**3. Preparation of SiO2-Li2O-Nb2O5 glasses by sol-gel – Experimental description**

The sol-gel method was used to prepare clear glasses of the ternary system SiO2-Li2O-Nb2O5. The choice of the molar compositions was based on the following criteria: equal molar amounts of lithium oxide and niobium oxide; obtain a clear gel. In this method, the starting materials were the lithium nitrate (LiNO3), niobium chloride (NbCl5), hydrogen peroxide (H2O2 - 30% V/V), tetraethylorthosilicate (TEOS) and ethanol (C2H5OH) as the mutual solvent.

The preparation steps are presented in the diagram of figure 2.

**Figure 2.** Diagram of the method used to prepare the glasses [16;17].

All samples were prepared using a molar ratio between (C2H5O)4Si : C2H5OH : H2O2 of 1:3:8. Hydrogen peroxide (H2O2) was used in the form of aqueous H2O2 (3% V/V). This dilution was carried out using deionized water. All solutions, placed in petri boxes were left to gelling at a constant temperature of 30 °C, during more than 1 week. The gel was submitted to a heat treatment which gave rise to the as-prepared sample. This treatment (Fig.3) consists of two steps. First, at a temperature of 120 °C for 48 °C, with the purpose of release the maximum number of free H2O groups. The second stage has two heat levels. The first at 250 °C, with the purpose of releasing some H2O groups, that probably still exist [4;5] and the second, at 500 °C, whose main aim is the liberation of the CO2 groups from the oxidation of organic radicals [nav91; sil90] . For this reason the heating rate must be as slow as possible.

**Figure 3.** Diagram of the drying process.

328 Heat Treatment – Conventional and Novel Applications

on the composition.

**description** 

solvent.

control the drying process. For example, Chou et al. [13] present a thermal process in which includes: (a) removal of ethanol (50 °C), water (90 °C, 4h), formamide (170 °C, 4h) and

The heating rate, usually less than or equal to 5 °C/min, the treatment temperature and the treatment duration time are critical factors that must be control, to prevent fracture. Furthermore, the use of long treatment times, at or near room temperature (20-50 °C), promote poly-condensation, which is an advantage to produce gels with well-defined microstructure, reducing the stresses [13]. Another important factor is to control the thickness of the gel. The greater the thickness of the gel greater the time required to complete the reaction [3-5]. The transition from gel to glass is usually accompanied through drying and sintering by an appropriate heat treatment process [2;13]. This process depends

**3. Preparation of SiO2-Li2O-Nb2O5 glasses by sol-gel – Experimental** 

The preparation steps are presented in the diagram of figure 2.

**Figure 2.** Diagram of the method used to prepare the glasses [16;17].

The sol-gel method was used to prepare clear glasses of the ternary system SiO2-Li2O-Nb2O5. The choice of the molar compositions was based on the following criteria: equal molar amounts of lithium oxide and niobium oxide; obtain a clear gel. In this method, the starting materials were the lithium nitrate (LiNO3), niobium chloride (NbCl5), hydrogen peroxide (H2O2 - 30% V/V), tetraethylorthosilicate (TEOS) and ethanol (C2H5OH) as the mutual

All samples were prepared using a molar ratio between (C2H5O)4Si : C2H5OH : H2O2 of 1:3:8. Hydrogen peroxide (H2O2) was used in the form of aqueous H2O2 (3% V/V). This dilution was carried out using deionized water. All solutions, placed in petri boxes were left to gelling at a constant temperature of 30 °C, during more than 1 week. The gel was submitted to a heat treatment which gave rise to the as-prepared sample. This treatment (Fig.3) consists of two steps. First, at a temperature of 120 °C for 48 °C, with the purpose of release the maximum number of free H2O groups. The second stage has two heat levels.

glycerol (230 °C, 14-18h); (2) burning of organic waste; (3) elimination of pores.

## **4. Glass-ceramics preparation**

#### **4.1. Thermal treatments (TT)**

The dried gels were submitted to heat treatments in order to obtain glass-ceramics with the LiNbO3 crystalline phase. It is important to refer that the samples prepared by this method present a thickness of 1 mm, approximately. Figure 4 depicts the profile of the heat treatment used. These treatments were performed in a horizontal tube furnace. The value of the threshold temperature parameter (Tp) was chosen based on the information obtained from the thermal analysis of each composition. This thermal analysis was performed using a Linseis Aparatus [18].

**Figure 4.** Diagram of the thermal treatment process.

#### **4.2. Thermoelectric treatments (TET)**

The heat treatments with the presence of an external electrical field, named as thermoelectric treatment, were carried out in a vertical tube furnace, designed and constructed for this purpose. Figure 5 shows a schematic draw of the oven.

Lithium Niobiosilicate Glasses Thermally Treated 331

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 °

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

Figure 6 shows the macroscopic aspect of the 92Si samples, TT at the temperatures of 650, 700 and 750 °C. The based glass, completely transparent, becomes translucent for

The 92Si samples, TTE, were named as: 650A (sample TTE at 650 °C with an electric field of 100 kV/m), 650B (sample TTE 650 °C with an electric field of 500 kV/m) and 650C (TTE sample at 650 °C with an electric field of 1000 kV/m). The same designation was used in TTE samples at temperatures of 700 (700A, 700B ... ) and 750 °C. In figure 7, photographs of all

As-prepared TT700 TT750

The samples 650A, 700A and 750A (samples TTE with a field amplitude of 100 kV/m) have a macroscopic aspect very similar to the sample 650B. With the increase of the TET

temperature and applying 500 kV/m and 1000 kV/m, all samples become translucent.

**6. Samples preparation** 

characteristics of the dc voltage source.

temperatures above 700 °C.

**7. 92Si samples composition results** 

**Figure 6.** Photographs of the 92Si TT glasses (minor division = 0 1 mm).

samples subjected to those treatments can be seen.

C. [16;17]

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

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 specified and justified in following sections.

## **5. Samples composition**

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


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 characteristics. This composition was therefore not full characterized.

The preparation process of the glasses and glass ceramics, the results of structural and electrical analyzes and their discussion, are the following sections.
