The University of Tokyo and the Japan Institute of Industrial Technology (Industrial Comprehensive Research) have developed a combination of two types of carbon nanofiber (CNF) and carbon nanotube (CNT) fibrous carbon materials and cyclic polymer material polyrotaxane. It is a rubber composite material that is as soft as rubber and has a thermal conductivity comparable to that of metal. The new rubber composite material has a high thermal conductivity of 14W / mK in the direction of CNF arrangement and high flexibility. The materials developed this time are expected to be applied to thermal interlayer materials, heat sinks and heat sinks of flexible electronic devices.
Research Background
In recent years, soft thermal management materials that exhibit high heat dissipation, such as thermal interlayer materials for flexible electronic devices and heat sinks, have attracted attention. In addition to high thermal conductivity, these materials require mechanical properties such as low Young's modulus, high tensile strength, and high toughness. Therefore, as the next generation of thermally conductive and flexible materials, flexible rubber materials and composite materials with high thermal conductivity of CNF and CNT have been vigorously researched and developed.
Carbon nanofibers (Celluouse Nanofibers, CNF) are made by cellulose (Celluouse) nano-processed (ultra-fine), with the characteristics of "lightness, toughness, environmental protection". The reason for CNF's attention is one of its characteristics-"weight is one-fifth of steel, but its strength is more than five times that of steel." If it is mixed with resin and rubber, it is possible to make automotive parts with light weight and high strength.
Carbon nanotube CNT is called the ultimate fiber, which is a tube (single-wall carbon nanotube) coaxially wound by a single layer of graphite or a tube (multi-wall carbon nanotube). The diameter of carbon nanotubes is generally between one and tens of nanometers, and the length is much larger than its diameter. It has many extraordinary physical properties (mechanical, electrical, thermal) and chemical properties. It is a one-dimensional carbon nanomaterial. As the best mechanical material found by humans so far, carbon nanotubes have extremely high tensile strength, Young's modulus and strain at break.
However, although the thermal conductivity of CNT exceeds 2000 W / mK, in order to achieve a thermal conductivity of 2 W / mK of the composite material, 10 wt% needs to be added. In addition, if a large amount of CNF is added, the flexibility of the composite material will be lost and become brittle. In general, fibrous carbon has strong cohesiveness and is difficult to uniformly disperse in the composite material. Therefore, it is difficult to form a heat conduction network in which the fibrous carbon is in contact with each other and connected to the entire composite material. In addition, the interface between the large fibrous carbon agglomerates and the rubber material becomes the starting point of failure during deformation and becomes one of the main causes of embrittlement.
Innovation
The rubber composite material developed this time is distributed with two different sizes of fibrous carbon materials (CNF and CNT) as fillers in polyrotaxane. CNF is 200nm thick and 10-100μm long, CNT is 10-30nm thick and 0.5-2μm long. Improving the dispersion of fibrous carbon materials in rubber materials and the formation of thermally conductive networks in composite materials are considered to be the key to achieving high thermal conductivity. In order to improve the dispersibility, CNF and CNT (CNF: CNT weight ratio of 9: 1) were dispersed in an aqueous solution of sodium chloride, and surface modification was carried out using a self-developed circulating water plasma reformer.
Next, mix the surface-modified CNF / CNT mixture with polyrotaxane, catalyst, and cross-linking agent in toluene solvent, and then put it in a container for AC electric field treatment, and then apply an AC electric field to cause cross-linking reaction. Made into a gel. Then, the obtained gel was heated in an oven to remove the solvent, and a film-like composite material was obtained.
By performing surface modification, the cocoon-like aggregates are loosened, and the CNF is aligned in the direction of the applied electric field. In addition, smaller CNTs are wrapped around the larger CNF, connecting the CNF together. Research suggests that by connecting CNF with a small amount of CNT, a thermally conductive network is formed in the entire composite material, thereby achieving high thermal conductivity.
The newly developed rubber material still has high flexibility even with the addition of 50% by weight of fibrous carbon material, and repeated deformation does not cause embrittlement. The research team believes that the fibrous carbon material is cross-linked with the polyrotaxane ring molecules, and the movement of the ring molecules maintains high flexibility and inhibits embrittlement (Figure 3).
The five stars in Figure 4 represent the rubber composite material developed this time, the dots represent the boron nitride rubber composite material developed in the past, and the green square area is the intended development target for the substrate material for flexible electronic devices. By using fibrous carbon material modified by plasma surface in water, the thermal conductivity is one order of magnitude higher than that of boron nitride, and the Young's modulus is lower (softer) rubber composite material. As a thermal management material for flexible electronic devices, it has reached a practical level.
In Japan, CNF's research and development work has been active for many years and has now achieved significant results. The main force for research and development of CNF are Japanese paper companies that use pulp in their daily business, paper companies such as Prince Holdings (HD), and the University of Tokyo.
The research team led by Professor Yano Hiroyuki of the Research Institute of Survival Circle of Kyoto University is advancing the research of replacing iron car body and frame with CNF. If the weight of the vehicle can be reduced, the fuel economy will be improved. Carbon dioxide emissions will also be reduced. In the long run, it may even be used to make aircraft fuselages like carbon fiber.
At the end of 2019, an alliance of industrial, academic, and government agencies collaborated in the NCV (Nano Cellulose Automobile) project of the Japanese Ministry of the Environment, using cellulose nanofibers to make an NCV lightweight concept car. The concept car uses as many components as possible based on cellulose nanomaterials (CNF) in the interior and body panels to reduce the weight of the vehicle by more than 10%.