Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2001-01-02
2001-09-04
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C219S543000, C219S544000
Reexamination Certificate
active
06284692
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
(Not applicable)
FIELD OF THE INVENTION
This invention relates to stabilized zirconia compositions for high-temperature applications, such as high-temperature heating elements.
BACKGROUND OF THE INVENTION
The storage heater is an element in the air supply system of a hypersonic true temperature tunnel. Cored bricks are the most costly item in the heater. Most metallic and ceramic materials cannot survive at the high operation temperature of a storage heater (2200° C.). The requirements for high temperature storage applications are: (1) Excellent phase stability up to 2200° C., (2) High thermal shock resistance parameter (
T>350° C.), (3) Creep deformation <1% under a compressive stress of 10 psi at 1650° C. Currently, 6-8 mol % yttria (Y
2
O
3
) stabilized zirconias (ZrO
2
) are used for such applications due to their good phase stability at high temperatures. The poor mechanical properties, especially thermal shock resistance, and large amount of yttria required to stabilize zirconia (yttria is ten times more expensive than zirconia) make the cost of operation high. It is well known that fine grained yttria partially stabilized zirconias containing 3 to 4.5 mol % yttria have very good room temperature bending strength (as high as 1 GPa) and moderate toughness (about 6 MPam). However, these zirconia materials deform superplastically at temperatures higher than 1300° C. due to their small grain sizes. Creep resistance can be enhanced by increasing the grain size. But increasing the grain size causes destabilization of the tetragonal phase and attendant cracking. Thus, from the standpoint of applications in a storage heater, there are conflicting requirements; large grain size is needed for improved creep resistance; small grain size is needed for preventing spontaneous tetragonal to monoclinic transformation.
Zirconia, which has a melting point of 2700° C., is widely used at elevated temperatures as insulation, thermal barrier coatings, as solid electrolytes (because of its high ionic conductivity) in electrochemical devices and as heating elements (because of its high electronic conductivity, above about 1600° C.). In a pure form, zirconia exhibits three crystallographic polymorphs; cubic (c) above about 2370° C., tetragonal (t) between about 1100° C. and about 2370° C., and monoclinic (m) below about 1100° C. The phase transformation from tetragonal to monoclinic (t→m) is martensitic accompanied by a large (5%) volume increase and a large (10%) shear strain. This transformation is destructive and leads to micro and/or macrocracks which sometimes destroy the ceramic.
Thus, any application of zirconia requires that either the cubic or the tetragonal phase be stabilized at lower temperatures. This is usually achieved by the addition of alkaline earth or rare earth oxides. Smaller dopant concentrations stabilize the tetragonal phase while larger concentrations are required for the stabilization of the cubic phase. The commonly used stabilizers are MgO, CaO, CeO
2
, and Y
2
O
3
. For ultra-high temperature applications (above about 2000° C.), Y
2
O
3
is preferred since it has a lower vapor pressure which is important to prevent destabilization that can occur due to evaporative loss of the dopant, such as can occur with CeO
2
, MgO, and CaO as dopants. Thus, yttria-stabilized zirconia becomes one of the obvious choices for ultra high temperature applications. However, CeO
2
-doped tetragonal zirconia is known to exhibit excellent mechanical properties, and thus is a potential candidate for many applications.
There are three ways by which the tetragonal phase (t) can form; (1 ) by direct heating in the tetragonal stability regime, (2) precipitation from the cubic phase, or (3) by a cubic→tetragonal displacive transformation. The first two methods lead to the formation of tetragonal phase, which is commonly referred to as the t-phase. This phase is metastable and at low temperature, and undergoes a stress-induced transformation in the immediate vicinity of a crack tip to the monoclinic phase, which results in enhanced toughness by the well known transformation toughening mechanism. The propensity to transformation depends upon the grain size of the t-phase; the larger the grain size, the greater is the tendency to transformation. For this reason, grain size in t-phase materials must be carefully controlled to prevent spontaneous transformation. This is undesirable since it leads to cracking and usually very poor properties. The critical grain size above which spontaneous transformation occurs depends upon the dopant type and concentration. Typically, the critical grain size is about 1 to 5 &mgr;m. The requirement that the grain size be maintained below a few microns restricts the use of t-phase materials to low temperature applications only since these fine-grained materials are superplastic and readily deform under a small stress above about 1200° C.
The so-called t′-phase is formed by a cubic→tetragonal displacive transformation which is known to be a ferroelastic transformation. Crystallographically, the t′-phase is identical with the t-phase. However, its morphology is quite different. The t′-phase grains consist of domains in three mutually orthogonal directions wherein the domain size is on the order of 0.1 &mgr;m or less. The extremely fine size of the domains makes the t′-phase highly resistant to martensitic transformation. In t′-phase materials, therefore, it is not the grain size that matters insofar as martensitic transformation is concerned, but the critical parameter is the domain size. Since the cubic phase is stable at elevated temperatures, the formation of the t′-phase requires that the temperature be raised high enough, into the stability regime of the cubic phase. For samples containing about 3% Y
2
O
3
or about 12% CeO
2
, this temperature is about 2000° C. At such a high temperature, grain growth readily occurs leading to a grain size in excess of 100 &mgr;m. Despite such a large grain size, these materials are resistant to the formation of monoclinic phase due to the fine domain size. Additionally, despite such a large grain size, these materials also exhibit a relatively high strength (about 400 MPa). At the same time, high temperature creep resistance is also excellent due to the large grain size. The t′-phase materials thus have a unique combination of the following properties: (1) Excellent resistance to destructive martensitic transformation which is common in large-grained t-phase materials, (2) Excellent toughness, (3) Good strength, and (4) Excellent creep resistance unlike t-phase materials.
Because of it properties, the t′-phase materials would seem to be ideally suited for elevated temperature applications such as heating elements, and the like. However, t′-phase systems have been very difficult to manufacture in form that is suitable for applications. Most of the literature reported on the t′-phase is either on arc-melted samples or in samples containing over 6 mol % yttria. The arc-melted materials can only be used for microstructural characterization, which are not in a useful form for applications. The properties of the latter, high yttria content materials are not attractive. They contain t′-phase with too small of a tetragonality, and have properties not much better than the cubic phase. The principal limitation in manufacturing a t′-phase zirconia with low dopant content has been the high fabrication temperatures required. Because of the limitations in manufacturing, the only use to date of t-phase zirconias has been for use in thermal barrier coatings. The principal limitation is the high fabrication temperatures required.
Currently heating elements are made of CaO- or Y
2
O
3
-stabilized zirconia with cubic as the predominant phase. These elements have many shortcomings: (1) Low phase stability due to the high pressure of the stabilizer in CaO-stabilized materials; (2) Poor mechanical
Jue Jan-Fong
Virkar Anil Vasudeo
Group Karl
Materials and Systems Research, Inc.
Sonntag James L.
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