Ceramic extruders

In his field-oriented contribution entitled Wear protection for augers in ceramic extruders - state of the art , Walter Reisinger describes the most important kinds of protective layers that are presently in... 

The relevant developmental situation is investigated in Holger Wam-pers contribution Perspectives for wear reduction with ceramic extruder components . 

Have recent years seen any real progress in the further development of ceramic extruders Undoubtedly. 

Designing ceramic machinery, especial ceramic extruders, requires knowledge of certain process parameters such as throughput, pressure, force and power requirement. Everyday practice shows that such quantities are highly dependent on the rheological properties and, hence, the material parameters of the compounds. 

Perspectives for Wear Reduction with Ceramic Extruder Components... 

At this point of our discussion with e, p, q, T, v we still have fourteen unknown variables left (keeping in mind that T is a symmetric tensor). On the other hand only five equations to describe the flow field are available. To allow the solution of these equations, namely the Navier-Stokes equations, we have to state some additional equations which describe the specific material properties of the ceramic extrudate. 

FIGURE 37.10 Looking through two ceramic extruded cordierite honeycomb substrates for catalytic converters. 

For clay and ceramics extruders or pug mills clay, materials for bricks, tiles and ceramics. 

The catalyst is normally contained on a ceramic substrate. These ceramics are extruded in a malleable state and then fired in ovens. The process consists of... 

As with mineral wools, there are different types of ceramic fiber, but they are all made from a combination of alumina, silica and china clay and may be made by blowing or extruding the liquid melt. 

The process has been commercially implemented in Japan since 1977 [1] and a decade later in the U.S., Germany and Austria. The catalysts are based on a support material (titanium oxide in the anatase form), the active components (oxides of vanadium, tungsten and, in some cases, of molybdenum) and modifiers, dopants and additives to improve the performance, especially stability. The catalyst is then deposited over a structured support based on a ceramic or metallic honeycomb and plate-type structure on which a washcoat is then deposited. The honeycomb form usually is an extruded ceramic with the catalyst either incorporated throughout the stmcture (homogeneous) or coated on the substrate. In the plate geometry, the support material is generally coated with the catalyst. 

The domestic use pattern for lead in 1990 was as follows lead-acid storage batteries, used for motor vehicles, motive power, and emergency back-up power, accounted for 80% of total lead consumption ammunition, bearing metals, brass and bronze, cable covering, extruded products, sheet lead, and solder, represented 12.4% the remaining 7.6% was used for ceramics, type metal, ballast or weights, tubes or containers, oxides, and gasoline additives (USDOC 1992). 

A great potential for new compounds is provided by structures with two carbon and two silicon atoms around the central silicon. These polysilanes with organic groups lead to silicon-carbide ceramics. A wide field of application would be opened up if one could make a polysilane as a plastic mass which could be extruded and modeled and if after pyrolysis silicon-carbide is formed without a strong contraction (this means a high ceramic yield). Polysilane fibers are only one product in a range of many... 

The catalyst is normally contained on a ceramic substrate. These ceramics are extruded in a malleable state and then fired in ovens. The process consists of starting with a ceramic and depositing an aluminum oxide coating. The aluminum oxide makes the ceramic, which is fairly smooth, have a number of bumps. On those bumps a noble metal catalyst, such as platinum, palladium, or rubidium, is deposited. The active site, wherever the noble metal is deposited, is where the conversion will actually take place. An alternate to the ceramic substrate is a metallic substrate. In this process, the aluminum oxide is deposited on the metallic substrate to give the wavy contour. The precious metal is then deposited onto the aluminum oxide. Both forms of catalyst are called monoliths. 

Zeolite catalysts and adsorbents have also been incorporated into monolithic contactors by several routes, including extruded zeoHte/binder composites [70], wash-coated ceramic monoliths [71] and corrugated thin-sheet monoliths [72]. 

Although extruded ceramic honeycombs were extensively investigated in the earlier stages of development [6, 9], their use has been progressively abandoned, mainly due to inadequate resistance against thermal shocks. Most ceramics will fracture during the sudden temperature drop associated with fuel cut-off during turbine trips. 

Figure 7-16 A highly simplified sketch of an automohile engine and catalytic converter with typical gas compositions indicated before and after the automotive catalytic converter. The catalytic converter is a tube wall reactor in which a noble-metal-impregnated wash coat on an extruded ceramic monolith creates surface on which reactions occur.

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

Let us consider two hypothetical phases in our composite, A and B, without specifying their physical state. They conld be a polymer melt and a glass fiber reinforcement during melt infiltration processing, a metal powder and ceramic powder that are being snbjected to consolidation at elevated temperatnre and pressure, or two immiscible polymer melts that will be co-extruded and solidified into a two-phase, three-dimensional object. In any case, the surface that forms between the two phases is designated AB, and their individual surfaces that are exposed to their own vapor, air, or inert gas (we make no distinction here) are labeled either A or B. The following three processes are defined as these surfaces interact and form ... 

Materials which are usually extruded include some soft metals (lead, dn, copper, brass, aluminum, magnesium and various alloys) rubber, soaps, ceramics, foods, plastics, Dynamites, Explosives (like Amatol... 

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