Transport properties of a and c axis oriented (Y1?xCax)Ba2Cu3Oy thin films

Anisotropy and Hall effect measurements have been performed in calcium-doped, i.e., overdoped YBa₂Cu₃O_y ((Y_1−xCa_x)Ba₂Cu₃O_y) thin films witha andc axis orientations. In highly overdoped films (x=0.4), the anisotropy of the normal resistivity decreases and a drastic change in Hall conductivity in the mixed state is observed. The change in Hall conductivity in the overdoped region is consistent with recent experimental results for La_2−xSr_xCuO₄ films and seems to be common in highT _c superconductors.     

Computer simulation of brownian motion of a polyelectrolyte molecule (charged bead-spring model)

A Langevin equation of motion for a charged bead-spring statistical chain is written in difference form and the relaxation and equilibrium behavior of the chain is studied by computer simulation. Results are presented for the behavior of end-to-end length h, principal axes of the polymer ellipsoid L₁, L₂, L₃, and chain contour length c in terms of their averages, root mean square values, root mean square fluctuations, orientations, and relaxation strengths and times. The simulation was made with various sets of parameters, bead number N, charge on the bead q, and radius of ion atmosphere around the bead k^?1. It is found that 〈h²〉^1/2 and 〈L₁²〉^1/2 increase more strongly with increasing q and decreasing κ than 〈L₂²〉^1/2, 〈L₃²〉^1/2, and 〈c₁²〉^1/2, indicating that the chain is expanded in three dimensions and at the same time is extended along the end-to-end direction. The relaxation time τ_rot of rotation of the end-to-end vector, which is proportional to N² at q = 0, increases with increasing q and tends to be proportional to N³ for an extended chain, while the relaxation time τ_conf of the magnitude of h is almost independent of q and is always proportional to N². It is concluded that the extended chain possesses a well-defined end-to-end axis and the chain rotates as a whole with a relaxation time τ_rot which is much longer than τ_conf. The complex viscosity of the chain is calculated from the Fourier transform of the time–correlation function of momentum flux and is found to have a frequency spectrum similar to that observed for aqueous solutions of poly(acrylic acid). The dominant mode appearing in the low-frequency range is evidenced to arise from the rotation of the extended chain.     

Dynamic viscosity of dilute solutions of poly(L-glutamic acid)

The dynamic viscosity η′ of a dilute solution of poly(L-glutamic acid) (DP = 1370) in a mixed solvent made up of aqueous 0.2M NaCl and dioxane (2:1 by volume) is measured over the pH range 4.2–10 and in the frequency range 2–500 kHz. The frequency dependence of η′ in the helix region (low pH) is interpreted in terms of a model molecule consisting of n rigid helical segments connected by universal joints. The steady-flow viscosity, relaxation time, and high-frequency limiting viscosity at pH 4.75 (helical content 80%) are well explained by this model with n = 5. This value of n is consistent with that estimated from the nucleation parameter σ = 1.4 × 10^?3 obtained from the relation between reduced steady-flow viscosity and helical content. The high-frequency values of η′ in the coil region (high pH) are fitted by Peterlin's theory. The internal viscosity seems to arise in part from the polyelectrolytic character of the molecule. An additional relaxation at low frequencies in the coil region is ascribed to rotation of molecules elongated by the electrostatic interaction. The lower value of reduced steady-flow viscosity in the coil region in the mixed solvent compared with that in water is interpreted in terms of the lower degree of effective ionization and the selective solvation of water by the polypeptide. No anomaly is observed in the helix–coil transition region, indicating that the relaxation time for helix–coil equilibrium is less than 10^?6sec.     

The simultaneous determination of the kinetics and thermodynamics of Cu(OH)2 decomposition by means of TG and DSC

The kinetics and thermodynamics of the thermal decomposition of powdered copper(II) hydroxide to copper(II) oxide and water were studied by means of TG-DSC recorded simultaneously at constant temperatures as well as TG at linearly increasing temperature. The thermal decomposition is regulated by one of random nucleation and subsequent growth mechanisms (an Avrami-Erofeyev law). The activation energy for the decomposition was considerably larger than the corresponding enthalpy change. The kinetic parameters estimated isothermally are smaller than those estimated dynamically. The cause of these differences is discussed.     

Optical testing of star sensor (I): Defocus spot measuring technique for ground-based test

We developed a technique for measuring the intensity distribution of a point image in sub-pixel resolution even after assembly of a lens and an image sensor, which is especially suitable for defocus adjustment of a star sensor used to determine the attitude of a spacecraft. The star sensor optical system is usually aligned in a defocused position to calculate the luminance centroid of a star image in sub-pixel resolution. In the measurement technique, the point image is carefully scanned around a certain pixel on the image sensor, and the intensity change of the pixel is recorded during the scanning. We applied this technique to the alignment of our star sensor’s optical system, and we were able to suppress the spot size in a range between 1.2 and 2.2 pixels at 1/e² diameter at a certain defocus position, making it appropriate for luminance centroid calculation.     

Hydration structures of bromides on cationic micelles

The peripheral structures of bromides on dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (HTAB) micelles have been studied by X-ray absorption fine structure (XAFS) at the Br-K edge. The XAFS spectra indicate that water is a dominant scattering group for Br- even in these micellar solutions. However, the oscillation intensity decreases with increasing micellar concentration, suggesting that the bromides are dehydrated to some extent when they are bound to the micelles. A XAFS analysis routine gives unusually short Br-O (water) distances and is inapplicable to the present systems. This comes from the structure of the first coordination shell, in which two or more scattering paths are involved. The second scattering group is obviously the head group of the surfactants forming the micelles. The detailed analysis has allowed us to estimate the hydration number of the bromides bound on the DTAB and HTAB micelles (N = ca. 4.2). The assumption that all of the bromides form direct ion-associates with the head groups causes the contradiction to the results of the XAFS analyses. This strongly implies that some of the bromides partitioned into the micelle are completely hydrated as far as their first coordination shell is concerned. Assuming that the maximum hydration number of the bromides bound to the head groups of the micelle is three, 40% of the bromides partitioned into the micelle are completely hydrated.     

Theoretical study of reaction of trifluoromethyl radical with hydroxyl and hydrogen radicals

Ab initio molecular orbital theory and density functional theory calculations have been carried out on the reactions of the trifluoromethyl radical with the hydroxyl and the hydrogen radicals. These reactions are key reactions that underlie a new fire extinguishing mechanism of non-bromine-containing halon replacements. The activation energies calculated by the MP2 and QCISD methods are in good agreement with the experimental values. The B3LYP, as well as MP2 and QCISD, give good results for the calculations of the heats of reactions. The GAUSSIAN-1 and GAUSSIAN-2 theory calculations present the most acxcurate results on both the activation energies and the heats of reactions. The effects of the scaling factors on the activation energies and the heats of reactions are also evaluated. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 277–289, 1998     

Studies on rotational barriers of N-C axially chiral compounds: increase in the rotational barrier by aromatization

The rotational barriers in axially chiral quinolin-2-one and quinazolin-2-one possessing N-(ortho-tert-butyl)phenyl group were found to significantly increase in comparison with those of corresponding dihydroquinolin-2-one and dihydroquinazolin-2-one. Analysis of transition state structure during N-Ar bond rotation based on DFT calculation indicates that the increase in the rotational barrier is due to considerable distortion of the nitrogen-containing heterocyclic part.     

Chloride ion catalyzed conformational inversion of carbocation intermediates in the hydrolysis of a benzo[a]pyrene 7,8-diol 9,10-epoxide

A highly efficient procedure for converting 7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (1) to its trans-9,10-chlorohydrin (5) with excellent yield and purity by the reaction of anhydrous HCl in THF has been developed. The rate of reaction of 5 has been determined as a function of sodium chloride concentration in 1:1 dioxane-water solutions. A large common ion rate depression for the reaction of the chlorohydrin was observed, and the rate data are fit to a mechanism in which all of the tetrol products are formed by the reaction of water with the C-10 carbocation intermediate. Yet, the cis/trans ratio of tetrols from the reaction of the carbocation intermediate from the hydrolysis of chlorohydrin 5 is different than the cis/trans tetrol ratio from the acid-catalyzed hydrolysis of diol epoxide 1, which hydrolyzes via a carbocation with the same connectivity as that formed in the hydrolysis of 5. To rationalize these results, it is proposed that the S(N)1 reaction of chlorohydrin 5 yields a different distribution of carbocation conformations than that formed from the reaction of 1 with H(+). The energy barrier for the inversion of these carbocation conformations must be large relative to the energy barriers for the reaction of each carbocation conformation with water. In solutions containing sufficient concentrations of chloride ion, however, a lower energy pathway via a halohydrin exists for the interconversion of the carbocation conformations. Thus, chloride ion catalyzes the interconversion of these two carbocation conformations.     

Cornish-Fisher expansions using sample cumulants and monotonic transformations

General formulas of the asymptotic cumulants of a studentized parameter estimator are given up to the fourth order with the added higher-order asymptotic variance. Using the sample counterparts of the asymptotic cumulants, formulas for the Cornish-Fisher expansions with third-order accuracy are obtained. Some new methods of monotonic transformations of the studentized estimator are presented. In addition, similar transformations of a fixed normal deviate are proposed up to the same order with some asymptotic comparisons to the transformations of the studentized estimator. Applications to a mean and a binomial proportion are shown with simulations for estimation of the proportion.