Why Calcium Improves a High-Temperature Superconductor

Jun 08, 2004

UPTON, NY - Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have found evidence to prove why adding a small amount of calcium to a common high-temperature superconductor significantly increases the amount of electric current the material can carry. This research may be a first step toward developing commercial applications for high-temperature superconducting materials. The results appear in the May 15, 2004 issue of Physical Review Letters.

“Many materials classified as high-temperature superconductors exhibit good properties only in single-crystal form and are actually unsuitable for practical applications, such as high-efficiency electrical wire, because their bulk composition – individual crystalline grains – disrupts the flow of electrons,” said Yimei Zhu, a Brookhaven physicist who led the research.

“But for practical applications in which large electric currents need to be transported, such as power cables, the polycrystalline forms must be used. These polycrystalline materials carry a very low current compared to their single-crystal counterparts,” he said.

This is due to the problem of grain boundaries – the interfaces created between adjacent grains. At grain boundaries, incoming electrons slow down or change direction, thus losing momentum and releasing the lost energy as heat. This results in low electron flow across the boundaries – exactly the opposite of “good” superconductor behavior.

Researchers theorized that electric voltage barriers at the grain boundaries are the cause of this problem. Now, the Brookhaven scientists have found evidence to support this theory.

“We discovered why grain boundaries are the predominant factor that limits the current flow in these materials,” said Brookhaven physicist Marvin Schofield, the paper’s principle author.

“By understanding grain boundary behavior, we can engineer grain boundaries with improved properties. This is a major challenge in superconductor research, which may lead to the commercialization of high-temperature superconducting materials that could revolutionize our daily lives in the near future.”

Scientists worldwide have studied YBCO, a high-temperature superconductor named for the elements it contains – yttrium, barium, copper, and oxygen. They know that it conducts significantly better when it is “doped” with calcium, but have not known, until now, why this is true. The Brookhaven scientists determined this by comparing calcium-doped YBCO to undoped YBCO.

The evidence lies in the areas within grain boundaries in which adjacent grains are most mismatched. To visualize this, picture a centimeter-based ruler next to an inch-based one, where the tick marks on each ruler represent the positions of atoms in the crystal structure of two adjacent, slightly different grains. The marks will match in some cases, nearly match in others, and misalign completely in the rest.

In undoped YBCO, the scientists found, the electrons encounter the most electrical resistance at the most misaligned regions, where the voltage barrier is wide and high. Doping YBCO with calcium causes these regions to shrink, both in width and height. As a result, Schofield and his colleagues determined that calcium doping increases the current across the grain boundary by 35 percent.

To perform the research, the Brookhaven scientists used a YBCO “bicrystal,” a type of crystal grown to contain just one grain boundary, much like two very large grains merged together. The electromagnetic properties of bicrystals are well characterized, allowing the researchers to pinpoint what happens to the electrons at the boundary upon calcium doping, using the results as a model for the overall material. Bicrystals eliminate the impossible task of isolating one boundary out of thousands in the material sample.

To closely examine the bicrystal grain boundary, the scientists used a transmission electron microscope (TEM), a device that uses electrons as tiny probes to “see” inside materials. A sample is placed inside the TEM and bombarded with electrons. As the electrons pass through the sample, they are scattered away from the charged regions of the material. When they emerge, they carry information about the electric and magnetic fields within the sample. This information is then retrieved by a method known as electron holography.

“With electron holography,” Schofield explained, “we can see exactly what the electrons in the material see at the grain boundary. Thus, this method takes us a tremendous step closer to understanding the role grain boundaries play in the properties of real materials.”

Additional collaborators instrumental in this research were Marco Beleggia, of Brookhaven Lab, and Karsten Guth and Christian Jooss, both of the University of Gottingen in Germany. The work was funded by the Office of Basic Energy Sciences within the U.S. Department of Energy’s Office of Science and the German Research Foundation.

The original news release can be found here.

Explore further: Synchrotron upgrade to make X-rays even brighter

add to favorites email to friend print save as pdf

Related Stories

2-D materials' crystalline defects key to new properties

Sep 24, 2014

Understanding how atoms "glide" and "climb" on the surface of 2D crystals like tungsten disulphide may pave the way for researchers to develop materials with unusual or unique characteristics, according to an international ...

Conjecture on the lateral growth of Type I collagen fibrils

Sep 12, 2014

Whatever the origin and condition of extraction of type I collagen fibrils, in vitro as well as in vivo, the radii of their circular circular cross sections stay distributed in a range going from 50 to 100 nm for the most ...

Mars Curiosity Rover Arrives at Martian Mountain

Sep 11, 2014

(Phys.org) —NASA's Mars Curiosity rover has reached the Red Planet's Mount Sharp, a Mount-Rainier-size mountain at the center of the vast Gale Crater and the rover mission's long-term prime destination.

Phosphorus a promising semiconductor

Sep 08, 2014

(Phys.org) —Defects damage the ideal properties of many two-dimensional materials, like carbon-based graphene. Phosphorus just shrugs.

Recommended for you

Synchrotron upgrade to make X-rays even brighter

9 minutes ago

(Phys.org) —The X-rays produced by the Cornell High Energy Synchrotron Source (CHESS) are bright, but they will soon be even brighter, thanks to a major upgrade that will make the quality of CHESS' X-rays ...

Cold Atom Laboratory creates atomic dance

15 hours ago

Like dancers in a chorus line, atoms' movements become synchronized when lowered to extremely cold temperatures. To study this bizarre phenomenon, called a Bose-Einstein condensate, researchers need to cool ...

Wild molecular interactions in a new hydrogen mixture

21 hours ago

Hydrogen—the most abundant element in the cosmos—responds to extremes of pressure and temperature differently. Under ambient conditions hydrogen is a gaseous two-atom molecule. As confinement pressure ...

Scientists create possible precursor to life

22 hours ago

How did life originate? And can scientists create life? These questions not only occupy the minds of scientists interested in the origin of life, but also researchers working with technology of the future. ...

User comments : 0