E. W. A. Young

An18O tracer study on the growth mechanism of alumina scales on NiAl and NiAlY alloys

The effect of Y additions on the oxidation mechanism of NiAl at 1270 K has been investigated. Mass transport in the alumina scale was examined with18O tracers. Proton activation analysis (18O(p, α)15N) and SIMS were used to measure the18O distribution in the scale. On pure NiAl and low-doped (0.07% Y) NiAl mainly outward scale growth was observed. Addition of 0.5% Y induces oxide formation at both interfaces of the scale. Larger quantities of Y added to the alloy (>0.5%) do not dissolve completely in the NiAl, but form Y-rich segregates. Internal oxidation of these intergranular segregates was observed. The relation between the modification of the scale-growth mechanism and the improved adherence of the scale to the alloy due to the addition of Y is discussed.

The use of a 18O tracer and Rutherford backscattering spectrometry to study the oxidation mechanism of NiAl.

The oxidation mechanism of NiAl was investigated in the temperature range between 1170 K and 1420 K. Oxidation was performed by alternating exposure of the samples to natural oxygen and oxygen gas enriched with 18O. The 18O tracer was profiled by using the reaction with 800 keV protons, 18O(p,α)15N. Rutherford Backscattering Spectrometry of 2 MeV 4He+ was used to check the position of noble metal markers. The 18O tracer experiments clearly show that up to 1420 K, Al diffusion through the sclae is the major material transport mechanism during oxidation, resulting in outside scale growth. Noble metal “marker” experiments however give misleading results by incorrectly suggesting an inside scale growth mechanism.

Growth mechanism of Al2O3 scales developed on Fe Cr Al alloys

Abstract By combining classical techniques used for oxidation studies, electrochemical measurements on the oxide scale and 18 O profilings by using the 18 O( p , α) 15 N reaction, it was possible to obtain reliable data on the growth mechanisms of alumina scales developed at 1082°C on three Fe Cr Al alloys differing by their impurity amount. It was shown that aluminum diffusion through the scale is the major material transport mechanism during oxidation, resulting in an outside scale growth. Nevertheless, carbon and other impurities enhance the oxygen inward diffusion and improve the scale adherence. Alumina scales behave as a mixed ionic and electronic conductor, but the incorporated impurities decrease the values of t i , σ i and kp , thus accounting for a slower oxidation rate in an industrial alloy than in a high purity one.

Bounded diffusion in solid solution electrode powder compacts. Part III. The kinetic properties of AgxTiS2 and AgxNiPS3

The kinetic properties of AgxTiS2 and AgxNiPS3 have been determined with dc (long-time current pulse) and ac techniques, using symmetrical cells of the type: Ag/AgI/AgxSSE/AgI/Ag AgxTiS2 : 0.07 ⩽ × ⩽0.98, T= 193°C, AgxNiPS3 : 0.05 ⩽ x ⩽ 0.90, T = 300–350°C. Data for the chemical diffusion coefficient (DAg+), the thermodynamic factor (Kt), the ionic (σAg+) and electronic (σe) conductivity, and the activation enthalpy (in the case of Ag0.05NiPS3) for these quantities, are obtained. In AgxTiS2, both DAg+(= 0.8-5 × 10−6cm2s−1) and Kt increase with increasing x; while σe is very high te≅1). In Ag0.05NiPS3, DAg+ has the same order of magnitude as in AgxTiS2, but the electronic conductivity is comparatively low tAg+≅0.10). Kt(Ag0.05NiPS3) is relatively high, and the AgI/Ag0.05NiPS3 interface exhibits an ionic charge transfer resistance. This indicates that Ag0.05NiPS3 is not suitable to incorporate Ag+-ions. Because of 2-phase behaviour of AgxNiPS3 with x > 0.20 for T < 350°C, the experimental data could only be qualitatively analyzed. Furthermore it is shown that carbon does not behave as an inert electrode material for AgxNiPS3.

Several electrochemical methods for the simultaneous measurement of thermodynamic activity and effective kinetic properties of inserted ions in solid solution electrodes

Three electrochemical methods for thesimultaneous measurement of thermodynamic activity and the effective kinetic properties of inserted ions Solid Solution Electrode powder compacts are presented. The theoretical substructure of these methods, and the practical application on the systemsAg/AgxTiS2/Ag(0<x<1, T=193°C),Li/LixCoO2(0.96<x<1, T=20°C), resulting in values for both the chemical diffusion coefficient and the Darken factor of the inserted ions, are discussed.

The thermoelectric power in AgxTiS2 and AgxNiPS3. Part II. The ionic component of the thermoelectric power

The EMF of the isothermal cells: Ag/AgI/AgxTiS2: 0<x<1, T=150–200°C/AgxNiPS3: 0<x<3, T=150–350°C has been measured. From the EMF-x curves the existence ranges of the 2-phase (stage I and II) regions −0.16<x<0.32 for the Ag/AgxTiS2 system at 190°C; 0.20 < x < 0.50 and 1 < x < 2 for the Ag/AgxNiPS3 system at 400°C - have been determined. The results are sustained by X-ray diffraction and electrical conductivity measurements. From the EMF-T curves the partial enthalpy (ΔHAg) and entropy (ΔSAg) of dissolution of silver in the AgxSSE (solid solution electrode) materials were obtained. In the case of AgxTiS2, ΔHAg has a low absolute value, while ΔSAg is distinctly positive. The EMF of the Ag/AgxNiPS3 system also has a positive temperature coefficient. Furthermore, the ionic component of the thermoelectric power, ΔE/ΔT, of the thermogalvanic cells: Ag/AgI/AgxSSE/AgI/Ag AgxTiS2: 0 < x < 1, T = 150–200°C( T ) (T+ΔT) AgxNiPS3: 0 < x < 1, T= 150–350°C has been measured. The kinetically important heat of transport of silver ions in the AgxSSE materials has been determined in two ways: first from the dependence of the ionic Seebeck coefficient (ϵAg+) on reciprocal temperature; and second from direct calculation, using the data for ϵAg+ and ΔSAg. The heat of transport is much smaller than the activation enthalpy for Ag+-conduction, indicating a high ionic polaron binding energy in these materials.