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High concentration Ce3+ doped BaF2 transparent ceramics
Journal Pre-proof High Concentration Ce
3+
doped BaF2 transparent ceramics
Xianqiang Chen, Yiquan Wu PII:
S0925-8388(19)34321-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.153075
Reference:
JALCOM 153075
To appear in:
Journal of Alloys and Compounds
Received Date: 27 June 2019 Revised Date:
14 November 2019
Accepted Date: 17 November 2019
3+ Please cite this article as: X. Chen, Y. Wu, High Concentration Ce doped BaF2 transparent ceramics, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153075. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section
YQ Wu designed and directed the project; XQ Chen performed the experiments; YQ Wu and XQ Chen analyzed the results; YQ Wu and XQ Chen wrote the article.
High Concentration Ce3+ Doped BaF2 Transparent Ceramics Xianqiang Chen, Yiquan Wu* Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA
Abstract: BaF2 transparent ceramics with high Ce3+ dopant concentration were prepared by a high vacuum hot pressing technique. The highest Ce3+ doping concentration obtained was 50 at.%. For the first time, the BaCeF5 transparent ceramic was reported. As seen under Scanning Electronic Microscopy, all ceramic samples had an average grain size of <50 µm and were highly dense. The transmittances of 1, 10, and 50at.% Ce3+ doped BaF2 transparent ceramics were 77.8, 73.2, and 79.5% at a wavelength of 550nm. At the Ce3+ emission peak of 354nm, the transmittances were 51.0, 66.1, and 31.5% for the same samples. The density increases linearly with increasing Ce3+ doping concentration and was measured to be 4.86, 5.02, and 5.62 g/cm3 for 1, 10, and 50at.% Ce3+ doped ceramics.
Keywords: BaCeF5; Barium Fluoride; Transparent Ceramics; Optical Properties
Introduction: Barium fluoride (BaF2) crystals are known as a fast, dense (4.8 g/cm3), and radiationresistant scintillator material. [1-5] It has a considerably large effective atomic number (Zeff = 52.7), making it effective for γ-ray detection. [6] In the 1970s, Farukhi and Swinehart first reported scintillation by BaF2 crystal due to auger-free luminescence (AFL) and self-trapped excitation (STE), which appears around 190-220 nm (fast, τ≈0.8ns) and 310 nm (slow, τ ≈620ns), respectively. [6, 7] One serious drawback to BaF2 single crystals is their perfect
cleavage, which causes issues in BaF2 optical components. [6] Additionally, The light yield (LY) of the fast luminescence component is, not very high (1500 ph/MeV) compared with a conventional γ-ray scintillator (Lu2-xYxSiO5:Ce; 27,000 ph/MeV) used in PET (Positron Emission Tomography). [8] Two approaches have been proposed to overcoming this problem: doping BaF2 with an impurity capable of suppressing the slow component (for example, La3+ and Sc3+) [9-11] and the addition of an activator (usually Ce3+) that converts excitonic emission to faster activator emission [12, 13]. In fluoride compounds, electron transitions of a Ce3+ ion can cause a rapid luminescence, for instance a deexcitation time of 30–50 ns within a UV range at a wavelength less than 300 nm to produce a sufficient light yield [9]. In the past, some work has been done on Ce3+ doped BaF2 crystal. For example, Tailor et al. reported Ce3+ doped BaF2 single crystals have a peak scintillation emission at 365 nm with a decay constant of approximately 50 ns [1]; Dorenbos et al. reported the effects of Ce3+ doping on the scintillation properties of BaF2 single crystals [14]; Visser et al. studied the scintillation properties and decay time of Ce3+ doped BaF2 crystals with the quenching of slower BaF2 emission [2]. It is worth pointing out that optical ceramics have a number of advantages over single crystals. In particular, they have high mechanical strength, thermal stability, radiation resistant, and are free from cleavage [15-18]. Optical fluoride ceramics have now been known for half a century and have been the subject of extensive studies. [19-23] In recent years, attempts have been made to improve the kinetic and other characteristics of barium fluoride based scintillators by fabricating BaF2 ceramics [3, 5-7]. To the best of our knowledge, only a few studies have been reported on fabrication and properties of Ce3+ doped BaF2 ceramics. In 2008, Batygov et al reported a Ba0.99Ce0.01F2.01 solid-solution ceramic prepared by the hot-pressing method [3]; In 2010, Demidenko et al. compared the scintillation parameters of BaF2 and BaF2:Ce3+ ceramics [5]; In 2012, Zhai et
al. prepared Ce3+:BaF2 transparent ceramics by the hot pressing method using co-precipitation nanoparticles with Ce3+ concentration lower than 0.5% [24]. In 2016 and 2017, Luo et al. reported luminescence and scintillation properties of vacuum sintered BaF2:Ce transparent ceramics, and the effect of sintering temperature and soaking time on transparent ceramic transparency and fluorescence properties were also studied [25,26]. Most recently, Nd:BaF2 transparent ceramics have been developed through a one-step vacuum sintering method by current authors [27]. However, there are no reports on high Ce3+ concentration doping BaF2 transparent ceramics and the study on the influences of different Ce3+ concentrations on the optical properties of BaF2 transparent ceramics. In the present study, high concentration (up to 50at.%) Ce3+ doped BaF2 transparent ceramics were fabricated via a vacuum hot pressing (VHP) from homogeneous Ce:BaF2 powders synthesized through a co-precipitation route. It was determined by XRD that the sintered Ce:BaF2 ceramics were composed of the cubic phase. The microstructures were studied using SEM and were found to be well-consolidated and highly dense. The spectroscopic properties of the materials were characterized to investigate the transmittance and photoluminescence behaviors resulted by doping with Ce3+.
Experimental Methods Ce3+:BaF2 nanopowders were synthesized by a co-precipitation method using commercially-sourced products: barium nitrate [Ba(NO3)2, Alfa Aesar, 99.999%], potassium fluoride (KF, Alfa Aesar, 99.99%), and cerium nitrate hexahydrate [Ce(NO3)3·6H2O, Alfa Aesar, 99.99%]. The chemicals were used as-received with no further purification. Two solutions, one Ce3+ and Ba2+ the other F-,were prepared in deionized water, with salts added in the ratio needed to produce a final stoichiometry of CexBa1-xF2+x where x=0.01-0.5. The
entire cationic solution [Ba(NO3)2 and Ce(NO3)3·6H2O, 0.25 mol/L] was added drop wise into the anionic solution (KF, 1.0 mol/L) under continuous magnetic stirring (500 rpm) until the reaction completed. Fig.1 shows the sketch of the co-precipitation method. The formation of Ce:BaF2 nanoparticles and molar ratio of Ba2+ and Ce3+ were designed according to the assumption that particles were formed by the following chemical reaction: (1-x) Ba(NO3)2 + x Ce(NO3)3 + (2+x) KF → CexBa1-xF2+x↓+ (2+x) KNO3 The resulting precipitate suspension was subsequently aged for 24 h at room temperature, and the precipitates were washed 10 times with deionized water, via a pumping filtration method, in order to remove all traces of the starting solution and other impurities. The separated products were then oven dried at 80°C, ground in an agate mortar and pestle, and finally calcined at 450°C for 5h in flowing argon to prepare for sintering. The ceramic samples were prepared via VHP by loading the powders into a graphite die (diameter of 19 mm), with layers of graphite foil (thickness of 0.5 mm) used to separate the sample powders from the punches. The powders were consolidated at 900 °C for 2 h under a uniaxial pressure of 50MPa under a vacuum of 10−7 mbar (OTF-1200X-VHP4, MTI). In order to evaluate optical properties, the sintered samples were ground to a thickness of ~1.4mm and then mirror-polished on both faces using diamond slurry. The density of the sintered ceramics was determined by the Archimedes method using water as the immersion liquid. X-Ray Diffraction (XRD; Bruker D2- PHASER) using CuKα (λ = 0.154 nm) radiation at a voltage of 30 kV and current of 10 mA was used to identify the phases present in the synthesized powders and sintered ceramics. Measurement conditions of 0.03°2θ step size and 0.2s count time were employed over a measurement range of 20–80°2θ. The microstructure of the synthesized powders was observed by Transmission Electron Microscopy (TEM; FEI TECNAI F30); and microstructures of the sintered ceramics were
observed by Scanning Electron Microscopy (SEM; FEI Quanta 200F) at an accelerating voltage of 20 kV. The transmittance of the mirror polished ceramic pellets was measured using a spectrometer (Shimadzu; UV-2600) over the wavelength range of 240–800 nm. Photo-Luminescence (PL) emission spectra measurements were performed at room temperature by a PE-LS55 spectrofluorometer.
Results and Discussion Fig.2 shows the microstructure of 1.0at.% Ce3+ doped BaF2 powders calcined at 450°C for 5h in flowing argon. The representative high-magnified TEM image distinctly shows the nanocrystals, most of which are pseudospherical particles with a mean size of about 30 nm. Fig.3 presents a photo of the Ce:BaF2 transparent ceramics with 1, 10, and 50at.% Ce3+ doping concentration. These samples are visibly transparent, and the letters behind them can be observed clearly. All three samples were double polished to ~1.40mm thickness with a diameter of ~18.75mm. The microstructures of these VHP-consolidated Ce:BaF2 transparent ceramics are presented in Fig.4. It can be observed that the ceramics have an average grain size of <50 µm and are highly dense, likely due to the homogenous morphology of the raw powders and the effectiveness of the applied VHP technique. The average grain size was much smaller than the results reported in our previous study on Nd:BaF2 transparent ceramics by vacuum sintering method. [27] Also, the average grain size was smaller than the report on Ce:BaF2 ceramics by hot-pressing process at temperatures up to 1100°C and pressures up to 250 MPa. [3] Smaller grain size always corresponds to better mechanical properties. The densities were measured to be 4.86, 5.02, and 5.62 g/cm3 for 1, 10, and 50at.% Ce3+ doped transparent ceramics, respectively. Fig.5 displays that the density increases linearly with increasing Ce3+
doping concentration. For scintillators, higher density is beneficial in improving the stopping power of the detection rays. The energy E of detection ray through a scintillator pixel that impinges normal to the pixel is given by I/Io = exp{-µtot(E) ρt},
(1)
where ρ is the scintillator density, and t is the thickness (or height) of the scintillator pixel and µtot (E) is the total absorption coefficient. Fig.6 (a) presents the XRD pattern of the Ce:BaF2 ceramics consolidated via the VHP method. All the samples are single-phased, with the phase indexed to the fluorite structure (PDF#04-0452), and no impurity peaks can be detected in the XRD measurement. It can be observed that with increasing cerium content the XRD peaks shift to higher values (Fig. 6 (b)), corresponding to a decreasing unit cell parameter. The calculated lattice parameters are listed in Fig.7 (for the pure BaF2, the theoretical lattice parameter is 6.20nm). Following Vegard's law, the unit cell parameter decreases almost linearly with increasing Ce3+ concentration, which indicates the formation of homogeneous solid solutions. Such behavior can be attributed to the ionic radii difference between the Ba and Ce ions. The ionic radius of Ba2+ (0.135 nm) is larger than that of Ce3+ (0.103 nm), thus the incorporation of Ce3+ decreases the lattice parameter of the material. Both of scattering and absorption affect the performance of the ceramic scintillation material. The probability, P, of a scattering event is given by P = exp (-γs l)
(2)
Whereγs is the optical scattering length in the ceramic, and l is the path length. Optical scattering in a ceramic scintillator, the same as other transparent ceramics, can be resulted by birefringence at grain boundaries, second phases (particularly at grain boundaries), and residual pores entrapped within the grains.
Equally as important as scattering is the optical absorption coefficient of the ceramic scintillation material. Absorption can be classified into two categories: the first is intrinsic absorption that can occur at energy above the host material band edge, or as a result of absorptive transition in the activator itself. The second category is extrinsic absorption due to impurities or radiation induced color centers. The intrinsic absorption is difficult to reduce since they are due to the host or activator, which cannot be altered. However, emission at wavelengths near the band edge should be avoided since radiation can easily induce absorption centers at these wavelengths. Transitions involving f-f transitions within rare earth activators tend to demonstrate a low absorption due to the forbidden nature of these transitions. The extrinsic absorption can be minimized by ensuring high purity starting materials, and as well minimizing any contamination of impurities during calcining and sintering. Fig.8 displays the transmittance spectra of the Ce:BaF2 transparent ceramics with 1, 10, and 50at.% Ce3+ doping concentration. The thickness of the samples is about 1.4mm. The transmittance at the visible wavelength range of 400-750nm was approximately >60%; particularly, transmittances at 550nm were 77.8%, 73.2%, and 79.5% for 1, 10, and 50at.% Ce3+ doped BaF2 transparent ceramics respectively. This is the first report on the BaCeF5 (50at.% Ce3+ doping) transparent ceramic with high transparency at visible wavelengths. Tianqi Sheng et al. studied the luminescence properties of the BaCeF5-based nanopowders for WLED applications. [28, 29] In the ceramic form with a cubic structure, the BaCeF5 may also be applicable as a solid-state laser and scintillator host, etc. At the wavelength <400nm, the transmittance decreases for each sample, and the transmittances were 51.0%, 66.1%, and 31.5% for 1, 10, and 50at.% Ce3+ doped BaF2 samples at the Ce3+ emission peak of 354nm. A recent study on the Ce:BaF2 ceramics by the SPS technique reports the
transmittance at 350nm was <10%. [6] The absorption at around 250-330nm was attributed to the electron transitions from the ground state of Ce3+ (2F5/2 and 2F7/2) to the different crystal field splitting components of the excited 5d state of Ce3+. The theoretical band gap energy of BaF2 is about 6.6eV and the optical band gap energy of Ce:BaF2 can be determined according to Tauc’s equation, which relates the absorption coefficient (α) and the photon energy (hν) as: (αhν)n =β(hν-Eg)
(3)
Where, (hν) is the photon energy, Eg is the optical band gap energy, β is the Tauc’s constant or sometimes called the band-tail parameter. [30, 31] Fig.9(a) shows the photoluminescence emission spectra of the Ce:BaF2 transparent ceramics doped with 1, 10, and 50at.% Ce3+ ions. All the samples were excitated at 287nm, and the emission peak at 354nm is due to the transition from the lowest 5d band to the 4f ground state of Ce3+ ions. Fig.9(b) presents a photo of these samples under an UV lamp (peak at 254nm). The 10 and 50at.% Ce3+ doped samples emit mazarine blue light, the 1at.% Ce3+ doped sample emits wathet blue light.
Conclusions The high vacuum hot pressing (VHP) technique has been used to fabricate high Ce3+ concentration (up to 50at.%) doped BaF2 transparent ceramics using homogeneous Ce:BaF2 powders synthesized by a wet chemical co-precipitation route. BaCeF5 transparent ceramic with a transmission of 79.5% at 550nm was firstly reported. The XRD patterns indicated that the Ce:BaF2 ceramics were cubic in structure, and the unit cell parameter decreases almost linearly with increasing Ce3+ concentration. The SEM analysis displayed that all the Ce:BaF2 ceramics have an average grain size of <50 µm and are highly dense. The transmittances
were 51.0%, 66.1%, and 31.5% for 1, 10, and 50at.% Ce3+ doped BaF2 at the Ce3+ emission peak of 354nm, respectively. The emission peak at 354nm was observed by 287nm excitation in all samples due to the transition from the lowest 5d band to the 4f ground state of Ce3+ ions.
Acknowledgments The authors gratefully acknowledge the Office of Naval Research (N00014-17-1-2548) for funding this research.
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Figure Captions Fig.1 the sketch of the co-precipitation method; Fig.2 the microstructure of 1 at.% Ce3+ doped BaF2 powders calcined at 450°C for 5h in flowing argon; Fig.3 Photo of the Ce:BaF2 transparent ceramics with different Ce3+ doping concentration range from 1% to 50%; Fig.4 microstructures of VHP-consolidated Ce:BaF2 transparent ceramics with 1, 10 and 50at.% Ce3+ doping; Fig.5 the densities of 1, 10 and 50at.% Ce3+ doped Ce:BaF2 transparent ceramics; Fig.6 (a) the XRD pattern of the Ce:BaF2 ceramics consolidated via the VHP method；(b) the pattern at 25-26 2Theta degree of 1, 10 and 50at.% Ce3+ doped Ce:BaF2 transparent ceramics Fig.7 the calculated lattice parameters of 1, 10 and 50at.% Ce3+ doped Ce:BaF2 transparent ceramics; Fig.8 the transmittance spectra of the Ce:BaF2 transparent ceramics with 1, 10, and 50at.% Ce3+ doping concentration; Fig 9 (a) the photoluminescence emission spectra of the Ce:BaF2 transparent ceramics doped with 1, 10, and 50at.% Ce3+ ion; (b) photo of these samples under an UV lamp (peak at 254nm).
Highlights
1. High concentration Ce3+ (50 at.%) doped BaF2 powders were synthesized by a coprecipitation method 2. The transmittances of 1, 10, and 50at.% Ce3+ doped BaF2 transparent ceramics were 77.8, 73.2, and 79.5% at a wavelength of 550nm 3. The BaCeF5 transparent ceramic was reported for the first time
Declaration of Interest Statement
We wish to confirm that there are no known conflicts of interest associated with this publication entitled “High Concentration Ce3+ Doped BaF2 Transparent Materials” (ID: JALCOM-D-19-08506).
3+
doped BaF2 transparent ceramics
Xianqiang Chen, Yiquan Wu PII:
S0925-8388(19)34321-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.153075
Reference:
JALCOM 153075
To appear in:
Journal of Alloys and Compounds
Received Date: 27 June 2019 Revised Date:
14 November 2019
Accepted Date: 17 November 2019
3+ Please cite this article as: X. Chen, Y. Wu, High Concentration Ce doped BaF2 transparent ceramics, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153075. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section
YQ Wu designed and directed the project; XQ Chen performed the experiments; YQ Wu and XQ Chen analyzed the results; YQ Wu and XQ Chen wrote the article.
High Concentration Ce3+ Doped BaF2 Transparent Ceramics Xianqiang Chen, Yiquan Wu* Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA
Abstract: BaF2 transparent ceramics with high Ce3+ dopant concentration were prepared by a high vacuum hot pressing technique. The highest Ce3+ doping concentration obtained was 50 at.%. For the first time, the BaCeF5 transparent ceramic was reported. As seen under Scanning Electronic Microscopy, all ceramic samples had an average grain size of <50 µm and were highly dense. The transmittances of 1, 10, and 50at.% Ce3+ doped BaF2 transparent ceramics were 77.8, 73.2, and 79.5% at a wavelength of 550nm. At the Ce3+ emission peak of 354nm, the transmittances were 51.0, 66.1, and 31.5% for the same samples. The density increases linearly with increasing Ce3+ doping concentration and was measured to be 4.86, 5.02, and 5.62 g/cm3 for 1, 10, and 50at.% Ce3+ doped ceramics.
Keywords: BaCeF5; Barium Fluoride; Transparent Ceramics; Optical Properties
Introduction: Barium fluoride (BaF2) crystals are known as a fast, dense (4.8 g/cm3), and radiationresistant scintillator material. [1-5] It has a considerably large effective atomic number (Zeff = 52.7), making it effective for γ-ray detection. [6] In the 1970s, Farukhi and Swinehart first reported scintillation by BaF2 crystal due to auger-free luminescence (AFL) and self-trapped excitation (STE), which appears around 190-220 nm (fast, τ≈0.8ns) and 310 nm (slow, τ ≈620ns), respectively. [6, 7] One serious drawback to BaF2 single crystals is their perfect
cleavage, which causes issues in BaF2 optical components. [6] Additionally, The light yield (LY) of the fast luminescence component is, not very high (1500 ph/MeV) compared with a conventional γ-ray scintillator (Lu2-xYxSiO5:Ce; 27,000 ph/MeV) used in PET (Positron Emission Tomography). [8] Two approaches have been proposed to overcoming this problem: doping BaF2 with an impurity capable of suppressing the slow component (for example, La3+ and Sc3+) [9-11] and the addition of an activator (usually Ce3+) that converts excitonic emission to faster activator emission [12, 13]. In fluoride compounds, electron transitions of a Ce3+ ion can cause a rapid luminescence, for instance a deexcitation time of 30–50 ns within a UV range at a wavelength less than 300 nm to produce a sufficient light yield [9]. In the past, some work has been done on Ce3+ doped BaF2 crystal. For example, Tailor et al. reported Ce3+ doped BaF2 single crystals have a peak scintillation emission at 365 nm with a decay constant of approximately 50 ns [1]; Dorenbos et al. reported the effects of Ce3+ doping on the scintillation properties of BaF2 single crystals [14]; Visser et al. studied the scintillation properties and decay time of Ce3+ doped BaF2 crystals with the quenching of slower BaF2 emission [2]. It is worth pointing out that optical ceramics have a number of advantages over single crystals. In particular, they have high mechanical strength, thermal stability, radiation resistant, and are free from cleavage [15-18]. Optical fluoride ceramics have now been known for half a century and have been the subject of extensive studies. [19-23] In recent years, attempts have been made to improve the kinetic and other characteristics of barium fluoride based scintillators by fabricating BaF2 ceramics [3, 5-7]. To the best of our knowledge, only a few studies have been reported on fabrication and properties of Ce3+ doped BaF2 ceramics. In 2008, Batygov et al reported a Ba0.99Ce0.01F2.01 solid-solution ceramic prepared by the hot-pressing method [3]; In 2010, Demidenko et al. compared the scintillation parameters of BaF2 and BaF2:Ce3+ ceramics [5]; In 2012, Zhai et
al. prepared Ce3+:BaF2 transparent ceramics by the hot pressing method using co-precipitation nanoparticles with Ce3+ concentration lower than 0.5% [24]. In 2016 and 2017, Luo et al. reported luminescence and scintillation properties of vacuum sintered BaF2:Ce transparent ceramics, and the effect of sintering temperature and soaking time on transparent ceramic transparency and fluorescence properties were also studied [25,26]. Most recently, Nd:BaF2 transparent ceramics have been developed through a one-step vacuum sintering method by current authors [27]. However, there are no reports on high Ce3+ concentration doping BaF2 transparent ceramics and the study on the influences of different Ce3+ concentrations on the optical properties of BaF2 transparent ceramics. In the present study, high concentration (up to 50at.%) Ce3+ doped BaF2 transparent ceramics were fabricated via a vacuum hot pressing (VHP) from homogeneous Ce:BaF2 powders synthesized through a co-precipitation route. It was determined by XRD that the sintered Ce:BaF2 ceramics were composed of the cubic phase. The microstructures were studied using SEM and were found to be well-consolidated and highly dense. The spectroscopic properties of the materials were characterized to investigate the transmittance and photoluminescence behaviors resulted by doping with Ce3+.
Experimental Methods Ce3+:BaF2 nanopowders were synthesized by a co-precipitation method using commercially-sourced products: barium nitrate [Ba(NO3)2, Alfa Aesar, 99.999%], potassium fluoride (KF, Alfa Aesar, 99.99%), and cerium nitrate hexahydrate [Ce(NO3)3·6H2O, Alfa Aesar, 99.99%]. The chemicals were used as-received with no further purification. Two solutions, one Ce3+ and Ba2+ the other F-,were prepared in deionized water, with salts added in the ratio needed to produce a final stoichiometry of CexBa1-xF2+x where x=0.01-0.5. The
entire cationic solution [Ba(NO3)2 and Ce(NO3)3·6H2O, 0.25 mol/L] was added drop wise into the anionic solution (KF, 1.0 mol/L) under continuous magnetic stirring (500 rpm) until the reaction completed. Fig.1 shows the sketch of the co-precipitation method. The formation of Ce:BaF2 nanoparticles and molar ratio of Ba2+ and Ce3+ were designed according to the assumption that particles were formed by the following chemical reaction: (1-x) Ba(NO3)2 + x Ce(NO3)3 + (2+x) KF → CexBa1-xF2+x↓+ (2+x) KNO3 The resulting precipitate suspension was subsequently aged for 24 h at room temperature, and the precipitates were washed 10 times with deionized water, via a pumping filtration method, in order to remove all traces of the starting solution and other impurities. The separated products were then oven dried at 80°C, ground in an agate mortar and pestle, and finally calcined at 450°C for 5h in flowing argon to prepare for sintering. The ceramic samples were prepared via VHP by loading the powders into a graphite die (diameter of 19 mm), with layers of graphite foil (thickness of 0.5 mm) used to separate the sample powders from the punches. The powders were consolidated at 900 °C for 2 h under a uniaxial pressure of 50MPa under a vacuum of 10−7 mbar (OTF-1200X-VHP4, MTI). In order to evaluate optical properties, the sintered samples were ground to a thickness of ~1.4mm and then mirror-polished on both faces using diamond slurry. The density of the sintered ceramics was determined by the Archimedes method using water as the immersion liquid. X-Ray Diffraction (XRD; Bruker D2- PHASER) using CuKα (λ = 0.154 nm) radiation at a voltage of 30 kV and current of 10 mA was used to identify the phases present in the synthesized powders and sintered ceramics. Measurement conditions of 0.03°2θ step size and 0.2s count time were employed over a measurement range of 20–80°2θ. The microstructure of the synthesized powders was observed by Transmission Electron Microscopy (TEM; FEI TECNAI F30); and microstructures of the sintered ceramics were
observed by Scanning Electron Microscopy (SEM; FEI Quanta 200F) at an accelerating voltage of 20 kV. The transmittance of the mirror polished ceramic pellets was measured using a spectrometer (Shimadzu; UV-2600) over the wavelength range of 240–800 nm. Photo-Luminescence (PL) emission spectra measurements were performed at room temperature by a PE-LS55 spectrofluorometer.
Results and Discussion Fig.2 shows the microstructure of 1.0at.% Ce3+ doped BaF2 powders calcined at 450°C for 5h in flowing argon. The representative high-magnified TEM image distinctly shows the nanocrystals, most of which are pseudospherical particles with a mean size of about 30 nm. Fig.3 presents a photo of the Ce:BaF2 transparent ceramics with 1, 10, and 50at.% Ce3+ doping concentration. These samples are visibly transparent, and the letters behind them can be observed clearly. All three samples were double polished to ~1.40mm thickness with a diameter of ~18.75mm. The microstructures of these VHP-consolidated Ce:BaF2 transparent ceramics are presented in Fig.4. It can be observed that the ceramics have an average grain size of <50 µm and are highly dense, likely due to the homogenous morphology of the raw powders and the effectiveness of the applied VHP technique. The average grain size was much smaller than the results reported in our previous study on Nd:BaF2 transparent ceramics by vacuum sintering method. [27] Also, the average grain size was smaller than the report on Ce:BaF2 ceramics by hot-pressing process at temperatures up to 1100°C and pressures up to 250 MPa. [3] Smaller grain size always corresponds to better mechanical properties. The densities were measured to be 4.86, 5.02, and 5.62 g/cm3 for 1, 10, and 50at.% Ce3+ doped transparent ceramics, respectively. Fig.5 displays that the density increases linearly with increasing Ce3+
doping concentration. For scintillators, higher density is beneficial in improving the stopping power of the detection rays. The energy E of detection ray through a scintillator pixel that impinges normal to the pixel is given by I/Io = exp{-µtot(E) ρt},
(1)
where ρ is the scintillator density, and t is the thickness (or height) of the scintillator pixel and µtot (E) is the total absorption coefficient. Fig.6 (a) presents the XRD pattern of the Ce:BaF2 ceramics consolidated via the VHP method. All the samples are single-phased, with the phase indexed to the fluorite structure (PDF#04-0452), and no impurity peaks can be detected in the XRD measurement. It can be observed that with increasing cerium content the XRD peaks shift to higher values (Fig. 6 (b)), corresponding to a decreasing unit cell parameter. The calculated lattice parameters are listed in Fig.7 (for the pure BaF2, the theoretical lattice parameter is 6.20nm). Following Vegard's law, the unit cell parameter decreases almost linearly with increasing Ce3+ concentration, which indicates the formation of homogeneous solid solutions. Such behavior can be attributed to the ionic radii difference between the Ba and Ce ions. The ionic radius of Ba2+ (0.135 nm) is larger than that of Ce3+ (0.103 nm), thus the incorporation of Ce3+ decreases the lattice parameter of the material. Both of scattering and absorption affect the performance of the ceramic scintillation material. The probability, P, of a scattering event is given by P = exp (-γs l)
(2)
Whereγs is the optical scattering length in the ceramic, and l is the path length. Optical scattering in a ceramic scintillator, the same as other transparent ceramics, can be resulted by birefringence at grain boundaries, second phases (particularly at grain boundaries), and residual pores entrapped within the grains.
Equally as important as scattering is the optical absorption coefficient of the ceramic scintillation material. Absorption can be classified into two categories: the first is intrinsic absorption that can occur at energy above the host material band edge, or as a result of absorptive transition in the activator itself. The second category is extrinsic absorption due to impurities or radiation induced color centers. The intrinsic absorption is difficult to reduce since they are due to the host or activator, which cannot be altered. However, emission at wavelengths near the band edge should be avoided since radiation can easily induce absorption centers at these wavelengths. Transitions involving f-f transitions within rare earth activators tend to demonstrate a low absorption due to the forbidden nature of these transitions. The extrinsic absorption can be minimized by ensuring high purity starting materials, and as well minimizing any contamination of impurities during calcining and sintering. Fig.8 displays the transmittance spectra of the Ce:BaF2 transparent ceramics with 1, 10, and 50at.% Ce3+ doping concentration. The thickness of the samples is about 1.4mm. The transmittance at the visible wavelength range of 400-750nm was approximately >60%; particularly, transmittances at 550nm were 77.8%, 73.2%, and 79.5% for 1, 10, and 50at.% Ce3+ doped BaF2 transparent ceramics respectively. This is the first report on the BaCeF5 (50at.% Ce3+ doping) transparent ceramic with high transparency at visible wavelengths. Tianqi Sheng et al. studied the luminescence properties of the BaCeF5-based nanopowders for WLED applications. [28, 29] In the ceramic form with a cubic structure, the BaCeF5 may also be applicable as a solid-state laser and scintillator host, etc. At the wavelength <400nm, the transmittance decreases for each sample, and the transmittances were 51.0%, 66.1%, and 31.5% for 1, 10, and 50at.% Ce3+ doped BaF2 samples at the Ce3+ emission peak of 354nm. A recent study on the Ce:BaF2 ceramics by the SPS technique reports the
transmittance at 350nm was <10%. [6] The absorption at around 250-330nm was attributed to the electron transitions from the ground state of Ce3+ (2F5/2 and 2F7/2) to the different crystal field splitting components of the excited 5d state of Ce3+. The theoretical band gap energy of BaF2 is about 6.6eV and the optical band gap energy of Ce:BaF2 can be determined according to Tauc’s equation, which relates the absorption coefficient (α) and the photon energy (hν) as: (αhν)n =β(hν-Eg)
(3)
Where, (hν) is the photon energy, Eg is the optical band gap energy, β is the Tauc’s constant or sometimes called the band-tail parameter. [30, 31] Fig.9(a) shows the photoluminescence emission spectra of the Ce:BaF2 transparent ceramics doped with 1, 10, and 50at.% Ce3+ ions. All the samples were excitated at 287nm, and the emission peak at 354nm is due to the transition from the lowest 5d band to the 4f ground state of Ce3+ ions. Fig.9(b) presents a photo of these samples under an UV lamp (peak at 254nm). The 10 and 50at.% Ce3+ doped samples emit mazarine blue light, the 1at.% Ce3+ doped sample emits wathet blue light.
Conclusions The high vacuum hot pressing (VHP) technique has been used to fabricate high Ce3+ concentration (up to 50at.%) doped BaF2 transparent ceramics using homogeneous Ce:BaF2 powders synthesized by a wet chemical co-precipitation route. BaCeF5 transparent ceramic with a transmission of 79.5% at 550nm was firstly reported. The XRD patterns indicated that the Ce:BaF2 ceramics were cubic in structure, and the unit cell parameter decreases almost linearly with increasing Ce3+ concentration. The SEM analysis displayed that all the Ce:BaF2 ceramics have an average grain size of <50 µm and are highly dense. The transmittances
were 51.0%, 66.1%, and 31.5% for 1, 10, and 50at.% Ce3+ doped BaF2 at the Ce3+ emission peak of 354nm, respectively. The emission peak at 354nm was observed by 287nm excitation in all samples due to the transition from the lowest 5d band to the 4f ground state of Ce3+ ions.
Acknowledgments The authors gratefully acknowledge the Office of Naval Research (N00014-17-1-2548) for funding this research.
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Figure Captions Fig.1 the sketch of the co-precipitation method; Fig.2 the microstructure of 1 at.% Ce3+ doped BaF2 powders calcined at 450°C for 5h in flowing argon; Fig.3 Photo of the Ce:BaF2 transparent ceramics with different Ce3+ doping concentration range from 1% to 50%; Fig.4 microstructures of VHP-consolidated Ce:BaF2 transparent ceramics with 1, 10 and 50at.% Ce3+ doping; Fig.5 the densities of 1, 10 and 50at.% Ce3+ doped Ce:BaF2 transparent ceramics; Fig.6 (a) the XRD pattern of the Ce:BaF2 ceramics consolidated via the VHP method；(b) the pattern at 25-26 2Theta degree of 1, 10 and 50at.% Ce3+ doped Ce:BaF2 transparent ceramics Fig.7 the calculated lattice parameters of 1, 10 and 50at.% Ce3+ doped Ce:BaF2 transparent ceramics; Fig.8 the transmittance spectra of the Ce:BaF2 transparent ceramics with 1, 10, and 50at.% Ce3+ doping concentration; Fig 9 (a) the photoluminescence emission spectra of the Ce:BaF2 transparent ceramics doped with 1, 10, and 50at.% Ce3+ ion; (b) photo of these samples under an UV lamp (peak at 254nm).
Highlights
1. High concentration Ce3+ (50 at.%) doped BaF2 powders were synthesized by a coprecipitation method 2. The transmittances of 1, 10, and 50at.% Ce3+ doped BaF2 transparent ceramics were 77.8, 73.2, and 79.5% at a wavelength of 550nm 3. The BaCeF5 transparent ceramic was reported for the first time
Declaration of Interest Statement
We wish to confirm that there are no known conflicts of interest associated with this publication entitled “High Concentration Ce3+ Doped BaF2 Transparent Materials” (ID: JALCOM-D-19-08506).