Full characterization of gain and noise boundaries for NF<Subscript>min</Subscript> or unity SWR<Subscript>out</Subscript> operation

For a receiver sub-block , Low Noise Amplifier is the first stage after the receiving antenna and as a key device, its amplification and noise figure (NF) affects the whole performance of the receiving part. In this paper we present a full graphical visualization in terms of gain, standing wave ratio (SWR) and noise for a GaAs HJ-FET transistor in two operating cases; (1) NF_min, (2) Unity \(\hbox {SWR}_{\mathrm{out}}\). The set of curves and contours presented will provide the designer with enough visual information about the transistor operating boundaries and will also visually assist on choosing the appropriate matching points for a wideband operation according to the desired (transducer gain \(\hbox {G}_{\mathrm{T}},\hbox {SWR}\)) for case (1) and (\(\hbox {G}_{\mathrm{T}},\hbox {SWR}_{\mathrm{in}},\hbox {NF}\)) for case (2). Numerical examples are given for each operating case and verified via a microwave circuit design software package to demonstrate the adequacy of the proposed graphical techniques . The results from simulations compare favourably with the visually estimated values.     

Graphene/polypropylene nanocomposites with improved thermal and mechanical properties

Small amount of large surface area graphene (G) is expected to significantly alter functional properties of polymers. The property enhancement is a function of degree of exfoliation and dispersion of G as well as its compatibility with base polymer. However, nonpolar nature of polyolefins such as polypropylene (PP) restricts homogeneous dispersion of G, leading to significant agglomeration and properties reduction. In this work, two compatibilizers, poly (ethylene-co-butyl acrylate) (EBA) (new compatibilizer) and PP-grafted-maleic anhydride (MA-PP) (conventional compatibilizer) were compared to enhance the dispersion efficacy of G in PP. The EBA-compatibilized nanocomposites exhibited 44% increase in the Young's modulus compared to 32% increment in MA-PP-compatibilized nanocomposites. Higher elongation at break for EBA-compatibilized nanocomposites is attributed to lower degree of crystallinity in these nanocomposites. On the other hand, EBA-compatibilized nanocomposites showed significantly improved thermal stability compared to MA-PP-compatibilized nanocomposites. The results indicate that EBA may act as a potential compatibilizer for G/PP nanocomposites.     

Creep and durability of sand-coated glass FRP bars in concrete elements under freeze/thaw cycling and sustained loads

This paper presents the test results of 21 concrete beams (1800 × 130 × 180 mm) reinforced with sand-coated glass FRP composite bars. The individual and coupled effects of freeze/thaw cycles and sustained bending stresses on the long-term behaviour of concrete beams reinforced with GFRP composite bars were investigated. The beams were exposed to 100, 200 and 360 freeze/thaw cycles (−20 °C to +20 °C) either in an unstressed state or loaded in bending to cause a tensile stress equals to 27% of the ultimate tensile strength of the GFRP bar. The conditioned beams were tested up to failure in a four-point bending set-up over a clear span of 1500 mm. The test results showed that the single or coupled action of freeze/thaw cycles and sustained bending stresses has no significant effect on the behaviour of the tested beams in terms of deflections, strains, and ultimate capacity. It was also concluded that the long-term deflections and the creep strain limits specified by ACI 440.1R-06 are conservative.     

加劲不锈钢管混凝土柱的非线性性能

该文考察了加劲的细柔高强不锈方钢管及矩形钢管混凝土柱的非线性性能。加劲的细柔钢管的整体高厚比（D/t）都介于60～160之间。混凝土的强度介于普通混凝土和高强混凝土之间。考察的对象主要是轴向受压的短柱。建立了非线性有限元模型来研究加劲的钢管混凝土柱的受力性能。进行了参数分析来考察横截面几何尺寸、混凝土强度等参数对柱子性能及承载力的影响。还将加劲钢管混凝土柱的结果与无加劲钢管混凝土柱的结果进行了对比。     

Green Biogenic Synthesis of Silver Nanoparticles Using Aqueous Extract of Moringa Oleifera: Access to a Powerful Antimicrobial,Anticancer, Pesticidal and Catalytic Agents

In this study, we look into the biogenic synthesis of (AgNPs) utilizing a simple and environmentally friendly method based on an aqueous extract of Moringa Oleifera (MO). The synthesized MOAgNPs were characterized using a UV–Visible spectrophotometry, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra and TEM image which confirmed the spherical shape of MOAgNPs with particle size range of 5–50 nm with an average particle size of 38.7 nm. Significantly, the prepared MOAgNPs showed high pesticidal activity towards Spodoptera littoralis. MOAgNPs also exhibited strong antibacterial activities against Gram-positive and Gram-negative bacteria. The prepared MOAgNPs were screened for their cytotoxic effect against (HCT-116), (HepG-2) and (MCF-7) carcinoma cell lines. Finally, the synthesized MOAgNPs have been used as a catalyst for the reduction of 2,4-Dinitrophenol using NaBH₄ to 2,4-Diaminophenol. Taken together, the outstanding catalytic and biological activities of the synthesized MOAgNPs entitled them for applications as catalyst, pesticidal, antibacterial and anticancer agents in medical applications.

     

Structural performance of cold-formed high strength stainless steel columns

This paper describes an accurate finite element model for the structural performance of cold-formed high strength stainless steel columns. The finite element analysis was conducted on duplex stainless steel columns having square and rectangular hollow sections. The columns were compressed between fixed ends at different column lengths. The effects of initial local and overall geometric imperfections have been taken into consideration in the finite element model. The material nonlinearity of the flat and corner portions of the high strength stainless steel sections were carefully incorporated in the model. The column strengths and failure modes as well as the load-shortening curves of the columns were obtained using the finite element model. Furthermore, the effect of residual stresses in the columns was studied. The nonlinear finite element model was verified against experimental results. An extensive parametric study was carried out using the verified finite element model to study the effects of cross-section geometries on the strength and behaviour of cold-formed high strength stainless steel columns. The column strengths predicted from the parametric study were compared with the design strengths calculated using the American Specification, Australian/New Zealand Standard and European Code for cold-formed stainless steel structures. The results of the parametric study showed that the design rules specified in the American, Australian/New Zealand and European specifications are generally conservative for cold-formed high strength stainless steel square and rectangular hollow section columns, but unconservative for some of the short columns.     

Oxidation of Maize and Rice Starches Using Sodium Chlorite Along with Formaldehyde

Maize and rice starches were independently oxidized with sodium chlorite in absence and presence of formaldehyde. The treatment was carried out under different conditions including sodium chlorite and formaldehyde concentrations and duration. Since the treatment involves degradation of starch via oxidation, the treated starch samples were monitored for carboxyl and carbonyl contents as well as apparent viscosity at different rates of shear. Results obtained indicated that with both starches the percent chlorite decomposed increases as the formaldehyde concentration increases within the range studied (0.0–0.3 g/100 starch). The same holds true for the duration of oxidation (15–90min). The apparent viscosity of starch before and after oxidation decreases as the rate of shear increases. Maize starch is more susceptible to oxidation than rice starch. When applied as sizing agents for cotton textiles oxidized starches derived from maize and rice starch display better performance than the unoxidized starch but with the superiority of the sizeability and desizeability of oxidized maize starch.     

High-Temperature Deformation and Ductility of a Modified 5083 Al Alloy

The high-temperature deformation of a 5.5% Mg and 0.6% Ca modified 5083 aluminum alloy was investigated in the temperature range from 573 to 723 K at strain rates in the range of 10⁻⁵-10⁻¹ s⁻¹. Ca was added to form an insoluble second phase in the range of temperatures tested to improve the high-temperature characteristics of this alloy. It was shown that the deformation behavior of the alloy could be divided into two regions with stress exponent, n of 3.5 and 13 at low and high strain rates, respectively. The apparent activation energy determined in both regions suggested that the deformation process is diffusion controlled in both regions. The slightly high value of n at the low-strain rate region (viscous glide) was attributed to the presence of threshold stress. The values of threshold stress showed an exponential increase with decreasing temperature and a dependence with an energy term Qo = 16.5 kJ mol⁻¹. Analysis of creep data in terms of threshold stress and using diffusivity of Mg in normalizing the strain rates, revealed two types of deformation behavior. At high values of normalized strain rate a high value of stress exponent of n = 10 is observed, and the exponential law creep takes place. At low normalized strain rates ≤10⁻⁹, the n value is 3 and the true activation energy, Q, is equal to 123 kJ mol⁻¹ suggesting viscous glide of dislocations as rate-controlling mechanism. Enhanced ductility has been observed in the region of viscous-glide controlled deformation as a result of high strain-rate sensitivity.     

Dehydration of 2-(2-Arylethyl)-2-hydroxy-4-oxopentanoic Acids and their hydrazones to form heterocycles

Dehydration of 2-(2-arylethyl)-2-hydroxy-4-oxopentanoic acids 1 with hydrochloric acid/acetic acid, affords 3-(2-arylethyl)-5-hydroxy-5-methyl-2(5H)-furanones 4 . Compounds of type 1 and 4 represent suitable precursors for the formation of pyridazin-3-ones 7 as they smoothly react with hydrazine. A new series of s-triazolo[4,3-b]pyridazin-3-ones 12 and tetrazolo[1,5-b]pyridazines 15 are obtained from the 3-chloropyridazines 11 upon treatment with semicarbazide and sodium azide, respectively. Reaction of 11 with phenyl- acetyl-hydrazine provides 3-benzyl-6-phenyl-8-(2-phenyl-ethyl)-s-triazolo[4,3-b]pyridazine 13 via dehydrative cyclization of the intermediate 14 which was clarified to exhibit tautomeric equilibria between enol–hydrazine form A and keto–hydrazine form B by means of ¹H and ¹³C NMR spectroscopy. Attempts to synthesize 3-alloxy-pyridazines 18 by reacting 11 with sodium alloxide afford N-allyl compounds 17 .