1. Field of the Invention
Embodiments relate to preparation of large area graphene oxide films and the conversion of these films into graphene oxide fibers and electrically conductive graphene fibers.
2. Background of the Related Art
Graphene has become an attractive material to prepare fibers of improved mechanical properties and specific electric conductivity. It has been found that because of its low density, graphene-based materials can have higher specific electrical conductivity than many metals. In addition, graphene has excellent mechanical properties, good thermal conductivity and can be obtained from reduction of graphene oxide (“GO”). This powder material, GO, has been known for more than 150 years since it was synthesized by Benjamin Brodie by using a mixture of potassium chlorate and fuming nitric acid (Brodie, B. C. (1859), “On the Atomic Weight of Graphite” Philosophical Transactions of the Royal Society of London 149: 249.).
Graphene oxide is a bulk precursor of the 2D material graphene, which is electrically conductive. Due to their lower weight, graphene based conductors can be useful in aeronautical conductors, and its greater chemical stability as compared to copper, makes this material attractive when environmental exposure cannot be avoided.
Free-standing pure graphene films have been made typically by filtration of aqueous graphene oxide dispersions (Dikin, et al., Nature 2007, 448-457) followed by chemical or thermal reduction. Unfortunately, the filtration process limits the production of these films to batch production and typically, only small area films (Area<100 cm2) are obtained. These films are highly bi-axially oriented, resulting in great modulus and tensile resistance but low toughness. Due to the low toughness these films have a tendency to tear or break while folding or deformed by twisting, and have been barely used as structural materials.
Recently patent applications reporting preparation of pure graphene fibers by wet spinning have been filed (see, for example, “Method for preparing high-strength conductive graphene fiber by large-size graphene oxide sheet” CN102534869 A; “Graphene fiber and method for manufacturing same” WO2012124934 A3). Those applications report injecting an aqueous dispersion of graphene oxide through a spinneret into a coagulation bath. The change of phase causes the outer surface of the graphene fiber to coagulate, making a skin that provides mechanical stability to the wet spun fiber. These fibers later are dried and, sometimes, mechanically treated to improve their performance. However, during drying and coagulation, the formation of a skin with a large area results in an irregular cross sectional shape as the core of the fiber loses the solvent and decreases its volume.
In a different approach, the linear metallic substrates of graphene stripes prepared by chemical vapor deposition were chemically etched leaving a dispersion of graphene that were pulled from the liquid into a linear fiber (“Process for spinning graphene ribbon fibers” WO 2011134717 A1). However these methods rely on a liquid dispersion, which limits the process and adds complexity.