Current additive manufacturing methods present the potential to construct net-shape structures with complicated architectures, thus eliminating the need for multi-step processing and ...fasteners/joints. Combined with these features is the ability to ascribe material properties at the sub-millimeter scale, inspiring multi-material, functionally graded designs. These features make additive manufacturing an attractive option for composite materials development. In an effort to extend this family of technologies beyond nano- and micro-composites, we explore the additive manufacture of multi-directional composite preforms. This exercise has served to highlight the aspects of additive manufacturing critical to composite and general materials processing, as well as to demonstrate the high fidelity between modeled and additively manufactured structures. Within the scope of composites development, we review the state-of-the-art and discuss challenges facing the broad adoption of additive manufacturing for directionally reinforced composites processing.
Development of platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) is essential for affordable proton exchange membrane fuel cells. Herein, a new type of atomically ...dispersed Co doped carbon catalyst with a core–shell structure has been developed via a surfactant-assisted metal–organic framework approach. The cohesive interactions between the selected surfactant and the Co-doped zeolitic imidazolate framework (ZIF-8) nanocrystals lead to a unique confinement effect. During the thermal activation, this confinement effect suppressed the agglomeration of Co atomic sites and mitigated the collapse of internal microporous structures of ZIF-8. Among the studied surfactants, Pluronic F127 block copolymer led to the greatest performance gains with a doubling of the active site density relative to that of the surfactant-free catalyst. According to density functional theory calculations, unlike other Co catalysts, this new atomically dispersed Co–N–C@F127 catalyst is believed to contain substantial CoN 2+2 sites, which are active and thermodynamically favorable for the four-electron ORR pathway. The Co–N–C@F127 catalyst exhibits an unprecedented ORR activity with a half-wave potential ( E 1/2 ) of 0.84 V ( vs. RHE) as well as enhanced stability in the corrosive acidic media. It also demonstrated high initial performance with a power density of 0.87 W cm −2 along with encouraging durability in H 2 –O 2 fuel cells. The atomically dispersed Co site catalyst approaches that of the Fe–N–C catalyst and represents the highest reported PGM-free and Fe-free catalyst performance.
A clear understanding of carbon fiber (CF) microstructure is necessary for the development of high strength CFs. Here, we present an atomistic approach for generating and characterizing realistic mi- ...crostructures of CFs. Large-scale reactive molecular dynamics simulations are used to generate a set of distinct CF microstructures. Comprehensive characterization of the simulated microstructures is enabled by the development of a suite of computational structural analysis tools capable of evaluation of hy- bridization states of carbon atoms, populations and orientations of individual carbon rings, degree of graphitization, and pore size distribution. The calculation of X-ray diffraction profiles provides a direct link between the structural features of simulated samples and experimental data available for CFs. The CF generation algorithm is shown to produce microstructures with experimental densities and with structural characteristics matching those of PAN-based CFs. The key structural features affecting the properties of CFs, such as the relative fractions of graphitic, turbostratic and amorphous micro- constituents, degree of alignment, pore size distributions, and chemical cross-linking can be effectively controlled in simulations, thus enabling efficient exploration of structureeproperties relationships in CFs. The capabilities of the developed approach are illustrated by performing computational analysis of the mechanical deformation and fracture of CFs under axial tensile loading.
Heterostructured materials Zhu, Yuntian; Wu, Xiaolei
Progress in materials science,
January 2023, 2023-01-00, Letnik:
131
Journal Article
Recenzirano
Odprti dostop
Heterostructured (HS) materials are a new class of materials that are composed of heterogeneous zones with dramatically different (>100 %) mechanical or physical properties. The interactive coupling ...between these heterogeneous zones produces a synergistic effect in which the integrated property exceeds the prediction by the rule-of-mixtures. HS materials possess superior mechanical or physical properties that are not achievable by their conventional homogenous counterparts. This review focuses primarily on structural HS materials, whose superior mechanical properties are enabled by a new scientific principle: hetero-deformation induced (HDI) strengthening and HDI work hardening. Geometrically necessary dislocations (GNDs) in the soft zones pile up and accumulate near the zone boundaries, producing back stress in the soft zones and forward stress in the hard zones, which collectively produces the HDI stress. HS materials have a unique deformation behavior: formation of dispersive microscopic strain bands, which helps to distribute plastic strain over the whole gauge length, increasing uniform elongation. They can be readily produced using conventional industrial technologies and facilities at large scale and low cost. The superior properties, new materials science and great application potentials are driving the fast development of the HS materials field. This review is meant to introduce students and researchers to this emerging field, and to serve as an authoritative reference on HS materials.
The optical properties of metal nanoparticles have attracted wide interest. Recent progress in controlling nanoparticles with atomic precision (often called nanoclusters) provide new opportunities ...for investigating many fundamental questions, such as the transition from excitonic to plasmonic state, which is a central question in metal nanoparticle research because it provides insights into the origin of surface plasmon resonance (SPR) as well as the formation of metallic bond. However, this question still remains elusive because of the extreme difficulty in preparing atomically precise nanoparticles larger than 2 nm. Here we report the synthesis and optical properties of an atomically precise Au279(SR)84 nanocluster. Femtosecond transient absorption spectroscopic analysis reveals that the Au279 nanocluster shows a laser power dependence in its excited state lifetime, indicating metallic state of the particle, in contrast with the nonmetallic electronic structure of the Au246(SR)80 nanocluster. Steady-state absorption spectra reveal that the nascent plasmon band of Au279 at 506 nm shows no peak shift even down to 60 K, consistent with plasmon behavior. The sharp transition from nonmetallic Au246 to metallic Au279 is surprising and will stimulate future theoretical work on the transition and many other relevant issues.
The solubility of selenium on the sulfur site in tetrahedrite Cu12Sb4S13 has been explored by theoretical calculations here, and the results have been verified by X-ray diffraction and X-ray ...synchrotron studies on Cu12Sb4S13–xSex with x ranging from zero to 3. Density-functional theory calculations predict that Se substitution on the tetrahedral 24g site is preferred, and this is found to be consistent with Rietveld refinement of the crystal structure. High temperature thermoelectric property measurements on Cu12Sb4S13-xSex reveal that Se substitution results in a decrease in electrical resistivity without diminution of the Seebeck coefficient. The “decoupling” of these parameters leads to a 30% enhancement in power factor of the x = 1 sample compared to that of pure Cu12Sb4S13. Furthermore, in spite of an increased electronic thermal conductivity, alloy scattering of phonons caused by Se substitution reduces both the lattice and total thermal conductivities, leading to a large increase in the thermoelectric figure of merit.
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PEM (Polymer Electrolyte Membrane) fuel cells have the potential to reduce our energy use, pollutant emissions, and dependence on fossil fuels. In the past decade, significant ...advances have been achieved for commercializing the technology. For example, several PEM fuel cell buses are currently rated at the technical readiness stage of full-scale validation in realistic driving environments and have met or closely met the ultimate 25,000-h target set by the U.S. Department of Energy. So far, Toyota has sold more than 4000 Mirai PEM fuel cell vehicles (FCVs). Over 30 hydrogen gas stations are being operated throughout the U.S. and over 60 in Germany. In this review, we cover the material, design, fundamental, and manufacturing aspects of PEM fuel cells with a focus on the portable, automobile, airplane, and space applications that require careful consideration in system design and materials. The technological status and challenges faced by PEM fuel cells toward their commercialization in these applications are described and explained. Fundamental issues that are key to fuel cell design, operational control, and material development, such as water and thermal management, dynamic operation, cold start, channel two-phase flow, and low-humidity operation, are discussed. Fuels and fuel tanks pertinent to PEM fuel cells are briefly evaluated.
The objective of this review is three fold: (1) to present the latest status of PEM fuel cell technology development and applications in the portable and transportation power through an overview of the state of the art and most recent technological advances; (2) to describe materials and water/thermal transport management for fuel cell design and operational control; and (3) to outline major challenges in the technology development and the needs for fundamental research for the near future and prior to fuel cell world-wide deployment.
On the origin of contact-electrification Wang, Zhong Lin; Wang, Aurelia Chi
Materials today (Kidlington, England),
November 2019, 2019-11-00, Letnik:
30
Journal Article
Recenzirano
This paper reviews the physics mechanisms of contact electrification (or triboelectrification) among solids, liquids, and gases. It is concluded that electron transfer is the dominant mechanism for ...contact electrification and is even responsible for initiating the electric double layer at solid–liquid interfaces.
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Although contact electrification (triboelectrification) (CE) has been documented since 2600 years ago, its scientific understanding remains inconclusive, unclear, and un-unified. This paper reviews the updated progress for studying the fundamental mechanism of CE using Kelvin probe force microscopy for solid–solid cases. Our conclusion is that electron transfer is the dominant mechanism for CE between solid–solid pairs. Electron transfer occurs only when the interatomic distance between the two materials is shorter than the normal bonding length (typically ∼0.2 nm) in the region of repulsive forces. A strong electron cloud overlap (or wave function overlap) between the two atoms/molecules in the repulsive region leads to electron transition between the atoms/molecules, owing to the reduced interatomic potential barrier. The role played by contact/friction force is to induce strong overlap between the electron clouds (or wave function in physics, bonding in chemistry). The electrostatic charges on the surfaces can be released from the surface by electron thermionic emission and/or photon excitation, so these electrostatic charges may not remain on the surface if sample temperature is higher than ∼300–400 °C.
The electron transfer model could be extended to liquid–solid, liquid–gas and even liquid–liquid cases. As for the liquid–solid case, molecules in the liquid would have electron cloud overlap with the atoms on the solid surface at the very first contact with a virginal solid surface, and electron transfer is required in order to create the first layer of electrostatic charges on the solid surface. This step only occurs for the very first contact of the liquid with the solid. Then, ion transfer is the second step and is the dominant process thereafter, which is a redistribution of the ions in solution considering electrostatic interactions with the charged solid surface. This is proposed as a two-step formation process of the electric double layer (EDL) at the liquid–solid interface. Charge transfer in the liquid–gas case is believed to be due to electron transfer once a gas molecule strikes the liquid surface to induce the overlapping electron cloud under pressure. In general, electron transfer due to the overlapping electron cloud under mechanical force/pressure is proposed as the dominant mechanism for initiating CE between solids, liquids and gases. This study provides not only the first systematic understanding about the physics of CE, but also demonstrates that the triboelectric nanogenerator (TENG) is an effective method for studying the nature of CE between any materials.
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As polymer networks infiltrated with water, hydrogels constitute the major components of the human body; and hydrogels have been widely used in applications that closely interact with ...biological organisms, such as tissue engineering, drug delivery, and biological research. More recently, owing to their superior softness, wetness, responsiveness, biocompatibility, and bioactivity, hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, coatings, optics, electronics, and water harvesters. A nascent field named hydrogel machines rapidly evolves, exploiting hydrogels as key components for devices and machines. While there are reviews on individual categories of hydrogel machines in literature, a comprehensive discussion on various categories of hydrogel machines that systematically correlate hydrogels’ properties and machines’ functions is still missing in the field. This review is aimed to provide such a panoramic overview. We first classify various hydrogel machines into a number of categories according to their applications. For each category, we discuss (i) the working principles of the hydrogel machines, (ii) the specific properties of hydrogels that enable the key functions of the machines, and (iii) challenges faced by hydrogel machines and recent developments to address them. The field of hydrogel machines will not only translate fundamental understanding of hydrogels into new applications, but also shift the paradigm in machine design by integrating hydrogels that can potentially minimize physical and physiological mismatches with biological organisms.
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Photocatalytic carbon dioxide (CO2) reduction to obtain hydrocarbon solar fuels is one of the promising strategies to solve energy crisis and complement carbon cycle. However, the low ...activity and poor product selectivity greatly limit its practical application. Tuning product selectivity is of great significance to improve the yield of target product and deepen the understanding of CO2 reduction reaction mechanism. In this review, we firstly summarize the widely accepted pathways of photocatalytic CO2 reduction reactions. Secondly, important factors affecting product selectivity are analyzed, mainly including light-excitation attributes, band structure of photocatalysts, separation of photogenerated charge carriers, adsorption/activation of reactants, surface active sites of catalytic reaction, and adsorption/desorption of intermediates. Finally, the challenges and perspectives in developing photocatalysts with high CO2 reduction efficiency and product selectivity are presented.
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