Kenta Kirimoto

Metallic–nonmetallic transition on the conductivity temperature dependence of multiwall carbon nanotubes

We applied a noncontacting surface acoustic wave technique to measure the conductivity of aligned multiwall carbon nanotube films. By changing intensities of the parallel and perpendicular electric field components at the surface of the film sample, we observed different temperature dependencies of the conductivity. It was found that the temperature dependence of the conductivity parallel to the tube axis is comparable to that of a single-wall carbon nanotube, namely, there is a metallic–nonmetallic transition at temperature of 280 K. On the other hand, the temperature dependence of the conductivity perpendicular to the tube axis is similar to that of a graphene sheet.

Localized relaxation in stabilized zirconia

Abstract Stabilized zirconia is well known for long-range transport of oxygen ions which is caused by diffusion relaxation of oxygen vacancies. We used torsional vibrations to measure the temperature dependence of internal friction in yttria-stabilized zirconia (YSZ) doped with 9.5 mol % Y2O3 and calcia-stabilized zirconia (CSZ) doped with 12 mol % CaO. In the temperature range 300– 700 K , the internal friction peak exhibits anisotropy, different in YSZ from CSZ, which we attribute to localized relaxation of oxygen vacancies. The results imply that some oxygen vacancies are bound within the local structure, a greater number in CSZ than in YSZ, and suggest that the defect symmetry of local structure depends on the type of dopant ion.

Dielectric loss in a C60 film observed by coupling with the external electromagnetic field of a surface acoustic wave

The dielectric loss in C60 films was studied by a noncontacting technique utilizing the external electric fields associated with surface acoustic waves (SAW) on a piezoelectric crystal. A sharp increase in loss was observed at temperatures below 220 K together with other structure not found with standard SAW measurements. We believe that these features are due to induced current in C60, causing joule loss, and to the formation of localized dipole moments by charge transfer between adsorbed O2 and C60 molecules, giving rise to thermally activated relaxation.

Internal Friction Due to Localized Relaxation around Y-ions in Single Crystal Yttria-Stabilized Zirconia

The internal friction in single crystal yttria-stabilized zirconia (YSZ) doped with 9.5 mol% Y2O3 was measured for longitudinal sound waves in the frequency range from 10 Hz to 800 Hz using a vibrating reed technique. In the temperature range from 300 K to 700 K, observations of internal friction reveal two closely overlapping absorption peaks, confirming the existence of two relaxation modes in YSZ. One of the peaks is due to diffusion relaxation, which is known to be responsible for long-range transport of O-ions. The second peak shows clearly the existence of localized relaxation, which is attributed to bound oxygen vacancies within the local structure which is formed around an Y-ion due to symmetry breaking. The absorption peak caused by the localized relaxation exhibits anisotropy resulting from the asymmetric local structure, and the strength of this peak changes with temperature reflecting the number of bound oxygen vacancies.

Fabrication of Nanoscale Cubic SiC Particle Film

Nanoscale cubic SiC particle film is grown on Si substrate by hydrogen plasma sputtering of a SiC target. Before the film growth an amorphous SiC buffer layer of about 100 nm thickness is prepared on the Si substrate. By annealing the buffer layer in hydrogen atmosphere, the nanoscale cubic SiC particle film can be grown on the buffer layer on Si. Particle size, composition and crystallinity of the film depend on the composition, the crystallinity and the surface morphology of the buffer layer.

ULTRASONIC ATTENUATION DUE TO LOCALIZED RELAXATION IN CUBIC YTTRIA-STABILIZED ZIRCONIA

Longitudinal ultrasonic attenuation in yttria-stabilized zirconia [ZrO2(Y)] was measured at frequencies of 10 MHz and 120 MHz and at temperatures between 300 and 600 K. The temperature variation was strongly anisotropic, the maximum effect being observed in the direction. At 10 MHz a relaxation peak was observed from which an activation energy of 0.7 ±0.07 eV could be estimated. A similar figure was obtained from dielectric constant measurements, but the ionic conductivity yielded a higher value of 0.92 ±0.1 eV. Our conclusion is that there are two separate hopping processes for oxygen ions, one between Zr-related sites which can percolate the crystal and hence produce conduction, and another between Y-related sites which are localized.

Carrier transport properties of nanocrystalline Er3N@C80

Electrical transport properties of the nanocrystalline Er3N@C80 with fcc crystal structure were characterized by measuring both temperature-dependent d.c. conductance and a.c. impedance. The results showed that the Er3N@C80 sample has characteristics of n-type semiconductor and an electron affinity larger than work function of gold metal. The Er3N@C80/Au interface has an ohmic contact behavior and the contact resistance was very small as compared with bulk resistance of the Er3N@C80 sample. The charge carriers in the sample were thermally excited from various trapped levels and both acoustic phonon and ionic scatterings become a dominant process in different temperature regions, respectively. At temperatures below 250 K, the activation energy of the trapped carrier was estimated to be 35.5 meV, and the ionic scattering was a dominant mechanism. On the other hand, at temperatures above 350 K, the activation energy was reduced to 15.9 meV, and the acoustic phonon scattering was a dominant mechanism. In addi...

Current-voltage characteristics of C70 solid near Meyer-Neldel temperature

The current-voltage characteristics of the C70 solid with hexagonal closed-packed structures were measured in the temperature range of 250–450 K. The current-voltage characteristics can be described as a temporary expedient by a cubic polynomial of the voltage, i = a v 3 + b v 2 + c v + d. Moreover, the Meyer-Neldel temperature of the C70 solid was confirmed to be 310 K, at which a linear relationship between the current and voltage was observed. Also, at temperatures below the Meyer-Neldel temperature, the current increases with increasing voltage. On the other hand, at temperatures above the Meyer-Neldel temperature a negative differential conductivity effect was observed at high voltage side. The negative differential conductivity was related to the electric field and temperature effects on the mobility of charge carrier, which involve two variations in the carrier concentration and the activation energy for carrier hopping transport.The current-voltage characteristics of the C70 solid with hexagonal closed-packed structures were measured in the temperature range of 250–450 K. The current-voltage characteristics can be described as a temporary expedient by a cubic polynomial of the voltage, i = a v 3 + b v 2 + c v + d. Moreover, the Meyer-Neldel temperature of the C70 solid was confirmed to be 310 K, at which a linear relationship between the current and voltage was observed. Also, at temperatures below the Meyer-Neldel temperature, the current increases with increasing voltage. On the other hand, at temperatures above the Meyer-Neldel temperature a negative differential conductivity effect was observed at high voltage side. The negative differential conductivity was related to the electric field and temperature effects on the mobility of charge carrier, which involve two variations in the carrier concentration and the activation energy for carrier hopping transport.

Electric field induced effects in Y3N@[C80]6− anionic solid

By measuring the i−v characteristics of Y3N@[C80]6− anionic solid at various temperatures and electric field strengths, four conducting phases with i∝v, i∝v2 with low resistance, i∝v4 and i∝v2 with high resistance were observed. First, at temperatures below 100 K and field strengths below 30 Vcm−1, the current passing through the sample was a linear function of the d.c. bias voltage due to free moving charges. Second, at the same field strengths, the trapped carrier was thermally activated with activation energies of 13.6 meV for temperature range of 100−250 K and 88.7 meV for 250−450 K. In this conducting phase, the carrier transport was governed by space charge limited conduction mechanism. Third, when the electric field increased from 30 to 120 Vcm−1, the i became a quartic function of the v because the carrier mobility is a quadratic function of the field strength. A conducting phase with high resistance was observed at temperatures below 100 K. The trapped carrier was thermally activated with activat...

Transport properties of carriers through nanocrystalline Lu3N@C80/Au interface

The transport property of carriers passing through a nanocrystalline Lu3N@C80/Au interface has been studied by measuring impedance under various alternating voltages and DC biases. A depletion layer thicker than 110 µm is recognized in the Lu3N@C80/Au Schottky contact. Such a thick depletion layer indicates that the formation of space charge in the nanocrystalline Lu3N@C80 phase is difficult owing to an increase in LUMO energy, i.e., a decrease in the electron affinity of the Lu3N@C80 molecule. A high interfacial resistance is found to increase the transmission time of the carriers passing through the Lu3N@C80/Au Schottky contact. Such a high interfacial resistance may be ascribed to the low mobility of the carriers in the nanocrystalline Lu3N@C80 phase.