Volume 37, Issue 4
October 1991, pages 311-385
pp 311-319 October 1991
Micellar solutions of non-ionic surfactant triton X-100 (8% by weight) show phase separation at cloud pointTcp ∼ 335 K. This paper reports results of small angle neutron scattering (SANS) experiments from this solution as a function of temperature between 298 and 332 K. The range of wave-vector transferQ, covered in these experiments is from 0.02 to 0.15 Å−1. It is seen that as one approachesTcp, the neutron scattering cross section diverges in the region of lowQ (<0.06 Å−1) while it is independent of temperature in region of largeQ(>0.06 Å−1). We believe that the divergence of scattering at lowQ with an increase in temperature is because of changes in the structure factorS(Q) of the solution. The measured distributions have been analyzed using four different models for inter-micellar potential. The models used to calculate the structure factorS(Q) are (1) mean spherical approximation (MSA) with Yukawa tail for attractive potential, (2) MSA with an attractive square well potential, (3) random phase approximation (RPA) with an attractive square-well potential and (4) Sticky hard sphere model (attractive square-well potential with Percus-Yevick approximation). The strengths of the attractive potential required to fit the SANS data are (−6.6 to − 14.4)/kt for model (1), (− 6.6 to − 15.0)/kt for model (2), (− 3.8 to − 7.3)/kB T for model (3) and (−2 to −2.7)/kt for model (4). On the basis of reasonableness of the derived strength of the potential near the phase separation temperature and its relative temperature dependence, it is concluded that present data favour the Sticky hard sphere model.
pp 321-326 October 1991
A study of aggregation of sulphur particles in colloidal suspension of sulphur in water-methanol mixture using TEM and electron diffraction is reported. From the micrographs the aggregates formed have been found to be random and tenuous indicating a fractal structure. The electron diffraction patterns of the aggregates are used to study the mechanism of diffusion and reaction limited aggregation.
pp 327-332 October 1991
Using Wilson’s F-G matrix method a normal co-ordinate analysis of the spectral frequencies and form of the zero-wave vector vibrations of the high temperature superconductor, orthorhombic DyBa2Cu3O7 has been performed. The vibrational frequencies and the potential energy distribution of the 21 infrared-active and 15 Raman-active modes are presented. The potential constants employed here are presented and evaluated vibrational frequencies are compared with the available experimental values.
pp 333-344 October 1991
Silver-clad tapes of highTc Bi-Pb-Sr-Ca-Cu-O superconductors have been fabricated through the powder-in-tube method. The critical current density, Jc, of a thick tape was 534 A.cm−2 (77 K, 0 T). Subsequent rolling and sintering of the tapes led to a gradual decrease inJc, instead of the expected increase. This was caused by the microcracks developed in the core material by a rather drastic reduction during the rolling of the tapes. A modified and well controlled rolling technique, on the other hand, resulted in much improvedJc values. Repeated rolling and sintering resulted in a good grain alignment and no microcracks were observed. In the present studies, maximumJc of 1846 A.cm−2 (77 K, 0 T) and 2.43 × 104 A.cm−2 (4.2 K, 0 T) have been obtained Optimization of the processing and sintering parameters are expected to lead to still higherJc values.
pp 345-351 October 1991
The acoustic signals generated in solids due to interaction with pulsed laser beam is used to determine the ablation threshold of bulk polymer samples of teflon (polytetrafluoroethylene) and nylon under the irradiation from a Q-switched Nd:YAG laser at 1.06µm wavelength. A suitably designed piezoelectric transducer is employed for the detection of photoacoustic (PA) signals generated in this process. It has been observed that an abrupt increase in the amplitude of the PA signal occurs at the ablation threshold. Also there exist distinct values for the threshold corresponding to different mechanisms operative in producing damages like surface morphology, bond breaking and melting processes at different laser energy densities.
pp 353-356 October 1991
Three pairs of dyes which operate as efficient energy transfer dye lasers in ethanol have been investigated to understand the dominant energy transfer mechanisms taking place in them. Special attention is given to the experimental configuration which is expected to influence the energy transfer rate constants over an order of magnitude or two. Fluorescence lifetimes have also been studied using time correlated single photon counting technique. It is found that apart from the excitation source and the concentration of dyes, the experimental configuration has a crucial role to play in determining the dominant energy transfer mechanism.
pp 357-362 October 1991
We present exact numerical results for a symmetric layered medium (prism/Ag film/nonlinear dielectric/Ag film/prism) where the middle dielectric slab is assumed to have a saturation-type nonlinearity. We show bistable behaviour in the power dependence of the reflectivity ofp-polarized light under the condition when coupled surface modes are excited in the structure. Moreover, we study the effect of saturation on the bistable behaviour to show that multivalued character is inhibited by saturation effects. The field distributions corresponding to the minimum reflectivity states of the nonlinear structure are also presented.
pp 363-371 October 1991
A systematic theoretical investigation has been carried out to study the effect of viscosity of the pressure-transmitting fluids on the measurement of pressure up to 1 GPa using piston gauges. The fluid flow equation is modified to determine the fall rate (v) with pressure (P), taking the pressure-dependent viscosityη(P) and clearance between piston and cylinder [H(P)] terms into account. Above 0.4 GPa, the fall rate curve shows the tendency to be pressure independent. The near-constancy ofv withP can be avoided with less viscous fluid or by increasingH(P). Finally. the initial clearance obtained from the experimental data of fall rate, shows a weak dependence of pressure, although theoretically it is assumed to be independent of pressure. This weak pressure dependence is attributed to the effect of viscosity of the pressure transmitting fluid.
pp 373-385 October 1991
An automated linear laboratory EXAFS spectrometer of the Johansson type has been indigenously developed. Only two translational motions are required to achieve the necessary Rowland circle configuration for the (fixed) X-ray source, the dispersing and focusing bent crystal and the receiving slit. With the available crystals the spectral region from 5 to 25 keV can be scanned. The linear motions of the crystal and receiving slit including the detector assembly are achieved by employing software-controlled DC motors and utilizing optical encoders for position sensing. The appropriate rotation of the crystal is achieved by the geometry of the instrument. There is a facility to place the sample alternately in the path of the X-ray beam and out of the path to record both the incident X-ray intensityI0 and the transmitted intensityI employing the scintillation detector. An arrangement with a two-window proportional detector before the sample to measureI0 and the scintillation detector to recordI is also developed; in this case it is not necessary to oscillate the sample. Fast electronic circuits are employed to minimize counting errors. The instrument is user-friendly and it is operated through a menu-driven IBM compatible PC. EXAFS spectra of high resolution have been recorded using the spectrometer and employing the Si(111) reflecting planes; the X-ray source being a Rigaku 12 kW rotating anode with Cu target. We describe the spectrometer and discuss its performance with a few representative spectra.
Volume 93 | Issue 6
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