I have no clue about your particular material system. But in general, I'd think any compound that will create a lower temperature melting phase than the processing temperature to act as a "glue" between sintered particles could be considered "sintering aid"?

One of the important points is that a sintering aid should not be reactive to your desired ceramics or you can choose such a sintering aid that can act as a sintering aid as well as a reactant. Note that a small amount of doping of sintering aid can also improve some properties of your desired materials. 

Dear Pratic:

For selecting sintering aids, it is better that the dopant can increase the point defects so that transportation will be enhanced to promot sintering. Second phase will be decteriate to the transparency of Y2O3.

I'd be excited to look at a wafer of this type of material using NDE real time high resolution Bragg XRD Microscopy!

What is the typical diffracting domain size and particle/grain size in such sintered materials for optical/elector-optical applications?

Is SiC 6H or 4H produced thus? Through sintering and grain growth, I mean.

https://www.researchgate.net/post/What_is_the_best_way_to_interpret_REAL_TIME_3D_X_Y_o-2TH_o_ph_ch_reciprocal_space_relative_intensity_data_from_HRXRD_rocking_curve_analyses

dear pratik

i work  on transparent ceramic too such as Y2O3. Al2O3 and YAG. we use sintering aid such as TEOS (0.5% wt) and it works so well.