(202d) Design of a Laser Assisted Aerosol Reactor for Production of Ceramics on Semi-Industrial Scale

Full dense ceramics are of interest because of their high temperature strength, higher hardness, lower density and lower thermal conductivity compared to metals. The sensitivity of structural ceramics towards small defects, leading to a lower strength, is a disadvantage. The powder has to have certain characteristics to achieve superior properties and avoiding defects in the sintered product. The powders should be spherical, not agglomerated, highly pure and need to have a narrow size distribution in the nanorange (Haggerty and Cannon, 1981).

Aerosol reactors are used to produce nanoparticles of various compounds because they are clean, energy efficient and capable of producing chemically pure particles. Several types of aerosol reactors can be distinguished but only flame and tubular flow reactors are used for industrial application so far. Compared to these reactors, laser-assisted aerosol reactors offer additional advantages like a confined, well defined and a wall-less reaction zone. In principle with this technique any oxidic or non-oxidic ceramic can be generated from gaseous or liquid vapour precursors.

In the late 1970's Haggerty and Cannon (Haggerty & Cannon, 1981) started to develop a laser synthesis process to produce silicon containing nanoparticles (e.g., Si, SiC, Si3N4). Many other researchers continued in that direction, however, the majority of the research was focussed on the development of new materials and not on the design of the reactor. In this research a new design for a (CO2) laser assisted aerosol reactor is made, which will be able to produce nanoparticles up to kg/hr rate. Next to up-scaling, the reactor will also be improved such that powder characteristics, e.g. size distribution, are controllable.

The reactor is shown in the figure; the top part shows the top-view and the bottom-part shows the vertical cross-section of the reactor. The reactor assembly consists of a body, a nozzle and a flushing system. The body is plane symmetric, has an opening on top and bottom and, on the side, 8 identical viewing ports. On the bottom a flange with any nozzle design can be attached with the diameter of the opening as limiting factor. On the top different parts can be attached, e.g. a chimney. The 8 ports are equally sized, so that any window, lens or piece of equipment can be exchanged between each port. Also a wide range of angles can be covered when laser measurement techniques are used, e.g. PCS and Raman spectroscopy.

In the new system the nozzle is rectangular, has a mixing chamber and a porous metal screen on top. This design has several advantages over the round nozzle design previously used. In the mixing chamber any reactants can be mixed as long as they do not react at room temperature. The rectangular shaped nozzle has also an advantage when used in conjunction with a rectangular shaped laser beam. This system creates uniform heating conditions (time-temperature history) for each gas element in the fluid stream leading to uniform particle size and composition.

Particle sampling from the flame will be done by means of a thermophoretic sampler. The sampler will move a bar, holding a TEM grid, through the flame. On the other side of the flame the grid is deposited in a nitrogen flushed room and the bar is retracted. In this way the TEM grid passed the flame only once. The sampler can be operated such that new TEM grids can be placed on the bar without shutting the process down. The speed of the bar, and thus the TEM grid, through the flame can be regulated.

The product powder collection will be done by using a system containing ceramic candle filters. The system is currently under design.

Haggerty, J. S. and Cannon, W. R. (1981) 3. Sinterable powders from laser-driven reactions in Laser-Induced Chemical Processes 165-241. (ed by Steinfeld, J. I.). New York: Plenum Press.