Without doubt, our entry into the world of nano is currently one of the most exciting topics in science and technology. According to most experts, nanotechnology will be the key technology of the 21st century.
Nanomaterials are the harbingers of this development, which could have an effect on our lives as profound as the invention of the computer. At a size of one billionth of a metre, they can have completely different properties to anything previously known from the macroworld and can therefore open up a number of new markets and fields of application.
Scientific and commercial interest in the manufacture of nanostructured materials has greatly increased since the discovery of their importance for numerous applications. With the success of the Aerosil fumed oxide business and the establishment of the Advanced Nanomaterials internal start-up in January 2003, Degussa now occupies a leading technology position in the supply of new nanostructured materials via gas-phase synthesis.
The focal point of development is on multi-functional materials exhibiting combinations of properties, such as transparency and anti-static/conductivity, IR absorption and UV protection, and transparency. The highly flexible, scaleable and versatile gas-phase processes further developed by Degussa play a key role in this development.
Although a number of gas-phase synthesis processes exists, they all have in common the fundamental aspects of particle formation mechanisms that occur once the product species is generated.
The product quality and application characteristics of nanostructured materials depend strongly on the size distribution, morphology and state of aggregation, i.e. the size and number of primary particles defining the degree of aggregation. In gasphase reactors the final product characteristics are determined by fluid mechanics and particle dynamics within a few milliseconds at the early stages of the synthesis process.
Within this short timeframe, three major formation mechanisms dominate particle formation (Figure 1). The chemical reaction of the precursor leads to the formation of product monomers (clusters) by nucleation or direct inception and to the growth of particles via the reaction of precursor molecules on the surface of newly formed particles, which is called surface growth.
Figure 1. Particle formation mechanisms.
Coagulation is an intrinsic mechanism which inevitably occurs at high particle concentrations and therefore in all industrial aerosol processes. Particles dispersed in a fluid move randomly, due to Brownian motion, and, along their trajectories, they collide with each other. Assuming strong adhesive forces, which are characteristic for small particles, these collisions result in coagulation.
Finally, coalescence and fusion are sufficiently fast in the high temperature zones of the reactor to effect a reduction in the level of aggregation or even the formation of spherical particles, due to sintering processes.
Gas-phase reactions are characterised by very high temperatures and extremely short residence times. The manufacturing process and conditions determine the size and morphology of the product and hence their suitability in given fields of application.
Degussa investigated several processes, such as flame, hotwall, plasma and laser evaporation reactors. Each process delivered unique capabilities but the flame process is Degussa’s preferred option because of its scalability, versatility and cost-efficient production.
Flame reactors are one of the most common reactor designs for the production of high-purity nanostructured materials in large quantities, especially for the production of silica, titania and alumina. Powders, liquids and vapours can be used as precursors.
Due to the high energy density in the flame, the precursor concentration can also be quite high, with flame temperatures reaching 1,000-2,400°C.
The residence time in the highest temperature region is very short, usually between 10 and 100ms. This hot zone is crucial for the formation of the primary particles, which in this process range from only a few nm up to 500 nm. Beyond this zone, only the size and the morphology of the aggregates can be influenced.
The specific surface areas of these materials is up to 400m2/g.
The shape of the flame can be influenced by the type of fuel and air inlet used. If the gases are pre-mixed, the flame reaction takes place right at the burner mouth, creating a very short, homogeneous flame. With diffusion flames, the fuel and the air or oxygen are fed separately to the burner mouth. The reactants have to diffuse together before they can combust, which creates a significantly longer flame.
The control of reactor parameters, such as temperature profile, residence time in the hot zone and concentration of the reaction partners, is of great importance in producing tailor-made nanostructured materials. However, linking process parameters measurably to product characteristics requires an excellent understanding of the physico-chemical fundamentals of gas phase synthesis.
Although the basic process was developed several decades ago and extensive empirical know-how has been accumulated, the formation mechanisms are not yet fully understood.
Measurements in gas-phase reactors are quite problematic, as time scales are very small, temperatures are extremely high and the gaseous atmosphere is often aggressive. Therefore, computer simulation can be employed in fundamental investigations and may provide a better roadmap for product and process optimisation.
Processes with particulate matter increasing and decreasing in amount and size over time can be described mathematically by so-called particle population balance models (PPBs).
Numerous models have been developed that describe particle formation and growth in flames: monodisperse, moment, sectional and Monte Carlo-type models.
Gas-phase reactors usually produce non-homogeneous profiles for temperature, velocities, concentrations of species etc. Therefore, when simulating complex reactor geometries, the PPB technique has to be coupled with computational fluid dynamics (CFD). Depending on the information needed, Degussa chooses different algorithms ranging from a basic but fast monodisperse model to enhance the knowledge of the synthesis mechanisms and their effect on the products to a CFD coupled technique to emulate real production reactors.
For example, particle formation in a pre-mixed flame reactor has been modelled by approximating the average time-temperature-history of the particles’ trajectories. The adiabatic flame temperature is assumed to drop exponentially. The precursor mole fraction entering the reactor is set to 0.01 and reacts at an extremely small time-scale (<1 ms).
Therefore, the inception of particles occurs almost instantaneously, driving up the number of particles in the system. Consequently, an intense coagulation process sets in and the aggregate volume grows continuously, forming either larger spherical particles or agglomerates. Coalescence induces the primary particle diameter to grow in regions of high temperatures.
Downstream, or at larger residence times, the temperature drops and causes the coalescence to cease until coagulation dominates the formation process (t>20 ms). As Figure 2 shows, the evolution of the primary particle size levels out at this point. However, the number of primaries within the agglomerates continues to grow, due to coagulation.
Figure 2. Comparison of simulation results & experiment.
The calculated particle size evolution is compared to experimental data obtained by thermophoretic sampling, which makes possible direct measurement in the flame at various distances (residence times) from the burner mouth.
Figure 2 also shows TEM-picture sections of particles collected from the flame after residence times of 20, 50 and 90 milliseconds and the corresponding average aggregates obtained from simulation. With increasing residence time, the advancing degree of aggregation can clearly been seen, but there is hardly any change in primary size. The results from experiment and simulation are in excellent conformity, validating the simulation technique.
Degussa has developed several products based on flame synthesis. These include nanostructured zinc oxide, which can be used as a UV filter in sunscreen lotions or transparent coatings, and extremely fine particles of ceria (cerium dioxide), which are ideally suited to polishing integrated circuit wafers in the chemical mechanical planarisation (CMP) process, due to their high abrasivity.
Indium tin oxide (ITO) can equip polymer surfaces with transparent, antistatic properties, and can also be used for IR absorption.
Finally, the newly developed nanocomposite ‘MagSilica’, with its superparamagnetic behaviour, is an excellent example of the high customising potential of nanomaterials through gas-phase synthesis.
Transparent UV protection is currently dominated by organic UV-absorbing materials. However, mineral nanostructured UV filtering materials such as zinc oxide can overcome some of the disadvantages of their organic counterparts, including long-term and high-temperature stability, as well as having food/pharma compatibility.
By using well-engineered mineral UV-absorbers, the durability of polymeric systems exposed to UV irradiation can be increased dramatically. Figure 3 demonstrates how an optimised particle size distribution maximises UV absorption while maintaining the visible transparency.
Figure 3. UV-vis spectrum of nanostructured zinc oxide
Mineral UV filters are becoming more widely used in cosmetic applications to protect against the irradiation intensity of UVA light, which causes long-lasting damage to deep skin tissues.
Nanostructured zinc oxide can provide the desired protection values without whitening the skin. Further technical applications for nanostructured zinc oxide are connected to its biostatic or odour-absorbing capability.
Additionally, its excellent catalytic activity or absorbing capacity in sulphur-containing gas processes is related to the high chemically available surface of the pyrogenic material. The porosity is very low compared to materials from most liquid phase processes.
Recent developments to improve formulation compatibility further broaden the potential of this material in UV-absorbing applications. Surface treatment and dispersion technology are tools to adapt the mineral surface to the surrounding matrix and make systems easy to use.
In the past few years ceria, which was originally used in glass polishing, has become a very important abrasive in the CMP of interlayer dielectrics (ILD) and especially shallow trench isolation (STI) for semiconductor device manufacturing. Although its mechanical abrasivity is rather low compared to conventional abrasive particles like silica or alumina, ceria is particularly interesting for polishing SiO2 surfaces. Due to its chemical affinity to silica, it combines the mechanical and chemical aspects of CMP. This affinity also makes ceria the favourable abrasive for STI processes where a high SiO2/Si3N4 selectivity is very desirable.
The nature of the ceria - its morphology, crystallographic structure, particle size, purity etc. - is extremely important for superior polishing results. Its particle size, aggregate structure and degree of crystallinity directly influence parameters such as the roughness of the polished surfaces and the removal rate during CMP. In order to get a high degree of planarity, small particles or aggregates with a narrow particle size distribution are desirable.
Figure 4 shows a typical TEM micrograph of the pyrogenic ceria developed by Degussa. The primary particles are slightly aggregated and highly crystalline.
Figure 4. TEM micrographs of pyrogenic ceria.
The average primary particle size is in the range of 8-15 nm. The aggregate size is typically below 200 nm. Due to the small particle sizes, this material is an ideal abrasive for polishing SiO2 wafers, guaranteeing an extremely low degree of roughness while maintaining a high removal rate.
Moreover, due to the high BET surface area of up to 100 m2/g, it can be used in different catalytic applications, such as the catalytic reduction of harmful diesel fuel exhaust emissions.
ITO is widely used in the manufacture of transparent conductive coatings. Whilst ITO is usually deposited using evaporation or sputtering techniques, it is also possible to generate the desired thin film layer by dispersing nanostructured particulate ITO material in organic solvents, water or traditional coating materials, such as UV coatings or PU coating material, and then applying wet chemical coating techniques like spraying, dip coating or flow coating.
These techniques are not only less expensive and easier to handle than sputtering processes but also facilitate additional layer attributes, such as scratch resistance, thereby extending the intrinsic electrical conductive, antistatic and/or IR-absorbing properties of the ITO.
The optical and electronic properties of ITO films are highly dependent on the deposition parameters and the composition of material used. For good conductivity, the layer must contain a high density of charge carriers, i.e. free electron and oxygen gaps. However, high conductivity (or low sheet resistance) must be balanced against high transmission in the visible region. Wet chemical coating techniques permit a sheet resistance below 1,000 Ohm/sq and a visible transmission of >80%.
The nanostructured ITO developed by Degussa shows a specific surface area (BET) of about 45 m2/g with an average primary particle size of 15-20 nm and a mean aggregate size of about 70 nm (D50), e.g. in an ethanolic dispersion.
Due to the small aggregate sizes, it is possible to produce highly transparent coatings (>85%) without haze (<1%). In addition, the coating not only combines a good IR absorption with a high transmittance but also features scratch resistance and anti-static properties (R <107 Ohm/sq).
Typical applications of ITO-coated substrates include a large variety of electro-optical devices such as flat panel displays, solar cells and electrodes for LCDs, as well as defogging aircraft and automobile windows, heat-reflecting or heat-absorbing coatings, gas sensors and anti-static window coatings.
The versatility of the flame process is also behind MagSilica (Fe2O3-SiO2). MagSilica couples large specific surface and high purity with ‘tunable’ magnetic properties. In a flame process, nanostructured, magnetic iron oxide crystals are isolated from each other within a silicon dioxide matrix.
Due to the nanoscale size of the crystal domains, the internal thermal energy is sufficient to prevent a remanent alignment of the magnetic moment in the absence of an external magnetic field. In a field, however, these materials can be effectively magnetised, resulting in a ‘tunable’ magnetism (also known as superparamagnetic behaviour).
The properties, the matrix and the crystallites of MagSilica can be adjusted independently in relation to each other over a wide range - an important criterion for custom-tailoring the material for future applications. The material’s potential is being evaluated and samples are being tested in various applications.
These include additives for adhesives that are cured or deactivated by electromagnetic fields. By using MagSilica, it is possible to heat the adhesive directly and homogeneously, with heating rates of over 15 K/second. Curing time can thus be reduced from hours to minutes and the energy-consuming heating of the surrounding substrate is no longer necessary.
Another potential application is in magneto-rheological fluids for clutches and tunable shock absorbers (Figure 5).
Figure 5. TEM micrograph of MagSilica & magnetorheological fluid attracted by an electro-magnetic field
Gas-phase synthesis is a well-known technique for the production of an extensive variety of nanostructured materials. Current commercial products generated by this method are mainly reinforcing fillers, rheology control additives and pigments.
However, the growing need for multifunctional materials (scratch-resistant and transparent, transparent and conductive etc.) is driving new developments to highly specialised functional materials.
Fundamental research by process simulation can give us a good understanding of the particle formation mechanisms. Combined with detailed analysis of particle size and morphology and their impact on the function at hand, it is possible to customise nanostructured products, providing distinct performance advantages for specific applications.
The development of such technical innovations, however, comprises only the first step of the route to successful commercialisation of new nanoscale products. The real challenge lies in the discovery and development of markets and applications that can benefit from the technology. In practice, this requires a focused and dedicated marketing and applied technology effort, coupled with the willingness and ability to fit the product precisely to the needs of the potential customer.