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Aluminate glass based phosphors for LED applications
Journal of the European Ceramic Society 36 (2016) 2969–2973
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Aluminate glass based phosphors for LED applications K. Haladejová a , A. Prnová a , R. Klement a , W.-H. Tuan b , S.-J. Shih c , D. Galusek a,∗ a
Vitrum Laugaricio, Joint Glass Center of the IIC SAS, TnU AD, and FChFT STU, Trenˇcín, Slovak Republic Department of Materials Science & Engineering, National Taiwan University, Taipei, Taiwan c Department of Materials Science & Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan b
a r t i c l e
i n f o
Article history: Received 7 August 2015 Received in revised form 19 November 2015 Accepted 24 November 2015 Available online 11 December 2015 Keywords: Aluminate glass Luminescence Crystallization Emission
a b s t r a c t Aluminate glasses are transparent in IR, UV and vis, and represent an ideal host matrix for optically active dopants. Due to their lower phonon energies in comparison to silicate glasses, non-radiative transitions are suppressed and high efficiency of luminescence is expected. We present a summary of the most important results of a study of luminescence properties of aluminate glasses prepared in the form of microspheres by flame synthesis in binary and ternary systems Al2 O3 –RE2 O3 –(SiO2 ), RE = Y, Yb, doped with optically active elements, (Er, Ce). The influence composition and crystallization of the host glass on luminescence was studied. The glasses were crystallized under controlled conditions, and the influence of phase composition (glass-to-crystalline phase ratio, fraction of various crystalline phases) on wavelength and intensity of luminescence was studied. Case studies of various systems revealed the luminescence intensities can be efficiently tuned by controlled crystallization of the host glass. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Aluminate glasses are interesting candidates as host matrices for optically active rare earth dopants, with potential applications as phosphors for HB-LED lighting. They possess a number of advantages in comparison both to common silicate glasses and polycrystalline aluminates. First, they are able to accommodate higher concentrations of rare-earth dopants in comparison to their single- or polycrystalline counterparts of identical composition, such as yttrium- or ytterbium aluminium garnets, or the respective rare earth aluminate perovskites. The glasses also exhibit good mechanical properties, especially hardness, and high chemical and thermal resistance [1,2]. Most importantly, however, lower phonon energies are characteristic for the aluminate glasses in comparison to common silicate glasses that reduce the nonradiation losses due to multiphonon relaxation and non-radiative transitions, with resulting higher emission intensities [3]. They are also transparent in an ultraviolet range of spectra [4], which is of particular importance for LED phosphors excited by UV light. Recently, up-conversion (UC) phosphors, which can up-convert the near-infrared (NIR) light to visible light have attracted significant attention for their potential application in display monitors, optical data storage, medical diagnostics, solid-state visible lighting,
∗ Corresponding author. E-mail address: [email protected] (D. Galusek). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.027 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
and solar cells [5]. Particular interest is paid to the host materials with low phonon energies that favor up-conversion luminescence. Aluminate glasses can be considered as good candidates for UC phosphors due to their characteristic properties such as wide transparency in the NIR region (up to 5 m) [6], low phonon energy (∼800 cm−1 ) [7], high refractive indices (1.7–1.8 or higher) [8], high hardness and good corrosion and thermal resistance. The main disadvantage of aluminate glasses is that they are usually relatively difficult to prepare, especially in bulk. The Al2 O3 as the main component of these glasses is not a typical glass former. Preparation of the aluminate glasses thus requires intense source of heat due to their high melting temperatures. Specific precautions, such as high cooling rates of the melt, and prevention of heterogeneous nucleation by the use of containerless melting techniques are also required during their preparation due to their high tendency to crystallization. Weber et al. describe the preparation of aluminate glasses by costly and time consuming containerless melting techniques with the use of AAL (aero-acoustic levitator) or CNL (conical nozzle levitator) where a drop of melt is kept floating by a stream of inert gas while heated by high power laser source [9]. McMillan et al. prepared CaO–Al2 O3 glasses containing 50 mol% Al2 O3 via splat quenching technique [10]. Rosenflanz et al. describes a simple method for preparation of glass microspheres with high alumina content from polycrystalline precursor powder by flame synthesis [11]. Bulk glasses were then prepared by hot pressing of the glass microspheres in the temperature interval between the glass transition temperature and the onset of crystallization tempera-
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ture. In our previous works we reported on successful application of flame synthesis for preparation of aluminate glass microspheres of various compositions in the pseudo-binary and ternary systems Al2 O3 –Y2 O3 [12,13], Al2 O3 –Y2 O3 –SiO2 [14,15], Al2 O3 –La2 O3 [16], Al2 O3 –Yb2 O3 [17], and Al2 O3 –CaO–SiO2 [18], in some instances doped with rare earth optically active additives. In the present work we summarize the results of some selected case studies aimed at the investigation of the influence of chemical composition of the host glass, content of the activator ion, and of the controlled crystallization of these glasses on their luminescence properties.
Table 1 Denominations and compositions of studied alumina glasses. Sample
AYS5Er1 AYS15Er1 AYS15Er5 AYS15Yb2Er1 AYS15Yb3Er1 AYS15Yb5Er1 AYS15Yb7Er1 AYCe0.5
Composition/mol% Al2 O3
Y2 O3
SiO2
Yb2 O3
Er2 O3
Ce2 O3
72.5 65.1 63.1 65.1 65.1 65.1 65.1 76.6
21.5 18.9 16.9 16.9 15.9 13.9 11.9 23.08
5 15 15 15 15 15 15 –
– – – 2 3 5 7 –
1 1 5 1 1 1 1 –
– – – – – – – 0.25
2. Experimental All studied glasses have been prepared by flame synthesis from polycrystalline precursor powders of corresponding compositions by flame synthesis. The precursor powders were synthesized with the use of a modified Pechini synthesis [19]. Required metal nitrates (i.e., aluminium nitrate and the respective rare earth nitrates, all of the p.a. quality) were dissolved in deionized water and mixed. Then, an aqueous solution of citric acid and ethylene glycol (in the molar ratio 1:1) was added to the solution of nitrates. The resulting solution was refluxed at 85 ◦ C for 2 h and then heated to 150 ◦ C to promote polymerization (solution viscosity increased rapidly) and solvent evaporation until an aerated resin was formed. Finally, the organic compounds were removed by heating to 800 ◦ C for 6 h. The synthesized precursor powders were then crushed, sieved through a 40 m polyethylene mesh screen and fed into methane–oxygen flame [12]. In the flame the powder particles melted. The molten droplets were sprayed with distilled water ensuring the estimated cooling rate of 1000 ◦ C s−1 . Spherical glassy particles formed, which were then collected in a sedimentation tank, separated, dried and calcined at 650 ◦ C. The morphology of the microspheres was examined by optical microscopy (Nikon ECLIPSE ME 600) in transmitted light, and by SEM analysis (JEOL 7600f) at the 20 kV accelerating voltage. The crystallization properties of prepared glasses were studied by simultaneous thermal analysis (STA), and high temperature X-Ray diffraction (HT-XRD). The HTXRD experiments were carried out in ambient atmosphere in the temperature range between the RT and 1200 ◦ C, at the heating rate of 5 ◦ C min−1 with the use of an X-ray diffractometer Panalytical Empyrean equipped with Cu X-ray lamp (CuK␣ radiation with 0.15405 nm wavelength), and a high temperature cell Anton Paar HTK 16. A diffraction pattern in a selected 2 range between 20 and 40◦ was recorded every 10 ◦ C: the use of a position-sensitive PIXCELL® detector facilitated real-time recording of temperature development of the phase composition (time required for acquiring of diffraction pattern at any respective temperature <60 s). The HT-XRD measurements were complemented by DTA measurements (NETZSCH STA 449 F1 Jupiter) of the microspheres in the temperature range 35–1200 ◦ C performed at the rate of 10 ◦ C/min, in the atmosphere of flowing nitrogen. The photoluminescence spectra was recorded by Fluorolog FL3-21 spectrometer (Horiba Jobin Yvon) using Xe (450 W) arc lamp as an excitation source. All presented photoluminescence spectra have been corrected for the instrument response. For up-conversion spectra measurement the laser diode (980 nm) was used as an excitation source. 3. Results and discussion The compositions and denominations of all studied glasses are listed in Table 1. All compositions were derived from the eutectic composition in the pseudobinary phase system Al2 O3 –Y3 Al5 O12 , i.e., 60 wt% (or 76.8 mol%) of Al2 O3 and 40 wt% (or 23.2 mol%) of Y2 O3 with the temperature of melting 1640 ◦ C. With the exception of Yb2 O3 doping, where ytterbia was added on account of yttria, the
Al2 O3 /Y2 O3 molar ratio was held identical to the alumina-to-yttria ratio of the eutectic, and both alumina and yttria were replaced by silica and/or erbia equimolarly. Silica was also added in some instances to increase the glass forming ability of aluminate glasses. Ytterbia was added as a sensitizer, while erbia and ceria were used as optical activators. X-ray powder diffraction examination (the results are not shown) confirmed amorphous character of all prepared compositions. No crystalline phases were detected. The prepared glasses were used for evaluation of their luminescence properties. The influence of phase composition of glasses crystallized under controlled conditions on their luminescent properties was also examined.
3.1. Activator content and glass composition Both the host matrix and the content of activator ion play an important role in luminescence properties of a material. The following results demonstrate how the optical properties of aluminate glasses doped with Er3+ activator are influenced by the chemical composition of the host matrix, and the content of the activator ion [15]. The influence was not expected to be high: in the rare earth activators the emission in visible wavelength range is based on f → f transitions. The f orbitals are below the valence orbitals: any influence of the coordination sphere is therefore expected to be small. When monitored at 1530 nm the excitation spectra (not shown here) exhibit 10 bands with barycenters at 358, 366, 378, 407, 443, 452, 488, 522, 543 and 656 nm corresponding to the excited levels 2G 4 4 2 4 4 4 2 4 4 7/2 , G9/2 , G11/2 , G9/2 , F3/2 , F5/2 , F7/2 , H11/2 , S3/2 , and F9/2 . The strongest band at 378 nm corresponds to the 4 I15/2 → 4 G11/2 transition. The excitation spectra thus provide different possibilities of excitation of green and IR luminescence in the studied Er3+ doped glasses. Fig. 1 shows the photoluminescence (emission and excitation) spectra of the Er3+ -doped glasses with various contents of silica, which was added as a glass former, enhancing the glass forming ability and thermal stability of the glass. The emission spectra (excited at 378 nm by a Xe lamp) exhibit two green emissions: one weaker green emission centered at 525 nm (2 H11/2 → 4 I15/2 ) and one strong emission centred at 548 nm (4 S3/2 → 4 I15/2 ). Only a weak red emission corresponding to 4 F9/2 → 4 I15/2 transition was observed at 645 nm. The intensities of the emission bands decreased with the content of the activator ion. The effect was attributed to concentration quenching due to Er3+ clustering in the aluminate host matrix with high activator ion content. The intensities of the emission bands decreased also with increasing SiO2 content (Fig. 2): the emission intensities of YAS15Er1 glass are about 1/3 lower than emission intensities of the YAS5Er1. The effect was attributed to higher losses through non-radiative transitions in the glass matrix with silicate host sub-lattice [3].
K. Haladejová et al. / Journal of the European Ceramic Society 36 (2016) 2969–2973
Fig. 1. The excitation (recorded at the em = 548 nm) and emission (recorded at the excitation wavelength exc = 378 nm) spectra of the AYS15 glass doped with 1 and 5 mol% of Er2 O3 .
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Fig. 3. The intensity of emission at the wavelengths 525, 545, and 655 nm of Al2 O3 –Y2 O3 –Yb2 O3 –SiO2 glasses doped with 1 mol% Er2 O3 as a function of replacement Y3+ /Yb3+ and measured at the excitation wavelength 980 nm (up-conversion).
Fig. 2. The emission spectra recorded at the excitation wavelength exc = 378 nm of two yttrium aluminosilicate glasses with different SiO2 content (5 and 15 mol%).
3.2. Aluminate glasses with up-conversion luminescence The up-conversion luminescent aluminate glasses were derived from the composition of the basic yttrium aluminosilicate glass, containing 65.6 mol% Al2 O3 , 19.5 mol% Y2 O3 and 15.0 mol% SiO2 . The doping level of Er3+ was 2 mol% (1 mol% of Er2 O3 ). The effect of Yb3+ concentration (Y3+ substitution by Yb3+ ions in the Yb3+ range between 2 and 14 mol%) on up-conversion emission intensity was studied (glasses AYS15Yb2Er1 to AYS15Yb7Er1 in Table 1). The glasses exhibit typical absorption bands in their UV–vis-NIR absorption spectra due to Er3+ ions in the spectral range between 300 and 1600 nm. The additional spectral band at 980 nm was attributed to the 2 F7/2 → 2 F5/2 electron transition of the Yb3+ ions: the intensity of that band increased with increasing concentration of Yb3+ ions. Photoluminescence spectra was recorded at the excitation wavelengths of 378 and 975 nm. Excitation at 378 nm initiated strong emission in visible region, with maxima of the emission bands at 524 nm and 547 nm, and an additional strong band in NIR region, centered at 1532 nm. The emission was attributed to the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 I13/2 → 4 I15/2 transitions of the Er3+ ions, respectively. The intensity of the emission bands decreased with increasing concentration of Yb3+ ions. The excitation in the NIR region at 975 nm significantly increased the intensity of emission bands corresponding to 4 I13/2 → 4 I15/2 transition: the
Fig. 4. Schematic description of the up-conversion mechanism proposed for the Al2 O3 –Y2 O3 –Yb2 O3 –SiO2 glasses.
increase varied with the Yb3+ ions concentrations, with the maximum at 6 mol% Yb3+ (glass AYS15Yb3Er1). Up-conversion spectra of the glasses excited by the IR laser at 980 nm exhibited two intense emission bands at 525, and 545 nm (green emission) and one lower intensity band at 655 nm (red emission). The intensity of UC emission was strongly Yb3+ dependent (Fig. 3). The proposed mechanism of up-conversion is shown in Fig. 4. 3.3. Influence of crystallization To study the influence of crystallization on luminescence properties of aluminate glasses doped with two different activators, Er3+ and Ce3+ , the thermal properties of the glasses AYS15Er1, and AYCe0.5 were examined by DSC. The DSC records of the AYCe0.5 show two crystallization effects, both attributed to crystallization of YAG. The characteristic temperatures are listed in Table 2. Addition of silica slightly decreased the glass transition temperature, and increased the onset of crystallization temperature in com-
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Table 2 Characteristic temperatures of studied glasses as determined by the DSC: glass transition temperature (Tg ), onset of crystallization temperatures (Tx1 , Tx2 ), and the temperature of maxima of the exothermic crystallization peaks (Tp1 , Tp2 ). Sample Temperature/◦ C
AYS15Er1
AYCe0.5
Tg Tx1 Tp1 Tx2 Tp2
882 935 960 1000 1018
893 923 936 989 1000
Fig. 6. The emission spectra of the AYS15 glass doped with 1 mol% of Er2 O3 , and crystallized at various temperatures.
Fig. 5. Relative integral intensities of diffraction maxima of the two phases (YAG, cristobalite) identified in the course of crystallization of the AYS15 glass doped with 1 mol% of Er2 O3 by high temperature X-ray diffraction.
parison to silica-free glass: the result indicated increased thermal stability of the AYS15Er1 glass. Two exothermic effects with the maxima at 960 and 1018 ◦ C were attributed to crystallization of the YAG phase, and possibly cristobalite. Based on the results of the DSC analysis time-temperature regimes for heat treatment of the glasses were selected in order to prepare glasses with various contents of crystalline phase(s) embedded in residual yttrium aluminate or yttrium aluminosilicate glass (3–5 h annealing in the temperature range between 880 and 1200 ◦ C), and fully crystallized material (annealing 1500 ◦ C for 3 h). As confirmed by the X-ray powder diffraction, the major phase in the AYCe 0.5 glass crystallized in the temperature region up to 1200 ◦ C was YAG. The second phase, ␣-Al2 O3 , was reliably detected only in the sample treated at 1500 ◦ C for 5 h. The high temperature X-ray diffraction study of the phase development in the AYS15Er1 glass revealed transitive formation of cristobalite in the temperature interval between 1000 and 1150 ◦ C, with the maximum phase content at 1100 ◦ C (Fig. 5). The luminescence spectra of the AYS15Er1 glass crystallized under various conditions is shown in Fig. 6. The emission spectra (excited at 378 nm by a Xe lamp) exhibit two green emissions: one weaker green emission centered at 525 nm (2 H11/2 → 4 I15/2 ) and one strong emission centred at 548 nm (4 S3/2 → 4 I15/2 ). Only a weak red emission corresponding to 4 F9/2 → 4 I15/2 transition was observed at 645 nm. Annealing at 944 ◦ C resulted in slight increase of intensity of green luminescence. The reason for such behaviour is unclear: the luminescence might be affected by partial ordering of the glass structure, and more homogeneous distribution of the activator ions in the glass matrix. Increase of the annealing temperature resulted in gradual decrease of the green emission and splitting of the green emission band centred at 545 nm to two bands with maxima at 541 and 555 nm. The effect was attributed to a combination of several effects. The band splitting was most likely the result of the
Fig. 7. The emission spectra of the binary Al2 O3 –Y2 O3 glass doped with 0.5 mol% Ce, and crystallized at various temperatures.
presence of Er3+ ions with various coordination: those remaining in the residual glass, and those bound in the form of crystalline aluminates, either built in the structure of crystalline YAG structure or as a Er3+ -containing phase, such as Er4 Al2 O9 . The latter might also contribute to decrease of emission intensity: clustering of Er3+ ions in the crystalline erbium aluminate phase might result in concentration quenching. Other factors contributing to decrease of the emission intensity include light scattering due to the formation of new phases, such as YAG and cristobalite, embedded in residual glass with different refraction index, and formation of residual glass with increased silica content, which results in higher losses by non-radiative transitions. The luminescence spectra of the glass AYCe0.5 and of the heattreated samples after controlled crystallization are shown in Fig. 7. The excitation spectra (not shown) exhibit two broad absorption bands centered at 343 and 456 nm, respectively, that belong to the Ce3+ 4f-5d configuration. The emission spectra consist of a peak centered at 535 nm and a shoulder at longer wavelength side ∼575 nm, which can be attributed to electronic transitions of the Ce3+ . The former is assigned to the 5d1 → 2 F5/2 transition and the latter to the 5d1 → 2 F7/2 transition, respectively. The emission intensity increases with the temperature of heat-treatment, reaching the maximum for the sample treated at 1200 ◦ C. The effect was attributed to the higher packing densities of Ce3+ ions in the crys-
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tallites than in glass; crystallinity of the samples increased with increasing treatment temperature, as documented by X-ray diffraction. However, the PL intensity of the sample treated at 1500 ◦ C decreased significantly in comparison with the sample treated at 1200 ◦ C, despite the presence of the second crystalline phase, ␣Al2 O3 , that could also accommodate Ce3+ ions. The decrease was attributed to oxidation Ce3+ → Ce4+ , with possible contribution of light scattering due to the presence of several phases with different refraction indices. 4. Conclusions The potential of aluminate glasses as host matrices for optically active dopants has been demonstrated. The studied yttrium aluminate and aluminosilicate glasses doped with rare earth activators, such as Er, and Ce, exhibit strong luminescence in visible wavelength range excited by near ultraviolet radiation, or in some cases when a combination of dopants is used, also by infrared (up-conversion). The addition of silica improves the glass forming ability of binary RE2 O3 –Al2 O3 glasses, but impairs luminescence. The influence of controlled crystallization of the host glass on luminescent properties was also studied. The emission wavelength is not influenced by partial crystallization, and the resulting change of phase composition of the host matrix. Emission intensities at various wavelengths are markedly influenced by crystallization of host glass as the result of several possible mechanisms, including: (1) light scattering due to formation of crystalline phases with different refraction indices in residual glass; (2) concentration quenching due to accumulation of activator ions in residual glass or in crystalline phases; (3) in aluminosilicate glasses higher losses by non-radiative transitions in residual glass with increased silica content. Er3+ band splitting is also observed in crystallized glasses, most likely due to the presence of Er3+ ions with various coordinations in glass and in crystalline phases. Acknowledgments The financial support of this work to the project SAS-NSC JRP 2012/14, and by the grants VEGA 1/0631/14 and VEGA 2/0058/14 is gratefully acknowledged. This publication was created in the frame of the project “Centre of excellence for ceramics, glass, and silicate materials” ITMS code 262 20120056, based on the Operational Program Research and Development funded from the ERDF.
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References [1] E. Garskaite, D. Jasaitis, A. Kareiva, Sol–gel preparation and electrical behaviour of Ln:YAG (Ln = Ce, Nd, Ho, Er), J. Serb. Chem. Soc. 68 (2003) 677–684. [2] H. Gong, D.Y. Tang, H. Huang, M.D. Han, T. Sun, J. Zhang, et al., Crystallization kinetics and characterization of nanosized Nd:YAG by a modified sol–gel combustion process, J. Cryst. Growth 362 (2013) 52–57. [3] A.D. Sontakke, K. Annapurna, Study on Tb3+ containing high silica and low silica calcium aluminate glasses: impact of optical basicity, Spectrochim. Acta A 94 (2012) 180–185. [4] L.G. Hwa, G.W. Lee, Influence of water on the physical properties of calcium aluminate oxide glasses, Mater. Chem. Phys. 58 (1999) 191–194. [5] A. Shalav, Rare-earth doped up-converting phosphors for an enhanced silicon solar cell response, in: PhD Thesis, University of New South Wales, Australia, 2006. [6] P.L. Higby, R.J. Ginther, I.D. Aggarwal, E.J. Friebele, Glass formation and thermal properties of low-silica calcium aluminosilicate glasses, J. Non-Cryst. Solids 126 (1990) 209–215. [7] S.L. Oliveira, S.M. Lima, T. Catunda, L.A.O. Nunes, J.H. Rohling, A.C. Bento, et al., High fluorescence quantum efficiency of 1.8 m emission in Tm-doped low silica calcium aluminate glass determined by thermal lens spectrometry, Appl. Phys. Lett. 84 (2004) 359–361. [8] M.V. Kumar, T. Ishikawa, B. Basavalingu, J.T. Okada, Y. Watanabe, Thermal and optical properties of glass and crystalline phases formed in the binary R2 O3 –Al2 O3 (R = La-Lu and Y) system, J. Appl. Phys. 113 (2013) 193503. [9] J.K.R. Weber, J.H. Abadie, A.D. Hixson, P.C. Nordine, G.A. Jerman, T.E. Mitchell, Glass formation and polyamorphism in rare-earth oxide–aluminum oxide compositions, J. Am. Ceram. Soc. 83 (2000) 1868–1872. [10] P.F. McMillan, B. Piriou, Raman spectroscopy of calcium aluminate glasses and crystals, J. Non-Cryst. Solids 195 (1996) 261–271. [11] A. Rosenflanz, M. Frey, B. Endres, T. Anderson, E. Richards, C. Schardt, Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature 430 (2004) 761–764. [12] A. Prnová, R. Karell, D. Galusek, The preparation of binary Al2 O3 –Y2 O3 glass microspheres by flame synthesis from powder oxide precursors, Ceram. Silik. 52 (2008) 109–114. [13] A. Prnová, R. Karell, D. Galusek, Y2 O3 –Al2 O3 binary glass microspheres: synthesis and characterisation, Adv. Mater. Res. 39–40 (2008) 189–192. [14] A. Prnová, A. Domanická, R. Klement, J. Kraxner, M. Polovka, M. Pentrák, et al., Er- and Nd-doped yttrium aluminosilicate glasses: preparation and characterization, Optical Mater. 33 (2011) 1872–1878. [15] A. Domanická, R. Klement, A. Prnová, K. Bodiˇsová, D. Galusek, Luminescent rare-earth ions doped Al2 O3 –Y2 O3 –SiO2 glass microspheres prepared by flame synthesis, Ceram. Int. 40 (2014) 6005–6012. [16] A. Haliaková, A. Prnová, R. Klement, D. Galusek, W.H. Tuan, Flame-spraying synthesis of aluminate glasses in the Al2 O3 –La2 O3 system, Ceram. Int. 38 (2012) 5543–5549. ˇ [17] A. Prnová, K. Bodiˇsová, R. Klement, M. Migát, P. Veteˇska, M. Skrátek, et al., Preparation and characterization of Yb2 O3 –Al2 O3 glasses by the Pechini sol–gel method combined with flame synthesis, Ceram. Int. 40 (2014) 6179–6184. [18] E. Bernardo, L. Fiocco, A. Prnová, R. Klement, D. Galusek, Gehlenite: Eu3+ phosphors from a silicone resin and nano-sized fillers, Optical Mater. 36 (2014) 1243–1249. [19] M.P., Pechini, 1967. US Pat. No. 3330697.
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Aluminate glass based phosphors for LED applications K. Haladejová a , A. Prnová a , R. Klement a , W.-H. Tuan b , S.-J. Shih c , D. Galusek a,∗ a
Vitrum Laugaricio, Joint Glass Center of the IIC SAS, TnU AD, and FChFT STU, Trenˇcín, Slovak Republic Department of Materials Science & Engineering, National Taiwan University, Taipei, Taiwan c Department of Materials Science & Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan b
a r t i c l e
i n f o
Article history: Received 7 August 2015 Received in revised form 19 November 2015 Accepted 24 November 2015 Available online 11 December 2015 Keywords: Aluminate glass Luminescence Crystallization Emission
a b s t r a c t Aluminate glasses are transparent in IR, UV and vis, and represent an ideal host matrix for optically active dopants. Due to their lower phonon energies in comparison to silicate glasses, non-radiative transitions are suppressed and high efficiency of luminescence is expected. We present a summary of the most important results of a study of luminescence properties of aluminate glasses prepared in the form of microspheres by flame synthesis in binary and ternary systems Al2 O3 –RE2 O3 –(SiO2 ), RE = Y, Yb, doped with optically active elements, (Er, Ce). The influence composition and crystallization of the host glass on luminescence was studied. The glasses were crystallized under controlled conditions, and the influence of phase composition (glass-to-crystalline phase ratio, fraction of various crystalline phases) on wavelength and intensity of luminescence was studied. Case studies of various systems revealed the luminescence intensities can be efficiently tuned by controlled crystallization of the host glass. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Aluminate glasses are interesting candidates as host matrices for optically active rare earth dopants, with potential applications as phosphors for HB-LED lighting. They possess a number of advantages in comparison both to common silicate glasses and polycrystalline aluminates. First, they are able to accommodate higher concentrations of rare-earth dopants in comparison to their single- or polycrystalline counterparts of identical composition, such as yttrium- or ytterbium aluminium garnets, or the respective rare earth aluminate perovskites. The glasses also exhibit good mechanical properties, especially hardness, and high chemical and thermal resistance [1,2]. Most importantly, however, lower phonon energies are characteristic for the aluminate glasses in comparison to common silicate glasses that reduce the nonradiation losses due to multiphonon relaxation and non-radiative transitions, with resulting higher emission intensities [3]. They are also transparent in an ultraviolet range of spectra [4], which is of particular importance for LED phosphors excited by UV light. Recently, up-conversion (UC) phosphors, which can up-convert the near-infrared (NIR) light to visible light have attracted significant attention for their potential application in display monitors, optical data storage, medical diagnostics, solid-state visible lighting,
∗ Corresponding author. E-mail address: [email protected] (D. Galusek). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.027 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
and solar cells [5]. Particular interest is paid to the host materials with low phonon energies that favor up-conversion luminescence. Aluminate glasses can be considered as good candidates for UC phosphors due to their characteristic properties such as wide transparency in the NIR region (up to 5 m) [6], low phonon energy (∼800 cm−1 ) [7], high refractive indices (1.7–1.8 or higher) [8], high hardness and good corrosion and thermal resistance. The main disadvantage of aluminate glasses is that they are usually relatively difficult to prepare, especially in bulk. The Al2 O3 as the main component of these glasses is not a typical glass former. Preparation of the aluminate glasses thus requires intense source of heat due to their high melting temperatures. Specific precautions, such as high cooling rates of the melt, and prevention of heterogeneous nucleation by the use of containerless melting techniques are also required during their preparation due to their high tendency to crystallization. Weber et al. describe the preparation of aluminate glasses by costly and time consuming containerless melting techniques with the use of AAL (aero-acoustic levitator) or CNL (conical nozzle levitator) where a drop of melt is kept floating by a stream of inert gas while heated by high power laser source [9]. McMillan et al. prepared CaO–Al2 O3 glasses containing 50 mol% Al2 O3 via splat quenching technique [10]. Rosenflanz et al. describes a simple method for preparation of glass microspheres with high alumina content from polycrystalline precursor powder by flame synthesis [11]. Bulk glasses were then prepared by hot pressing of the glass microspheres in the temperature interval between the glass transition temperature and the onset of crystallization tempera-
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ture. In our previous works we reported on successful application of flame synthesis for preparation of aluminate glass microspheres of various compositions in the pseudo-binary and ternary systems Al2 O3 –Y2 O3 [12,13], Al2 O3 –Y2 O3 –SiO2 [14,15], Al2 O3 –La2 O3 [16], Al2 O3 –Yb2 O3 [17], and Al2 O3 –CaO–SiO2 [18], in some instances doped with rare earth optically active additives. In the present work we summarize the results of some selected case studies aimed at the investigation of the influence of chemical composition of the host glass, content of the activator ion, and of the controlled crystallization of these glasses on their luminescence properties.
Table 1 Denominations and compositions of studied alumina glasses. Sample
AYS5Er1 AYS15Er1 AYS15Er5 AYS15Yb2Er1 AYS15Yb3Er1 AYS15Yb5Er1 AYS15Yb7Er1 AYCe0.5
Composition/mol% Al2 O3
Y2 O3
SiO2
Yb2 O3
Er2 O3
Ce2 O3
72.5 65.1 63.1 65.1 65.1 65.1 65.1 76.6
21.5 18.9 16.9 16.9 15.9 13.9 11.9 23.08
5 15 15 15 15 15 15 –
– – – 2 3 5 7 –
1 1 5 1 1 1 1 –
– – – – – – – 0.25
2. Experimental All studied glasses have been prepared by flame synthesis from polycrystalline precursor powders of corresponding compositions by flame synthesis. The precursor powders were synthesized with the use of a modified Pechini synthesis [19]. Required metal nitrates (i.e., aluminium nitrate and the respective rare earth nitrates, all of the p.a. quality) were dissolved in deionized water and mixed. Then, an aqueous solution of citric acid and ethylene glycol (in the molar ratio 1:1) was added to the solution of nitrates. The resulting solution was refluxed at 85 ◦ C for 2 h and then heated to 150 ◦ C to promote polymerization (solution viscosity increased rapidly) and solvent evaporation until an aerated resin was formed. Finally, the organic compounds were removed by heating to 800 ◦ C for 6 h. The synthesized precursor powders were then crushed, sieved through a 40 m polyethylene mesh screen and fed into methane–oxygen flame [12]. In the flame the powder particles melted. The molten droplets were sprayed with distilled water ensuring the estimated cooling rate of 1000 ◦ C s−1 . Spherical glassy particles formed, which were then collected in a sedimentation tank, separated, dried and calcined at 650 ◦ C. The morphology of the microspheres was examined by optical microscopy (Nikon ECLIPSE ME 600) in transmitted light, and by SEM analysis (JEOL 7600f) at the 20 kV accelerating voltage. The crystallization properties of prepared glasses were studied by simultaneous thermal analysis (STA), and high temperature X-Ray diffraction (HT-XRD). The HTXRD experiments were carried out in ambient atmosphere in the temperature range between the RT and 1200 ◦ C, at the heating rate of 5 ◦ C min−1 with the use of an X-ray diffractometer Panalytical Empyrean equipped with Cu X-ray lamp (CuK␣ radiation with 0.15405 nm wavelength), and a high temperature cell Anton Paar HTK 16. A diffraction pattern in a selected 2 range between 20 and 40◦ was recorded every 10 ◦ C: the use of a position-sensitive PIXCELL® detector facilitated real-time recording of temperature development of the phase composition (time required for acquiring of diffraction pattern at any respective temperature <60 s). The HT-XRD measurements were complemented by DTA measurements (NETZSCH STA 449 F1 Jupiter) of the microspheres in the temperature range 35–1200 ◦ C performed at the rate of 10 ◦ C/min, in the atmosphere of flowing nitrogen. The photoluminescence spectra was recorded by Fluorolog FL3-21 spectrometer (Horiba Jobin Yvon) using Xe (450 W) arc lamp as an excitation source. All presented photoluminescence spectra have been corrected for the instrument response. For up-conversion spectra measurement the laser diode (980 nm) was used as an excitation source. 3. Results and discussion The compositions and denominations of all studied glasses are listed in Table 1. All compositions were derived from the eutectic composition in the pseudobinary phase system Al2 O3 –Y3 Al5 O12 , i.e., 60 wt% (or 76.8 mol%) of Al2 O3 and 40 wt% (or 23.2 mol%) of Y2 O3 with the temperature of melting 1640 ◦ C. With the exception of Yb2 O3 doping, where ytterbia was added on account of yttria, the
Al2 O3 /Y2 O3 molar ratio was held identical to the alumina-to-yttria ratio of the eutectic, and both alumina and yttria were replaced by silica and/or erbia equimolarly. Silica was also added in some instances to increase the glass forming ability of aluminate glasses. Ytterbia was added as a sensitizer, while erbia and ceria were used as optical activators. X-ray powder diffraction examination (the results are not shown) confirmed amorphous character of all prepared compositions. No crystalline phases were detected. The prepared glasses were used for evaluation of their luminescence properties. The influence of phase composition of glasses crystallized under controlled conditions on their luminescent properties was also examined.
3.1. Activator content and glass composition Both the host matrix and the content of activator ion play an important role in luminescence properties of a material. The following results demonstrate how the optical properties of aluminate glasses doped with Er3+ activator are influenced by the chemical composition of the host matrix, and the content of the activator ion [15]. The influence was not expected to be high: in the rare earth activators the emission in visible wavelength range is based on f → f transitions. The f orbitals are below the valence orbitals: any influence of the coordination sphere is therefore expected to be small. When monitored at 1530 nm the excitation spectra (not shown here) exhibit 10 bands with barycenters at 358, 366, 378, 407, 443, 452, 488, 522, 543 and 656 nm corresponding to the excited levels 2G 4 4 2 4 4 4 2 4 4 7/2 , G9/2 , G11/2 , G9/2 , F3/2 , F5/2 , F7/2 , H11/2 , S3/2 , and F9/2 . The strongest band at 378 nm corresponds to the 4 I15/2 → 4 G11/2 transition. The excitation spectra thus provide different possibilities of excitation of green and IR luminescence in the studied Er3+ doped glasses. Fig. 1 shows the photoluminescence (emission and excitation) spectra of the Er3+ -doped glasses with various contents of silica, which was added as a glass former, enhancing the glass forming ability and thermal stability of the glass. The emission spectra (excited at 378 nm by a Xe lamp) exhibit two green emissions: one weaker green emission centered at 525 nm (2 H11/2 → 4 I15/2 ) and one strong emission centred at 548 nm (4 S3/2 → 4 I15/2 ). Only a weak red emission corresponding to 4 F9/2 → 4 I15/2 transition was observed at 645 nm. The intensities of the emission bands decreased with the content of the activator ion. The effect was attributed to concentration quenching due to Er3+ clustering in the aluminate host matrix with high activator ion content. The intensities of the emission bands decreased also with increasing SiO2 content (Fig. 2): the emission intensities of YAS15Er1 glass are about 1/3 lower than emission intensities of the YAS5Er1. The effect was attributed to higher losses through non-radiative transitions in the glass matrix with silicate host sub-lattice [3].
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Fig. 1. The excitation (recorded at the em = 548 nm) and emission (recorded at the excitation wavelength exc = 378 nm) spectra of the AYS15 glass doped with 1 and 5 mol% of Er2 O3 .
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Fig. 3. The intensity of emission at the wavelengths 525, 545, and 655 nm of Al2 O3 –Y2 O3 –Yb2 O3 –SiO2 glasses doped with 1 mol% Er2 O3 as a function of replacement Y3+ /Yb3+ and measured at the excitation wavelength 980 nm (up-conversion).
Fig. 2. The emission spectra recorded at the excitation wavelength exc = 378 nm of two yttrium aluminosilicate glasses with different SiO2 content (5 and 15 mol%).
3.2. Aluminate glasses with up-conversion luminescence The up-conversion luminescent aluminate glasses were derived from the composition of the basic yttrium aluminosilicate glass, containing 65.6 mol% Al2 O3 , 19.5 mol% Y2 O3 and 15.0 mol% SiO2 . The doping level of Er3+ was 2 mol% (1 mol% of Er2 O3 ). The effect of Yb3+ concentration (Y3+ substitution by Yb3+ ions in the Yb3+ range between 2 and 14 mol%) on up-conversion emission intensity was studied (glasses AYS15Yb2Er1 to AYS15Yb7Er1 in Table 1). The glasses exhibit typical absorption bands in their UV–vis-NIR absorption spectra due to Er3+ ions in the spectral range between 300 and 1600 nm. The additional spectral band at 980 nm was attributed to the 2 F7/2 → 2 F5/2 electron transition of the Yb3+ ions: the intensity of that band increased with increasing concentration of Yb3+ ions. Photoluminescence spectra was recorded at the excitation wavelengths of 378 and 975 nm. Excitation at 378 nm initiated strong emission in visible region, with maxima of the emission bands at 524 nm and 547 nm, and an additional strong band in NIR region, centered at 1532 nm. The emission was attributed to the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 I13/2 → 4 I15/2 transitions of the Er3+ ions, respectively. The intensity of the emission bands decreased with increasing concentration of Yb3+ ions. The excitation in the NIR region at 975 nm significantly increased the intensity of emission bands corresponding to 4 I13/2 → 4 I15/2 transition: the
Fig. 4. Schematic description of the up-conversion mechanism proposed for the Al2 O3 –Y2 O3 –Yb2 O3 –SiO2 glasses.
increase varied with the Yb3+ ions concentrations, with the maximum at 6 mol% Yb3+ (glass AYS15Yb3Er1). Up-conversion spectra of the glasses excited by the IR laser at 980 nm exhibited two intense emission bands at 525, and 545 nm (green emission) and one lower intensity band at 655 nm (red emission). The intensity of UC emission was strongly Yb3+ dependent (Fig. 3). The proposed mechanism of up-conversion is shown in Fig. 4. 3.3. Influence of crystallization To study the influence of crystallization on luminescence properties of aluminate glasses doped with two different activators, Er3+ and Ce3+ , the thermal properties of the glasses AYS15Er1, and AYCe0.5 were examined by DSC. The DSC records of the AYCe0.5 show two crystallization effects, both attributed to crystallization of YAG. The characteristic temperatures are listed in Table 2. Addition of silica slightly decreased the glass transition temperature, and increased the onset of crystallization temperature in com-
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Table 2 Characteristic temperatures of studied glasses as determined by the DSC: glass transition temperature (Tg ), onset of crystallization temperatures (Tx1 , Tx2 ), and the temperature of maxima of the exothermic crystallization peaks (Tp1 , Tp2 ). Sample Temperature/◦ C
AYS15Er1
AYCe0.5
Tg Tx1 Tp1 Tx2 Tp2
882 935 960 1000 1018
893 923 936 989 1000
Fig. 6. The emission spectra of the AYS15 glass doped with 1 mol% of Er2 O3 , and crystallized at various temperatures.
Fig. 5. Relative integral intensities of diffraction maxima of the two phases (YAG, cristobalite) identified in the course of crystallization of the AYS15 glass doped with 1 mol% of Er2 O3 by high temperature X-ray diffraction.
parison to silica-free glass: the result indicated increased thermal stability of the AYS15Er1 glass. Two exothermic effects with the maxima at 960 and 1018 ◦ C were attributed to crystallization of the YAG phase, and possibly cristobalite. Based on the results of the DSC analysis time-temperature regimes for heat treatment of the glasses were selected in order to prepare glasses with various contents of crystalline phase(s) embedded in residual yttrium aluminate or yttrium aluminosilicate glass (3–5 h annealing in the temperature range between 880 and 1200 ◦ C), and fully crystallized material (annealing 1500 ◦ C for 3 h). As confirmed by the X-ray powder diffraction, the major phase in the AYCe 0.5 glass crystallized in the temperature region up to 1200 ◦ C was YAG. The second phase, ␣-Al2 O3 , was reliably detected only in the sample treated at 1500 ◦ C for 5 h. The high temperature X-ray diffraction study of the phase development in the AYS15Er1 glass revealed transitive formation of cristobalite in the temperature interval between 1000 and 1150 ◦ C, with the maximum phase content at 1100 ◦ C (Fig. 5). The luminescence spectra of the AYS15Er1 glass crystallized under various conditions is shown in Fig. 6. The emission spectra (excited at 378 nm by a Xe lamp) exhibit two green emissions: one weaker green emission centered at 525 nm (2 H11/2 → 4 I15/2 ) and one strong emission centred at 548 nm (4 S3/2 → 4 I15/2 ). Only a weak red emission corresponding to 4 F9/2 → 4 I15/2 transition was observed at 645 nm. Annealing at 944 ◦ C resulted in slight increase of intensity of green luminescence. The reason for such behaviour is unclear: the luminescence might be affected by partial ordering of the glass structure, and more homogeneous distribution of the activator ions in the glass matrix. Increase of the annealing temperature resulted in gradual decrease of the green emission and splitting of the green emission band centred at 545 nm to two bands with maxima at 541 and 555 nm. The effect was attributed to a combination of several effects. The band splitting was most likely the result of the
Fig. 7. The emission spectra of the binary Al2 O3 –Y2 O3 glass doped with 0.5 mol% Ce, and crystallized at various temperatures.
presence of Er3+ ions with various coordination: those remaining in the residual glass, and those bound in the form of crystalline aluminates, either built in the structure of crystalline YAG structure or as a Er3+ -containing phase, such as Er4 Al2 O9 . The latter might also contribute to decrease of emission intensity: clustering of Er3+ ions in the crystalline erbium aluminate phase might result in concentration quenching. Other factors contributing to decrease of the emission intensity include light scattering due to the formation of new phases, such as YAG and cristobalite, embedded in residual glass with different refraction index, and formation of residual glass with increased silica content, which results in higher losses by non-radiative transitions. The luminescence spectra of the glass AYCe0.5 and of the heattreated samples after controlled crystallization are shown in Fig. 7. The excitation spectra (not shown) exhibit two broad absorption bands centered at 343 and 456 nm, respectively, that belong to the Ce3+ 4f-5d configuration. The emission spectra consist of a peak centered at 535 nm and a shoulder at longer wavelength side ∼575 nm, which can be attributed to electronic transitions of the Ce3+ . The former is assigned to the 5d1 → 2 F5/2 transition and the latter to the 5d1 → 2 F7/2 transition, respectively. The emission intensity increases with the temperature of heat-treatment, reaching the maximum for the sample treated at 1200 ◦ C. The effect was attributed to the higher packing densities of Ce3+ ions in the crys-
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tallites than in glass; crystallinity of the samples increased with increasing treatment temperature, as documented by X-ray diffraction. However, the PL intensity of the sample treated at 1500 ◦ C decreased significantly in comparison with the sample treated at 1200 ◦ C, despite the presence of the second crystalline phase, ␣Al2 O3 , that could also accommodate Ce3+ ions. The decrease was attributed to oxidation Ce3+ → Ce4+ , with possible contribution of light scattering due to the presence of several phases with different refraction indices. 4. Conclusions The potential of aluminate glasses as host matrices for optically active dopants has been demonstrated. The studied yttrium aluminate and aluminosilicate glasses doped with rare earth activators, such as Er, and Ce, exhibit strong luminescence in visible wavelength range excited by near ultraviolet radiation, or in some cases when a combination of dopants is used, also by infrared (up-conversion). The addition of silica improves the glass forming ability of binary RE2 O3 –Al2 O3 glasses, but impairs luminescence. The influence of controlled crystallization of the host glass on luminescent properties was also studied. The emission wavelength is not influenced by partial crystallization, and the resulting change of phase composition of the host matrix. Emission intensities at various wavelengths are markedly influenced by crystallization of host glass as the result of several possible mechanisms, including: (1) light scattering due to formation of crystalline phases with different refraction indices in residual glass; (2) concentration quenching due to accumulation of activator ions in residual glass or in crystalline phases; (3) in aluminosilicate glasses higher losses by non-radiative transitions in residual glass with increased silica content. Er3+ band splitting is also observed in crystallized glasses, most likely due to the presence of Er3+ ions with various coordinations in glass and in crystalline phases. Acknowledgments The financial support of this work to the project SAS-NSC JRP 2012/14, and by the grants VEGA 1/0631/14 and VEGA 2/0058/14 is gratefully acknowledged. This publication was created in the frame of the project “Centre of excellence for ceramics, glass, and silicate materials” ITMS code 262 20120056, based on the Operational Program Research and Development funded from the ERDF.
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References [1] E. Garskaite, D. Jasaitis, A. Kareiva, Sol–gel preparation and electrical behaviour of Ln:YAG (Ln = Ce, Nd, Ho, Er), J. Serb. Chem. Soc. 68 (2003) 677–684. [2] H. Gong, D.Y. Tang, H. Huang, M.D. Han, T. Sun, J. Zhang, et al., Crystallization kinetics and characterization of nanosized Nd:YAG by a modified sol–gel combustion process, J. Cryst. Growth 362 (2013) 52–57. [3] A.D. Sontakke, K. Annapurna, Study on Tb3+ containing high silica and low silica calcium aluminate glasses: impact of optical basicity, Spectrochim. Acta A 94 (2012) 180–185. [4] L.G. Hwa, G.W. Lee, Influence of water on the physical properties of calcium aluminate oxide glasses, Mater. Chem. Phys. 58 (1999) 191–194. [5] A. Shalav, Rare-earth doped up-converting phosphors for an enhanced silicon solar cell response, in: PhD Thesis, University of New South Wales, Australia, 2006. [6] P.L. Higby, R.J. Ginther, I.D. Aggarwal, E.J. Friebele, Glass formation and thermal properties of low-silica calcium aluminosilicate glasses, J. Non-Cryst. Solids 126 (1990) 209–215. [7] S.L. Oliveira, S.M. Lima, T. Catunda, L.A.O. Nunes, J.H. Rohling, A.C. Bento, et al., High fluorescence quantum efficiency of 1.8 m emission in Tm-doped low silica calcium aluminate glass determined by thermal lens spectrometry, Appl. Phys. Lett. 84 (2004) 359–361. [8] M.V. Kumar, T. Ishikawa, B. Basavalingu, J.T. Okada, Y. Watanabe, Thermal and optical properties of glass and crystalline phases formed in the binary R2 O3 –Al2 O3 (R = La-Lu and Y) system, J. Appl. Phys. 113 (2013) 193503. [9] J.K.R. Weber, J.H. Abadie, A.D. Hixson, P.C. Nordine, G.A. Jerman, T.E. Mitchell, Glass formation and polyamorphism in rare-earth oxide–aluminum oxide compositions, J. Am. Ceram. Soc. 83 (2000) 1868–1872. [10] P.F. McMillan, B. Piriou, Raman spectroscopy of calcium aluminate glasses and crystals, J. Non-Cryst. Solids 195 (1996) 261–271. [11] A. Rosenflanz, M. Frey, B. Endres, T. Anderson, E. Richards, C. Schardt, Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature 430 (2004) 761–764. [12] A. Prnová, R. Karell, D. Galusek, The preparation of binary Al2 O3 –Y2 O3 glass microspheres by flame synthesis from powder oxide precursors, Ceram. Silik. 52 (2008) 109–114. [13] A. Prnová, R. Karell, D. Galusek, Y2 O3 –Al2 O3 binary glass microspheres: synthesis and characterisation, Adv. Mater. Res. 39–40 (2008) 189–192. [14] A. Prnová, A. Domanická, R. Klement, J. Kraxner, M. Polovka, M. Pentrák, et al., Er- and Nd-doped yttrium aluminosilicate glasses: preparation and characterization, Optical Mater. 33 (2011) 1872–1878. [15] A. Domanická, R. Klement, A. Prnová, K. Bodiˇsová, D. Galusek, Luminescent rare-earth ions doped Al2 O3 –Y2 O3 –SiO2 glass microspheres prepared by flame synthesis, Ceram. Int. 40 (2014) 6005–6012. [16] A. Haliaková, A. Prnová, R. Klement, D. Galusek, W.H. Tuan, Flame-spraying synthesis of aluminate glasses in the Al2 O3 –La2 O3 system, Ceram. Int. 38 (2012) 5543–5549. ˇ [17] A. Prnová, K. Bodiˇsová, R. Klement, M. Migát, P. Veteˇska, M. Skrátek, et al., Preparation and characterization of Yb2 O3 –Al2 O3 glasses by the Pechini sol–gel method combined with flame synthesis, Ceram. Int. 40 (2014) 6179–6184. [18] E. Bernardo, L. Fiocco, A. Prnová, R. Klement, D. Galusek, Gehlenite: Eu3+ phosphors from a silicone resin and nano-sized fillers, Optical Mater. 36 (2014) 1243–1249. [19] M.P., Pechini, 1967. US Pat. No. 3330697.