Rare earth ion-doped Y₃Al₅O₁₂ (YAG)-based transparent ceramics have been used as important laser gain media for a long time, yet the doping concentration of active ions is limited due to concentration quenching, wherein the inflexion concentration quenching of Nd³⁺ is recognized as 1.0 at%. In this work, YAG–Al₂O₃ nanocrystalline transparent ceramics with a concentration of Nd³⁺ (0–5.0 at%) were fabricated via amorphous crystallization, and the crystal structure evolution, morphology, and optical properties were systematically investigated by differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), magnetic resonation (MAS), nuclear magnetic resonation (NMR), and fluorescence spectroscopy. The doping of Nd³⁺ can promote the transition of Al^[5] and Al^[6] to Al^[4], indicating improvements in the ability of the amorphous material to form Nd³⁺:Y₂O₃–Al₂O₃ vitrified beads, and 1.5 at% Nd³⁺:YAG–Al₂O₃ nanocrystalline transparent ceramics can be obtained by crystallization at 1050 °C with a matrix composed of YAG and concomitant δ-Al₂O₃ and θ-Al₂O₃. The nanocrystalline transparent ceramics show an internal transmittance of 89.56% at 1064 nm, and the strongest emission peak corresponds to the energy transfer from ⁴F_3/2 to ⁴I_11/2 of Nd³⁺ with a fluorescence lifetime of 231 μs when pumped by an 808 nm laser. Specifically, spectral broadening begins to occur, indicating the onset of concentration quenching, when the concentration of Nd³⁺ exceeds 1.5 at%, substantially higher than the 1.0 at% observed in YAG ceramics. YAG–Al₂O₃ nanocrystalline transparent ceramics obtained by amorphous crystallization can be utilized as the matrix to increase the inflexion point of doping concentration quenching of Nd³⁺, and this material may have great potential as a laser gain medium.

(1-x)CoTiNb₂O₈-xZnNb₂O₆ microwave dielectric ceramics were prepared via the conventional solid-state reaction route with the aim of reducing the τ_f value and improving the thermal stability. The phase composition and the microstructure were investigated using X-ray diffraction, Raman spectra, and scanning electron microscopy. A set of phase transitions which were induced by composition had been confirmed via the sequence: rutile structure→coexistence of rutile and columbite phase→columbite phase. For (1-x)CoTiNb₂O₈-xZnNb₂O₆ microwave dielectric ceramics, the addition of ZnNb₂O₆ content (x = 0-1) led to the decrease of ε_r from 62.98 to 23.94. As a result of the high Q × ƒ of ZnNb₂O₆ ceramics, the increase of ZnNb₂O₆ content also led to the lower sintering temperatures and the higher Q × ƒ values. The τ_f value was reduced from +108.04 (x = 0) to - 49.31 ppm/℃ (x = 1). Among them, high density 0.5CoTiNb₂O₈-0.5ZnNb₂O₆ ceramics were obtained at 1175 ℃ with excellent microwave dielectric properties of ε_r 39.2, Q × ƒ 40013 GHz, and τ_f + 3.57 ppm/℃.