Hydrogen bubbles were generated as cavitation nuclei and their distributions were measured. The number and size distribution of bubble cavities generated on axisymmetric bodies was calculated and compared with experimental results. The measured size distribution of bubble cavities agreed qualitatively with the calculated value, but the total number of cavities was about one half of the calculation. The role of stream nuclei on the inception of sheet cavity was investigated experimentally. Without added nuclei, the value of the incipient cavitation number σi showed a large scattering, whereas with added nuclei the scattering became fairly small and σi converged to the upper limit of that when no nuclei were added. σi with added nuclei also coincided with the desinent cavitation number σd, and σd remained unchanged by adding nuclei.
Frequency-dependent carrier dynamics in dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)-based organic field-effect transistors and metal-insulator-semiconductor capacitors were studied by using admittance measurements. Both the real (conductance) and imaginary (capacitance) components of admittance, which have a mutually complementary relationship, were analyzed in the same way. On the basis of an experimental study of devices with different contact geometries, peaks observed in the conductance spectra corresponding to slopes in the capacitance spectra were classified into two groups. The high-frequency peak was attributed to the carrier injection from the top contact to the DNTT/insulator interface just underneath the contact while the low-frequency peaks were attributed to the drift current spreading all over the interface. The model calculation of carrier diffusion reproduces the low-frequency peaks very well. It is shown that the field-effect mobility from the accumulation region to the subthreshold region can be estimated by the fitting.
The frequency and time dependence of capacitance of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT)-based metal-insulator-semiconductor (MIS) capacitors were studied. The frequency spectra show a long and gradual slope over many orders of magnitude in frequency, and the time evolution in the subthreshold region exhibits a logarithmic dependence. These two phenomena are analyzed from a unified point of view on the basis of charge trapping models that were originally developed for Si-based MIS capacitors. The charge trapping model with a Gaussian-distributed surface potential in DNTT successfully reproduces both the frequency and time dependences with the same parameter set.Index Terms-Charge traps, metal-insulator-semiconductor (MIS) capacitor, organic semiconductor, random surface potential.
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