Showing papers in "Journal of Materials Research in 2012"
TL;DR: In this paper, the authors present a review of the features distinguishing HiPIMS from other deposition methods along with how they influence the deposition conditions, such as the plasma parameters and the sputtered material, as well as the resulting thin film properties.
Abstract: High-power impulse magnetron sputtering (HiPIMS) is a promising sputtering-based ionized physical vapor deposition technique and is already making its way to industrial applications. The major difference between HiPIMS and conventional magnetron sputtering processes is the mode of operation. In HiPIMS the power is applied to the magnetron (target) in unipolar pulses at a low duty factor (<10%) and low frequency (<10 kHz) leading to peak target power densities of the order of several kilowatts per square centimeter while keeping the average target power density low enough to avoid magnetron overheating and target melting. These conditions result in the generation of a highly dense plasma discharge, where a large fraction of the sputtered material is ionized and thereby providing new and added means for the synthesis of tailor-made thin films. In this review, the features distinguishing HiPIMS from other deposition methods will be addressed in detail along with how they influence the deposition conditions, such as the plasma parameters and the sputtered material, as well as the resulting thin film properties, such as microstructure, phase formation, and chemical composition. General trends will be established in conjunction to industrially relevant material systems to present this emerging technology to the interested reader.
258 citations
TL;DR: In this paper, surface morphological changes were examined using scanning electron microscopy after heat treatment in the temperature range of 600-900 °C for solid oxide fuel cell cathodes.
Abstract: Elemental enrichment behavior on the surface of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) was investigated in order to understand potential degradation mechanism of solid oxide fuel cell cathodes. Surface morphological changes were examined using scanning electron microscopy after heat treatment in the temperature range of 600–900 °C. Submicron-sized precipitates were formed on grain surfaces after heat treatment. Their shapes appeared to be aligned along the surface orientations of the underlying grains. Auger electron spectroscopy and transmission electron microscopy characterization revealed that the precipitate was strontium (Sr)-oxygen (O) based. The formation of Sr–O precipitates was found to increase with increasing temperature and oxygen partial pressure. A defect chemistry model is presented based on the observed phenomena.
180 citations
TL;DR: In this article, the anisotropic indentation response of α-titanium was characterized by electron backscatter diffraction, which was performed for a number of different crystallographic orientations.
Abstract: This study reports on the anisotropic indentation response of α-titanium. Coarse-grained titanium was characterized by electron backscatter diffraction. Sphero-conical nanoindentation was performed for a number of different crystallographic orientations. The grain size was much larger than the size of the indents to ensure quasi-single-crystal indentation. The hexagonal c-axis was determined to be the hardest direction. Surface topographies of several indents were measured by atomic force microscopy. Analysis of the indent surfaces, following Zambaldi and Raabe (Acta Mater. 58(9), 3516–3530), revealed the orientation-dependent pileup behavior of α-titanium during axisymmetric indentation. Corresponding crystal plasticity finite element (CPFE) simulations predicted the pileup patterns with good accuracy. The constitutive parameters of the CPFE model were identified by a nonlinear optimization procedure, and reproducibly converged toward easy activation of prismatic glide systems. The calculated critical resolved shear stresses were 150 ± 4, 349 ± 10, and 1107 ± 39 MPa for prismatic and basal 〈a〉-glide and pyramidal〈c + a〉-glide, respectively.
163 citations
TL;DR: In this article, a review of recent studies on the confinement of poly(ethylene oxide), poly(e-caprolactone), polypropylene, and poly(vinylidene fluoride) polymers in multilayered films is presented.
Abstract: Recent advances utilizing forced assembly multilayer coextrusion have led to the development of a new approach to study the structure–property relationships of confined polymer crystallization. Confinement of crystalline polymer materials in layer thicknesses ranging from hundreds to tens of nanometers thick, resulted in multilayer films possessing enhanced gas barrier properties. The enhanced gas barrier has been attributed to nanolayer confinement of the crystalline polymer resulting in a highly ordered intralayer lamellae orientation extending over micron or larger scale areas. Research into the confined crystallization mechanism of the multilayered polymer films has resulted in several material case studies as well as an understanding of the chemical and thermodynamic parameters that control the degree and rate of the confinement in multilayer polymer systems. This review highlights our recent studies on the confinement of poly(ethylene oxide), poly(e-caprolactone), polypropylene, and poly(vinylidene fluoride) polymers in multilayered films.
149 citations
TL;DR: In this article, the effect of Li+-solvation sheath structure on the interphasial processes at graphitic anode is discussed, and a new strategy of forming a more desirable interphase is also discussed.
Abstract: In any electrochemical device, the interface between electrolyte and electrode should be the only “legitimate” location where redox reactions happen. Particularly in Li ion batteries, these interfaces become “interphases” due to the reactivity of the electrode materials used, and they mainly consist of chemical species from the sacrificial decomposition of electrolyte components. Since the emergence of Li ion technology, it has been recognized that interphase on graphitic anodes, usually referred as SEI (solid electrolyte interphase) after its electrolyte attributes, is the key component supporting the reversibility of Li+-intercalation chemistry. Research attention focused on this component during the past two decades has led to substantial understanding about both its chemistry and mechanism. This article summarizes these progresses, and elaborates on the relatively recent insights, including the effect of Li+-solvation sheath structure on the interphasial processes at graphitic anode. A new strategy of forming a more desirable interphase is also discussed.
149 citations
TL;DR: In this paper, the authors explored the effect of grain boundary relaxation on the deformation of Ni-W alloys and showed that higher annealing temperatures result in faster, more pronounced strengthening.
Abstract: The hardening effect caused by the relaxation of nonequilibrium grain boundary structure has been explored in nanocrystalline Ni–W alloys. First, the kinetics of relaxation hardening are studied, showing that higher annealing temperatures result in faster, more pronounced strengthening. Based on the temperature dependence of relaxation strengthening kinetics, triple junction diffusion is suggested as a plausible kinetic rate limiter for the removal of excess grain boundary defects in these materials. Second, the magnitude of relaxation strengthening is explored over a wide range of grain sizes spanning the Hall–Petch breakdown, with an apparent maximum hardening effect found at a grain size below 10 nm. The apparent activation volume for plastic deformation is unaffected by annealing for grain sizes down to ∼10 nm, but increases with annealing for the finest grain sizes, suggesting a change in the dominant deformation mechanism for these structures.
148 citations
TL;DR: In this paper, the authors presented a method for the determination of the fracture toughness of materials from scratch data, which is based on the recorded forces and depth of penetration of the scratch data.
Abstract: This article presents a novel microscratch technique for the determination of the fracture toughness of materials from scratch data. While acoustic emission and optical imaging devices provide quantitative evidence of fracture processes during scratch tests, the technique proposed here provides a quantitative means to assess the fracture toughness from the recorded forces and depth of penetration. We apply the proposed method to a large range of materials, from soft (polymers) to hard (metal), spanning fracture toughness values over more than two orders of magnitude. The fracture toughness values so obtained are in excellent agreement with toughness values obtained for the same materials by conventional fracture tests. The fact that the proposed microscratch technique is highly reproducible, almost nondestructive, and requires only small material volumes makes this technique a powerful tool for the assessment of fracture properties for microscale materials science and engineering applications.
120 citations
TL;DR: In this paper, the influence of negative ions on the electrical properties of transparent conductive oxides (TCO) films was reviewed and compared with their results, and ways for reducing the effect of negative ion bombardment were discussed.
Abstract: Transparent conductive oxides (TCOs) are degenerately doped compound semiconductors with wide band gaps (Eg > 3 eV), which are used as transparent electrodes in optoelectronic devices. Reports on the influence of negative ions on the electrical properties of TCO films are reviewed and compared with our results. It was reported that the radial resistivity distributions depend (i) on the excitation mode of the magnetron (direct current or radio frequency), (ii) on the erosion state of the sputtering target, and (iii) on the density of the ceramic targets. This can be explained by the fact that the negative ions in magnetron discharges (in our case O−) are generated at the target surface and accelerated toward the growing films. Their energy and their radial distribution depend on the discharge voltage and the shape of the emitting surface, i.e., of the erosion groove. Ways for reducing the effect of negative ion bombardment are discussed.
117 citations
TL;DR: In this paper, the authors investigated the origin of the NIR absorption by analyzing dielectric constants of CsxWO3 (x = 0.15, 0.25, and 0.33) and WO2.
Abstract: Dispersions of reduced tungsten oxide and tungsten bronze nanoparticles are known to show a remarkable absorption of near-infrared (NIR) light applicable to solar control filters for automotive and architectural windows. Origin of the NIR absorption has been investigated by analyzing dielectric constants of CsxWO3 (x = 0.15, 0.25, and 0.33) and WO2.72, and using Mie scattering theory. The optical analysis and Mie scattering theory analysis indicate that a localized surface plasmon resonance and polarons of localized electrons contribute alongside to the observed NIR absorption at different wavelengths.
113 citations
TL;DR: In this article, the authors show that nanoindentation can be a powerful tool for understanding the structure-mechanical property relationship of biological materials and illustrate this for fish teeth and scales.
Abstract: Most biological materials are hierarchically structured composites that often possess exceptional mechanical properties. We show that nanoindentation can be a powerful tool for understanding the structure‑mechanical property relationship of biological materials and illustrate this for fish teeth and scales, not heretofore investigated at the nanoscale. Piranha and shark teeth consist of enameloid and dentin. Nanoindentation measurements show that the reduced modulus and hardness of enameloid are 4‑5 times higher than those of dentin. Arapaima scales are multilayered composites that consist of mineralized collagen fibers. The external layer is more highly mineralized, resulting in a higher modulus and hardness compared with the internal layer. Alligator gar scales are composed of a highly mineralized external ganoin layer and an internal bony layer. Similar design strategies, gradient structures, and a hard external layer backed by a more compliant inner layer are exhibited by fish teeth and scales and seem to fulfill their functional purposes.
97 citations
TL;DR: In this paper, a simple glucose hydrothermal process was used to produce carbon spheres with regular shapes and smooth surfaces, and the chemical properties of the carbon spheres were affected by temperature.
Abstract: Carbon spheres (CSs) with regular shapes (O 300–1200 nm) and numerous oxygen groups (–OH, C6H5–C=O, and C=O) were prepared by a simple glucose hydrothermal process. CSs were studied by x-ray powder diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, x-ray photoelectron spectra, and elemental analysis. Their size was directly proportional to temperature and glucose concentration. Their shape could be controlled by temperature and reaction time. To prepare CSs (O 300–800 nm) with regular shapes and smooth surfaces, temperature and reaction time in the range of 180–230 °C and 3–4 h, respectively, were optimal. The chemical properties of the CSs were affected by temperature. A phase transformation from amorphous to turbostratic structure took place at T > 230 °C. The number of oxygen groups decreased as the temperature increased, and T ≤ 230 °C were optimal to prepare oxygen-rich CSs. Comparison of oxygen contents and O/C ratios indicated a further carbonization, and the degree was directly related to temperature. A possible formation mechanism for the CSs is proposed.
TL;DR: In this paper, a compilation of measured effective rate constants for oxygen surface exchange on mixed conducting perovskites, which covers a great variety of compositions ranging from (La,Sr)MnO3−δ to (La Sr)(Co,Fe)O3 −δ, demonstrates the importance of ionic conductivity as a key factor for the surface oxygen exchange rate.
Abstract: The compilation of measured effective rate constants for oxygen surface exchange on mixed conducting perovskites, which covers a great variety of compositions ranging from (La,Sr)MnO3−δ to (La,Sr)(Co,Fe)O3−δ and (Ba,Sr)(Co,Fe)O3−δ, demonstrates the importance of ionic conductivity—i.e., high oxygen vacancy concentration as well as vacancy mobility—as a key factor for the surface oxygen exchange rate. This interpretation is corroborated by ab initio calculations, which indicate that the approach of an oxygen vacancy to oxygen intermediates adsorbed on the surface is the rate determining step for a number of perovskites.
TL;DR: In this article, the effect of H2 plasma treatment on amorphous indium-gallium-zincoxide (a-IGZO) thin-film transistor (TFT) was evaluated under thermal stress.
Abstract: We report the effect of H2 plasma treatment on amorphous indium–gallium–zinc–oxide (a-IGZO) thin-film transistor (TFT). The changes in electrical characteristics and stability of the a-IGZO TFT treated by H2 plasma were evaluated under thermal stress. Each device exhibited a change in the subthreshold swing, turn on voltage shift, and hysteresis depending on the amount of hydrogen atom. It was found that there occurred a decrease of oxygen deficiency and an increase of hydrogen content in channel layer and channel/dielectric interface with increasing treatment time. The proper hydrogen dose well passivated the oxygen vacancies; however, more hydrogen dose acted as excessive donors. The change of oxygen vacancy and total trap charge were explained by the activation energy from Arrhenius plot. Through this study, we found that the optimized H2 plasma treatment brings device stability by affecting oxygen vacancy and trap content in channel bulk and channel/dielectric interface.
TL;DR: In this article, the authors performed different proton irradiations and critically evaluated the results obtained from nanoindentation and pillar compression, both performed parallel and perpendicular to the irradiation direction.
Abstract: Small-scale testing techniques such as nanoindentation and micro-/nanocompression are promising methods for addressing mechanical properties of ion-beam-irradiated materials. We performed different proton irradiations and critically evaluated the results obtained from nanoindentation and pillar compression, both performed parallel and perpendicular to the irradiation direction. Experiments parallel to beam direction suffer from variation of material properties with penetration depth. This is improved by cross-sectional experiments, thereby probing the effect of different doses along the beam penetration depth on mechanical properties. Finally, we demonstrate that, compared with nanoindentation, miniaturized uniaxial compression experiments offer a more reliable and straightforward interpretation of the mechanical data, as they impose a nominally uniaxial stress on a well-defined volume at a specific position. Moreover, the exposed pillar geometry is not influenced by surface contamination and enables in situ observation of the governing mechanical processes, which is typically not possible during indentation experiments in a half-space geometry.
TL;DR: In this article, measurements of the temperature changes in a modified nanoindenter have been made and their influence on the displacement and load measurements is discussed, showing that controlling the temperatures of the indenter tip and the sample enabled flat punch indentations of gold, a good thermal conductor, to be carried out over several minutes at 665 °C in vacuum, as well as permitting thermal stability to be quickly re-established in site-specific microcompression experiments.
Abstract: In small-scale testing at elevated temperatures, impurities in inert gases can pose problems so that testing in vacuum would be desirable. However, previous experiments have indicated difficulties with thermal stability and instrument noise. To investigate this, measurements of the temperature changes in a modified nanoindenter have been made and their influence on the displacement and load measurements is discussed. It is shown that controlling the temperatures of the indenter tip and the sample enabled flat punch indentations of gold, a good thermal conductor, to be carried out over several minutes at 665 °C in vacuum, as well as permitting thermal stability to be quickly re-established in site-specific microcompression experiments. This allowed compression of nickel superalloy micropillars up to sample temperatures of 630 °C with very low levels of oxidation after 48 h. Furthermore, the measured Young moduli, yield and flow stresses were consistent with literature data.
TL;DR: A review of the existing nanoindentation creep methods and related issues can be found in this article, where the authors also suggest possible novel ways to better estimate the small-scale creep properties.
Abstract: Recent extensive nanomechanical experiments have revealed that the instantaneous strength and plasticity of a material can be significantly affected by the size (of sample, microstructure, or stressed zone). One more important property to be added into the list of size-dependent properties is time-dependent plastic deformation referred to as creep; it has been reported that the creep becomes more active at the small scale. Analyzing the creep in the small scale can be valuable not only for solving scientific curiosity but also for obtaining practical engineering information about the lifetime or durability of advanced small-scale structures. For the purpose, nanoindentation creep experiments have been widely performed by far. Here we critically review the existing nanoindentation creep methods and the related issues and finally suggest possible novel ways to better estimate the small-scale creep properties.
TL;DR: In this paper, the authors developed the highest conductivity solid electrolyte, stabilized bismuth oxide (Dy0.08W0.04Bi0.88O0.36).
Abstract: Low temperature solid oxide fuel cells (SOFCs) are a promising solution to revolutionize stationary, transportation, and personal power energy conversion efficiency. Through investigation of fundamental conduction mechanisms, we have developed the highest conductivity solid electrolyte, stabilized bismuth oxide (Dy0.08W0.04Bi0.88O0.36). To overcome its inherent thermodynamic instability in the anode environment, we invented a functionally graded bismuth oxide/ceria bilayered electrolyte. For compatibility with this bilayared electrolyte, we developed high performance bismuth ruthenate–bismuth oxide composite cathodes. Finally, these components were integrated into an anode-supported cell with an anode functional layer, resulting in an exceptionally high power density of ∼2 W/cm2 at moderate temperatures (650 °C) and sufficient power down to 300–400 °C for most applications. Moreover, because SOFCs can operate on conventional fuels, these low temperature SOFCs provide one of the most efficient energy conversion technologies available without relying on a hydrogen infrastructure.
TL;DR: In this paper, the effects of time, temperature, solution concentration and concentration ratios of the precursor chemicals are investigated for the growth of zinc oxide (ZnO) nanowires.
Abstract: Hydrothermal approach is widely used for the synthesis of zinc oxide (ZnO) nanowires. Zinc nitrate hexahydrate, zinc acetate and zinc chloride are three common salts that are used for synthesis. Among these, zinc nitrate hexahydrate is primarily used in many studies and zinc chloride is preferred for electrodeposition. In this work, zinc acetate dihydrate salt is used for the growth of ZnO nanowires and the effects of time, temperature, solution concentration and concentration ratios of the precursor chemicals are investigated. It is found that the growth time and solution concentration control the lengths of the nanowires, whereas the precursor concentration ratio and solution concentration control their diameter. High solution concentrations and high zinc acetate dihydrate concentrations lead to the development of thin film morphology. Optimum growth parameters are obtained and suggested for the use of zinc acetate dihydrate as a zinc source for growing ZnO nanowires with high aspect ratio (AR). The use of zinc acetate dihydrate leads to the formation of ZnO nanowires without impurities and eliminates the need for using extra capping agents.
TL;DR: In this paper, the authors quantify the time-dependent mechanical properties of a linear viscoelastoplastic material under contact loading and show that the relaxation modulus can be measured independently of concurrent plasticity exhibited during the loading phase.
Abstract: Here we quantify the time-dependent mechanical properties of a linear viscoelastoplastic material under contact loading. For contact load relaxation, we showed that the relaxation modulus can be measured independently of concurrent plasticity exhibited during the loading phase. For indentation creep, we showed that the rate of change of the contact creep compliance $$\dot L(t)$$
can be measured independently of any plastic deformation exhibited during loading through $$\dot L(t) = 2a(t)\dot h(t)/P_{\max } $$
, where a(t) is the contact radius, h(t) is the displacement of the contact probe, and Pmax is the constant applied load during the creep phase. These analytical relations were compared with numerical simulations of conical indentation creep for a viscoelastoplastic material and validated against sharp indentation creep experiments conducted on polystyrene. The derived relations enable extraction of viscoelastic material characteristics, even if sharp probes confer concurrent plasticity, applicable for a general axisymmetric contact probe geometry and a general time-independent plasticity.
TL;DR: In this article, the many roles of hydrogen in zinc oxide, tin dioxide, titanium dioxide, indium (III) oxide, gallium oxide, and strontium titanate are reviewed.
Abstract: Oxide semiconductors exhibit a range of physical properties and have potential optical, electronic, and energy applications. Transparent conducting oxides (TCOs) are currently used in products such as flat-panel displays. The prevailing n-type conductivity in these materials has historically been attributed to native defects such as oxygen vacancies. Recent calculations and experiments, however, have provided evidence that native defects are actually not responsible in majority of the cases. Hydrogen, on the other hand, does act as a shallow donor and can dramatically affect the electrical properties of oxides. In addition to contributing to n-type doping, hydrogen also passivates dangling bonds in cation vacancies and passivates acceptor dopants. Some oxides contain “hidden hydrogen,” perhaps H2 molecules, which dissociate at elevated temperatures. In this article, the many roles of hydrogen in zinc oxide, tin dioxide, titanium dioxide, indium (III) oxide, gallium (III) oxide, and strontium titanate are reviewed. The emphasis is on fundamental electronic, structural, and vibrational properties of hydrogen complexes, as determined by experiment and theory.
TL;DR: In this article, a hybrid structure consisting of graphitic foliates grown along the length of aligned multiwalled CNTs was proposed for charge storage and a model for their nucleation and growth was proposed.
Abstract: Graphene and carbon nanotubes (CNTs) are fascinating materials, both scientifically and technologically, due to their exceptional properties and potential use in applications ranging from high-frequency electronics to energy storage devices. This manuscript introduces a hybrid structure consisting of graphitic foliates grown along the length of aligned multiwalled CNTs. The foliate density and layer thickness vary as a function of deposition conditions, and a model is proposed for their nucleation and growth. The hybrid structures were studied using electron microscopy and Raman spectroscopy. The foliates consist of edges that approach the dimensions of graphene and provide enhanced charge storage capacity. Electrochemical impedance spectroscopy indicated that the weight-specific capacitance for the graphenated CNTs was 5.4× that of similar CNTs without the graphitic foliates. Pulsed charge injection measurements demonstrated a 7.3× increase in capacitance per unit area. These data suggest that this unique structure integrates the high surface charge density of the graphene edges with the high longitudinal conductivity of the CNTs and may have significant impact in charge storage and related applications.
TL;DR: In this article, the intrinsic strength between epoxy and silica derived from the molecular level is used to predict the structural behavior of epoxy-silica interface at the macroscopic length scale by invoking a finite element approach using a cohesive zone model.
Abstract: Organic–inorganic interfaces exist in many natural or synthetic materials, such as mineral–protein interfaces found in bone and epoxy–silica interfaces found in concrete construction. Here, we report a model to predict the intrinsic strength between organic and inorganic materials, based on a molecular dynamics simulation approach combined with the metadynamics method, used to reconstruct the free energy surface between attached and detached states of the bonded system and scaled up to incorporate it into a continuum model. We apply this technique to model an epoxy–silica system that primarily features nonbonded and nondirectional van der Waals and Coulombic chemical interactions. The intrinsic strength between epoxy and silica derived from the molecular level is used to predict the structural behavior of epoxy–silica interface at the macroscopic length scale by invoking a finite element approach using a cohesive zone model which shows a good agreement with existing experimental results.
TL;DR: In this paper, the authors used instrumented indentation to characterize the mechanical and transport behavior of a pH-sensitive hydrogel in various aqueous buffer solutions, and they demonstrated the utility of indentation techniques in the characterization of pH sensitive hydrogels.
Abstract: We use instrumented indentation to characterize the mechanical and transport behavior of a pH-sensitive hydrogel in various aqueous buffer solutions In the measurement, an indenter is pressed to a fixed depth into a hydrogel disk and the load on the indenter is recorded as a function of time By analyzing the load–relaxation curve using the theory of poroelasticity, the elastic constants of the hydrogel and the diffusivity of water in the gel can be evaluated We investigate how the pH and ionic strength of the buffer solution, the hydrogel cross-link density, and the density of functional groups on the polymer backbone affect the properties of the hydrogel This work demonstrates the utility of indentation techniques in the characterization of pH-sensitive hydrogels
TL;DR: In this paper, the local mechanical properties of duplex stainless steel were studied by nanoindentation and the elastic and plastic properties of the two phases were determined, and the ferrite phase was found to supersede austenite in the elastic modulus, hardness, and strain-hardening exponent, while both phases possess similar yield strength.
Abstract: The local mechanical properties of ferritic and austenitic domains in a duplex stainless steel are locally studied by nanoindentation. The elastic and plastic properties of the two phases are determined. Without any specific surface treatment (chemical or electrochemical), the austenitic and ferritic domains present in the duplex stainless steel are distinguished using magnetic force microscopy. The magnetic scans allow nanoindentation results to be assigned to the respective phase, yielding the local mechanical properties of the duplex steel. The magnetic scans also show a sharp transition between the phases that is maintained even inside indentations. The ferrite phase is found to supersede austenite in the elastic modulus, hardness, and strain-hardening exponent, while both phases possess similar yield strength. Interface properties are a weighted average of the phase properties.
TL;DR: In this paper, the authors describe cracking during microcompression of Si, InAs, MgO, and MgAl2O4 crystals and compare this with previous observations on Si and GaAs micropillars.
Abstract: This article describes cracking during microcompression of Si, InAs, MgO, and MgAl2O4 crystals and compares this with previous observations on Si and GaAs micropillars. The most common mode of cracking was through-thickness axial splitting, the crack growing downward from intersecting slip bands in pillars above a critical size. The splitting behavior observed in all of these materials was quantitatively consistent with a previous analysis, despite the differences in properties and slip geometry between the different materials. Cracking above the slip bands also occurred either in the side or in the top surface of some pillars. The driving forces for these modes of cracking are described and compared with observations. However, only through-thickness axial splitting was observed to give complete failure of the pillar; it is, therefore, considered to be the most important in determining the brittle-to-ductile transitions that have been observed.
TL;DR: In this article, the authors extend the results for Pr0.1Ce0.9O2-δ to include values of x 5 0, 0.002, 0.,008, 0, 1.1, and 0.20 (in PrxCe1-xO2δ).
Abstract: Nonstoichiometric mixed ionic and electronic conductors (MIECs) find use as oxygen permeation membranes, cathodes in solid oxide fuel cells, oxygen storage materials in three-way catalysts, and chemoresistive gas sensors. Praseodymium–cerium oxide (PrxCe1-xO2-δ) solid solutions exhibit MIEC behavior in a relatively high and readily accessible oxygen partial pressure (PO2) regime and as such serve as model systems for investigating the correlation between thermodynamic and kinetic properties as well as exhibiting high performance figures of merit in the above applications. In this paper, we extend recently published results for Pr0.1Ce0.9O2-δ to include values of x 5 0, 0.002, 0.008, 0.1, and 0.20 (in PrxCe1-xO2-δ) to test how both defect and transport parameters depend on Pr fraction. Important observed trends with increasing x include increases in oxygen ion migration energy and MIEC and reductions in vacancy formation and Pr ionization energies. The implications these changes have for potential applications of PrxCe1-xO2-δ are discussed.
TL;DR: In this article, the feasibility of the use of boron nitride nanotubes (BNNTs) as reinforcement in aluminum matrix composites for structural application is analyzed.
Abstract: Nature and mechanism of interfacial reactions between boron nitride nanotubes (BNNTs) and aluminum matrix at high temperature (650 °C) are studied using high-resolution transmission electron microscopy (HRTEM). This study analyzes the feasibility of the use of BNNTs as reinforcement in aluminum matrix composites for structural application, for which interface plays a critical role. Thermodynamic comparison of aluminum (Al)-BNNT with analogous Al-carbon nanotube (Al-CNT) system reveals lesser amount of reaction in the former. Experimental observation also reveals thin (~7 nm) reaction-product formation at Al-BNNT interface even after 120 min of exposure at 650 °C. The spatial distribution of the reaction-product species at the interface is governed by the competitive diffusion of N, Al, and B. Morphology of the reaction products are influenced by their orientation relationship with BNNT walls. A theoretical prediction on Al-BNNT interface in macroscale composite suggests the formation of strong bond between the matrix and reinforcement phase.
TL;DR: In this paper, the effects of temperature and alloying elements on deformation in the high-strain-rate regime were investigated by testing fine-grained magnesium alloys with an average grain size of 2 ∼ 3 μm by a nanoindentation technique.
Abstract: The effects of temperature and alloying elements on deformation in the high-strain-rate regime were investigated by testing fine-grained magnesium alloys with an average grain size of 2 ∼ 3 μm by a nanoindentation technique The dynamic hardness measurements aligned well with existing quasistatic data, together spanning a wide range of strain rates, 10−3 ∼ 150/s The high-rate hardness was influenced by various alloying elements (Al, Li, Y and Zn) to different degrees, consistent with expectations based on solid solution strengthening Transmission electron microscopy observations of the indented region revealed no evidence for deformation twins for any alloying elements, despite the high strain-rate The activation energy for deformation in the present alloys was found to be 85 ∼ 300 kJ/mol within the temperature range of 298 ∼ 373 K, corresponding to a dominant deformation mechanism of dislocation glide
TL;DR: In this paper, microcompression tests were performed on the Al/Nb multilayers of incoherent interfaces with the layer thicknesses of 5 nm Al/5 nm Nb and 50 nm AL/50nm Nb.
Abstract: Microcompression tests were performed on the Al/Nb multilayers of incoherent interfaces with the layer thicknesses of 5 nm Al/5 nm Nb and 50 nm Al/50 nm Nb. The Al-Nb multilayers showed increase in strength as the layer thickness was reduced; the average flow stresses at 5% plastic strain from the 5 nm Al/5 nm Nb and 50 nm Al/50 nm Nb layer thickness specimens were determined to be 2.1 GPa and 1.4 GPa respectively. The results from this Al-Nb microcompression study were compared with those of the previous report on Cu-Nb multilayer microcompression results that indicated that the flow stresses of the Al-Nb multilayer are lower than those of Cu-Nb with the same bilayer spacing. The observed difference in strength was attributed to a potential difference in the interfacial strength of the two incoherent multilayer systems.
TL;DR: In this paper, the ionic conductivity of gel polymer electrolytes was increased, up to a maximum value of (1.64 ± 0.01) x 10−4 S/cm by adding 60 wt% of BmlmTFSI.
Abstract: The preparation of l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmlmTFSI)-based poly(methyl methacrylate)-poly(vinyl chloride), PMMA-PVC, gel polymer electrolytes was done by solution casting technique. The ionic conductivity of gel polymer electrolytes was increased, up to a maximum value of (1.64 ± 0.01) x 10−4 S/cm by adding 60 wt% of BmlmTFSI. Conductivity-frequency dependence, dielectric relaxation, and dielectric moduli formalism were also further analyzed. These studies assert the ionic transportation mechanisms in the polymer matrix. Occurrence of polarization electrode-electrolyte interface is also observed. This leads to the formation of electrical double layer and hence indicates the non-Debye characteristic of the polymer matrix in the dielectric studies. Based on the changes in shift, changes in intensity, changes in shape, and existence of new peaks, attenuated total reflectance-Fourier transform infrared divulged the complexation between PMMA, PVC, lithium bis(trifluoromethanesulfonyl)imide, and BmlmTFSI, as shown in the infrared spectra.