**4. Transparent Polycrystalline Al2O3 Produced by using PECS**

Many reports on PECS for sintering transparent Al2O3 have been published as well as other transparent polycrystalline oxides. As process technology for ceramic powder is progressed, oxide ceramic powders with fine grain size and less agglomeration have been developed. Transparent polycrystalline Al2O3 with fine grains have been able to be prepared with such advanced oxide powder by using PECS.

Recently transparent oxide ceramics with fine grains such as 300 nm have been reported with different the PECS techniques as well as Al2O3. Munir and his colleagues promote PECS with ultra-high pressure such as 500 MPa [82]. High-pressure PECS is effective for preparing highly transparent polycrystalline Al2O3 [83], and also Y2O3-doped ZrO2 [82] and Y2O3 [84]. Highpressure PECS is very useful for eliminate closed pores. However sample size is likely limited in high-pressure PECS.

Kim et al. proposed slow-heating PECS for densifying Al2O3 with less grain growth [57]. PECS with slow healing rate is available for not only Al2O3 but also MgAl2O4 [85]. Kim studied kinetics of densification and grain growth with stress rate in the point of view on "dynamic grain growth" [86]. He mentioned that slow stress rate in PECS is preferred in order to densification of Al2O3 with less grain growth. On the other hand, Makino and his colleagues reported that transparent polycrystalline Al2O3 can successfully obtained by PECS with fast heating rate such as 200 K/min [73]. However transparency of the sample with fast heating rate was not good in homogeneity. In order to densify Al2O3 without significant grain growth, influences of heating rate is still in discussion.

Goto and his colleagues reported PECS of transparent Lu2O3 with two-step pressure profile [87]. Lu2O3 is one of the candidates on laser host materials for high-power and ultra-short pulse lasers. However it is difficult for densification by conventional sintering. Taking account of advanced studies on transparent oxides given by Kim and Goto, a sintering profile is very important even in a process of PECS.

Thus PECS provides transparent polycrystalline oxides. Besides the oxides described here, there are many examples of transparent oxides sintered by using PECS. Table 1 shows a variety of transparent polycrystalline oxides prepared by using PECS.



**Table 1.** Transparent polycrystalline oxides produced by PECS

Goto and his colleagues reported PECS of transparent Lu2O3 with two-step pressure profile [87]. Lu2O3 is one of the candidates on laser host materials for high-power and ultra-short pulse lasers. However it is difficult for densification by conventional sintering. Taking account of advanced studies on transparent oxides given by Kim and Goto, a sintering profile is very

Thus PECS provides transparent polycrystalline oxides. Besides the oxides described here, there are many examples of transparent oxides sintered by using PECS. Table 1 shows a variety

**Materials Dopants Remarks Ref.**

As-received Slow-heating [57] Cr2O3-doped [76] As-received High-pressure [88]

Cr2O3-doped Slow-heating [77]

MgO-doped High-pressure [83] As-received Fast-heating [73] As-received Slow-heating [80] La2O3-Doped High-pressure [81] As-received Two-step temperature [74, 75]

Undoped & LiF-doped Two-step Pressure & Temperature [89]

As-received Slow-heating [85] As-received [90] As-received [91] As-received Slow-heating [92] Lab-made [93]

As-received High-pressure [82] As-received [94] As-received [95] Lab-made [96]

Y2O3 As-received, undoped High-pssure [97]

MgO, Y2O3 and La2O3-doped [56]

ZrO2, La2O3 and MgO- doping [78]

important even in a process of PECS.

10 Sintering Techniques of Materials

Al2O3

MgAl2O4

Y2O3-Doped ZrO2

of transparent polycrystalline oxides prepared by using PECS.
