Part I. Kinetics of coarsening of the gamma-prime precipitate in nickel-silicon alloys. Part II. Rate of crystallization of an amorphous Fe-P-C alloy

<p>The coarsening kinetics of Ni₃ Si(γ') precipitate in a binary Ni-Si alloy containing 6.5 wt. % silicon was studied by magnetic techniques and transmission electronmicroscopy. A calibration curve was established to determine the concentration of silicon in the matrix. The variation of the Si content of the Ni-rich matrix as a function of time follows Lifshitz and Wagner theory for diffusion controlled coarsening phenomena. The estimated values of equilibrium solubility of silicon in the matrix represent the true coherent equilibrium solubilities.p> <p>The experimental particle-size distributions and average particle size were determined from dark field electron micrographs. The average particle size varies linearly with tp>-1/3p> as suggested by Lifshitz and Wagner. The experimental distributions of particle sizes differ slightly from the theoretical curve at the early stages of aging, but the agreement is satisfactory at the later stages. The values of diffusion coefficient of silicon, interfacial free energy and activation energy were calculated from the results of coarsening kinetics. The experimental value of effective diffusion coefficient is in satisfactory agreement with the value predicted by the application of irreversible the rmodynamics to the process of volume constrained growth of coherent precipitate during coarsening. The coherent γ' particles in Ni-Sialloy unlike those in Ni-Al and Ni-Ti seem to lose coherency at high temperature. A mechanism for the formation of semi-coherent precipitate is suggested.p>

Ancillary ligand effects : from zirconium(IV)-catalyzed homogeneous propylene polymerization to platinum(II)-mediated C-H bond activation

<p>A series of C<sub>s- and C<sub>1-symmetric doubly-linked ansa-metallocenes of the general formula {1,1'-SiMe₂-2,2'-E-('ƞp>5p>-C<sub>5H₂-4-Rp>1p>)-(ƞp>5p>-C<sub>5H-3',5'-(CHMe₂)₂)}ZrC<sub>2 (E = SiMe₂ (1), SiPh₂ (2), SiMe₂ -SiMe₂ (3); Rp>1p> = H, CHMe₂, C<sub>5H₉, C<sub>6H₁₁, C<sub>6H₅) has been prepared. When activated by methylaluminoxane, these are active propylene polymerization catalysts. 1 and 2 produce syndiotactic polypropylenes, and 3 produces isotactic polypropylenes. Site epimerization is the major pathway for stereoerror formation for 1 and 2. In addition, the polymer chain has slightly stronger steric interaction with the diphenylsilylene linker than with the dimethylsilylene linker. This results in more frequent site epimerization and reduced syndiospecificity for 2 compared to 1. p> <p>C<sub>1-Symmetric ansa-zirconocenes [1,1 '-SiMe₂-(C<sub>5H₄)-(3-R-C<sub>5H₃)]ZrCl₂ (4), [1,1 '-SiMe₂-(C<sub>5H₄)-(2,4-R₂-C<sub>5H₂)]ZrCl₂ (5) and [1,1 '-SiMe₂-2,2 '-(SiMe₂-SiMe₂)-(C<sub>5H₃)-( 4-R-C<sub>5H₂)]ZrCl₂ (6) have been prepared to probe the origin of isospecificity in 3. While 4 and 3 produce polymers with similar isospecificity, 5 and 6 give mostly hemi-isotactic-like polymers. It is proposed that the facile site epimerization via an associative pathway allows rapid equilibration of the polymer chain between the isospecific and aspecific insertion sites. This results in more frequent insertion from the isospecific site, which has a lower kinetic barrier for chain propagation. On the other hand, site epimerization for 5 and 6 is slow. This leads to mostly alternating insertion from the isospecific and aspecific sites, and consequently, a hemi-isotactic-like polymers. In comparison, site epimerization is even slower for 3, but enchainment from the aspecific site has an extremely high kinetic barrier for monomer coordination. Therefore, enchainment occurs preferentially from the isospecific site to produce isotactic polymers. p> <p>A series of cationic complexes [(ArN=CR-CR=NAr)PtMe(L)]p>+p>[BF<sub>4]p>+p> (Ar = aryl; R = H, CH₃; L = water, trifluoroethanol) has been prepared. They react smoothly with benzene at approximately room temperature in trifluoroethanol solvent to yield methane and the corresponding phenyl Pt(II) cations, via Pt(IV)-methyl-phenyl-hydride intermediates. The reaction products of methyl-substituted benzenes suggest an inherent reactivity preference for aromatic over benzylic C-H bond activation, which can however be overridden by steric effects. For the reaction of benzene with cationic Pt(II) complexes, in which the diimine ligands bear 3,5-disubstituted aryl groups at the nitrogen atoms, the rate-determining step is C-H bond activation. For the more sterically crowded analogs with 2,6-dimethyl-substituted aryl groups, benzene coordination becomes rate-determining. The more electron-rich the ligand, as reflected by the CO stretching frequency in the IR spectrum of the corresponding cationic carbonyl complex, the faster the rate of C-H bond activation. This finding, however, does not reflect the actual C-H bond activation process, but rather reflects only the relative ease of solvent molecules displacing water molecules to initiate the reaction. That is, the change in rates is mostly due to a ground state effect. Several lines of evidence suggest that associative substitution pathways operate to get the hydrocarbon substrate into, and out of, the coordination sphere; i.e., that benzene substitution proceeds by a solvent- (TFE-) assisted associative pathway. p>

Structure and properties of an amorphous ferromagnetic alloy

<p>The structure and the electrical and magnetic properties of an amorphous alloy containing approximately 80 at .% iron, 13 at.% phos phorus and 7 at.% carbon (Fe_(80)Fe_(13)C_7) obtained by rapid quenching from the liquid state have been studied. Transmission electron diffraction data confirm the amorphous nature of this alloy. An analysis of the radial distribution function obtained from X-ray diffraction data indicates that the number of nearest neighbors is approximately seven, at a distance of 2.6A. The structure of the alloy can be related to that of silicate glasses and is based on a random arrangement of trigonal prisms of Fe_2P and Fe_3C types in which the iron atoms have an average ligancy of seven. Electrical resistance measurements show that the alloys are metallic. A minimum in the electrical resistivity vs. temperature curve is observed between 10° K to 50° K depending on the specimen, and the temperature at which the minimum occurs is related to the degree of local ordering. The Fe-P-C amorphous alloys are ferromagnetic. The Curie temperature measured by the induction method and by Mossbauer spectroscopy is 315° C. The field dependence of the magneto-resistance at temperatures from liquid helium to room temperature is similar to that found in crystalline iron. The ordinary Hall coefficient is approximately 10^(-11) volt-cm/amp-G. The spontaneous Hall coefficient is about 0.6 x 10^(-9) volt-cm/amp-G and is practically independent of temperature from liquid helium temperature up to 300° c. p>

Pulsed neutron measurements in two adjacent finite media

<p>The pulsed neutron technique has been used to investigate the decay of thermal neutrons in two adjacent water-borated water finite media. Experiments were performed with a 6x6x6 inches cubic assembly divided in two halves by a thin membrane and filled with pure distilled water on one side and borated water on the other side. p> <p>The fundamental decay constant was measured versus the boric acid concentration in the poisoned medium. The experimental results showed good agreement with the predictions of the time dependent diffusion model. It was assumed that the addition of boric acid increases the absorption cross section of the poisoned medium without affecting its diffusion properties: In these conditions, space-energy separability and the concept of an “effective” buckling as derived from diffusion theory were introduced. Their validity was supported by the experimental results. p> <p>Measurements were performed with the absorption cross section of the poisoned medium increasing gradually up to 16 times its initial value. Extensive use of the IBM 7090-7094 Computing facility was made to analyze properly the decay data (Frantic Code). Attention was given to the count loss correction scheme and the handling of the statistics involved. Fitting of the experimental results into the analytical form predicted by the diffusion model led top> <p>Ʃ_av = 4721 sec<sup>-1p> (±150)p> <p>D_o = 35972 cmp>2p>sec<sup>-1p> (±800) for water at 21˚C</p> <p>C (given) = 3420 cmp>4p>sec<sup>-1p>p> <p>These values, when compared with published data, show that the diffusion model is adequate in describing the experiment. p>

I. Site effects and exciton structure in molecular crystals - Benzene. II. The first and second triplet states of solid benzene

<p>The effect of intermolecular coupling in molecular energy levels (electronic and vibrational) has been investigated in neat and isotopic mixed crystals of benzene. In the isotopic mixed crystals of C<sub>6H₆, C<sub>6H₅D, m-C<sub>6H₄D₂, p-C<sub>6H₄D₂, sym-C<sub>6H₃D₃, C<sub>6D₅H, and C<sub>6D₆ in either a C<sub>6H₆ or C<sub>6D₆ host, the following phenomena have been observed and interpreted in terms of a refined Frenkel exciton theory: a) Site shifts; b) site group splittings of the degenerate ground state vibrations of C<sub>6H₆, C<sub>6D₆, and sym-C<sub>6H₃D₃; c) the orientational effect for the isotopes without a trigonal axis in both the p>1p>B_2u electronic state and the ground state vibrations; d) intrasite Fermi resonance between molecular fundamentals due to the reduced symmetry of the crystal site; and e) intermolecular or intersite Fermi resonance between nearly degenerate states of the host and guest molecules. In the neat crystal experiments on the ground state vibrations it was possible to observe many of these phenomena in conjunction with and in addition to the exciton structure.p> <p>To theoretically interpret these diverse experimental data, the concepts of interchange symmetry, the ideal mixed crystal, and site wave functions have been developed and are presented in detail. In the interpretation of the exciton data the relative signs of the intermolecular coupling constants have been emphasized, and in the limit of the ideal mixed crystal a technique is discussed for locating the exciton band center or unobserved exciton components. A differentiation between static and dynamic interactions is made in the Frenkel limit which enables the concepts of site effects and exciton coupling to be sharpened. It is thus possible to treat the crystal induced effects in such a fashion as to make their similarities and differences quite apparent.p> <p>A calculation of the ground state vibrational phenomena (site shifts and splittings, orientational effects, and exciton structure) and of the crystal lattice modes has been carried out for these systems. This calculation serves as a test of the approximations of first order Frenkel theory and the atom-atom, pair wise interaction model for the intermolecular potentials. The general form of the potential employed was V(r) = Bep>-Crp> - A/rp>6p> ; the force constants were obtained from the potential by assuming the atoms were undergoing simple harmonic motion.p> <p>In part II the location and identification of the benzene first and second triplet states (p>3p>B_1u and p>3p>E_1u) is given.p>