Creating Continuously Graded Microstructures with Electric Fields via Locally Altering Grain Boundary Complexions

Tailoring microstructures represents a daunting goal in materials science. Here, an innovative proposition is to utilize grain boundary (GB) complexions (a.k.a. interfacial phases) to manipulate microstructural evolution, which is challenging to control via only temperature and doping. Herein, we use ZnO as a model system to tailor microstructures using applied electric fields as a new knob to control GB structures locally via field-driven stoichiometry (defects) polarization. Specifically, continuously graded microstructures are created under applied electric fields. By employing aberration-corrected scanning transmission electron microscopy (AC STEM) in conjunction with density functional theory (DFT) and ab initio molecular dynamics (AIMD), we discover cation-deficient, oxygen-rich GBs near the anode with enhanced GB diffusivities. In addition, the field-driven redistribution of cation vacancies is deduced from a defect chemistry model, and subsequently verified by spatially resolved photoluminescence spectroscopy. This bulk stoichiometry polarization leads to preferential formation of cation-deficient (oxidized) GBs near the anode to gradually promote grain growth towards the anode. This mechanism can be utilized to create continuously graded microstructures without abnormal grain growth typically observed in prior studies. This study exemplifies a case of tailoring microstructural evolution via altering GB complexions locally with applied electric fields, and it enriches fundamental GB science

Comparative observations on the squamous-columnar junction of Von Ebner’s glandular duct at the bottom of vallate papillae in dogs, rats, mice and human

Background: This paper aims to comparatively observe similarities of squamous-columnar junction (SCJ) at the opening of Von Ebner's glandular ducts at the vallate papillae in dogs, mice, rats and humans, lay a foundation for the selection of the model in future study of the carcinogenesis in SCJ at vallate papillae. Materials and methods: The localization of the vallate papillae in three laboratory animals and humans was comparatively observed. The differences of SCJ at vallate papillae were comparatively observed by Alcian blue, immunohistochemistry and HE staining. Results: Anatomically, the canine vallate papillae were most similar to those of humans in location, whereas mice and rats only had a single, Ω-shaped, vallate papilla lying directly anterior to the posterior border of the intermolar eminence. In histology, the SCJ of dogs lacked a transition zone similar to that of the human SCJ, and there was glandular epithelium secreting acidic mucus at the opening of the rats’ Von Ebner's glandular ducts. All of this suggested that the histological structure of SCJ in rats and dogs is more distinct from that of humans, whereas the histological structure of SCJ at vallate papilla in mice was more similar. Conclusions: The structure of SCJ at vallate papilla in mice is most similar to that of humans, so we conclude that mouse is the most suitable model for studying tumorigenesis in SCJ at vallate papillae in these three common laboratory animals

Grain Boundary Engineering in Li-ion Batteries and Ceramic Materials

Alloy-type anodes are promising next-generation electrodes for Li-ion batteries because of their high specific capacity, but the severe volume expansion causes fast capacity decay. Here, two methods were explored to improve alloy-type anodes. First, a thermodynamically driven grain boundary engineering was proposed as a potential strategy. SnSb with Bi addition fabricated through ball milling and annealing was selected as a model system. The Bi-doped SnSb demonstrated improved cycling performance with < 1% porosity. Transmission electron microscopy shows grain boundaries Bi segregation, and thermodynamic modeling further indicates the stabilization of a nanoscale liquid-like interfacial phase. In situ X-ray microscopy shows the crack suppression effect for the Bi-doped sample, suggesting a potential grain boundary sliding as a stress relief mechanism. Second, cryogenic milling was demonstrated as a novel method to engineer alloy-type anodes. This process can suppress cold welding, exfoliate bulk graphite into multilayer graphene, and evenly disperse them between the grains to form nanostructured electrodes. Based on the cross-section electron microscopy and X-ray microscopy, the dispersed graphene between the nanosized grains can effectively alleviate the volume expansion upon lithiation. Compared to the traditional ball milling methods under room temperature, the cryomilled SnSb-C composite anode showed improved cycling stability and rate capability. For ceramic processing methods, electric field assisted sintering process can have a lower sintering temperature and faster sintering time compared to conventional sintering. For the second part of the thesis, nonthermal electric field effects on ZnO polycrystalline specimen was investigated. In undoped ZnO, electric field can induce defect polarization and subsequently alter the grain boundary structure. We also demonstrated the possibility of creating and controlling graded microstructure via electric fields. In Bi2O3-doped ZnO, electric field can drive the migration of Bi-rich secondary liquid phase. In summary, the first part of the dissertation introduces grain boundary engineering and cryogenic milling as two novel approaches to improve alloy-type battery electrodes. The second part of the dissertation investigated electric field effects on ceramic microstructure evolution. The feasibility of tailoring ceramic microstructure with electric field was demonstrated

Discovery of electrochemically induced grain boundary transitions

Electric fields and currents, which are used in innovative materials processing and electrochemical energy conversion, can often alter microstructures in unexpected ways. However, little is known about the underlying mechanisms. Using ZnO-Bi2O3 as a model system, this study uncovers how an applied electric current can change the microstructural evolution through an electrochemically induced grain boundary transition. By combining aberration-corrected electron microscopy, photoluminescence spectroscopy, first-principles calculations, a generalizable thermodynamic model, and ab initio molecular dynamics, this study reveals that electrochemical reduction can cause a grain boundary disorder-to-order transition to markedly increase grain boundary diffusivities and mobilities. Consequently, abruptly enhanced or abnormal grain growth takes place. These findings advance our fundamental knowledge of grain boundary complexion (phase-like) transitions and electric field effects on microstructural stability and evolution, with broad scientific and technological impacts. A new method to tailor the grain boundary structures and properties, as well as the microstructures, electrochemically can also be envisioned

Analysis of Current and Qualified Major Grain Producing Areas in China in the Last 30 Years

China’s grain production has been on a pathway parallel to urbanization in the last 30 years. When the balance between the grain supply and demand is considered, contradiction between farming suitability and actual deviation provides warning of a crisis regarding China’s food security. In this study, we constructed a set of topologic maps to summarize the basic distribution of the farming conditions in China, and Kernel density and Local Moran’s I analyses were con-ducted to investigate the spatial-temporal pattern of China’s regional grain production based on the grain output at the county level from 1985 to 2019. The results show that the high-output zones were concentrated in the regions with superior physical conditions in 1985, and by 2019, the high-output zones had increased in the northern regions (i.e., Northeast China Plain) and decreased in the southern regions (i.e., Southern China). The surplus zones of per capita grain output were concentrated in the regions with high total outputs during 1985–2019. The shortage zones were distributed in the regions with low total outputs and low total outputs or large populations. Based on the above three results, several typical commodity grain bases have lost their dominant role (i.e., the Pearl River Delta); furthermore, the qualified commodity grain bases were compiled at both the national and regional level based on overlay analysis (i.e., the Tarbagatay Prefecture as well as eastern and central Inner Mongolia)

Analysis of Current and Qualified Major Grain Producing Areas in China in the Last 30 Years

China’s grain production has been on a pathway parallel to urbanization in the last 30 years. When the balance between the grain supply and demand is considered, contradiction between farming suitability and actual deviation provides warning of a crisis regarding China’s food security. In this study, we constructed a set of topologic maps to summarize the basic distribution of the farming conditions in China, and Kernel density and Local Moran’s I analyses were con-ducted to investigate the spatial-temporal pattern of China’s regional grain production based on the grain output at the county level from 1985 to 2019. The results show that the high-output zones were concentrated in the regions with superior physical conditions in 1985, and by 2019, the high-output zones had increased in the northern regions (i.e., Northeast China Plain) and decreased in the southern regions (i.e., Southern China). The surplus zones of per capita grain output were concentrated in the regions with high total outputs during 1985–2019. The shortage zones were distributed in the regions with low total outputs and low total outputs or large populations. Based on the above three results, several typical commodity grain bases have lost their dominant role (i.e., the Pearl River Delta); furthermore, the qualified commodity grain bases were compiled at both the national and regional level based on overlay analysis (i.e., the Tarbagatay Prefecture as well as eastern and central Inner Mongolia)

Avoiding dendrite formation by confining lithium deposition underneath Li-Sn coatings

The use of interfacial layers to stabilize the lithium surface is a popular research direction for improving the morphology of deposited lithium and suppressing lithium dendrite formation. This work considers a different approach to controlling dendrite formation where lithium is plated underneath an interfacial coating. In the present research, a Li-Sn intermetallic was chosen as a model system due to its lithium-rich intermetallic phases and high Li diffusivity. These coatings also exhibit a significantly higher Li exchange current than bare Li thus leading to better charge transfer kinetics. The exchange current is instrumental in determining whether lithium deposition occurs above or below the Li-Sn coating. High-resolution transmission electron microscopy and cryogenic focused ion beam scanning electron microscopy were used to identify the features associated with Li deposition. Atomic scale simulations provide insight as to the adsorption energies determining the deposition of lithium below the Li-Sn coating.</p

Avoiding Dendrite Formation by Confining Lithium Deposition Underneath Li-Sn Coatings

The use of interfacial layers to stabilize the lithium surface is a popular research direction for improving the morphology of deposited lithium and suppressing lithium dendrite formation. This work considers a different approach to controlling dendrite formation where lithium is plated underneath an interfacial coating. In the present research, a Li-Sn intermetallic was chosen as a model system due to its lithium-rich intermetallic phases and high Li diffusivity. These coatings also exhibit a significantly higher Li exchange current than bare Li thus leading to better charge transfer kinetics. The exchange current is instrumental in determining whether lithium deposition occurs above or below the Li-Sn coating. High-resolution transmission electron microscopy and cryogenic focused ion beam scanning electron microscopy were used to identify the features associated with Li deposition. Atomic scale simulations provide insight as to the adsorption energies determining the deposition of lithium below the Li-Sn coating. </p