Shingo Akao

Ultramultiple roundtrips of surface acoustic wave on sphere realizing innovation of gas sensors

A thin beam of wave usually diverges due to diffraction, which is a limitation of any device using such waves. However, a surface acoustic wave (SAW) on a sphere with an appropriate aperture does not diverge but is naturally collimated, realizing ultramultiple roundtrips along an equator of the sphere. This effect is caused by the balance between diffraction and focusing on a spherical surface, and it enables realization of high-performance ball SAW sensors. The advantage of ball SAW is most fully appreciated when applied to a very thin sensitive film for which the multiple-roundtrip enhances the sensitivity, but the attenuation loss is not very large. It is exemplified in a hydrogen gas sensor that realizes a wide sensing range of 10 ppm to 100% for the first time, and realizes relatively fast response time of 20 s without heating the sensitive film.

Evaluation of Response Time in Ball Surface-Acoustic-Wave Hydrogen Sensor using Digital Quadrature Detector

Hydrogen leak detection sensors must have high sensitivity and a short response time of 1 s or less. A ball surface-acoustic-wave (SAW) hydrogen sensor has a high sensitivity and can detect hydrogen in a very wide concentration range of 10 ppm to 100%. Moreover, a fast response can be expected because of the very thin sensitive film used. In this study, we developed a digital quadrature detector (DQD) to measure responses of less than 1 s, and measure phases in 1 ms intervals with excellent sensitivity. We evaluated the response time of the ball SAW hydrogen sensor where the signal was averaged 256 times in 0.256 s using the DQD. As a result, the response time was found to be 1 s or less for 3.0 vol % hydrogen gas in nitrogen.

Temperature Compensation for Ball Surface Acoustic Wave Devices and Sensor Using Frequency Dispersion

We propose a new temperature compensation method for ball surface acoustic wave (SAW) devices using frequency dispersion. This method distinguishes the temperature effect independent of frequency and surface effects such as mass loading or elastic effects linearly dependent on frequency. After stating the principle of the method, we verify it by fabricating ball SAW devices with an interdigital transducer (IDT) that works at two frequencies, and apply them to the measurement of the coating of albumin on the surface of the device and on a hydrogen gas sensor with a surface-coated sensing film. Delay time measurements are carried out at two frequencies, and the difference in delay time is calculated to eliminate temperature-independent effects. Because the results clearly show surface effects without temperature disturbance, it is a difficult to realize sensors using ball SAW devices.

Development of Microseparation Column for Ball Surface Acoustic Wave Gas Chromatograph

For the safety and security of a society, ubiquitous multiple-gas sensors are required. The gas chromatograph (GC) based on the separation of gases according to the elution time is frequently used for multiple-gas sensing, but it is not portable. In this situation, we have developed a ball surface acoustic wave (SAW) sensor, where SAW on a sphere with a specific width is naturally collimated and makes multiple roundtrips without diffusing by diffraction. In this study, in order to develop a portable GC, we developed a gas separation column using micro-electro-mechanical-system (MEMS) technology and evaluated it, combining it with a ball SAW sensor.

Analysis of ball surface acoustic wave sensor response to wide variety of gases using gas chromatography

It is necessary to detect a wide variety of dangerous gases for environment assessment and security. Gas chromatography using a gas separation column has been used for this purpose; however, a compact and high-performance system has not yet been established. Thus, we propose a new approach to measure gases using a ball surface acoustic wave (SAW) sensor, after separating them using gas separation columns. In this work, amplitude responses were observed for mixed alcohol gases. With water–ethanol mixed gas, both delay time and amplitude responses were observed, and different response and recovery times were found for these responses. Also, an amplitude response reflecting the leaky loss of SAW was observed and an approximate equation of leaky loss was derived to quantitatively evaluate the roles of leaky loss and other parameters observed in the ball SAW sensor. These results suggest that the proposed approach is promising for developing compact and high-performance multigas sensors.

Observation of the Roundtrips of Surface Acoustic Waves on a Single Crystal LiNbO3 Ball

Propagation of surface acoustic waves (SAWs) was investigated on a single lithium niobate (LiNbO3) ball of 1 inch diameter. We found ten specific routes, on which the roundtrips of SAWs are observed, and one or more specific routes, on which the multiple reflections of bulk waves are observed. Since the temperature coefficient is similar on each route of the SAW, one route can be used to correct the temperature dependency of all other routes, and the other routes can be used as multiple-gas sensors by coating various reactive thin films along each route.

Development of High Precision Metal Micro-Electro-Mechanical-Systems Column for Portable Surface Acoustic Wave Gas Chromatograph

In the field of environmental measurement and security, a portable gas chromatograph (GC) is required for the on-site analysis of multiple hazardous gases. Although the gas separation column has been downsized using micro-electro-mechanical-systems (MEMS) technology, an MEMS column made of silicon and glass still does not have sufficient robustness and a sufficiently low fabrication cost for a portable GC. In this study, we fabricated a robust and inexpensive high-precision metal MEMS column by combining diffusion-bonded etched stainless-steel plates with alignment evaluation using acoustic microscopy. The separation performance was evaluated using a desktop GC with a flame ionization detector and we achieved the high separation performance comparable to the best silicon MEMS column fabricated using a dynamic coating method. As an application, we fabricated a palm-size surface acoustic wave (SAW) GC combining this column with a ball SAW sensor and succeeded in separating and detecting a mixture of volatile organic compounds.

Acoustic dispersion in a ball-shaped surface acoustic wave device

Theoretical model for ball surface acoustic wave (SAW) sensors has been developed. Wave front distortion encountered in planar SAW sensors caused by diffraction or reflection can be neglected because of the diffraction-free propagation of a naturally collimated beam. Wave form at each turn is expressed by an integral of terms defined by the array factor and the elementary charge density of electrodes as well as the frequency-dependent velocity and attenuation caused by the sensitive film. Irrespective of the significant simplicity of the theory, excellent agreement has been observed between the simulated and experimental wave forms of a double-electrode ball SAW sensor.

Development of Packed Column for Surface Acoustic Wave Gas Chromatograph Using Anodically Bonded Silicon–Glass Structure with a Compression Jacket

Gas or liquid chromatographs (GC/LCs) are frequently used for multiple-compound sensing, but they are too large and heavy to be portable. To provide a handy GC/LC, many groups have proposed micro GC/LCs in which large columns are replaced by small micro-electro-mechanical-system (MEMS) columns. However, because of the high packing pressure, a packed MEMS column using an anodically bonded structure has not been realized so far. In this study, we realized such a MEMS column for the first time by introducing a compression jacket to reduce the tensile stress at the silicon–glass boundary. Consequently we succeeded in the separation and detection of lower hydrocarbons using a ball surface acoustic wave sensor.

Temperature compensation of ball surface acoustic wave sensor by two-frequency measurement using undersampling

To realize a practical two-frequency measurement (TFM) system for precise temperature compensation in a ball surface acoustic wave (SAW) sensor, the application of undersampling (US) was investigated. The subtraction coefficient for the temperature compensation in US was theoretically explained. The principle of the TFM system using US was simulated by the decimation of the oversampling (OS) waveform after applying a narrow band-pass filter, and the delay time was measured using a wavelet transform. In the application of the method to trace moisture measurement, the delay time response due to US matched that due to OS with a correlation coefficient higher than 0.9999. Although rms noise was increased by US, the response to the concentration change of 4?17 nmol/mol was measured with a signal-to-noise ratio higher than 20. From these results, it was shown that the precise output of the ball SAW sensor could be obtained even when using US, which was equivalent to that using OS.