Membrane temperature

Liu, S.C., Fairbanks, G., and Palek, J. (1977) Spontaneous reversible protein cross-linking in the human erythrocyte membrane. Temperature and pH dependence. Biochemistry 16, 4066. 

Although SPMDs are simple in design, the mechanisms governing their performance as passive samplers of HOCs can be quite complex (see Chapter 3). The underlying principle of molecular-size discrimination in the uptake and loss of chemicals by SPMDs is shown in Eigure 2.1. The sizes of the molecules shown in the illustration are scaled to the postulated 10 A diameter of the transient pores in the membrane. Temperature and the presence of plasticizers/solvent will affect the effective pore sizes. 

The temperature sensor on the hotplate measures the membrane temperature, Tm, at a certain location, Xg, such as the membrane center ... 

The reference temperature. To, refers to ambient temperature, i.e., the membrane temperature before applying any heating power. The thermal resistance, t], can be generally defined as ... 

The thermal resistance will be temperature-dependent as canbe seen in Eq. (3.24), which is not only a consequence of the temperature dependence of the thermal heat conduction coefficients. The measured membrane temperature, Tm, is related to the location of the temperature sensor, so that the temperature distribution across the heated area will also influence the thermal resistance value. The nonlinearity in Eq. (3.24) is, nevertheless, small. The expression thermal resistance consequently often refers to the coefficient t]o only, which is used as a figure of merit and corresponds, according to Eqs. (3.24) and (3.25), to the thermal resistance or thermal efficiency of the microhotplate at ambient temperature, Tq. The temperature Tm can be determined from simulations with distinct heating powers. The thermal resistance then can be extracted from these data. 

The circular microhotplate was thermally characterized, and the results were compared with simulations carried out according to the approach discussed in Chap. 3. Applying FEM simulations as described in Sect. 3.3 generate a temperature field, and the temperature in the membrane center represents the overall membrane temperature according to Eq. (3.21). The values that have been used for the simulation are summarized in Table 4.2. 

The comparison of simulation and measurement data of an uncoated membrane is shown in Fig. 4.7. The temperature curves, T to T4, were measured with the on-membrane temperature sensors. The graphs of the simulated temperatures are denoted Si to S4. The temperature discrepancy between simulation and experiment was less than 5% for all sensors. The general shape of the temperature distribution was correctly modeled within measurement accuracy. It has to be noted that no additional fitting parameters were used for these simulations. 

Fig. 4.21. Ratio of the leakage current of the reverse-biased drain/n-weU junction and the drain current as a function of the drain current. A drain current of 14 niA corresponds to a membrane temperature of 375 °C...

Fig. 5.9. Membrane temperature, Tm, vs. reference voltage, Vr. The reference voltage is given in digital code. The full range of digital values from 1 to 1023 corresponds to a temperature range ofl70°Cto 310 °C...

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. 

We report here on the structure and gas transport properties of asymmetric membranes created by the Langmuir-Blodgett deposition of ultra-thin polymeric lipid films on porous supports. Transmission and grazing angle FTIR spectroscopy provide a measure of the level of molecular order in the n-alkyl side-chains of the polymeric lipid. The level of orientational order was monitored as a function of the temperature. Gas permeation studies as a function of membrane temperature are correlated to the FTIR results. 

The rate at which a substance passes through a semipermeable membrane is determined by the diffu-sivity 0(cm /s) of the gas. D varies with the membrane temperature T K) according to the Arrhenius equation ... 

Thermal insulation effects by limiting the substrate and membrane temperature to prevent thermal damage and (3) Reduce permeation of corrosive fluid to the substrate, thus minimizing its corrosion rate. CRM linings, such as acid brick and monolithic cements, also prevent "wash", which is the removal of the membrane or substrate corrosion products by the circulating medium. Even when the fluid eventually reaches the membrane or substrate surface, the amount is relatively small, thus limiting chemical attack, and any corrosion products are trapped beneath the masonry shield. 

The PDMS and POMS membranes and process parameters were investigated using experimental studies for comparison (Sampranpiboon et al., 2000). The following operational conditions were applied for both membranes temperature, 303.15 K downstream pressure (permeate side), 0.3997 kPa membrane thickness, 10 pm. 

Figure 8. Radiation target analyses of thylakoids. High energy y-radiation was provided by a Co-source of a dose rate at the sample position of0.803 Mrad/h. The membrane temperature was maintained at -18 1 C. Initial PSII activity of the control Fv/Fm averaged 0.856 0.005.

FIGURE 21.44 Methanol permeation rate of the membranes as a function of thickness and at different temperatures (a, a, o, ) commercial samples N112, N115, and N117 (b, bj, , ) composite membranes. Temperature at 25°C, plots a and b (open symbols), and at 65°C, plots a and bl (full symbols). (Reprinted from J. Electroanal. Chem., 532(1-2), Dimitrova, R, Friedrich, K.A., Vogt, B., and Stimming, U., Transport properties of ionomer composite membranes for direct methanol fuel cells, 75-83, Copyright 2002, with permission from Elsevier.)... 

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