In recent years, rare-earth doped lutetium oxide (Lu2O3) has been recognized as a promising material for applications as scintillators and laser gain media because of its wide band gap (5.8 eV) and favorable properties such as high density (9.42 g/cm3), low thermal expansion, high thermal shock resistance, phase and chemical stability [1]. Fabrication of Lu2O3 materials at temperatures below the melting point using ceramic technology is an alternative approach to the crystal growth which is difficult due to the high melting temperatures above 2400 °C. Besides the lower fabrication temperature, transparent ceramics provide feasibility for larger size samples, multilayered structures and gradient doping profile for better laser performance. A review of recent literature shows that transparent Lu2O3 ceramics can be fabricated using cubic phase powders and presureless sintering under vacuum or flowing H2 atmosphere at the temperature as high as 1850 °C. It was also demonstated that the monoclinic phase nanoparticles (for instance, Y2O3 particles) act as an efficient driving force for low-temperature sintering and neck formation because of higher surface free energy than that of the cubic phase. However, the densification behavior of monoclinic Lu2O3 nanopopowder as well as the optical properties of transparent ceramics fabricated from these particles were not reported so far. In this work, we report on the synthesis of monoclinic Yb3+-doped Lu2O3 nanoparticles using a laser ablation and the fabrication of Yb:Lu2O3 transparent ceramics from these nanoparticles. Scanned Scanning electron microscopy indicated that the obtained nanopowder was mainly in the form of aggregates composed with of 15-25 nm spherical particles. From BET analysis, we determined the specific surface area of Yb:Lu2O3 nanoparticles as 35.2 m2/g and equivalent particle size as 17 nm. The XRD pattern of as-synthesized particles was in good agreement with XRD pattern of monoclinic B-type Lu2O3 (space group C2/m) reported for nanocrystalline films prepared by pulsed laser deposition and powders synthesized by radio frequency plasma spraying or flame spray pyrolysis. No significant phase changes were detected with an increase in calcination temperature up to 800 °C. The most intense peaks of C-type Lu2O3 at around 30° and 35° become narrower at 900 °C and the particles were fully converted into pure cubic phase after calcination at 1000 °C. The monoclinic phase Yb:Lu2O3 powder compact exhibited a minor shrinkage in the temperature range 200-500 °C. This feature could be attributed to low temperature sintering and neck formation due to apparently higher surface free-energy of monoclinic phase nanoparticles than compared to that of the cubic phase one. The next interesting feature was observed in the vicinity of 700 °C. An expansion of compact is explained by phase transformation, which is accompanied by a significant density decrease between the monoclinic B-type Lu2O3 (10.23 g/cm3), and the cubic C-type Lu2O3 phases (9.42 g/cm3). It is notable that the temperature for of phase transformation depends on the packing density of particles, i.e. in case of 39% dense compact, B->C transition occurs ~300 °C earlier than that of filled powder. Translucent Yb:Lu2O3 ceramics exhibiting an optical transmittance of 48.5% at the wavelength of 1080 nm was were obtained after conventional vacuum sintering of monoclinic phase compact at 1780 °C for 20 h. The optical quality of the obtained ceramics was greatly improved when the phase transformation occurred before sintering by calcination of powder at 1100 °C for 3 h in air. In this case, an optical transmittance of 76.6% at 1080 nm was achieved for 1.5-mm-thick transparent Yb:Lu2O3 sample. Rapid neck formation and phase conversion of monoclinic nanoparticles leads to limited densification of compact resulting in transparent edges and translucent inner part of ceramic sample. Consequently, the heating profile during vacuum sintering of monoclinic particles could be further optimized in order to obtain homogeneous, uniform and high optical quality Yb:Lu2O3 ceramics. References: 1. M. Guzik, J. Pejchal, A. Yoshikawa, A. Ito, T. Goto, M. Siczek, T. Lis, and G. Boulon, Cryst. Growth Des. 14 (2014) 3327.
Densification and phase transition of monoclinic Yb-doped Lu2O3 nanoparticles
L Esposito;J Hostasa;
2015
Abstract
In recent years, rare-earth doped lutetium oxide (Lu2O3) has been recognized as a promising material for applications as scintillators and laser gain media because of its wide band gap (5.8 eV) and favorable properties such as high density (9.42 g/cm3), low thermal expansion, high thermal shock resistance, phase and chemical stability [1]. Fabrication of Lu2O3 materials at temperatures below the melting point using ceramic technology is an alternative approach to the crystal growth which is difficult due to the high melting temperatures above 2400 °C. Besides the lower fabrication temperature, transparent ceramics provide feasibility for larger size samples, multilayered structures and gradient doping profile for better laser performance. A review of recent literature shows that transparent Lu2O3 ceramics can be fabricated using cubic phase powders and presureless sintering under vacuum or flowing H2 atmosphere at the temperature as high as 1850 °C. It was also demonstated that the monoclinic phase nanoparticles (for instance, Y2O3 particles) act as an efficient driving force for low-temperature sintering and neck formation because of higher surface free energy than that of the cubic phase. However, the densification behavior of monoclinic Lu2O3 nanopopowder as well as the optical properties of transparent ceramics fabricated from these particles were not reported so far. In this work, we report on the synthesis of monoclinic Yb3+-doped Lu2O3 nanoparticles using a laser ablation and the fabrication of Yb:Lu2O3 transparent ceramics from these nanoparticles. Scanned Scanning electron microscopy indicated that the obtained nanopowder was mainly in the form of aggregates composed with of 15-25 nm spherical particles. From BET analysis, we determined the specific surface area of Yb:Lu2O3 nanoparticles as 35.2 m2/g and equivalent particle size as 17 nm. The XRD pattern of as-synthesized particles was in good agreement with XRD pattern of monoclinic B-type Lu2O3 (space group C2/m) reported for nanocrystalline films prepared by pulsed laser deposition and powders synthesized by radio frequency plasma spraying or flame spray pyrolysis. No significant phase changes were detected with an increase in calcination temperature up to 800 °C. The most intense peaks of C-type Lu2O3 at around 30° and 35° become narrower at 900 °C and the particles were fully converted into pure cubic phase after calcination at 1000 °C. The monoclinic phase Yb:Lu2O3 powder compact exhibited a minor shrinkage in the temperature range 200-500 °C. This feature could be attributed to low temperature sintering and neck formation due to apparently higher surface free-energy of monoclinic phase nanoparticles than compared to that of the cubic phase one. The next interesting feature was observed in the vicinity of 700 °C. An expansion of compact is explained by phase transformation, which is accompanied by a significant density decrease between the monoclinic B-type Lu2O3 (10.23 g/cm3), and the cubic C-type Lu2O3 phases (9.42 g/cm3). It is notable that the temperature for of phase transformation depends on the packing density of particles, i.e. in case of 39% dense compact, B->C transition occurs ~300 °C earlier than that of filled powder. Translucent Yb:Lu2O3 ceramics exhibiting an optical transmittance of 48.5% at the wavelength of 1080 nm was were obtained after conventional vacuum sintering of monoclinic phase compact at 1780 °C for 20 h. The optical quality of the obtained ceramics was greatly improved when the phase transformation occurred before sintering by calcination of powder at 1100 °C for 3 h in air. In this case, an optical transmittance of 76.6% at 1080 nm was achieved for 1.5-mm-thick transparent Yb:Lu2O3 sample. Rapid neck formation and phase conversion of monoclinic nanoparticles leads to limited densification of compact resulting in transparent edges and translucent inner part of ceramic sample. Consequently, the heating profile during vacuum sintering of monoclinic particles could be further optimized in order to obtain homogeneous, uniform and high optical quality Yb:Lu2O3 ceramics. References: 1. M. Guzik, J. Pejchal, A. Yoshikawa, A. Ito, T. Goto, M. Siczek, T. Lis, and G. Boulon, Cryst. Growth Des. 14 (2014) 3327.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
