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John A. Newman Professor of Physical Science,
Director Cornell Center for Materials Research
Ph.D., Stanford University, 1971
B.S., Massachusetts Institute of Technology, 1966
The focus of our research is the synthesis and characterization of solid state compounds. Our objective is to discover and understand materials with novel crystal structures and/or new or enhanced properties. A variety of solid state synthesis techniques (typically carried out at temperatures as low as 200 oC to as high as 2500 oC) as well as by solution techniques (0 to 200 oC) are used to produce single crystal or polycrystalline products. The crystal structure of new compounds is determined using the CCD based X-ray diffractometers in our departmental facility or by powder diffractometers in our laboratories. Physical properties, such as electrical resistivity, thermal conductivity, thermopower, Hall effect and magnetic susceptibility, as well as selected chemical properties, such as electrochemical behavior, are examined and compared to properties expected on the basis of electronic structure calculations or similarity to known compounds.
Current materials of interest include nitrides, intermetallic compounds, molybdenum or tungsten based cluster compounds, and chalcogenide (S, Se or Te) compounds.
Binary nitride compounds, such as TiN, AlN, GaN and Si3N4, are used as wear resistant coatings, ceramics, laser diodes, moisture barriers, electrical insulation, etc. In spite of the usefulness of such binaries, ternary and quaternary nitride phases are relatively unexplored. We are filling that gap and developing the novel synthetic chemistry necessary to prepare these phases. Our research shows that nitrides are often structurally and chemically unique and quite different from compounds based on anions of neighboring elements, such as oxygen. For example, the compound Ba3Ge2N2 is quite unexpected. With Ba+2 and N-3, the average oxidation state of Ge is zero! When the crystal structure is examined (figure 1), it becomes apparent that there are two distinct Ge sites, one with Ge+2, the other with Ge-2. While Ge-2 is apparently stable in the presence of N2 under the synthetic conditions employed, it is never found in oxide, sulfide or even phosphide chemistry.
Intermetallic compounds, substances containing only metallic elements but in an ordered crystalline arrangement, are a very large class of materials. Such compounds are usually metallic and display a wide variety of properties from magnetism to superconductivity to catalytic behavior. In particular, in collaboration with the Abruña group, we have discovered that some of these compounds are superb electrocatalysts for fuel cell electrodes. For example, PtBi or PtPb (figure 2) are highly active for the oxidation of formic acid and methanol as well as hydrogen, all at room temperature. These materials are much less sensitive to CO "poisoning" and to S and Cl impurities (again at room temperature) that may enter the fuel cell in the fuel or from the air. Thus they enable the use of rather impure hydrogen in fuel cells. Through collaborations with Prof. R. Bruce van Dover (in MS&E), we are expanding our search for even more active electrocatalysts for use as fuel cell anodes and cathodes using combinatorial methods. Through other collaborations, we are also exploring novel architectures for such electrodes or for fuel cell membranes. This work is supported through the Cornell Fuel Cell Institute.
In reduced Mo and W compounds, metal-metal bonding produces metal clusters. We are exploiting the chemistry of M6X8 clusters (M = Mo or W, X = Cl or S) to make catalyst systems attached to polymer supports and, in a separate project, to link the clusters together with ditopic organic ligands to prepare cluster networks in one-, two-, or three dimensions. Such systems are unique and novel behavior of such nano-structured materials is expected. Figure 3 shows the structure of one cluster that we prepare to use as the starting point for the synthesis of linked clusters.
Chalcogenide compounds are a large class of materials with a wide spectrum of physical behavior. Currently, we focus on small bandgap semiconductors based on tellurides and selenides that have potential use as thermoelectric materials.
Further information can be found on our group WEB page.
Our research is primarily supported by DOE, NSF, and ONR.