Researchers at the Cambridge Graphene Centre at the University of Cambridge, UK, have designed a method for producing high quality conductive graphene inks with high concentrations. Conductive inks are useful for a range of applications, including printed and flexible electronics, transistors, and more.
The method uses ultrahigh shear forces in a microfluidization process to exfoliate graphene flakes from graphite. The process is said to convert 100% of the starting graphite material into usable flakes for conductive inks, avoiding the need for centrifugation and reducing the time taken to produce a usable ink. The research also describes optimization of the inks for different printing applications, as well as giving detailed insights into the fluid dynamics of graphite exfoliation.
The inks produced by the microfluidization process have high concentrations of up to 100g of graphene flakes per litre. Using the most efficient rheology modifiers and stabilizers, the microfluidized graphene mixture is optimized for screen printing.
As well as graphene, this method can easily be applied to other layered materials, such as hexagonal boron nitride or transition metal dichalcogenides. This could create a family of printable circuit components – conductor, insulators and semiconductors – with which to build a variety of printed electronics with different functionalities.
The inks contain a high concentration of chemically unmodified few-layer graphene, leading to excellent conductivity of the final printed material. The inks also give an excellent sheet resistance below 2 Ω/sq, suitable for RFID antennas and electrodes in optoelectronic or energy storage devices. These inks are ideal for applications where low-cost is important.
In the microfluidization process, graphite powder is mixed with water and a surfactant is added to prevent flakes from aggregating. The mixture is passed through a microfluidizer; Turbulent flow through the diamond-coated microchannel leads to ultra-high shear rates of 108 s-1, exfoliating the graphite into few-layer flakes. Importantly, all of the input mixture flows through the microchannel and experiences the high shear, and the process can be repeated to achieve the required graphene flake sizes. “All of the starting mixture experiences the same uniform intensive shear levels, converting it into a usable ink with high concentration. There is no wastage of material or time consuming post-processing,” added the team.
An important issue for the use of graphene inks in printed electronics and other applications is scalability – producing inks and dispersions in large enough quantities for industrial applications. With the 100% yield of the microfluidization method, the team says that it is now possible to produce high quality graphene in sufficient quantities for commercial products.
Inks produced using this method have already been commercialized via a University of Cambridge spin out company, Cambridge Graphene, which was recently acquired by engineering solutions company Versarien. The inks are also supplied to Graphene Flagship partner Novalia, UK for use in their interactive touch-based printed electronic demos.