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CNT’s are old stuff. Make room for BNNT – it is stronger and can take the heat!

Posted on January 28th, 2016 by in New Materials & Applications

BNNT

BNNT (Source: BNNT, LLC, www.bnnt.com)

Before boron nitride nanotubes (BN2T) there was carbon nanotubes (CNT’s). Carbon nanotubes are said to have supplanted fullerenes (Bucky Balls) as the hottest research topic of the 20th century in 1991. That year, an article in the journal Nature placed the name of S. Iijima in recent history as the discoverer of CNT’s.  Little is remembered of Radushkevich and Lukyanovich who first published that carbon filaments could be hollow and have a nanometer-size diameter in 1952 [Source: J. Phys. Chem. Russia, vol 1, page 88, 1952].  Who said life was fair!

CNT’s are thirty times stronger than Kevlar® polyaramide and impart strength to nanocomposites made with lightweight polymers such as epoxy.  The tiny tubes reinforce the polymer like the steel rods in concrete, promising lightweight and strong materials for transportation and sports equipment.  However, there is a catch in the interface of CNT’s with polymers.  According to the American Institute of Physics, Professor Changhong Ke from the Mechanical Engineering Department of State University of NY at Binghamton discovered that the weakest link in these nanocomposites is the interface between the polymer and the nanotubes. When the composite is broken, the CNT’s sticking out have clean surfaces, in contrast to chunks of polymer being still stuck to them. The clean break indicates that the interface of CNT’s and the polymer failed. This is, of course, a big problem for polymer nanocomposites.

CNT’s are yesterday’s story. Enter, boron nitride nanotubes (BN2T)

Boron nitride (BN) is a versatile inorganic compound with a wide range of industrial applications in coatings, ceramic composites, lubricants, insulators, etc. because of a combination of unique properties including high oxidation resistance, large thermal conductivity, good electrical insulation, chemical inertness, excellent lubricity, non-toxicity and environmental friendliness.

BN appears in a variety of crystalline structures of which the hexagonal (h) polymorph is the most stable.  It exhibits a structure similar to graphite.  Boron and nitrogen atoms form strong covalent bonds, forming hexagonal B3N3 cells.  In contrast, the multiple two-dimensional BN layers are stacked together by weak van der Waals forces.  The color of h BN is white, an advantage for some uses, thus its nickname “white Graphene”.  It has unique properties. For example, when a few hundred parts per million is added to a polymer it enhances productivity of processes such as extrusion significantly.

But enough of carbon and onto BN nanotubes…

In 1994, Dr. Alex Zettl of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) synthesized boron nitride nanotubes (BN2T) in his lab.  It has all the properties of BN plus it is the strongest, lightest, most thermally conducting, most chemically resistant fiber known to exist and it can stand up to 800ºC (vs <400ºC for CNT’s).

In December 2015, the American Institute of Physics reported on the polymer nanocomposite studies of Professor Ke and his colleagues with BN2T. They measured the force required to pluck a BN nanotube out of its nanocomposite with polymers and found that polymer-BN nanotube bond strengths were higher than those reported for CNT – 35% higher for a poly methyl methacrylate interface and approximately 20% higher for the epoxy interface.

Boron nitride nanotubes adhere more strongly to polymers because of the electronic arrangements in the molecules. In CNT’s all carbon atoms have equal charges in their nucleus thus there are no dipoles. In boron nitride, the B-N bond is polarized resulting in a permanent dipole that makes BN2T a good piezoelectric material. The unequal charge distribution (polarity of B-N bond) leads to a stronger attraction between the boron nitride and the polymer molecules, as verified by molecular dynamics simulations performed by Professor Ke’s colleagues in Dr. Xianqiao Wang’s group at the University of Georgia.

Boron nitride nanotubes have other advantages over carbon nanotubes.  They are more stable at high temperatures and can absorb neutron radiation more efficiently — both useful properties in extreme environments like in outer space. In addition, boron nitride nanotubes are piezoelectric and can thus generate an electric charge when stretched. This property implies that the material offers energy harvesting as well as sensing and actuation capabilities.  As shown in Figure 1, we can also add self-healing to the advantages of BN nanotubes:

Figure 1 (a) BN nanotube bent under pressure inside a transmission electron microscope, (b) its walls self-healed when pressure was removed [Source: Golberg, D., Costa, P. M. F. J., Mitome, M., Bando, Y., "Properties and engineering of individual inorganic nanotubes in a transmission electron microscope". J of Materials Chem 19 (7): 909, 2009]

Figure 1 (a) BN nanotube bent under pressure inside a transmission electron microscope, (b) its walls self-healed when pressure was removed
[Source: Golberg, D., Costa, P. M. F. J., Mitome, M., Bando, Y., “Properties and engineering of individual inorganic nanotubes in a transmission electron microscope”. J of Materials Chem 19 (7): 909, 2009]

The main drawback to boron nitride nanotubes is the cost. In 2014-2015 the cost approaches about one thousand dollars per gram, nearly fifty times more than the price of CNT’s.  Increase in consumption of BN2T is bound to lower its price, as has been the case with CNT’s.  At current prices, only aerospace, military and space are viable market segments for BN2T polymer nanocomposites.

Is BN2T the Yin to the Yang of CNT’s? YinYang

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