Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2001-10-01
2003-05-20
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S080000, C055S523000, C055SDIG003, C264S630000
Reexamination Certificate
active
06566290
ABSTRACT:
BACKGROUND OF THE INVENTION
The instant invention relates to ceramic bodies or structures having compositions within the Li
2
O—Al
2
O
3
—SiO
2
(lithium aluminosilicate) system. Specifically, the present invention relates to lithium aluminosilicate ceramics having a low coefficient of thermal expansion (CTE), high heat capacity, high refractoriness, and high thermal shock resistance.
In the industry cordierite (2MgO—2Al
2
O
3
—5SiO
2
) has been the cost-effective material of choice for high temperature filtering applications, such as flow-through and wall-flow filters, due to its combination of good thermal shock resistance, filtration efficiency, and durability under most operating conditions.
However, under certain circumstances cordierite filters are susceptible to damage and have even catastrophically failed.
A need therefore exists for a ceramic suitable for high temperature filtering applications without the shortfalls of cordierite.
The present invention provides such a ceramic and a method of fabricating the same.
SUMMARY OF THE INVENTION
The instant invention is founded upon the discovery of a predominately two-phase ceramic within the Li
2
O—Al
2
O
3
—SiO
2
system which has high refractoriness, high resistance to thermal shock, and high heat capacity properties which make the inventive ceramic extremely desirable in high temperature applications, such as filters for diesel exhaust engines.
Specifically the invention is a ceramic article which consists essentially, by weight on the oxide basis, of 10-25% SiO
2
, 65-85% Al
2
O
3
, and 2-12% Li
2
O and is composed of a first phase having anisotropic thermal expansion behavior (widely differing expansions along the crystallographic axes) with an average coefficient of thermal expansion from room temperature to 1000° C. of −5×10
−7
/° C. and being less than 50% by weight of the ceramic article, and a second phase having a higher melting point than the melting point of the first phase. The melting point of the second phase is preferably at least 1800° C.
The inventive ceramic structures have 32 to 50 weight % of beta-eucryptite (LiAlSiO
4
) as a first phase having a melting point T
m1
, and 50 to 68 weight % of a second phase having a positive component in thermal expansion which is higher than the component in thermal expansion of the first phase and a melting point T
m2
, wherein T
m2
>T
m1
. The second phase is selected from the group consisting of lithium aluminate spinel (LiAl
5
O
8
), lithium aluminate (LiAlO
2
), corundum (Al
2
O
3
), and combinations thereof.
The inventive ceramic structures exhibit a coefficient of thermal expansion (CTE) from room temperature to 800° C. of −30×10
−7
/° C. to +30×10
−7
/° C., preferably −20×10
−7
/° C. to +10×10
−7
/° C.; a permeability of at least 0.5×10
−12
m
2
, preferably 1.0×10
−12
m
2
to 5.0×10
−12
m
2
; a total porosity of 35-65%, preferably 45-55%; a median pore size of 8-25 micrometers, preferably 15-20 micrometers; and, a high refractoriness at temperatures of between 1550° C. to 1650° C.
The inventive ceramic structures are suitable in high temperature applications such as filters for diesel exhaust and automotive catalytic converters. In particular the inventive structure is especially suitable as a honeycomb diesel particular filter having an inlet end and an outlet end and a multiplicity of cells extending from the inlet end to the outlet end, the cells having porous walls, wherein part of the total number of cells at the inlet end are plugged along a portion of their lengths, and the remaining part of cells that are open at the inlet end are plugged at the outlet end along a portion of their lengths, so that an engine exhaust stream passing through the cells of the honeycomb from the inlet end to the outlet end flows into the open cells, through the cell walls, and out of the structure through the open cells at the outlet end.
The invention is also a method of making the ceramic article. A mixture of lithium carbonate, alumina, clay and/or sand, solvent, optionally binders, lubricants and plasticizers are formed into a plasticized batch, shaped into a green body, optionally dried, and fired at temperatures of 1300° C.-1400° C. and for a time sufficient to form the product structure.
DETAILED DESCRIPTION OF INVENTION
The invention is a ceramic which is largely biphasic, having as a first phase a low CTE phase and as a second phase a high melting temperature phase (the high temperature phase may include more than a single phase as further described herein below). This unique phase duality renders the inventive structure, highly refractory with a near-zero CTE, thus making it suitable for high temperature applications such as filtering of particulate matter from diesel exhaust streams.
The inventive composition area lies within the Li
2
O—Al
2
O
3
—SiO
2
(LAS) system and consists essentially, by weight on the oxide basis, of about 10-25 SiO
2
, 65-85 Al
2
O
3
, and 2-20 Li
2
O. The preferred compositional area consists essentially, by weight on the oxide basis, of about 13-20 SiO
2
, 70-80 Al
2
O
3
, and 3.5-10 Li
2
O. Minor amounts of other refractory oxides, such as ZrO
2
, Cr
2
O
3
, V
2
O
3
, and Ta
2
O
5
may optionally be present.
In a preferred embodiment the inventive structure includes 32 to 50% by weight a first phase of beta-eucryptite having a melting point T
m1
, and 50 to 68% by weight a second phase having a positive component in thermal expansion which is higher than the component in thermal expansion of the first phase and a melting point T
m2
, wherein T
m2
>T
m1
.
The low CTE phase is beta-eucryptite (LiAlSiO
4
) which has an average CTE from room temperature to 1000° C. of about −5×10
−7
/° C., and with a highly anisotropic CTE (i.e., widely differing expansions along the crystallographic axes) at the a-axis of about +80×10
−7
/° C. and at the c-axis of about −170×10
−7
/° C.
However, beta-eucryptite also has a low melting point of about 1410° C. Therefore, the amount of beta-eucryptite in the final body is less than about 50 percent by weight, and more preferably between about 32 to 45 weight percent to insure that the effective melting temperature of the final body is not compromised. In other words the majority of the ceramic is composed of the high temperature phase.
The high temperature phase has a melting point higher than that of beta-eucryptite, preferably higher than 1800° C. The high temperature phase is selected from the group consisting of lithium aluminate spinel (LiAl
5
O
8
), lithium aluminate (LiAlO
2
), corundum (Al
2
O
3
), and combinations thereof. Lithium aluminate spinel has a melting point of about 1960° C. Corundum has a melting point of about 2020° C. LiAlO
2
has a melting point of about 1850° C.
All three of these phases have a high CTE. Lithium aluminate spinel has a CTE from room temperature to 1000° C. of about 85×10
−7
/° C., while corundum has a CTE from room temperature to 1000° C. of 84×10
−7
/° C. It is preferred that the second high temperature phase be lithium aluminate spinel because it is in thermodynamic equilibrium with LiAlSiO
4
in the solid state, and also forms a rigid network in combination with liquids near this composition in the partially molten state. Therefore, in an especially preferred embodiment the inventive ceramic comprises about 35 weight % beta-eucryptite and 65 weight % lithium aluminate spinel.
The large CTE mismatch between the beta-eucryptite phase and the high temperature phase promotes microcracking either along grain boundaries between beta-eucryptite crystals or between the beta-eucryptite and the high temperature phases which leads to a CTE over a temperature range from room temperature to 800° C. of from −30×10
−7
/° C. to 30×10
−7
/° C., preferably −20×10
−7
/° C. to 10×10
−7
/° C., resulting in excellent therma
Beall Douglas M.
Beall George H.
Corning Incorporated
Gheorghiu Anca C.
Group Karl
LandOfFree
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