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Both H2S and CO2 are removed from raw natural gas sources for product quality and safety reasons. If a cryogenic method is used for separation, the acid gas components (H2S + CO2) may be recovered ...

In this work, we fabricated a Pt/SiN/TaN memristor device and characterized its resistive switching by controlling the compliance current and switching polarity. The chemical and material properties of SiN and TaN were investigated by X-ray photoelectron spectroscopy. Compared with the case of a high compliance current (5 mA), the resistive switching was more gradual in the set and reset processes when a low compliance current (1 mA) was applied by DC sweep and pulse train. In particular, low-power resistive switching was demonstrated in the first reset process, and was achieved by employing the negative differential resistance effect. Furthermore, conductance quantization was observed in the reset process upon decreasing the DC sweep speed. These results have the potential for multilevel cell (MLC) operation. Additionally, the conduction mechanism of the memristor device was investigated by I-V fitting.

Junction networks made of longitudinally connected metal oxide nanowires (MOx NWs) have been widely utilized in resistive-type gas sensors because the potential barrier at the NW junctions leads to improved gas sensing performances. However, conventional MOx–NW-based gas sensors exhibit limited gas access to the sensing sites and reduced utilization of the entire NW surfaces because the NW networks are grown on the substrate. This study presents a novel gas sensor platform facilitating the formation of ZnO NW junction networks in a suspended architecture by growing ZnO NWs radially on a suspended carbon mesh backbone consisting of sub-micrometer-sized wires. NW networks were densely formed in the lateral and longitudinal directions of the ZnO NWs, forming additional longitudinally connected junctions in the voids of the carbon mesh. Therefore, target gases could efficiently access the sensing sites, including the junctions and the entire surface of the ZnO NWs. Thus, the present sensor, based on a suspended network of longitudinally connected NW junctions, exhibited enhanced gas response, sensitivity, and lower limit of detection compared to sensors consisting of only laterally connected NWs. In addition, complete sensor structures consisting of a suspended carbon mesh backbone and ZnO NWs could be prepared using only batch fabrication processes such as carbon microelectromechanical systems and hydrothermal synthesis, allowing cost-effective sensor fabrication.

This paper proposes a design optimization method for the insulation structure of a High Voltage Direct Current (HVDC) converter transformer, which is exposed to AC, DC, and DC polarity reversal (DCPR) stresses. Since DC and DCPR stresses provide significantly different electric field distributions, conventional transformer insulations designed for AC systems cannot guarantee safety against electrical breakdown. Focusing on main-winding and end-winding insulation including pressboard barriers and oil between windings, this paper presents a design solution based on genetic algorithm (GA). By exploiting modern computing power, the proposed approach can save a considerable amount of effort and time compared to conventional approaches which depend on experimental data and human experience. From the comparison with example reference insulation structures, we verified that the optimized structures maintain the required insulation performance with the reduction of length by 2.8% for the main-winding insulation and by 13.8% for the end-winding insulation.

Introduction These days, the use of various industrial and personal equipment using hazardous gases is on the rising such as H2 for fuel cells. To utilize these gases, the low concentration sensor(~ppm) for leakage detection as well as the high concentration sensor (~%) for monitoring hazardous gasses in the industrial sites are essential. Semiconductor type and electrochemical type sensors are commonly used for gas detection but they are not suitable for high concentration because they are easily saturated in high concentration. In contrast, thermal conductivity type sensors are used as high concentration sensors because they measure resistance change of the heat loss of heated wire(heater) to a gas environment without sensor signal saturation in high concentration. However conventional thermal conductivity type sensors require large power (hundreds of milliwatts to watts) and relatively large size. In the previous study, a microscale bridge-type heater-based gas sensor was developed using MEMS technology for low power consumption and small size [1, 2]. This sensor used the 3ω-method to measure accurately heat loss to the gas with a high signal to noise ratio. Here, we developed a 3ω-method based high concentration gas sensor using a suspended nano-sized wire heater to minimize the required power and size. The suspended nano-sized wire heater consists of a suspended nanowire backbone, an eave structure and a thin gold heater on the suspended nanowire. The suspended nanowire backbone structure was fabricated by pyrolyzing suspended photoresist wire, and a thin gold layer as a heater material, which ensures high sensitivity due to its high-temperature coefficient of resistance, was selectively coated on the suspended carbon nanowire by virtue of the eave structure. The 3ω-method based gas sensor exhibited high sensitivity and wide linear range with ultra-small power consumption because of its suspended architecture, small size, high aspect ratio and high surface to volume ratio. In addition, all the processes of the 3D nanostructures were carried out at a wafer-level enabling cost-effective manufacturing owing to novel eave structures and carbon-MEMS processes (consisting of photolithography and pyrolysis) [3]. Method The suspended nanowire heaters were fabricated by a three-step process. First, eave structures for the selective metal coating on a suspended carbon nanowire was fabricated by oxide etching and isotropic silicon etching. Then, suspended microscale suspended polymer wires were patterned by two successive photolithography processes and the micro polymer wire was converted into a carbon nanowire by a dramatic volume reduction in pyrolysis. The pyrolysis temperature was set to 700℃ for low carbon electrical conductivity so that the electrical charge flows only through the gold. The last step was the deposition of gold as a heater line. A 50-nm-thick gold was deposited using evaporation. Owing to the eave structure and anisotropic evaporation, the gold layer is solely connected through the suspended wire as shown in Figure a. Results and Conclusions The gold-coated suspended carbon nanowire and the eave structure were well-defined as shown in Figure b. The gold-coated nanowire exhibited a very stable 3ω voltage output signal compared to bare carbon nanowire with high conductivity (Figure c). The thermal penetration depth reduces as the input frequency increases, the 3ω voltage decreases with increasing input frequency (Figure d). The 3ω voltage linearly changed with gas concentration depending on the relative thermal conductivity of target gas in comparison to that of N2 (Figure e). Therefore, selective gas detection is feasible. Figure f showed the response and recovery time of the sensor when it is exposed to 100% Ar and 5% H2. The sensor measures a gas concentration based on the thermal equilibrium between heater structure and gas environment only. Thus, very fast response and recovery time within 3s can be achieved even at high gas concentrations. In addition, the power consumption was only 0.107 mW due to suspended nanowire-type heater configuration. References [1] Kommandur, Sampath, et al. "A microbridge heater for low power gas sensing based on the 3-Omega technique." Sensors and Actuators A: Physical 233 (2015): 231-238. [2] Kommandur, Sampath, et al. "Metal-coated glass microfiber for concentration detection in gas mixtures using the 3-Omega excitation method." Sensors and Actuators A: Physical 250 (2016): 243-249. [3] Lim, Yeongjin, et al. "Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform." Nanoscale research letters 8.1 (2013): 492. Figure 1

Introduction Over the last few decades, research has been conducted to build sensitive, reliable and high-performance gas monitoring systems that meet the demands of industrial sites. In particular, 1D metal oxide (MOx) nanomaterials have been actively studied as a gas sensing material because of its excellent sensing performance, low cost, and suitability for a micro/nano device [1]. Besides, the MOx 1D nanostructures are relatively easy to synthesize, easy to control morphology and have various options including surface functionalization to make even further enhancement of the sensing performance [2]. In particular, when the MOx 1D nanostructures linked together and form a junction structure, an additional current path is generated through the formed junction and this forms additional potential barriers. Thus, an additional gas sensing mechanism can be utilized to further improve sensing performance as shown in Figure 1 [3]. In general, however, the fabrication process of MOx nanostructure networks including dense junctions requires complex and precise techniques. In addition, the nanostructure networks are positioned on the substrates. Therefore, gas access to the junctions and mass transfer to the sensing sites are limited. In this study, we report a novel gas sensor architecture including a suspended carbon mesh integrated with ZnO nanowires networks. The nanowires are grown on single carbon wires of the mesh and meet at the void regions of mesh structures. Thus, dese nanowire junctions can be formed in a suspended form (Figure 2). Owing to the suspended architecture, gas access to the sensing sites is effective and the sensing signal is free from the effect of the substrate [4]. The suspended carbon nanomesh was fabricated using the carbon-MEMS process consisting of simple photolithography and pyrolysis processes. The ZnO nanowire networks were precisely patterned at the suspended mesh using the hydrothermal method. The length of the nanowires was adjusted for good junction formation and the advantage of the suspended junctions was evaluated. Method Suspended carbon nanomesh was fabricated by photolithography and pyrolysis (Vacuum, 700°C) processes. Via two successive exposure steps in photolithography, suspended polymer mesh was defined without complex lithography techniques. After the pyrolysis process, microscale polymer mesh was converted into nano-sized carbon mesh because of the large volume reduction (up to 90%) in pyrolysis. After a thin ZnO seed layer was selectively coated on the suspended carbon mesh, ZnO nanowires were grown on the patterend seed layer by a hydrothermal process (Zn(NO3)2 25mM, HMTA 25mM ). The pristine ZnO nanostructures were annealed using RTA (N2 atmosphere, 300°C) for better adhesion and connection. Results and Conclusions Figure 2 shows the SEM images of the fabricated suspended carbon nanomesh (width ~140μm, thickness ~400nm, line spacing ~6μm) and ZnO nanowire networks (diameter ~100 nm, length ~3.5 μm). ZnO nanowires were well grown to form uniform and dense junctions. To characterize the effect of the nanowire junctions, NO2 gas sensing responses were compared between a nanomesh-based gas sensor and a single suspended carbon wire-based sensor. As shown in Figure 3. the suspend mesh-based sensor exhibited higher response and wider linear range due to the effect of junctions. This novel sensor architecture can be widely utilized because all the fabrication processes of the presented sensor were carried out at a wafer-level making it cost-effective and various MOx nanowires can be simple integrated with a bunch of junctions. References [1] A. Dey, Semiconductor metal oxide gas sensors, Materials Science and Engineering: B, 229 (2018) 206 – 217. [2] Y. Lim, S. Kim, Y. M. Kwon, J. M. Baik, H. Shin, A highly sensitive gas-sensing platform based on a metal-oxide nanowire forest grown on a suspended carbon nanowire fabricated at a wafer level, Sensors and Actuators B : Chemical. 260, (2018) 55-62. [3] R. Khan, H.W. Ra, J. Kim, W. Jang, D. Sharma, Y. Im, Nanojunction effects in multiple ZnO nanowire gas sensor, Sensors and Actuators B: Chemical, 150 (2010) 389 – 393. [4] Y. Lim, J. I. Heo, M. J. Madou, H. Shin, Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform, Nanoscale Research Letter. 8 (2013) 492. Figure 1

This paper reports a novel metal oxide nanowire (MOx NW) based gas senor built on a suspended carbon nanowire heater allowing ultra-low power consumption. This sensing platform was fabricated using only wafer level batch fabrication processes such as carbon-MEMS, atomic layer deposition, and hydrothermal growth processes. Owing to the mechanical rigidity, the suspended carbon nanowire allows selective circumferential growth of MOx NWs on its surface. The suspended carbon NW and MOx NWs are electrically separated with a thin HfO 2 layer. In addition, the suspended high-aspect-ratio geometry of the carbon NW enables MOx NWs to be heated with ultra-low power Joule heating. The architecture of the suspended nanowire forest ensures efficient gas access to the sensing sites, detecting NO2 as low as 100 ppb.

One important issue related to the standard CMOS technology is a low-resistivity silicon substrate that results in low Q-factor for the passive spiral inductor. Therefore, active inductors have widely studied and applied to different range of RF circuit applications due to high-Q, low insertion loss, small size, and inductance tenability [1]. However, the conventional active inductors have also some drawbacks such as high noise figure, poor linearity, and high power consumption [2]. Moreover, most of the previously studied active inductors were basically grounded-type 1-port network. In this paper, a design of novel active 2-port inductor using LC parallel resonator is presented. The parasitic capacitances are decreased by adding parallel L f and C f to gates of M 1 and M 2 , as a result the Q-factor of overall circuit can be improved. The simulation was performed in Cadence Spectre by using SP simulation. The overall circuit size of the proposed active inductor is 0.45mm × 0.5mm including the pads, where the active area occupies only 0.2mm × 0.15 mm. The proposed circuit consumped 3.6mW DC power at 1.8V supply voltage. On wafer probing was used to characterize 2-port S-parameters of the proposed circuit and also a pad de-embedding method was used for accurate measurements. Figs. 1 and 2 show the simulated and measured inductances and Q-factors of the fabricated circuit. From an experiment, it is found that the inductance and Q-factor are higher than 2 nH and 50, respectively, in the frequency range of 1–6 GHz. The simulated and measured noise figure of the fabricated circuit is shown in Fig. 3. The measured noise figure is less then 12 dB in constant inductance range. The proposed circuit provides high inductance, Q-factor, low DC power consumption, and low noise figure over wide frequency range. The proposed 2-port active inductor is applicable in various RF circuits and systems such as coupler, RFIC filters, power dividers, and LC-VCO.