A novel dual‐band bandpass filter using co‐directional split ring resonators with closely spaced passbands and wide upper stopband

In this article, interdigital capacitor loaded co‐directional split ring resonators (CDSRRs) and their dual‐band bandpass filter applications are proposed. The proposed resonator is formed by nested open loop resonators having open ends at the same place unlike conventional split ring resonators (SRRs). In addition, the inner open loop resonator has interdigital capacitor located between the open ends. The proposed resonator exhibits dual resonance behavior with a small center frequency ratio. Both of resonance frequencies can be controlled due to the changes in the interdigital capacitor and the electrical length of the outer resonator. A dual‐band microstrip bandpass filter is designed by using the proposed CDSRR. Two CDSRRs are used to obtain two poles in each passband. Overall electrical length of the designed filter is 0.23 λ_g × 0.14 λ_g (0.0329 λ_g²), where λ_g is the guided wavelength for the used substrate at the lowest passband center frequency of 1.8 GHz. A small center frequency is obtained by adjusting the second passband at 2.27 GHz. A very wide upper stopband, closely spaced passbands, low insertion losses and high selectivity at both passbands can be obtained by means of the proposed structure. The designed filter was also fabricated and tested. The measured results show a very good agreement with the predicted results.     

Synthesis and enhanced gas sensing properties of crystalline CeO2/TiO2 core/shell nanorods

Crystalline CeO₂/TiO₂ core/shell nanorods were fabricated by a hydrothermal method and a subsequent annealing process under the hydrogen and air atmosphere. The thickness of the outer shell composed of crystal TiO₂ nanoparticles can be tuned in the range of 5-11 nm. The crystal core/shell nanorods exhibited enhanced gas-sensing properties to ethanol vapor in terms of sensor response and selectivity. The calculated sensor response based on the change of the heterojunction barrier formed at the interface between CeO₂ and TiO₂ is agreed with the experimental results, and thus the change of the heterojunction barrier at different gas atmosphere can be used to explain the enhanced ethanol sensing properties.     

Miniaturized frequency‐agile bandpass filter integrated single‐pole double‐throw switch

This article presents a miniaturized frequency‐agile bandpass filter (BPF) integrated single pole double throw (SPDT) switch using common LC resonator. The BPF‐integrated on‐sate channel is constituted by the capacitively coupled LC resonators with loaded varactor diodes and reverse‐biased p‐i‐n diodes. The off‐state channel with high suppression is built when the p‐i‐n diodes loaded LC resonators is forward‐biased. As an example, a frequency‐agile BPF‐integrated SPDT switch with constant 3‐dB fractional bandwidth of 18.4% is designed. Its fabricated circuit area including bias circuit but excluding feeding lines is 0.105 λ_g × 0.079 λ_g. Measured results show that its 3 dB operating frequency bandwidth covers from 0.499 to 1.077 GHz with in‐band insertion loss varying from 4.15 to 3.15 dB. Off‐state suppression better than 37.8 dB, port‐to‐port isolation better than 51.1 dB, and good return loss can be also observed.     

High-performance liquefied petroleum gas sensing based on nanostructures of zinc oxide and zinc stannate

Toxic and combustible gas detection plays a major role in environmental air quality monitoring. Real-time monitoring of hazardous gases and signal of accidental leakages is of great importance owing to the concern for safety requirements in industries and household applications. A simple and economical method for the fabrication of highly sensitive zinc oxide (ZnO) nanorods based gas sensors for detecting low concentrations of Liquefied Petroleum Gas (LPG) was studied in this work. Platinum (Pt) nanoparticles were deposited on the sensing medium which acts as catalysts to improve the sensor performance. The change in electrical resistance of the metal oxide semiconductor for varying concentrations of LPG was measured. Maximum response of 59% was achieved for 9000 ppm LPG at 250 °C. Further to improve the sensing performance of the sensor towards LPG, surface modification of ZnO nanorods using zinc stannate (Zn₂SnO₄) microcubes was performed. High response of 63% was observed for 3000 ppm LPG at 250 °C. Significant improvement in response of the sensor with Zn₂SnO₄ microcubes on ZnO nanorods was observed when compared to sensor with ZnO nanorods.     

Microwave subsurface imaging of dielectric structures using fractal geometries of complementary split ring resonators

In this paper, newly proposed complementary split ring resonator (CSRR) structures of fractal geometries are used for microwave imaging of coated dielectric structures in order to detect concealed voids or similar inclusions. The proposed fractal‐based CSRR structures are found to display improved sensitivity as compared to the conventional square CSRR sensors of same cross‐sectional dimensions, and thus appear to be more appropriate for imaging applications. Detailed analyses of different orders of square Sierpinski fractals have been carried out. Raster scanning of the test region is performed and various parameters such as the magnitude and phase at unloaded resonant frequency, along with the shift in resonant frequency when loaded with the test structure are measured. Furthermore, both qualitative and quantitative images of the test structure are retrieved in terms of all the three measured parameters. Simulation and experimental results demonstrate the applicability of the proposed sensor for microwave imaging.     

ZnO纳米棒修饰的QCM气体传感器检测NH_3研究

主要介绍了ZnO纳米棒修饰的石英晶体微天平(QCM)气体传感器的制备与测试。采用两步法在石英晶振片表面制备直径为100 nm的ZnO纳米棒敏感膜,构成QCM NH3传感器。检测系统为自主研发的基于LabVIEW平台的QCM气体传感器频率测试软件。检测NH3的体积分数为5×10-6~50×10-6,响应时间均在10 s以内,最大频差值为10.9 Hz,响应最大频差值与NH3体积分数呈现良好的线性关系。室温条件下,ZnO纳米棒敏感膜可以完全实现吸附解吸过程,具有可逆性。该传感器性能稳定,响应灵敏,具有重复性。     

α-MoO3/TiO2 core/shell nanorods: Controlled-synthesis and low-temperature gas sensing properties

Crystalline α-MoO₃/TiO₂ core/shell nanorods are fabricated by a hydrothermal method and subsequent annealing processes under H₂/Ar flow and in the ambient atmosphere. The shell layer is composed of crystalline TiO₂ particles with a diameter of 2-6 nm, and its thickness can be easily controlled in the range of 15-45 nm. The core/shell nanorods show enhanced sensing properties to ethanol vapor compared to bare α-MoO₃ nanorods. The sensing mechanism is different from that of other one-dimensional metal oxide core/shell nanostructures due to very weak response of TiO₂ nanoparticles to ethanol. The enhanced sensing properties can be explained by the change of type II heterojunction barrier formed at the interface between α-MoO₃ and TiO₂ in the different gas atmosphere. The present results demonstrate a novel sensing mechanism available for gas sensors with high performance.     

Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods

Gas sensors based on a quartz crystal microbalance (QCM) coated with ZnO nanorods were developed for detection of NH₃ at room temperature. Vertically well-aligned ZnO nanorods were synthesized by a novel wet chemical route at a low temperature of 90 °C, which was used to grow the ZnO nanorods directly on the QCM for the gas sensor application. The morphology of the ZnO nanorods was examined by field-emission scanning electron microscopy (FE-SEM). The diameter and length of the nanorods were 100 nm and 3 μm, respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent performance to NH₃ gas. The frequency shift (Δf) to 50 ppm NH₃ at room temperature was about 9.1 Hz. It was found that the response and recovery times were varied with the ammonia concentration. The fabricated gas sensors showed good reproducibility and high stability. Moreover, the sensor showed a high selectivity to ammoniac gas over liquefied petroleum gas (LPG), nitrous oxide (N₂O), carbon monoxide (CO), nitrogen dioxide (NO₂), and carbon dioxide (CO₂).     

Ultrahigh ethanol response of SnO2 nanorods at low working temperature arising from La2O3 loading

The influences of La₂O₃ loading on the ethanol sensing properties of SnO₂ nanorods were investigated. An obvious enhancement of response was obtained. The response of 5 wt% La₂O₃ loaded SnO₂ nanorods was up to 213 for 100 ppm ethanol at low working temperature of 200 °C, while that of pure SnO₂ nanorods is 45.1. The improvement in response might be attributed to the presence of basic sites, which facilitated the dehydrogenation process. While the working temperature was increased to 300 °C, the sensor response decreased to 16 for 100 ppm ethanol. Additionally, the La₂O₃ loaded SnO₂ nanorods sensors showed good selectivity to ethanol over methane and hydrogen. Our results demonstrated that the La₂O₃ loaded SnO₂ nanorods were promising in fabricating high performance ethanol sensors which could work at low temperature.     

Compact dual‐band bandpass filter and diplexer using hybrid resonant structure with independently controllable dual passbands

In this article, a compact dual‐band bandpass filter (BPF) is developed using a hybrid resonant structure, which consists of a microstrip stub‐loaded dual‐mode resonator and a slotline stub‐loaded dual‐mode resonator. These two resonators, both having two controllable resonant modes and one transmission zero (TZ), are analyzed and used to construct two desired passbands of a dual‐band BPF. Multiple TZs are generated by introducing a source‐load coupling, thus improving the selectivity of the passbands. Then, the dual‐band BPF is reshaped to configure a compact diplexer. The inherent TZs of the two proposed resonators are designed to improve the frequency property and port isolation of the diplexer. Finally, a dual‐band BPF and a diplexer with the lower and upper passbands centered at 2.45 and 3.45 GHz, respectively, are designed, fabricated, and measured to verify the proposed structure and method.     

A metamaterial hepta‐band antenna for wireless applications with specific absorption rate reduction

Present article embodies the design and analysis of slotted circular shape metamaterial loaded multiband antenna for wireless applications with declination of SAR. The electrical dimension is 0.260 λ × 0.253 λ × 0.0059 λ (35 × 34 × 0.8 mm³) of proposed design, at lower frequency of 2.23 GHz. The antenna consists of circular shape rectangular slot as the radiation element loaded with metamaterial split ring resonator (SRR) and two parallel rectangular stubs, etched rectangular single complementary split‐ring resonator (CSRR) and reclined T‐shaped slot as ground plane. Antenna achieves hepta bands for wireless standards WLAN (2.4/5.0/5.8 GHz), WiMAX (3.5 GHz), radio frequency identification (RFID) services (3.0 GHz), Upper X band (11.8 GHz—for space communication) and Lower K_U band (13.1 GHz—for satellite communication systems operating band). Stable radiation patterns are observed for the operating bands with low cross polarization. The SRR is responsible for creating an additional resonating mode for wireless application as well as provide the declination in SAR about 13.3%. Experimental characteristic of antenna shows close agreement with those obtained by simulation of the proposed antenna.     

A selective room temperature formaldehyde gas sensor using TiO2 nanotube arrays

A new gas sensor using TiO₂ nanotube arrays was fabricated and explored for formaldehyde detection at room temperature. Highly ordered vertically grown TiO₂ nanotube arrays were synthesized by using the conventional electrochemical anodization process. The sensor using the fabricated nanotube arrays as the sensing elements demonstrated a good response to different concentrations of formaldehyde from 10 to 50 ppm and a very good selectivity over other reducing gas species such as ethanol and ammonia at room temperature. While the exact sensing mechanism is unclear, some possibilities are briefly discussed.     

Selective gas sensing response from different loading of Ag in sol-gel mesoporous titania powders

Bare and Ag loaded TiO₂ (0.05, 0.5, and 5.0 mol% Ag) powders prepared by sol-gel process have been used for the detection of ethanol, LPG, acetone and toluene gases. These materials were well characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) specific surface area analysis techniques. Specific surface area increased with increasing Ag loading in these mesoporous materials. A thorough TEM/HRTEM investigation with X-ray maps of the modified particles shows the extent of TiO₂ surface coverage by Ag nanoparticles. From the comparative study of the sensor response of Ag-TiO₂ powders for various gases, it is observed that each Ag loading resulted in the highest response towards a particular gas or in other words, every gas responded best towards a particular Ag loading of the sensor material, i.e. 0.05 mol% Ag-TiO₂ sensor showed highest response towards ethanol, 0.5 mol% Ag-TiO₂ sensor showed best response towards toluene, whereas, bare TiO₂ proved to be the best sensor for both acetone and LPG gases. This establishes that Ag loading is not required for detection of acetone and LPG gases. On the basis of detailed materials characterization, a mechanism for the gas sensing response of each analyte has been discussed.     

LPG sensing properties of Pd-sensitized vertically aligned ZnO nanorods

One-dimensional (1-D) vertically aligned ZnO nanorods are synthesized on glass substrate through a simple chemical route and their liquefied petroleum gas (LPG) sensing properties are studied. The morphology and structure of vertically aligned ZnO nanorods has been characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The LPG sensing properties of the vertically aligned ZnO nanorods are improved significantly after palladium (Pd) sensitization. The unsensitized vertically aligned ZnO nanorods exhibited the maximum response of 37% at 573 K upon exposure to 2600 ppm LPG, which improved to 60% at operating temperature of 498 K after the Pd sensitization. The Pd-sensitized vertically aligned ZnO nanorods showed more selectivity towards LPG as compared to CO₂. Our results demonstrate that the chemically grown vertically aligned ZnO nanorods along with Pd sensitization are promising material for the fabrication of cost effective and high performance gas sensors.     

A thin‐layer dielectric and metamaterial unit‐cell stack loaded miniaturized SRR‐based antenna for triple narrow band 4G‐LTE applications

This article presents the design of a miniaturized dual‐band antenna for long‐term evolution (LTE) application is presented. In the basic antenna design, split ring resonator was loaded in the radiating plane of the patch and frequency of resonance was further modified with the help of E‐shaped stub. The antenna has been fabricated using FR‐4 substrate and the measured dual bands at 2.11 and 2.665 GHz are found in a close match with the simulated data. By placing a thin dielectric resonator of permittivity ε _r = 10.2 and thickness of 1.27 mm, two closely spaced narrow bands are obtained at 2.217 and 2.28 GHz. A novel metamaterial unit‐cell having near‐zero refractive index is designed and mounted above the dielectric resonator. This stack configuration generates triple narrow frequency band in the LTE 2 GHz spectrum range. The overall size of the proposed antenna is 20 × 25 mm².     

Highly sensitive tuning of coupled optical ring resonators by microfluidics

The authors develop optofluidic coupled ring resonator (OCRR) system in which one of the ring resonators serves as a microfluidic channel. Highly sensitive tuning of the OCRR is demonstrated by making small changes in the refractive index of the fluid. A refractometric sensing scheme using the OCRR is proposed and demonstrated, allowing for measuring a refractive index change down to 10⁻⁹ refractive index units, two orders of magnitude better than a single ring resonator.     

Design and development of a MEMS butterfly resonator using synchronizing beam and out of plane actuation

This paper presents beam modeling techniques for maximizing mechanical sensitivity of a butterfly resonator for gyroscopic applications. We investigate the geometric aspects of synchronizing beam that connects the wings of a butterfly resonator. Our results show that geometric variation in the synchronizing beam can have a large effect on the frequency split and sensitivity of the device. The model simulation shows a sensitivity of \( 10^{ - 12} \)\( (m/^\circ /s) \) for a frequency split of 10 Hz resulting from the optimized synchronized beam. Out of plane actuation was developed to drive and sense the resonators displacement. Fabricated butterfly resonators were tested, and the experimental results show a frequency split of 305 Hz and 400 Hz while the model illustrated a split of 195 Hz and 330 Hz respectively. The design and analysis presented in this paper can further aid the development of MEMS butterfly resonators for inertial sensing applications.

     

Low profile four‐port super‐wideband multiple‐input‐multiple‐output antenna with triple band rejection characteristics

A quad‐port planar multiple‐input‐multiple‐output (MIMO) antenna possessing super‐wideband (SWB) operational features and triple‐band rejection characteristics is designed. The proposed MIMO configuration consists of four modified‐elliptical‐self‐complementary‐antenna (MESCA) elements, which are excited by tapered co‐planar waveguide (TCPW) feed lines. A radiator‐matched complementary slot is present in the ground conductor patch of each MESCA element. The proposed MIMO antenna exhibits a bandwidth ratio of 36:1 (|S₁₁| < ?10 dB; 0.97‐35 GHz). Further, a step‐like slit‐resonator is etched in the radiator to eliminate interferences at 3.5 GHz. A hexagonal shaped complementary split ring resonator (CSRR) is also loaded on the MESCA radiator to remove interferences at 5.5 and 8.5 GHz. The MIMO antenna is fabricated on FR‐4 substrate of size 63 × 63 mm² and experimental results are found in good agreement with the simulated results. The MIMO antenna exhibits inter‐element isolation >17 dB and envelope correlation coefficient (ECC) <0.01 at all the four ports.     

Thin-film sensor based tip-shaped split ring resonator metamaterial for microwave application

The artificially constructed materials based split ring resonators (SRRs) may have exotic electromagnetic properties and have received growing interest in recent years. Moreover, the resonance frequency shift of this material is extraordinarily sensitive to the changes in the capacitance of SRR, which makes SRR suit for microwave thin-film sensing applications. Based on such principle, the tip-shaped SRR metamaterial is presented as thin-film sensor in this paper to reduce device size and resonance frequency as well as to improve the Q-factor. The structure is placed inside an X-band waveguide with dimensions of 22.86 mm × 10.16 mm × 12.8 mm to investigate resonance frequency shift in different cases by numerical method. In contrast to the traditional structures, the tip-shaped design exhibits a miniaturization and sharper dip on resonance in their transmission spectra. Furthermore, the proposed sensor can deliver the sensitivity level of 16.2 MHz/μm and less than a 2 μm nonlinearity error when the uniform benezocyclobutene films from 100 nm to 50 μm thick are coated onto the fixed structure. These results indicate that the proposed thin-film sensor has high sensitivity and low nonlinearity error, and make it great promising application for wireless sensors in future.     

Love wave hydrogen sensors based on ZnO nanorod film/36°YX-LiTaO3 substrate structures operated at room temperature

Love wave hydrogen sensors based on ZnO nanorod layers deposited on 36°YX-LiTaO₃ substrates have been studied. The ZnO nanorod layers are prepared by two steps: first, the seed layers, as well the guiding layers of the Love wave devices, are deposited by RF magnetron sputtering; second, the nanostructural layers, as well the sensing layers of the sensors, are grown by hydrothermal synthesis. Two kinds of ZnO layers have been analyzed by XRD, SEM and XPS. The XRD shows that both ZnO layers have (0 0 2) oriented wurtzite structures. The SEM results reveal that the morphologies of the deposited ZnO seed layers are continuous and compact, while the hydrothermal treated layers are with nanorods almost perpendicular to the substrate surfaces. Finally, the hydrogen sensing responses of the Love wave sensors activated by Pt catalysts are measured for various concentrations of hydrogen in synthetic air at room temperature. The results show that the sensors have high sensitivity and repeatability as the nanorod layers are optimized, such as the frequency shift 8 kHz toward 0.04% of H₂ in synthetic air is obtained while the height of the nanorod layer is about 2.1 μm and the central frequency of the sensor is about 125.5 MHz. The XPS analyses of the sensitive layers show that there are oxygen vacancies in the layers, so the oxygen vacancy model is used to explain the hydrogen sensing mechanism of the Love wave sensors.