Preparation of Dense Metallic Membranes

Based on the different compositions, structures and configurations, the dense metallic membranes can be prepared using various methods. As described earlier, the dense metallic hydrogen separation membrane can be 

It is obvious that the pure-metal membranes can be easily prepared by conventional metallurgical processes in the configurations of tube and disk, which have a thickness greater than 25 pm and can be used as unsupported H2 permeation membranes. Because of the two obvious weaknesses of embrittlement and slow surface kinetic, the preparation of these metal-based H2 permeation membranes mainly focuses on preparation of alloys in order to improve the embrittlement and modification of the surface for improving the surface kinetic. The preparation of an extra modified layer on these pure non-Pd metal membranes is the subject of the current discussion. 

Makrides et al (1967) patented a H2-extraction membrane where refractory metals, such as Ta, Nb and V, were coated with Pd to facilitate H2 ingress and egress and to prevent oxidation of the refractory metal surfaces. They started with commercially available foils. After being etched electrolytically in hydrofluoric acid and washed with acetone, the wet foils were placed in a vacuum chamber where they were further dried by evacuation. After 

The thick-wall unsupported Pd and Pd-based alloys can also be prepared by conventional metallurgical processes. Self-supporting dense Pd-based membranes possess wall thicknesses greater than 50-100 rm to keep a sufficient mechanical strength. These membranes are too thick to obtain a 

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The manufacture of dense metal membranes or thin films can be effected by a number of processes casting/rolling, vapor deposition by physical and chemical means, electroplating (or electroforming) and electroless plating. By far, casting in combination with rolling is the predominant preparation and fabrication technique. It is noted that many of these processes have been demonstrated with palladium and its alloys because of their low oxidation propensity. Preparation of dense metal membranes is summarized in some detail as follows. 

To conclude this section, it is necessary to state that Pd and Pd-based membranes are currently the membranes with the highest hydrogen permeability and selectivity. However, the cost, availability, their mechanical and thermal stabilities, poisoning, and carbon deposition problems have made the large-scale industrial application of these dense metal membranes difficult, even when prepared in a composite configuration [26,29,33-37],... 

This chapter focuses mainly on Pd-based MRs with respect to the gas permeation mechanism, membrane preparation, MR construction and operation, as well as applications in a variety of chemical reactions. In addition to a general description of Pd membranes and MRs, recent progress and critical issues in the dense metal membrane area will also be presented at the end of this chapter. 

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

Electroplating. Basically in electroplating, a substrate is coated with a metal or its alloy in a plating bath where the substrate is the cathode and the temperature is maintained constant Membranes from a few microns to a few millimeters thick can be deposited by carefully controlling the plating time, temperature, current density and the bath composition. Dense membranes made of palladium and its various alloys such as Pd-Cu have been prepared. Porous palladium-based membranes have also been made by deposition on porous support materials such as glass, ceramics, etc. 

Now considering dense membranes, attention will be focused only on ceramic membranes, since a detailed description of the preparation and properties of the interesting and promising metal membranes have been described in detail in the preceding chapter of this book. Data concerning the permeability of Ag and Pd-alloy membranes, though, are listed in Table 2 for comparison. 

A number of novel applications of zeolites depend on the ability to create thin, adhesive films on various substrates. While zeolite films or layers are commonly prepared on dense substrates such as silicon wafers, zeolite membranes are made on porous supports in order to permit permeation through the zeolite layer. Numerous synthetic studies have addressed the goal of obtaining adhesive layers of zeolites on various substrates such as noble and nonnoble metals, glass, ceramics, silicon, and even biological substrates such as cellulose fibers. For a more detailed discussion of zeolite membranes the reader is referred to the article by Julbe in this book. Pertinent reviews to this subject are given in the following references.[57,58]... 

Tong J, Suda H, Haraya K, Matsumura Y (2005) A novel method for the preparation of thin dense Pd membrane on macroporous stainless steel tube filter. J Memb Sci 260 10-18 Ryi SK, Park JS, Kim SH, Cho SH, Park JS, Kim DW (2006) Development of a new porous metal support of metallic dense membrane for hydrogen separation. J Memb Sci 279 439-445... 

Concerning the preparation of thin membranes directly on porous supports, a lower thickness limit seemingly exists for which a dense metal layer can be obtained. This thickness limit increases with increasing surfaee roughness and pore size in the support s top layer." " Clearly, this relation puts strong demands on the support quality in terms of narrow pore size distribution, and the amount of surface defects. Therefore both pore size and roughness of the support surface are often reduced by the application of meso-porous intermediate layers prior to deposition of the permselective metal layer. This procedure facilitates the preparation of thin defect-free membranes beeause it is relatively easier to cover small pores by filling them with metal. It is therefore conceivable that for a certain low Pd-alloy thickness and support pore size, the H2 flux becomes limited by the support resistance. ... 

Currently, the membranes incorporated in MMRs are mainly zeolite and Pd-based dense metal ones. Incorporation of these membranes in microreactors can be achieved using one of the preparation methods described in previous chapters. 

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