Delft IMP uses gas phase deposition technologies for nanostructuring of particles. This includes atomic layer deposition (ALD) and a variant, called molecular layer deposition (MLD). The principle of ALD is shown in figure 1.
Figure 1: ALD principle, showing a surface (A) which is exposed to a gaseous reactant, for example tri-methyl aluminium. The gas will react with the surface, forming a monolayer. To allow multiple coatings, the monolayer (B) is exposed to a second reactant, in this case water, returning the surface to state (A). The number of reaction repetitions determines the deposited coating thickness.
These techniques result in a homogeneous coating on particles in the nanometer scale, allowing control of particle properties (controlled release), and intensification of coating material (catalyst intensification). ALD can be used to deposit a wide range of metal oxides, pure metals, and other inorganic materials, and has come a long way since it’s invention in the early ’50’s.
History of ALD – Russian invention, Finnish development
Figure 2: History of ALD
The principle of ALD originates in the former Soviet Union in the 50’s, whereby Prof. Aleskovskii (1) theorized that when using a two-staged adsorption – reaction process, atomic monolayers could be formed on the surface of particles. Together with Prof. Kol’tsov he showed in 1968 that multilayers of aluminium oxide could be formed onto silicagel particles (2) (3). In 1979, Yakouvlev (4) demonstrated for the first time that fluidized bed reactors can be used for process intensification of atomic layer deposition. Already in the ’60’s the Russians demonstrated atomic layer deposition on particles ranging in size from 10 to 1000 micrometer, including mica, quartz, but also on nanopowder agglomerates such as silica gel and Aerosil (2).
The technology development of atomic layer deposition occurred later in Finland, starting in the late 70’s with the development for flat surface applications such as displays, and later for computer chips. This was done by the joint efforts of Microchemistry LTD, University of Helsinky and Neste Oy. In the beginning of the 90’s, interest for catalyst production by ALD started to arise, especially by Neste Oy, a large Finnish oil company, who intended to use the process for catalysis in oil refining processes. For example, in 1994 Lindblad (5) showed production of nickel catalysts using ALD. In parallel, between 1990 and 2000 the University of Joensuu developed an alternative ALD method to produce ruthenium, molybdenum, tungsten and cobalt catalysts in a fluidized bed (6).
There has been an ongoing effort to intensify the process of particle nanostructuring. Figure 3 shows a short summary of some developed reactor concepts throughout the history of ALD.
Figure 3: History of reactor concepts, showing: a. the research reactor of Aleskovskii (1) ; b. the flat substrate equipment developed in Finland (7) ; c. the fluidized bed reactor as used in Finland (6) ; d. the continuous pneumatic transport reactor as developed by Delft IMP and Delft Technical University (8)
Currently, Delft IMP offers fluidized beds and pneumatic transport reactors to customers, as these concepts allow best scalability toward commercial scale production.
Fluidized beds for process intensification
Already in 1979 Yakouvlev (4) showed that fluidized beds would be a beneficial method for process intensification. Indeed, since 2000 there has been a strong trend toward using fluidized bed reactors for ALD in both research as in industry. Figure 4 shows the ALD process as performed in a fluidized bed.
Figure 4: Schematic of ALD in a fluidized bed. Starting from the lower left corner and moving clockwise: at the particle surface, material is deposited in two subsequent steps; this is called atomic layer deposition (ALD). The material can either grow as a conformal film (i.e. for encapsulation), or as islands (i.e. for catalyst intensification). The particles are moving around in the fluidized bed ensuring a homogeneous coating process
In the Delft IMP coating process, particles are fluidized under atmospheric conditions with nitrogen gas, enabling perfect mixing of the particles. Standard, commercially available fluidized beds can be used for this purpose. To perform ALD in these reactors, precursors are added to the nitrogen gas, which react with the particles and form an uniform coating on the particles over time. Depending on the nature of the coating and the size of the batch, the process takes between minutes and hours.
This process is ideal when batchwise production is required, such as in pilot production, ingredient encapsulation for the food industry, medication encapsulation for pharma, etcetera.
Pneumatic flow reactor for continuous production
For catalysis, required volumes are much higher, and stringent product control over the entire production is needed. For this purpose, Delft IMP and Technical University of Delft have developed a new patent protected reactor concept, a based on pneumatic transport, as shown in figure 5.
Figure 5: Schematic of ALD in a pneumatic transport reactor
In this method, particles are transported through a tubular reactor. By positioning of ports into the tubular reactor, we can perform the ALD reactions while the particles are being transported through the reactor. This allows continuous production of the desired functionalized particles. The approach is modular; depending on the required coating thickness we can extend the reactor length, and depending on the required volume of produced particles we can increase the reactor diameter.
The pneumatic flow reactor has enormous advantages, especially for the catalysis industry:
- Easily scalable, both in coating thickness as in produced volume
- True continuous, no downtime
- Based on existing pneumatic transport equipment
- No / limited servicing required due to lack of moving parts
For more information, don’t hesitate to contact us.
The ALD process is highly customizable to apply many different types of surface functionalities. Please see our Services page for more information.
This technology section is based on the following excellent ALD reviews: