Graphene Flagship tests material in zero gravity for out-of-this-world applications
The Graphene Flagship is a 10-year, €1bn project funded by the European Union, aiming to take graphene from the lab to a stage where it could be considered for applications ranging from photonics to electronics, composites, biomedicine, energy and coatings.
Our so-called zero-gravity flights are funded by the European Space Agency (ESA) and the scientific work by the Flagship, which has four main partners in this activity – us at the University of Cambridge, Université Libre de Bruxelles, CNR in Italy, and Leonardo, an aerospace company based in Italy.
Leonardo has a branch that builds satellites, and one issue in satellites is heat balancing – from the outside to the inside, from the dark part to the one exposed to the sun, and from the heat generated by the electronics inside. To take the heat around, you typically use pumps called loop heat pipes. These do not have mechanical parts because the last thing you want in a satellite is a pump with a part moving around that can be blocked. Then what do you do? It’s extremely expensive to go and catch the satellite and fix it.
To avoid these issues, the concept being put forward is a pump that works based on a “porous wick,” which enables a pressure difference between liquid and vapour, bringing about the transmission of the heat.
By including graphene in this wick, you can get positive effects. One is the increase of the capillary pressure, making it easier to bring the fluid around the pipe. Graphene also increases thermal conductivity, so it makes it easier for the fluid to evaporate – this again makes the heat transfer better.
Graphene also has better wetting capability, meaning it maintains better contact with the liquid. Usually these heat pumps have issues starting up because there is no switch, no mechanical part, so they have to self-start-up when the temperature of the wick reaches a certain level and the liquid is evaporating. If you have a better wetting capability then it improves the start-up.
Eventually these pipes will be mounted on satellites, so there is a roadmap for qualification including testing them in space-like environments – not only zero gravity but also hyper-gravity, because when you launch a rocket it experiences gravity that is much bigger than 1g – it goes up to 1.5, 1.6g, and in space it experiences zero g.
The zero g experiments are conducted on a plane run by Novespace and the ESA. The plane flies on a fraction of an elliptical orbit, essentially a parabola, for 22-24 seconds at a time and during this you experience weightlessness. This is repeated 31 times per flight, so you get 11.5 minutes of weightlessness per flight.
It is challenging, and there are loads of ESA and Novespace rules that you need to obey. It takes a long time to prepare all the instrumentation – you need to prepare locks in a certain way, everything has to be enclosed and computer-controlled. During a flight, you can’t use a screwdriver because, if it flies off and you go to 2g, it could be a weapon. So there are lots of health and safety rules.
It takes a week to do testing on the plane before flights. Novespace also provides workshops and lab space for the last adjustments. Once everything is done and you pass all the health and safety procedures, during the flight everything is quite simple because most of the interaction with the experiment is changing parameters on the laptop. You are not allowed to open up the experiment during zero g or hyper-gravity.
There is a one-minute break between each parabola, and then every five parabolas you have a five-minute break for checks and adjustments. In the two campaigns we did, there was an additional 15-minute break after the first 15 parabolas.
A few people get sick, but yesterday we didn’t have any problems and I was quite OK in both flights. When the plane stops you are OK, and you take an injection of scopolamine before each flight.
In 2020 we may have the opportunity to test the pump on a real satellite. If that goes well we aspire to test it on the space station in 2022 or later. So we have a roadmap but this is just the beginning, we need to see what the results are. In each of the experiments we collect several gigabytes of data so once we finish the campaign we need to start analysing everything, which is what we are going to do for the next couple of months.