Preparation and Thermo-physical Properties of Gd3+/Ga3+ Co-doped Garnet-type Yb3Al5O12

Gd3+ and.Ga3+ co-doped garnet-type Yb3-xGdxAl5-xGaxO12 solid solution (x=0, 0.1, 0.2, 0.3, 0.4, 0.5; YGAGO) ceramic materials were synthesized by a citric acid sol-gel method. XRD results demonstrate that all the Yb3-xGdAl5-xGaxO12 samples are composed of pure garnet Yb3Al5O12 phase. Increasing the Gd3+ and Ga3+ doping content leads to a shift of the X-ray pattern to lower 2 theta values. SEM morphology shows that microstructure is composed of nano-sized crystal grains and highly dense grain boundary. The substitution of larger Gd3+ and Ga3+ in smaller Yb3+ and Al6+ sites, respectively, can lower the thermal conductivity, and jointly facilitate higher coefficients of thermal expansion (CET) due to the relaxation of distorted Yb3-xGdxAl5-xGaxO12 structure with substitutional point defects. The phonon mean free path of Yb3-xGdxAl5-xGaxO12 ceramics decreases with increasing Gd3+ and Ga3+ content at identical temperature levels because of the lattice distortion and local imperfection scaling generated by incorporating large dopant Gd3+ and Ga3+ cations. Gd3+ and Ga3+ co-doped garnet-type Yb3-xGdxAl5-xGaxO12 composition shows a minimum thermal conductivity value (lambda=1.67 W/(m.K), T=1273 K) and a maximum CET (alpha(1)=11.71 x10(-6) K-1, T=1273 K) when x = 0.5.