Real-Time Monitoring of Aluminum Oxidation Through Wide Band Gap MgF2 Layers for Protection of Space Mirrors

Because of its extraordinary and broad reflectivity, aluminum is the only logical candidate for advanced space mirrors that operate deep into the UV. However, aluminum oxidizes rapidly in the air, and even a small amount of oxide (as little as a nanometer) can have a noticeable, detrimental impact on its reflectivity at short wavelengths. Thin films of wide band gap materials like MgF2 have previously been used to protect aluminum surfaces. Here we report the first real-time, spectroscopic ellipsometry (SE) study of aluminum oxidation as a function of MgF2 over layer thickness, which ranged from 0 – 6 nm. SE data analysis was performed vis-à-vis a multilayer optical model that included a thick silicon nitride layer. The optical constants for evaporated aluminum were initially determined using a multi-sample analysis (MSA) of SE data from MgF2 protected and bare Al surfaces. Two models were then considered for analyzing the real-time data obtained from Al/MgF2 stacks. The first used the optical constants of aluminum obtained in the MSA with two adjustable parameters: the thicknesses of the aluminum and aluminum oxide layers. The thicknesses obtained from this model showed the expected trends (increasing Al2O3 layer thickness and decreasing Al layer thickness with time), but some of the Al2O3 thicknesses were unphysical (negative). Because the optical constants of very thin metals films depend strongly on their structures and deposition conditions, a second, more advanced model was employed that fit the optical constants for Al, and also the Al and Al2O3 thicknesses, for each data set. In particular, the Al and Al2O3 thicknesses and optical constants of Al were determined in an MSA for each of 50 evenly spaced analyses in each four-hour dynamic run performed. The resulting optical constants for Al were then fixed for that sample and the thicknesses of the Al and Al2O3 layers were determined. While the first and second models yielded similar Al and Al2O3 thickness vs. time trends, the film thicknesses obtained in this manner were more physically reasonable. Thicker MgF2 layers slow the oxidation rate of aluminum. The results from this work should prove useful in protecting space mirrors prior to launch

Revealing Anisotropic Spinel Formation on Pristine Li- and Mn-Rich Layered Oxide Surface and Its Impact on Cathode Performance

Surface properties of cathode particles play important roles in the transport of ions and electrons and they may ultimately dominate cathode's performance and stability in lithium-ion batteries. Through the use of carefully prepared Li1.2Ni0.13Mn0.54Co0.13O2 crystal samples with six distinct morphologies, surface transition-metal redox activities and crystal structural transformation are investigated as a function of surface area and surface crystalline orientation. Complementary depth-profiled core-level spectroscopy, namely, X-ray absorption spectroscopy, electron energy loss spectroscopy, and atomic-resolution scanning transmission electron microscopy, are applied in the study, presenting a fine example of combining advanced diagnostic techniques with a well-defined model system of battery materials. The present study reports the following findings: (1) a thin layer of defective spinel with reduced transition metals, similar to what is reported on cycled conventional secondary particles in the literature, is found on pristine oxide surface even before cycling, and (2) surface crystal structure and chemical composition of both pristine and cycled particles are facet dependent. Oxide structural and cycling stabilities improve with maximum expression of surface facets stable against transition-metal reduction. The intricate relationships among morphology, surface reactivity and structural transformation, electrochemical performance, and stability of the cathode materials are revealed

Correction to: Revealing Anisotropic Spinel Formation on Pristine Li- and Mn-Rich Layered Oxide Surface and Its Impact on Cathode Performance (Advanced Energy Materials, (2017), 7, 11, (1602010), 10.1002/aenm.201602010)

Adv. Energy Mater. 2017, 7, 1602010 In the originally published article, the red circle in Figure b showed the wrong spot. The corrected Figure b along with the corrected figure caption is shown below. The authors apologize for any inconvenience this may have caused. (Figure presented.) a) HAADF STEM image collected on the S-Poly crystal sample, b) SAED pattern collected from the particle shown in (a), c) simulated SAED pattern corresponding to the combination of C2/m and a spinel structure, and d) dark field image taken with the reflection shown in red in (b)

Revealing Anisotropic Spinel Formation on Pristine Li- and Mn-Rich Layered Oxide Surface and Its Impact on Cathode Performance

Surface properties of cathode particles play important roles in the transport of ions and electrons and they may ultimately dominate cathode's performance and stability in lithium-ion batteries. Through the use of carefully prepared Li1.2Ni0.13Mn0.54Co0.13O2 crystal samples with six distinct morphologies, surface transition-metal redox activities and crystal structural transformation are investigated as a function of surface area and surface crystalline orientation. Complementary depth-profiled core-level spectroscopy, namely, X-ray absorption spectroscopy, electron energy loss spectroscopy, and atomic-resolution scanning transmission electron microscopy, are applied in the study, presenting a fine example of combining advanced diagnostic techniques with a well-defined model system of battery materials. The present study reports the following findings: (1) a thin layer of defective spinel with reduced transition metals, similar to what is reported on cycled conventional secondary particles in the literature, is found on pristine oxide surface even before cycling, and (2) surface crystal structure and chemical composition of both pristine and cycled particles are facet dependent. Oxide structural and cycling stabilities improve with maximum expression of surface facets stable against transition-metal reduction. The intricate relationships among morphology, surface reactivity and structural transformation, electrochemical performance, and stability of the cathode materials are revealed

A physical large-signal model for GaN HEMTs including self-heating and trap-related dispersion

We show results of a self-consistent large-signal electro-thermal GaN HEMT model that includes trap related and self-heating dispersion effects. Both self-heating and trap dynamics are treated with a strictly physical approach that makes it easier to link the model parameter with the physical HEMT structure and material characteristics. The model, implemented in ADS, is applied to measured DC data taken at ambient temperatures between 200 K and 400 K, with excellent results. Several examples are given of dynamic HEMT simulation, showing the co-existence and the interaction of temperature- and trap related dispersive effects