Microchanneled design, membrane

Another way to use silicon wafers as DLs was presented by Meyers and Maynard [77]. They developed a micro-PEMFC based on a bilayer design in which both the anode and the cathode current collectors were made out of conductive silicon wafers. Each of fhese componenfs had a series of microchannels formed on one of their surfaces, allowing fhe hydrogen and oxygen to flow through them. Before the charmels were machined, a layer of porous silicon was formed on top of the Si wafers and fhen fhe silicon material beneath the porous layer was electropolished away to form fhe channels. After the wafers were machined, the CEs were added to the surfaces. In this cell, the actual diffusion layers were the porous silicon layers located on top of the channels because they let the gases diffuse fhrough fhem toward the active sites near the membrane. 

Micropumps based on piezoelectrics are made of pumping chambers that are actuated by three piezoelectric lead zirconate titanate disks (PZT). The pump consists of an inlet, pump chambers, three silicon membranes, three normally closed active valves, three bulk PZT actuators, three actuation reservoirs, flow microchannels, and outlet. The actuator is controlled by the peristaltic motion that drives the liquid in the pump. The inlet and outlet of the micropump are made of a Pyrex glass, which makes it biocompatible. Gold is deposited between the actuators and the silicon membrane to act as an upper electrode. Silver functions as a lower electrode and is deposited on the sidewalls of the actuation reservoirs. In this design, three different pump chambers can be actuated separately by each bulk PZT actuator in a peristaltic motion. 

To achieve all of the benefits of the microchanneled, flat plate design, multiple membrane wafers must be stacked together to form multiwafer modules. 

Membrane MSR for the dehydrogenation of cyclohexane to benzene were designed [67]. This is an endothermic reaction whose equilibrium conversion is 18.9% at 200 °C. The conversion can increase beyond equilibrium up to 99% if the hydrogen is removed from the system. Therefore, a Pd-membrane with microchannels has been used to continuously remove hydrogen out of the reaction zone in order to enhance the conversion. The reactors were made of silicon using photo-etching technique, and Pt was used as a catalyst, which was sputtered onto the reaction chamber [67]. Out of two reactors, one example is shown in Figure 6.14. 

Figure 7.13 (a) compact reactor design based on stacked microchannel membrane modules... 

Figure 7.16 Design and synthesis of a chemically functional polymer membrane by an interfacial polycondensation reaction and multilayer flow inside a microchannel.

Zeolite membranes can be prepared on a porous substrate with precisely designed microchannels to form an MMR, as shown in Figure 3.7 [31]. This configuration promises to provide rapid mass and heat transfer rates, high efficiency, and safe operation. A more detailed description of MMRs can be found in Chapter 8. 

Figure 3.7 Membrane microreactor design and top view SEM pictures of (a) the 3 pm thick CsNaX catalyst deposited on the microchannel wall and (b) the 6pm thick NaA grown on the back of the stainless steel plate. Reproduced from [5], With permission from Elsevier.

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