| 研究課題/領域番号 |
17K14654
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| 研究機関 | 東京大学 |
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研究代表者 |
張 亜 東京大学, 生産技術研究所, 特任助教 (80779637)
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| 研究期間 (年度) |
2017-04-01 – 2019-03-31
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| キーワード | phonon spectroscopy / Phononic crystal / MEMS / thermoelectrics |
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| 研究実績の概要 |
Sustainable energy systems become more and more important since the fossil fuel is running up. The thermoelectric devices directly convert waste heat into useful electricity, which are very important for building an energy-efficient society. Since the thermoelectric coefficient is inversely proportional to the thermal conductance of materials, reducing the thermal conductance is very crucial for improving the efficiency of thermoelectric devices.
It has been known that phonon is responsible for the transmission of sound and heat. By using the wave nature of phonons, the phononic crystal, which is an array of holes in a membrane structure, has been proposed to modulate the thermal properties of materials. However, phonon bandgap, which is the core concept of phononic crystal, has not been observed experimentally, due to the great difficulties in generating and detecting the thermal phonons (0.1-100 THz).
Here, we aim at using an ultra-sensitive MEMS thermometer as a novel tool for investigating the coherent thermal transport in phononic crystals. In FY2017, we have succeeded in integrating phononic crystal structures of different designs on MEMS thermometers (GaAs MEMS bean resonators). BY using the thermistor, we measured the reduction of thermal conductance of the GaAs thin beam. The result shows that smaller hole size and neck size gives more decreasing in the thermal conductance of the GaAs beam.
This work gives us a large step in approaching the final target of this project, observing coherent phonon transport in the thermal phonon range.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
In FY2017, as we planned in the research proposal, we fabricated phononic crystal structures on the MEMS thermistors, and measured the reduction of thermal conductance of the GaAs thin beam by the fabricated phononic crystal.
For the fabrication, we succeeded in fabricating the phononic crystals of different hole sizes and neck sizes (the edge-to-edge distance between two holes, from 200 nm to 500 nm). One of the most crucial parameters for the phononic crystal is the smoothness of the hole structure. Although it is very different to do a quantitative estimation for the smoothness, from the SEM observation, the holes are very smoothly and homogeneously formed on the MEMS beam.
For the measurement, by using the MEMS thermometer, we have measured the reduction of thermal conductance of the beam by the phononic crystals. We found the reduction of the thermal conductance is highly related with the hole size and neck size. When the hole/neck size is large (500 nm), the reduction of thermal conductance is only caused by the reduction of volume of material. However, when the hole/neck size is small (200nm), the reduction of thermal conductance is sufficiently larger than that expected from volume reduction.
Although the main part of the plan was finished smoothly, due to the unexpected breaking of the femtosecond laser, the construction of the optical system was delayed. It also resulted in a delay in using the budget. Now the laser has been repaired, the optical system is in construction. This delay will not affect the plan of FY2018.
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| 今後の研究の推進方策 |
In FY2018, we will focus on performing phonon spectroscopy with MEMS thermometer and femtosecond laser. Phononic spectroscopy can be divided into four steps: phonon generation, time delay, phonon detection, and phonon spectra calculation.
Thermal phonons are generated in the center of the MEMS beam by two heat pulses from a femtosecond laser. Then we modulate the time delay between two laser pulses to induce an interference in the phonon transport, which will be detected as a modulated temperature rise on the beam. Simply speaking, we measure the temperature rise of the beam caused by two laser pulses as a function of the time delay between the two pulses. If coherent transport takes place in the beam, we should observe an interference pattern. Then by doing the Fourier Transform of the interference pattern, we could derive the thermal phonon spectrum of the MEMS beam. When a phononic crystal structure is fabricated on the beam, a phonon bandgap may be observed in the phonon spectrum. Note that since phonons whose energies are within the bandgap are more localized in the beam, the phonon bandgap is supposed to show a peak-like feature in the spectrum.
Furthermore, during last year’s work, we found this research may be much more meaningful than what we expected. It may be possible to develop a general technique or device for measuring the thermal phonon transport in any micro/nano scale materials, giving a great improvement in the study of thermal transport. In FY2018, we will work on making a plan for achieving this in the future work.
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| 次年度使用額が生じた理由 |
Because the femtosecond laser was broken unexpectedly in FY2017, the construction of the optical system was delayed. Therefore, the purchasing of some necessary optical components (beam splitter, femtosecond laser attenuators, etc.) are postpone to FY2018.
At present, the femtosecond laser has been repaired. The amount of money will be used to purchase these optical components to construct the laser auto-correlation system for phonon spectroscopy measurement.
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