Showing papers in "Journal of Materials Research in 2014"

The importance of carbon fiber to polymer additive manufacturing

Abstract: Additive manufacturing (AM) holds tremendous promise in terms of revolutionizing manufacturing. However, fundamental hurdles limit the widespread adoption of this technology. First, production rates are extremely low. Second, the physical size of the parts is generally small, less than a cubic foot. Third, the mechanical properties of the polymer parts are generally poor, limiting the potential for direct part replacement and functional use of the polymer components. This article describes various ways in which carbon fibers (CFs) can be used to address these fundamental hurdles. First, CF-reinforced polymers developed for AM have demonstrated specific strengths approaching aerospace-quality aluminum. Second, CF additions can radically reduce the distortion and warping of the material during deposition, which enables large-scale, out-of-the-oven, high deposition rate manufacturing. Finally, the complementary nature of CF technology and AM is discussed, showing how merging the two manufacturing processes enables the construction of complex components that would not be possible with either technology alone.

On the fatigue properties of metals manufactured by selective laser melting — The role of ductility

Abstract: The selective-laser-melting (SLM) technique is an outstanding new production technology that allows for time-efficient fabrication of highly complex components from various metals. SLM processing leads to the evolution of numerous microstructural features strongly affecting the mechanical properties. For enabling application in envisaged fields the development of a robust production process for components subjected to different loadings is crucially needed. With regard to the behavior of SLM components subjected to cyclic loadings, the damage evolution can be significantly different depending on the raw material that is used, which is, in this case, highly ductile austenitic stainless steel 316L and high-strength titanium alloy TiAl6V4. By means of a thorough set of experiments, including postprocessing, mechanical testing focusing on high-cycle fatigue and microstructure analyses, it could be shown that the behavior of TiAl6V4 under cyclic loading is dominated by the process-induced pores. The fatigue behavior of 316L, in contrast, is strongly affected by its monotonic strength.

Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting

Abstract: Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (μ phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations.

Thermal effects on microstructural heterogeneity of Inconel 718 materials fabricated by electron beam melting

Abstract: Additive manufacturing technologies, also known as 3D printing, have demonstrated the potential to fabricate complex geometrical components, but the resulting microstructures and mechanical properties of these materials are not well understood due to unique and complex thermal cycles observed during processing. The electron beam melting (EBM) process is unique because the powder bed temperature can be elevated and maintained at temperatures over 1000 °C for the duration of the process. This results in three specific stages of microstructural phase evolution: (a) rapid cool down from the melting temperature to the process temperature, (b) extended hold at the process temperature, and (c) slow cool down to the room temperature. In this work, the mechanisms for reported microstructural differences in EBM are rationalized for Inconel 718 based on measured thermal cycles, preliminary thermal modeling, and computational thermodynamics models. The relationship between processing parameters, solidification microstructure, interdendritic segregation, and phase precipitation (δ, γ′, and γ″) are discussed.

Materials perspective of polymers for additive manufacturing with selective laser sintering

Abstract: The fundamental factors of polymer powders, their importance for successful selective laser sintering (SLS) processing, and the outstanding position of polyamide 12 (PA12) powders in this connection are presented. Considering key factors, the combination of intrinsic and extrinsic properties necessary to generate a powder likely for SLS application is emphasized. Only a specific combination of indicated points leads to success. This is one reason for fewer materials commercially available to date for SLS application. PA12 and some dry blends based on PA12 are today the materials that are used to generate almost all commercial SLS parts. The specific performance of particular PA12 for SLS processing is unmatched from other polymers so far. Reasons are the precise molecular control of SLS polymers for thermal behavior (enlargement of sintering window) and the open chain structure. This is for generation of sufficient mechanical properties and to induce interlayer bonding of successively sintered layers to reduce anisotropic parts.

Two-dimensional layered materials: Structure, properties, and prospects for device applications

Abstract: Graphene’s layered structure has opened new prospects for the exploration of properties of other monolayer-thick two-dimensional (2D) layered crystals. The emergence of these inorganic 2D atomic crystals beyond graphene promises a diverse spectrum of properties. For example, hexagonal-boron nitride (h-BN), a layered material closest in structure to graphene is an insulator, while niobium selenide (NbSe2), a transition metal dichalcogenide, is metallic, and monolayers of other transition metal dichalcogenides such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are direct band gap semiconductors. The rich spectrum of properties exhibited by these 2D layered material systems can potentially be engineered on-demand and creates exciting prospects for using such systems in device applications ranging from electronics, photonics, energy harvesting, flexible electronics, transparent electrodes, and sensing. A review of the structure, properties, and the emerging device applications of these materials is presented in this paper. While the layered structure of these materials makes them amenable to mechanical exfoliation for quickly unveiling their novel properties and for fabricating proof-of-concept devices, an overview of the synthesis routes that can potentially enable scalable avenues for forming these 2D atomic crystals is also discussed.

Compositionally graded metals: A new frontier of additive manufacturing

Abstract: The current work provides an overview of the state-of-the-art in polymer and metal additive manufacturing and provides a progress report on the science and technology behind gradient metal alloys produced through laser deposition. The research discusses a road map for creating gradient metals using additive manufacturing, demonstrates basic science results obtainable through the methodology, shows examples of prototype gradient hardware, and suggests that Compositionally Graded Metals is an emerging field of metallurgy research.

Additive manufacturing of nickel-based superalloy Inconel 718 by selective electron beam melting: Processing window and microstructure

Abstract: Cube-shaped IN718 samples were produced by selective electron beam melting (SEBM) with varying beam power, deflection speed, and beam spot size. Process parameter windows were identified where fully dense samples are obtained with no surface unevenness. High deflection speeds were demonstrated to result in smaller demand of area energy. This result is explained by the reduced time for heat dissipation into the substrate during hatching. The grain structure was strongly affected by SEBM process parameters. Under certain conditions, epitaxial growth over many layers and well-developed columnar grain structures were obtained with a polycrystalline substrate plate. A more defocused beam led to a lower melt pool temperature and shallower melt pool geometry where maximum temperature gradients and solidification rates were more or less in parallel with the building direction and primary dendrite arm orientation. These conditions help to suppress grain nucleation ahead of the nucleation front as vigorous melt movement, fragmentation of dendrites, and tertiary arm growth are suppressed.

A look into Cu-based shape memory alloys: Present scenario and future prospects

Abstract: Cu-based shape memory alloys (SMAs) and among these copper–zinc (Cu–Zn), copper–aluminum (Cu–Al), and copper–tin (Cu–Sn) alloys both with and without ternary additions have shown potential due to their good shape recovery, ease of fabrication, excellent conductivity of heat and electricity. However, their applications are still limited because of the shortcomings of thermal stability, brittleness, and mechanical strength, which are closely related with microstructural characteristic of Cu-based SMAs, such as coarse grain sizes, high elastic anisotropies, and the congregation of secondary phases or impurities along the grain boundaries. Efforts are being made to overcome these drawbacks with proper ternary additions, adopting alternative processing routes and also optimizing the heat treatment cycles. The present article will deal with the current status of research and commercialization of Cu-based SMAs and dwell upon the future directions in which research should be targeted and future prospects of converting the research into components for commercial use.

Novel ABS-based binary and ternary polymer blends for material extrusion 3D printing

Abstract: Material extrusion 3D printing (ME3DP) based on fused deposition modeling (FDM) technology is currently the most commonly used additive manufacturing method. However, ME3DP suffers from a limitation of compatible materials and typically relies upon amorphous thermoplastics, such as acrylonitrile butadiene styrene (ABS). The work presented here demonstrates the development and implementation of binary and ternary polymeric blends for ME3DP. Multiple blends of acrylonitrile butadiene styrene (ABS), styrene ethylene butadiene styrene (SEBS), and ultrahigh molecular weight polyethylene (UHMWPE) were created through a twin screw compounding process to produce novel polymer blends compatible with ME3DP platforms. Mechanical testing and fractography were used to characterize the different physical properties of these new blends. Though the new blends possessed different physical properties, compatibility with ME3DP platforms was maintained. Also, a decrease in surface roughness of a standard test piece was observed for some blends as compared with ABS.

Tribological and corrosion properties of Al–12Si produced by selective laser melting

Abstract: The effect of annealing on the tribological and corrosion properties of Al–12Si samples produced by selective laser melting (SLM) is evaluated via sliding and fretting wear tests and weight loss experiments and compared to the corresponding material processed by conventional casting. Sliding wear shows that the as-prepared SLM material has the least wear rate compared to the cast and heat-treated SLM samples with abrasive wear as the major wear mechanism along with oxidation. Similar trend has also been observed for the fretting wear experiments, where the as-prepared SLM sample displays the minimum wear loss. On the other hand, the acidic corrosion behavior of the as-prepared SLM material as well as of the cast samples is similar and the corrosion rate is accelerated by increasing the heat treatment temperature. This behavior is due to the microstructural changes induced by the heat treatment, where the continuous network of Si characterizing the as-prepared SLM sample transforms to isolated Si particles in the heat-treated SLM specimens. This shows that both the wear and corrosion behaviors are strongly associated with the change in microstructure of the SLM samples due to the heat-treatment process, where the size of the hard Si particles increases, and their density decreases with increasing annealing temperature.

Role of dopants as a carrier suppressor and strong oxygen binder in amorphous indium-oxide-based field effect transistor

Abstract: In this review, we discuss the recent developments of high-performance and improved-stability of indium-oxide-based transparent amorphous-oxide semiconductor (TAOS) thin-film transistors (TFTs) properties. TAOSs are widely explored with the aim of producing high-performance semiconductors suitable for the channel layer of TFTs which enable to survive under light and thermal-bias-induced stress conditions. Numerous TAOSs have been invented with some improved performance characteristics of TFTs such as mobility, light and thermal induced bias stress. However, there has been no clear elucidation of the mechanisms driving these improvements. In this review, we discuss the progression of innovations of high performance indium-oxide-based TAOS TFTs from its first reported amorphous indium gallium zinc oxide (a-IGZO) to present, and their properties that are correlated with the Lewis acid strength (L) and bonding strength of dopant and oxygen as a carrier suppressor and strong binder. The proposed mechanism can be practical to develop novel TAOS TFTs with high mobility and stability.

Additive nanomanufacturing – A review

Abstract: Additive manufacturing has provided a pathway for inexpensive and flexible manufacturing of specialized components and one-off parts. At the nanoscale, such techniques are less ubiquitous. Manufacturing at the nanoscale is dominated by lithography tools that are too expensive for small- and medium-sized enterprises (SMEs) to invest in. Additive nanomanufacturing (ANM) empowers smaller facilities to design, create, and manufacture on their own while providing a wider material selection and flexible design. This is especially important as nanomanufacturing thus far is largely constrained to 2-dimensional patterning techniques and being able to manufacture in 3-dimensions could open up new concepts. In this review, we outline the state-of-the-art within ANM technologies such as electrohydrodynamic jet printing, dip-pen lithography, direct laser writing, and several single particle placement methods such as optical tweezers and electrokinetic nanomanipulation. The ANM technologies are compared in terms of deposition speed, resolution, and material selection and finally the future prospects of ANM are discussed. This review is up-to-date until April 2014.

Comparative study of microstructures and mechanical properties of in situ Ti–TiB composites produced by selective laser melting, powder metallurgy, and casting technologies

Abstract: This study presents results of selective laser melting (SLM), powder metallurgy (PM), and casting technologies applied for producing Ti–TiB composites from Ti–TiB2 powder. Diffraction patterns and microstructural investigations reveal that chemical reaction occurred between Ti and TiB2 during all the three processes, leading to the formation of Ti–TiB composites. The ultimate compressive strength of SLM-processed and cast samples are 1421 and 1434 MPa, respectively, whereas the ultimate compressive strengths of PM-processed 25%, 29%, and 36% porous samples are 510, 414, and 310 MPa, respectively. The Young’s moduli of porous composite samples are 70, 45, and 23 GPa for 25%, 29%, and 36% porosity levels, respectively, and are lower than those of SLM-processed (145 GPa) and cast (142 GPa) samples. Fracture analysis of the SLM-processed and cast samples shows shear fracture and microcracks across the samples, whereas failure of porous samples occurs due to porosities and weak bonds among particles.

Synthesis strategies for improving the performance of doped-BaZrO 3 materials in solid oxide fuel cell applications

Abstract: Solid oxide fuel cells (SOFCs) offer an efficient energy conversion technology for alleviating current energy problems. High temperature proton-conducting (HTPC) oxides are promising electrolytes for this technology, since their activation energy is lower than that of conventional oxygen-ion conductors, enabling the operating temperature reduction at 600 °C. Among HTPC oxides, doped BaZrO3 materials possess high chemical stability, needed for practical applications. Though, poor sinterability and the resulting large volume of highly resistive grain boundaries hindered their deployment for many years. Nonetheless, the recently demonstrated high proton conductivity of the bulk revived the attention on doped BaZrO3, stimulating research on solving the sintering issues. The proper selection of dopants and sintering aids was demonstrated to be successful for improving the BaZrO3 electrolyte sinterability. We here briefly review the synthesis strategies proposed for preparing BaZrO3-based nanostructured powders for electrolyte and electrodes, with the aim to improve the SOFC performance.

The formation of α + β microstructure in as-fabricated selective laser melting of Ti–6Al–4V

Abstract: Ti–6Al–4V parts made by the additive manufacturing (AM) technique selective laser melting (SLM) generally show poor ductility due to their fine martensitic microstructure. This study was designed to assess whether a more suitable microstructure can be obtained when long laser/material interaction times are used. As-fabricated components with an α + β microstructure were produced and characterized with various microscopy techniques. The microstructural evolution was discussed in relation to the build platform temperature, the cyclic reheating, and the thermal stresses that developed during the process. The hardness of the samples was also evaluated and discussed. The hardness varied in relation to the different microstructure morphologies observed in the samples and different partitioning of the alloying elements. This study indicates a methodology through SLM to obtain Ti–6Al–4V with an as-deposited α + β microstructure which is more desirable than that the typical fully martensitic microstructure typically obtained after SLM.

Compressive strain-induced metal–insulator transition in orthorhombic SrIrO3 thin films

Abstract: Orthorhombic SrIrO3 is a correlated metal whose electronic properties are highly susceptible to external perturbations due to the comparable interactions of spin–orbit interaction and electronic correlation. We have investigated the electronic properties of epitaxial orthorhombic SrIrO3 thin-films under compressive strain using transport measurements, optical absorption spectra, and magnetoresistance. The metastable, orthorhombic SrIrO3 thin-films are synthesized on various substrates using an epi-stabilization technique. We have observed that as in-plane lattice compression is increased, the dc-resistivity (ρ) of the thin films increases by a few orders of magnitude, and the dρ/d T changes from positive to negative values. However, optical absorption spectra show Drude-like, metallic responses without an optical gap opening for all compressively strained thin films. Transport measurements under magnetic fields show negative magnetoresistance at low temperature for compressively strained thin-films. Our results suggest that weak localization is responsible for the strain-induced metal–insulator transition for the orthorhombic SrIrO3 thin-films.

Graphene synthesis and application for solar cells

Abstract: To date graphene and graphene-derived materials have created an immense research interests due to its extraordinary physical, chemical, and physiochemical properties, which delineated graphene as an outstanding material for future electronics, optics, and energy-harvesting devices. Typically, graphene has high mobility and optical transparency along with excellent mechanical properties and chemical inertness. Single-layer graphene exhibits ultrahigh optical transmissivity (∼98%), which allows passing through wide range of light wave lengths, thus designated as an ever-reported material for an optically conducting window. Furthermore, graphene’s optical, electrical, and electrocatalytic properties can be tuned by applying different chemical functionalization protocols, which make it one of the most suitable candidates for advanced applications in optoelectronic and energy-harvesting devices. This review is intended to summarize the most important experimental results from the recent publications concerning the fascinating properties of graphene electrodes and their applications in various types of solar cells. Furthermore, the state of the art of different graphene synthesis processes and functionalization for the applications in solar cells are also discussed in this review.

Enhancing microwave absorption of TiO2 nanocrystals via hydrogenation

Abstract: TiO2 has attracted tremendous research interest for photocatalytic water splitting, solar hydrogen generation, environmental pollution removal, dye-sensitized solar cells, lithium-ion batteries, supercapacitors, and field emission. Microwave absorption materials (MAMs) play important roles in many military (e.g., the stealth coating on the B-2 bomber) and civil (e.g., telecommunications, noise reduction, information security, signal, and data protection) applications. However, TiO2 is not a good MAM due to its poor absorption in the microwave region. Here, we report that via hydrogenation excellent and tunable microwave absorption is achieved with hydrogenated TiO2 nanocrystals. After hydrogenation, 4.3x and 103x improvements have been obtained in storing and dissipating the electric energy of the microwave electromagnetic field. Their permittivity values are higher than those of the current carbonaceous MAMs. Instead of relying on the dipole rotation or ferromagnetic resonance mechanisms for traditional MAMs, the hydrogenated TiO2 nanocrystals work as good MAMs based on a newly proposed collective-movement-of-interfacial-dipole (CMID) mechanism. Although there is still no direct physical evidence of the interface effects of the CMID mechanism, the CMID as a hypothesis at this point successfully explained the origin of the enhanced microwave absorption of the hydrogenated TiO2 nanoparticles. This study thus may open new applications for TiO2 nanocrystals and also stimulate new approaches for new MAM development.

Energy dissipation mechanisms in hollow metallic microlattices

Abstract: When properly designed at ultra-low density, hollow metallic microlattices can fully recover from compressive strains in excess of 50%, while dissipating a considerable portion of the elastic strain energy. This article investigates the physical mechanisms responsible for energy loss upon compressive cycling, and attributes the most significant contribution to a unique form of structural damping, whereby elastic local buckling of individual bars releases energy upon loading. Subsequently, a simple mechanical model is presented to capture the relationship between lattice geometry and structural damping. The model is used to optimize the microlattice geometry for maximum damping performance. The conclusions show that hollow metallic microlattices exhibit exceptionally large values of the damping figure of merit, (Young’s modulus)1/3(loss coefficient)/(density), but this performance requires very low relative densities (<1%), thus limiting the amount of energy that can be dissipated.

Benchmarking spintronic logic devices based on magnetoelectric oxides

Abstract: Active research is ongoing in logic devices beyond complementary metal–oxide–semiconductor electronics. One of the most promising classes of such devices is spintronic/nanomagnetic devices. Switching of magnetization by spin torque (ST) demonstrated in spintronic devices results in relatively high switching energy. An attractive option for lowering switching energy is magnetoelectric (ME) switching achieved by placing other materials (mostly oxides) adjacent to ferromagnets. We review recent experiments on ME switching, classify them according to the ME phenomena into surface anisotropy, exchange bias, and magnetostrictive, and compare switching parameters for these classes. Then, we perform micromagnetic simulations of switching by the effective ME field of both stand-alone nanomagnets and spintronic interconnects. We determine the threshold values of ME field for switching and the resulting switching time. These switching requirements are incorporated into the previously developed benchmarking framework for spintronic logic devices and circuits. We conclude that ME switching results in 1 to 2 orders of magnitude improvement of switching energy and several time improvement of switching delay compared with ST switching across various schemes of spin logic devices.

Shaping of ceramic parts by selective laser melting of powder bed

Abstract: Laser additive manufacturing allows the production of polymeric or metallic parts with complex shapes. A major advantage of this contactless technology is that it allows reaching very high energy densities with an excellent precision in short times. This is very suitable for processing hard refractory metals for instance. Unfortunately, current results are less satisfactory for ceramics as a consequence of their intrinsic properties such as a low thermal shock resistance and very high refractoriness. Another significant limitation is related to the poor absorptivity of oxide ceramics in the near-infrared region which is typical for most commercial selective laser melting (SLM) machines. This study considers an alternative to overcome the above-mentioned limitations, especially the lack of absorptivity. SLM of oxide ceramics has become possible. Large parts with complex shapes and relative densities up to 90% have been manufactured on a commercial SLM machine.

Tunability and enhancement of mechanical behavior with additively manufactured bio-inspired hierarchical suture interfaces

Abstract: In nature, biological structures often exhibit complex geometries that serve a wide range of specific mechanical functions. One such example are the ammonites, a large group of extinct mollusks, which produced elaborate, fractal-like hierarchical suture interface patterns. This report experimentally explores the influence of hierarchical suture interface designs on mechanical behavior by taking advantage of additive manufacturing and its ability to fabricate complex geometries. In addition, structure/property relationships of additively manufactured multi-material prototypes are investigated. It is shown that increasing the order of hierarchy amplifies stiffness by more than an order of magnitude. Tensile strength can also be tailored by changing the order of hierarchy, which alters the normal to shear stress ratio of the interfacial layer. The addition of failure mechanisms with increased order of hierarchy also significantly increases the toughness. Therefore, hierarchical suture interfaces can be used to diversify the mechanical behavior of additively manufactured materials.

Microcantilever bending experiments in NiAl — Evaluation, size effects, and crack tip plasticity

Abstract: For a better understanding of the local fracture behavior of semi-brittle materials, we carried out bending experiments on notched microcantilevers of varying sizes in the micrometer range using NiAl single crystals. Smaller and larger beams were milled with a focused ion beam in the so-called “soft” and “hard” orientation and were tested in situ in a scanning electron microscope and ex situ with a nanoindenter, respectively. The measurements were evaluated using both linear-elastic fracture mechanics and elastic–plastic fracture mechanics. The results show that (i) the fracture toughness is in the same range as the macroscopically determined one which is around 3.5 MPa for the soft orientation and around 8.5 MPa for the hard orientation, that (ii) there is a strong influence of the anisotropic behavior of NiAl on the fracture toughness values, and that (iii) the J-integral technique is the most accurate quantification method.

Beam speed effects on Ti–6Al–4V microstructures in electron beam additive manufacturing

Abstract: The effect of the beam scanning speed on part microstructures in the powder-bed electron beam additive manufacturing (EBAM) process was investigated in this research. Four levels of the beam speed were tested in building EBAM Ti–6Al–4V samples. The samples were subsequently used to prepare metallographic specimens for observations by optical microscopy and scanning electron microscopy. During the experiment, a near-infrared thermal imager was also used to acquire build surface temperatures for melt tool size estimates. It was found that the X-plane (side surface) shows columnar prior β grains, with the width in the range of about 40–110 µm, and martensitic structures. The width of columnar grains decreases with the increase of the scanning speed. In addition, the Z-plane (scanning surface) shows equiaxed grains, in the range of 50–85 µm. The grain size from the lowest beam speed (214 mm/s) is much larger compared to other samples of higher beam speeds (e.g., 376–689 mm/s). In addition, increasing the beam scanning speed will also result in finer α-lath. However, the porosity defect on the build surface also becomes severe at the highest scanning speed (689 mm/s).

Characterization of microstructure and residual stress in a 3D H13 tool steel component produced by additive manufacturing

Abstract: H13 tool steel was deposited using the additive manufacturing technique Direct Metal Deposition to produce a part having a wedge geometry. The wedge was characterized both in terms of microstructure and residual stress. It was found that phase transformations were significantly influencing the microstructure, which was then linked to the residual stress distribution as seen in Fig. 8. The residual stress distribution was found to be opposite to that reported in the literature. This was attributed to the low temperature martensitic phase transformation of the H13 tool steel and the subsequent tempering of the microstructure with an increasing number of layers of deposited material. The high hardness and compressive residual stress of the top 4 mm of the wedge are ideal in die casting and forging dies, as it will resist thermal fatigue. It also has a hardness higher than that produced by typical heat treatment processes.

Toward efficient solar water splitting over hematite photoelectrodes

Abstract: Hematite has been considered as one of the most promising materials for solar water splitting, although its photoelectrochemical performance is still not very high and limited by its intrinsic properties. In the past few years, sizable advances in the development of hematite photoelectrodes for enhanced water splitting activities have been achieved by a variety of rational modification strategies, including nanostructure design for efficient charge collection, metal ion doping for promoted charge carrier transfer, heterojunctions for efficient charge separation, and surface and/or interface modification for retarded charge recombination and enhanced light absorption. In this article, research work and milestone achievement actually focused on hematite photoelectrodes for water splitting is reviewed in detail. A review on this topic by answering the key question, “how to modify or design hematite photoelectrode to improve its conductivity, enhance charge separation as well as catalyze surface water oxidation,” in authors' view, can be potentially helpful to enable hematite for further efficient solar energy conversion, which will be very inspiring and important to this field.

Nonisotropic experimental characterization of the relaxation modulus for PolyJet manufactured parts

Abstract: Mechanical properties of parts constructed with additive manufacturing (AM) technologies are highly influenced by raw material and process characteristics. It is widely assumed that a certain degree of anisotropy should be expected in AM parts due to their layer-upon-layer nature. Present work focuses on the PolyJet process, where each layer is built by selective jetting of photopolymers upon flat surfaces and subsequent UV radiation curing. An extensive experimental program was carried out to find out if the so-constructed parts present viscoelastic behavior and if their mechanical characteristics also depend on part orientation. Both hypotheses have been proven true, so a viscoelastic orthotropic-like behavior shall be expected in PolyJet manufactured part. Nevertheless, a significant improvement on material properties has been found for nearly vertical building orientations. This unexpected behavior is related to a shielding effect upon UV curing caused by support material.

Copper oxide as a “self-cleaning” substrate for graphene growth

Abstract: Commonly used techniques for cleaning copper substrates before graphene growth via chemical vapor deposition (CVD), such as rinsing with acetone, nitric, and acetic acid, and high temperature hydrogen annealing still leave residual adventitious carbon on the copper surface. This residual carbon promotes graphene nucleation and leads to higher nucleation density. We find that copper with an oxidized surface can act as a self-cleaning substrate for graphene growth by CVD. Under vacuum conditions, copper oxide thermally decomposes, releasing oxygen from the substrate surface. The released oxygen reacts with the carbon residues on the copper surface and forms volatile carbon monoxide and carbon dioxide, leaving a clean copper surface free of carbon for large-area graphene growth. Using oxidized electropolished copper foil leads to a reduction in graphene nucleation density by over a factor of 1000 when compared to using chemically cleaned oxygen free copper foil.

Optimized welding parameters for Al 6061 ultrasonic additive manufactured structures

Abstract: Ultrasonic additive manufacturing (UAM) is a solid state manufacturing process that combines additive joining of thin metal tapes and subtractive computer numerical control milling operations to generate near-net shape metallic parts. We conducted a design of experiments study with the goal to optimize UAM process parameters for aluminum 6061. Weld force, weld speed, amplitude, and temperature were varied based on a Taguchi L18 experimental design matrix and tested for mechanical strength using a shear test and a comparative push-pin test. Statistical methods including analysis of variance (ANOVA), mean effects plots, and interaction effects plots were conducted to determine optimal process parameters. Results indicate that weld amplitudes of 32.76 µm and weld speeds of 84.6 mm/s yield maximum mechanical strength while temperature and force are statistically insignificant for the parameter levels tested. Annealing of cold-worked foil stock produces a 13% strength increase for UAM samples over homogeneous annealed material.