**1. Introduction**

322 Heat Treatment – Conventional and Novel Applications

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The term glass comes from the latin word, vitrium, and refers to one of the oldest known material. It is defined in accordance with the ASTM C-169-92 norm, as an inorganic product obtained by quenching a melt until hardness conditions, without crystallization. This definition is however too restrictive because it only applies to glasses prepared by fusion method. A broader definition is that proposed by A. Paul: the glass shows the elastic behavior characteristic of the crystalline state and the behavior of the viscous liquid state. The most common properties of the glasses are the transparency to visible radiation, mechanical stability, inert at the biological level and dielectric material. However, due to the possibility of controlling the microstructure by changing the composition or by applying heat treatments, controlling the process of nucleation and crystallization, the properties of glasses can be modified.

The initial chemical composition is a factor, controllable, which allows to shape some of the properties of the glasses. In any glass, the units that define its structure can be divided into three categories, defined according to their structural function, network former; modifier and intermediate species. The formers are units that, without the addition of other elements, can form glass such as SiO2, B2O3, GeO2 and P2O5. The network modifiers, do not form glass by itself, but are often combined with a former. Examples of modifying elements are the alkaline elements (Li, Na, K, etc.) and alkaline-earth metals (Mg, Ca, etc.). The intermediate species are elements that can both play a role in forming or modifying the network (Al, Nb, etc.).

The formation of glass ceramics, for example by heat treatment of the as-prepared glass, shows at the technological level great advantages, for the single crystals and sintered ceramics, as the possibility of their properties (optical, electrical, mechanical, chemical, etc. ) be controlled via the volume fraction of the active phase dispersed in the matrix. For example, to maintain optical transparency, the process of nucleation and crystal growth

© 2012 Graça and Valente, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Graça and Valente, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

requires a great degree of control being achieved when the size of the crystals dispersed in the glassy matrix is not high enough to cause light scattering. However, for most electrical applications it is necessary that the crystals are large enough to present, for example, a ferroelectric response. This commitment is not easy to perform. Another condition that can maintain the optical transparency of a glass ceramic is the small difference between the refractive indices of the crystals and of the glassy matrix. If this difference is insignificant it allows, regardless of the size of the crystals, to keep the optical transparency. In recent years there has been a growing interest in the preparation, characterization and technology implementation of glasses and glass ceramics in different type of devices. However, it is important to note that, in general, the electrical and optical properties of glass ceramics are not as good as their single crystal when not embedded in a matrix. This is due to the fact that the glass ceramics present at least two distinct phases, the crystal (considered the active phase) and the amorphous (support). The electric polarization of the crystals embedded in a glassy matrix is more difficult due to the low value of the dielectric constant of the glassy phase. On the other hand and because of the single crystal growth processes present a high economic cost, have now start to been replaced by glass ceramics. Some glass ceramics have the additional advantage of being dense and not porous materials.

Lithium Niobiosilicate Glasses Thermally Treated 325

devices. Thus the preparation, structural, electrical and optical characterization of glass ceramic showing ferroelectric properties is of great importance for potential technological

Of the various ferroelectric known materials, lithium niobate (LiNbO3), first synthesized in 1949 by Matthias and Remeika, had attracted attention from many researchers due to their excellent piezoelectric properties, electro optics, electroacoustic, pyroelectric and photorefractive. In late 1960, due to the appearance and development of various applications of fiber optics, several research centers, including Bell Laboratories, studied in detail the structural characteristics and properties of LiNbO3 crystal, especially its electro

The fact that the preparation of LiNbO3 single crystals, by the traditional growth techniques is technically difficult and economically costly, the scientific research about the preparation methods of inorganic glasses containing LiNbO3 crystals is the main topic of this chapter. Their structural, morphological, optical and electrical properties will be discussed in

The as-prepared glasses were prepared by sol-gel method. This method allowed to prepare glasses of molar composition (100-2x)SiO2-xLi2O-xNb2O5, with x < 10, that through the meltquenching method are of extreme difficulty to prepare. The synthesis of the glass ceramics was performed by heat treatment of the as-prepared glass and the nucleation and growth of crystals in the glass matrix controlled by the heating rate, temperature and treatment time. It was introduced for the first time, a new variable in this type of glass treatments that was the presence of a external dc electric field, during the heat-treatment cycle.. These heat treatments, with external electrical field applied treatments were called thermoelectric (TET). The main objective of the TET is the hypothesis of promote the precipitation of nanosize crystals in a crystalline preferred orientation. In all prepared compositions, LiNbO3 particles were precipitated due to the thermal treatments, in the glass surfaces. The electric analysis showed the presence of conduction, relaxation and polarization mechanisms.

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,

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

applications.

optics properties.

function of the glass treatment parameters.

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

resulting in glasses, that this method start to developed [1-2].

The growth and crystal orientation can generally be achieved through different processes, such as: mechanical deformation, thermodynamic control, kinetic control (electrochemical induced nucleation). In glasses the use of thermodynamic control is the most common. However, control of crystallization, with the desired crystalline phase is usually difficult because crystallization is a complex process affected by various factors such as composition, surface conditions and heat treatment parameters.

The crystallization process, in a glass, of a particular crystalline phase oriented in a predefined direction is usually a desirable objective but difficult to implement. One way that can induce the crystallization of oriented particles in a glass is the application, together with the thermal process of external fields (magnetic / electric). In the presence of an electric field polar nuclei should be oriented parallel to the field and the existing nuclei and / or crystallites can rotate until reach the same direction. However, this can only occur if the value of the electric field is high enough to allow the resultant force to overcome the viscous medium, enabling the rotation. Currently, glass ceramics containing ferroelectric crystals are a class of materials with high technological interest because of the ferroelectric crystals presenting a structural anisotropy results in the formation of electric dipoles and therefore a spontaneous electric polarization.

A large number of ferroelectric materials is presented in the form of crystalline ceramic. The scientific and technological development that photonics has been presenting, has required, particularly in terms of applications, new materials which exhibit characteristics such as optical transparency and are optically active so they can be used as amplifiers, switches, sensors, transducers, filters (…) , that is because there is a need for materials that use light to perform functions already implemented in electronic devices (mainly in the sectors of communications, energy, instrumentation, etc.), but more efficiently or giving rise to new devices. Thus the preparation, structural, electrical and optical characterization of glass ceramic showing ferroelectric properties is of great importance for potential technological applications.

324 Heat Treatment – Conventional and Novel Applications

requires a great degree of control being achieved when the size of the crystals dispersed in the glassy matrix is not high enough to cause light scattering. However, for most electrical applications it is necessary that the crystals are large enough to present, for example, a ferroelectric response. This commitment is not easy to perform. Another condition that can maintain the optical transparency of a glass ceramic is the small difference between the refractive indices of the crystals and of the glassy matrix. If this difference is insignificant it allows, regardless of the size of the crystals, to keep the optical transparency. In recent years there has been a growing interest in the preparation, characterization and technology implementation of glasses and glass ceramics in different type of devices. However, it is important to note that, in general, the electrical and optical properties of glass ceramics are not as good as their single crystal when not embedded in a matrix. This is due to the fact that the glass ceramics present at least two distinct phases, the crystal (considered the active phase) and the amorphous (support). The electric polarization of the crystals embedded in a glassy matrix is more difficult due to the low value of the dielectric constant of the glassy phase. On the other hand and because of the single crystal growth processes present a high economic cost, have now start to been replaced by glass ceramics. Some glass ceramics have

The growth and crystal orientation can generally be achieved through different processes, such as: mechanical deformation, thermodynamic control, kinetic control (electrochemical induced nucleation). In glasses the use of thermodynamic control is the most common. However, control of crystallization, with the desired crystalline phase is usually difficult because crystallization is a complex process affected by various factors such as composition,

The crystallization process, in a glass, of a particular crystalline phase oriented in a predefined direction is usually a desirable objective but difficult to implement. One way that can induce the crystallization of oriented particles in a glass is the application, together with the thermal process of external fields (magnetic / electric). In the presence of an electric field polar nuclei should be oriented parallel to the field and the existing nuclei and / or crystallites can rotate until reach the same direction. However, this can only occur if the value of the electric field is high enough to allow the resultant force to overcome the viscous medium, enabling the rotation. Currently, glass ceramics containing ferroelectric crystals are a class of materials with high technological interest because of the ferroelectric crystals presenting a structural anisotropy results in the formation of electric dipoles and therefore a

A large number of ferroelectric materials is presented in the form of crystalline ceramic. The scientific and technological development that photonics has been presenting, has required, particularly in terms of applications, new materials which exhibit characteristics such as optical transparency and are optically active so they can be used as amplifiers, switches, sensors, transducers, filters (…) , that is because there is a need for materials that use light to perform functions already implemented in electronic devices (mainly in the sectors of communications, energy, instrumentation, etc.), but more efficiently or giving rise to new

the additional advantage of being dense and not porous materials.

surface conditions and heat treatment parameters.

spontaneous electric polarization.

Of the various ferroelectric known materials, lithium niobate (LiNbO3), first synthesized in 1949 by Matthias and Remeika, had attracted attention from many researchers due to their excellent piezoelectric properties, electro optics, electroacoustic, pyroelectric and photorefractive. In late 1960, due to the appearance and development of various applications of fiber optics, several research centers, including Bell Laboratories, studied in detail the structural characteristics and properties of LiNbO3 crystal, especially its electro optics properties.

The fact that the preparation of LiNbO3 single crystals, by the traditional growth techniques is technically difficult and economically costly, the scientific research about the preparation methods of inorganic glasses containing LiNbO3 crystals is the main topic of this chapter. Their structural, morphological, optical and electrical properties will be discussed in function of the glass treatment parameters.

The as-prepared glasses were prepared by sol-gel method. This method allowed to prepare glasses of molar composition (100-2x)SiO2-xLi2O-xNb2O5, with x < 10, that through the meltquenching method are of extreme difficulty to prepare. The synthesis of the glass ceramics was performed by heat treatment of the as-prepared glass and the nucleation and growth of crystals in the glass matrix controlled by the heating rate, temperature and treatment time. It was introduced for the first time, a new variable in this type of glass treatments that was the presence of a external dc electric field, during the heat-treatment cycle.. These heat treatments, with external electrical field applied treatments were called thermoelectric (TET). The main objective of the TET is the hypothesis of promote the precipitation of nanosize crystals in a crystalline preferred orientation. In all prepared compositions, LiNbO3 particles were precipitated due to the thermal treatments, in the glass surfaces. The electric analysis showed the presence of conduction, relaxation and polarization mechanisms.
