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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 7 — Jul. 1, 2013
  • pp: 908–912
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Two-step sintering of Gd0.3Lu1.6Eu0.1O3 transparent ceramic scintillator

Zachary Seeley, Nerine Cherepy, and Stephen Payne  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 7, pp. 908-912 (2013)
http://dx.doi.org/10.1364/OME.3.000908


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Abstract

Transparent ceramic scintillators with the composition Gd0.3Lu1.6Eu0.1O3 (GLO:Eu) have been prepared by different sintering profiles: a traditional profile consisting of a slow ramp followed by a dwell, and a two-step profile consisting of a fast ramp and short dwell followed by a long dwell at a lower temperature. A subsequent Hot Isostatic Press (HIP) step was used to achieve full density and transparency. Two-step sintering allowed full transparency to be achieved after HIPing at 1525°C, while traditionally sintered samples required 1850°C in the HIP to achieve high transparency indicating that two-step sintering is successful in maintaining a small grain size and therefore allowing densification to be decoupled from grain growth during the low temperature HIP step. HIPing at elevated temperatures between 1525 and 1850°C resulted in rapid grain growth from sub-micron to ~300 µm grains. Radioluminescence spectra show negligible difference between samples with sub-micron grain size and those with 300 µm grains.

© 2013 OSA

1. Introduction

Transparent polycrystalline ceramics are fully dense monoliths comprised of small crystallites in random orientations. Usually formed by sintering and densifying ceramic nanoparticles, transparency is achieved by selecting optically isostropic cubic crystal structures and minimizing scattering defects such as residual porosity and the presence of secondary phases [1

1. A. Ikesue, Y. L. Aung, T. Taira, T. Kamimura, K. Yoshida, and G. Messing, “Progress in ceramic lasers,” Annu. Rev. Mater. Res. 36(1), 397–429 (2006). [CrossRef]

]. Due to their fabrication via a solid-state route, transparent ceramics unveil potential material compositions for optics that would otherwise be difficult to manufacture in the single crystal form due to incongruency and/or high melting points [2

2. H. Lin, S. Zhou, and H. Teng, “Synthesis of Tb3Al5O12 (TAG) transparent ceramics for potential magneto-optical applications,” Opt. Mater. 33(11), 1833–1836 (2011). [CrossRef]

,3

3. S. F. Wang, J. Zhang, D. W. Luo, F. Gu, D. Y. Tang, Z. L. Dong, G. E. B. Tan, W. X. Que, T. S. Zhang, S. Li, and L. B. Kong, “Transparent ceramics: processing, materials and applications,” Prog. Solid State Chem. 41(1-2), 20–54 (2013). [CrossRef]

].

Achieving full density is traditionally accomplished via a careful high temperature sintering schedule, whereby the ceramic achieves transparency [4

4. A. Ikesue, A. Furusato, and K. Kamata, “Fabrication of polycrystalline, transparent YAG ceramics by solid-state reaction method,” J. Am. Ceram. Soc. 78(1), 225–228 (1995). [CrossRef]

]. The main driving force causing this densification is reduction in surface energy, and is usually accompanied by particle coarsening. Grain boundaries are the predominant pathway for atomic diffusion in ceramics and therefore as particles coarsen, densification slows. This phenomenon has traditionally led to slow, high temperature sintering profiles resulting in large-grained transparent ceramics.

More recently, a novel two-step sintering profile has been adopted for several different transparent ceramics materials to reach full density without significant grain growth [5

5. I. W. Chen and X. H. Wang, “Sintering dense nanocrystalline ceramics without final-stage grain growth,” Nature 404(6774), 168–171 (2000). [CrossRef] [PubMed]

7

7. H. R. Khosroshahi, H. Ikeda, K. Yamada, N. Saito, K. Kaneko, K. Hayashi, and K. Nakashima, “Effect of cation doping on mechanical properties of yttria prepared by an optimized two-step sintering process,” J. Am. Ceram. Soc. 95(10), 3263–3269 (2012). [CrossRef]

]. In this approach, ceramics are rapidly heated to a high temperature (T1), but with only a short dwell time before cooling to a lower temperature (T2), for an extended dwell time. During the first step, the grain size is established, but the dwell is not long enough to allow significant grain growth. Subsequently, in the second step the grain size is frozen at the lower temperature dwell, however the temperature is sufficient to allow atomic diffusion and densification. This profile allows the densification to be decoupled from grain growth resulting in fine-grained, fully dense transparent ceramics.

Grain size reduction in transparent ceramics will have several beneficial implications. A higher concentration of grain boundaries will strengthen the material as well as provide diffusion pathways allowing more efficient densification. Also, scattering defects such as pores or secondary phases tend to coarsen with the grains during sintering; therefore a smaller grain size will inevitably lead to smaller defects improving transparency. And finally, more specific to bixbyites in particular, these ceramics go through a rapid grain growth temperature region where pore-grain boundary breakaway can occur leading to trapped residual porosity and reduced transparency [8

8. Z. M. Seeley, J. D. Kuntz, N. J. Cherepy, and S. A. Payne, “Transparent Lu2O3:Eu ceramics by sinter and HIP optimization,” Opt. Mater. 33(11), 1721–1726 (2011). [CrossRef]

]. Two-step sintering avoids this higher temperature region where densification is plagued by rapid grain growth and improves the probability of fabricating highly transparent ceramics. We previously reported that Lu2O3-based ceramics doped with europium can be fabricated to high transparency via the traditional sintering route [8

8. Z. M. Seeley, J. D. Kuntz, N. J. Cherepy, and S. A. Payne, “Transparent Lu2O3:Eu ceramics by sinter and HIP optimization,” Opt. Mater. 33(11), 1721–1726 (2011). [CrossRef]

], and have optimized the transparency with the composition Gd0.3Lu1.6Eu0.1O3 (GLO:Eu) which creates a stable cubic lattice parameter [9

9. Z. M. Seeley, Z. R. Dai, J. D. Kuntz, N. J. Cherepy, and S. A. Payne, “Phase stabilization in transparent Lu2O3:Eu ceramics by lattice expansion,” Opt. Mater. 35(1), 74–78 (2012). [CrossRef]

]. This material displays attractive performance for use as an x-ray detector scintillator screen used in radiography [10

10. J. W. T. Heemskerk, R. Kreuger, M. C. Goorden, M. A. N. Korevaar, S. Salvador, Z. M. Seeley, N. J. Cherepy, E. van der Kolk, S. A. Payne, P. Dorenbos, and F. J. Beekman, “Experimental comparison of high-density scintillators for EMCCD-based gamma ray imaging,” Phys. Med. Biol. 57(14), 4545–4554 (2012). [CrossRef] [PubMed]

,11

11. T. Martin, P.-A. Douissard, Z. Seeley, N. Cherepy, S. Payne, E. Mathieu, and J. Schuladen, “New high stopping power thin scintillators based on Lu2O3 and Lu3Ga5-xInxO12 for high resolution x-ray imaging,” IEEE Trans. Nucl. Sci. 59(5), 2269–2274 (2012). [CrossRef]

]; however these samples were processed at very high temperatures, sufficient to grow grains as large as 500 µm. In the present study, we have found two-step sintering to be successful in achieving sub-micrometer grain size while maintaining transparency and scintillation properties for the bixbyite Gd0.3Lu1.6Eu0.1O3. Smaller grain size furthermore is likely to offer improved mechanical properties, desirable in fabrication of thin (<3 mm) sheets.

2. Experimental procedure

Nanoparticles with the composition Gd0.3Lu1.6Eu0.1O3 (GLO:Eu) were synthesized via the flame spray pyrolysis (FSP) method by NanoceroxTM having a particle size of 30 nm and a specific surface area of 18 m2/g. Nanoparticles were suspended in an aqueous solution containing polyethylene glycol (PEG) and ammonium polymethacrylate (Darvan C-N) using an ultrasonic probe and a high shear mixer. This suspension was spray-dried at 210°C into flowing nitrogen to protect the organics. The dried powder was then sieved (<50µm) resulting in uniform agglomerates of nanoparticles with an even distribution of organic additives. Formulated nanoparticles were then uniaxially pressed at 50 MPa and isostatically pressed at 200MPa to form green compacts approximately 50% dense, followed by a heat treatment at 1050°C in air to burn out the organics. Calcined compacts were then loaded into a tungsten element vacuum furnace and sintered under a vacuum of <2 × 10−6 Torr to reach closed porosity and densities of approximately 97%. Profiles for traditional and two-step sintering are shown in Fig. 1
Fig. 1 Profiles for traditional (blue), and 2-step (red) sintering.
. The traditional profile consists of a slow ramp (1°/min) from 1050 to 1600°C followed by a 2 hour dwell. The two-step profile is a fast ramp (10°/min) to 1575°C with a 5 min dwell immediately followed by a 20 hour dwell at 1500°C. These were the lowest two-step temperatures determined experimentally to produce samples with closed porosity and therefore should result in minimal grain size. The sintered samples were then hot isostatically pressed (HIP’ed) under 200 MPa of inert argon gas pressure at temperatures ranging between 1525 and 1850°C for 4 h in a tungsten element HIP. Since the samples were closed porosity after vacuum sintering, no canning was necessary during the HIP step.

Ceramic surfaces were ground flat and parallel, given an inspection polish, and wipe-cleaned with acetone and methanol. Beta radioluminescence employed a 90Sr/90Y source (~1 MeV average beta energy). Radioluminescence spectra were collected with a Princeton Instruments/Acton Spec 10 spectrograph coupled to a thermoelectrically cooled CCD camera. Samples were thermally etched in air at 1450°C for 4h to show grain boundaries and analyzed for grain size with electron and optical microscopy.

3. Results and discussion

4. Summary

Transparent ceramic scintillators with the composition Gd0.3Lu1.6Eu0.1O3 have been prepared by different sintering profiles: a traditional profile consisting of a slow ramp followed by a dwell, and a two-step profile consisting of a fast ramp and short dwell followed by a long dwell at a lower temperature. Both sintering profiles resulted in approximately 97% of theoretical density, and a subsequent HIP step was used to achieve full density and transparency. Two-step sintering allowed full transparency to be achieved after HIPing at 1525°C, while traditionally sintered samples required 1850°C in the HIP to achieve high transparency indicating that two-step sintering is successful in maintaining a small grain size and therefore allowing densification to be decoupled from grain growth during the low temperature HIP step. However, HIPing too hot resulted in rapid grain growth from sub-micron to ~300 µm between 1525 and 1850°C. While radioluminescence spectra show negligible difference between samples with sub-micron grain size and those with 300 µm grain size, the smaller grain size makes for lower temperature HIP processing, and will likely lead to a more robust, more transparent ceramic. Therefore, two-step sintering is a viable method for producing transparent GLO:Eu ceramic scintillators for radiography screens.

Acknowledgments

Thanks to Keith Lewis, Todd Stefanik of Nanocerox Inc., Kiel Holliday, and the Confined Large Optical Scintillator Screen and Imaging System (CoLOSSIS) team including Patrick Allen, James Trebes, Daniel Schneberk, Roger Perry and Gary Stone. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the US DOE, Office of NNSA, Enhanced Surveillance Subprogram. LLNL-JRNL-632393

References and links

1.

A. Ikesue, Y. L. Aung, T. Taira, T. Kamimura, K. Yoshida, and G. Messing, “Progress in ceramic lasers,” Annu. Rev. Mater. Res. 36(1), 397–429 (2006). [CrossRef]

2.

H. Lin, S. Zhou, and H. Teng, “Synthesis of Tb3Al5O12 (TAG) transparent ceramics for potential magneto-optical applications,” Opt. Mater. 33(11), 1833–1836 (2011). [CrossRef]

3.

S. F. Wang, J. Zhang, D. W. Luo, F. Gu, D. Y. Tang, Z. L. Dong, G. E. B. Tan, W. X. Que, T. S. Zhang, S. Li, and L. B. Kong, “Transparent ceramics: processing, materials and applications,” Prog. Solid State Chem. 41(1-2), 20–54 (2013). [CrossRef]

4.

A. Ikesue, A. Furusato, and K. Kamata, “Fabrication of polycrystalline, transparent YAG ceramics by solid-state reaction method,” J. Am. Ceram. Soc. 78(1), 225–228 (1995). [CrossRef]

5.

I. W. Chen and X. H. Wang, “Sintering dense nanocrystalline ceramics without final-stage grain growth,” Nature 404(6774), 168–171 (2000). [CrossRef] [PubMed]

6.

K. Serivalsatit and J. Ballato, “Submicrometer grain-sized transparent erbium-doped Scandia ceramics,” J. Am. Ceram. Soc. 93(11), 3657–3662 (2010). [CrossRef]

7.

H. R. Khosroshahi, H. Ikeda, K. Yamada, N. Saito, K. Kaneko, K. Hayashi, and K. Nakashima, “Effect of cation doping on mechanical properties of yttria prepared by an optimized two-step sintering process,” J. Am. Ceram. Soc. 95(10), 3263–3269 (2012). [CrossRef]

8.

Z. M. Seeley, J. D. Kuntz, N. J. Cherepy, and S. A. Payne, “Transparent Lu2O3:Eu ceramics by sinter and HIP optimization,” Opt. Mater. 33(11), 1721–1726 (2011). [CrossRef]

9.

Z. M. Seeley, Z. R. Dai, J. D. Kuntz, N. J. Cherepy, and S. A. Payne, “Phase stabilization in transparent Lu2O3:Eu ceramics by lattice expansion,” Opt. Mater. 35(1), 74–78 (2012). [CrossRef]

10.

J. W. T. Heemskerk, R. Kreuger, M. C. Goorden, M. A. N. Korevaar, S. Salvador, Z. M. Seeley, N. J. Cherepy, E. van der Kolk, S. A. Payne, P. Dorenbos, and F. J. Beekman, “Experimental comparison of high-density scintillators for EMCCD-based gamma ray imaging,” Phys. Med. Biol. 57(14), 4545–4554 (2012). [CrossRef] [PubMed]

11.

T. Martin, P.-A. Douissard, Z. Seeley, N. Cherepy, S. Payne, E. Mathieu, and J. Schuladen, “New high stopping power thin scintillators based on Lu2O3 and Lu3Ga5-xInxO12 for high resolution x-ray imaging,” IEEE Trans. Nucl. Sci. 59(5), 2269–2274 (2012). [CrossRef]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(260.3800) Physical optics : Luminescence
(290.5930) Scattering : Scintillation

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: April 4, 2013
Revised Manuscript: May 20, 2013
Manuscript Accepted: May 29, 2013
Published: June 4, 2013

Virtual Issues
Optical Ceramics (2013) Optical Materials Express

Citation
Zachary Seeley, Nerine Cherepy, and Stephen Payne, "Two-step sintering of Gd0.3Lu1.6Eu0.1O3 transparent ceramic scintillator," Opt. Mater. Express 3, 908-912 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-7-908


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References

  1. A. Ikesue, Y. L. Aung, T. Taira, T. Kamimura, K. Yoshida, and G. Messing, “Progress in ceramic lasers,” Annu. Rev. Mater. Res.36(1), 397–429 (2006). [CrossRef]
  2. H. Lin, S. Zhou, and H. Teng, “Synthesis of Tb3Al5O12 (TAG) transparent ceramics for potential magneto-optical applications,” Opt. Mater.33(11), 1833–1836 (2011). [CrossRef]
  3. S. F. Wang, J. Zhang, D. W. Luo, F. Gu, D. Y. Tang, Z. L. Dong, G. E. B. Tan, W. X. Que, T. S. Zhang, S. Li, and L. B. Kong, “Transparent ceramics: processing, materials and applications,” Prog. Solid State Chem.41(1-2), 20–54 (2013). [CrossRef]
  4. A. Ikesue, A. Furusato, and K. Kamata, “Fabrication of polycrystalline, transparent YAG ceramics by solid-state reaction method,” J. Am. Ceram. Soc.78(1), 225–228 (1995). [CrossRef]
  5. I. W. Chen and X. H. Wang, “Sintering dense nanocrystalline ceramics without final-stage grain growth,” Nature404(6774), 168–171 (2000). [CrossRef] [PubMed]
  6. K. Serivalsatit and J. Ballato, “Submicrometer grain-sized transparent erbium-doped Scandia ceramics,” J. Am. Ceram. Soc.93(11), 3657–3662 (2010). [CrossRef]
  7. H. R. Khosroshahi, H. Ikeda, K. Yamada, N. Saito, K. Kaneko, K. Hayashi, and K. Nakashima, “Effect of cation doping on mechanical properties of yttria prepared by an optimized two-step sintering process,” J. Am. Ceram. Soc.95(10), 3263–3269 (2012). [CrossRef]
  8. Z. M. Seeley, J. D. Kuntz, N. J. Cherepy, and S. A. Payne, “Transparent Lu2O3:Eu ceramics by sinter and HIP optimization,” Opt. Mater.33(11), 1721–1726 (2011). [CrossRef]
  9. Z. M. Seeley, Z. R. Dai, J. D. Kuntz, N. J. Cherepy, and S. A. Payne, “Phase stabilization in transparent Lu2O3:Eu ceramics by lattice expansion,” Opt. Mater.35(1), 74–78 (2012). [CrossRef]
  10. J. W. T. Heemskerk, R. Kreuger, M. C. Goorden, M. A. N. Korevaar, S. Salvador, Z. M. Seeley, N. J. Cherepy, E. van der Kolk, S. A. Payne, P. Dorenbos, and F. J. Beekman, “Experimental comparison of high-density scintillators for EMCCD-based gamma ray imaging,” Phys. Med. Biol.57(14), 4545–4554 (2012). [CrossRef] [PubMed]
  11. T. Martin, P.-A. Douissard, Z. Seeley, N. Cherepy, S. Payne, E. Mathieu, and J. Schuladen, “New high stopping power thin scintillators based on Lu2O3 and Lu3Ga5-xInxO12 for high resolution x-ray imaging,” IEEE Trans. Nucl. Sci.59(5), 2269–2274 (2012). [CrossRef]

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