Gas-Phase Production of Nanoparticles †

Gas-phase synthesis is a well-known chemical manufacturing technique for an extensive variety of nano-sized particles. Since the potential of ultra-fine and especially nano-sized particles in high-per-formance applications has been identified, the scientific and commercial interest has increased immensely, disclosing this field as a most important technology of the future. The paper will present the basics of the gas-phase synthesis and particle formation process includ-ing the relation between the principal process conditions and the product characteristics. Moreover, several reactor technologies such as flame, hot-wall, plasma and laser reactors will be introduced


Introduction
Nanomaterials and ultra-thin functional coatings of nanoparticles will determine the utility of many products in the future: superhard materials and superfast computers, dirt-repellent surfaces and new cancer treatments, scratch-proof coatings and environmentally friendly fuel cells with highly effective catalysts. The market for products manufactured by nanotechnology is already registering double-digit growth rates and will amount to hundreds of billions US$ by 2002.
Nanoparticles are at the core of this technology. These are particles ranging in size from 1 millionth to 100 millionths of a millimetre Ҁ more than 1,000 times smaller than the diameter of a hair. In this order of magnitude, it is not only the chemical composition but also the size and the shape of the particles that determine their properties. Optical, electric and magnetic properties, but also hardness, toughness or the melting point of nanomaterials differ substantially from the properties of the macroscopic solids. Table 1 gives an overview of effects associated with decreasing particle size.
Degussa possesses many years of experience in the manufacture and marketing of extremely fine powders. Examples are the pyrogenic silica (AEROSIL ® ), titania, alumina and industrial carbon black. Fumed • lower the electrical conductivity of metals (Cu, Ni, Fe, Co, Cu alloys) • initially increasing and later decreasing magnetic coercivity, finally superparamagnetic behaviour (Fe 2 O 3 ) • higher the hardness and strength of metals and alloys • higher the ductility, hardness and formability of ceramics; the lower the sintering and superplastic forming temperature of ceramics (TiO 2 ) • higher the blue-shift of optical spectra of quantum dots (quantum confinement of Si) • higher the luminescence of semiconductors (Si, GaAs, ZnS:Mn 2ѿ ) silica, for which Degussa is the world market leader, is used in particular as a reinforcing filler in silicone rubber and to control the rheology of coatings and colorants. Industrial carbon black, in which Degussa ranks second in the world, is used particularly in the tyre industry and as a pigment in printing inks, coatings and plastics. In industrial products, primary nanoscale particles are normally not isolated but build up aggregates and agglomerates. In aggregates, the primary particles contact each other at surfaces or edges and Ҁ as a rule Ҁ they cannot be broken down further by shear forces applied in the application. Agglomerates are formed when aggregates and/or primary particles contact each other at points. If nanoparticles are dispersed in a liquid, the agglomerates are destroyed and the surface chemical groups of the aggregates interact with each other (see Fig. 1).  In the case of fumed silica, this affinity is attributed to the hydrogen bridge linkages and results in a temporary, three-dimensional lattice structure becoming macroscopically "visible" in the form of thickening and thixotropy. To control the application behaviour often means to generate tailor-made aggregates with a tailor-made surface chemistry. In Table 2, some parameters during and "after" production are shown which have an influence on the aggregates and hereby also an inf luence on the application effects.

Scope and structure
Degussa utilises the know-how from years of experience and the resultant market knowledge to develop the global, technology-and innovation-driven market for nanoscale materials. This is done in the research groups of the business units Silica & Silanes and Advanced Fillers & Pigments, which are focusing on developing new or improved types of fumed silica, titania, alumina, carbon black and dispersions thereof.
The Project House Nanomaterials was created to develop new nanomaterials and also new gas phase processes. To this end, an interdisciplinary team with experienced people from different business units was formed. Chemists, materials scientists, process engineers and industrial management experts from the Project House Nanomaterials develop together with customers and universities innovative technologies and novel nanoscale materials such as special types of carbon black, zirconia, indium tin oxide, zinc oxide, ceria and superparamagnetic composite materials. Some examples are described in Chapter 5.
These new materials are manufactured in the gas phase at temperatures up to 10,000 K. The manufacturing process and conditions determine size and morphology of the particles and hence their application properties. Seven new pilot plants have been built comprising three f lame reactors, two hot-wall reactors, a plasma reactor, and very new, a laser evaporation reactor. They are described in detail later on (see Chapter 4). One special feature of the Project House Nanomaterials is the close cooperation both with potential customers and with academia, the latter providing results from fundamental research. Some projects carried out together with the universities are described in the following section.

Cooperation with universities
The Project House Nanomaterials has nine cooper- Prof. Ebert evaluates the feasibility of an adiabatic quench system in the low-pressure f lame reactor to rapidly cool particle-gas mixtures by adiabatic expansion. This system also affects particle morphology.
Prof. Fissan further develops his SMPS (scanning mobility particle sizer) with respect to corrosion resistance against chlorine-containing atmospheres and operation of the SMPS connected to reactors, which are operated at reduced pressure.
Prof. Hahn is checking the scalability of his concept to synthesise non-aggregated oxidic nanoparticles using hot-wall reactor technology at reduced pressure. The experiments are done in a dedicated pilot reactor mainly producing SiO 2 .
Prof. Roth is proving the feasibility of the TEM-grid sampling system to take samples out of running reactors. The particles deposited by thermophoresis can be directly investigated by means of transmission electron microscopy (TEM). Thus particle size and morphology can be correlated with reactor settings and the particle formation can be monitored as a function of residence time and chemical conversion rate.

Gas-Phase Synthesis of Nanoparticles
Although a number of variations exist for gas-phase synthesis processes, they all have in common the fundamental aspects of particle formation mechanisms that occur once the product species is generated [2], [3], [4]. Product quality and application characteristics of nanoscaled particles depend strongly on the particle size distribution and on the morphology of the particles, i.e. the size and number of primary particles defining the degree of aggregation. In gasphase reactors, the final characteristics of particulate products are determined by f luid mechanics and particle dynamics within a few milliseconds at the early stages of the synthesis process. Within this short time frame, three major formation mechanisms dominate the particle formation.
Chemical reaction of the precursor leads to the formation of product monomers (clusters) by nucleation or direct inception, and to the growth of particles by reaction of precursor molecules on the surface of newly formed particles; this is called surface growth [5], [6]. Coagulation is an intrinsic mechanism which inevitably occurs at high particle concentrations and therefore in all industrial aerosol processes. Particles dispersed in a f luid move randomly due to Brownian motion and collide with each other along their trajec-ation partners from eight universities (see Fig. 2). This tight cooperation is funded by the Deutsche Forschungsgemeinschaft, DFG (German Science Foundation).
The universities provide state-of-the-art measuring technology and the results from their fundamental research in the fields of particle synthesis and processing, particle characterisation and simulation of particle formation. On the other hand, the universities are granted access to dedicated pilot reactors to test their equipment or to prove their results on larger, industrial-scale equipment. Following are some examples: Prof. Bockhorn's aim is to model the formation of particles, especially of soot, in flames. To gather basic information on particle formation, he developed the RAYLIX method, which combines laser-induced incandescence (LII), Rayleigh scattering and extinction measurement, whereas the Rayleigh scattering and the LII-signal are detected in two dimensions. This set-up provides locally resolved information on soot volume fractions and on the size of the soot particles in f lames [1].
Prof. Leipertz uses time-resolved laser-induced incandescence to monitor on-line the average particle size of just-formed soot particles. Both systems have been successfully transferred to monitor the carbon black formation in a plasma reactor. We also successfully adapted both systems to monitor the particle formation of carbon-free systems such as titania and zirconia in our laser evaporation reactor. This unique reactor is the result of the cooperation with Dr. tories. Assuming strong adhesive forces, characteristic for small particles, or chemical bonds, these collisions result in coagulation [7], [8]. 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 [9], [10]. Figure 3 shows the mechanisms' influence on particle formation, growth, and final morphology.
According to these particle formation mechanisms, the product quality can be inf luenced by a sensible selection of the process parameters. However, linking process parameters such as temperature, reactant state or reactor geometry to product characteristics requires a good understanding of the physico-chemical fundamentals of gas-phase synthesis. Measurements in gas-phase reactors are quite problematic as time scales are extremely small, temperatures very high and the gaseous atmosphere is often aggressive. Therefore, process simulation is a useful tool and can significantly improve the general understanding of particle formation and moreover can support product and process optimisation.
Numerous models based on the so-called particle population balance have been developed and applied to the simulation of particle formation and growth [11], [12]. A robust and time-efficient model is the simple monodisperse model developed by Kruis et al. [13], which has been coupled to fluid dynamics (CFD) successfully by Schild et al. [14]. While CFDcoupled simulations of the particle formation reveal specific reactor characteristics, the plain application of population balances, i.e. simulations reduced to time-temperature calculations for particle trajectories, is also very helpful in terms of increasing the basic understanding of the synthesis process.
As an example, the particle formation in a premixed f lame reactor has been modelled by approximating the average time-temperature history of the particles' trajectories. The calculated particle size evolution was then compared to experimental data obtained by thermophoretic sampling on a TEM grid which enables direct measurements of particle sizes and morphology in the f lame at various distances (residence times) from the burner mouth [15]. Figure 4 presents the principle of the sampling device, sections of the TEM pictures taken and the corresponding average aggregates obtained from simulation. With increasing residence time, the expected advancing degree of aggregation can clearly be seen. Both results, from experiment and simulation, are in excellent conformity and validate the simulation technique.

Reactor Technologies
Researchers of the Project House Nanomaterials have improved existing processes and developed new technologies for the production of tailor-made nanoscaled materials.

Flame reactor
Flame reactors are one of the most common reactor designs for the production of high-purity nanoscale powders in large quantities [16][17][18][19][20]. The capability for full-production scale has been demonstrated for many metal oxides such as silica, titania, alumina and others. Powders, liquids and vapours can be used as precursors. The f lame provides the energy to evaporate   the precursors and to drive the chemical reactions. Due to the high energy density in the f lame, the precursor concentration can be quite high. In the flames, temperatures from 1000°C up to 2400°C can be realised. The residence time in the highest temperature region is very short and usually ranges between 10 and 100 ms. This region is crucial for the formation of the primary particles. After this zone only the size and the morphology of the aggregates can be inf luenced. With f lame synthesis, primary particle sizes from a few nm up to 500 nm are accessible. The specific surface areas of these powders can go up to 400 m 2 /g and higher.
The shape of the flame can be influenced by using "premixed" or "diffusion" f lames. In premixed f lames the fuel and the air/oxygen are already mixed and the reaction takes places right at the burner mouth. These premixed f lames are typically very short. In diffusion f lames the fuel and the air/oxygen are fed separately to the burner mouth. The reactants have to diffuse together and combust in the diffusion zone [21].
The control of the three reactor parameters temperature profile, reactor residence time and reactant concentration is of great importance to generate tailor-made particles. Unfortunately, these parameters cannot be changed in f lame reactors independently. Every adjustment of the feed flow causes a change in all three parameters.
The Project House Nanomaterials runs two f lame reactors (Fig. 5, 6). One of these flame reactors is especially designed for low pressures down to p min ҃ 20 kPa. The control of the reactor pressure is interesting for two reasons: 1. Decreasing the pressure causes a dilution effect, which allows for the synthesis of less aggregated particles. 2. By controlling the pressure one can change the residence time independently from the f lame temperature. Fig. 7 shows the f low sheet of a flame reactor. Flame reactors consist usually of the burner, a f lame tube, a particle collection unit (e.g. a bag filter) and an off-gas treatment unit (e.g. an alkaline scrubber). 28 KONA No.20 (2002)

Hot-wall reactor
Hot-wall systems employ tubular furnace-heated reactors for initiating the synthesis reaction [20]. The construction is relatively simple and process parameters are moderate in comparison to other gas-phase reactors. Temperatures range below 1700°C, concentrations are variable, the gas composition is freely selectable and the system pressure is usually atmospheric but can also be used as a process parameter. The technique allows for precise process control and therefore allows for particle production with specific characteristics. Due to their high energy requirements, hot-wall systems have mainly been investigated on a lab scale. But there are also industrial applications. One important example is the production of Al-doped TiO 2 as a pigment. Precursors that are most often used for particle formation are metal chlorides and organometallic precursors. The mixing of the reactants and the carrier gas is, besides the above-mentioned process parameters, the most important instrument for controlling product characteristics. A variety of mixing arrangements has been investigated on a laboratory and industrial scale. Premixed systems, often used on a lab scale, are avoided in industrial applications as the reactant streams could react spontaneously before entering the reactor. The feeding location is also crucial. The reactants can be introduced concurrently at the reactor entrance or somewhat downstream, thus inf luencing the residence time within the reaction zone. Alternatively, the reactants can be fed separately at different locations, affecting the particle formation mechanisms and enabling even composites or coated particles.
To summarise, the hot-wall technology involves the following Advantages: Disadvantages: Figure 8 presents the schematic set-up of a pilotscale hot-wall reactor developed in cooperation with the Department of Material Science of the Technical University of Darmstadt (Prof. Hahn) and operated at the Project House Nanomaterials. The hot zone is divided into three parts, allowing for specific temperature profiles, various precursor feeding locations and also for product sampling during particle formation.  • often requires volatile precursors with significant vapour pressure and stability below reaction temperature, often resulting in high precursor costs • high degree of aggregation at high aerosol concentrations • high energy requirement • simplicity of design • precise control of process parameters and f lexibility with respect to gas composition and system pressure • production of oxides, non-oxides, semiconductors and metals in the range from atomic to micrometre dimensions The carrier gas is preheated and the precursor is vaporised by a commercial liquid precursor delivery system (LPDS). Exiting the last furnace section, the aerosol stream is quenched and the powder is collected in a bag filter. So far, the pilot plant has been investigated intensely for the production of tailormade silica particles and will be tested for mixed metal oxides in the near future (Fig. 9).

Plasma reactor
Thermal or hot plasmas are in or close to the local thermodynamic equilibrium, i.e. the temperature of heavy species in the plasma and the temperature of the electrons are mostly identical. Hot plasmas are characterised by a high electron density of 10 21 Ҁ10 26 m Ҁ3 . These plasmas can be generated by gaseous discharge between electrodes, by microwaves, by laser or high-energy particle-beams or by electrodeless radio frequency (RF) discharge. An RF discharge can be maintained either by capacitive or by inductive coupling. The most widely used electrical methods for producing plasmas are high-intensity arcs and inductively coupled high-frequency discharge. An inductively coupled high-frequency discharge is maintained by a time-varying magnetic field operated at 3 MHz to 30 MHz [22].
Today, the plasma deposition of coatings and films by plasma spraying, thermal plasma chemical vapour deposition (TPCVD) and thermal plasma physical vapour deposition (TPPVD) is quite common. The production of nanoparticles by means of thermal plasma is a less evaluated field. One investigated example is the production of carbon black by means of highintensity arc plasmas [23].
The electrodes necessary for arc discharge are typically made from thermally quite stable graphite. They are nevertheless eroded or evaporated within a short time, thus polluting the desired product. To avoid product contamination, plasma processes which do not use electrodes can be used, e.g. "cold" microwave plasma or the thermal RF plasma. But the microwave plasma requires reduced pressure, thus strongly limiting production rate and particle temperature. Therefore we started to evaluate the nanoparticle synthesis by means of an inductively coupled plasma. Due to the missing electrodes, contamination in the product can be minimised.
Our high-frequency plasma reactor is operated at normal pressure with a frequency of 3Ҁ4 MHz, providing 40 kW of thermal power (efficiency 30Ҁ40%) and yielding locally up to 10 000 K. Particles are col-lected by means of a cyclone or a filter, the waste gas can be washed with alkaline or acidic solutions. Up to 6 Nm 3 /h of typical plasma gases such as nitrogen or argon as well as air can be used; solid, liquid or gaseous precursors can be fed up-or downstream of the plasma f lame. The residence time in the hot zone is less than 1 s; cooling rates of 10 6 Ҁ10 9 K/s are feasible. Depending on the gas composition the precursors are physically or chemically converted to nanoparticles. The reactor set-up and a picture thereof are given in Figures 10 and 11.

Laser reactor
In cooperation with the German Science Foundation (DFG) and the University of Jena, the Project  House Nanomaterials has built a pilot-scale laser evaporation reactor (see Fig. 12 aѿb). The reactor is a scale-up of the reactor from the University of Jena [24,25] and features three CO 2 lasers of 2 kW laser power each at a wavelength of 10.59 mm, a reaction chamber and a product separation unit. Figure 13 shows the schematic process f low diagram. The laser beams are directed and focused into the reaction chamber, resulting in an intensity of 6 kW in a volume of only a few cubic millimetres. This high-intensity volume is located in the centre of the reaction chamber where the precursor, typically a coarse metal oxide powder, is f luidised. The gas-dispersed precursor powder is evaporated by the focused laser, and nanopowder is formed by condensation of the vapours. Figure 14 is the view into the reactor during operation. The so-formed particles are separated from the gas stream by conventional filtration methods. Figure 15 shows a TEM image of laser-synthesised particles. The high-power intensity of the laser opens a wide field of solid precursor options having high vaporisation temperatures, e.g. ceramics and metal oxides, which are not suitable for the synthesis of nanoscaled powders by standard gas-phase processes. Due to the a) Laser unit with reaction chamber to the right and filter to the left. b) Reaction chamber.   high cooling rates, the morphologies of the lasersynthesised particles differ significantly from typical pyrogenic oxides, which opens new fields of potential applications.

Zirconia
The production of pyrogenic oxides by means of f lame hydrolysis of metal chloride vapours is well known. Degussa has been using this approach successfully for decades producing, e.g. silica (AEROSIL ® ), alumina and titania. The synthesis of pyrogenic zirconia via the chloride route on a large scale is difficult and very costly. The chloride precursors make high demands on the materials of construction for the reactors, especially for the evaporation unit. Furthermore, the chloride in the product hinders its suitability toward several potential fields of applications. To reduce the chloride in the powder, a lot of effort has to be put into the process. One cannot use excessive temperatures for the deacidification because high temperature equals loss of specific surface area (BET). To overcome all these disadvantages, a new process was developed for the production of lowchloride (500ppm) high-BET (30m 2 /g) zirconia.
In contrast to the existing chloride route, the new gas-phase process is based on metallorganic or nonchloride salt solutions.
The process uses a flame spray pyrolysis technique to yield nanoscaled high-purity (mixed) metal oxides. The precursor is sprayed into a flame reactor where the conversion into the oxide material takes place followed by the particle separation. Variations in the time-temperature history of the particles in the combustion zone and the spraying parameters result in different product grades, e.g. specific surface area (BET). With this reactor set-up, various metal oxides and mixed metal oxide nanopowders are accessible. Zirconia and different yttrium-stabilised forms thereof (YSZ) but also indium tin oxide (ITO), alumina, etc. have been synthesised in the pilot-scale reactor . Figure 16 shows a TEM image of ZrO 2 with a BET of 74 m 2 /g. In Table 2 typical physico-chemical data of ZrO 2 and YSZ are listed. As the average primary particle size is in the 12-nm region, the sintering process starts at low temperatures (see also Fig. 17).

Indium tin oxide
For the production of transparent electrically conductive polymer composites, nanoscaled indium tin oxide (ITO) is being developed for use as a raw material. The ITO nanoparticles should be characterised by the following parameters: • Colourless or slightly blue • Specific resistance: 10 2 Ҁ10 8 Ωcm • Average primary particle size: 20 nm • Aggregate size: 100 nm • Good dispersibility • Anisotropic self-organisation in the matrix Such an ITO can be produced by spray pyrolysis (see Figure 18). It was demonstrated that the conductivity could be clearly improved by doping the ITO samples. Therefore the technical term DITO (doped indium tin oxide) is reasonable and may be applied. Under optimal circumstances it is possible to reduce the specific resistance from 660 ⍀cm to 400 ⍀cm with DITO. Further post-treatment additionally reduces the resistance to 10 ⍀cm, whilst BET surface area remains constant. The colour of the material can also be controlled by the addition of certain dopants.

Superparamagnetic nanocomposites
Superparamagnetic nanocomposites show the characteristic magnetic properties of paramagnetic as well as of ferromagnetic materials. Like paramagnets, these materials possess no permanently aligned structure of the elementary magnetic dipoles when no magnetic field is applied. Otherwise, the shape of the magnetisation curve looks like a ferromagnetic curve without hysteresis (see Fig. 19). Nanoscale magnetic particles display this superparamagnetic behaviour because the tiny magnetic domains are small enough to immediately become disordered due to entropy [26,27].
A TEM image of a superparamagnetic material is shown in Fig. 20. Via f lame synthesis, nanoscaled iron oxide particles were embedded in an amorphous silica matrix. The dark dots represent the iron oxide crystals. The TEM picture indicates that the magnetic domains of iron oxide are well distributed through the silica matrix and that they are smaller than 20 nm. These are important requirements to obtain singledomain and well-decoupled magnetic centres.
X-ray diffraction of the produced powders shows mainly the magnetic iron oxide phases magnetite and maghemite with a low content of non-magnetic hematite (Fig. 21). The specific surface area of the composite (BET) was varied from 114 m 2 /g to 214 m 2 /g by adjusting process parameters. A SQUID magnetometer was used for magnetic characterisation. The particles display superparamagnetic behaviour above 50 K and the iron oxide domains are magnetically well decoupled. The so far highest saturation magnetisation achieved was 18 Am 2 /kg. The advantage of the described superparamagnetic nanocomposites is their high chemical, mechanical and thermal stability, which is often well beyond that of traditional superparamagnetic materials with organic coatings.

Conclusion
Gas-phase synthesis is a well-known technique for the production of an extensive variety of nano-sized particles. The "original" commercial nanopowders generated by this method comprise the largest share of the market, e.g. as reinforcing fillers, for rheology control and as pigments. The growing need for multifunctional materials (scratch-resistantѿtransparent, transparentѿconductive, etc.) drives the transition from powders to highly specialised functional materials.
Based on fundamental research, the detailed analysis of particle size and morphology together with a good understanding of the particle formation mechanisms, it is possible to design reactors and to precisely control the processes. Results thereof are technical innovations consisting of new synthesis routes and new forms of nanoparticles with customtailored characteristics which provide 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 precisely fit your product to the needs of the customer. Degussa plans to implement such an approach in order to fully realise the commercial potential of their innovative nanomaterials and synthesis technologies resulting from their strategic research.

Andreas Gutsch
Dr. Gutsch studied chemical engineering in Karlsruhe, Germany, and began his career working as a free-lance consultant in the power engineering field. While he was doing his doctorate, he gained international experience at the University of Cincinnati, Ohio. In 1995, he joined former Degussa. Here he worked in the process engineering field, where he was appointed Chief of Section in 1998. He has been head of the Nanomaterials Project House since 2000. In April 2002 Dr. Gutsch has been appointed head of Creavis Technologies & Innovation. Dr. Gutsch belongs to a number of advisory committees at the German Research Union, acts as special adviser for the Land North Rhine-Westphalia, and is an appointed member of various scientific organisations.

Heike Mühlenweg
Heike Mühlenweg received her PhD-degree in Mechanical Process Engineering from the Technical University of Clausthal in 1995. The following year she accepted a scholarship from the German National Science Foundation (DFG) and spent eighteen month as a Post-Doc researcher at the Department of Mechanical and Aerospace Engineering, Arizona State University, USA. In 1998 she started at the Degussa AG. Following nine month of scientific work concerning population balances at the University of Cincinnati, USA, she worked as R&D engineer in Degussa's Department of Process Technology and Engineering. Her main topic was the simulation of particle synthesis in gas-phase reactors and the collaboration in preparation of the Project House Nanomaterials as a new strategic project for the development of advanced nano-scaled powders. In January 2000 she has been transferred to the starting Project House Nanomaterials and since she is responsible as R&D manager for the coordination of a joint venture project with nine leading German Universities funded by the DFG. Furthermore, based on her simulation know-how she developed and constructed a gas-phase reactor that is now operated for the generation of new nano-scaled powders.

Günther Michael
Dr. Michael studied chemistry at the University of Kaiserslautern where he received his PhD-degree on "Agostic complexes of chromium" in 1986. From 1987 on he has been working with the Aerosil Applied Technology Group of Degussa AG. He was also the quality supervisor for Aerosil. Since 2000 he is working with the Project House Nanomaterials as senior manager.

Markus Pridöhl
Dr. Pridöhl studied chemistry at the Technical University of Berlin where he received his PhD-degree on "Synthesis, 77Se-NMR spectroscopy and HPLC analysis of Bis-organyl-sulfur-selen-chains" in 1995. In the same year he started with the former Cerdec AG in Frankfurt as a quality control manager. After a trainee assignment in the field of carbon black production and site management with Degussa AG in 1996 he worked as R&D manager with Degussa's Fillers and Pigments Division from 1998 on. In 2000 Dr. Pridöhl joined the Project House Nanomaterials as R&D manager.