanroom photolithography. We demonstrate the large scale patterning of CNT growth catalyst via adaptation of a laser-based electrostatic printing process that uses magnetic ink character recognition (MICR) toner. The MICR toner contains iron oxide nanoparticles that serve as the catalyst for CNT growth, which are printed onto a flexible polymer (polyimide) and then transferred to a rigid substrate (silicon or alumina) under heat and mechanical pressure. Then, the substrate is processed for CNT growth under an atmospheric pressure chemical vapor deposition (CVD) recipe. This process enables digital control of patterned CNT growth via the laser intensity, which controls the CNT density; and via the grayscale level, which controls the pixelation of the image into arrays of micropillars. Moreover, virtually any pattern can be designed using standard software (e.g., MS Word, AutoCAD, etc.) and printed on demand. Using a standard office printer, we realize isolated CNT microstructures as small as 140 μm and isolated catalyst ″pixels″ as small as 70 μm (one grayscale dot) and determine that individual toner microparticles result in features of approximately 5-10 μm . We demonstrate that grayscale CNT patterns can function as dry adhesives and that large-area catalyst patterns can be printed directly onto metal foils or transferred to ceramic plates. Laser printing therefore shows promise to enable high-speed micropatterning of nanoparticle-containing thin films under ambient conditions, possibly for a wide variety of nanostructures by engineering of toners containing nanoparticles of desired composition, size, and shape.

Hierarchical Composite Structures Prepared by Electrophoretic Deposition of Carbon Nanotubes onto Glass Fibers

Carbon nanotube/glass fiber hierarchical composite structures have been produced using an electrophoretic deposition (EPD) approach for integrating the carbon nanotubes (CNTs) into unidirectional E-glass fabric, followed by infusion of an epoxy polymer matrix. The resulting composites show a hierarchical structure, where the structural glass fibers, which have diameters in micrometer range, are coated with CNTs having diameters around 10-20 nm. The stable aqueous dispersions of CNTs were produced using a novel ozonolysis and ultrasonication technique that results in dispersion and functionalization in a single step. Ozone-oxidized CNTs were then chemically reacted with a polyethyleneimine (PEI) dendrimer to enable cathodic EPD and promote adhesion between the CNTs and the glass-fiber substrate. Deposition onto the fabric was accomplished by placing the fabric in front of the cathode and applying a direct current (DC) field. Microscopic characterization shows the integration of CNTs throughout the thickness of the glass fabric, where individual fibers are coated with CNTs and a thin film of CNTs also forms on the fabric surfaces. Within the composite, networks of CNTs span between adjacent fibers, and the resulting composites exhibit good electrical conductivity and considerable increases in the interlaminar shear strength, relative to fiber composites without integrated CNTs. Mechanical, chemical and morphological characterization of the coated fiber surfaces reveal interface/interphase modification resulting from the coating is responsible for the improved mechanical and electrical properties. The CNT-coated glass-fiber laminates also exhibited clear changes in electrical resistance as a function of applied shear strain and enables self-sensing of the transition between elastic and plastic load regions.

Single-step anodization for synthesis of hierarchical TiO2 nanotube arrays on foil and wire substrate for enhanced photoelectrochemical water splitting

A hierarchical nanoporous-TiO2 nanotube composite structure consisting of a nanoporous top layer with smooth underlying nanotubes was obtained by a single-step anodization technique. The growth of such composite structure has also been applied to Ti wire substrate for the first time. The photoelectrochemical performance was examined under AM 1.5 simulated solar irradiation in 1 M KOH electrolyte. In general, the hierarchical architecture demonstrated improved photoelectrochemical activity over plain TiO2 nanotubes. Furthermore, the composite structure on a wire substrate was also found to enhance photoelectrochemical activity over a foil substrate. Under optimized conditions, over a 40% increase in hydrogen generation for hierarchical nanotubes over plain nanotubes is observed and over 25% increase in hydrogen generation using a wire substrate over a foil substrate is observed.

Fabrication and potential applications of CaCO3–lentinan hybrid materials with hierarchical composite pore structure obtained by self-assembly of nanoparticles

Chemically designed, hierarchical composite pore materials, with two or more different sized pores arranged regularly and high specific surface area, have attracted considerable attention because of possessing improved functions and applications in chemistry, pharmaceutics, environmental protection and so on. In this study, two biogenic materials, calcium carbonate (CaCO3), the most widely studied biomineral, and lentinan, showing prominent antitumor activity, are incorporated into the biomineralization process to produce CaCO3–lentinan microspheres with hierarchical composite pore structure by the hierarchical assembly of nanoparticles. The pore structure analysis indicated that a wormhole mesoporous structure was present in the framework of the CaCO3–lentinan microspheres and a microporous structure was present in the core part of the mesoporous walls, rendering an unusual hierarchical composite pore structure of CaCO3. The results revealed that the hierarchical composite pore structure could obviously reduce the release rate and prolong the release time of the anticancer drug, which led to minimization of poisonous side effects and decreased bodily injuries due to the stepwise release of the drug from mesoporous pores to micropores. The component CaCO3 with pH-sensitivity could controllably release DOX at the acidic tumour site, which minimizes the spread of toxic drugs around the normal tissues while maximizing tumor-directed drug delivery. In particular, the CaCO3–lentinan microspheres have ideal biocompatibility and biodegradability which are of critical importance for the clinical application. Further investigation also revealed the desirable properties of the CaCO3–lentinan microspheres for the removal of Congo red (CR) pollutant from waste water, with the maximum removal capacity of 213.2 mg g−1, which will be a great potential material in real applications.

A Practical Activity Recognition Approach Based on the Generic Activity Framework

In spite of the obvious importance of activity recognition technology for human centric applications, state-of-the-art activity recognition technology is not practical enough for real world deployments because of the insufficient accuracy and lack of support for programmability. The authors introduce a generic activity framework to address these issues. The generic activity framework is a refined hierarchical composition structure of the traditional activity theory. New activity recognition algorithms that can cooperate with the proposed activity framework and model are proposed. To be practical, activity recognition technology should also be programmable. The hierarchical aspects of our generic activity framework help to improve activity recognition programmability. The generic activity framework decouples the observation subsystem (i.e., the sensor set) from the rest of the activity model. The authors demonstrate the value of this decoupling by experimentally comparing the level of user effort needed in making sensor changes and the ramifications of such changes on model updates. They compare the level of effort required by the authors’ model to the requirements of previously reported approaches.

(Invited) Manufacturing of Hierarchical Composite Structures via Controlled Ceramic Nanoparticle Incorporation in Electrodeposited Thin Films

Electrochemical deposition has continued to grow as an important industry tool not only for depositing traditional metal structures, but also for manufacturing more complex composite thin films. Previous work has demonstrated the ability to manufacture metal matrix composites with precisely controlled oxide nanoparticle incorporation using a rotating disk electrode (RDE) setup. In this method the speed of the RDE is used to directly control the rate of particle incorporation. It is feasible to manufacture higher order composite structures using this novel technique by simply varying the rotation rate of the RDE during deposition. Specific attention is given to periodically layered composite structures. These hierarchical structures are demonstrated with nanometer scale interface resolution and sub-micron to millimeter characteristic length scales (layer thicknesses). These ordered composites should yield unique mechanical properties compared to those of randomly dispersed composite structures, and have immediate industry applications.

Electrospun three-dimensional porous CuO/TiO2 hierarchical nanocomposites electrode for nonenzymatic glucose biosensing

Hierarchical nanocomposites of single-crystalline CuO nanoplates attached on the surface of polycrystalline TiO2 nanotubes were first prepared by electrospinning. In application to nonenzymatic glucose determination, the precusor was electrospun onto fluorine-doped tin oxide (FTO) glass directly. After hot press and calcination, a three-dimensional porous CuO/TiO2 modified electrode was fabricated. Because of the combination of hierarchical composite and three-dimensional structure, it exhibits a high sensitivity of 1321μAmM−1cm−2 in the range from 10μM to 2mM, a detection limit of 390nM (S/N=3) and a good stability.

Self-assembled 3D flower-like hierarchical Fe3O4/KxMnO2 core–shell architectures and their application for removal of dye pollutants

A simple method for fabricating hierarchical composite structures consisting of K-birnessite (KxMnO2) nanosheets on Fe3O4 microspheres was reported in this paper. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements indicated that the size of the three-dimensional (3D) flower-like hierarchical microspheres was about 420 ± 5 nm and the shell was composed of nanosheets with a thickness of about 5 nm. The experimental parameters such as reaction time, reaction temperature and the concentration of KMnO4 used in the solution were investigated for the growth of the core–shell hierarchical structures. These core–shell composites exhibited excellent removal efficiency (over 98% for rhodamine B (RB) and 94% for methylene blue (MB)) and rapid separation from aqueous solution by an external magnetic field, taking advantage of the combined benefits of hierarchical structured KxMnO2 nanosheets and magnetic microspheres. In addition, the adsorption behavior of this new adsorbent fitted well with both Langmuir and Freundlich isotherms and the pseudo-second-order kinetic model. All the results demonstrate that the core–shell composites are effective adsorbents for the removal of dye pollutants and thus could provide a new platform for dye decontamination.

Superhydrophobicity of Bionic Alumina Surfaces Fabricated by Hard Anodizing

Bionic alumina samples were fabricated on convex dome type aluminum alloy substrate using hard anodizing technique. The convex domes on the bionic sample were fabricated by compression molding under a compressive stress of 92.5 MPa. The water contact angles of the as-anodized bionic samples were measured using a contact angle meter (JC2000A) with the 3 μL water drop at room temperature. The measurement of the wetting property showed that the water contact angle of the unmodified as-anodized bionic alumina samples increases from 90° to 137° with the anodizing time. The increase in water contract angle with anodizing time arises from the gradual formation of hierarchical structure or composite structure. The structure is composed of the micro-scaled alumina columns and pores. The height of columns and the depth of pores depend on the anodizing time. The water contact angle increases significantly from 96° to 152° when the samples were modified with self-assembled monolayer of octadecanethiol (ODT), showing a change in the wettability from hydrophobicity to super-hydrophobicity. This improvement in the wetting property is attributed to the decrease in the surface energy caused by the chemical modification.

The hierarchical structure and properties of multifunctional carbon nanotube fibre composites

The axial mechanical, electrical and thermal properties of carbon nanotubes (CNTs) can be exploited macroscopically by assembling them parallel to each other into a fibre during their synthesis by chemical vapour deposition. Multifunctional composites with high volume fraction of CNT fibres are then made by direct polymer infiltration of an array of aligned fibres. The fibres have a very high surface area, causing the polymer to infiltrate them and resulting in a hierarchical composite structure. The electrical and thermal conductivities of CNT/epoxy composites are shown to be superior to those of equivalent specimens with T300 carbon fibre (CF) which is widely used in industry. From measurements of longitudinal coefficient of thermal expansion (CTE) of the composites we show that the CTE of CNT fibres is approximately −1.6 × 10 −6 K −1, similar to in-plane graphite. The combination of electrical, thermal and mechanical properties of CNT fibre composites demonstrates their potential for multifunctionality.

Manufacturing of Hierarchical Composite Structures via Controlled Ceramic Nanoparticle Incorporation in Electrodeposited Thin Films

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Biodegradable Nanofibers-Reinforced Microfibrous Composite Scaffolds for Bone Tissue Engineering

Native bone extracellular matrix (ECM) is a complex hierarchical fibrous composite structure, resulting from the assembling of collagen fibrils at several length scales, ranging from the macro to the nanoscale. The combination of nanofibers within microfibers after conventional reinforcement methodologies seems to be a feasible solution to the rational design of highly functional synthetic ECM substitutes. The present work aims at the development of bone ECM inspired structures, conjugating electrospun chitosan (Cht) nanofibers within biodegradable polymeric microfibers [poly(butylene succinate)-PBS and PBS/Cht], assembled in a fiber mesh structure. The nanofibers-reinforced composite fiber mesh scaffolds were seeded with human bone marrow mesenchymal stem cells (hBMSCs) and cultured under osteogenic differentiation conditions. These nanofibers-reinforced composite scaffolds sustained ECM deposition and mineralization, mainly in the PBS/Cht-based fiber meshes, as depicted by the increased amount of calcium phosphates produced by the osteogenic differentiated hBMSCs. The osteogenic genotype of the cultured hBMSCs was confirmed by the expression of osteoblastic genes, namely Alkaline Phosphatase, Osteopontin, Bone Sialoprotein and Osteocalcin, and the transcription factors Runx2 and Osterix, all involved in different stages of the osteogenesis. These data represent the first report on the biological functionality of nanofibers-reinforced composite scaffolds, envisaging the applicability of the developed structures for bone tissue engineering.

A new model to simulate the elastic properties of mineralized collagen fibril

Bone, because of its hierarchical composite structure, exhibits an excellent combination of stiffness and toughness, which is due substantially to the structural order and deformation at the smaller length scales. Here, we focus on the mineralized collagen fibril, consisting of hydroxyapatite plates with nanometric dimensions aligned within a protein matrix, and emphasize the relationship between the structure and elastic properties of a mineralized collagen fibril. We create two- and three-dimensional representative volume elements to represent the structure of the fibril and evaluate the importance of the parameters defining its structure and properties of the constituent mineral and collagen phase. Elastic stiffnesses are calculated by the finite element method and compared with experimental data obtained by synchrotron X-ray diffraction. The computational results match the experimental data well, and provide insight into the role of the phases and morphology on the elastic deformation characteristics. Also, the effects of water, imperfections in the mineral phase and mineral content outside the mineralized collagen fibril upon its elastic properties are discussed.

Biomaterials by freeze casting

The functional requirements for synthetic tissue substitutes appear deceptively simple: they should provide a porous matrix with interconnecting porosity and surface properties that promote rapid tissue ingrowth; at the same time, they should possess sufficient stiffness, strength and toughness to prevent crushing under physiological loads until full integration and healing are reached. Despite extensive efforts and first encouraging results, current biomaterials for tissue regeneration tend to suffer common limitations: insufficient tissue-material interaction and an inherent lack of strength and toughness associated with porosity. The challenge persists to synthesize materials that mimic both structure and mechanical performance of the natural tissue and permit strong tissue-implant interfaces to be formed. In the case of bone substitute materials, for example, the goal is to engineer high-performance composites with effective properties that, similar to natural mineralized tissue, exceed by orders of magnitude the properties of its constituents. It is still difficult with current technology to emulate in synthetic biomaterials multi-level hierarchical composite structures that are thought to be the origin of the observed mechanical property amplification in biological materials. Freeze casting permits to manufacture such complex, hybrid materials through excellent control of structural and mechanical properties. As a processing technique for the manufacture of biomaterials, freeze casting therefore has great promise.

Synthesis of metallic glass composites using phase separation phenomena

Phase separation phenomena in metallic glass systems have been explored by thermodynamic calculation and experimental method in the present study. Miscibility gap and spinodal curve for liquid phase in Gd–Ti–Al–Co/Cu systems have been calculated using the CALPHAD method. Based on the calculation result, various types of phase separation phenomena have been observed experimentally. Basically two types of structure are observed: interconnected-type structure by spinodal decomposition; and droplet-type structure by nucleation and growth mechanism. Multi-step phase separation occurs during cooling from the liquid, forming a hierarchical length scale composite structure. Depending on the glass-forming ability of the separated liquid phase, amorphous/amorphous or amorphous/crystalline composite structure can be obtained. The present study shows that there are several factors such as critical temperature, asymmetry and composition range of the spinodal curve which affect scale and morphology of the microstructure after phase separation.