What are nanomaterials?
Most particulate materials consist of particles of with variable sizes, but the majority of particles are between a certain minimal and maximal size range. The variation in size is called ‘particle size distribution’. Most particulate materials are thus characterized by their particle size distribution and this also applies to nanomaterials.
In 2011 the European Commission published the “Recommendation on the definition of a nanomaterial”. Herein it is recommended to define a nanomaterial as a material containing particles, of which 50 % or more of the particles are in the size range of 1 – 100 nanometres. Thus, while nanomaterials can also contain larger particles, the definition aims to distinguish nanomaterials from non-nanomaterials.
Size of nanoparticles
Nanoparticles are between 1 and 100 nanometres. To put this into perspective, the diameter of a hair is 80.000 times larger than 1 nanometer. Nanoparticles are nanosized (1 – 100 nanometres) in one or more external dimensions. Thus flakes (for example clay), tubes (like carbon nanotubes) as well as rounded nanoparticles (TiO2) can be nanomaterials.
Particles between 1 and 100 nanometres tend to cluster. This clustering is known as agglomeration or aggregation. Loosely clustered particles are called agglomerates; fused or almost fused particles are called aggregates.
Agglomerates and aggregates are within the definition of nanomaterials, because the starting material is nanoparticles and they are thus not identical to solid particles of similar size. Larger particles may behave differently than nanoparticles; agglomerates and aggregates may behave like larger particles or like nanoparticles, depending on the situation.
Three categories of nanomaterials
Nanomaterials may be classified roughly into three categories:
- Naturally occurring materials like (vulcanic) ash, minerals etc.
- By-products of processes with high temperature like combustion, some industrial processes, welding etc.
- Synthetic nanomaterials.
Synthetic nanomaterials are produced intentionally, because of their special properties.
Widely used nanomaterials
A broad range of nanomaterials is available and their functionalities are diverse. Nanomaterial-containing products may be for example ultra-strong, extremely economical, or dirt-repellent.
Widely-used nanomaterials are metal oxides like TiO2, SiO2, ZnO, CeO2, metals like silver and aluminum, and carbon compounds like carbon nanotubes (CNT). Nanomaterials may be used for their antibacterial properties (silver and TiO2), ‘easy-to-clean’ surfaces (TiO2), UV-protection (TiO2, ZnO, CeO2), increased corrosion resistance (SiO2), flame retardant nature (TiO2, SiO2, clay), improved conduction (CNT), purification of water (aluminum, silver, TiO2) and stronger and lighter materials (CNT). These frequent applications are only few of many.