Air‐Stable Cesium Lead Iodide Perovskite for Ultra‐Low Operating Voltage Resistive Switching

CsPbX₃ (X = halide, Cl, Br, or I) all‐inorganic halide perovskites (IHPs) are regarded as promising functional materials because of their tunable optoelectronic characteristics and superior stability to organic–inorganic hybrid halide perovskites. Herein, nonvolatile resistive switching (RS) memory devices based on all‐inorganic CsPbI₃ perovskite are reported. An air‐stable CsPbI₃ perovskite film with a thickness of only 200 nm is successfully synthesized on a platinum‐coated silicon substrate using low temperature all‐solution process. The RS memory devices of Ag/polymethylmethacrylate (PMMA)/CsPbI₃/Pt/Ti/SiO₂/Si structure exhibit reproducible and reliable bipolar switching characteristics with an ultralow operating voltage (<+0.2 V), high on/off ratio (>10⁶), reversible RS by pulse voltage operation (pulse duration < 1 ms), and multilevel data storage. The mechanical flexibility of the CsPbI₃ perovskite RS memory device on a flexible substrate is also successfully confirmed. With analyzing the influence of phase transition in CsPbI₃ on RS characteristics, a mechanism involving conducting filaments formed by metal cation migration is proposed to explain the RS behavior of the memory device. This study will contribute to the understanding of the intrinsic characteristics of IHPs for low‐voltage resistive switching and demonstrate the huge potential of them for use in low‐power consumption nonvolatile memory devices on next‐generation computing systems.     

Identification of cubic stiffness nonlinearity by linearity-conserved NARMAX modeling

A modeling technique to estimate a NARMAX model is developed to identify nonlinearities which are contained in linear-based nonlinear systems. Considering great contributions by linear parts of the NARMAX model on describing nonlinearities, a linear model, which is estimated from small amplitude input and the corresponding output is taken as the linear part of the NARMAX model. Hence, the capabilities of the model to predict nonlinear behaviors for any input within stable region are fairly improved, and multiplicity problem in selecting a nonlinear regression model is also resolved. As an illustration, one degree of freedom system with cubic stiffness is identified in terms of NARMAX modeling technique using the procedure proposed in this work and conventional one, respectively. By extraction higher order FRFs from the NARMAX models, dominant nonlinearities of the system are predicted, and the results by the two methods are compared with analytic one, which shows the priority of the modeling technique proposed.     

Estimation of Part Waiting Time and Fleet Sizing in AGV Systems

As manufacturing systems have grown in size and complexity, material flow control has become one of the key issues for system efficiency, and determination of the number of vehicles required is an important issue in the design of the AGV (automatic guided vehicle) systems for automated material flow control. In an AGV system, a part issues a delivery request for its transportation, and then an empty vehicle is assigned based on a pre-determined vehicle selection rule and provides delivery service. This research presents a fleet sizing procedure for an AGV system with multiple pickup and delivery stations. A queueing model is used to estimate part waiting times. The fleet sizing procedure estimates the minimum number of vehicles needed to ensure a predefined part waiting time limit. While most stochastic models assume first-come-first-served or random vehicle selection rules for the selection of an empty vehicle, this model considers such additional rules as the nearest vehicle selection rule, which is the most popular among all vehicle selection rules. The performance of the proposed model is examined through computational experiments. This work was supported by Korea Research Foundation Grant (KRF-2003-042-D20136).     

Chatter analysis of a parallel mechanism-based universal machining center

Extensive researches have been carried out on machine tool chatter to obtain assessment procedure and improvement measures. In this study, chatter limit is predicted on a newly fabricated universal machining center by the combination of structural dynamic characteristics and cutting mechanics. We showed the unstable cutting conditions, and from them we could plot the unstable borderlines. From the chatter simulations we could say that the newly built universal machining center can be well used in the finishing machining of steel as other common machine tools.     

Broadband electron spin resonance at low frequency without resonant cavity

We have developed a nonconventional broadband electron spin resonance (ESR) spectrometer operating continuously in the frequency range from 0.5 to 9 GHz. Dual antenna structure and the microwave absorbing environment differentiate the setup from the conventional one and enable broadband operation with any combination of frequency or magnetic field modulation and frequency or magnetic field sweeping. Its performance has been tested with the measurements on a 1,1-diphenyl-2-picrylhydrazyl (DPPH) sample and with the measurements on the single molecular magnet, V6, in solid state at low temperature.     

A study on the control characteristics of ER valve-FHA system and durability test

In this paper, making the best use of the features of the electro-rheological (ER) valve, a two-port pressure control valve using ER fluids is proposed and manufactured. The ER-Valve characteristics are evaluated by changing the intensity of the electric field and the number of electrode. As only with electrical signal change to the ER-Valve in which ER fluid flowing, ER fluid flow is controlled, so development of simple ER-Valves have been tried. The ER-Valves and pressure drop check method are considered to be applied to the fluid power industry. Using the manufactured pressure control valve, a one-link manipulator with FHA (Flexible Hydraulic Actuator) is driven. As a result, it is experimentally confirmed that the pressure control valve using ER fluids is applicable to use in driving actuator. If it applies characteristics of the ER fluids, it will be able to apply in the control system for the ER Valve which occurs from industrial controller. After having durability test, shear stress increased regularly because of starch particles crushed by pump and particle size that was almost the same. Moreover, Ra of copper electrode increased about 1.56 times rather than before those of performing durability test, and Rz increased about 2.2 times.     

Zn-substitution effects on the normal optical conductivity and the superconducting gap

The Zn-substitution effects on the c-axis optical spectra have been investigated for highly oxygenated YBa₂ Cu₃O_y crystals with y = 6.88 – 6.92. In the normal state, the Zn-substitution does not affect the high frequency conductivity but changes the -dependence of the scattering rate at low frequencies, leading to the increase in the dc resistivity in the c-direction. In the superconducting state, we observed a pronounced effect of Zn-substitution that a large number of unpaired carriers form a Drude-like absorption band within the gap, keeping the maximum gap amplitude unchanged. The missing area which gives the superfluid density is found to correlate with T_c.     

Anion Constructor for Atomic-Scale Engineering of Antiperovskite Crystals for Electrochemical Reactions

Among the Pt group metals, Pd has been considered the most efficient for application in electrocatalysts as an alternative to Pt. Despite the comparable electrochemical activities of Pd and Pd-metal alloys, they are vulnerable to liquid acidic electrolytes, leading to degradation of catalytic activity. Pd–Ni alloys have been used to enhance catalytic activity because the electronic structure of Pd can be easily changed by adding Ni. In other studies, N atoms have been introduced for more stable M–Ni catalysts by inducing the formation of Ni₄N species; however, the structural analysis and the role of nitrogen have not been fully understood yet. Herein, the Pd–Ni alloy nitride with a unique crystal structure shows a promising catalytic activity for oxygen reduction reaction (ORR). The nitride PdNi nanoparticles have a novel monolithic antiperovskite structure of chemical formula (Pd_xNi_1−x)NNi₃. The unique antiperovskite crystal (Pd_xNi_1−x)NNi₃ possesses superior ORR activity and stability, originating from the downshifted d-band center of the monolayer Pd/antiperovskite surface and the lower formation energy of the antiperovskite core nanocrystal. Consequently, (Pd_xNi_1−x)NNi₃, as a Pt-free Pd-based electrocatalyst, overcomes the stability issue of Pd under acidic conditions by achieving 99-times higher mass activity than commercial Pd/C, as shown by the durability test.     

Direct Observation of Fe-Ge Ordering in Fe5−xGeTe2 Crystals and Resultant Helimagnetism

Microscopic structures and magnetic properties are investigated for Fe_5−xGeTe₂ single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet. An Fe atom (Fe(1)) located in the outermost Fe₅Ge sublayer has two possible split-sites which are either above or below the Ge atom. Scanning tunneling microscopy shows √3 × √3 superstructures which are attributed to the ordering of Fe(1) layer. The √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe_5−xGeTe₂ due to its non-centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties and the origin of the new vdW ferromagnet, Fe_5−xGeTe₂ for future topology-based spin devices.     

Visibly Clear Radiative Cooling Metamaterials for Enhanced Thermal Management in Solar Cells and Windows

The study of transparent daytime radiative cooling with no additional energy consumption is a promising area of research. Its applications include solar cells and building and automobile windows that are prone to heating issues. Ubiquitous applications necessitate the development of metamaterials with high mechanical flexibility in a scalable manner while overcoming translucence. In this study, visibly clear and flexible radiative cooling metamaterials have been developed using a newly designed optical modulator filled into randomly distributed silica aerogel microparticles in a silicone elastomer. The optical modulator effectively suppresses visible light scattering, thus enabling higher loading of silica aerogel microparticles while securing visible clarity. The significant suppression of the rise in temperature by the metamaterial is verified using both indoor and outdoor experiments. The visibly clear metamaterials deployed in solar cells and windows can effectively suppress the rise in temperature under solar irradiation, thereby mitigating the performance degradation of solar cells by heating issues and suppressing the rise in temperature of indoor air.