The present invention relates generally to a method for fabricating ceramic articles from moldable powdered mixtures that are formed by mixing inorganic particulate raw materials with a binder system that includes a water-soluble organic binder and an inorganic binder, and water as a solvent. The method particularly relates to the manufacturing of cordierite articles having improved strength in a temperature region between 300° C.-900° C. to resist cracking and damage during sintering.
Popular cellular ceramic monoliths which are generally formed by extrusion, such as cordierite honeycomb substrates which find applications in catalytic converters, diesel particulate filters, electrically heated catalyst, and chemical processing catalyst, require binders and other similar aids for proper processing. Typically, the binder which is an organic material must meet a number of requirements.
For example, the binder must be compatible with the ceramic material such that a flowable dispersion comprising a relatively high loading of the ceramic material in the binder may be provided. Also, the “green” perform produced by shaping the dispersion of ceramic powder in the binder should have reasonable strength such that it can be handled.
For desirable burnout, the binder should be removable from the shaped ceramic part without incurring distortion or breakage of the part. Also, the binder-free perform should be strong enough to undergo defect-free consolidation. The formulations of binders meeting these requirements is complex and a large number of different binder formulations are known in the art.
Typically, water-soluble cellulose ether binders are used for cordierite-forming batches. These binders result in subsequently formed green bodies having good “wet” strength, as well as good integrity in size and shape. “Wet” strength is used to describe the strength of the body after extrusion but before drying. “Green” strength refers to the strength of the body after drying but before firing.
Cellulose ether binders which burnout in the temperature region between 100° C.-600° C., and more specifically around 300° C., are difficult to remove without incurring distortion or breakage of the ceramic part. Removal of organic components during firing involves a sequence of simultaneous reactions which are fairly complex, including, for example, oxidation, volatilization, and thermal degradation.
Therefore, the major obstacle in working with plasticized mixture including organic binders is that the subsequently formed green ceramic article cracks when fired, particularly in thin walled-honeycomb structures. The cracking is a result of internal stresses developed during the removal of large amounts of organics which causes excessive temperature or pressure gradients internal to the bodies.
Accordingly, special consideration must be undertaken during firing to avoid cracking of the ceramic body. Long and complicated firing cycles, specially designed kilns, and similar means have been used to control the burnout of organic binders and reduce the thermal stresses, differential shrinkage and high cracking frequency. These methods however, require expensive and sophisticated equipment and increase the cost of firing and manufacturing.
In light of the foregoing problems experience in the art, there remains a need for a method of fabricating ceramic articles, and in particular cordierite ceramic bodies having improved strength in the temperature region between 300° C.-900° C. to withstand the thermal stresses and shrinkage which form in the body during sintering from organic burnout, therefore, such articles being able to be fired with less cracks and defects, in a cost-effective and efficient manner.