Cyclic oxidation data

A total of 131 datasets were reported for materials falling within the categories of structural materials covered by COTEST. The distribution of cycle duration at temperature for all the datasets is shown in Fig. 17.3. This falls well into line with the defined categories of ultra-short ( 10min), short (10min t 8h) and long cycle ( 8h). 

The number of cycles employed is shown in Table 17.16. The median values of the time at temperature were 636 h for long cycle testing, 930 h for short cycle testing and 500 h for ultra-short cycle testing. This gives an indication of the test duration that the participants have found necessary to assess the cyclic high temperature performance of materials, and as such provides a basis for future recommendations. 

3 Cycle duration for all 131 reported data series and division into 

Number of cycles Long cycle Short cycle Ultra-short cycle Total 

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Figure 8.29 Comparison of the cyclic oxidation data obtained for Ni-25 wt% Cr-6 wt% A1 specimens that were coated with Na2S04 to those pre-sulphidized in an H2S-H2 gas mixture. Approximately 5 mg cm of Na2S04 was added to one specimen after every 5 h of exposure up to 20 h and then after every 10 h interval beyond 20 h. The pre-sulphidation was performed at the same time intervals that the Na2S04 was applied and the sulphur picked up was equivalent to the sulphur in a 5 mg cm Na2S04 deposit. The specimens were oxidized in air at 1000 °C.

Figure 10.19 Cyclic oxidation data for a straight aluminide and a platinum aluminide on IN-738 at 1200 °C in air.

Barrett, C. A. (1992), A Statistical Analysis of Elevated Temperature Gravimetric Cyclic Oxidation Data of 36 Ni- and Co-base Superalloys Based on an Oxidation Attack Parameter, Technical Memorandum 105934. Washington, DC National Aeronautics and Space Administration. 

Figure 6-27. Unpublished data of Brindley (1996) (reported in Brady et al. (1996a)) showing that although the oxidation-resistance of the orthorhombic-based alloy Ti-22Al-20Nb-2Ta-lMo at.% is superior to that of Ti-48Al-2Cr-2Nb at.% at 800°C in air, it still suffers from extensive hardening during elevated-temperature exposure in air. (a) Cyclic oxidation data (b) Knoop microhardness data (25 g/15 s).

Figure 6-39. Cyclic oxidation data showing excellent long-term oxidation behavior of y-i- Laves phase Ti-Al-Cr alloys such as Ti-51Al-12Cr at.% (1-h cycles, 1000°C in air). (Ti 8Al-2Cr-2Nb at.% data of McKee and Huang (1991) Ti-51Al-12Cr data of Brady et al. (1996b)). McCarron et al. (1993) also showed excellent cyclic oxidation-resistance for Ti-44Al-28Cr at.% after more than 2000 1-h cycles at 900 °C in air.

Figure 6-43. Cyclic oxidation data of MoSi2 in dry air (Meschter, 1992).

Evaluation of cyclic oxidation data available in the hterature and from the participating laboratories. 

Work package 2 Evaluation of existing cyclic oxidation data available from the literature and those supplied by the various project contractors... 

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

Several recent general papers on the properties of quinoxaline N-oxides have reported studies on the crystal structures of quinoxaline 1,4-dioxide,380 its 2,3-dimethyl derivative,380 ethyl 7-chloro-3-methyl-2-quinoxalinecarboxylate 1,4-dioxide,40 and N-(2-hydroxyethyl)-3-methyl-2-quinoxalinecarboxamide 1,4-dioxide 931 the NMR spectral data of quinoxaline 1,4-dioxide for comparison with those of related dioxides 348 the NMR data for biologically active quinoxalinecarboxamide dioxides 381 thermochemical data for several quinoxaline dioxides 183 and polaro-graphic or cyclic voltammetric data for 2,3-disubstituted quinoxaline dioxides.239 894... 

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

Although Taube s pyrazine Ru"—Ru dimer was produced by the Ag oxidation of [(NHjljRu—NC4H4N—Ru(NH3)5] , attempts to prepare similar Ru"-Ru " complexes from [(NH3)5Ru(C5H4N)2Ru(NH3)5]" and [(NHjljRu—NC5H4C2H4C5H4N—Ru(NH3)5]" were unsuccessful. Cyclic voltammetric data indicated a two-electron oxidation to Ru" -Ru " dimers. In view of the identical ligands around each Ru atom, Mayoh and Day have questioned the localization of the Ru valencies in Taube s dimer into discrete Ru" and Ru " centres. However, a theoretical calculation of the conditions necessary for valence trapping in any mixed valence system, showed that the condition is indeed satisfied by the above Ru compound. Other workers have suggested that the available data on this complex could also be explained by a molecular orbital scheme in which the Ru ion and pyrazine-filled n (or k ) molecular orbitals are mixed, and the unpaired electron is mainly but un-symmetrically shared by the two cations. ... 

Anodic oxidation of heterocyclic thiones leads generally to disulfides. Thus cyclic voltammetric data at a pyrolytic graphite anode of purine-2,6-dithione show three peaks. The first and second correspond to a disulfide formation from the 6- and 2-thione groups, respectively, whereas the third is due to an oxidation to purine-2,6-disulfonic acid.432 Similarly, the electrochemical oxidation of benzthiazole-2-thione and benzimidazole-2-thione in CH3CN-NaC104 at a platinum electrode afforded the corresponding disulfides in good yield.433... 

Fig. 6.17 Cyclic voltammetry data (solid line) for the oxidation of 2,4-dimethyl-3-ethylpyrrole in MeCN at v = 0.2 V s-1 and the simulated curve (dash-dot, DigiSim ) corresponding to the reaction sequence in Scheme 6.10. Reprinted with permission [39].

Fleischmann et al s 34 report cyclic voltammetry data for the oxidation of a series of aromatic hydrocarbons in a molten salt electrolyte, AlCl3-NaCl-KCl at 150°. Electrooxidation in this medium occurs at unusually low oxidation potentials. Tris-(p-substituted phenyl)amines, with the exception of tri (p-nitrophenyl) amine, yield very stable radical cations by all electrochemical criteria 380>S42 Mono- and bis-p-substituted triphenylamines, however, dimerize with rate constants ranging from 101 to 10s M 1 sec 1 to benzidines 176 (Eq. (237)), which subsequently are oxidized to the radical cations 177, whose ESR-spectra are observed. Dimerization is fastest with the p-N02 andp-CN-derivative, in accordance with HMO calculations, which predict the highest spin sensity in the p-position of these compounds 542 ... 

Table 4.5 Cyclic voltammetric data for copolymers 86a, 88a and 91a and for the corresponding oxides 86b, 88b and 91b[a].

Oxidation of 56 to dication gives irreversible cyclic voltammetry, which was qualitatively similar to that found by Kochi and coworkers for the achiral tetra-arylene 75 [113]. The cyclic voltammetric data were consistent with the reversible CC bond formation. Dication 752+.2SbCl6 was isolated and its hexacyclic structure was unequivocally determined by X-ray crystallography (Fig. 15.28) [113]. 

The comparison between redox properties data taken from different literature sources is also somewhat complicated because of the use of different experimental conditions solvent, working electrode, reference electrode, sweep rate, electrolyte, and so on. On the other hand, it is quite helpful that most of the authors cite their data with respect to a reference compound under the same experimental conditions (usually compared to unsubstituted TTF or TCNQ). Cyclic voltammetry data for 1Tb, under various experimental conditions, are given in Table 3. The particular influence of solvent on TTF oxidation potentials was discussed in Ref. 28. 

There is increasing evidence that several oxides, especially electroch-emically formed hydrous oxides, exhibit potential-pH behavior, as derived from cyclic voltammetry data, close to 1.5(2.303R71/F) rather than the more commonly accepted 2.303RTIF. Such behavior has been associated, for example, with increasing acidity of the oxy species at the oxide surface accompanying the increase in degree of oxidation of the central metal ion [12,142],... 

Confirmation of the reaction mechanism is provided by kinetic data dependent upon the same pK for the N- l coordinated ruthenium(III) complex (see Figure 6) Owing to the instability of l-[(Ado)(NH3)3Ru(II)l and severe restrictions required of the oxidizing partner isomerization kinetic rates were derived from cyclic voltammetric data using the method of Nicholson and Shain " " after forming the N-1 coordinated Ru(II) complex at the electrode surface by electrolytic reduction of the N- 6 bound Ru(III) species. Since the specific rates estimated by this method were independent of concentration the rate law is taken to be first order in the complex. 

The cyclic oxidation plot in Figure 22 indicates an additional feature of the TiCrAl alloys, the excellent adherence of the alumina layers formed on them. The alloys for which the data are plotted include a wide range of compositions, as indicated by the... 

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