Densification and phase transition of Yb-doped Lu2O3 nanoparticles synthesized by laser ablation

doi:10.1016/j.matlet.2016.09.038

Materials Letters

Volume 185, 15 December 2016, Pages 396-398

https://doi.org/10.1016/j.matlet.2016.09.038 Get rights and content

Highlights

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Morphology, phase evolution and densification of Yb: Lu₂O₃ nanoparticles were studied.
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Phase conversion during vacuum sintering led to a porous microstructure of ceramic.
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Transparent Yb: Lu₂O₃ ceramic was obtained using calcined powder.

Abstract

In this work we report on the effects induced by calcining monoclinic ytterbium-doped lutetium oxide (Yb: Lu₂O₃) nanoparticles on their phase transformation and densification behavior. Morphology and phase evolution of powder synthesized by laser ablation were studied by scanning electron microscopy and high temperature X-ray diffraction, respectively. The powder consisted of soft agglomerates comprising 15–25 nm spherical particles with monoclinic crystal structure. The beginning of phase transformation depends on the particle packing density, resulting in the temperatures of 700 °C and 1000 °C for loose and pressed powder, respectively. Transformation-assisted vacuum sintering of as-synthesized powder at 1780 °C for 20 h resulted in a core-shell and porous microstructure of Yb: Lu₂O₃ ceramic due to significant density decrease and obstructed densification during phase conversion. On the other hand, transparent Yb: Lu₂O₃ ceramic exhibiting an optical transmittance of 76.6% at the wavelength of 1080 nm was obtained using additional calcining of powder.

Introduction

In recent years, rare earth doped lutetium oxide (Lu₂O₃) has been recognized as a promising material for application 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], [2], [3], [4]. The most important advantage of Lu₂O₃ over the other sesquioxides (Y₂O₃ and Sc₂O₃) and yttrium-aluminum garnet (YAG) is its ability to keep a high thermal conductivity after Yb-ion doping since the atomic mass and ionic radii of Lu³⁺ are almost the same as those of Yb³⁺. Fabrication of Lu₂O₃ materials at temperatures below the melting point using ceramic technology is as an alternative approach to the crystal growth which is difficult due to the high melting temperatures above 2400 °C.

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 [5], [6], [7], [8]. It was also demonstrated that the monoclinic phase nanoparticles act as an efficient driving force for low-temperature sintering and neck formation because of higher surface free energy compared to that of the cubic phase [9], [10]. Consequently, it is expected that the use of monoclinic Lu₂O₃ particles can make it possible to lower the fabrication temperature of high optical quality ceramics.

The purpose of this work is to compare Yb-doped Lu₂O₃ nanoparticles with different phase composition in terms of densification behavior and optical properties of ceramics sintered at 1780 °C for 20 h under a vacuum.

Section snippets

Sample preparation

Monoclinic 1 at% Yb-doped Lu₂O₃ nanoparticles synthesized by laser ablation [11] were used as a starting material. To investigate the effects induced by different phase composition, a small part of powder was annealed at up to 1100 °C in order to reach full conversion into cubic phase. Then as-synthesized and calcined powders were uniaxially dry-pressed under a pressure of 200 MPa into 15-mm-diameter disks with a thickness of 3 mm. After a complete removal of residual organic components by

Results and discussion

The X-ray diffraction patterns of Yb:Lu₂O₃ nanopowder heat treated at various temperatures are shown in Fig. 1a. The XRD pattern of as-synthesized particles is in good agreement with XRD pattern of monoclinic B-type Lu₂O₃ (space group C2/m) reported for nanocrystalline films prepared by pulsed laser deposition [12] and powders synthesized by radio frequency plasma spraying [13] or flame spray pyrolysis [10]. No significant phase changes were detected with an increase in calcining temperature up

Conclusions

The nanosized Yb:Lu₂O₃ particles synthesized by laser ablation were studied and used as a starting material for the fabrication of transparent ceramics. The nanopowder was mainly in the form of soft aggregates consisting of 15–25 nm spherical particles composed of a metastable monoclinic phase. High temperature XRD showed that the phase transformation of the monoclinic phase starts at around 800 °C and the particles have fully converted to the cubic phase after calcining at 1000 °C. Rapid neck

Acknowledgments

The work was supported by RFBR (research projects Nos. 16-03-00193 A and 16-33-00826 mol_a) and by Act 211 Government of the Russian Federation, Contract no 02. A03.21.0006. The authors gratefully acknowledge Dr V.V. Platonov and Dr A.N. Orlov from Institute of Electrophysics UrB RAS for the synthesis of nanopowder by laser ablation and transmission spectra measurements, respectively.

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Densification and phase transition of Yb-doped Lu2O3 nanoparticles synthesized by laser ablation

Highlights

Abstract

Introduction

Section snippets

Sample preparation

Results and discussion

Conclusions

Acknowledgments

Opt. Mater.

Opt. Mater.

Mater. Lett.

Mat. Sci. Eng. A

Mater. Lett.

J. Lumin.

J. Eur. Ceram. Soc.

Densification and phase transition of Yb-doped Lu₂O₃ nanoparticles synthesized by laser ablation