Lutetium oxide (Lu₂O₃) transparent ceramics have a great potential in high-power or high-efficiency solid-state laser applications due to the excellent thermal-mechanical properties, low fabrication temperature, high absorption and emission cross-sections when doped with trivalent rare earth laser ions. Among them, Lu₂O₃ doped with Yb³⁺ stands out as the best combination for applications in high-efficiency and high-power lasers, considering a high thermal conductivity due to the similar ionic radii and bonding forces of lutetium and ytterbium ions. The combination of vacuum pre-sintering with HIP post-treatment, along with the powder synthesized by a co-precipitation method, highly favors to fabricate highly transparent sesquioxide ceramics. Some previous reports demonstrated the high efficiency and high power laser outputs of Yb:Lu₂O₃ transparent ceramics. Thermal properties of Yb:Lu₂O₃ ceramics are significant for the high-efficiency and high-power laser applications. In ceramic gain media, the significant factors affecting the thermal conductivity include the doping content of active ions, porosity and grain size. Although increasing Yb³⁺ concentration could not cause a decrease in thermal conductivity in theory, little work has reported the related experimental investigations yet.

The commercial oxide powders (i.e., Lu₂O₃ and Yb₂O₃) were utilized as raw materials. Ammonium bicarbonate (NH₄HCO₃) and ammonium sulfate ((NH₄)₂SO₄) were used as a precipitant and a dispersant, respectively. The precursors of Yb:Lu₂O₃ were synthesized by a co-precipitation method. All the precursors with different Yb³⁺ doping contents were calcined at 1050 ℃ to obtain Yb:Lu₂O₃ powders, which were dry-pressed at a low pressure in a steel die and then cold-isostatically pressed at 250 MPa. All the green bodies were vacuum sintered at 1500 ℃ for 2 h to fabricate pre-sintered ceramics that then were HIP post-treated in Ar atmosphere pressure of 176 MPa at 1750 ℃ for 3 h. Finally, the HIP post-treated ceramics were annealed in air at 900 ℃ for 10 h and then polished on the both sides to a thickness of 1.3 mm for the coming optical characterizations. Some annealed transparent ceramic disks were then machined to the sizes of (Φ = 10.0 mm, d = 1.3 mm) and (Φ=5.5 mm, d<1.0 mm) for measuring the thermal diffusivity and specific heat, respectively.

The phase compositions of the synthesized precursors and powders were identified by X-ray diffraction (XRD, Cu Kα1 radiation, λ = 0.154 18 nm). The microstructures of all the powders and ceramics were determined by field emission scanning electron microscopy (FESEM, SU8220, Hitachi, Japan). The in-line transmittance and absorption spectra of Yb∶Lu₂O₃ transparent ceramics were characterized by ultraviolet-visible-near infrared spectroscopy (UV-VIS-NIR, Carry-5000, Varian, USA). The thermal diffusivity was measured by a laser-flash method, and the specific heat was measured by differential scanning calorimetry (DSC).

Pure Lu₂O₃ precursor consists of spherical particles with various sizes, which range from tens of nanometers to hundreds of nanometers in diameter, but the micro-morphology of the precursor with Yb³⁺ is made of thin stacked layers with nano-sized thickness, indicating that Yb³⁺ doping has a great impact on the morphologies of Lu₂O₃ precursor, but its concentration level does not affect the morphologies of the precursors apparently. The particles of Yb³⁺-doped nano-powders agglomerate in two-dimensional space and are in the shape of regular polyhedra. The phases of all the calcined nano-powders are consistent with those of cubic Lu₂O₃. The lattice parameters of Yb:Lu₂O₃ nano-powders are 10.384 4、10.389 4、10.390 2、10.394 9 Åand 10.396 5 Å, respectively, with increasing the Yb³⁺ concentration. The corresponding average gain sizes are 57.7, 51.9, 53.6, 61.8 and 51.4 nm, respectively. The relative densities of all the Yb:Lu₂O₃ pre-sintered ceramics are 95.3%, 96.1%, 95.6%, 95.4% and 95.7%, with average grain sizes of 0.92, 0.97, 0.99, 1.09 μm and 1.14 μm, respectively, as Yb³⁺ concentration increases. 3% Yb:Lu₂O₃ transparent ceramic sample has the maximum in-line transmittance with the values of 83.0% at 1080 nm and 78.8% at 600 nm. The absorption coefficients at 948 nm of these ceramics are linearly correlated to Yb³⁺ doping content, which follows the Lambert-Beer Law. Four main emission peaks are located at 948, 976, 1033 and 1079 nm, respectively, among which the largest emission cross-section is at 976 nm with a value of 1.148×10^-20 cm². The laser emission wavelengths include 1079 nm and 1033 nm. The maximum thermal conductivity of 3% Yb∶Lu₂O₃ ceramics is 13.1 W·m^-1·K^-1 at room temperature, which is similar to the value of Yb:Lu₂O₃ single crystal, while the thermal conductivity of 15% Yb∶Lu₂O₃ ceramics is 11.6 W·m^-1·K^-1, which is greater than that of pure YAG material. The result indicates that Yb:Lu₂O₃ transparent ceramics are a gain medium with a high thermal conductivity, even doped with a high Yb³⁺ doping content, demonstrating a great application potential in high-power solid-state lasers.

Yb:Lu₂O₃ transparent ceramics with Yb³⁺ doping content of 0-15% were fabricated by vacuum sintering and HIP post-treating from co-precipitated nano-powders. The nano-powders calcined at 1050 ℃ had the similar particle sizes with different morphologies as the precursors. 3% Yb∶Lu₂O₃ nano-powder showed a better dispersivity, and the corresponding transparent ceramics possessed the maximum optical quality with the in-line transmittance of 83% at 1080 nm. The spectral characteristics of all the Yb:Lu₂O₃ transparent ceramics were calculated. At β of > 0.059, there were two main lasing emission wavelengths, peaked at 1033 nm and 1079 nm, respectively. 3% Yb:Lu₂O₃ transparent ceramics possessed better thermal properties rather than others, with the room-temperature thermal conductivity of 13.1 W·m^-1·K^-1. The results indicated a great application potential of Yb:Lu₂O₃ transparent ceramics in high-power solid-state lasers.