New rubber composite material: its thermal conductivity is comparable to that of metal, and it is as soft as rubber.
By combining carbon nanofibers (CNF) and carbon nanotubes (CNT), two fibrous carbon materials, with the ring polymer material polyrotaxane, the University of Tokyo and the Japan Institute of Industrial Technology (JIT) have developed a rubber composite that is as soft as rubber and has a thermal conductivity equal to that of metal. The new rubber composite has a high thermal conductivity of 14W/mK along the CNF direction, and also has high flexibility. The materials developed this time are expected to be applied to thermal sandwich materials, heat sinks and heat sinks of flexible electronic devices.
In recent years, soft thermal management materials with high heat dissipation, such as thermal interlayer materials and heat sinks, used in flexible electronic devices, have attracted much attention. In addition to high thermal conductivity, these materials also need low Young's modulus, high tensile strength, high toughness and other mechanical properties. Therefore, as the next generation of thermal conductive flexible materials, flexible rubber materials and CNF and CNT composites with high thermal conductivity have been energetically researched and developed.
Celluose nanofibers (CNF) are made from cellulose by nano (ultra fine) treatment, which is characterized by "lightness, toughness and environmental protection". The reason why CNF is concerned is that it has a characteristic that "its weight is one fifth of that of steel, but its strength is more than five times that of steel". If mixed with resin and rubber, automobile parts with light weight and high strength can be made.
Carbon nanotube CNT is called the ultimate fiber, which is a tube (multi wall carbon nanotube) formed by coaxially winding a single layer of graphite (single wall carbon nanotube) or nesting a single wall carbon nanotube layer by layer along the coaxially. The diameter of carbon nanotubes is generally between one to dozens of nanometers, and the length is far greater than its diameter. With many extraordinary physical properties (mechanical, electrical, thermal) and chemical properties, carbon nanotubes are one-dimensional carbon nanomaterials. As the best material with mechanical properties discovered by human beings so far, carbon nanotubes have extremely high tensile strength, Young's modulus and fracture strain.
However, although the thermal conductivity of CNT exceeds 2000W/mK, 10wt% needs to be added in order to achieve the thermal conductivity of 2W/mK composites. In addition, if a large amount of CNF is added, the flexibility of the composite will be lost and become brittle. Generally speaking, fibrous carbon has strong agglutination and is difficult to be evenly dispersed in the composite, so it is difficult to form a heat conduction network connected by fibrous carbon contacting each other on the whole composite.
In addition, the interface between large fibrous carbon aggregate and rubber material becomes the starting point of failure during deformation and one of the main reasons for embrittlement.
The rubber composite developed this time has two different sizes of fibrous carbon materials distributed in polyrotaxane（ CNF and CNT) as filler. CNF is 200nm thick and 10-100 long μ m. CNT is 10-30nm thick and 0.5-2 long μ m。 Improving the dispersion of fibrous carbon materials in rubber materials and forming a heat conduction network in composites are considered to be the key to achieving high thermal conductivity. In order to improve the dispersibility, CNF and CNT (CNF: CNT weight ratio is 9:1) were dispersed in sodium chloride aqueous solution, and surface modification was carried out by using the self-developed circulating water plasma weight conditioner.
Next, the surface modified The CNF/CNT mixture is mixed with polyrotaxane, catalyst and cross-linking agent, and then put into the container for AC electric field treatment, and then the AC electric field is applied to cause cross-linking reaction to produce gel. After that, the gel obtained by heating in the oven is removed from the solvent, and the film like composite material is obtained.
The internal electron microscope image of the composite developed this time. By surface modification, the cocoon like aggregates are loose, The CNF is arranged in the direction of the applied electric field. In addition, the smaller CNT is wrapped around the larger CNF to connect the CNFs together. It is believed that by connecting CNF with a small amount of CNT, a thermal conductivity network is formed in the whole composite, thus achieving high thermal conductivity.
In Japan, The research and development work of CNF has been active for many years and has achieved significant results. The main force in the research and development of CNF is Japanese paper making companies, Prince Holding (HD) and other paper companies that use pulp in their daily business, as well as the University of Tokyo.
The research team led by Professor Hiroyuki Yano of the Kyoto University Institute for Conservation of Circles is promoting the use of Research on the substitution of CNF for iron car body and frame. If the vehicle can be lightweight, 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 fuselage like carbon fiber.
At the end of 2019, an alliance composed of industrial, academic and government institutions made use of cellulose nanofibers to produce an NCV lightweight concept car in the NCV (Nano Cellulose Vehicle) project of the Ministry of Environment of Japan. The interior and body panels of the concept car use as many components based on cellulose nano materials (CNF) as possible, reducing the vehicle weight by more than 10%.
