Hiroyuki Obara

Ultrasound backscatter from blood in the 30 to 70 MHz frequency range

New intravascular ultrasound imaging methods for the visualization of the vascular wall and plaque at frequencies greater than 20 MHz must overcome the obscuring influence of backscatter from blood. This effect can reduce the contrast between the vessel lumen and soft plaque or media adversely affecting the diagnosis. We have extended the approach developed by de Kroon at the Thorax Centre, Rotterdam to study backscatter from blood in the frequency range from 30 to 70 MHz. At a frequency of 35 MHz the transition from low shear stress to high shear stress was accompanied by a reduction of 14.3 dB in the backscatter coefficient. However at 70 MHz this transition resulted in only a 2.9 dB reduction in backscatter coefficient. This is consistent with a breakdown in the Rayleigh scattering assumption. Details of the viscometer design are described and quantitative comparisons of backscatter from blood as a function of shear rate are made to previously measured backscatter properties of media, adventitia, and plaque

Investigation of Impulse Response for an Interdigital Surface-Acoustic-Wave Transducer

The improved equivalent circuit model for a SAW interdigital transducer which we proposed in a previous paper contains an energy storage effect in addition to usual second order effects, and appropriately expresses the metallization effect. Using this model, this paper describes the relations between the impulse responses and the various second order effects for an IDT with an arbitrary metallization ratio on LiNbO3. To investigate in detail the influence of the second order effects on the performance of an IDT, figures of the characteristic responses are contrasted in both the frequency and time domains.

Equivalent four-port networks for series connected SAW-IDT and their application

We have developed the new equivalent four-port network of an N-pair of IDT for electric series connection of IDTs. From this equivalent circuit, we have derived the four-port transfer matrix, which is very useful for the analysis of the SAW devices using series connected IDTs. As a practically useful example, we have analyzed the one-port SAW resonator using connected IDTs. Finally, by numerical calculation, resonance characteristics of this type of resonators on LiNbO/sub 3/ (128/spl deg/ Y,X) are investigated.

Two-port SAW resonator using series connected IDTs

A two-port SAW resonator using series connected IDTs is proposed. This type of resonator has an advantage over a conventional two-port SAW resonator in that the design condition of the number of finger pairs of an IDT for high performance is not strict, since the impedance level of series connected IDTs is very high. We have analyzed this type of resonator using six-port equivalent circuits and their transfer matrices of an N-pair of IDT for series connection. Finally, by numerical calculation, the resonance characteristics of these types of resonator on LiNbO/sub 3/ (128/spl deg/ Y,X) are investigated.

Simple analysis of two-port SAW resonators composing of parallel and series connected IDTs

A simple analysis of two types of two-port SAW resonators composed of series-connected IDTs and parallel-connected IDTs are presented. One type is where the series-connected IDTs are used for transmitter and parallel-connected IDTs are used for receiver. Another type is where parallel-connected IDTs are used for transmitter and series-connected IDTs are used for receiver. These types of resonator have a feature of impedance transformer performance. We have analyzed these types of resonator by six-port equivalent circuits and transfer matrices of N-pair IDT using force factors developed in previous papers.

Evaluation of acoustic wave velocity in thin diamond films by thermal measurement

In this report, a new method to evaluate the acoustic wave velocity in thin dielectric films by thermal measurement is proposed and an experimental result for diamond thin film is presented. The principle is to use the relation between the thermal conductivity, the average acoustic wave velocity, and the mean free path of the phonon after measuring the propagating velocity of the thermal wave. This method was applied to a diamond thin film grown on a Si substrate. The evaluated acoustic wave velocity seems very reasonable, but how this is related to the surface acoustic wave velocity of a layered structure such as ZnO/diamond/Si remains to be investigated.

Evaluation of acoustic wave velocities in piezoelectric, dielectric and intrinsic semiconductor materials using thermal wave propagation

In the development of various SAW devices, it is very important to evaluate some qualities of materials before we start any fabrication process of the devices. Last year, we proposed a new non-destructive method to evaluate the acoustic wave velocity by thermal measurement and evaluated the acoustic wave velocity in a thin diamond film grown on Si. We extended this technique to other materials such as intrinsic Si and yz lithium niobate wafers and re-examined the approximations used in the previous report. We could evaluate the acoustic wave velocities in these materials within about 10% error of the known values measured by other accurate methods. Thus, we could demonstrate the feasibility of this simple technique again for quick evaluation of the materials. The detail of the experimental results, the evaluation processes and related problems are discussed.

Numerical Simulation of an Axisymmetric Ethanol Reforming Reactor for Hydrogen Fuel Cell Applications

Hydrogen fuel cell technology is currently capable of providing adequate power for a wide range of stationary and mobile applications. Nonetheless, the sustainability of this technology rests upon the production of hydrogen from renewable resources. Among the techniques under current study, the chemical reforming of alcohols and other bio-hydrocarbon fuels, appears to offer great promise. In the so called autothermal reforming process, a suitable combination of total and partial oxidation supports hydrogen production from ethanol with no external addition of energy required. Furthermore, the autothermal reforming process conducted in a well insulated reactor, produces temperatures that promote additional hydrogen production through the endothermic steam reforming and the water-gas shift reactions, which may be catalyzed or uncatalyzed, with the added benefit of lowered carbon monoxide concentrations. In this study, an adiabatic ethanol reforming reactor was simulated assuming the reactants to be air (21% O2 and 79% N2 ) and ethanol (C2 H5 OH) and the products to be H2 O, CO2 , CO and H2 , with all constituents taken to be in the gaseous state. The air was introduced uniformly through a ring around the side of the reactor and the gaseous ethanol was injected into the center of one end, with products withdrawn from the center of the opposite end, to create an axisymmetric flow field. The gas flows within the reactor were assumed to be turbulent, and the chemical kinetics of a simple four reaction system was assumed to be controlled by turbulent mixing processes. Air and fuel flow rates into the reactor were varied to obtain six different levels of oxidation (air-fuel ratios) while maintaining the same total gaseous mass flow out of the reactor. The numerical results for the reacting flow show that hydrogen production is maximized when the air-fuel ratio on a mass basis is held at approximately 2.8. These findings are in qualitative agreement with observations from previous experimental studies.Copyright

Observation of anisotropic thermal diffusivities in yz LiNbO/sub 3/ using transient heat pulse diffusion measurement [SAW devices]

In previous reports, we proposed and studied a new technique to estimate acoustic wave velocities in various materials using the heat pulse diffusion measurement and estimated the acoustic wave velocity in a thin diamond film grown on Si and intrinsic Si wafer. We also tried a similar measurement on LiNbO/sub 3/. In this report we improved this technique and re-examined intrinsic Si and yz LiNbO/sub 3/ wafers used in the previous report. We find the evidence of the anisotropic diffusivities in LiNbO/sub 3/ for the first time.

Simple six-port equivalent circuits for surface-acoustic-wave interdigital transducers

Abstract We have derived new simple and concise six-port equivalent circuit models and their transfer matrices of an N-pair of interdigital transducers (IDT) for analyzing two-port surface-acoustic-wave (SAW) devices such as filters and resonators. The present models include various secondary effects such as electrical shorting and mass loading effect (MEL) and energy storage effect. Therefore, they can predict characteristics close to the actual performances of devices.