WO2017066361A1

WO2017066361A1 - Rare earth phosphate ceramic article and process for making same

Info

Publication number: WO2017066361A1
Application number: PCT/US2016/056706
Authority: WO
Inventors: Ralph Alfred Langensiepen; Paul Maynard Schermerhorn
Original assignee: Corning Incorporated
Priority date: 2015-10-13
Filing date: 2016-10-13
Publication date: 2017-04-20
Also published as: TW201731802A

Abstract

Disclosed is a method of manufacturing a large rare earth or rare-earth-like ceramic body, for example a forming body in a glass making process, and in particular a method for producing a dry particulate in a reduced number of steps compared to conventional methods, the dry particulate suitable for use in an iso-pressing operation used to manufacture the ceramic body.

Description

RARE EARTH PHOSPHATE CERAMIC ARTICLE AND PROCESS FOR MAKING SAME

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Serial No. 62/240,762, filed on October 13, 2015, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

Field

[0002] The present disclosure is generally directed to rare earth phosphate-based ceramic articles and methods for producing rare earth or rare-earth-like materials, for example YP0₄ materials, and from which such ceramic articles can be made.

Technical Background

[0003] Refractory rare earth phosphate ceramic materials have been in use in various industries for many years. Typically, these materials are chemically derived binder phases formed by heating phosphoric acid-wetted masses of ceramic constituents, with the aqueous solution of phosphoric acid reacting with a fine ceramic powder like alumina or aluminum hydrate to form AIPO₄. In the present case of rare earth phosphate ceramic material fabrication, such as xenotime (YPO₄), a wet process has traditionally been used to form fine precipitates of rare earth phosphate precursors, for example in the form of YPO₄-2H₂O, the mineral churchite. Calcination of the dried reaction product YPO₄-2H₂O does result in the mineral xenotime, but the morphology of the particles is highly acicular. These fine needle-shaped particles do not process well to dense ceramics, and residual water or remnant pore channels in the crystals enhance proton conductivity in the formed ceramic. This phenomenon has been related to water weakening in the ceramic and enhanced high temperature creep, i.e., deformation. Accordingly, a dry reaction process is needed to achieve satisfactory creep properties.

SUMMARY

[0004] In accordance with the present disclosure, a means to densify the reactant powder mix to compact particles in the size range desired for final ceramic processing, which particles form the YPO₄ reaction product that is sequentially sintered at temperatures higher than required for the exothermic reaction to be initiated, is described. Thus, granulated dense compacts of rare earth phosphates can be manufactured, for example, compacts comprising such rare earths as lanthanum oxide, cerium oxide or the rare earth-like yttrium oxide. In one example, such compacts can be sized to be US Sieve size No. -50 (-48 mesh) agglomerates of greater than 50% dense Y₂O₃/P₂O₅ mixed powders with a minimum of interior void space. Reaction of the Y₂O₃ and the P₂O₅ to YPO₄ will proceed rapidly on heating the mixture to 1200°C, exhibiting a reaction volume shrinkage of at least 16%. Sintering shrinkage as the temperature of the particles is further raised to 1600°C can further reduce the particle volume by up to 40% to yield No. -60 (-60 mesh) particles with greater than 90% density. These particles can be reduced in size by milling, for example hammer milling, to a No. -140 /No. +325 mesh (-150 mesh/+325 mesh) size fraction that represents 55% of the desired mix for an example ceramic body formulation. In other examples finer particles for the matrix can be created by dry vibratory milling to a desired 20% by weight No. -325 (-325 mesh) and 25% by weight 2 micrometer D₅₀ particles. Yields can be as high as 100% with milling times reduced by 50% or more over previous methods. Contamination levels may also decrease with the reduced handling of the powders.

[0005] In respect of such rare earth or rare earth-like oxides, compaction of dry reactants serves to eliminate excess void space between the reactant particles and to enhance the reaction rate and the crystallite size, for example in the exothermic reaction of P₂O₅ and Y₂O₃ to produce YPO₄. Elimination of the surface area afforded by the void space also eliminates the potential for excessive atmospheric water attachment to the P₂O₅ particles that could affect both the reaction kinetics, morphology and phase purity of the compact so produced. Mixed powders can be compacted in a suitable atmospherically protected tablet press, or an atmospherically protected iso-pressing operation. However, the resulting reactant compacts would form large dense compacts requiring extensive particle size reduction for the end product needs. Continuous roll compaction and dry granulation is possible with Pharmaceutical-grade roll compaction systems available from suppliers like Freund- Vector or Fitzpatrick Chilsonator. Intermediate bulk containers (IBCs) of dry blended powders, filled and tumble-mixed under vacuum, dry nitrogen or dry air can be loaded into the sealed discharge system of a roll compactor feed screw. A metered flow of dry blended powder can then be directed into the sealed roll pair. A dense tape of compacted powder can be formed under the action of the compactor rolls, which tape would fall into the rotating arms of a granulator device to form sieve-sized dense particles that fall either into another atmosphere protected IBC or directly into the entrance of a tumbling furnace, for example a tubular rotary furnace operating under an inert or dry air cover gas with the tube. In the instance of YP0₄, reaction to the desired xenotime mineral form may then ensue at a temperature approaching 1200°C, while sintering to dense crystalline particles suitable for subsequent rapid sizing to the desired particle size distribution would be attained at temperatures between 1500°C and 1700°C. Pin mills or dry or wet ball mills may be employed to create the final size reduction from sub-millimeter particle size to the graded sub-millimeter particle size distribution necessary to produce optimal spray-dried powders for very large iso-pressing operations to form the controlled shrinkage compacts desired.

[0006] It should be noted that while the following detailed description is presented in respect of a ceramic body formed using a yttrium oxide precursor material, other suitable precursor materials include without limitation Y2O3, SC2O3, Er₂0₃, Lu₂0₃, Tm₂0₃, Ho₂0₃, Dy₂0₃, Tb₂0₃, Gd₂0₃, La₂0₃ and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a schematic view of an example glass making process employing a ceramic forming body;

[0008] FIG. 2 is a perspective view, in cross section, of a ceramic block according to embodiments of the present disclosure;

[0009] FIG. 3 is a perspective view, in cross section, of the forming body of FIG. 1 ; and

[0010] FIG. 4 is a process flow chart outlining a method of forming a ceramic block according to the present disclosure, including the forming bodies of FIGS 1 and 3.

DETAILED DESCRIPTION

[0011] Apparatus and methods will now be described more fully hereinafter with reference to the accompanying drawings in which example embodiments of the disclosure are shown.

Whenever possible, the same reference numerals are used throughout the drawings to refer to the same or like parts. However, this disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. [0012] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0013] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0014] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order, nor that with any apparatus, specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification.

[0015] As used herein, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a" component includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0016] In the present application, the terms xenotime, YP0₄ and Υ₂0₃ ·Ρ₂0₅ are used

interchangeably to mean a yttrium phosphate material. A YPC based material is a material comprising primarily YPO4, with or without an excess amount of Y2O3 or P₂0₅ over the stoichiometry required for YPO4, and other minor components as may be found therein.

[0017] As used herein, the term (Υ₂0₃)χ Ρ₂0₅ means a yttrium phosphate material comprising Y2O3 and P₂C"5 wherein the molar ratio of Y₂0₃ to P₂0₅ is x. [0018] As used herein, the term "sintering" refers to a thermally activated process in which solid state diffusion, driven by a reduction in surface energy, creates a solid body from a compacted mass of particles. The body thus formed is referred to as a sintered body.

[0019] As used herein, the term "compact" refers to a compacted body formed by compression, without sintering, of a particulate material.

[0020] All particle size ranges are described as sieve and/or mesh sizes, for example between one mesh size and another mesh size, and are presented as US sieve sizes and expressed as No. SS, where SS denoted the sieve size, with equivalent Tyler mesh sizes included in parentheses and expressed as (TT mesh), where TT represents the Tyler mesh number. A negative number (e.g., -SS or -TT) indicates the particles are capable of passing through the indicated screen. A positive number indicates the particles are blocked by the indicated screen.

[0021] As used herein, D₅₀ refers to the median diameter of the particles in a sample of particles. Thus, for a D₅₀ of 5 micrometers, 50% of the particles are larger than 5 micrometers and 50% of the particles are smaller than 5 micrometers.

[0022] Shown in FIG. 1 is an example glass manufacturing apparatus 10. In some examples, the glass manufacturing apparatus 10 can comprise a glass melting furnace 12 that can include a melting vessel 14. In addition to melting vessel 14, glass melting furnace 12 can optionally include one or more additional components such as heating elements (e.g., combustion burners or electrodes) configured to heat raw materials and convert the raw material into molten glass. In further examples, glass melting furnace 12 may include thermal management devices (e.g., insulation components) configured to reduce heat lost from a vicinity of the melting vessel. In still further examples, glass melting furnace 12 may include electronic devices and/or electromechanical devices configured to facilitate melting of the raw materials into a glass melt. Still further, glass melting furnace 12 may include support structures (e.g., support chassis, support member, etc.) or other components.

[0023] Glass melting vessel 14 is typically comprised of refractory material, such as a refractory ceramic material. In some examples, glass melting vessel 14 may be constructed from refractory ceramic bricks, for example refractory ceramic bricks comprising alumina or zirconia.

[0024] In some examples, the glass melting furnace may be incorporated as a component of a glass manufacturing apparatus configured to fabricate a glass ribbon. In some examples, the glass melting furnace of the disclosure may be incorporated as a component of a glass manufacturing apparatus comprising a slot draw apparatus, float bath apparatus, down-draw apparatus including a fusion process, up-draw apparatus, press-rolling apparatus, tube drawing apparatus or any other glass manufacturing apparatus. By way of example, FIG. 1 schematically illustrates glass melting furnace 12 as a component of a fusion down-draw glass manufacturing apparatus 10 for fusion drawing a glass ribbon for subsequent processing into glass sheets.

[0025] The glass manufacturing apparatus 10 (e.g., fusion down-draw apparatus 10) can optionally include an upstream glass manufacturing apparatus 16 positioned upstream relative to glass melting vessel 14. In some examples, a portion of, or the entire upstream glass manufacturing apparatus 16, may be incorporated as part of the glass melting furnace 12.

[0026] As shown in the illustrated example, the upstream glass manufacturing apparatus 16 can include a raw material storage bin 18, a raw material delivery device 20 and a motor 22 connected to the raw material delivery device. Storage bin 18 may be configured to store a quantity of raw materials 24 that can be fed into melting vessel 14 of glass melting furnace 12, as indicated by arrow 26. In some examples, raw material delivery device 20 can be powered by motor 22 to deliver a predetermined amount of raw materials 24 from the storage bin 18 to melting vessel 14. In further examples, motor 22 can power raw materials delivery device 20 to introduce raw materials 24 at a controlled rate based on a level of molten glass sensed downstream from melting vessel 14. Raw materials 24 within melting vessel 14 can thereafter be heated to form molten glass 28.

[0027] Glass manufacturing apparatus 10 can also optionally include a downstream glass manufacturing apparatus 30 positioned downstream of glass melting furnace 12 relative to the flow direction of molten glass. In some examples, a portion of the downstream glass manufacturing apparatus 30 may be incorporated as part of glass melting furnace 12. For instance, first connecting conduit 32 discussed below, or other portions of the downstream glass manufacturing apparatus 30, may be incorporated as part of the glass melting furnace 12. Elements of the downstream glass manufacturing apparatus, including first connecting conduit 32, may be formed from a precious metal. Suitable precious metals include platinum group metals selected from the group of metals consisting of platinum, iridium, rhodium, osmium, ruthenium and palladium, or alloys thereof. For example, downstream components of the glass manufacturing apparatus may be formed from a platinum-rhodium alloy including 70 to 90% by weight platinum and 10 to 30% by weight rhodium. However, other suitable metals can include molybdenum, palladium, rhenium, tantalum, titanium, tungsten and alloys thereof.

[0028] The downstream glass manufacturing apparatus 30 can include a first conditioning (i.e. processing) vessel such as fining vessel 34, located downstream from melting vessel 14 and coupled to melting vessel 14 by way of the above-referenced first connecting conduit 32. In some examples, molten glass 28 may be gravity fed from melting vessel 14 to fining vessel 34 by way of first connecting conduit 32. For instance, gravity may cause molten glass 28 to pass through an interior pathway of first connecting conduit 32 from melting vessel 14 to fining vessel 34. It should be understood, however, that other conditioning vessels may be positioned downstream of melting vessel 14, for example between melting vessel 14 and fining vessel 34. In some embodiments, a cooling vessel (not shown) may be employed between the melting vessel and the fining vessel such that molten glass from the melting vessel is cooled to a temperature lower than the temperature of the molten glass in the melting vessel before entering the fining vessel.

[0029] Bubbles may be removed from molten glass 28 within fining vessel 34 by various techniques. For example, raw materials 24 may include multivalent compounds (i.e. fining agents) such as tin oxide that, when heated, undergo a chemical reduction reaction and release oxygen. Other suitable fining agents include without limitation arsenic, antimony, iron and cerium. Fining vessel 34 is heated to a temperature greater than the melting vessel temperature, thereby heating the fining agent. Oxygen bubbles produced by the temperature-induced chemical reduction of the fining agent(s) rise through the molten glass within the fining vessel, wherein gases in the melt produced in the melting furnace can coalesce into the oxygen bubbles produced by the fining agent. The enlarged gas bubbles can then rise to a free surface of the molten glass in the fining vessel and thereafter be vented out.

[0030] Downstream glass manufacturing apparatus 30 can further include a second conditioning vessel such as mixing vessel 36 for mixing the molten glass that may be located downstream from fining vessel 34. Mixing vessel 36 can be used to provide a homogenous glass melt composition, thereby reducing or eliminating inhomogeneity that may otherwise exist within the fined molten glass exiting the fining vessel. As shown, fining vessel 34 may be coupled to mixing vessel 36 by way of a second connecting conduit 38. In some examples, molten glass 28 may be gravity fed from fining vessel 34 to mixing vessel 36 by way of second connecting conduit 38. For instance, gravity may cause molten glass 28 to pass through an interior pathway of second connecting conduit 38 from fining vessel 34 to mixing vessel 36. It should be noted that while mixing vessel 36 is shown downstream of fining vessel 34, mixing vessel 36 may be positioned upstream from fining vessel 34. In some embodiments, downstream glass manufacturing apparatus 30 may include multiple mixing vessels, for example a mixing vessel upstream from fining vessel 34 and a mixing vessel downstream from fining vessel 34. These multiple mixing vessels may be of the same design, or they may be of a different design from one another.

[0031] Downstream glass manufacturing apparatus 30 can further include another conditioning vessel such as delivery vessel 40 that may be located downstream from mixing vessel 36. Delivery vessel 40 may condition molten glass 28 to be fed into a downstream forming device. For instance, delivery vessel 40 can act as an accumulator and/or flow controller to adjust and provide a consistent flow of molten glass 28 to forming body 42 by way of exit conduit 44. As shown, mixing vessel 36 may be coupled to delivery vessel 40 by way of third connecting conduit 46. In some examples, molten glass 28 may be gravity fed from mixing vessel 36 to delivery vessel 40 by way of third connecting conduit 46. For instance, gravity may act to drive molten glass 28 to pass through an interior pathway of third connecting conduit 46 from mixing vessel 36 to delivery vessel 40.

[0032] Downstream glass manufacturing apparatus 30 can further include forming apparatus 48 comprising the above-referenced forming body 42 including inlet conduit 50. Exit conduit 44 can be positioned to deliver molten glass 28 from delivery vessel 40 to inlet conduit 50 of forming apparatus 48. In a fusion forming process, forming body 42 can comprise a trough 52 positioned in an upper surface of the forming body and converging forming surfaces 54 that converge along a bottom edge (root) 56 of the forming body. Molten glass delivered to the forming body trough via delivery vessel 40, exit conduit 44 and inlet conduit 50 overflows the walls of the trough and descends along the converging forming surfaces 54 as separate flows of molten glass. The separate flows of molten glass join below and along the root to produce a single ribbon of glass 58 that is drawn from root 56 by applying tension to the glass ribbon, such as by gravity and pulling rolls (not shown), to control the dimensions of the glass ribbon as the glass cools and viscosity increases such that the glass ribbon 58 goes through a visco-elastic transition and has mechanical properties that give the glass ribbon 58 stable dimensional characteristics. The glass ribbon may subsequently be separated into individual glass sheets 60 by a glass separation apparatus (not shown).

[0033] The forming body, for example forming body 42, is responsible for forming a viscous molten glass into a largely finished product (e.g., a glass sheet), and is therefore central to a fusion down-draw process. Thus, geometric stability of the forming body is a benefit for making glass sheets with consistent thickness, and thickness variation, over a long production campaign. Since the forming body is a long heavy object typically operating at an elevated temperature, e.g., 1200°C, for prolonged periods of time while supported only at the ends, the forming body is subjected to creeping, i.e., geometric deformation due to the weight of the forming body and the molten glass it contains, and the temperature of the forming body. Stringent requirements may therefore be imposed on the refractory materials for manufacturing the forming body.

Accordingly, a large continuous and unitary block of ceramic material 45 having substantially homogeneous composition and physical properties throughout its volume is desired to fabricate a single forming body.

[0034] Although the following description is presented in the context of a densified YP0₄ body, it should be understood that the processes described here are applicable more generally to the production of both rare earth or rare earth-like densified bodies.

[0035] Large YPC based ceramic blocks having low levels of contaminants and desirable attributes, especially creep rate at 1250°C, have been shown to meet the requirements for production of a forming body for making large-size glass sheets. A process for making such ceramic blocks constitutes various aspects of the present disclosure.

[0036] The chemical composition of the ceramic block 45 according to embodiments disclosed herein is substantially uniform, i.e., the major components Y₂O₃ and P₂O₅ are distributed substantially uniformly throughout the full volume of the bulk material. Thus, the material constituting the ceramic block may comprise essentially a single phase, such as the YPO₄ phase, throughout the bulk of the ceramic block in certain embodiments. However, it is possible that the material may comprise multiple phases that are all substantially uniformly distributed inside the bulk material. For example, the material constituting the ceramic block may comprise, in addition to a main YPO₄ phase, a minor Y₂O₃ phase distributed substantially uniformly in the YPO₄ phase. Trace amounts of impurities, such as A1₂0₃, BaO, B₂0₃, CaO, MgO, MnO, Zr0₂, and the like, may be present in the bulk body at various amounts. Since these impurities are typically present at very low concentrations, the distribution thereof may exhibit an irregular pattern. For example, the concentration of sodium, [Na], may be higher in the vicinity of a surface of ceramic block 45, and lower in regions far from the surface of the block, due to the high-temperature diffusion of sodium in furnaces from the surface to the center of the block.

[0037] The chemical composition of the ceramic block may be represented by a formula (Y₂0₃)_x Ρ₂0₅, where x is the molar ratio between Y₂0₃ and Ρ₂0₅, and 0.95 < x < 1.05. Thus the material may be a stoichiometric YP0₄ material, or may comprise an excess amount of either Y₂0₃ or P₂0₅. Nonetheless, it is more advantageous that the Y₂0₃ molar amount is not lower than P₂C"5, and thus it is desired that 1.00 < x < 1.05, in certain embodiments 1.00 < x < 1.03 and in certain other embodiments 1.00 < x < 1.02. This is because excessive P₂0₅ can lower the melting temperature of YPO₄ faster than the same amount of Y₂0₃.

[0038] The dimensions of the ceramic block can be described in terms of length L, width W and height H. In certain embodiments, the block can have a large size, where L > 20 centimeters, W > 20 centimeters, and H > 20 centimeters. The block can be a full solid block having a cubic, cuboidal, spherical, spheroidal, or other geometry. For example, the ceramic block 45 shown in FIG. 2 comprises a rectangular cross sectional shape. However, in other embodiments the ceramic block may take a shape comprising a trough-shaped top part and a wedge-shaped bottom part connected with each other as illustrated by forming body 42 shown in FIG. 3, for example by machining. In some examples the ceramic block 42, 45 can have a shape where L > 50 centimeters, W > 30 centimeters, and H > 50 centimeters. For larger size bodies, the block 42, 45 may have a length L > 100 centimeters, in certain embodiments L > 200 centimeters, in certain other embodiments L > 300 centimeters. As used hereinafter, ceramic block 45 shall be intended to represent a ceramic block of any shape or size, including the forming body 42, unless otherwise explicitly indicated.

[0039] Low levels of contamination, among other factors, provide for a low creep rate at 1250°C and 6.89 MPa. Accordingly, in certain embodiments, the ceramic block of the present disclosure may comprise calcium at a concentration by weight of [Ca], where [Ca] < 100 ppm, in certain embodiments [Ca] < 80 ppm, in certain embodiments [Ca] < 50 ppm, and in certain other embodiments [Ca] < 40 ppm. In certain embodiments, the ceramic block of the present disclosure may comprise zirconium at a concentration by weight of [Zr], where [Zr] < 50 ppm, in certain embodiments [Zr] < 40 ppm, in certain embodiments [Zr] < 30 ppm, in certain embodiments [Zr] < 20 ppm, and in certain other embodiments [Zr] < 10 ppm. In certain embodiments, the ceramic block of the present disclosure may comprise aluminum at a concentration by weight of [Al], where [Al] < 60 ppm, in certain embodiments [Al] < 50 ppm, in certain embodiments [Al] < 40 ppm, in certain embodiments [Al] < 30 ppm, and in certain other embodiments [Al] < 20 ppm. In certain embodiments, the ceramic block of the present disclosure may comprise barium at a concentration by weight of [Ba], where [Ba] < 100 ppm, in certain embodiments [Ba] < 80 ppm, in certain embodiments [Ba] < 50 ppm, and in certain embodiments [Ba] < 40 ppm. In certain embodiments, the ceramic block of the present disclosure may be characterized by the sum total of [Al], [B], [Ba], [Ca], [Fe], [Hf], [K], [Li], [Mg], [Mn], [Na] and [Zr] being at most 500 ppm by weight, in certain embodiments at most 400 ppm by weight, in certain other embodiments at most 300 ppm by weight, in certain other embodiment at most 200 ppm by weight, and in certain other embodiments at most 100 ppm by weight.

[0040] In addition to the lack of metal contaminants, the ceramic block of the present disclosure may further contain a low level of carbon. Carbon can be entrapped in any dense material made by sintering because organic matter can be introduced into the material prior to firing, either unintentionally due to material handling issues or intentionally because organic binders may be used in processes for making them. The existence of carbon can affect the mechanical properties of the ceramic block of the present disclosure, and can cause undesired outgassing during normal use thereof.

[0041] The ceramic block of the present disclosure can be used for handling alkali-free glass materials aimed for applications in opto-electronic devices, such as glass substrates for a liquid crystal display (LCD). For example, as described supra, the ceramic block can be formed into a large-size forming body, e.g., forming body 42, used in overflow down-draw processes for making alumino silicate glass-based LCD glass substrates. It is desired that for these

applications, ceramic block 45 is essentially free of alkali metal ions because such ions are particularly detrimental to semiconductor manufacturing processes conducted on the glass substrates. "Essentially free", as used herein, means ceramic block 45 comprises, by weight, equal to or less than 5 ppm of any alkali metal. In certain embodiments, ceramic block 45 according to the present disclosure can comprise equal to or less than 3 ppm on an elemental basis, for example equal to or less than 1 ppm, of any alkali metal. [0042] The ceramic block of the present disclosure may exhibit a nominal density of at least 85%, in certain embodiments at least 87%, in certain embodiments at least 89%, in certain embodiments at least 90%, in certain embodiments at least 93%, in certain embodiments at least 95%), and in certain other embodiments at least 97%, of the theoretical density of YP0₄ under standard conditions (1 atmosphere pressure, 0°C). The theoretical density of YPO₄ under standard conditions is typically considered as being 4.27 g em^"3. The higher the density of the ceramic block, the lower the porosity thereof. Given that ceramic block 45 comprises multiple grains, a certain level of porosity is expected to exist between the grain boundaries. The porosity at the grain boundary can significantly affect the mechanical properties of the ceramic block, such as, e.g., creep rate at an elevated temperature and modulus of rupture (MOR) at both room temperature and at the elevated operating temperature.

[0043] The ceramic block of the present disclosure further exhibits a low level of cracks in the crystalline grains inside the bulk material. It is believed the existence of a high level of micro- cracks in the micro-structure of ceramic block 45 can reduce the MOR and creep rate of the material under high temperature operating conditions. Such micro-cracks can propagate under load and stress, leading to material failure.

[0044] The ceramic block of the present disclosure is further characterized by low water content. Water in ceramic block 45 can take various forms, e.g., free water present in H₂0 form trapped at the grain boundary, or in the form of OH bonded to the surface or bulk of the material and/or the crystal grains. Without intending to be bound by a particular theory, it is believed the existence of water inside the bulk of the material can cause the formation of micro-cracks and propagation thereof under high operating temperatures. Therefore, the total H₂0 content in the ceramic block in certain embodiments is maintained in a range equal to or less than 300 ppm by weight, in certain other embodiments equal to or less than 200 ppm by weight, in certain other embodiments equal to or less than 100 ppm by weight, and in still other embodiments equal to or less than 50 ppm by weight.

[0045] A ceramic block of the present disclosure, having the composition and properties enumerated above, can be made by a carefully controlled synthesis method. Such large, high- purity, high-performance ceramic materials have not been found in nature, and therefore must be synthesized. As mentioned above, due to the high melting point of YPO₄, a high-temperature step would be unavoidable to make the ceramic block of the present disclosure. On the other hand, also due to the high melting point of YP0₄, directly forming such a large ceramic block 45 by melting the precursor material into a fluid, followed by cooling as is typically used in forming glass materials and some crystalline materials, would be impractical. Thus, the present disclosure describes a sintering method, i.e., heating densely packed precursor particles having the intended final composition into a densified ceramic body 45 containing multiple crystal grains.

[0046] As mentioned above, the existence of a large amount of H₂0 in a ceramic block of the present disclosure can lead to compromised mechanical properties. Therefore, the synthesizing method disclosed herein for making a large-size ceramic block 45 includes forming a precursor ceramic material using a "dry" synthesis approach, e.g., by reacting anhydrous P₂0₅ with dry Y₂O₃ in the desired amounts to form the precursor material with the desired end composition. Due to the highly hygroscopic nature of P₂0₅, the two powders are intimately mixed and reacted in a substantially closed container with a dry atmosphere cover gas or gases to prevent water absorption. The dry atmosphere cover may comprise an atmosphere of any suitable dry inert gas, for example nitrogen or any of the noble gases including helium, neon, argon, krypton, xenon, radon and mixtures thereof. The dry atmosphere may be continually replenished by using a continuous flow of the dry atmosphere. By dry what is mean is a moisture content equal to or less than 200 ppm water, for example in a range from about 100 to about 200 ppm water. To achieve a low level of metal contaminants, the raw materials of Y₂0₃ and P₂0₅ should be as pure as possible, and handling of both raw materials should not introduce the unwanted metals. This requires the use of clean containers made of materials substantially inert to Y₂0₃ and P₂0₅, even at elevated reaction temperatures. For example, the containers may be a glass-lined container.

[0047] In accordance with the present disclosure and referring to FIG. 4, a process 100 for dry processing YPO₄ is disclosed. In a first step 102, dry powders of Y₂0₃ and P₂0₅ are obtained and stored in a manner such that the dry powders do not absorb moisture. In a second step 104, the dry powders of Y₂0₃ and P₂0₅ are mixed in a suitable intermediate bulk container (IBC). Again, a dry, inert cover gas is included in the IBC during step 104 to prevent water absorption, particularly by the P₂0₅. A suitable IBC may be, for example, a Nalgene® container, available from Nalge Nunc International, a subsidiary of Thermo Fisher Scientific. However, any container capable of excluding moisture for the requisite processing time may be used. It is not necessary, however, that the container be a hermetic container. Moisture exclusion can entail continuing a flow of dry inert gas into the IBC to maintain a positive pressure within the container and prevent ingress of moisture. In certain embodiments, Y2O3 may be prepared by drying and batching the Y2O3 separately from the P2O5. Y2O3 is less absorptive of moisture than P2O5, and may, if desired, be initially processed without the need for a dry, inert cover gas. P2O5 may be transferred under a dry, inert cover gas to the IBC, and upon completion of the Y2O3 processing, the Y2O3 may be transferred to the IBC. While both the Y2O3 and the P2O5 are in the IBC, the content of the IBC can be mixed, such as by tumble mixing. The contents of the IBC are maintained under a dry, inert cover gas during the mixing.

[0048] Upon completion of the mixing step, the content of the IBC (the Y2O3 - P2O5 mixture) is transferred, still under a dry, inert cover gas, to a roll compactor-granulator (mill). The roll compactor-granulator may comprise separate devices, for example a separate roll compactor and a separate granulator, however, combination roll compactors-granulators are commercially available and may be employed if desired. Moreover, combined units greatly simplify the task of maintaining a dry, inert cover gas over the granular material. Although not required, good results can be obtained with a pharmaceutical-grade roll compactor-granulator such as, without limitation, the Chilsonator® roll compactor produced by the Fitzpatrick Company, available with an integral mill, or the Freund- Vector TFC roll compactor.

[0049] In step 106, the roll compactor compacts the mixed Y2O3 - P2O5 content of the IBC. A feed system conveys the dry powder mix to a compaction area. In the compaction area the powder mix is compacted between counter-rotating rolls, wherein the force applied to the powder mix by the rolls compacts the powder mix to produce a compacted powder mix in ribbon form. The compacted ribbon may then be broken up, for example by milling, in a granulating area, to an appropriate particle size. For example, in embodiments described herein, the compacted powder mix may be granulated to a particle size in a range from about No. -40 (-40 mesh) to about No. -70 (-65 mesh), for example from about No. -45 (-42 mesh) to about No. -60 (-60 mesh). For example, a suitable particle size is No. -50 (-48 mesh). Like compaction, the granulating is performed under a dry, inert cover gas.

[0050] The granulated compact is then fed in step 108 into a rotary tube furnace operating at a temperature effective to sinter the granulated compact, for example in a range from about 1550°C to about 1650°C. [0051] Upon complete sintering, the Y2O3 and P2O5 raw materials are substantially completely reacted to form a networked material where both components are substantially evenly distributed throughout the sintered precursor material, and wherein the sintered precursor material has a mean particle size in a range from about No. -50 (-48 mesh) to about No. -80 (-80 mesh), for example No. -60 (-60 mesh).

[0052] The sintered precursor material may then be transferred to a series of particle sizing steps. In a first such particle sizing step, step 110, the precursor material is milled, for example by hammer or pin milling, to a mean particle size of approximately No. -140 (-150 mesh), wherein 90% of the particles pass through a screen with an opening size of 0.104 millimeters, for example in a range from about No. -120 (-115 mesh) to about No. -170 (-170 mesh). In a next particle sizing step, step 112, the milled precursor material can be screened to a course batch particle size in a range from about No. -140 (-150 mesh) to about No. +325 (+325 mesh). The product resulting from step 112 may then be subjected to a first vibratory milling process in step 1 14 to a particle size in a range from about No. +270 (+270 mesh) to about No. +400 (+400 mesh), for example No. +325 (+325 mesh). The product resulting from either one or both of step 112 and/or step 114 may optionally be cycled back through steps 110, 112 and 114 if desired. A portion of the product resulting from step 112 may be retained as course batch material.

[0053] Product of step 114 may be subsequently screened during step 116 to form particulate in a range from about No. -270 (-270 mesh) to about No. -400 (-400 mesh), for example No. -325 (- 325 mesh), with a D₅₀ particle size of about 5 micrometers, for example in a range from about 4 micrometers to about 6 micrometers. A portion of product resulting from step 116 may be retained as medium batch material. The remainder of product from step 116 may then be milled in step 118, for example via a second vibratory milling process, to a D₅₀ particle size in a range from about 1 micrometer to about 3 micrometers, for example about 2 micrometers. Product from step 118 may be designated as fine batch material.

[0054] In a next step 120, coarse batch material, medium batch material and fine batch material are blended in a ratio suitable for the intended final product. For example, for the manufacture of a forming body as described herein, a ratio of about 50 to about 60 wt. % course batch material, about 15 to about 25 wt. % medium batch material and about 20 to about 30 wt. % fine batch material. A suitable ratio may include, for example, 55 weight % course batch material, 20 weight % medium batch material and 25 weight % fine batch material. The blended particulate may then be mixed at step 122, for example in a high shear mixer, then dried in spray drying step 124. For example, spray drying step 124 may include a pulse combustion spray drying step.

[0055] The spray dried intermediate product of step 124 can then be processed into a forming body suitable for use in a glass making process, for example a fusion down draw glass making process, although other glass making processes that employ large ceramic bodies may benefit from the ceramic bodies of the present disclosure. In some examples, the spray dried product of step 124 can be isostatically pressed, as represented in step 126, to form a green body. The green body can then be fired at step 128 under conditions suitable to provide a fired (sintered), fully consolidated ceramic block, then optionally machined at step 130 into the final body to be used in a glass making process, although it should be understood that the sintered block may be used, for example either before or after machining, in any other apparatus or process where a large, formed ceramic body can be employed.

[0056] To obtain a green body with the necessary strength to withstand subsequent handling, one or more organic binders may be used with the particulate. Such organic binder may include, for example, poly(meth)acrylates, Methocel™, and the like. However, the use of organic binders in the green body can result in a residual level of carbon in the final ceramic block. Residual carbon in the final ceramic block can be detrimental to the final sintering step 128 and negatively impact the resultant mechanical properties of the ceramic block. Thus, the total amount of organic binders should be limited to at most 5% (i.e. in a range from about 0% to about 5%) by weight of inorganic constituents of the green body, and in certain embodiments at most 3%, in certain other embodiments at most 2%, and in still other embodiments at most 1%.

[0057] The nominal density of the green body affects the final nominal density of the sintered ceramic block and hence the mechanical properties thereof. Accordingly, the density of the green body is at least 60% of the theoretical density of YP0₄ crystals under standard conditions, in certain embodiments at least 63%, in certain other embodiments at least 65%, in certain other embodiments at most 67%, in certain other embodiments at least 69%, and in still other embodiments at least 70%.

[0058] Dense packing of the particles is needed to achieve a high nominal density of the green body at the end of step 126. To achieve a dense packing, the particles should have a particle distribution facilitating packing thereof. To that end, in step 126, the particles can comprise the following components: (pi) from 45% - 65% wt.% of coarse batch;

(p2) from 10% to 30% wt.% of medium batch; and

(p3) from 15% to 35% wt.% of fine batch.

[0059] Accordingly, particles with the desired particle size distribution and amounts, and optionally the at least one organic binder material, are mixed thoroughly and uniformly with each other, put into a flexible bag (e.g. urethane) that can be shaken to promote compaction, vacuumed out and then sealed for isopressing in step 126.

[0060] In step 126, the flexible bag containing the particulate is subjected to a high pressure isopressing process to densify the particulate entered into the isopressing bag and obtain a green body with a high nominal density, and to achieve an isotropic density profile throughout the body. An anisotropic density profile of the green body would be detrimental to the final properties of the final ceramic block, as an isotropic density profile of the final ceramic block would be highly desired.

[0061] The isopressing is typically carried out in a machine called an isopress that comprises a container and a fluid that can be pressurized to as high as 124 MPa (18000 psi). The particulate in a sealed bag is then placed into the fluid, and pressurized from all directions to a pressure of at least 50 MPa, in certain embodiments at least 80 MPa, in certain other embodiments at least 100 MPa, and in still other embodiments at least 120 MPa.

[0062] The temperature profile of the final sintering step 128 significantly impacts the final properties and composition of the resultant ceramic block. If the temperature rises too fast, a high thermal gradient will occur from the surface to the core of the green body, resulting in substantial thermal stress that could crack the green body before the isopressing process ends. In addition, too steep a temperature rising curve can cause undesirable early-stage closure of the pores in the vicinity of the surface, resulting in the entrapment of water, carbon, carbon monoxide and carbon dioxide inside the ceramic body, which can cause undesired properties and/or later-stage cracking due to pressure built-up from these gaseous species. Therefore, it is desired that step 128 comprises the following:

(a) increasing the temperature of the furnace from 200°C to 1500°C at an average temperature elevation rate of not higher than 50°C/hour, in certain embodiments not higher than 40°C/hour, in certain embodiments not higher than 30°C/hour, in certain embodiments not higher than 20°C/hour, and in certain other embodiments not higher than 10°C/hour; and (b) maintaining the temperature of the furnace at over 1500°C for at least 100 hours, in certain embodiments at least 200 hours, in certain embodiments at least 300 hours, in certain other embodiments at least 400 hours, and in still other embodiments at least 500 hours.

[0063] Thus, in part (a), due to the slow temperature elevation rate, the green body is subjected to a low temperature gradient from the surface to the core of the green body, and it is allowed to degas through the open pores which close slowly, reducing the entrapment of water and carbon- based contaminants as well as the chance of cracking due to thermal gradients. To remove any organic materials substantially completely, at least in the initial stage of step 128, the sintering may be carried out in an oxidizing environment, such as air, so that organic materials can be oxidized substantially completely and removed from the bulk of the green body before the full consolidation (densification) thereof.

[0064] During the final stage of sintering, the green body densifies through the elimination of the body's porosity. Such densification is manifested by the reduction of volume, i.e., shrinkage. The higher the nominal density of the green body, the lower the total shrinkage of the green body at the end of the sintering step to achieve a given final density of the resultant ceramic block 45. The initial speed of shrinking is largely determined by the temperature of the green body. The higher the initial temperature, the faster the shrinking the green body will undergo. However, the higher the initial furnace temperature, the more likely a large temperature gradient from the surface to the core of the green body will result. Accordingly, a slow temperature elevation rate, especially at the beginning stage of the sintering step, is beneficial to reducing detrimental thermal gradients, and to reducing the initial shrinking speed of the green body. Therefore, the amount of shrinkage as well as the shrinking speed both can impact the final porosity, density and properties of the resultant ceramic block.

[0065] At the end of sintering step 128, after the green body has been substantially completely densified, it is allowed to cool down at a low cooling rate, e.g., not over 200°C/hour from 1600°C to 200°C, in certain embodiments not over 100°C/hour, in certain other embodiments advantageously not over 50°C/hour, in still other embodiments not over 10°C/hour, so that the cooling does not result in a substantial thermal gradient inside the bulk of the ceramic block, which can cause cracking.

[0066] Once ceramic block 45 has been formed, the ceramic body may be shaped to suit the particular use to which the ceramic block will be applied, including without limitation as a forming body in a glass making process, for example forming body 42 for use in a fusion down draw process. Shaping may include machining, grinding or any other shaping process known to those skilled in the art.

[0067] It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments of the present disclosure without departing from the spirit and scope of the disclosure. Thus, it is intended that the present disclosure cover the modifications and variations of such embodiments provided they come within the scope of the appended claims and their equivalents.

Claims

>5 What is claimed is:

1. A method comprising:

in a dry inert atmosphere:

mixing a rare earth oxide and P₂O₅ to form an oxide mixture;

roll compacting the oxide mixture to produce a ribbon of the oxide mixture;

granulating the ribbon; and

tumble sintering the granulated oxide mixture to produce a rare earth phosphate particulate material.

2. The method according to claim 1, wherein the rare earth oxide is selected from the group consisting of lanthanum oxide, cerium oxide and yttrium oxide and mixtures thereof.

3. The method according to claim 1 or claim 2, wherein a particle size of the rare earth phosphate particulate material after the tumble sintering is in a range from about No. -100 to No. -200 expressed as a US sieve size.

4. The method according to claim 1 or claim 2, further comprising, after the tumble sintering, milling the rare earth phosphate particulate material in a first milling step.

5. The method according to claim 4, further comprising, after the first milling step, screening the rare earth phosphate particulate material in a first screening step to produce a course batch material with a particle size in a range from about No. -140 to about No. +325 expressed as a US sieve size.

6. The method according to claim 5, further comprising milling the course batch material to produce a milled course batch material with a particle size in a range from about No. -270 to about No. -400 expressed as a US sieve size.

7. The method according to claim 6, further comprising screening the milled course batch material in a second screening step to produce a medium batch material with a D₅₀ particle size in a range from about 3 micrometers to about 7 micrometers.

8. The method according to claim 7, further comprising milling the medium batch material in a third milling step to produce a fine batch material with a D₅₀ particle size in a range from about 1 micrometer to about 3 micrometers.

9. The method according to claim 8, further comprising blending course batch material, medium batch material and fine batch material in a ratio of about 50 to about 60 wt. % course batch material, about 15 to about 25 wt. % medium batch material and about 20 to about 30 wt. % fine batch material to produce a blended batch material.

10. The method according to claim 9, further comprising spray drying the bended batch material to obtain a dry blended batch material having a particle size in a range from about No. -80 to about No. -120 expressed as a US sieve size.

11. The method according to claim 10, wherein the spray drying comprises pulse combustion spray drying.

12. The method according to claim 10, further comprising:

isopressing the dry blended batch material to form a green body; and

sintering the green body to produce a consolidated ceramic body.

13. The method according to claim 12, further comprising using the consolidated ceramic body in a glass making process.

14. The method according to claim 13, wherein the glass making process is a fusion glass making process.

15. A consolidated ceramic body made by the method of any one of claims 1 to 12.

16. A method of manufacturing a ceramic body comprising:

in a dry inert atmosphere:

mixing a rare earth oxide and P₂O₅ to form an oxide mixture;

roll compacting the oxide mixture to produce a ribbon of the oxide mixture;

granulating the ribbon; and

tumble sintering the granulated oxide mixture to produce a rare earth phosphate particulate material;

processing the rare earth phosphate particulate material to obtain a course particulate material with a particle size in a range from about No. -140 to about No. +325 expressed as a US sieve size, a medium particulate material with a D₅₀ particle size in a range from about 3 micrometers to about 7 micrometers and a fine particulate material with a D₅₀ particle size in a range from about 1 micrometer to about 3 micrometers;

isopressing the course, medium and fine rare earth particulate materials to form a green body; and

sintering the green body in a furnace to produce the ceramic body.