Yttrium Segregation and YAG Precipitation at Surfaces of Yttrium‐ Doped α‐A1₂O₃

Abstract

presented in Figs. 3(B) and (C), indicate that the phase in bright contrast in Fig. 3(A) contains both yttrium. and aluminum while the phase in dark contrast in Fig. 3(A) is rich in aluminum but devoid of yttrium.The work of Loudjani et al., ' un ublished work of McCune and Donlon, and the YZ03-A1203 phase diagram3 all suggest that the phase in bright contrast is Y3AI5Ol2. Apparently the solubility of yttrium in N203 is extremely low and the dopant was rejected to the melt during crystal growth until the local concentration was sufficient to solidify the eutectic.In summary, these results indicate that even in single-crystal alumina with an yttrium content roughly 2 orders of magnitude lower than that investigated by Loudjani et al. virtually no yttrium goes P into solution. The emission spectroscopy results indicate that the upper limit for the solubility of yttrium is <lo ppm and reinforce the position that the chemical state of yttrium must be carefully determined before interpreting its role in modifying the behavior of A1203. For example, El-Aiat and Kroger4 investigated the electrical properties of nominally yttrium-doped crystals from the same lot as those whose chemical analysis is presented in Table I. Their conductivity results were consistent with donor doping and from this they inferred that yttrium was an isoelectric donor in alumina. However, since the yttrium does not, in fact, dissolve in alumina, it is far more likely that the background impurity level in the crystal is responsible for the behavior.Auger electron spectroscopy and low-energy ion scattering were used to assess surface composition of yttrium-doped polycrystalline and single-crystal a-aluminas which were heated under vacuum to temperatures above 1500 "C. Extensive surjGace and intergranular precipitation of yttrium aluminum garnet was observed in a compact of nominal Y content of 210 wt ppm, while apparent Y segregation to surjGace cation sites predominated in a second compact with a Y content of 160 wt ppm. In the latter case, the segregation enthalpy of yttrium to the alumina surjGace in the temperature range 1700" to 1900°C was estimated to be of the order -44 W fmol with a maximum su$ace cation site occupancy of ~1 3 % .

Atomistic Simulation of Y‐Doped α‐Alumina Interfaces

“…The underlying atomic scale mechanisms are, however, still speculated upon, the following literature review showing possible explanations for this so‐called Y effect. It is well known that Y strongly segregates to α‐alumina surfaces and that its solubility in the bulk is very limited (∼10 atomic ppm) 3,4 . Imaging‐SIMS confirmed Y dopants in α‐alumina ceramics to be located mainly at grain boundaries and free surfaces in pores 5 .…”

Grain Boundary Segregation of Yttrium in Chromia Scales

“…First, Cawley and Halloran (49) demonstrated recently, using yttrium-doped sapphire from the same lot employed by EI-Aiat and Kroger (48), that the solubility of yttrium in these A1203 single crystals is lower than that of other impurities. (50) also demonstrated that the solubility of yttrium in A1203 is extremely low. Measurements by McCune et at.…”

Grain growth suppression in alumina via doping and two-step sintering

“…Due to its limited solubility in alumina crystal lattice ($ 10 at ppm) yttrium segregates to alumina/alumina interfaces [32,33]. Large yttria cations segregated at the interfaces block the motion of Al 3 þ and O 2À ions along grain boundaries, which results in reduced grain-boundary diffusivity and decreased densification rate [34].…”