Thermocouple cell technique

Mench et al. developed a technique to embed microthermocouples in a multilayered membrane of an operating PEM fuel cell so that the membrane temperature can be measured in situ. These microthermocouples can be embedded inside two thin layers of the membrane without causing delamination or leakage. An array of up to 10 thermocouples can be instrumented into a single membrane for temperature distribution measurements. Figure 32 shows the deviation of the membrane temperature in an operating fuel cell from its open-circuit state as a function of the current density. This new data in conjunction with a parallel modeling effort of Ju et al. helped to probe the thermal environment of PEM fuel cells. 

It is of interest primarily for very uniform ultra-thin films and coatings (0.002-5 mils) in applications such as electrical resistors, thermistors, thermocouples, stator cores, connectors, fast-sensing probes, photo cells, memory units, dropwise steam condensers for recovery of sea water, pellicles for beam splitters in optical instruments, windows for nuclear radiation counters, panels for micrometeorite detection, dielectric supports for planar capacitors, encapsulation of reactive powders, and supports in x-ray and optical work. Any significant growth would depend upon a major breakthrough in process techniques and a consequent lowering in price. 

The spatial uniformity of temperature in the cell is difficult to determine, and we are not aware of a careful study of this problem. In most experiments, it is the temperature of the electrode-solution interface or that of the diffusion layer that is relevant. A possible internal thermometer could be created by measuring a temperature-sensitive voltammetric function, for example, the peak separation in the cyclic voltammogram of a reversible reaction, which is 2.22RT/ F. The resolution is not likely to be outstanding, but such a technique would probably allow detection of serious differences between the thermocouple reading and the actual temperature of the electrode-solution interface. 

As was explained in the previous section, when an adsorbate contacts an adsorbent, heat is released. The thermal effect produced can be measured with the help of a thermocouple placed inside the adsorbent and referred at the room temperature (see Figure 6.3) [3,31,34,49], This is a version of the Tian-Calvet heat-flow calorimeter [50], This calorimetric technique is distinguished by the fact that the temperature difference between the tested adsorbent and a thermostat is measured. Consequently, in the Tian-Calvet heat-flow calorimeter, the thermal energy released in the adsorption cell is allowed to flow without restraint to the thermostat [3,31,34,49],... 

In all cases, the commonest problem is the introduction of electrical leads, or thermocouples, into the high-pressure cell. Different techniques are used in large-volume fluid cells at pressures up to 2 GPa and in multianvil systems (including DACs) above 2 GPa. 

Technique. The irradiation vessels were — 250 cc. cylindrical boro-silicate glass cells equipped with a thermocouple well and break-seal. Before filling, the vessels were heated in air at 500°C. for 16 hours, cooled,... 

Ford et al, (1969) have described a new technique for accurately calibrating the temperature of low-temperature infrared cells. A liquid of known melting point is introduced into the cell as a capillary film between NaCl or AgCl windows. The cell is assembled with the thermocouple in the usual position, and the temperature lowered until the liquid freezes. The monochromator is set at the frequency at which the... 

The temperature of the cell must be uniform, and the measuring device must accurately reflect the temperature within the cell. Two methods are commonly used for measuring temperatures— thermocouples and pyrometry. In alloy studies, pyrometry is the more reliable technique, but at temperatures below about 800°C, the use of thermocouples is necessary. In both cases it is critical to measure the temperature as close to the cell as possible. 

An unusual technique was used when adding CO2 for solid phase equilibrium studies. The equilibrium cell was first filled with gaseous nitrogen and the cell cooled to operating temperatures. Then, the thermocouple seal directly above the cell was removed and crushed lumps of solid CO2 were dropped into the cell. A wire screen at the bottom of the cell prevented the solid particles from plugging the vapor inlet line. The seal was then replaced and the system was evacuated... 

The Linkam DSC is of the single specimen design and is described in detail elsewhere [36]. The cell comprises a silver enclosure around a thermocouple plate, the plate has a 3 X 0.5 mm slot and the sample is held in contact with the plate by a low thermal mass spring. A reference (cdibration) sample of the same thermal mass was first subjected to the temperature ramp and the thermal response of the neutral system recorded. The sample was then run and its thermal response recorded. The differential response was subsequently calculated from the electronic recordings, the single pan technique relies on the accuracy and reproducibility of the temperature control system and is comparable to that of a conventional, two-pan, heat-flux DSC (for example a Du Pont 990) [1]. 

This dynamic technique was initially developed by Mathonat and coworkers at University of Clermont-Ferrand (France). It is a flow technique where the heat of mixing is measured in a mixing cell located inside a Calvet type calorimeter (Fig. 14.5a). The Calvet sensors are a thermopiles constituted of thermocouples surrounding the mixing cell and measuring the heat power exchanged with a thermostated calorimetric block. The mixing cell represented in Fig. 14.5b consists of an hastelloy tube... 

Temperature Distribution Measurement The temperature distribution in fuel cells can be of critical importance to the kinetics, electrolyte conductivity, material compatibility issues (high-temperature fuel cells), internal reformation process (high-temperature fuel cells), and other kinetic and transport phenomena known to be functionally dependent on temperature. Since the SOFC is dominated by the electrolyte resistance, which is a strong function of temperature, the current distribution in these systems closely follows the temperature profile. Several techniques can be used to measure the temperature distribution in a fuel cell. An embedded thermistor or thermocouple can be used when carefully placed. Additionally, infrared temperature measurement is a fascinating way to observe real-time temperature variation in a fuel cell. Infrared scanners can be used to look at temperature distribution in a specially modified single fuel cell and have been useful to see the phase change processes from ice to liquid in a low-temperature fuel cell [31]. 

For the PEFC, the water balance is highly coupled to the temperature distribution, as discussed. However, direct measurement of localized temperature is difficult, due to the two-phase nature of flow in the gas channels and the small through-plane dimensions of a typical electrolyte. Besides infrared measurement, the most conunonly applied technique is the direct embedding of a thermocouple or thermistor within the bipolar plate. This approach is acceptable for most fuel cell varieties. If all thermal transport parameters, such as specific heat, thermal conductivity, and contact resistance, are known, calculation of the temperature profiles within the fuel cell can be accomplished using embedded thermocouple data and analytical or computational heat transfer models. 

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