Haido S. Karayianni

Impedance spectroscopy study of nickel electrodeposits

Abstract Nickel electrodeposits prepared in NiSO4 and NiCl2 electrolytes with thicknesses of 43–49.4 μm were examined by the impedance spectroscopy (IS) method for a–c frequencies 1 Hz–100 kHz. The Nyquist diagrams were single, almost perfect semicircles, suggesting the applicability of the in parallel combination of the Ra–Ca elements characterised by a single relaxation time, 8.27×10−6 s, combined in series with another Rb element, Rb≪Ra. This relaxation time predicted that the incorporated hydrogen atoms are involved in the process of conduction by a hopping/diffusion mechanism, which is differentiated in the cases of NiSO4–Ni and NiCl2–Ni deposits. The diffusion coefficient of hydrogen, 1.25×10−11 cm2 s−1, was determined. The microstructure of Ni electrodeposits was also studied and characterised by electron microscopy. Finally, a general model for the microstructure of Ni electrodeposits, consistent with the results of impedance spectroscopy, was formulated.

An insight into the disorder properties of the α-cyclodextrin polyiodide inclusion complex with Sr2+ ion: dielectric, DSC and FT-Raman spectroscopy studies

At T < 250 K, the polyiodide inclusion complex (α-cyclodextrin)2·Sr0.5·I5·17H2O displays two separate relaxation processes due to both the frozen-in proton motions in an otherwise ordered H-bonding network and the order–disorder transition of some normal H-bonds to flip-flop ones. At T>250 K, the AC-conductivity is dominated by the combinational contributions of the disordered water network, the mobile Sr2+ ions, the polyiodide charge-transfer interactions and the dehydration process. The evolution of the Raman spectroscopic data with temperature reveals the coexistence of four discrete pentaiodide forms. In form (I) (I− 3·I2 ↔ I2·I− 3), the occupancy ratio (x/y) of the central I− ion differs from 50/50. In form (IIa) (I2·I− ·I2) x/y = 50/50, whereas in its equivalent form (IIb) (I2·I− ·I2) * as well as in form (III) (I− 3·I2), x/y = 100/0 (indicative of full occupancy). Through slow cooling and heating, the inverse transformations (I) → (IIa) and (IIa) → (I) occur, respectively.

MOCVD Cobalt Oxide Deposition from Inclusion Complexes: Decomposition Mechanism, Structure, and Properties

D MOCVD Cobalt Oxide Deposition from Inclusion Complexes: Decomposition Mechanism, Structure, and Properties N. D. Papadopoulos, H. S. Karayianni, P. E. Tsakiridis, M. Perraki, E. Sarantopoulou, and E. Hristoforou Laboratory of Physical Metallurgy, School of Mining and Metallurgical Engineering, and Laboratory of Physical Chemistry, Section of Materials Science and Engineering, School of Chemical Engineering, National Technical University of Athens, Zografou 157 80, Athens, Greece National Institute of Research, Athens, Greece

Interpretation of Electrical Conductance Transition of Hematite in the Spin-Flip Magnetic Transition Temperature Range

Using polycrystalline pure hematite at frequencies of 100 Hz-100 kHz and temperatures of 190-350 K impedance spectroscopy was employed to investigate the electrical conductance transition and correlate it with magnetic transition occurring within this temperature range. A background of slight impurity donor or acceptor semiconductivity was observed above which a peak of conductivity appeared in the temperature range of magnetic transition. The results suggested that the transition from antiferromagnetically to weak ferromagnetically coupled Fe 3+ and vice versa takes place through their transformation to uncoupled Fe 3+ and an equilibrium between these types of coupled and uncoupled pairs of Fe 3+ is established. A model, thermodynamically sustained, involving bulk concentration of both types of coupled and of uncoupled Fe 3+ was formulated precisely predicting the dependence of magnetic transition on temperature and the appearance of peaks in both the uncoupled Fe 3+ concentration and conductivity within the transition temperature range. The conductivity, mainly due to intrinsic semiconductance coming from the activation of uncoupled Fe 3+ , depends on both temperature and concentration of uncoupled Fe 3+ . The heretofore elusive semiconductive character of hematite is explained.

Significant modification of the I(-)3 Lewis base character in the β-cyclodextrin polyiodide inclusion complex with Co2+ ion: an FT-Raman investigation.

The β-cyclodextrin (β-CD) polyiodide inclusion complex (β-CD)(2)·Co(0.5)·I(7)·21H(2)O has been synthesized, characterized and further investigated via FT-Raman spectroscopy in the temperature range of 30-120°C. The experimental results point to the coexistence of I(-)(7) units (I(2)·I(-)(3)·I(2)) that seem not to interact with the Co(2+) ions and I(-)(7) units that display such interactions. The former units exhibit a disorder-order transition of both their I(2) molecules above 60°C due to a symmetric charge-transfer interaction with the central I(-)(3) [I(2)←I(-)(3)→I(2)], whereas in the latter units only one of the two I(2) molecules becomes well-ordered above 30°C. The other I(2) molecule remains disordered presenting no charge-transfer phenomena. The Co(2+) ion induces a considerable asymmetry on the geometry of the I(-)(3) anion and a significant modification of its Lewis base character. Complementary dielectric measurements suggest no important involvement of H···I contacts in the observed modification of the I(-)(3) electron-transfer properties.

THE MECHANISM OF ZN CORROSION IN BOTH AERATED AND DEAERATED AQUEOUS KNO3 SOLUTIONS

The corrosion of Zn in both aerated by air (O2 + N2), and deaerated by N2, solutions of KNO3 at concentrations largely varying, 10‐5‐1 M, is studied. The corrosion process is followed in time by potentiometry and the potential of Zn electrode (vs. calomel) vs. time plots is derived. In both cases, a transition period is observed until a steady state is achieved where the rate of Zn2+ and OH‐ production becomes equal to the rate of Zn(OH)2 precipitate formation. In the aerated solutions the potential and corrosion rate decrease with time while in the deaerated solutions they pass successively through a minimum and a maximum before a steady state is achieved. By a suitable potentiometric analysis the results are explained. The most important factors found to be affecting the mechanism of corrosion process are presented. From the discovery of the mechanism and the factors affecting the Zn corrosion, predictions for promoting or slowing down the corrosion process may be derived.