Size Dependence of Nanoscale Wear of Silicon Carbide

Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius around 370 nm), the wear resistances of both sc-SiC and nc-SiC are higher than that of Si. This result is expected from the Archard's equation because SiC is harder than Si. However, with the smaller AFM tips (tip radius around 20 nm), the wear resistances of sc-SiC and nc-SiC are lower than that of Si, despite the fact that the contact pressures are comparable to those applied with the nanoindenter tips, and the plastic zones are well-developed in both sets of wear experiments. We attribute the decrease in the relative wear resistance of SiC compared to that of Si to a transition from a wear regime dominated by the materials' resistance to plastic deformation (i.e., hardness) to a regime dominated by the materials' resistance to interfacial shear. This conclusion is supported by our AFM studies of wearless friction, which reveal that the interfacial shear strength of SiC is higher than that of Si. The contributions of surface roughness and surface chemistry to differences in interfacial shear strength are also discussed

Directed plant cell-wall accumulation of iron: embedding co-catalyst for efficient biomass conversion

Plant lignocellulosic biomass is an abundant, renewable feedstock for the production of biobased fuels and chemicals. Previously, we showed that iron can act as a co-catalyst to improve the deconstruction of lignocellulosic biomass. However, directly adding iron catalysts into biomass prior to pretreatment is diffusion limited, and increases the cost of biorefinery operations. Recently, we developed a new strategy for expressing iron-storage protein ferritin intracellularly to accumulate iron as a catalyst for the downstream deconstruction of lignocellulosic biomass. In this study, we extend this approach by fusing the heterologous ferritin gene with a signal peptide for secretion into Arabidopsis cell walls (referred to here as FerEX). The transgenic Arabidopsis plants. FerEX. accumulated iron under both normal and iron-fertilized growth conditions; under the latter (iron-fertilized) condition, FerEX transgenic plants showed an increase in plant height and dry weight by 12 and 18 %, respectively, compared with the empty vector control plants. The SDS- and native-PAGE separation of cell-wall protein extracts followed by Western blot analyses confirmed the extracellular expression of ferritin in FerEX plants. Meanwhile, Perls' Prussian blue staining and X-ray fluorescence microscopy (XFM) maps revealed iron depositions in both the secondary and compound middle lamellae cell-wall layers, as well as in some of the corner compound middle lamella in FerEX. Remarkably, their harvested biomasses showed enhanced pretreatability and digestibility, releasing, respectively, 21 % more glucose and 34 % more xylose than the empty vector control plants. These values are significantly higher than those of our recently obtained ferritin intracellularly expressed plants. This study demonstrated that extracellular expression of ferritin in Arabidopsis can produce plants with increased growth and iron accumulation, and reduced thermal and enzymatic recalcitrance. The results are attributed to the intimate colocation of the iron co-catalyst and the cellulose and hemicellulose within the plant cell-wall region, supporting the genetic modification strategy for incorporating conversion catalysts into energy crops prior to harvesting or processing at the biorefinery.https://doi.org/10.1186/s13068-016-0639-

Best Practices for Quasistatic Berkovich Nanoindentation of Wood Cell Walls

For wood and forest products to reach their full potential as structural materials, experimental techniques are needed to measure mechanical properties across all length scales. Nanoindentation is uniquely suited to probe in situ mechanical properties of micrometer-scale features in forest products, such as individual wood cell wall layers and adhesive bondlines. However, wood science researchers most commonly employ traditional nanoindentation methods that were originally developed for testing hard, inorganic materials, such as metals and ceramics. These traditional methods assume that the tested specimen is rigidly supported, homogeneous, and semi-infinite. Large systematic errors may affect the results when these traditional methods are used to test complex polymeric materials, such as wood cell walls. Wood cell walls have a small, finite size, and nanoindentations can be affected by nearby edges. Wood cell walls are also not rigidly supported, and the cellular structure can flex under loading. Additionally, wood cell walls are softer and more prone to surface detection errors than harder inorganic materials. In this paper, nanoindentation methods for performing quasistatic Berkovich nanoindentations, the most commonly applied nanoindentation technique in forest products research, are presented specifically for making more accurate nanoindentation measurements in materials such as wood cell walls. The improved protocols employ multiload nanoindentations and an analysis algorithm to correct and detect errors associated with surface detection errors and structural compliances arising from edges and specimen-scale flexing. The algorithm also diagnoses other potential issues arising from dirty probes, nanoindenter performance or calibration issues, and displacement drift. The efficacy of the methods was demonstrated using nanoindentations in loblolly pine (Pinus taeda) S2 cell wall layers (S2) and compound corner middle lamellae (CCML). The nanoindentations spanned a large range of sizes. The results also provide new guidelines about the minimum size of nanoindentations needed to make reliable nanoindentation measurements in S2 and CCML

CELL WALL DOMAIN AND MOISTURE CONTENT INFLUENCE SOUTHERN PINE ELECTRICAL CONDUCTIVITY

Recent work has highlighted the importance of movement of chemicals and ions through the wood cell wall.  Movement depends strongly on moisture content and is necessary for structural damage mechanisms such as fastener corrosion and wood decay.  Here we present the first measurements of electrical resistance of southern pine at the subcellular level as a function of wood moisture content by using a 1 µm diameter probe.  Measurements were taken with the probe contacting the S2 layer of the cell wall and the cell corner compound middle lamella in the latewood and the cell corner compound middle lamella in the earlywood.  The resistance decreased with increasing relative humidity in all locations.  The resistance decreased more rapidly with relative humidity in the S2 layer than in any of the middle lamellae.  These results give insight into how some moisture-dependent wood properties affecting ion movement may be partitioned across cell wall layers

Spontaneous Phase Transformation and Exfoliation of Rectangular Single-Crystal Zinc Hydroxy Dodecylsulfate Nanomembranes

Free-standing two-dimensional (2D) nanostructures, exemplified by graphene and semiconductor nanomembranes, exhibit exotic electrical and mechanical properties and have great potential in electronic applications where devices need to be flexible or conformal to nonplanar surfaces. Based on our previous development of a substrate-free synthesis of large-area, free-standing zinc hydroxy dodecylsulfate (ZHDS) hexagonal nanomembranes, herein, we report a spontaneous phase transformation of ZHDS nanomembranes under extended reaction time. The hexagonal ZHDS sheets transformed into rectangular single crystal nanomembranes with sizes of hundreds of micrometers. They contain long-range-ordered zinc vacancies that can be fitted into an orthorhombic superlattice. A surplus of dodecylsulfate ions and a deficit of Zn²⁺ diffusion near the water surface are believed to be the factors that drive the phase transformation. The phase transformation starts with the formation of zinc vacancies at the topmost layer of the hexagonal hillock, and propagates along the spiral growth path of the initial hexagonal sheets, which bears a great resemblance to the classic “periodic slip process”. Mechanical property characterization of ZHDS nanomembranes by nanoindentation shows they behave much like structural polymers mechanically due to the incorporation of surfactant molecules. We also developed a one-step exfoliation and dehydration method that converts ZHDS nanomembranes to ZnO nanosheets using <i>n</i>-butylamine. This work provides a further understanding of the growth and stability of ZnO-based nanomembranes, as well as advisory insight for the further development on solution-based synthesis of free-standing, single-crystalline 2D nanostructures

Effect of Moisture Content on Ion Diffusion and Glass Transition Temperature in Wood Cell Walls

Understanding and controlling the diffusion of ions and chemicals within the secondary plant cell walls are pivotal in various applications of biomasses. Recent studies have shown that inorganic ion diffusion through secondary cell walls is controlled by a moisture-induced glass transition in amorphous polysaccharides, including amorphous cellulose and hemicelluloses. Understanding the diffusion of ions in these structures has been the subject of numerous recent experiments; however, a deep understanding of the underlying mechanisms of interactions between ion atoms and water/hemicellulose molecules is still lacking. This study uses molecular dynamics simulations to elucidate the diffusion mechanisms of potassium and chloride ions in the cell walls under varying moisture content. The results reveal that a higher moisture content leads to the formation of solvent layers around the ions and reduces the charge interaction between the functional groups of wood polymers and ions. Hence, a higher moisture content results in an improved diffusion rate of ions within the domain. The simulation results also show that higher moisture content lowers the glass transition temperature, promoting diffusion of ions in the system. In contrast, increases in the ion concentration increase the glass transition temperature of the system and degrade the diffusion of ions in the system

Analysis of Adhesive Penetration into Wood using Nano-X-ray Computed Tomography

This study focused on the penetration of adhesive into cell lumens and cell walls. Douglas-fir samples containing an iodinated phenol-formaldehyde adhesive were analyzed using nano-X-ray computed tomography (XCT), X-ray fluorescence microscopy, and energy-dispersive X-ray spectroscopy to observe adhesive penetration in cell walls. A gradient of penetration was observed within the cell wall structure. In addition, these nano-XCT datasets were indexed to previous micro-XCT datasets, which gave the ability to link the nanoscale cell wall penetration to microscale penetration in to the porous network of cell lumens.