**1.2 Y2O3 as a host material and fabrication methods for the potential applications**

### *1.2.1 Y2O3 structure and dopant*

Y2O3 has a C-type cubic structure of rare earth oxides. The C-type structure is due to the bixbyite (space group Ia3 (Th7) with X = 16) and is similar to the fluorite

**61**

**Figure 1.**

*Crystal structure of Y2O3.*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

(CaF2)-type structure, in which there is a quarter of empty anion sites and they are

Three quarters of the cations are in cubes, in which the missing oxygen ions are along the face diagonal. One quarter of the cations are in cubes, in which the missing oxygen ions are along the body diagonal. The unit cell contains 48 oxygen and 32 yttrium ions. Yttrium ions are located at the center of an approximate cube, and oxygen ions are located at six of the eight vertices [1, 18]. In three quarters of these sites, the missing oxygen ions are located on a face diagonal of the mini-cube, and the rest are located at the ends of a body diagonal. The full unit cell contained 4 × 4 × 4 of these mini-cubes. Here, only a 2 × 2 × 1 part is shown for clarity of presentation. As an oxide, yttria showed broad transmittivity from 280 nm to 8 μm. Y2O3 melts at 2430°C, but the C-type structure was only stable up to 2325°C [6]. The H-type structure has a hexagonal form at high temperatures. The high melting temperature and this polymorphic transition make the growth of yttria single crystals difficult. Like fluorite-structured oxides, oxygen vacancies and interstitials

In fluorite-structured oxides, Y2O3 can dissolve a large amount of aliovalent cations. This is accompanied by the formation of charge-compensation oxygen vacancies when accepted dopants are present and donor dopants are present in the oxygen interstitials. These anion defects facilitate oxygen diffusion. Indeed, oxygen anion diffusion was much faster than yttrium cation diffusion in pure Y2O3. Therefore, cation diffusion is the rate control step for grain boundary migration at all compositions. Because cation doping can be conducted readily within the solubility limit, Y2O3 like CeO is a good candidate for examining the cation dopant effects on grain boundary mobility [18]. The cubic symmetry of this oxide further ensured a lack of strong anisotropy in grain

boundary mobility, which could otherwise complicate grain growth behavior.

Dopants of both donor and acceptor types as additives for sintering yttria at lower temperatures include Mg2+, Sr2+, Sc3+,Yb3+, Gd2+, La3+, Ti4+, Zr4+, Ce4+, and Nb5+ [19, 20]. The role of the dopant in yttria is similar to fluorite-structured oxide estimated from several important features. The grain boundary mobility by cation diffusion involves an interstitial mechanism. Solute drag could suppress grain boundary mobility at high dopant concentrations. Grain boundary mobility is influenced by a dopant-defect interaction, which is charge and size dependent. Moreover, undersized dopants have a tendency to enhance grain boundary mobility markedly due to distortion of the surrounding lattice that apparently facilitates

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

are the major defects in pure yttrium oxide [19].

arranged regularly (**Figure 1**).

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*

(CaF2)-type structure, in which there is a quarter of empty anion sites and they are arranged regularly (**Figure 1**).

Three quarters of the cations are in cubes, in which the missing oxygen ions are along the face diagonal. One quarter of the cations are in cubes, in which the missing oxygen ions are along the body diagonal. The unit cell contains 48 oxygen and 32 yttrium ions. Yttrium ions are located at the center of an approximate cube, and oxygen ions are located at six of the eight vertices [1, 18]. In three quarters of these sites, the missing oxygen ions are located on a face diagonal of the mini-cube, and the rest are located at the ends of a body diagonal. The full unit cell contained 4 × 4 × 4 of these mini-cubes. Here, only a 2 × 2 × 1 part is shown for clarity of presentation. As an oxide, yttria showed broad transmittivity from 280 nm to 8 μm. Y2O3 melts at 2430°C, but the C-type structure was only stable up to 2325°C [6]. The H-type structure has a hexagonal form at high temperatures. The high melting temperature and this polymorphic transition make the growth of yttria single crystals difficult. Like fluorite-structured oxides, oxygen vacancies and interstitials are the major defects in pure yttrium oxide [19].

In fluorite-structured oxides, Y2O3 can dissolve a large amount of aliovalent cations. This is accompanied by the formation of charge-compensation oxygen vacancies when accepted dopants are present and donor dopants are present in the oxygen interstitials. These anion defects facilitate oxygen diffusion. Indeed, oxygen anion diffusion was much faster than yttrium cation diffusion in pure Y2O3. Therefore, cation diffusion is the rate control step for grain boundary migration at all compositions. Because cation doping can be conducted readily within the solubility limit, Y2O3 like CeO is a good candidate for examining the cation dopant effects on grain boundary mobility [18]. The cubic symmetry of this oxide further ensured a lack of strong anisotropy in grain boundary mobility, which could otherwise complicate grain growth behavior.

Dopants of both donor and acceptor types as additives for sintering yttria at lower temperatures include Mg2+, Sr2+, Sc3+,Yb3+, Gd2+, La3+, Ti4+, Zr4+, Ce4+, and Nb5+ [19, 20]. The role of the dopant in yttria is similar to fluorite-structured oxide estimated from several important features. The grain boundary mobility by cation diffusion involves an interstitial mechanism. Solute drag could suppress grain boundary mobility at high dopant concentrations. Grain boundary mobility is influenced by a dopant-defect interaction, which is charge and size dependent. Moreover, undersized dopants have a tendency to enhance grain boundary mobility markedly due to distortion of the surrounding lattice that apparently facilitates

**Figure 1.** *Crystal structure of Y2O3.*

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

reaches the melting temperature, 2340°C [6].

*RE*(H2O)*<sup>n</sup>*

*1.2.1 Y2O3 structure and dopant*

respectively.

to be derived from the cubic fluorite-type structure by removing one quarter of the oxygen atoms [5]. Yttria has 32 yttrium and 48 oxygen ion sites per unit cell. This structure has large interstitial sites with the same size as an oxygen ion in the anion sublattice. Yttria has a cubic or alpha-type crystal structure until the temperature reaches 2325°C. The phase transition to a tetragonal crystal structure also occurs at 2325°C. A stable phase, known as hexagonal or beta-type, is maintained until it

Recently, yttria, which is used widely in partially stabilized zirconia and sintering aids, was the center of attention in the semiconductor industry because yttria is superior to quartz, Al2O3, ZrO2, BN, and SiC in terms of its radical or cationic

Nevertheless, when the synthesis of nanoparticles using coprecipitation was carried out and the influence of precursors or additives was examined, it was reported that yttria, sintered under vacuum or hydrogen conditions, was close to the theoretical density [8–10]. The additives affected the grain boundary migration. This compensated for the charge, cation diffusion, solute transport, and additivedefect interaction [11, 12]. According to Huang et al. [13], the theoretical density of yttria ceramic was obtained by the two-step sintering and vacuum sintering of a lanthanum-doped yttria ceramic combination. Previous studies [14–16] attempted to improve the yttria transmittance and electrical conduction of the body sintered using special techniques, such as spark plasma sintering, microwave-flash combustion synthesis, hot isostatic pressing, and the addition of tri- or tetravalent additives. Therefore, it is necessary to examine the intrinsic sintering characteristics of yttria itself. More recently, Choi et al. [17] reported not only the behavior, color, and density of yttria ceramics but also the weight change due to oxygen vacancies and oxygen diffusion in a sintered body as a function of the sintering temperature. They suggested that the changes in color, density, weight, and microstructure of grains according to the sintering temperature were related to the volatilization of yttrium ions at oxygen vacancies in the lattice site at high temperatures [8, 10, 11]. On the other hand, the sintering property of the starting material, calcined yttria, from which the hydration reaction had been eliminated, as expressed in Eq. (1), but oxygen diffusion in oxygen vacancies and yttria powder, which removes gas spouting of the precursor, occurred continuously up to 1250°C, is unknown [13, 15]:

3+ + H2O[*RE*(OH) (H2O)*<sup>n</sup>*

where *RE* and *n* represent the coordination number of rare earth and cation,

To determine the effects of oxygen vacancies and hydration on the sintering properties of yttria, this study studied the sintering characteristics of yttria calcined without a gas or hydration reaction containing a hydration reagent of precursors. In addition, when oxygen diffusion occurred in the oxygen vacancies, the yttria powder was heated repeatedly to adopt the result of density and increasing weight. The plasma resistivity of the yttria ceramic was compared with that of the control group.

**1.2 Y2O3 as a host material and fabrication methods for the potential applications**

Y2O3 has a C-type cubic structure of rare earth oxides. The C-type structure is due to the bixbyite (space group Ia3 (Th7) with X = 16) and is similar to the fluorite

−1 ] 2+

+ H3O<sup>+</sup> (1)

resistance activated by plasma [7] and sintering-limiting property [6].

**60**

defect migration [12, 18]. According to Kingery et al. [21], the diffusion of oxygen anions in yttria is much faster than that of yttrium cations. Hence, yttrium cation interstitial diffusion is the rate control step for grain boundary migration.

#### *1.2.2 Yttria ceramic fabrication*

Yttria has potential applications, such as missile domes and bulb envelopes because of its optical transparency and a host material for solid-state lasers and high-resistance materials for etching plasma. Several studies have focused on obtaining full-density or high-transparency polycrystalline yttria ceramics. In the former, several methods have attempted to control the particle size and its distribution, coprecipitation, dopants or additives, and homogeneity, through hydrothermal methods. In the latter, a novel sintering method was studied. A combination of vacuum sintering and hot isostatic pressing methods was based on the low-temperature sintering process to suppress grain growth without sacrificing densification. The pores can be removed completely after hot isostatic pressing. These results are based on the addition of additives in yttria. Ikegami et al. [22] reported the fabrication of transparent ceramics by the two-temperature sintering of yttrium hydroxide with near theoretical total transmittance. Haung et al. [13] synthesized lanthanum-doped yttria ceramic by a combination of two-step sintering and vacuum sintering. A twostep strategy was proposed to produce materials with near theoretical total transmittance, and these studies focused on transparent ceramics used as infrared windows.

#### **1.3 Sample fabrication**

The sample preparation was as follows. Yttria powder (99.9%, UUHP, Shin-Etsu Chemical Co. Ltd., Japan) was used as the starting material of as-received yttria. ASY and CY powder was prepared by calcination at 1250°C for 48 h. The powder of fused yttria, FY (Mirae Materials Co. Korea), by induction melting was ball milled with 300 μm zirconia ball media for 24 h. After milling, the powder mixtures were dried at 500°C for 24 h and sieved with 325 mesh to achieve uniform particle sizes. Here, the graphite mold having a diameter of 60 mm was filled with 100 g of the starting material. After forming one axis at 200 kg, cold isostatic pressing (CIP) was performed at a 300 bar pressure to increase the green density. The heating and cooling rates were 5°C/min. The sintered specimens listed in **Table 1** were obtained by the thermal history shown in **Figure 2**.

In the crystal phase, a coarse grain microstructure, and a microstructure of specimen, the specimens were observed using X-ray diffraction (XRD, D/MAX-2550V, Rigaku, Japan), polarizing microscope (Eclipse LV100, Nikon, Japan), and scanning electron microscope (SEM, JSM-6700F, Jeol, Japan), respectively, while bulk density of the sintered specimens was measured according to the ASTM D6683-14 method. The modulus of elasticity of the sintered specimens was observed using a pulse echo device (5800 Pulser/Receiver, Panametrics, Japan) with an oscilloscope (Tektronix, TDS3012, Japan) of X- and Y-modulation, longitudinal, and transverse waves. KIC (HV-100, Mitutoyo, Japan) was measured by applying a load of 500 g on average of 10 times or more after the specimen surface was polished with 0.2 μm diamond paste. The etching depth and the weight reduction rate were obtained by surface roughness (Surfcorder ET3000, Kosaka Lab., Japan) after plasma etching for 60 min by using an inductively coupled plasma system (Versiline, UNAXIS Co., USA). Sintering properties and plasma evaluation were compared with yttria ceramics sintered, Al2O3 (99.6%, Semiconductor Wafer, Inc., Taiwan), ZrO2 (Semiconductor Wafer, Inc., Taiwan), Si wafer (Sumco Co., Japan), and quartz (General Electric (GE), USA), respectively.

**63**

YO1.335.

**Figure 2.**

**Table 1.**

the 48 standard positions [5, 11, 15].

*Sintering schedule of the three-repeated cycle in air.*

to high-temperature XRD equipment.

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

**1.4 Characteristic evaluation of yttria ceramics sintered by three-repeated cycle**

**Table 1** lists the names of yttria ceramic specimens. **Figure 3** shows the XRD crystal phase of the yttria ceramic sintered under the conditions in **Figure 2**. X-ray diffraction analysis (XRD, D/MAX-2550V, Rigaku, Japan) was conducted to determine the crystal structure of the prepared yttria ceramics sintered using CuKα1 radiation between 5 and 70° (2θ) at room temperature. The crystal phase of CY, the calcined starting material, was identified as Y2O3, but ASY contained both Y2O3 and

The difference in the crystallographic phase depending on the starting material is believed to be the effect of the oxygen vacancies, which was about one-fourth of

> cm2 /s

The starting material calcined at 1250°C for 48 h was exposed to hydration and precursor gases, whereas ASY exhibited continuous volatilization of precursors up to 1250°C [10, 16]. Rhodes et al. [23] examined the continuous volatilization of methane gas. They observed not only the release of carbon or water impurities but also carbonization and hydration reactions until the temperature reached 1200°C. At higher temperatures, the calcined starting material was found to be free from the influence of the adsorption gas and hydration reaction of the precursor [5, 9, 10, 16]. A mixture phase of Y2O3 and YO1.335 in ASY was obtained after the sintering, and the relative oxygen filling was less than CY [13]. Based on this, similar study results were reported; the self-diffusion coefficient and activation energy at 1050–1250°C were 6 × 10<sup>−</sup><sup>6</sup>

and 82 kJ/mole, respectively [24]. Swamy et al. [25] suggested the possibility of a fluorite crystal phase formation due to the irregular oxygen vacancies in the unit lattice according

In addition, there was a difference between the results in that the additive affected only the change in transmittance, YO1.401 or YO1.335 phase formation, and the crystalline phase of the monoclinic were mixed depending on the heat treatment conditions. On the other hand, these results are common in that it originated from oxygen vacancies in

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

CY Calcined yttria, at 1250°C for 24 h in air FY Fused yttria by induction melting

HY [17] Hot-pressed, at 1600°C for 8 h under Ar gas

**Name Starting materials** ASY As-received yttria

*Yttria ceramics named and prepared.*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*


**Table 1.**

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

*1.2.2 Yttria ceramic fabrication*

**1.3 Sample fabrication**

by the thermal history shown in **Figure 2**.

(General Electric (GE), USA), respectively.

defect migration [12, 18]. According to Kingery et al. [21], the diffusion of oxygen anions in yttria is much faster than that of yttrium cations. Hence, yttrium cation

Yttria has potential applications, such as missile domes and bulb envelopes because of its optical transparency and a host material for solid-state lasers and high-resistance materials for etching plasma. Several studies have focused on obtaining full-density or high-transparency polycrystalline yttria ceramics. In the former, several methods have attempted to control the particle size and its distribution, coprecipitation, dopants or additives, and homogeneity, through hydrothermal methods. In the latter, a novel sintering method was studied. A combination of vacuum sintering and hot isostatic pressing methods was based on the low-temperature sintering process to suppress grain growth without sacrificing densification. The pores can be removed completely after hot isostatic pressing. These results are based on the addition of additives in yttria. Ikegami et al. [22] reported the fabrication of transparent ceramics by the two-temperature sintering of yttrium hydroxide with near theoretical total transmittance. Haung et al. [13] synthesized lanthanum-doped yttria ceramic by a combination of two-step sintering and vacuum sintering. A twostep strategy was proposed to produce materials with near theoretical total transmittance, and these studies focused on transparent ceramics used as infrared windows.

The sample preparation was as follows. Yttria powder (99.9%, UUHP, Shin-Etsu Chemical Co. Ltd., Japan) was used as the starting material of as-received yttria. ASY and CY powder was prepared by calcination at 1250°C for 48 h. The powder of fused yttria, FY (Mirae Materials Co. Korea), by induction melting was ball milled with 300 μm zirconia ball media for 24 h. After milling, the powder mixtures were dried at 500°C for 24 h and sieved with 325 mesh to achieve uniform particle sizes. Here, the graphite mold having a diameter of 60 mm was filled with 100 g of the starting material. After forming one axis at 200 kg, cold isostatic pressing (CIP) was performed at a 300 bar pressure to increase the green density. The heating and cooling rates were 5°C/min. The sintered specimens listed in **Table 1** were obtained

In the crystal phase, a coarse grain microstructure, and a microstructure of specimen, the specimens were observed using X-ray diffraction (XRD, D/MAX-2550V, Rigaku, Japan), polarizing microscope (Eclipse LV100, Nikon, Japan), and scanning electron microscope (SEM, JSM-6700F, Jeol, Japan), respectively, while bulk density of the sintered specimens was measured according to the ASTM D6683-14 method. The modulus of elasticity of the sintered specimens was observed using a pulse echo device (5800 Pulser/Receiver, Panametrics, Japan) with an oscilloscope (Tektronix, TDS3012, Japan) of X- and Y-modulation, longitudinal, and transverse waves. KIC (HV-100, Mitutoyo, Japan) was measured by applying a load of 500 g on average of 10 times or more after the specimen surface was polished with 0.2 μm diamond paste. The etching depth and the weight reduction rate were obtained by surface roughness (Surfcorder ET3000, Kosaka Lab., Japan) after plasma etching for 60 min by using an inductively coupled plasma system (Versiline, UNAXIS Co., USA). Sintering properties and plasma evaluation were compared with yttria ceramics sintered, Al2O3 (99.6%, Semiconductor Wafer, Inc., Taiwan), ZrO2 (Semiconductor Wafer, Inc., Taiwan), Si wafer (Sumco Co., Japan), and quartz

interstitial diffusion is the rate control step for grain boundary migration.

**62**

*Yttria ceramics named and prepared.*

**Figure 2.**

*Sintering schedule of the three-repeated cycle in air.*

#### **1.4 Characteristic evaluation of yttria ceramics sintered by three-repeated cycle**

**Table 1** lists the names of yttria ceramic specimens. **Figure 3** shows the XRD crystal phase of the yttria ceramic sintered under the conditions in **Figure 2**. X-ray diffraction analysis (XRD, D/MAX-2550V, Rigaku, Japan) was conducted to determine the crystal structure of the prepared yttria ceramics sintered using CuKα1 radiation between 5 and 70° (2θ) at room temperature. The crystal phase of CY, the calcined starting material, was identified as Y2O3, but ASY contained both Y2O3 and YO1.335.

The difference in the crystallographic phase depending on the starting material is believed to be the effect of the oxygen vacancies, which was about one-fourth of the 48 standard positions [5, 11, 15].

The starting material calcined at 1250°C for 48 h was exposed to hydration and precursor gases, whereas ASY exhibited continuous volatilization of precursors up to 1250°C [10, 16]. Rhodes et al. [23] examined the continuous volatilization of methane gas. They observed not only the release of carbon or water impurities but also carbonization and hydration reactions until the temperature reached 1200°C. At higher temperatures, the calcined starting material was found to be free from the influence of the adsorption gas and hydration reaction of the precursor [5, 9, 10, 16]. A mixture phase of Y2O3 and YO1.335 in ASY was obtained after the sintering, and the relative oxygen filling was less than CY [13]. Based on this, similar study results were reported; the self-diffusion coefficient and activation energy at 1050–1250°C were 6 × 10<sup>−</sup><sup>6</sup> cm2 /s and 82 kJ/mole, respectively [24]. Swamy et al. [25] suggested the possibility of a fluorite crystal phase formation due to the irregular oxygen vacancies in the unit lattice according to high-temperature XRD equipment.

In addition, there was a difference between the results in that the additive affected only the change in transmittance, YO1.401 or YO1.335 phase formation, and the crystalline phase of the monoclinic were mixed depending on the heat treatment conditions. On the other hand, these results are common in that it originated from oxygen vacancies in

#### **Figure 3.**

*XRD patterns of yttria ceramics sintered. (a) ASY, as-received yttria; (b) CY, calcined yttria; (c) FY, fused yttria; and (d) HY [17], hot-pressed yttria.*

the yttria regular site [13, 18, 24]. The density of the CY specimen was 4.9 g/cm3 , which is higher than that of ASY, 4.8 g/cm3 , as shown in **Figure 4**. On the other hand, CY, calcined yttria, and ASY, repeated-cycle sintering yttria, corresponding to the density of commercial yttria (Y0100A, Kyocera, Japan) and 4.9 g/cm3 were effective sintering methods, despite being lower than the theoretical density (5.01 g/cm3 ) of the HY specimen.

**Figure 5** shows a coarse grain microstructure of the CY specimen, in which the surface of the specimen was polished with a 0.2 μm diamond paste, regardless of the starting material condition. The microstructure after repeated-cycle sintering revealed a large number of open pores, in which the hydration reactant and precursor gas of formula (1) were observed, but no coarse particles were observed in the ASY specimen in **Figure 6(a)** [9, 24].

In addition, the CY specimen showed a relatively dense microstructure, as shown in **Figure 6(b)**. The coarse particles in the initial stage changed to uniform particles after repeat-cycle sintering of yttria in air (see **Figures 5** and **6(a)**). Because Y2O3 is an oxygen-deficient complex, it is easy to deviate from the stoichiometric Y:O ratio of 40:60 (at.%) due to oxygen vacancies [13, 25]. The majority

**65**

**Figure 6.**

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

phase of YO1.335 with oxygen deficiencies appeared in only the ASY specimen

*Optical photograph of the polished surface of CY, yttria ceramics, the first step sintered at 1700°C for 6 h in air. Different granules in the matrix represent abnormal grain growth. The black dots show the remaining* 

A thermodynamically stable liquid phase formed locally at 1522°C when the Y:O ratio was 60:40 (at.%), which resulted in pyrolysis of the liquid phase in the repeated heat treatment [25]. The reasons why the rough particles of **Figure 5**, which appeared at the initial stages of less-pressure sintering of yttria, changed to homogeneous particles like the microstructure in **Figure 6(a)** after repeated sintering are as follows. As Y2O3 is an oxygen-deficient body, it is easy to deviate from the stoichiometric Y:O ratio range of 40:60 (at.%) due to oxygen vacancies [13]. In **Figure 3(a)**, which showed the XRD crystal phase, only the YO1.335 crystal phase with oxygen deficiencies appeared in the ASY specimen. The liquid phase was thermodynamically stable, formed locally at 1522°C and decomposed thermally in the repeated heat treatment, resulting in a crystal when the Y:O ratio was in 60:40 (at.%). The coarse particles in **Figure 5** were prepared by the liquid phase formation

This liquid phase changed stoichiometrically at a Y:O ratio of 40:60 (at.%) due to oxygen diffusion in the lattice during repeated sintering. Therefore, the coarse particles in **Figure 4** disappear because it could change into a crystalline phase rather than exist in the liquid phase [25, 26]. **Figure 7** shows the Y-O phase diagram at 1522°C when the Y:O ratio was 60:40 (at.%); coarse particles support the validity of liquid phase formation hypothesis. This liquid phase changed stoichiometrically at a Y:O ratio (at.%) of 40:60 due to oxygen diffusion in the lattice during the repeated

*SEM images of (a) ASY, as-purchased Y2O3 powder; (b) CY, calcined Y2O3 powder at 1250°C for 48 h in air;* 

*(c) fused yttria by induction melting; and (d) HY, hot-pressed yttria [17].*

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

observed in XRD patterns in **Figure 3**.

**Figure 5.**

*polishing powders.*

when the Y:O ratio was 60:40 (at.%) at 1522°C.

**Figure 4.** *Bulk density of yttria ceramics sintered.*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*

#### **Figure 5.**

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

the yttria regular site [13, 18, 24]. The density of the CY specimen was 4.9 g/cm3

ing methods, despite being lower than the theoretical density (5.01 g/cm3

of commercial yttria (Y0100A, Kyocera, Japan) and 4.9 g/cm3

calcined yttria, and ASY, repeated-cycle sintering yttria, corresponding to the density

*XRD patterns of yttria ceramics sintered. (a) ASY, as-received yttria; (b) CY, calcined yttria; (c) FY, fused* 

**Figure 5** shows a coarse grain microstructure of the CY specimen, in which the surface of the specimen was polished with a 0.2 μm diamond paste, regardless of the starting material condition. The microstructure after repeated-cycle sintering revealed a large number of open pores, in which the hydration reactant and precursor gas of formula (1) were observed, but no coarse particles were observed in the

In addition, the CY specimen showed a relatively dense microstructure, as shown in **Figure 6(b)**. The coarse particles in the initial stage changed to uniform particles after repeat-cycle sintering of yttria in air (see **Figures 5** and **6(a)**). Because Y2O3 is an oxygen-deficient complex, it is easy to deviate from the stoichiometric Y:O ratio of 40:60 (at.%) due to oxygen vacancies [13, 25]. The majority

, as shown in **Figure 4**. On the other hand, CY,

, which

) of the HY

were effective sinter-

**64**

**Figure 4.**

*Bulk density of yttria ceramics sintered.*

is higher than that of ASY, 4.8 g/cm3

*yttria; and (d) HY [17], hot-pressed yttria.*

ASY specimen in **Figure 6(a)** [9, 24].

specimen.

**Figure 3.**

*Optical photograph of the polished surface of CY, yttria ceramics, the first step sintered at 1700°C for 6 h in air. Different granules in the matrix represent abnormal grain growth. The black dots show the remaining polishing powders.*

phase of YO1.335 with oxygen deficiencies appeared in only the ASY specimen observed in XRD patterns in **Figure 3**.

A thermodynamically stable liquid phase formed locally at 1522°C when the Y:O ratio was 60:40 (at.%), which resulted in pyrolysis of the liquid phase in the repeated heat treatment [25]. The reasons why the rough particles of **Figure 5**, which appeared at the initial stages of less-pressure sintering of yttria, changed to homogeneous particles like the microstructure in **Figure 6(a)** after repeated sintering are as follows. As Y2O3 is an oxygen-deficient body, it is easy to deviate from the stoichiometric Y:O ratio range of 40:60 (at.%) due to oxygen vacancies [13]. In **Figure 3(a)**, which showed the XRD crystal phase, only the YO1.335 crystal phase with oxygen deficiencies appeared in the ASY specimen. The liquid phase was thermodynamically stable, formed locally at 1522°C and decomposed thermally in the repeated heat treatment, resulting in a crystal when the Y:O ratio was in 60:40 (at.%). The coarse particles in **Figure 5** were prepared by the liquid phase formation when the Y:O ratio was 60:40 (at.%) at 1522°C.

This liquid phase changed stoichiometrically at a Y:O ratio of 40:60 (at.%) due to oxygen diffusion in the lattice during repeated sintering. Therefore, the coarse particles in **Figure 4** disappear because it could change into a crystalline phase rather than exist in the liquid phase [25, 26]. **Figure 7** shows the Y-O phase diagram at 1522°C when the Y:O ratio was 60:40 (at.%); coarse particles support the validity of liquid phase formation hypothesis. This liquid phase changed stoichiometrically at a Y:O ratio (at.%) of 40:60 due to oxygen diffusion in the lattice during the repeated

#### **Figure 6.**

*SEM images of (a) ASY, as-purchased Y2O3 powder; (b) CY, calcined Y2O3 powder at 1250°C for 48 h in air; (c) fused yttria by induction melting; and (d) HY, hot-pressed yttria [17].*

sintering. Therefore, it is believed that the coarse particles shown in **Figure 4** disappear because the crystalline phase was more stable than the liquid phase, but this hypothesis will be proven through further studies. **Figure 8(a)** and **(b)** showed the Vickers indentation results of the ASY and CY specimens. Stress propagation along the cleavage plane and the low-level mist shape were distinct from the HY specimen, which was similar to the glass-like microstructure in **Figure 8(c)**.

The Young's modulus increased with decreasing particle size, as listed in **Table 2**, where average values of the indentation of the specimens, KIC, were measured to apply a 500 g load more than ten times. On the other hand, the HY specimen KIC appeared to be relatively high regardless of the particle size because the dense microstructure, crystal phase, and the amorphous phase were mixed as shown in **Figures 6(c)** and **8(c)** [26, 27]. **Figure 9** presents the plasma resistivity profiles of the sintered specimens and the control group. From Eqs. (2) and (3), the etching depth and weight reduction rate were obtained by surface roughness after plasma etching for 60 min using an inductively coupled plasma system by masking the abrasive specimen.

The sintering properties and plasma evaluation of yttria ceramic were compared with Al2O3 (Semiconductor Wafer, Inc., Taiwan), ZrO2 (Semiconductor Wafer, Inc., Taiwan), Si wafer (Sumco Co., Japan), and quartz (General Electric) [11]. The etch rate and weight loss rate, which are related directly to the plasma resistance, were calculated as follows. The evaluation of plasma etching was performed using simple equations:

**67**

and

**Table 2.**

**Figure 8.**

weight loss rate (g/min cm<sup>2</sup>

*Hardness, Young's modulus, and KIC of yttria ceramics sintered.*

three times higher than that of the control.

between the specimens according to the starting materials.

) = weigh variation (g)/exposure area (cm2

exposure time (min) × weight before exposure (g) (3)

*Vickers indents in sintered polycrystalline yttria ceramics of varying sinter method at 500 g load. Note the general tendency for both more cracking and greater complexity of cracking, particularly near the indent: (a) ASY, as-received yttria; (b) CY, calcined yttria; (c) HY, hot-pressed yttria [17]; and (d) FY, fused yttria.*

**Name Grain size (μm) Young's modulus (GPa) KIC (GPa)** ASY <2 146 3.27 ± 0.6 CY 1 153 2.28 ± 0.5 FY 0.8 — — HY [17] >5 168 1.2 ± 0.5

Quartz and Si wafer (100) were etched at a depth of 2.94–3.14 μm within 10 min of plasma irradiation, and then the etching evaluation was completed. On the other hand, the etching depth of Al2O3 and ZrO2 irradiated with plasma for 60 min was 1.31–1.45 μm. The resistivity of the yttria was 0.41–0.54 μm, which was

The etching depth and etch rate of the control and yttria specimens was in the order of Si wafer, quartz, ZrO2, Al2O3, and Y2O3, as shown in **Figure 10(a)**. The weight loss rate of Al2O3 and ZrO2 was 12 times higher than that of Y2O3, as shown in **Figure 10(b)**. In the present study, there was no difference in plasma resistance

) ×

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

**Figure 7.** *Phase diagram of yttrium-oxygen [6].*

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*

#### **Figure 8.**

*Ceramic Materials - Synthesis, Characterization, Applications and Recycling*

which was similar to the glass-like microstructure in **Figure 8(c)**.

abrasive specimen.

performed using simple equations:

sintering. Therefore, it is believed that the coarse particles shown in **Figure 4** disappear because the crystalline phase was more stable than the liquid phase, but this hypothesis will be proven through further studies. **Figure 8(a)** and **(b)** showed the Vickers indentation results of the ASY and CY specimens. Stress propagation along the cleavage plane and the low-level mist shape were distinct from the HY specimen,

The Young's modulus increased with decreasing particle size, as listed in **Table 2**,

where average values of the indentation of the specimens, KIC, were measured to apply a 500 g load more than ten times. On the other hand, the HY specimen KIC appeared to be relatively high regardless of the particle size because the dense microstructure, crystal phase, and the amorphous phase were mixed as shown in **Figures 6(c)** and **8(c)** [26, 27]. **Figure 9** presents the plasma resistivity profiles of the sintered specimens and the control group. From Eqs. (2) and (3), the etching depth and weight reduction rate were obtained by surface roughness after plasma etching for 60 min using an inductively coupled plasma system by masking the

The sintering properties and plasma evaluation of yttria ceramic were compared with Al2O3 (Semiconductor Wafer, Inc., Taiwan), ZrO2 (Semiconductor Wafer, Inc., Taiwan), Si wafer (Sumco Co., Japan), and quartz (General Electric) [11]. The etch rate and weight loss rate, which are related directly to the plasma resistance, were calculated as follows. The evaluation of plasma etching was

Etch rate (μm min) = etch depth (μm)/exposure time (min) (2)

**66**

**Figure 7.**

*Phase diagram of yttrium-oxygen [6].*

*Vickers indents in sintered polycrystalline yttria ceramics of varying sinter method at 500 g load. Note the general tendency for both more cracking and greater complexity of cracking, particularly near the indent: (a) ASY, as-received yttria; (b) CY, calcined yttria; (c) HY, hot-pressed yttria [17]; and (d) FY, fused yttria.*


#### **Table 2.**

*Hardness, Young's modulus, and KIC of yttria ceramics sintered.*

#### and

 weight loss rate (g/min cm<sup>2</sup> ) = weigh variation (g)/exposure area (cm2 ) × exposure time (min) × weight before exposure (g) (3)

Quartz and Si wafer (100) were etched at a depth of 2.94–3.14 μm within 10 min of plasma irradiation, and then the etching evaluation was completed. On the other hand, the etching depth of Al2O3 and ZrO2 irradiated with plasma for 60 min was 1.31–1.45 μm. The resistivity of the yttria was 0.41–0.54 μm, which was three times higher than that of the control.

The etching depth and etch rate of the control and yttria specimens was in the order of Si wafer, quartz, ZrO2, Al2O3, and Y2O3, as shown in **Figure 10(a)**. The weight loss rate of Al2O3 and ZrO2 was 12 times higher than that of Y2O3, as shown in **Figure 10(b)**. In the present study, there was no difference in plasma resistance between the specimens according to the starting materials.

#### **Figure 9.**

*Coverage of the etching profile for the topographies of the different samples by a plasma test: (a) Al2O3, (b) ZrO2, (c) CY, (d) FY, and (e) HY [17].*

#### **Figure 10.**

*Coverage of the plasma-deposited yttria ceramics versus quartz, silicon, Al2O3, and ZrO2. (a) Etch depth and rate and (b) weight loss rate and relative weight loss rate. Deposition time of Si wafer (100) and quartz: 10 min. ZrO2, Al2O3, and yttria (CY and HY [17]): 60 min. W. L. R. means the weight loss rate.*

#### **1.5 Summary**

The plasma resistance and characteristics of the yttria ceramics were investigated in terms of calcination and three-step sintering. The crystal phase of the

**69**

*Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using…*

calcined yttria ceramics was identified as Y2O3, but as-received yttria ceramics contained both Y2O3 and YO1.335 phase on the XRD pattern have been demonstrated. The coarse particles in the initial stage changed to uniform particles after repeatcycle sintering of yttria in air. In the repeated heat treatment process, the liquid phase with local differences in the Y:O ratio was converted to a uniform crystal grain. The stress propagation along the cleavage plane and the low-level mist shape

appeared to the hot-pressed specimen. The Young's modulus increased with decreasing particle size, and KIC appeared to be relatively high regardless of the particle size because the dense microstructure, crystal phase, and the amorphous phase were mixed. The etching depth and etch rate of the control and yttria specimens were in the order of Si wafer, quartz, ZrO2, Al2O3, and Y2O3. The weight loss rate of Al2O3 and ZrO2 was 12 times higher than that of yttria ceramics. There was no difference in plasma resistance between the specimens according to the starting materials. The calcination and three-repeated cycle could be suggested as one of

**2. Characteristics of thermal oxidation on hot-pressed pure yttria** 

of yttria ceramics and powder synthesis techniques to obtain full density have attracted considerable interest. In particular, the sintering temperature has direct effects on the porosity, density, microstructure or crystal phase, and grain boundary migration. Yttria is used widely as a liquid sintering additive in AlN, SiC, SiAlON, and ZrO2 [7, 12]. Yttria itself, however, is a sintering-limit material when heated and can induce abnormal grain growth during the sintering process [6].

Fundamental studies on the forming process that affects the sintering behavior

Finely divided yttria, having a purity at least 99.9%, was pressed into compacts, sintered in a dry hydrogen atmosphere or a partial vacuum to 2150–2300°C, and refired in wet hydrogen at 1950–2300°C to redoxidize any yttria that had been reduced to yttrium during sintering [8]. The resulting polycrystalline yttria ceramics were examined to determine if there was any improvement in the intrinsic

The previously studied yttria ceramics were divided into three groups. The first was developed from the control of yttria particles, such as 5 nm Y2O3 synthesized from a 50 to 120 nm Y2O3 powder and Y(OH)3 using a coprecipitation method [9, 24]. The second group was a transparent yttria ceramic used as an infrared window to examine the electrical properties of yttria ceramics with additives, Th, Nd, and Er [15, 28, 29]. The last group involved the development of unconventional sintering techniques, such as triaxial isostatic press, hot isostatic pressing, or vacuum sintering, to fabricate full-

Most studies reported that the synthesis of nano-yttria particles was dependent only on the process variables. On the other hand, derivation of a common denominator for the properties of yttria is more complex because of the sintering methods of yttria ceramics, sintering property, and grain boundary migration by dopants

Nevertheless, the purity of the starting material and packing behavior as a function of the particle size used in the granule preparation together with the particle surface area affects the sintering characteristics [13]. These are important factors in the synthesis of dense sintered bodies. When yttria powders are induced on the nanoscale and have high purity, the fine particles or submicron-sized

*DOI: http://dx.doi.org/10.5772/intechopen.81750*

effective sintering methods of yttria.

**ceramics**

**2.1 Introduction**

properties of yttria.

density polycrystalline yttria ceramics [13, 30].

such as La, Sr, and Sc [25, 31].

#### *Plasma Resistance Evaluation and Characteristics of Yttria Ceramics Sintered by Using… DOI: http://dx.doi.org/10.5772/intechopen.81750*

calcined yttria ceramics was identified as Y2O3, but as-received yttria ceramics contained both Y2O3 and YO1.335 phase on the XRD pattern have been demonstrated.

The coarse particles in the initial stage changed to uniform particles after repeatcycle sintering of yttria in air. In the repeated heat treatment process, the liquid phase with local differences in the Y:O ratio was converted to a uniform crystal grain.

The stress propagation along the cleavage plane and the low-level mist shape appeared to the hot-pressed specimen. The Young's modulus increased with decreasing particle size, and KIC appeared to be relatively high regardless of the particle size because the dense microstructure, crystal phase, and the amorphous phase were mixed. The etching depth and etch rate of the control and yttria specimens were in the order of Si wafer, quartz, ZrO2, Al2O3, and Y2O3. The weight loss rate of Al2O3 and ZrO2 was 12 times higher than that of yttria ceramics. There was no difference in plasma resistance between the specimens according to the starting materials. The calcination and three-repeated cycle could be suggested as one of effective sintering methods of yttria.
