Cold isopressing method

Details Cold isopressing method Cold isopressing method

2000-09-22

2003-02-25

Ball, Michael W. (Department: 1733)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S242000, C156S089110, C156S089280, C425S393000, C428S034400, C264S628000, C264S632000, C264S635000, C264S650000, C210S500250

Reexamination Certificate

active

06524421

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a cold isopressing method in which a green material is compacted within an isopressing mold. More particularly, the present invention relates to such a method in which two or more layers of green ceramic material are laminated within the isopressing mold and one of the layers is a tape-cast film. Even more particularly, the present invention relates to such a method in which the laminated layers are used to form a ceramic membrane element capable of selectively transporting oxygen or hydrogen ions.
BACKGROUND OF THE INVENTION
Cold isopressing is a well-known technique that is used to form filters, structural elements and membranes. In isostatic pressing, a granular form of the material to be compacted is placed within an elastic isopressing mold. The granular material can be a ceramic or metallic powder or in case of ceramics can be a mixture of powder, binder and plasticizing agents. The isopressing mold is then positioned within a pressure vessel and slowly subjected to a hydrostatic pressure with cold or warm water to compact the granular material into a green form which subsequently, as appropriate, can be fired and sintered. An example of such a process that is applied to the formation of Tungsten rods is disclosed in U.S. Pat. No. 5,631,029. In this patent, fine Tungsten powder is isostatically pressed into a tungsten ingot.
Various green ceramics have been manufactured by isostatically pressing green ceramic materials. The isopressing molds can be cylindrical, as has been described above with reference to Tungsten ingots, or can be flat to produce plate-like articles. An important application for ceramic materials is to produce ceramic membrane elements to separate oxygen or hydrogen from feed streams. Such ceramic materials, while impermeable to the oxygen or hydrogen, conduct ions of oxygen or hydrogen to effect the separation. In practice, the ceramic is subjected to a high temperature and the oxygen or hydrogen is ionized at one surface of the membrane. The ions travel through the membrane and recombine at the other side thereof to emit electrons. The electrons are conducted through the ceramic material itself or through a separate electrical pathway for ionization purposes.
For example, a class of such materials, known as mixed conductors conduct both oxygen ions and electrons. These materials are well suited for oxygen separation since they can be operated in a pressure driven mode, that is a difference in oxygen activity on the two sides of the ceramic drives the oxygen transport. Perovskites such as La
1−x
Sr
x
CoO
3−y
, La
x
Sr
1−x
FeO
3−y
, La
x
Sr
1−x
Fe
1−y
Co
y
O
3−z
are examples of mixed conductors. At elevated temperatures, these materials contain mobile oxygen-ion vacancies [V
O
. . . ] that provide conduction sites for transport of oxygen ions through the material. Oxygen ions are transported selectively, and can thus act as a membrane with an infinite selectivity for oxygen. The oxygen transport involves the following chemical reactions:
1/2O
2
+2
e
− −
⇄O
− −
Surface reaction
O
− −
+[V
O
. . . ]⇄nil Reaction within the electrolyte
The oxygen ions annihilate the highly mobile oxygen ion vacancies in the electrolyte. Electrons must be supplied (and removed at the other end of the membrane) for this reaction to proceed.
An oxygen partial pressure differential across the membrane gives rise to an electromotive force (emf) termed as Nernst potential, and is given by the following equation:
V
=(
RT/zF
)
ln
(
Po
2
,2/
Po
2
, 1)
where,
R=the gas constant (8.314 J/gmole-K)
T=temperature (K)
F=Faraday's constant (96488 Coulomb/gmole)
Po
2
, 1 and Po
2
, 2=partial pressure of oxygen on the opposite sides of the membrane
z=the number of electrons given up by one oxygen molecule, i.e. 4
The Nernst potential is developed internally, and it drives the flux of oxygen vacancies against the ionic resistance of the electrolyte. Thin films are therefore highly desirable because the ideal oxygen flux is inversely proportional to the thickness of the membrane. Thus, thinner films can lead to higher oxygen fluxes, reduced area, lower operating temperatures and smaller O
2
pressure differentials across the electrolyte.
The thin films of ceramic are, however, fragile and must be supported. Therefore, efforts have been aimed at development of the thin film technology involving the deposition of a dense oxygen transport membrane film on a suitable porous substrate.
Solid state gas separation membranes, formed by depositing a dense mixed conducting oxide layer onto a porous mixed conducting support are disclosed in Yasutake Teraoka et al. “Development of Oxygen Semipermeable Membrane Using Mixed Conductive Perovskite-Type Oxides” Jour. Ceram. Soc. Japan. International Ed, Vol. 97, No. 4, pp 458-462, 1989 and Yasutake Teraoka et al. “Preparation of Dense Film of Perovskite-Type Oxide on Porous Substrate”, Jour. Ceram. Soc. Japan, International Ed. Vol. 97, No. 5, pp 523-529, 1989. The relatively thick porous mixed conducting supports disclosed in these references provide mechanical stability for the thin, relatively fragile, dense mixed conducting layers. In these references, thin films composed of La
0.6
Sr
0.4
CoO
3
were deposited onto porous supports of the same material by rf sputtering and liquid suspension spray deposition. The films produced by sputtering proved to be cracked and porous. Thin films (less than 15 &mgr;m in thick thickness) made by liquid suspension spraying followed by sintering at 1400° C. were dense and crack-free. Pal et al. “Electrochemical Vapor Deposition of Yttria-Stabilized Zirconia Films” from the Proceedings of the First International Symposium on Solid Oxide Fuel Cells, Vol. 89-11, pp 41-56, 1989 discloses an EVD process in which yttria-stabilized zirconia electrolyte films are deposited onto a porous substrate. EVD is a modification of the conventional chemical vapor deposition process which utilizes a chemical potential gradient to grow thin, gas impervious layers of either electronically, or ionically conducting metal oxides on porous substrates. The process involves contacting a mixture of metal halides on one side of a porous substrate and a mixture of hydrogen and water on the opposite side. The reactants diffuse into the substrate pores and react to form the multicomponent metal oxide that is deposited on the pore wall. Continued deposition, however, causes pore narrowing until eventually the pores are plugged with the multicomponent metal oxide.
U.S. Pat. No. 5,240,480 discloses an organometallic chemical deposition (OMCVD) method to prepare thin films of muticomponent metallic oxides for use as inorganic membranes. The inorganic membranes are formed by reacting organometallic complexes corresponding to each of the respective metals and an oxidizing agent under conditions sufficient to deposit a thin membrane onto the porous substrate.
Both EVD and OMCVD process involve expensive and complex equipment and often toxic and expensive precursor materials. Furthermore, it is difficult to control the stoichiometry of multicomponent metallic oxides (e.g. mixed conducting perovskites) deposited by such processes.
U.S. Pat. No. 5,494,700 discloses a precipitate- free aqueous solution containing a metal ion and a polymerizable organic solvent to fabricate dense crack- free thin films (<0.5 &mgr;m/coating) on dense/porous substrates for solid oxide fuel cell and gas separation applications. The method comprises first preparing a precipitate-free starting solution containing cations of oxide's constituents dissolved in an aqueous mixture comprising a polymerizable organic solvent. The precursor film is deposited on the substrate by a spin-coating technique. The deposition is followed by drying and calcining in the presence of oxygen and at the temperatures below 600° C. to convert the film of polymeric precursor into

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