The uniform growth of single-crystal graphene over wafer-scale areas remains a challenge in the commercial-level manufacturability of various electronic, photonic, mechanical, and other devices based ...on graphene. Here, we describe wafer-scale growth of wrinkle-free single-crystal monolayer graphene on silicon wafer using a hydrogen-terminated germanium buffer layer. The anisotropic twofold symmetry of the germanium (110) surface allowed unidirectional alignment of multiple seeds, which were merged to uniform single-crystal graphene with predefined orientation. Furthermore, the weak interaction between graphene and underlying hydrogen-terminated germanium surface enabled the facile etch-free dry transfer of graphene and the recycling of the germanium substrate for continual graphene growth.
Spatial light modulators are essential optical elements in applications that require the ability to regulate the amplitude, phase and polarization of light, such as digital holography, optical ...communications and biomedical imaging. With the push towards miniaturization of optical components, static metasurfaces are used as competent alternatives. These evolved to active metasurfaces in which light-wavefront manipulation can be done in a time-dependent fashion. The active metasurfaces reported so far, however, still show incomplete phase modulation (below 360°). Here we present an all-solid-state, electrically tunable and reflective metasurface array that can generate a specific phase or a continuous sweep between 0 and 360° at an estimated rate of 5.4 MHz while independently adjusting the amplitude. The metasurface features 550 individually addressable nanoresonators in a 250 × 250 μm
area with no micromechanical elements or liquid crystals. A key feature of our design is the presence of two independent control parameters (top and bottom gate voltages) in each nanoresonator, which are used to adjust the real and imaginary parts of the reflection coefficient independently. To demonstrate this array's use in light detection and ranging, we performed a three-dimensional depth scan of an emulated street scene that consisted of a model car and a human figure up to a distance of 4.7 m.
An unprecedented approach to the generation of an N-centered radical via a photocatalytic energy-transfer process from readily available heterocyclic precursors is reported, which is distinctive of ...the previous electron transfer approaches. In combination with singlet oxygen, the in-situ-generated nitrogen radical from the oxadiazoline substrate in the presence of fac-Ir(ppy)3 undergoes a selective ipso addition to arenes to furnish remotely double-functionalized spiro-azalactam products. The mechanistic studies provide compelling evidence that the catalytic cycle selects the energy-transfer pathway. A concurrent activation of molecular oxygen to generate singlet oxygen by energy transfer is also rationalized. Furthermore, the occurrence of the electron transfer phenomenon is excluded on the basis of the negative driving forces for one-electron transfer between oxadiazoline and the excited state of fac-Ir(ppy)3 with a consideration of their redox potentials. The necessity of singlet oxygen as well as the photoactivated oxadiazoline substrate is clearly supported by a series of controlled experiments. Density functional studies have also been carried out to support these observations. The scope of substrates is explored by synthesizing diversely functionalized cyclohexadienone moieties in view of their utility in complex organic syntheses and as potential targets in pharmacology.
The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The ...zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi0.5Sb1.5Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.
According to an understanding of wearable robot systems, this article suggests an appropriate design process for surface-mounted permanent magnet synchronous motor (SPMSM) for the joints of wearable ...robots. The major requirements of SPMSM for the wearable robot are investigated and categorized into the dimensional constraints, electromagnetic performances, and thermal restrictions. Given the defined requirements, the electric motor design process for an actual robot is proposed, considering electromagnetic and thermal characteristics. For thermal analysis, the lumped parameter thermal network (LPTN) is adopted, and the process to compose a precise LPTN for the SPMSM is presented. With given controller and electric motor specifications, the numbers of poles and slots are determined, and fractional-slot concentrated winding is adopted considering the overall behavior of the SPMSM such as torque density and noise and vibration. As a preliminary design, the shape of the rotor including permanent magnets, shape of the stator, and the number of turns are designed via space harmonic analysis which is fast. Subsequently, a detailed design process is performed via finite-element analysis. At this stage, the thermal characteristics considering the driving cycle are analyzed via the LPTN, which is appropriate for parametric design. The final model is determined from electromechanical and thermal viewpoint. Finally, experiments are conducted to validate the proposed design process.
This article proposes a fast and accurate coupled electromagnetic-thermal analysis method for a permanent magnet synchronous motor. In conventional design methods, the electric and thermal ...characteristics are calculated simultaneously using a finite element analysis (FEA). However, FEA requires considerable computational time. Therefore, in order to reduce the computational time, mathematical models of the electric parameters and characteristics were proposed as an alternative. Accordingly, the models of the electric parameter, such as d - and q -axis inductance and flux linkage, were obtained using a modified lumped parameter magnetic circuit containing a reluctance of an iron core. Furthermore, the models were fitted according to the formula type to consider nonlinearity according to variations in current and temperature. The electric parameters calculated by FEA were used for curve fitting. A greater number of FEA points were required to determine the effect of current and temperature on the electric parameters. Curve fitting using an appropriate formula type was performed considering the accuracy and minimum number of analysis points. Mathematical modeling of the electric characteristics, including efficiency and losses, was performed. The models of losses were coupled as heat sources to the lumped parameter thermal network, which is well known for its low computational time. The proposed coupled analysis method was applied to a reference motor; through this analysis, the electric characteristics and temperature distribution were calculated simultaneously, considering changes in losses and temperature distribution. The experimental validation was conducted with an acceptable error of 4.4%.
In order to improve the performance of the wound field synchronous motor (WFSM), the permanent magnet assist (PM-assist) is investigated in this paper. The effect of the PM-assist is dependent on the ...inserted position of the PMs. Thus, four models whose PMs are inserted at each different position are suggested. Then, the most effective position of the PM is decided by the mean torque per employed amount of the PM. By inserting the PMs into the decided position of a WFSM designed for a small electric vehicle traction, the PM-assisted WFSM is designed. By comparing the torque and the efficiency of the original WFSM and the PM-assisted WFSM, the effectiveness of the PM-assist is figured out. Finally, the validity of this research is verified by the experiment using the manufactured WFSM.
The electrocatalytic activity of a CuO flower-like nanostructured electrode was investigated in terms of its application to enzyme-less amperometric H
2O
2 sensors. The CuO nanoflowers film was ...directly formed by chemical oxidation of copper foil under hydrothermal condition and then used as active electrode material of non-enzymatic electrochemical sensors for H
2O
2 detection under alkaline conditions. The sensitivity of the sensor with CuO nanoflowers electrode was 88.4
μA/mM
cm
2 with a linear response in the range from 4.25
×
10
−5 to 4
×
10
−2
M and a detection limit of 0.167
μM (
S/
N
=
3). Excellent electrocatalytic activity, large surface-to-volume ratio and efficient electron transport property of CuO nanoflowers electrode have enabled stable and highly sensitive performance for the non-enzymatic H
2O
2 sensor.
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