Reactive Inkjet Printing (RIJ) & Controlled Crystallisation

Inkjet printing is an appealing choice of production process on account of its additive nature.
Its particular strength is its ability to precisely place pico-litres of ink at predetermined locations on a substrate, which results in thin films, two-dimensional patterns and even three-dimensional structures.
Of particular importance to this research is the fact that inkjet printing can place different materials either side by side, or one on top of the other.
A logical step is to combine reactants, which has led to the emergence of a new field: reactive inkjet printing.
The research proposed here intends to establish a dominant position for the UK in this nascent field.

The main aims of the proposed research are:
- Generate functional materials in situ using an inkjet printer as a synthesis tool. In a single process step, novel film coatings and nanostructures can be patterned and integrated into electronic devices such as conductors, transistors, sensors, printed batteries or displays.
- Control the crystallisation of inorganic systems using inkjet printing.

The principle questions to be answered are:
- Can the overall energy consumption involved in producing printed electronic devices be reduced?
- Are there performance increases to be gained as a consequence of synthesising material in-situ?
- Many crystalline systems have been produced by inkjet printing but can properties such as size and degree of crystallinty be controlled?

Reactive inkjet printing (RIJ) and inkjet-controlled crystallisation both build upon inkjet printing, which is an additive technique. Briefly, the advantages of inkjet printing are that it uses the 'direct write' approach that removes the need for masks, which leads to a reduction in the number of process, which leads to cost-savings, efficient use of materials and reduction of waste. Inkjet printing also allows a process to be easily scaled-up.

RIJ can be compared to microreactor chips. However, instead of pumping raw materials through channels (300-3000 microns wide) towards a reaction channel, droplets that are 30 - 60 microns wide are added to a reaction site. Advantages such as increased yield of reactions and reduced material consumption are offered since mixing and heat transfer are improved. RIJ can open a new way of performing science and is clearly multi-disciplinary. RIJ has already been used to synthesise DNA, it could also be used in tissue engineering to produce cell guides, composites technology to produce functionally graded materials and printed electronics. As an exciting new area of science, with appeal to many disciplines, RIJ offers an excellent opportunity for the PDRA who will be recruited and the PhD students who will work on this theme. There are many publishing opportunities.

RIJ offers potential benefits to industry since it leads to a further reduction in process steps. It is a more energy efficient process since the energy used to synthesise the material is the same energy used to deposit and pattern the functional material into a usable device. RIJ also offers the possibility of generating materials, and devices, with higher performances, since stabilising surfactants are not needed. (Inkjet printed tracks formed from silver nano-particles generated in-situ from MOD inks exhibit much higher conductivities than tracks formed from inks that contain stabilised nanoparticles.)

RIJ also offers environmental benefits in that the overall energy demand of a process is reduced, and, of course, it also offers the advantages of inkjet in that material is used very efficiently. RIJ could help in the generation of strong acids, e.g. HF and structures based on heavy metals, such as Cd, since none of these materials are wasted. Less waste means less pollution, and a more energy efficient process is clearly appealing. The reduction in process steps, reduces overall process time, which should lead to increased production rates and cheaper prices.

Producing crystals with controlled sizes will appeal to industry as demonstrated very recently (July 21st 2011) in Japan, where an inkjet-based printing technique has been used to make high-performance, single-crystal thin-film transistors. The process is performed at room temperature, and is aimed towards large-area printed electronics, such as flexible displays, solar cells, electronic paper and sensor sheets. IDTechEx predicts that the market for printed electronics and electrics will be $335 billion within twenty years, just for devices primarily made by printing with electronic inks. By 2015, they suggest that the market for organic LED displays and lighting will be $8.20 billion.