In situ electron microscopy studies of electric field assisted sintering of oxide ceramics

A wide range of studies shows a dramatic effect of applied electric fields or currents on the sintering behavior of oxide ceramic powders. However, the mechanisms accounting for the so-called flash sintering remain elusive despite the wide application potential. Using in-situ scanning and transmission electron microscopy, we aim to gain insight into the atomic origins of sintering behavior, as well as of the high conductivity states that occur in conjunction with flash events during field-assisted sintering. We investigate the sintering dynamics of ZnO green bodies with a density between 50% and 70% and ZnO thin films with and without electric fields and under different oxidizing and reducing gas pressures. Specifically, we use a specially designed SEM heating stage to study the evolution of microstructure and morphology, including grain/void morphology, segregation, and precipitation, both with and without applied fields and with and without gas pressures up to 2 mbar. The in-situ TEM sintering studies, also under controlled electric field and gas pressure, allow us to detect chemical segregation and valence changes (using EDX and EELS) near the sintering boundaries. By gaining access to structural and chemical information down to the atomic scale, we hope to determine how the electric field causes flash sintering. Please click Additional Files below to see the full abstract

The effect of high current densities on iron-carbon alloy thin films

The recently discovered flash sintering method for preparing high quality oxide materials can be applied to the preparation of high performance nanocrystalline metals as well. Just as for the oxide materials, it is possible to use electric fields and currents to enhance densification of metal powders while limiting grain growth, however, the exact mechanism is still under discussion. The goal of our study is to understand how electric currents effect impurity redistribution and grain growth in fine grained metals. Please click Additional Files below to see the full abstract

Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn2_2O4_4

Manganese dissolution during the oxygen evolution reaction (OER) has been a persistent challenge that impedes the practical implementation of Mn-based electrocatalysts including the LiMn$_x$O$_4$ system in aqueous alkaline electrolyte. The investigated LiMn$_2$O$_4$ particles exhibit two distinct Mn dissolution processes; one independent of OER and the other associated to OER. Combining the bulk sensitive X-ray absorption spectroscopy, surface sensitive X-ray photoelectron spectroscopy and electrochemical detection of Mn dissolution using rotating ring-disk electrode, we explore the less understood Mn dissolution mechanism during OER. We correlate near-surface oxidation with the charge attributed to dissolved Mn, which demonstrates increasing Mn dissolution with the formation of surface Mn4+ species under anodic potential. The observed stronger dissolution during the OER is attributed to the formation of additional Mn$^{4+}$ from Mn$^{3+}$ during OER. We show that control over the amount of Mn4+ in Li$_x$Mn2O$_4$ before the onset of the OER can partially mitigate the OER-triggered dissolution. Overall, our atomistic insights into the Mn dissolution processes are crucial for knowledge-guided mitigation of electrocatalyst degradation, which can be broadly extended to manganese-based oxide systems

In-Situ TEM study of the effect of hydrogen on crack propagation in steel

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\u3cem\u3eIn Situ\u3c/em\u3e Nanomechanical Testing in Focused Ion Beam and Scanning Electron Microscopes

The recent interest in size-dependent deformation of micro- and nanoscale materials has paralleled both technological miniaturization and advancements in imaging and small-scale mechanical testing methods. Here we describe a quantitative in situ nanomechanical testing approach adapted to a dualbeam focused ion beam and scanning electron microscope. A transducer based on a three-plate capacitor system is used for high-fidelity force and displacement measurements. Specimen manipulation, transfer, and alignment are performed using a manipulator, independently controlled positioners, and the focused ion beam. Gripping of specimens is achieved using electron-beam assisted Pt-organic deposition. Local strain measurements are obtained using digital image correlation of electron images taken during testing. Examples showing results for tensile testing of single-crystalline metallic nanowires and compression of nanoporous Au pillars will be presented in the context of size effects on mechanical behavior and highlight some of the challenges of conducting nanomechanical testing in vacuum environments

Polaronic Contributions to Friction in a Manganite Thin Film

Despite the huge importance of friction in regulating movement in all natural and technological processes, the mechanisms underlying dissipation at a sliding contact are still a matter of debate. Attempts to explain the dependence of measured frictional losses at nanoscale contacts on the electronic degrees of freedom of the surrounding materials have so far been controversial. Here, it is proposed that friction can be explained by considering damping of stick-slip pulses in a sliding contact. Based on friction force microscopy studies of La$_{(1-x)}$Sr$_x$MnO$_3$ films at the ferromagnetic-metallic to paramagnetic-polaronic conductor phase transition, it is confirmed that the sliding contact generates thermally-activated slip pulses in the nanoscale contact, and argued that these are damped by direct coupling into phonon bath. Electron-phonon coupling leads to the formation of Jahn-Teller polarons and a clear increase in friction in the high temperature phase. There is no evidence for direct electronic drag on the atomic force microscope tip nor any indication of contributions from electrostatic forces. This intuitive scenario, that friction is governed by the damping of surface vibrational excitations, provides a basis for reconciling controversies in literature studies as well as suggesting possible tactics for controlling friction