Materials Chemistry最新文献

Resistance measurements were performed on thin nickel foils exposed to argon-oxygen gas mixtures at elevated temperatures. The resistance initially increased very rapidly then at oxygen pressures less than 0.08 atm, the resistance change decreased substantially before slowly increasing. At higher oxygen pressures no decrease in the resistance change occurred. These results are interpreted in light of hydrogen-oxygen-nickel interactions which occur as a result of hydrogen dissolution in nickel prior to oxidation The hydrogen originates from long-term hydrogen anneals prior to the oxidation.

The acid-base dissociation behaviour of the surface of γ-FeOOH is investigated by potentiometric acid-base titrations of the oxide suspensions in aqueous solutions. The zero point of charge (p.z.c.) of the electrical double layer of the reversible oxide solution interface is determined: in KNO₃ titrations at various ionic strenghts cross over at pH = 7.6; by coincidence with the pH of the isoelectric point, determined by microelectrophoresis, the cross over of titrations in KNO₃ is identified as the point of zero charge of the surface. Specific effects regarding monovalent anions (Cl⁻, ClO₄⁻) are found to be absent, while SO₄⁻⁻ strongly adsorbs on the γ-FeOOH surface, thus modifying the balance of the positive and negative groups on the surface. The sequence of interaction of monovalent cations with the surface is reported: K⁺ < Na⁺ < Li⁺.

Surprising and of particular interest are the strong adsorption of Na⁺, which has always been reported to interact only electrostatically with oxide surfaces, and the absence of specific interactions with Cl⁻ ions.

Experimental intrinsic-viscosity data for randomly branched polydisperse polymer samples of poly(butyleneterephthalate), prepared by polycondensation of dimethylterephthalate (DMT) and 1,4-butanediol with small amounts of 1,3,5-tricarboxymethylbenzene (TMT) (from 0.5 to 1.5 mol% of trifunctional units), were used to test different theoretical treatments for different g-rules.

It was found that the $\text{g}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ -rule gave the best fits for the results.

Correlation of intrinsic viscosity with the weight-average degree of branching, B_w, was obtained by an empirical equation; intrinsic viscosity was also correlated with melt-flow index.

The sintering behaviour of powder mixtures Fe20 CrxMn (0 ⩽ x ⩽ 25 w %) was considered. In the case of the basic mixture Fe20 Cr (heated under hydrogen) chromium diffuses partially into α-iron, then mainly into austenite, once the surface oxyde is reduced; on the other hand under argon, chromium diffusion is more limited, and operates only at high temperature into γ-iron. Whatever the treatment atmosphere, an addition of manganese leads to a noticeable diffusion of this element, first into iron (T < 730°C), then into chromium and iron (T > 730°C). Due to the greater mobility of manganese, disturbing KIRKENDALL effects are observed during sintering, especially in the case of the higher manganese contents. However, homogeneous and dense alloys can be obtained by sintering at high temperature; according to the manganese content, the structure consists of a single-phase α when x < 6.5 wt % Mn, or of two or three phases when x > 7 wt % Mn.

Samples of randomly branched poly(butyleneterephthalate) with different degrees of branching were prepared by polycondensation of dimethylterephthalate (DMT), 1,4-butanediol (BD) and small amounts of 1,3,5-trycarboxymethylbenzene (TMT) (from 0.005 to 0.015 (mol TMT)/(mol DMT)).

The values of $\text{M}$_w were calculated according to Stockmayer from the reaction conversion, p, and correlated with melt-flow index.

Bate's empirical equation provided a good correlation between the branching parameter, $\text{B}$_w, and the viscosity ratio (<η_b>/<η₁>).