In the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests exhibit a localized “rattling” vibrational mode that provides an efficient mechanism for reducing the thermal conductivity. Inelastic neutron scattering and nuclear inelastic scattering measurements have yielded the phonon density of states in R8Ga16Ge30; the line width of the localized vibrational modes is found to be an important parameter in determining the lattice thermal conductivity. Neutron diffraction studies on R8Ga16Ge30 have shown that the guests in the larger cage are located off-center, and it was proposed that their jumping about the four off-center locations is responsible for the observed glass-like thermal conductivity at temperatures below 10 K. The detection of such slow guest motion is challenging because the typical time and energy scales involved are ca. 4 ns and 1 µeV, respectively. We have studied the slow europium tunneling dynamics in Eu4Sr4Ga16Ge30 by both Mössbauer and microwave absorption spectroscopy.

Resonant Ultrasound Spectroscopy and low-temperature ultrasonic attenuation measurements are reported for filled and unfilled skutterudites and for Ge-clathrates. These data reveal that an unusual elastic behavior complements the thermal properties of the filled skutterudites and indicate the presence of low-energy vibrational modes. The attenuation at low-temperatures in the single-crystalline Ge-clathrate is glasslike and can be described by the same phenomenological Tunneling Model that has been developed to describe the low-temperature properties of amorphous solids.

In materials that conduct both electricity and heat, the thermal and electrical currents are coupled. This thermoelectric coupling can be used to construct devices that act as temperature sensors, heat pumps, refrigerators, or power generators. A temperature difference ΔT across any electrical conductor will generate a corresponding voltage difference ΔV The ratio ΔV/ΔT is defined as the Seebeck coefficient S after Thomas See-beck who first discovered the effect in 1823. Probably the most familiar use of this effect is the thermocouple in which the union of two dissimilar metals generates a voltage in response to an imposed temperature difference. Interestingly an electrical current I passing through the junction of two dissimilar conductors results in the absorption or release of heat in the vicinity of the junction depending on the direction of the current. The ability to heat or cool in this manner was first discovered by Peltier and explained by Lord Kelvin. The latter showed that the amount of heat produced (or absorbed) near the junction is given by ΠI = STI where Π is called the Peltier coefficient and T is the temperature. It is primarily this effect that makes thermoelectric (Peltier) refrigeration possible. Thermoelectric refrigerators and power generators are attractive for many applications as they have no moving parts (except electrons and holes), use no liquid refrigerant, and last indefinitely.

We present initial assessments of the thermoelectric properties of two ternary tellurides with known crystal structures, Tl2GeTeM5 and Tl2SnTe5. Tl2SnTe5 appears to have a p-type figure of merit about the same as that of Bi2Te3, the best thermoelectric material among binary compounds. A good figure of merit is possible because the lattice thermal conductivity is very low. Based on neutron diffraction data, we have calculated atomic displacement parameters and thermal expansion coefficients. The atomic displacement parameters give some understanding of the low lattice thermal conductivity.

Polycrystalline CoSb3 films were grown on a variety of electrically insulating substrates by pulsed laser ablation from a stoichiometric hot-pressed target. These films are fully crystallized in the skutterudite structure, and the grains exhibit a strongly preferred alignment of the cubic [310]-axis perpendicular to the substrate surface. The film quality is studied for different single-crystal substrates and as a function of growth temperature and background gas.

Hall measurements show that the films are p-type semiconducting with a room-temperature carrier density of 3×1020 holes/cm3. The Hall mobility is found to be 50 to 60 cm2 /Vs, which is high for such a heavily-doped material. The Seebeck coefficient and the resistivity are measured as a function of temperature and are compared to bulk measurements.

Until recently, phosphate glasses were regarded as somewhat of a scientific curiosity—materials limited in technological application but nevertheless exhibiting interesting fundamental properties. Although the basic properties of many phosphate glasses have been studied on a laboratory scale during the past 50 years, very few of these materials proved to be of commercial interest. This lack of extensive practical application was primarily related to their poor chemical durability and a tendency to crystallize during processing.

The pace of modern technological development continues to result in an increased demand for new materials with specific characteristics, and this demand has led to renewed interest in a number of previously neglected materials—including phosphate glasses. The combined physical and chemical characteristics of phosphate glasses make them relatively unique among the families of inorganic glasses. These characteristics include: large thermal expansion coefficients, low preparation and softening temperatures, low melt viscosities, chemical compatibility with living bone, and a polymeric structure similar to that in organic polymers. Most recent research on phosphate glasses has emphasized one or more of these characteristics.

The major scientific questions pertinent to technological applications of phosphate glasses are related to utilizing the desirable characteristics of these materials while maximizing their corrosion resistance and thermal stability. In order to tailor a phosphate glass for a particular application, the corrosion mechanism of the glass must often be understood in detail, along with the relationship between the microscopic structure of the glass and its macroscopic properties.

Lead-iron phosphate glasses have recently been proposed as a new primary disposal medium for both commercial nuclear reactor wastes and some types of high-level radioactive U.S. defense wastes [1–41. The initial work on the characterization of lead-iron phosphate nuclear waste glasses, hereafter named phosphate glasses, concentrated on the preparation, thermal properties, and devitrification behavior (both during and after preparation) of these glasses. Aqueous corrosion rates were measured at 90°C in distilled water, acidic and basic solutions, low eH distilled water, and in the “reference’ natural ground water appropriate to disposal in a tuffaceous formation (i.e., J-13 well water [5]). The results of these initial investigations were extremely encouraging and indicated that homogeneous, highly leach resistant (at 90° C) phosphate glasses loaded with either commercial or high-level U.S. defense wastes could be prepared at relatively low temperatures (process temp.:≤ 1050°C). The chemical stability of the phosphate glasses was found to be primarily due to the stabilizing effect of iron on the structure of lead phosphate glass. This effect is illustrated in Fig. 1 where the leachate conductivity at 90°C is plotted versus time for phosphate glasses with varying iron concentrations. The addition of 9 wt.% iron oxide to lead metaphosphate glass [Pb(PO3)2] increases the chemical durability of the glass in water by a factor of about 104. The tendency of pure lead phosphate glasses to crystallize is also greatly suppressed by the addition of iron oxide [3,4]. Lead metaphosphate glass can be completely crystallized by heating the glass in air at 30O°C for several hours. Lead-iron phosphate glasses, however, can be heated in air for several hundred hours at a temperature as high as 500°C without exhibiting any evidence of crystallization.