Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
2000-01-03
2002-11-05
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C501S014000, C501S015000, C501S021000, C501S032000, C065S033100
Reexamination Certificate
active
06475938
ABSTRACT:
The present invention relates to a self-supporting glass-ceramic material possessing high softening temperature (~1000° C. or above) and a thermal expansion coefficient in excess of 10×10
−6
° C.
−1
(20-950° C.).
The invention further comprises use of the glass ceramic material for joining different types of material.
There are many technologically important applications where the special properties of ceramic materials are utilised, e.g. to provide electrical insulation, wear resistance, stability against oxidation etc. Frequently these ceramics are used in direct combination with other materials with very different physical properties, and they are often required to operate at high temperatures. In applications where a ceramic component is joined rigidly to a dissimilar material and the assembly is subject to temperature changes after the joining operation, there is a strong need to match the thermal expansion characteristics of the two materials. Failure to do so will reduce the reliability of the joint and will often lead to fracture.
Glass-ceramics, a class of materials produced by the controlled crystallisation of vitreous precursors, have proved particularly successful when employed in applications where they are bonded directly to other materials, primarily due to the ease with which their thermal expansion coefficients can be tailored to match a diverse range of substrate materials. As a general rule, though, the glass-ceramics with high thermal expansion coefficients (CTE) (above 10×10
−6
° C.
−1
) (20-950° C.) are limited in terms of their maximum temperature of application because they soften at relatively low temperatures, often below 850° C. This is especially true for glass-ceramics with high coefficients of thermal expansion which are commonly used for sealing to stainless steels and other metals with moderate to high expansion such as those based on the Li
2
O—SiO
2
or Li
2
O—ZnO—SiO
2
systems.
There are some applications, however, where both high thermal expansion coefficient and high softening temperature (~1000° C. or above) are required in combination with excellent oxidation/reduction resistance in various atmospheres. In particular, there is a demand for this combination of properties within the field of high temperature electrochemical cells, e.g solid oxide fuel cells (SOFCs), oxygen separators operating with oxygen ion conducting ceramic membranes etc. In addition to the requirements regarding expansion and chemical stability, there is usually also a need for good microstructural stability and low electrical conductivity at the operating temperature.
The use of porous, oxide-based supports for gas-separation membranes is generally known, and supports based on alumina, silica and microporous glass (e.g. Vycor®) are available commercially and have been widely used, as referred to in EP 0 515 936 A1. These porous support materials, however, are designed to support thin (<5 &mgr;m) membranes which themselves contain micropores up to a few nanometers in diameter and which separate gases by a physical filtering process. The inherent microstructural instability of these membrane materials with extremely small pores imposes an upper temperature limit of about 500° C. on their operation, since above this temperature pore collapse is possible. The thermal stability of the membrane support material at temperature above 500° C. is therefore irrelevant in this particular application area. In addition, the requirement of matching the thermal expansion of the support to that of the membrane is far less stringent because the membrane is so thin and because the highest operating temperature is limited to ~500° C.
In other disclosures, e.g. U.S. Pat. Nos. 2,920,971 and 3,157,522 and GB Patent 1,402,960, porous glass-ceramic bodies with good thermal stability at temperatures as high as 1400° C. are described. The materials of these inventions are intended for use as catalyst supports, with particular application in vehicle exhaust control systems. These materials are required to have very high resistance to thermal shock and are therefore based on glass-ceramics with low thermal expansion coefficients, with cordierite, celsian, &bgr;-spodumene and mullite as the preferred crystal phases. Although the refractoriness of these glass-ceramic supports is excellent, they are not suitable for use in contact with dense, ion/electron conducting ceramic membranes because of the severe mismatch in thermal expansion coefficients.
High expansion glass-ceramics based on the MgO—BaO—SiO
2
—B
2
O
3
are described by Chyung in U.S. Pat. No. 4,385,127 and by Hang et al. in U.S. Pat. No. 4,256,796. The application of these materials, however, is restricted to insulating or protective coatings on metal alloy substrates. The glass-ceramics described by Chyung and by Hang et al. have boron oxide contents of at least 5% by weight. High expansion glass-ceramics with improved refractoriness are described by Andrus & MacDowell in U.S. Pat. No. 5,250,360. These are essentially based on the barium silicate and strontium silicate systems with various oxide additions, but with little or no B
2
O
3
or alkali metal oxides such as Na
2
O and K
2
O. In the invention of Andrus & MacDowell the application of the glass-ceramics is again limited to the coating of metal alloy substrates to provide protection from oxidizing atmospheres and a barrier to heat transport. One important feature of the glass-ceramics described by Andrus & MacDowell is that cristobalite forms in the glass-ceramic coating immediately adjacent to the metal alloy interface, as this is found to enhance coating quality.
The main object of the present invention is to obtain solid, self-supporting glass-ceramic materials with a combination of high thermal expansion coefficient (above 10×10
−6
° C.)(20-950° C.) and high softening temperatures (~1000° C. or above).
Another object of the present invention is to obtain glass-ceramic material with level of open porosity tailored to suit specific modes of application.
A further object of the invention is to obtain glass ceramic material suited for use as means for joining different type of material.
A further object of the invention is to obtain glass-ceramic material suited for use in combination with dense, ion/electron conducting ceramic membranes where the operating temperatures are typically 800-1000° C. and where there is a strong need to match the high thermal expansion coefficient of the membrane material.
The inventor found that by processing certain glass raw materials according to a selected route, a glass ceramic material having the properties mentioned above could be obtained.
The glass-ceramics of this invention are formed by the controlled sintering and crystallisation of glass powders from the MgO—BaO—SiO
2
system, and in being produced by a powder route, they are amenable to many of the processing techniques employed in ceramic engineering, such as isostatic pressing, tape casting, extrusion, injection moulding etc. The applicability of existent forming technology means that fabrication of relatively large and if necessary, intricately shaped components is possible with these materials.
The glass-ceramics of the present invention have compositions on a weight percent basis in the general range 10-35% MgO, 10-55% BaO and 25-50% SiO
2
. Additionally, the glass-ceramic may contain up to 5% B
2
O
3
and up to 15% of other metal oxides and other components known in the field of glass ceramics such as fluorides, nitrides etc. The major crystal phases developed in these glass-ceramics are magnesium barium silicate (2MgO.BaO.2SiO
2
) and enstatite (MgO.SiO
2
). The magnesium barium silicate phase (2MgO.BaO.2SiO
2
) is considered to be responsible for the high thermal expansion coefficient, in particular at temperatures above 700° C.
The preferred composition for these high expansion glass-ceramics lies within the range 12-30% by weight of MgO, 15-50% by weight of BaO and 30-45% by weight of SiO
2
with up to 3% by weight of B
2
O
Group Karl
Norsk Hydro ASA
Wenderoth , Lind & Ponack, L.L.P.
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