Ce: YAG ceramics: effect of synthesis technology on luminescent and optical properties

The majority of scintillator materials used in ionisation detectors are applied in the singlecrystal form. It is known that growing scintillation crystals with high melting temperature is
a labour consuming and expensive process [1]. One of the promising approaches is replacement of single crystals by ceramic materials, such as Ce-doped yttrium aluminium garnet Y₃Al₅O₁₂, which have been studied for quite a long time [2][3]. Due to its unique scintillation properties, this material is applied in detection of X-radiation and soft gamma-radiation [4][5] and is characterised by high efficiency and mechanical and thermal resistance. Potential application of Ce:YAG as a luminophore for visualisation of X-ray images has been shown [6][7].

Unlike the technologies for creating singlecrystal materials, the ceramic technology offers a possibility to manufacture highly transparent and large-size samples (including those based
on YAG) applying lower synthesis temperatures, with homogeneous or controlled-gradient distribution of dopant ions [8][9], which is its indisputable advantage. However, specific technological features of the development of transparent ceramic materials are still the subject of active study today. This paper presents the results of investigation into the dependence of the optical and luminescent properties of yttrium aluminium garnet Y₃Al₅O₁₂, doped with Се³⁺  ions with concentration of 0.1 and 1 at.%  (Се³⁺: YAG), on the synthesis technology features.

Preparation of samples

An implementation chart according to which ceramics synthesis was performed is given in Table 1. Ceramic samples Се³⁺: YAG were synthesised from nanopowders Al₂O₃, Y₂O₃, 1 at.% Ce³⁺:Y₂O₃ diameter of the order of 10–15 nm, of in-house production, obtained by the method of laser ablation of target [10]. Taken as the initial coarse powders were commercial powders with purity > 99.99 %. The powders were synthesised in metastable phases, and for their transfer to the major phase, atmospheric annealing was applied, performed in electric muffle furnaces with silicon carbide heating elements at temperatures of 1200 and 900 °С for 3 hours for nanopowders Al₂O₃ and Y₂O₃, 1 at. % Ce³⁺:Y₂O₃ respectively. For the synthesis of Се³⁺: YAG with a desired concentration, nanopowders Al₂O₃, Y₂O₃, 1 at. % Ce³⁺:Y₂O₃ in the necessary stoichiometric proportion were taken. The nanopowders were mixed on a ball mill with inclined axis, in a plastic reservoir, in ethyl alcohol medium for 48 hours. The powder : balls : alcohol mass ratio was 1 : 4 : 8. As a sintering additive, 0.5 mass % of TEOS (tetraethoxysilane) was added during mixing of the powders. After that, the mixture was evaporated in a rotary vacuum evaporator, followed by atmospheric annealing at a temperature of 600 °С for 3 hours to remove residual alcohol.

The powder mixtures were then pressed into compacts by the single-axis static pressing method, applying pressure of 200 MPa, in a metal mould with working diameter of 14 mm, to a density making 40–50 % of YAG theoretical density (4.55 g/cm³ ). To remove organic admixtures from the ceramics, the powder mixtures were annealed in air at a temperature of 800 °С for 3 hours. Sintering of compacts was performed in a high-vacuum oven with graphite heating elements at a pressure of 5⋅10^–5 mbar and temperature of 1700–1780 °С for 20 hours [1]. After vacuum sintering, the samples were of black colour due to the lack of oxygen in the structure. To make up for that lack, the samples were annealed in air at a temperature of 1300 °С for 5 hours.

Within the framework of this procedure, samples were synthesised in two different ways. The fundamental difference between them consists in the synthesis of yttrium aluminium garnet major phase: immediately in the process of sintering (the first method) and prior to sintering (the second method). In the first method, the mixture of powders after annealing was additionally screened through a 200 mesh sieve and then annealed (samples No. 2406 – 1 at. % Се, No. 2421 – 0.1 at. % Се) or not annealed (samples No. 2405 – 1 at. % Се, No. 2420 – 0.1 at. % Се) in air at a temperature of 600 °С for 3 hours for removal of organic admixtures. In so doing, the temperature of vacuum sintering was 1700 °С for 20 hours.

When using the second method, (samples No. 1851 – 1 at. % Се, No. 1854 – 0.1 at. % Се), the mixture of powders after annealing was converted into YAG in the form of compacts with
a density making 20 % of the theoretical density at a temperature of 1200 °С for 3 hours in an atmospheric furnace. Then the compacts were crushed and milled, same as in the procedure of powders mixing. In so doing, the temperature of vacuum sintering was 1780 °С for 20 hours. Further, the ceramics were ground and polished using a buffing wheel and diamond polishing pastes (for final polishing, paste ASM 1/10 was used) as per a proven technology. Finished surface quality was checked by means of an optical microscope (BX51TRF-5, Olympus Сorp., Japan). Lying in the base of this ceramic synthesis are the methods by which the authors would obtain optical ceramics before [8][11].

Experiment

Measurements of the surface area of powders (BET analysis) were made on an automated gas adsorption analyser (TriStar 3000, Micromeritics Instrument Corporation, USA). The structures of powder and synthesised ceramics were determine by the method of X-ray diffraction analysis (XDA) on diffractometer D8 Discover GADDS, Bruker AXS, Germany. The morphology of ceramic surfaces was studied with the use of optical microscope BX51TRF-5, Olympus Сorp., Japan. Optical characteristics of the synthesised ceramic samples Се³⁺:YAG were measured on a double-beam optical spectrophotometer Shimadzu UV-2450 (200–900 nm). The X-ray luminescence (XL) spectra were measured on a monochromator-based installation МDR-23 with X-ray source URS-1,0 (W-anode, 20 kV, 14 mА) and FEU-106.

Results and discussion

The synthesis yielded six samples (Ø10×2 mm) with Се³⁺ dopant concentration of 0.1 and 1 at. % (Fig. 1). The samples are characterised by bright yellow colouring and high transparency [11].

As mentioned above, in this paper the fundamental difference between the methods consists in synthesis of the yttrium aluminium garnet major phase from the powders of yttrium and aluminium oxides: immediately during sintering (the first method) and prior to sintering (the second method). This difference can be visually observed on X-ray images of XDA of the compacts: the first method is shown in Fig. 2, a, and the second – Fig. 2, b.

Under the first method, the compact was annealed at 800 °С for 10 hours. According to the technology, a required annealing time for removal of organics is 3 hours, but for more confident stating of the absence of reactions between yttrium and aluminium oxides, the time was extended to 10 hours. The research results did not show any reactions between the oxides (see Fig. 2, a). At the same time, XDA demonstrates presence of cubic phase Y₂O₃ in amount of 60 wt. % and crystallite mean size (CSR) = 43 (5) nm, presence of various aluminium oxide phases: rhombohedral corundum in amount of 10 wt. % and CSR = 48 (10) nm, monoclinic θ-phase Al₂O₃ in amount of 20 wt. % and CSR = 22 (2) nm, cubic θ-phase Al₂O₃ in amount of 10 wt. % and CSR = 29 (3) nm. The presence of a large amount of different aluminium oxide phases is caused by preliminary annealings of the nanopowder and compact.

Under the second method, the compact was also annealed at 800 °С for 10 hours. According to the XDA data (see Fig. 2, b), it contains the following phases: perovskite orthorhombic phase YAlO3 in amount of 65 wt. % and CSR = 90 (20) nm, rhombohedral corundum in amount of 17 wt. % and CSR = 70 (10) nm, cubic garnet Y₃Al₅O₁₂ in amount of 14 wt. % and CSR = 78 (6) nm, cubic Y₂O₃ in amount of 4 wt. % and CSR = 54 (6) nm, unidentified crystalline phase < 1 wt. %. It is probably one of the aluminium oxide modifications. It can be seen from XDA (see Fig. 2, b) that the initial compact was obtained from already reacted oxide powders, which formed the necessary cubic garnet phase Y₃Al₅O₁₂ and an intermediate perovskite orthorhombic phase YAlO3. It was found too that a portion of the initial material (yttrium and aluminium oxide powders) never reacted.

As a result of using both methods, after vacuum sintering, ceramics with 100 % YAG phase were synthesised, which is confirmed by typical XDA X-ray images (see Figs. 2, c, d). All the studied ceramics had 100 % YAG phase. An important stage of ceramics synthesis is powder pressing, therefore it is worth mentioning that compactibility of nanopowders under the same pressure values of pressing (in the given case, 200 MPa) differs notably. Thus, for instance, density of the compact for the first method was about 2.02 g/сm3 (44.4 % of YAG density), and for the second method – 2.18 g/сm³ (47.9 % of YAG density). In the second case, the powders were coarser after sintering and milling than the initial powders in the first approach. The nanopowder compactibility problem does exist, and it is investigated [12][13]: the smaller the size of the particles, the worse their compactibility. The initial density of the powder particles cannot but tell on the final characteristics of the optical ceramics.

The final particle fineness was also considered in selection of appropriate YAG synthesis temperature. For the first method, a temperature of 1700 °С was selected, because a fine powder was used, which is more active in terms of sintering due to more developed specific surface of the particles. For the second option, a temperature of 1780 °С was selected, as the powder was coarser even after milling because of agglomeration of particles and phase transformations. According to the BET analysis data, specific surfaces of the powders used in the first method (before the pressing stage) were as follows: yttrium oxide – 17.92 m² /g, aluminium oxide – 57.5 m² /g, and in the second method (before the pressing stage) specific surface of the particles was equal to 11.7 m² /g. The data of BET analysis of the powders are consistent with the CSR of XDA given above (see Figs. 2, а, b), where the CSR of the phases of materials in the compact is considerably higher than the size of initial nanopowder particles. To study the macrostructure (Fig. 3), the ceramics were polished and then subjected to thermal etching at a temperature of 1350 °С for 5 hours. The studies showed that the mean size of ceramic crystallites does not depend on the production method. A dependence can be
observed in the samples with different concentration of Се³⁺ ions (see Fig. 3). Crystallite mean size decreases from 24–27 µm (0.1 at. % Се) to 17 µm (1 at. % Се).

The optical absorption and transmission spectra of the studied samples are given in Fig. 4, a, and Fig. 4, b, respectively. The spectra feature distinct absorption bands with the maxima in the region of 228, 260, 304, 338, and 457 nm, associated with 4 f → 5 d transitions in Ce³⁺ ions.

As can be seen from the optical absorption spectra (see Fig. 4, a), the intensity of said absorption bands of the samples synthesised by any one given method largely depends on the dopant concentration. However, the intensity of the absorption bands of samples with the same design concentration of dopant but synthesised by different method differs (e.g., spectra for samples Nos. 2420, 2421 (the first synthesis method) and No. 1854 (the second synthesis method); see Fig. 4, а). Hence, it can be concluded that samples synthesised by the second method contain smaller amount of Ce3+ ions. Proceeding from this, the first approach is more preferable from the viewpoint of exact dosing of dopant.

From the optical transmission spectra (see Fig. 4, b) it can be seen that in the transparency region (540–900 nm) transmission factor for the ceramic samples amounted to 60...81 %. The transmission factor value in this region (see Fig. 4, b), determining optical quality of the samples, depends on the concentration of Ce³⁺ ions in YAG lattice. The lower the concentration of cerium ions, the higher the optical quality of samples. One of the reasons for this effect occurrence can be the influence of crystallite sizes on the optical properties of samples. As was pointed out above, with Ce³⁺ concentration increasing, the mean size of crystallites in a sample decreases. With smaller crystallite size, more light scattering centres appear, which leads to deterioration of the optical quality of samples. It can be presumed that an increase of annealing time results in the growth of crystallite sizes and improves transparency of ceramics with high concentration of Се³⁺ dopant.

There is an effect worth mentioning. Samples with the same dopant concentration, synthesised by the first method (samples Nos. 2420, 2421 (0.1 % Се) and Nos. 2405, 2406 (1 % Се)), have different transmission factors in the transparency region (see Fig. 4, b). The size of crystallites in said pairs of samples is equal. The difference in pairs consists in the presence or absence of additional annealing of ceramic samples after screening through a 200 mesh sieve (see Table 1). Samples that were not subjected to additional annealing contain more pores and defects, caused by auxiliary organic substances getting into the initial powder.

The XL spectra of the samples were measured in a range of 200–800 nm (Fig. 5). All the samples feature an intensive luminescence band, with its maximum lying in the region of 525–538 nm, corresponding to d–f emitting transitions ( ²D → ²F_5/2 and ²D → ²F_7/2) in Ce³⁺ ions. It can be seen on the normalised XL spectra (see Fig. 5, b) that with dopant concentration in samples increasing, the luminescence spectrum maximum shifts from 525 to 538 nm. In the 240–460 nm region, there is also a luminescence band, associated with decay of autolocalised excitons (ALE) in YAG matrix. The complex shape of the ALE band is explained by the presence of absorption bands of the Ce³⁺ centres in this region. The XL spectra additionally feature luminescence bands of uncontrollable REE admixtures Gd³⁺ and Tb³⁺ (see Fig. 5), whose presence in negligible quantities does not affect the main luminescence of Ce³⁺ ions.

The findings of the research made it possible to compile a table with comparative characteristics of the samples synthesised by the two methods (Table 2). Irrespective of the synthesis method, the obtained results represent single-phase samples of transparent ceramics with fairly high optical quality (transparency no worse than 60 %) (see Table 2). Samples Nos. 2405 and 2406, synthesised by the first method, with 1 % Ce, have the brightest luminescence. Among the samples with cerium concentration of 0.1 %, the brightest one was sample No. 2420. Notably, this sample has the worst optical quality as compared with similar samples with 0.1 % Ce.

For more accurate estimate of the relative content of Ce³⁺ in YAG ceramics, attention should be paid to the ALE luminescence band (see Table 2). The ALE band intensity in this case is implicitly indicative of the relative concentration of cerium ions in YAG (insert in Fig. 5, b, Table 2). The higher the concentration of cerium ions in YAG, the lower the intensity of ALE luminescence band. The intensity of ALE luminescence band for sample No. 2420 turns out lower than for samples Nos. 2421 and 1854, hence, the concentration of Ce3+ ions in sample No. 2420 is higher. On account of that, the content of Ce³⁺ ions in the YAG lattice under the second method is lower than under the first.

Conclusion

Single-phase samples of ceramics 0.1 at. % Се:YAG and 1 at. % Се:YAG, sized Ø10×2 mm, were synthesised by two different methods from nanopowders with grain size 10–15 nm.
It is shown that with dopant concentration in the samples increased from 0.1 to 1 at. %, the mean size of crystallites decreases from 24–27 µm to 17 µm, which, in its turn, is one of the causes for transmission factor in the wavelength band of 500–900 nm decreasing from 81 to 60 %. When synthesis of the yttrium aluminium garnet major phase in YAG ceramics takes place immediately during sintering of samples (the first method), the absence of intermediate annealing after powder screening and prior to pressing leads to transparency decrease in the ready ceramics and appearance of transparency dependence on the dopant concentration. The higher the dopant concentration, the worse the transparency of samples. If YAG phase is synthesised prior to sintering (the second approach), the dependence is inverse.

However, the luminescence intensity decreases in this case, which may be due to additional escape of dopant from samples when the powder mixture being turned into YAG is in the form of compacts and/or during vacuum sintering, whose temperature is notably higher (1700 °С – the first method, and 1780 °С – second method). This issue is to be investigated yet.