The work of polymer chemist Vincent Voet looks set to lead the way to a new type of data storage. A brief summary of his work goes something like this: turn a gas into a polymer, turn the polymer into a block copolymer, turn the block copolymer into self-organized nanodomains, partially remove the domains, which leaves you with a nanofoam, and fill the empty space with nickel. Voet will defend his PhD thesis on the subject on 30 January.
‘Look, here’s the gas cylinder. The gas goes into this high pressure reaction chamber with some fluids and a starter compound, and is turned into a polymer.’ Vincent Voet likes to create things in the lab. ‘I definitely enjoy synthetic chemistry, but checking my samples under a special microscope, to see if the electric properties are right, is also very exciting. The versatility makes this a great project.’
Voet turned vinylidene fluoride gas into the polymer polyvinylidene fluoride (PVDF), which exhibits ferroelectric properties. This behaviour occurs because the electric charge in the polymer molecules is separated slightly: one side is a bit positive and the other is a bit negative. ‘We call this a dipole’, Voet explains.
Next, he prepared what is called a block copolymer. This is a long molecule composed of alternating blocks of two different polymers: one of PVDF and the other polystyrene. The resulting block copolymer has the tendency to stack, leading to neatly aligned PVDF and polystyrene blocks. ‘You therefore get nanoscale domains of both polymers.’
Now, remember PVDF is a ferroelectric compound. With an electric field, it is possible to align all the PVDF dipoles in a domain, which means the entire domain will have a positive and a negative side, comparable to the two poles of a magnet. This charge separation remains when the electric field is removed.
‘This means you have a “bit”, a memory unit, that can be switched to plus or minus (0 or 1) with an electric field’, says Voet. Writing to this type of memory is faster than to a standard magnetic bit on a hard drive, and takes less energy.
However, reading from such a ferroelectric bit isn’t as simple and can destroy the material. Voet consequently tried to combine it with a magnetic bit. ‘We removed the polystyrene domains and filled the resulting space with nickel, which has magnetic properties.’ Once the polystyrene had been removed, an open structure called a nanofoam remained. Filling the holes with nickel wasn’t easy. ‘Basically, the samples became completely coated with nickel, and this caused short circuits.’
To circumvent this problem, a colleague of Voet is now using different magnetic compounds. The ultimate goal is to link a ferroelectric and a magnetic bit. ‘The ferroelectric bit can deform a little when you apply a current, and we can use this to switch the magnetic bit.’ The advantage is that you use an electric field to write to the bit, which is fast, and you use a non-destructive magnetic field to read from it. ‘Then you would have a fast, small and energy-efficient memory system, ideal for mobile applications like a smartphone or camera.’
Voet’s PhD thesis forms the basis of such an application. Some of his colleagues will continue his work, since he will not be working on it in the near future. He will spend the next year working in Lesotho (Africa), where he will be teaching the science curriculum in a training centre for nurses. His wife, who recently qualified as a doctor, will be working in a nearby hospital. ‘Friends asked us to go there, to help in the country’s development. And I really like teaching!’
defended his thesis
on 30 January at 12.45 p.m. in the Martinikerk.
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