Track-Etch Method

An example of the track-etch membrane was given in Section 2.2 (Quinn et al. 1972). Booman and Delmastro (1974) have also described the layer deposition method to produce a microporous membrane by a track-etch method. 

The track-etch membrane can be used in reverse osmosis and electrodialysis separation processes where it consists of a thin metal layer with a thin layer of insulator material on each side. The membrane pore diameters were in the range 0.5-10 nm. 

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Pores with a very regular, linear shape can be produced by the track-etch method (Quinn et al. 1972). Here a thin layer of a material is bombarded with highly energetic particles from a radioactive source. The track left behind in the material is much more sensitive to an etchant in the direction of the track axis than perpendicular to it. So etching the material results in straight pores of uniform shape and size with pore diameters ranging between 6 nm and 1200 nm. To avoid overlap of pores only 2-5% of the surface can be occupied by the pores. This process has been applied on polymers (e.g. Nuclepore membranes) and on some inorganic systems like mica. Membranes so obtained are attractive as model systems for fundamental studies. 

The scope of applications of MF has been broadened in recent years by the introduction of inert membranes, particularly polypropylene, polycarbonate, and Teflon . In general, these materials cannot be made by the methods developed for the cellulosics because of their insolubility. Both Teflon and pol)propylene MF membranes have been made by a controlled stretching procedure in which microtears are introduced Microporous polycarbonate membranes have been prepared by a unique radiation-track-etch method A thin polycarbonate film is exposed to ionizing radiation which leaves labile sites that can later be chemically etched to produce straight-throi channels. The pore size can be controlled by the etching conditions. The pores in these membranes, contrary to those in cellulosic membranes, are quite uniform in diameter. 

Gingrich, J.E. and Fisher, J.C., 1976. Uranium exploration using the Track-Etch method. In Exploration for Uranium Ore Deposits. IAEA, Vienna, pp. 213-227. 

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique, which can also be used to make PVDF membranes. On the other hand, other hydrophobic microflltration membranes are made by the thermally induced phase separation technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned in the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold called the liquid entry pressure of water (LEPw). These membranes can therefore be used for membrane distillation. The track-etching method is applied to make microfiltration membranes from PC. 

The first procedure reported for the production of metal nanorods is called template synthesis.This method entails the preparation or deposition of the desired material within the cylindrical and monodisperse pores of a nanopore membrane. Martin and coworkers used polycarbonate filters, prepared by the track-etch method, and nanopore aluminas prepared electrochemically from Al foil, as template materials. This method allows the preparation of cylindrical nanostructures with monodisperse diameters and lengths, and depending on the nature of the membrane and the synthetic method used, these may be solid nanowires or hollow nanotubes. 

A number of companies (e.g, Nuclepore and Poretics) sell micro- and nanoporous polymeric filtration membranes that have been prepared via the track-etch" method [36]. As shown in Fig. 16.1 A and 16.IB, these membranes contain cylindrical pores of uniform diameter. The pores are randomly distributed across the mem-... 

Track-etching method. (Adapted from H. Stathmann ef a/. ... 

Several papers present reviews of measurement methods or improvements in existing methods. Yamashita et al. (1987) present the description of a portable liquid scintillation system that can be used for thoron (Rn-220) as well as radon (Rn-222) in water samples. Thoron measurements have not been made for houses where radon in water may be a significant source. Such an instrument could be useful in making such determinations as well as in studying geochemical problems as described in this report. A review of measurement methods by Shimo et al. (1987) and of development and calibration of track-etch detectors (Yonehara et al., 1987) are also included. Samuelsson... 

Measurements were made using two types of passive track-etch alpha dosimeters. One of them was the bare detector of CR-39. After exposure these dosimeters were etched by 30 % NaOH at 70°C for 5 hours. The number of pits was scored under a microscope with a television camera in Shiga University of Medical Science. Methods of calibration and adjustment for deposition of radon daughters introduced by Yonehara (Yonehara et al., 1986) were adopted. The second detectors were Terradex type SF (Alter and Price, 1972). These detectors consist of a plastic cup, covered by a filter to allow entry only of gases, with a track-etch detector inside. The reading of results was carried out by Terradex Corp. in Walnut Creek, California, U.S.A.. The measurements of radon concentration were carried out by both methods in each location, except for Hokkaido where the measurements were done only by Terradex. However, the data obtained by CR-39 detectors will be mainly presented in this paper, because the two methods did not give identical results as separately reported in this proceedings by Yonehara et al. (Yonehara et al., 1986). 

In order to assess the accuracy of the present method, we compared it with two other methods. One was the Track Etch detector manufactured by the Terradex Corp. (type SF). Simultaneous measurements with our detectors and the Terradex detectors in 207 locations were made over 10 months. The correlation coefficient between radon concentrations derived from these methods was 0.875, but the mean value by the Terradex method was about twice that by our detectors. The other method used was the passive integrated detector using activated charcoal which is in a canister (Iwata, 1986). After 24 hour exposure, the amount of radon absorbed in the charcoal was measured with Nal (Tl) scintillation counter. The method was calibrated with the grab sampling method using activated charcoal in the coolant and cross-calibrated with other methods. Measurements for comparison with the bare track detector were made in 57 indoor locations. The correlation coefficient between the results by the two methods was 0.323. In the case of comparisons in five locations where frequent measurements with the charcoal method were made or where the radon concentration was approximately constant, the correlation coefficient was 0.996 and mean value by the charcoal method was higher by only 12% than that by the present method. 

An alternate method to produce templated electrodes is the use of chemical reduction of the monomer in the presence of a track-etched or alumina membrane. Parthasarathy et al. [46] have produced enzyme-loaded nanotubules by a combination of both electrochemical and chemical deposition. Initially, the alumina membrane was sealed at one end with a thick Au film (Figure 1.9a), after which the membrane was placed into a mixture of pyrrole and Et4NBF4. The pyrrole was then electropolymerized to form a small plug of polypyrrole at the closed end of the alumina membrane (Figure 1.9b). Subsequently, the membrane was placed into a... 

Two well known processes have been used to prepare inorganic membranes with nearly straight pores anodic oxidation and track etching. The former method has been practiced commercially while the latter remains as a laboratory investigation. 

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

Figure 4 The 96-well permeability testing method. The support membrane is a 3-p.m track etched polycarbonate of 10-p.m thick the artificial membrane in hexadecane and the recommended incubation time is four to six hours. Source Courtesy of Millipore Corporation, Billerica, Massachusetts, U.S.A.

Method of manufacture phase inversion, sintering, stretching, or track etching... 

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