Abstract
Active learning—the field of machine learning (ML) dedicated to optimal experiment design—has played a part in science as far back as the 18th century when Laplace used it to guide his ...discovery of celestial mechanics. In this work, we focus a closed-loop, active learning-driven autonomous system on another major challenge, the discovery of advanced materials against the exceedingly complex synthesis-processes-structure-property landscape. We demonstrate an autonomous materials discovery methodology for functional inorganic compounds which allow scientists to fail smarter, learn faster, and spend less resources in their studies, while simultaneously improving trust in scientific results and machine learning tools. This robot science enables science-over-the-network, reducing the economic impact of scientists being physically separated from their labs. The real-time closed-loop, autonomous system for materials exploration and optimization (CAMEO) is implemented at the synchrotron beamline to accelerate the interconnected tasks of phase mapping and property optimization, with each cycle taking seconds to minutes. We also demonstrate an embodiment of human-machine interaction, where human-in-the-loop is called to play a contributing role within each cycle. This work has resulted in the discovery of a novel epitaxial nanocomposite phase-change memory material.
Chemical and structural heterogeneity and the resulting interaction of coexisting phases can lead to extraordinary behaviours in oxides, as observed in piezoelectric materials at morphotropic phase ...boundaries and relaxor ferroelectrics. However, such phenomena are rare in metallic alloys. Here we show that, by tuning the presence of structural heterogeneity in textured Co1-x Fex thin films, effective magnetostriction λeff as large as 260 p.p.m. can be achieved at low-saturation field of ~10 mT. Assuming λ100 is the dominant component, this number translates to an upper limit of magnetostriction of λ100 approximate5λeff >1,000 p.p.m. Microstructural analyses of Co1-x Fex films indicate that maximal magnetostriction occurs at compositions near the (fcc+bcc)/bcc phase boundary and originates from precipitation of an equilibrium Co-rich fcc phase embedded in a Fe-rich bcc matrix. The results indicate that the recently proposed heterogeneous magnetostriction mechanism can be used to guide exploration of compounds with unusual magnetoelastic properties.
New results are reported suggesting that with appropriate levels of Fe doping Mg can rapidly and reversibly absorb up to 7 mass fraction (%) hydrogen at moderate temperatures and pressures useful for ...hydrogen storage applications. Hydrogenation kinetics and thermodynamics of Mg–4Fe at.% (+/− 1 at.%) thin films capped with Pd at temperatures ranging from 363 K to 423 K were studied by a number of different methods:
in situ infrared imaging, volumetric pressure-composition isotherm (PCI) measurements, and
ex situ X-ray diffraction and transmission electron microscopy. The hydride growth rate was determined by utilizing wedge-shaped films and infrared imaging; assuming formation of a continuous hydride layer, the growth rate was found to range from ≈3.8 nm/s at lower temperature to ≈36.7 nm/s at higher temperature. The apparent activation energy of the thermally activated hydrogenation kinetics was measured to be 56 kJ/mol; this value suggests that at low temperatures hydrogen diffusion along grain boundaries of MgH
2 is the mechanism controlling the hydride layer growth.
Reproducible PCI measurements of 600 nm-thick uniform films showed a pressure plateau and large hysteresis; from these measurements enthalpy and entropy were estimated as 66.9 kJ/mol and 0.102 kJ/(mol∗K), respectively, which are both slightly less than values for pure magnesium (as either films or bulk). The extremely rapid and cyclable kinetics of Mg-4 at.% Fe films suggest that properly grown Mg–Fe powders of 1–2 μm size can be fully charged with hydrogen within 1 min at temperature near 150 °C (423 K), with possible practical hydrogen storage applications.
► Hydrogenation of Mg-4 at.% Fe films at low temperatures (363 K–423 K) shows very fast kinetics. ► A new method based on IR imaging of a wedge-shaped film was used to evaluate kinetics of the hydride growth. ► Pressure-composition isothermal measurement of a single film was used to evaluate thermodynamic properties. ► Apparent activation energy of hydrogenation kinetics, 56 kJ/mol, suggests hydrogen diffusion along grain boundaries as a controlling mechanism.
The microstructure of an annealed alloy with a Zr8Ni21 composition was studied by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence of three phases, ...Zr8Ni21, Zr2Ni7, and Zr7Ni10, was confirmed by SEM/X-ray energy dispersive spectroscopy compositional mapping and TEM electron diffraction. Distribution of the phases and their morphology can be linked to a multi-phase structure formed by a sequence of reactions: (1) L → Zr2Ni7 + L’; (2) peritectic Zr2Ni7 + L’ → Zr2Ni7 + Zr8Ni21 + L”; (3) eutectic L” → Zr8Ni21 + Zr7Ni10. The effect of annealing at 960 °C, which was intended to convert a cast structure into a single-phase Zr8Ni21 structure, was only moderate and the resulting alloy was still multi-phased. TEM and crystallographic analysis of the Zr2Ni7 phase show a high density of planar (001) defects that were explained as low-energy boundaries between rotational variants and stacking faults. The crystallographic features arise from the pseudo-hexagonal structure of Zr2Ni7. This highly defective Zr2Ni7 phase was identified as the source of the broad X-ray diffraction peaks at around 38.4° and 44.6° when a Cu-K was used as the radiation source.
The electrochemical performance and microstructure of positive electrodes are intimately linked. As such, developing batteries resistance to capacity and voltage fade requires understanding these ...underlying structure–property relationships and their evolution with operation. Epitaxial films of a Li-rich manganese–nickel–cobalt oxide cathode material were deposited on (100)- and (111)-oriented SrRuO
3
/SrTiO
3
substrates. Cyclic voltammetry and impedance spectroscopy tracked the response of these positive electrode materials, while the microstructure of the pristine and cycled films was characterized using transmission electron microscopy. Energy-dispersive X-ray spectroscopy identifies compositional fluctuations in as-deposited films. Phase transformations and dissolution were observed after electrochemical testing. There is a correlation between both local composition and substrate orientation (i.e., surface faceting) and what degradation pathways are active. Regions with comparatively higher concentrations of Ni and Co were more resistant to dissolution and unfavorable phase transformations than those with relatively more Mn. As such, a global composition metric may not be an accurate predictor of degradation and performance. Rather possessing the synthetic ability to engineer the chemical profile as well as characterizing it, pose a challenge and opportunity.
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