Leo Brewer - Research Focus

Research Focus

Although Brewer's research covered an unusually wide range of subjects and employed a multitude of techniques from theory to spectroscopy, his primary focus was on high-temperature thermodynamics, materials science (including refractory containment materials), studies of metallic phases, and the development of metallic bonding theory, incorporating the concepts of electron promotion and generalized acid-base theory. He was also involved at different points in his career with astrophysics and ceramics.

Brewer's early high-temperature work also showed that the equilibrium vapor above CuCl was mainly Cu3Cl3 molecules at normal pressures. This simple observation led to what became known as Brewer's Rule. He showed that when vapor and condensed phases are in equilibrium, the vapor species become more complex as the temperature is raised. This includes the formation of polymers and unusual oxidation states. His rule became the foundation of the field of high-temperature chemistry.

Much of his research focused on resolving discrepancies between reported experimental values and values predicted by chemical bonding models. In many instances, the reported data were shown to be in error, and the reliability of the model was confirmed. Examples are the demonstrations that the enthalpies of formation of C(g) and N(g) were much larger than the widely accepted values. Brewer's compilation of the thermodynamic properties and phase diagrams of 101 binary systems of molybdenum provides many examples of use of predictive models when no reliable experimental data are available.

In some instances, the experimental results were confirmed and it was necessary to improve the models. An example would be the neglect of gaseous polymer species at high temperatures. The war-time study uncovered evidence of polymerization in high-temperature vapors. This led to a general theory which predicted that saturated high-temperature vapors would be complex mixtures of species and that the complexity would increase with increasing temperature. These predictions have been confirmed by high-temperature workers for many systems. The refractory studies initiated with the sulfides were extended to studies of silicides and borides and other refractory phases. The experience on the Manhattan Project on the use of platinum to reduce the volatility of lanthanides and actinides were extended to a wide range of transition metal intermetallic compounds through use of the Engel correlation of electronic and crystal structures that has led to the prediction of the structures and compositions of the phases of most of the two billion multi-component phase diagrams of the transition metals.

Brewer devoted major effort to the characterization of the thermodynamic properties at high temperatures, and the critical evaluations of the thermodynamic properties from the Manhattan Project were updated periodically. One of Brewer's compilations covered the thermodynamic properties of the solid, liquid and gaseous phases of the elements and their oxides between room temperature and temperature to above 3000 K. The thermodynamic applications of these data were well-illustrated by the 2nd edition of Lewis and Randall's Thermodynamics, which Brewer and Kenneth Pitzer revised in 1961. Brewer's global interest in all of the elements is illustrated by a paper in 1951 on the equilibrium distribution of the elements in the Earth's gravitational field.

Brewer conducted a wide range of spectroscopic studies both at high temperatures and in matrices to fix the thermodynamic properties of high-temperature vapors. From 1950 to 1970, Brewer published many papers on the analysis of the spectra produced by high-temperature gaseous molecules. Several of these papers described a molecular beam method for determining their ground electronic states. When low temperature matrix isolation was developed by George Pimentel at UC Berkeley, Brewer produced many papers on the spectra of his high-temperature molecules in a frozen inert matrix. Brewer also had a long-term interest in the electronic states of I2, and he had several papers on its remarkable complexities.

Much of Brewer's later research was aimed at characterizing the extremely strong generalized Lewis acid-base interactions between lanthanides, actinides and left-hand transition metals with the platinum group metals. A combination of high-temperature solid electrolyte cells, equilibration with oxides, carbides and nitrides, and vapor pressure measurements were used. These intermetallics were shown to be among the most stable of all types of compounds, as predicted by the Engel theory. Engel had suggested a correlation between the number of conduction electrons and the crystal structure of the metals. Brewer extended this concept to include the nature of d and f electrons, and the concept of acid-base interactions. Starting investigations with undergraduate students, he tested these ideas by heating ZrC with the noble metal platinum, and found that the formation of ZrPt3 released a great deal of energy despite the great stability of ZrC. Over several years Brewer developed the Brewer-Engel theory for such bonds, and he published many papers about its application.