Graduate School of Engineering
Department of Materials Processing

Microsystems Design and Processing
Materials Evaluation and Sensing (Prof. Mihara)

Research Topics

In tunnels, nuclear power plants and many important structures, accidents occurred from defects of materials recently became serious problems. In our laboratory, we study novel ultrasonic methods for defect detection of structural materials such as metals and ceramics, and for highly sensitive gas sensors.

(1) Nonlinear ultrasound - subharmonics -

Cracks in solids can be detected by ultrasound if they are open. However, their detection is not easy when they are closed with a closure stress, and thus it is a fundamental problem in ultrasonic testing. Subharmonics with half the input frequency is potentially useful in the detection and evaluation of such cracks, although quantitative analysis has not been established. In our laboratory, we developed analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, for the first time. We proved their validity by comparison with experiments on a well-defined fatigue crack in aluminum alloy, finding reasonable agreements. Based on these achievements, we are developing a novel method for accurate sizing of closed cracks by the time of flight of subharmonics, which solves the fundamental problem in ultrasonic testing of cracks.

(2) Ball SAW - spherical SAW gas sensor technology -

New mode of surface acoustic wave (SAW) on a sphere realizes a sensitive gas sensor. We discovered that SAW on a sphere is naturally collimated to form a thin parallel beam and multiple roundtrips of SAW along an equator is realized without diffraction loss. Taking advantage of the resultant much longer propagation distance than on a planar substrate, a sensitive gas sensor can be developed, where the delay time and amplitude change due to interaction with atmospheric gas is significantly enhanced. This new principle is being applied to develop a breakthrough in hydrogen gas sensor.

(3) Ultrasonic atomic force microscopy (UAFM) - nondestructive characterization for nanotechnology -

Combining AFM and ultrasound, we have developed UAFM. We are now applying it to investigate the elasticity variation on domain boundary (DB) in lead zirconate titanate (PZT). The UAFM imaged the change in contact stiffness not only among grains but also on the DB. According to an analysis, the contact stiffness of the DB was approximately 10% lower than that within the domain. This is the first direct evidence of the variation of the elasticity due to the DB. The implication of this finding is that the low stiffness at the DB may affect the piezoelectricity of PZT and the easy mobility of the DB under a stress and electric field, which are important for not only actuator applications but also high-speed writing memory applications.

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