Ceramics Phase Equilibrium Data

Our objective is to compile, evaluate, determine, and disseminate phase equilibrium data to facilitate and optimize the development, processing, and usage of advanced ceramic materials. By delineating the conditions (chemical composition, temperature, pressure, and atmosphere) under which pure compounds and their mixtures occur, phase equilibrium data provide essential thermochemical guidance for the technical exploitation of ceramic materials.

Phase equilibrium data are used throughout the ceramics industry to understand and control the complex phenomena that underlie the production and performance of advanced materials. Phase diagrams serve as maps of the equilibrium chemical and structural behaviors exhibited by materials and provide critical starting information for the rational design of materials processing schemes, quality assurance efforts, and optimization of the physical and chemical properties of advanced materials. The Phase Equilibria Diagrams series (NIST Standard Reference Database 31) provides critical written commentaries, evaluated graphical representations, bibliographic data, and analytical capabilities. The published portion of the database now includes over 20,000 phase diagrams contained in 21 books and a CD-ROM. Activities in the Ceramics Division Data Center include continuous addition of new database content (about 1000 new entries per year), dissemination of the data in printed and digital formats, and development of scientific software to facilitate access to and use of the data.

Impact and Customers:

• Advanced ceramics are critical enabling materials for a wide spectrum of devices and systems for electronic, communications, energy, medical, and chemical purification applications.

• According to one industrial customer, "…the availability of the diagrams, particularly in the CD-ROM format, has resulted in decreased development costs, decreased time to market, decreased manufacturing costs, improved quality, and increased product functionality for our customers."

• Since 1933, NIST scientists have collaborated with the American Ceramic Society (ACerS) to meet the need for reliable phase diagram data by jointly publishing a series of critically evaluated collections of phase equilibrium diagrams.

Recent activities in the Data Center have concentrated on a re-design of the Content Management System (CMS) to enable continuous publication of new data, in contrast to batch releases in the form of topically oriented books. With the phase-out of hard-copy products, the Data Center is now poised to continuously release new content for advanced materials in a range of classes including oxides, chalcogenides, nitrides, borides, carbides, phosphides, and salts, as well as for mixed systems including two or more of these classes. Publication will now take place as periodic updates of a comprehensive CD-ROM developed on-site at NIST, and on-demand uploads to a web version of the database developed off-site by the American Ceramic Society.

Using the redesigned CMS, we are currently in the final stages of producing the content of (virtual) Volume 15 of the series, consisting of approximately 1000 phase diagrams for many new oxide systems as well as mixed and non-oxide systems. This new content will be released for publication on a continuous basis for the web version, and when completed, will be added to an updated and improved CD-ROM (v.3.2) which will be available for purchase in 2008. Recent experimental phase equilibria research has focused on a number of Bi₂O₃-based dielectric systems of interest for embedded elements such as capacitors, filters, and resonators.

Analogous to the technically important phase in the Bi-Zn-Nb-O system, pyrochlore stability fields were determined in detail for the Bi-M-Nb-O systems, where M = Fe, Mn, and Co. These systems offer alternatives to the Zn phase, which is difficult to process due to volatility of ZnO at elevated temperatures. The compositional location of the pyrochlore phase fields, combined with crystal structure analyses, demonstrated that the pyrochlores in these systems, like the Zn analog, are of the "misplaced-displaced" type. That is, they form at M-cation-rich stoichiometries that force smaller M-type metals to mix with large Bi³+ ions on the same crystallographic site. This highly unusual crystal chemical behavior is accompanied by displacive disorder on these sites, which appears to lead to the unusual and potentially useful dielectric properties of the pyrochlores.

We also determined phase stability fields for the analogous Bi₂O₃-Al₂O₃-Nb₂O₅ system, considered attractive since Al₂O₃ is both inexpensive and nonvolatile. However, contrary to published reports of pyrochlore formation, no distinct ternary compounds were found. The published claims likely arose from confusion of fluorite-type phases in the binary sub-systems with pyrochlore, which is structurally related to fluorite.

We reported this work in an invited plenary lecture at the 13th U.S.-Japan Seminar on Dielectric and Piezoelectric Ceramics, held in Awaji City, Japan, in November 2007. This meeting is held biannually by the Penn State Center for Dielectric Studies, an NSF-supported industry-university cooperative research center with 24 member companies. This forum provides an efficient venue for NIST researchers to remain current with emerging technology directions and critically important ceramic systems.

Phase equilibrium diagram for the Bi₂O₃-Al₂O₃-Nb₂O₅ system in air. Formation of the reported pyrochlore phase, "Bi₂AlNbO₇", (open red circle) was not observed.