Extruded cordierite honeycombs

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

Extruded Cordierite Honeycomb Ceramics for Environmental Applications... 

Extruded cordierite honeycombs also have applications in other fields because of their unique material and structural properties such as high porosity, low thermal expansion, high geometric surface area, and low gas flow restriction [2]. Utilizing their porous ceramic wall as filters, extruded honeycombs can be used as trap oxidizers to eliminate toxic particulate matter from diesel engine exhaust. 

This paper describes the properties and applications of extruded cordierite honeycombs. Euture tasks to upgrade performance of extruded cordierite honeycombs are also discussed. 

Table 13.1.2 shows the material properties of standard extruded cordierite honeycombs. Extruded cordierite honeycombs have an extremely low coefficient of thermal expansion from 40 to 800°C, hence, high thermal shock resistance is expected. The softening temperature is above 1410°C. Porosity is 35%, and mean pore diameter is 5 (jtm (typical value). 

TABLE 13.1.2 Material Properties of Extruded Cordierite Honeycombs... 

Figure 13.1.4 shows the thermal expansion of extruded cordierite from 40 to 800°C, in comparison with other ceramics. No material is equal to cordierite in thermal expansion properties. Hence, extruded cordierite honeycombs having extremely low thermal expansion, are widely used as automotive catalyst substrates, diesel particulate filters, and heat exchangers, where high thermal shock resistance is required. 

Due to durability requirements of catalysts for long-term vehicle use such as 120 000 miles or so, extruded cordierite honeycombs with low thermal... 

Figure 13.1.12 shows the electric furnace test results for thermal shock resistance of extruded cordierite honeycombs for automotive catalyst substrates. The withstanding temperature of the substrates is more than 700° C. 

Samples A to H were washcoated on leached cordierite honeycombs and sample I was extruded with 16% alumina binder. 

Figure 13.1.1 shows the manufacturing process flow chart for cordierite honeycomb. The raw materials for cordierite are kaolin, alumina, and talc, which are pulverized to suitable sizes for extrusion. These raw material powders are mixed with organic binders and water, then kneaded. The kneaded clay mixture is extruded through a die, dried and cut to specified length. These dried bodies are then fired at around 1400°C. 

Figure 13.1.3a shows the low thermal expansion scheme of extruded cordierite due to the orientation of the crystals. The c-axes of flat-shaped kaoli-nite crystals are oriented vertical to the honeycomb wall by the shearing force generated during extrusion through the narrow slit of the die. The c-axis of... 

Since exhaust gas temperature changes rapidly during engine operation, ceramic honeycomb substrates must have thermal shock resistance. Thermal shock resistance of ceramic honeycombs is determined by electric furnace or gas burner testing. Thermal shock resistance of ceramic material is generally represented by the following equation [8]. As the coefficient of thermal expansion of extruded cordierite is extremely low, high thermal shock resistance is expected. 

Cordierite [12182-53-5/, Mg4Al4Si5018, is a ceramic made from talc (25%), kaolin (65%), and A Oj (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and more recently for automotive exhaust catalyst substrates. In the latter, the cordierite raw materials are mixed as a wet paste, extruded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

Ceramic, The ceramic substrate is made from a mixture of silicon dioxide, talc, and kaolin to make the compound cordierite [12182-53-5]. Cordierite possesses a very low coefficient of thermal expansion and is thermal-shock resistant. The manufacturing process involves extruding the starting mixture (which is mixed with water and kneaded into a sort of dough) through a complex die to form the honeycomb structure. The extmded piece is dried and fired in a kiln to form the cordierite. The outside or circumferential dimension is formed by the die, and the length is cut later with a ceramic saw. 

Extrusion is also used to produce the alumina shells for sodium vapor lamps and the honeycomb-shaped catalyst supports for automotive emission-control devices (see Chapter 37). The catalyst supports are designed to give a high surface area and can consist of hundreds of open cells per square centimeter with wall thicknesses <100 pna. To produce these shapes, cordierite ceramic powder is mixed with a hydraulic-setting polyurethane resin. The mix is extruded into a water bath at a rate that matches the rate of cure of the polyurethane (about 2mm/s). It is then fired to produce the final ceramic. 

Honeycomb monoliths are obtained by extruding a paste made by catalytic material, whereas plate catalysts are made by depositing the catalytic material onto a stainless steel net or a perforated metal plate. Composite ceramic monolith catalysts, consisting of a monolith matrix made of cordierite coated with metal oxide SCR material, are also offered. However, they may suffer adhesion problems in the presence of dust and their use may be preferably limited to clean environment. Coated metal monoliths are constructed of thin metal foil and are characterized by large cell densities. In view of this, they are used exclusively in dust-free applications. 

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