Small world, big business – manufacturing nanoparticles

Materials World magazine
,
1 Jun 2010
Schematic of the flow reactor with the nozzle geometry simulation

Ed Lester, Technical Director of Promethean Particles, a spin-out company from the University of Nottingham, UK, describes the process of continuous hydrothermal synthesis for manufacturing inorganic nanoparticles.

Inorganic nanoparticles are increasingly finding uses in applications such as sunscreens, displays, MRI contrast agents and enhanced body armour. They enter early in the value chain and often are not visible in the final product. For example, they could be part of the dye added to a composite for use in a car.

The market is dominated by business-to-business transactions, particularly in the chemical industry, which can lead to the perception of relatively few ‘nano-enabled’ products. Consequently Lux Research, independent research and advisory body for emerging technologies, forecasts that nanoparticles will touch US$3.1trillion worth of products across the value chain by 2015, with the intermediates market worth US$432bln.

There are three things commercial end-users of nanoparticles want, enhanced product functionality, dispersed nanoparticles and an assured supply of large quantities of material.

Routes to manufacture

There are two fundamental approaches to producing inorganic nanoparticles, wet and dry methods. Dry processes are generally based on transporting of ultrafine droplets that carry metal precursors through a high-energy zone where solid nanoparticles are created. This zone could be a plasma, flame or laser beam, but the final product is a dry powder.

Dry methods have been the most successful in penetrating the marketplace despite the need for additional downstream steps of redispersion and formulation.

Wet processes, including sol-gel, hydro-or solvo-thermal synthesis (pressurised hot water or solvent), by contrast produce nanoparticles in dispersion. While the products tend to be highly controlled, in terms of specification, these processes are not as easy to scale because they are normally batch operations.

Engineering issues

Batch hydrothermal processing is straightforward in engineering terms – chemicals are sealed in a vessel, heated and then cooled after an appropriate residence time (hours to days), then the products are retrieved.

In general, process engineers favour continuous processing as the best route to large scale manufacturing so the need to develop a continuous hydrothermal synthesis was apparent. Professor Adschiri from Tohoku University, Japan, first described this process. He explained the manufacture of cerium oxide powder and other materials using a T-piecereactor where supercritical water, above 374oC and 218 atmospheres, was mixed continuously with an aqueous metal salt stream. The two fluids are combined rapidly resulting in metal salt (MLx) hydrolysis immediately followed by a dehydration step.

Hydrolysis MLx + xOH- M(OH)x + xL- Dehydration M(OH)x MOx/2 + x/2H2O.

The way the process allows the superheated water to mix with the metal-salt solution is the most important feature of the process. Poor mixing can lead to particle build up and agglomeration, which will quickly lead to blockages. Mixing sub-, near- or supercritical water with an aqueous metal salt is a significant engineering challenge because superheated water can have a low density of lessthan 0.3g/cc, and a high degree of buoyancy.

Our approach to inhibit blocking was to compare the T-reactor to other alternatives, using enhanced image analysis modelling. The model was based on pseudo-reactors that recreated the mixing regimes of the high pressure reactor, but in a transparent Perspex scale model with methanol simulating the heated flow and sugar water simulating the metal salt flow. By adding a dye to one of the fluids it is possible to see if they are mixing, and where. If the mixing occurs in the initial inlet pipes it creates stagnant zones with little net flow/mass transfer, resulting in dead spots where particle accumulation can occur. Other issues occur with fluid partitioning or back mixing inside the inlet pipes.

A new nozzle-based design has been developed at Nottingham to take advantage of the fundamental properties of the fluid flows. This counter-current reactor feeds the superheated water stream downwards into the up-flow of cold aqueous metal salt. This design allows the metal salt feed to remain cold until mixing and the downstream flow to be heated externally through the outer pipe.

The prototype was modelled and then built at bench scale (g/hr basis). The bench-scale rig has been found to produce a higher quality product than with the T-piece and could be run for extended periods without blockages or pressure surges. A patent was filed by The University of Nottingham in 2005 to cover this design which represents a step change in hydrothermal technology.

One of the most significant benefits of this continuous system is the production of a number of different samples in a single run (potentially 50-100 per day). This is achievable by altering parameters such as flow rate, precursor concentration, temperature and flow ratio. These small samples could be enough for testing with techniques such as Transmission Electron Microscopy, X-ray diffraction, Ultraviolet-visible spectroscopy and Raman in order to determine characteristic properties.

Scaling up

When the technology was pitched to potential end users, nanomanufacturing technologies were not perceived to be significant if they were not inherently scalable. In 2007, the bench-scale reactor was scaled up 30 times and a pilot plant constructed, this is capable of producing 150g/hr of nanomaterials on a dry weight basis. The kinetics of formation are consistent with the bench-scale system, when operating at the same conditions. This is primarily due to the rapid nucleation rates.

Promethean Particles Ltd uses a bench-scale system for rapid prototyping of materials for clients, and a pilot-scale system for mass production, once the target material is developed. The pilot-scale system has been scaled up a further 12 times to achieve 5-10 tonnes per annum production rates, which is commercial scale for many end users.

The reactor is now used to produce high quality materials in dispersion and is finding uses in the production of new materials and novel morphologies. For example, hydroxyapatite can now be produced as sheets or mesoporous tubes with other nanomaterials inside such as fluorescent, magnetic, or antimicrobial materials. Work is also underway to configure this material as slow release drug carriers. These could be considered as second generation nanomaterials where complex composite structures are formed with novel arrangements of other nanostructures that have additional or complimentary functionality. New materials include complex oxides where the composition is designed to add functionality for end use. These materials, in interesting morphologies, are useful in a range of industrial applications, with particular interest coming from the green energy sector, polymer and healthcare companies. The energy sector, for example, needs applications in fuel cells, hydrogen storage,batteries, super capacitors and photovoltaics and the technology can supply solutions to these sectors.

Further information: Promethean Particles