Fabrication and Characterization of New Er-Doped Yttrium–Scandium–Aluminum–Garnet Ceramics ^†

Abstract

We report the fabrication and characterization of yttrium–aluminum–garnet (Er:YAG) and yttrium–scandium–aluminum–garnet (Er:YSAG) ceramics for the implementation of analysis as an active medium for 1500 nm lasing. High erbium content Er:YAG and Er:YSAG ceramics are fabricated from Er:YAG and Er:YSAG powders, respectively. All ceramic samples belong to the garnet-type cubic structure (space group Ia3d) without any traceable impure phases. Including Sc³⁺ in the Er:YAG crystal structure leads to improved mechanical characteristics and elastic–plastic properties of the materials. The optical transmittance of ceramics is affected strongly by including Sc³⁺ and increases up to 60% at about 1500 nm.

1. Introduction

The rare-earth-doped yttrium–aluminum–garnet, RE³⁺:Y₃Al₅O₁₂ (RE³⁺:YAG), is well-known as an active media for high-power lasers [1,2,3]. Transparent ceramics were initially developed to replace single crystals in cases of disk geometry [4], as well as multilayer and concentration gradient architectures. A number of authors have studied YAG and YSAG ceramics doped by Tm³⁺ [5], Yb³⁺ [6,7], and Er³⁺ [8]. The ceramics obtained show good laser and optical properties, in particular, transparency of up to 83%. We synthesize (Y_0.5Er_0.5)₃Al₅O₁₂ (Er³⁺:YAG) and (Y_0.5Er_0.5)₃(Al_0.8Sc_0.2)₅O₁₂ (Er³⁺:YSAG) ceramics and investigate their microstructures and transmittance depending on the presence of Sc³⁺ ions.

2. Materials and Methods

Precursor powders were synthesized by the reverse precipitation of metal chlorides (MeCl₃·6H₂O, where Me includes Al, Sc, Y, and Er) into a cooled aqueous ammonia solution of 25% through spraying [6,9,10]. The disaggregated oxyhydrate powders were calcined at 1200 °C for 2 h in an oven. Additionally, some of the powder was calcined at 1600 °C. TEOS was used as a sintering addition at the milling powder stage. Then, the ceramic powders were processed by uniaxial pressing, cold isostatic pressing, and sintering in a vacuum at 1760–1780 °C for 10 h. After vacuum sintering, the samples were processed through lighting annealing and polished on both sides.

The phase composition and unit cell parameters of the ceramic powders were determined with an Empyrean X-ray diffractometer (PANalytical, Almelo, The Netherlands) using CuKα radiation (1.5406 Å). The experimental powder diffraction patterns were compared with the XRD pattern of Y₃Al₅O₁₂ from the ICSD database (№ 01-072-1853).

The particle size distribution of the oxyhydrate powders was measured with a laser diffraction technique using the Analysette 22 MicroTecPlus laser particle sizer (Fritsch, Markt Einersheim, Germany).

The microstructure of the ceramic surfaces was scanned on an electron microscope EVO 10 (GmbH Zeiss Microscopy, Jena, Germany). The samples were examined at a 20 kV accelerating voltage with a 9 mm working distance in low vacuum operation (EP = 70 Pa); cathode LaB6. Digital images were captured in tiff format with resolution 1024 × 768 px (0.09 nm/px).

Before SEM imaging, the samples of dry particles were placed onto carbon-adhesive tape. The observations of samples were carried out with a beam current on the samples of 370 pA. The images were captured in backscattered electron mode (BSE).

An examination of chemical composition was carried out on detector Smart EDX (AMETEK, Berwyn, PA, USA) with a beam current of 626 pA for analyses and at a 20 kV accelerating voltage with a 9 mm working distance.

The refractive index was determined with the total internal reflection method with the Metricon refractometer for three wavelengths.

The room-temperature transmittance spectra of the Er³⁺:YAG and Er³⁺:YSAG ceramics were recorded in the wide range of 250 to 1700 nm on a Shimadzu UV-3101PC spectrophotometer (Shimadzu, Kyoto, Japan) controlled by a desktop computer.

3. Results and Discussion

Characteristics of the Er³⁺:YSAG ceramic powders are summarized in

Table 1. Characteristics of the Er³⁺:YSAG ceramic powder obtained with calcinations at 1200 °C and 1600 °C.

Annealing Temperature	Particle Size Distribution, μm			Phase Composition		dXRD, nm	YSAG unit Cell Parameters, a, Å
	d10	d50	d90	Er:YSAG	Y2O3
1200 °C	0.22	0.88	2.13	99.2%	0.8%	66 ± 2	11.9904(2)
1600 °C	-	-	-	100%	-	>150	11.9958(2)

. The average grain size was evaluated as d₅₀ = 0.88 μm. X-ray diffraction data (Figure 1) point to the presence of Y₂O₃ as an impurity phase after annealing at 1200 °C. To exclude the impurity phase, the samples were annealed at a higher temperature of 1600 °C. X-ray data of the repeatedly annealed powders included picks corresponding to YSAG only and pointed to the compliance of the initial cationic composition with stoichiometric one. Moreover, differences in unit cell parameters of the powders before and after annealing at 1600 °C implied a particular formation of garnets at 1200 °C and full formation at 1600 °C. The average crystallite size d_XRD was about 66 nm, and the ceramic powder may be classified as a nanocrystalline one.

Experimental data of refractive indices for three wavelengths (

Table 2. Measured refractive indices of the ceramic samples and coefficients in the Sellmeier dispersion equation.

Ceramic Samples	Refractive Indices			Coefficients in the Sellmeier Dispersion Equation
	633.5 nm	969.0 nm	1539.5 nm	A	B
Er:YAG	1.8386(5)	1.8259(5)	1.8167(5)	6.9(1.4)·103	4.36(2)·10−1
Er:YSAG	1.8487(5)	1.8359(5)	1.8257(5)	6.9(1.1)·103	4.30(2)·10−1

) were subsequently fitted using a least-squares fitting program to the Sellmeier dispersion Equation (Figure 2):

$$\frac{1}{{\mathrm{n}}^2-1}=-\frac{\mathrm{A}}{{\mathsf{\lambda }}^2}+\mathrm{B}$$

(1)

where (–A), the slope of the plot of (n² − 1)⁻¹ versus λ⁻², gives a measure of dispersion and B, the intercept of the plot at λ = ∞, gives n_∞ = (1 + 1/B)^1/2.

Figure 3a,b show the SEM micrographs of the Er³⁺:YAG and Er³⁺:YSAG ceramics, respectively. Er³⁺:YAG includes many pores that might be scattering centers. The surfaces of the Er³⁺:YSAG ceramics demonstrate a homogenious texture, and the number of pores decreases significantly. Energy-dispersive X-ray spectroscopy (EDX) analysis shows overstated Al³⁺ content in the ceramics, in particular, 5.1(4) and 5.4(5) atoms per formula unit (apfu) in Er:YSAG and Er:YAG, respectively. This is explained by an instrument error up to 10%. Thus, Sc³⁺ can occupy dodecahedral and octahedral structural position and substitute Y³⁺ and Al³⁺, respectively. The empirical formulas calculated based on twelve oxygen atoms per formula unit are Er_1.26Y_1.17Sc_0.49Al_5.08O₁₂ and Er_1.36Y_1.23Al_5.41O₁₂.

Probably, the presence of Sc³⁺ in the YAG crystal structure leads to decreasing melting temperature and the improved elastic–plastic properties of the materials. As a result, the Er:YSAG ceramic sample has a more perfect microstructure and higher optical transmittance (Figure 4).