Fabrication and Microstructures of YAG Transparent Ceramics

YAG transparent ceramics with different microstructures were fabricated via solidstate reaction of high-purity powders. Influences of grain size, grain-boundary phases on the transmittance of the fabricated YAG ceramics were experimentally investigated. Our results confirm that the optical scattering loss in YAG ceramics is mainly caused by pores. Grain size did not influence the transmittance, and the grain-boundary phases with similar refractive index to the host only affected slightly the transmittance of the YAG transparent ceramics.


Introduction
Recently, transparent ceramics have attracted considerable attention of research due to their potential applications in solid-state lasers [1][2][3][4][5], medical devices [6][7][8], and optoelectronic devices [9].Among the transparent ceramics produced for application as laser gain host, Yttrium aluminum garnet (YAG) ceramics has been intensively investigated.The YAG single crystal has a high fracture strength, good chemical stability, excellent thermal and optical properties.It was widely used as solid-state laser gain hosts.However, producing single crystal YAG has the drawback that it not only takes a long time to grow the crystal but also is difficult to grow large sizes.Producing transparent YAG ceramics has non of these drawbacks.Furthermore, one can also use the advanced ceramic fabrication technology to fabricate composite ceramic structures, which makes the development of a multifunctional laser feasible.
So far, various techniques and procedures for fabricating transparent YAG ceramics have been reported [10][11][12][13][14][15][16].In the 1980s, DeWith and VanDijk [12], and Seketa et al. [13] reported efforts made for fabricating transparent YAG ceramics, although only translucent ceramics were obtained.In 1995, Ikesue et al. [21] first demonstrated transparent Nd:YAG ceramics for laser application.In the early 2000s, Lu et al. [14,15] fabricated large-scale Nd:YAG ceramic rods using precipitated powders, and achieved high efficient and high power laser emission.It was generally believed that in order to obtain transparent ceramics, pure phase, big grain size and few pores [17] are the crucial requirements on the ceramics.However, recently A. Ikesue et al studied the optical scattering in Nd:YAG transparent ceramics.They found that optical scattering was caused solely by pores [18].The existence of a grain boundary in a ceramic has little relationship to optical scattering [19].In this paper, YAG transparent ceramics with different microstructures were fabricated by the solid-state reaction sintering method using high-purity Al 2 O 3 , Y 2 O 3 and rare earth oxide powders as the starting materials.The effects of grain size, grain-boundary phases and pores on the transmittance of the fabricated ceramics were investigated.We showed that the existence of grain-boundary phases only affect slightly the transmission of the ceramics.

Experimental procedure
Al 2 O 3 , Y 2 O 3 and Rare Earth oxide powders were used as the starting materials.We have synthesized the Rare Earth doped Y 2 O 3 nanopowders by co-precipitation using the mixture of ammonium hydrogen carbonate and ammonia as precipitator [4].To fabricate the specimens, the starting powders were weighed so that the ratio of (Y, RE) 2 O 3 to Al 2 O 3 was about 3:5.After that Al 2 O 3 and the Rare Earth doped Y 2 O 3 powders were mixed with 0.5 wt% tetraethyl orthosilicate (TEOS) as the sintering acid, the mixture was milled for 15 h with high-purity ZrO 2 balls.Then the milled slurry was dried at 70 o C for 24 h.The milled powders were isostatically pressed at 200 MPa into pellets that were 20 mm in diameter.Finally the pellet was sintered at 1760 o C for 6 h in vacuum (~1.2×10 -3 Pa) using a high-vacuum furnace, and annealed at 1500 o C for 10 h in an air atmosphere.
Phase composition in the sintered sample was identified with a X-ray diffractometer (XRD, Model D/MAX-2550V, Rigaku Co., Tokyo, Japan) using nickel-filtered CuKα radiation in the range of 2θ from 10 o -70 o .The microstructure of the samples after sintering was observed with Electron Microprobe Analysis (EPMA, Model JXA-8100, JEOL Co., Japan) as well as Energy Disperse Spectroscopy (EDS) and high resolution transmission electron microscopy (HRTEM, Model JEM-2010/2100F, JEOL Co., Japan).The transmission spectra of mirror-polished samples (1 mm thickness) were measured over the wavelength region from 200 to 2000 nm, using a spectrophotometer (Model CARY 500 Scan, Vavian Co., America).

Results and Discussions
Grain size, grain-boundary phase and pores are usually considered as the main factors affecting transmittance of ceramics [17].Provided that the residual pores were free, the bigger the grain size was, the higher the transmittance of a ceramic.Fig. 1 shows a comparison of microstructures of the YAG ceramics fabricated by Konishima Company [20], the authors of the article and A. Ikesue et al [21], respectively.Apparently, no pores and grain-boundary phases exist in the samples.The grain size of the samples was about 1.5, 10 and 50 µm, respectively.Despite their grain size differences, the transmittance of all the three YAG samples could reach above 80% at 1064 nm, which is near the theoretical transmittance of 83.8%.The result clearly confirms Ikesus et al's conclusion that the optical transmittance of the transparent ceramics has little relationship with the grain size.Although the size of a grain determines the length of its grain boundary, the length of a grain boundary has no relationship to optical scattering.What affects the strength of optical scattering is the thickness of the grain-boundary.As far as two grains are connected with boundary thickness far less than the optical wavelength, the effect of grain-boundary on the optical scattering could be neglected.From the EPMA image of the YAG sample, shown in Fig. 3, it can be seen that the grain size of the ceramic is quite uniform, with an average size of about 10 µm.However, EDS of the YAG transparent ceramic show that a Al 2 O 3 grain-boundary phase exists in the sample.It is therefore to conclude that YAG transparent ceramics could still be obtained even if the Al 2 O 3 grain-boundary phase exists in the ceramics.The EPMA image in Fig. 5 showed that the sample consisted of a well-defined microstructure having a uniform grain distribution of about 20 µm in size.Furthermore, no pores were observed.However, the table accompanying Fig. 5 showed that the grainboundary phases comprised of 85 wt% Y 2 O 3 and 15 wt% Al 2 O 3 , suggesting the existence of Y 2 O 3 in the grain boundary.To explain the above results, we note that the refractive index of the Al 2 O 3 and Y 2 O 3 grain-boundary phase are 1.70~1.80and 1.89, respectively, which are near to that of the YAG host (1.82).Based on the optical theory, the lower the difference between the refractive index of the host and the grain-boundary phases, the lower the optical scattering and higher the transmittance, therefore, little light would be scattered by the grain boundary phases.

Conclusions
The important results obtained from the present experiment are summarized as follows: (1) Grain size has no influence on the transmittance of YAG transparent ceramics.
(2) Grain-boundary phases Al 2 O 3 and Y 2 O 3 only affect slightly the transmittance of YAG transparent ceramics, and the lower the difference between the refractive indexes of host and the grain-boundary phase is, the higher the transmittance.
(3) YAG transparent ceramics (1mm thickness) with transmittance of 81.8% at 1064 nm were produced.The sample consisted of a well-defined microstructure with a uniform grain distribution of about 10 µm in size, and no pore and grain-boundary phases were observed.

Fig. 3 .
Fig. 3. EPMA and EDS images of the YAG transparent ceramic with Al 2 O 3 grain-boundary phase

Fig. 7 .YFig. 8 (
Fig. 7. (a) The microstructure of the thermal etched surface and (b) HRTEM of the grain boundary of sample sintered at 1760 o C for 6 hours

Fig. 8 .
Fig. 8. (a) The picture and (b) the transmission curve of YAG:Er sample