Scott D. Walck

Pyrite FeS2–C composite as a high capacity cathode material of rechargeable lithium batteries

Pyrite FeS2 is a promising cathode material for rechargeable lithium batteries because of its high theoretical capacity (894 mA h g−1), low cost and near-infinite earth abundance. However, the progress in developing viable Li/FeS2 batteries has been hampered by the poor cyclability of the FeS2 cathode. Aiming to improve the cyclability of the FeS2 cathode, we here report a facile method for the synthesis of FeS2–C composites by a one-pot hydrothermal reaction of FeSO4 and Na2S2 in the presence of carbon black, and examine the effect of composition on the structure of FeS2–C composites and the cycling performance of Li/FeS2 cells. It is shown that the added carbon not only surrounds the FeS2 surface but also penetrates into the entire FeS2 particle, forming continuously conductive networks throughout the FeS2 particle. However, introduction of carbon meanwhile increases the particle size of the FeS2 active material. These two factors lead to an improvement in the rate capability of Li/FeS2 cells while having little effect on the specific capacity and capacity retention of the FeS2 cathode. On the other hand, we show that the electrolyte plays an important role in affecting the cyclability of Li/FeS2 cells, and that the ether- and carbonate-based electrolytes affect the cycling performance of Li/FeS2 cells in their unique manners.

Structural and electrical analysis of epitaxial 2D/3D vertical heterojunctions of monolayer MoS2 on GaN

Integration of two-dimensional (2D) and conventional (3D) semiconductors can lead to the formation of vertical heterojunctions with valuable electronic and optoelectronic properties. Regardless of the growth stacking mechanism implemented so far, the quality of the formed heterojunctions is susceptible to defects and contaminations mainly due to the complication involved in the transfer process. We utilize an approach that aims to eliminate the transfer process and achieve epitaxial vertical heterojunctions with low defect interfaces necessary for efficient vertical transport. Monolayers of MoS2 of approximately 2 μm domains are grown epitaxially by powder vaporization on GaN substrates forming a vertical 2D/3D heterojunction. Cross-sectional transmission electron microscopy (XTEM) is employed to analyze the in-plane lattice constants and van der Waals (vdW) gap between the 2D and 3D semiconductor crystals. The extracted in-plane lattice mismatch between monolayer MoS2 and GaN is only 1.2% which correspon...

A multi-step transmission electron microscopy sample preparation technique for cracked, heavily damaged, brittle materials.

A new technique for the preparation of heavily cracked, heavily damaged, brittle materials for examination in a transmission electron microscope (TEM) is described in detail. In this study, cross-sectional TEM samples were prepared from indented silicon carbide (SiC) bulk ceramics, although this technique could also be applied to other brittle and/or multiphase materials. During TEM sample preparation, milling-induced damage must be minimized, since in studying deformation mechanisms, it would be difficult to distinguish deformation-induced cracking from cracking occurring due to the sample preparation. The samples were prepared using a site-specific, two-step ion milling sequence accompanied by epoxy vacuum infiltration into the cracks. This technique allows the heavily cracked, brittle ceramic material to stay intact during sample preparation and also helps preserve the true microstructure of the cracked area underneath the indent. Some preliminary TEM results are given and discussed in regards to deformation studies in ceramic materials. This sample preparation technique could be applied to other cracked and/or heavily damaged materials, including geological materials, archaeological materials, fatigued materials, and corrosion samples.

TEM Characterization of the Deformed Region Beneath Knoop Indents in Boron Carbide

Understanding the deformation mechanisms in ceramic materials is crucial for developing and optimizing next-generation ceramic materials in body and vehicle armor systems. In an effort to better understand the mechanistic response of polycrystalline boron carbide to large contact stresses, transmission electron microscopy (TEM) methods were used to examine thin cross-sections of the inelastically deformed regions beneath Knoop indents of various loads and load-dwell times. Indentation allows the study of deformation of microstructural features as a function of distance and depth from the loading and allows for comparison to mechanistic modeling. TEM cross-sections were prepared from 0.3, 1, and 2 kgf Knoop indents in a hot-pressed polycrystalline boron carbide. Due to excessive spallation of material surrounding 2 kgf indents, load-dwell times of 15 and 45 s were only used for the 0.3 and 1 kgf indent loads. Although FIB preparation of nano-indentation methods have been used to examine the areas under indents with great success [1], the higher loads applied in this study allows a more comprehensive examination of the nature of the sub-surface inelastic region and the extended cracking. However, the extensive cracking presents problems for sample preparations that were solved by Brennan et al. using a multi-step sample preparation approach [2]. For the work in this study, an improved technique that was less labor intensive was used [3]. The process involves first preparing the indents in an appropriately spaced array across the surface near the edge, sufficiently close to use a masked-ion milling system (MIMS), but not close enough that the fracture mechanics were compromised. The cross sections of the indents were examined and indexed in an SEM, as shown in Fig 1, and then vacuum infiltrated with a low viscosity epoxy in order to fill the open cracks. The TEM samples were then prepared by in-situ lift-out technique after specific indents were located in the FIB by using the index distances found previously.

A Multi-Step Transmission Electron Microscopy Sample Preparation Technique for Indented Ceramics having Extensive Sub-Surface Cracking

Understanding the deformation mechanisms in ceramic materials is crucial for optimizing and implementing next-generation ceramic materials in body and vehicle armor systems. One particularly powerful tool for studying deformation mechanisms is transmission electron microscopy (TEM) of impacted materials. Indentation is useful for studying deformation mechanisms, since this method offers the ability to track microstructural features as a function of depth from the indent. Preparing an indented sample for examination in TEM is extremely challenging since the sample preparation process may introduce additional cracks and artifacts. Ion beam milling processes can also induce surface damage and artifacts, such as amorphization and re-deposition of amorphous material in and on indented ceramics [1]. Amorphization should particularly be minimized, since amorphization is an inelastic deformation mechanism that has been observed in some ceramics [2]. All of these effects will change the appearance of the indentation-induced cracking and other microstructural features, therefore obscuring microstructure of the mechanically damaged area. The goal of this work was to develop a sample preparation technique for heavily cracked ceramics that preserves the true microstructure for TEM imaging and analysis.

HAADF STEM of Phase Separated Anion Exchange Membranes Prepared by Ultracryomicrotomy

The study of the morphology of anion exchange membranes (AEMs) is important for developing more efficient alkaline fuel cells, sensors and purification systems. Both alkaline fuel cells and proton exchange membrane fuel cells (PEMFC) have high energy and power densities that make them great alternatives to the combustion engine in transportation applications. Alkaline fuel cells are cheaper than PEMFCs, but the mechanical and charge transport properties of AEMs are insufficient for widespread commercialization. To improve the properties of AEMs, new research is focused on optimizing the morphologies of the AEM.[1-3]