Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2003-04-01
2004-02-10
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S125000, C501S127000, C264S630000
Reexamination Certificate
active
06689707
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to calcium aluminate compositions, articles made from these compositions and methods of making such articles. More particularly, the invention relates to low thermal expansion calcium aluminate compositions and articles and methods of making low expansion articles.
BACKGROUND OF THE INVENTION
Low thermal expansion ceramic bodies are desirable in a wide variety of applications. For example, low expansion bodies have been used as filters for fluids, in particular, as diesel particulate filters and as substrates for catalytic converters, an example of which is known in the art as a honeycomb substrate. Additionally, low thermal expansion bodies are desirable in applications where the thermal shock resistance and the ultimate use temperature are high. Substrates used under conditions of high thermal gradients are examples of this application. For example, structures such as honeycomb and cellular substrates are subjected to harsh environments, which require high thermal shock resistance, low thermal expansion, and high mechanical shock properties. Maintaining these properties for extended periods of time in their intended environments eliminates many potentially useful refractory materials.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to high thermal shock resistance of cordierite ceramics. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion. That is, honeycombs with a low thermal expansion have a good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application. Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to optimize the thermal shock resistance and other properties of the cordierite substrates.
Another property of certain types of catalysts is the ability to purify exhaust gases and the ability to convert carbon monoxide, hydrocarbons and nitrogen oxides (NO
x
) produced during engine operation into less environmentally harmful gases. Some catalyst systems utilize alkali metals contained on the catalyst support store nitrogen oxides, and such catalysts are referred to in the art as NO
x
adsorbers. A disadvantage of presently available catalyst and purification systems is that most of the alkali metals contained on the catalyst support for storing NO
x
readily react with cordierite within the temperature range of interest for NO
x
adsorber use. For example, potassium, a widely used alkali adsorber material, appears to readily react with cordierite, which draws the potassium out of the high surface area washcoat and prevents it from performing its adsorber function. Furthermore, the potassium reacts with cordierite to form relatively high coefficient of thermal expansion (CTE) phases that make the substrate and the catalyst system much less thermal shock resistant.
There is a need to provide alternative low CTE materials that are useful in high temperature applications. It would be desirable to provide materials that have a low CTE and excellent thermal shock resistance.
SUMMARY OF THE INVENTION
One embodiment of the invention relates to a ceramic article and methods of forming a ceramic article consisting essentially of a main phase of CaAl
4
O
7
and an additive which forms binary eutectic or lower melting component with the main phase when fired to a temperature of less than about 1600° C. and exhibiting a CTE of less than about 25×10
−7
/° C. over the temperature range of about 25° C. to 800° C. Other embodiments relate to a calcium aluminate article and a method of forming the article consisting essentially of a main phase of CA2 and an additive, the article having a thermal expansion less than about 20×10
−7
/° C. over the temperature range 25° C. to 800° C., preferably less than about 15×10
−7
/° C. over the temperature range 25° C. to 800° C. In certain embodiments, the articles contain a network of microcracks and include grains having a median grain size of between about 10 microns and 100 microns. The articles of the present invention can be used in a wide variety of applications, including but not limited to manufacturing honeycomb substrates for high temperature applications and honeycomb substrates for NO
x
adsorption applications.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.
REFERENCES:
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patent: 3969542 (1976-07-01), Tomita et al.
patent: 4204878 (1980-05-01), Nudelman et al.
patent: 4960737 (1990-10-01), Guile et al.
patent: 5374392 (1994-12-01), Skelcey et al.
patent: 2-160661 (1990-06-01), None
S. Jonas et al..,A New Non-silicate Refractory of Low Thermal Expansion, Ceramics International 24, 211-216 (1998), no month.
S. Jonas et al.,Low Thermal Expansion Refractory Composites Based on CaAl4O7, Ceramics International 25, 77-84 (1999), no month.
Y. Suzuki et al.,In Situ Synthesis and Microstructure of Porous CaAl4O7/CaZrO3Composite, J. Ceram. Soc. Japan 109, 205-209 (2001), no month.
R. D. Shannon et al.,Effective Ionic Radii in Oxides and Fluorides*, Acta Cryst. 925-946 (1969), no month.
Beall Douglas M.
Lakhwani Shahid G.
Pinckney Linda R.
Corning Incorporated
Gheorghiu Anca C.
Group Karl
Servilla Scott
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