Spinel composition

Unfortunately, this approach for electrochemical stability determination has not been widely adopted. The few exceptions include the seminal electrolyte work by Guyomard and Tarascon. In the formulation of new electrolytes for lithium ion technology, the spinel composite electrode was used as the standard working surface in all of the voltammetric measurements. The oxidative decomposition limits of the new electrolytes thus determined are summarized in Table 7 along with a handful of stability data that were determined in a similar approach for other electrolyte systems. 

Fourcroy predicted that this mineral would give chemists the opportunity to make a more thorough study of the properties of chromium and perhaps to discover compounds of it which, because of their rich and varied colors, would be useful in painting and in the manufacture of glass and enamel (90). He also encouraged study of the chromium alloys. The chrome-iron ore is now known as chromite. It is not a chromate, but has the spinel composition, Fe(Cr02)2-... 

The data for several Li/Ti/O spinel compositions are presented in Table 9 with their reported transition temperatures. Unfortunately many of these samples were not homogeneous. 

Peigney, A., Rul, S., Lefevre-Schlick, F. and Laurent, Ch., Densification during hot-pressing of carbon nanotube-metal-magnesium aluminate spinel composites , to be submitted, 2006. 

Mechanical properties and thermal shock behaviour of magnesia-spinel composite refractory materials... 

Young s moduli of spinel composites decrease with additions of spinel, for all particle sizes, where the influence of the spinel is greater, the larger the... 

SEM micrograph showing the microstructure of a 20% 22 pm preformed spinel composite. 

The mean critical defect size (c), calculated from the Griffith equation,107108 increases markedly with increasing spinel content (Fig. 14.7). For 0.5 pm in situ spinel composites, the defect size increases up to -20%, and then decreases slightly with further additions of spinel. However, for the preformed spinel composites there is a gradual increase in defect size up to 30% spinel additions. The increase in defect size is in general much more marked at 20%, in... 

For the in situ formed 0.5 pm spinel composites, the work of fracture (TWof) value increases only slightly with up to 10% additions, but at 20% it increases by a factor of 2.5 (Fig. 14.8). There is a general but less marked increase in TWof> by a factor of 2, at 30% additions of preformed spinel (Fig. 14.8). It seems that the effect of preformed spinel content is more important than particle size, within the larger scatter data. 

Fracture surfaces of pure MgO show a large proportion of transgranular cracks, with a few intergranular cracks (Fig. 14.9). At low additions of spinel, transgranular cracks are still present with some intergranular cracks, in the fracture surfaces of each spinel composite. However, at higher additions of spinel (> 20%), mostly intergranular fracture occurs. For example, 20%... 

The fracture of the magnesia-spinel composites is either semi-stable or stable, but never catastrophic.105 It may be concluded that crack propagation is a much greater energy-consuming process than crack initiation in these materials. For many industrial applications, the initiation of fracture is less... 

Figure 14.11 shows that the i parameter for 22 pm spinel composites decreases with additions of up to -20%, and then increases with further additions of... 

Figure 14.12 shows that R"" increases with additions of 0.5 pm in situ formed spinel to a maximum at 20% loading, and an improvement by a factor of 4, as compared to MgO. The R"" parameter for the 3 pm and 11 pm preformed spinel composites shows similar values to the 0.5 pm spinel composites up to 20% additions, but is more sensitive to particle size with further additions. The 22 pm preformed spinel composites shows a similar trend but there is a larger effect on R"" above 10% additions, as... 

The thermal shock damage resistance of 0.5 pm in situ formed composites will be expected to be much less than that of preformed composites. For 3 pm and 11 pm spinel composites, addition of 30 10% spinel may provide some improvements. Further deterioration of strength in the coarsest spinel composite as a result of thermal shock should be a minimum at 20% (Fig. 14.12). In general coarser (22 pm) preformed spinel powders appear to be more beneficial than finer powders, but there is no obvious advantage with additions of more than 30%.74,103... 

Relative strengths of pure MgO and preformed spinel composites, as a function of quench temperature. 

Spinel composites should be more useful than pure MgO in terms of resistance to thermal shock damage and further loss of strength, because the relative strengths of all composite materials after shocking remain much higher than those of MgO (Fig. 14.13). These results show clearly that the resistances to thermal shock damage for all these materials have the trends expected on the basis of the calculated R" and R" parameters.74,103... 

Figure 14.14(b) demonstrates that at a quench temperature of 600°C for 20% 22 pm spinel composite, cracks start developing, and the number on polished surfaces increases smoothly up to a quench temperature of 1000°C. Cracks do not progress into the bar centres, and the distance of crack penetration through the bar remains very small, even at the highest quench temperature of 1000°C. 

No particular quench temperature at which crack density increased markedly is observed using dye penetrant for the spinel composites, in contrast to the MgO materials. It is clearly seen that there is a gradual development of a dense microcrack network in the spinel composites, with increase in the quench temperature. 

Fraction retained strengths of MgO, and spinel composites, after a quench from 800°C, as a function of Ff " parameter. 

The relationships between the K1C and fracture surface energy are similar to each other, and generally decrease with increasing spinel content, apart from the in situ formed composites which show different microstructures and fracture paths. It is therefore clear that the development of thermal shock resistance in the magnesia-spinel composites cannot be linked to any increased fracture toughness. 

After quenching at 600°C, a large number of cracks appear on the polished surfaces of MgO bars, and a small proportion of cracks progress towards the centres of the bars. In contrast, the 22 pm spinel composites do not show a critical quench temperature a gradual development of microcrack networks is observed with increasing quench temperature, and cracks do not progress into the cross-section of the bars. 

For pure MgO, cracks propagate catastrophically after shocking from above the critical quench temperature, with a large decrease in strength. However, spinel composites of all types have higher relative strengths after quenching than pure MgO. 

The 20% 22 pm spinel composites show a significant improvement in shock damage in terms of increased difficulty of crack propagation. In general, coarser spinel particles are more beneficial than fine, but there is no advantage with additions of >30% for all the particle sizes. 

Aksel, C., Spinel formation, reaction conditions and densification properties of magnesia-spinel composites , Key Eng. Mater, 2004 264-8 1071 -4. 

Aksel, C., Thermal Shock Behaviour and Mechanical Properties of Magnesia-Spinel Composites , PhD Thesis, Department of Materials Engineering, University of Leeds, Leeds, UK, 1998. 

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