FUNCTIONALLY GRADED FIRING SETTERS AND PROCESS FOR MANUFACTURING THESE SETTERS

Abstract

A functionally graded firing setter that includes a substrate layer of cubic oxide; a top layer of unstabilized zirconium dioxide/hafnium dioxide; and a continuous transitional gradient layer disposed between the substrate layer and the top layer. The continuous transitional gradient layer includes cubic oxide stabilized zirconium dioxide/hafnium dioxide. The cubic oxide can be calcium oxide (CaO) or magnesium oxide (MgO).

Claims

1. A functionally graded firing setter comprising: a substrate layer consisting essentially of cubic oxide; a top layer consisting essentially of unstabilized zirconium dioxide, unstabilized hafnium dioxide, or a combination thereof; and a continuous transitional gradient layer disposed between the substrate layer and the top layer, wherein the continuous transitional gradient layer comprises cubic oxide stabilized zirconium dioxide, cubic oxide stabilized hafnium dioxide, or a combination thereof.

2. The functionally graded firing setter according to claim 1, wherein the cubic oxide is selected from a group consisting of calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y.sub.2O.sub.3), and cerium dioxide (CeO.sub.2).

3. The functionally graded firing setter according to claim 1, wherein the cubic oxide is selected from a group consisting of calcium oxide (CaO) and magnesium oxide (MgO).

4. The functionally graded firing setter according to claim 1, wherein the continuous transitional gradient layer is configured to chemically bond the substrate layer and the top layer.

5. The functionally graded firing setter according to claim 1, wherein the cubic oxide is calcium oxide (CaO).

6. The functionally graded firing setter according to claim 1, wherein the cubic oxide is magnesium oxide (MgO).

7. A method for producing a functionally graded firing setter, the method comprising: preparing a first slurry consisting essentially of a cubic oxide; preparing a second slurry consisting essentially of unstabilized zirconium dioxide, unstabilized hafnium dioxide, or a combination thereof; depositing the first slurry over a planar carrier; maintaining the first slurry deposited over the planar carrier for a predetermined duration; after the predetermined duration, depositing the second slurry over the maintained first slurry; upon depositing the second slurry, drying the deposited second slurry and the maintained first slurry; and upon drying, sintering the dried second slurry and the first slurry at a predetermined temperature range to obtain the functionally graded firing setter.

8. The method according to claim 7, wherein the cubic oxide is selected from a group consisting of calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y.sub.2O.sub.3), and cerium dioxide (CeO.sub.2).

9. The method according to claim 7, wherein the cubic oxide is selected from a group consisting of calcium oxide (CaO) and magnesium oxide (MgO).

10. The method according to claim 7, wherein the predetermined temperature range is about 1560° C. to 1650° C.

11. The method according to claim 7, wherein the first slurry is deposited by tape casting, the first slurry is maintained for partially drying, and the second slurry is deposited by co-casting over the partially dried first slurry.

12. The method according to claim 11, wherein the method further comprises: upon drying and before sintering, removing the planar carrier.

13. The method according to claim 11, wherein the planar carrier is silicone-coated Mylar film.

14. The method according to claim 11, wherein the first slurry is tape casted in a layer of about 1.0 mm - 2.0 mm thickness.

15. The method according to claim 14, wherein the co-casted second slurry has a thickness of about 0.012 inches.

16. A functionally graded firing setter prepared by a method comprising: preparing a first slurry consisting essentially of a cubic oxide; preparing a second slurry consisting essentially of unstabilized zirconium dioxide, unstabilized hafnium dioxide, or a combination thereof; depositing the first slurry over a planar carrier; maintaining the first slurry deposited over the planar carrier for a predetermined duration; after the predetermined duration, depositing the second slurry over the maintained first slurry; upon depositing the second slurry, drying the deposited second slurry and the maintained first slurry; and upon drying, sintering the dried second slurry and the first slurry at a predetermined temperature range to obtain the functionally graded firing setter.

17. The functionally graded firing setter according to claim 16, wherein the cubic oxide is selected from a group consisting of calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y.sub.2O.sub.3), and cerium dioxide (CeO.sub.2).

18. The functionally graded firing setter according to claim 16, wherein the cubic oxide is selected from a group consisting of calcium oxide (CaO) and magnesium oxide (MgO).

19. The functionally graded firing setter according to claim 16, wherein the predetermined temperature range is about 1560° C. to 1650° C.

20. The functionally graded firing setter according to claim 16, wherein the first slurry is deposited by tape casting, the first slurry is maintained for partially drying, and the second slurry is deposited by co-casting over the partially dried first slurry.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying figures, which are incorporated herein, form part of the specification and illustrate embodiments of the present invention. Together with the description, the figures further explain the principles of the present invention and to enable a person skilled in the relevant arts to make and use the invention.

[0036] FIG. 1 is a schematic diagram that depicts a functionally graded firing setter, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

[0037] Subject matter will now be described more fully hereinafter. Subject matter may, however, be embodied in a variety of different forms and, therefore, covered or claimed subject matter is intended to be construed as not being limited to any exemplary embodiments set forth herein; exemplary embodiments are provided merely to be illustrative. Likewise, a reasonably broad scope for claimed or covered subject matter is intended. Among other things, for example, the subject matter may be embodied as apparatus and methods of use thereof. The following detailed description is, therefore, not intended to be taken in a limiting sense.

[0038] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiments of the present invention” does not require that all embodiments of the invention include the discussed feature, advantage, or mode of operation.

[0039] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of embodiments of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0040] The following detailed description includes the best currently contemplated mode or modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention will be best defined by the allowed claims of any resulting patent.

[0041] The following detailed description is described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, specific details may be set forth in order to provide a thorough understanding of the subject innovation. It may be evident, however, that the claimed subject matter may be practiced without these specific details. In other instances, well-known structures and apparatus are shown in block diagram form in order to facilitate describing the subject innovation.

[0042] As defined herein, the term “conventional setters” refers to setters with firing surfaces that contain alumina, stabilized zirconia, magnesia, silicates, or other glassy phases.

[0043] As defined herein, the term “pure” refers to any oxide compound or mixture of oxide compounds that is essentially pure, with the exception of minor or trace impurities that may be unavoidable in commercially available sources.

[0044] As defined herein, the term “pure unstabilized ZrO.sub.2/HfO.sub.2” refers to unstabilized ZrO.sub.2, unstabilized HfO.sub.2, and any mixtures thereof, that are essentially pure, with the exception of minor or trace impurities that may be unavoidable in commercially available sources. Commercial sources of unstabilized ZrO.sub.2 typically contain a small naturally occurring HfO.sub.2 phase in the range of 1 - 5 wt.%. Because the chemical and physical properties of HfO.sub.2 are very similar to ZrO.sub.2, low percentages of HfO.sub.2 do not significantly influence the properties or crystallographic behavior of the ZrO.sub.2/HfO.sub.2 mixture compared with the pure ZrO.sub.2 phase.

[0045] As defined herein, the term “co-casting” refers to any of several methods for depositing a viscous paste or castable slurry to form a substrate layer, and subsequently coating the substrate layer with a second viscous paste or castable slurry consisting of a dissimilar material. Preferred co-casting methods include doctor blade casting or tape casting a slurry from an extruding hopper that rides above the casting area. Co-casting can be accomplished with two or more doctor blades in staged tandem arrangements, or by making multiple passes over preexisting layers with a single doctor blade. Other deposition methods known as micro extrusion, robocasting, and direct ink writing (DIW) are also considered suitable co-casting methods for the purposes of this invention. Compared to spray coating techniques, co-casting methods generally provide better uniformity and fewer voids.

[0046] It is clear that many important ceramic and powder metal components are not compatible with conventional setters that contain alumina, stabilized zirconia, magnesia, and silicates. Novel setters are disclosed that can provide chemically inert, non-stick, stable firing surfaces for heat treating and sintering these important components at higher temperatures and in various atmospheres.

[0047] In some embodiments, the novel setters of the present invention provide firing surfaces that are nonreactive with solid oxide fuel cell (SOFC) components, anodes containing strontium, iron, cobalt, and molybdenum (SCFM), Ni-based cermet anodes, vanadium-doped strontium titanate (SVT) anodes, vanadium and sodium-doped strontium niobate (SNNV) anodes, Lanthanum Strontium Manganite (LSM) components, and components containing any of the volatile species consisting of Co, Mn, Mo, Ni, Cr, V, Bi, Li, Na, and Cu.

[0048] In some embodiments, the novel setters of the present invention provide firing surfaces that are nonreactive with soft and hard ferrites, nickel manganate ceramics, barium titanate and other titanate-based electronic components, piezoelectric lead zirconium titanates (PZT), zinc oxide varistors, NTC and PTC components, YBCO and superconducting materials, dental and bio-medical ceramics, and beryllium compounds (BeO).

[0049] In some embodiments, the novel setters of the present invention provide firing surfaces that are nonreactive with metal-based components, including powder metallurgy components obtained by injection molding (MIM) and additive manufacturing processes (DIW) that contain alloys of titanium, tungsten, molybdenum, vanadium, magnesium, stainless steel, and alloys that contain silicon.

[0050] In some preferred embodiments, the novel setters of the present invention are characterized by a pure MgO base substrate that transitions to a pure unstabilized ZrO.sub.2/HfO.sub.2 surface layer through a continuous, functionally graded region containing MgO-stabilized ZrO.sub.2/HfO.sub.2. The graded transition region includes approximately 4 - 30 mol% MgO-stabilized ZrO.sub.2/HfO.sub.2 and provides a strong reaction-based chemical bond between the pure MgO substrate and the unstabilized ZrO.sub.2/HfO.sub.2 surface layer, which greatly increases the resistance to cracking and peeling. The graded MgO-stabilized ZrO.sub.2/HfO.sub.2 transition region also provides critical TCE strain compensation between the pure MgO substrate and the unstabilized ZrO.sub.2/HfO.sub.2 surface layer which further enhances resistance to peeling and cracking at high temperatures.

[0051] In some embodiments, the novel setters of the present invention may include a base substrate that consists of MgO, CaO, Y.sub.2O.sub.3, CeO.sub.2, or any of the cubic oxides that are soluble in ZrO.sub.2/HfO.sub.2, however MgO and CaO are preferred. It has been found that CaO-stabilized and MgO-stabilized ZrO.sub.2/HfO.sub.2 are more resistant to phase migration than Y.sub.2O.sub.3-stabilized ZrO.sub.2/HfO.sub.2, and they exhibit superior high-temperature strength and corrosion resistance. Additionally, the cost of Y.sub.2O.sub.3, CeO.sub.2, and other rare earth oxides may be prohibitively high for many applications.

[0052] Referring to FIG. 1 which shows an exemplary embodiment of the disclosed firing setter. The firing setter 100 includes a top layer 110 consisting essentially of unstabilized zirconium dioxide, unstabilized hafnium dioxide, or a combination thereof, a substrate layer 130 consisting essentially of cubic oxide, and a continuous transitional gradient layer 120 disposed between the substrate layer and the top layer. The continuous transitional gradient layer includes cubic oxide stabilized zirconium dioxide, cubic oxide stabilized hafnium dioxide, or a combination thereof. The top surface 140 of the firing setter 100 includes pure unstabilized zirconium dioxide, unstabilized hafnium dioxide, or a combination thereof.

[0053] In some preferred embodiments of the present invention, a casting slurry is prepared by mixing together pure MgO powders with suitable solvents, dispersants, surfactants, organic binders, and plasticizers. Another casting slurry consisting of pure unstabilized ZrO.sub.2/HfO.sub.2 powders with suitable solvents, dispersants, surfactants, organic binders, and plasticizers is prepared separately. The MgO slurry is then tape cast onto a suitable carrier, such as a silicone-coated polyethylene terephthalate (Mylar) film, to produce a well-dispersed uniform layer of MgO slurry situated on top of the carrier. After waiting a predetermined time, and before the MgO slurry layer has dried, the pure ZrO.sub.2/HfO.sub.2 slurry is then co-cast over the pure MgO substrate layer. After fully drying, the carrier can then be removed, and the co-cast tape bilayer can be sintered at approximately 1560° C. - 1650° C. to produce a functionally graded setter with a reaction-bonded pure unstabilized ZrO.sub.2/HfO.sub.2 surface that is peel resistant and crack-free. Surprisingly, it has been found that this novel process produces a firing setter with a pure unstabilized ZrO.sub.2/HfO.sub.2 surface that is chemically inert, warp resistant, and structurally stable up to temperatures well above 1650° C.

[0054] In some embodiments of the present invention, a casting slurry is prepared by mixing together pure CaO powders with suitable solvents, dispersants, surfactants, organic binders, and plasticizers. Another casting slurry consisting of pure unstabilized ZrO.sub.2/HfO.sub.2 powders with suitable solvents, dispersants, surfactants, organic binders, and plasticizers is prepared separately. The CaO slurry is then tape cast onto a suitable carrier, such as a silicone-coated polyethylene terephthalate (Mylar) film, to produce a well-dispersed uniform layer of CaO slurry situated on top of the carrier. After waiting a predetermined time, and before the CaO slurry layer has dried, the pure unstabilized ZrO.sub.2/HfO.sub.2 slurry is then co-cast over the pure CaO substrate layer. After fully drying, the carrier can then be removed, and the co-cast tape bilayer can be sintered at approximately 1560° C. - 1650° C. to produce a functionally graded setter with a reaction-bonded pure unstabilized ZrO.sub.2/HfO.sub.2 surface that is peel resistant and crack-free. Surprisingly, it has been found that this novel process produces a firing setter with a pure unstabilized ZrO.sub.2/HfO.sub.2 surface that is chemically inert, warp resistant, and structurally stable up to temperatures well above 1650° C.

[0055] The formation of the substrate slurries can be accomplished by mixing together suitable cubic oxide powders with solvents, dispersants, surfactants, organic binders, and plasticizers. Suitable cubic oxide powders include MgO, CaO, CeO.sub.2, and Y.sub.2O.sub.3, preferably pure MgO and pure CaO, and most preferably pure MgO. High purity, low silica, cubic oxide powders are preferred, and most preferably an electrically fused version of MgO due to its lower chemical reactivity. Cubic oxide powders with sieve sizes between 325 and 140 mesh are preferred.

[0056] Formation of the surface co-casting slurries can be accomplished by mixing together suitable pure unstabilized ZrO.sub.2/HfO.sub.2 powders with solvents, dispersants, surfactants, organic binders, and plasticizers. Powders with unstabilized ZrO.sub.2 + HfO.sub.2 > 99.6 wt.% and silica impurities < 0.3 wt.% are preferred. Unstabilized ZrO.sub.2/HfO.sub.2 powders with sieve sizes between 325 and 140 mesh are preferred.

[0057] In some preferred embodiments, the solvents and organic binders comprising both the substrate and surface co-casting slurries are chosen so that they are mutually soluble to a high degree, thereby promoting intermixing of particles and mass transport at the co-casting interface.

[0058] In some embodiments of the present invention, preferred solvents for producing tape casting slurries include n-butyl acetate, tert-Butyl acetate, ethyl acetate, isoparaffinic solvents, cyclohexanone, water, and other solvents that are considered to have low toxicity or environmental hazards.

[0059] In some embodiments of the present invention, preferred organic binders for non-aqueous tape casting slurries include polyvinyl butyral (60,000 - 1,000 ,000 MW), ethyl methacrylate copolymers, poly(propylene carbonate) and poly(methyl methacrylate).

[0060] In some embodiments of the present invention, preferred organic binders for aqueous tape casting slurries include polyvinyl alcohol-based polymers, polyvinylpyrrolidone, cellulose ethers, polyethyloxazoline (5,000 - 500,000 MW), acrylic copolymers, and acrylic latex emulsions.

[0061] In some embodiments of the present invention, preferred dispersants for non-aqueous tape casting slurries include Menhaden fish oil, stearic acid, oleic acid, glycerol trioleate, poly(12-hydroxystearic acid), polyethylene glycol tert-octylphenyl ether, and polyoxyethylenesorbitan monooleate.

[0062] In some embodiments of the present invention, preferred dispersants for aqueous tape casting slurries include oleic acid, glycerol trioleate, Dolapix CE64 (ammonium salt of a polycarboxylic acid, Zschimmer and Schwarz GmbH Co.), citric acid, stearic acid, polyethylenimine (1300 - 2,000 ,000 MW), ammonium salt of poly(acrylic acid), and poly(oxyethylene nonylphenol ether).

[0063] In some embodiments of the present invention, preferred plasticizers for non-aqueous tape casting slurries include polyethylene glycol, polyalkylene glycol, dioctyl phthalate and benzyl butyl phthalate.

[0064] In some embodiments of the present invention, preferred plasticizers for aqueous tape casting slurries include poly(propylene glycol), glycerol, dibutyl phthalate, and propylene glycol.

[0065] The carrier can be any flexible or rigid layer capable of supporting the slurries or viscous paste including plastic, metals, glass, ceramics, or silicone-coated polyethylene terephthalate (Mylar) film.

[0066] The green (unfired) co-cast multilayer can be reaction sintered at temperatures from about 1200° C. to about 1850° C., preferably from about 1560° C. to about 1650° C., in air or inert atmospheres.

[0067] Without wishing to be limited by theory, it is believed that the co-casting process creates a liquid interface which facilitates intimate mixing between cubic oxide particles from the substrate and pure unstabilized ZrO.sub.2/HfO.sub.2 particles from the coating. Before drying is complete, the otherwise abrupt transition at the substrate/coating interface is thereby converted into a continuous gradient by material transport. Gradients of the volume content of phases and gradients in chemical composition of a single phase can both occur, depending on the solubility of the components. It is believed that gradient formation can be enhanced by forced and thermal convection, and the depth of the graded interface is predominantly controlled by the degree of solidification of the first (substrate) cast at the time when the second (coating) cast is applied. Particle intermixing is further enhanced by using an organic binder for formulating both the substrate slurry and the pure unstabilized ZrO.sub.2/HfO.sub.2 slurry that is readily dissolved by a common solvent, preferably water.

[0068] During the reaction sintering process, diffusion results in the formation of a continuous transitional gradient that includes ZrO.sub.2/HfO.sub.2 phases that are stabilized by cubic oxides from particle intermixing that has taken place.

[0069] It is further believed that this transitional cubic oxide stabilized ZrO.sub.2/HfO.sub.2 gradient is highly beneficial for accommodating the thermal coefficient of expansion (TCE) mismatch between the pure cubic oxide substrate and the pure unstabilized ZrO.sub.2/HfO.sub.2 surface coating. While phase diagrams for all stabilized ZrO.sub.2/HfO.sub.2 systems are not fully established, it is known that certain cubic oxides including MgO, CaO, Y.sub.2O.sub.3, and CeO.sub.2 are soluble in ZrO.sub.2/HfO.sub.2. By alloying pure ZrO.sub.2/HfO.sub.2 with these cubic oxides, it is possible to stabilize the alloy “partially” or “fully” such that the cubic zirconia phase is retained upon cooling from high temperatures, thereby preventing the destructive crystallographic volume change that would otherwise occur. It is known that fully stabilized zirconia compositions retain the wholly cubic structure on cooling, while partially stabilized zirconia produces large cubic grains within which are dispersed tetragonal precipitates that transform under stress and prevent crack propagation. According to most references, 8 - 12 mol% addition of MgO results in a partially stabilized zirconia structure, and more than 13 mol% of MgO results in a fully stabilized cubic zirconia structure.

Example 1

[0070] The processing of the novel firing setters starts with the preparation of essentially pure MgO and pure unstabilized ZrO.sub.2/HfO.sub.2 slurries. The compositions of the MgO and unstabilized ZrO.sub.2/HfO.sub.2 slurries can be found in Table 1.

TABLE-US-00001 Compositions of the MgO and pure unstabilized ZrO.sub.2/HfO.sub.2 slurries: Category/Purpose Ingredient MgO slurry wt. % ZrO.sub.2/HfO.sub.2 slurry wt. % Ceramic powder MgO (99.9%), -200 mesh 74.06 – Unstabilized ZrO.sub.2/HfO.sub.2 (ZrO.sub.2 + HfO.sub.2 > 99.6%) d50 = 3.0 - 6.0 micron – 67.17 Dispersants Menhaden fish oil 0.74 – Menhaden fish oil 0.67 Solvents n-Butyl acetate 19.81 23.03 Plasticizers Benzyl butyl phthalate 1.15 1.84 Binders Ethyl methacrylate copolymer.sup.1 4.24 7.29 1. Paraloid™ B-72, The Dow Chemical Company

[0071] The solvents, dispersants, and ceramic powders are ball-milled overnight for approximately 16 hours, before adding the binders and plasticizers, and ball-milling the slurries again for another 16 to 24 hours to obtain uniform well dispersed slurries. The MgO slurry is tape cast in a relatively thick layer, 1.0 mm - 2.0 mm, on a silicone-coated Mylar film with a 10-inch-wide doctor blade. After the MgO substrate layer is partially dried on the casting bed, usually about 15 minutes, the ZrO.sub.2/HfO.sub.2 slurry is cast over the MgO substrate layer using a doctor blade with a gap that is adjusted to give a ZrO.sub.2/HfO.sub.2 slurry thickness of about 0.012 inches. The resulting co-cast bilayer tape is then dried on the casting bed, and after about 4 - 6 hours, it is cut using a metal punch into the desired shapes. Sintering of the co-cast tape bilayers is carried out in a high temperature furnace. The furnace temperature is raised to a temperature of 400° C., at a rate of 1° C. per minute, and held for 4 hours to decompose and vent organic components. The temperature is finally raised to 1600° C., at a rate of 1° C. per minute, and held for 2 hours to reaction sinter the co-cast tape bilayer and produce the functionally graded firing setter.

[0072] While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above-described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention as claimed.