The dissertation is dedicated to the investigation of the formation processes, structural-phase state, morphology, optical and luminescent properties of highly doped YAG:Sm3+ ceramics obtained through the consolidation of nano powder under the phase transformation conditions. The fundamental physical-technological parameters of the synthesis of optical highly doped YAG:Sm3+ ceramics were determined. The conditions for obtaining YAG:Sm3+ ceramics by using a complex dopant MgO+SiO2 with functional parameters at the single crystal analog level were implemented.
In the first chapter, the optical properties of Sm3+ ions in YAG ceramics are described, the potential use of YAG:Sm3+ as a sensitizer for forced spontaneous emission in YAG:Nd3+ lasers is highlighted.
In the second chapter, the experimental methodologies for powder compaction and fabrication of optical ceramic samples through reactive sintering of powder mixtures with a stoichiometric composition of the garnet in vacuum are described.
In the third chapter, the influence of consolidation conditions on the optical properties and structural-phase state of YAG:Sm3+ (5 at.%) ceramics is studied. The results of a detailed study of the formation patterns of microstructure and optical properties of YAG:Sm3+ (5 at.%) ceramics sintered in the temperature range of 1700–1800°C are presented. It is shown that at the consolidation temperature of 1700°C, the densification of ceramics is not complete despite the high branching of grain boundaries. It is established that the bimodal grain growth in YAG:Sm3+ (5 at.%) ceramics observed at the temperature of 1800°C is connected with the simultaneous coexistence of two or more grain boundaries types with different mobility. Optimal consolidation conditions (T=1725°C, 10 hours) are determined, ensuring complete removal of pores and the formation of YAG:Sm3+ (5 at.%) ceramics with optical losses of 0.08 cm-1 at the wavelength of 808 nm, an average grain size of ~20 μm, and the optical absorption coefficient α≈2.25 cm-1 at the wavelength of 1064 nm.
In the fourth chapter, the influence of Sm3+ ion concentration on the structural-phase state and optical properties of reactively sintered transparent YAG:Sm3+ ceramics (3-15 at.%) are analyzed. The formation conditions of highly doped optical ceramics YAG:Sm3+ with a samarium ion content ≤9 at.% are established. In YAG:Sm3+ ceramics with doping concentrations of 11 and 15 at.%, the secondary phase formation at grain boundaries occurs due to the partial decomposition of the supersaturated substitutional solid solution Y1-xSmxAG into perovskite Y1-xSmxAlO3 and Al2O3. It is shown that the substitutional solid solution (Y1-xSmx)3Al5O12 is formed throughout the entire concentration range under investigation. The YAG:Sm3+ ceramics lattice parameter linearly increases along with the activator concentration increase from 3 to 15 at.% according to Vegard’s rule.
The satellite intensity of the main spectral lines, corresponding to the NN pairs of RE3+ ions in the dodecahedral position, increases with the Sm3+ ions concentration. The satellite intensity enhancement leads to the broadening of absorption spectral lines in the range of 1060-1070 nm. The Sm3+ ions absorption coefficient at 1064 nm increases with the concentration of Sm3+ from 1.27 cm−1 for 3 at.% to 7.91 cm−1 for 15 at.%. Monophasic highly doped YAG:Sm3+ ceramics (9 at.%) is characterized by the optical absorption coefficient α1064nm=4.5 cm−1 and optical losses α808nm=0.07 cm−1.
In the fifth chapter, the mutual influence of Si4+ and Mg2+ ions different concentrations on the consolidation features, microstructure formation, and optical properties of YAG and YAG:Sm3+ ceramics is studied. The consolidation processes in the YAG (Si4+, Mg2+) model system are investigated to control solid-state sintering processes by changing the concentration of defects in both the anionic and cationic sublattices. It is demonstrated that the concentration ratio CSi/CMg can be used as an effective criterion for the densification processes of YAG (Si4+, Mg2+) ceramics. The formation of electrically neutral complexes 〖Mg〗_(Al(octa))^'+〖Si〗_(Al(tetra))^∙ in adjacent nodes of the crystal lattice with equal concentrations of Si4+ and Mg2+ ions leads to a significant reduction in the diffusion coefficient, hinders the densification processes, and is accompanied by the entrapment of pores within the grain volume. The optimal composition of the Si4++Mg2+ complex dopant is determined, which minimizes the concentration of optically active defects in the YAG structure and simultaneously enhances the interdiffusion of constituent ions during consolidation. It is shown that the use of optimized the MgO+SiO2 complex dopant composition reduces the optical losses of YAG:Sm3+ ceramics by three times compared to the traditional SiO2 dopant, allowing the production of ceramics comparable to single-crystal counterparts.
