Cordierite, properties

The stmcture of cordierite-like materials is based on a frame of font- and six-member rings composed of aluminum-silicon-oxygen tetrahedral. Mg cations are localized in octahedrons between tetrahedrons of six- and four-member rings. A frame stmcture allows to obtain modified materials with 3d cations instead of Mg having similar ionic radiuses, for example, Mn, Co Ni, Cu, Zn [4]. The presence of transition elements is of interest because these elements may strongly influence on the cordierite properties and catalytic activity. 

While the discovery of the catalytic properties of zeolites was driven by the desire to improve industrial prcKessing, the development of emission control catalysts was necessitated by governmental fiat. The first requirement was for 90+% removal of CO and of hydrocarbons, a goal which could not be met by oxidation with base metal oxides. To achieve the required spedfications during automobile operations, it was necessary to develop supported platinum catalysts. Originally the support was alumina in pellet form. Later platinum on cordierite was used in honeycomb form, containing 200-400 square channels per square inch. 

The first composite system is a barium osumilite composition reinforced with 30 wt% (25 vol%) No. 1 whiskers. The second composite system evaluated is a barium-stuffed cordierite matrix 30 wt% No. 1 whisker composite. The room-temperature properties of these two composites are given in Table 3.16. 

Table 3.16 Properties of Ba-osumilite and Ba-stuffed cordierite matrix composites containing 30 wt% No. 1 whiskers10...

Property Ba-osumilite matrix Ba-stuffed cordierite matrix... 

Variation of properties of Ba-stuffed cordierite matrix composite with temperature (reproduced by permission of Am. Ceram. Soc. Bull.) . 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

Table 2.5 Summary of elevated temperature mechanical properties for zirconia, cordierite, and spinel matrix composites...

Cordierite. This material possesses a rare combination of properties, i.e., relatively high strength and hardness, low thermal expansion coefficient, high heat conductivity and plasticity it conserves mechanical strength under thermal shocks. Therefore, its application areas are varied. 

The initial substrates in 1975, with square cell configuration, were designed to have 200 cells/in. with a wall thickness of 0.012 in. They were extruded from cordierite composition (EX-20), which had a fired wall porosity of 35%. Similarly, the triangular cell substrates had a cell density of 236 cells/in. and a wall thickness of 0.0115 in. They were extruded from a lowcr-CTE cordierite composition (EX-32), with a fired wall porosity of 40%. The geometric properties of these two substrates, as well as current versions of square cell substrates with cell density of 3(X) and 4(X) cells/in. are summarized in Table 1. 

Table 12 Nominal Properties of Standard and Thin-Wall Cordierite Substrates...

Different applications generally require different filter properties (temperature resistance, pressure drop, filtration efficiency, etc.), and therefore a number of different types of filters, based on either ceramic (SiC, mullite, cordierite, etc.) or metallic (stainless steel, Hastelloy, Fecralloy, etc.) materials, are currently produced and commercialized. Table 1 summarizes such filter types, their producers, and their major application fields. 

Table 4 Properties of Extruded Cordierite Diesel Filters with 100/17 Cell Structure and Checkerboard Plugging Pattern...

The main representatives of this group are steatite, cordierite and forsterite ceramics. The prevailing crystalline phases in these materials are the minerals enstatite, cordierite and forsterite, which are also responsible for the main properties. All the materials find use in electrical and electronic applications where their insulating ability, as well as further specific properties are exploited. These are ... 

Ceramics with a wide sintering interval include, for instance, porcelain for which temperature variations within limits of about 20 ""C have little effect on the final properties. In contrast, for example, with cordierite ceramics a change in temperature of several degrees may damage the ware seriously as a result of excessive melt formation. 

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