With a little physics ingenuity, engineers have designed a way to redistribute electricity on a small scale—a negative capacitor—potentially opening new avenues of research into more energy-efficient computing.
In a new study, researchers at Argonne National Laboratory, together with collaborators in France and Russia, have created a permanent, static negative capacitor, a device thought to have been in violation of physical laws until about a decade ago.
While previously proposed designs for negative capacitors worked on a temporary, transient basis, the newly developed one works as a steady-state, reversible device.
The researchers found that by pairing a negative capacitor in series with a positive capacitor, they could locally increase the voltage on the positive capacitor to a point higher than the total system voltage. In this way, they could distribute electricity to regions of a circuit requiring higher voltage while operating the entire circuit at lower voltage.
“The objective is to get electricity where it is needed while using as little as possible in a controlled, static régime,” says Argonne researcher Valerii Vinokur.
In traditional capacitors, the capacitor’s electric voltage is proportional to the stored electrical charge and increasing the stored charge increases the voltage. In negative capacitors, the opposite happens. Increasing the charge decreases the voltage. Because the negative capacitor is a part of a larger circuit, this does not violate conservation of energy.
“One way you can think about it is like having a refrigerator,” says one researcher from University of Picardie (France), Igor Lukyanchuk, “Inside the refrigerator, of course, it is much colder than the outside environment, but that is because we are heating up the rest of the environment by expending energy to cool the refrigerator.”
A prime component of the negative capacitor put forward by Vinokur and his colleagues involves a filling made of a ferroelectric material, which is similar to a magnet except that it has an internal electric polarization rather than a magnetic orientation.
“In a ferroelectric nanoparticle, on one surface you will have a positive charge, and at the other surface you will have negative charges,” Vinokur said. “This creates electric fields that try to depolarize the material.”
By splitting a nanoparticle into two equal ferroelectric domains of opposite polarization, separated by a boundary called a domain wall, Vinokur and his colleagues minimized the effect of the total depolarizing electric field. Then, by adding charge to one of the ferroelectric domains, researchers shifted the position of the domain wall between them.
Because of the cylindrical nature of the nanoparticle, the domain wall began to shrink, causing it to displace beyond the new electric equilibrium point. “Essentially, you can think of the domain wall like a fully extended spring,” says Lukyanchuk. “When the domain wall displaces to one side because of the charge imbalance, the spring relaxes, and the released elastic energy propels it further than expected. This effect creates the static negative capacitance.”
