Temperature dependence of Mott transition in VO_2 and programmable critical temperature sensor

The temperature dependence of the Mott metal-insulator transition (MIT) is studied with a VO_2-based two-terminal device. When a constant voltage is applied to the device, an abrupt current jump is observed with temperature. With increasing applied voltages, the transition temperature of the MIT current jump decreases. We find a monoclinic and electronically correlated metal (MCM) phase between the abrupt current jump and the structural phase transition (SPT). After the transition from insulator to metal, a linear increase in current (or conductivity) is shown with temperature until the current becomes a constant maximum value above T_{SPT}=68^oC. The SPT is confirmed by micro-Raman spectroscopy measurements. Optical microscopy analysis reveals the absence of the local current path in micro scale in the VO_2 device. The current uniformly flows throughout the surface of the VO_2 film when the MIT occurs. This device can be used as a programmable critical temperature sensor.Comment: 4 pages, 3 figure

Fracture behavior and thermal durability of lanthanum zirconate-based thermal barrier coatings with buffer layer in thermally graded mechanical fatigue environments

The effects of buffer layer on the fracture behavior and lifetime performance of lanthanum zirconate (La2Zr2O7; LZO)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) tests, which are designed to simulate the operating conditions of rotating parts in gas turbines. To improve the thermal durability of LZO-based TBCs, composite coats consisting of two feedstock powders of LZO and 8 wt% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing different volume ratios (50:50 and 25:75, respectively). The composite coat of 50:50 volume ratio was employed as the top coat, and two types of buffer layers were introduced (25:75 volume ratio in LZO and 8YSZ, and 8YSZ only). These TBC systems were compared with a reference TBC system of 8YSZ. The TGMF tests with a tensile load of 60 MPa were performed for 1000 cycles, at a surface temperature of 1100 °C and a dwell time of 10 min, and then the samples were cooled at room temperature for 10 min in each cycle. For the single-layer TBCs, the composite top coat showed similar results as for the reference TBC system. The triple-layer coating (TLC) showed the best thermal cycle performance among all samples, suggesting that the buffer layer was efficient in improving lifetime performance. Failure modes were different for the TBC systems. Delamination and/or cracks were created at the interface between the bond and top coats or above the interface in the single-layer TBCs, but the TBCs with the buffer layer were delaminated and/or cracked at the interface between the buffer layer and the top coat, independent of buffer layer species. This study allows further understanding of the LZO-based TBC failure mechanisms in operating conditions, especially in combined thermal and mechanical environments, in order to design reliable TBC systems

A potential role of coumestrol in soybean leaf senescence and its interaction with phytohormones

Coumestrol is a natural organic compound synthesized in soy leaves and functions as a phytoalexin. The coumestrol levels in plants are reported to increase upon insect attack. This study investigates the correlation between coumestrol, senescence, and the effect of phytohormones on the coumestrol levels in soybean leaves. Our analysis involving high-performance liquid chromatography and 2-D gel electrophoresis indicated a significant difference in the biochemical composition of soybean leaves at various young and mature growth stages. Eight chemical compounds were specifically detected in young leaves (V1) only, whereas three different coumestans isotrifoliol, coumestrol, and phaseol were detected only in mature, yellow leaves of the R6 and R7 growth stage. MALDI-TOF-MS analysis was used to identify two proteins 3,9 -dihydroxypterocarpan 6A-monooxygenase (CYP93A1) and isoflavone reductase homolog 2 (IFR2) only in mature leaves, which are key components of the coumestrol biosynthetic pathway. This indicates that senescence in soybean is linked to the accumulation of coumestrol. Following the external application of coumestrol, the detached V1-stage young leaves turned yellow and showed an interesting development of roots at the base of the midrib. Additionally, the application of phytohormones, including SA, methyl jasmonate (MeJA), and ethephon alone and in various combinations induced yellowing within 5 days of the application with a concomitant significant increase in endogenous coumestrol accumulation. This was also accompanied by a significant increase in the expression of genes CYP81E28 (Gm08G089500), CYP81E22 (Gm16G149300), GmIFS1, and GmIFS2. These results indicate that various coumestans, especially coumestrol, accumulate during leaf maturity, or senescence in soybean

The seeded growth of graphene

In this paper, we demonstrate the seeded growth of graphene under a plasma chemical vapor deposition condition. First, we fabricate graphene nanopowders (~5 nm) by ball-milling commercial multi-wall carbon nanotubes. The graphene nanoparticles were subsequently subject to a direct current plasma generated in a 100 Torr 10%CH(4) - 90%H(2) gas mixture. The plasma growth enlarged, over one hour, the nuclei to graphene sheets larger than one hundred nm(2) in area. Characterization by electron and X-ray diffraction, high-resolution transmission electron microscopy images provide evidence for the presence of monolayer graphene sheets

Monoclinic and Correlated Metal Phase in VO_2 as Evidence of the Mott Transition: Coherent Phonon Analysis

In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 THz and 6.0 THz indicating the rutile metal phase of VO_2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68^oC. The monoclinic and correlated metal(MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photo-assisted hole excitation which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate non-equilibrium state.Comment: 4 pages, 2 figure

Kinematics Characteristic of Lower Extremity during Simulated Skiing Exercise

We analyze the kinematic factors of sectional and total movements in healthy participants to providing group dependent information during simulated exercise. Participants in this study's experiment were 26 male adults, the elapsed times in each phase, the difference in the lower extremity angles, and muscle activity were computed through analysis of kinematic factors. We revealed that motions of the experts took shorter to perform than non-experts, and showed larger variation of lower limb joint angle in most events during simulated skiing. There were also significant group dependent differences in the peak and mean EMG values during simulated skiing. Referring to these results, such a non-expert's posture leads to enhance muscle activity to keep the lower body in balance. Non-experts should maintain appropriate ROM with lower-intensity exercise to improve muscle endurance initially, and it can be useful in providing preliminary data for future training and rehabilitation studies, as well as improvements in muscle strength