Nav: Home

Research dispels misconception of superconductivity in niobium compound

December 07, 2017

For over 65 years, niobium boride (NbB) has been considered a classic example of a superconducting material. This assumption, recorded in manuals on the physics of condensed matter and articles in scientific journals, has now been contested in a study conducted by researchers at the University of São Paulo (USP) in Brazil and at San Diego State University in the United States.

In an article published in Physical Review Materials, the researchers show that the superconductivity detected hitherto was not due to NbB. The superconducting properties were associated with filaments of almost pure niobium that "meandered around" the grains of NbB in the samples studied.

The principal investigator for the study was Renato de Figueiredo Jardim, Full Professor in the University of São Paulo's Physics Institute (IF-USP) and Director of its Lorena School of Engineering (EEL-USP). The study was conducted under the aegis of the Center for Research & Development of Functional Materials (CDMF), one of 17 Research, Innovation & Dissemination Centers (RIDCs) funded by the Sao Paulo Research Foundation - FAPESP.

"We know the element niobium (Nb) on its own is superconductive when chilled to very low temperatures in the range of 9.2 kelvins," Jardim said. "Now, we've discovered that this is not the case for NbB. Samples of NbB contain a large volumetric fraction of NbB but also a small amount of almost pure Nb. Two distinct crystalline phases coexist in the materials studied. This minority phase, comprising approximately 98% niobium and 2% boron, is what behaves as a superconductor."

In the electron microscope images reproduced in the article, the white filaments correspond to the minority phase consisting of approximately 98% niobium and 2% boron. The notation used to characterize this composition is Nb0.98B0.02. The gray areas, corresponding to the larger volumetric fraction, are NbB.

The authors note that even if it occurs in a small volumetric fraction, the minority phase (Nb0.98B0.02) is superconductive and forms a three-dimensional mesh through which the electrical current can transit from one extremity of the material to the other.

This feature is highly likely to have misled the researchers who previously investigated NbB, so that they found the material to be superconductive at temperatures below approximately 9 kelvins.

As Jardim explains, the identification of NbB lattice structure by scanning electron microscopy provided a qualitative proof of the property based on "visual evidence". "But this point alone was insufficient to confirm our hypothesis", he noted. "We had to go further in search of quantitative proof. We did so by applying a thermodynamic model to the data taken from the materials studied, and in this way, we obtained the proof we sought."

Contributions for new technological applications

From the macroscopic standpoint, superconductivity is a property of certain materials that, when cooled below a given temperature, conduct electricity without any energy loss - i.e., with zero electrical resistance.

The technological applications of superconductivity are fairly well known today. The main application is in coils made with superconducting wire. When such a coil is cooled and thermally insulated, an applied electrical current flows through it indefinitely, generating magnetic fields without energy dissipation. This kind of device is used in magnetic resonance imaging (MRI) equipment, which has become commonplace.

"The technology has advanced a great deal in recent years," Jardim said. "A special type of vacuum flask called a dewar is used for cryogenic storage with an inner temperature at the level of liquid helium, which is 4.2 kelvins (approximately minus 270 °C). These dewars are commercially available and can be used to refrigerate superconducting coils."

According to Jardim, no technological applications are currently foreseen for NbB, "but a 'cousin' of NbB, magnesium diboride (MgB2), has aroused strong interest since the start of the last decade. Our research may contribute to its technological application."

Superconductors and diamagnetism

Alongside this macroscopic property, Jardim says, there's another property, which is also macroscopic, called "perfect", by which the superconductor's interior magnetic field is completely excluded when the material is placed in an external magnetic field.

Diamagnetism is present in all materials. However, it is often so weak that its manifestation is masked by other, more robust magnetic responses, such as ferromagnetism, in which the material is attracted by an external magnetic field, and paramagnetism, in which the material's atomic magnetic dipoles align parallel to the external magnetic field.

When the diamagnetic response is sufficiently strong, as in a superconductor, the repulsion due to the magnetic field can cause the material to levitate. This phenomenon has recently become famous.

"Diamagnetism can be viewed as the generation of a current on the surface of the material that results in a magnetic field of the same magnitude as the external magnetic field that's being applied but acting in the opposite direction. It's as if the material expels from its interior the magnetic field in which it is immersed," Jardim explained.
About São Paulo Research Foundation (FAPESP)

The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships, and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. For more information:

Fundação de Amparo à Pesquisa do Estado de São Paulo

Related Magnetic Field Articles:

Earth's last magnetic field reversal took far longer than once thought
Every several hundred thousand years or so, Earth's magnetic field dramatically shifts and reverses its polarity.
A new rare metals alloy can change shape in the magnetic field
Scientists developed multifunctional metal alloys that emit and absorb heat at the same time and change their size and volume under the influence of a magnetic field.
Physicists studied the influence of magnetic field on thin film structures
A team of scientists from Immanuel Kant Baltic Federal University together with their colleagues from Russia, Japan, and Australia studied the influence of inhomogeneity of magnetic field applied during the fabrication process of thin-film structures made from nickel-iron and iridium-manganese alloys, on their properties.
'Magnetic topological insulator' makes its own magnetic field
A team of U.S. and Korean physicists has found the first evidence of a two-dimensional material that can become a magnetic topological insulator even when it is not placed in a magnetic field.
Scientists develop a new way to remotely measure Earth's magnetic field
By zapping a layer of meteor residue in the atmosphere with ground-based lasers, scientists in the US, Canada and Europe get a new view of Earth's magnetic field.
More Magnetic Field News and Magnetic Field Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...