**Author details**

294 Heat Treatment – Conventional and Novel Applications

**5. Conclusion** 

method also varied with heating rate.

thermal conductivity of the material.

Finally, as we have already noted the material properties of the 350°C material are similar to that of the 400°C sample in terms of composition, although key differences are noted in the structure and morphology, as highlighted in Figure 10. These differences are significant in light of the superior electrochemical characteristics of the 350°C material. Figure 10 shows that heat treatment at 350°C has resulted in the greatest contraction of the unit cell and also a lowering of the BET surface area relative to the other HEMD materials. The smaller unit cell is the main structural difference between the 350°C and 400°C materials, which otherwise have very similar Pr values (0.83 and 0.84, respectively). This suggests that the proportion of each phase is not the only structural feature influencing the movement of lithium ions through the material, and that the unit cell volume is also significant. The high performance of the 350°C material is an indication that lower unit cell volumes are favoured for lithium diffusion in HEMD. The morphology differences between these materials are also likely to play a significant role in determining their relative electrochemical performance. As previously noted, the lower BET surface area for the 250°C sample due to a lower volume of micropores, was found to enhance its capacity over the 200°C material. Similarly, the change in morphology brought about by the removal of these micropores in

the 350°C material has clearly improved the material capacity across discharge rates.

In conclusion, this work has shown that kinetic analysis can be employed in optimising the thermal processing of positive electroactive materials. As an example, kinetic analysis using a first-order kinetic analysis method and an incremental integral approach was performed for the water loss process from electrolytic manganese dioxide, the precursor material for use in non-aqueous Li/MnO2 cells. Both methods determined the pre-exponential factor and the activation energy for this process as a function of the extent of conversion, hence allowing for variation in the values over the process studied, while the incremental integral

Comparison and investigation of the two methods revealed the incremental integral method as the preferred means for obtaining the kinetic parameters describing this process. This result stems back to the use of a single TG experiment and the exclusive use of a slower scan rate using the incremental integral method (multiple scan rates, and thus experiments, are required in the first order kinetic analysis method). A single experiment avoids inherent variability between multiple experiments and limits errors induced in the somewhat arbitrary application of the background correction. Using slower scan rates reduces the possible undesired effects arising from thermal gradients across the sample as a result of the

Using the kinetic parameters determined for this process, and assuming first order kinetics, a family of thermal treatment regimes based on heating temperature and duration were calculated to, in theory, completely remove water from the manganese dioxide structure.

Characterization of the resulting HEMD materials revealed considerable differences in their properties. It was found that materials undergo a mainly kinetically limited thermal Wesley M. Dose and Scott W. Donne *Discipline of Chemistry, University of Newcastle, Callaghan, Australia* 
