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Posted: Nov 19, 2013
Crystals for tomorrow's mobile phones
(Nanowerk News) When it comes to wireless communication, the amount of data that needs to be transferred keeps growing ever faster. The basis for this are highly perfected crystals with very specific features. An international team of researchers has now developed a material system that meets the highest requirements, and the results of that research were published in the scientific journal Nature ("Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics"). The rare earth scandate crystals required for this purpose were contributed by the Leibniz Institute for Crystal Growth (IKZ) – because it is the only institute worldwide that is able to produce them.
Ten years ago not every person necessarily owned a mobile phone. The people that were reachable via mobile phone were particularly progressive or business people. Today even school kids have mobile phones, and even the elderly oftentimes are depended on these devices – the trend is definitely going towards second mobile phones. In order to keep up with this enormous extent of mobile communication it is not only necessary that every person has a mobile phone, but the cell networks must be able to keep up with the transmission as well.
The special brick wall structure of the material creates enormously great perfection, as the “mortar” layer” basically “soaks up” any defects. (Image: Ye Zhu/Muller group)
Mobile cell devices, similarly to radio receivers, use various frequencies. These are coordinated by applying a certain voltage to a capacitor. In order to prevent the cell network from overloading, network providers want to improve the control system in such a way that as many signal as possible can be transferred at the same time.
An international team of researchers has now developed the best material in the world for tuneable capacitors. Its core element is a specific dielectric, whose ability to store a charge can be altered by applying a voltage to it. This also decreases the loss of energy, which so far drastically shortens the battery capacity.
The new material can significantly improve the performance of capacitors, which are part of every mobile phone, and thus open up new options for wireless communication at higher frequencies.
The new dielectric is based on fibre-thin strontium titanate layers. They were practically sprayed on by way of a molecular stream epitaxy, one atomic layer after another, by project partner Darrell G Schlom from the Cornell University (USA). This modified epitaxy process caused the strontium titanate to have a special layer structure - one that is unknown in nature. Rare earth scandate crystals grown in the IKZ served as a substrate (crystalline basis with a structure that is as similar as possible) for these layers. As for why the international consortium chose to involve the IKZ of all institutes for growing the crystals? “Because we are the only institute in the world that can grow the rare earth scandate crystals!” emphasised Dr Reinhard Uecker, head of the Oxide Crystal group at the IKZ. In contrast to what their name indicates, rare earth elements are not rare; when they were first discovered they were merely found in rare minerals. At the time, oxides were described as “earths”.
In contrast to the tuneable dielectrics that are commercially available today, the new strontium titanate layers produced here show a much lower density of defects than what has caused energy and performance losses hitherto. The material owes its high perfection to the special structure of layers: The scanning electron microscope shows that the atomic layers are not evenly developed, and that they actually look like a brick wall: bricks with layers of mortar in between. Normally, strontium titanate has many lattice defects. However, when layered like in a brick wall, the thin “mortar layer“ soaks up the defects, and in turn the “bricks” reach a level of perfection that is that much higher. Darrell Schlom explains: “This material allows capacitors to reach at least five times the power compared to the materials that are currently being used.”