28 Dec 2015
|A group of researchers in Japan and China identified the requirements for the development of new types of extremely low power consumption electric devices by studying Cr-doped (Sb,Bi)2Te3 thin films. This study has been reported in Nature Communications (“Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3“).|
|At extremely low temperatures, an electric current flows around the edge of the film without energy loss, and under no external magnetic field. This attractive phenomenon is due to the material’s ferromagnetic properties; however, so far, it has been unclear how the material gains this property. For the first time, researchers have revealed the mechanism by which this occurs. “Hopefully, this achievement will lead to the creation of novel materials that operate at room temperature in the future,” said Akio Kimura, a professor at Hiroshima University and a member of the research group.|
|Ferromagnetism mediated by Sb or Te atoms. (Image: Hiroshima University)|
|Their achievement can be traced back to the discovery of the quantum Hall effect in the 1980’s, where an electric current flows along an edge (or interface) without energy loss. However, this requires both a large external magnetic field and an extremely low temperature. This is why practical applications have not been possible. Researchers believed that this problem could be overcome with new materials called topological insulators that have ferromagnetic properties such as those found in Cr-doped (Sb,Bi)2Te3.|
|A topological insulator, predicted in 2005 and first observed in 2007, is neither a metal nor an insulator, and has exotic properties. For example, an electric current is generated only at the surface or the edge of the material, while no electric current is generated inside it. It looks as if only the surface or the edge of the material has metallic properties, while on the inside it is an insulator.|
|At extremely low temperatures, a thin film made of Cr-doped (Sb,Bi)2Te3 shows a peculiar phenomenon. As the film itself is ferromagnetic, an electric current is spontaneously generated without an external magnetic field and electric current flows only around the edge of the film without energy loss. However, it was previously unknown as to why Cr-doped (Sb,Bi)2Te3 had such ferromagnetic properties that allowed it to generate electric current.|
|“That’s why we selected the material as the object of our study,” said Professor Kimura.|
|Because Cr is a magnetic element, a Cr atom is equivalent to an atomic-sized magnet. The N-S orientations of such atomic-sized magnets tend to be aligned in parallel by the interactions between the Cr atoms. When the N-S orientations of Cr atoms in Cr-doped (Sb,Bi)2Te3 are aligned in parallel, the material exhibits ferromagnetism. However, the interatomic distances between the Cr atoms in the material are, in fact, too long to interact sufficiently to make the material ferromagnetic.|
|The group found that the non-magnetic element atoms, such as the Sb and Te atoms, mediate the magnetic interactions between Cr atoms and serve as the glue to fix the N-S orientations of Cr atoms that face one direction. In addition, the group expects that its finding will provide a way to increase the critical temperature for relevant device applications.|
|The experiments for this research were mainly conducted at SPring-8. “We would not have achieved perfect results without the facilities and the staff there. They devoted themselves to detecting the extremely subtle magnetism that the atoms of non-magnetic elements exhibit with extremely high precision. I greatly appreciate their efforts,” Kimura said.|
|Source: Hiroshima University|
29 Sep 2015
Consumers aren’t embracing electric cars and trucks, partly due to the dearth of charging stations required to keep them moving. Even the conservation-minded are hesitant to go electric in some states because, studies show, if fossil fuels generate the electricity, the car is no greener than one powered with an efficient gasoline.
Charging cars by solar cell would appear to be the answer. But most cells fail to meet the power requirements needed to directly charge lithium-ion batteries used in today’s all-electric and plug-in hybrid electric vehicles.
Researchers at Case Western Reserve University, however, have wired four perovskite solar cells in series to enhance the voltage and directly photo-charged lithium batteries with 7.8 percent efficiency–the most efficient reported to date, the researchers believe.
The research, published in the Aug. 27 issue of Nature Communications, holds promise for cleaner transportation, home power sources and more.
“We found the right match between the solar cell and battery,” said Liming Dai, the Kent Hale Smith Professor of macromolecular science and engineering and leader of the research. “Others have used polymer solar cells to charge lithium batteries, but not with this efficiency.”
In fact, the researchers say their overall photoelectric conversion and storage outperformed all other reported couplings of a photo-charging component with lithium-ion batteries, flow batteries or super-capacitors.
Perovskite solar cells have active materials with a crystalline structure identical to the mineral perovskite and are considered a promising new design for capturing solar energy. Compared to silicon-based cells, they convert a broader spectrum of sunlight into electricity.
In short order, they have matched the energy conversion of silicon cells, and researchers around the world are pursuing further advances.
Dai’s lab made multilayer solar cells, which increases their energy density, performance and stability. Testing showed that, as desired, the three layers convert into a single perovskite film.
By wiring four lab-sized cells, about 0.1 centimeter square each, in series, the researchers further increased the open circuit voltage. The solar-to-electric power conversion efficiency was 12.65 percent.
To charge button-sized lithium-ion batteries, they used a lithium-ion-phosphate cathode and a lithium-titanium-oxide anode. The photoelectric conversion and storage efficiency was 7.8 percent. Through 10 photo-charge/galvanostatic (steady current) discharge cycles lasting nearly 18 hours, the technology maintained almost identical discharge/charge curves over all cycles, showing high cycling stability and compatibility of the components.
“We envision, in the not too distant future, this is a system that you could have at home to refuel your car and, eventually, because perovskite solar cells can be made as a flexible film, they would be on the car itself,” said Jiantie Xu, who, with Yonghua Chen, is an equally contributing first author of the study. Both are macromolecular science and engineering research associates in Case School of Engineering.
The researchers are developing small-scale prototypes and working to further improve the perovskite cell’s stability and optimize the system.
- Jiantie Xu, Yonghua Chen, Liming Dai. Efficiently photo-charging lithium-ion battery by perovskite solar cell. Nature Communications, 2015; 6: 8103 DOI: 10.1038/ncomms9103