Ceramic maximum operating temperature

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

Most fiber-matrix composites (FMCs) are named according to the type of matrix involved. Metal-matrix composites (MMCs), ceramic-matrix composites (CMCs), and polymer-matrix composites (PMCs) have completely different structures and completely different applications. Oftentimes the temperatnre at which the composite mnst operate dictates which type of matrix material is to be nsed. The maximum operating temperatures of the three types of FMCs are listed in Table 1.27. 

Metal monoliths have interesting properties, such as high tolerance for mechanical stress and vibrations and high thermal conductivity. Moreover, the cell walls may be thinner as compared to their ceramic counterparts. However, the maximum operating temperature of metal monoliths is not as high as for vanous ceramics. This is not a problem for certain combustor designs that limit the temperature in (part of) the catalyst... 

The DieMate manifold is insulated to improve thermal uniformity and to prevent exposure of the DieMate socket to excess temperature. (The DieMate socket maximum temperature specihcation is 150 °C.) Figure 12.4 shows the insulated DieMate manifold before attachment of the 1 /16-inch transfer lines. The insulation consists of a ceramic hber strip that has a maximum operating temperature of 2300 °F (1260° C). 

Several different materials have been studied. Metallic monoliths have been used extensively since their first application for automobile converters. They allow very thin walls and have a very high thermal conductivity. However, their thermal expansion gives rise to some problems when looking at the coating and stability of the washcoat on the metallic surface, compared with the ceramic monolith. Furthermore, their maximum operation temperature is limited to 1200-1400 C, cf. Table 1. Probably, the maximum temperature is somewhat lower for long-time exposure. However, several ceramic monoliths that can stand higher thermal conditions have been developed, as reported in Table 1. 

However, the maximum operating temperature is lower compared with some ceramic materials, cf. Table 1. Nevertheless, they can be used in combustor designs where the catalyst temperature is limited, like in the hybrid combustor described in Section 7.2.3. In that case, the catalyst temperature is limited to 900-1000°C. 

Table F.3. Maximum operating temperature (°C) of ceramics for handling liquid metals under inert atmosphere (A = Attacked)...

Ceramic membranes have been studied for esteriflcation/condensation reaction, where one integrates reaction and separation. Zeolite is seen as an alternative in different processes, and this is put directly into the reaction mixtures to perform the separation. The maximum operating temperature could be a few hundred degrees centigrade, this being a function of the membrane material. 

The carbon-fiber/polymer composites reviewed in the previous section have excellent mechanical properties but limited temperature resistance. Maximum operating temperature is presently 370°C (Table 9.2). These composites cannot meet the increasingly exacting requirements of many aerospace applications which call for a material with low density, excellent thermal-shock resistance, high strength, and with temperature resistance as high or higher than that of refractory metals or ceramics. These requirements are met by the so-called carbon-carbon materials. 

The maximum operating temperature of the insulation would have been approximately 1150 K (1610 °F). At this temperature, the most promising insulation materials for this application were ceramic or metallic foams. Figure 9-44 depicts Inconel 617 hollow sphere foam. 

Monolith substrate materials suitable for use in high-temperature combustion are either various ceramics or certain alloys. In Section II.C, the demands on a combustion catalyst were already summanzed. The substrate should have a high thermal shock resistance, but at the same time the melting temperature should be higher than the maximum anticipated operating temperature A number of interesting materials are summarized in Table 2, accompanied by thermal expansion coefficients and thermal conductivities. 

Temperature Limit Like any other ceramic material, many factors affect the maximum use temperature of high purity silica products. In general, 2000°F is the highest temperature limit for cyclic service. When the temperature goes above 2000°F, the vitreous/fused silica grains will crystallize to cristobalite and quartz. If the operating temperature is then cycled, the various silica inversions can take place which will tear the brick apart. When operation is restricted to continuous service only, then the maximum use temperature is approximately 3000°F. 

The maximum ionic conductivity in ZrO2-based systems is observed when the content of acceptor-type dopant cations with the smallest radii (Sc, Yb, Y) is close to the minimum necessary to completely stabilize the cubic fiuorite-type phase in the operating temperature range [9,11,16, 32-35]. This concentration (often referred to as the low stabilization limit) and the conductivity of the ceramic electrolytes are dependent, to a finite extent, on the pre-history and various micro structural features. In addition to the metastable states discussed above, critical microstructural factors... 

A major multinational company hires you as a consultant because of your knowledge of ceramics. You are asked to recommend a ceramic that will have the maximum possible creep resistance at an operating temperature of 1200°C. What material would you select and why Also consider how you would process it. 

Purchasable microreactors are limited in their temperature and pressure resistance, depending mainly on the reactor material used and the fabrication of the reactor. Most metallic microreactors operate at a maximum temperature of 500 °C, whereas ceramic microreactors offer temperature resistance up to 1100°C at ambient pressure and high chemical resistance. Metallic microreactors, in contrast, can withstand higher pressures generally. Further, it has to be considered whether the reactor material has any influence on the performance of the reaction, for instance unwanted catalytic activity. Normally most metallic microreactors can be provided in several materials. 

In summary, the braze material undergoes large deformations at both room and operating temperatures and the interlayer provides a mechanism for ensuring that the largest maximum principal stress occurs outside of the ceramic component at room temperature. Also, some stress relaxation occurs in the structure when heated from room to operating temperature, permanent deformations notwithstanding. 

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