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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 Optical Society of America
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]. 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 [2,3].
Achieving full density is traditionally accomplished via a careful high temperature sintering schedule, whereby the ceramic achieves transparency [4]. 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 [5–7]. 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]. 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], and have optimized the transparency with the composition Gd0.3Lu1.6Eu0.1O3 (GLO:Eu) which creates a stable cubic lattice parameter [9]. This material displays attractive performance for use as an x-ray detector scintillator screen used in radiography [10,11]; 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. 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
Figure 2 shows a photograph comparing the final transparencies of GLO:Eu processed with two-step sintering and 1525°C HIP, and traditional sintering with 1650 and 1850°C HIP. The two-step sintered sample displays a high degree of transparency similar to our previously reported traditional sintered samples with a high temperature HIP (Fig. 2 right); however traditional sintering combined with a lower HIP temperature results in a degraded transparency caused by residual porosity (Fig. 2 middle). This result not only confirms that two step sintering is a viable process for achieving transparent GLO:Eu, but that it also allows full densification at a lower HIP temperature than traditional sintering.Figure 3 shows micrographs of the thermally etched surface of two-step sintered GLO:Eu samples after HIPing between 1525 and 1750°C. After HIPing at 1525°C, the grains are sub-micrometer size (average of 900 nm), but by 1650°C they have grown to about 50 µm and continue to grow to about 100 µm by 1750°C. This result is consistent with our previous findings that this material is subject to rapid grain growth at temperatures near 1650°C [8,9]; however, in this work, it becomes evident that this grain growth can occur in the HIP step as well as the vacuum sintering step. It is also evident that densification is occurring in the HIP at 1525°C without significant grain growth indicating that grain boundary diffusion is active at this temperature but that grain boundaries are not mobile. Therefore, in order to minimize grain size in the final transparent ceramic, it is critical to maintain a low sintering temperature by utilizing the two-step sintering method, but it is equally important to optimize the HIP temperature to allow for densification without grain growth.Finally, as the main application for GLO:Eu is as a radiography scintillator screen, it is important to verify that the reduction in grain size by 2.5 orders of magnitude achieved by this two-step sintering method does not impact the scintillation light yield. Figure 4 shows the radioluminescence spectra and integrated light yield for a traditionally sintered GLO:Eu sample with grain size of ~300 µm in comparison to a two-step sintered sample with sub-micron grain size. The two samples have almost identical spectra and total integrated light yield of 57,000 and 52,000 Photons/MeV, respectively, indicating the smaller grain size has negligible effect on the scintillation performance.
Fig. 2 Photograph of GLO:Eu samples after (left) two-step sintering and HIP at 1525 °C, (middle) traditional sintering with 1650°C HIP, and (right) traditional sintering with 1850°C HIP. Samples are 2 mm thick.
Fig. 3 Electron microscope images from the surface of GLO:Eu after thermal etching to show grain size. Two-step sintering followed by HIPing at (A) 1525°C, (B) 1650°C, and (C) 1750°C (optical image).
Fig. 4 Radioluminescence spectra and integrated light yield for traditional and two step sintered GLO:Eu.
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
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