Bulk Crystal Growth of Cr-doped MgAl2O4 spinel by Czochralski method and properties characterization
Introduction
Spinel oxides comprise an important class of compounds with a variety of interesting electrical, magnetic and optical properties. Among these oxides, the magnesium aluminum spinel (MgAl2O4) has long been considered as an important material, because it has a combination of desirable properties of high melting point, high strength, chemical resistance and unusual inertness to high-fluence neutron irradiation. The space group of MgAl2O4 is Fd3m (Oh7) and its lattice parameter is equals to 8.08435 Å [1], which can be easily doped by transition metal ions. Although, it is difficult to obtain large size crystal with good quality due to the high melting temperature (2130 °C) of MgAl2O4, some researchers have tried to grow MgAl2O4 crystals with different method such as Vierneuil method [2], [3], Float-zone [4], [5], Micro-pulling-down [6], [7] and Czochalski method [8], [9], [10].
Rencently, MgAl2O4 doped with Cr3+ has been reported, The-Long Phan et al. [11] reported the time resolved photoluminescence spectra of natural MgAl2O4 spinel doped with Cr3+ ions, M.G. Brik et al. [12], A.R. Molla et al. [13], P. Gluchowski et al. [14] and F. Rossi et al. [15] prepared Cr:MgAl2O4 with the form of nanocrystalline powders, glass and nanoceramics, respectively, and J. Someya et al. [16] grown Cr:MgAl2O4 crystal by the Floating-zone technique, in these reports photoluminescence were studied, which revealed that the Cr:MgAl2O4 is a new potential candidate for gain matrices as lamp pumped lasers and luminescent solar concentrators.
Czochralski is widely used for lager size crystals with the fewest flaws intended for optical applications. The melt flow in the Czochralski crystal growth contains the natural convection and the forced convection, which are generated by thermal gradients and rotation of crystal and crucible, respectively, and effective transport of impurities can be obtained by optimizing the two convections. In the paper of Bulk Crystal Growth – Methods and Materials, Czochralski is considered as an important method for greatest perfection crystal growth [17]. Our goal is to obtain large size, high-quality MgAl2O4 laser crystals. In the present paper, the growth of bulk Cr:MgAl2O4 crystal by the Czochralski growth method was first reported, the effective segregation coefficient of Cr3+ was determined, the quality was investigated by high-resolution X-ray diffractometry, and the thermal conductivity was measured from 270 K to 370 K. Furthermore, its transmission spectra, emission spectra, excitation spectra and photoluminescence decay curve were also measured.
Section snippets
Crystal growth
The softening temperature of iridium crucible is about 2250 °C, in order to ensure the safety of the crucible, the vertical and transverse temperature gradient must be kept as small as possible. On the other hand, crystal growth requires a certain temperature gradient, which is closely related to the growth rate, doped ion segregation, melt state and crystalline quality. So it is a challenge to grow the crystal by the Czochralski method. The starting material was prepared from the
Crystal structure
The concentration of doped ion in crystal can be expressed by the following equation:where keff is the effective segregation coefficient of doped ion in crystal, WT is the weight of the starting material and C0 is the concentration of doped ion of the starting material, Ws is the weight of the grown crystal and Cs is the concentration of doped ion of the grown crystal. The values of WT is 350 g, the C0 is calculated to be 0.0143 atom% for the composition of Cr0.001MgAl
Conclusion
Bulk single crystal Cr:MgAl2O4 was firstly successfully grown by the Czochralski method and the size is Ø30 mm × 70 mm, and the effective segregation coefficient of Cr3+ was determined to be 1.22. X-ray rocking curve indicates a high crystalline quality in terms of the low level of crystal domain twining, its thermal conductivity is 22.16 W/mK at the temperature of 300 K. The crystal has broad absorption bands with the peaks at 393 nm and 545 nm respectively, and the emission peaks are located
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51502292, 51272254, 61405206, 61205173), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. CXJJ-16M251), and Anhui Provincial Natural Science Foundation of China (Grant No. 1608085ME91).
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