Showing papers in "Materials Science & Engineering R-reports in 2018"

Additive manufacturing of multi-material structures

Abstract: Additive manufacturing (AM) or 3D printing has revolutionized the manufacturing world through its rapid and geometrically-intricate capabilities as well as economic benefits. Countless businesses in automotive, aerospace, medical, and even food industries have adopted this approach over the past decade. Though this revolution has sparked widespread innovation with single material usage, the manufacturing world is constantly evolving. 3D printers now have the capability to create multi-material systems with performance improvements in user-definable locations. This means throughout a single component, properties like hardness, corrosion resistance, and environmental adaptation can be defined in areas that require it the most. These new processes allow for exciting multifunctional parts to be built that were never possible through traditional, single material AM processes. AM of metals, ceramics, and polymers is currently being evaluated to combine multiple materials in one operation and has already produced never-before-produced parts. While multi-material AM is still in its infancy, researchers are shifting their mindset toward this unique approach showing that the technology is beginning to advance past a research and development stage into real-world applications. This review is intended to highlight the range of 3D printed polymer-based, metal-metal, and metal-ceramic applications while discussing advantages and challenges with additively manufactured multi-material structures.

Thermal conductivity of polymers and polymer nanocomposites

Abstract: Polymers are widely used in industry and in our daily life because of their diverse functionality, light weight, low cost and excellent chemical stability. However, on some applications such as heat exchangers and electronic packaging, the low thermal conductivity of polymers is one of the major technological barriers. Enhancing the thermal conductivity of polymers is important for these applications and has become a very active research topic over the past two decades. In this review article, we aim to: 1). systematically summarize the molecular level understanding on the thermal transport mechanisms in polymers in terms of polymer morphology, chain structure and inter-chain coupling; 2). highlight the rationales in the recent efforts in enhancing the thermal conductivity of nanostructured polymers and polymer nanocomposites. Finally, we outline the main advances, challenges and outlooks for highly thermal-conductive polymer and polymer nanocomposites.

Structural evolutions of metallic materials processed by severe plastic deformation

Abstract: Bulk nanostructured (ns)/ultrafine-grained (UFG) metallic materials possess very high strength, making them attractive for high strength, lightweight and energy efficient applications. The most effective approach to produce bulk ns/UFG metallic materials is severe plastic deformation (SPD). In the last 30 years, significant research efforts have been made to explore SPD processing of materials, SPD-induced microstructural evolutions, and the resulting mechanical properties. There have been a few comprehensive reviews focusing mainly on SPD processing and the mechanical properties of the resulting materials. Yet no such a review on SPD-induced microstructural evolutions is available. This paper aims to provide a comprehensive review on important microstructural evolutions and major microstructural features induced by SPD processing in single-phase metallic materials with face-centered cubic structures, body-centered cubic structures, and hexagonal close-packed structures, as well as in multi-phase alloys. The corresponding deformation mechanisms and structural evolutions during SPD processing are discussed, including dislocation slip, deformation twinning, phase transformation, grain refinement, grain growth, and the evolution of dislocation density. A brief review on the mechanical properties of SPD-processed materials is also provided to correlate the structure with mechanical properties of SPD-processed materials, which is important for guiding structural design for optimum mechanical properties of materials.

Conductive nitrides: Growth principles, optical and electronic properties, and their perspectives in photonics and plasmonics

Abstract: The nitrides of most of the group IVb-Vb-VIb transition metals (TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN) constitute the unique category of conductive ceramics. Having substantial electronic conductivity, exceptionally high melting points and covering a wide range of work function values, they were considered for a variety of electronic applications, which include diffusion barriers in metallizations of integrated circuits, Ohmic contacts on compound semiconductors, and thin film resistors, since early eighties. Among them, TiN and ZrN are recently emerging as significant candidates for plasmonic applications. So the possible plasmonic activity of the rest of transition metal nitrides (TMN) emerges as an important open question. In this work, we exhaustively review the experimental and computational (mostly ab initio) works in the literature dealing with the optical properties and electronic structure of TMN spanning over three decades of time and employing all the available growth techniques. We critically evaluate the optical properties of all TMN and we model their predicted plasmonic response. Hence, we provide a solid understanding of the intrinsic (e.g. the valence electron configuration of the constituent metal) and extrinsic (e.g. point defects and microstructure) factors that dictate the plasmonic performance. Based on the reported optical spectra, we evaluate the quality factors for surface plasmon polariton and localized surface plasmon for various TMN and critically compare them to each other. We demonstrate that, indeed TiN and ZrN along with HfN are the most well-performing plasmonic materials in the visible range, while VN and NbN may be viable alternatives for plasmonic devices in the blue, violet and near UV ranges, albeit in expense of increased electronic loss. Furthermore, we consider the alloyed ternary TMN and by critical evaluation and comparison of the reported experimental and computational works, we identify the emerging optimal tunable plasmonic conductors among the immense number of alloying combinations.

Naturally-derived biopolymer nanocomposites: Interfacial design, properties and emerging applications

Abstract: Naturally derived materials with hierarchical organization are attractive candidates for high-performance and functional bionanocomposites because of their renewability, biocompatibility, biodegradability, flexibility, and the availability of multiple reactive sites for introducing novel functionalities. Complementary to these inherent properties, the synergistic combination of biological and synthetic components facilitated by strong interfacial interactions, can substantially enhance the structural performance and facilitate added functionalities of these bio-enabled nanocomposites. In this review, we discuss the current progress of naturally-derived bionanocomposites that has advanced our understanding of the biological/synthetic interfaces. In these bionanocomposites, various novel synthetic nanomaterials (e.g., graphene, carbon nanotube, mineral nanoparticles and metallic nanoparticles) are efficiently integrated with biological components to achieve novel properties such as superior electrical and thermal conductivity, controlled gas barrier properties, complex actuation, and unique optical properties. Two popular biocomponents, nanocellulose and silk, are mainly discussed in this review as representatives of classes of polysaccharides and polypeptides with some other biopolymers briefly discussed as well. The structures and morphologies, processing strategies, tailored functionalities and emerging applications of these bionanocomposites are analyzed and summarized. Finally, we present an outlook on the current trends in providing structural and interfacial enhancement as well as emerging applications which will employ new optical, sensing, structural, and actuating functionalities and properties.

Electrically conducting fibres for e-textiles: An open playground for conjugated polymers and carbon nanomaterials

Abstract: Conducting fibres and yarns promise to become an essential part of the next generation of wearable electronics that seamlessly integrate electronic function into one of the most versatile and most widely used form of materials: textiles. This review explores the many types of conducting fibres and yarns that can be realised with conjugated polymers and carbon materials, including carbon black, carbon nanotubes and graphene. We discuss how the interplay of materials properties and the chosen processing technique lead to fibres with a wide range of electrical and mechanical properties. Depending on the choice of conjugated polymer, carbon nanotube, graphene, polymer blend, or nanocomposite the electrical conductivity can vary from less than 10−3 to more than 103 S cm−1, accompanied by an increase in Young's modulus from 10 s of MPa to 100 s of GPa. Further, we discuss how conducting fibres can be integrated into electronic textiles (e-textiles) through e.g. weaving and knitting. Then, we provide an overview of some of the envisaged functionalities, such as sensing, data processing and storage, as well as energy harvesting e.g. by using the piezoelectric, thermoelectric, triboelectric or photovoltaic effect. Finally, we critically discuss sustainability aspects such as the supply of materials, their toxicity, the embodied energy of fibre and textile production and recyclability, which currently are not adequately considered but must be taken into account to ready carbon based conducting fibres for truly practical e-textile applications.

A comprehensive review of lithium salts and beyond for rechargeable batteries: Progress and perspectives

Abstract: The increasing need for energy storage has been the motivation for intensive research in batteries with different chemistries in the recent past. Among the elements of the batteries, the salts and their solvent play an important role. In particular, the cathodic stability at low potential depends importantly on the choice of the cation, while the stability at high potentials is mainly due to oxidation of anions and the ion mobility and dissociation depend primarily on the delocalization of the anion, so that many attempts are made to find the optimum choice of both the cations and anions of the salts, and their solvents. Although lithium-based batteries are almost exclusively used today, efforts are currently made to explore batteries based on sodium, aluminum, magnesium, calcium, potassium. The purpose of the present work is to review the salts and solvents that have been proposed in these different batteries and discuss their properties and their ability to be used in the near future and in the next generation of batteries.

High-entropy alloys and metallic nanocomposites: Processing challenges, microstructure development and property enhancement

Abstract: Two classes of new materials, i.e. high entropy alloys (HEAs) and metallic nanocomposites offer processing related challenges, while showing significant promise for an array of technological applications requiring wear and irradiation resistance. This review addresses those challenges together with microstructure-property correlations. In particular, the main focus of this review is to demonstrate the efficacy of the superfast densification route, i.e., spark plasma sintering (SPS) as an effective consolidation route for these two classes of materials. To start with, this review will critically analyze the influence of conventional solidification on the microstructure and property of Cu-based bearing alloys. Using nanocrystalline Cu-Pb and Cu-Pb-TiB2, Cu-Pb-cBN (cubic boron nitride) as model systems, various aspect of the microstructure-property correlation and enhancement of the tribological properties will be highlighted. A thorough understanding of the processing related issues and stability of the nanoscale/ultrafine microstructure obtained via SPS will be illustrated. A significant part of this review will further discuss the development of novel nanostructured HEAs and HEA-based composites for wear and irradiation resistance applications. The efficacy of SPS route to prepare bulk HEAs with high sinter density will be demonstrated, together with property enhancement in single phase and two-phase HEAs. HEA-based nanocomposites containing soft metallic dispersoids (Bi, Pb, Ag) and ceramic lubricating phases (MoS2, CaF2/BaF2) for wear resistance application will be highlighted. Finally, oxide dispersed and refractory HEAs via MASPS route for irradiation resistance application will be discussed to elucidate the effective usage of the design and development of HEAs for technologically critical applications.

Recent progress in ultrathin two-dimensional semiconductors for photocatalysis

Abstract: Benefiting from the unique electronic structures and high specific surface areas, two-dimensional (2D) semiconductors present significant advantages in photocatalytic researches for realizing efficient solar energy utilization. As compared with the bulk counterparts, 2D semiconductors tend to possess distinct photoexcitation processes and thus diverse photocatalytic behaviors, owing to the abundant surface structural factors and strong quantum confinement effects. Accordingly, enormous efforts have been devoted to exploring the relationship between the structure and photoexcitation processes in 2D semiconductors, for the sake of the optimization of photocatalytic performance. The attempts not only establish various new 2D semiconductor-based photocatalysts, but also provide in-depth understanding on the involved relevant mechanisms. Herein, an overview of recent advances in the exploitation of 2D semiconductors in photocatalytic applications is presented. By summarizing the investigations on structural characterization and photoexcitation processes, we highlight the relationship between structural factors and photocatalytic performance; in particular, the impacts of excitonic effects mediated by Coulomb interactions on photocatalytic behaviors are discussed. We conclude the review by exploring the future challenges and opportunities for the developments of 2D materials on photocatalytic areas.

Solution-processable organic and hybrid gate dielectrics for printed electronics

Abstract: Over the few past decades, printed electronics as an emerging technology have achieved tremendous progress in material design and device fabrication. OTFTs are the key building blocks of many electronic devices. Printable OTFTs have promising applications in flexible circuits, sensors and backplanes for active-matrix displays. Gate dielectrics play key roles in OTFTs to afford electrical insulating properties and interfaces for charge transport. In this paper, we review the recent progress of polymer and hybrid dielectrics for printable OTFTs. The requirement and mechanism of the gate dielectrics, different types of materials and remaining challenges for this field are presented. This review will provide comprehensive and timely guidelines for dielectric design and device engineering towards the fabrication of printed low-voltage and high-performance OTFTs.

Organic polymeric and small molecular electron acceptors for organic solar cells

Abstract: In organic solar cells (OSCs), the electron donor (D) and electron acceptor (A) blended active layer is the most crucial component for governing the power conversion efficiency (PCE). Various efficient donor materials with wide structural variations have been developed to couple with high-electron mobility fullerene-based acceptors, giving PCEs beyond 12%. However, fullerene-embedded OSCs encounter great challenges of low flexibility for structural modifications, poor absorption and blend morphological stability. The demand for alternative acceptors drives current OSC research towards non-fullerene acceptors (NFAs). Tailor-made NFAs of polymer or small molecule (SM) can typically exhibit tunable optical and electrochemical properties, high solubility, air stability, and favorable intermolecular interactions leading to compact packing and good nano-phase segregation in the active blend. In this review, we systematically depict the effects of molecular structures on the physical properties and device performances. The promising/most popular cores and general molecular design strategies of NFAs are outlined. The polymeric and SM NFAs were classified into several sub-groups based on their structural features, and in every sub-group, the structural evolution, current status, the champion case as well as the future challenges were highlighted and discussed. For polymeric NFAs, naphtalene diimide (NDI) and perylene diimide (PDI) are most promising and widely explored due to their easy synthesis, high electron affinity and mobility, leading to promising PCE when NDI and PDI units are conjugated with various electron-rich/deficient aromatics. Various electron-deficient core-based polymeric NFAs were also employed. Aromatic diimides (NDI and PDI) were also widely employed as the central core or terminal unit for SM NFAs. In particular, PDI was interested in electron deficient core, and their monomers, dimers, and trimers gave various degrees of success. PDI trimeric NFA showed superior PCE (∼9.3%) because of its twisted 3D or fused geometry capable of interlocking the polymer donor allows optimum molecular packing, morphology and, therefore, efficient charge separation and transport. The excellent photochemical stability, strong absorption and synthetic flexibility of diketopyrrolopyrrole (DPP) produced promising SM NFAs. The rigid and co-planar indacenodithiophene (IDT) cores bearing various electron-deficient terminal groups were extensively explored, and the structural engineering on both the core and side chain groups together with post-treatments produced the highest PCE (∼13.2%). These results conclude that NFAs possess the better possibility for tuning absorption profile, matched energy levels and optimal D/A nano-morphology for delivering promising PCEs. We highlighted the structure-property-performance relationships and future challenges, and hope this article can trigger new ideas for designing more promising NFAs.

Strain engineering and mechanical assembly of silicon/germanium nanomembranes

Abstract: Silicon (Si) and/or germanium (Ge) nanomembranes (NMs) play crucial roles in various applications, including conventional microelectronics, as well as recently emerging high-performance flexible/stretchable electronics. Because of their superior mechanical properties, such as flexibility, strain-ability, and bond-ability, Si/GeNMs can be strain-engineered, functionalized, and assembled into two/three-dimensional (2D/3D) micro/nano-architectures and devices. These features offer significant opportunities in nanoscience and for the development of nanotechnology. Strain engineering of semiconductor NMs enables the modification of their physical properties, in particular those of Si and Ge (e.g., carrier mobility, band structure), thus creating enormous potential for use in high-speed rigid/flexible electronics, optoelectronics, and nanophotonics. The mechanical properties of NMs allow large deformations at the micro/nano-scale, via self-assembly or guided self-assembly, leading to 3D micro/nano-architectures, including tubes, wrinkles, buckles, and mesostructures. The transformation from 2D planar NMs to 3D micro/nano-architectures again strongly influences physical properties (e.g., mechanics, optics, and electronics), providing fascinating applications in sensing, energy harvesting, bio-integration, and flexible/stretchable electronics. In this Review, the recent progress in strain engineering and mechanical assembly of Si/GeNMs is reviewed, ranging from fundamental principles to device applications.