Optical Materials 31 (2009) 729–733

Contents lists available at ScienceDirect

Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Processing and scintillation properties of Eu3+ doped Lu2O3 transparent ceramics

Y. Shi a,*, Q.W. Chen b, J.L. Shi b

a School of Materials Science and Engineering, Department of Electronics and Information Materials, Shanghai University, 149 Yanchang Road, Shanghai 200072, Chinab Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

a r t i c l e i n f o

Article history:Received 12 December 2007Received in revised form 9 April 2008Accepted 29 April 2008Available online 20 June 2008

Keywords:LutetiaTransparent ceramicsScintillatorProcessing

0925-3467/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.optmat.2008.04.017

* Corresponding author. Tel.: +86 21 56331793.E-mail address: yshi@shu.edu.cn (Y. Shi).

a b s t r a c t

A novel processing was developed to fabricate transparent europium (Eu3+) ion doped lutetia (Lu2O3)scintillation ceramics. The microstructural evolution of Eu3+:Lu2O3 phosphor powder under different cal-cining temperatures was investigated by FTIR and TEM. Highly transparent polycrystalline Lu2O3 ceram-ics were densified from as-prepared powder by pressureless sintering under 1850 �C for 6 h in flowing H2

atmosphere. Optical linear transmittances in the visible wavelength region for Lu2O3 ceramic could reachas high as above 80%. Integration of the X-ray excited emission spectra showed that Lu2O3:5at%Eu3+ cera-mic scintillator provided about 10 times overall emission intensity with respect to BGO single crystal atroom temperature.

� 2008 Elsevier B.V. All rights reserved.

3+

1. Introduction

Lu2O3 is a kind of attractive sesquioxide with cubic crystallinestructure which favors its applications as a promising host materialfor high-performance scintillation detectors and laser gain media.The most important superiority of Lu2O3 is its phenomenally highdensity (�9.42 g/cm3) combined with the high Z-number of Lu (71)element, which endows Lu2O3 material with exceptionally highstopping power for ionizing radiation detection. It was reportedthat europium-doped Lu2O3 ceramic scintillator has found itsapplications in the field of digital X-ray imaging technology [1–4]. However, fabrication of Lu2O3 single crystals is quite difficultbecause of its very high melting point (2490 �C) and strict require-ments for crystal-growing facilities. Recently, transparent ceramicsprocessing including nanosized ceramic powders and advanceddensification technology provides a possible approach to overcom-ing the disadvantages and limits of conventional single-crystalgrowth methods. It would be much easier to elaborate polycrystal-line Lu2O3 ceramics with a full densification state and a complexchemical composition homogeneously under sintering tempera-ture about 700 �C lower than its melting point with a relativelow cost and size flexibility. Lempicki has reported the fabricationof Eu:Lu2O3 ceramics by hot-pressing from Eu:Lu2O3 powder pre-pared by oxalate precipitation method [1]. The combustion synthe-sis combined vacuum sintering was also conducted by Zych tofabricate translucent Eu:Lu2O3 ceramics [3–6], which showed anotable radioluminescence intensity.

ll rights reserved.

In this paper, Eu -doped transparent lutetia ceramics were fab-ricated by pressureless sintering in flowing H2 atmosphere fromthe as-prepared nanosized Eu3+:Lu2O3 powder, which was ob-tained by a co-precipitation method using ammonium hydroxideand ammonium hydrogen carbonate as the mixed precipitant.The linear optical transmittances and grain boundary microstruc-ture of obtained Eu3+:Lu2O3 polycrystalline scintillators were char-acterized. The radioluminescent characteristics of obtainedEu3+:Lu2O3 transparent ceramics were investigated and comparedwith BGO crystal under X-ray excitation.

2. Experimental

Lutetium (Lu3+) and europium (Eu3+) ion nitrate solution wasprepared with corresponding oxide (purity > 99.9%) dissolved inslightly excessive nitric acid according to proper chemical compo-sition. Ammonium hydroxide (NH4OH) and ammonium hydrogencarbonate (NH4HCO3) were mixed according to a proper ratio tobe used as a complex precipitant. The precursor precipitate wasproduced by adding 250 ml of the precipitant solution into1000 ml of the mother solution containing Lu3+ and Eu3+ ion(0.06 M for Lu3+) under mild agitation with a magnetic stirrer.The initial pH and final pH values of the solution were about 1and 9, respectively. After aging for 24 h, the resultant precipitationprecursor was filtered using a suction filter, washed four timeswith deionized water and rinsed twice with ethyl alcohol to re-move impurity anions. After being dried at 100 �C over 24 h inair, the precipitation precursor was crushed and calcined at tem-peratures between 600 �C and 1200 �C for 2 h, respectively.

The obtained Eu3+ doped Lu2O3 powder was dry-pressed to agreen compact disc with a diameter of 20 mm in a double action

500 1000 1500 2000 2500 3000 3500 4000

0

1

2

3

4

5

1200ºC

850ºC

1000ºC

750ºC

Inte

nsity

(a.u

.)

Wavenumber (Cm-1)

650oC

Fig. 1. FTIR spectra of the 5%Eu3+-doped Lu2O3 phosphors calcined at differenttemperatures.

60

80

100

DS

SBET

le s

ize

DS (

nm)

30

40

Specific surface730 Y. Shi et al. / Optical Materials 31 (2009) 729–733

die at the pressure of 30 MPa followed by a cold isostatic pressingunder the pressure of 200 MPa. The densification of powder com-pacts was carried out by a pressureless sintering under 1850 �Cfor 6 h in H2 atmosphere, using a furnace with the tungsten-heat-ing elements (Model FDB-14-19, NEMS, Japan). The pure hydrogengas was adopted to keep the partial pressure of H2 in furnacechamber to be at around 1 atm and the flowing rate was 0.5 l/min. The heating-up rate was 2 �C/min. Transparent Eu3+:Lu2O3

ceramics obtained were carefully polished on both sides by dia-mond pastes.

Fourier transform infrared spectra (FTIR) were recorded using aThermo Nicolet Nexus infrared spectrometer with KBr pellets. Thegrain sizes of Eu3+:Lu2O3 powder were calculated by Scherrer’sequation from X-ray diffraction patterns, which were measuredwith X-ray diffractometer (D/max 2550 V, Rigaku, Japan) utilizingnickel-filtered Cu Ka radiation (1.5406 Å) in the range of 2h = 10–70�. The specific surface areas of as-prepared phosphor powderswere measured from Brunauer–Emmett–Teller (BET) analysis(Micrometrics, ASAP2010, USA).

Microstructures of obtained Lu2O3 powder and grain boundarystructure of transparent Lu2O3 ceramics were observed by field-emission transmission electron microscopy (JEM-2100 F, JEOL,Japan). The linear optical transmittances of transparent Lu2O3

ceramics were measured on an UV–Vis spectrometer (UV-2501PC, SHIMADZU, Japan) in wavelength from 280 nm to1100 nm. Radioluminescence spectra measurement of obtainedtransparent Eu3+:Lu2O3 ceramics was carried out at modified Spec-trofluorometer 199S (Edinburgh Instrument, Great Britain)equipped with an X-ray tube (W cathode) operated at 40 kV.

600 700 800 900 1000 1100 1200

20

40

Temperature (ºC)

Prim

ary

part

ic

10

20

area SB

ET (m2/g)

Fig. 2. Dependence of specific surface areas (SBET) and primary particle sizes (Ds) of5%Eu3+-doped Lu2O3 phosphor powders on their calcination temperatures.

3. Results and discussion

3.1. Dependence of characteristics of Eu3+:Lu2O3 powders oncalcination temperatures

The details of preparation of Eu3+:Lu2O3 phosphor powder by awet chemical co-precipitation have been reported in our previouspapers [7–8], which demonstrated that the mixed precipitant ofNH3 � H2O + NH4HCO3 solutions was an effective precipitant toachieve well-dispersed nanosized Lu2O3 powder. The additional re-sults on the microstructural changes in the course of crystallizationof precipitation precursor have been presented in this paper.

Fig. 1 depicts the Fourier transformed infrared (FTIR) absorptionspectra in the range of 4000–400 cm�1 for the decomposed prod-ucts of the precipitation precursor calcined at different tempera-tures. The broad absorption band on the IR spectra at 3410 cm�1

was assigned to –OH stretching. The two intense peaks at 1530and 1400 cm�1, as well as absorption peaks at 1090 and840 cm�1 might be due to C–O band stretching and bending indi-cating the presence of the carbonate group. The peaks at577 cm�1 are the characteristics of Lu–O stretching (Lu2O3 host lat-tice vibration) [9], which are attributed to the crystallized lutetiumoxide absorption bands. With the increase of calcination tempera-tures, the intensity of the absorption bands at 3410, 1530, and1400 cm�1 decreased gradually. When the calcination temperatureis raised to 1000 �C, the absorption bands as mentioned above al-most disappeared.

It could be illustrated from Fig. 2 that average primary particlesize calculated from Scherrer’s equation and specific surface areasof Eu3+-doped Lu2O3 powder were greatly dependent on its calci-nation temperatures. As calcination temperature increased from650 �C to 1200 �C, the primary particle size of Lu2O3 powder in-creased from less than 20 nm to about 90 nm, coupled with anobvious declining of specific surface area from 38 m2/g to less than10 m2/g. The micrographs in Fig. 3 shows the morphology of the

lutetia powders calcined at 750 �C, 1000 �C, and 1200 �C for 2 h,respectively, demonstrating that narrowly distributed in particlesize, weakly agglomerated state, and uniform fine spherical Eu3+-doped Lu2O3 powder were obtained. Moreover, it could be seenthat the sizes of primary lutetia particles increased from about20 nm to 100 nm as calcination temperature ascended from750 �C to 1200 �C, which was identical to data in Fig. 2. Under firingcondition of 1000 �C, the primary particle size was in the range of30–40 nm, specific surface area was 18 m2/g, and no sintering oc-curred between the adjacent Lu2O3 particles. These microstructurefeatures would be greatly beneficial to obtain fully densified poly-crystalline Eu:Lu2O3 specimen with high transparency. Consideringthat a smaller primary grain size was beneficial to maintain thehigh sinterability of Lu2O3 powder and to achieve the desired den-sification for the transparent ceramics, we chose 1000 �C/2 h as theoptimized calcining conditions to accomplish the transformation ofamorphous precipitation precursors to 5at%Eu3+-doped Lu2O3

phosphor powder.

3.2. Fabrication and microstructure of 5at%Eu3+:Lu2O3 basedscintillation ceramics

The as-prepared 5at%Eu3+:Lu2O3 powder was formed into diskshape with a diameter of 20 mm by a dry pressing and a cold iso-

Fig. 3. Transmittance-electron micrographs of 5mol%Eu3+:Lu2O3 powders calcined at (a) 750 �C, (b) 1000 �C and (c) 1200 �C for 2 h, respectively.

Y. Shi et al. / Optical Materials 31 (2009) 729–733 731

static pressing. The relative density of obtained Lu2O3 green bodywas measured to be about 47% theoretically. After being sinteredat 1850 �C in flowing H2 atmosphere without any additives, poly-crystalline transparent Lu2O3 ceramics was obtained with a rela-tive density of 99.8% theoretically. Compared to hot-presssintering or vacuum sintering in previously reported works, ourpressureless sintering was much practical and easy to conduct. Ithas been recognized that H2 atmosphere was favourable to achievepolycrystalline transparent oxide ceramic [10]. A much highergrain boundary mobility of Y2O3 was measured under H2 atmo-sphere [11], which was believed to be similar for lutetia sinteringdue to the same crystal structure.

Fig. 4a gives the appearance of 5at%Eu3+:Lu2O3 transparentceramic samples, which have been polished on both sides. Thediameter and thickness of the specimens were about 14 mmand 1 mm, respectively. Fig. 4b shows the dependence of theoptical linear transmittance of 5at%Eu3+:Lu2O3 ceramic specimenon wavelengths in the visible wavelength region. It was foundthat the optical transmittance increased rapidly as the wave-length increased from 300 nm to 550 nm. There existed severalsmall peaks at 265 nm, 395 nm, 466 nm and 534 nm, which cor-responded to the light absorption of Eu2O3. The broad absorptionpeak at 266.5 nm was known to have resulted from the charge-transfer transitions between O2� and Eu3+, which contributed tothe electronic transition from the 2p orbital of O2� to the 4forbital of Eu3+, and the other three peaks reflected the intra-con-figurational partially forbidden 4f ? 4f transitions within theEu3+ [8]. Within the wavelength range of 600–800 nm, the in-

line transmittances of specimen could maintain at a high levelof approximately 80%, which was quite satisfactory for the appli-cation of Eu3+:Lu2O3 transparent ceramics as a kind of high-per-formance scintillator.

According to the following equation, the optical transmittances(T) are dependent on attenuation coefficient (a) of transparentceramics with the thickness of t:

T ¼ I=I0 ¼ ð1� RÞ2 expð�atÞ ð1Þ

where R = (n � 1)2/(n + 1)2, in which n = 1.9 was the refraction indexfor Lu2O3 ceramics [12]. From the linear optical transmittance value79.4% of Lu2O3 ceramic at 611 nm (Fig. 4b), it could be calculatedthat attenuation coefficient obtained for 5at%Eu3+:Lu2O3 transpar-ent ceramic was 0.28 cm�1 at the wavelength of 611 nm.

It was well recognized that the residual pores and impurities atgrain boundary would play a key role in damaging the opticalproperties of transparent ceramics [13–14]. The TEM micrographshown in Fig. 5a presented a typical microstructure morphologyof 5%Eu3+:Lu2O3 ceramics, exhibiting that there was no secondarygrain boundary phase existing at grain boundary. The high-resolu-tion micrograph in Fig. 5b revealed the lattice image of adjacentLu2O3 grains clearly, which further proved that no residual impu-rity was observed at grain boundary. The energy-dispersed spec-trum of Lu2O3 grains demonstrated that Eu, Lu, and O were thethree elements existing in our specimen, and the molar ratio ofEu/Lu was 4.9%, which coincided with the designed chemical com-position (5at%Eu3+) quite well. It could be suggested that the goodchemical homogeneity and ‘‘clean” grain boundary structure were

200 300 400 500 600 700 800

0

10

20

30

40

50

60

70

80

90

534466.5

395

266.5

Tran

smitt

ance

(%)

Wavelength (nm)

(a) (b)

Fig. 4. (a) Photograph of mirror-polished transparent 5mol%Eu3+:Lu2O3 ceramic specimens (sintered at 1850 �C for 6 h, thickness: 1 mm), (b) optical in-line transmittance ofspecimen shown in (a).

Fig. 5. (a) TEM micrograph of 5mol%Eu3+:Lu2O3 transparent ceramic and (b) high-resolution lattice image of adjacent Lu2O3 grains showing the morphology of grainboundary.

732 Y. Shi et al. / Optical Materials 31 (2009) 729–733

quite favorable to reducing scattering losses arising from voids andimpurities when light passed through polycrystalline Lu2O3

ceramics.

3.3. Radioluminescence of transparent 5at%Eu3+:Lu2O3 ceramics

In order to evaluate the light output of 5%Eu3+-doped Lu2O3

ceramics as-made, the radioluminescence spectra of transparentLu2O3:Eu ceramic were measured with a comparison to those ofBGO crystal with the same dimension under excitation of X-rayof 40 kV at room temperature. Fig. 6 shows typical Eu3+ sharpemission lines without any other parasitic emission. The mainemission peak was located at 611 nm, which was attributed tothe 5D0 ?

7F2 transition of Eu3+. The broad characteristics emissionpeak of Eu2+ around 590 nm was not detected [15]. Combiningwith our previous results of excitation and emission spectra underUV excitation [8], it could be concluded that Eu3+ was stable inLu2O3 during sintering at H2 atmosphere and no transition toEu2+ occurred.

It is found in Fig. 6 that the main emission peak of Lu2O3:Euceramics was much more intense than that of BGO crystal,which was observed at around 500 nm. Integration of the spectra(in the inset of Fig. 6 shows that the 5at%Eu3+:Lu2O3 ceramicsprovided about ten times higher overall intensity with respectto BGO single crystal. Assuming that light yield of BGO is about9000 phot/MeV [16], it could be estimated that steady-state lightoutput of 5at%Eu:Lu2O3 transparent ceramics would be about90,000 phot/MeV. This relative high light yield value will enabletransparent Eu3+:Lu2O3 ceramics to be a promising polycrystal-line scintillation material with high stopping power, which couldfind its applications in stationary digital imaging andfluoroscopy.

4. Conclusion

In this paper, nanosized, well dispersed, and spherical Eu3+

doped Lu2O3 phosphor powders with particle size of about 30–40 nm could be synthesized by a novel co-precipitation processafter being calcined at 1000 �C for 2 h. The specific surface areaof resultant powder reached 18 m2/g and could be densified totransparent Eu3+:Lu2O3 ceramics at a temperature of 1850 �C for6 h in flowing H2 atmosphere. Optical linear transmittance in thevisible light region (600–800 nm) for 5at%Eu3+:Lu2O3 ceramics

200 300 400 500 600 700

0.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

0.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

200 300 400 500 600 700 800

0.0

2.0x10

4.0x10

6.0x10

8.0x10

1.0x10

1.2x10

0.0

2.0x10

4.0x10

6.0x10

8.0x10

1.0x10

1.2x10

Spec

tral

Inte

gral

Wavelength (nm)

Eu:Lu2O3 CeramicsBGO Crystal

(×10)

Inte

nsity

(arb

.uni

ts)

Wavelength (nm)

Eu:Lu2O3 Ceramic

BGO Crystal*10

611nmIntegral of radioluminescence spectraat room temperature

Fig. 6. A comparison of radioluminescence spectra of 5at%Eu:Lu2O3 scintillation ceramic to those of BGO crystal under excitation of X-ray of 40 kV at room temperature.

Y. Shi et al. / Optical Materials 31 (2009) 729–733 733

(1 mm in thickness) could maintain around 80% owing to full den-sification and ‘‘clean” grain boundary structure, which was identi-fied by HETEM observation. Radioluminescence spectra showedthat 5at%Eu3+:Lu2O3 ceramics exhibited a strong and sharp mainemission peak at 611 nm and the light yield was estimated to beabout 10 times higher with respect to BGO single crystal, indicatingthat it was a potential ceramic scintillator for stationary digitalimaging and fluoroscopy.

Acknowledgments

We gratefully acknowledge the financial support from the Nat-ural Science Foundation of China under Grant No. 50572115 andBasic Research Key Project of Shanghai Municipal under GrantNo. 06JC14029.

Prof. Martin Nikl (Institute of Physics, Prague, Czech Republic) isgreatly appreciated for his help on the characterization of radiolu-minescence spectral properties under X-ray excitation. Ms. MeilingRuan (Shanghai Institute of Ceramics, Chinese Academy of Sci-ences) is also greatly appreciated for her help on HRTEMobservation.

References

[1] A. Lempicki, C. Brecher, P. Szupryczynski, H. Lingertat, V.V. Nagarkar, S.V.Tipnis, S.R. Miller, Nucl. Instrum. Meth. A 488 (2002) 579.

[2] V.V. Nagarkar, S.V. Tipnis, S.R. Miller, A. Lempicki, C. Brecher, P. Szupryczynski,H. Lingertat, IEEE Trans. Nucl. Sci. 50 (3) (2003) 297.

[3] E. Zych, D. Hreniak, W. Strek, L. Kepinski, K. Domagala, J. Alloys Comp. 341(2002) 391.

[4] E. Zych, D. Hreniak, W. Strek, J. Alloys Comp. 341 (2002) 385.[5] E. Zych, D. Hreniak, W. Strek, J. Phys. Chem. B 106 (2002) 3805.[6] E. Zych, J.T. Piegza, Chem. Mater. 18 (2006) 2194.[7] Q.W. Chen, Y. Shi, L.Q. An, S.W. Wang, J.Y. Chen, J.L. Shi, J. Eur. Ceram. Soc. 27

(2007) 191.[8] Q.W. Chen, Y. Shi, L.Q. An, J.Y. Chen, J.L. Shi, J. Am. Ceram. Soc. 89 (6) (2006)

2038.[9] N.T. McDevitt, W.L. Baun, Spectrochim. Acta 20 (1964) 799.

[10] R.L. Coble, J. Am. Ceram. Soc. 45 (3) (1962) 123.[11] P.L. Chen, I.W. Chen, J. Am. Ceram. Soc. 79 (7) (1996) 1801.[12] J. Lu, K. Takaichi, T. Uematsu, A. Shirakawa, M. Musha, K. Ueda, H. Yagi, T.

Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 81 (23) (2002) 4324.[13] A. Ikesue, A. Yoshida, J. Am. Ceram. Soc. 81 (8) (1998) 2194.[14] A. Ikesue, Y.L. Aung, T. Taira, T. Kamimura, K. Yoshida, G. Messing, Annu. Rev.

Mater. Res. 36 (2006) 397.[15] R.J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, J. Phys. Chem. B 108

(2004) 12027.[16] C.W.E. van Eijk, Nucl. Instrum. Meth. A 509 (2003) 17.