Amplitude Bounds on Periodic and Aperiodic Oscillations: Second-order Systems

The global behaviour of the control systems described by thepair of differential equations x{dot} = –f(x)±g(y)+p(t), y{dot} = –f(x)±g(y)+p(t) has been investigated. Here f(x), g(y), h(x) and k(y) are polynomialsof odd degree with leading coefficients positive and p(t) andq(t) are bounded functions of time. Sufficient conditions havebeen found under which the trajectories of the above systemmay eventually be confined in a subset of (x, y, t)-space, thusgiving bounds on the amplitude of periodic as well as aperiodicoscillations. Further bounds on the amplitude of oscillationshave been investigated by finding regions in (x,y,t)-space fromwhich all trajectories eventually leave and into which no trajectoriesenter. Thus sufficient conditions have been derived for theexistence of an annulus in which oscillatory behaviour may beconfined.     

Comparison of spectrum normalization techniques for univariate analysis of stainless steel by laser-induced breakdown spectroscopy

Analytical performance of six different spectrum normalization techniques, namely internal normalization, normalization with total light, normalization with background along with their three-point smoothing methods were studied using LIBS for quantification of Cr, Mn and Ni in stainless steel. Optimization of the number of laser shots per spectrum was carried out to obtain the best analytical results. Internal normalization technique model was used for selecting the best emission lines having sufficient intensity and spectral purity for Cr, Mn and Ni for comparison of different normalization techniques. For detailed evaluation of these normalization techniques, under optimized experimental conditions, three statistical parameters i.e., standard error of prediction, relative standard deviation and average bias, were compared for these techniques using the selected emission lines. Results show that the internal normalization technique produces the best analytical results followed by total light normalization. The smoothing of the raw spectra reduces the random error and produces better analytical results provided the peak under study has sufficient (≥7) number of pixels.     

Trace determination of uranium in fertilizer samples by total reflection X-ray fluorescence

Uranium is reported to be present in phosphate fertilizers. The recovery of uranium from the fertilizers is important because it can be used as fuel in nuclear reactors and also because of environmental concerns. For both these activities suitable method of uranium determinations at trace levels in these fertilizers are required. Studies have been initiated for such TXRF determination of uranium and the results are reported in the present paper. For TXRF determinations the fertilizer samples were processed with nitric acid and the uranium present in it was removed by solvent extraction using tri-n-butyl phosphate as the extractant. The organic phase containing uranium was equilibrated with 1.5% suprapure nitric acid to bring out uranium in aqueous phase. This aqueous phase was mixed with internal standard Y and the TXRF spectra were measured by depositing samples on float glass supports. The amounts of uranium in four fertilizer samples of Hungarian origin were determined by processing these TXRF spectra. Uranium concentrations in two fertilizer samples were found to be in the range of 4–6 μg/g, whereas two fertilizer samples did not show the presence of uranium. The precision of the TXRF determination of uranium was found to be better than 8% (1σ).     

Thermal carbonization of nanoporous silicon: Formation of carbon nanofibres without a metal catalyst

An interesting phenomenon is observed while carrying out thermal carbonization of porous silicon (PS) with an aim to arrest the natural surface degradation, and it is a burning issue for PS-based device applications. A tubular carbon structure has been observed on the PS surface. Raman, Fourier transform infrared spectroscopy (FTIR) and electron microscope studies, revealed that the tubular structure is nothing but amorphous carbon nanofibres sprouted within the pores in the absence of a metal catalyst, for which a suitable explanation is proposed.     

ChemInform Abstract: Heterogeneous Platinum Nucleation and Crystallization of a Heavy-Metal Fluoride Glass.

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