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ScienceWise - Winter 2013

Increased capacity

Article Illustration
A capacitor can be as simple as this - though a good one will require some far more sophisticated materials science
Article Illustration
Ray Withers and Yun Liu. Other members of the research team include Wanbiao Hu, Mandy Snashall, Lasse Noren, Terry Franckombe, Hua Chen, Frank Brink and Jennifer Wong-Leung

New materials that may revolutionise green energy

One of the problems with all energy generation, be it green or not, is that the demand from households and industry varies enormously throughout the day. At five o’clock in the afternoon when the population start to arrive home and begin cooking dinner there may be a demand for three or even four times as much power as at five o’clock in the morning.

Even worse are unexpected heat-waves or cold-snaps, during which everyone turns on climate control.  Clearly the simple answer is to just store the power as it’s generated then release it again when it’s needed. The trouble is that this simple answer isn’t so simple to engineer. Conventional batteries such as the lead acid accumulator in your car, are simply not economic on the scales that would be required. They also have a limited life span. Imagine changing the entire national power infrastructure as often as you change your car battery! 

A better way to store electricity is with a capacitor – a device that collects electrical charge on two closely separated plates. 

More or less any two conducting plates will do. For example two five cent coins with a piece of paper between will act as a capacitor. And like many other types of capacitor, the two coin device would never need replacing because unlike a battery, it has no wet chemical systems to deteriorate over time. However the reason Australia’s power stations aren’t coupled to a mountain of five cent coins is that such capacitors are only capable of storing a miniscule amount of electricity.

There are two ways to increase the storage capability of a capacitor. One is to make the plates larger and/or have more of them. The other way is to change the material that separates those plates. The effectiveness of that separating material is determined by a property known as the dielectric constant. The bigger the dielectric constant, the more energy that can be stored in a capacitor of a given size.  

Professors Yun Liu and Ray Withers of the ANU Research School of Chemistry and their research groups have recently published a paper in the prestigious journal Nature Materials, in which they describe a totally new class of dielectric materials. 

“Metals have an enormous dielectric constant but they’re useless as separators in capacitors because they would simply conduct the stored charge, losing all the energy in the process,” Professor Liu explains, “Insulators don’t leak the charge away but neither do they tend to have such high dielectric constants. So what we’ve been trying to create is a dielectric that combines the best properties of both.”  

They’ve achieved that using clever adaption of a very common material, the mineral rutile (titanium dioxide). By adding small amounts of niobium and indium to the rutile the scientists were able to create tiny defects in the lattice. Rather like adding a few tennis balls to a hopper full of golf balls, the smaller balls have to sit awkwardly around the larger ones.  

“The additional elements coupled with the defects they induce create what are in effect nanoscale regions that have the properties of a metal. But because these are dispersed in an insulating lattice you don’t get the massive losses that would occur using solid metal” Professor Withers says.  

The result is a super dielectric material with up to a million times the storage capacity of paper as well as low loss. But the material’s excellent properties don’t end there. Because rutile is a stable solid mineral its dielectric properties are very tolerant of high and low temperatures which makes it ideal for harsh industrial environments. It’s also cheap and abundant which means that in principle, such storage devices could be mass produced very economically.  

A huge amount of the cost of electricity is what’s known as “gold plating” the supply. That is having vast spare generating capability on standby to cope with peaks in demand. Perhaps one day capacitive storage may offer a far more economical alternative and dramatically reduce our greenhouse emissions in the process.



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