1. a Ceramics Division, National Institute of Standards and Technology, Technology Administration, U.S. Department of Commerce, Gaithersburg, MD 20899, USA
2. b Ceramics Division, National Institute of Standards and Technology, Technology Administration, U.S. Department of Commerce, Gaithersburg, MD 20899, USA
3. c Nalco Chemical Company, Naperville, IL 60563, USA
4. d Henry Krumb School of Mines, Columbia University, New York, NY 10027, USA