Research

Flux growth of new intermetallics

Molten metals have been used as solvents for the growth of a variety of compounds, ranging from large single crystals of silicon to exploratory synthesis of new intermetallic materials. An unexplored aspect of growth of materials from metal fluxes is the modification of the reactivity of the medium by mixing flux components. Some metal fluxes are often incorporated into the product, acting as reactive solvents; reactions in aluminum flux often yield aluminide products. Other metal fluxes act as inert reaction media. Tin flux can often be used to grow compounds which contain no tin at all.

This variation in flux behavior indicates the possibility of using a combination of different metals to create a unique reaction solvent for new materials. There remain a large number of unusual low-melting flux systems (such as highly reactive metals and eutectic mixtures) that are yet unexplored and are likely to yield new phases. Our investigation into metal flux reactions includes the production of new intermetallic phases with interesting magnetic behavior or with potential use as hydrogen storage materials.

Magnetic intermetallics

The hard magnetic materials used in computer hard drives are intermetallics such as SmCo5 and Nd2Fe14B. Their strong ferromagnetism is derived from the coupling of the local moments of the unpaired electrons on the rare earth ion with the itinerant magnetism of the transition metal valence electrons. The binary phase diagrams of early rare earths (such as La, Ce, Pr) combined with late transition metals (Ni, Fe) all feature low melting eutectics in the rare earth-rich region which may act as fruitful synthesis media for magnetic compounds containing two paramagnetic elements. We have grown new materials containing clusters and layers of iron separated by lanthanum ions from La/Ni eutectic (melting point 532 C).

Hydrogen storage intermetallics

Hydrogen is often absorbed by alloys of an electropositive metal combined with an electronegative metal. A 3:1 mixture of the electropositive metals calcium and zinc forms a low melting eutectic (391ºC). Addition of electronegative late transition metals to this mixture produces a number of new intermetallic phases (such as Ca21Ni2Zn36, CaNi2Zn3, and Ca6Pt3Zn5) with structures and compositions indicative of promise for H2 uptake. Reactions of various elements in the Mg/Ni eutectic are also being investigated for the synthesis of lighter-weight hydrogen storage alloys such as Mg6Ni16Si7.

A John Deere vehicle with a hydrogen fuel cell.

Polymers as Ion Exchange Solvents

Poly(ethylene oxide), or PEO, is a polyether commonly used in lithium batteries as an electrolyte. It solvates lithium salts and transports Li cations between the battery electrodes during operation. Given these capabilities, PEO should be a feasable solvent for ion exchange processes in porous or layered systems which degrade in the usual ion exchange solvent (water). We are exploring the use of short chain oligomers of PEO (sometimes referred to as poly(ethylene glycol) or PEG) as reaction media for ion exchange of zeolites and other porous and layered oxide solids.

Zeolites consist of a negatively charged aluminosilicate framework defining cages that contain cations, most commonly Na+. A cage found in both the sodalite and Zeolite X structures is shown below. The most common method of exchanging the sodium ions in the as-synthesized compounds for more useful ions is by immersing the solid in an aqueous solution containing the desired ion, or in a molten salt containing the desired ion. This is effective for many cations, but can lead to the degradation of the zeolite framework with other cations. We have found that using PEG oligomers as an ion exchange solvent prevents this degradation.

In addition to the necessary structural and compositional analysis of the ion-exchanged zeolites, the products may have interesting optical, magnetic, and catalytic properties requiring study. Fluorescence has been observed from europium and silver ions exchanged into zeolites; it is likely that other heavy metal ions or rare earth ions (or clusters thereof) will also exhibit unusual optical or magnetic properties.