WITH IMPROVED SPALLING RESISTANCE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims priority to pending U.S. Provisional Patent Application No. 62/096,327 filed December 23, 2014 which is incorporated herein by reference.

BACKGROUND

[0002] The present invention is related to a ceramic filter for molten metal and a method of forming a ceramic filter for molten metal. More specifically, the present invention is related to a ceramic filter with improved structural integrity and therefore improved spalling resistance.

[0003] Ceramic filters for molten metal have historically been created by foam replication techniques. In foam replication techniques an open cell foam, typically a polyurethane foam, with the desired porosity is impregnated with a ceramic precursor such that the interstitial struts of the foam are coated in ceramic precursor. The coated foam is then heated to the extent necessary to dry the ceramic precursor, vaporize the foam and sinter the ceramic thereby forming a ceramic replica of the original foam. Filters prepared by foam replication techniques, referred to as ceramic foam filters, have been widely used and are still used in large numbers. One disadvantage of ceramic foam filters is the propensity for small particles of ceramic to become dislodged from the filter, referred to in the art as spalling. Since it is difficult to manufacture a ceramic foam filter with consistent structure, the surface typically contains weak struts that tend to break during filtration. Furthermore, it is very difficult to prepare filters with a repeatable structure since the nature of the foam is inherently inconsistent. [0004] More recently, techniques have been developed for the formation of ceramic filters by a 3D-printing process wherein a ceramic precursor is deposited in a systematic manner and the ceramic precursor is then sintered to form the ceramic article. U.S. Pat. No. 7,527,671 , which is included herein by reference, exemplifies filters formed by 3-D printing techniques. An advantage of the filters prepared in this manner is the high surface to volume ratio and the precisely spaced geometric elements in the structured periodic lattice. As can be realized from the description therein, each layer has a layer thickness approximately equal to the thickness of the extruded filaments.

[0005] The process for extruding is exemplified in U.S. Pat. No. 7,597,835 to Marsac, which is incorporated herein by reference, wherein described is a process of extruding ceramic precursor with a given cross-sectional shape to print the extruded ceramic precursor in the intended pattern. The ceramic precursor must have rheological properties that allow for extrusion through an orifice of very small cross- sectional size yet be sufficiently rigid to be self-supporting after extrusion, thereby maintaining structural integrity through the firing process. Many suitable materials are taught as a ceramic precursor including pseudoplastic formulations, as taught in U.S. Pat. Nos. 6,027,326 and 6,401 ,795 to Cesarano et al., which are incorporated herein by reference; biphasic inks and sol-gels as taught in U.S. Pat. Nos. 8, 187,500 and 7,790,661 to Lewis et al., which is incorporated herein by reference; and by the use of viscosity modifiers as taught in U.S. Pat. No. 7,790,661 to Covitch which is incorporated herein by reference.

[0006] The formation of ceramic filters by 3-D printing provides a significant advantage in the art, relative to foam replication techniques, as the structures have more repeatable porosity, and therefore more repeatable flow, and the incidence of ceramic filter particles releasing into the metal during filtration is reduced. In fact, 3D printed ceramic filters are used commonly in less aggressive applications.

[0007] When attempting to expand the technology to more difficult applications demanding larger filter sizes and/or for use in those applications that do not allow pre-heating of the filter, the printed filters tended to fail catastrophically, presumably due to thermal shock. The use of zirconia-toughened mullite with a considerably lower coefficient of thermal expansion versus alumina provides some improvements, but the filters were still not sufficient. The filters still exhibit the loss of particles, or a spalling defect, at a level which is unacceptable.

[0008] After significant research, a surprising failure mode has been realized. In practice, when the metal is first introduced to the filter, the filaments from the top one or two layers tend to release into the melt. It is hypothesized that once a crack propagates through a given filament, it is easy for the filament to peel away from the filter, essentially unzipping from the body, and then releasing into the metal via an additional crack. This spalling tends to occur on the top of the filter, but can also occur on the bottom. When struts release from the top or bottom layer, subsequent layers are free to fail in the same fashion.

[0009] The previous deficiencies of 3-D printed filters are mitigated herein thereby allowing for the use of 3-D printed filters in wider applications such as those requiring larger sizes or in applications where pre-heating is not available.

SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide a ceramic filter with a repeating lattice and sufficient structural integrity to be suitable for use in a large size and in applications where pre-heating is not available. [0011] It is another object of the invention to provide a method for forming a ceramic filter with repeatable lattice structure and controlled pore size.

[0012] These and other advantages, as will be realized, are provided in a ceramic filter. The filter comprises a plurality of layers wherein each layer comprises filaments wherein each filament has a filament thickness wherein each layer comprises an overlap ratio defined as a layer thickness of the layer divided by the filament thickness of the filaments forming the layer wherein the overlap ratio of at least one layer is no more than 0.85.

[0013] Yet another embodiment is provided in a method of forming a ceramic filter. The method includes extruding a ceramic precursor into a continuous filament on a surface; forming a first layer of continuous filament; forming a second layer of continuous filament over the first layer wherein the continuous filament in the second layer is not parallel to the continuous filament in the first layer and the second layer has an overlap ratio of no more than 0.85.

FIGURES

[0014] Figs. 1 A and 1 B are schematic view of an embodiment of the invention.

[0015] Fig. 2 is a schematic partial cut-away view of filaments of prior art filter.

[0016] Fig. 3 is a schematic partial cut-away view of filaments of an embodiment of the invention.

[0017] Fig. 4 is a schematic top cut-away view of an embodiment of the invention.

[0018] Fig. 5A illustrates a prior art deposition of filaments.

[0019] Fig. 5B illustrates an embodiment of the invention.

DESCRIPTION

[0020] The instant invention is specific to an improved ceramic filter and a method of forming the improved ceramic filter. More specifically, the present invention is related to a method of forming a ceramic filter with improved strength as evidenced by reduced structural breakdown during interaction with molten metal.

[0021] The invention will be described with reference to the various figures forming an integral non-limiting component of the disclosure. Throughout the disclosure similar elements will be numbered accordingly.

[0022] An embodiment of the invention will be described with reference to Figs. 1A and 1 B wherein a ceramic filter, generally represented at 10, is illustrated schematically in perspective view top view, Fig. 1 A, and perspective view, Fig 1 B. In Figs. 1A and 1 B, a series of filaments, 12, are arranged in layers of inner filaments wherein the filaments of each layer are separated from each other and preferably parallel, and are typically not parallel to the filaments of adjacent layers thereby providing a general lattice structure. At times, adjacent layers may be drawn directly on top of subsequent layers in two or more layers to create a more open structure. The outer extent of the ceramic filter is bound by an optional, but preferred, layered outer filaments, 14, arranged in parallel fashion with a gross external shape being that of the intended filter. The ceramic filter of Figs. 1 A and 1 B are illustrated with a round gross external shape for convenience with the understanding that the gross external shape could be any desired shape. It is preferable that the filter be printed in a single pass with an entrance filament, 16, and terminating with an exit filament, 18, wherein the entrance filament and exit filament are removed prior to use. As would be realized, each inner filament spans across the layer, preferably linearly, and continues to a return filament, 15, such that the lattice is a continuous element. At either the initiation or completion of a layer, the extrusion transitions between an inner filament and an outer filament at a transition filament, 20. [0023] An embodiment of the invention will be described with reference to Figs. 2 and 3, wherein Fig. 2 represents the prior art and Fig. 3 illustrates an embodiment of the invention. In Figs. 2 and 3, a schematic cut-away cross-sectional view of inner filaments is illustrated at a junction. With reference to Fig. 2, the upper filament, 30, and lower filament, 32, form junctions, 34, with a central filament, 36. As would be realized, each layer thickness, represented as L ¹ , L ² and L ³ , is essentially the thickness of the cross-sectional size of the filament forming the layer measured perpendicular to the central filament. L ¹ , for example, will be essentially the same as the diameter of upper filament, 30, when the filament is round. With other shapes, the thickness measured perpendicular to the central filament is the same as the layer thickness L ¹ . Therefore, the thickness of the sintered ceramic filter is defined by Formula I:

Filter Thickness =∑ Tn Formula I

where n is the number of layers and Tn is the thickness of the n ^th layer. Therefore, the thickness of the filter is approximately the summation of the thickness of each inner filament.

[0024] In an embodiment of the invention, as illustrated in schematic cross- sectional view in Fig. 3, the layer thickness of layers L ⁴ and L ⁵ are less than the thickness of the filament forming the layer. For the purposes of comparison, L ² is the thickness of the central filament without perturbation. The ratio of the layer thickness, L ⁴ or L ⁵ for example, to the thickness of the filament forming the layer is defined as the overlap ratio, O. By way of example and with reference to Fig. 3 and assuming each layer is the same thickness represented by L ² , the overlap ratio for the top layer is defined as L /L ² , for the central layer the overlap ratio is L ² /L ² and for the lower layer the overlap ratio is L ⁵ /L ² and the average overlap ratio is defined by Formula II to be:

AO = (∑?(0n))/n

Formula II

wherein n is the number of layers and On is the overlap ratio for the n ^th layer.

[0025] The thickness, T, of the sintered ceramic is therefore defined by Formula III:

T = AO x∑? Tn

Formula III

Where T, AO and Tn are as defined above.

[0026] By way of example, the AO in Fig. 2 is 1 since the thickness of each layer is theoretically the thickness of the filament. It would be understood to those of skill in the art that deposition of one lattice layer on top of the other has some natural overlap ratio, due to gravitational forces, even though this is undesirable since the desire in the prior art is to have maximum surface area and porosity per unit of ceramic. In practice, sintered ceramic filters formed without intentional overlap have a typical average overlap ratio of about 0.90. For the purposes of the invention the overlap ratio is preferably no more than 0.85. With an overlap ratio higher than about 0.85, the formation of ceramic gussets is insufficient to provide adequate strength in aggressive molten metal filtration applications. In some embodiments an overlap ratio of zero is suitable, particularly, on the outermost layer since this forms a support lattice which is hypothesized to increase the strength of the entire lattice. With an overlap ratio lower than about 0.10 the density of the filter becomes large which decreases the filtering capability. It is more preferable that the sintered ceramic filter have an overlap ratio of at least 0.2, and preferably at least 0.5, to no more than 0.8 and preferably no more than 0.75. By way of example, with an overlap ratio of 0.2 the layer thickness is 20% of the thickness of the filament forming the layer.

[0027] A particular feature of the present invention is the ability to tailor the filter, thereby optimizing the spalling to filtering capability. As discussed herein, the spalling defect primarily occurs on the outer layers, or top and bottom layers, in use. Therefore, a filter can be prepared with regions of high overlap and regions of low overlap. By way of example, the filter can be defined to have zones wherein a zone comprises less than all of the layers in a filter. A first zone, for example, can be a top zone comprising a defined number of lattice layers from the uppermost including some of those towards the center of the filter. A second zone, for example a bottom zone, can comprise a defined number of layers of lattice layers from the bottom-most including some of those toward the center of the filter. A center zone, for example, can include up to all of the layers between the top zone and bottom zone wherein each zone may have a unique average overlap ratio. At least one of the top zone or bottom zone may independently have an average overlap ratio of no more than 0.85 and more preferably an overlap ratio of at least 0.1 , more preferably at least 0.2, and even more preferably at least 0.5, to no more than 0.8 and preferably no more than 0.75. The central zone may have an average overlap ratio of at least 0.90 or the average overlap ratio of at least one central zone may have an average overlap ratio of at least 0.10 to no more than 0.90 and more preferably an overlap ratio of at least 0.2, and preferably at least 0.5, to no more than 0.8 and preferably no more than 0.75.

[0028] While not limited to any theory, it is hypothesized that by forcing the filaments into compressed engagement at the junction, the material contained therein spreads laterally thereby providing ceramic gussets, 36, representing increased overlap of the filaments at the junction as illustrated in schematic top view in Fig. 4. While the upper filament and lower filament are illustrated in Figs. 2 and 3 as being parallel and in the same plane perpendicular to the central lattice element, this is for convenience of illustration and the upper filament and lower filament may be in different planes perpendicular to the central filament and it is not necessary that they be parallel.

[0029] An embodiment of the invention will be described with reference to Fig. 5 wherein a process of the invention is illustrated in schematic cross-sectional side view. In Figs. 5A and 5B, a virtual deposition surface, 40, illustrated for the purposes of discussion, is defined as the upper plane of the previous layer of internal filaments, 12, upon which an additional layer of internal filaments, 12', will be deposited. A nozzle, 42, extrudes a ceramic precursor, 44, which forms the filament. The height of the nozzle above the deposition surface is the sum of the thickness of the filament, defined as Y, and the height of the nozzle above the upper surface of the filament, defined as X. At high X, the ceramic precursor is allowed to flow freely onto the deposition surface as illustrated in Fig. 5A thereby essentially laying on the virtual deposition surface. As X is reduced to zero in Fig. 5B, the ceramic precursor is no longer in free fall and is instead forced into the previous layer and therefore extends below the deposition surface by a distance Z. As would be understood, Z and X are inversely proportional.

[0030] In one embodiment, the nozzle is moved in a more dynamic fashion. The head is moved not only parallel to the virtual deposition surface, but also repeatedly moved vertically, with varying X, closer to and away from the surface of deposition to deposit material in between the internal filaments of the previous level without drawing through or disturbing the previous layer. This mechanism of tying the internal filaments to a previous layer creates a more robust, criss-crossed layer such that when molten metal is introduced to the filter, the filaments are less likely to unzip and release into the metal.

[0031] Various techniques are known in the art for extruding solid filaments including the use of biphasic ink as the extruded material wherein the biphasic ink comprises a gel phase with a plurality of flocculated and non-flocculated particles in a carrier liquid as set forth in U.S. Pat. No. 8, 187,500, which is incorporated herein by reference. The biphasic ink employs different dispersants such that particles can attain flocculated or non-flocculated states within a carrier liquid. The non-flocculated particles remain repulsive through the use of a particularly preferred comb polymer dispersant with ionizable and nonionizable side-chains.

[0032] Slurries may also be employed as the extruded material wherein the slurry has a sufficiently high concentration of particles to be pseudoplastic and very little, if any, organic binder.

[0033] Sol-gel inks can be employed as the extruded material particularly for forming metal oxide structures. The sol-gel ink preferably has a metal oxide precursor with at least one member selected from the group consisting of Ti, Sn, Zr, and In. A stress reliever is typically included with polyvinylpyrrolidone, poly(N,N- dimethylacrylamide), poly(2-methyl-oxazoline), poly(ethylene glycol), poly(propylene glycol) or polyvinyl alcohol) being suitable stress relievers for demonstration of the invention. A solvent is typically included and optionally a polymerization inhibitor.

[0034] Chemically reactive suspensions can be used to form the extruded material. Chemically reactive suspensions employ two fluids in separate containers which are fed into a cylindrical mixing chamber. The two liquids are mixed and deposited from the mixing chamber onto a platform that moves relative to the mixing chamber to form the ceramic filter. The mixture gels shortly after being deposited. Alternatively, the platform may be stationary and the mixing chamber can move to form the pattern.

[0035] The slurry can be extruded into a bath containing solution, solvent or oil with high concentration of acid, base or salt that induces flocculation of the particles within the slurry. The rate at which the slurry flocculates can be controlled by the concentration of acid, base or salt relative to the slurry and the materials used.

[0036] The extruded material may eventually be dried, if necessary, to remove any volatile components which is referred to herein as a green filter. The green filter is then heated to a temperature necessary to sinter the ceramic precursor thereby forming a ceramic filter.

[0037] The invention has been described with reference to the preferred embodiments without limit thereto. One of skill in the art would realize additional embodiments and improvements which are not specifically set forth herein but which are within the scope of the invention as more specifically set forth in the claims appended hereto.