A. Thermal shrinkage behavior of LSO:Ce green body from as-synthesized LSO powder

Figure 1(a) shows the SEM micrograph of synthetic LSO:Ce 0.5 mol% powders. It is clearly noted that as-synthesized LSO powders consisted of well-dispersed spherical particles with average primary grain size around 100 nm. There existed slight agglomeration where some particles are overlaid by other particles. From the particle size distribution result shown in Fig. 1(b), it is demonstrated that the synthetic LSO powders are composed of secondary particles arranging from 50 to 600 nm with median particle diameter of 183 nm, which is in good agreement with SEM micrograph.

FIG. 1.

(a) SEM micrograph and (b) the particle size distribution of as-synthesized LSO:Ce powder.

To evaluate sintering behavior of LSO:Ce green compact, the thermal shrinkage of specimen was measured by a thermal expansion analyzer. Figure 2 shows the dependence of relative density of the LSO compact on temperatures and linear thermal shrinkage from room temperature to 1500 °C. The relative densities of the green body compacts after cold isostatic pressing were determined from the initial bulk density and the measured linear shrinkage from room temperature to 1500 °C in air by Eq. (1):^{Reference Li, Ikegami and Mori17}

(1)$$\rho \% = {{{\rho _0}} \mathord{\left/ {\vphantom {{{\rho _0}} {{{{{{\rm{(}}1 - \Delta L{\rm{/}}{L_0}{\rm{)}}}^3}} \mathord{\left/ {\vphantom {{{{{\rm{(}}1 - \Delta L{\rm{/}}{L_0}{\rm{)}}}^3}} {{\rho _{\rm{T}}}}}} \right. \kern-\nulldelimiterspace} {{\rho _{\rm{T}}}}}}}} \right. \kern-\nulldelimiterspace} {{{{{{\rm{(}}1 - \Delta L{\rm{/}}{L_0}{\rm{)}}}^3}} \mathord{\left/ {\vphantom {{{{{\rm{(}}1 - \Delta L{\rm{/}}{L_0}{\rm{)}}}^3}} {{\rho _{\rm{T}}}}}} \right. \kern-\nulldelimiterspace} {{\rho _{\rm{T}}}}}}}\quad ,$$

where ρ is the density of the columnar compact at specific temperatures, ρ₀ (3.72 g/cm³) is the initial density of the green compact after cold isostatic pressing, ΔL/L ₀ is the linear shrinkage rate under different temperatures, and ρ_T is theoretical density (7.40 g/cm³) of LSO phase. From Fig. 2, the thermal shrinkage behavior can be divided into two distinct parts. It is obvious that compacts exhibited a slight expansion till the temperature reached near upon 1200 °C and relevant relative density shows a little decrease. Obvious shrinkage is detected in the temperature range from 1200 to 1500 °C. The linear shrinkage of compact could reach 4% as the temperature ascended up to 1500 °C and relevant relative density of the specimen increased to about 60%. It could be deduced that the best densification temperature should be higher than 1500 °C. Due to the limits of thermal expansion analyzer, thermal shrinkage characteristic of green compacts under temperatures above 1500 °C cannot be obtained.

FIG. 2.

Dependences of relative density and linear thermal shrinkage on temperature for LSO:Ce green compacts.

Dependence of relative densities of LSO:Ce ceramics on different sintering temperatures range from 1500 to 1780 °C (holding time: 4 h) in flowing H₂ atmosphere is displayed in Fig. 3. As sintering temperature increased from 1500 to 1650 °C, relative density of LSO ceramics increased dramatically from less than 60% up to above 90%. A fully densified LSO ceramics with a relative density of 99.5% is attained as sintering temperature attained 1720 °C. Moreover, as sintering temperature was raised to 1780 °C, relative density of LSO ceramics has no significant increase compared with that of specimen of 1720 °C. So the optimized sintering temperature is 1720 °C.

FIG. 3.

Dependence of relative densities of LSO:Ce ceramics on different sintering temperatures (holding time: 4 h).

B. Microstructures of as-fabricated LSO ceramics

The XRD patterns in Fig. 4 illustrate the positions of diffraction peaks of LSO powder and as-sintered LSO:Ce ceramics which are compatible well with the standard diffraction data (ICSD #279584) of monoclinic LSO phase. No other second phase is found, indicating that LSO phase remains stable and no decomposition occurs during the sintering course. Morphology of fracture surfaces of LSO:Ce ceramics sintered at different sintering conditions is observed by SEM to investigate microstructure variation of polycrystalline LSO:Ce ceramics during sintering process. SEM micrograph shown in Fig. 5(a) revealed that the grain size in the sample was only around 2–3 μm, suggesting that grains growth of LSO:Ce ceramics sintered at 1650 °C for 4 h is not complete, and considerable residual pores at the grain boundaries were observed, hindering achievement of the fully dense polycrystalline LSO ceramics. Figure 5(b) shows the fracture micrograph of LSO ceramics sintered at 1720 °C for 4 h. It shows that grain size reaches about 5 μm and that residual pores have been eliminated completely as sintering temperature reached 1720 °C. This microstructure is quite beneficial to achieve good optical transmittance in polycrystalline sample, which has been proved by following photograph of LSO ceramics under visible light. From fracture micrograph of LSO ceramics sintered at 1780 °C for 4 h shown in Fig. 5(c), it is obvious that more big pores appear in grains and grain boundaries relative to the LSO ceramics sintered at 1720 °C for 4 h. This phenomenon may indicate that excessively fast grain growth due to high sintering temperature causes pores growing and secondary recrystallization, which result in reduce of transparency of obtained LSO ceramics. Figure 5(d) presents the fracture micrograph of LSO ceramics sintered at 1720 °C for 8 h. When holding time was extended to 8 h at the sintering temperature of 1720 °C, LSO ceramics undergo remarkable grain coarsening. As-attained translucent LSO ceramics sintered at 1720 °C for 4 h was mechanically polished on both sides. SEM micrograph of polished surface of the polished LSO ceramics presents in Fig. 6(a). It further manifests that no minor second phases or decomposition of the LSO was found in the sintered LSO ceramics. Corresponding energy-dispersive x-ray spectrum (EDS) of rectangle range of the SEM micrograph is shown in Fig. 6(b). The EDS results suggest that the atom ratio of Lu and Si in the sintered LSO ceramics is in accord well with chemical formula of LSO.

FIG. 4.

XRD patterns of the synthetic LSO powder and as-fabricated ceramics.

FIG. 5.

SEM micrographs of fracture surface of LSO ceramics obtained at different sintering conditions: (a) 1650 °C for 4 h, (b) 1720 °C for 4 h, (c) 1780 °C for 4 h, (d) 1720 °C for 8 h.

FIG. 6.

SEM micrograph (a) and corresponding EDS (b) of polished surface of LSO ceramics obtained at 1720 °C for 4 h.

Appearance of the polished LSO ceramics is presented in Fig. 7(a). The polished specimen with a diameter of 14.5 mm and a thickness of 1.0 mm shows a translucent state, and the characters under the LSO ceramics can be observed. Unlike YAG, LuAG, and Lu₂O₃ matrix with isotropical cubic crystal structure, LSO has a monoclinic structure leading to an optical anisotropy in different crystal orientation. Owing to existence of birefringence, it will bring about light scattering when light cross grain boundaries. Hence, it is much more difficult to obtain transparent polycrystalline LSO ceramics by sintering under normal pressure. Figure 7(b) shows fluorescent photograph of the LSO ceramics under UV light of 365 nm, which emit strong blue luminescence demonstrating good luminescent property of the LSO ceramics.

FIG. 7.

Photographs under (a) visible light and (b) UV light of 365 nm of the translucent LSO ceramics sintered at 1720 °C for 4 h.

TEM micrograph of the LSO ceramics sintered at 1720 °C for 4 h is shown in Fig. 8(a). The grain boundary was clean and no intergranular second phase was observed between adjacent grains. The selected area electron diffraction (SAED) pattern is shown in Fig. 8(b).

FIG. 8.

TEM micrograph (a) and SAED pattern (b) of the translucent LSO:Ce ceramics.

C. Luminescent properties of the fabricated LSO:Ce ceramics

Figure 9(a) presents a VUV-UV excitation spectrum of the LSO:Ce ceramics sintered at 1720 °C for 4 h by monitoring the emission band at 420 nm. There are four strong excitation bands respectively centered at 189, 262, 297, and 356 nm in the excitation spectrum. The excitation band centered at 189 nm is considered to be the host absorption band, which correspond to the band gap (E _g = 6.5 eV, ∼190 nm) of LSO matrix.^{Reference Cooke, Bennett, McClellan, Roper, Whittaker and Portis18} Moreover, 4f → 5d electron transition of Ce³⁺ is responsible for the excitation bands centered at 262, 297, and 356 nm.^{Reference Shi, Liu, Zhang, Qi, Ren, Tao and Xu19} Emission spectra of the LSO:Ce ceramics under 6.58 eV (189 nm) and 3.49 eV (356 nm) excitation both shows a broad emission band with the peak centered at about 2.96 eV (420 nm) as shown in Fig. 9(b), but the emission spectrum under 189 nm excitation has a broader emission band than emission spectrum under 356 nm excitation. Emission spectrum under 356 nm excitation can be divided into three components (red dot lines) centered at 3.14 eV (396 nm), 2.97 eV (419 nm), and 2.74 eV (454 nm) by Gaussian fitting, which is closely related to the Ce occupation in LSO lattice.^{Reference Cooke, Bennett, McClellan, Roper, Whittaker and Portis18,Reference Suzuki, Tombrello, Melcher and Schweitzer20} The emission peaking at 396 nm and 419 nm ascribes transition of 5d lowest level to 4f ground state (²F_5/2 and ²F_7/2) of Ce1 (Ce³⁺ located in one of two independent lattice sites), whereas the emission of peaking at 454 nm originates from Ce2 (Ce³⁺ located in the other site in LSO crystal) emission.^{Reference Suzuki, Tombrello, Melcher and Schweitzer20,Reference Liu, Shi, Yin, Fu, Zhang and Ren21} Percentage of Ce2 emission intensity reaches 27.1% in emission spectrum obtained from 356 nm excitation, which is much lower than 36.5% in emission spectrum obtained from 189 nm excitation. It may indicate that efficiency of host absorbing (189 nm) transfer to Ce2 is higher than Ce1 absorbing (356 nm) transfer to Ce2.

FIG. 9.

(a) Excitation and (b) emission spectra of the translucent LSO:Ce ceramics.

XEL spectra of the LSO:Ce ceramics are depicted in Fig. 10(a), exhibiting a broad emission band in the range of 350–650 nm. Radioluminescence intensity of LSO:Ce ceramics can reach twice of that of BGO single crystal, suggested that the as-sintered LSO:Ce ceramics has been provided with good light yield property. Figure 10(b) shows decay profile of the LSO:Ce ceramics under excitation of 356 nm and emission of 420 nm. The decay profile contains only a fast decay component of 21.2 ns by a single exponential function fitting, which is obviously faster than that (28.7 ns) of LSO:Ce single crystal,^{Reference Ren, Qin, Lu and Li22} Lempicki et al. proposed that this phenomenon was due to that the ceramics fabrication process had shifted more of the emitters into the faster site than is normally found in the single crystal,^{Reference Lempicki, Brecher, Lingertat, Miller, Glodo and Sarin12} but we deem it is due to oxygen vacancies and grain boundaries existed in LSO:Ce ceramics, both of them will bring about nonradiative transitions of Ce³⁺ in LSO lattice, which is much faster than radiative transitions.

FIG. 10.

(a) XEL spectra of the translucent LSO:Ce ceramics and BGO single crystal and (b) luminescence decay curve of the LSO:Ce ceramics sintered at 1720 °C for 4 h under excitation of 356 nm and emission of 420 nm.

Photopeak measurement data of the samples are depicted in Fig. 11. It shows that the light yield of the fabricated LSO ceramics is 21,300 ph/MeV under the ¹³⁷Cs excitation source, which reaches 91% that of LSO single crystal. The energy resolutions of the ceramics and crystal are 19 and 12%, respectively. Both light yield and energy resolution of the fabricated LSO ceramics are comparable to that of bulk LSO single crystal and the previously reported results.^{Reference Wang, Loef, Rhodes, Glodo, Brecher, Nguyen, Lempicki, Baldoni, Higgins and Shah14,Reference Roy, Lingertat, Brecher and Sarin23}

FIG. 11.

Comparison of the light yield of LSO:Ce ceramics with BGO and LSO bulk single crystals.