Fabrication of Transparent Yb:Sc2O3 Ceramics by Hot Isostatic Pressing Without Sintering Additive

Highly transparent Yb:Sc2O3 ceramics were successfully fabricated using commercially powders without any sintering additive by a hot-isostatic-press (HIP) method. The ceramics were sintered at 1700°C in vacuum followed by HIP at 1680°C to obtain a relatively high transmittance. The effects of Yb doping concentrations on the microstructure and transmittance were investigated, respectively. The samples with a doping concentration of 10 at% achieved the highest transmittance, which showed 79.4% at a wavelength of 800 nm. The fluorescence emission spectroscopy suggests that it is promising to achieve laser output at around 1080 nm. These results may afford a enchiridion for comprehending the microstructural development of transparent Yb:Sc2O3 ceramics without sintering additive.


Introduction
Yb:Sc 2 O 3 is considered to be an ideal gain medium for high power laser applications due to its high thermal conductivity (17W/mK at 300 K) and large splitting of the electronic states in sesquioxides such as Y 2 O 3 , Lu 2 O 3 and Sc 2 O 3 [1][2][3]. Compared with the Yb:Sc 2 O 3 single crystal [4][5][6][7], the polycrystalline Yb:Sc 2 O 3 transparent ceramic can possess similar optical properties at a lower preparation temperature. Moreover, it is possible to achieve large size and uniform doping at a high concentration. Therefore, it is easier to select a suitable Yb doping concentration for laser oscillation compared with single crystal [8][9][10][11].
To obtain transparent ceramic with high optical quality, it is vital to eliminate the scattering centers such as pores and secondary phases as much as possible [12][13][14]. Many attempts have been made to remove the pores and improve the laser quality of the Yb:Sc 2 O 3 transparent ceramics. The one hand is the preparation of nano-powders with high sintering activity, many efforts have been carried out by wet chemical method, such as co-precipitation [15,16], sol-gel process [9], self-propagating hightemperature synthesis [17] and so on. The powders prepared by these methods can be with high sinterability. However, these processes are complicated and exhausting, resulting in cost too much, very low efficiency and very hard to achieve actual production. Ball milling is a simple and efficient way to obtain homogeneous ultrafine powders compared to these intricate methods. In 2011, C. Gheorghe et al. [18] successfully fabricated Tm:Sc 2 O 3 ceramics with 75% transmittance using powders prepared by solid-state reaction. On the other hand, sintering aids were widely used in the  [17] obtained 2% Yb:Sc 2 O 3 with a transmittance of 78% by hot-press sintering with LiF as sintering aids. But the introduction of sintering additive can easily cause excessive grain growth and the formation of impure phases [14], which is not conductive to the mechanical and optical properties of the ceramics. As the advent of hot isostatic pressing with high-efficiency sintering ability [19], such as YAG can be achieved without any sintering additive. However, there are very few articles about the efficient preparation of Yb:Sc 2 O 3 ceramics without sintering aids.
In this paper, we reported on an efficient way to fabricate high transparent Yb:Sc 2 O 3 ceramics without any sintering additive, via a vacuum sintering plus hot isostatic pressing method using accessible commercial powders. The effects of Yb doping concentrations on the microstructure and optical properties were investigated. It would help understanding the microstructural development of Yb:Sc 2 O 3 transparent ceramics without sintering additive.

Experiment
High-purity Yb 2 O 3 (99.99%)and Sc 2 O 3 (99.99%)powders were used as starting materials. Both powders were mixed according to the formula (Sc (1-x) Yb x ) 2 O 3 (x=0, 0.02, 0.03, 0.05, 0.07, 0.10). Then the mixture was ball milled for 24 h in Teflon jars with a speed of 200 r/min. After drying the slurry at 80°C for 24 h, the powders were sifted with a 200-mesh screen and calcinated at 400°C for 4 h. The green ceramic samples were dry pressed into a disk with diameter of 15 mm at 20 MPa, and further cold isostatically pressed at 200 MPa to increase the density. Before the densification sintering, the specimens were calcining at 1000°C for 10h to remove organic impurities and residual carbon. And then, the Yb:Sc 2 O 3 disks were vacuum sintered at 1700°C followed by hot isostatic pressed at 1680°C and 200 MPa in argon atmosphere for 2 h. Next, the obtained Yb:Sc 2 O 3 ceramics were annealed at 1500°C for 8 h in air to remove oxygen vacancies. Finally, the transparent Yb:Sc 2 O 3 ceramics were obtained by mirror-polished into 2.0mm thickness for measurement.
The phase compositions of the ceramics were identified by X-ray diffraction (XRD) using CuKα radiation at 40 kV and 30 mA (DX-1000CSC, Tongda Co. Ltd). The 2θ for all the datas ranged from 20° to 65° with the scan speed of 10°/min and 0.05° step size. Morphologies of the powders and the ceramics were examined using field emission scanning electron microscopy (SEM, Inspect F, FEI). The average grain size of the sintered ceramic was estimated by Nano measurement software [14] using SEM images. The optical transmittance was measured by a UV-VIS-NIR spectrometer (Lambda950, PerkinElmer) in the range of 200-1100 nm. The fluorescence spectrum was measured at room temperature using an FLS920 Edinburgh Analytical Instruments Fluorescence Spectroscopy Life Spec PS Spectrophotometer.  with differen eaks of all s t no impurit d in the Sc 2 3+ (0.745Å), All characte pacing, as is mic systems. e of the therm a pore-free In addition, from hot is the mass tr eby increasin n Figure 2 [25], and YAG [26] ceramics. The full-width at halfmaximum (FWHM) of the emission peak at 1044 nm is about 20.4 nm, which is greater than the FWHM of previous Yb:Sc 2 O 3 ceramics [1]. And the wide emission band benefits for ultra-short pulse laser operation [8].