Grain orientation control of cerium doped lutetium oxyorthosilicate ceramics in a strong magnetic field

Materials Letters 198 (2017) 85–88

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Grain orientation control of cerium doped lutetium oxyorthosilicate ceramics in a strong magnetic field Lingcong Fan a,b, Menghan Jiang a, Debao Lin a, Ying Shi a,⇑, Yiquan Wu b,⇑, Li Pi c, Jun Fang c, Jianjun Xie a, Fang Lei a, Lei Zhang a, Yunbo Zhong a, Jieyu Zhang a a b c

School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred 14802, NY, United States High Magnetic Field Laboratory, Chinese Academy of Sciences and University of Science and Technology of China, Hefei 230031, China

a r t i c l e

i n f o

Article history: Received 17 February 2017 Received in revised form 23 March 2017 Accepted 1 April 2017 Available online 4 April 2017 Keywords: Lutetium oxyorthosilicate Texture Strong magnetic field Optical property

a b s t r a c t Cerium-doped lutetium oxyorthosilicate (LSO:Ce) scintillator ceramics were fabricated by slip casting in a 9 T vertical magnetic field, followed by hot isostatic pressing (HIP) at 1650 °C under 150 MPa applied pressure. XRD and Electron Backscatter Diffraction (EBSD) mapping results reveal that the grains of the  LSO:Ce ceramics are aligned along the ½402 crystallographic direction. The degree of texturing (Lotgering factor, f) of the ceramics was determined to be 0.28. Measurements revealed the in-line transmittance of the textured LSO:Ce ceramics to be 4.7 times that of the untextured ceramics, attaining a transmittance of 6.6% at 420 nm (emission maximum). Ó 2017 Published by Elsevier B.V.

1. Introduction Cerium-doped lutetium oxyorthosilicate (Lu2SiO5:Ce, LSO:Ce) is a very important scintillator material in the family of fast, efficient, and high density scintillators, especially for nuclear medicine imaging [1]. Although large, high quality LSO:Ce single crystals are commercially available from a few industrial and research institutions [2], the price of LSO:Ce single crystals is still too high for application in Positron Emission Tomography (PET), high energy physics, and safety inspection applications, due to the high processing temperatures required to fabricate LSO:Ce single crystals (melting point >2050 °C). In addition, even using the most sophisticated processing techniques, a variation in Ce3+ concentration along the length of LSO:Ce crystal boules is seemingly unavoidable, occurring even in single crystals from prestigious companies like Saint-Gobain Inc [3]. A ceramic version of LSO:Ce could circumvent a number of the problems associated with single crystal growth. Previous reports have demonstrated that the light yield and decay time of LSO:Ce ceramics fabricated by pressureassisted sintering [4] and pressureless sintering [5] are comparable to that of LSO:Ce single crystals. Translucent LSO:Ce ceramics with a thickness of 1 mm could be used in X-ray Computer Tomography (CT) scanners [6]. However, to completely absorb c-ray radiation ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Shi), [email protected] (Y. Wu). http://dx.doi.org/10.1016/j.matlet.2017.04.003 0167-577X/Ó 2017 Published by Elsevier B.V.

(511 keV), which results from the annihilation of an electronpositron pair, LSO:Ce scintillators of 20 mm thickness or greater are required [7]. LSO possesses a monoclinic crystal structure, and thus is an optically anisotropic material. The maximum difference in refractive index between crystal directions is 0.028 [8], which is approximately 4 times greater than that of hexagonal a-alumina. Calculations using the Rayleigh–Gans–Debye (RGD) theory [9] predict the in-line transmittance of LSO ceramics with an average grain size of 200 nm and random grain orientation to be 1.6% at the wavelength of emission maximum (420 nm). Hence, the transparency of LSO:Ce ceramics must be improved to make expand its range of suitable applications. In LSO:Ce ceramics, grain alignment is likely to be the only practical approach to achieve enhanced transparency. Materials that possess non-cubic crystal structures also are magnetically anisotropic, i.e. the magnetic susceptibility of the material is different along different crystallographic directions. Because of this, ceramic particles dispersed in a slurry can be aligned along a certain direction by applying a strong magnetic field, which serves to overcome the viscous force of the slurry, as well as the thermal motion of the particles. Hence, green bodies with grains oriented along a certain crystallographic direction can be obtained by slip or gel casting in a strong magnetic field. Subsequent sintering treatment transforms the grain-oriented green body into a densified textured ceramic. In this letter, we demonstrate that the transparency of LSO:Ce ceramics can be

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improved through grain orientation control, achieved by slip casting in a strong magnetic field and subsequent post-Hot Isostatic Pressing (HIPing).

Fig. 1. XRD patterns of LSO:Ce ceramics casted by slip casting in a strong magnetic field (9 T), and without magnetic field (0 T), and subsequently sintered at 1650 °C.

2. Experimental procedure LSO:Ce powder with 0.5 at.% cerium concentration relative to lutetium was synthesized by a sol-gel method [10]. The LSO:Ce powder was then planetary ball milled for 20 h, using zirconia balls as the ball milling media and distilled water as dispersion medium. Post milling, the average particle size of the LSO:Ce powder was measured to be 0.2 mm. The LSO:Ce slurry was then slip casted in a 9 T vertical magnetic field at the High Magnetic Field Laboratory (HMFL), Hefei, China. The slip casting direction was parallel to magnetic field direction. A reference sample (untextured) was also fabricated by slip casting without an applied magnetic field. The slip casted LSO:Ce green compacts were subsequently sintered at 1650 °C for 4 h in air, then HIPed at 1650 °C under 150 MPa argon for 1 h. The HIPed LSO:Ce ceramics were then lapped and polished to a mirror finish with 0.25 mm diamond slurry. The phase composition and degree of crystalline texture of the ceramics were determined by analysis of X-ray Diffraction (XRD) patterns measured using an X-ray Diffractometer (Dnmax-2550, Rigaku, Japan) using Cu Ka radiation (0.15406 nm).The grain orientation distributions of the polished LSO:Ce ceramics were determined through Electron Backscatter Diffraction (EBSD) measurements made on a Scanning Electron Microscope (SEM, Magellan400, FEI, The Netherlands) The in-line transmittance of the LSO: Ce ceramics were measured on a UV–vis-NIR spectrophotometer (UV-2900, Hitachi, Japan).

Fig. 2. EBSD orientational maps and pole figures of the LSO:Ce ceramics casted by slipcasting without (a, b) and with(c, d) a 9 T magnetic field applied. The insets of (b) and (d) shows the color codes for the pole figures, blue and red bars indicate the lowest and highest intensity, respectively.

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3. Results and discussions Fig. 1 presents the XRD patterns of the textured (9 T) and untextured (0 T) LSO:Ce ceramics. Both XRD patterns can be matched to a standard PDF card (JCPDS card 41-0239) for LSO, with no second phases observed. In the pattern of the untextured ceramic, diffraction peaks from various planes can be detected. However, in the textured ceramic, most observed diffraction peaks correspond to  the ð402Þ lattice plane, with the relative intensity of other reflec tions all much lower than that of the ð402Þ reflections. This sug gests that the ð402Þ lattice planes parallel to the surface of the textured LSO:Ce ceramic, and that most grains are aligned parallel  to the ½402 direction. The Lotgering factor [11], a semi-quantitative technique to evaluate the degree of texturing of ceramics, was calculated from the XRD data. The Lotgering factor f, is defined as f = (P  P0)/(1  P0), P where P = I(h0l)/ I(hkl) for monoclinic LSO, was calculated from the sum of the h0 l reflections overall of the hkl reflections, and P P0 = I0(h0l)/ I0(hkl) was calculated from the JCPDS card of a randomly oriented LSO sample. I(hkl) and I0(hkl) are the peak intensities of the hkl reflections over a 2h range of 10–32°. The f factor of anisotropic sample is 0, which is the case for a sintered ceramic fabricated without an applied magnetic field, while f = 1 stands for a ceramic fully oriented along one direction. The f value of the textured LSO:Ce ceramics was determined to be 0.28. The degree of texturing of LSO:Ce ceramics can be influenced partially by the magnetic properties of original LSO:Ce powder, which can be driven by its grain characteristics. It has been recently demonstrated that the magnetic properties of pure and doped fine-grained oxides strongly depend on the presence of defects like interphase boundaries and grain boundaries [12] and on the presence of doping atoms in the XRD-invisible amorphous surficial, interfacial and intergranular layers [13]. The EBSD orientational map of the surface perpendicular to the magnetic field direction of the untextured ceramics (Fig. 2a) shows a range of colors. This suggests that the grains of the LSO:Ce ceramic are randomly oriented in various crystallographic orientations. The orientational map of the textured ceramic, however, shows a significantly larger fraction of red colored grains (Fig. 2c), which indicates that considerable grain growth has occurred along the  magnetic field direction. The f101g and f402g pole figure (Fig. 2b) computed from the orientation distribution of the untextured LSO: Ce ceramics exhibits random orientations, while a central

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maximum in the f101g pole figure and a circular ring pattern in  the f402g pole figure were observed in the textured LSO:Ce  ceramic (Fig. 2d).These features correspond to a prominent f402g orientation perpendicular to the ceramic’s surface, with the orientation density showing a distribution maximum around 7. This  indicates that the ½402 crystallographic direction of the textured LSO:Ce ceramic is aligned with the magnetic field direction. Fig. 3 presents the in-line transmittance curves of the LSO:Ce ceramics in the wavelength range from 200 to 800 nm. The inline transmittance of the untextured LSO:Ce ceramic is only 1.4% at 420 nm, due to light scattering from grain boundaries, stemming from the random orientation of grains with different refractive indices. In the LSO:Ce ceramic casted in a 9 T magnetic field, with  grain alignment along the ½402, the in-line transmittance of the textured LSO:Ce ceramics is higher, 6.6% at 420 nm, which is 4.7 times that of the untextured LSO:Ce ceramics. The increased transmittance of the textured ceramic is visually apparent from photographs of the LSO:Ce ceramics shown in Fig. 3. These results demonstrate that the in-line transmittance of monoclinic LSO:Ce ceramics can be dramatically improved through grain orientation control. 4. Conclusion Monoclinic LSO:Ce ceramics was textured along the crystallo graphic direction of ½402 by combining slip casting in a 9 T vertical magnetic field and HIPing at 1650 °C under 150 MPa. Calculation of XRD data revel the degree of texturing of the textured LSO:Ce ceramics is 0.28. The orientational map and pole figure of the textured ceramics shows grains are well aligned along the crystal lographic direction of ½402. The in-line transmittance of the textured LSO:Ce ceramics is 6.6% at 420 nm, which is 4.7 times of that of the untextured ceramics. Acknowledgments This work is funded by the National Natural Science Foundation of China under grant numbers of 51172139 and 21301115. The authors are grateful to Miss Xuemei Song and Prof. Yi Zeng at Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS, Shanghai, China) for their help in EBSD measurement and fruitful discussion. L.C. Fan gratefully acknowledges the Air Force Office of Scientific Research (contract FA9550-14-1-0155) for partially funding his work. Y.Q. Wu also gratefully acknowledges the AFOSR (STTR II: Transparent non-cubic ceramics using magnetic fields to control microstructural orientation) for partially funding their work. References

Fig. 3. In-line transmittance curves of the textured (red dash-dot line) and untextured (blue solid line) LSO:Ce ceramics, the insets are photographs of the LSO:Ce ceramics.

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