Membrane, thermal conductivity

Equation 19.5 is often utilized in MD, while Equations 19.6 and 19.7 are the alternate expressions to calculate the membrane thermal conductivity, where e is the porosity, k and kg are thermal conductivities for solid (polymer) and gases in the pores (air and water vapor), respectively. The values of thermal conductivity of polyvinylidenedifluoride (PVDF), Polytetrafluoroethylene (PTFE), and Polypropylene (PP) have been reported in a narrow range 0.17-0.29 W m [48]. The thermal conductivities... 

In Eq. 4.35 Omem is the membrane thermal conductivity, and 4nem,o are the internal and external membrane diameter, and hp xm is the convective heat transport coefficient in the permeation zone. 

With the thermally insulated cathode, local overheat in the MEA increases by a factor of 40 (Figure 3.6(a)) and reaches 2 K. Physically, membrane thermal conductivity is not high enough to efficiently remove the heat produced in the CCL and the temperature on the cathode side rises dramatically. In this situation, the value of thermal conductivity of the catalyst layer plays a minor role a reduction of A by a factor of 10 changes the local overheat by a factor of less than two (Figure 3.6(b)). 

It is advisable to evaluate the discussed above heat crossover through the membrane. In low-T cells, membrane thermal conductivity is two to five times smaller than the thermal conductivity of the catalyst layers as an estimate we take kx = 5. With the 50-nm membrane and 10- im catalyst layers we have km = 5 (Table 3.3). With the parameters from Table 3.3 we get... 

If the anode side is thermally insulated, the heat flux through the membrane is zero (Figure 3.7). In other words, heat generated in the CCL is removed through the CCL/backing layer interface. In that case, membrane thermal conductivity is of no significance and it does not appear in Eq. (3.93). 

In the case of the thermally insulated cathode, the heat flux is directed in the opposite way, through the membrane and the ACL for that reason the relation (3.94) involves k = A /A. Suppose that A is fixed according to (3.94) the decrease in A then dramatically increases T. In a DMFC with a thermally insulated cathode, the spots of low membrane thermal conductivity are very dangerous heat of the reaction (3.67) appears to be trapped and the adjacent domain of the CCL may be strongly overheated. 

Flow stoichiometry thermal conductivity (W m K ) Membrane thermal conductivity (W m K )... 

Gas channel and shoulder width GDL thickness Catalyst layer thickness Number of fuel cell in the stack Porosity of GDL/catalyst layer Thermal conductivity of membrane Thermal conductivity of catalyst layers Thermal conductivity of GDL Thermal conductivity of bipolar pressure Permeability of GDL Electronic conductivity in the GDL/land Anode/cathode inlet pressure Stoichiometry... 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

Another possibihty to improve the temperature homogeneity is to introduce an additional polysiHcon plate in the membrane center. The thermal conductivity of polysilicon is lower than that of crystalline siHcon but much higher than the thermal conductivity of the dielectric layers, so that the heat conduction across the heated area is increased. Such an additional plate constitutes a heat spreader that can be realized without the use of an electrochemical etch stop technique. Although this device was not fabricated, simulations were performed in order to quantify the possible improvement of the temperature homogeneity. The simulation results of such a microhotplate are plotted in Fig. 4.9. The abbreviations Si to S4 denote the simulated temperatures at the characteristic locations of the temperature sensors. At the location T2, the simulated relative temperature difference is 5%, which corresponds to a temperature gradient of 0.15 °C/pm at 300 °C. 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

It is important to note that Vie and Kjelstrup [250] designed a method of measuring fhe fhermal conductivities of different components of a fuel cell while fhe cell was rurming (i.e., in situ tests). They added four thermocouples inside an MEA (i.e., an invasive method) one on each side of the membrane and one on each diffusion layer (on the surface facing the FF channels). The temperature values from the thermocouples near the membrane and in the DL were used to calculate the average thermal conductivity of the DL and CL using Fourier s law. Unfortunately, the thermal conductivity values presented in their work were given for both the DL and CL combined. Therefore, these values are useful for mathematical models but not to determine the exact thermal characteristics of different DLs. 

Solving the energy equation provides prediction of the temperature distribution and its effect on cell performance in a PEFC. Figure 12 presents a temperature distribution in the middle of the membrane for a single-channel PEFC. The maximum temperature rise in this case is 4 °C, which will only fect cell performance slightly. However, the temperature variation depends strongly on the thermal conductivities of the GDL and flow plate as well as thermal boundary conditions. 

The excellent insulating and dielectric properties of BN combined with the high thermal conductivity make this material suitable for a huge variety of applications in the electronic industry [142]. BN is used as substrate for semiconductor parts, as windows in microwave apparatus, as insulator layers for MISFET semiconductors, for optical and magneto-optical recording media, and for optical disc memories. BN is often used as a boron dopant source for semiconductors. Electrochemical applications include the use as a carrier material for catalysts in fuel cells, electrodes in molten salt fuel cells, seals in batteries, and BN coated membranes in electrolysis cells for manufacture of rare earth metals [143-145]. 

Combination of the dehydrogenation on one surface of the Pd-based catalytic active membrane and hydrogenation by the diffused hydrogen on the other surface has been proposed [33]. Thanks to the excellent thermal conductivity of these membranes, the heat released by the hydrogenation can be utilized to drive the endothermic dehydrogenation (Figure 12.8). 

Moreover, in direct contact membrane distillation, a minimum value ofthickness is required to keep the difference of temperature across the membrane. Generally, in membrane distillation materials with low thermal conductivity are also required to reduce the heat loss through the membrane-self. Pore-size distribution also plays an... 

If silicon technology is involved all thermal sensors suffer from the high thermal conductivity of silicon, which dramatically decrease their sensitivity [12]. However, by use of micromachining and integrated silicon technology a powerful thermal biosensor can be realized. Using a thermopile integrated on a thin micromachined silicon membrane reduces thermal loss due to the substrate and so excellent performance can be accomplished [13]. 

A different analyser method based upon the same effect (deviating thermal conductivities of gaseous components) is shown in Fig. 6.127. The gas to be analysed diffuses into the measuring cell. Here, a thermal conductivity sensor made of three superimposed silicon chips shows a balanced (zero) output of two thin film resistors fitted on a membrane on the chip in the middle of this stack. One of these thin film resistors is exposed to the gas to be measured. Due to its thermal conductivity, the pair of thin film resistors show an unbalanced signal output. 

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