**3. Sintering of transparent polycrystalline alumina**

Transparent polycrystalline Al2O3 has increasingly become the focus of recent investigations primarily because of their unique combination of properties. Single crystals of Al2O3 are highly transparent in visible and IR region. However polycrystalline Al2O3 ceramics are usually opaque because of light scattering of pores and grain boundaries as well as impurities. High density is the most important factor to produce polycrystalline transparent ceramics, as well as grain size. Because of the high efficiency of pores for light scattering, transparency in polycrystalline materials requires extremely low level in porosity, less than 0.01 vol.%. Samples with such low porosity could only be produced under proper sintering conditions involving high temperatures and long sintering time. Residual porosity is much more important than grain boundaries for obtaining the transparency, even in crystallographically anisotropic materials in optical properties. The scattering efficiency for spherical pores, however, decreas‐ es dramatically when the pore size in the nanometric range could be achieved [38-40]. It is believed that nanostructured polycrystalline materials would possess higher transparency than ones with the micrometric grain size range. The sintering process at high temperature causes extensive grain growth and then seriously degrades the mechanical properties of the material. What is more important, the higher/bigger grain size larger than 410 μm leads to significant light scattering coming from the birefringence of coarse Al2O3 grains [41].

After Coble developed transparent polycrystalline Al2O3 [42], many studies for producing transparent polycrystalline Al2O3 by sintering techniques were reported [40, 41, 43-82]. Recently, fine-grained transparent polycrystalline Al2O3 has attracted much attention due to its superior mechanical and optical properties. This material is prepared by sintering using HP and HIP at low temperature ranging from 1150 to 1400°C. The formation of nanostructure (< 1 μm) results in a significant improvement in both the mechanical strength and the optical transparency. It is reported that the mechanical strength of the fine-grained transparent Al2O3 is reached up to 400 - 600 MPa together with a high in-line transmission up to 60 % for visible light [41, 45]. Thus far, the addition of small amount of MgO is known to suppress normal and abnormal grain growth. The MgO concentration needed to inhibit abnormal grain growth depends on other impurities, CaO, SiO2 etc.. [42, 46, 47, 70, 72]. Coble opened a new chapter that positive effect of 250 ppm MgO addition in sintering of Al2O3 is accompanied by dissolution into Al2O3 and excess MgO beyond its solid solubility limit exists as non-stoichio‐ metric MgAl2O4 spinel at the grain boundaries of Al2O3 [42]. Hence, MgO strongly segregates into Al2O3 grain boundaries and produces a solute drag effect. The resultant microstructure is finer in grain size with higher final density. The transparent MgO doped Al2O3 ceramics was sintered to full density and had an in-line transmission of 40-50 % between 400 and 600 nm of the wavelength.

On the other hand, HPed Al2O3 yielding better transparency than pressureless-sintered samples was reported long back [38, 48-50]. Those reports showed that the increase of the transparent Al2O3 with a much smaller grain size of 1 μm could be obtained by a continuous hot-pressing process at 1400°C under pressures of 120 MPa in different atmosphere.

predominated by sample temperature. The pulse electric current waveform had effects on the sample temperature, but did not have direct influence on the densification, grain growth and

Transparent polycrystalline Al2O3 has increasingly become the focus of recent investigations primarily because of their unique combination of properties. Single crystals of Al2O3 are highly transparent in visible and IR region. However polycrystalline Al2O3 ceramics are usually opaque because of light scattering of pores and grain boundaries as well as impurities. High density is the most important factor to produce polycrystalline transparent ceramics, as well as grain size. Because of the high efficiency of pores for light scattering, transparency in polycrystalline materials requires extremely low level in porosity, less than 0.01 vol.%. Samples with such low porosity could only be produced under proper sintering conditions involving high temperatures and long sintering time. Residual porosity is much more important than grain boundaries for obtaining the transparency, even in crystallographically anisotropic materials in optical properties. The scattering efficiency for spherical pores, however, decreas‐ es dramatically when the pore size in the nanometric range could be achieved [38-40]. It is believed that nanostructured polycrystalline materials would possess higher transparency than ones with the micrometric grain size range. The sintering process at high temperature causes extensive grain growth and then seriously degrades the mechanical properties of the material. What is more important, the higher/bigger grain size larger than 410 μm leads to

significant light scattering coming from the birefringence of coarse Al2O3 grains [41].

the wavelength.

After Coble developed transparent polycrystalline Al2O3 [42], many studies for producing transparent polycrystalline Al2O3 by sintering techniques were reported [40, 41, 43-82]. Recently, fine-grained transparent polycrystalline Al2O3 has attracted much attention due to its superior mechanical and optical properties. This material is prepared by sintering using HP and HIP at low temperature ranging from 1150 to 1400°C. The formation of nanostructure (< 1 μm) results in a significant improvement in both the mechanical strength and the optical transparency. It is reported that the mechanical strength of the fine-grained transparent Al2O3 is reached up to 400 - 600 MPa together with a high in-line transmission up to 60 % for visible light [41, 45]. Thus far, the addition of small amount of MgO is known to suppress normal and abnormal grain growth. The MgO concentration needed to inhibit abnormal grain growth depends on other impurities, CaO, SiO2 etc.. [42, 46, 47, 70, 72]. Coble opened a new chapter that positive effect of 250 ppm MgO addition in sintering of Al2O3 is accompanied by dissolution into Al2O3 and excess MgO beyond its solid solubility limit exists as non-stoichio‐ metric MgAl2O4 spinel at the grain boundaries of Al2O3 [42]. Hence, MgO strongly segregates into Al2O3 grain boundaries and produces a solute drag effect. The resultant microstructure is finer in grain size with higher final density. The transparent MgO doped Al2O3 ceramics was sintered to full density and had an in-line transmission of 40-50 % between 400 and 600 nm of

homogeneity of the sample sintered by the PECS process.

8 Sintering Techniques of Materials

**3. Sintering of transparent polycrystalline alumina**

The major contribution to the less transparency for undoped polycrystalline Al2O3 originates from scattering caused by the remaining pores, and the difference between translucent and transparent Al2O3 could come entirely from the difference in pore-size and its distribution. Until quite recently, HIP is the most widely used technique for developing transparent alumina as it eliminates residual porosity and prevents grain growth leading to high transmission. For post-HIP treated samples, results for undoped [52], single doped (Mg2+, Ti4+) [39, 45, 46] transparent polycrystalline Al2O3 have been reported so far. This method gave fairly repro‐ ducible in-line transmittance between different groups with values up to 65 %.

Recently, some new sintering techniques such as microwave sintering [65] have also been studied for transparent crystalline Al2O3. Pressure-sintering such as HP and HIP is usually expensive in process cost. Microwave sintering is expected to realize homogeneous heating of the whole of ceramic sample.
