Permeability ceramic substrates

Permeable layer reactors consist of thin, porous metal or ceramic substrates onto which titanium dioxide is coated in a manner that allows for flowing fhrough the porous substrates. The flow can be either perpendicular to the surface of the porous media or alternatively may combine perpendicular and parallel vectors. An example of such a reactor was presented by Tsuru et al. (2006). Here, a titania membrane (pore sizes 2.5-22 nm) was prepared... 

Several researchers are investigating the feasibility of Pd-based membranes deposited onto highly permeable, porous metal and/or ceramic substrates. Fig. 10.7a [76,116-119]. Currently, the most common method used for the deposition of the active membrane layer onto the porous substrate is electroless plating, although some researchers have utilized physical and chemical vapor deposition techniques. 

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. 

Ceramic and semiconductor thin films have been prepared by a number of methods including chemical vapor deposition (CVD), spray-coating, and sol-gel techniques. In the present work, the sol-gel method was chosen to prepare uniform, thin films of titanium oxides on palladium Titanium oxide was chosen because of its versatility as a support material and also because the sol-gel synthesis of titania films has been clearly described by Takahashi and co-workers (22). The procedure utilized herein follows the work of Takahashi, but is modified to take advantage of the hydrogen permeability of the palladium substrate. Our objective was to develop a reliable procedure for the fabrication of thin titania films on palladium, and then to evaluate the performance of the resulting metalloceramic membranes for hydrogen transport and ethylene hydrogenation for comparison to the pure palladium membrane results. 

In selecting the most appropriate membrane, a much wider choice is available for polymeric than for either metallic or ceramic membranes. Polydi-methylsiloxane (PDMS) is by far the most commonly used, thanks to its high permeability and stability. Polyvinylalcohol (PVA) and Nation have also been described, especially for the more hydrophilic substrates. Operation of a polymer based MR at relatively... 

Glazes for ceramic bodies and porcelain enamels for metallic substrates are coatings that are applied to these surfaces with a variety of purposes chemical inertness, zero permeability to liquids and gases, cleanability, smoothness and resistance to abrasion and scratching, mechanical strength, and decorative and aesthetic considerations [12]. 

Clay nanocomposites are also being developed as barrier coatings for film and for containers. The nanocomposite is deposited on the film from a solution of PVOH/ EVOH copolymer in a mix of water and isopropyl alcohol which has been used in a supersonic dispersion system to nano-disperse 7 nm diameter silica and titanium dioxide particles. The ratio of polymer to silica depends on the barrier properties required. Typical microgravure equipment can be used to coat the solution onto a plastic substrate. The result reportedly is a transparent barrier coating which is superior to silica- and alumina-coated films, and is comparable to aluminum-coated materials. Oxygen permeability at a coating thickness of 2 pm is less than 1 cc/m d atm, and moisture permeation less than f g/m d. Costs are reported to be competitive with ceramic coatings [4]. 

In recent years there has been tremendous interest in porous ceramics because of their applications as filters, membranes, catalytic substrates, thermal insulation, gas-burner media and refractory materials. These are due to their superior properties, such as low bulk density, high permeability, high temperature stability, erosion/corrosion resistance and excellent catalytic activity. One branch of this field is porous SiC ceramics, owing to their low thermal expansion coefficient, high thermal conductivity and excellent mechanical properties. However, it is difficult to sinter SiC ceramics at moderate temperatures due to their covalent nature. In order to realize the low temperature fabrication of porous SiC ceramics, secondary phases may be added to bond SiC. Oxidation bonded porous SiC ceramics have been found to exhibit good thermal shock resistance owing to the microstructure with connected open pores. 

Novel polymer-ceramic nanocomposite membranes were fabricated, characterized, and tested for their barrier performance. Atomic layer deposition (ALD) was used to deposit alumina films on primary, micron-sized (16 and 60 pm) high-density polyethylene (HOPE) particles at a rate of 0.5 nm/cyde at 77 °C. Well-dispersed polymer-ceramic nanocomposites were obtained by extruding aluminapolymer particle size. The diffusion coefficient of fabricated nanocomposite membranes can be reduced to half with the inclusion of 7.29vol.% alumina flakes. However, a corresponding increase in permeability was also observed due to the voids formed at or near the interface of the polymer and alumina flakes during the extrusion process, as evidenced by electron microscopy [1]. 

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