The allotropes of carbon are at the forefront of advanced materials research, but carbon’s elemental neighbours are now combining to offer exciting new possibilities, as Simon Frost reports.
Combine boron and nitrogen, carbon’s immediate neighbours on the periodic table, and you get a fascinating inorganic compound with numerous similarities to carbon, underpinned by a comparable crystal structure and equal number of outer shell electrons. The Encyclopaedia Britannica describes boron nitride (BN) as ‘an industrial ceramic material of limited but important application’, but those limits are beginning to recede, thanks to recent developments in nanotechnology. Scientists worldwide are now synthesising a range of diverse BN-based materials that can out-do their carbon equivalents by some measures, and counteract them by others.
Aside from being the basis of all life that we know, carbon makes countless materials possible – from long-harnessed commodities such as coal and diamond, to the emerging ‘wonder materials’ of graphene and carbon nanotubes. Boron nitride has several similar allotropes but, although it has been synthesised commercially since the 1940s, its available allotropes were until recently limited to hexagonal (HBN) – an expensive but highly effective lubricant, and cubic (CBN) – a super-hard diamond-like material most commonly used in high-end cutting tools.
Changhong Ke, an associate professor in mechanical engineering at the State University of New York, USA, is part of a team that is among the first to investigate boron nitride nanotubes (BNNTs) produced using the high-temperature pressure (HTP) method developed by NASA and the US Department of Energy Jefferson Lab. In 2014, a patent was granted for the apparatus – an integrated production system for the production of BNNTs via the pressure vapour-condenser (PVC) method, producing highly flexible, high aspect ratio, few-to-single walled BNNTs with high crystallinity.
‘The limited availability of BNNTs has been a big hurdle for the research and applications for quite a while,’ Ke tells Materials World – BNNTs were first theorised in 1994, but it wasn’t until 2009 when the HTC/PVC method was the first to create BNNTs scalable to gramme quantities with few defects in the lab. ‘The so-called HTP/PVC method is capable of growing long, highly crystalline, small diameter BNNTs using a high-power laser in a high temperature and pressure environment. These high-quality BNNTs provide opportunities to investigate their intrinsic structural and material properties and their applications.’
Ke and colleagues’ research, published in Applied Physics in December 2015, is focused on the integration of BNNTs with lightweight polymers. ‘These BBNT-reinforced polymer composites could be used in many industries, such as aerospace, automotive and sports equipment. In particular, aerospace has a high demand for lightweight, high-strength, multi-functional materials,’ he says. ‘A plate made of a BNNT-polymer composite could be deployed as a shell or skull material for high-speed aircraft to reduce structural weight, protect against high temperatures, shield radiation and, due to its piezoelectric nature, it could also offer sensing and morphing functions.’
‘BNNTs have better thermal stability than CNTs and can survive much higher temperatures in air – up to 800°C, twice that of carbon nanotubes,’ he explains. ‘They can also absorb neutron radiation much better than carbon nanotubes, making them good radiation shielding materials.’
A stronger weakest link
Significantly, Ke found that BNNT-reinforced polymers could offer a greater strength-to-weight ratio than their CNT-reinforced equivalents while improving on their key flaw. ‘The weakest link in these nanocomposites is the interface between the polymer and the nanotubes,’ he explains. Testing the bond between BNNTs and polymers, they discovered that the binding strength of BNNTs with poly(methyl methacrylate) was 35% higher than with CNTs, and 20% higher for BNNTs when paired with an epoxy interface.
CNTs still have an unsurprising advantage over the relatively young BNNTs, though – their cost, at US$10–20 per gramme is minimal compared with the prohibitive US$1,000 per gramme estimated for BNNTs. ‘The major reason for the high cost of BNNTs is their low production yield so far,’ Ke explains. ‘They are quite difficult to synthesise because they have a curved cylindrical geometry. In contrast, the production of flat boron nitride sheets is easier and the corresponding cost is much lower. Researchers have been working on efficient production methods, in particular those that can be scaled up for mass production.’
Ke notes, however, that the same was once true for CNTs. ‘If we use the history of CNTs as a reference, their cost in the 1990s and early 2000s was orders of magnitude higher than their cost today. It is optimistically expected that more BNNT materials will be produced and available on the market, which will dramatically drive down the price. The cost of BNNTs will be lower in the future.’
Soaking up spills
Nanosheets, as Ke mentions, are another key area of research for BN materials. Published in Nature Communications in November 2015, a joint study by the Australian Research Council and Drexel University College of Engineering, USA, describes a method for transforming BN nanosheets into sponge-like aerogels that could adsorb up to 33 times their weight in oils and organic solvents. Lead author of Boron Nitride Colloidal Solutions, Ultralight Aerogels and Freestanding Membranes through One-step Exfoliation and Functionalisation, Professor Ying Chen, called it the most exciting advancement in oil spill remediation technology for decades.
‘Boron nitride is lighter and more oxidation resistant compared to any form of carbon,’ co-researcher Professor Vadym Mochalin told Materials World. ‘Using the techniques described in our paper, it can be fabricated into ultra-low density aerogels, featuring fully accessible surfaces for efficient adsorption of viscous and sticky liquids such as oil. In contrast to many carbon adsorbents – which have very small pores and, therefore, suffer from diffusion limitations – the mesoporous structure of BN aerogel is ideal in these situations.’
They processed the hexagonal BN, which is milled with urea, resulting in partial exfoliation and chemical functionalisation of BN nanosheets by amino groups, making the material highly hydrophilic. ‘This is the key to producing high concentrations of colloidal BN nanosheets in water,’ Mochalin explains. ‘The suspension was then gelated and freeze-dried, yielding free-standing solid aerogels.’
Production of BN nanosheets, he says, is very simple and can be done with any standard mill. ‘There are large capacity mills used in industry that can also be employed from mechanochemical exfoliation and modification of BN. The process does not require any toxic or aggressive reagents – just BN and urea.’
Distinct from graphene
It is not only the adsorption of BN nanosheet that interests Mochalin – it offers a host of capabilities that distinguish it from its carbon equivalent, graphene. ‘Offering all advantages of 2D materials, it has some properties different from graphene. For example, in contrast to graphene, BN is a good electrical insulator. Thus, it can be used as composite nanofiller, especially in electronics packaging, reinforcing the polymer without making it conductive, which would be catastrophic for electric circuits.
‘BN is also more resistant to oxidation and can be used in air atmosphere at higher temperatures than graphene. The highly porous structure of BN aerogel renders it a very poor heat conductor, so the material can be used as a thermal insulator at high temperatures, for example, in aerospace applications. Finally, 2D BN is fluorescent, and therefore could have interesting applications in optoelectronic devices, laser emitters, bioimaging, and drug delivery.’
Hexagonal BN could also complement graphene as a substrate in electronic applications, according to researchers at the US Department of Energy’s Oak Ridge National Laboratory (ORNL), Northwestern University and Stony Brook University, USA, who claim that thanks to their unique hexagonal BN synthesis method, graphene set upon a substrate of their single-layer hexagonal BN would offer several thousand times higher electron mobility than other substrates, enabling, for example, much faster data transfer.
Their process is based upon atmospheric pressure chemical vapour deposition but uses, as ORNL researcher Yijing Stehle describes, ‘a more gentle, controllable way to release the reactant into the furnace […] to take advantage of inner furnace conditions.’
They investigated the growth stages of the hexagonal BN single crystals and found that they change their shape from triangular to truncated triangular and further to hexagonal depending on copper substrate distance from the precursor. This variation, they believe, is affected by the ratio of boron to nitrogen active species concentrations on the copper surface inside the reactor. ‘Imagine batteries, capacitors, solar cells, video screens and fuel cells as thin as a piece of paper,’ said Stehle. The graphene and hexagonal BN capacitor and fuel cell prototype they are developing is not only super thin, but transparent.
In December 2015, Science published Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs, marking the first successful synthesis of 2D boron – borophene. While graphite is composed of stacks of 2D sheets from which graphene can be exfoliated, no such analogous process exists for making 2D boron. The authors of the paper, from the US Department of Energy’s Argonne National Laboratory, Stony Brook and Northwestern, have now developed the first working method.
Argonne researcher Nathan Guisinger told Materials World, ‘We had recently used our e-beam evaporators to do the first growth of graphene on a silver rod, evaporating directly from a carbon rod. The key in doing so was controlling the atomic flux to a low enough level to form graphene. From that experience, Andrew Mannix of Northwestern was inspired to try the same evaporation techniques with a rod of elemental boron and was successful. It may seem trivial, but a lot of effort went into optimising the process. A key advantage is that this method is relatively safe and environmentally friendly, whereas many boron-based materials are synthesised with nasty precursors.’
Unlike most 2D materials, which typically appear extremely smooth and in even planes, borophone exhibits ridges, like that of corrugated cardboard, which result in anisotropy – directionally dependent electrical and mechanical properties. Another property is a tensile strength that could be higher than any material known, as Guisinger explains, ‘Our theory collaborators calculated the tensile strength along one axial direction that was comparable, if not stronger than known values for graphene, which has the highest tensile strength measured. We have not yet experimentally confirmed these predictions.’
Guisinger notes that the applications of borophene could be in electrodes of Li-ion batteries, and radiation-hardened electronics for space applications, sensors and cancer therapy, but in the near term, integration with other 2D materials would be most achievable.
No work in vain
There is one overriding question with any new class of material – how will it affect our daily lives? While Professor Mochalin hopes that boron nitride materials will become significant, he offers an optimistic perspective either way. ‘An important thing to keep in mind is, no matter which material eventually finds its way into our daily lives, its invention, development and implementation into applications would hardly be possible without researching hundreds of other materials, among which that single one is identified as the best for the task. No work goes in vain. We study and systematise different materials and it all contributes to building our knowledge and eventually will be useful – maybe in surprising applications.’
For full interviews with the researchers quoted in this article, download the Materials World app.