Luminescent Er3+doped transparent alumina ceramics

Luminescent Er3+doped transparent alumina ceramics DRDLÍKOVÁ, K.; KLEMENT, R.; DRDLÍK, D.; SPUSTA, T.; GALUSEK, D.; MACA, K. Analog Integrated Circuits and Signal Processing 2017, vol. 37, iss. 7, pp. 2695-2703 ISSN : 0955-2219 DOI: http://dx.doi.org/10.1016/j.jeurceramsoc.2017.02.017 Accepted manuscript © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license (http://creativecommons.org/licenses/by-nc-nd/4.0/), doi: 10.1016/j.jeurceramsoc.2017.02.017 Final version available from http://www.sciencedirect.com/science/article/pii/S0955221917300833 dspace.vutbr.cz LUMINESCENT Er3+ DOPED TRANSPARENT ALUMINA CERAMICS Katarína Drdlíková 1, Róbert Klement 2, Daniel Drdlík 1, Tomáš Spusta 1, Dušan Galusek 2, Karel Maca 1 1 CEITEC - Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic 2 Joint Glass Centre of the IIC SAS, TnU AD, and FChFT STU, Trenčín, Slovakia Corresponding author: K. Drdlíková ([email protected]) Keywords: alumina, dopants/doping, erbium/erbium compounds, luminescence, transparent ceramics ABSTRACT We report on successful preparation of Er3+ doped transparent alumina (0.1-0.17 at. %) exhibiting visible light photoluminescence using wet shaping method and hot isostatic pressing. The effects of dopant amount, type of doping powder and powder pre-treatment on final microstructure, real in-line transmittance and photoluminescence characteristics were studied. The real in-line transmittance ranged between 28-56 %, depending on processing parameters. The transparency decreased with increased amount of dopant. The decrease is dependent on the type of doping powder and its pre-treatment. The photoluminescence spectra measured in both visible and NIR region showed typical emission bands due to the presence of Er3+ ions. The decay profiles of the 4S3/2→4I15/2 transition were fitted with a 2-exponential function, with faster component in the range of 360-700 ns and slower component around 1.6-2.4 μs. The intensity of emissions and lifetime of the 4S3/2 level decrease significantly with increasing concentration of Er3+ ions. 1. INTRODUCTION Transparent polycrystalline alumina ceramics have been studied for decades as a material for optical applications and a cheaper alternative to sapphire single crystals [1]. Besides the economic and ecological benefits of such replacement, their applications could be extended due to better mechanical properties of polycrystalline materials and possibility of production complex shaped parts. Moreover, doping with certain elements could lead to new interesting functional properties, thus expanding the application potential of transparent aluminas even further (e.g. dosimeters, phosphors for high-brightness LEDs or laser host materials). However, adding the required amount of dopant (mostly with very low solubility in alumina lattice) inevitably leads to formation of second phase inclusions, which impair transparency, and make the preparation of transparent alumina with luminescence properties a challenging topic. There are only few works dealing with transparent or translucent alumina doped with optically (photoluminescence - PL) active rare earth (RE) or transition metal (TM) ions. Crdoped transparent alumina exhibiting thermoluminescence (TL) and optically stimulated luminescence was designed as a potential TL dosimetry material [2]. Doping of alumina with Ti and Mg was proposed for the same purpose [3]. However, the mean grain size of prepared material is well within the micrometer size range as the result of pressure-less sintering at relatively high temperature, with adverse impact on transparency. Penilla et al. [4] prepared by spark plasma sintering (SPS) Tb3+ doped transparent alumina with fine-grained microstructure with 75 % of total transmission at a wavelength of 800 nm and with characteristic Tb3+ emission. According to the authors, this was the first instance when a birefringent (non-cubic) polycrystalline oxide bulk ceramic possessed both high transparency and induced light emission properties. The authors also suggested that only high heating and cooling rates accessible by SPS technique and operating out of thermodynamic balance could result in achieving high concentration of dopant while maintaining the transparency. Alvarez-Clemares et al. [5] showed that doping of alumina with up to 0.7 wt. % of ceria hinders the growth of alumina grains during SPS and enables the preparation of transparent alumina with total transmission of up to 70 % in the IR-VIS range. Although ceria-doped alumina exhibits photoluminescent properties [5], no measurement of the photoluminescence was performed in this work. Sanamyan et al. [6] reported on preparation of Er - doped alumina powders (0.1 - 0.3 at. % of Er) subsequently sintered by SPS to transparent ceramics with submicron microstructure. All samples prepared under optimized conditions exhibited sharp, well-resolved emission lines characteristic for RE3+ ions, indicating predominantly crystalline single-site Er3+ occupation. Obtained IR spectra were similar to those previously observed in Er3+-doped Al2O3 powders prepared by sol–gel technique [7]. However, the effect of Er concentration on the PL intensity was not discussed, and no data of optical transmission were provided. Wang et al. [8] studied the influence of the phase structure and the amount of dopant on PL intensity of alumina powders. The strongest emission intensity was detected at the lowest examined concentrations (0.5 and 1 mol % Er). Dramatic concentration quenching effect took place above 1.5 mol % Er3+ doping concentration. At 1 mol. % Er3+ doping, the PL intensity of the Er3+-doped Al2O3 powders increased with the phase structure changed in the sequence γ → θ → α-(Al, Er)2O3. These findings indicate that relatively low amount of Er with controllable impact on the transparency are sufficient for obtaining adequate PL intensity of doped transparent Er3+-doped alumina (α-(Al, Er)2O3). The present study follows and extends our recent work dealing with rare-earth doped transparent alumina [9], with the aim to enhance important material properties significantly (real in-line transmission, PL intensity) by modifying of the processing parameters. More precisely, it is aimed at alternative way of producing Er3+-doped transparent and photoluminescent alumina, combining colloidal processing (slip casting) for green body preparation, advanced pre-sintering in a conventional electric furnace and subsequent hot isostatic pressing. Our approach differs from all cited works concerned on preparation of transparent alumina with PL properties where spark plasma sintering was applied to achieve full densification. Transparent alumina ceramics prepared in this study are characterized in terms of their microstructure (mean grain size, presence and size of secondary phase(s) inclusions), real in-line transmittance and photoluminescence properties. Contrary to all mentioned studies dealing with transparent PL alumina, the present work examines also the effect of dopant concentration and pre-treatment of doping powder on all studied characteristics. 2. EXPERIMENTAL High purity 99.99 % commercial alumina powder (Taimicron TM-DAR, Taimei Chemicals Co., Ltd., Tokyo, Japan), with the primary particle size of 150 nm and specific surface area of 13.7 m2 g−1 was used as a starting material (the values were determined by the producer from SEM micrographs and by BET analysis, respectively). Aqueous suspensions with the solid content of 45 vol. % of Al2O3 were stabilized electrosterically with the use of a commercial dispersant Darvan CN. De-ionized water with room temperature electric conductivity 25 ± 5.10−4 S m−1 was used as a dispersion medium. The optically active dopant (Er) in the amount equivalent to 0.1 - 0.17 at. % of Er with respect to Al2O3 was added in the form of oxide powders. Two different Er2O3 powders were used as starting materials. First, a commercial coarse-grained Er2O3 powder (Treibacher Industrie AG, Austria) was used. In order to obtain a nano-powder, the commercial powder was dissolved in 65 % HNO3. Citric acid dissolved in deionized water and ethylene glycol was added to the nitrate solution and heated in oil bath for 2 h at 85–90 °C. The solvent was evaporated under continuous stirring. The product was dried and calcined at 800 °C for 1h to remove organic residua. The powder was then milled in planetary mill (vessel and grinding balls from alumina) in order to reduce the number of aggregates. Residual aggregates were removed from diluted water suspension by centrifugal sedimentation at 800 rpm for 2 min. Only the particles, which remained in the suspension were dried and further used as dopants. This type of doping powder was further throughout the text denoted as P_S powder. Doped samples are denoted as P_S_x, The abbreviations are explained in Table 1. The nano-Er2O3 powder (purity 99.9 %, particle size 20-30 nm, GNM – Getnanomaterials, USA) was used as received. Part of the nano-powder was also centrifugated, under the conditions described above, to remove aggregates. These types of doping powder were further throughout the text denoted as C and C_S powder, respectively. The samples doped with untreated commercial nano-powder were denoted as C_x and the samples doped with centrifuged commercial nano-powder were denoted as C_S_x. The suspensions with added Er2O3 were homogenized on rollers for 24 h. 40 g of alumina milling balls with diameter 4 mm were added to 10 ml of the suspension. After homogenization, the suspensions were poured into PVC dishes and allowed to dry at ambient conditions for three days. Green bodies were then dried at 80°C for 5 h. The samples were pre-sintered to 95–96 % of the theoretical density, which means only closed porosity was present [10, 11]. The samples doped with the P_S powder were pre- sintered using two different regimes: single step pre-sintering (SSP) – and two-step presintering (TSP) in order to establish more appropriate pre-sintering regime. Due to superior results achieved by the TSP regime only TSP was used in later experiments. SSP was performed by heating the samples at the rate of 20 °Cmin−1 up to the maximal temperature 1480 °C without isothermal dwell, with subsequent cooling at the same rate. When the two-step pre-sintering (TSP) was applied, the specimens were first heated to a temperature T1 (1430-1440 °C) without isothermal dwell whereby achieving the relative density of 88 - 90 %, and subsequently cooled down to a temperature T2 (1280 °C) at the rate of 20 °Cmin−1. The isothermal dwell at the temperature T2 was 10 h. The samples pre-sintered to closed porosity were hot isostatically pressed (HIP ABRA Shirp, Switzerland) at the temperature of 1280 °C for 3 h and the pressure of 200 MPa with argon used as a pressure medium. Molybdenum heating and shielding enables sintering without carbon contamination, which is usual in furnaces with graphite heating elements. The density of sintered specimens was determined according to the Archimedes’ principle. The theoretical density for all samples was calculated from the theoretical density of Al2O3 and Er2O3 by the rule of mixtures. The microstructures were examined by scanning electron microscopy on polished and thermally etched cross sections (Lyra 3, Tescan, Czech Republic). The mean grain size was determined using the linear intercept method with a correction factor of 1.56. Minimum of 200 grains were measured in order to obtain statistically robust set of data. Distribution of Er in sintered alumina was determined using a high resolution (scanning) Transmission Electron Microscope FEI Titan Themis 60-300 (FEI, Czech Republic) equipped by Quantax EDS (Bruker Nano, Germany). The real in-line transmission (RIT) of polished samples was measured with a non-polarized He–Ne laser (λ = 632.8 nm). The distance from the sample to the detector was 860 mm, with an opening angle of 0.5°. The real in-line transmission was measured in at least five different positions on each sample and was recalculated to constant thickness of 0.8 mm. The absorption spectra in the visible and NIR spectral range were recorded in the reflection mode using the Cary 5000 spectrometer equipped with the integrating sphere (module DRA2500). The excitation and emission fluorescence spectra were measured using Fluorolog FL321 spectrometer (Horiba Jobin Yvon) with Xe-lamp (450 W) as an excitation source. For luminescence detection in visible and NIR spectral range, the PMT (R928) and InGaAs detector cooled with liquid nitrogen were used, respectively. The luminescence spectra of studied samples were recorded at room temperature. The luminescence decay curves were recorded at RT with the same instrument using time correlated single photon counting technique (TCSPC) with the pulsed laser diode at excitation wavelength 375 nm. 3. RESULTS Pre-sintering regime Green bodies of Er3+-doped alumina were prepared by mixing fine alumina powder with two types of Er2O3 nano-powders, in some cases further pre-treated by centrifugation in order to decrease the amount of aggregates present in starting Er2O3 powder. The designation of individual samples is related to the preparation process and is explained in Table 1. Green bodies were pre-sintered using two different regimes: single step pre-sintering (SSP) – and two-step pre-sintering (TSP) in order to establish more appropriate pre-sintering regime. For full densification hot isostatic pressing (HIP) was performed. Polycrystalline alumina exhibits strong dependence of optical transmittance on grain size due to its birefrigent nature [12]. Therefore, the thorough optimization of all processing parameters is needed, with special emphasis on sintering conditions. Erbium oxide was observed to retard significantly the densification of alumina: the effect was more pronounced at higher dopant concentrations. Fig. 1 shows the comparison of sintered densities of Er3+- doped samples with samples doped with negligible amount of Zr prepared in the same way in dependence on sintering temperature. The differences resulting from various amounts of the dopant were amplified at lower relative densities (RD); above 96 % of the theoretical density (TD) the sintering behavior was almost identical. This allows the use of similar sintering regimes irrespective of the amount of the dopant. Porosity in polycrystalline alumina ceramics is usually considered to be closed if the relative density exceeds 95 % of the theoretical density [10, 11]. In order to minimize the grain size the samples pre-sintered to relative densities ranging between 95.8 and 97.8 % of TD were selected for HIP experiments. The used pre-sintering regimes and achieved relative densities are summarized in Table 2. Based on the results from previous work [10], the temperature 1280 °C and a holding time of 3 h was selected as the most suitable conditions for hot isostatic pressing (allowing full densification without observable grain growth). All samples sintered to full density (> 99.9 % of TD); no differences could be recorded using the applied measuring method. The values of the mean grain size (MGS) of sintered Er3+-doped ceramics after HIP are summarized in the Table 3. The mean grain size of two-step pre-sintered samples seems to be slightly lower in comparison to single-step pre-sintered ones, but any difference lies within the range of the measurement error. Microstructure of all HIP-ed samples is fine-grained and homogeneous without any second phase inclusions. Representative microstructure is shown in Fig. 2 and observable also in Fig. 3. However, some imperfections of used doping process resulted in rarely occurring aggregates of erbium oxide (Fig. 3a). This type of aggregates was found in samples doped with the P_S powder pre-sintered using single-step regime (the size of 10 – 20 µm taking into account the largest dimension). The aggregates in TSP samples were several fold smaller (3 – fact that the emission intensity decreases with increasing concentration, point to the fact that local environment around the Er3+ ions is relatively disordered. The Er3+ ions aggregates are also possibly formed during the sample preparation that lead to the concentration quenching of luminescence due the direct interaction among Er3+ ions in the Al2O3 matrix. Lifetime The lifetime corresponding to 4S3/2 level (4S3/2 → 4I15/2 transition) exhibits strong dependence on the Er3+ concentration in Al2O3 host matrix (see Fig. 8, Table 5). All decay curves show non-mono-exponential profiles, even for the lowest concentrating sample containing 0.1 at. % of Er3+ ions (2.38.1019 cm-3), and were well fitted with double exponential decay function. It is well known that at very low concentrations of optically active ions, with negligible interactions between them, the decay of the luminescence can be described by a single exponential [20]. However, at higher concentrations the distances between active ions are shorter and the energy transfer processes become more efficient, resulting in a nonexponential decay [21]. All decay profiles thus exhibit a fast exponential decay in the initial part with a decay time of about 500 ns, followed by a slower decay with a decay time of  2 μs. Similar double-exponential behavior of 4S3/2 level decay was observed and reported by Agazzi [18] for Er3+-doped Al2O3 films with dopant concentration in the range of 2.10194.1020 cm-3, with the lifetimes of 12.6 μs for faster and 30.8 μs for shorter decay process, respectively. The observed non-exponential nature of the green luminescence decays, accompanied by shortening of the lifetimes with increasing Er3+ concentration is related to energy transfer processes (cross-relaxation processes) between Er3+ ions at higher concentrations and/or to different quenching traps (impurities and/or structural defective sites) [22]. During this process, an excited Er3+ ion (donor) is deactivated by transferring a part of its energy to another neighboring Er3+ ion (acceptor). The energy transfer is possible if there is a certain overlap in the fluorescence emission and absorption spectra of the donor and acceptor, which is the case of concentrated Er3+ systems; this means that there are matching differences in acceptor and donor energy levels. Some of the possible cross-relaxation channels that depopulate thermalized 4S3/2 level can be described as (denoted as (initial sate Er3+(I), Er3+(II)final state Er3+(I), Er3+(II)): (4S3/2, 4I15/2  4I13/2, 4I9/2), (4S3/2, 4I15/2  4I9/2, 4I13/2), (4I9/2, 4I9/2  4I11/2, 4F9/2), (4I9/2, 4I11/2  4I13/2, 4F9/2) and other [19]. For example, in the case of (4S3/2, 4I15/2  4I9/2, 4I13/2) cross-relaxation channel, the donor is deactivated without emitting green fluorescence light and as a result the intensity and lifetime are diminished. However, as mentioned above, the structural defects may also play important role in the luminescence quenching. 5. CONCLUSION Transparent photoluminescent aluminas doped with 0.1 – 0.17 at. % Er were successfully prepared by a combination of wet shaping technique (slip casting), pressure-less pre-sintering, and hot isostatic pressing (HIP). Fully dense samples with grain size less than 400 nm exhibited the RIT values ranging between 28-56 %. The samples doped with Er2O3 nanopowder prepared by Pechini method exhibited slightly lower transparency (up to 52 %) due to presence of Er agglomerates in the microstructure. The RIT value of 56 % established for sample doped with commercial Er2O3 nano-powder is the highest real in-line transparency so far reported in the literature for luminescent RE doped aluminas. The positive influence of suspension centrifugation prior to slip casting on RIT and photoluminescence intensity was demonstrated. Upon excitation of the Er3+ ions in the UV range, intensive green, weak red and infrared emissions were observed, whose intensities and lifetime were described in dependence on the processing route. Based on the obtained results, future work to further improve the optical characteristics of this type of material will be focused on optimization of dopant concentration below 0.1 at% Er. ACKNOWLEDGEMENT This work is part of the project 5SA14857, which has acquired the financial contribution from the EU Framework Programme for Research and Innovation Horizon 2020 within the scope of the Marie Sklodowska-Curie Actions co-financed by the South Moravian Region according to the Grant Agreement no. 665860. The financial support of this work by the grant VEGA 1/0631/14 and GAČR 15-06390S is also gratefully acknowledged. This research has also been financially supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC 2020 (LQ1601). Part of the work was carried out with the support of core facilities of CEITEC open access project, ID number LM2011020, funded by the Ministry of Education, Youth and Sports of the Czech Republic under the activity „Projects of major infrastructures for research, development and innovations”. REFERENCES [1] W.H. Rhodes, D.J. Sellers, T. Vasilos, Hot-Working of Aluminum-Oxide .2. OpticalProperties, J Am Ceram Soc 58(1-2) (1975) 31-34. [2] Q. Liu, Q.H. Yang, G.G. Zhao, S.Z. Lu, H.J. Zhang, The thermoluminescence and optically stimulated luminescence properties of Cr-doped alpha alumina transparent ceramics, J Alloy Compd 579 (2013) 259-262. [3] Q. Liu, Q.H. Yang, G.G. Zhao, S.Z. Lu, Titanium effect on the thermoluminescence and optically stimulated luminescence of Ti,Mg:alpha-Al2O3 transparent ceramics, J Alloy Compd 582 (2014) 754-758. [4] E.H. Penilla, Y. Kodera, J.E. Garay, Blue-Green Emission in Terbium-Doped Alumina (Tb:Al2O3) Transparent Ceramics, Adv Funct Mater 23(48) (2013) 6036-6043. [5] I. Alvarez-Clemares, G. Mata-Osoro, A. Fernandez, S. Lopez-Esteban, C. Pecharroman, R. Torrecillas, J.S. Moya, Ceria doped alumina by Spark Plasma Sintering for optical applications, J Eur Ceram Soc 32(11) (2012) 2917-2924. [6] T. Sanamyan, R. Pavlacka, G. Gilde, M. Dubinskii, Spectroscopic properties of Er3+- doped alpha-Al2O3, Opt Mater 35(5) (2013) 821-826. [7] A.A. Kaplyanskii, A.B. Kulinkin, A.B. Kutsenko, S.P. Feofilov, R.I. Zakharchenya, T.N. Vasilevskaya, Optical spectra of triply-charged rare-earth ions in polycrystalline corundum, Phys Solid State+ 40(8) (1998) 1310-1316. [8] X.J. Wang, M.K. Lei, T. Yang, H. Wang, Phase structure and photoluminescence Al2O3 powders prepared by the properties of Er3+-doped sol-gel method, Opt Mater 26(3) (2004) 247-252. [9] K. Bodišová, R. Klement, D. Galusek, V. Pouchlý, D. Drdlík, K. Maca, Luminescent rareearth-doped transparent alumina ceramics, J Eur Ceram Soc 36(12) (2016) 2975-2980. [10] M. Trunec, K. Maca, R. Chmelik, Polycrystalline alumina ceramics doped with nanoparticles for increased transparency, J Eur Ceram Soc 35(3) (2015) 1001-1009. [11] T. Spusta, J. Svoboda, K. Maca, Study of pore closure during pressure-less sintering of advanced oxide ceramics, Acta Mater 115 (2016) 347-353. [12] R. Apetz, M.P.B. van Bruggen, Transparent alumina: A light-scattering model, J Am Ceram Soc 86(3) (2003) 480-486. [13] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Energy level structure and transitions probabilities of the trivalent lanthanides in LaF3 Argonne National Laboratory, Special Report1977. [14] M.J. Weber, Probabilities for Radiative and Nonradiative Decay of Er3+ in Laf3, Phys Rev 157(2) (1967) 262-&. [15] C.M. Dodson, R. Zia, Magnetic dipole and electric quadrupole transitions in the trivalent lanthanide series: Calculated emission rates and oscillator strengths, Phys Rev B 86(12) (2012). [16] P. Dorenbos, The 4f(n)<-> 4f(n-1)5d transitions of the trivalent lanthanides in halogenides and chalcogenides, J Lumin 91(1-2) (2000) 91-106. [17] G. Liu, B. Jacquier, Spectroscopic Properties of Rare Earths in Optical Materials, Springer Berlin Heidelberg2006. [18] L. Agazzi, Spectroscopic Excitation and Quenching Processes in Rare-Earth-Ion-Doped Al2O3 and their Impact on Amplifier and Laser Performance, University of Twente, The Netherlands, 2012. [19] V. Venkatramu, S.F. Leon-Luis, U.R. Rodriguez-Mendoza, V. Monteseguro, F.J. Manjon, A.D. Lozano-Gorrin, R. Valiente, D. Navarro-Urrios, C.K. Jayasankar, A. Munoz, V. Lavin, Synthesis, structure and luminescence of Er3+-doped Y3Ga5O12 nano-garnets, J Mater Chem 22(27) (2012) 13788-13799. [20] P. Babu, H.J. Seo, C.R. Kesavulu, Y.H. Jang, C.K. Jayasankar, Thermal and optical properties of Er3+-doped oxyfluorotellurite glasses, J Lumin 129(5) (2009) 444-448. [21] B. Dibartolo, Energy Transfer Processes in Condensed Matter, Springer US1984. [22] V. Venkatramu, M. Giarola, G. Mariotto, S. Enzo, S. Polizzi, C.K. Jayasankar, F. Piccinelli, M. Bettinelli, A. Speghini, Nanocrystalline lanthanide-doped Lu3Ga5O12 garnets: interesting materials for light-emitting devices, Nanotechnology 21(17) (2010). Figure captions Fig. 1. Dependence of relative density of Er3+-doped alumina on the sintering temperature (pressure-less sintering). For comparison, the densification data of alumina doped with 250 ppm Zr are also present. Fig. 2. The representative microstructure of HIPed alumina doped by Er. Fig. 3. The Er-enriched aggregates and grains found in the HIPed samples doped using: a) P_S powder pre-sintered by single step pre-sintering b) P_S powder pre-sintered by two-step pre-sintering c) C_S powder pre-sintered by two-step pre-sintering Fig. 4. TEM image of Er3+-doped alumina (0.1 at. % of Er3+) microstructure and EDS mapping for Al, O and Er elements on the grain boundary. Fig. 5. Transparent Er3+-doped alumina. The samples are ca 3 cm in diameter and they are positioned 1 cm above the text. Fig. 6. Diffuse reflectance spectra of the Al2O3:Er3+ ceramics in the UV-VIS-NIR range recorded at RT (a). The expanded segment of the spectra with corresponding absorption bands assignment (b). All transitions start from 4I15/2 ground state to the indicated levels. Fig. 7. Luminescence spectra of the Al2O3:Er3+ doped ceramics: (a) The representative excitation spectrum monitored at 1529 nm with corresponding band assignment; Emission spectra of the Er3+ doped Al2O3 ceramics monitored in VIS (b) and NIR (c) spectral region after sample excitation at 379 nm. Fig. 8. Er3+ luminescence decay curves of the green emission (2H11/2, 4S3/2 4I15/2) recorded at RT under laser diode excitation at 375nm: (a) The decay curves were fitted with 2exponential decay function. (b) The dependence of ave on Er3+ concentration. Fig. 9. The real in-line transmittance – Er content dependence. Table captions Table 1. Abbreviation in designation of samples Table 2. Pre-sintering regimes for individual samples and achieved relative densities Table 3. Mean grain size of rare earth doped alumina after HIP (1280°C / 3 h, 200 MPa) Table 4. Real in-line transmittance of rare earth doped alumina after HIP (1280°C / 3 h, 200 MPa) Table 5. Lifetimes of the 4S3/2 level at RT obtained under excitation at 375 nm Table 1 Abbreviation in designation of samples Meaning in relation to the preparation of samples P Nano-powder prepared by modified Pechini method C Commercial nano-powder S Sedimentation used for removal of the aggregates x at. % of Er with respect to Al2O3 Table 2 Sample SSP – regime a) SSP – RD [%] TSP – regime a) TSP – RD [%] P_S_0.1 1480 °C 97.2 1430 °C → 1280 °C / 10 h 96.9 P_S_0.17 1480 °C 96.4 1440 °C → 1280 °C / 10 h 95.8 C_S_0.1 - - 1440 °C → 1280 °C / 10 h 97.4 C_S_0.125 - - 1440 °C → 1280 °C / 10 h 96.9 C_S_0.15 - - 1440 °C → 1280 °C / 10 h 96.2 C_ 0.11 - - 1440 °C → 1280 °C / 10 h 97.8 a) SSP – without dwell; TSP – without dwell in the first step Fig. 4 Fig. 5 Fig. 6a Fig. 6b Fig. 7a Fig. 7b Fig. 7c Fig. 8a Fig. 8b Fig. 9