This work investigates the effects of doping on both the thermodynamics and kinetics of sintering in aluminum-doped yttrium oxide nanoparticles (Al-doped Y2O3), with the objective of delineating their interdependent effects at different stages of the process. Direct measurements of surface and grain boundary energies using differential scanning calorimetry showed that Al-doping decreases both interfacial energies, leading to an increase in dihedral angle (from 152.7 +/- 5.6 degrees to 165.8 +/- 5.5 degrees) and, therefore, sintering stress. Densification and grain growth analyses showed that despite this increase in sintering stress, the onset of sintering is delayed for the Al-doped samples, demonstrating that a large dihedral angle is a necessary but not sufficient condition for densification. The measurements of activation energies for densification and grain growth point out that Al suppresses grain boundary mobility by increasing the activation energy from 400 to 448 kJ/mol, hindering densification at the intermediate stages of sintering. At temperatures above 1150celcius, grain growth is activated in the Al-doped samples, which rapidly releases the accumulated sintering stress and exhibits a higher densification rate than in undoped Y2O3. This study demonstrates a complex interconnectivity between the thermodynamics and kinetics at different temperature ranges of sintering and reinforces the need for a comprehensive description for proper design of sintering aids.

Nanomaterials can exhibit improved electrochemical performance in cathode applications, but their inherently high surface areas cause unconventional instability, leading to capacity fading after a limited number of battery cycles. This is because of their high surface reactivity, which makes them more susceptible to phenomena such as grain growth, sintering, solubilization, and phase transformations. Thermodynamically, these can be attributed to an increased contribution of interfacial enthalpies to the total free energy of the system. The lack of experimental data on the interfacial thermodynamics of lithium-based materials has hindered strategies to mitigate such degradation mechanisms. In this study, interfacial energies of LiMn2O4 nanoparticles were directly measured for the first time using calorimetry, and the possibility of thermodynamically manipulating both surface and grain boundary energies using a dopant (scandium) was explored. We show that undoped LiMn2O4 nanoparticles have a surface energy of 0.85 J/m2, which is significantly lower than that of LiCoO2. Moreover, introducing scandium further lowered the LiMn2O4 surface energy, leading to a demonstrated improved stability against coarsening and reactivity to water, which can potentially result in more stable cathode materials for battery applications.

Leakage currents in InAlN/GaN‐based high‐electron‐mobility transistors considered for normally‐off devices critically depend on the oxidation behavior of InAlN thin films. Herein, lattice‐matched InAlN thin films deposited on GaN (0001) are rapid thermally annealed at 800 °C in an oxygen‐rich environment. Aberration‐corrected scanning transmission electron microscopy combined with electron energy‐loss spectroscopy is used to systematically characterize the oxidation behavior of InAlN films as a function of annealing time. Initial growth of oxide layers is found to be reaction limited, which is replaced by a diffusion limited growth process once a critical thickness of the oxide layer is obtained. Growing oxide layers are amorphous and become porous with increasing annealing time.

Fully dense transparent zinc aluminate ceramics with nanoscaled grain sizes were fabricated by Deformable Punch Spark Plasma Sintering (DP-SPS). Optical transmission spectra showed high transparency, with up to 70% transmitted light in the visible spectrum. Vickers hardness was measured and grain boundary strengthening observed, showing hardness increase from 18.2 GPa up to 22.5 GPa as the grain sizes decreased from 60.3 to 10.1 nm. The trend followed the Hall-Petch relationship, with hardness linearly proportional to the inverse of square root of grain size. A low grain size limit reported in previous literature below which hardness decreases, known as inverse Hall-Petch relationship, was not observed within the studied grain size range. Cross-sections of the hardness tests' indentations were prepared by focused ion beam and observed by electron microscopy and showed radically different crack patterns underneath the indentation imprint when contrasting samples with dissimilar grain sizes, shedding light on the mechanisms behind the observed grain boundary hardening mechanisms.

Journal Article Phase Stability of Iron Oxide Evaluated Through Selected Area Electron Diffraction During In-Situ Heating Experiments Get access Boyi Qu, Boyi Qu University of California Davis, Department of Materials Science and Engineering, Davis, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Andrew M Thron, Andrew M Thron University of California Davis, Department of Materials Science and Engineering, Davis, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Klaus van Benthem Klaus van Benthem University of California Davis, Department of Materials Science and Engineering, Davis, CA, USA Corresponding author: benthem@ucdavis.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 1914–1915, https://doi.org/10.1017/S1431927619010304 Published: 01 August 2019

GaN based high mobility transistors (HEMT) are a promising alternative to Si-based power transistors due to higher operating voltages, higher operating temperatures, and a significant reduction in Ohmic loss at high frequencies. AlGaN-GaN HEMTS have gained attention due to the formation a 2-dimensional electron gas with a high charge carrier mobility at the AlGaN/GaN interface. Power efficiency, especially at high operating frequencies, can further be improved by replacing the AlGaN layer with lattice matched InAlN [1]. In order achieve a high switching frequency for GHz applications a narrow gate and a thin barrier height must be achieved. Recent studies have shown that the charge carrier density in the 2DEG for InAlN/GaN based HEMTs can be maintain for barrier thicknesses as low as 9nm, and optimal performance can be maintained for barrier thicknesses as low as 3nm [2]. This is an improvement upon AlGaN based devices who’s performance degrades at such thin barrier thicknesses.

Nanoporous hydroxyapatite (HA) ceramics were consolidated using conventional or free sintering, spark plasma sintering (SPS) and flash sintering (FS). Microstructures formed during electric field assisted sintering, which includes SPS and FS revealed that nanovoids were retained within HA grains during processing. After free sintering, however, no nanovoids were detected by transmission electron microscopy (TEM). The observed nanovoids were confirmed not induced by electron beam damaging, and showed either a round or faceted shape with diameters ranging between 5 and 10 nm. In-situ TEM heating experiments demonstrated that nanovoids remained stable up to 900 degrees C but disappeared at a temperature as high as 1100 degrees C.

This work presents a thorough analysis of the grain growth behavior of gadolinium doped yttria stabilized zirconia (YSZ). The effect of the dopant on the thermodynamics as well as on the kinetics of the process is reported by providing extensive experimental data. While normal grain growth following a parabolic growth relation was observed, gadolinium inhibited the process proportionally to its concentration. By using microcalorimetry, we showed that the dopant decreases the grain boundary energy of YSZ, and hence reduces the driving force for growth. Analysis of the growth profile at different temperatures and times indicate that gadolinium does not significantly affect the grain boundary mobility or the activation energy for grain growth. The results rationalize that the dopant is acting on a mostly thermodynamic basis and opens good perspective for design of coarsening control focused on the system energetics.

Solid state dewetting of Au/Ni bilayer films was investigated by in-situ cross-sectional TEM heating with energy-dispersive X-ray spectroscopy. The early stage of dewetting process revealed both grain boundary grooving at the surface of the Ni layer and void nucleation along the Au/Ni interface between Au grains. During annealing, inter-diffusion occurred and enhanced solubility was found between Au and Ni. Au diffused predominantly through Ni grain boundaries and decorated the Ni/substrate interface. The energetically preferred adsorption of Au at the Ni/SiO2 interface and at Ni grain boundaries, coupled with some alloying in the volume of the metal films delays the break-up of the bilayer film compared to pure Au or Ni films. The experimental observations confirm that dewetting kinetics of metal bilayer films are strongly affected by both partial alloying of the metals as well as the metal/metal interface.

Helium gas accumulation from alpha decay during extended storage of spent fuel has potential to compromise the structural integrity the fuel. Here we report results obtained with surrogate nickel particles which suggest that alumina formed by atomic layer deposition can serve as a low volumefraction, uniformly-distributed phase for retention of helium generated in fuel particles such as uranium oxide. Thin alumina layers may also form transport paths for helium in the fuel rod, which would otherwise be impermeable. Micron-scale nickel particles, representative of uranium oxide particles in their low helium solubility and compatibility with the alumina synthesis process, were homogeneously coated with alumina approximately 3-20 nm by particle atomic layer deposition (ALD) using a fluidized bed reactor. Particles were then loaded with helium at 800 degrees C in a tube furnace. Subsequent helium spectroscopy measurements showed that the alumina phase, or more likely a related nickel/alumina interface structure, retains helium at a density of at least 10(17) atoms/cm(3). High resolution transmission electron microscopy revealed that the thermal treatment increased the alumina thickness and generated additional porosity. Results from Monte Carlo simulations on amorphous alumina predict the helium retention concentration at room temperature could reach 10(21) atoms/cm(3) at 400 MPa, a pressure predicted by others to be developed in uranium oxide without an alumina secondary phase. This concentration is sufficient to eliminate bubble formation in the nuclear fuel for long-term storage scenarios, for example. Measurements by others of the diffusion coefficient in polycrystalline alumina indicate values several orders of magnitude higher than in uranium oxide, which then can also allow for helium transport out of the spent fuel. (C) 2017 Elsevier B.V. All rights reserved.

The solid state dewetting of Au/Ni bilayer films from SiO2/Si substrates exhibits both homogeneous and localized dewetting of Ni and long-edge retraction for Au under isothermal annealing condition. The top Au layer retracted up to 1 mm from the edge of the substrate wafer to reduce the energetically unfavored Au/Ni interface. In contrast, Ni dewetted and agglomerated locally due to its limited diffusivity compared to Au. Film morphology and local chemical composition varied significantly across hundreds of micrometers along the direction normal to the retracting edge. The accumulation of Au from tong-range edge retraction resulted into the separation of Au and Ni, which are affected by alloying between the two elements. The experimental observations suggest that alloying combined with unequal self-diffusion coefficient for the metal components control the bilayer dewetting process. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Thin films deposited at low temperatures are often kinetically constrained and will dewet the underlying substrate when heat-treated. Dewetting can be a serious concern in microelectronics reliability [1], while it can also be utilized for engineering of nanostructures with potentials in storage [2], catalysis [3], or optical/magnetic applications [4]. Mechanisms for dewetting of single layer films have been studied extensively. However little work has been reported on the cross-sectional characterization of dewetting processes for multilayer or alloyed thin films. In this study, we report cross-sectional characterization of Au/Ni bilayer dewetting at early stage. Changes in the morphology and chemistry were observed by cross-sectional transmission electron microscopy (TEM) techniques, including scanning transmission electron microscopy (STEM), Procession Electron Diffraction (PED), electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDXS).

Systematic statistical analysis of the microstructural changes in 3mol% yttria-stabilized zirconia was performed after flash sintering by alternating current (AC). The micrographs in the gauge section of the specimen were identical to those from DC flash sintered samples while no evident electrode effect was present for AC flash sintered samples. However, finite element modeling revealed a temperature gradient from the surface to the volume of the sintered body. Microstructure gradients, across the width of gauge section, were revealed for the AC flash sintered sample. Classical grain growth models due to Joule heating were insufficient in justifying the microstructural evolution under the simulated temperature distribution. Bimodal grain diameter distributions in flash sintered samples were observed. Therefore, it is proposed that faster grain growth mechanisms activated on a fraction of the grains by electric field/current occurred during flash sintering, and is responsible for the instantaneous grain growth.

The solid state dewetting of Au/Ni bilayer films was investigated by cross-sectional transmission electron microscopy techniques, including energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy and precession electron diffraction. After annealing under high vacuum conditions the early stage of film agglomeration revealed significant changes in film morphology and chemical distribution. Both Au and Ni showed texturing. Despite the initial deposition sequence of the as-deposited Au/Ni/SiO2/Si interface structure, the majority of the metal/SiO2 interface was Au/SiO2 after annealing at 675 °C for 1 h. Void nucleation was predominantly observed at Au/Ni/SiO2 triple junctions, rather than grain boundary grooving at free surface of the metal film. Detailed cross-sectional characterization reveals that the Au/Ni interface in addition to small amounts of metal alloying strongly affects film break-up and agglomeration kinetics. The formation of Au/SiO2 interface sections is found to be energetically preferred over Ni/SiO2 due to compressive stress in the as-deposited Ni layer. Void nucleation is observed at the film/substrate interface, while the formation of voids at Ni/Au phase boundaries inside the metal film is caused by the Kirkendall effect.

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Dewetting of ultra-thin Ni films deposited on SiO2 layers was observed, in cross-section, by in situ scanning transmission electron microscopy. Holes were observed to nucleate by voids which formed at the Ni/SiO2 interface rather than at triple junctions at the free surface of the Ni film. Ni islands were observed to retract, in attempt to reach equilibrium on the SiO2 layer. SiO2 layers with 120 nm thickness were found to limit in situ heating experiments due to poor thermal conductivity of SiO2. The formation of graphite was observed during the agglomeration of ultra-thin Ni films. Graphite was observed to wet both the free surface and the Ni/SiO2 interface of the Ni islands. Cr forms surface oxide layers on the free surface of the SiO2 layer and the Ni islands. Cr does not prevent the dewetting of Ni, however it will likely alter the equilibrium shape of the Ni islands.

The consolidation of crystalline powders to obtain dense microstructures is typically achieved through a combination of volume and grain boundary diffusion. In situ transmission electron microscopy was utilized to study neck formation between adjacent nickel particles during the early stages of sintering. It was found that the presence of carbon during consolidation of Ni lowers the reduction temperature of nickel oxides on the particle surface and therefore has the potential to accelerate consolidation. In the absence of carbon, the surface oxides remain present during the early stage of sintering and neck formation between particles is limited by self-diffusion of nickel through the oxide layer. This study provides direct experimental evidence that corroborates related earlier hypotheses of self-cleaning on the surface of the nanoparticles that precedes neck formation and growth.

Using Field Assisted Sintering Technique/Spark Plasma Sintering the effect of heating rate on the sintering of zinc oxide at a temperature of 400°C has been investigated. For the highest heating rate of 100°C/min, relative density larger than 95% was achieved whereas at low heating rates only little shrinkage occurred. Hardness measurements, Transmission Electron Microscopy, and impedance spectroscopy revealed clear differences between heating rates. It was found that residual water is responsible for this behavior, enhancing particle rearrangement and diffusion kinetics.

Ultrathin films of nickel deposited onto (1 0 0) Si substrates were found to form kinetically constrained multilayered interface structures characterized by structural and compositional gradients. The presence of a native SiO2 on the substrate surface in tandem with thickness-dependent intrinsic stress of the metal film limits the solid-state reaction between Ni and Si. A roughly 6.5 nm thick Ni film on top of the native oxide was observed regardless of the initial nominal film thickness of either 5 or 15 nm. The thickness of the silicide layer that formed by Ni diffusion into the Si substrate, however, scales with the nominal film thickness. Cross-sectional in situ annealing experiments in the transmission electron microscope elucidate the kinetics of interface transformation towards thermodynamic equilibrium. Two competing mechanisms are active during thermal annealing: thermally activated diffusion of Ni through the native oxide layer and subsequent transformation of the observed compositional gradient into a thick reaction layer of NiSi2 with an epitaxial orientation relationship to the Si substrate; and, secondly, metal film dispersion and subsequent formation of faceted Ni islands on top of the native oxide layer.