**Author details**

192 Heat Treatment – Conventional and Novel Applications

magnesium spinel.

**4. Conclusions** 

**Figure 12.** TEM image from magnesium doped Al2O3. It is considered that the high temperature annealing makes magnesium atoms to diffuse to the surface of the clusters, creating a thin layer of

at higher temperatures, and an increase in the crystallite size was already perceptible.

size is about 300 nm in the same calcination conditions.

Polycrystalline powder of α-Al2O3 was successfully obtained using sol-gel and Pechini process. A second phase was found in doped samples, forming nanocrystals of aluminates and lanthanides oxides on the surface of alumina grains. We did not observe the incorporations of the dopants inside the alumina structure. Strongly dependent of temperature calcinations, the nanocrystallinity of the sample was retained after calcinations

The average diameter of nanocrystals depended on the dopant specie: for Yb and Er doped samples, it was D = (36 ± 2) nm for the sample calcinated at 1200 °C and D= (182 ± 8) nm for one calcinated at 1600 °C, therefore increased by 5 times. The approximately size of the AlNd is 200 nm for sample calcinated at 1600 °C during 4 h and for Tb3Al5O12 crystals the

The TL emission mechanism in the visible region can be related to F center and to the lanthanide (Ln) relaxation. During the irradiation the Ln3+ ion is reduced to Ln2+. It is not completely known if the reduction is due to the transfer of an electron or a hole; however, the process of reduction of trivalent lanthanide ions by irradiation was previously verified in literature [18, 19]. During thermal stimulation an electron can recombine with the Ln2+ forming Ln3+\* in excited state, which emits a photons returning on this way to the ground state. In the case of Mg spinel, probably the Mg ions promoted the oxygen vacancies Sonia Hatsue Tatumi \* , Alexandre Ventieri, José Francisco Sousa Bitencourt, Katia Alessandra Gonçalves, Juan Carlos Ramirez Mittani, René Rojas Rocca and Shiva do Valle Camargo *Universidade Federal de São Paulo, Universidade de São Paulo, Centro Estadual de Educação Tecnológica Paula Souza, Brazil* 
