- Email: [email protected]
Hot pressing of Yb:Sc2O3 laser ceramics with LiF sintering aid
Optical Materials 100 (2020) 109701
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Hot pressing of Yb:Sc2O3 laser ceramics with LiF sintering aid D.A. Permin a, S.S. Balabanov a, I.L. Snetkov b, O.V. Palashov b, A.V. Novikova a, *, O.N. Klyusik a, I.V. Ladenkov c a
G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the RAS, Nizhny Novgorod, Russia Institute of Applied Physics of the RAS, Nizhny Novgorod, Russia c JSC “RPE “Salut”, Nizhny Novgorod, Russia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser ceramics Hot pressing Scandium oxide Self-propagaring high-temperature synthesis
The fabrication of transparent Sc2O3 ceramics by hot pressing of self-propagating high-temperature synthesized powders was developed. The effect of lithium fluoride aid and calcination temperature on the powders properties was investigated. After calcination at 1100 � C, uniformly shaped Sc2O3 nanopowders with a primary particle size of less than 50 nm were obtained. Using the prepared powders doped with LiF fully dense optical quality Yb: Sc2O3 transparent ceramics were fabricated by hot pressing. The in-line transmittance of the sample reached 78% at a wavelength of 800 nm. Laser generation with a maximum efficiency of 24% was achieved using a 2% Yb: Sc2O3 sample.
1. Introduction Recently, ytterbium doped rare earth oxides such as Y2O3, Lu2O3 and Sc2O3 have attracted great interest as potential candidates for creating high power solid state laser media [1–3]. Among these materials Sc2O3 probably holds the greatest promise due to the highest thermal con ductivity (for undoped material), low phonon energy and large splitting of the ground state of ytterbium dopant ions [4]. There are several ref erences with regard to the successful preparation of laser elements based on scandium oxide single crystals doped with rare earth ions [2,3,5]. However, the obvious technological difficulties in growing Sc2O3 crys tals, associated with a high melting point (~2430 � C), make this approach less attractive compared to ceramic technology. The main approach to the preparation of transparent scandium oxide ceramics is the free vacuum sintering of nanodispersed powders ob tained by the solution precipitation method [6–9]. The low driving force of the free sintering makes it necessary to use long soaking times at high temperatures (>1800 � C) to achieve a high level of light transmission (sufficient for laser generation) [11]. This leads to grain growth and the contamination of the ceramics by the equipment materials. Additives such as yttrium oxide [10] could be applied to enhance the sinterability of scandia ceramics, but the dilution of Sc2O3 by rare earths results in a significant drop in the thermal conductivity of the sample, worsening its performance. The required level of optical properties can be also
achieved by combining free sintering and hot isostatic pressing [12]. Another effective approach to obtain highly transparent material is a hot uniaxial pressing of highly dispersed Sc2O3 powders. In this case lithium fluoride is used as a sintering additive in an amount of up to 1 wt %. But the number of publications with regard to this method is limited [13,14]. The patent [14] states that high-quality Sc2O3 ceramics can be achieved by using uniform nano-sized powders having no hard ag glomerates, uniform doping with a LiF sintering additive and the se lection of barothermal processing conditions. However, the characteristics of the Sc2O3 ceramics so obtained are not given. It was previously shown that the nanodispersed scandium oxide powders could be produced by self-propagating high-temperature syn thesis (SHS) method [15,16]. In addition, this method allows one to uniformly introduce sintering additives, in particular, lithium fluoride [16]. In the available literature we did not find any references regarding the effect of a LiF additive on the properties of the Sc2O3 powders or on the sintering process and the microstructural properties of Yb:Sc2O3 ceramics. The aim of this work was to investigate the effect of a lithium fluoride additive on the properties of scandium oxide SHS-powders, the process of a consolidation by hot pressing, and the study of the properties (microstructural and optical) of the Yb:Sc2O3 ceramics.
* Corresponding author. G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, 49 Tropinina Str., Nizhny Novgorod, 603950, Russia. E-mail address: [email protected] (A.V. Novikova). https://doi.org/10.1016/j.optmat.2020.109701 Received 14 November 2019; Received in revised form 10 January 2020; Accepted 14 January 2020 Available online 25 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
D.A. Permin et al.
Optical Materials 100 (2020) 109701
2. Experimental
ceramics was recorded using a TII S150-2 (SOLAR, Belarus) spectrom eter at an excitation wavelength of 940 nm using a laser diode source. To ascertain the possibility of the generation of laser radiation, dielectric coatings (antireflective on one side and mirror on the opposite one) were applied to the end surfaces of the ceramic sample for two wavelengths (pumping λp ¼ 940 nm and laser generation λl ~ 1040 nm). The pre pared sample was mounted on a copper heat sink and placed in a laser cavity formed by the back surface of the sample, a totally reflecting spherical mirror with a focus of 120 cm and an output mirror. The pump radiation was focused by spherical mirror on the studied sample into a spot of 2 mm in diameter and after one V-shaped pass through the sample was deflected to an absorber. The optical scheme of this exper iment is described in detail in Ref. [17]. A fibre-coupled laser diode, Laserline LDM 2000, emitting at a wavelength of 940 nm, was used as a pump source. The laser power was measured using Ophir 3A and Ophir 30A-BB-18 bolometric power meters.
The nanopowders of ytterbium doped scandium oxide were obtained with the use of metal nitrates as an oxidant, and glycine as a fuel. The starting materials for the synthesis of SHS-precursors were commercially available low dispersed scandium Sc2O3 and ytterbium Yb2O3 oxides (99.99% Lanhit, Russia), nitric acid HNO3 (99.9999%, Khimreaktiv, Russia), glycine NH2CH2COOH (99.9%, Khimreaktiv, Russia) and lithium fluoride LiF (“pure”, Khimreaktiv, Russia). The water solutions of metal nitrates were obtained by dissolving ~10 g of the scandium oxide (or a mixture of scandium and yttrbium oxides) in a stoichiometric amount of nitric acid upon heating. Glycine was added to the solution in a molar ratio of 1:1 in respect to the metal nitrates. To introduce a sintering additive, a solution of LiF in dilute nitric acid with a concentration of ~0.01 g/ml had been previously prepared as described in Ref. [16]. The resulting solution was added to a mixture of scandium nitrate and glycine, which ensured a uniform dis tribution of LiF in the synthesized powder. Then water was evaporated at a temperature of ~110 � C. A portion of such a mixture in a quartz flask was placed in a furnace preheated to 400 � C, where the oxidation-reduction exothermic reactions were initiated and propagated over the entire sample volume. As a result, scandium oxide was obtained in the form of a bulk foamy mass. The SHS-product was held at a tem perature of 750� С for 30 min for the complete oxidation of organic products. X-ray diffraction analysis was performed with the use of a XRD-6000 diffractometer, (Shimadzu, Japan) equipped with graphite mono chromator (CuKα radiation λ ¼ 1.54178 Å) in the 2θ range of 10–60 deg. The scanning step for 2θ was 0.02 deg., the scanning rate was 2 deg/min. An ICDD database was used in the analysis. The theoretical density of the materials was calculated from the results of the XRD analysis by the formula:
ρXRD ¼
3. Results and discussion 3.1. The properties of synthesized powders The results of the XRD analysis of the SHS-powders annealed at different temperatures are presented in Fig. 1 in comparison to the diffraction angles data for the cubic Sc2O3 phase (ICDD No. 43–1028). On the basis of a full-profile analysis of the X-ray diffraction patterns the powders were shown to have a cubic C-type crystal lattice of RE2O3 sesquioxides (space group Ia3-, No. 206, Z ¼ 16). No peaks of other phases or traces of impurities were observed on the diffraction patterns. We did not find any information in the literature regarding the sol ubility of lithium fluoride in the scandium oxide matrix or their chemical interaction. One can assume either the formation of a solid solution in the LiF:Sc2O3 system, or the formation of a uniformly distributed LiF phase which is non-detectable using the XRD method due to lower X-ray scattering intensity, or amorphization caused by the size effects and the similarity of the Bragg reflections of scandium oxide and lithium fluo ride. The lattice parameter a, the theoretical density ρXRD, and the average crystalline size dXRD of the Sc2O3 and LiF:Sc2O3 powders calculated on the basis of XRD analysis are given in Table 1. The density values of Sc2O3 and LiF:Sc2O3 powders coincide within the confidence interval and are independent of the calcination temper ature. With an increase in the heat treatment temperature, a noticeable increase in the crystallite size (coherently diffracting domain size) of the powders is observed. For the LiF:Sc2O3 samples, a higher degree of crystallinity is observed both after synthesis and upon calcination. Thus, even at relatively low temperatures, lithium fluoride accelerates the diffusion processes in the scandium oxide. Micrographs in Fig. 2 (a, c) demonstrate the morphology of the asprepared Sc2O3 and LiF:Sc2O3 powders consisting of irregular agglom erates with a porous structure caused by the synthesis mechanism. The propagation of the reaction front first causes foaming of the precursor, followed by the onset of the combustion reaction, which is accompanied by the evolution of a large amount of gaseous products. The LiF doped sample offers clearly visible primary particles, while in the pure scandia the bubble walls look solid. A significant change in the morphology of both powders occurred after annealing at 1100 � C (Fig. 2 b,d). The particles become more homogeneous and rounded. In accordance with the higher crystallinity of LiF:Sc2O3 powders, shown by XRD analysis, a noticeably higher average size was achieved in the case of LiF-doped powders. The infrared spectra (Fig. 3) confirm the contamination of Sc2O3 and LiF:Sc2O3 nanopowders by hydroxide and carbonate groups. The sources of OH and CO23 are the incomplete combustion of the SHS precursor, as well as interaction of the nanoparticles with atmospheric water and carbon dioxide. A similar process was observed earlier in the case of holmium oxide particles with a developed surface [18]. A significant decrease in impurities content occurs after powders annealing at 1000
Z⋅M⋅1:66 V
where Z is a number of structural units in the unit cell (16 for the cubic crystal structure of bixbyite), М is the average molar mass, and V is the unit cell volume. The morphology of the powders and the microstructure of the sin tered ceramics were studied using a Auriga CrossBeam (Carl Zeiss, Germany) scanning electron microscope at an accelerating beam voltage of EHT ¼ 3 kEV with a secondary electron detector. The Fourier-transform infrared (FT-IR) spectra of the scandium oxide powders were measured with a FT-IR Spectrometer Tenzor-27 (Bruker, USA). For that purpose the powders were mixed with KBr in a ratio of 3:1000 and compacted to a pellet at 500 MPa. The powders were consolidated by hot pressing in vacuum in a graphite mold (Ø 13 mm) at a maximum temperature of 1600 � C and a uniaxial pressure of 50 MPa on homemade equipment. Prior to sintering, the powder was compacted in a stainless steel mold at a pressure of ~10 MPa. The compacted material was isolated with graphite paper in order to reduce the contamination effect of the equipment. Heating was car ried out with the use of graphite heaters; the residual pressure in the chamber was 10 Pa. The sinterability of the scandium oxide ceramic samples was studied using dilatometric analysis (DLA) using a displacement sensor (resolution 5 μm) embedded in a hot press at a heating rate of 5 � C/min to a temperature of 1600 � C in vacuum under a loading of 5 MPa. Before the DLA experiment, the baseline of the ther mal expansion of the equipment was determined. The obtained ceramic samples were ground on both sides to a thickness of 1 mm and polished. A density of the ceramic samples was determined by hydrostatic weighing in water. The light transmission of the samples was measured using SF-2000 spectrophotometer (LOMO, Russia) in a wavelength range of 190–1100 nm. The luminescence spectrum of the ytterbium doped scandium oxide 2
D.A. Permin et al.
Optical Materials 100 (2020) 109701
Fig. 1. XRD spectra of the “as made” Sc2O3 (a) and LiF:Sc2O3 (b) SHS-powders and additionally annealed at temperatures up to 1100� C.
C. Given that the nanopowders appear to be quite a promising starting material for sintering transparent ceramics, it is crucial to pay attention to the formation of carbonates in the calcined nanopowders exposed to air. We should also note the lower impurities content in powders con taining lithium fluoride. A possible reason for this may be the higher powder crystallinity and, consequently, their lower reactivity to air moisture and carbon dioxide.
�
Table 1 The lattice parameter a, theoretical density ρXRD and coherently diffracting domain size dXRD of the prepared powders. Material
Annealing temperature, � С
a, Å
ρXRD, g/cm3
dXRD, nm
Sc2O3
–
9.888 � 0.01
12 � 1
900
9.877 0.006 9.874 0.006 9.877 0.006 9.898 0.008 9.879 0.006 9.881 0.007 9.875 0.007
3.790 � 0.005 3.803 � 0.002 3.807 � 0.002 3.803 � 0.002 3.778 � 0.003 3.801 � 0.002 3.799 � 0.002 3.805 � 0.002
1000 1100 LiF: Sc2O3
– 900 1000 1100
� � � � � � �
20 � 1
3.2. Sintering and properties of the scandia ceramics hot pressed using SHS-powders
26 � 2 31 � 3
The results of the dilatometric analysis of the ceramics are presented in Fig. 4. In general, the sintering of LiF:Sc2O3 ceramics occurs much more intensively. The shrinkage of undoped scandium oxide proceeds in a single stage in a temperature ranging from 700 � C to 1510 � C. Shrinkage of the 1 wt%LiF:Sc2O3 sample occurs in two stages. The first, as in material without LiF, is associated with powder recrystallization and a change in their morphology from plate to spherical-like [18]. The second, more intense, is in the area of action of the LiF sintering aid. LiF forms a liquid phase at a temperature of ~850 � C, significantly in tensifies mass transfer and reduces the temperature of plastic deforma tion. Upon reaching a temperature of ~1200 � C (at which LiF is almost
15 � 1 25 � 2 29 � 2 37 � 4
Fig. 2. SEM micrographs of SHS derived powders: Sc2O3 (a) as prepared and (b) annealed at 1100� C; LiF:Sc2O3 (c) as prepared and (d) annealed at 1100� C. 3
D.A. Permin et al.
Optical Materials 100 (2020) 109701
Fig. 3. IR Fourier spectra of the “as made” Sc2O3 (a) and LiF:Sc2O3 (b) SHS-powders and additionally annealed at temperatures up to 1100� C.
The microstructure of Sc2O3 and LiF:Sc2O3 ceramics display signifi cant differences (Fig. 5.). A rather narrow single-mode grain size dis tribution, with an average grain size of about 3 μm, is observed in the Sc2O3 sample (Fig. 5a). Pores with a diameter of 0.5–1 μm are visible at the triple junctions, which are the reasons for the ceramics opacity and low density (97.7% of the theoretical). The fracture surface shows a predominantly intragranular fracture, which indicates a high strength of the grain boundary, comparable with the volume of the grain. Another structure is observed for the ceramics with the addition of lithium fluoride (Fig. 5b). The grain size distribution is bimodal, indi cating the occurrence of a secondary recrystallization process. The average grain size is more than an order of magnitude greater than in the case of Sc2O3 ceramics without the sintering aid. This is due to the liquid-phase sintering mechanism and is typical for many ceramic ma terials when using lithium fluoride as a sintering aid [19–22]. The predominantly intergranular ceramics fracture is caused by impurities consolidation (most likely, by-products of the LiF) and, respectively, a decrease in bond strength compared to grain volume. Both in the grain volume and at the grain boundaries one can observe the presence of residual pores and a secondary phase, presumably LiF, which was not removed at high-temperature soaking. Nevertheless, the ceramic struc ture is almost completely dense (with a density of 99.2%), which ensures the final transparency of such ceramics. The optical properties of the ceramic samples were studied on LiF: Sc2O3 doped with 2mol.% Yb2O3. According to the literature data, the thermal conductivity of 2% Yb: Sc2O3 is not inferior to that of 2% Yb: Y2O3 and 2% Yb: Lu2O3 [23]. A further increase in the doping degree noticeably worsens the thermophysical properties of scandium oxide ceramics. No significant differences were found in the microstructure of Yb-doped ceramics compared to undoped one. Fig. 6 shows the optical in-line transmission and the luminescent spectra of the LiF,2%Yb:Sc2O3 sample hot pressed at 1600 � C for 1 h. The inset of Fig. 6a also shows the appearances of the LiF doped and undoped
Fig. 4. Shrinkage of Sc2O3 and 1wt% LiF:Sc2O3 compacts.
completely removed) the LiF: Sc2O3 compact reaches nearly full density, while the shrinkage of Sc2O3 goes on up to temperatures of ~1500 � C. In addition to the intensified densification, LiF plays an important role in preventing ceramics contamination with carbon impurity. The mecha nism of its action is complex and is associated with a decrease in the temperature of the closed porosity formation and the creation of a slight gas stream from the sample which prevents the penetration of hydro carbons and carbon oxides into compact at the open porosity stage due to the evaporation of lithium fluoride. LiF also reacts with carbon-containing compounds to form volatile CFn derivatives [18–20] which are less prone to pyrolysis or recombination reactions with the release of elemental carbon in the sample.
Fig. 5. SEM Micrographs of the Sc2O3 (a) and 1wt.%LiF:Sc2O3 (b) ceramics. 4
D.A. Permin et al.
Optical Materials 100 (2020) 109701
Fig. 6. UF-Vis-IR transmission spectrum (a) and the luminescent spectrum of 2%Yb:Sc2O3 ceramics (b).
samples. The undoped sample was non-transparent in the measured wavelength range that is why we did not plot its spectra in the figure. The absorption bands in the range of 900–1000 nm are attributed to the 2 F7/2 → 2F5/2 transition of Yb3þ ions in the scandium oxide matrix [16]. In the range of the laser generation of ytterbium ions (at a wavelength of 1.04 μm), the transmittance of 2% Yb:Sc2O3 ceramics does not change compared to undoped Sc2O3 material, and amounts to 78%, which is still less than the calculated value based on the Sc2O3 refractive index given in Ref. [24] (~80%). A decrease in transmittance may be due to the scattering on residual nanopores and/or second-phase LiF nanoparticles. Apparently further improvement in optical transmittance is associated with optimization of both the powder preparation for pressing (deag glomeration and granulation operations) and the sintering conditions that ensure the complete removal of the sintering additive. A ceramic sample with the best optical properties was chosen to determine the possibility of generating laser radiation. The transmission of the output mirrors were 5%, 10% and 20% at the generation wave length. Generation in a pulse-periodic regime (pump pulse length 3 ms, pulse repetition rate 10 Hz) was obtained when the absorbed pump power exceeded a threshold of 2.2 W. A maximum wavelength of laser generation was appeared to be at 1040 nm. The maximum differ ential efficiency of 24% was obtained with a 20% transmission mirror (Fig. 7).
4. Conclusions The effect of a LiF sintering aid on the properties of Sc2O3 powders obtained by the SHS glycine-nitrate method, as well as on the densifi cation characteristics of scandium oxide ceramics prepared by hot pressing was established. It was shown that doping with LiF enhances both the crystallinity and the average particle size of scandium oxide SHS-nanopowders. Lithium fluoride significantly improves consolida tion during hot-pressing and allows for achievement of a density that is close to the theoretical. Generation in a pulse-periodic regime at 1040 nm with a maximum differential efficiency of 24% was obtained on Sc2O3 ceramics doped with 2%Yb ions. The addition of LiF ensures the fabrication of high quality of Sc2O3 ceramics, which makes it possible to obtain laser-quality samples with rare-earth ions for use as promising lasers with high average power. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement D.A. Permin: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Super vision, Writing - review & editing. S.S. Balabanov: Methodology, Project administration, Writing - review & editing. I.L. Snetkov: Investigation, Methodology, Writing - original draft. O.V. Palashov: Investigation, Methodology, Supervision. A.V. Novikova: Investigation, Writing - original draft, Visualization. O.N. Klyusik: Investigation. I.V. Ladenkov: Investigation. Acknowledgements This research was supported within the frame of the program of fundamental scientific research of the state academies of sciences (project No.0095-2019-0005). References [1] A. Pirri, G. Toci, M. Nikl, M. Vannini, High efficiency laser action of 1% at. Yb3þ: Sc2O3 ceramic, Opt. Express 20 (2012) 22134–22142, https://doi.org/10.1364/ OE.20.022134. [2] A. Pirri, G. Toci, B. Patrizi, M. Vannini, An overview on Yb-doped transparent polycrystalline sesquioxide laser ceramics, IEEE J. Sel. Top. Quantum Electron. 24 (5) (2018) 1–8, https://doi.org/10.1109/JSTQE.2018.2799003. [3] V. Peters, Growth and Spectroscopy of Ytterbium-Doped Sesquioxides, PhD thesis, University of Hamburg, Shaker, 2001.
Fig. 7. Dependence of the average power of the laser radiation on the average absorbed pump power for hot pressed 2%Yb:Sc2O3 ceramic sample. 5
D.A. Permin et al.
Optical Materials 100 (2020) 109701
[4] I.L. Snetkov, D.E. Silin, O.V. Palashov, E.A. Khazanov, H. Yagi, T. Yanagitani, H. Yoneda, A. Shirakawa, K. Ueda, A.A. Kaminskii, Study of the thermo-optical constants of Yb doped Y2O3, Lu2O3 and Sc2O3 ceramic materials, Opt. Express 21 (2013) 21254–21263, https://doi.org/10.1364/OE.21.021254. [5] L. Fornasier, E. Mix, V. Peters, K. Petermann, G. Huber, New oxide crystals for solid state lasers, Cryst. Res. Technol. 34 (2) (1999) 255–260. [6] J.-G. Li, T. Ikegami, T. Mori, Solution-based processing of Sc2O3 nanopowders yielding transparent ceramics, J. Mater. Res. 19 (2004) 733–736, https://doi.org/ 10.1557/jmr.2004.19.3.733. [7] B. Jiang, C. HU, J. Li, H. Kou, Y. Shi, W. Liu, Y. Pan, Synthesis and properties of Yb: Sc2O3 transparent ceramics, J. Rare Earths 29 (10) (2011) 951–953, https://doi. org/10.1016/S1002-0721(10)60576-5. [8] Z. Dai, Q. Liu, D. Hreniak, J. Dai, W. Wang, J. Li, Fabrication of Yb:Sc2O3 transparent ceramics from co-precipitated nanopowders: the effect of ammonium hydrogen carbonate to metal ions molar ratio, Opt. Mater. 75 (2018) 673–679, https://doi.org/10.1016/j.optmat.2017.11.035. [9] X. Lu, B. Jiang, J. Li, W. Liu, L. Wang, X. Ba, C. Hu, B. Liu, Y. Pan, Synthesis of highly sinterable Yb:Sc2O3 nanopowders for transparent ceramic, Ceram. Int. 39 (4) (2013) 4695–4700, https://doi.org/10.1016/j.ceramint.2012.10.268. [10] S. Lu, Q. Yang, H. Zhang, Y. Wang, D. Huang, Fabrication and spectral properties of Yb:(Sc0.9Y0.1)2O3 transparent ceramics, Opt. Mater. 35 (4) (2013) 793–797, https://doi.org/10.1016/j.optmat.2012.09.041. [11] Z. Dai, Q. Liu, G. Toci, M. Vannini, A. Pirri, V. Babin, M. Nikl, W. Wang, H. Chen, J. Li, Fabrication and laser oscillation of Yb:Sc2O3 transparent ceramics from coprecipitated nano-powders, J. Eur. Ceram. Soc. 38 (4) (2018) 1632–1638, https:// doi.org/10.1016/j.jeurceramsoc.2017.10.027. [12] K. Serivalsatit, J. Ballato, Submicrometer grain-sized transparent erbium-doped scandia ceramics, J. Am. Ceram. Soc. 93 (11) (2010) 3657–3662, https://doi.org/ 10.1111/j.1551-2916.2010.03954.x. [13] G.E. Gazza, D. Roderick, B. Levine, Transparent Sc2O3 by hot-pressing, J. Mater. Sci. 6 (8) (1971) 1137–1139, https://doi.org/10.1007/BF00980612. [14] J.S. Sanghera, G.R. Villalobos, W.H. Kim, S.S. Bayya, B. Sadowski, I.D. Aggarwal, Hot-pressed Transparent Ceramics and Ceramic Lasers, U.S. Patent No. US8105509B2 (2008). [15] D.A. Permin, E.M. Gavrishchuk, O.N. Klyusik, S.V. Egorov, A.A. Sorokin, Selfpropagating high-temperature synthesis of Sc2O3 nanopowders using different
[16]
[17] [18]
[19] [20]
[21]
[22]
[23] [24]
6
precursors, Adv. Powder Technol. 27 (6) (2016) 2457–2461, https://doi.org/ 10.1016/j.apt.2016.08.025. D.A. Permin, E.M. Gavrishchuk, O.N. Klyusik, A.V. Novikova, A.A. Sorokin, Preparation of optical ceramics based on highly dispersed powders of scandium oxide, J. Opt. Technol. 85 (1) (2018) 58–62, https://doi.org/10.1364/ JOT.85.000058. I.L. Snetkov, I.B. Mukhin, S.S. Balabanov, D.A. Permin, O.V. Palashov, Efficient lasing in Yb:(YLa)2O3 ceramics, Quantum Electron. 45 (2015) 95–97, https://doi. org/10.1070/QE2015v045n02ABEH015652. S.S. Balabanov, S.V. Filofeev, M.G. Ivanov, E.G. Kalinina, D.K. Kuznetsov, D.A. Permin, E.Y. Rostokina, Self-propagating high-temperature synthesis of (Ho1xLax)2O3 nanopowders for magneto-optical ceramics, Heliyon. 5, e01519. https:// doi.org/10.1016/j.heliyon.2019.e01519. L. Esposito, A. Piancastelli, P. Miceli, S. Martelli, A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing, J. Eur. Ceram. Soc. 35 (2) (2015) 651–661, https://doi.org/10.1016/j.jeurceramsoc.2014.09.005. M. Rubat du Merac, H.-J. Kleebe, M.M. Muller, I.E. Reimanis, Fifty years of research and development coming to fruition: unraveling the complex interactions during processing of transparent magnesium aluminate (MgAl2O4) spinel, J. Am. Ceram. Soc. 96 (11) (2013) 3341–3365, https://doi.org/10.1111/Jace.12637. S.S. Balabanov, A.V. Belyaev, A.V. Novikova, D.A. Permin, E.Y. Rostokina, R. P. Yavetskiy, Densification peculiarities of transparent MgAl2O4 ceramics – effect of LiF sintering additive, Inorg. Mater. 54 (10) (2018) 1045–1050, https://doi.org/ 10.1134/s0020168518100023. S.S. Balabanov, R.P. Yavetskiy, A.V. Belyaev, E.M. Gavrishchuk, V.V. Drobotenko, I.I. Evdokimov, A.V. Novikova, O.V. Palashov, D.A. Permin, V.G. Pimenov, Fabrication of transparent MgAl2O4 ceramics by hot-pressing of sol-gel-derived nanopowders, Ceram. Int. 41 (2015) 13366–13371, https://doi.org/10.1016/j. ceramint.2015.07.123. J. Sanghera, W. Kim, G. Villalobos, et al., Ceramic Laser Materials, Materials 5 (2) (2012) 258–277, https://doi.org/10.3390/ma5020258. K. Takaichi, H. Yagi, P. Becker, et al., New data on investigation of novel laser ceramic on the base of cubic scandium sesquioxide: two-band tunable CW generation of Yb3þ:Sc2O3 with laser-diode pumping and the dispersion of refractive index in the visible and near-IR of undoped Sc2O3, Laser Phys. Lett. 4 (7) (2007) 507–510, https://doi.org/10.1002/lapl.200710020.
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Hot pressing of Yb:Sc2O3 laser ceramics with LiF sintering aid D.A. Permin a, S.S. Balabanov a, I.L. Snetkov b, O.V. Palashov b, A.V. Novikova a, *, O.N. Klyusik a, I.V. Ladenkov c a
G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the RAS, Nizhny Novgorod, Russia Institute of Applied Physics of the RAS, Nizhny Novgorod, Russia c JSC “RPE “Salut”, Nizhny Novgorod, Russia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser ceramics Hot pressing Scandium oxide Self-propagaring high-temperature synthesis
The fabrication of transparent Sc2O3 ceramics by hot pressing of self-propagating high-temperature synthesized powders was developed. The effect of lithium fluoride aid and calcination temperature on the powders properties was investigated. After calcination at 1100 � C, uniformly shaped Sc2O3 nanopowders with a primary particle size of less than 50 nm were obtained. Using the prepared powders doped with LiF fully dense optical quality Yb: Sc2O3 transparent ceramics were fabricated by hot pressing. The in-line transmittance of the sample reached 78% at a wavelength of 800 nm. Laser generation with a maximum efficiency of 24% was achieved using a 2% Yb: Sc2O3 sample.
1. Introduction Recently, ytterbium doped rare earth oxides such as Y2O3, Lu2O3 and Sc2O3 have attracted great interest as potential candidates for creating high power solid state laser media [1–3]. Among these materials Sc2O3 probably holds the greatest promise due to the highest thermal con ductivity (for undoped material), low phonon energy and large splitting of the ground state of ytterbium dopant ions [4]. There are several ref erences with regard to the successful preparation of laser elements based on scandium oxide single crystals doped with rare earth ions [2,3,5]. However, the obvious technological difficulties in growing Sc2O3 crys tals, associated with a high melting point (~2430 � C), make this approach less attractive compared to ceramic technology. The main approach to the preparation of transparent scandium oxide ceramics is the free vacuum sintering of nanodispersed powders ob tained by the solution precipitation method [6–9]. The low driving force of the free sintering makes it necessary to use long soaking times at high temperatures (>1800 � C) to achieve a high level of light transmission (sufficient for laser generation) [11]. This leads to grain growth and the contamination of the ceramics by the equipment materials. Additives such as yttrium oxide [10] could be applied to enhance the sinterability of scandia ceramics, but the dilution of Sc2O3 by rare earths results in a significant drop in the thermal conductivity of the sample, worsening its performance. The required level of optical properties can be also
achieved by combining free sintering and hot isostatic pressing [12]. Another effective approach to obtain highly transparent material is a hot uniaxial pressing of highly dispersed Sc2O3 powders. In this case lithium fluoride is used as a sintering additive in an amount of up to 1 wt %. But the number of publications with regard to this method is limited [13,14]. The patent [14] states that high-quality Sc2O3 ceramics can be achieved by using uniform nano-sized powders having no hard ag glomerates, uniform doping with a LiF sintering additive and the se lection of barothermal processing conditions. However, the characteristics of the Sc2O3 ceramics so obtained are not given. It was previously shown that the nanodispersed scandium oxide powders could be produced by self-propagating high-temperature syn thesis (SHS) method [15,16]. In addition, this method allows one to uniformly introduce sintering additives, in particular, lithium fluoride [16]. In the available literature we did not find any references regarding the effect of a LiF additive on the properties of the Sc2O3 powders or on the sintering process and the microstructural properties of Yb:Sc2O3 ceramics. The aim of this work was to investigate the effect of a lithium fluoride additive on the properties of scandium oxide SHS-powders, the process of a consolidation by hot pressing, and the study of the properties (microstructural and optical) of the Yb:Sc2O3 ceramics.
* Corresponding author. G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, 49 Tropinina Str., Nizhny Novgorod, 603950, Russia. E-mail address: [email protected] (A.V. Novikova). https://doi.org/10.1016/j.optmat.2020.109701 Received 14 November 2019; Received in revised form 10 January 2020; Accepted 14 January 2020 Available online 25 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
D.A. Permin et al.
Optical Materials 100 (2020) 109701
2. Experimental
ceramics was recorded using a TII S150-2 (SOLAR, Belarus) spectrom eter at an excitation wavelength of 940 nm using a laser diode source. To ascertain the possibility of the generation of laser radiation, dielectric coatings (antireflective on one side and mirror on the opposite one) were applied to the end surfaces of the ceramic sample for two wavelengths (pumping λp ¼ 940 nm and laser generation λl ~ 1040 nm). The pre pared sample was mounted on a copper heat sink and placed in a laser cavity formed by the back surface of the sample, a totally reflecting spherical mirror with a focus of 120 cm and an output mirror. The pump radiation was focused by spherical mirror on the studied sample into a spot of 2 mm in diameter and after one V-shaped pass through the sample was deflected to an absorber. The optical scheme of this exper iment is described in detail in Ref. [17]. A fibre-coupled laser diode, Laserline LDM 2000, emitting at a wavelength of 940 nm, was used as a pump source. The laser power was measured using Ophir 3A and Ophir 30A-BB-18 bolometric power meters.
The nanopowders of ytterbium doped scandium oxide were obtained with the use of metal nitrates as an oxidant, and glycine as a fuel. The starting materials for the synthesis of SHS-precursors were commercially available low dispersed scandium Sc2O3 and ytterbium Yb2O3 oxides (99.99% Lanhit, Russia), nitric acid HNO3 (99.9999%, Khimreaktiv, Russia), glycine NH2CH2COOH (99.9%, Khimreaktiv, Russia) and lithium fluoride LiF (“pure”, Khimreaktiv, Russia). The water solutions of metal nitrates were obtained by dissolving ~10 g of the scandium oxide (or a mixture of scandium and yttrbium oxides) in a stoichiometric amount of nitric acid upon heating. Glycine was added to the solution in a molar ratio of 1:1 in respect to the metal nitrates. To introduce a sintering additive, a solution of LiF in dilute nitric acid with a concentration of ~0.01 g/ml had been previously prepared as described in Ref. [16]. The resulting solution was added to a mixture of scandium nitrate and glycine, which ensured a uniform dis tribution of LiF in the synthesized powder. Then water was evaporated at a temperature of ~110 � C. A portion of such a mixture in a quartz flask was placed in a furnace preheated to 400 � C, where the oxidation-reduction exothermic reactions were initiated and propagated over the entire sample volume. As a result, scandium oxide was obtained in the form of a bulk foamy mass. The SHS-product was held at a tem perature of 750� С for 30 min for the complete oxidation of organic products. X-ray diffraction analysis was performed with the use of a XRD-6000 diffractometer, (Shimadzu, Japan) equipped with graphite mono chromator (CuKα radiation λ ¼ 1.54178 Å) in the 2θ range of 10–60 deg. The scanning step for 2θ was 0.02 deg., the scanning rate was 2 deg/min. An ICDD database was used in the analysis. The theoretical density of the materials was calculated from the results of the XRD analysis by the formula:
ρXRD ¼
3. Results and discussion 3.1. The properties of synthesized powders The results of the XRD analysis of the SHS-powders annealed at different temperatures are presented in Fig. 1 in comparison to the diffraction angles data for the cubic Sc2O3 phase (ICDD No. 43–1028). On the basis of a full-profile analysis of the X-ray diffraction patterns the powders were shown to have a cubic C-type crystal lattice of RE2O3 sesquioxides (space group Ia3-, No. 206, Z ¼ 16). No peaks of other phases or traces of impurities were observed on the diffraction patterns. We did not find any information in the literature regarding the sol ubility of lithium fluoride in the scandium oxide matrix or their chemical interaction. One can assume either the formation of a solid solution in the LiF:Sc2O3 system, or the formation of a uniformly distributed LiF phase which is non-detectable using the XRD method due to lower X-ray scattering intensity, or amorphization caused by the size effects and the similarity of the Bragg reflections of scandium oxide and lithium fluo ride. The lattice parameter a, the theoretical density ρXRD, and the average crystalline size dXRD of the Sc2O3 and LiF:Sc2O3 powders calculated on the basis of XRD analysis are given in Table 1. The density values of Sc2O3 and LiF:Sc2O3 powders coincide within the confidence interval and are independent of the calcination temper ature. With an increase in the heat treatment temperature, a noticeable increase in the crystallite size (coherently diffracting domain size) of the powders is observed. For the LiF:Sc2O3 samples, a higher degree of crystallinity is observed both after synthesis and upon calcination. Thus, even at relatively low temperatures, lithium fluoride accelerates the diffusion processes in the scandium oxide. Micrographs in Fig. 2 (a, c) demonstrate the morphology of the asprepared Sc2O3 and LiF:Sc2O3 powders consisting of irregular agglom erates with a porous structure caused by the synthesis mechanism. The propagation of the reaction front first causes foaming of the precursor, followed by the onset of the combustion reaction, which is accompanied by the evolution of a large amount of gaseous products. The LiF doped sample offers clearly visible primary particles, while in the pure scandia the bubble walls look solid. A significant change in the morphology of both powders occurred after annealing at 1100 � C (Fig. 2 b,d). The particles become more homogeneous and rounded. In accordance with the higher crystallinity of LiF:Sc2O3 powders, shown by XRD analysis, a noticeably higher average size was achieved in the case of LiF-doped powders. The infrared spectra (Fig. 3) confirm the contamination of Sc2O3 and LiF:Sc2O3 nanopowders by hydroxide and carbonate groups. The sources of OH and CO23 are the incomplete combustion of the SHS precursor, as well as interaction of the nanoparticles with atmospheric water and carbon dioxide. A similar process was observed earlier in the case of holmium oxide particles with a developed surface [18]. A significant decrease in impurities content occurs after powders annealing at 1000
Z⋅M⋅1:66 V
where Z is a number of structural units in the unit cell (16 for the cubic crystal structure of bixbyite), М is the average molar mass, and V is the unit cell volume. The morphology of the powders and the microstructure of the sin tered ceramics were studied using a Auriga CrossBeam (Carl Zeiss, Germany) scanning electron microscope at an accelerating beam voltage of EHT ¼ 3 kEV with a secondary electron detector. The Fourier-transform infrared (FT-IR) spectra of the scandium oxide powders were measured with a FT-IR Spectrometer Tenzor-27 (Bruker, USA). For that purpose the powders were mixed with KBr in a ratio of 3:1000 and compacted to a pellet at 500 MPa. The powders were consolidated by hot pressing in vacuum in a graphite mold (Ø 13 mm) at a maximum temperature of 1600 � C and a uniaxial pressure of 50 MPa on homemade equipment. Prior to sintering, the powder was compacted in a stainless steel mold at a pressure of ~10 MPa. The compacted material was isolated with graphite paper in order to reduce the contamination effect of the equipment. Heating was car ried out with the use of graphite heaters; the residual pressure in the chamber was 10 Pa. The sinterability of the scandium oxide ceramic samples was studied using dilatometric analysis (DLA) using a displacement sensor (resolution 5 μm) embedded in a hot press at a heating rate of 5 � C/min to a temperature of 1600 � C in vacuum under a loading of 5 MPa. Before the DLA experiment, the baseline of the ther mal expansion of the equipment was determined. The obtained ceramic samples were ground on both sides to a thickness of 1 mm and polished. A density of the ceramic samples was determined by hydrostatic weighing in water. The light transmission of the samples was measured using SF-2000 spectrophotometer (LOMO, Russia) in a wavelength range of 190–1100 nm. The luminescence spectrum of the ytterbium doped scandium oxide 2
D.A. Permin et al.
Optical Materials 100 (2020) 109701
Fig. 1. XRD spectra of the “as made” Sc2O3 (a) and LiF:Sc2O3 (b) SHS-powders and additionally annealed at temperatures up to 1100� C.
C. Given that the nanopowders appear to be quite a promising starting material for sintering transparent ceramics, it is crucial to pay attention to the formation of carbonates in the calcined nanopowders exposed to air. We should also note the lower impurities content in powders con taining lithium fluoride. A possible reason for this may be the higher powder crystallinity and, consequently, their lower reactivity to air moisture and carbon dioxide.
�
Table 1 The lattice parameter a, theoretical density ρXRD and coherently diffracting domain size dXRD of the prepared powders. Material
Annealing temperature, � С
a, Å
ρXRD, g/cm3
dXRD, nm
Sc2O3
–
9.888 � 0.01
12 � 1
900
9.877 0.006 9.874 0.006 9.877 0.006 9.898 0.008 9.879 0.006 9.881 0.007 9.875 0.007
3.790 � 0.005 3.803 � 0.002 3.807 � 0.002 3.803 � 0.002 3.778 � 0.003 3.801 � 0.002 3.799 � 0.002 3.805 � 0.002
1000 1100 LiF: Sc2O3
– 900 1000 1100
� � � � � � �
20 � 1
3.2. Sintering and properties of the scandia ceramics hot pressed using SHS-powders
26 � 2 31 � 3
The results of the dilatometric analysis of the ceramics are presented in Fig. 4. In general, the sintering of LiF:Sc2O3 ceramics occurs much more intensively. The shrinkage of undoped scandium oxide proceeds in a single stage in a temperature ranging from 700 � C to 1510 � C. Shrinkage of the 1 wt%LiF:Sc2O3 sample occurs in two stages. The first, as in material without LiF, is associated with powder recrystallization and a change in their morphology from plate to spherical-like [18]. The second, more intense, is in the area of action of the LiF sintering aid. LiF forms a liquid phase at a temperature of ~850 � C, significantly in tensifies mass transfer and reduces the temperature of plastic deforma tion. Upon reaching a temperature of ~1200 � C (at which LiF is almost
15 � 1 25 � 2 29 � 2 37 � 4
Fig. 2. SEM micrographs of SHS derived powders: Sc2O3 (a) as prepared and (b) annealed at 1100� C; LiF:Sc2O3 (c) as prepared and (d) annealed at 1100� C. 3
D.A. Permin et al.
Optical Materials 100 (2020) 109701
Fig. 3. IR Fourier spectra of the “as made” Sc2O3 (a) and LiF:Sc2O3 (b) SHS-powders and additionally annealed at temperatures up to 1100� C.
The microstructure of Sc2O3 and LiF:Sc2O3 ceramics display signifi cant differences (Fig. 5.). A rather narrow single-mode grain size dis tribution, with an average grain size of about 3 μm, is observed in the Sc2O3 sample (Fig. 5a). Pores with a diameter of 0.5–1 μm are visible at the triple junctions, which are the reasons for the ceramics opacity and low density (97.7% of the theoretical). The fracture surface shows a predominantly intragranular fracture, which indicates a high strength of the grain boundary, comparable with the volume of the grain. Another structure is observed for the ceramics with the addition of lithium fluoride (Fig. 5b). The grain size distribution is bimodal, indi cating the occurrence of a secondary recrystallization process. The average grain size is more than an order of magnitude greater than in the case of Sc2O3 ceramics without the sintering aid. This is due to the liquid-phase sintering mechanism and is typical for many ceramic ma terials when using lithium fluoride as a sintering aid [19–22]. The predominantly intergranular ceramics fracture is caused by impurities consolidation (most likely, by-products of the LiF) and, respectively, a decrease in bond strength compared to grain volume. Both in the grain volume and at the grain boundaries one can observe the presence of residual pores and a secondary phase, presumably LiF, which was not removed at high-temperature soaking. Nevertheless, the ceramic struc ture is almost completely dense (with a density of 99.2%), which ensures the final transparency of such ceramics. The optical properties of the ceramic samples were studied on LiF: Sc2O3 doped with 2mol.% Yb2O3. According to the literature data, the thermal conductivity of 2% Yb: Sc2O3 is not inferior to that of 2% Yb: Y2O3 and 2% Yb: Lu2O3 [23]. A further increase in the doping degree noticeably worsens the thermophysical properties of scandium oxide ceramics. No significant differences were found in the microstructure of Yb-doped ceramics compared to undoped one. Fig. 6 shows the optical in-line transmission and the luminescent spectra of the LiF,2%Yb:Sc2O3 sample hot pressed at 1600 � C for 1 h. The inset of Fig. 6a also shows the appearances of the LiF doped and undoped
Fig. 4. Shrinkage of Sc2O3 and 1wt% LiF:Sc2O3 compacts.
completely removed) the LiF: Sc2O3 compact reaches nearly full density, while the shrinkage of Sc2O3 goes on up to temperatures of ~1500 � C. In addition to the intensified densification, LiF plays an important role in preventing ceramics contamination with carbon impurity. The mecha nism of its action is complex and is associated with a decrease in the temperature of the closed porosity formation and the creation of a slight gas stream from the sample which prevents the penetration of hydro carbons and carbon oxides into compact at the open porosity stage due to the evaporation of lithium fluoride. LiF also reacts with carbon-containing compounds to form volatile CFn derivatives [18–20] which are less prone to pyrolysis or recombination reactions with the release of elemental carbon in the sample.
Fig. 5. SEM Micrographs of the Sc2O3 (a) and 1wt.%LiF:Sc2O3 (b) ceramics. 4
D.A. Permin et al.
Optical Materials 100 (2020) 109701
Fig. 6. UF-Vis-IR transmission spectrum (a) and the luminescent spectrum of 2%Yb:Sc2O3 ceramics (b).
samples. The undoped sample was non-transparent in the measured wavelength range that is why we did not plot its spectra in the figure. The absorption bands in the range of 900–1000 nm are attributed to the 2 F7/2 → 2F5/2 transition of Yb3þ ions in the scandium oxide matrix [16]. In the range of the laser generation of ytterbium ions (at a wavelength of 1.04 μm), the transmittance of 2% Yb:Sc2O3 ceramics does not change compared to undoped Sc2O3 material, and amounts to 78%, which is still less than the calculated value based on the Sc2O3 refractive index given in Ref. [24] (~80%). A decrease in transmittance may be due to the scattering on residual nanopores and/or second-phase LiF nanoparticles. Apparently further improvement in optical transmittance is associated with optimization of both the powder preparation for pressing (deag glomeration and granulation operations) and the sintering conditions that ensure the complete removal of the sintering additive. A ceramic sample with the best optical properties was chosen to determine the possibility of generating laser radiation. The transmission of the output mirrors were 5%, 10% and 20% at the generation wave length. Generation in a pulse-periodic regime (pump pulse length 3 ms, pulse repetition rate 10 Hz) was obtained when the absorbed pump power exceeded a threshold of 2.2 W. A maximum wavelength of laser generation was appeared to be at 1040 nm. The maximum differ ential efficiency of 24% was obtained with a 20% transmission mirror (Fig. 7).
4. Conclusions The effect of a LiF sintering aid on the properties of Sc2O3 powders obtained by the SHS glycine-nitrate method, as well as on the densifi cation characteristics of scandium oxide ceramics prepared by hot pressing was established. It was shown that doping with LiF enhances both the crystallinity and the average particle size of scandium oxide SHS-nanopowders. Lithium fluoride significantly improves consolida tion during hot-pressing and allows for achievement of a density that is close to the theoretical. Generation in a pulse-periodic regime at 1040 nm with a maximum differential efficiency of 24% was obtained on Sc2O3 ceramics doped with 2%Yb ions. The addition of LiF ensures the fabrication of high quality of Sc2O3 ceramics, which makes it possible to obtain laser-quality samples with rare-earth ions for use as promising lasers with high average power. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement D.A. Permin: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Super vision, Writing - review & editing. S.S. Balabanov: Methodology, Project administration, Writing - review & editing. I.L. Snetkov: Investigation, Methodology, Writing - original draft. O.V. Palashov: Investigation, Methodology, Supervision. A.V. Novikova: Investigation, Writing - original draft, Visualization. O.N. Klyusik: Investigation. I.V. Ladenkov: Investigation. Acknowledgements This research was supported within the frame of the program of fundamental scientific research of the state academies of sciences (project No.0095-2019-0005). References [1] A. Pirri, G. Toci, M. Nikl, M. Vannini, High efficiency laser action of 1% at. Yb3þ: Sc2O3 ceramic, Opt. Express 20 (2012) 22134–22142, https://doi.org/10.1364/ OE.20.022134. [2] A. Pirri, G. Toci, B. Patrizi, M. Vannini, An overview on Yb-doped transparent polycrystalline sesquioxide laser ceramics, IEEE J. Sel. Top. Quantum Electron. 24 (5) (2018) 1–8, https://doi.org/10.1109/JSTQE.2018.2799003. [3] V. Peters, Growth and Spectroscopy of Ytterbium-Doped Sesquioxides, PhD thesis, University of Hamburg, Shaker, 2001.
Fig. 7. Dependence of the average power of the laser radiation on the average absorbed pump power for hot pressed 2%Yb:Sc2O3 ceramic sample. 5
D.A. Permin et al.
Optical Materials 100 (2020) 109701
[4] I.L. Snetkov, D.E. Silin, O.V. Palashov, E.A. Khazanov, H. Yagi, T. Yanagitani, H. Yoneda, A. Shirakawa, K. Ueda, A.A. Kaminskii, Study of the thermo-optical constants of Yb doped Y2O3, Lu2O3 and Sc2O3 ceramic materials, Opt. Express 21 (2013) 21254–21263, https://doi.org/10.1364/OE.21.021254. [5] L. Fornasier, E. Mix, V. Peters, K. Petermann, G. Huber, New oxide crystals for solid state lasers, Cryst. Res. Technol. 34 (2) (1999) 255–260. [6] J.-G. Li, T. Ikegami, T. Mori, Solution-based processing of Sc2O3 nanopowders yielding transparent ceramics, J. Mater. Res. 19 (2004) 733–736, https://doi.org/ 10.1557/jmr.2004.19.3.733. [7] B. Jiang, C. HU, J. Li, H. Kou, Y. Shi, W. Liu, Y. Pan, Synthesis and properties of Yb: Sc2O3 transparent ceramics, J. Rare Earths 29 (10) (2011) 951–953, https://doi. org/10.1016/S1002-0721(10)60576-5. [8] Z. Dai, Q. Liu, D. Hreniak, J. Dai, W. Wang, J. Li, Fabrication of Yb:Sc2O3 transparent ceramics from co-precipitated nanopowders: the effect of ammonium hydrogen carbonate to metal ions molar ratio, Opt. Mater. 75 (2018) 673–679, https://doi.org/10.1016/j.optmat.2017.11.035. [9] X. Lu, B. Jiang, J. Li, W. Liu, L. Wang, X. Ba, C. Hu, B. Liu, Y. Pan, Synthesis of highly sinterable Yb:Sc2O3 nanopowders for transparent ceramic, Ceram. Int. 39 (4) (2013) 4695–4700, https://doi.org/10.1016/j.ceramint.2012.10.268. [10] S. Lu, Q. Yang, H. Zhang, Y. Wang, D. Huang, Fabrication and spectral properties of Yb:(Sc0.9Y0.1)2O3 transparent ceramics, Opt. Mater. 35 (4) (2013) 793–797, https://doi.org/10.1016/j.optmat.2012.09.041. [11] Z. Dai, Q. Liu, G. Toci, M. Vannini, A. Pirri, V. Babin, M. Nikl, W. Wang, H. Chen, J. Li, Fabrication and laser oscillation of Yb:Sc2O3 transparent ceramics from coprecipitated nano-powders, J. Eur. Ceram. Soc. 38 (4) (2018) 1632–1638, https:// doi.org/10.1016/j.jeurceramsoc.2017.10.027. [12] K. Serivalsatit, J. Ballato, Submicrometer grain-sized transparent erbium-doped scandia ceramics, J. Am. Ceram. Soc. 93 (11) (2010) 3657–3662, https://doi.org/ 10.1111/j.1551-2916.2010.03954.x. [13] G.E. Gazza, D. Roderick, B. Levine, Transparent Sc2O3 by hot-pressing, J. Mater. Sci. 6 (8) (1971) 1137–1139, https://doi.org/10.1007/BF00980612. [14] J.S. Sanghera, G.R. Villalobos, W.H. Kim, S.S. Bayya, B. Sadowski, I.D. Aggarwal, Hot-pressed Transparent Ceramics and Ceramic Lasers, U.S. Patent No. US8105509B2 (2008). [15] D.A. Permin, E.M. Gavrishchuk, O.N. Klyusik, S.V. Egorov, A.A. Sorokin, Selfpropagating high-temperature synthesis of Sc2O3 nanopowders using different
[16]
[17] [18]
[19] [20]
[21]
[22]
[23] [24]
6
precursors, Adv. Powder Technol. 27 (6) (2016) 2457–2461, https://doi.org/ 10.1016/j.apt.2016.08.025. D.A. Permin, E.M. Gavrishchuk, O.N. Klyusik, A.V. Novikova, A.A. Sorokin, Preparation of optical ceramics based on highly dispersed powders of scandium oxide, J. Opt. Technol. 85 (1) (2018) 58–62, https://doi.org/10.1364/ JOT.85.000058. I.L. Snetkov, I.B. Mukhin, S.S. Balabanov, D.A. Permin, O.V. Palashov, Efficient lasing in Yb:(YLa)2O3 ceramics, Quantum Electron. 45 (2015) 95–97, https://doi. org/10.1070/QE2015v045n02ABEH015652. S.S. Balabanov, S.V. Filofeev, M.G. Ivanov, E.G. Kalinina, D.K. Kuznetsov, D.A. Permin, E.Y. Rostokina, Self-propagating high-temperature synthesis of (Ho1xLax)2O3 nanopowders for magneto-optical ceramics, Heliyon. 5, e01519. https:// doi.org/10.1016/j.heliyon.2019.e01519. L. Esposito, A. Piancastelli, P. Miceli, S. Martelli, A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing, J. Eur. Ceram. Soc. 35 (2) (2015) 651–661, https://doi.org/10.1016/j.jeurceramsoc.2014.09.005. M. Rubat du Merac, H.-J. Kleebe, M.M. Muller, I.E. Reimanis, Fifty years of research and development coming to fruition: unraveling the complex interactions during processing of transparent magnesium aluminate (MgAl2O4) spinel, J. Am. Ceram. Soc. 96 (11) (2013) 3341–3365, https://doi.org/10.1111/Jace.12637. S.S. Balabanov, A.V. Belyaev, A.V. Novikova, D.A. Permin, E.Y. Rostokina, R. P. Yavetskiy, Densification peculiarities of transparent MgAl2O4 ceramics – effect of LiF sintering additive, Inorg. Mater. 54 (10) (2018) 1045–1050, https://doi.org/ 10.1134/s0020168518100023. S.S. Balabanov, R.P. Yavetskiy, A.V. Belyaev, E.M. Gavrishchuk, V.V. Drobotenko, I.I. Evdokimov, A.V. Novikova, O.V. Palashov, D.A. Permin, V.G. Pimenov, Fabrication of transparent MgAl2O4 ceramics by hot-pressing of sol-gel-derived nanopowders, Ceram. Int. 41 (2015) 13366–13371, https://doi.org/10.1016/j. ceramint.2015.07.123. J. Sanghera, W. Kim, G. Villalobos, et al., Ceramic Laser Materials, Materials 5 (2) (2012) 258–277, https://doi.org/10.3390/ma5020258. K. Takaichi, H. Yagi, P. Becker, et al., New data on investigation of novel laser ceramic on the base of cubic scandium sesquioxide: two-band tunable CW generation of Yb3þ:Sc2O3 with laser-diode pumping and the dispersion of refractive index in the visible and near-IR of undoped Sc2O3, Laser Phys. Lett. 4 (7) (2007) 507–510, https://doi.org/10.1002/lapl.200710020.