Pyrolyzed membranes

There are researchers involved in the study of flat sheet homogeneous membrane particularly focused on the development of entropic contributions to diffusiv-ity selectivity as the polymer matrix evolved to a rigid caibon matrix [22]. Polymer precursor membranes pyrolyzed at intermediate steps in the pyrolysis process and finally pyrolyzed membranes were tested for the purpose to study the development... 

Fig. 4.28 Plots of permeation rates at 120°C against kinetic diameter for pure gases. ( ) Asymmetric polyimide hollow fiber membrane, (o) carbon membrane pyrolyzed at TOC C, and (A) carbon membrane pyrolyzed at 850°C. (From [74])...

Fig. 6.9 WAXD patterns of the CMS membranes pyrolyzed at (a) 550°C and (b) 700°C as a fune-tion ofPVPeontent (From [6])...

Peterson et al. (1997) and Suda and Haraya (1995) have also prepared asymmetric capillary PMDA-ODA membranes by first coating a polytetrafluoroethylene (PTFE) tube with polyamic acid, then immersing the capillary in a water and/or ethanol bath, followed by drying and imidization. The PTFE capillary was removed and the asymmetric membranes pyrolyzed at 1223 K. The selectivities for these membranes approached or exceeded... 

Not only hydrocarbon systems, but also silicon rubbers (Lee 1986), can be pyrolyzed to obtain silicon-based membranes. Details of the pyrolysis are mainly reported for nonmembrane applications. A recent example is the paper of Boutique (1986) for the preparation of carbon fibers used in aeronautical or automobile constructions. 

A completely different approach was taken by Koresh and Soffer (1980, 1986, 1987). Their preparation procedure involves a polymeric system like polyacrylonitrile (PAN) in a certain configuration (e.g. hollow fiber). The system is then pyrolyzed in an inert atmosphere and a dense membrane is obtained. An oxidation treatment is then necessary to create an open pore structure. Depending on the oxidation treatment typical molecules can be adsorbed and transported through the system. 

In gas separation applications, polymeric hollow fibers (diameter X 100 fim) are used (e.g. PAN) with a dense skin. In the skin the micropores develop during pyrolyzation. This is also the case in the macroporous material but is not of great importance from gas permeability considerations. Depending on the pyrolysis temperature, the carbon membrane top layer (skin) may or may not be permeable for small molecules. Such a membrane system is activated by oxidation at temperatures of 400-450 C. The process parameters in this step determine the suitability of the asymmetric carbon membrane in a given application (Table 2.8). 

The hollow-fiber systems for gas separation or the tubular microfiltration systems have to be pyrolyzed before mounting in the membrane housing, because of the large shrinkage during pyrolysis. That is the most critical step in the fabrication of a separation system. 

Coming, diameter 1 mm, wall thickness about 0.2 mm) have been used to make pyrolyzed silicon-based membranes. Pyrolysis was carried out in a closed chamber in N2 or He atmosphere at temperatures between 600 and 800°C followed by an oxidation step in air at temperatures of 500-900°C. 

Koros, W.J. et al. (2002) Mixed matrix membranes with pyrolyzed carbon sieve particles and methods of making and... 

The carbon materials attract the increasing interest of membrane scientists because of their high selectivity and permeability, high hydrophobicity and stability in corrosive and high-temperature operations. Recently many papers were published considering last achievements in the field of carbon membranes for gas separation [2-5]. In particular, such membranes can be produced by pyrolyzing a polymeric precursor in a controlled condition. The one of most usable polymer for this goal is polyacrylonitrile (PAN) [6], Some types of carbon membranes were obtained as a thin film on a porous material by the carbonization of polymeric predecessors [7]. Publications about carbon membrane catalysts are not found up to now. 

Figure 4.15 Use of three molecules to determine the evolving pore size distributions of partially pyrolyzed polysilastyrene membranes [Grosgogeat et al., 1991]...

Other techniques utilize lasers for sample evaporation/pyrolysis and excitation such as laser induced desorption (LID) or laser microprobe mass analysis (LAMMA) (see e g. [1]). Some of the sample introduction procedures in Py-MS enhance the information obtained from Py-MS by the use of time-resolved, temperature-resolved, or modulated molecular beams techniques [10]. In time-resolved procedures, the signal of the MS is recorded in time, and the continuous formation of fragments can be recorded. Temperature-resolved Py-MS allows a separation and ionization of the sample from a platinum/rhodium filament inside the ionization chamber of the mass spectrometer based on a gradual temperature increase [11]. The technique can be used either for polymer or for additives analysis. Attempts to improve selectivity in Py-MS also were done by using a membrane interface between the pyrolyzer and MS [12]. 

E. Grosgogeat, J.R. Fried, R.G. Jenkins and S.T. Hwang, A method for the determination of the pore size distribution of molecular sieve materials and its application to the characterization of partially pyrolyzed polysilastyrene/porous glass composite membranes. /. Membr. Sci., 57 (1991) 237. 

Carbon, which has stability in aqueous as well as non-aqueous solutions, is also a candidate for porous inorganic membranes. At present, a few manufacturers have commercialized carbon UF membranes, the pore sizes of which are larger than 10 nm. No carbon nanofiltration membranes have been reported, while carbon gas separation membranes, which have pore sizes less than 1 nm, have been reported by several groups [7]. Nomura et al. [40] coated poly (vinylidene chloride) on a-alumina supports and pyrolyzed them at 825° C... 

Some applications require cheap electroconductive material based on small silicon particulates of high surface area. Authors of paper (Thakur et al. 2012) report on ultrasonically fiuctured macroporous membranes, mixing silicon particulates (size range 10-50 pm) with pyrolyzed polyacrylonitrile and application of the mixture for produeing long life anodes of lithium ion batteries. 

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