Binary sequences with low aperiodic autocorrelation for synchronization purposes

An evolutionary algorithm is used to find three sets of binary sequences of length 49-100 suitable for the synchronization of digital communication systems. Optimization of the sets are done by taking into consideration the type of preamble used in data frames and the phase-lock mechanism of the communication system. The preamble is assumed to be either a pseudonoise (PN) sequence or a sequence of 1s. There may or may not be phase ambiguity in detection. With this categorization, the first set of binary sequences is optimized with respect to aperiodic autocorrelation which corresponds to the random (PN) preamble without phase ambiguity case. The second and third sets are optimized with respect to a modified aperiodic autocorrelation for different figures of merit corresponding to the predetermined preamble (sequence of 1s) with and without phase ambiguity cases.     

Multilayer Graphene Broadband Terahertz Modulators with Flexible Substrate

Fabrication of terahertz modulators as simple devices with high modulation depth across a broad bandwidth is still very challenging. In this study, four different chemical vapor deposition grown multilayer graphene (MLG) modulators based on MLG/ionic liquid/gold sandwich structures have been investigated. Flexible substrates (PVC and PE) were chosen as host materials, and devices were fabricated with three different thicknesses. The resultant MLG devices can be operated at low voltages between 0 and 3.4 V providing nearly complete modulation between 0.2 and 1.5 THz with low insertion losses. Even with such low gate voltages, the devices have been doped significantly inducing 7–11-fold improvement in their sheet conductivities depending on device thickness. In addition, sheet conductivity has been improved more than three times as the graphene layer number increased from 30 to 100. With the demonstration of promising device performances, the proposed modulators can be potential candidates for applications in terahertz and related optoelectronic technologies.     

Spectrophotometric and electrochemical behavior of a novel azocalix[4]arene derivative as a highly selective chromogenic chemosensor for Cr

In this study, a novel azocalix[4]arene derivative, 5,11,17-tris[(1-naphtyl)azo]-25,26,27,28-tetrahydroxy-calix[4]arene (NAC4) bearing napthyl groups on the upper rim was synthesized. Its complexation behavior for alkali, alkaline-earth and various heavy metal ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Hg²⁺, Ni²⁺, Cd²⁺, Cu²⁺, Zn²⁺, Co²⁺, Fe²⁺, Cr³⁺, Ag⁺) was investigated by spectroscopic and voltammetric methods. This chemosensor exhibits decreased absorbance in the presence of Hg²⁺ and a unique absorbance quenching effect only for Cr³⁺. In addition, a new absorption band centered at 565 nm with the formation of the 1:1 host–guest complex (Cr³⁺-NAC4) was observed in the case of Cr³⁺, leading to an obvious color change from light orange to dark lilac. In voltammetric experiments, Cr³⁺ ions decreased voltammetric peaks of NAC4, whereas no significant changes occurred in the presence of the other metal ions. The Benesi–Hildebrand method was used to determine a logarithmic value of 3.76 for the association constant of the complex between Cr³⁺ and NAC4.     

Design and analysis of an integrated optical sensor for scanning force microscopies

In this paper, a novel probe for displacement sensing will be introduced. It is based on a conventional GaAs cantilever, integrated with a Bragg grating as a photo-elastic strain sensor. The deflection of the cantilever is measured directly from the intensity modulation of the reflected light. The principle of the experimental setup and the sensor, as well as the theoretical investigation of the force and displacement sensitivity of the probe, is presented. Finite-element method simulations were performed to get the optimum sensor design. Transfer matrix method simulation of the waveguide grating have been described in detail. In order to enhance the sensitivity, different types of grating structures are discussed. Using this new design, it should be possible to achieve sensitivities, defined as the fractional change in detected optical power per unit displacement of the cantilever, as high as 10/sup -4/ /spl Aring//sup -1/ of cantilever deflection.     

Guided growth of large-scale, horizontally aligned arrays of single-walled carbon nanotubes and their use in thin-film transistors

A convenient process for generating large-scale, horizontally aligned arrays of pristine, single-walled carbon nanotubes (SWNTs) is described. The approach uses guided growth, by chemical vapor deposition (CVD), of SWNTs on miscut single-crystal quartz substrates. Studies of the growth reveal important relationships between the density and alignment of the tubes, the CVD conditions, and the morphology of the quartz. Electrodes and dielectrics patterned on top of these arrays yield thin-film transistors that use the SWNTs as effective thin-film semiconductors. The ability to build high-performance devices of this type suggests significant promise for large-scale aligned arrays of SWNTs in electronics, sensors, and other applications.     

Molecular scale buckling mechanics in individual aligned single-wall carbon nanotubes on elastomeric substrates

We have studied the scaling of controlled nonlinear buckling processes in materials with dimensions in the molecular range (i.e., approximately 1 nm) through experimental and theoretical studies of buckling in individual single-wall carbon nanotubes on substrates of poly(dimethylsiloxane). The results show not only the ability to create and manipulate patterns of buckling at these molecular scales, but also, that analytical continuum mechanics theory can explain, quantitatively, all measurable aspects of this system. Inverse calculation applied to measurements of diameter-dependent buckling wavelengths yields accurate values of the Young's moduli of individual SWNTs. As an example of the value of this system beyond its use in this type of molecular scale metrology, we implement parallel arrays of buckled SWNTs as a class of mechanically stretchable conductor.     

Design and analysis of a 35 kVA medium frequency power transformer with the nanocrystalline core material

Medium frequency power transformers embedded into power electronics converters are frequently encountered in many applications such as electrical transportation and renewable energy systems and power supplies. Thus, researchers have been focused on soft magnetic materials such as amorphous and nanocrystalline materials to obtain smaller and more efficient transformer designs with the improvements on manufacturing technologies of the high frequency core materials. In this study, the transformer design methodology is proposed with the finite element analysis method, and a 35 kVA medium frequency transformer with nanocrystalline core material is designed. After the sizing stage, three-dimensional model of the transformer is created with finite element analysis software, and then co-simulations of this electromagnetic transformer model with a power electronics converter circuit are performed for practical operation conditions. Furthermore, thermal behavior of the prototype transformer is determined with the thermal coupling analysis, and temperature distribution of the prototype transformer is visualized with a thermal imaging camera. The transformer efficiency, exact equivalent circuit of the transformer and flux distributions in the transformer core are obtained from these simulation studies. In addition, the prototype of the designed transformer is produced and tested. The design conditions and simulation results are validated with experimental studies.