Planar stacks

It has been established that, when mesophase pitch is carbonized, the morphology of the pitch is the primary factor [20] in determining the microstructure of the resulting graphitic material. This may be attributed to the stacking behavior of mesophase molecules (quite similar to the planar stacking in turbostratic graphite), which may be visualized as shown in Fig. 5. 

We have now adjusted our molecular systems to provide a model in which both forces can operate simultaneously. The U-shaped relationship that exists between the imide function and amides of aryl amines creates a hydrogen bonding edge and a planar stacking surface that converge from perpendicular directions as in 44 to provide a microenvironment complementary to nucleic acid components. A large number of aromatic rings can be functionalized with this simple scaffold, and spacers (R) can also be incorporated. The imide function is a mimic of the thymine residues. 

SOFC are produced with either tubular or planar stack configurations investments for planar design are a rough estimate, as no prototypes exist. Specific investments for PAFC are in the range 4000- 4500/kW (IEA, 2007). For further fuel-cell R D needs see IEA (2005). 

M. Hsu, "Zirconia Fuel Cell Power System Planar Stack Development," Fuel Cell Abstracts, 1986 Fuel Cell Seminar, Tucson, AZ, October 26-29, 1986. 

The study of vapour-deposited organic films has been applied to a large number of molecular systems. However, most of the work has concentrated on the study of the growth and optoelectronic characteristics of planar stacking molecules such as the Pc and polycyclic aromatic compounds based on naphthalene, and perylene. In particular, PTCDA has become extensively studied. 

Fig. 1.6 Illustration of a planar-stack, solid-oxide fuel cell (SOFC), where an membrane-electrode assembly (MEA) is sandwiched between an interconnect structure that forms fuel and air channels. There is homogeneous chemical reaction within the flow channels, as well as heterogeneous cehmistry at the channel walls. There are also electrochemical reactions at the electrode interfaces of the channels. A counter-flow situation is illustrated here, but co-flow and cross-flow configurations are also common. Channel cross section dimensions are typically on the order of a millimeter.

Typically, TSOFC use co- and counter-flow configurations whereas planar stacks sometimes favour cross flow simplifying manifolds attachment. The flow of air usually provides cooling to a stack in either design as does internal reforming (Sulzer Hexis). The flow regime strongly affects the distribution of gas composition, mechanical stress, stack temperature and ultimately current density. 

However, the mechanical self support of cells is basically provided by the thickest PEN layer either one of the electrodes or the electrolyte (Sulzer Hexis, MHI, RR, CFCL). Thick porous ceramic (RR, MHI) and metallic substrates and interconnects (Ceres Power) onto which a thin PEN is applied have also been suggested to provide the mechanical support required. The electrolyte and anode supported cells are nowadays preferred in tubular and planar stacks. 

Different shapes serve different purposes. Block stacks are convenient to use in some applications and planar stacks are easier in others For instance, a laptop computer would do well to use a planar configuration rather than a block configuration. Generally, fuel cells can come in any imaginable shape and size, and be designed specifically for a wide variety of applications. 

We decided to work with a planar design in order to compare the efficiency of block stacks to planar stacks. 

The result of this endeavor was the L79 planar stack. Although we are still experimenting and tweaking the design, we believe it is a good design to present with the rest of the hydrogen system. 

Each cell in this planar stack is connected in series, that is, the positive electrode of one cell is connected to the negative of the next cell and so on. In a block stack this is accomplished by using bipolar plates, but in the L79 we use circuit board traces with tab wire connects to perform the same function as the bipolar plates. The L79 can also be built in a parallel configuration or a series-parallel configuration depending on the current and voltage desired. 

To construct the power supply based on the L79 planar stack will require ten stacks of twelve MEAs each, and will have an output of 6 volts at 2 amperes per stack. Five series connected pairs of stacks produce 12 volts at 2 amperes per pair. The five pairs are then connected in parallel for a total output of 12 volts at 10 amperes for the power supply (see below). 

These planar stacks can be racked in a variety of ways. The above illustration shows a simple tubing manifold system for feed lines and gas exit lines. On the next page is another view of the rack of stacks. 

Planar stacking faults, twins, and grain boundaries ... 

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