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Characterizations of a hot-pressed Er and Y codoped CaF2 transparent ceramic
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Characterizations of a hot-pressed Er and Y codoped CaF2 transparent ceramic Weiwei Li a , Zuodong Liu a , Zhiwei Zhou a , Jinghong Song b , Bingchu Mei a,∗ , Liangbi Su c a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Center of Materials Research and Analysis, Wuhan University of Technology, Wuhan 430070, China Key Laboratory of Transparent and Opto-Functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China b c
a r t i c l e
i n f o
Article history: Received 23 November 2015 Received in revised form 30 January 2016 Accepted 2 February 2016 Available online xxx Keywords: Calcium fluoride Erbium Yttrium Transparent ceramic Luminescence
a b s t r a c t In this work, Er:CaF2 transparent ceramic codoped with non-active ions (Y3+ ions) was fabricated by hotpressed (HP) sintering method. Er and Er/Y codoped CaF2 nanoparticles were synthesized by a simple coprecipitation method, the phase composition and the shape of obtained nanoparticles were analysed by X-ray diffraction patterns and transmission electron microscopy, respectively. Fluorite transparent ceramics were fabricated by HP sintering method at a temperature of 800 ◦ C. For 2 mm thickness ceramic samples, the optical transmission of Er, Y:CaF2 transparent ceramic in the near-infrared range reached about 90%, and the optical transmission of Er, Y:CaF2 transparent ceramic was much higher than Er:CaF2 ceramic sample. Microstructures were characterized using SEM analysis, the two ceramic samples exhibited nearly pore-free microstructure and the average grain sizes were less than 1.0 m. The absorption, upconversion and infrared emission spectras of ceramic samples were also measured and discussed. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As one of the optical material, calcium fluoride (CaF2 ) compounds doped with trivalent lanthanide ions has gained much attention as solid-state laser and other luminescence materials for many years. Such as, U:CaF2 single crystal was the second material used as laser host material by Sorokin and Stevenson in 1960 [1], and the first laser polycrystalline ceramic was fabricated based on Dy-doped CaF2 compounds by Hatch et al. in 1964 [2]. The CaF2 compounds seems to be an ideal medium for preparation of luminescent materials because of their high transparency in a wide wavelength region (from vacuum UV to 9 m), low linear and nonlinear refractive index, as well as low phonon energy [3,4]. The optical spectral properties of CaF2 doped with trivalent rare-earth ions have been attracted more and more attention. So far, the fabrication and optical properties of rare earth ions doped CaF2 transparent ceramics have been investigated extensively. According to the available literature, two synthesis routes were utilized to produce fluorite polycrystalline ceramics. In 2008, Basiev et al. [5] reported the fabrication of CaF2 –SrF2 –YbF3 fluoride laser ceramics by hot-forming methods based on deformation
∗ Corresponding author. E-mail address: [email protected] (B. Mei).
of the single crystals at elevated temperature. This method has been successfully utilized for CaF2 :Yb, CaF2 :Er, SrF2 :Nd and other fluoride laser ceramics [6–8]. Another approach, fluorite transparent ceramics were fabricated from chemical synthesized nanoparticles after vacuum sintering. Yb:CaF2 transparent ceramics were fabricated from the chemical synthesized nanoparticles after a vacuum sintering at 900 ◦ C and a hot-isostatically pressed (HIP) post-treatment under an argon pressure of 160 MPa by Aubry et al. in 2009 [9]. Up to now, many studies on CaF2 transparent ceramics fabricated from the nanoparticles have been reported [10–12]. In recent years, the upconversion and infrared luminescence properties of erbium (Er3+ ) ions in different host materials have been intensively investigated [13–15], and Er:CaF2 transparent ceramics were also fabricated [16,17]. It was reported that Er ions doped materials tend to form clusters at high doping concentration. In our earlier work [16], we had shown that strong cross-relaxation processes between Er3+ ions happens at high doing level, and favoring the red upconversion emission. In order to suppress the cross-relaxation processes between Er3+ ions, it is necessary to avoid the large clustering formation and enlarge the separation of the active ions (Er3+ ions) in the matrix materials. Zhao et al. systematically studied the effect of YF3 addition on the formation and optical properties of Er3+ ions doped fluoride nanocrystals in glassceramics [18]. It was found that codoping non-active ions (Y3+ ions) into fluoride nanocrystals can dilute the effective concentration of
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Er3+ ions, and suppress some detrimental energy transfer process between Er3+ ions. For the same reason, a Nd, Y codoped CaF2 single crystal has been developed by Wang et al. [19], and codoping with Y3+ was considered to improve the detrimental concentration quenching effect that results from the clustering of the rare earth ions, in addition to modulate the spectra properties of Nd3+ ions. So the buffer ions M3+ (such as Gd3+ , Y3+ and Sc3+ ) were codoped into CaF2 matrix lattice to break the rare earth ions clusters. In this work, to overcome the strong cross-relaxation process of Er3+ ions at high doping concentration, Y3+ ions were selected as buffer ions to be added into the Er:CaF2 transparent ceramics. The results presented here, to our knowledge, were the first experimental demonstration of trivalent lanthanide ions spectroscopic properties modulated by buffer ions in fluorite transparent ceramics. In this paper, we synthesized the Er and Er/Y codoped fluorite nanoparticles by a coprecipitation method. High quality fluorite transparent ceramics were fabricated by hot pressed sintering method. The optical and luminescence characteristics of the fabricated transparent ceramic samples were also investigated. 2. Experimental 2.1. Fluorite nanoparticles synthesis Commercially available chemical reagents including calcium nitrate tetrahydrate (Ca(NO3 )2 ·4H2 O, AR), erbium trinitrate pentahydrate (ErN3 O9 ·5H2 O, 99.9%), yttrium nitrate hexahydrate (YN3 O9 ·6H2 O, 99.9%), and potassium fluoride dihydrate (KF·2H2 O, AR) were used as raw materials to synthesize fluorite nanoparticles by a coprecipitation method. Water was distilled. In the typical synthesizing procedure, solutions containing the cationic precursors (Ca2+ , Er3+ and Y3+ , totally 0.5 mol/L) and KF·2H2 O (F− , 1.2 mol/L) were separately made in the same volume distilled water. These two solutions were stirred separately for 5 min, and then the cationic solutions were added dropwise to the anion solutions. The mixture ration of the chemical reagents was determined by the following chemical reaction:
Fig. 1. XRD patterns of as-synthesized fluorite nanoparticles.
were with a Cu K␣ radiation ( = 1.5405 Å) in the 2 range from 20◦ to 80◦ . Transmission electron microscopy (TEM; JEM-2100F, JEOL, Tokyo, Japan) was employed to determine the morphology of the obtained fluorite nanoparticles. The average nanoparticle and grain sizes were measured by the linear intercept method, taking at least 300 nanoparticles into account. The optical transmission and absorption spectra were measured by a UV–vis-NIR spectrophotometer (Lambda 750S, PerkinElmer, Waltham, USA) in the wavelength range of 200–2500 nm. Microstructure of the ceramics were characterized by a Field-emission Scanning Electron Microscope (FE-SEM; ULTRA PLUS-43-13, Zeiss, Oberkochen, Germany). The visible upconversion and near-infrared emission spectras were excited under 980 nm laser from a Ti:Sapphire solid-state laser (3900S, Spectra Physics, Mountain View, USA), and detected using a photomultiplier tube detector and an In GaAs detector, respectively. All the spectral measurements were performed at room temperature.
(1 − x − y)Ca(NO3 )2 + xEr(NO3 )3 + yY(NO3 )3 + (2 + x + y)KF → Ca1 − x − y Erx Y y F2 + x + y ↓ + (2 + x + y)KNO3 where x (x = 2 mol%) is Er doping level and y (y = 0, 3 mol%) is Y doping level. The mixed solutions were stirred for 30 min and then stayed for 24 h at room temperature. The opaque aqueous solutions were centrifuged at 11,000 rpm for 20 min and washed two times with distilled water. The obtained nanoparticles were oven dried at 80 ◦ C for 24 h. 2.2. Ceramics fabrication The synthesized fluorite nanoparticles were filled into a graphite mold with a cavity diameter of 16 mm directly without any specific pretreatments, and then hot pressed at 800 ◦ C in a vacuum environment for 120 min. Fluorite transparent ceramics were fabricated using a hot pressed sintering method, and details of the fabrication procedure have been described in the previous works [11,16]. After hot pressed at 800 ◦ C for 120 min, the ceramic samples were ground and polished for subsequent evaluation. In order to protect the ceramic sample from carbon contamination, graphite die was covered by a embedding high melting point of alumina powder layer. 2.3. Measurements Structures of the obtained fluorite nanoparticles were characterized using powder X-ray diffraction (XRD) measurements
3. Results and discussion The XRD patterns of the as-synthesized fluorite nanoparticles are shown in Fig. 1. All the obtained XRD diffraction peaks of the nanoparticles are indexed into CaF2 cubic phase with the typical fluorite structure (space group: Fm3m) and no extra diffraction peaks are detected. It indicates that the single fluorite cubic crystallinity of the as-synthesized nanoparticles with no detectable secondary phase or other impurities. The average nanoparticle size D are calculated using the Scherrer equation [20]: D = /[(2) × cos()]
(1)
where is the wavelength of Cu K␣ radiation ( = 1.5405 Å), 2 is the peak position and (2) is the corrected peak-width at half-maximum of the diffraction peaks. The calculated average nanoparticle size of Er:CaF2 and Er, Y:CaF2 nanoparticles are about 27.4 nm and 23.1 nm, respectively. Fig. 2 shows the TEM photographs of as-synthesized fluoride nanoparticles. The TEM results reveal that the fluorite nanoparticles were slight agglomerated. For both two nanoparticle samples, some nanoparticles were much bigger with nanocube shapes, and others were much smaller with nanosphere shapes. The nanoparticle size presented in Fig. 2(b) was much smaller than presented in Fig. 2(a). The measured average nanoparticles sizes of Er:CaF2 and Er, Y:CaF2 nanoparticles were about 25.6 nm and 20.2 nm, respectively. This result was close to that values calculated from the XRD patterns, showing a good agreement between these two measure-
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Fig. 2. TEM micrographs of the as-synthesized (a) Er:CaF2 and (b) Er, Y:CaF2 nanoparticles by the coprecipitation method.
Fig. 3. Photograph of (a) Er, Y:CaF2 and (b) Er:CaF2 transparent ceramics hot pressed at 800 ◦ C 120 min (2 mm thickness).
ments. This phenomena can be attributed to the slight effect of trivalent yttrium ions on alkaline fluorite nanoparticles growth rate through surface charge modification [21,22]. Each substitution of Ca2+ by Y3+ in CaF2 matrix materials requires one more F− for charge compensation, and the introduction of these extra F− ions may induce transient electric dipoles with their negative poles outward in the fluoride nanoparticle surface. These dipoles substantially hinder the diffusion of the F− ions from the solution to the nanoparticles owing to the charge repulsion, consequently retarding the growth of CaF2 nanoparticles, as observed in Er:CaF2 and Er, Yb:CaF2 systems [17,23]. Fig. 3 shows the photograph of polished Er:CaF2 and Er, Y:CaF2 transparent ceramics with 2 mm thickness. The ceramic samples shows almost the same appearance. From the figure, it is evident that the fabricated ceramic samples are transparent and the letters under the ceramic samples can be recognized clearly. Optical transmissivity is the main parameter for evaluating the optical properties of obtained fluorite ceramics. The optical transmissivity spectra of the obtained fluorite transparent ceramics (2 mm thickness) in the range of 250–2500 nm is shown in Fig. 4. The optical transmissivity spectra of ceramic samples have similar slopes and their transmissivity increase with increasing wavelength from 250 to 2500 nm, and the optical transmissivity of Er, Y:CaF2 is much higher than Er:CaF2 transparent ceramic. It seems that codoping Y3+ ions have the positive effect on the improving optical transmission in the fabricating process. The optical transmissivity of Er, Y:CaF2 ceramic sample was reached about 90% in the infrared wavelength range. Compared with the theoretical transmittance of CaF2 polycrystal ceramic [17], there were also have several percent optical losses for the fabricated ceramic samples.
In addition, the optical transmissivity of ceramic samples decrease significantly at visible wavelength range, which was closely related to light scattering within the ceramic microstructure. It is well known that the optical transmissivity of polycrystalline materials is sensitive to several microstructure factors such as grain size, second phase, grain boundaries and residual pores [4,24–26]. Fig. 5 shows fracture surface of the fluorite transparent ceramics hot-pressed at 800 ◦ C in a vacuum environment. The fracture mode of the single Er doped ceramic sample was mainly intergranular (Fig. 5(a)), and the ceramic sample codoped with Y3+ was mainly transgranular (Fig. 5(b)). It can be seen that the fabricated fluorite ceramic samples exhibited a nearly pore-free microstructure. It is worth to notice that the grain size in Fig. 5(a) is much larger than the grain size in Fig. 5(b). The average grain size of Er:CaF2 and Er, Y:CaF2 transparent ceramics were about 0.73 m and 0.57 m, respectively. Similar phenomena have also observed in the Er:CaF2 and Er, Yb:CaF2 transparent ceramics [17,23]. In the CaF2 matrix material, when Ca2+ was replaced by the trivalent yttrium ions (Y3+ ), the valence mismatch is compensated by interstitial fluoride ions (Fi − ), and the additional Fi − retards the diffusion of cations, which decreased grain boundaries mobility of CaF2 ceramics during sintering process and led to the reduction of grain size. So, the transparent ceramics codoped with Y3+ have a smaller grain size. The
Fig. 4. Optical transmissivity spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics hot pressed at 800 ◦ C for 120 min (2 mm thickness).
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Fig. 5. Fracture surfaces of (a) Er:CaF2 and (b) Er, Y:CaF2 transparent ceramics; the pictures in the right side showing the EDS spectra of the corresponding ceramic sample.
Fig. 6. Absorption spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics with the same Er doing concentration (2 mm thickness).
optical transmissivity (T) of a single phase polycrystalline ceramic can be represented by [27]: T = (1 − Rs)exp[−( pore + gb )L]
(2)
where Rs describes the reflection losses at the two sample surfaces at normal incidence, which is independent of the ceramic thickness, pore is the residual pores scattering, gb is the grain boundaries scattering, and L is the ceramic samples thickness. Due to the nearly pore free microstructure of the fabricated fluoride transparent ceramics, we here assume that pore can be set equal to zero. Transparent ceramics consist of grains and grain boundaries. If there is a difference in optical properties between grains and grain boundaries, the interfaces between them will become the light scat-
Fig. 7. Upconversion emission spectrum of Er3+ doped in Er:CaF2 and Er, Y:CaF2 transparent ceramics from 450 nm to 720 nm under 980 nm excitation.
tering defects. Lyberis et al. [4] have identified two types of defects localized at grain boundaries within fluorite ceramic samples fabricated form chemical synthesized nanoparticles, and the oxygenized grain boundaries was considered as the major scattering resource. It can be believed that a similar phenomena also occurred in fluorite ceramic samples fabricated in this article, due to we used a similar experimental procedure with Lyberis et al. described in the literature [4]. So the mainly scattering source in fluorite transparent ceramics fabricated in this work was also considered as oxygenized grain boundaries. According to Apetz and Bruggen analysis based on Raleigh–Gans–Debye theory, where only considering light scattering at the grain boundaries, the optical transmissivity (T) of polycrystalline ceramic can be represented by [27]: T = (1 − Rs)exp[−(32 rn2 L)/2 ]
(3)
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Fig. 8. Simplified energy levels for Er3+ ions in fluorite transparent ceramic under 980 nm excitation.
where r is the grain size, n is the refractive index difference between oxygenized grain boundaries and the calcium fluoride matrix material, is the wavelength of the incident light in the medium. As represented from Eq. (3), the optical transmissivity strongly depends on the grain size (r), wavelength (), and sample thickness (L). Here, due to the low light scattering, several microstructure factors such as residual pores, impurities and grain shape were neglected, as identified by Kim et al. in the literature [25]. The equation explain precisely why the better transmittance results in the case of Er,Y:CaF2 ceramics in comparison to Er:CaF2 . This also explain why the optical transmissivity of the ceramic samples decreases significantly with the decrease of wavelength. The EDS spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramic samples are also shown in Fig. 5. The spectrum of the EDS exhibited Ca, F, Y and Er signals. This result confirms that Er and Y ions are incorporated into the fluorite matrix lattice. The absorption spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics with 2 mm thickness are presented in Fig. 6, with all the absorption bands labeled with their corresponding energy levels. The absorption peaks can be seen near 378, 530, 650, 980, and 1530 nm, corresponding to the transitions from the 4 I15/2 ground state to the excited states of Er3+ ions, respectively. Y3+ codoping leads to a series much stronger absorption peaks. The absorption peak position of Er, Y:CaF2 transparent ceramic were not significantly shift by codoping Y3+ ion. The absorption band in the wavelength range from 1450 to 1600 nm corresponding to 4I 4 3+ ions, and the absorption band is 15/2 → I13/2 transition of Er centered at about 1525 nm. There are two absorption peaks located at 1507.4 nm and 1527.3 nm, and the relative intensity between these two peaks are changed with Y3+ codoping into the fluorite transparent ceramic. So, the absorption spectra of Er3+ ions in CaF2 transparent ceramic can be slightly altered by codoping Y3+ ions, which likely results from the formation of new [Er3+ –Y3+ ] clusters. Fig. 7 shows the upconversion spectra of fabricated fluorite transparent ceramic samples under 980 nm excitation in the range of 420–720 nm. There are two main upconversion emission bands in the spectra, green emission (525 and 550 nm) and red emission (660 nm), which are corresponding to (2 H11/2 , 4 S3/2 ) → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions of Er3+ ions, respectively. The emission intensity of two ceramic samples has been normalized on the red 660 nm emission band. The upconversion mechanism of Er3+
Fig. 9. Infrared emission spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics around 1530 nm under 980 nm excitation.
ions in Er:CaF2 transparent ceramic has been discussed in previous literature [16], and it is including ground state absorption (GSA) and excited state absorption (ESA). A simplified energy levels and the possible upconversion mechanisms for Er3+ ions in fluorite transparent ceramic under 980 nm excitation is shown in Fig. 8. In addition, the cross-relaxation (CR) processes between two nearby Er3+ ions cannot be ignored at higher doping concentration. The CR processes between (4 F7/2 → 4 F9/2 ) and (4 I15/2 → 4 I13/2 ) transitions of Er3+ ions at high doping concentration play an important role on depopulating the 4 S3/2 /2 H11/2 levels, which induces the intense red upconversion emissions with relative weaken green upconversion emission. A significant increase of the green upconversion emission in the Er, Y:CaF2 transparent ceramic may be due to the presence of Y3+ ions in fluorite transparent ceramics increase the average distance of Er3+ ions, which hinders the creation of large erbium clusters and circumvention of the cross relaxation process (4 F7/2 → 4 F9/2 ) and (4 I15/2 → 4 I13/2 ) transitions in Er clusters for the same doping concentration. Therefore, 4 F7/2 levels mostly decay nonradiatively to the lower 4 S3/2 and 2 H11/2 levels, and an overall increase in green emission intensity was observed. The infrared emission spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics at around 1530 nm under 980 nm excitation are
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shown in Fig. 9. The emission intensity of two ceramic samples has been normalized on the 1530 nm emission intensity. The infrared emission band range from 1450 to 1600 nm, corresponding to the 4I 4 3+ ions in the fluorite matrix lattice. 13/2 → I15/2 transition of Er Besides the main peak located at 1530 nm, there are also presence several emission peaks, located at around 1507, 1543, 1569 and 1585 nm, respectively. The 4 I13/2 level was populated owing to the non-radiative from 4 I11/2 state and the cross-relaxation (CR) process between Er3+ ions, as shown in Fig. 8. The infrared emission of Er3+ ions have been greatly discussed in literatures [11,16]. The width of emission band was became norrow with Y3+ ions codoping. 4. Conclusions Er and Er/Y ions codoped fluorite nanoparticles were synthesized by a coprecipitation method. The obtained fluorite nanoparticles were single fluorite phase with an average diameter about 30 nm. Fluorite transparent ceramics were fabricated by hotpressed sintering method at 800 ◦ C. For a 2 mm thickness sample, the transmittance of Er, Y:CaF2 transparent ceramic in the infrared range reached about 90%. The microstructure of the ceramic sample were almost densification without residual pores, Er and Y ions were incorporated into the fluorite lattice site and confirmed by EDS spectra. Intense visible upconversion and infrared emission under 980 nm excitation were observed. With Y3+ ions codoping, the green emission intensity much stronger than the red emission band, the characterizations of infrared emission spectra was also modulated. The broad and weakly structured infrared emission bands were caused by their rich multisite structure. Acknowledgements This work was financially supported by the State Key Program of National Natural Science Foundation of China (No. 51432007). The authors would like to express their gratitude to Prof. Chao Liu and Dr. Zhiyong Zhao from State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology for their upconversion and near-infrared luminescence measurement support. References [1] P.P. Sorokin, M.J. Stevenson, Stimulated infrared emission from trivalent uranium, Phys. Rev. Lett. 5 (1960) 557–559. [2] S.E. Hatch, W.F. Parsons, R.J. Weagley, Hot-pressed polycrystalline CaF2 :Dy2+ laser, Appl. Phys. Lett. 5 (1964) 153–154. [3] J.L. Doualan, P. Camy, R. Moncorgé, E. Daran, M. Couchaud, B. Ferrand, Latest developments of bulks crystals and thin films of rare-earth doped CaF2 for laser applications, J. Fluor. Chem. 128 (2007) 459–464. [4] A. Lyberis, G. Patriarche, P. Gredin, D. Vivien, M. Mortier, Origin of light scattering in ytterbium doped calcium fluoride transparent ceramic for high power lasers, J. Eur. Ceram. Soc. 31 (2011) 1619–1630. [5] T.T. Basiev, M.E. Doroshenko, P.P. Fedorov, V.A. Konyushkin, S. Kuznetsov, V.V. Osiko, M.Sh. Akchurin, Efficient laser based on CaF2 -SrF2 -YbF3 nanoceramics, Opt. Lett. 33 (2008) 521–523.
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Characterizations of a hot-pressed Er and Y codoped CaF2 transparent ceramic Weiwei Li a , Zuodong Liu a , Zhiwei Zhou a , Jinghong Song b , Bingchu Mei a,∗ , Liangbi Su c a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Center of Materials Research and Analysis, Wuhan University of Technology, Wuhan 430070, China Key Laboratory of Transparent and Opto-Functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China b c
a r t i c l e
i n f o
Article history: Received 23 November 2015 Received in revised form 30 January 2016 Accepted 2 February 2016 Available online xxx Keywords: Calcium fluoride Erbium Yttrium Transparent ceramic Luminescence
a b s t r a c t In this work, Er:CaF2 transparent ceramic codoped with non-active ions (Y3+ ions) was fabricated by hotpressed (HP) sintering method. Er and Er/Y codoped CaF2 nanoparticles were synthesized by a simple coprecipitation method, the phase composition and the shape of obtained nanoparticles were analysed by X-ray diffraction patterns and transmission electron microscopy, respectively. Fluorite transparent ceramics were fabricated by HP sintering method at a temperature of 800 ◦ C. For 2 mm thickness ceramic samples, the optical transmission of Er, Y:CaF2 transparent ceramic in the near-infrared range reached about 90%, and the optical transmission of Er, Y:CaF2 transparent ceramic was much higher than Er:CaF2 ceramic sample. Microstructures were characterized using SEM analysis, the two ceramic samples exhibited nearly pore-free microstructure and the average grain sizes were less than 1.0 m. The absorption, upconversion and infrared emission spectras of ceramic samples were also measured and discussed. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As one of the optical material, calcium fluoride (CaF2 ) compounds doped with trivalent lanthanide ions has gained much attention as solid-state laser and other luminescence materials for many years. Such as, U:CaF2 single crystal was the second material used as laser host material by Sorokin and Stevenson in 1960 [1], and the first laser polycrystalline ceramic was fabricated based on Dy-doped CaF2 compounds by Hatch et al. in 1964 [2]. The CaF2 compounds seems to be an ideal medium for preparation of luminescent materials because of their high transparency in a wide wavelength region (from vacuum UV to 9 m), low linear and nonlinear refractive index, as well as low phonon energy [3,4]. The optical spectral properties of CaF2 doped with trivalent rare-earth ions have been attracted more and more attention. So far, the fabrication and optical properties of rare earth ions doped CaF2 transparent ceramics have been investigated extensively. According to the available literature, two synthesis routes were utilized to produce fluorite polycrystalline ceramics. In 2008, Basiev et al. [5] reported the fabrication of CaF2 –SrF2 –YbF3 fluoride laser ceramics by hot-forming methods based on deformation
∗ Corresponding author. E-mail address: [email protected] (B. Mei).
of the single crystals at elevated temperature. This method has been successfully utilized for CaF2 :Yb, CaF2 :Er, SrF2 :Nd and other fluoride laser ceramics [6–8]. Another approach, fluorite transparent ceramics were fabricated from chemical synthesized nanoparticles after vacuum sintering. Yb:CaF2 transparent ceramics were fabricated from the chemical synthesized nanoparticles after a vacuum sintering at 900 ◦ C and a hot-isostatically pressed (HIP) post-treatment under an argon pressure of 160 MPa by Aubry et al. in 2009 [9]. Up to now, many studies on CaF2 transparent ceramics fabricated from the nanoparticles have been reported [10–12]. In recent years, the upconversion and infrared luminescence properties of erbium (Er3+ ) ions in different host materials have been intensively investigated [13–15], and Er:CaF2 transparent ceramics were also fabricated [16,17]. It was reported that Er ions doped materials tend to form clusters at high doping concentration. In our earlier work [16], we had shown that strong cross-relaxation processes between Er3+ ions happens at high doing level, and favoring the red upconversion emission. In order to suppress the cross-relaxation processes between Er3+ ions, it is necessary to avoid the large clustering formation and enlarge the separation of the active ions (Er3+ ions) in the matrix materials. Zhao et al. systematically studied the effect of YF3 addition on the formation and optical properties of Er3+ ions doped fluoride nanocrystals in glassceramics [18]. It was found that codoping non-active ions (Y3+ ions) into fluoride nanocrystals can dilute the effective concentration of
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Er3+ ions, and suppress some detrimental energy transfer process between Er3+ ions. For the same reason, a Nd, Y codoped CaF2 single crystal has been developed by Wang et al. [19], and codoping with Y3+ was considered to improve the detrimental concentration quenching effect that results from the clustering of the rare earth ions, in addition to modulate the spectra properties of Nd3+ ions. So the buffer ions M3+ (such as Gd3+ , Y3+ and Sc3+ ) were codoped into CaF2 matrix lattice to break the rare earth ions clusters. In this work, to overcome the strong cross-relaxation process of Er3+ ions at high doping concentration, Y3+ ions were selected as buffer ions to be added into the Er:CaF2 transparent ceramics. The results presented here, to our knowledge, were the first experimental demonstration of trivalent lanthanide ions spectroscopic properties modulated by buffer ions in fluorite transparent ceramics. In this paper, we synthesized the Er and Er/Y codoped fluorite nanoparticles by a coprecipitation method. High quality fluorite transparent ceramics were fabricated by hot pressed sintering method. The optical and luminescence characteristics of the fabricated transparent ceramic samples were also investigated. 2. Experimental 2.1. Fluorite nanoparticles synthesis Commercially available chemical reagents including calcium nitrate tetrahydrate (Ca(NO3 )2 ·4H2 O, AR), erbium trinitrate pentahydrate (ErN3 O9 ·5H2 O, 99.9%), yttrium nitrate hexahydrate (YN3 O9 ·6H2 O, 99.9%), and potassium fluoride dihydrate (KF·2H2 O, AR) were used as raw materials to synthesize fluorite nanoparticles by a coprecipitation method. Water was distilled. In the typical synthesizing procedure, solutions containing the cationic precursors (Ca2+ , Er3+ and Y3+ , totally 0.5 mol/L) and KF·2H2 O (F− , 1.2 mol/L) were separately made in the same volume distilled water. These two solutions were stirred separately for 5 min, and then the cationic solutions were added dropwise to the anion solutions. The mixture ration of the chemical reagents was determined by the following chemical reaction:
Fig. 1. XRD patterns of as-synthesized fluorite nanoparticles.
were with a Cu K␣ radiation ( = 1.5405 Å) in the 2 range from 20◦ to 80◦ . Transmission electron microscopy (TEM; JEM-2100F, JEOL, Tokyo, Japan) was employed to determine the morphology of the obtained fluorite nanoparticles. The average nanoparticle and grain sizes were measured by the linear intercept method, taking at least 300 nanoparticles into account. The optical transmission and absorption spectra were measured by a UV–vis-NIR spectrophotometer (Lambda 750S, PerkinElmer, Waltham, USA) in the wavelength range of 200–2500 nm. Microstructure of the ceramics were characterized by a Field-emission Scanning Electron Microscope (FE-SEM; ULTRA PLUS-43-13, Zeiss, Oberkochen, Germany). The visible upconversion and near-infrared emission spectras were excited under 980 nm laser from a Ti:Sapphire solid-state laser (3900S, Spectra Physics, Mountain View, USA), and detected using a photomultiplier tube detector and an In GaAs detector, respectively. All the spectral measurements were performed at room temperature.
(1 − x − y)Ca(NO3 )2 + xEr(NO3 )3 + yY(NO3 )3 + (2 + x + y)KF → Ca1 − x − y Erx Y y F2 + x + y ↓ + (2 + x + y)KNO3 where x (x = 2 mol%) is Er doping level and y (y = 0, 3 mol%) is Y doping level. The mixed solutions were stirred for 30 min and then stayed for 24 h at room temperature. The opaque aqueous solutions were centrifuged at 11,000 rpm for 20 min and washed two times with distilled water. The obtained nanoparticles were oven dried at 80 ◦ C for 24 h. 2.2. Ceramics fabrication The synthesized fluorite nanoparticles were filled into a graphite mold with a cavity diameter of 16 mm directly without any specific pretreatments, and then hot pressed at 800 ◦ C in a vacuum environment for 120 min. Fluorite transparent ceramics were fabricated using a hot pressed sintering method, and details of the fabrication procedure have been described in the previous works [11,16]. After hot pressed at 800 ◦ C for 120 min, the ceramic samples were ground and polished for subsequent evaluation. In order to protect the ceramic sample from carbon contamination, graphite die was covered by a embedding high melting point of alumina powder layer. 2.3. Measurements Structures of the obtained fluorite nanoparticles were characterized using powder X-ray diffraction (XRD) measurements
3. Results and discussion The XRD patterns of the as-synthesized fluorite nanoparticles are shown in Fig. 1. All the obtained XRD diffraction peaks of the nanoparticles are indexed into CaF2 cubic phase with the typical fluorite structure (space group: Fm3m) and no extra diffraction peaks are detected. It indicates that the single fluorite cubic crystallinity of the as-synthesized nanoparticles with no detectable secondary phase or other impurities. The average nanoparticle size D are calculated using the Scherrer equation [20]: D = /[(2) × cos()]
(1)
where is the wavelength of Cu K␣ radiation ( = 1.5405 Å), 2 is the peak position and (2) is the corrected peak-width at half-maximum of the diffraction peaks. The calculated average nanoparticle size of Er:CaF2 and Er, Y:CaF2 nanoparticles are about 27.4 nm and 23.1 nm, respectively. Fig. 2 shows the TEM photographs of as-synthesized fluoride nanoparticles. The TEM results reveal that the fluorite nanoparticles were slight agglomerated. For both two nanoparticle samples, some nanoparticles were much bigger with nanocube shapes, and others were much smaller with nanosphere shapes. The nanoparticle size presented in Fig. 2(b) was much smaller than presented in Fig. 2(a). The measured average nanoparticles sizes of Er:CaF2 and Er, Y:CaF2 nanoparticles were about 25.6 nm and 20.2 nm, respectively. This result was close to that values calculated from the XRD patterns, showing a good agreement between these two measure-
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Fig. 2. TEM micrographs of the as-synthesized (a) Er:CaF2 and (b) Er, Y:CaF2 nanoparticles by the coprecipitation method.
Fig. 3. Photograph of (a) Er, Y:CaF2 and (b) Er:CaF2 transparent ceramics hot pressed at 800 ◦ C 120 min (2 mm thickness).
ments. This phenomena can be attributed to the slight effect of trivalent yttrium ions on alkaline fluorite nanoparticles growth rate through surface charge modification [21,22]. Each substitution of Ca2+ by Y3+ in CaF2 matrix materials requires one more F− for charge compensation, and the introduction of these extra F− ions may induce transient electric dipoles with their negative poles outward in the fluoride nanoparticle surface. These dipoles substantially hinder the diffusion of the F− ions from the solution to the nanoparticles owing to the charge repulsion, consequently retarding the growth of CaF2 nanoparticles, as observed in Er:CaF2 and Er, Yb:CaF2 systems [17,23]. Fig. 3 shows the photograph of polished Er:CaF2 and Er, Y:CaF2 transparent ceramics with 2 mm thickness. The ceramic samples shows almost the same appearance. From the figure, it is evident that the fabricated ceramic samples are transparent and the letters under the ceramic samples can be recognized clearly. Optical transmissivity is the main parameter for evaluating the optical properties of obtained fluorite ceramics. The optical transmissivity spectra of the obtained fluorite transparent ceramics (2 mm thickness) in the range of 250–2500 nm is shown in Fig. 4. The optical transmissivity spectra of ceramic samples have similar slopes and their transmissivity increase with increasing wavelength from 250 to 2500 nm, and the optical transmissivity of Er, Y:CaF2 is much higher than Er:CaF2 transparent ceramic. It seems that codoping Y3+ ions have the positive effect on the improving optical transmission in the fabricating process. The optical transmissivity of Er, Y:CaF2 ceramic sample was reached about 90% in the infrared wavelength range. Compared with the theoretical transmittance of CaF2 polycrystal ceramic [17], there were also have several percent optical losses for the fabricated ceramic samples.
In addition, the optical transmissivity of ceramic samples decrease significantly at visible wavelength range, which was closely related to light scattering within the ceramic microstructure. It is well known that the optical transmissivity of polycrystalline materials is sensitive to several microstructure factors such as grain size, second phase, grain boundaries and residual pores [4,24–26]. Fig. 5 shows fracture surface of the fluorite transparent ceramics hot-pressed at 800 ◦ C in a vacuum environment. The fracture mode of the single Er doped ceramic sample was mainly intergranular (Fig. 5(a)), and the ceramic sample codoped with Y3+ was mainly transgranular (Fig. 5(b)). It can be seen that the fabricated fluorite ceramic samples exhibited a nearly pore-free microstructure. It is worth to notice that the grain size in Fig. 5(a) is much larger than the grain size in Fig. 5(b). The average grain size of Er:CaF2 and Er, Y:CaF2 transparent ceramics were about 0.73 m and 0.57 m, respectively. Similar phenomena have also observed in the Er:CaF2 and Er, Yb:CaF2 transparent ceramics [17,23]. In the CaF2 matrix material, when Ca2+ was replaced by the trivalent yttrium ions (Y3+ ), the valence mismatch is compensated by interstitial fluoride ions (Fi − ), and the additional Fi − retards the diffusion of cations, which decreased grain boundaries mobility of CaF2 ceramics during sintering process and led to the reduction of grain size. So, the transparent ceramics codoped with Y3+ have a smaller grain size. The
Fig. 4. Optical transmissivity spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics hot pressed at 800 ◦ C for 120 min (2 mm thickness).
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Fig. 5. Fracture surfaces of (a) Er:CaF2 and (b) Er, Y:CaF2 transparent ceramics; the pictures in the right side showing the EDS spectra of the corresponding ceramic sample.
Fig. 6. Absorption spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics with the same Er doing concentration (2 mm thickness).
optical transmissivity (T) of a single phase polycrystalline ceramic can be represented by [27]: T = (1 − Rs)exp[−( pore + gb )L]
(2)
where Rs describes the reflection losses at the two sample surfaces at normal incidence, which is independent of the ceramic thickness, pore is the residual pores scattering, gb is the grain boundaries scattering, and L is the ceramic samples thickness. Due to the nearly pore free microstructure of the fabricated fluoride transparent ceramics, we here assume that pore can be set equal to zero. Transparent ceramics consist of grains and grain boundaries. If there is a difference in optical properties between grains and grain boundaries, the interfaces between them will become the light scat-
Fig. 7. Upconversion emission spectrum of Er3+ doped in Er:CaF2 and Er, Y:CaF2 transparent ceramics from 450 nm to 720 nm under 980 nm excitation.
tering defects. Lyberis et al. [4] have identified two types of defects localized at grain boundaries within fluorite ceramic samples fabricated form chemical synthesized nanoparticles, and the oxygenized grain boundaries was considered as the major scattering resource. It can be believed that a similar phenomena also occurred in fluorite ceramic samples fabricated in this article, due to we used a similar experimental procedure with Lyberis et al. described in the literature [4]. So the mainly scattering source in fluorite transparent ceramics fabricated in this work was also considered as oxygenized grain boundaries. According to Apetz and Bruggen analysis based on Raleigh–Gans–Debye theory, where only considering light scattering at the grain boundaries, the optical transmissivity (T) of polycrystalline ceramic can be represented by [27]: T = (1 − Rs)exp[−(32 rn2 L)/2 ]
(3)
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Fig. 8. Simplified energy levels for Er3+ ions in fluorite transparent ceramic under 980 nm excitation.
where r is the grain size, n is the refractive index difference between oxygenized grain boundaries and the calcium fluoride matrix material, is the wavelength of the incident light in the medium. As represented from Eq. (3), the optical transmissivity strongly depends on the grain size (r), wavelength (), and sample thickness (L). Here, due to the low light scattering, several microstructure factors such as residual pores, impurities and grain shape were neglected, as identified by Kim et al. in the literature [25]. The equation explain precisely why the better transmittance results in the case of Er,Y:CaF2 ceramics in comparison to Er:CaF2 . This also explain why the optical transmissivity of the ceramic samples decreases significantly with the decrease of wavelength. The EDS spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramic samples are also shown in Fig. 5. The spectrum of the EDS exhibited Ca, F, Y and Er signals. This result confirms that Er and Y ions are incorporated into the fluorite matrix lattice. The absorption spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics with 2 mm thickness are presented in Fig. 6, with all the absorption bands labeled with their corresponding energy levels. The absorption peaks can be seen near 378, 530, 650, 980, and 1530 nm, corresponding to the transitions from the 4 I15/2 ground state to the excited states of Er3+ ions, respectively. Y3+ codoping leads to a series much stronger absorption peaks. The absorption peak position of Er, Y:CaF2 transparent ceramic were not significantly shift by codoping Y3+ ion. The absorption band in the wavelength range from 1450 to 1600 nm corresponding to 4I 4 3+ ions, and the absorption band is 15/2 → I13/2 transition of Er centered at about 1525 nm. There are two absorption peaks located at 1507.4 nm and 1527.3 nm, and the relative intensity between these two peaks are changed with Y3+ codoping into the fluorite transparent ceramic. So, the absorption spectra of Er3+ ions in CaF2 transparent ceramic can be slightly altered by codoping Y3+ ions, which likely results from the formation of new [Er3+ –Y3+ ] clusters. Fig. 7 shows the upconversion spectra of fabricated fluorite transparent ceramic samples under 980 nm excitation in the range of 420–720 nm. There are two main upconversion emission bands in the spectra, green emission (525 and 550 nm) and red emission (660 nm), which are corresponding to (2 H11/2 , 4 S3/2 ) → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions of Er3+ ions, respectively. The emission intensity of two ceramic samples has been normalized on the red 660 nm emission band. The upconversion mechanism of Er3+
Fig. 9. Infrared emission spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics around 1530 nm under 980 nm excitation.
ions in Er:CaF2 transparent ceramic has been discussed in previous literature [16], and it is including ground state absorption (GSA) and excited state absorption (ESA). A simplified energy levels and the possible upconversion mechanisms for Er3+ ions in fluorite transparent ceramic under 980 nm excitation is shown in Fig. 8. In addition, the cross-relaxation (CR) processes between two nearby Er3+ ions cannot be ignored at higher doping concentration. The CR processes between (4 F7/2 → 4 F9/2 ) and (4 I15/2 → 4 I13/2 ) transitions of Er3+ ions at high doping concentration play an important role on depopulating the 4 S3/2 /2 H11/2 levels, which induces the intense red upconversion emissions with relative weaken green upconversion emission. A significant increase of the green upconversion emission in the Er, Y:CaF2 transparent ceramic may be due to the presence of Y3+ ions in fluorite transparent ceramics increase the average distance of Er3+ ions, which hinders the creation of large erbium clusters and circumvention of the cross relaxation process (4 F7/2 → 4 F9/2 ) and (4 I15/2 → 4 I13/2 ) transitions in Er clusters for the same doping concentration. Therefore, 4 F7/2 levels mostly decay nonradiatively to the lower 4 S3/2 and 2 H11/2 levels, and an overall increase in green emission intensity was observed. The infrared emission spectra of Er:CaF2 and Er, Y:CaF2 transparent ceramics at around 1530 nm under 980 nm excitation are
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shown in Fig. 9. The emission intensity of two ceramic samples has been normalized on the 1530 nm emission intensity. The infrared emission band range from 1450 to 1600 nm, corresponding to the 4I 4 3+ ions in the fluorite matrix lattice. 13/2 → I15/2 transition of Er Besides the main peak located at 1530 nm, there are also presence several emission peaks, located at around 1507, 1543, 1569 and 1585 nm, respectively. The 4 I13/2 level was populated owing to the non-radiative from 4 I11/2 state and the cross-relaxation (CR) process between Er3+ ions, as shown in Fig. 8. The infrared emission of Er3+ ions have been greatly discussed in literatures [11,16]. The width of emission band was became norrow with Y3+ ions codoping. 4. Conclusions Er and Er/Y ions codoped fluorite nanoparticles were synthesized by a coprecipitation method. The obtained fluorite nanoparticles were single fluorite phase with an average diameter about 30 nm. Fluorite transparent ceramics were fabricated by hotpressed sintering method at 800 ◦ C. For a 2 mm thickness sample, the transmittance of Er, Y:CaF2 transparent ceramic in the infrared range reached about 90%. The microstructure of the ceramic sample were almost densification without residual pores, Er and Y ions were incorporated into the fluorite lattice site and confirmed by EDS spectra. Intense visible upconversion and infrared emission under 980 nm excitation were observed. With Y3+ ions codoping, the green emission intensity much stronger than the red emission band, the characterizations of infrared emission spectra was also modulated. The broad and weakly structured infrared emission bands were caused by their rich multisite structure. Acknowledgements This work was financially supported by the State Key Program of National Natural Science Foundation of China (No. 51432007). The authors would like to express their gratitude to Prof. Chao Liu and Dr. Zhiyong Zhao from State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology for their upconversion and near-infrared luminescence measurement support. References [1] P.P. Sorokin, M.J. Stevenson, Stimulated infrared emission from trivalent uranium, Phys. Rev. Lett. 5 (1960) 557–559. [2] S.E. Hatch, W.F. Parsons, R.J. Weagley, Hot-pressed polycrystalline CaF2 :Dy2+ laser, Appl. Phys. Lett. 5 (1964) 153–154. [3] J.L. Doualan, P. Camy, R. Moncorgé, E. Daran, M. Couchaud, B. Ferrand, Latest developments of bulks crystals and thin films of rare-earth doped CaF2 for laser applications, J. Fluor. Chem. 128 (2007) 459–464. [4] A. Lyberis, G. Patriarche, P. Gredin, D. Vivien, M. Mortier, Origin of light scattering in ytterbium doped calcium fluoride transparent ceramic for high power lasers, J. Eur. Ceram. Soc. 31 (2011) 1619–1630. [5] T.T. Basiev, M.E. Doroshenko, P.P. Fedorov, V.A. Konyushkin, S. Kuznetsov, V.V. Osiko, M.Sh. Akchurin, Efficient laser based on CaF2 -SrF2 -YbF3 nanoceramics, Opt. Lett. 33 (2008) 521–523.
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Please cite this article in press as: W. Li, et al., Characterizations of a hot-pressed Er and Y codoped CaF2 transparent ceramic, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.006