Conference Reports: SIXTH INTERNATIONAL CONFERENCE ON HIGH TEMPERATURES-CHEMISTRY OF INORGANIC MATERIALS, Gaithersburg, MD, April 3–7, 1989

This conference was the sixth of a series, sponsored by the International Union of Pure and Applied Chemistry (lUPAC) Commission II.3 on High Temperature and Solid State Chemistry, and which is held about every 3 years. The NIST meeting represented only the second occasion that this conference series had been held in the U.S.A. Attendance, exceeding 170, included participants from 19 countries, and 130 papers were presented.


About the Conference
The conference program emphasized the basic chemical science and measurement issues underly-ing the characterization, processing, and performance of materials at high temperatures. Each of the major classes of materials was considered, including high performance alloys, ceramics, composites, and specialized forms such as films, coatings, clusters, powders, slags, fluxes, etc. in addition, individual substances, namely the elements and their compounds, were discussed in detail. Seven plenary lectures and 68 invited talks were given as well as 61 poster presentations and computer-based demonstrations. Also, Prof. Leo Brewer, one of the foremost pioneers of the field, gave an overview of the conference proceedings together with his perspective on the "Role of Chemistry in High-Temperature Materials Science and Technology." During the conference sessions, many of the hot issues of the day were also discussed, including cold fusion, high-temperature superconductors, low pressure production of diamond films, etc.
Participation by the leading international researchers in the field was particularly strong in the materials-related areas of measurement techniques, thermochemistry and models, processing and synthesis, and performance under extreme environments. Of special interest were the topics on databases and phase equilibria models, processingmainly from the vapor phase, and high power laser-materials interactions.
The conferees were welcomed by Dr. Lyle Schwartz, Director of the Institute for Materials Science and Engineering (IMSE) (now Materials Science and Engineering Laboratory), who also gave an overview of pertinent NIST and IMSE research activities. Prof. Jean Drowart of the Free University of Brussels, Belgium, addressed the meeting on behalf of lUPAC and gave a fascinating account of "7000 Years of High Temperature Materials Chemistry." A few representative technical highlights from each of the main conference sessions are given in the following discussion.

Advances in Measurement Techniques
Three areas were given special emphasis. These were spectroscopic probes, difFractometry, and physicochemical methods. The types of spectroscopic probes discussed included Raman and related laser spectroscopic methods for in situ molecular-level or phase-specific monitoring of hot surfaces. Examples were considered in the areas of corrosion, oxide superconductor processing, and in Raman imaging of ceramic crack suppression due to phase transformation toughening (see fig. 1). An interesting novel application of in situ optical emission spectroscopic analysis of molten steel, using a laser-induced plasma-forming technique, was also discussed (see fig. 2). These effectively nonintnisive methods also have potential as process monitoring probes for intelligent processing in addition to their utility in experimental systems.
In the area of diffractometry, in situ analysis of material structures at high temperatures, using x-ray and neutron sources, was described. Atom probe chemical analysis on alloy surfaces using field-ion microscopy was also discussed.
Physicochemical techniques have traditionally been key to the characterization of materials at high temperatures and significant recent advances have occurred in this area. Methods have been developed which effectively eliminate containment problems. For instance, with liquid metals, transient microsecond time scale techniques have been applied to accurate measurements of melting points and heat capacities at very high temperatures. For steady state measurements, electromagnetic levitation may be used as, for instance, with emissivity and optical constant measurements. Another transient technique that was discussed by a number of researchers throughout the conference is the pulsed laser-heating approach to the production of vapor species for mass and optical spectroscopic characterization.

Thermochemistry and Models
This session was particularly well represented by the leaders in the field. Progress on development of thermodynamic databases was reviewed by researchers from the United States, U.S.S.R., Canada, France, Sweden and the United Kingdom. While the databases developed thus far are incomplete they are still sufficiently extensive to allow their use in thermochemical and phase equilibria models for many high-temperature alloy, ceramic, composite, slag, glass, and other systems. A key element in these models is the description of nonideal mixing, present in many practical systems. Among the various models considered, those accounting for ordering or formation of Uquid associates appear particularly promising (see fig. 3). In one of the presentations, direct experimental (neutron diffraction) evidence was presented for ordering in Uqud alloys (see fig. 4). Many papers were presented dealing with experimental determinations of thermochemical data and applications of the data to materials process development.

Processing and Synthesis
The chemical basis for high temperature processing and synthesis of materials is a rapidly growing area of research and representative work in the field was discussed. An area of significant promise for the design of new or improved materials is that of molecular/atomic clusters. These species, with properties intermediate between molecular and bulk material, are key reaction intermediates to most deposition and condensation processes. They also serve as model structures for surfaces owing to their intrinsic high ratio of surface to bulk atoms. Their unique reactivity as a function of cluster size was indicated by several speakers (see fig. 5).
The session on CVD and other vapor phasebased processes was particularly exciting. Thermochemical, kinetic, transport models, whereby the processing of films (diamond, semiconductor, ceramic, alloy, etc.) could be optimized, were described (see fig. 6).

Performance Under Extreme Environments
The important related areas of hot and high temperature corrosion were discussed for both alloy and ceramic materials. In particular, the key role of chemical reaction and solubility was demonstrated (see fig. 7).
Another area where materials are subject to extreme conditions is that of laser-materials interactions. There are many areas of science and technology that require an improved understanding of this interaction, including design of laser resistant materials, laser deposition of films, laser etching for electronic devices, laser stimulated chemical processing, laser welding, and laser heating for containerless studies of thermochemistry at ultra-high temperatures. This latter case has special significance to providing thermodynamic data for nuclear reactor excursions (see fig. 8) and for materials data for advanced aerospace applications.

Additional Information
A three volume proceedings (1350 pages) is being published by Humana Press, Clifton, NJ. Many of the conference presentations will appear in these volumes. Also included are a few articles, not presented at the conference, in order to provide a more complete coverage of certain topics. This will be the first generally available publication for this subject area and the proceedings should be of considerable interest to researchers, students, and others interested in the scientifically challenging, and technologically indispensable, interplay between materials and high temperatures.
The next meeting in the series is scheduled to be held in 1991 in Orleans, France and will be chaired by J. P. Coutures.    Figure 8. Maximum UOj"'' signals from the mass spectrometer for laser pulses of varying strength. Qp is the peak absorbed power density, and Tsmai is the measured maximum surface temperature in the pulse. The scale designating the maximum number density in the ionizer of the mass spectrometer was calculated from measured ion intensities and the vapor pressure (Torr) is that of UO2 at the peak surface temperature. The hatched area represents the range of results of the steady-state calibrations, (Taken from Olander, paper 123.)