**1. Introduction**

526 Sintering of Ceramics – New Emerging Techniques

Woermann, E. (1985) Die Thermische Stabilität von Pseudobrookit-Mischkristallen; DFG-

Wohlfromm, H.; Moya T. S. & Pena P. (1990) "Effect of ZrSiO4 and MgO Additions on

Yoleva, A.; Djambazov, S.; Arsenov, D. & Hristov, V. (2010) "Effect of SiO2 addition on

Reaction Sintering and Properties of Al2TiO5 Based Materials". *J. Mater. Sci* 25 3753-

thermal hysteresis of Aluminum titanate" J. Univ. of Chem. Tech. and Met. 45 3

Abschlu-bericht Tialit (AZ. W. 81/23).

3764.

269-274.

There is currently a high demand for advanced materials for different types of applications (see Fig. 1.1) in which besides a high mechanical performance, a partial or total transparency in a given spectral range is required. Transparent ceramics become more and more important for applications in which materials are subject to extremely high mechanical and thermal stress in combination with optical properties. More recently, interest has focused on the development of transparent armor materials (ceramic) for both military and civil applications. Also, the development of new optoelectronic devices has extended the use of ordinary optical materials to new applications and environments such as temperature (IR) sensor, optical fiber communications, laser interferometers, etc. A considerable fraction of these new devices operates in aggressive environments, such as ovens, radiation chambers and aerospace sensors. In such cases, the sensitive electronic component must be preserved from the extreme external condition by a transparent window. Transparent and coloured ceramics are also often used as wear and scratch resistant parts such as bearings and watch glasses as well as for their aesthetic Properties in synthetic opals and rubies.

In science and technology the word transparent is used for those components that show clear images regardless of the distance between the object and the transparent window. Clear transparency is achieved when after transmission through the window the light does not undergo noticeable absorption or scattering. This applies, for example, to some glasses, single-crystalline and polycrystalline transparent ceramics.

Most of ordinary optical transparent materials, glasses, polymers or alkali hydrides are soft, weak and/or brittle. Figure 1.2 shows the transmittance spectrum of BK7, a commercial glass widely used for visible optics. However, this material presents a strong absorption in the IR range making it of no use for this spectral range. Furthermore, this material shows a very low melting point (559ºC) so it cannot be used at high temperature.

Sintering to Transparency of Polycrystalline Ceramic Materials 529

Most of these problems can be solved with the use of polycrystalline materials as they show similar mechanical, chemical and thermal stability compared to single crystals and can be produced in complex shapes and are not size limited as single crystals. Among them, only a small number of materials such as MgF2, ZnS and ZnSe are used for optical applications, but generally their use is limited to coatings and thin films and their mechanical behavior is not appropriate. For this reason, the current trend moves towards the production of polycrystalline transparent ceramics which, by providing more versatility to properties and to the production of complex shapes, will broaden the fields of application. Compared to single crystals, polycrystalline materials have a complex microstructure consisting on grains, grain boundaries, secondary phases and pores. These factors have a great influence on the optical properties, resulting in transparent, translucent or opaque materials, as can be seen

In order to become transparent, the ceramic should be poreless and have optically perfect crystal boundaries and crystals. The surface of a pore is a boundary between phases with sharply different optical properties, and therefore, it intensely reflects and refracts light. A large number of pores makes ceramics opaque. Pores may be intercrystalline or intracrystalline. The elimination of intracrystalline pores, even if they are submicron in size, is a much longer process than elimination of closed intercrystalline pores. Intercrystalline pores occur at crystal boundaries which are sinks of vacancies, and this makes their removal easier. The presence of a second phase on the crystal boundaries which has different optical properties from the main crystalline phase leads to reflection and refraction of light and makes the ceramic less transparent. For this reason, transparent ceramics are obtained from raw material of high purity and the amount of additive is chosen so that they completely

The crystals in ceramics should be optically perfect, this is the absence of optical defects: pores, inclusions of the second solid phases, aggregate boundaries, and dislocations. In

in figure 1.3.

Fig. 1.3. Light scattering on polycrystalline ceramics.

dissolve in the solid solution with the main phase.

Fig. 1.1. Applications in which transparent materials are required.

However, some ceramic materials, such as corundum, spinel, yttria and YAG, do not show any absorption in a large range between IR and UV (see Fig. 1.2) and are suitable to work under extreme conditions due to their chemical stability and high mechanical performance. Many transparent ceramics are single crystal materials grown from the melt or by flame processes. However, the growth and machining of single crystals is an expensive task, which largely limits their scale up of production and, therefore, their range of applications.

Fig. 1.2. Transmittance spectra of different materials.

However, some ceramic materials, such as corundum, spinel, yttria and YAG, do not show any absorption in a large range between IR and UV (see Fig. 1.2) and are suitable to work under extreme conditions due to their chemical stability and high mechanical performance. Many transparent ceramics are single crystal materials grown from the melt or by flame processes. However, the growth and machining of single crystals is an expensive task, which largely limits their scale up of production and, therefore, their range of applications.

Personal vehicle

TRANSPARENT

 Al2 O3 MgAl2 O4

 Y2 O3 BK7

Armor vehicle

0 2 4 6 810

**Wavelength (**microns)

Fig. 1.1. Applications in which transparent materials are required.

0.0

Fig. 1.2. Transmittance spectra of different materials.

0.2

0.4

**Transmittance**

0.6

0.8

1.0

Office, shops, etc

Most of these problems can be solved with the use of polycrystalline materials as they show similar mechanical, chemical and thermal stability compared to single crystals and can be produced in complex shapes and are not size limited as single crystals. Among them, only a small number of materials such as MgF2, ZnS and ZnSe are used for optical applications, but generally their use is limited to coatings and thin films and their mechanical behavior is not appropriate. For this reason, the current trend moves towards the production of polycrystalline transparent ceramics which, by providing more versatility to properties and to the production of complex shapes, will broaden the fields of application. Compared to single crystals, polycrystalline materials have a complex microstructure consisting on grains, grain boundaries, secondary phases and pores. These factors have a great influence on the optical properties, resulting in transparent, translucent or opaque materials, as can be seen in figure 1.3.

Fig. 1.3. Light scattering on polycrystalline ceramics.

In order to become transparent, the ceramic should be poreless and have optically perfect crystal boundaries and crystals. The surface of a pore is a boundary between phases with sharply different optical properties, and therefore, it intensely reflects and refracts light. A large number of pores makes ceramics opaque. Pores may be intercrystalline or intracrystalline. The elimination of intracrystalline pores, even if they are submicron in size, is a much longer process than elimination of closed intercrystalline pores. Intercrystalline pores occur at crystal boundaries which are sinks of vacancies, and this makes their removal easier. The presence of a second phase on the crystal boundaries which has different optical properties from the main crystalline phase leads to reflection and refraction of light and makes the ceramic less transparent. For this reason, transparent ceramics are obtained from raw material of high purity and the amount of additive is chosen so that they completely dissolve in the solid solution with the main phase.

The crystals in ceramics should be optically perfect, this is the absence of optical defects: pores, inclusions of the second solid phases, aggregate boundaries, and dislocations. In

Sintering to Transparency of Polycrystalline Ceramic Materials 531

incorporation of nitrogen into aluminium oxide stabilizes a crystalline spinel phase, which due to its cubic crystal structure and unit cell, is an isotropic material which can be produced as a transparent ceramic nanomaterial. T. M. Hartnett et al (Hartnett et al., 1998) have obtained transparent AlON in the IR region by sintering at 1880ºC in a graphite oven. However, this material is not stable under low N2 pressure and at temperatures higher than 1640ºC. Furthermore, this material is susceptible to oxide in air atmosphere at temperatures

AlON 300 1950 323 3.68 2425 Sapphire 700 1500-2200 345-386 3.98 2300 Yttria 150 720 174 5.03 2430 Spinel 100-200 1400 273 3.58 2400 YAG 200-250 1215 300 4.55 1950 ZrO2 210 1100 180-200 6.52 2128

Zircona (ZrO2) is a material with a cubic structure that can be used as a transparent material in different fields: domestic, industrial and military. It is not amongst the hard ceramics (table 1), and its high refractive index near 2.2 is similar to some perowskites. This may explain why few investigations have been published on sintered (polycrystalline) transparent ZrO2. Single crystals of cubic zirconia are, however, important as artificial gemstones. Their high refractive index gives rise to a high degree of brilliance that comes close to diamonds. In the future the flexibility of powder technology in producing more complex shapes may stimulate the substitution of zirconia single crystals by sintered transparent decorative and optical products. Peuchert et al., (2009) have obtained a transparent material with 65% transmittance at 600 nm using vacuum atmosphere followed

Yttrium oxide (Y2O3) is another material with a cubic structure that can be used on pipes for discharge lamps or heat-resistant windows due to its corrosion resistance, thermal stability and transparency in a wide range of the electromagnetic spectrum (figure 2.1). Similarly, when introducing rare earths in its structure, it can be used as the active medium in solidstate lasers. Lukin et al., (1980) have obtained transparent yttria after sintering at 1900ºC under vacuum. Anderson (1998) has developed a ceramic Nd:Y2O3 by conventional sintering. Greskovish et al., (1973) have developed a laser Y2O3:Nd3+ doped with ThO2. La2O3 doped Y2O3 is of interest for IR applications because it is one of the longest wavelength transmitting oxides. It is a refractory with a melting point of 2430ºC and has a moderate thermal expansion coefficient (table 1). A major consideration is the low emissivity of yttria, which limits background radiation upon heating. In particular, lasers with ytterbium as dopant allow the efficient operation both in CW (Kong et al., 2005) and in

Modulus (GPa)

Density (g/ml)

Melting point (K)

higher than 1200ºC making it not useful as projection lamps (Wei, 2009).

Knoop hardness (Kg/mm2)

Material Strength

(MPa)

Table 1. Physical properties of ceramic materials.

by hot isostatic pressing using titanium oxide as sintering aid.

pulsed regimes (Tokurakawa et al., 2007).

ceramics from optically anisotropic crystals an additional scattering of light arises on the boundaries because of their arbitrary crystallographic orientation.

The amount of light scattering therefore depends on the wavelength of the incident radiation, or light. For example, since visible light has a wavelength scale on the order of hundreds of nanometers, scattering centers will have dimensions on a similar spatial scale. Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque as opposed to transparent materials. The degree of transparency also depends on the thickness of the component. These losses increase with growing thickness and result in translucent behavior. Transparency, which is independent of the thickness, is only possible for materials with an in-line transmission close to the theoretical maximum. These materials do not cause losses by absorption or scattering. These considerations lead us to the requirements to be observed by manufacturers of transparent ceramics.

#### **2. Ceramic materials**

Different materials have been proposed to prepare transparent ceramics such as alumina, spinel, yttria, AlON, YAG, etc (see Fig. 2.1).

Fig. 2.1. Transmittance spectra of ceramic materials.

Aluminium oxynitride spinel (Al23O27N5, AlON) is a cubic material and one of the leading candidates for transparent armor. This material is very useful for aircraft, missiles, IR and laser windows, etc (Mccauley et al., 2009) as it is stable up to 1200ºC, relatively light weight and resistance to damage due to oxidation or radiation. AlON is optically transparent (≥80%) in the near ultraviolet, visible and near infrared regions of the electromagnetic spectrum (see Fig. 2.1). It is four times harder than fused silica glass, 85% as hard as sapphire and nearly 15% harder than magnesium aluminate spinel (table 1). The

ceramics from optically anisotropic crystals an additional scattering of light arises on the

The amount of light scattering therefore depends on the wavelength of the incident radiation, or light. For example, since visible light has a wavelength scale on the order of hundreds of nanometers, scattering centers will have dimensions on a similar spatial scale. Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque as opposed to transparent materials. The degree of transparency also depends on the thickness of the component. These losses increase with growing thickness and result in translucent behavior. Transparency, which is independent of the thickness, is only possible for materials with an in-line transmission close to the theoretical maximum. These materials do not cause losses by absorption or scattering. These considerations lead us to the requirements to be observed

Different materials have been proposed to prepare transparent ceramics such as alumina,

 Al23O27N5 Al2 O3 MgAl2 O4

 Y2 O3 Y3 Al5 O12

2 3 4 5 6 7 8 9 10

Wavelength (microns)

Aluminium oxynitride spinel (Al23O27N5, AlON) is a cubic material and one of the leading candidates for transparent armor. This material is very useful for aircraft, missiles, IR and laser windows, etc (Mccauley et al., 2009) as it is stable up to 1200ºC, relatively light weight and resistance to damage due to oxidation or radiation. AlON is optically transparent (≥80%) in the near ultraviolet, visible and near infrared regions of the electromagnetic spectrum (see Fig. 2.1). It is four times harder than fused silica glass, 85% as hard as sapphire and nearly 15% harder than magnesium aluminate spinel (table 1). The

boundaries because of their arbitrary crystallographic orientation.

by manufacturers of transparent ceramics.

spinel, yttria, AlON, YAG, etc (see Fig. 2.1).

0.0

Fig. 2.1. Transmittance spectra of ceramic materials.

0.2

0.4

Transmittance

0.6

0.8

1.0

**2. Ceramic materials** 

incorporation of nitrogen into aluminium oxide stabilizes a crystalline spinel phase, which due to its cubic crystal structure and unit cell, is an isotropic material which can be produced as a transparent ceramic nanomaterial. T. M. Hartnett et al (Hartnett et al., 1998) have obtained transparent AlON in the IR region by sintering at 1880ºC in a graphite oven. However, this material is not stable under low N2 pressure and at temperatures higher than 1640ºC. Furthermore, this material is susceptible to oxide in air atmosphere at temperatures higher than 1200ºC making it not useful as projection lamps (Wei, 2009).


Table 1. Physical properties of ceramic materials.

Zircona (ZrO2) is a material with a cubic structure that can be used as a transparent material in different fields: domestic, industrial and military. It is not amongst the hard ceramics (table 1), and its high refractive index near 2.2 is similar to some perowskites. This may explain why few investigations have been published on sintered (polycrystalline) transparent ZrO2. Single crystals of cubic zirconia are, however, important as artificial gemstones. Their high refractive index gives rise to a high degree of brilliance that comes close to diamonds. In the future the flexibility of powder technology in producing more complex shapes may stimulate the substitution of zirconia single crystals by sintered transparent decorative and optical products. Peuchert et al., (2009) have obtained a transparent material with 65% transmittance at 600 nm using vacuum atmosphere followed by hot isostatic pressing using titanium oxide as sintering aid.

Yttrium oxide (Y2O3) is another material with a cubic structure that can be used on pipes for discharge lamps or heat-resistant windows due to its corrosion resistance, thermal stability and transparency in a wide range of the electromagnetic spectrum (figure 2.1). Similarly, when introducing rare earths in its structure, it can be used as the active medium in solidstate lasers. Lukin et al., (1980) have obtained transparent yttria after sintering at 1900ºC under vacuum. Anderson (1998) has developed a ceramic Nd:Y2O3 by conventional sintering. Greskovish et al., (1973) have developed a laser Y2O3:Nd3+ doped with ThO2. La2O3 doped Y2O3 is of interest for IR applications because it is one of the longest wavelength transmitting oxides. It is a refractory with a melting point of 2430ºC and has a moderate thermal expansion coefficient (table 1). A major consideration is the low emissivity of yttria, which limits background radiation upon heating. In particular, lasers with ytterbium as dopant allow the efficient operation both in CW (Kong et al., 2005) and in pulsed regimes (Tokurakawa et al., 2007).

Sintering to Transparency of Polycrystalline Ceramic Materials 533

with different crystalline orientations. Two approaches can be followed to minimize/reduce this scattering: (1) increasing the texture of the polycrystalline material during the forming stage to minimize the misorientations between grains (2) decreasing the grain size as much as possible, which usually implies starting with nanometer sized alumina particles and often making use of non conventional sintering techniques, such as microwave (Jiping Cheng et al., 2002) or spark plasma sintering. However, since Coble´s work in the 60´s (Coble, 1962), the possibility of using other ceramic materials than the cubic ones for optical applications

One way to avoid grain growth during sintering and obtain denser materials with reduced porosity is based on the modification of the diffusion mechanisms at high temperature by the introduction of second phases in grain boundaries or by doping with various elements that change the state of the charges at the grain boundary. In the case of alumina many additives have been proposed, mainly metal oxides (NiO, CoO, ZnO, SnO2, Y2O3, MgO, ...) (Budworth, 1970). Magnesia (MgO) (Shuzhi et al., 1999) is one of the most used doping agents in the case of alumina as it inhibits the grain growth and promotes the development of a more uniform grain structure. The role of MgO has been widely studied since the work of Coble in the 60's, indicating that small additions of MgO (≤ 0.25%) allowed obtaining alumina with a density close to theoretical. Bennison and Harmer (1990a) have also studied the role of MgO in the sintering of alumina, highlighting the effect it has on all the parameters that control the sintering of alumina. Handwerker et al. (1989) have also proposed that the addition of magnesia to alumina reduces the chemical heterogeneity due to the presence of impurities, reducing or eliminating the liquid phase generated by the abnormal growth of grains. Jiping Cheng et al., (2002) studied the influence of the presence of MgO on the transparency of alumina and observed that using MgO as sintering aid allows obtaining alumina samples with a more uniform grain size (equiaxed) and no

was demonstrated.

porosity after microwave sintering.

λ/10 in order to obtain transparent materials.

effect on the optical properties of the final material.

**3. Processing and sintering of transparent ceramic materials** 

producing complex shapes could broaden the fields of application.

Although many transparent ceramics are single crystal materials, transparent polycrystalline ceramics have different advantages such as low price, ease of manufacture, mass-production and more versatility in properties. Furthermore, the capability of

The classical issues of ceramic processing are related with powder quality, purity, defect free processing and elimination of minor defects and pores. When the aim is to reach transparency, requirements will be similar from a qualitative point of view but noticeably more restrictive from a quantitative point of view. In addition, microstructural features such as maximum pore and grain size (the latter for birefringent materials) are critical in scattering process. The requirements for these parameters depend on the wavelength at which transparency is desired. In general, as it will be discussed in section 4, it is assumed that defect size must be <

Then, in this chapter the processing and sintering techniques most widely used in ceramic manufacturing will be revised, describing the key parameters to be controlled and their

Magnesium aluminate spinel (MgAl2O4) is a transparent ceramic with a cubic crystal structure. This material shows an excellent optical transmission from 0.2 to 5.5 micrometers and high values of hardness (table 1) making it useful for a wide range of optical applications, including electronic and structural windows in the IR region (see Fig. 2.1). Optical quality transparent spinel has been produced by sinter/HIP, hot pressing, and hot press/HIP operations, and it has been shown that the use of a hot isostatic press can improve its optical and physical properties (Patel et al., 2000). Several authors have worked on obtaining materials of transparent spinel, as the case of Cook et al., (2005). These authors have synthesized spinel powders which subsequently formed by cold isostatic pressing and sintered in a hot pressing machine by applying a mechanical pressure of 4 MPa obtaining a transparent material. Lu et al., (2006) have obtained a transparent spinel material after sintering at low temperature and high pressure (2-5 GPa). Morita et al., (2008) have obtained a transparent spinel material after sintering at 1300ºC in plasma sintering equipment (Spark Plasma Sintering, SPS), reaching 47% of transmittance at 550nm. Spinel offers some processing advantages over AlON such as the fact that spinel is capable of being processed at much lower temperatures than AlON, and has been shown to possess superior optical properties within the infrared (IR) region.

Yttrium aluminium garnet (Y3Al5O12, YAG) is a polycrystalline material with has a cubic structure that can be used in structural (table 1) and functional applications. YAG has been considered a prime candidate for its use as a matrix in oxide-oxide composites in gas turbine engines (Mah et al., 2004). Also, it can be used as substrate for dielectric components, prisms and mirrors, as a part of discharge lamps, high intensity lamps as well as an active medium for the production of lasers (Ikesue et al., 1995), since it is able to accept trivalent cations in its structure, especially rare earths and transition metals. In this sense, YAG can be doped with Yb3+ for diode-laser (Takaichi et al., 2003), with Er3+ (Qin et al., 2004) widely used in medical applications or Eu3+ (Shikao et al., 2001) used as cathode ray tube. Finally, Nd3+ doped YAG is one of the most popular laser materials (Savastru et al., 2004). Due to its high thermal conductivity has been widely used in commercial, medical (ophthalmology, cosmetic, dental, etc), military and industry since its discovery in 1964. Three stable phases exist in the Y2O3–Al2O3 system: an orthorhombic perowskiteYAlO3 ("YAP") with an Y/Al ratio 1:1, an alumina-rich cubic garnet Y3Al5O12 ("YAG"), and an yttria rich monoclinic phase with composition Y4Al2O9 ("YAM"). Wen et al., (2004) have obtained a transparent polycrystalline material with a transmittance of 63% in the visible range and 70% in the IR region, using a solid state reaction of alumina and yttria and after sintering at 1750ºC in vacuum atmosphere (see Fig. 2.1 for transparency in the IR). Reverse-strike coprecipitation is a common synthesis route for multicationic systems (Li et al., 2000a). Li et al., (2000b) have synthesized a material by the reverse coprecipitation route using aluminum and yttrium nitrates as precursors. The material was sintered at 1700ºC in vacuum.

Traditionally, alpha alumina (α-Al2O3) has been considered one of the most widely used structural ceramics as basic matrix in many industrial applications, due to its good mechanical properties (table 1), refractory character and its chemical stability in harsh environments. Also, α-Al2O3 presents no absorption (see Fig. 2.1) from the near UV (>0.2 μm) until the IR range (> 5 μm). However, due to the birefringent character of alumina, an additional light scattering directly related to the grain size takes place in polycrystalline materials. This light scattering is originated at the boundaries between two alumina grains

Magnesium aluminate spinel (MgAl2O4) is a transparent ceramic with a cubic crystal structure. This material shows an excellent optical transmission from 0.2 to 5.5 micrometers and high values of hardness (table 1) making it useful for a wide range of optical applications, including electronic and structural windows in the IR region (see Fig. 2.1). Optical quality transparent spinel has been produced by sinter/HIP, hot pressing, and hot press/HIP operations, and it has been shown that the use of a hot isostatic press can improve its optical and physical properties (Patel et al., 2000). Several authors have worked on obtaining materials of transparent spinel, as the case of Cook et al., (2005). These authors have synthesized spinel powders which subsequently formed by cold isostatic pressing and sintered in a hot pressing machine by applying a mechanical pressure of 4 MPa obtaining a transparent material. Lu et al., (2006) have obtained a transparent spinel material after sintering at low temperature and high pressure (2-5 GPa). Morita et al., (2008) have obtained a transparent spinel material after sintering at 1300ºC in plasma sintering equipment (Spark Plasma Sintering, SPS), reaching 47% of transmittance at 550nm. Spinel offers some processing advantages over AlON such as the fact that spinel is capable of being processed at much lower temperatures than AlON, and has been shown to possess superior optical

Yttrium aluminium garnet (Y3Al5O12, YAG) is a polycrystalline material with has a cubic structure that can be used in structural (table 1) and functional applications. YAG has been considered a prime candidate for its use as a matrix in oxide-oxide composites in gas turbine engines (Mah et al., 2004). Also, it can be used as substrate for dielectric components, prisms and mirrors, as a part of discharge lamps, high intensity lamps as well as an active medium for the production of lasers (Ikesue et al., 1995), since it is able to accept trivalent cations in its structure, especially rare earths and transition metals. In this sense, YAG can be doped with Yb3+ for diode-laser (Takaichi et al., 2003), with Er3+ (Qin et al., 2004) widely used in medical applications or Eu3+ (Shikao et al., 2001) used as cathode ray tube. Finally, Nd3+ doped YAG is one of the most popular laser materials (Savastru et al., 2004). Due to its high thermal conductivity has been widely used in commercial, medical (ophthalmology, cosmetic, dental, etc), military and industry since its discovery in 1964. Three stable phases exist in the Y2O3–Al2O3 system: an orthorhombic perowskiteYAlO3 ("YAP") with an Y/Al ratio 1:1, an alumina-rich cubic garnet Y3Al5O12 ("YAG"), and an yttria rich monoclinic phase with composition Y4Al2O9 ("YAM"). Wen et al., (2004) have obtained a transparent polycrystalline material with a transmittance of 63% in the visible range and 70% in the IR region, using a solid state reaction of alumina and yttria and after sintering at 1750ºC in vacuum atmosphere (see Fig. 2.1 for transparency in the IR). Reverse-strike coprecipitation is a common synthesis route for multicationic systems (Li et al., 2000a). Li et al., (2000b) have synthesized a material by the reverse coprecipitation route using aluminum and yttrium

nitrates as precursors. The material was sintered at 1700ºC in vacuum.

Traditionally, alpha alumina (α-Al2O3) has been considered one of the most widely used structural ceramics as basic matrix in many industrial applications, due to its good mechanical properties (table 1), refractory character and its chemical stability in harsh environments. Also, α-Al2O3 presents no absorption (see Fig. 2.1) from the near UV (>0.2 μm) until the IR range (> 5 μm). However, due to the birefringent character of alumina, an additional light scattering directly related to the grain size takes place in polycrystalline materials. This light scattering is originated at the boundaries between two alumina grains

properties within the infrared (IR) region.

with different crystalline orientations. Two approaches can be followed to minimize/reduce this scattering: (1) increasing the texture of the polycrystalline material during the forming stage to minimize the misorientations between grains (2) decreasing the grain size as much as possible, which usually implies starting with nanometer sized alumina particles and often making use of non conventional sintering techniques, such as microwave (Jiping Cheng et al., 2002) or spark plasma sintering. However, since Coble´s work in the 60´s (Coble, 1962), the possibility of using other ceramic materials than the cubic ones for optical applications was demonstrated.

One way to avoid grain growth during sintering and obtain denser materials with reduced porosity is based on the modification of the diffusion mechanisms at high temperature by the introduction of second phases in grain boundaries or by doping with various elements that change the state of the charges at the grain boundary. In the case of alumina many additives have been proposed, mainly metal oxides (NiO, CoO, ZnO, SnO2, Y2O3, MgO, ...) (Budworth, 1970). Magnesia (MgO) (Shuzhi et al., 1999) is one of the most used doping agents in the case of alumina as it inhibits the grain growth and promotes the development of a more uniform grain structure. The role of MgO has been widely studied since the work of Coble in the 60's, indicating that small additions of MgO (≤ 0.25%) allowed obtaining alumina with a density close to theoretical. Bennison and Harmer (1990a) have also studied the role of MgO in the sintering of alumina, highlighting the effect it has on all the parameters that control the sintering of alumina. Handwerker et al. (1989) have also proposed that the addition of magnesia to alumina reduces the chemical heterogeneity due to the presence of impurities, reducing or eliminating the liquid phase generated by the abnormal growth of grains. Jiping Cheng et al., (2002) studied the influence of the presence of MgO on the transparency of alumina and observed that using MgO as sintering aid allows obtaining alumina samples with a more uniform grain size (equiaxed) and no porosity after microwave sintering.
