Porous doping dependence

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

Sensors based on the above reaction scheme have been developed for Al3+, Zn2+, Cu2+, Ca2+, Pb2+, Hg2"1", K+, Li+, etc. A polycation, protamine sensor has also been developed using 2/7/-dichlorofluorescein octadecyl ester (DCFOE) doped in polymer membranes. However, most of these sensors are pH dependent due to the pH dependence of the cation complexation reactions. The cation ion indicators can be immobilized on any solid support, such as silica, cellulose, ion-exchange resin, porous glass, sol-gel, or entrapped in polymer membranes. 

Classical characterization methods (gas sorption, TEM, SEM, FTIR, XPS and elemental analysis) were used to describe the resulting porous carbon structures. Temperature-dependent experiments have shown that all the various materials kept the nitrogen content almost unchanged up to 950 °C, while the thermal and oxidation stability was found to be significantly increased with N-doping as compared to all pure carbons. Last but not least, it should be emphasized that the whole material synthesis occurs in a remarkably energy and atom-efficient fashion from cheap and sustainable resources. 

Similarly, impervious yttria-stabilized zirconia membranes doped with titania have been prepared by the electrochemical vapor deposition method [Hazbun, 1988]. Zirconium, yttrium and titanium chlorides in vapor form react with oxygen on the heated surface of a porous support tube in a reaction chamber at 1,100 to 1,300 C under controlled conditions. Membranes with a thickness of 2 to 60 pm have been made this way. The dopant, titania, is added to increase electron How of the resultant membrane and can be tailored to achieve the desired balance between ionic and electronic conductivity. Brinkman and Burggraaf [1995] also used electrochemical vapor deposition to grow thin, dense layers of zirconia/yttria/terbia membranes on porous ceramic supports. Depending on the deposition temperature, the growth of the membrane layer is limited by the bulk electrochemical transport or pore diffusion. 

Fig. 4 [7, 8], At anodic potentials near J the electrode behavior is characterized by an exponential dependence of current on potential and by the uneven dissolution of silicon surface leading to the formation of porous silicon (PS) [9]. The values of the characteristic currents J to J4 are a function of electrolyte composition but are largely independent of doping. At potentials more positive than the second plateau current J4, current oscillation may occur [8]. The surface resulting from dissolution at potentials higher than the second peak is brightly smooth, while that produced between the first and second peak is relatively less smooth [10].

Most of the research was done with 0.2 micrometer rated porous polypropylene (Accurel ) membrane, and the concentration of polyacetylene in the composite could be varied from 4 to 43 percent. Larger percentages should be possible. The membranes did not lose their flexibility, and membrane properties such as flux rates and bubble point pressure were not altered (see Experimental Procedure 1). As is the case for polyacetylene alone, the conductivity of these membranes could be varied depending upon the type and amount of dopant. Iodine doped laminates were the most stable of the two doped laminates investigated in this study. 

With regard to porous materials, it should be noted that more or less restricted ionic conductivity is a general property that can vary significantly depending on doping, type and concentration of defects, and temperature. Interestingly, several porous materials, such as hydrated aluminosilicates, can behave as liquid electrolyte-like conductors, whereas such materials behave as solid ionic conductors when dry. 

No significant variation in pore diameter was observed in moderately doped SiC substrates with an increase in current density from 10 to 60 m A cm-2. However, the pore density increased by more than one order of magnitude. The lowest pore density of 3 x 107cm-2 was obtained at the anodization current density of 10 mA cm-2 (Figure 2.11). An increase of the current density to 20 mA cm-2 led to an increase of the pore density to 1 x 108 cm-2. A further doubling of the current density (up to 40 mA cm-2) formed a porous network with a pore density of 1.5 x 108 cm-2, and then, the pore density does not depend on current density significantly. 

The formation of porous Si critically depends on the type of conductivity and doping level of the Si electrode, H F concentration in the electrolyte, nature of solvent in which H F is diluted, and applied current density (orvoltage) [5,7,10,11,71,85-87]. 

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