Influence of sintering conditions on the microstructure and optical properties of Eu: CaF2 transparent ceramic

Influence of sintering conditions on the microstructure and optical properties of Eu: CaF2 transparent ceramic

Materials Research Bulletin 95 (2017) 138–145 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.c...

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Materials Research Bulletin 95 (2017) 138–145

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage:

Influence of sintering conditions on the microstructure and optical properties of Eu: CaF2 transparent ceramic Feng Xionga , Jinghong Songb , Weiwei Lia , Bingchu Meia,* , Liangbi Suc a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China b Center of materials Research and Analysis, Wuhan University of Technology, Wuhan 430070, China c Key Laboratory of Transparent and Opto-Functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China


Article history: Received 2 May 2017 Received in revised form 30 June 2017 Accepted 18 July 2017 Available online 23 July 2017 Keywords: A. Ceramic A. Fluorides B. Microstructure B. Optical properties


Nanopowders of the 5 at.% Eu doped CaF2 have been synthesized by co-precipitated method. X-ray diffraction (XRD) indicated that the samples corresponds to the cubic CaF2 phase and scanning electron microscopy (SEM) showed that the morphology of nanopowders were approximately spherical. The Eu: CaF2 transparent ceramics were fabricated by hot-pressing (HP) method at different P-T conditions in a vacuum environment. The fracture surface, the relative density, average grain size and shrinkage displacements were also studied to analysis the influence of sintering conditions on Eu: CaF2 ceramic. In comparison with other samples, the proper sintering temperature and pressure was 600  C/30 MPa. The transmittance of ceramic was up to 79.7% and 88.85% at the wavelength of 760 nm and 2400 nm. Furthermore, the emission spectra of Eu: CaF2 transparent ceramics under 393.5 nm excitation was also discussed. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Scintillator is a kind of optical material which is different from common laser materials. With the different radiation intensity, the luminescence of scintillation is discontinuous [1]. Ever since the invention of ZnS and CaWO4 scintillator in the late 19th century, scintillator materials have been attracted increasing interests due to their unique optical properties. In the 1980s, the GE company has successfully developed the Eu:(Y,Gd)2O3 transparent ceramics, and were used as medical X-CT detector [2]. After that, there was a growing number of literature on scintillator: in Zych’s [3] research, the fluorescence and scintillation properties of Ce: YAG transparent ceramic have been studied; According to Yanagida’s [4] study, Pr: LuAG transparent ceramic scintillator was first successfully synthesized using sintering process and the basic properties of ceramic were also discussed. As a group of innovative functional material, scintillator can radiate visible or ultraviolet light after absorbing a high-energy particles and it has been applied extensively in medical diagnosis [5], industrial detection [6], radiation detection [7], high energy physics [8] and other areas.

* Corresponding author. E-mail address: [email protected] (B. Mei). 0025-5408/© 2017 Elsevier Ltd. All rights reserved.

Classically, the scintillator materials can be divided into three general type: glass, ceramic and single-crystal [9]. Compared with scintillation ceramics, growing large size single-crystals are more difficult ascribe to the long growth time, the great possibility of grain boundary formation, and the cracks after the cooling process [10]. Whereas, ceramic, which has many good properties such as low production cost, simple but mature production technology and good processing performance, is a promising material used to produce scintillator and has drawn great attention in related fields [11]. With regard to optical ceramics, previous studies have mainly focused on host materials such as, YAG [12–14], Y2O3 [15], ZnO [16], etc. However, as one of the emerging luminescence host materials, fluoride compounds ceramic, particularly CaF2, is known for many superb performances. Unlike other oxide matrix material, it has a higher refractive index [17], lower phonon energy [18], higher transmittance and broader transmittance range [19]. Rare earth ions are renowned for its unique optical properties, which can be used as doping elements. For instance, Nd: CaF2 [19], Er: CaF2 [20,21] and Eu: CaF2. Eu doped ceramic materials have wavelength conversion function, and show characteristic ion transitions in the visible region which correspond to f-f energy level [22]. Besides, the Eu: CaF2 is known to have high light yield (24,000 photons/ Mev) [23] in scintillator materials. Therefore, Eu: CaF2 has been

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widely used in scintillation counter and other industrial applications. In order to satisfy the applicable conditions of industrial application, the emphasis is to increase the light yield and the transmittance of materials. Thus, the temperature control during the sintering process has a great impact on the manufacture of high performance ceramic materials, such as density, porosity and transmittance. Many prior studies have put attention on the optical performance of Eu: CaF2 transparent ceramics and Eu: CaF2 nanoparticles [24,25]. However, there are few researches about the impact of sintering conditions, especially sintering temperature on the microstructure and optical properties of the Eu: CaF2 transparent ceramics. In this paper, 5 at.% Eu: CaF2 nanoparticles were synthesized by co-precipitation method, and the transparent ceramics were fabricated by hot pressed method at different temperatures (400, 500, 600, 700  C). Then, the performances of ceramics were investigated by microstructure test and spectrum measurement to find the proper sintering process. 2. Experimental 2.1. Nanoparticle synthesis Nanoparticles of Eu: CaF2 were synthesized by co-precipitation method using commercial chemicals: calcium nitrate tetrahydrate (99.9%), potassium fluoride dihydrate (99.9%) and europium nitrate hexahydrate. The water in this experiment was fully distilled. There was no further purification of any of the chemicals used in this study. The formation of Eu: CaF2 nanoparticles and molar ratio of Ca2+ and Eu3+ were designed according to the following chemical reaction: (1-X) Ca(NO3)2 + X + (2 + X) KNO3

Eu(NO3)3 + (2 + X)

KF ! Ca(1-X)EuXF(2+X)#

In this equation, the X is the concentration (X = 1, 3, 5, 7 at.%) of Eu doping content. For the synthesis of bulk Eu: CaF2 nanoparticle, 0.064 mol KF2H2O and 3.4 mmol Eu(NO3)36H2O were dissolved in 64 ml deionized water. In addition, 0.141 mol KF2H2O were also added into 141 ml deionized water. The amount of KF was appropriately excessive and the F were used as charge compensation ions. Subsequently, the anionic solution (KF, 1.0 mol/L) was added to the cationic solution (Eu(NO3)3 and Ca (NO3)2, 1.0 mol/L) at a speed of 7 ml/min. The mixed solution was stirred 30 min and stayed for 3 h at room temperature. Then, the opaque aqueous solutions were centrifuged at 11,000 rpm for 10 min. The precipitate were centrifuged and washed with deionized water several times to removal K+ and NO3. Finally, the precipitate were oven dried at 85  C for 24 h and lightly crushed in an agate mortar.


2.3. Characterizations The XRD measurements of as-synthesized powders and ceramics sintered at 600  C were carried out using a X-Ray diffractometer (D/Max-RB, Rigaku, Tokyo, Japan) with Cu-Ka radiation source (l = 0.154056 nm) at 40KV and 30 mA in the scan range 20 –80 . In order to observed the morphology of the samples, the Transmission Electron Microscopy images were taken on TEM (JEM-2100F, JEOL, Japan). The microstructures of the fracture surface of samples were observed by Field-emission Scanning electron microscope (FE-SEM, Zeiss ULTRA PLUS-43-13, German) and the Nanomeasure software was used to measure the mean size of the ceramic from the SEM pictures [21]. The density of the sintered ceramics was obtained by Archimedes method. The transmittance of Eu: CaF2 transparent ceramics were measured by a spectrophotometer (Lambda 750, Perkin Elmer, Fremont, CA). The emission spectrum was recorded by fluorescence spectrophotometer (FLS920; Edinburgh Instruments, Edinburgh, UK) and the 393.5 nm laser diode was used as the pumping source to excite the electrons. The emitted photons were collected by NIR-PMT (nearinfrared photomultiplier tube) detector. Every measurement in this study was performed at room temperature. 3. Results and discussion The XRD patterns of the 5 at.% Eu-doped CaF2 nanoparticles and the ceramic sintered at 600  C/30 MPa are shown in Fig. 1. It can be noticed that all the diffraction peaks between 20 and 80 of the nanoparticles and ceramic are corresponding to the cubic CaF2 phase, which in comparison with standard XRD patterns (PDF 650535), there are no any second phase appeared. The average size of nanoparticles is related to the peak-width at half-maimum (D(2u)) and the position of the peak (2u). The nanoparticle size can be calculated by using the Debye-Scherrer equation: L = 0.89l/[D(2u)  cos(u)]


Eliminating the influence of the instrumental broadening, the average size of nanoparticle is about 22 nm. Comparing with the Eu: CaF2 nanoparticles, the full width at half maximum (FWHM) of the transparent ceramic is much smaller, in addition, the intensity of diffraction peaks get stronger and the (200) peak becomes more evident, indicating that the degree of crystallization improves during sintering process.

2.2. Ceramics fabrication A series of CaF2 transparent ceramics doped Eu ions were fabricated by hot-press method. The as-synthesized nanoparticles were introduced into a graphite mold without any treatment or binder. Then, the samples were put into the vacuum furnace. The powders were sintered at 400  C, 500  C, 600  C, 700  C in a vacuum environment under uniaxial pressures of 0 MPa, 10 MPa, 30 MPa and 50 MPa. The soaking time was 120 minThe sintering temperature can be gradually increased in the whole sintering progress, the heating rate were 5  C/min and the sintering time can be controlled by program. Fig. 1. X-ray diffraction patterns of Eu: CaF2 ceramic and nanopowder (a. 600  C/ 30 MPa HP-sintered transparent ceramic; b. Nanoparticle; c. PDF 65-0535).


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Fig. 2. (a) SEM micrographs of Eu: CaF2 nanoparticles, (b)TEM image of Eu: CaF2 nanoparticles, (c) HRTEM image of nanoparticles, (d) the corresponding SAED pattern.

Fig. 2(a) and (b) shows SEM micrographs (100 K) and TEM image of CaF2 nanoparticles with 5 at.% Eu-doped concentration. It can be clearly seen that the Eu: CaF2 powders are approximately spherical in shape and have no obvious agglomeration. The nanomeasure shows that the size of particles range from approximately 19 nm to 26 nm, and the results about particles diameter are consistent with XRD theoretical calculation. Fig. 2(c) and (d) exhibit the high-resolution images and corresponding electron diffraction patterns of selected region. When the Eu3+ ions are incorporated into crystal lattice of CaF2, the line defects are caused by the interstitial F ions which are used as the charge compensation ions [26] and the difference of ionic radius between Eu3+ and Ca2+ are leading to the decrease of crystallinity. Therefore, the lattice distortion of the CaF2 matrix can be observed from the HRTEM images. Through statistical analysis of the SAED pattern (Fig. 2(d)) by software, the lattice spacing is about 3.17 Å, which is roughly corresponding to the spacing between (111) planes of cubic CaF2 (3.14 Å). Fig. 3 presents the photographs of the Eu:CaF2 ceramics sintered at 600  C with different pressures. As is shown in the picture, when the pressure is 0 MPa, only shrinkage deformation occurred in sintering process and the sample diameter decreased. Along with the rising of pressures, the transparency of ceramics increased at first (0–30 MPa) and kept stable finally (30–50 MPa). However, the cracks appeared in samples that sintered at 50 MPa, which mainly caused by the elastic after-effect of particles when the uniaxial pressure exceeds the maximum that may be sustained by specimen.

The SEM micrographs of the Eu: CaF2 ceramics (10 MPa and 30 MPa) are shown in Fig. 4. After the observation of the fracture surface microstructure, it can be clearly seen that residual pores exist in the microstructure of the sample sintered at 10 MPa, yet the ceramic at 30 MPa has pore-free structure. In addition, as the pressure reached 30 MPa, the grain size increased and the grain boundary turned clear. The comparison of Fig. 4(a) and (b) indicates that the high uniaxial pressure can increase sintering

Fig. 3. Photographs of sintered Eu: CaF2 ceramics at various uniaxial pressures.

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Fig. 4. SEM micrograph of Eu: CaF2 ceramics sintered at 600  C with different uniaxial pressures: a. 10 MPa, b. 30 MPa.

Fig. 5. Photographs of the Eu (5 at.%): CaF2 transparent ceramics sintered at 30 MPa with different temperatures under: room light (a) and UV light (b).

stress, promote the particle flow and grain growth [27]. In the sintering process, the structure of nanoparticles will occur deformation and shrinkage due to the grain contracts, which can cause the local high-stress concentration in the surface shell, leading to the pores close and the difficulties of atomic diffusion. Thus, the loading pressure can play active roles in diminishing the high-stress concentration and accelerating the sintering of ceramic [28,29]. Apart from the pressure, the effect of temperature is also important. The images of the polished Eu: CaF2 transparent ceramic samples at different temperatures are shown in Fig. 5. The diameter of all samples in the pictures is typically 16.0 mm and the thickness are 2.0 mm. The Fig. 5(a) shows that the samples under room light, and the words on the papers can be clearly identified through all ceramics except the far left one. The picture of ceramics under UV irradiation is shown in Fig. 5(b), and the samples show red PL emission. All the ceramic were mirror-polished. From the photographs of samples, it can be found that the ceramic sintered at 600  C has a highest optical transmittance among all samples and other samples shows poor quality due to the optical losses. The optical losses can be divided into two types, one is light reflection on two material faces, the other is sample thickness and material itself [30]. After taking into account these factors, the transmittance can be obtained by the following equation [31]: T = I/I0 = (1-Rs)exp(-gd)

Where d, n and g describe the sample thickness, refractive index of CaF2 matrix, and total scattering coefficient, individually. In Malitson IH’s work, the refractive index of calcium fluoride was calculated by the minimum-deviation method from UV to infrared, and the refractive index values range from 1.30541 (220 nm) to 1.42168 (2400 nm) [32]. So, the value of transmittance can reach 92.75% to 94.11% on the basis of ignores the scattering effects. Fig. 6 shows the in-line transmittance spectra of samples sintered at 30 MPa with different temperatures. The ceramic sintered at 400  C has extremely low transmittance and characterized as a horizontal line (no mention) in the spectra. The inset image exhibits the transmittance spectra in visible light region. As can be seen from


The I and I0 indicates the intensity of transmitted light and incident light, respectively. The Rs represents the reflection losses at the two material surfaces at normal incidence. With: Rs = 2R'/(1 + R')


R' = [(n  1)(n + 1)1]2


Fig. 6. In-line transmittance spectra of mirror-polished Eu: CaF2 ceramic sintered at different temperatures for 2 h under vacuum environment (2.0 mm thickness).


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the spectra, the transmittance curves are increased and then decreased along with the increasing of temperature in the visible region, the sample sintered at 600  C is reaching a maximal value of 88.85% in the 2400 nm and nearly 80% over 780 nm. Compared with the theoretical value, the transmittance of sample has about 5% loss in the infrared range which is ascribed to the light scattering of ceramic. As described in Eq. (2), the actual transmittance is restricted by d and g, the absorption coefficient g can be subsumed into g op, g0 and g in [30]. The g0 is the absorption term characteristic of electron transition and g op is the absorption scattering term because of optical anisotropy. According to the XRD results, the CaF2 sample has cubic crystal structure, it means the g op can be neglected. However, the g in is caused by the defects in the ceramic, having a crucial influence on transmittance. As shown in Fig. 2(c), the rare-earth ions doped into matrix can result in the line defects and the decreased of crystallinity, that can cause the losses of transmittance though grain boundary refraction. Furthermore, the second phase such as oxygenized grain boundaries and the pores may exist in the CaF2 materials (shown in Fig. 7) which can also be the source about light scattering [31]. The influence of temperatures on transmittance will be discussed with the microstructure evolution. In order to prepare high quality ceramic, the defects in structure should be reduced as much as possible and this purpose can be achieved by the regulation of microstructure. Fig. 7 shows the fracture surface of Eu: CaF2 ceramics obtained at different temperatures (30 MPa). It can be clearly seen that the grain size of ceramic sintered at 400  C is so small that the combination between nanoparticles becomes loose (Fig. 7(a)). The grains are not fully developed and many pores can be observed in the image. In addition, the intergranular fractures have become a major part in fracture behavior due to the poor compaction of ceramic. Along with the raise of sintering temperature (Fig. 7(b)), the densification of the ceramic is mainly proceeded through the grains growth and

the pores are connected because of the grain boundary migration. Meanwhile, the majority of the pores are expelled by bulk diffusion effect and only a few pores are found in the picture, contributing to the greatly improve of the microstructure performance. Fig. 7(c) shows the fracture surface of ceramic sintered at 600  C, and no conspicuous pores can be observed from the picture. The tiny amounts of enclosed pores are mutually isolated and the structure becomes more dense. Thus, both transgranular and intergranular fractures can be found in SEM images (Fig. 7(b)) and (c)). The microstructure of oversintering sample can be observed from Fig. 7(d). The grain-boundary damages, which is mainly related to the oxygenized grain boundaries phase [30], and the abnormal grain growth appeared in the fracture surface image. It is generally known that a large number of pores can exist in the grain boundary at the initial stage of sintering. With the temperatures rise, the grain of ceramic grown up and boundary migration effect can cause the pores agglomerates into channels, leading to the removal of pores. However, when the temperature comes to 700  C, the abnormal grain growth, which has great interfacial curvature, will pass through the defects and continue to grow. Besides, the rates of grain growth are more quickly than pores mobility owing to the excessive temperature and the pores are wrapped into the grain, resulting in many intragranular pores. As a result, the intragranular pores play an active role in stress concentration, inducing the transgranular fracture which is dominant for damage mechanisms. As is well known, the high transmittance and well structure performances are two necessary conditions for optical application. Through analysis of the microstructure and optical properties at different sintering stage, it found that various P-T conditions have significant effects on the ceramic structure evolution and transmittance. As explained in transmittance, the grain boundary pores (400, 500  C and low pressure), intragranular pores (700  C) and secondary phase can lead to the decrease of transmittance. When the temperature raise to 600  C, the ceramic has the high transmittance, fully dense and pore-free structure. Thus, the

Fig. 7. SEM micrographs of the fracture surface of the Eu (5 at.%): CaF2 ceramics sintered at 30 MPa with different temperatures: (a) 400  C, (b) 500  C, (c) 600  C and (d) 700  C.

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Fig. 8. Effect of sintering temperature on the average grain size and relative density of 5 at.% Eu: CaF2 ceramics.

Fig. 9. Shrinkage curve at 5  C/min between room-temperature and 700  C.

fracture surface and In-line transmittance of ceramics indicates that the suitable temperature and pressure for the Eu: CaF2 ceramic is 600  C/30 MPa. The relative density and grain sizes can be used to measure the densification process and microstructure evolution of the materials. Fig. 8 shows the relative density and the average grain size of different samples. With the temperature increasing from 400  C to 700  C, the grain size has increased from 136 nm to 955 nm. During sintering process, the interparticle contact area enlarged through neck growth, which results in the average grain size varies directly as temperature. As for density, it can be noticed that the relative


density increased from 90.2% to 99.1% with the sintering temperature rising from 400 to 600  C. At the initial stage, the relative density of ceramic sintered at 400  C is far below the theory value (3.382 g cm3) as a result of the small grain size and great amount of the grain boundary pores. Then, the densification process accomplished by grain boundary migration and surface diffusion leading to the decrease in defects and increase in density. Nevertheless, the relative density declines instead of increasing at the final stage, and this phenomenon is caused by the existence of intragranular pores at the overheating temperature. The relationship between shrinkage displacement, which is recorded from the numeroscope during the whole sintering process, and temperature are shown in Fig. 9. It can be found that the displacements increased with the temperature development, and the shrinkage rate reaches a maximum between 350  C and 500  C. It is well known that the main driving force for sintering process is the reduction of surface energy [33], and the proper temperature are the necessary condition of ceramic preparation. Through the comparison between Fig. 5(a) (400  C) and Fig. 5(b) (500  C), the shrinkage displacements in this two stages can be attributed to the diffusion and densification effects. Therefore, to further optimize the sintering process and reduce defects, the heating rate between 350  C and 500  C should be reduced appropriately and the soaking time should be increased. The appearance of the polished Eu:CaF2 transparent ceramic samples with different ions doped concentrations sintered at 600  C are shown in Fig. 10. It can be clearly seen that all the obtained samples are highly transparent. The Fig. 10(b) represents the red PL emission of samples and the red color of the specimens is greatly increased with the rising of Eu ions doped concentration. Fig. 11 shows the transmittance spectra of ceramics and the inset enlarges the spectral region of 300–800 nm. The spectra indicate that the 5% Eu doped ceramic shows the highest optical transmittance among all samples in the visible region and the absorption peaks of samples is related with Eu3+ ions transitions [9]. As explained in Fig. 5, the 1 at.% and 3 at.% Eu doped ceramics exhibits relatively low transmittance (below 70%) in the short wavelength, which mainly due to the size of pores in samples is similar to the wavelength. The emission spectra of the Eu: CaF2 transparent ceramic with different Eu doped concentrations are shown in Fig. 12. It can be clearly seen that the emission bands are greatly increased with the rising of Eu3+ ions doped concentration. Many strong emission peaks, for instance the orange (590 nm) and red (690 nm) luminescence, are noticed from the image and the peaks are corresponding to the characteristic peaks of Eu3+ ions. Besides the main emission peaks, there are many relative low intensity emission peaks are observed and all emissions are caused by the consecutive transitions from 5D0 level to 4FJ ground state multiplets. The emissions at 590 nm and 576 nm are corresponding

Fig. 10. Photographs of the Eu (1, 3, 5, 7 at.%): CaF2 transparent ceramics under. Room light (a) and UV light (b).


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pressure can promote the evolution of microstructures. The fracture surface revealed that the influence of temperature and pressure in sintering process: in the low temperature and pressure, the grains are not fully developed and the pores were also observed; however, the extra-high temperature can cause the oxygenized phase on grain boundary and abnormal grain growth, which can be the major problem affecting the transmittance. In order to prepare high quality ceramic, the heating rate between 350  C and 500  C should be reduced whereas the soaking time needed to be expended appropriately. As for ceramic sintered at 600  C/30 MPa, the transmittance of visible and near-infrared region can reach about 79.7% and 88.9%, respectively. Compared with pure CaF2, the characteristic peaks of Eu3+ ions appeared in the Eu: CaF2 emission spectrum and all the emission peaks were caused by the transitions of 5D0 excited level to 7FJ ground level. Acknowledgement

Fig. 11. In-line transmittance spectra of mirror-polished Eu: CaF2 ceramics with different Eu ions doped concentrations (2.0 mm thickness).

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51432007). References

Fig. 12. Emission spectrum of 5% Eu doped CaF2 ceramic (column fitting image).

to the transitions of 5D0 excited level to 7F0 and 7F1 level, these transitions are independent of the host matrix. In addition, the peaks at 613.5 nm, 627 nm and 689.5 nm are due to the 5D0 ! 7F2, 5 D0 ! 7F3 and 5D0 ! 7F4 transitions, respectively [9]. It is well known that the charge compensation of rare-earth ions (Re3+) doped fluoride matrix is achieved by the interstitial F. The differences in substitution position of interstitial F can bring about the differences in site symmetries and configurations of Eu: CaF2 lattice, which also results in the variety of spectroscopic properties [34]. 4. Conclusions To sum up, the Eu:CaF2 ceramics with different P-T sintering conditions were obtained from co-precipitation nanoparticles by vacuum hot-pressing method and the samples have been analyzed for microstructure and optical properties. The samples were single cubic phase and the average diameter of nanoparticles was about 22 nm, the morphology of nanopowders were uniform distributed, which can be used as raw materials for ceramic-making. Through analyzing pressure data indicating the proper high uniaxial

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