Transformation ofn-hexane over microporous molecular sieves: Studies over MeAPO-11 and MeAPSO-11

CoAPO-11, MgAPO-11, ZnAPO-11, COAPSO-11, ZnAPSO-11 and MgAPSO-11 samples were synthesized by hydrothermal methods and characterized. The samples were loaded with 0.5 wt.% Pt and tested for their catalytic activities in the isomerization ofn-hexane. All the samples exhibited catalytic activities and selectivities for isomerization comparable to that of SAPO-11. The selectivities for the different monomethyl and dimethyl isomers are rationalized on the basis of shape selectivity and acidities of different samples.     

Thin film processing of axially modified phthalocyanine derivatives

A new class of hydroxyl terminated silicon phthalocyanine (SiPc) derivatives was prepared by capping dihydroxy SiPc with a variety of compounds containing two or more hydroxyl groups. The unique feature of these polyhydroxy SiPc derivatives is their function as crosslinkers that allows their processability into polyurethane films on reacting with isophorone diisocyanate. The films possess high impact resistance, thermostability at 125°C for over 10 h, and low absorption in the visible region. Solid polymeric films incorporating over 20% (w/w) of Pc rings were prepared without phase segregation. The kinetic parameters of polymerization, such as reaction conversion versus time and temperature, heat of reaction (ΔH), activation energy (ΔE), and reaction order (n) were investigated. Some of the polyhydroxy SiPc derivatives exhibit very high solubility in common organic solvents and possess adequate optical transparency for use as nonlinear optical materials in vision protection against laser light. © 1996 John Wiley & Sons, Inc.     

Electromagnetic signals of quark gluon plasma

Successive equilibration of quark degrees of freedom and its effects on electromagnetic signals of quark gluon plasma are discussed. The effects of the variation of vector meson masses and decay widths on photon production from hot strongly interacting matter formed after Pb + Pb and S + Au collisions at CERN SPS energies are considered. It has been shown that the present photon spectra measured by WA80 and WA98 Collaborations can not distinguish between the formation of quark matter and hadronic matter in the initial state.     

Incomplete fusion reactions in 16O+165Ho

Excitation functions for evaporation residues of the system ¹⁶O + ¹⁶⁵Ho have been measured up to 100 MeV. Recoil range distribution of long lived reaction products were measured at ¹⁶O beam energy of 100 MeV. Detailed Monte Carlo simulation of recoil range distributions of products were performed with the help of PACE2 code, in order to extract the contributions of incomplete fusion in the individual channels. The results clearly show the incomplete fusion contributions in the tantalum and thulium products. This is confirmed by the predictions of breakup fusion model of the incomplete fusion.     

New physics with beauty

We review the effects of new physics on CP asymmetries and decays of B mesons. Possible sources and corresponding signals for new physics are studied briefly. We discuss how the decay mode b → s ℓℓ (and B → K*ℓℓ) will enable us to understand the nature of new physics. We also examine the possibility of truly clean signature of new physics — a signature based on observables alone and without hadronic uncertainties.     

Structural, thermal and deformation-induced point defects in PdIn

Perturbed angular correlation of gamma rays (PAC) was applied to measure concentrations of point defects in quenched and milled intermetallic compounds. Two systems with high ordering energies were studied, PdIn and NiAl, with principal results obtained for PdIn. The defects, lattice vacancies and antisite atoms, were detected by quadrupole interactions induced at nuclei of nearby In/Cd probe atoms. Measurements on annealed, quenched and milled samples are compared to identify quadrupole interaction signals with specific configurations of defects near probe atoms. Müller and Hahn had previously identified signals with a Pd-vacancy in the first near-neighbor shell of the In probe and a Pd-antisite atom in the second shell. Six additional signals are identified in this work, including two distinct configurations of two Pd-vacancies in the first shell, a Pd-vacancy in the fourth shell, an In-vacancy in the second or third shell, and an In-antisite atom in the first shell. After quenching, observation of both Pd and In-vacancies after quenching from 1100-1500 K demonstrates that the high temperature equilibrium defect is the Schottky vacancy pair. An activation energy of 0.91(18) eV was determined for formation of a Schottky pair. After milling, the site fraction of probes having one Pd-vacancy in the first shell increased rapidly and saturated at a value of about 20%. This implies a Pd-vacancy concentration of about 3.3 at.%, with a corresponding enthalpy of the order of 3 kJ?mol^?1, making a significant fraction of the excess enthalpy in the milled intermetallic. After milling NiAl, a Ni-vacancy concentration of order 1–10 at.% was observed. A comparison is made of defect combinations observed after mechanical milling of PdIn and NiAl by PAC and of other intermetallics by Mössbauer spectroscopy. Dominant defects produced by milling are the Schottky pair for PdIn, triple defect for NiAl and FeRh and antisite atom pair for FeAl.     

First principles study of electronic structure and optical properties of CaTiO3

The electronic-energy band structure, site and angular momentum decomposed density of states (DOS) and charge-density contours of perovskite CaTiO ₃ are calculated by the first principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows an indirect (R-Γ) band gap of 1.5 eV. The total DOS as well as the partial density of states (PDOS) are compared with the experimental photoemission spectra. The calculated DOS are in reasonable agreement with the experimental energy spectra and the features in the spectra are interpreted by a comparison of the spectra with the PDOS. The origin of the various experimentally observed bands have been explained. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ca and TiO ₃ is mainly ionic and that the TiO ₃ entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of CaTiO ₃ . The real and imaginary parts of the dielectric function and hence the optical constants such as refractive index and extinction coefficient are calculated. The calculated spectra are compared with the experimental results for CaTiO ₃ and are found to be in good agreement with the experimental results. The effective number of electrons per unit cell participating in the interband transitions are calculated. The role of band structure calculation as regards the optical properties of CaTiO ₃ is discussed. Received 1 February 2000 and Received in final form 21 July 2000     

Positron impact ionisation of He ion

Triple differential cross-sections (TDCS) of a hydrogenic (He⁺) ion has been studied by positron impact using coplaner geometry for both symmetric and asymmetric kinematics in the intermediate and medium high incident energy region. TDCS has also been studied of He⁺ ion by electron impact for symmetric kinematics taking account of the electron exchange effect. The final state wavefunction is chosen as the correlated 3-body Coulomb wavefunction satisfying the exact asymptotic boundary condition. The long range Coulomb interaction in the initial channel between the ionic target and the projectile has also been taken into account properly. For positron impact, the collision is found to be almost recoilless at lower incident energies, in contrast to the strong recoil peak noted in the case of electron impact ionisation. For electron impact, the exchange effect is found to be significantly high for equal energy sharing in the final channel. Received 10 July 1999 and Received in final form 7 December 1999     

Thermo-Mechanical Analysis of Single-Disc Edge-Cooled Silicon Nitride Millimeter Wave Window for 200 kW CW Gyrotrons

This paper investigates the possibility of using Silicon Nitride Composite (Kyocera SN-287) as single-disc, edge-cooled window for gyrotrons operating below 200 kW CW in the frequency range 28-42 GHz. Rotationally symmetric TE_0n, and TEM₀₀ Gaussian modes of rf transmission through the window have been considered. Analysis performed using a one dimensional (1D) finite difference (FD) code reveals that thermal stresses developed due to non-uniform temperature distribution on the disc surface are well within manageable limits for a 200 kW 42 GHz gyrotron proposed for some ECRH applications. For industrial gyrotrons, for microwave material processing and operating at a maximum power level of 100 kW CW, Si₃N₄ windows may be a cost effective replacement for sapphire windows. It is found that a TE₀₂ profile results in lower peak temperature at the window disc in comparison to a Gaussian beam profile and allows the use of smaller discs.