**5. Two-step PECS for transparent polycrystalline alumina**

The authors study PECS with two-step temperature profile, that is, two-step PECS (referred as to TS-PECS), in order to fabricate transparent oxide ceramics with fine grains [74, 75]. Figure 3 shows the sintering profile of TS-PECS with other PECS techniques. TS-PECS can provide well-transparent oxides with shorter sintering period in comparison with slow-heating PECS.

Figure 4 shows appearance, fracture surface and density of polycrystalline Al2O3 prepared by using TS-PECS with 1st different temperature for 60 min and 1200°C for 20 min under 100 MPa in vacuum. A sample prepared by slow-heating PECS at 1200°C is shown for comparison. Importance of the 1st step temperature can be understood in Figure 4. The sample sintered at 1000o C in the 1st step has high transparency and less grain growth. The meaning of the 1st step is densification without significant grain growth. Sintering at 1000o C can provide densification without grain growth, however, full densification cannot be achieved. In order to reach to the full densification of the sample, the 2nd step with higher sintering temperature is necessary.

**Figure 3.** Schematic diagrams on sintering profiles of TS-PECS with other PECS techniques.

**Figure 4.** Appearance, fracture surface and density of polycrystalline Al2O3 prepared by using TS-PECS with 1st differ‐ ent temperature for 60 min and 1200°C for 20 min under 100 MPa in vacuum.

TS-PECS is also useful for other transparent oxides such as MgAl2O4. This polycrystalline oxide has better transparency because of isotropic crystal structure. Figure 5 shows appearance of polycrystalline MgAl2O4 produced by using TS-PECS. Even regular PECS such as 1300°C for 20 min with 100 K /min can provide transparent MgAl2O4 with fine grain size. However TS-PECS can increase transparency of the sintered sample.

**Figure 5.** Appearance of polycrystalline MgAl2O4 produced by using (a) PECS and (b) TS-PECS under 100 MPa in vac‐ uum.

Table 2 shows mechanical properties of transparent Al2O3 of TS-PECS. Bending strength of the samples is approximately 400 MPa, which is comparable with any commercial opaque Al2O3. This is caused by the existence of macroscopic defects as large as a few tens micrometers. Figure 6 shows an optical microscopic image of the inside of the transparent Al2O3 prepared by TS-PECS. Many black dots are observed in the sample. Figure 7 represents a scanning electron microscopic image of the cross-section of a black dot in transparent Al2O3 prepared by TS-PECS. Size of the black dot in Al2O3 is approximately 50 μm in diameter. The black dot is pores although the surrounding is fully densified. The microstructure of the black dots implies that the black dots were derived from the agglomeration of the initial particles of the Al2O3 powder. Such a larger defect leads low mechanical strength, as given by the Griffith Criteria. Even PECS with high pressure, the powder properties such as the agglomeration is very important. PECS under 100 MPa in uniaxial pressure cannot eliminate the agglomeration of the initial particles. In particular elimination of the agglomeration of the initial particles is very important in even PECS for structural ceramics and transparent ceramics.


**Table 2.** Mechanical Properties of Transparent Al2O3 prepared by TS-PECS (1000°C for 60 min, 1200°C for 20 min, 100MPa 100 K/min)

**Figure 4.** Appearance, fracture surface and density of polycrystalline Al2O3 prepared by using TS-PECS with 1st differ‐

(a) General PECS (b) Slow‐heating PECS

(c) Two‐step Pressure PECS (d) Two‐step PECS

**Figure 3.** Schematic diagrams on sintering profiles of TS-PECS with other PECS techniques.

12 Sintering Techniques of Materials

ent temperature for 60 min and 1200°C for 20 min under 100 MPa in vacuum.

Figure 6 An optical microscopic image of the inside of the transparent Al2O3 prepared by TS-PECS Average Grain Size 0.31 μm Vickers Hardness 20.8 GPa Bending Strength 400 MPa

Density 99.8%

Fracture Toughness 3.3 MPam1/2

(a) PECS: 1300°C for 20 min (b) TS-PECS: 1150°C for 60 min and 1350°C for 20 min

Figure 5 Appearance of polycrystalline MgAl2O4 produced by using (a) PECS and (b) TS-PECS under 100 MPa in vacuum.

agglomeration of the initial particles is very important in even PECS for structural ceramics and transparent ceramics.

 Table 2 shows mechanical properties of transparent Al2O3 of TS-PECS. Bending strength of the samples is approximately 400 MPa, which is comparable with any commercial opaque Al2O3. This is cause by the existence of macroscopic defects as large as a few tens micrometers. Figure 5 shows an optical microscopic image of the inside of the transparent Al2O3 prepared by TS-PECS. Many black dots are observed in the sample. Figure 6 represents a scanning electron microscopic image of the cross-section of a black dot in transparent Al2O3 prepared by TS-PECS. Size of the black dot in Al2O3 is approximately 50 μm in diameter. The black dot is porous although the surrounding is fully densified. The microstructure of the black dots implies that the black dots were derived from the agglomeration of the initial particles of the Al2O3 powder. Such a larger defect leads low mechanical strength, as given by the Griffith Criteria. Even PECS with high pressure, the powder properties such as the agglomeration is very important. PECS under 100 MPa in uniaxial pressure cannot eliminate the agglomeration of the initial particles. In particular elimination of the

> Figure 5 An optical microscopic image of the inside of the transparent Al2O3

prepared by TS-PECS

8 Book Title

**Figure 6.** An optical microscopic image of the inside of the transparent Al2O3 prepared by TS-PECS

**Figure 7.** A scanning electron microscopic image of the cross-section of a black dot in transparent Al2O3.

TS-PECS is also available for preparing transparent colored-Al2O3 and MgAl2O4. Figure 7 shows appearance of various transparent Al2O3 and MgAl2O4 added with different dopants. A red color in Al2O3 and MgAl2O4 is caused by doping Cr2O3. In polycrystalline Al2O3, MnO causes the colour of orange or brown, however less transparency. Doping MnO into MgAl2O4 shows yellow in color and good transparency.

**Figure 8.** Appearance of various transparent Al2O3 and MgAl2O4 added with different dopants.
