New Insights into Superconductivity of Niobium Boride

For more than six and a half decades, niobium boride (NbB) has been regarded a typical example of a superconducting material. This presumption has been noted down in manuals related to physics of condensed matter and scientific articles journals, and has at present been challenged in a research carried out by scientists from the University of São Paulo (USP), Brazil, and from San Diego State University, United States.

The team has demonstrated that the superconductivity observed earlier could not be credited to NbB. The superconducting characteristics were intrinsic to filaments of nearly pure niobium that “meandered around” the NbB grains in the samples investigated. The results of the study have been reported in an article published in the Physical Review Materials journal.

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), was the principal investigator of the study. The research was carried out through support from the Center for Research & Development of Functional Materials (CDMF), one of 17 Research, Innovation & Dissemination Centers (RIDCs) funded by the Sao Paulo Research Foundation, or 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, 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.

Professor Renato de Figueiredo Jardim, in the IF-USP and  Director of  EEL-USP

Analysis of the electron microscope images seen in the article shows that the white filaments indicate the minority phase including about 2% boron and 98% niobium. The notation adopted to denote this composition is Nb0.98B0.02. The gray areas indicating the larger volumetric fraction are NbB.

The researchers point out that even when the phenomenon occurs in a tiny volumetric fraction, the minority phase, that is, Nb0.98B0.02, is superconductive and creates a three-dimensional mesh for the electrical current to move between the extremities of the material.

It is highly probable that this characteristic could have misguided the scientists who tested NbB earlier, such that they discovered the material to exhibit superconductivity under the temperatures of about 9 K.

According to Jardim, the recognition of NbB lattice structure through scanning electron microscopy offered a qualitative proof of the characteristic depending upon “visual evidence.” “But this point alone was insufficient to confirm our hypothesis,” noted Jardim. “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

In a macroscopic view, superconductivity is a characteristic of specific materials that conduct electricity upon being cooled below a specific temperature, without any loss in energy, or with zero electrical resistance.

At present, there is reasonable knowledge on the technological uses of superconductivity. It is mainly used in coils made of superconducting wire. Cooling as well as thermal insulation of such a coil results in the indefinite flow of an applied electrical current through it, thereby producing magnetic fields without the loss of energy. Such a device is commonly applied in magnetic resonance imaging, or MRI, equipment.

The technology has advanced a great deal in recent years,” stated Jardim. “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.”

Jardim stated that at present, there are no technological uses 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

According to Jardim, apart from this macroscopic characteristic, there is one more macroscopic characteristic known as “perfect,” through which the interior magnetic field of the superconductor is entirely ruled out upon placing the material in an external magnetic field.

Diamagnetism can be found in all materials. Yet, it is normally so feeble that its presence is concealed by other stronger magnetic properties such as ferromagnetism through which the material is attracted to an external magnetic field, as well as paramagnetism through which the atomic magnetic dipoles of the material get aligned parallel to the external magnetic field.

If the diamagnetic response is adequately robust, similar to the case of a superconductor, the repulsion caused by the magnetic field can enable levitation of the material. This aspect has recently gained much attention.

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,” explained Jardim.

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