Associate Professor Susan Krumdieck from the University of Canterbury describes a process she developed to create new ceramic materials that could be used on hypersonic vehicles. She explains her process of pulsed-pressure chemical vapour deposition and describes how the ceramic particles are deposited one by one inside her specially designed reactor.
Why has this group of top researchers in the US come to me to do the process development for them? Seems a long way to go, but the thing is I developed a process that works in a whole new way that nobody had thought of before, and it solves a lot of the problems of being able to do this process of getting a chemical into the vapour form, getting it to the surface that you want it to deposit it on and getting it to deposit there evenly. This is my baby, this is a new version of what I designed and developed during my PhD programme, and this is what I call pulsed-pressure chemical vapour deposition processing.
And it’s quite simple really, which is a good thing because the standard chemical vapour deposition machines are at least a million dollars to start with, but this we built here in New Zealand for under 20,000 New Zealand dollars.
The whole principle of it is really simple. For chemical vapour deposition, we need a chemical, and so the chemical we have in a liquid form, which is very easy to handle – chemicals that are in a gas form need a lot more equipment, so I use the liquid form – and then all we need are little tubes, and a little bit of pressure from a gas into the bottle will push the liquid through the tubes.
And this bit up here is the mechanical engineering genius of the whole thing, and that’s a way to take that liquid and control that we take a small amount of it that we can control how much that is, and we shoot it into the reactor when we want to, all under control. This is what we call the reactor – it’s a glass tube, but it’s still a reactor – and it’s being pulled down to a vacuum by a vacuum pump. And that liquid that comes in through an ultrasonic atomiser gets atomised into tiny droplets, finds itself in a vacuum and becomes vapour, so that is the vapour part of chemical vapour deposition.
Once the liquid, the chemical is now in a vapour form, it bounces around in this vacuum cause it’s still quite low pressure, and it will find the surface where we want it to deposit. And we have a little heater down in this area here that heats the surface we want to deposit on. And our chemical, when it lands on that hot surface, will get unzipped – it’s a thermally activated reaction – and when it unzips, it leaves behind a ceramic molecule which finds a little crystal to start growing in, and the crystals then grow on that surface. And we can then look at those in a scanning electron microscope after we have deposited for a length of time and cooled it down and taken the sample out.
So it’s really quite simple, and that is why here in New Zealand I’ve been able to work on quite a few different kinds of materials and why the group from the United States asked me to work on developing the sort of heat-shield material for the hypersonic vehicles.