Comprehensive diagnosis of growth rates of the ablative Rayleigh-Taylor instability

The growth rate of the ablative Rayleigh-Taylor instability is approximated by gamma = square root[kg/(1 + kL)] - beta km/rho(a), where k is the perturbation wave number, g the gravity, L the density scale length, m the mass ablation rate, and rho(a) the peak target density. The coefficient beta was evaluated for the first time by measuring all quantities of this formula except for L, which was taken from the simulation. Although the experimental value of beta = 1.2+/-0.7 at short perturbation wavelengths is in reasonably good agreement with the theoretical prediction of beta = 1.7, it is found to be larger than the prediction at long wavelengths.     

Silver nanoparticle size–dependent measurement of quantum efficiency of Rhodamine 6G

The plasmonic absorption band of silver nanoparticles in the visible range of electromagnetic spectrum has been successfully exploited to alter the emission characteristics of the Rhodamine 6G dye molecule. The influence of the nanoparticle size on the fluorescence quantum yield of Rhodamine 6G is interrogated via steady state fluorescence as well as dual beam thermal lens technique. The potential of the thermal lens technique that probe nonradiative path in contrast to radiative path exhibited in the fluorescence spectra as a complementary method to measure the quantum yield of a dye molecule is exploited. Analysis of the results clearly indicates that the particle size and the spectral overlap between the emission spectra of Rhodamine 6G, and absorption spectra of the silver nanoparticles determine the quantum yield value of dye–nanoparticle mixture.     

Magnetic and Mössbauer study of ε-Fe<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>N (2 < <Emphasis Type="Italic">y</Emphasis> < 3) nanoparticles

ε-Fe _y N (2 < y < 3) nanoparticles were prepared by the nitridation of spindle type hematite nanoparticles using ammonia at a temperature of 550 °C. Chemical analysis for the elements brings out the composition as y = 2.56. These nanoparticles show ferromagnetic behaviour even though it has higher nitrogen content than the stoichiometric Fe₃N. The quenching of the particles from higher temperature and the higher nitrogen content introduces disorder in the system unlike the ordered ε-Fe₃N. Mössbauer spectroscopy at lower temperature clearly shows the existence of two sextets corresponds to the two types of iron environments. The presence of more than one site excludes perfect ordering. This is consistent with the peak widths which result from variations in the distributions of next nearest interstitial neighbours. Existence of two different Debye temperatures for two iron sites shows that it varies according to the number and type of neighbouring atoms.     

Mössbauer studies and magnetic properties of spinel lead ferrite

In the present communication, we have reported the synthesis of nanocrystalline lead ferrite (PbFe₂O₄) by citrate mediated autocombustion method. X-ray diffraction pattern reveals the single phase formation in cubic (spinel) structure. The particle size and the surface morphology of the samples are characterized by TEM and SEM analysis. Magnetic studies are carried out using vibrating sample magnetometer (VSM) shows a very high coercive field for the material. Mössbauer studies were performed to investigate the local symmetry i.e. Fe is in octahedral/tetrahedral site and the charge states of Fe ions.     

Spectroscopic and thermal studies on the decomposition of l,3,5-triamino-2,4,6-trinitrobenzene (TATB)

The kinetics of the thermal decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) in condensed state has been investigated by high temperature infrared spectroscopy (IR) and thermogravimetry (TG) in conjunction with pyrolysis gas analysis, differential thermal analysis (DTA) and hot stage microscopy. The decomposition proceeds in two main stages under isothermal conditions and the initial stage involving about 24% loss in weight obeys Avrami-Erofe'ev equation (n= 1), and is governed by an activation energy (E) of 150.58 kJ·mol^–1 and log(A in s^–1) 12.06. The second stage corresponding to 24 to 90% loss in weight gave best fit for Avrami-Erofe'ev equation,n=2, withE=239.56 kJ·mol^–1 and log(A in s^–1) 19.88 by isothermal TG. The effect of additives, on the initial thermolysis of TATB has also been studied. Evolved gas analysis by IR showed that NH₃, CO₂, NO₂, HCN and H₂O are produced in the initial stage of decomposition. The decomposition in KBr matrix in the temperature range 272 to 311.5°C shows relative preferential loss in the -NH₂ to -NO₂ band intensity which indicates that the rupture of C-NH₂ bond, weakened also by the interaction of the NH₂ with the neighbouring NO₂ group, appears to be the primary step in the thermolysis of TATB.     

Novel tricomponent membranes containing poly(ethylene glycol)/poly(pentamethylcyclopentasiloxane)/poly(dimethylsiloxane) domains

The synthesis and characterization of novel tricomponent networks consisting of well‐defined poly(ethylene glycol) (PEG) and poly(dimethylsiloxane) (PDMS) strands crosslinked and reinforced by poly(pentamethylcyclopentasiloxane) (PD₅) domains are described. Network synthesis occurred by dissolving α,ω‐diallyl PEG and α,ω‐divinyl PDMS prepolymers in a common solvent (toluene), introducing a stoichiometric excess of pentamethylcyclopentasiloxane (D₅H) to the charge, inducing the cohydrosilation of the prepolymers by Karstedt's catalyst and completing network formation by the addition of water. Water in the presence of the Pt‐based catalyst oxidizes the SiH groups of D₅H to SiOH functions that immediately polycondense and bring about crosslinking. The progress of cohydrosilation and polycondensation was followed by monitoring the disappearance of the SiH and SiOH functions by Fourier transform infrared spectroscopy. Because cohydrosilation and polycondensation are essentially quantitative, overall network composition can be controlled by calculating the stoichiometry of the three network constituents. The very low quantities of extractable (sol) fractions corroborate efficient crosslinking. The networks swell in both water and hexanes. Differential scanning calorimetry showed three thermal transitions assigned, respectively, to PEG (melting temperature: 46–60 °C depending on composition), PDMS [glass‐transition temperature (T_g) = ～?121 °C], and PD₅ (T_g = ～?159 °C) and indicated a phase‐separated tricomponent nanoarchitecture. The low T_g of the PD₅ phase is unprecedented. The strength and elongation of PEG/PD₅/PDMS networks can be controlled by overall network composition. The synthesis of networks exhibiting sufficient mechanical properties (tensile stress: 2–5 MPa, elongation: 100–800%) for various possible applications has been demonstrated. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3093–3102, 2002