(117d) Atomic Layer Deposition and Other Applications of the Littleford Plow Mixer As a Mechanically Fluidized Bed

The Littleford plow mixer is a well-known workhorse in commercial operations ranging from pharmaceuticals to iron ore. The unit operations involved include blending, dissolution, spray coating, vacuum drying, agglomeration, direct-contact cooling, and interfacial chemical reactions. Each of these applications is enhanced by the use of the plow mixer as a mechanically-fluidized bed.

The geometry and operating parameters of the mixer will be briefly reviewed, and compared to other types of fluidized beds. In particular, the ability of the unit to operate under high vacuum will be explained.

Several operations using the Littleford plow mixer will be discussed to illustrate the utility of the unit as a fluidized bed. One area of interfacial chemical reactions of particular interest is atomic layer deposition.

A typical procedure is as follows: The substrate was first loaded into the 22-liter mixer. The plow speed was then set, and the hot-oil unit was started and recirculated to heat the product. Once the material reached the target temperature, it was held at temperature to completely remove any moisture. The active chemical cylinders were weighed and then attached to the vapor header. The nitrogen supply was used to check for leakage, assuring that the flow was zero when the appropriate valves were closed. Vacuum was then applied to the unit, and an initial nitrogen purge (300 sccm) was put through each addition line for several minutes. A rotary lobe blower was connected in series with the 2-stage liquid-ring vacuum pump to achieve pressures below 25 torr. Dosing time was determined experimentally by periodically stopping the dosing and re-weighing the cylinder(s) to determine rate. Once the desired dosing cycles were completed, the material was cooled using the recirculating system. The vacuum was shut off and the nitrogen purge was continued until the vessel reached atmospheric pressure. The material was discharged through the bottom valve, with the plows running, and weighed. Degree of coverage was determined by analytical means.

Other unit operations will be discussed, as time and interest allow.