The 83rd JSAP Autumn Meeting 2022 Highlighted Presentations
October 24, 2022

Ultrafine-fiber ceramic thermistors

Ultrafine-fiber ceramic thermistor capable of measuring
the temperature of a single cell at high speed, realized
via photocrystallization
National Institute of Advanced Industrial Science and Technology (AIST)
Tomohiko Nakajima, Tetsuo Tsuchiya

Summary•

Realization of an ultrafine-fiber ceramic thermistor by forming a spinel-type
manganese oxide thermistor film of 1-µm thickness on a 15-µm ultra-fine resin fiber.

•Development of applications of a unique photocrystallization process using achemical solution method and crystallization with a laser is advancing.

•This research will open new possibilities for ceramic film formation and is expected
to be applied to next-generation wearable devices for biological monitoring and threedimensional electronic devices.

A research group led by Tomohiko Nakajima and Tetsuo Tsuchiya at the National Institute of Advanced
Industrial Science and Technology (AIST) has been developing a unique ceramic film formation process using
a chemical solution method and crystallization using a laser. This process has enabled the formation of ceramicoxide films on flexible substrates, such as organic substrates, with very low heat resistance. As a result, theresearch group succeeded in forming a spinel-type manganese oxide thermistor film with a thickness of 1 μm
on a 15-μm fine resin fiber, thereby realizing an ultrafine-fiber ceramic thermistor. The process used in thisresearch will open new possibilities for ceramic film formation and is expected to be applied to next-generation
wearable devices for biological monitoring and three-dimensional electronic devices.

Detail

Evolving Ceramics into Next-Generation Materials via Low-Temperature Processes
When compared with organic and metallic materials, ceramic materials exhibit higher chemical
stability under different temperatures and humidity levels. Owing to their high reliability, they areused in various digital devices including smartphones. However, ceramic materials havecharacteristics of electronic materials. In exchange for their high stability, they require high formation
temperatures and cannot be applied in flexible organic electronic devices made of resins and othermaterials.
A research group led by Tomohiko Nakajima and Tetsuo Tsuchiya at the National Institute ofAdvanced Industrial Science and Technology (AIST) has been conducting research on lowtemperature processes for ceramic film formation. They developed a unique photocrystallization
process that combines a chemical solution method with crystallization using a laser. In their method,
ceramic materials were coated onto a substrate made of resin or other materials and irradiated withlight to form a film. This technology involved the formation of ceramic films by irradiating laser pulsesof 10 – 20 ns and locally raising the temperature of the film. To date, it has been thought thatceramics only crystallize at high temperatures, but in this research, “we have succeeded in producing
the same effect (formation and growth of crystal nuclei) as high-temperature sintering using a laser",says Nakajima, who is one of the research group leaders.
The breakthrough in this process was the development of nanoparticle ink materials that can growcrystals in 10 – 1000 laser pulses. Base materials, such as resins, are sensitive to heat. Whenforming films via laser irradiation or other means, they are easily destroyed upon exposure to highenergy for long periods. To reduce the damage to the base material, it is necessary to use a material
with a high optical absorption coefficient. Therefore, the research group developed a special
nanoparticle ink that allowed ceramic crystals to grow at the particle interface. The nanoparticles
dispersed in the ink enabled the crystallization of ceramics over a short laser irradiation time, thereby
reducing damage to the base material and enabling the process to be applied to resin-basedmaterials.
The ceramic film produced using this process has high functionality, high durability, and easy
resource recyclability. Furthermore, a wide range of applications can be found including conductive
films for flexible solar cells, semiconductor films for flexible organic light-emitting diodes, andelectrodes for fuel cells.

Development of a temperature sensor capable of measuring the temperature of asingle cell
The research group developed an ultrafine-fiber ceramic thermistor by applying a low-temperaturefilm-forming process to a three-dimensional (3D) structure (Figure 1) and succeeded in forming aspinel-type manganese oxide thermistor film with a thickness of 1 μm on a 15-μm fine resin fiber.
Thermistors are widely used as temperature sensors in electronic devices. Fine-fiber ceramic
thermistors can accurately measure the temperatures of very small objects. For example, they canmeasure the temperature of a single cell that constitutes a human being.
The advantage of very small sensors is that they can perform accurate measurements without
affecting the object being measured. If the object being measured is very small and the temperaturesensor is relatively large, this results in a difference in heat capacities, causing heat to flow from thesensor to the object being measured. This would inhibit the temperature change of the object being
measured, making accurate measurement impossible. Therefore, the smaller the sensor, the fasterthe response time. Fine-fiber ceramic thermistors can be used as a surface function for medical
capillary catheters for monitoring cells and local biological sites as well as for other applications suchas 3D electronics.
Nakajima stated that the technology for creating 3D structures using a low-temperature ceramic filmforming process will enable the development of new devices such as printed electronics. In thedevelopment of wearable devices, designers will be able to intuitively add the necessary functions
to the surface of a 3D shape, which will then be printed for use. This low-temperature process canbe used in a variety of applications involving ceramic materials in the development of next-generation
devices.

FiguresFigure 1. Ultrafine-fiber ceramic thermistor (top); Co-, Ni-, and Cu-doped Mn3O4 thin film deposited on a 15-μm diameter
resin ultrafine-fiber (bottom).

Figure 2. Temperature responses of an ultrafine-fiber ceramic thermistor, showing that the increase and decrease intemperature due to LED irradiation are fast and at comparable rates.