**6. Samples preparation**

330 Heat Treatment – Conventional and Novel Applications

specified and justified in following sections.

**5. Samples composition** 

• 92SiO2-4Li2O-4Nb2O5 (mole%); • 88SiO2-6Li2O-6Nb2O5 (mole%); • 84SiO2-8Li2O-8Nb2O5 (mole%);

**Figure 5.** Schematic draw of the oven used for the thermoelectric treatments [19].

By the sol-gel method the following two compositions were prepared:

characteristics. This composition was therefore not full characterized.

electrical analyzes and their discussion, are the following sections.

The dc external electric field was produced using a high dc voltage source (PS325 Stanford Research System), which could apply a potential difference between 25 V and 2500 V, with a maximum current of 10.5 mA. The temperature was controlled by a Digi-Sense Temperature Controller R/S. All the treatment process is controlled by computer. In these treatments, the thermoelectric cycles used (heating ramp, threshold temperature, treatment time and cooling ramp) were equal to those used in the treatments without the presence of an external field (horizontal tube furnace). In the TET treatments the dc electric field was applied during the periods of heating and dwell, and switched off at the beginning of the cooling step. The parameters: temperature level (Tp) and electric field amplitude are

referenced from here by 92Si, 88Si and 84Si, respectively. The 84Si composition did not form a transparent and amorphous gel and glass, indicating the composition limit for those

The preparation process of the glasses and glass ceramics, the results of structural and

The preparation of the based glass with molar composition 92SiO2-4Li2O-4Nb2O5 and 88SiO2-6Li2O-6Nb2O5 followed the procedure described in figure 2. With the aim of obtain glass ceramics containing LiNbO3 crystallites, heat treatments (TT) were carried out on the as-prepared glass samples (treated at 120 ° C for 48 h and subsequently at 500 ° C for 4 h), which present a thickness between 0.6 and 1.0 mm. For the TT, carried out in a horizontal tubular furnace, the threshold temperature (Tp) choice, differential thermal analyzes (DTA) was performed to the base glass of each composition. The temperatures at the observed exothermic effects, which can indicate the occurrence of crystallization, lead to the definition of the threshold temperatures, which in the 92Si composition case were the following: 650, 700, 750 and 800 °C. The 88Si based glass was TT at 600, 650, 700 and 800 ° C. [16;17]

The 92Si based glass was also subjected to thermoelectric treatments (TTE), which followed the same thermal profile of the TT. The 92Si based samples were therefore TTE at 650, 700 and 750 °C, for 4 hours. For each temperature, three different TTE were performed, differing in the amplitude value of the electric field applied: i) 100 kV/m, ii) 500 kV/m and iii) 1000 kV/m. These values were selected based on the thickness of the samples and the characteristics of the dc voltage source.
