1. G.R. Frank, K.A. Canfield, T.R. Wright, Alumina-based core containing yttria, U.S. Patent no. 4837187 (1989).
2. J. Ronald, S. Rodney, Ceramic core and method of making, U.S. Patent no. 6578623 (2003).
3. R.R. Robb, C. Yaker, L. Burd, Ceramic cores for manufacturing hollow metal castings, U.S. Patent no. 4190450 (1980).