Porous metals

Several types of aggregate-bed filters are available which provide in-depth filtration. Both gravel and particle-bed filters have been developed for removal of dry particulates but have not been used extensively. Filters have also been developed using a porous ceramic or porous metal filter surface. 

Porous metal stmctures can also be created by spraying molten metal onto a base. Porosity is controlled by spraying conditions or by an additive that may be removed later. 

Porous Metals Guidebook, Metal Powder Industries Eederation, Princeton, N.J., 1980. 

Porous metal oxide deposits also permit the development of high boiler water concentrations. Water flows into the deposit and heat appHed to the tube causes the water to evaporate, leaving a concentrated solution. Again, corrosion may occur. Caustic attack creates irregular patterns, often referred to as gouges. Deposition may or may not be found in the affected area. 

Table 7. Operating Limits for Porous Metal Bearings...

Porous metal Nominal composition, wt % Foadlimit, P, MN./m " Static Dynamic Speed limit, m/ s P limit MN/(m-s)... 

As a generahty, porous metal sleeve bearings tolerate Pp levels up to 1.8 MN/(m-s) (50, 000 psift/min). Pp levels for thmst bearings should not exceed about 20% of the sleeve bearing limit. Variations of oil viscosity, oil content, graphite content, and other material and property details also influence the approximate operating limits given in Table 7. 

As with porous metals, service limits for nonmetallic bearings commonly include a Pp load-speed limit as a measure of maximum tolerable surface ... 

V. T. Morgan, Porous Metal Bearings and Their Application, MEP-213, Mechanical Engineering Pubhcations, Ltd., Workiagton, UK, 1984. 

S has been approximated for flames stabili2ed by a steady uniform flow of unbumed gas from porous metal diaphragms or other flow straighteners. However, in practice, S is usually determined less directly from the speed and area of transient flames in tubes, closed vessels, soap bubbles blown with the mixture, and, most commonly, from the shape of steady Bunsen burner flames. The observed speed of a transient flame usually differs markedly from S. For example, it can be calculated that a flame spreads from a central ignition point in an unconfined explosive mixture such as a soap bubble at a speed of (p /in which the density ratio across the flame is typically 5—10. Usually, the expansion of the burning gas imparts a considerable velocity to the unbumed mixture, and the observed speed will be the sum of this velocity and S. ... 

Subperiosteal. The subperiosteal implants are placed on the residual bony ridge and are not osseointegrated. This implant is most commonly used in the mandible but sometimes is used in the maxilla. Subperiosteal implants have been installed since the 1940s (311) and still have a success rate after five years of only 50 to 60%. A success rate of over 90% for five years and 50% for 15 years also has been quoted (312). Subperiosteal implants are fitted by casting, which is an individual procedure. The casting can be coated with a porous metal coating or other coating and then put in the patient. This may result in an improvement for these implants. 

Other uses include impregnation of wood to improve dimensional stability and reduce water absorption, sealing of porous metal castings by impregnation, and coil impregnation, to give a rigid structure both heat and water resistant. 

Metal A filter constructed from metal mesh, fibers, or sintered porous metal. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

These replaceable cartridges or packs are the most commonly used however, there are cartridges of wire mesh, sintered or porous metal which can be removed, cleaned, and replaced. Usually, the fine pores of the metal become progressively plugged and the cartridges lose capacity. They are often used for filtering hot fluids, or polymers with suspended particles, pharmaceuticals, and foods (liquids). In the case of polymers and other applications a special solvent and blow-back cleaning system may be employed. 

With some types of particles the porous metal tends to plug, but they can usually be backwashed or washed with a solvent or acid/alkali to remove the particles from within the metal pores. This is one reason why manufacturer s testing or plant testing can be important to the proper selection. Once the internal plug-gage has reached a point of reduction in flow-through capacity, it must be discarded. The actual cost of this type of cartridge is several times that of the non-metal-... 

Filter elements may be of the 5-micron, woven mesh, micronic, porous metal, or magnetic type. The micronic and 5-micron elements have non-cleanable filter media and are disposed of when they are removed. Porous metal, woven mesh and magnetic filter elements are usually designed to be cleaned and reused. 

Most of the electrocatalysts we will discuss in this book are in the form of porous metal films deposited on solid electrolytes. The same film will be also used as a catalyst by cofeeding reactants (e.g. C2H4 plus 02) over it. This idea of using the same conductive film as a catalyst and simultaneously as an electrocatalyst led to the discovery of the phenomenon of electrochemical promotion. 

Since electrochemical promotion (NEMCA) studies involve the use of porous metal films which act simultaneously both as a normal catalyst and as a working electrode, it is important to characterize these catalyst-electrodes both from a catalytic and from an electrocatalytic viewpoint. In the former case one would like to know the gas-exposed catalyst surface area A0 (in m2 or in metal mols, for which we use the symbol NG throughout this book) and the value, r0, of the catalytic rate, r, under open-circuit conditions. 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

An important question frequently raised in electrochemical promotion studies is the following How thick can a porous metal-electrode deposited on a solid electrolyte be in order to maintain the electrochemical promotion (NEMCA) effect The same type of analysis is applicable regarding the size of nanoparticle catalysts supported on commercial supports such as Zr02, Ti02, YSZ, Ce02 and doped Zr02 or Ti02. What is the maximum allowable size of supported metal catalyst nanoparticles in order for the above NEMCA-type metal-support interaction mechanism to be fully operative ... 

We consider the porous metal catalyst film shown in Figure 11.12 which is interfaced with an O2" conductor. When a positive current, I, is applied between the catalyst and a counter electrode, oxide ions O2 are supplied from the solid electrolyte to the three phase boundaries (tpb) solid electrolyte-metal-gas at a rate I/2F. Some of these O2 will form 02 at the tpb and desorb ... 

They first reported and studied permanent NEMCA and via cyclic voltammetry established the dependence of metal/solid electrolyte capacitance on porous metal film mass, which confirms the O2" backspillover promoting mechanism. 

Barendrecht et al. show that the charged polymer films are similar to porous metal... 

Besides lateral (parallel) growth of MFI layers onto a porous metal support, also axial growth of MFI crystals onto (dense) metal supports is possible leading to interesting new composite catalysts. 

The field of surface-mediated synthesis of metal carbonyl clusters has developed briskly in recent years [4-6], although many organometallic chemists still seem to be unfamiliar with the methods or consider themselves ill-equipped to carry them out. In a typical synthesis, a metal salt or an organometallic precursor is brought from solution or the gas phase onto a high-area porous metal oxide, and then gas-phase reactants are brought in contact with the sample to cause conversion of the surface species into the desired products. In these syntheses, characteristics such as the acid-base properties of the support influence fhe chemisfry, much as a solvenf or coreactant influences fhe chemisfry in a convenfional synfhesis. An advanfage of... 

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