Bioprocess‐Inspired Microscale Additive Manufacturing of Multilayered TiO2/Polymer Composites with Enamel‐Like Structures and High Mechanical Properties

Natural structure‐forming processes found in biological systems are fantastic and perform at ambient temperatures, in contrast with anthropogenic technologies that commonly require harsh conditions. A new research direction “bioprocess‐inspired fabrication” is proposed to develop novel fabrication techniques for advanced materials. Enamel, an organic–inorganic composite biomaterial with outstanding mechanical performance and durability, is formed by repeating the basic blocks consisting of columnar hydroxyapatite or fluorapatite and an organic matrix. Inspired by the enamel formation process, a microscale additive manufacturing method is proposed for achieving a multilayered organic–inorganic columnar structure. In this approach, rutile titanium dioxide (TiO₂) nanorods, polymers, and graphene oxide (GO) are sequentially assembled in a layer‐by‐layer fashion to form an organic–inorganic structure. In particular, GO serves as a substrate for TiO₂ nanorods and interacts with polymers, jointly leading to the strength of the composites. Impressively, this enamel‐like structure material has hardness (1.56 ± 0.05 GPa) and ultrahigh Young's modulus (81.0 ± 2.7 GPa) comparable to natural enamel, and viscoelastic property (0.76 ± 0.12 GPa) superior to most solid materials. Consequently, this biomimetic synthetic approach provides an in‐depth understanding for the formation process of biomaterials and also enables the exploration of a new avenue for the preparation of organic–inorganic composite materials.     

Stepped Moduli in Layered Composites

This paper describes adaptive composites that respond to mechanical stimuli by changing their Young's modulus. These composites are fabricated by combining a shorter layer of elastic material (e.g., latex) and a longer layer of stiffer material (e.g., polyethylene and Kevlar), and fixing them together at their ends. Tension along the layered composite increases its length, and as the strain increases, the composite changes the load‐bearing layer from the elastic to the stiff material. The result is a step in the Young's modulus of the composite. The characteristics of the step (or steps) can be engineered by changing the constituent materials, the number of layers, and their geometries (e.g., sinusoidal, hierarchical, two‐dimensional web‐like, rod‐coil, embedded, and ring structures). For composites with more than two steps in modulus, the materials within the composites can be layered in a hierarchical structure to fit within a smaller volume, without sacrificing performance. These composites can also be used to make structures with tunable, stepped compressive moduli. An adaptation of these principles can generate an electronic sensor that can monitor the applied compressive strain. Increasing or decreasing the strain closes or opens a circuit and reversibly activates a light‐emitting diode.     

High Performance Nanotube‐Reinforced Plastics: Understanding the Mechanism of Strength Increase

Polymer–multiwalled carbon nanotube composite films were fabricated using two types of polymer matrices, namely poly(vinyl alcohol), (PVA) and chlorinated polypropylene. In the first case, the PVA was observed to form a crystalline coating around the nanotubes, maximising interfacial stress transfer. In the second case the interface was engineered by covalently attaching chlorinated polypropylene chains to the nanotubes, again resulting in large stress transfer. Increases in Young's modulus, tensile strength, and toughness of × 3.7, × 4.3, and × 1.7, respectively were observed for the PVA‐based materials at less than 1 wt.‐% nanotubes. Similarily for the polypropylene‐based composites, increases in Young's modulus, tensile strength and toughness of × 3.1, × 3.9, and × 4.4, respectively, were observed at equivalent nanotube loading levels. In addition, a model to describe composite strength was derived. This model shows that the tensile strength increases in proportion to the thickness of the interface region. This suggests that composite strength can be optimized by maximising the thickness of the crystalline coating or the thickness of the interfacial volume partially occupied by functional groups.     

Bioinspired Nanocomposites: Ordered 2D Materials Within a 3D Lattice

Composites, materials composed of two or more materials—metallic, organic, or inorganic—usually exhibit the combined physical properties of their component materials. The result is a material that is superior to conventional monolithic materials. Advanced composites are used in a variety of industrial applications and therefore attract much scientific interest. Here the formation of novel carbon‐based nanocomposites is described via incorporation of graphene oxide (GO) into the crystal lattice of single crystals of calcite. Incorporation of a 2D organic material into single‐crystal lattices has never before been reported. To characterize the resulting nanocomposites, high‐resolution synchrotron powder X‐ray diffraction, electron microscopy, transmission electron microscopy, fluorescence microscopy and nanoindentation tests are employed. A detailed analysis reveals a layered distribution of GO sheets incorporated within the calcite host. Moreover, the optical and mechanical properties of the calcite host are altered when a carbon‐based nanomaterial is introduced into its lattice. Compared to pure calcite, the composite GO/calcite crystals exhibits lower elastic modulus and higher hardness. The results of this study show that the incorporation of a 2D material within a 3D crystal lattice is not only feasible but also can lead to the formation of hybrid crystals exhibiting new properties.     

ZnO压敏/BaTiO3电容双功能多层器件

研究了ZnO压敏/BaTiO3电容双功能多层器件,在共烧过程中瓷体分层和收缩的问题。通过调节BaTiO3瓷料和粘合剂的质量比为1:0.8,压敏陶瓷/粘合剂质量比为1:0.6,烧结温度1100℃和瓷体/瓷体的交界层,使BaTiO3体系电容器和ZnO压敏陶瓷电阻器的共烧器件,在宏观上匹配很好。这为其它相似元件的共烧提供了参考。     

A General Approach for Fabricating Arc‐Shaped Composite Nanowire Arrays by Pulsed Laser Deposition

Here, a new method is demonstrated that uses sideways pulsed laser deposition to deliberately bend nanowires into a desired shape after growth and fabricate arc‐shaped composite nanowire arrays of a wide range of nanomaterials. The starting nanowires can be ZnO, but the materials to be deposited can be metallic, semiconductor, or ceramic depending on the application. This method provides a general approach for rational fabrication of a wide range of side‐by‐side or “core–shell” nanowire arrays with controllable degree of bending and internal strain. Considering the ZnO is a piezoelectric and semiconductive material, its electrical properties change when deformed. This technique has potential applications in tunable electronics, optoelectronics, and piezotronics.     

A Delivery System for Self‐Healing Inorganic Films

Multilayer composites that utilize polymeric and brittle inorganic films are essential components for extending the lifetimes and exploiting the flexibility of many electronic devices. However, crack formation within the brittle inorganic layers that arise from defects as well as the flexing of these multilayer composite materials allows the influx of atmospheric water, a major source of device degradation. Thus, a composite material that can initiate self‐healing upon the influx of environmental water through defects or stress‐induced cracks would find potential applications in multilayer composite materials for permeation barriers. In the present study, the reactive metal oxide precursor TiCl₄ is encapsulated within the pores of a degradable polymer, poly(lactic acid) (PLA). Electrospun PLA fibers are found to be reactive to atmospheric water leading to the hydrolysis of the degradable polymer shell and subsequent release of the reactive metal oxide precursor. Release of the reactive TiCl₄ from the pores results in hydrolysis of the metal oxide precursor, forming solid titanium oxides at the surface of the fibers. The efficacy of this self‐healing delivery system is also demonstrated by the integration of these reactive fibers in the polymer planarization layer, poly(methyl methacrylate), of a multilayer film, upon which an alumina barrier layer is deposited. The introduction of nanocracks in the alumina barrier layer lead to the release of the metal oxide precursor from the pores of the fibers and the formation of titanium dioxide nanoparticles within the crack and upon the thin film surface. In this study the first delivery system that may find utility for the self‐healing of multilayer barrier films through the site‐specific delivery of metal oxide nanoparticles through smart reactive composite fibers is established.     

Probing the Effects of Nanoscale Architecture on the Mechanical Properties of Hexagonal Silica/Polymer Composite Thin Films

We examine the effects of controlling nanoscale architecture on the tensile properties of honeycomb‐structured silica/polymer composite films. The hexagonal films are produced using evaporation‐induced self‐assembly and uniaxially strained using a home‐built tensile testing apparatus. Significant differences in the yield strain, failure strain, and tensile moduli between the axes parallel and perpendicular to the film‐deposition direction are observed for the thinnest films examined and are attributed to anisotropy in the film nanostructure that is further characterized with transmission electron microscopy and atomic force microscopy. For properly oriented composites, these films have tensile moduli comparable to the Young's modulus of bulk silica but exhibit failure strains that are about an order of magnitude larger than those seen in typical bulk‐silica systems. The yielding and failure processes are explored using X‐ray diffraction and optical microscopy and are characterized by irreversible changes in the nanoscale architecture. We show that tuning the nanoscale architecture can provide control over the tensile properties of composites, allowing for materials with combinations of stiffness and elasticity unachievable in the analogous bulk systems.     

Interaction Scheme and Temperature Behavior of Energy Transfer in a Light‐Emitting Inorganic‐Organic Composite System

Determining and controlling the inter‐component excitation conversion in light‐emitting nanocomposite materials is a key factor for predicting the composite luminescence properties and for the operation of many opto‐electronic devices. Here we present an extensive study of the inter‐component energy transfer in the composite system given by ZnO particles interacting with the conjugated polymer, poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene]. The composite emission is studied upon varying the acceptor concentration, and the system temperature in the range 50–300 K. The temperature dependence of the energy transfer rate is described by a rate model, taking into account the temperature dependence of the single components nonradiative decay rates, and a dipole–surface interaction scheme in the hybrid material. The proposed model accounts very well for the experimental observation of energy transfer and can be used to predict the temperature behavior of the emission from light‐emitting nanocomposite materials.     

Increased Interface Strength in Carbon Fiber Composites through a ZnO Nanowire Interphase

One of the most important factors in the design of a fiber reinforced composite is the quality of the fiber/matrix interface. Recently carbon nanotubes and silicon carbide whiskers have been used to enhance the interfacial properties of composites; however, the high growth temperature degrade the fiber strength and significantly reduce the composite's in‐plane properties. Here, a novel method for enhancing the fiber/matrix interfacial strength that does not degrade the mechanical properties of the fiber is demonstrated. The composite is fabricated using low‐temperature solution‐based growth of ZnO nanowires on the surface of the reinforcing fiber. Experimental testing shows the growth does not adversely affect fiber strength, interfacial shear strength can be significantly increased by 113%, and the lamina shear strength and modulus can be increased by 37.8% and 38.8%, respectively. This novel interface could also provide embedded functionality through the piezoelectric and semiconductive properties of ZnO.     

A General Strategy to Fabricate Flexible Oxide Ceramic Nanofibers with Gradient Bending-Resilience Properties

Inorganic materials assembled with rigid elements such as crystals or graphitized carbon generally show brittleness and hardness. However, it is found that both TiO₂ ceramic crystal nanofibers (NFs) and carbon NFs show superior flexibility, in which the former are surprisingly knottable and the latter exhibit excellent bending-resilience property. The different flexure mechanisms are revealed by fabricating composite NFs of these two constituents and find that the carbon NFs can be recovered to the original states after releasing the external force, while the bending-resilience is weakened and the softness of the composite NFs is enhanced upon increasing the TiO₂ content. The graphitized carbon can store mechanical deformation energy that enables the NFs with bending-resilience, while both the homogeneous interfaces between TiO₂ crystals and the heterogeneous interfaces between TiO₂ and carbon can alleviate stress concentration, which reduce the flexural modulus of the composite NFs. By filling different contents of elastic carbon into TiO₂ NFs, a series of flexible NFs that exhibit gradient bending-resilience properties are fabricated. This study provides a deeper understanding of the mechanical properties of inorganic materials.     

Shape Anisotropy Influencing Functional Properties: Trigonal Prismatic ZnO Nanoparticles as an Example

The shape of crystalline particles is recognized as one important parameter for the adjustment of functional properties of inorganic materials. The surfaces of a thermodynamically stable crystal correspond to a set of lattice planes determined by the minimum interface energy. Thus, a morphology deviating from the most stable state correlates to either a change of the proportion of those surfaces to each other or ultimately a new set of surfaces emerges. At the nanoscale, when the surface‐to‐volume ratio is large, it is expected that a change in morphology implies a measurable alteration of properties. Here, the synthesis of nanocrystalline ZnO nano­particles possessing a new non‐equilibrium shape is presented. The reaction of special organometallic precursors at the interface of a water‐in‐oil emulsion facilitates the synthesis of fairly monodisperse prismatic ZnO nanocrystallites with an adjustable aspect ratio in gram amounts. It is found that the special morphology influences the bulk properties of the ZnO materials. Contrary to the well‐known quantum size effect (smaller particles produce a blue‐shift), a shortening of the ZnO nanoprisms induces a decrease in the bandgap (red‐shift). This effect is due to the influence of an electric field inside the particles caused by the polarity of the surfaces terminating the nanoprisms (the quantum‐confined Stark effect).     

高性能片式多层氧化锌压敏电阻器材料研究

对非Bi系氧化锌压敏电阻材料进行了系统研究。研究结果表明:在ZnO基体材料中,添加适量PbO、Co2O3、Cr2O3、MnO2、ZrO2、TiO2、Sb2O3 、B2O3等非Bi系添加剂,采用传统陶瓷制备工艺和合适烧结工艺,可获得a >50、IL<1 mA、烧结温度低于1 100℃的实用非Bi系氧化锌电阻瓷料。采用该瓷料,利用MLC工艺,选用Pd30/Ag70电极浆料,制作出V1mA<30 V、a >30、IL<1 mA的片式多层压敏电阻器。     

叠层片式ZnO压敏电阻器配方研究

研究了以ZnO-Bi2O3-SiO2系为基础,适当添加Co、Mn、Sb、Cr、Ni等金属氧化物的叠层片式ZnO压敏电阻器配方。通过严格控制Bi2O3及SiO2的含量,较好地解决了瓷料与银鈀内电极的共烧问题,且电性能优良。其瓷料的特点是烧结温度低(<1050℃),产品非线性系数高(≥25),泄漏电流小(<5μA),限制电压低(V1A/V1mA<1.70),通流能量大(≥1800A/cm2)。     

Tuning the Rigidity of Silk Fibroin for the Transfer of Highly Stretchable Electronics

The transfer of stretchable electrodes or devices from one substrate to another thin elastomer is challenging as the elastic stamp often yields a huge strain beyond the stretchability limit of the electrodes at the debonded interface. This will not happen if the stamp is rigid. However, a rigid material cannot be used as the substrate for stretchable electrodes. Herein, silk fibroin with tunable rigidity (Young's modulus can be changed from 134 kPa to 1.84 GPa by controlling the relative humidity) is used to transfer highly stretchable metal networks as highly conformable epidermal electrodes. The silk fibroin stamp is tuned to be rigid during peeling, and then be soft and highly stretchable as a substrate when laminated on moisturized human skin. In addition, the epidermal electrodes exhibit no skin irritation or inflammation after attaching for over 10 d. The high compliance results in a lower interface impedance and lower noises of the electrode in measuring electromyographic signals, compared with commercial Ag‐AgCl gel electrodes. The strategy of tuning the rigidity at different stages of transfer is a general method that can be extended to the transfer of other stretchable electrodes and devices for epidermal electronics, human machine interfaces, and soft robotics.     

Nanometer‐Scale Mapping of Elastic Modules in Biogenic Composites: The Nacre of Mollusk Shells

In this study, a newly developed nanoscale modulus mapping is applied in order to visualize the 2D‐distribution of mechanical characteristics in the aragonitic nacre layer of Perna canaliculus (green mussel) shells. Modulus maps provide lateral resolution of about 10 nm. They allow the aragonitic mineral (CaCO₃) tablets and the interfaces between them to be clearly resolved, which are filled by an organic substance (mainly beta‐chitin). The experimental data are compared with finite element simulations that also take into account the tip radius of curvature and the thickness of organic layers, as measured by means of scanning electron microscopy with back‐scattered electrons. Based on this comparison, the Young modulus of beta‐chitin is extracted. The obtained number, E_β = 40 GPa, is higher than previously evaluated. The collected maps reveal that the elastic modules in the nacre layer change gradually across the ceramic/organic interfaces within a spatial range four times wider than the thickness of the organic layers. This is possibly due to inhomogeneous distribution of organic macromolecules within ceramic tablets. According to the data, the concentration of macromolecules gradually increases when approaching the organic/ceramic interfaces. A behavior of this type is unique to biogenic materials and distinguishes them from synthetic composite materials. Finally, three possible mechanisms that attempt to explain why gradual changes of elastic modules significantly enhance the overall resistance to fracture of the nacre layer are briefly discussed. The experimental findings support the idea that individual ceramic tablets, comprising the nacre, are built of the compositionally and functionally graded ceramic material. This sheds additional light on the origin of the superior mechanical properties of biogenic composites.     

Functional Organic Semiconductors Assembled via Natural Aggregating Peptides

Specific peptide sequences designed by inspection of protein–protein interfaces have been identified and used as tectons in hybrid functional materials. Here, an 8‐mer peptide derived from an interface of the peroxiredoxin family of self‐assembling proteins is exploited to encode the assembly of the perylene imide‐based organic semiconductor building blocks. By augmenting the peptide with additional functionality to trigger aggregation and manipulate the directionality of peptide‐semiconductor coupling, a series of hybrid materials based on the natural peptide interface is presented. Using spectroscopic probes, the mode of self‐assembly and the electronic coupling between neighboring perylene units is shown to be strongly affected by the number of peptides attached, and by their backbone directionality. The disubstituted material with peptides extending in the N to C direction away from the perylene core exhibits strong coupling and long‐range order, both attractive properties for electronic device applications. A bio‐organic field‐effect transistor is fabricated using this material, highlighting the possibilities of exploiting natural peptide tectons to encode self‐assembly in other functional materials and devices.     

Methods and Applications of Multilayer Silk Fibroin Laminates Based on Spatially Controlled Welding in Protein Films

Recent use of biopolymers as interface materials between planar, inorganic electronics and biological tissues has required the adaptation of micro‐ and nanofabrication techniques for use with these nontraditional materials. In this work, a method which builds on this principle for spatial control of adhesion in multilayer silk fibroin laminates is investigated. This is accomplished through the addition of a spatially patterned amorphous silk adhesive layer in between the films to be adhered, before thermally processing them with heat (120 °C) and pressure (80 Psi) according to established procedures. A one‐step method for rapid, high‐throughput fabrication is demonstrated, which establishes a strong (1100 kPa) bond between the layers independent of the initial processing conditions of the films. The adhesive layers can be patterned using existing silk fabrication techniques, allowing for the assembly of complex geometries including bilayers and microbubbles. Additionally, the utility of this method is demonstrated for potential applications in drug delivery and transient electronics. This approach provides a versatile method for construction of complex multilayer structures in silk, which with future work may ultimately improve the utility of this material as a bridge between high technology and the biomedical sciences.     

Hybrid ZnO–Dye Hollow Spheres with New Optical Properties by a Self‐Assembly Process Based on Evans Blue Dye and Cetyltrimethylammonium Bromide

A facile one‐step process for the fabrication of hybrid ZnO–dye hollow spheres with novel optical properties has been discovered. Addition of Evans blue (EB) dye to cetyltrimethylammonium bromide (CTAB) results in the formation of CTAB‐EB micelles through an ionic self‐assembly process, and the resulting material acts as a soft template for the crystallization of ZnO upon addition of a zinc salt and ammonia under mild refluxing conditions. The formation mechanism of such hollow spheres has been investigated. These new hybrid ZnO–dye hollow spheres display distinct optical properties that differ from properties observed for the pure ZnO and dye components. This approach is a new and effective method for fabricating novel semiconductor–dye hybrids with unique electronic and optical properties and is expected to provide access to additional inorganic–organic materials with novel structures and unusual functionalities.     

Scaling the Stiffness,Strength, and Toughness of Ceramic‐Coated Nanotube Foams into the Structural Regime

Natural materials such as bone and tooth achieve precisely tuned mechanical and interfacial properties by varying the concentration and orientation of their nanoscale constituents. However, the realization of such control in engineered foams is limited by manufacturing‐driven tradeoffs among the size, order, and dispersion uniformity of the building blocks. It is demonstrated how to manufacture nanocomposite foams with precisely controllable mechanical properties via aligned carbon nanotube (CNT) growth followed by atomic layer deposition (ALD). By starting with a low density CNT forest and varying the ALD coating thickness, we realize predictable ≈1000‐fold control of Young's modulus (14 MPa to 20 GPa, where E ～ ρ ^2.8), ultimate compressive strength (0.8 MPa to 0.16 GPa), and energy absorption (0.4 to 400 J cm^–3). Owing to the continuous, long CNTs within the ceramic nanocomposite, the compressive strength and toughness of the new material are 10‐fold greater than commercially available aluminum foam over the same density range. Moreover, the compressive stiffness and strength equal that of compact bone at 10% lower density. Along with emerging technologies for scalable patterning and roll‐to‐roll manufacturing and lamination of CNT films, coated CNT foams may be especially suited to multifunctional applications such as catalysis, filtration, and thermal protection.