Excitation threshold for subharmonic generation from contrast microbubbles

Six models of contrast microbubbles are investigated to determine the excitation threshold for subharmonic generation. The models are applied to a commercial contrast agent; its characteristic parameters according to each model are determined using experimentally measured ultrasound attenuation. In contrast to the classical perturbative result, the minimum threshold for subharmonic generation is not always predicted at excitation with twice the resonance frequency; instead it occurs over a range of frequencies from resonance to twice the resonance frequency. The quantitative variation of the threshold with frequency depends on the model and the bubble radius. All models are transformed into a common interfacial rheological form, where the encapsulation is represented by two radius dependent surface properties-effective surface tension and surface dilatational viscosity. Variation of the effective surface tension with radius, specifically having an upper limit (resulting from strain softening or rupture of the encapsulation during expansion), plays a critical role. Without the upper limit, the predicted threshold is extremely large, especially near the resonance frequency. Having a lower limit on surface tension (e.g., zero surface tension in the buckled state) increases the threshold value at twice the resonance frequency, in some cases shifting the minimum threshold toward resonance.     

Amperometric CO<Subscript>2</Subscript> gas sensor based on interconnected web-like nanoparticles of La<Subscript>2</Subscript>O<Subscript>3</Subscript> synthesized by ultrasonic spray pyrolysis

Thin films of La₂O₃ were deposited onto glass substrates by ultrasonic spray pyrolysis. Their structural and morphological properties were characterized by X-ray diffraction, Fourier transform Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photo-electron spectroscopy, Brunauer-Emmett-Teller and optical absorption techniques. The sensor displays superior CO₂ gas sensing performance at a low operating temperature of 498 K. The signal change on exposure to 300 ppm of CO₂ is about 75%, and the signal only drops to 91% after 30 days of operation.

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Graphical abstract Schematic diagram of the CO₂ gas sensing mechanism of an interconnected web-like La₂O₃ nanostructure in presence of 300 ppm of CO₂ gas and at an operating temperature of 498 K.

     

Effect of montmorillonite filler on structural and electrical properties of polymer nanocomposite electrolytes

Polymer–clay nanocomposites consisting of polymer (polyethylene oxide) and NaI as salt with different concentration of organically modified Na⁺-montmorillonite (DMMT) have been fabricated and characterized. X-ray diffraction analysis shows that the polymer–salt complexes have been intercalated into the nanometric silicate layers of DMMT. Fourier transform infrared analysis shows that the polymer structure in the clay interlayer is similar to that of the polymer–salt complexes, and there is a strong interaction between the polymer–salt complexes and clay layers. A study of surface morphology using scanning electron microscopy reveals that microstructure of composites is affected by clay addition. Complex impedance analysis was used to calculate the bulk resistance of the composites. An enhancement in the conductivity of about one order of magnitude has been observed on 5% clay addition compared to that of the polymer–salt complexes, and it decreases monotonically for higher clay concentration. The effect of clay concentration on the structural and physical properties of polymer nanocomposites is well correlated.     

A new algorithm for nonlinear least squares

A new algorithm for the nonlinear least-squares problems is introduced and illustrated in this article. It is shown that the new algorithm is relatively more efficient compared to the other algorithms in current use and it works for problems where other methods fail. The new method is illustrated by solving a number of classical test problems. In light of the present method, improvements for some other methods in current use are also suggested in this article. Bibliography: 22 titles. Published inZapiski Nauchnykh Seminarov POMI, Vol. 207, pp. 143–157, 1993.     

The distribution and the exact percentage points for Wilks'L mvc criterion

Summary Wilks'L _mvc is the likelihood ratio criterion for testing the hypothesis that the mean values are equal, the variances are equal and the covariances are equal, in ap-variate normal population. In this article the exact null distribution as well as the exact percentage points are given for the first time. The distribution is obtained for the most general cases and the inverse tables, namely, the values ofu for given values ofF(u) are computed for the values ofF(u)=0.01, 0.02, 0.05 and for the various values ofn andp whereF(u) is the exact distribution function of the test statistic,n=N−1 andN is the sample size. The exact tables are given forp=2, 3, 4, 5, 6, 7, 8, 9.     

Compound glass preparation for optical fibers using Brazilian natural quartz

We present studies related to the investigation of Brazilian natural quartz as a basic raw material for production of optical multicomponent glasses for optical fibers. The work includes characterization and selection of quartz, preparation, the melting process and glass characterization.     

One-pot Amberlite IR-120 Catalysed Synthesis of Glycosyl Dihydropyridones

Summary. The one-pot reaction of β-glycosyl amino acids with β-ketoesters in the presence of Amberlite IR-120 resin and 4 ? molecular sieve in refluxing toluene resulted in the respective dihydropyridones in fair to good yields. Present address: Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221005, India.