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New rubber composite: thermal conductivity comparable to metal, soft as rubber

By combining two kinds of fiber-like carbon materials, CNF and CNT, the rubber composite material, which is as soft as rubber and has the same thermal conductivity as metal, has been developed by combining two kinds of fiber-like carbon materials (CNF) and carbon nanotubes (CNT) with the cyclic polymer polyrotane. The new rubber composite has high thermal conductivity of 14w/mk in the direction of CNF arrangement, and has high softness. The materials developed are expected to be used in the thermal sandwich materials, fins and heat sinks of flexible electronic devices.


In recent years, the soft heat management materials, such as the interlayer materials and fins used in flexible electronic devices, have attracted much attention. In addition to high thermal conductivity, these materials also need mechanical properties such as low Young's modulus, high tensile strength and high toughness. Therefore, as the next generation of thermal flexible materials, flexible rubber materials and high thermal conductivity CNF and CNT composites are developed with energy.


The nanofibers (CNF) are made from cellulose after nanoparticle treatment, which has the characteristics of "light, strong and environmental protection". The reason CNF is concerned is one of its characteristics - "weight is one fifth of steel, and its strength is more than five times of steel". If mixed with resin and rubber, we can make light and high strength auto parts.


CNT is called the ultimate fiber, which is a tube (multi wall carbon nanotubes) which is made of single-layer graphite wound into a tube (single wall carbon nanotubes) or by the single wall carbon nanotubes (MWCNTs) which are coated in a coaxial layer. The diameter of carbon nanotubes is usually between one and dozens of nanometers, and the length is much larger than its diameter. It has many extraordinary physical properties (mechanics, electricity, thermal) and chemical properties. It is one-dimensional carbon nano materials. As the best material of mechanical properties discovered by human, carbon nanotubes have high tensile strength, Young's modulus and fracture strain.


However, although the thermal conductivity of CNT exceeds 2000w/mk, 10wt% is needed to achieve the thermal conductivity of the composite materials by 2w/mk. In addition, if a large amount of CNF is added, the softness of the composite will be lost and become brittle. Generally speaking, fiber-like carbon has strong agglutination and is difficult to disperse evenly in the composite, so it is difficult to form a heat conduction network connected by fiber-like carbon contact on the whole.


In addition, the interface between large fiber-like carbon coagulation and rubber materials becomes the starting point of deformation failure and one of the main reasons for embrittlement.


Two kinds of fiber-like carbon materials (CNF and CNT) were distributed in the polyrotane as fillers. CNF is 200nm thick and 10-100 long μ m. CNT is 10-30nm thick, 0.5-2 long μ m。 Improving the dispersion of fiber-like carbon in rubber and forming a heat conduction network in composite materials are considered to be the key to achieve high thermal conductivity. In order to improve the dispersion, CNF and CNT (cnf:cnt weight ratio 9:1) were dispersed in sodium chloride aqueous solution. The surface modification was carried out by using the self-developed circulating water plasma weight integrator.


Next, in the toluene solvent, the surface modified CNF/CNT mixture was mixed with polyalkanes, catalysts and crosslinking agents, then placed in an AC electric field treatment container, and then AC electric field was applied to crosslink the reaction to form gels. After that, the gel obtained from the oven was heated and the solvent was removed to obtain the film composite material.


The development of the composite internal electron microscope image. By surface modification, cocoon like aggregates are loose and CNF is arranged along the applied electric field. In addition, the smaller CNTs are wrapped outside the larger CNF and connect the CNF together. The research shows that the high thermal conductivity is achieved by connecting CNF with a small amount of CNT, and forming a heat conduction network in the whole composite.


In Japan, CNF has been active in research and development for many years, and has achieved great achievements. The main research and development of CNF are Japanese paper making companies, such as Prince Holdings (hd) and Tokyo University, which use pulp in daily business.


The research team led by Professor yanohao of Kyoto University Students' Research Institute of the conservation is promoting the research on replacing the body and frame of iron car with CNF. If vehicle lightweight is achieved, the fuel economy will be improved. Carbon dioxide emissions will also be reduced. In the long run, it is even possible to use it as carbon fiber to make aircraft fuselage.


At the end of 2019, an alliance of industrial, academic and government agencies worked together to build a concept vehicle for lightweight NCV using cellulose nanofibers in the NCV (nanocellulose vehicle) project of Japan's environmental ministry. Concept vehicles use as many components as possible based on cellulose nanomaterials (CNF) on interior and body panels, which can reduce vehicle weight by more than 10%.


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