Fe/nanoporous carbon hybrid derived from metal–organic framework for highly effective microwave absorption

A novel Fe/nanoporous carbon (Fe/NPC) composite was synthesized via Fe₃O₄ nanoparticles coating in metal–organic framework. The pyrolysis conditions are carefully optimized, and the effects of pyrolysis temperature on electromagnetic parameters have been systematically analyzed. Among these candidates, the Fe/NPC composite pyrolyzed at 800 °C (Fe/NPC‐800) shows the eminent microwave absorption due to interface polarization and well matched characteristic impedance. The results indicate that the effective reflection loss absorption bandwidth of Fe/NPC‐800 reaches 5.6 GHz (12.4–18 GHz) with a thickness of 1.8 mm. It provides an exciting clue for the fabrication of lightweight and highly effective microwave absorbers in the future.     

Miniaturization of GPS patch antennas based on novel dielectric ceramics Zn(1−x)MgxAl2O4 by sol–gel method

The need for miniaturization and weight reduction of GPS patch antennas has prompted the search for new microwave dielectric materials. In this study, a sol–gel method was used to prepare Zn_(1?x)Mg_xAl₂O₄ thin films and fabricate GPS patch antennas at a low annealing temperature (700 °C). X-ray diffraction (XRD) patterns, field emission scanning electron microscopy images, Fourier transform infrared spectra, and optical band gap analyses confirmed the nanostructure of (Mg/Zn)Al₂O₄. The XRD patterns displayed the characteristic peaks of (Mg/Zn)Al₂O₄ with a face-centered cubic structure. Mg addition decreased the crystallite size, surface morphology, and lattice parameters of the resultant films, evidently affecting their density and dielectric constant (? _r ). Based on the material investigated and microwave antenna theory, GPS patch antennas were fabricated using Zn_(1?x)Mg_xAl₂O₄ and then studied using a PNA series network analyzer. The fabricated patch antennas with different ? _r ceramics decreased in size from 12.5 to 10.8 cm². The patch antennas resonated at a frequency of 1.570 GHz and provided a return loss bandwidth between ?16.6 and ?20.0 dB; their bandwidth also improved from 90 to 255 MHz. The GPS patch antenna fabricated from Zn_0.70Mg_0.30Al₂O₄ showed an excellent combination of return loss (?20.0 dB), small size (10.8 cm²), and wide bandwidth (255 MHz). Therefore, addition of Mg improves antenna performance and decreases the dimensions of the device.     

Wide-frequency-range dielectric response of polystyrene latex dispersions

In this work we analyze the dielectric properties of dilute colloidal suspensions of nonconducting spherical particles with a thin electrical double layer from experimental data obtained by performing impedance spectroscopy experiments over a broad frequency range, from 20 Hz to 1 GHz. The electrode polarization correction was made by fitting a circuit model in the complex impedance plane (impedance spectrum) using a constant phase angle (CPA) element to fit the electrode polarization in series with the sample impedance. This simple procedure is found to be effective in eliminating the electrode contribution. The dielectric response shows two different dispersions, the alpha relaxation (counterion relaxation) that occurs at low kilohertz frequencies, and the delta relaxation (Maxwell-Wagner effect) found in the MHz range. These are reasonably well fitted over a broad frequency range by the theoretical expressions given by a simplified standard model (not including anomalous conduction) and a generalized model (including anomalous conduction) for the low-frequency dispersion, plus Maxwell-Wagner-O'Konski theory for the delta relaxation in the mid-frequency range. An analysis was also made of the need to include, for these latices, the effects of ion mobility in the Stern layer in order for the values of the zeta-potential obtained from electrophoretic and dielectric data to be compatible with each other.     

Multiplexing microelectrodes for dielectrophoretic manipulation and electrical impedance measurement of single particles and cells in a microfluidic device

This work presents a microfluidic device, which was patterned with (i) microstructures for hydrodynamic capture of single particles and cells, and (ii) multiplexing microelectrodes for selective release via negative dielectrophoretic (nDEP) forces and electrical impedance measurements of immobilized samples. Computational fluid dynamics (CFD) simulations were performed to investigate the fluidic profiles within the microchannels during the hydrodynamic capture of particles and evaluate the performance of single‐cell immobilization. Results showed uniform distributions of velocities and pressure differences across all eight trapping sites. The hydrodynamic net force and the nDEP force acting on a 6 μm sphere were calculated in a 3D model. Polystyrene beads with difference diameters (6, 8, and 10 μm) and budding yeast cells were employed to verify multiple functions of the microfluidic device, including reliable capture and selective nDEP‐release of particles or cells and sensitive electrical impedance measurements of immobilized samples. The size of immobilized beads and the number of captured yeast cells can be discriminated by analyzing impedance signals at 1 MHz. Results also demonstrated that yeast cells can be immobilized at single‐cell resolution by combining the hydrodynamic capture with impedance measurements and nDEP‐release of unwanted samples. Therefore, the microfluidic device integrated with multiplexing microelectrodes potentially offers a versatile, reliable, and precise platform for single‐cell analysis.     

Assembly and performance of a 6.4 T cryogen‐free dynamic nuclear polarization system

We report on the assembly and performance evaluation of a 180‐GHz/6.4 T dynamic nuclear polarization (DNP) system based on a cryogen‐free superconducting magnet. The DNP system utilizes a variable‐field superconducting magnet that can be ramped up to 9 T and equipped with cryocoolers that can cool the sample space with the DNP assembly down to 1.8 K via the Joule–Thomson effect. A homebuilt DNP probe insert with top‐tuned nuclear magnetic resonance coil and microwave port was incorporated into the sample space in which the effective sample temperature is approximately 1.9 K when a 180‐GHz microwave source is on during DNP operation. ¹³C DNP of [1‐¹³C] acetate samples doped with trityl OX063 and 4‐oxo‐TEMPO in this system have resulted in solid‐state ¹³C polarization levels of 58 ± 3% and 18 ± 2%, respectively. The relatively high ¹³C polarization levels achieved in this work have demonstrated that the use of a cryogen‐free superconducting magnet for ¹³C DNP is feasible and in fact, relatively efficient—a major leap to offset the high cost of liquid helium consumption in DNP experiments.     

EIS study on passive films of AISI 304 stainless steel in oxygenous sulfuric acid solution

Electrochemical impedance spectroscopy (EIS) and Kramers–Kronig (K–K) transforms were made on American Iron and Steel Institute (AISI) 304 stainless steel (SS) in naturally aerated sulfuric acid solution at room temperature. The K–K relations testify that the systems under investigation comply with the linearity, causality, and stability constraints of linear system theory (LST) and thereby validate the EIS data. The polarization resistance decreased with removing of passive film and pitting formed in the passive layer, due to layer thinning or pitting caused by chloride. The impedance data for 304 SS with passive films can be accurately modeled using individual components of the equivalent circuits. The polarization resistances (Rp) of the 304 SS can be confirmed by Nyquist plots and estimated from the anodic polarization curves. The reaction model of the dissolution‐passivation process of 304 SS in acid solution is proposed. Copyright © 2009 John Wiley & Sons, Ltd.     

Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal

We report on circularly polarized light emitted from a chiral nematic liquid crystal doped with a luminescent organolanthanide dye. The organolanthanide emission displays an extremely narrow spectral bandwidth of ΔλE =8 nm. This is considerably narrower than the CNLC selective reflection bandwidth ΔλR =60 nm. When conventional dyes with broader emission bandwidths are dissolved into CNLCs, the average degree of circular polarization g of emitted light is reduced from the maximum degree g MAX ; this is due to the overlap of the emission band with the reflection band edges, and spectral regions outside the reflection band. Here, however, we can place the entire emission band inside the reflection band and achieve g ≈g MAX =1.27. Furthermore, a high degree of circular polarization is maintained under off-axis viewing up to a viewing angle of ≈30° to the normal.     

Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal

We report on circularly polarized light emitted from a chiral nematic liquid crystal doped with a luminescent organolanthanide dye. The organolanthanide emission displays an extremely narrow spectral bandwidth of Δ λ_E≈ 8 nm. This is considerably narrower than the CNLC selective reflection bandwidth Δ λ_R≈60 nm. When conventional dyes with broader emission bandwidths are dissolved into CNLCs, the average degree of circular polarization g of emitted light is reduced from the maximum degree g _MAX ; this is due to the overlap of the emission band with the reflection band edges, and spectral regions outside the reflection band. Here, however, we can place the entire emission band inside the reflection band and achieve g ≈ g _MAX=1.27. Furthermore, a high degree of circular polarization is maintained under off-axis viewing up to a viewing angle of ≈ 30° to the normal.     

Characterization of Electrode/Electrolyte Interface of ECIS Devices

Electric cell‐substrate impedance sensing requires low electrode/electrolyte interface impedance for effective biomedical and biophysical applications. Thus a complete understanding of physical processes involved in the formation of an electric double layer is required to design a low interface impedance device. This paper presents the numerical simulation of the impedance for the electrode/electrolyte interface of three‐electrode devices along with the practical realization for the effective workout of impedance sensing devices. The three‐electrode based impedance sensing devices along with phosphate buffered saline as electrolyte is simulated using COMSOL Multiphysics to evaluate the impedance of the electrode/electrolyte interface. Microfabrication technology is used to realize three‐electrode impedance sensing devices with diverse configuration which are used to measure the electrode/electrolyte interface impedance. The measured impedance data were then compared with the COMSOL simulated results and it is found that both the data sets fitted well with less than 5 % RSE. The results obtained from simulation and experiments indicate that the impedance due to double layer diffusion dominates in the low frequency region up to few kHz whereas electrolytic bulk resistance plays a major role in the higher frequency range. The experimental impedance data were further interpreted by electrochemical impedance spectroscopy analysis software to model the equivalent circuit of the electrochemical system.     

Cross‐Polarization Electron‐Nuclear Double Resonance Spectroscopy

Magnetic nuclei in the proximity of a paramagnetic center can be polarized through electron‐nuclear cross‐polarization and detected in electron‐nuclear double resonance (ENDOR) spectroscopy. This principle is demonstrated in a single‐crystal model sample as well as on a protein, the β2 subunit of E.coli ribonucleotide reductase (RNR), which contains an essential tyrosyl radical. ENDOR is a fundamental technique to detect magnetic nuclei coupled to paramagnetic centers. It is widely employed in biological and materials sciences. Despite its utility, its sensitivity in real samples is about one to two orders of magnitude lower than conventional electron paramagnetic resonance, thus restricting its application potential. Herein, we report the performance of a recently introduced concept to polarize nuclear spins and detect their ENDOR spectrum, which is based on electron‐nuclear cross polarization (eNCP). A single‐crystal study permits us to disentangle eNCP conditions and CP‐ENDOR intensities, providing the experimental foundation in agreement with the theoretical prediction. The CP‐ENDOR performance on a real protein sample is best demonstrated with the spectra of the essential tyrosyl radical in the β2 subunit of E.coli RNR.     

A Chiral Reduced‐Dimension Perovskite for an Efficient Flexible Circularly Polarized Light Photodetector

Chiral quasi‐2D perovskite single crystals (SCs) were investigated for their circular polarized light (CPL) detecting capability. Quasi‐2D chiral perovskites, [(R)‐β‐MPA]₂MAPb₂I₇ ((R)‐β‐MPA=(R)‐(+)‐β‐methylphenethylamine, MA=methylammonium), have intrinsic chirality and the capability to distinguish different polarization states of CPL photons. Corresponding quasi‐2D SCs CPL photodetector exhibit excellent detection performance. In particular, our device responsivity is almost one order of magnitude higher than the reported 2D perovskite CPL detectors to date. The crystallization dynamics of the film were modulated to facilitate its carrier transport. Parallel oriented perovskite films with a homogeneous energy landscape is crucial to maximize the carrier collection efficiency. The photodetector also exhibits superior mechanical flexibility and durability, representing a promising candidate for sensitive and robust CPL photodetectors.     

Improved Efficiency and Stability of Flexible Dye Sensitized Solar Cells on ITO/PEN Substrates Using an Ionic Liquid Electrolyte

Flexible dye‐sensitized solar cells (DSSCs) built on plastic substrates have attracted great interest as they are lightweight and can be roll‐to‐roll printed to accelerate production and reduce cost. However, plastic substrates such as PEN and PET are permeable to water, oxygen and volatile electrolyte solvents, which is detrimental to the cell stability. Therefore, to address this problem, in this work, an ionic liquid (IL) electrolyte is used to replace the volatile solvent electrolyte. The initial IL‐based devices only achieved around 50% of the photovoltaic conversion efficiency of the cells using the solvent electrolyte. Current‐voltage and electrochemical impedance spectroscopy (EIS) analysis of the cells in the dark indicated that this lower efficiency mainly originated from (i) a lack of blocking layer to reduce recombination, and (ii) a lower charge collection efficiency. To combat these problems, cells were developed using a 12 nm thick blocking layer, produced by atomic layer deposition, and 1 μm thick P25 TiO₂ film sensitized with the hydrophobic MK‐2 dye. These flexible DSSCs utilizing an IL electrolyte exhibit significantly improved efficiencies and a <10% drop in performance after 1000 h aging at 60°C under continuous light illumination.     

Light driving force for surface patterning on azobenzene-containing polymers

In this paper, we investigated the effect of light driving force induced surface deformation on azobenzene-containing polymers. The surface deformation is attributed to light-induced mass migration inside the polymers. Circular cap arrays are firstly fabricated by high power laser ablation via polarization controlled three-beam interference. The circular caps are subsequently exposed to polarization controlled two-beam interfering field. The results illuminate that when the interfering laser beams are both set to P polarization, the circular caps are deformed. While the laser beams are of other interfering modes like (S, S) and (+45° , -45°), the caps are seldom deformed. The circular caps are also exposed to single intensity-homogeneous linearly polarized laser beam. The deformation of the caps keeps the same direction as the irradiating polarization. A model based on the focusing effect of the circular caps is addressed to explain the origin of the light driving force for mass migration in azopolymers. The all-optical approach for the production of deformed caps can be used to generate aspherical lens, which may be applied to many domains.     

圆偏振发光高分子的研究

综合评述了近年来有关圆偏振发光高分子的研究，包括圆偏振发光高分子的潜在应用及其在发光高分子理论研究上的意义，圆偏振发光高分子的类型与性能表征等。并提出了三条获得高度圆偏振发光高分子的可能途径。     

一种新型联萘基旋光共轭聚合物的圆二色谱和圆偏振荧光光谱

众所周知 ,聚合物的光电性质依赖于聚合物链的构象和 (或 )组成 ,通过在聚合物上引入手性单元 ,采用圆二色谱 ( CD)和圆偏振荧光光谱 ( CPL)等方法可表征聚合物结构 [1] .近年来 ,由于圆偏振光可用作光数据存储和液晶显示器背景光 [2 ] ,人们开始注重共轭聚合物圆偏振光材料的研究 .共轭聚合物的光致和电致圆偏振光的现象由一种带手性侧链的聚噻吩[3 ] 和一种带手性侧链的聚 (对苯撑乙烯 ) [4 ]产生 ,但它们的圆偏振荧光度 (用不对称因子 glum=2 ( IL-IR) / ( IL+IR)表示 ,IL 和 IR 分别指左圆偏振光强度和右圆偏振光强度 )相对较低 …     

Constructing holey γ-Fe2O3 nanosheets with enhanced capability for microwave absorption

Iron-based nanostructured materials are desirable for various applications owing to their high magnetization. Herein, we presented a facile fabrication of free-standing γ-Fe₂O₃ nanosheets by a molten salt method for microwave absorption. Furthermore, Al component was incorporated to serve as the template for hole production. The designed holey γ-Fe₂O₃ nanosheets were effective to maximize the usage of nanomaterials, thereby providing larger surface area, richer defects, and more polarization centers. Both experimental measurements and electric field simulation showed an enhanced capability for microwave absorption using holey γ-Fe₂O₃ when compared with its intact counterpart. Oxygen vacancy was generated during the hole evolution, which significantly improved the electrical conductivity and thus promoted the conductive loss mechanism. In addition, the holey configuration may extend the transmission path of microwave and confer it with multiple reflection and scattering within the absorbent matrix. Meantime, the polarization loss was strengthened owing to Al modulation associated with the induced defect sites. With the improvement in conductive loss, polarization, impedance matching, and attenuation constant, the as-synthesized holey γ-Fe₂O₃ exhibited promising microwave absorbability, with a maximum reflection loss of 52.4 dB and an effective bandwidth of 5.12 GHz at a thickness of 2.2 mm, overperforming most pure Fe-based materials.     

Crystal structure,Hirshfeld surface analysis,Thermal study and Conduction mechanism of [(C4H9)4P]3Bi2Cl9 compound

A novel tris (tetrabutylphosphonium) nonachlorodibismuthate (III) compound has been synthesized and characterized by a single‐crystal X‐ray diffraction, calorimetric, IR spectroscopy and electrical measurements. X‐ray diffraction analysis at room temperature reveals that the title compound belongs to the monoclinic system with P2₁/c space group. The unit cell dimensions are: a = 19.201(6) Å, b = 16.743 (5) Å, c = 22.396 (8) Å, β = 98.96 (2)° and Z = 4. The crystal structure was solved down to R equal to 0.035 for 5597 independent reflections. The crystal package is provided by electrostatic interactions and hydrogen bonds (C‐H ^…. Cl). Intermolecular interactions present in the grown single crystal were analyzed by Hirshfeld surface and 2‐dimensional fingerprint plot. The differential scanning calorimetry reveals one order–disorder phase transition at 400 ± 5 K.The electrical conductivity were carried out in the frequency range 200 Hz–1 MHz at various temperatures 343–413 K using impedance spectroscopy technique. The obtained results were analyzed by fitting the experimental data to an equivalent circuit model. The temperature dependence of alternating and direct current conductivities confirms the observed phase transition in the calorimetric study; they were described in terms of Arrhenius relation. AC conductivity measurements reveal that the conduction depends on both frequency and temperature, this agrees well with Correlated Barrier Hopping model in phase I and II.     

Co-Ni Electromagnetic Coupling in Hollow Mo2C/NC Sphere for Enhancing Electromagnetic Wave Absorbing Performance†

For enhancing the electromagnetic wave (EW) attenuation and adsorption, rational constructing and homogeneously distributing bimetallic electromagnetic coupling units in hollow structure is an effective way, but hard to achieve. Herein, a CoNi-doped hybrid zeolite imidazole framework was synthesized as precursor, which was further converted into a hollow CoNi-bimetallic doped molybdenum carbide sphere (H-CoNi@MoC/NC) through a two-step etching and calcination strategy. At the loading amount of 15 wt%, a strong absorption of minimum reflection loss (RL_min) of –60.05 dB at 7.2 GHz with the thickness of 3.1 mm and a wide effective adsorption bandwidth (EAB) of 3.52 GHz at the thickness of 2.5 mm were achieved, which was far beyond the reported MoC-based metallic hybrids. The crucial synergistic Co-Ni electromagnetic coupling effect in the composite was characterized, not only enhancing the dipolar/interfacial polarization, but also promoting the impedance matching, displaying the optimized EW absorbing performance.      

Plasma Characteristics of a Ni–Zn Ferrite Enhanced Internal-Type Inductively Coupled Plasma Source Operated at 2 and 13.56 MHz

Plasma and electrical characteristics of an internal-type inductively coupled plasma source with a Ni–Zn ferrite module installed near the antenna were investigated for different rf power frequencies of 2 and 13.56 MHz. Due to the lower heating of the Ni–Zn ferrite module on the antenna for the operation at 2 MHz compared to the operation at 13.56 MHz, higher plasma density and lower rf rms antenna voltage were resulted for the operation at 2 MHz in addition to more stable plasma characteristics. By the application of 500 W of rf power to the source, a high plasma density of 8 × 10¹¹ cm⁻³ which is about four times higher than that with 13.56 MHz could be obtained at the pressure of 10 mTorr Ar. When photoresist etch uniformity was measured for the operation with 2 MHz by etching photoresist on a 300 mm diameter substrate using 10 mTorr Ar/O₂ (9:1) mixture, the etch uniformity of about 5.5% could be obtained.     

A study on degradation of germanium coating on Kapton used for spacecraft sunshield application

Sunshield membranes made of germanium‐coated black polyimide (GBP) or Kapton are often used on the reflector/transmitter antenna of satellites for thermal control applications. However, the germanium top layer is prone to degrade during ground storage and implementation. Hence, vacuum/inert gas‐sealed packaging is required for storing the membranes, followed by a staggered fabrication schedule as the shelf‐life of the GBP is identified as only ~6 months. In the present study, microstructural, thermo‐optical, and electrical properties along with X‐ray photoelectron spectroscopy (XPS) studies for evaluating oxidation states of the as‐received and degraded GBP films have been investigated thoroughly. The radio frequency (RF) loss behavior of both the films has also been studied for S band (2.5–3.5 GHz), Ku band (10.5–14.5 GHz), and Ka band (30–35 GHz). Copyright © 2015 John Wiley & Sons, Ltd.