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The goal of this research was improving the mechanical and physical properties of poly lactic acid (PLA) using cellulose nanofiber (CNF) as reinforcing and maleated PLA (PLA-g-MA) as a compatibilizer. PLA/nanocellulose composites were prepared through melt mixing with maleated PLA (5 wt%) and two levels of cellulose nanofibers (CNFs) (3 and 5 wt%) using twin screw extrusion. Nanocomposites fracture morphology, thermal properties, crystallization behavior, and mechanical behavior were determined as a function of cellulose nanofibers and PLA grafted on maleic anhydride content using scanning electron microscopy (SEM), atomic force microscopy, heat distortion temperature (HDT), impact, and tensile testing. The SEM micrographs confirmed the uniform dispersion of CNF within PLA matrix in the presence of maleated PLA. Among nanocellulose and compatibilizer, the latter one has better role in enhancement of nanocomposites HDT. Compatibilized nanocomposites (PLA/CNF5/PLAgMA5) exhibited maximum impact strength which was 131% higher than that of neat PLA. Compared to pure PLA, 138 and 40% improvements for the tensile strength and Young’s modulus can be obtained for the resulting nanocomposite with PLA/CNF5/PLAgMA5, respectively.

The detection of piezoelectric current in one-dimensional semiconductor materials has been a controversial issue due to the possibility of charge annihilation at nanoscale dimensions. We report here, the mechanically triggered electrical current in uniaxially compressed individual ZnO nanobelts under no applied bias. The measurements were carried out in situ by using a transmission electron microscope. In contrast to the bending mode, the magnitude of the electrical current increased with the increase of uniaxial compression, which indicates the load-mode dependency of the detected current. The flow of electrical current through the ZnO nanobelts under applied stress was explained based on the separation of ionic charges along the two ends of the nanobelts due to the applied compressive force. The charge separation is expected to induce an internal electric field inside the nanobelt and facilitate the movement of free charge carriers through the nanobelt. Due to the separation and accumulation of charges, the metal–semiconductor system becomes forward biased when contact is established, leading to the flow of electrons through the Schottky barrier.

Nanolayers of Al2O3 and TiO2 coatings were applied to lithium- and manganese-rich cathode powder Li1.2Ni0.13Mn0.54Co0.13O2 using an atomic layer deposition (ALD) method. The ALD coatings exhibited different surface morphologies; the Al2O3 surface film appeared to be uniform and conformal, while the TiO2 layers appeared as particulates across the material surface. In a Li-cell, the Al2O3 surface film was stable during repeated charge and discharge, and this improved the cell cycling stability, despite a high surface impedance. The TiO2 layer was found to be more reactive with Li and formed a LixTiO2 interface, which led to a slight increase in cell capacity. However, the repetitive insertion/extraction process for the Li+ ions caused erosion of the surface protective TiO2 film, which led to degradation in cell performance, particularly at high temperature. For cells comprised of the coated Li1.2Ni0.13Mn0.54Co0.13O2 and an anode of meso-carbon-micro-beads (MCMB), the cycling stability introduced by ALD was not enough to overcome the electrochemical instability of MCMB graphite. Therefore, protection of the cathode materials by ALD Al2O3 or TiO2 can address some of the capacity fading issues related to the Li-rich cathode at room temperature.

The size scale effect on the piezoelectric response of bulk ZnO and ZnO nanobelts has been studied using molecular dynamics simulation. Six molecular dynamics models of ZnO nanobelts are constructed and simulated with lengths of 150.97 A and lateral dimensions ranging between 8.13 and 37.37 A. A molecular dynamics model of bulk ZnO has also been constructed and simulated using periodic boundary conditions. The piezoelectric constants of the bulk ZnO and each of the ZnO nanobelts are predicted. The predicted piezoelectric coefficient of bulk ZnO is 1.4 C m −2 , while the piezoelectric coefficient of ZnO nanobelts increases from 1.639 to 2.322 C m −2 when the lateral dimension of the ZnO NBs is reduced from 37.37 to 8.13 A. The changes in the piezoelectric constants are explained in the context of surface charge redistribution. The results give a key insight into the field of nanopiezotronics and energy scavenging because the piezoelectric response and voltage output scale with the piezoelectric coefficient.

A series of in situ transmission electron microscopy combined with scanning tunneling microscopy measurements were carried out to investigate the effect of mechanical deformation on the electrical transport properties of amorphous TiO2 nanotubes. Under no mechanical straining, it was found that the TiO2 nanotubes behave as electrical insulators. However, the nanotubes show semiconducting behavior under a highly deformed state. On the basis of a metal–semiconductor–metal model, it was suggested that in-shell defects, surface defect-driven conduction modes, are responsible for the appearance of the semiconducting behavior.

The gas diffusion layer (GDL) is a critical component of a proton exchange membrane fuel cell, and can play a key role in fuel cell performance. In order to design reliable and durable fuel cells, knowledge of the GDL microstructure is necessary. Currently, characterization of GDLs is generally based on porosity measurements to obtain a pore size distribution. However, the pore size distribution in GDLs may not be the only factor that affects the fuel cell performance. Additional microstructural characterization of GDLs manufactured by three different vendors (Toray, SGL, and Freudenberg) has been investigated. In addition to the pore size distribution, other statistical information of GDL microstructure including size, shape, orientation, and distribution of pores have been characterized and compared. Among these GDLs, the Freudenberg sample was found to have the smallest pore size and orientation analysis indicated that the pores were randomly distributed. Pore roundness was the lowest and pore clustering was highest in Toray sample. The effect of threshold setting on pore size data was also studied and found to have negligible influence on the calculated distributions. The microstructures of the GDLs were reconstructed in three-dimension using computer simulations and good agreement with the two-dimensional image analysis data was observed. The present work opens new opportunities for experimentalists and modelers in the area of fuel cell research to take into account the statistical characteristics of GDL microstructure.

The nanoscale transitional zone between a nanofiber and surrounding matrix (interphase) defines the ultimate mechanical characteristics in nanocomposite systems. In spite of this importance, one can hardly find quantitative data on the mechanical properties of this transitional zone in the cellulose–nanofiber composites. In addition, most of the theoretical models to predict the mechanical properties of interphase are developed with the assumption that this transitional zone is independent of the nanofiber size. In the current study, we show that the mechanical properties of interphase in cellulose nanocrystal (CNC) composites can be quantitatively characterized and the correlation with the size of CNCs can be mapped. The peak force tapping mode in atomic force microscope (AFM) was used to characterize deformation, adhesion, and modulus gradient of the interphase region in poly(vinyl alcohol) (PVA)–poly (acrylic acid) (PAA)–cellulose nanocrystal (CNC) composites. In comparison to the polymer matrix, the adhesion force of CNC was lower. The average elastic modulus in the interphase varied from 12.8 GPa at the interface of CNC to 9.9 GPa in PVA–PAA matrix. It was observed that the existence of PAA increased the gradient of mechanical and adhesion properties of the interphase zone. This occurs due to the variation in the ester linkage density from the CNC interface to the polymer matrix. Finally, it is shown that interphase thickness is higher for CNCs with larger diameter.

As the applications for inorganic nanowires continuously grow, studies on the stability of these structures under high electrical/thermal stress conditions are needed. ZnTe nanowires are grown by the vapor-liquid-solid technique and their breakdown under Joule heating is studied through in situ monitoring in a transmission electron microscope (TEM). The experimental setup, consisting of a scanning tunneling microscope (STM) and a movable piezotube inside the TEM, allows the manipulation of a single nanowire. A voltage applied to the STM tip in contact with a ZnTe nanowire leads to the breakdown of the nanowire into Zn and Te particles or balls which is observed in real time. These balls grow by Ostwald ripening, rendering the surface morphology of the ZnTe nanowire progressively rough. Diffraction patterns along the stem of the wire after the partial breakdown showed substantially smaller lattice spacing compared to 0.35 nm for pristine ZnTe nanowires.

We investigate the anode performance of Si-based nanorods by tuning its composition using an oblique (co)deposition technique. Our results show that pure Si nanorods have a higher initial anodic capacity of 1500 mAh g−1, but the capacity diminishes after 50 cycles due to the morphological change and pulverization. By introducing approximately 70 at.% Cu into Si nanorods, the Si–Cu composite nanorods demonstrate 500 mAh g−1 of capacity sustainable in 100 cycles, which is attributed to the flexibility and improved toughness of Si–Cu composite nanorods.

The ultimate properties of a fibrous composite system depend highly on the transverse mechanical properties of the fibers. Here, we report the size dependency of transverse elastic modulus in cellulose nanocrystals (CNCs). In addition, the mechanical properties of CNCs prepared from wood and cotton resources were investigated. Nanoindentation in an atomic force microscope (AFM) was used in combination with analytical contact mechanics modeling (Hertz model) and finite element analysis (FEA) to estimate the transverse elastic moduli (Et) of CNCs. FEA modeling estimated the results more accurately than the Hertz model. Based on the AFM–FEA calculations, wood CNCs had higher transverse elastic moduli in comparison to the cotton CNCs. Additionally, Et was shown to increase with a reduction in the CNCs’ diameter. This size-scale effect was related to the Ia/Ib ratio and crystalline structure of CNCs.

We report here investigations of crystal and electronic structure of as-synthesized and annealed ZnO nanobelts by an in-situ high-resolution transmission electron microscope equipped with a scanning tunneling microscopy probe. The in-situ band gap measurements of individual ZnO nanobelts were carried out in scanning tunneling spectroscopy mode using the differential conductance dI/dV–V data. The band gap value of the as-synthesized ZnO nanobelts was calculated to be ∼2.98 eV, while this property for the annealed nanobelts (∼3.21 eV) was close to the band gap value for bulk ZnO materials (∼3.37 eV). The difference in the band gap value of the as-synthesized ZnO nanobelts and annealed ones was attributed to the planar defects (e.g. stacking faults and twins). These defects can alter the electronic structure by producing localized resonant states that result in band gap reduction.

Rice husk (RH), an abundant agricultural residue, was reacted with 2-bromoisobutyryl bromide, to convert it to a heterogeneous polyfunctional macroinitiator for Atom Transfer Radical Polymerization (ATRP). The number of active sites placed on the RH surface was small, but they were ATRP active. Non-polar methyl methacrylate (MMA) and polar acrylonitrile (AN) were polymerized from the RH, and a sequential monomer addition was used to prepare an amphiphilic PMMA-b-PAN copolymer on RH surface. FTIR qualitatively confirmed the grafting. Gravimetric and XPS analysis of the different RH surface compositions indicated thin layers of oligomeric PMMA, PAN, and PMMA-b-PAN. The modified surfaces were mapped by nanomechanical AFM to measure surface roughness, and adhesion and moduli using the Derjaguin-Muller-Toropov model. RH grafted with MMA possessed a roughness value of 7.92, and a hard and weakly adhering surface (13.1 GPa and 16.7 nN respectively) while RH grafted with AN yielded a roughness value of 29 with hardness and adhesion values of 4.0 GPa and 23.5 nN. The PMMA-b-PAN modification afforded a surface with a roughness value of 51.5 nm, with hardness and adhesion values of 3.0 GPa and .75.3 nN.

Silicon is a promising anode material for Li-ion batteries in regarding of high capacity, low cost and safety, but it suffers poor cycling stability due to the pulverization induced by severe volume expansion/shrinkage (297%) during lithium insertion/extraction. In our previous investigation on aluminum nanorods anodes, it is found the selection of substrates in which Al nanorods grown plays the role in prevention of pulverization resulting in the increase of cycling life. Adapting this knowledge, we investigated the Si based nanorods anodes by tuning its composition and element distribution. Our results show that although the Si nanorods demonstrated higher initial anodic capacity of 1500 mAh/g, it diminished after 50 cycles due to morphology change and pulverization. By codepositing Cu, the Si-Cu composite nanorods demonstrated sustainable capacity of 500 mAh/g in 100 cycles attributing to its flexible and less brittle nature.

Summary Properties of composite materials are directly affected by the spatial arrangement of reinforcement and matrix. In this research, partially hydrolysed cellulose microcrystals were used to fabricate polycaprolactone microcomposites. The spatial distribution of cellulose microcrystals was characterized by a newly developed technique of X-ray ultra microscopyandmicrotomography.Thephaseandabsorption contrast imaging of X-ray ultra microscopy revealed twodimensional and three-dimensional information on CMC distribution in polymer matrices. The highest contrast and flux (signal-to-noise ratio) were obtained using vanadium foil targets with the accelerating voltage of 30 keV and beam current of >200 nA. The spatial distribution of cellulose microcrystals was correlated to the mechanical properties of the microcomposites. It was observed that heterogeneous distribution and clustering of cellulose microcrystals resulted in degradation of tensile strength and elastic modulus of

This paper presents a parametric finite element analysis using a history-dependent internal state variable model for a hydroforming process. Experiments were performed for the internal state variable model correlation and for validating a 2-in. copper tee hydroforming process simulation. The material model constants were determined from uniaxial stress–strain responses obtained from tensile tests on the tube's material. In the finite element simulations, the mesh and boundary conditions were integrated with the geometry and process parameters currently used in industry. The study provides insights for the variation of different process parameters (velocity and pressure profiles, and bucking system characteristics) related to the finished product.

Computational design of materials processes has received great interests during the past few decades. Successful designs require accurate assessment of material properties, which can be influenced by the internal microstructure of materials. This work aim to develop a novel computational model based on dislocation structures to predict the flow stress properties of metallic materials. To create sufficient training data for the model, the flow stress of a precipitation–hardening aluminum alloy was measured by characterizing the dislocation structure of specimens from interrupted mechanical tests using a high resolution electron backscatter diffraction technique. The density of geometrically necessary dislocations was calculated based on analysis of the local lattice curvature evolution in the crystalline lattice. For three essential features of dislocation microstructures – substructure cell size, cell wall thickness, and density of geometrically necessary dislocations – statistical parameters of their distributions were used as the input variables of the predictive model. An artificial neural network (ANN) model was used to back-calculate the in situ non-linear material parameters for different dislocation microstructures. The model was able to accurately predict the flow stress of aluminum alloy 6022 as a function of its dislocation structure content. In addition, a sensitivity analysis was performed to establish the relative contribution of individual dislocation parameters in predicting the flow stress. The success of this approach motivates further use of ANNs and related methods to calibrate and predict inelastic material properties that are often too cumbersome to model with rigorous dislocation-based plasticity models.

Carbon Nanotubes (CNTs) have been a subject of interest for most of the researches after their discovery, due to their superior mechanical properties and ability to be used as the reinforcement phase in nanocomposites. The other carbon structure which was discovered a few years after the discovery of CNTs was Carbon Nanohorns (CNHs). The structure of CNH is cone-shaped compared to the cylindrical shape of CNT’s structure. This cone-shaped structure causes difficulties in finding the stress distribution in CNHs when embedded in the matrix phase of nanocomposites. In this paper the governing differential equation of the stress distribution on the CNHs placed in a nanocomposite matrix is derived using some simplifying assumptions. It has been shown here that while the stress distribution is symmetric for the special case of CNT, it is non-symmetric for the general CNHs and the maximum stress shifts toward the tip of CNH.

The future of microelectronic and nanoelectronics devices could lie in one-dimensional nanomaterials including boron nitride nanotubes (BNNTs). In such applications, however, the flow of electrical current may induce structural failure resulting in reduction of component Service life. Here, we utilized scanning tunneling microscopy inside a transmission electron microscope to study the thermal failure of individual multi-walled BNNTs via Joule heating. At elevated temperatures, the nanotube failed by the formation of amorphous nanoclusters and progression of structural defects. These Clusters have various sizes and initially form on the outermost shell layers of BNNTs.

The passive films play an important role in corrosion and stress corrosion cracking of austenitic stainless steels. The current research investigates the relationship between alloy chemistry, microstructure, and mechanical behavior of passive films formed on 316, 304, and 904L stainless steels (SS). X-ray photoelectron spectroscopy and transmission electron microscopy were used to investigate the effect of alloy chemistry and microstructure constituents on the thin film fracture properties determined by nanoindentation tests. The analyses showed that fracture loads are directly related to the crystallography of the thin films. It was found that decreasing the ratio of iron to other metallic elements in the film led to an increase in the load required to fracture the film. It was also found that films grown on 304, 316, and 904L stainless steels were the cubic polymorph of Cr2O3, rather than the lower energy rhombohedral form. In the case of 904L SS the film formed as an epitaxial layer. In the other two cases it consisted of small crystalline islands in an amorphous matrix. A dichromate treatment of 316 SS decreased the iron content in the oxide film and increased the hardness. It also resulted in an epitaxial film.

The in-grain orientation gradient and grain fragmentation are essential features of substructure development under cold deformation up to large strains. In the previous study [P. Trivedi, D.P. Field, H. Weiland, Alloying effects on dislocation substructure evolution of aluminum alloys, Int. J. Plasticity 20 (2004) 459–476], we showed that the Mg and Si elements can influence the development of dislocation structures. The present study extends the above-mentioned work to the materials where effective hardening particles are distributed throughout the polycrystalline structure, and the interaction of dislocation substructures and these particles is significant. We focus upon the small strain regime and compare the in-grain orientation gradient of a precipitation hardened aluminum alloy as a function of precipitate morphologies during deformation. The precipitate morphologies were characterized using transmission electron microscopy, and the dislocation structure was analyzed using electron backscatter diffraction analysis of deformed specimens. The results clearly show that the deformation response was a function of the precipitate characteristics. Although, grain fragmentation increased during deformation, the effect of precipitates on the in-grain orientation gradient, and dislocation substructure evolution was more pronounced in the presence of semi-coherent β″ precipitates. This investigation offers motivation to include the precipitate parameters in the deformation framework of physics based computational modeling of crystals containing hardening particles.

Cellulose nanofibers are known to possess aspect ratios larger than 200 and mechanical properties comparable to carbon nanotubes. Combined with other significant properties including low cost, low density, and biocompatibility, cellulose nanofibers are an attractive reinforcement material for nanocomposites. The load transfer between embedded fibers and matrix play a major role in designing nanocomposites with ultimate mechanical properties. In this work, we studied a general case where a simple axial loading exists along the axis of a cellulose fiber embedded in a polymer matrix. Then analytical relation between the applied load, the longitudinal stress along the fiber, and shear stresses along the interface of fiber and matrix was derived. It is shown that the maximum longitudinal stress occurs at the middle of the fiber, while maximum shear stress occurs at the extreme ends. Also, it is shown that the shear stress along the cellulose fibers can be approximated as a linear function of applied load. The derived relationships are useful for design of cellulose-based nanocomposites with enhanced mechanical properties.

We propose a micromechanical model for elastic–plastic deformation of crystals containing elastic inclusions. The model is based on a novel method for calculation of elastic strain energy associated with incompatible deformation in the matrix and in the inclusion, whereby this energy is interpreted as the interaction energy of geometrically necessary dislocation loops (Mesarovic, S.Dj., 2005. Energy, configurational forces and characteristics lengths, associated with the continuum description of geometrically necessary dislocations. Int. J. Plasticity 21, 1855–1889), which are geometrically equivalent to the incompatible strain. The method is general, easily applied to complex geometries and particle interactions, and – it often yields analytic solutions. In this communication, we analyze the simple case of dilute concentration of inclusions. The resulting model is remarkably simple; it falls within the framework of classical crystal plasticity with a specific elastic–plastic constitutive law that accounts for shape of inclusions.

The precipitate sequence in a 6022 aluminum alloy was investigated by means of differential scanning calorimetry (DSC), transmission electron microscopy, and Vickers hardness measurements. The solution-treated samples were quenched and then immediately subjected to DSC and isothermal aging experiments. It was observed that in the early stages of aging there are some unknown small precipitates that form prior to the formation of β″ precipitates. Studies on isothermally aged and DSC heated samples suggest that some of the β″ needles transform during growth to lath-shaped precipitates. An alternative precipitation sequence for AA6022 is proposed.

The precipitation of metastable phases during differential scanning calorimetry experiments of cold deformed AA6022 is investigated by means of transmission electron microscopy. It is found that β″ precipitates are replaced by β′ and Q′ in the predeformed samples. The results show that at higher temperatures only Q′ precipitates are present in the microstructure of the predeformed samples.

Kinetics of β″ precipitation in an Al-0.55 pct Mg-1.1 wt pct Si (AA6022) in both deformed and undeformed conditions were investigated by means of differential scanning calorimetry (DSC). Transmission electron microscopy (TEM) analysis on predeformed samples was also performed to study the effect of deformation on β″ precipitates. It was found that the introduction of dislocations changes the precipitate type from β″ to β′ and Q′. Also the measured activation energy for precipitate formation in the case of predeformed and undeformed samples was compared. The results show that dislocations facilitate precipitation by decreasing the activation energy.

In proton exchange membrane fuel cells, the stacks of anode, cathode, and membrane layers including gas diffusion layer (GDL) are held together by a compressive force applied through a bipolar plate. In this work, we studied the electrical properties of a carbon fiber of a GDL under deformation using four-point measurement methods inside a scanning electron microscope (SEM). We found out that through bending deformation the electrical resistivity of carbon fibers will be reduced. The drop in resistance during deformation may be the result of increasing conduction channels in the carbon fiber and parallel transport through them. Our finding offers a new insight on the effect of deformation on tuning the electrical properties of GDL materials.

The electrical properties of boron nitride (BN) nanostructures, particularly BN nanotubes (NTs), have been studied less in comparison to the counterpart carbon nanotubes. The present work investigates the field emission (FE) behavior of BNNTs under multiple cycles of FE experiments and demonstrates a strain-engineering pathway to tune the electronic properties of BNNTs. The electrical probing of individual BNNTs were conducted inside a transmission electron microscope (TEM) using an in situ electrical holder capable of applying a bias voltage of up to 110 V. Our results indicate that in the first cycle a single BNNT can exhibit the current density of 1 mA cm 2 at 110 V and the turn-on voltage of 325 V m 1 . However, field emission properties reduced considerably in subsequent cycles. Real-time imaging revealed the structural degradation of individual BNNTs during FE experiments. The electromechanical measurements show that the conductivity of BNNTs can be tuned by means of mechanical straining. The resistance of individual BNNTs reduced from 2000 to 769 M and the carrier concentration increased from 0:35 10 17 to 1:1 10 17 cm 3 by straining the samples up to 2.5%. (Some figures may appear in colour only in the online journal)

One-dimensional nanostructures such as ZnTe, CdTe, Bi(2)Te(3) and others have attracted much attention in recent years for their potential in thermoelectric devices among other applications. A better understanding of their mechanical properties is important for the design of devices. A combined experimental and computational approach has been used here to investigate the size effects on the Young's modulus of ZnTe nanowires (NWs). The mechanical properties of individual ZnTe nanowires in a wide diameter range (50-230 nm) were experimentally measured inside a high resolution transmission electron microscope using an atomic force microscope probe with the ability to record in situ continuous force-displacement curves. The in situ observations showed that ZnTe NWs are flexible nanostructures with the ability to withstand relatively high buckling forces without becoming fractured. The Young's modulus is found to be independent of nanowire diameter in the investigated range, in contrast to reported results for ZnO NWs and carbon nanotubes where the modulus increases with a decrease in diameter. Molecular dynamics simulations performed for nanowires with diameters less than 20 nm show limited size dependence for diameters smaller than 5 nm. The surface atoms present lower Young's modulus according to the simulations and the limited size dependency of the cylindrical ZnTe NWs is attributed to the short range covalent interactions.

In situ electrochemical lithiation and delithiation processes inside a nanobattery consisting of an individual amorphous Si nanorod and ionic liquid were explored. Direct formation of the crystalline Li(22)Si(5) phase due to the intercalation of Li ions was observed. In addition, the role of the electrolyte-nanorod interface was examined. It was observed that the lithiation of Si nanorods is dominated by surface diffusion. Upon the delithiation process, partial decomposition of Li(22)Si(5) particles was observed which can explain the irreversible capacity loss that is generally seen in Si anodes. This study shows that the radial straining due to lithiation does not cause cracking in nanorods as small in diameter as 26 nm, whereas cracks were observed during the lithiation of 55 nm Si nanorods.

In this investigation, the size-scale in mechanical properties of individual [0001] ZnO nanowires and the correlation with atomic-scale arrangements were explored via in situ high-resolution transmission electron microscopy (TEM) equipped with atomic force microscopy (AFM) and nanoindentation (NI) systems. The Young's modulus was determined to be size-scale-dependent for nanowires with diameter, d, in the range of 40 nm ≤ d ≤ 110 nm, and reached the maximum of ∼ 249 GPa for d = 40 nm. However, this phenomenon was not observed for nanowires in the range of 200 nm ≤ d ≤ 400 nm, where an average constant Young's modulus of ∼ 147.3 GPa was detected, close to the modulus value of bulk ZnO. A size-scale dependence in the failure of nanowires was also observed. The thick ZnO nanowires (d ≥ 200 nm) were brittle, while the thin nanowires (d ≤ 110 nm) were highly flexible. The diameter effect and enhanced Young's modulus observed in thin ZnO nanowires are due to the combined effects of surface relaxation and long-range interactions present in ionic crystals, which leads to much stiffer surfaces than bulk wires. The brittle failure in thicker ZnO wires was initiated from the outermost layer, where the maximum tensile stress operates and propagates along the (0001) planes. After a number of loading and unloading cycles, the highly compressed region of the thinner nanowires was transformed from a crystalline to an amorphous phase, and the region near the neutral zone was converted into a mixture of disordered atomic planes and bent lattice fringes as revealed by high-resolution images.

The recent observation of high flexibility in buckled boron nitride nanotubes (BNNTs) contradicts the pre-existing belief about BN nanotube brittleness due to the partially ionic character of bonding between the B and N atoms. However, the underlying mechanisms and relationships within the nanotube remained unexplored. This study reports for the first time the buckling mechanism in multi-walled BNNTs upon severe mechanical deformation. Individual BNNTs were deformed inside a transmission electron microscope (TEM) equipped with an in situ atomic force microscopy holder. High-resolution TEM images revealed that bent BNNTs form multiple rippling upon buckling. The critical strain to form the first ripple was measured as 4.1% and the buckling process was reversible up to 26% strain. As opposed to carbon nanotubes, the BNNTs buckled into V-shaped ripples rather than smooth wavy shapes. The rippling wavelength was quantified in terms of the outer diameter and thickness of the nanotubes. The BNNTs showed a larger rippling wavelength compared to that of CNTs with the same number of walls. This difference was explained by the tendency of BN structures to reduce the number of thermodynamically unfavorable B‐B and N‐N bonds at the sharp corners in the rippling regions. The BNNTs’ structure also exhibited a higher fracture strain compared to their counterpart. (Some figures in this article are in colour only in the electronic version)

In this program, Rochester Institute of Technology (RIT), General Motors (GM) and Michigan Technological University (MTU) have focused on fundamental studies that address water transport, accumulation and mitigation processes in the gas diffusion layer and flow field channels of the bipolar plate. These studies have been conducted with a particular emphasis on understanding the key transport phenomena which control fuel cell operation under freezing conditions. Technical accomplishments are listed below: • Demonstrated that shutdown air purge is controlled predominantly by the water carrying capacity of the purge stream and the most practical means of reducing the purge time and energy is to reduce the volume of liquid water present in the fuel cell at shutdown. The GDL thermal conductivity has been identified as an important parameter to dictate water accumulation within a GDL. • Found that under the normal shutdown conditions most of the GDL-level water accumulation occurs on the anode side and that the mass transport resistance of the membrane electrode assembly (MEA) thus plays a critically important role in understanding and optimizing purge. • Identified two-phase flow patterns (slug, film and mist flow) in flow field channel, established the features of each pattern, and created a flow patternmore » map to characterize the two-phase flow in GDL/channel combination. • Implemented changes to the baseline channel surface energy and GDL materials and evaluated their performance with the ex situ multi-channel experiments. It was found that the hydrophilic channel (contact angle   10⁰) facilitates the removal of liquid water by capillary effects and by reducing water accumulation at the channel exit. It was also found that GDL without MPL promotes film flow and shifts the slug-to-film flow transition to lower air flow rates, compared with the case of GDL with MPL. • Identified a new mechanism of water transport through GDLs based on Haines jump mechanism. The breakdown and redevelopment of the water paths in GDLs lead to an intermittent water drainage behavior, which is characterized by dynamic capillary pressure and changing of breakthrough location. MPL was found to not only limit the number of water entry locations into the GDL (thus drastically reducing water saturation), but also stabilizes the water paths (or morphology). • Simultaneously visualized the water transport on cathode and anode channels of an operating fuel cell. It was found that under relatively dry hydrogen/air conditions at lower temperatures, the cathode channels display a similar flow pattern map to the ex-situ experiments under similar conditions. Liquid water on the anode side is more likely formed via condensation of water vapor which is transported through the anode GDL. • Investigated the water percolation through the GDL with pseudo-Hele-Shaw experiments and simulated the capillary-driven two-phase flow inside gas diffusion media, with the pore size distributions being modeled by using Weibull distribution functions. The effect of the inclusion of the microporous layer in the fuel cell assembly was explored numerically. • Developed and validated a simple, reliable computational tool for predicting liquid water transport in GDLs. • Developed a new method of determining the pore size distribution in GDL using scanning electron microscope (SEM) image processing, which allows for separate characterization of GDL wetting properties and pore size distribution. • Determined the effect of surface wettability and channel cross section and bend dihedral on liquid holdup in fuel cell flow channels. A major thrust of this research program has been the development of an optimal combination of materials, design features and cell operating conditions that achieve a water management strategy which facilitates fuel cell operation under freezing conditions. Based on our various findings, we have made the final recommendation relative to GDL materials, bipolar design and surface properties, and the combination of materials, design features and operating conditions: • GDL materials: use lower thermal conductivity cathode GDL and decrease the anode GDL thickness. • Bipolar plate design: use a channel geometry that can be produced using a high-speed manufacturing process, with a hydrophilic coating. • Shutdown and gas purge protocol: incorporate above findings in developing cost effective and energy efficient shutdown purge protocol. It should be noted that a comprehensive fuel cell operating strategy must consider the entire range of operating conditions under which the system needs to perform. Although the recommendations above will benefit fuel cell performance under conditions where liquid water is expected to be present, they must also be fully assessed to understand their impact under relatively dry conditions.« less

Boron nitride (BN) nanotubes have structural and mechanical properties similar to carbon nanotubes and are known to be the strongest insulators. Great interest has been focused on understanding the mechanical properties of BN nanotubes as a function of their structural and physical properties. Yet, the published data have not been reviewed and systematically compared. In this paper, we critically review the mechanical properties of BN nanotubes from both experimental and simulation perspectives. The experimental reports include thermal vibrations, electric induced resonance method, and in situ force measurements inside transmission electron microscopy. The modeling and simulation efforts encompass tight bonding methods and molecular dynamics. Replacing the covalent sp2 bond (C–C) by ionic bond (B–N) results in differences in the mechanical properties of BN nanotubes in comparison to carbon nanotubes. The experimental and computational simulations indicate that BN nanotubes are highly flexible. High necking angles in BN nanotubes are assumed to be correlated with unfavorable bonding in B–B and N–N atoms.

The fabrication of cellulose nanocomposites with ultimate mechanical properties has received tremendous attention during the past decade. However, the published data has not been reviewed and systematically compared from mechanical point of view. The current study aims to fill this gap by providing a critical review on the published data on the mechanics of cellulose nanocrystals and their composites. The studies on individual cellulose nanocrystals show that their strength depends on the number and type of inter and intra hydrogen bonds on the cellulose chains, which are affected by the cellulose type and origin. It has been shown that the tensile modulus, yield strength and creep resistance are higher in cellulose nanocomposites than in unfilled polymers. However, above optimum cellulose content, the agglomeration of nanocrystals degrades the mechanical properties. Furthermore, cellulose nanocrystals enhance the structural stiffness of polymer composites at elevated temperatures. Formation of rigid nanocrystal network causes increase in the storage modulus (E′) and glass transition temperature.

From a scientific viewpoint, direct comparison between mechanical tests and computational simulations on a one-to-one basis has the potential to lead to substantial development in the concept of virtual testing of materials. Successful application of virtual testing methodology in our daily life basis requires the use of high-fidelity computational models that are being validated through accurate characterization techniques. The content of this chapter is prepared to cover some of the most recent developments in the area of materials characterizations with great potential for virtual testing and modeling applications. During the last decade, atomic force microscopy (AFM) has evolved into an essential tool for direct measurements of intermolecular forces that can be employed for verification of first-principle and molecular dynamic models. Novel techniques in the area of in situ electron microscopy have been developed in the last decade for investigating the structure–mechanical property relationship of advanced materials. X-ray ultra-microscopy (XuM) and microelectromechanical systems (MEMS) are among the two newest in situ microscopy developments. These techniques provide an excellent platform for direct correlation between structure and properties of nanoscale materials. These systems contain a limited number of atoms and possible equilibrium configurations, which can be identified in real time by means of in situ electron microscopy techniques. In addition, because of the limited number of atoms, these systems can be atomistically modeled within the reach of currently available computational power. This chapter provides a comprehensive review on the above-mentioned characterization techniques that can be used to validate computational models at nanometer length scales.

In this research, cellulose micro-crystals (CMC) were used to reinforce a bio-polymer, polycaprolactone (PCL). Mechanical properties were tested using nanoindentation. Electron microscopy imaging and a new technique called x-ray ultra microscopy and microtomography (XuM) were used to investigate the distribution of the filler in the matrix. We could demonstrate a clear correlation between the spatial distribution of CMC-PCL composites and their nanomechanical properties.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Boron nitride nanotubes (BNNTs) are not only structurally similar to carbon nanotubes (CNTs),[1] but their mechanical properties are also calculated and measured to be comparable to that of the CNTs.[2] Unlike carbon nanotubes (CNTs), the band gap of BNNTs is merely uniform, not sensitive to the change of chirality, diameter, and number of nanotubular walls. Furthermore, BNNTs are of advantageous to nanowires as they are ideally free of dangling bonds at their surfaces. Thus BNNTs are promising nanostructures for nanoscale electronic and photonic devices. However, BNNTs show insulating behavior due to wide band gap of 5.[4] eV[3]. Theoretical simulations predict that the electronic properties in BNNTs can be tuned by means of mechanical deformation or chemical alteration. Here, we report the electrical probing of individual Fe doped BNNTs inside a transmission electron microscope (TEM) using an in-situ electrical holder. All the measurements were carried out on a single tilt STM-TEM holder (Nanofactory Instruments) in a JEM 4000FX TEM system that operated at 200kV. Fig. 1display the image of a typical Fe doped functionalized BNNT in contact with the STM tip and the corresponding I-V data is shown in Fig. 2. It is evident from the I-V curve that in comparison to the insulating behavior of the pure BNNT nanotube, Fe doped BNNT shows a semiconducting behavior with high current of 80 nA. Results indicate that functionalized BNNTs are a new class of electronic materials showing the quantum tunneling behaviors. Details of these results will be discussed in the meeting.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

The coupled piezoelectric and semiconducting properties of the NWs and NBs accommodate charge creation, accumulation, and discharge processes. Here, we report the mechanically triggered generation of piezoresponse in the uniaxially compressed piezoelectric ZnO nanobelts. In order to correlate the structure-piezoresponse property of a nanobelt, we conducted structural imaging simultaneously with the piezoresponse measurements using a Nanoindentor (NI) /TEM system (capacitance < 3 pF) with a conducting probe. These nanobelts self-ignites electrically once subjected to an externally applied uniaxial compressive load without any external bias.

In spite of extensive studies on the preparation and characterization of nanocomposite materials, the correlation of their properties at the nanoscale with those in bulk is a relatively unexplored area. This is of great importance, especially for materials with potential biomedical applications, where surface properties are as important in determining their applicability as bulk characteristics. In this study, the nanomechanical characteristics of thin poly(vinyl alcohol) (PVOH)‐poly(acrylic acid) (PAA)‐cellulose nanocrystal (CNC) membranes were studied using the nanoindentation module in an atomic force microscope (AFM) and the properties were compared with the macro-scale properties obtained by tensile tests. In general, the elastic properties measured by nanoindentation followed the same trend as macro-scale tensile tests except for the PVOH 85-PAA 0-CNC 15 sample. In comparison to the macro-scale elastic properties, the measured elastic moduli with AFM were higher. Macro-scale tensile test results indicated that, in the presence of PAA, incorporation of CNCs up to 20 wt% improved the elastic modulus of PVOH, but when no PAA was added, increasing the CNC content above 10 wt% resulted in their agglomeration and degradation in mechanical properties of PVOH. The discrepancy between macro-scale tensile tests and nanoindentation in the PVOH 85-PAA 0-CNC 15 sample was correlated to the high degree of inhomogeneity of CNC dispersion in the matrix. It was found that the composites reinforced with cellulose nanocrystals had smaller indentation imprints and the pile-up effect increased with the increase of cellulose nanocrystal content. (Some figures may appear in colour only in the online journal)