John G. Dillard

The oxidation of cobalt(II) adsorbed on manganese dioxide

X-ray photoelectron spectroscopy (XPS) measurements of cobalt adsorbed on MnO2 reveal strong evidence that Co(II) has been oxidized to Co(III). The manganese spectra are characteristic of Mn(IV). Model calculations suggest that Co(II) cannot be oxidized by O2 to Co(III) in bulk solution at seawater concentrations but that the oxidation can proceed in the presence of the strong electric field at the MnO2-solution interface. Ni(II), however, cannot be oxidized at the interface except at very high concentrations. These calculations suggest that the oxidation of Co(II) can explain the geochemical separation of cobalt from nickel.

Surface analysis and the adsorption of Co(II) on goethite

Abstract The adsorption of aqueous Co(II) on goethite (α-FeOOH) was studied as a function of pH. Adsorption reactions were carried out in solutions with cobalt concentrations in the range 1 × 10−2 to 1 × 10−1M and at pH between 4 and 10. The quantity of cobalt adsorbed increases with increasing pH. The chemical nature of adsorbed cobalt was probed using X-ray photoelectron spectroscopy (XPS). The general features of the cobalt XPS spectra indicate that cobalt(II) is adsorbed at pH 6–7.5. At pH 8 and 10 cobalt is present as a mixture of Co(II) and Co(III). Analysis of X-ray diffraction patterns and interpretation of electrophoretic mobility results lead to the suggestion that cobalt-containing surface species are best represented as a mixture of Co(OH)2 and CoOOH at pH 8 and 10.

The mechanisms of Co(II) oxidation on synthetic birnessite

Studies of the adsorption of Co(II) on synthetic birnessite have been carried out at pH 4, 6, 7, 8 and 10. At pH values of 4, 6 and 7 cobalt(II) is oxidized to Co(III) while at pH 8 and 10 surface cobalt corresponds to Co(II). The Co(II) produced at pH 8 and 10 appears to be Co(OH)2 produced via precipitation on the MnO2 surface. The oxidizing agent is identified as surface Mn(IV) from a comparison of x-ray photoelectron spectroscopic results for samples prepared at pH 6.5 under anaerobic and aerobic conditions. The identification of Mn(III) is accomplished by comparing the Mn 2p core electron binding energies and the Mn 3s multiplet splitting values with the results for a variety of manganese oxides.

The oxidation states of cobalt and selected metals in Pacific ferromanganese nodules

Abstract A study of the oxidation states of metals in selected Pacific ferromanganese nodules by x-ray photoelectron spectroscopy (XPS) indicates the presence of Co(III), Pb(II), Pb(IV), Fe(III) and Ti(IV). These findings are based on measurements of core electron binding energies and an evaluation of the core electron shake-up satellite features.

Environmental aging effects on the durability of electrically conductive adhesive joints

Abstract The mechanical behavior of electrically conductive adhesive (ECA) joints exposed to elevated temperature and relative humidity conditions has been investigated, and failure mechanisms of conductive adhesive joints have been determined. Three silver-filled, epoxy-based model adhesive systems have been studied in conjunction with printed circuit board (PCB) substrates with metallizations of Au/Ni/Cu and Cu. Double cantilever beam (DCB) tests have been adopted to investigate the effects of environmental aging on ECA joints. This study reveals that conductive adhesives as well as substrate metallizations both play important roles in the durability of conductive adhesive joints. The rate of water attack on the interface of conductive adhesive joints with Cu-plated PCB substrates is faster than for those with Au/Ni/Cu metallization. A possible explanation of this phenomenon is based on considerations of surface free energy and interfacial free energy. Following drying of the aged conductive adhesive joints, the fracture energy recovered to some extent. This recovery in the fracture energy could be attributed to the reversible effect of plasticization of the bulk adhesives, as well as the regaining of bond strength between the adhesive and the substrate during drying at 150°C. XPS analysis of DCB failure surfaces suggested that diffusion of Cu to the Au surface might have occurred on the Au/Ni/Cu-plated PCB substrates during aging. Copper oxide was detected on the substrate surface upon exposure of the conductive adhesive joints to the hot/wet environment.

Surface characterization of synthetic vulcanized rubber treated with oxygen plasma

The surface of a synthetic vulcanized styrene-butadiene rubber (R2) was treated in an oxygen plasma to improve adhesion in joints prepared with a one-component solvent-based polyester-urethane adhesive. The modifications produced on the rubber surface by plasma treatment were assessed using advancing and receding contact angle measurements, x-ray photoelectron spectroscopy, (XPS), infrared-attenuated total reflection spectroscopy and scanning electron microscopy. Adhesion was obtained from T-peel tests of treated R2 rubber/polyurethane adhesive joints. Several experimental variables were considered, such as the radio-frequency power and the length and lifetime of the plasma treatment. The treatment in the oxygen plasma produced a noticeable decrease in contact angle, which can be mainly ascribed to the creation of C-O and C=O moieties on the rubber surface. Advancing and receding contact angles only differed by ∼10°. Depending on the experimental conditions used, some ablation was produced on the surface, which was more noticeable as the length and power of the treatment increased. An adequate performance of adhesive joints was obtained using a power of 50 W and a time for oxygen plasma treatment of <10 min. The changes in the rubber surface remained for 2 h after plasma treatment, as indicated by the variation in peel strength and XPS data. Although improved adhesion was obtained by treating the rubber in an oxygen plasma, the T-peel strength values are not sufficient to assure technical use, probably due to the migration of waxes and zinc stearate to the surface once the treatment was carried out. Finally, sulfur oxidation was produced by the plasma treatment, and for severe conditions solid crystals of a sodium salt of an oxidized sulfur compound (sodium sulphate or an organic sulphate) appeared on the treated rubber surface.

X-ray photoelectron spectroscopic (xps) surface characterization of cobalt on the surface of kaolinite

Abstract The adsorption of cobalt(II) on kaolinite was studied at selected pH values in the range 3–10. The chemical nature of adsorbed cobalt was characterized by XPS spectral features, binding energy measurements, Co 2p satellite characteristics, and L3VV Auger (X-ray induced) kinetic energy measurements. That the Co 2p spectral features are not identical for samples prepared at a series of pH values, provides experimental evidence that the chemical nature of the cobalt-clay interaction changes with pH. The presence of satellite features and the Co 2 p 1 2 -Co 2 p 3 2 separation indicate Co(H) on kaolinite. The Co 2 p 3 2 binding energies decrease from 782.2 eV for kaolinite treated at pH 3 to 781.0 eV for the pH 10 sample. The binding energies for pH 7.8–10.00 samples are in close agreement with that for authentic Co(OH)2. Satellite structure in transition metal 2p XPS spectra is due to simultaneous excitation of electrons and corresponds to charge transfer processes from ligand to the metal. The satellite intensity and energy separation from the main photopeak are related to the extent of covalent bonding in the metal compound. Intense satellite peaks were observed for high spin covalent Co(II) compounds. The ratio of the Co 2p satellite peak intensity to the main Co 2p photopeak decreases from 2.2 for the pH 5 sample to 1.21 for the pH 10 sample. The large satellite intensity ratio at low pH values suggests that Co(II) is more covalently bound to the clay at pH 5 than at higher pH values. The Co L3VV Auger kinetic energy results also show a variation for samples prepared at different pHs. In model systems Auger transitions occur at high kinetic energies for elements in environments of high electron density. The L3VV Auger transitions occur at 770.3 eV for the pH 6 sample and 768.6 eV for samples prepared above pH 7.8. These results are consistent with the interpretation of the satellite intensity ratios and binding energy measurements and support the notion of covalent Co(II)- clay bonding at low pH values.

Failure analysis of surface-treated unvulcanized SBS rubber/polyurethane adhesive joints

Abstract The surface of an unvulcanized styrene-butadiene-styrene (SBS) rubber (S0) was chlorinated with 0.5–7wt% trichloroisocyanuric acid (TCI) solutions. The adhesion strength of surface-treated SO rubber/polyurethane adhesive joints was obtained from T-peel tests. The failed surfaces after peeling were characterized by ATR-IR spectroscopy, contact angle measurements, SEM coupled with EDX analysis and XPS. The untreated SO rubber shows no adhesion because of a lack of compatibility with the polyurethane adhesive. Chlorination of SO rubber with O.5wt% TCl produced a noticeable increase in T-peel strength which was due to the formation of CCl and CO species on the rubber surface, the failure of the joint being produced at the chlorinated surface (weak boundary layer). Treatment of S0 rubber with greater concentrations of TCI, up to 2wt%, resulted in a decrease in the joint strength because the locus of failure was directed deeper into the chlorinated layer. On the other hand, the treatment of S0 rubber with 7wt% TCI produced different joint strength values in replicate joints that were tested under similar experimental conditions, mainly owing to differences in the locus of failure in the joints. Failure occurred randomly throughout the adhesive, the interface or the surface chlorinated layer.

An X-ray photoelectron spectroscopic (XPS) study of cobalt adsorbed on the clay mineral chlorite

Adsorption of Co(H2O)62+ and Co(NH3)63+ on chlorite has been studied. For Co(H2O)62+ the adsorption has been studied at pH 3 and 7, while adsorption of Co(NH3)63+ was initiated at pH 3. In oxygen-free solutions Co(H2O)6+ is adsorbed as Co(H2O)62+. When Co(NH3)63+ interacts with chlorite the pH of the solution increases and Co3+ is reduced to Co2+. These conclusions are obtained from X-ray photoelectron spectroscopy (XPS or ESCA) studies of the binding energies of the Co 2p levels. A probable mechanism for adsorption and subsequent reduction of Co3+ is presented and discussed.

A Stress Singularity Approach for the Prediction of Fatigue Crack Initiation in Adhesive Bonds. Part 2: Experimental

Abstract Using the epoxy-aluminum wedge specimens defined and analyzed in Part 1, we measure the number of cycles required to initiate an interfacial fatigue crack near the apex, where the stress field is predicted to be singular. The eigenvalue, λ, and the generalized stress intensity factor, Q, are varied via the wedge angle, and via the beam deflection, respectively. Crack initiation is detected using a strain gage bonded near the tip of the wedge. Following the methodology developed in Part 1, the fatigue data are then used to construct a fatigue initiation criterion characteristic of the bimaterial interface. This criterion is a 3-D surface, with the ordinate representing the generalized stress intensity factor and the two horizontal axes representing the number of cycles to initiation and the eigenvalue, respectively. Three key assumptions of the model are found to be satisfied in the specimens tested herein: (1) geometric imperfections at the apex are smaller than the singular region, (2) the plast...