Gong, Cheng; Kim, Eun Mi; Wang, Yuan; Lee, Geunsik; Zhang, Xiang(Nature Communications, Springer Science and Business Media LLC, 2019-06-14)[Article]
Materials that are simultaneously ferromagnetic and ferroelectric – multiferroics – promise the control of disparate ferroic orders, leading to technological advances in microwave magnetoelectric applications and next generation of spintronics. Single-phase multiferroics are challenged by the opposite d-orbital occupations imposed by the two ferroics, and heterogeneous nanocomposite multiferroics demand ingredients’ structural compatibility with the resultant multiferroicity exclusively at inter-materials boundaries. Here we propose the two-dimensional heterostructure multiferroics by stacking up atomic layers of ferromagnetic Cr2Ge2Te6 and ferroelectric In2Se3, thereby leading to all-atomic multiferroicity. Through first-principles density functional theory calculations, we find as In2Se3 reverses its polarization, the magnetism of Cr2Ge2Te6 is switched, and correspondingly In2Se3 becomes a switchable magnetic semiconductor due to proximity effect. This unprecedented multiferroic duality (i.e., switchable ferromagnet and switchable magnetic semiconductor) enables both layers for logic applications. Van der Waals heterostructure multiferroics open the door for exploring the low-dimensional magnetoelectric physics and spintronic applications based on artificial superlattices.
his paper reports the influence of dysprosium ion (Dy3+) substitution on the structural and magnetic properties of NiDyxFe2−xO4 (0.0 ≤ x ≤ 0.1) nanoparticles (NPs) prepared using a hydrothermal method. The structure and morphology of the as-synthesized NPs were characterized via X-ray diffraction (XRD), scanning and transmission electron microscope (SEM, and TEM) analyses. 57Fe Mössbauer spectra were recorded to determine the Dy3+ content dependent variation in the line width, isomer shift, quadrupole splitting, and hyperfine magnetic fields. Furthermore, the magnetic properties of the prepared NPs were also investigated by zero-field cooled (ZFC) and field cooled (FC) magnetizations and AC susceptibility measurements. The MZFC (T) results showed a blocking temperature (TB). Below TB, the products behave as ferromagnetic (FM) and act superparamagnetic (SPM) above TB. The MFC (T) curves indicated the existence of super-spin glass (SSG) behavior below Ts (spin-glass freezing temperature). The AC susceptibility measurements confirmed the existence of the two transition temperatures (i.e., TB and Ts). Numerous models, e.g., Neel–Arrhenius (N–A), Vogel–Fulcher (V–F), and critical slowing down (CSD), were used to investigate the dynamics of the systems. It was found that the Dy substitution enhanced the magnetic interactions.
Ganigué, Ramon; Naert, Pieter; Candry, Pieter; de Smedt, Jonas; Stevens, Christian V.; Rabaey, Korneel(Bioresource Technology, Elsevier BV, 2019-05-28)[Article]
Valeric acid and its ester derivatives are chemical compounds with a high industrial interest. Here we report a new approach to produce them from crude glycerol, by combining propionic acid fermentation with chain elongation. Propionic acid was produced by Propionibacterium acidipropionici (8.49 ± 1.40 g·L−1). In the subsequent mixed population chain elongation, valeric acid was the dominant product (5.3 ± 0.69 g·L−1) of the chain elongation process. Residual glycerol negatively impacted the selectivity of mixed culture chain elongation towards valeric acid, whereas this was unaffected when Clostridium kluyveri was used as bio-catalyst. Valeric acid could be selectively isolated and upgraded to ethyl valerate by using dodecane as extractant and medium for esterification, whereas shorter-chain carboxylic acids could be recovered by using a 10 wt% solution of trioctylphosphine oxide (TOPO) in dodecane. Overall, our work shows that the combined fermentation, electrochemistry and homogeneous catalysis enables fine chemical production from side streams.
Shahid, Hamza; Alzaher, Hussain(Arabian Journal for Science and Engineering, Springer Science and Business Media LLC, 2019-05-25)[Article]
A low-voltage and ultra-low power sub-Hertz timer using transistor operating in sub-threshold region is proposed. The sub-Hertz operation is achieved by controlling the amount of currents charging and discharging the timer’s capacitor instead of using large passive components. Pulse width modulation is accomplished by sizing the transistors in charging and discharging control blocks. The timer is working from a single supply voltage of as low as 0.4 V. The circuit is designed in a standard CMOS 150 nm and simulated using Cadence. Simulation results show an oscillation frequency of as low as 0.0217 Hz (a period of 46 s) while using integrable capacitor (100 pF). Its average power consumption for one period is 13.91 pW.
Pedro, Ricardo Pablo; Paulose, Jayson; Souslov, Anton; Dresselhaus, Mildred; Vitelli, Vincenzo(Physical Review Letters, American Physical Society (APS), 2019-03-21)[Article]
Topological quantum and classical materials can exhibit robust properties that are protected against disorder, for example, for noninteracting particles and linear waves. Here, we demonstrate how to construct topologically protected states that arise from the combination of strong interactions and thermal fluctuations inherent to soft materials or miniaturized mechanical structures. Specifically, we consider fluctuating lines under tension (e.g., polymer or vortex lines), subject to a class of spatially modulated substrate potentials. At equilibrium, the lines acquire a collective tilt proportional to an integer topological invariant called the Chern number. This quantized tilt is robust against substrate disorder, as verified by classical Langevin dynamics simulations. This robustness arises because excitations in this system of thermally fluctuating lines are gapped by virtue of interline interactions. We establish the topological underpinning of this pattern via a mapping that we develop between the interacting-lines system and a hitherto unexplored generalization of Thouless pumping to imaginary time. Our work points to a new class of classical topological phenomena in which the topological signature manifests itself in a structural property observed at finite temperature rather than a transport measurement.
Tang, Henry Yue Hin; Shin, David S; Hura, Gregory L.; Yang, Yue; Hu, Xiaoyu; Lightstone, Felice C; McGee, Matthew D; Padgett, Hal S; Yannone, Steven M; Tainer, John A.(Biochemistry, American Chemical Society (ACS), 2018-10-30)[Article]
Protein engineering to alter recognition underlying ligand binding and activity has enormous potential. Here, ligand binding for E. coli phosphoenolpyruvate carboxykinase (PEPCK), which converts oxaloacetate into CO2 and phosphoenolpyruvate as the first committed step in gluconeogenesis, was engineered to accommodate alternative ligands as an exemplary system with structural information. From our identification of bicarbonate binding in the PEPCK active site at the supposed CO2 binding site, we probed binding of nonnative ligands with three oxygen atoms arranged to resemble bicarbonate geometry. Crystal structures of PEPCK and point mutants with bound nonnative ligands thiosulfate and methanesulfonate along with strained ATP plus reoriented oxaloacetate intermediates and unexpected bicarbonate were solved and analyzed. The mutations successfully altered the bound ligand position and orientation, as well as its specificity: mutated PEPCKs bound either thiosulfate or methanesulfonate, but never both. Computational calculations predicted a methanesulfonate binding mutant and revealed that release of active site ordered solvent exerts a strong influence on ligand binding. Besides nonnative ligand binding, one mutant altered the Mn2+ coordination sphere: instead of the canonical octahedral ligand arrangement, the mutant in question only had a five-coordinate arrangement. From this work, critical features of ligand binding, position, and metal ion co-factor geometry required for all downstream events can be engineered with small numbers of mutations to provide insights into fundamental underpinnings of protein-ligand recognition. Through structural and computational knowledge, the combination of designed and random mutations aids robust design of predetermined changes to ligand binding and activity in order to engineer protein function.
The development of novel porous composite materials for organic dye degradation and removal has received increasing attention due to water contamination problem. In this paper, hydrothermal synthesized nano zeolite A have been encapsulated with porous periodic mesoporous organosilica (PMO) through a simple modified StÖber method an organosilane-directed growth-induced etching strategy, the obtained yolk-shell structured sample was further functionalized by the impregnation of copper, realizing the composite material with hierarchical porous and catalytic properties. The morphology, porosity and metal content of the zeolite Cu/A and Cu/A@Et-PMO were fully characterized. As compared to the parent material, the composite Cu/A@Et-PMO have an efficient adsorption and catalytic degradation performance on methylene blue (MB), the removal efficiency reached as high as 95% of 60 mg/L MB with 10min. These novel structured porous composites may have great potential for adsorption and degradation application including waste effluents.
Carbon nanosheets are widely used in electrocatalysis. Structure control is a great challenge in preparation of carbon materials. Here, a facial solid state assembly method is applied to prepare carbon nanosheets. By choosing amino acids as precursors, structure regulation of carbon nanosheets could be realized via switching the side chains of organic linkers, showing structure evolution from dense monolithic to porous nanosheets. Graphene-like carbon nanosheet constructed foam superstructure was also obtained. Owing to high porosity (BET surface area 1680 m2 g−1 with hierarchical pore structure) and nitrogen (8.23 wt%) modified graphitic frameworks, glutamic acid derived nitrogen doped porous carbon nanosheet superstructure showed high ORR activity (onset potential 0.9 V vs. RHE).
The active regions of ultraviolet light emitting diodes (UVLEDs) for UVB and ultra-violet band C wavelengths are composed of AlGaN alloy quantum barriers (QBs) and quantum wells (QWs). The use of alloy QBs and QWs facilitates the formation of percolative paths for carrier injection but also decreases carrier confinement within the QWs. We applied the recently developed Localization Landscape (LL) theory for a full 3D simulation of the LEDs. LL theory describes the effective quantum potential of the quantum states for electrons and holes in a random disordered system with a high computational speed. The results show that the potential fluctuations in the n-AlGaN buffer layer, QWs, and QBs provide percolative paths for carrier injection into the top (p-side) QW. Several properties due to compositional disorder are observed: (1) The peak internal quantum efficiency is larger when disorder is present, due to carrier localization, than for a simulation without fluctuations. (2) The droop is larger mainly due to poor hole injection and weaker blocking ability of the electron blocking layer caused by the fluctuating potentials. (3) Carriers are less confined in the QW and extend into the QBs due to the alloy potential fluctuations. The wave function extension into the QBs enhances TM emission as shown from a k·p simulation of wave-functions admixture, which should then lead to poor light extraction.
The rotary chemical looping combustion reactor design - which utilizes oxygen carriers in a matrix of micro channels for indirect fuel conversion - provides a viable path for fossil-based electric power generation with CO2 capture. Its thermally integrated matrix of micro channels minimizes irreversibilities associated with heat transfer in the reactor, and establishes multiscale coupling between oxygen carrier kinetics, reactor geometry and plant operating conditions. In this study, we implement an optimization framework that exploits this multiscale coupling for simultaneous reactor design and power plant economic optimization. Results for the methane-fueled power plant reveal optimized thermal efficiencies of 54–56% for a rotary chemical looping recuperative Brayton cycle plant, with compressor pressure ratio in the 3–7 range. By switching from an efficiency to an economic objective, we identified solutions that reduced electricity cost by about 11%; by performing scaling and technology maturity projections, we show competitive economics for the rotary chemical looping plant with CO2 capture.
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