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Composite YAG/Nd:YAG/YAG transparent ceramics for planar waveguide laser were fabricated by non-aqueous tape casting and solid-state reactive sintering. The slurry made from the oxide powder mixtures shows a shear thinning behavior. The morphologies of the tapes were homogeneous in structure, and the tapes had appropriate strength and toughness. After calcining at 600°C for 10h in air, the samples contained less than 0.05wt.% of carbon. No gaps were found between the layers on the fracture surface of the green body compacted by cold isostatic pressing. The composite YAG/Nd:YAG/YAG transparent ceramics with in-line transmittance of 82.5% at 1064nm were obtained by vacuum-sintering at 1760°C for 30h, whose average grain size is 36.8μm. The diffusion distance of the Nd3+ ions was about 150μm along the thickness direction of the ceramics.
© 2014 Optical Society of America
1. Introduction
Nowadays, solid-state lasers continue to occupy a prominent place among different kinds of fields, especially the high power and large energy lasers for advanced manufacturing industry and laser weapons [1–6]. In order to meet the challenging requirements in the areas of laser power, beam quality, efficiency, size and weight, it is essential to optimize the structural designing of laser gain media. Planar waveguide structure (PWs) has peculiarities of low lasing thresholds, high gain, high heat transmission and optical confinement with respect to the traditional uniform bulk laser system [7,8], which makes waveguide lasers suitable for high power and compact devices. Some excellent planar waveguide lasers had been reported. A thermal-bonded PWs Nd:YAG laser crystal obtained an optical-to-optical conversion efficiency of 58% with an output power of 2.9 W at 1064nm [9]. Another PWs Nd:YAG laser crystal obtained a slope efficiency of 54% with an output power of 105 W at lasing wavelength of 946nm [10]. In recent years, polycrystalline Nd:YAG ceramics have attracted much attention owing to their flexible structural designing and excellent laser properties, which are believed to be the next-generation gain materials for advanced solid-state lasers [11–24]. Furthermore, larger neodymium ion concentration can be achieved by the ceramic fabrication technique [25–28]. Taking the advantages of both the Nd:YAG ceramic and the planar waveguide structure, it is very promising to construct a compact, efficient and high power solid-state laser.
Investigations on various composite laser ceramics have been carried out in recent years. Dry-pressing shows the shortages in the controlling of the thickness of single layer and the bonding between different layers. Though the composite ceramic made by the thermal diffusion-bonded method has very flat interface, it is an expensive and time-consuming process. Furthermore, it is difficult to polish the thin slice ceramic to micron level. Kupp et. al. [29,30] prepared composite Er:YAG ceramics by the tape casting method and the optical quality was comparable to the single crystals. Tang et. al. [31,32] prepared Yb:YAG and Nd:YAG composite ceramics and they also achieved laser output. Ba et. al. [33,34] prepared YAG transparent ceramics with high optical quality by both the aqueous and the non-aqueous tape casting methods. Such composite structure laser ceramics mentioned above are mainly designed to improve the thermal conductivity and decrease the thermal effects, and the thickness of the dopant part is relatively large. The present work is focused on composite YAG/Nd:YAG/YAG ceramics for planar waveguide laser. PWs is a special kind of composite structure, in which the thickness of the dopant part is only a dozen to hundreds microns. Utilizing the tape casting technology, which is widely used in electronic industry, solid oxide fuel cells and structural materials [35], the thickness of every tape can be controlled at micron level, which is suitable for preparing the PWs ceramics. To the best of our knowledge, no such a planar waveguide laser ceramics were obtained by tape casting.
In this work, we report the fabrication of the composite YAG/Nd:YAG/YAG transparent ceramics for planar waveguides laser by the combination of non-aqueous tape casting process and solid-state reactive sintering method. The rheological properties of the slurry were investigated. The microstructures of the green body and sintered ceramics were examined. The diffusion of dopant ions at the interface of the composite layers was observed and the optical property was also studied.
2. Experimental procedure
The high purity commercial α-Al2O3 (99.98%, Alfa Aesar, USA), Y2O3 (99.999%, Alfa Aesar, USA), Nd2O3 (99.99%, Alfa Aesar, USA), TEOS (tetraethyl orthosilicate, 99.999%, Alfa Aesar, USA), MgO (99.99%, Alfa Aesar, USA), ethanol (Analytical grade, Shanghai Zhenxing Chemical Co., Ltd., China), xylenes (ACS 98.5%, Alfa Aesar, USA), MFO (Menhaden fish oil, F8020-500ML, Sigma–Aldrich, USA), PVB (Polyvinylbutyral, B-98, Aladdin Chemical Ltd., China), PEG-400 (Polyethylene glycol, CP, Sinopharm Chemical Reagent Co., Ltd., China) and BBP (butyl benzyl phthalate, 98%, Alfa Aesar, USA) were used as starting materials. Y2O3, Al2O3 and Nd2O3 powders were weighted with chemical compositions of Y3Al5O12 and Nd0.03Y2.97Al5O12 (1.0at.% Nd:YAG). Appropriate amount of MgO and TEOS were added as sintering aids. These powders were mixed and milled for 10 h in the mixed solvent of ethanol and xylene (mass ratio of 1:1), with 1.0wt.% MFO as dispersant. The weight ratio of the powders to the solvents was 5:2. Then, 8.0wt.% PVB as binder, 4.0 wt.% PEG-400 and 4.0wt.% BBP as plasticizers were added into the slurry and milled for another 15h. In the tape casting process, the height between the blade and the substrate is 450μm with the casting speed of 1cm/s. The tapes were dried at room temperature for 24h and then cut to slices of 20 × 20mm. The thickness of a single layer dried-tape slice was about 150μm. 15 layers of YAG composite tape slice were stacked at both sides of a single layer of 1.0at.% Nd:YAG compose tape slice respectively (Fig. 1). The stacked composite samples were laminated at 70°C under 20MPa for 30min to obtain the green bodies. The added organics were removed by calcining at 600°C for 10h in air, and then the green body was compacted at 250MPa by cold isostatic pressing to achieve a relatively dense microstructure. The vacuum-sintering process was carried out at 1760°C for 30h, and then the as-sintered ceramics were annealed at 1450°C for 20h in air to eliminate the oxygen vacancies. Finally, the composite ceramics were mirror-polished on both surfaces to 1.5mm.
The rheological behavior of the slurry was monitored by the rheometer (Anton Paar Physica MCR-301, Germany). The microstructures of the tape, the green body and the final ceramics were observed by the field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan). The in-line transmittance curve of the polished ceramic was detected by a UV–VIS–NIR spectrophotometer (Cary-5000, Varian, USA). The distribution of Nd3+ ions along the thickness direction of the composite ceramics was tested by the inductively coupled plasma atomic emission spectra-mass spectrometry (ICP-MS, Thermo Scientific XSERIES 2, USA).
3. Results and discussion
Figure 2 shows the rheological behavior of the slurry prepared from the mixture of Al2O3 and Y2O3 powders. The upper curves (with arrow) represent the increasing speed of the shearing test process, while the below curves stand for the opposite process. The curves exhibit shear thinning behavior, viscosity decreases with increasing shear rate, and shear stress increases with increasing shear rate, which is suitable for tape casting. At low shear rate, a near network is formed by the interaction of binders on the surface of the particles, which holds the slurry not to flow and avoids sedimentation. In the high shear rate range, the near network breaks down, and this provides the slurry with high fluidity and low viscosity [36].
Figure 3(a) shows the photograph of the tape prepared by the non-aqueous tape casting method. It can be seen from the secondary electron image of the tape in Fig. 3(b) that the powders were mixed homogeneously. Figures 3(c) and 3(d) are the micrographs of the tape at the same place under higher magnification and they were obtained from different observation mode. Figure 3(d) is the partial enlarged back-scattered electron image of the tape. The powders were composed of Al2O3 particles (dark) and Y2O3 particles (bright). Because of the organics on the surface of the tape, the image looks not so clear. Further evidence on the existence of the organics can be confirmed in Fig. 3(c). It is obvious that the Y2O3 particles in the black circle were coated by organics. Owing to the organics, especially the binders and the plasticizers, the tapes have appropriate strength and toughness [35], which can be bent without flaw or damage, as shown in Fig. 3(b).
Fig. 3 (a) Photograph, (b,c) secondary electron image and (d) partial enlarged back-scattered electron image of the tape.
Figure 4 exhibits the partial enlarged back-scattered electron image of the tape and the fracture surface of the green body. The green body has been calcined to remove the organics and further compacted by cold isostatic pressing. After removing the organics, Fig. 4(b) looks much more clear than Fig. 4(a), and the content of the residual carbon was less than 0.05wt.%.
Fig. 4 Partial enlarged back-scattered electron image of (a) the tape and (b) the fracture surface of the green body.
Figures 5(a), 5(b), 5(c) and 5(d) represent the fracture morphology of the whole, the left, the middle and the right part of the green body, respectively. It can be seen that the green body had a relatively dense structure, and no gaps were found between the layers on the fracture surface.
Fig. 5 The fracture surface of the green body (a) the whole, (b) the left, (c) the middle, and (d) the right part of the green body.
The SEM image of the cross-section of the composite YAG/Nd:YAG/YAG transparent ceramics is shown in Fig. 6(a). Obviously, the ceramic was pore-free and no secondary phase was found. The grain size of the sample was obtained by the linear intercept method [37], and the calculated average grain size was 36.8μm,which was attributed to the long sintering time of 30h. It can be seen from Fig. 6(b) that the fracture mode of the ceramics was trans-granular.
Fig. 6 SEM images of the YAG/Nd:YAG/YAG ceramics (a) polished and thermally etched surface; (b) fracture surface.
The in-line transmission curve of the YAG/Nd:YAG/YAG ceramics with the thickness of 1.5mm is shown in Fig. 7. The transmittances reach 82.5% at the lasing wavelength of 1064nm and 81.0% at 400nm, respectively. The slight absorption peaks correspond to the energy level transitions of Nd3+ions. The peak at 253nm in the curve can be attributed to the absorption of the Fe3+ions [38].The Fe3+ions might be introduced from the starting materials or the tape casting process by the steel blade.
Fig. 7 Transmittance curve of the planar waveguide structure YAG/Nd:YAG/YAG ceramics (photo of the composite ceramics as inset).
As mentioned above, the planar waveguide structure was composed of 15 layers of YAG composite tape slice on both sides of a single layer of 1.0at.% Nd:YAG composite tape slice. The thickness of a single layer dried-tape was about 150μm. Considering the shrinkage, the theoretical thickness of the single layer of 1.0at.% Nd:YAG ceramics should be 120μm after sintering. However, Nd3+ ions diffused during the sintering process. This phenomenon can be detected by ICP-MS, and the distribution of Nd3+ ions along the thickness direction of the sample was determined by line scanning from one surface to another (1.5mm distance), as shown in Fig. 8.The result shows that the distribution of Nd3+ ions ranges from 0.5mm to 0.92mm, it means the total thickness of the Nd3+ ions containing part was about 420μm. Thus, the unidirectional diffusion distance of Nd3+ ions was about 150μm. Because the total mass of Nd3+ ions is a constant, and the thickness of the single layer Nd:YAG tape is very small before sintering, it may be approximately considered as a one-dimension non-steady diffusion with limited source. However, the diffusion coefficient of Nd3+ions is changed with the increase of temperature, and both the lattice and the grain boundary diffusion coefficients should be taken into account, so the actual situation is more complicated. Though the diffusion process is inevitable, in order to obtain the fine planar waveguide structure, the diffusion distance must be as short as possible. Under the premise of obtaining high optical quality ceramics, it requires a lower sintering temperature and shorter dwelling time.
Fig. 8 Concentration distribution of Nd3+ along the thickness direction of the YAG/Nd:YAG/YAG ceramics.
4. Conclusions
High optical quality composite YAG/Nd:YAG/YAG transparent ceramics for planar waveguide laser were fabricated by the non-aqueous tape casting and solid-state reactive sintering method. The slurry with shear thinning behavior is suitable for tape casting. The morphologies of the tapes and the green body were homogeneous in structure. The tapes with appropriate strength and toughness can be bent without damage. No gaps were found between the layers on the fracture surface of the green body compacted by cold isostatic pressing. The transmittance of the composite YAG/Nd:YAG/YAG ceramics sintered at 1760°C for 30h reaches 82.5% at 1064nm, whose average grain size was 36.8μm. The diffusion distance of Nd3+ ions was nearly 150μm. These results demonstrate that the non-aqueous tape casting method is a promising technology to prepare composite transparent ceramics for planar waveguide laser.
Acknowledgments
This work was supported by the Major Program of National Natural Science Foundation of China (Grant No. 50990301),the Key Program of National Natural Science Foundation of China (Grant No. 91022035) and the Project for Young Scientists Fund of National Natural Science Foundation of China (Grant Nos. 51002172 and 51302298).
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