**2. Preparation of glasses by the Sol-Gel method**

In the literature there is a considerable number of works on the preparation of glasses by melt-quenching. However, it does not exist, to our knowledge, a comparable number of publications on the same type of glasses prepared by sol-gel method. The preparation of materials by sol-gel, with relevant scientific and technological characteristics only began after 1930. However this method was discovered in late 1800. It was, only, after 1970, and with the preparation of inorganic monolithic gels and their subsequent thermal treatment, resulting in glasses, that this method start to developed [1-2].

The sol-gel method has opened a new path for the preparation of glasses of high technological interest, presenting a high degree of chemical purity and very high homogeneity. The preparation of a glass according to this method is performed by mixing, in the liquid state, its constituents until a homogeneous solution, at the molecular level, is

#### 326 Heat Treatment – Conventional and Novel Applications

reached. It is then subjected to polymerization and gelling (gel point is defined as the point, in time, where the mixture forms a rigid substance and can be removed from the original container [3]). The resulting gel is transformed into glass by heat treatment during which the volatile species are eliminated and the material undergoes a densification [4]. This method of preparation is known by the generic name sol-gel process. In this process there are two variants, which have the common passage through a gel phase (forming a 3D network [1]) but which differ in the starting products and the first steps of reaction. In one, it starts with a colloidal suspension and in the other with metal-organic compounds, which are dissolved in alcohols. Indeed, the scientific name sol-gel process can only be applied to the first case but it is also used to the second [4;5].

Lithium Niobiosilicate Glasses Thermally Treated 327

water, alcohol and acid to induce the hydrolysis. Parameters such as the pH, the molar ratio between H2O and the alkoxide and the presence of catalysts (acids) may, as desired, that the onset of condensation is given only after the end of the hydrolysis of the metal alkoxide. In addition to this fact and due to the immiscibility between the water and the alkoxide it is necessary to use a mutual solvent, such as ethanol (C2H5OH). The reactions of hydrolysis and polymerization induce an increase of the solution viscosity until it becomes a gel. In the

third stage the gel is heat treated until be converted into a glass [2;6].

**Figure 1.** Flowchart of the sol-gel process used in the formation of a SiO2-TiO2 glass [6].

high cost of starting materials, namely the metal alkoxides [7;8].

With this sol-gel process it is possible to have a high control in the interaction between the liquid precursors, minimizing the energy required for the process, obtaining a final product with high homogeneity and with a high control of its morphology [7]. In summary, the major advantages of this sol-gel process are the high purity of the end product, the low temperature processing (these characteristics are related to the nature of the starting materials), high homogeneity, novel compositions, which are very difficult or even impossible to synthesize by other processes [7;8] and the ability of the shape of final products to be wide (thin film, monolithic blocks, fibers, beads and powders) [6;9;10;11;12].

The major disadvantage of the sol-gel method, for the preparation of glasses, when compared to the melt quenching method (traditional method for glass preparation), is the

A major problem related with the preparation of monolithic blocks is the shrinkage of the gel during the drying process, leading to very high internal mechanical stresses and therefore fracture of the block can occur [13]. This shrinkage is related to the removal of fluids which are inside the pores, resulting in a stress on the capillary walls, which is inversely proportional to the pore diameter [14]. One way to minimize the gel fracture is to

The method using solutions of polymerizable species, which was used in the preparation of the gels presented in this chapter, starts with liquids or alcoholic solutions of an organometallic compound such as the metal alkoxides M(OR)n, where M is a metal and R an alkyl group, exposing them to reactions of hydrolysis, polymerization and dehydration [3;6]. In this method, the alkoxides usually used are the tetraethylorthosilicate (TEOS) and the tetrametoxisilicate (TMOS). However, in several papers other type of alkoxides can be found, such as boron, aluminum or titanium but, usually, mixed with the TEOS. In the case of the gels prepared in this work, the TEOS solution was the only used and the remained components were introduced in the form of nitrate (LiNO3) and chloride (NbCl5), mainly due to their high solubility in water and/or alcohol.

Regarding the chemical formation of a polymeric gel from metal alkoxides (M (OR) n), it is typically described by two consecutive reactions:

$$\text{--}\quad\text{hydrolysis (}\,M\text{(OR)}\_{n}\text{ + xH}\_{2}\text{O}\rightarrow\text{M(OH)}\_{x}\text{(OR)}\_{n-x}\text{ + xROH )}\text{ --}$$

and the condensation, which can be subdivided into two phases:


Briefly, the hydrolysis reaction is the substitution of the alkoxy group (OR) with a hydroxide group (OH). The subsequent condensation reactions involves, in the case of TEOS (Si(OC2H5)4), the silanol group (Si-OH) that gives rise to siloxane bonds Si-O-Si), water and alcohol. It must be noticed that these reactions are, however, a simplified version of which occurs during the hydrolysis and condensation of the alkoxide solutions [4;5].

The diagram showed in figure 1 represents, according to Sakka et al. [6], the preparation of a glass from metal alkoxides. This diagram shows the preparation of a binary glass of the SiO2-TiO2 system. The treatment temperatures found in the flowchart are only examples, because they depend directly on the glass composition.

According to figure 1, the preparation process can be divided into three steps. The first involves the mixing of the alkoxide in the amounts corresponding to the final composition of the glass, yielding a clear solution. In the second step it is added to the alkoxide solution water, alcohol and acid to induce the hydrolysis. Parameters such as the pH, the molar ratio between H2O and the alkoxide and the presence of catalysts (acids) may, as desired, that the onset of condensation is given only after the end of the hydrolysis of the metal alkoxide. In addition to this fact and due to the immiscibility between the water and the alkoxide it is necessary to use a mutual solvent, such as ethanol (C2H5OH). The reactions of hydrolysis and polymerization induce an increase of the solution viscosity until it becomes a gel. In the third stage the gel is heat treated until be converted into a glass [2;6].

326 Heat Treatment – Conventional and Novel Applications

it is also used to the second [4;5].

due to their high solubility in water and/or alcohol.

typically described by two consecutive reactions:


and the condensation, which can be subdivided into two phases:

because they depend directly on the glass composition.


occurs during the hydrolysis and condensation of the alkoxide solutions [4;5].


Briefly, the hydrolysis reaction is the substitution of the alkoxy group (OR) with a hydroxide group (OH). The subsequent condensation reactions involves, in the case of TEOS (Si(OC2H5)4), the silanol group (Si-OH) that gives rise to siloxane bonds Si-O-Si), water and alcohol. It must be noticed that these reactions are, however, a simplified version of which

The diagram showed in figure 1 represents, according to Sakka et al. [6], the preparation of a glass from metal alkoxides. This diagram shows the preparation of a binary glass of the SiO2-TiO2 system. The treatment temperatures found in the flowchart are only examples,

According to figure 1, the preparation process can be divided into three steps. The first involves the mixing of the alkoxide in the amounts corresponding to the final composition of the glass, yielding a clear solution. In the second step it is added to the alkoxide solution

reached. It is then subjected to polymerization and gelling (gel point is defined as the point, in time, where the mixture forms a rigid substance and can be removed from the original container [3]). The resulting gel is transformed into glass by heat treatment during which the volatile species are eliminated and the material undergoes a densification [4]. This method of preparation is known by the generic name sol-gel process. In this process there are two variants, which have the common passage through a gel phase (forming a 3D network [1]) but which differ in the starting products and the first steps of reaction. In one, it starts with a colloidal suspension and in the other with metal-organic compounds, which are dissolved in alcohols. Indeed, the scientific name sol-gel process can only be applied to the first case but

The method using solutions of polymerizable species, which was used in the preparation of the gels presented in this chapter, starts with liquids or alcoholic solutions of an organometallic compound such as the metal alkoxides M(OR)n, where M is a metal and R an alkyl group, exposing them to reactions of hydrolysis, polymerization and dehydration [3;6]. In this method, the alkoxides usually used are the tetraethylorthosilicate (TEOS) and the tetrametoxisilicate (TMOS). However, in several papers other type of alkoxides can be found, such as boron, aluminum or titanium but, usually, mixed with the TEOS. In the case of the gels prepared in this work, the TEOS solution was the only used and the remained components were introduced in the form of nitrate (LiNO3) and chloride (NbCl5), mainly

Regarding the chemical formation of a polymeric gel from metal alkoxides (M (OR) n), it is

**Figure 1.** Flowchart of the sol-gel process used in the formation of a SiO2-TiO2 glass [6].

With this sol-gel process it is possible to have a high control in the interaction between the liquid precursors, minimizing the energy required for the process, obtaining a final product with high homogeneity and with a high control of its morphology [7]. In summary, the major advantages of this sol-gel process are the high purity of the end product, the low temperature processing (these characteristics are related to the nature of the starting materials), high homogeneity, novel compositions, which are very difficult or even impossible to synthesize by other processes [7;8] and the ability of the shape of final products to be wide (thin film, monolithic blocks, fibers, beads and powders) [6;9;10;11;12].

The major disadvantage of the sol-gel method, for the preparation of glasses, when compared to the melt quenching method (traditional method for glass preparation), is the high cost of starting materials, namely the metal alkoxides [7;8].

A major problem related with the preparation of monolithic blocks is the shrinkage of the gel during the drying process, leading to very high internal mechanical stresses and therefore fracture of the block can occur [13]. This shrinkage is related to the removal of fluids which are inside the pores, resulting in a stress on the capillary walls, which is inversely proportional to the pore diameter [14]. One way to minimize the gel fracture is to 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 glycerol (230 °C, 14-18h); (2) burning of organic waste; (3) elimination of pores.

Lithium Niobiosilicate Glasses Thermally Treated 329

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

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

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

be as slow as possible.

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

**4. Glass-ceramics preparation** 

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

purpose. Figure 5 shows a schematic draw of the oven.

**4.2. Thermoelectric treatments (TET)** 

**4.1. Thermal treatments (TT)** 

Linseis Aparatus [18].

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 on the composition.
