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Effect of sintering temperature on the transparency and mechanical properties of lutetium aluminum garnet fabricated by spark plasma sintering
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Journal of the European Ceramic Society 32 (2012) 3097–3102
Effect of sintering temperature on the transparency and mechanical properties of lutetium aluminum garnet fabricated by spark plasma sintering Liqiong An a,b , Akihiko Ito a,∗ , Takashi Goto a a
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan b Japan Society for the Promotion of Science, Japan
Received 9 February 2012; received in revised form 2 April 2012; accepted 12 April 2012 Available online 30 April 2012
Abstract Transparent lutetium aluminum garnet (Lu3 Al5 O12 , LuAG) was fabricated by reactive spark plasma sintering. The effect of sintering temperature on the crystal phase, microstructure, transparency and mechanical properties of LuAG bodies was investigated. Fully dense and single-phase LuAG bodies were obtained at sintering temperatures 1573–1923 K. The average grain size increased from 0.18 to 0.52 m with increasing sintering temperature from 1573 to 1773 K, and grain growth became significant at 1823 K. Transmittance showed a maximum value of 77.8% at 2000 nm at a sintering temperature of 1773 K after annealing at 1423 K in air for 43.2 ks. The Vickers hardness increased from 14.2 to 17.2 GPa with decreasing grain size from 7.45 to 0.23 m. © 2012 Elsevier Ltd. All rights reserved. Keywords: Sintering; Optical properties; Hardness; Transparent ceramic
1. Introduction Lutetium aluminum garnet (Lu3 Al5 O12 , LuAG) is a promising host for scintillating materials owing to its high density (6.7 Mg m−3 ) and high effective atomic number (Zeff = 63). By doping with rare earth ions such as cerium (Ce3+ ) and praseodymium (Pr3+ ), LuAG transforms into an efficient and fast-response scintillator.1–3 LuAG, which is isostructural to yttrium aluminum garnet (YAG), is also a promising laser gain medium4 and a high-refractive index lens material for microlithographic imaging.5,6 Several studies have been conducted on single-crystal LuAG grown by Czochralski7,8 and micro-pulling-down methods.9 In particular, a transparent LuAG body has been prepared by pressureless sintering using powders prepared by coprecipitation10,11 and combustion12,13 methods. Reactive sintering in vacuum has also been used to fabricate Pr-doped LuAG.2,14 Hot isostatic pressing was performed to achieve high density and reduce pore size after vacuum sintering using a powder prepared by flame spray pyrolysis.15 Although highly
∗
Corresponding author. Tel.: +81 22 215 2106; fax: +81 22 215 2107. E-mail address: [email protected] (A. Ito).
0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.04.020
transparent LuAG bodies have been prepared, high sintering temperatures (over 1973 K) and prolonged sintering (greater than 14.4 ks) are drawbacks of pressureless sintering. These transparent LuAG ceramics usually have large grains in several to tens micrometers with poor mechanical properties. Spark plasma sintering (SPS) is a versatile technique to obtain fully dense bodies with fine grains because of the fast densification at a relatively low temperature and in a short time.16 By controlling sintering conditions, i.e., temperature, pressure and holing time, highly transparent ceramics have been fabricated with enhanced mechanical properties.17–20 However, there are no reports on the preparation of transparent LuAG bodies by SPS. In the present study, transparent LuAG bodies were prepared by reactive SPS. In addition, the effect of the sintering temperature on the crystal phase, microstructure, transparency and mechanical properties of LuAG bodies were investigated. 2. Experimental procedure Commercial Lu2 O3 powder (Shin-Etsu Rare Earth, Tokyo, Japan, 99.99% purity) and ␣-Al2 O3 powder (Wako Pure Chemical, Tokyo, Japan, 99.9% purity) were used as starting materials. These powders were mixed in the stoichiometric ratio
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Fig. 1. FESEM images of starting powders of (a) Lu2 O3 and (b) Al2 O3 .
Fig. 2. XRD patterns of the bodies sintered by SPS at (a) 1473 K and (b) 1573 K.
Lu:Al = 3:5 and ball milled with zirconia balls in ethanol for 43.2 ks (12 h). The milled slurry was dried at 333 K for 86.4 ks (24 h) and the powder mixture was ground and filtered through a 200-mesh sieve. The mixed powder was poured into a graphite
die with diameter 10 mm and then sintered using an SPS apparatus (SPS-210 LX, Fuji Electronic Industry, Japan) in vacuum. The sintering temperature was increased to 873 K in 180 s and to 1273 K in 240 s and then held for 300 s. The temperature was further increased to 1473–1923 K at 0.17 K s−1 and maintained for 2.7 ks (45 min). Pressure was maintained at 10 MPa up to 1273 K, and then it was increased to 100 MPa in 60 s. Both sides of as-sintered specimens were mirror polished using diamond slurry. The final thickness of the specimens was approximately 1 mm. Heat treatment was performed at 1423 K in air for 43.2 ks (12 h). Density was measured by the Archimedes method in distilled water, and the crystal phase was investigated by X-ray diffraction (XRD, RAD-2C, Rigaku, Japan). The polished surface was thermally etched in air for 3.6 ks (1 h) below the sintering temperature at 100–200 K. A field emission scanning electron microscope (FESEM, JSM-7500F, JEOL, Japan) and a scanning electron microscope (S-3100H, Hitachi, Japan) were used to observe the thermally etched surfaces and fracture surfaces of the sintered bodies. The average grain size was determined from the linear intercept length.21 The inline transmittance in the wavelength range 190–2500 nm was measured with a spectrophotometer (UV-3101PC, Shimadzu, Japan). Vickers hardness (HV ) and fracture toughness (KIC ) at room temperature were measured by a hardness tester (HM-221,
Fig. 3. FESEM images of thermally etched surfaces of LuAG bodies sintered at (a) 1723 K and (b) 1923 K. White circles refer to pores.
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3. Results and discussion
Fig. 4. Effect of sintering temperature on the relative density and average grain size of LuAG bodies.
Mitutoyo, Japan) at a load (P) of 9.8 N. Fracture toughness was calculated with Eq. (1) using the half-length of the crack (c) formed around the corners of indentations22 : P (1) KIC = 0.073 × c1.5
Fig. 1 shows the FESEM images of the starting powders. Both Lu2 O3 and Al2 O3 powders were nearly spherical and slightly agglomerated. The average grain sizes of Lu2 O3 and Al2 O3 were approximately 30 and 160 nm, respectively. Fig. 2 shows the XRD patterns of the bodies sintered by SPS at 1473 and 1573 K. At 1473 K, a LuAlO3 (LuAP) phase (JCPDS #24-0690) was identified with a small amount of LuAG phase (JCPDS #73-1368) (Fig. 2a). Single-phase LuAG was obtained at 1573 K (Fig. 2b). Furthermore, LuAG has no phase transformation up to the melting point (2316 K).23 Fig. 3 shows the FESEM images of thermally etched surfaces of the LuAG bodies sintered at 1723 and 1923 K. The grain size was 0.03–0.8 m and pores were rarely observed at 1723 K (Fig. 3a). In contrast, at 1923 K, the grains significantly grew and pores were located at the triple junctions of the grain boundaries (white circles in Fig. 3b). Fig. 4 shows the influence of the sintering temperature on the relative density and average grain size of the LuAG bodies. At 1573 K, the relative density was 99.5% of the theoretical value. With increasing sintering temperature to 1923 K, the relative density remained greater than 99.5% although a negligible amount of pores was observed, as shown in Fig. 3b. The grains grew steadily from 0.18 to 0.52 m with increasing sintering temperature from 1573 to 1773 K. Grain growth
Fig. 5. FESEM images of fracture surfaces of LuAG bodies sintered at (a) 1573 K, (b) 1673 K (c) 1723 K and (d) 1823 K. White circles refer to pores.
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Fig. 6. Photograph of as-sintered and annealed LuAG bodies sintered at (a) 1623 K, (b) 1673 K, (c) 1723 K, (d) 1773 K and (e) 1823 K. The text was 30 mm below the specimens.
became significant at 1823 K and the average grain size reached 7.45 m at 1923 K. Fig. 5 shows the fracture surfaces of the LuAG bodies sintered at 1573–1823 K. The fracture mode was mainly transgranular at a low sintering temperature of 1573 K and a high sintering temperature of 1823 K. At sintering temperatures of 1673 and 1723 K, the fracture mode was a mixture of transgranular and intergranular modes. Pores were observed at 1573 and 1673 K (white circle in Fig. 5a and b), whereas pores were rarely observed at 1723 and 1823 K (Fig. 5c and d) and grain growth became significant at 1823 K (Fig. 5d). Fig. 6 shows the photographs of as-prepared and the annealed LuAG bodies sintered at different temperatures. The text 30 mm below as-prepared LuAG bodies became more visible with increasing sintering temperature. As-sintered LuAG bodies had a yellow color, which might be due to a defect in aluminum garnet.24,25 LuAG bodies became colorless after annealing. Fig. 7 shows the transmittance spectra of as-prepared LuAG bodies. The transmittance increased with the sintering temperature up to 1773 K and then decreased. An absorption peak was located at around 420 nm in the specimens sintered at
Fig. 7. Transmittance spectra of as-sintered LuAG bodies at 1573–1923 K.
Fig. 8. Transmittance spectra of LuAG bodies after annealing at 1423 K in air for 43.2 ks. Dotted line indicates the transmittance of LuAG single crystal in the wavelength range of 410–1970 nm.26
1723–1823 K. This peak may be related to the yellow color of as-prepared specimens. Fig. 8 shows the transmittance spectra of the LuAG bodies annealed at 1423 K in air for 43.2 ks. The dotted line represents the transmittance of single-crystal LuAG in the wavelength range 410–1970 nm.26 The transmittance after annealing increased particularly in the ultraviolet and visible ranges. For example, the LuAG body sintered at 1773 K exhibited a transmittance of 8.4% before annealing and 25.2% after annealing, respectively, at a wavelength of 500 nm. Moreover, the absorption peak of as-prepared specimens diminished. Although the relative density was greater than 99.5% at 1823 and 1923 K (Fig. 3), significant grain growth and increase in porosity as shown in Fig. 3b may decrease the transparency of LuAG at high temperatures. It is reported that fully dense LuAG (greater than 99.6% of the theoretical value) prepared by vacuum sintering at 2073 K for 36 ks using powder precursors from the urea homogeneous precipitation method had the highest transmittance (71%) at 550 nm (the value was normalized to 1 mm thickness).11 In previously reported vacuum sintering, the sintering temperature was higher than 1973 K and holding time was greater than 14.4 ks.12,14 In the present study, the sintering temperature (1773 K) for obtaining fully dense bodies with the highest transmittance was about 200–400 K lower and the holding time (2.7 ks) was about 1/3–1/9 times shorter than that reported.10–15 Fig. 9 shows the effect of sintering temperature on the Vickers hardness (HV ) and fracture toughness of the annealed LuAG bodies. The Vickers hardness of the fully dense LuAG body sintered at 1573 K was 16.8 GPa, and it slightly decreased from 17.2 to 14.2 GPa with increasing sintering temperature from 1623 to 1923 K owing to grain growth. A similar behavior has been reported in various ceramics such as Al2 O3 ,27 Ca10 (PO4 )6 (OH)2 (calcium hydroxyapatite)28 and Lu2 Ti2 O7 .29 The highest hardness (17.2 GPa) was obtained for the specimen sintered at 1623 K with an average grain size of 0.23 m.
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Fig. 9. (a) Effect of sintering temperature on Vickers hardness and fracture toughness of annealed LuAG bodies and (b) the Vickers hardness as a function of the inverse square root of the grain size.
The fracture toughness was 1.2–1.6 MPa m1/2 and had no direct correlation with grain size. Fig. 9(b) shows HV as a function of the inverse square root of the grain size (d). HV shows linear dependency with d−1/2 , obeying the Hall–Petch relationship. This may be attributed to the reduced free path for dislocations with decreasing grain size.30 4. Conclusions Transparent LuAG was fabricated by reactive spark plasma sintering. Single-phase LuAG bodies with high density (>99.5%) were obtained at the sintering temperatures 1573–1923 K. The average grain size of the LuAG bodies was 0.18–0.52 m at 1573–1773 K and the optimal sintering temperature was 1773 K, showing a transmittance of 77.8% at 2000 nm after annealing at 1423 K in air for 43.2 ks. Grain growth became significant at 1823 K and pores were observed at the triple junctions of the grain boundaries at high sintering temperatures, which decreased transparency. By increasing the sintering temperature from 1623 to 1923 K, the Vickers hardness decreased from 17.2 GPa to 14.2 GPa as the grain size increased from 0.23 to 7.45 m. Acknowledgments This research was supported in part by the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. This research was also supported in part by the Global COE Program of Materials Integration, Tohoku University. References 1. Nikl M, Mihoková E, Mareˇs JA, Vedda A, Martini M, Nejezchleb K, et al. Traps and timing characteristics of LuAG:Ce3+ scintillator. Phys Status Solidi A 2000;181:R10–2. 2. Yanagida T, Yoshikawa A, Ikesue A, Kamada K, Yokota Y. Basic properties of ceramic Pr:LuAG scintillators. IEEE Trans Nucl Sci 2009;56:2955–9.
3. Yoshikawa A, Yanagida T, Kamada K, Yokota Y, Pejchal J, Yamaji A, et al. Positron emission mammography using Pr:LuAG scintillator – Fusion of optical material study and systems engineering. Opt Mater 2010;32:1294–7. 4. Duan XM, Yao BQ, Li G, Ju YL, Wang YZ, Zhao GJ. High efficient actively Q-switched Ho:LuAG laser. Opt Express 2009;17:21691–7. 5. Wei YY, Brainard RL. Advanced processes for 193-nm immersion lithography. SPIE Press; 2009. 6. Letz M, Gottwald A, Richter M, Liberman V, Parthier L. Temperaturedependent Urbach tail measurements of lutetium aluminum garnet single crystals. Phys Rev B 2010;81:155109. 7. Kamada K, Yanagida T, Tsutsumi K, Usuki Y, Sato M, Ogino H, et al. Scintillation properties of 2-inch-diameter (LuAG) single crystal. IEEE Trans Nucl Sci 2009;56:570–3. 8. Ogino H, Yoshikawa A, Nikl M, Krasnikov A, Kamada K, Fukuda T. Growth and scintillation properties of Pr-doped Lu3 Al5 O12 crystals. J Cryst Growth 2006;287:335–8. 9. Yoshikawa A, Ogino H, Lee JH, Nikl M, Solovieva N, Garnier N, et al. Growth and optical properties of Yb doped new scintillator crystals. Opt Mater 2003;24:275–9. 10. Liao YK, Jiang DY, Shi JL. Transparent lutetium aluminum garnet sintered from carbonate coprecipitated powders. Mater Lett 2005;59:3724–7. 11. Li HL, Liu XJ, Xie RJ, Zhou GH, Hirosasi N, Pu XP, et al. Cerium-doped lutetium aluminum garnet phosphors and optically transparent ceramics prepared from powder precursors by a urea homogeneous precipitation method. Jpn J Appl Phys 2008;47:1657–61. 12. Cherepy NJ, Kuntz JD, Tillotson TM, Speaks DT, Payne SA, Chai BHT, et al. Cerium-doped single crystal and transparent ceramic lutetium aluminum garnet scintillators. Nucl Instrum Methods A 2007;579:38–41. 13. Liu XJ, Li HL, Xie RJ, Hirosaki N, Xu X, Huang LP. Cerium-doped lutetium aluminum garnet optically transparent ceramics fabricated by a sol–gel combustion process. J Mater Res 2006;21:1519–25. 14. Shi Y, Nikl M, Feng XQ, Mares JA, Shen YQ, Beitlerova A, et al. Microstructure, optical, and scintillation characteristics of Pr3+ doped Lu3 Al5 O12 optical ceramics. J Appl Phys 2011;109:013522. 15. Kuntz JD, Roberts JJ, Hough M, Cherepy NJ. Multiple synthesis routes to transparent ceramic lutetium aluminum garnet. Scripta Mater 2007;57:960–3. 16. Munir ZA, Quach DV, Ohyanagi M. Electric current activation of sintering: a review of the pulsed electric current sintering process. J Am Ceram Soc 2011;94:1–19. 17. Wang C, Zhao Z. Transparent polycrystalline ruby ceramic by spark plasma sintering. Mater Res Bull 2010;45:1127–31. 18. Morita K, Kim BN, Hiraga K, Yoshida H. Fabrication of high-strength transparent MgAl2 O4 spinel polycrystals by optimizing spark-plasma-sintering conditions. J Mater Res 2009;24:2863–72.
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19. An LQ, Ito A, Goto T. Highly transparent lutetium titanium oxide produced by spark plasma sintering. J Eur Ceram Soc 2011;31:237–40. 20. An LQ, Ito A, Goto T. Transparent yttria produced by spark plasma sintering at moderate temperature and pressure profiles. J Eur Ceram Soc 2012;32:1035–40. 21. Mendelson MI. Average grain size in polycrystalline ceramics. J Am Ceram Soc 1969;52:443–6. 22. Lawn BR, Fuller ER. Equilibrium penny-like cracks in indentation fracture. J Mater Sci 1975;10:2016–24. 23. Wu P, Pelton AD. Coupled thermodynamic-phase diagram assessment of the rare earth oxide–aluminium oxide binary systems. J Alloys Compd 1992;179:259–87. 24. Springis M, Pujats A, Valbis J. Polarization of luminescence of colour centres in YAG crystals. J Phys 1991;3:5457–61.
25. Babin V, Laguta VV, Maaroos A, Makhov A, Nikl M, Zazubovich S. Luminescence of F+ -type centers in undoped Lu3 Al5 O12 single crystals. Phys Status Solidi B 2010;248:239–42. 26. Kuwano Y, Suda K, Ishizawa N, Yamada T. Crystal growth and properties of (Lu,Y)3 Al5 O12 . J Cryst Growth 2004;260:159–65. 27. Shen ZJ, Johnsson M, Zhao Z, Nygren M. Spark plasma sintering of alumina. J Am Ceram Soc 2002;85:1921–7. 28. Wang JW, Shaw LL. Grain-size dependence of the hardness of submicrometer and nanometer hydroxyapatite. J Am Ceram Soc 2010;93:601–4. 29. An LQ, Ito A, Goto T. Effects of sintering and annealing temperature on fabrication of transparent Lu2 Ti2 O7 by spark plasma sintering. J Am Ceram Soc 2011;94:3851–5. 30. Krell A, Blank P. Grain size dependence of hardness in dense submicrometer alumina. J Am Ceram Soc 1995;78:1118–20.
Journal of the European Ceramic Society 32 (2012) 3097–3102
Effect of sintering temperature on the transparency and mechanical properties of lutetium aluminum garnet fabricated by spark plasma sintering Liqiong An a,b , Akihiko Ito a,∗ , Takashi Goto a a
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan b Japan Society for the Promotion of Science, Japan
Received 9 February 2012; received in revised form 2 April 2012; accepted 12 April 2012 Available online 30 April 2012
Abstract Transparent lutetium aluminum garnet (Lu3 Al5 O12 , LuAG) was fabricated by reactive spark plasma sintering. The effect of sintering temperature on the crystal phase, microstructure, transparency and mechanical properties of LuAG bodies was investigated. Fully dense and single-phase LuAG bodies were obtained at sintering temperatures 1573–1923 K. The average grain size increased from 0.18 to 0.52 m with increasing sintering temperature from 1573 to 1773 K, and grain growth became significant at 1823 K. Transmittance showed a maximum value of 77.8% at 2000 nm at a sintering temperature of 1773 K after annealing at 1423 K in air for 43.2 ks. The Vickers hardness increased from 14.2 to 17.2 GPa with decreasing grain size from 7.45 to 0.23 m. © 2012 Elsevier Ltd. All rights reserved. Keywords: Sintering; Optical properties; Hardness; Transparent ceramic
1. Introduction Lutetium aluminum garnet (Lu3 Al5 O12 , LuAG) is a promising host for scintillating materials owing to its high density (6.7 Mg m−3 ) and high effective atomic number (Zeff = 63). By doping with rare earth ions such as cerium (Ce3+ ) and praseodymium (Pr3+ ), LuAG transforms into an efficient and fast-response scintillator.1–3 LuAG, which is isostructural to yttrium aluminum garnet (YAG), is also a promising laser gain medium4 and a high-refractive index lens material for microlithographic imaging.5,6 Several studies have been conducted on single-crystal LuAG grown by Czochralski7,8 and micro-pulling-down methods.9 In particular, a transparent LuAG body has been prepared by pressureless sintering using powders prepared by coprecipitation10,11 and combustion12,13 methods. Reactive sintering in vacuum has also been used to fabricate Pr-doped LuAG.2,14 Hot isostatic pressing was performed to achieve high density and reduce pore size after vacuum sintering using a powder prepared by flame spray pyrolysis.15 Although highly
∗
Corresponding author. Tel.: +81 22 215 2106; fax: +81 22 215 2107. E-mail address: [email protected] (A. Ito).
0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.04.020
transparent LuAG bodies have been prepared, high sintering temperatures (over 1973 K) and prolonged sintering (greater than 14.4 ks) are drawbacks of pressureless sintering. These transparent LuAG ceramics usually have large grains in several to tens micrometers with poor mechanical properties. Spark plasma sintering (SPS) is a versatile technique to obtain fully dense bodies with fine grains because of the fast densification at a relatively low temperature and in a short time.16 By controlling sintering conditions, i.e., temperature, pressure and holing time, highly transparent ceramics have been fabricated with enhanced mechanical properties.17–20 However, there are no reports on the preparation of transparent LuAG bodies by SPS. In the present study, transparent LuAG bodies were prepared by reactive SPS. In addition, the effect of the sintering temperature on the crystal phase, microstructure, transparency and mechanical properties of LuAG bodies were investigated. 2. Experimental procedure Commercial Lu2 O3 powder (Shin-Etsu Rare Earth, Tokyo, Japan, 99.99% purity) and ␣-Al2 O3 powder (Wako Pure Chemical, Tokyo, Japan, 99.9% purity) were used as starting materials. These powders were mixed in the stoichiometric ratio
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Fig. 1. FESEM images of starting powders of (a) Lu2 O3 and (b) Al2 O3 .
Fig. 2. XRD patterns of the bodies sintered by SPS at (a) 1473 K and (b) 1573 K.
Lu:Al = 3:5 and ball milled with zirconia balls in ethanol for 43.2 ks (12 h). The milled slurry was dried at 333 K for 86.4 ks (24 h) and the powder mixture was ground and filtered through a 200-mesh sieve. The mixed powder was poured into a graphite
die with diameter 10 mm and then sintered using an SPS apparatus (SPS-210 LX, Fuji Electronic Industry, Japan) in vacuum. The sintering temperature was increased to 873 K in 180 s and to 1273 K in 240 s and then held for 300 s. The temperature was further increased to 1473–1923 K at 0.17 K s−1 and maintained for 2.7 ks (45 min). Pressure was maintained at 10 MPa up to 1273 K, and then it was increased to 100 MPa in 60 s. Both sides of as-sintered specimens were mirror polished using diamond slurry. The final thickness of the specimens was approximately 1 mm. Heat treatment was performed at 1423 K in air for 43.2 ks (12 h). Density was measured by the Archimedes method in distilled water, and the crystal phase was investigated by X-ray diffraction (XRD, RAD-2C, Rigaku, Japan). The polished surface was thermally etched in air for 3.6 ks (1 h) below the sintering temperature at 100–200 K. A field emission scanning electron microscope (FESEM, JSM-7500F, JEOL, Japan) and a scanning electron microscope (S-3100H, Hitachi, Japan) were used to observe the thermally etched surfaces and fracture surfaces of the sintered bodies. The average grain size was determined from the linear intercept length.21 The inline transmittance in the wavelength range 190–2500 nm was measured with a spectrophotometer (UV-3101PC, Shimadzu, Japan). Vickers hardness (HV ) and fracture toughness (KIC ) at room temperature were measured by a hardness tester (HM-221,
Fig. 3. FESEM images of thermally etched surfaces of LuAG bodies sintered at (a) 1723 K and (b) 1923 K. White circles refer to pores.
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Fig. 4. Effect of sintering temperature on the relative density and average grain size of LuAG bodies.
Mitutoyo, Japan) at a load (P) of 9.8 N. Fracture toughness was calculated with Eq. (1) using the half-length of the crack (c) formed around the corners of indentations22 : P (1) KIC = 0.073 × c1.5
Fig. 1 shows the FESEM images of the starting powders. Both Lu2 O3 and Al2 O3 powders were nearly spherical and slightly agglomerated. The average grain sizes of Lu2 O3 and Al2 O3 were approximately 30 and 160 nm, respectively. Fig. 2 shows the XRD patterns of the bodies sintered by SPS at 1473 and 1573 K. At 1473 K, a LuAlO3 (LuAP) phase (JCPDS #24-0690) was identified with a small amount of LuAG phase (JCPDS #73-1368) (Fig. 2a). Single-phase LuAG was obtained at 1573 K (Fig. 2b). Furthermore, LuAG has no phase transformation up to the melting point (2316 K).23 Fig. 3 shows the FESEM images of thermally etched surfaces of the LuAG bodies sintered at 1723 and 1923 K. The grain size was 0.03–0.8 m and pores were rarely observed at 1723 K (Fig. 3a). In contrast, at 1923 K, the grains significantly grew and pores were located at the triple junctions of the grain boundaries (white circles in Fig. 3b). Fig. 4 shows the influence of the sintering temperature on the relative density and average grain size of the LuAG bodies. At 1573 K, the relative density was 99.5% of the theoretical value. With increasing sintering temperature to 1923 K, the relative density remained greater than 99.5% although a negligible amount of pores was observed, as shown in Fig. 3b. The grains grew steadily from 0.18 to 0.52 m with increasing sintering temperature from 1573 to 1773 K. Grain growth
Fig. 5. FESEM images of fracture surfaces of LuAG bodies sintered at (a) 1573 K, (b) 1673 K (c) 1723 K and (d) 1823 K. White circles refer to pores.
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Fig. 6. Photograph of as-sintered and annealed LuAG bodies sintered at (a) 1623 K, (b) 1673 K, (c) 1723 K, (d) 1773 K and (e) 1823 K. The text was 30 mm below the specimens.
became significant at 1823 K and the average grain size reached 7.45 m at 1923 K. Fig. 5 shows the fracture surfaces of the LuAG bodies sintered at 1573–1823 K. The fracture mode was mainly transgranular at a low sintering temperature of 1573 K and a high sintering temperature of 1823 K. At sintering temperatures of 1673 and 1723 K, the fracture mode was a mixture of transgranular and intergranular modes. Pores were observed at 1573 and 1673 K (white circle in Fig. 5a and b), whereas pores were rarely observed at 1723 and 1823 K (Fig. 5c and d) and grain growth became significant at 1823 K (Fig. 5d). Fig. 6 shows the photographs of as-prepared and the annealed LuAG bodies sintered at different temperatures. The text 30 mm below as-prepared LuAG bodies became more visible with increasing sintering temperature. As-sintered LuAG bodies had a yellow color, which might be due to a defect in aluminum garnet.24,25 LuAG bodies became colorless after annealing. Fig. 7 shows the transmittance spectra of as-prepared LuAG bodies. The transmittance increased with the sintering temperature up to 1773 K and then decreased. An absorption peak was located at around 420 nm in the specimens sintered at
Fig. 7. Transmittance spectra of as-sintered LuAG bodies at 1573–1923 K.
Fig. 8. Transmittance spectra of LuAG bodies after annealing at 1423 K in air for 43.2 ks. Dotted line indicates the transmittance of LuAG single crystal in the wavelength range of 410–1970 nm.26
1723–1823 K. This peak may be related to the yellow color of as-prepared specimens. Fig. 8 shows the transmittance spectra of the LuAG bodies annealed at 1423 K in air for 43.2 ks. The dotted line represents the transmittance of single-crystal LuAG in the wavelength range 410–1970 nm.26 The transmittance after annealing increased particularly in the ultraviolet and visible ranges. For example, the LuAG body sintered at 1773 K exhibited a transmittance of 8.4% before annealing and 25.2% after annealing, respectively, at a wavelength of 500 nm. Moreover, the absorption peak of as-prepared specimens diminished. Although the relative density was greater than 99.5% at 1823 and 1923 K (Fig. 3), significant grain growth and increase in porosity as shown in Fig. 3b may decrease the transparency of LuAG at high temperatures. It is reported that fully dense LuAG (greater than 99.6% of the theoretical value) prepared by vacuum sintering at 2073 K for 36 ks using powder precursors from the urea homogeneous precipitation method had the highest transmittance (71%) at 550 nm (the value was normalized to 1 mm thickness).11 In previously reported vacuum sintering, the sintering temperature was higher than 1973 K and holding time was greater than 14.4 ks.12,14 In the present study, the sintering temperature (1773 K) for obtaining fully dense bodies with the highest transmittance was about 200–400 K lower and the holding time (2.7 ks) was about 1/3–1/9 times shorter than that reported.10–15 Fig. 9 shows the effect of sintering temperature on the Vickers hardness (HV ) and fracture toughness of the annealed LuAG bodies. The Vickers hardness of the fully dense LuAG body sintered at 1573 K was 16.8 GPa, and it slightly decreased from 17.2 to 14.2 GPa with increasing sintering temperature from 1623 to 1923 K owing to grain growth. A similar behavior has been reported in various ceramics such as Al2 O3 ,27 Ca10 (PO4 )6 (OH)2 (calcium hydroxyapatite)28 and Lu2 Ti2 O7 .29 The highest hardness (17.2 GPa) was obtained for the specimen sintered at 1623 K with an average grain size of 0.23 m.
L. An et al. / Journal of the European Ceramic Society 32 (2012) 3097–3102
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Fig. 9. (a) Effect of sintering temperature on Vickers hardness and fracture toughness of annealed LuAG bodies and (b) the Vickers hardness as a function of the inverse square root of the grain size.
The fracture toughness was 1.2–1.6 MPa m1/2 and had no direct correlation with grain size. Fig. 9(b) shows HV as a function of the inverse square root of the grain size (d). HV shows linear dependency with d−1/2 , obeying the Hall–Petch relationship. This may be attributed to the reduced free path for dislocations with decreasing grain size.30 4. Conclusions Transparent LuAG was fabricated by reactive spark plasma sintering. Single-phase LuAG bodies with high density (>99.5%) were obtained at the sintering temperatures 1573–1923 K. The average grain size of the LuAG bodies was 0.18–0.52 m at 1573–1773 K and the optimal sintering temperature was 1773 K, showing a transmittance of 77.8% at 2000 nm after annealing at 1423 K in air for 43.2 ks. Grain growth became significant at 1823 K and pores were observed at the triple junctions of the grain boundaries at high sintering temperatures, which decreased transparency. By increasing the sintering temperature from 1623 to 1923 K, the Vickers hardness decreased from 17.2 GPa to 14.2 GPa as the grain size increased from 0.23 to 7.45 m. Acknowledgments This research was supported in part by the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. This research was also supported in part by the Global COE Program of Materials Integration, Tohoku University. References 1. Nikl M, Mihoková E, Mareˇs JA, Vedda A, Martini M, Nejezchleb K, et al. Traps and timing characteristics of LuAG:Ce3+ scintillator. Phys Status Solidi A 2000;181:R10–2. 2. Yanagida T, Yoshikawa A, Ikesue A, Kamada K, Yokota Y. Basic properties of ceramic Pr:LuAG scintillators. IEEE Trans Nucl Sci 2009;56:2955–9.
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