Kazuki Ihara

Flexible heat-flow sensing sheets based on the longitudinal spin Seebeck effect using one-dimensional spin-current conducting films

Heat-flow sensing is expected to be an important technological component of smart thermal management in the future. Conventionally, the thermoelectric (TE) conversion technique, which is based on the Seebeck effect, has been used to measure a heat flow by converting the flow into electric voltage. However, for ubiquitous heat-flow visualization, thin and flexible sensors with extremely low thermal resistance are highly desired. Recently, another type of TE effect, the longitudinal spin Seebeck effect (LSSE), has aroused great interest because the LSSE potentially offers favourable features for TE applications such as simple thin-film device structures. Here we demonstrate an LSSE-based flexible TE sheet that is especially suitable for a heat-flow sensing application. This TE sheet contained a Ni0.2Zn0.3Fe2.5O4 film which was formed on a flexible plastic sheet using a spray-coating method known as “ferrite plating”. The experimental results suggest that the ferrite-plated film, which has a columnar crystal structure aligned perpendicular to the film plane, functions as a unique one-dimensional spin-current conductor suitable for bendable LSSE-based sensors. This newly developed thin TE sheet may be attached to differently shaped heat sources without obstructing an innate heat flux, paving the way to versatile heat-flow measurements and management.

Highly Uniform Thin-Film Transistors Printed on Flexible Plastic Films with Morphology-Controlled Carbon Nanotube Network Channels

Carbon nanotube (CNT) transistor arrays were fabricated on plastic films by printing. All the device elements were directly patterned by maskless printing without any additional patterning process, and minimum materials were used. During fabrication, the morphology of the CNT random network was controlled by an adsorption mechanism on the surface to be printed, which resulted in excellent and uniform electrical properties. The field-effect mobility was further improved by post-treatment to modify the morphology of the CNT network. These results are promising for realizing printed electronics integrated with CNT transistors.

Electrical property of printed transistors fabricated with various types of carbon nanotube ink

Printed electronics is expected to be a low-cost, eco-friendly, and on-demand fabrication technology because it decreases the number of process steps and the amount of waste materials. Carbon nanotube (CNT) transistor arrays were fabricated on plastic films by maskless printing without any additional pattering process, and minimum materials were used. During fabrication, the morphology of the CNT random network was controlled by an adsorption mechanism on the surface to be printed, which resulted in excellent and uniform electrical properties. Based on the fabrication technology, various types of CNT ink were examined. A guiding principle for developing CNT ink was shown to improve the TFT performance.

Low variability with high performance in thin-film transistors of semiconducting carbon nanotubes achieved by shortening tube lengths

Extremely low variability with excellent device performances in a SWCNT-TFT array has been demonstrated in SWCNT-TFTs fabricated by using a semiconducting ink of short SWCNTs with an average length of 340 nm; the field-effect mobility of 3.9 ± 0.45 cm2 V−1 s−1, on/off ratio from 105 to 106, and hysteresis of ≈0.5 V. AFM observations revealed that the nonionic surfactant, Brij 700, adopted for dispersing SWCNTs during their extraction, causes the significant and homogeneous shortening of SWCNTs compared with sodium cholate, which is frequently used for the dispersion of SWCNTs as an ionic surfactant. Thus, it has been concluded that the shortening of SWCNTs in the dispersing process using the Brij 700 surfactant contributes to the observed uniformity of performance among the devices.

Spin-Seebeck thermoelectric converter

Thermoelectric conversion (TEC) technologies, which convert heat into electricity, have received a great attention, because they are expected to be a powerful approach to utilize wasted thermal energy. Here we present novel thermoelectric converters based on the spin Seebeck effect (SSE), and show their scaling law which is largely different from that of conventional TEC devices. We experimentally demonstrate that the TEC output signals straightforwardly increase with the size of the converters. This scaling law enables us to implement simple-structured thermoelectric converters by using productive film-coating methods. Such coating-based TEC techniques may pave the way for a wide range of applications using a variety of heat sources.

Printing technology and advantage of purified semiconducting carbon nanotubes for thin film transistor fabrication on plastic films

Fabrication technology for printed carbon nanotube (CNT) transistors was developed. All electrodes, insulators, and CNT channels were directly printed and CNT thin film transistors (TFTs) were fabricated on plastic films without additional patterning processes. In this fabrication, surface free energy was controlled and a mechanism for adsorbing CNTs was applied to form a random network channel. Purified semiconducting CNT ink was used and the TFT characteristics clearly demonstrated the advantages of the ink. This increased the on-current of the printed CNT-TFT without deterioration in the on/off ratio. A large field-effect mobility of 3.6 cm2/Vs was obtained for the printed CNT-TFT with an on/off ratio of about 1,000.

Gamma radiation resistance of spin Seebeck devices

Thermoelectric devices based on the spin Seebeck effect (SSE) were irradiated with gamma (γ) rays with the total dose of around 3 × 105 Gy in order to investigate the γ-radiation resistance of the devices. To demonstrate this, Pt/Ni0.2Zn0.3Fe2.5O4/Glass and Pt/Bi0.1Y2.9Fe5O12/Gd3Ga5O12 SSE devices were used. We confirmed that the thermoelectric, magnetic, and structural properties of the SSE devices are not affected by the γ-ray irradiation. This result demonstrates that SSE devices are applicable to thermoelectric generation even in high radiation environments.