H. J. Trodahl

A study of the phase diagram of (K,Na,Li)NbO3 determined by dielectric and piezoelectric measurements, and Raman spectroscopy

A composition-temperature phase diagram of the system (1−x)(K0.5Na0.5)NbO3–xLiNbO3 is presented for 0⩽x⩽0.1. Using dielectric and piezoelectric resonance measurements, and Raman spectroscopy, ceramic samples containing 2%–10% LiNbO3 were studied over a temperature range of 7–770K showing a complex sequence of phase transitions. Analysis of the different Raman, piezoelectric, and dielectric data shows distinct transitions from cubic to tetragonal to orthorhombic to rhombohedral phase for x=0.02–0.05. The symmetries of the phases were assigned using analogy to phase diagram of (K0.5Na0.5)NbO3 single crystals and ceramics (x=0). At x>0.07 only one transition between ferroelectric phases occurs where tetragonal phase transforms to another phase, possibly of rhombohedral, orthorhombic, or monoclinic symmetry. In the region between x=0.05 and x=0.08, the phase transition sequence is more complex. Below 100K this phase with unidentified symmetry creates a vertical boundary with the rhombohedral phase present nea...

Thermal conductivity of landfast Antarctic and Arctic sea ice

[1] We present final results from a program to measure the thermal conductivity of sea ice with in situ thermistor arrays using an amended analysis of new and previously reported ice temperatures. Results from landfast first-year (FY) ice near Barrow, Alaska, and McMurdo Sound, Antarctica, are consistent with predictions from effective-medium models but 10–15% higher than values from the parameterization currently used in most sea ice models. We observe no previously reported anomalous near-surface reduction, which is now understood to have been an artifact, nor a convective enhancement to the heat flow, although our analysis is limited to temperatures below � 5C at which brine percolation is restricted. Results for landfast multiyear (MY) ice in McMurdo Sound are also consistent with effective-medium predictions, and emphasize the density dependence. We compare these and historical measurements with effective-medium predictions and the representation commonly used in sea ice models, developed originally for MY Arctic ice. We propose an alternative expression derived from effective-medium models, appropriate for both MY and FY ice that is consistent with ��

Thermoelectric power and conductivity of different types of polypyrrole

We have measured the thermoelectric power and conductivity as a function of temperature of a wide range of polypyrrole samples, including a film of soluble polypyrrole synthesized chemically, and wrinkled films synthesized using indium–tin oxide electrodes; other samples investigated include high-conductivity polypyrrole films synthesized at different temperatures and current densities, films grown on nonconducting substrates, and polypyrrole gas sensors. The thermoelectric powers are remarkably similar and metal-like for the medium and high conductivity samples but show nonzero extrapolations to zero temperature for wrinkled samples. The temperature dependence of conductivity tends to be greater for samples of lower conductivity. In contrast to polyaniline and polyacetylene, a crossover to metallic sign for the temperature dependence of conductivity at higher temperatures is not observed in any of our samples; the fluctuation-induced tunnelling and variable-range hopping expressions account for nearly all our conductivity data except for low-temperature anomalies.

Ferroelectric domains and piezoelectricity in monocrystalline Pb(Zr,Ti)O3 nanowires

Monocrystalline lead zirconate titanate nanowires were grown by a polymer assisted solvothermal technique. X-ray and electron diffractions confirmed tetragonal perovskite structure and a [001] orientation along the wire axis, respectively. Raman scattering was used to analyze the structure and composition of single wires. Ferroelectric/ferroelastic domain walls were imaged by transmission electron microscopy, showing some domains with polarization directions along the wire axis and some perpendicular to it. The domain walls disappeared upon heating above the ferroelectric phase transition at 460°C. Ferroelectric switching, as well as piezoelectric activity and hysteresis, were shown locally using piezoelectric force microscopy.

Raman spectroscopy of (K,Na)NbO3 and (K,Na)1−xLixNbO3

We report Raman spectroscopy in all four phases of a (K0.5+δNa0.5−δ)NbO3 (δ∼0.03) crystal and ((K0.5+δNa0.5−δ)1−xLix)NbO3 for x=0.02 and 0.03. The phase transitions in the Li-free crystal are homogeneous and hysteretic, with no mixed-phase region between the rhombohedral and orthorhombic phases. Raman spectra in the lowest-temperature phase in the Li-doped material differ significantly from those at x=0 and suggest a structural phase boundary at a Li concentration of less than 2%.

Temperature-dependent conductivity of conducting polymers exposed to gases*

We have made measurements of the effect of water vapour, oxygen and nitrogen on the temperature-dependent conductivity of polypyrrole gas sensors. Oxygen and nitrogen had little effect, while water vapour decreased conductivity and increased conductivity temperature dependence, consistent with decreased hopping between localized states. No evidence was found of any metallic term in the conductivity of our polypyrrole samples, in strong contrast to the case of polyaniline. We analyze previous conductivity measurements for polyaniline exposed to the same gases, showing that the conductivity remains consistent with the general expression for polymer conductivity with a significant metallic term.

Ion-assisted deposition of amorphous GaN: Raman and optical properties

We report the preparation of amorphous GaN by ion-assisted deposition and studies of the Raman and optical response of the resulting films. The films are transparent across the visible and show an edge whose energy and structure are in close agreement with crystalline material, suggesting a low density of gap states and homopolar bonds. The Raman spectrum is similar to a broadened vibrational density of modes calculated for wurtzite GaN, with a Raman cross section which varies among the vibrational bands.

Ferromagnetic redshift of the optical gap in GdN

We report measurements of the optical gap in a GdN film at temperatures from

Rare-earth mononitrides

300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}6\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Crystallization and diffusion in progressively annealed a‐Ge/SiOx superlattices

, covering both the paramagnetic and ferromagnetic phases. The gap is