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Presents information on a study which carried out a crystal structural characterization of SrGaGe clathrate compound. Sample preparation and experimental analyses; Results of structural analysis; Discussion; Conclusions.

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectra are obtained from a donor in aluminum nitride (AlN) crystals. Although observed in as-grown crystals, exposure to x rays significantly increases the concentration of this center. ENDOR identifies a strong hyperfine interaction with one aluminum neighbor along the c axis and weaker equivalent hyperfine interactions with three additional aluminum neighbors in the basal plane. These aluminum interactions indicate that the responsible center is a deep donor at a nitrogen site. The observed paramagnetic defect is either a neutral oxygen substituting for nitrogen (ON0) or a neutral nitrogen vacancy (VN0). [ABSTRACT FROM AUTHOR]

The electromechanical coupling coefficient K2 for surface acoustic waves propagating on c and a surfaces of bulk AlN single crystals has been measured using the S11-parameter method in the frequency range of 160–360 MHz. The extracted values of K2 are 0.11% and 0.47% for the c and a surfaces, respectively. By fitting our experimental data to our numerical simulation results, we have estimated piezoelectric constants, which are in a reasonable agreement with literature data. Our results are consistent with the negative sign of the e15 constant. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The temperature coefficient of frequency (TCF) for surface acoustic waves (SAW) propagating on (0001) bulk A1N single crystal has been measured at 358 MHz in the temperature range 25-70°C. The TCF value -19 ppm/° is in agreement with the value estimated from the temperature coefficients of the AlN elastic constants. This value is considerably smaller than temperature coefficients of many materials currently used for SAW substrates. [ABSTRACT FROM AUTHOR]

We measured the surface electric potential distribution on individual, electrically contacted and biased, multiwall carbon nanotubes using scanning surface potential microscopy. The voltage varies linearly along the nanotube and the voltage drop is directly proportional to the bias applied between electrodes. Its resistance decreases four times when a 4.5 V bias is applied between the nanotube and the substrate. Under these conditions, we were able to resolve the voltage drop along the nanotube and at the contacts, providing a unique way of measuring contact resistance, which is observed in this case to be on the order of 50 kΩ. [ABSTRACT FROM AUTHOR]

Transport properties of polycrystalline Ge clathrates with general composition Sr[sub 8]Ga[sub 16]Ge[sub 30] are reported in the temperature range 5 K≤T≤300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 10[sup 17]–10[sup 18] cm[sup -3]. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is [FRACTION SHAPE="CASE"][NUM]1[/NUM][DEN]4[/DEN][/FRACTION] that of Bi[sub 2]Te[sub 3] alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T>700 K, thus exceeding that of most known materials. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]