Characterization of low pressure chemical vapor deposited polymeric fluorinated carbon m (C:FX)n thin films with low dielectric constant

Thermal chemical vapor deposition of fluorinated carbon thin films in the polymeric form is described by hot filament decomposition of the gaseous C₃F₆O precursor. Decomposition at filament temperatures, ≤450 °C produces films in the ordered (CF₂)_2n polymeric chain structure as in a tetrafluoroethylene polymer. A composite of (CF₂)_2n chains structure and crosslinked m(C:F_x)_n phases are formed in films deposited at filament temperature ≥600 °C. Polymerization of :CF₂ radicals results in (CF₂)_2n chain structure and the crosslinked phase emerges from a separate process involving reaction among the CF₃, CFO and CF₃CO radicals and including CF₂. Substrate temperature affects both the C-to-F bonding configuration and the relative ratio of the composite phases. Dominant C–CF bonding structure in the low (<-5 °C) substrate temperature films is thermally less stable compared to the C–F structure, which dominates the crosslinked structure in films deposited at high (∼70 °C) substrate temperatures. Dielectric properties of the composite films are studied using the electrical equivalent model and a correlation with the C-to-F bond structure is established. High polymeric (CF₂)_2n phase determines the electrical impedance and the dielectric constant of the film, and the crosslinked phase imparts structural stability. PACS 81.15.Gh; 73.61.Ph; 77.84.Jd; 79.60.Fr     

Effect of annealing conditions on high temperature superconductivity in La2−xSrxCuO4

Abstract

Besides investigating a number of oxides of the series La_2?xM_xCuO₄ (M=Ba, Pb, Sr), we have, in this investigation emphasised on the studies of the effect of annealing conditions on superconducting properties of composition La_1.8Sr_0.2CuO₄. The effect is remarkable and is di'scuss'ed in the light of X-ray diffraction results. These samples, studied after a gap of two months, showed interesting changes which has been discussed in the present paper.     

Design of segmented cladding fiber for femtosecond laser pulse delivery at 1550 and 1064 nm wavelengths

Segmented cladding fiber (SCF) is capable of single mode operation over an extended range of wavelengths while maintaining large mode area. In this paper we report the design of an SCF with mode area as large as 1,825  $\upmu \hbox {m}^{2}$ , suitable for delivery of high peak power femtosecond laser pulses at 1550 and 1064 nm wavelengths. An SCF with such a large-mode area is a few-moded fiber and its design requires careful choice of design parameters to have robustness against mode-coupling effects and bend loss. In this paper we address these issues and report a design of an SCF showing near distortion-free propagation of 100-fs, 53-kW peak power pulses at 1550-nm wavelength with 1,825- $\upmu \hbox {m}^{2}$ mode area through fundamental mode. The same fiber can also deliver 250-fs, 15-kW peak power pulses at 1064-nm wavelength with 1,793- $\upmu \hbox {m}^{2}$ mode area. The fiber has been analyzed by using the radial effective-index method in conjunction with transfer matrix method and the pulse propagation has been studied by solving the nonlinear Schroedinger equation by split-step Fourier method. Such a fiber would find applications in multiphoton microscopy and in biomedical engineering.     

A routine gas chromatographic method for the analysis of chlorofluorocarbons in aerosol cans

Summary A gas chromatographic (GC) method for the routine analysis of fully halogenated chlorofluorocarbons (CFCs) in aerosol cans is described. The identification of CFCs by GC was found to be in full agreement with those by GC-mass-spectrometery. The method has been applied to the analysis of CFCs in 448 aerosol products. The most commonly used fully halogenated CFC propellants in aerosol cans were found to be CFC11, CFC12 and CFC114.