**6. Summary**

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

prepared by TS-PECS

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

Figure 6 A scanning electron microscopic image of the cross-section of a black dot in transparent Al2O3.

**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.

MgAl2O4 shows yellow in color and good transparency.

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

Density 99.8% Average Grain Size 0.31 μm Vickers Hardness 20.8 GPa Bending Strength 400 MPa 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

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

8 Book Title

PECS

K/min)

14 Sintering Techniques of Materials

Transparent Polycrystalline Al2O3 can be produced by using PECS with advanced Al2O3 powder. The preferred techniques of PECS to obtain better transparency in Al2O3 are classified into the following: (1) high-pressure PECS, (2) slow-heating PECS, (3) fast-heating PECS, and (4) two-step PECS, as well as PECS with preferred additives. Influences in sintering parameters in PECS for transparent polycrystalline Al2O3 are still not clear. At least, PECS with slow heating rate and two-step heating profile is preferred to produce transparent polycrystalline Al2O3. Using advanced Al2O3 powder and PECS, agglomeration of the particles is still signif‐ icant issue in transparent polycrystalline Al2O3. Management of Al2O3 powder to reduce the agglomeration is necessary to increase transparency of Al2O3 prepare by using PECS.
