My PhD was concerned with high temperature superconducting ceramics, which are so called because they operate at -196 °C. The casual observer may well wonder why a temperature so low is called high, suspecting Orwellian double-think. In particular, a reader with a keen interest in inkjet printing may additionally wonder what particular direction this article is taking given this opening, to which I say bear with me, there is a point.
When I first read about superconductors in the late eighties, there was considerable excitement as Georg Bednorz and Alex Mueller had just discovered that superconductivity in a cuprate, specifically lanthanum barium copper oxide, was occurring several degrees above the upper limit! Other cuprates were studied and the critical temperature, below which superconductivity occurs, was increased further. Before long there were articles about how such materials would form our next generation power lines.
But, of course, when the hype died down and people realised that flooding the country with liquid nitrogen to enable the new power lines to act as superconductors, instead of resistors, was both expensive and suffocating, a ‘hangover’ effect occurred.
I had already gained much experience with hangovers when I started my PhD in 1997, little aware that I was about to embark on a period of intensive experimentation to discover the optimal conditions necessary to fabricate the perfect hangover. The next morning when one grips a head too sore and realises that even the dull leaden skies of an English morning are too bright a cold realisation dawns.
For me, with superconductors this realisation was fault current limiters, a self-healing fuse. Put simply, when a large current passes through a superconductor it heats up and becomes a resistor, protecting everything downstream from the surge. Cooling the superconductor restores its superconducting function. This was marvellous, the superconductor was playing to its strengths; doing something it was uniquely suited to do. It no longer had to compete with power cables that can comfortably operate at 30 °C; it had other purposes for which it was eminently fit.
With my research into reactive inkjet printing, I believe we are playing to the strengths of inkjet printing. Around the world, inkjet printers routinely use a combination of four inks and the substrate to produce a staggeringly wide palette of colour. Droplets are merged to create an infinite variety of shade, hues and intensities.
The best example of reactive printing and, for me, the moment when cold realisation dawned is Alison Lennon’s use of a printer to synthesise hydrofluoric (HF) acid, which is so strong it dissolves glass. HF is an ideal etchant, and is used for cleaning silicon wafers. As Alison was using a printer, she could produce etch patterns on silicon surfaces, allowing metal to contact the underlying silicon for solar cells. By using a combination of polyacrylic acid and ammonium fluoride, Alison avoided handling large quantities of HF.
For me, Alison’s use of reactive inkjet printing is a perfect example of inkjet printing’s strengths. The ability to produce a limitless variety of patterns combined with the capacity to add controlled volumes of one reactant to a second reactant. When you think of an inkjet printer as a chemistry set where each nozzle is a pipette and combine that with the potential to print any pattern you want then the possibilities open up a field that is wider enough for rich, productive exploration.
The main strand of my research is simply to play inkjet printing to its strengths. Of course, it took a few hangovers to realise that.
If you’re interested in reactive inkjet printing, then perhaps the following two papers may be of interest.
Alison Lennon’s wonderful paper: Alison J. Lennon, Anita W.Y. Ho-Baillie, Stuart R. Wenham, “Direct patterned etching of silicon dioxide and silicon nitride dielectric layers by inkjet printing,” Solar Energy Materials & Solar Cells 93 (2009) 1865–1874
A deceptively titled review of reactive inkjet printing: Patrick J. Smith and Aoife Morrin, “Reactive Inkjet Printing,” Journal of Materials Chemistry, 2012, 22, 10965
Patrick will be speaking at the InPrint/IMI showcase in Munich on the 24th June and will be speaking at InPrint in November 2015. He is an Ambassador for InPrint.
To discover more about Patrick and his work at the University of Sheffield