A "buckypaper" is a randomly oriented self-supporting film of carbon nanotubes (CNTs), resembling flexible black paper. It is an assembly that represents the nanoscale material properties of the CNTs at a macroscopic scale, whilst avoiding all the inherent handling and processing issues of a nanomaterial.
Buckypapers (BPs) are exceptionally lightweight (10-50 gsm), flexible free-standing films with nanoscale porous structures, that typically range in thickness from 20-100 microns. The development of these sheets leads to the creation of strong, lightweight, foldable and highly conductive materials.
When was it first made, and why the name "buckypaper"?
In 1985, Nobel Laureates Curl, Kroto and Smalley identified a round molecule consisting of 60 carbon atoms. Its shape reminded the investigators of the geodesic domes promoted by architect Richard Buckminster “Bucky” Fuller and so the molecule was named “buckminsterfullerene”, or “buckyball” for short, becoming the first of the “fullerene” family of carbon allotropes. In 1991, Sumio Iijima (NEC) presented a significant work on the discovery of hollow carbon fullerenes, known as carbon nanotubes (CNTs), which were given the nickname “buckytubes”.
It was later discovered that there had been several preceding discoveries of nanotubes, notably in 1952 by Russian scientists L.V. Radushkevich and V.M. Lukyanovich, in the late 1950s, by Roger Bacon of Union Carbide and by Morinobu Endo in the 1970s. However, it was the work by Iijima that kick started the interest in these tube-like materials.
Later in the 1990s, following his discovery of the C60 fullerene, Richard Smalley filtered suspensions of carbon nanotubes into flat sheets to create samples for testing. These flat sheets of “buckytubes” became known as “buckypapers”.
What are its main properties and benefits?
The CNT buckypaper material provides a wide range of benefits including large specific surface areas, high electrical conductivity, flexibility, biocompatibility, nanometric scale porosity, scalable production and the ability for efficient electron transfer with enzymes. Buckypapers are ideal self-supporting frameworks for both enzymes and guest molecules such as metals, polymers and redox molecules, permitting the development of a wide range of catalytic bioelectrode interfaces.
Buckypapers are also essential for the introduction of high localised loadings of CNTs into composite structures. Whilst individual carbon nanotubes exhibit electrical conductivity values similar to those of metals (such as copper) and thermal conductivity values greater than diamond, the main issue has always been how to transfer these properties into composites.
Early research involved direct mixing of the nanotubes with polymer matrices to create the nanocomposites. However, high van der Waals forces led to agglomeration of the nanotubes, leading to poor CNT dispersion which resulted in a huge increase in the viscosity of the resin system. A lack of nanotube alignment in these nanocomposites was another major drawback, specifically because nanotubes are highly anisotropic.
The most important properties are along the axial direction which meant that the random alignment of the CNTs did not benefit the composite structures. However, it is possible to achieve good dispersion and alignment of the nanotubes using buckypapers, thereby avoiding the previous resin viscosity issues. The BPs enable high CNT loadings, as well as providing the potential for industrially suitable nanocomposite part and device production.
How are buckypapers manufactured?
Four main production methods have been identified and investigated to manufacture these thin film sheets and can be grouped as follows.
- Direct BP manufacture during CNT growth
- CNT powder compaction / frit-compression
- Casting methods
- Vacuum filtration
In the first method, the buckypaper is directly produced during CNT growth by chemical vapor deposition - also known as CVD method or post-CNT synthesis. This process uses dry techniques of shear pressing, alcohol drenching and pressing, domino pushing and CNT drawing. The disadvantages of this process are that it is not roll-to-roll, as well as that the resulting BP is typically not very thick. These techniques are better suited to thin film applications and smaller scale processes.
In the second method, the buckypaper is produced by powder compaction or frit-compression, which both require some form of mold, and ultimately result in BPs that are less flexible or easy to fold. Samples with thicknesses greater than 500 μm are typically referred to as "buckydiscs". The process is rapid but requires high cost membranes which can hinder scalability.
The third method involves techniques such as tape-casting, drop casting, rod coating, and air spraying. It is often difficult to detach the CNT layer or film from the supporting substrate, due to the low surface energy of the CNTs. Specially surface modified substrates are needed to assist in the separation of the films, such as the Surface-Engineered Tape Casting (SETC) technique, which uses modified surface structure morphology to assist the BP film separation.
The fourth method is pressure membrane filtration. This is typically easy to perform in the laboratory and involves the ultrasonication of CNTs with ionic/non-ionic surfactants in an aqueous solution, which improves the dispersibility of the carbon nanotubes and prevents their re-agglomeration. These dispersions are then filtered through a porous membrane under positive or negative pressure to create uniform films. Despite the wide use of this method, significant challenges remain including the high cost of the filter membranes, the need for high vacuum or pressure, relatively long filtration times, and (at lab-scale) relatively small BP diameters.
Nevertheless, this was the method selected by TECNALIA for the development of a new open pilot line for the industrial production of carbon nanotube-based nano-enabled products (CNT-based NEPs) within the EU funded PLATFORM project (H2020, GA 646307). Further development and access to the pilot line and its materials is continuing through the EU funded OASIS project (H2020, GA 814581).
The OASIS project “Open Access Single entry point for scale-up of Innovative Smart lightweight composite materials and components” offers access to pilot lines for the industrial production of nanoscale structures in unprocessed form, intermediate products with nanoscale features and nano-enabled products, as well as other complementary technical and non-technical services. These modular services will be provided to companies, particularly to SMEs, to gain access to unique facilities and knowledge without high capital investment.
As part of this European nano-based ecosystem, the Tecnalia pilot plant offers the production of continuous filtered films at scales suitable for industrial uptake and enables the manufacturing of buckypapers with widths up to 300 mm and lengths up to 100m.
The process enables the control of the BP thickness in the range of 10-200 microns and areal weights in the range of 30-200 g/m2. The material has a typical porosity of 45 - 60 %, and exhibits characteristic pore sizes of 2-6nm at the surface and 60nm in the sheet core. Through careful control of the thickness, porosity, pore size and areal weight it is possible to manufacture material with a range of thermal and electrical conductivities that can be tailored to a wide range of applications.
What are the industrial applications for "buckypapers"?
Recent activities at TECNALIA have focused on the creation of nano-enabled products (NEPs) based on carbon nanotubes (CNTs) for applications aimed primarily at composite structures for the transport sector. There is now an increasing demand for these products in other sectors such as energy, construction, water treatment and health.
As a member of OASIS, TECNALIA is actively involved in the development of a wide range of industrial solutions based on the CNT buckypaper material for applications such as:
- Enhanced electrical properties for composite aircraft (e.g. lightning strike protection)
- Free standing electrode for flexible supercapacitor cells
- Integrated lightweight composite heater element for public transport vehicles
- Flexible heater blanket for the repair and curing of aeronautic composite structures
- Integrated SHM sensor for heavy vehicle composite leaf springs
- Flexible SHM sensor for rubber bearing pads for bridges
- Anti-Icing/De-Icing layer for large composite structures (aircraft, wind turbines etc)
- Buckypaper interlayer with 3D rGO cathode for more efficient Li-S batteries
- Nanofiltration layer for water purification
TECNALIA will continue to generate ideas, identify new opportunities and work closely with companies to ensure that the advantages of these continuous CNT "buckypaper" sheets become known as the solution to problems across a wide range of industries.