Study On Cordierite Honeycomb Ceramics

Cordierite honeycomb ceramics have excellent thermal-shock resistance, adhering ability, thermal resistance and good mechanical strength. These properties make cordierite honeycomb ceramics an ideal material for many applications, such as substrates for automobile exhaust catalysts and filters for diesel particulate. Because of the important role that cordierite honeycomb ceramics plays in the protection of environment, especially in controlling air pollution, it has become one of the main subjects on environment-engineering materials. As the applications of cordierite ceramics are continuously expanding, better understanding of the basic aspects related to cordierite ceramics and further enhancing of its properties are widely pursued. The paper first studied experimentally on several fundamental topics of cordierite ceramics, including the effects of chemical compositions, impurities, talc-particles size and firing technology on its properties. Emphasis was given on the investigation of the correlation between the orientation of cordierite crystals and the related technology. The results revealed that the processing route of cordierite honeycomb ceramics played a crucial role in determining whether its CTE could reach a super low value. The experiment results showed that the CTE of extruded cordierite honeycomb bodies prepared directly with raw materials of Kaolin, talc and alumina can be as low as 1.2×10-6℃-1, far lower than the theoretical CTE of the common cordierite ceramics. However, the CTEs of all the honeycomb ceramics prepared with cordierite powder that were pre-synthesized with Kaolin, talc and alumina, were higher than 1.60×10-6℃-1. Analysis based on XRD patterns revealed that the basic difference of the two preparation method was that the one-step firing could lead to the orientation of cordierite crystals, while there was no preferred orientation in the samples prepared with the two-step firing. The effects of the talc particle size were investigated. In the present experimental range of 4-7μm,the increase of particle size could lead to a lower CTE and higher porosity. This was related to the fact that larger talc particles were preferable for the orientation of cordierite crystals during the forming. In addition, the CTE was also affected by chemical compositions and impurities. The experimental results showed that the cordierite ceramics containing slightly richer Al2O3 and MgO had lower CTE than stoichiometric cordierite ceramics. Moreover,the Al2O3-rich samples also exhibited better refractoriness. CaO not only affected the coefficient of thermal expansion (CTE) seriously, but also impaired the apparent porosity and the water absorption. On the other hand,Fe2O3 displayed different effects. As the content of Fe2O3 is in the range of 0.3% to 0.55wt%, it did not appreciably affect the CTE. However,when Fe2O3 content was higher than 0.55wt%, the CTE rose sharply with increasing of Fe2O3. The CTEs of the cordierite samples prepared by one-step and two-step were 1.32×10-6℃-1 and 1.84×10-6℃-1, respectively. The results of firing experiments showed that 1410℃is the optimal firing-temperature,at which the CTE of the samples fired was significantly lower than that of the sample fired at any other temperature. The effect of chemical treatments on the properties of cordierite was systematically studied. The mechanisms of chemical treatments affecting properties were also discussed. In order to overcome the drawbacks of CTE rebounding observed in conventional acid-treatment samples, a novel technique,the acid-alkali combination treatment,was developed. The key feature of this approach was that it not only lowered the CTE markedly, but also effectively depressed the CTE rebounding resulted from heat treatment. Inductively coupled plasma (ICP) analysis and XRD results showed that the reaction of acid with the cordierite ceramics was of selectivity. Acid preferentially dissolved Al3+ and Mg2+ ions. "Si", in the form of amorphous SiO2, seemed to be more "anti-acid"and remained in the acid-treated cordierite structures. Furthermore, such impurities in cordierite ceramics as Na+, K+ and Ca2+ were also dissolved. The ensuing alkali treatment mainly dissolved the "excessive"SiO2 left by the acid-treatment, which was considered to be responsible for the CTE rebounding of the acid-treated samples. Further study on the microstructures of the acid-treated samples showed that the apparent porosity was increased and pores became larger after the acid-treatment. Cracks could be observedon the treated samples. These changes in composition and microstructure caused by the acid-treatment were all favorable for lowering the CTE. The study on wall-flow cordierite ceramics was focused on the forming of pores and the control of porosity. In addition, the influence of firing schedule on its properties was studied. The results showed that there was remarkable difference in pore-forming ability between the organic pore-forming agent (starch) and the inorganic pore-forming agent (graphite). Experiments showed that a desired high porosity,say over 50%,could not be easily obtained with any one of the two pore-forming agents if added separately. A new pore-forming method was introduced, by which starch and graphite were added simultaneously as pore-forming,resulting in a desired porosity. It was also found that porosity had a strong influence on CTE. As apparent porosity was in the range of less than 40%, increasing of porosity resulted in a decreasing CTE. The results of firing experiment showed that firing schedule had a marked influence on the CTE, while it hardly affected porosity.