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Figure 40. XRD traces of ZnAl2O4 ceramic calcined at different temperatures. Reprinted with permission from [286], S. Mathur et al., J. Am. Ceram. Soc. 84, 1921 (2001). © 2001, American Ceramics Society.

The absorbance spectra (Fig. 42) of ZnAl2O4 nano-particles heat-treated (6 h) at 700, 800, 1000, 1200, and 1400 °C reveal a systematic shift in the absorption maxima. The absorbance edge became steeper with increasing the calcination temperature, and a "red shift" (4.5 to 4.0 eV) was observed (Fig. 42). Since the bandgap, Eg, of a nano-crystalline specimen scales with its diameter, d, as Eg ~ 1/d,

Figure 41. TEM micrographs of ZnAl2O4 ceramic calcined at 600 (a), 800 (b), 1000 (c), and 1200 "C (d). Reprinted with permission from [286], S. Mathur et al., J. Am. Ceram. Soc. 84, 1921 (2001). © 2001, American Ceramics Society.
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