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• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Definitions

• the present invention relates to synthetic YPO 4 material, method for making such material and article comprising such material.
• the present invention relates to compositionally and structurally modified YPO 4 material comprising yttria, article comprising such material for high-temperature applications and method for making such article.
• the present invention is useful, e.g., for making isopipe for fusion draw glass making processes.
• LCDs Liquid crystal displays
• Certain isopipes currently used to produce LCD glass sheets are made from a zircon refractory. High temperature creep of the isopipe is believed to limit its operational lifetime. Display glass drawn on an isopipe with excessive creep cannot produce a sheet of uniform thickness. Creep deformation leads to an uneven distribution of glass flow. Hence, a reduction in the intrinsic creep rate of the refractory material used to manufacture the isopipe can result in the use of a wider isopipe, extend the fusion draw process to higher temperature glasses, and extend service life of the isopipe in the current process to minimize process down time and replacement costs. [0006] In the case of an isopipe, the creep rate is a function of the material itself as well as the length and height of the isopipe.
• the creep rate of a polycrystalline ceramic body such as an isopipe is influenced by factors that can be categorized as either microstructural (e.g., porosity, grain size, grain shape, and arrangement of second phases) or chemical such as stoichiometry and concentration of impurities.
• microstructural e.g., porosity, grain size, grain shape, and arrangement of second phases
• chemical such as stoichiometry and concentration of impurities.
• the microstructure and chemical composition of the ceramic body are themselves dependent upon processing history, which includes initial particle size of the powders, sintering temperature, and contaminants. These same factors play a role in the ability of the material to meet other requirements for an isopipe such as strength.
• the refractory material would be used to produce isopipes and other components used in the glass- making process. The methods and compositions described herein address this need.
• the sintered YPO 4 composite material comprises at least 95% (in certain embodiments at least 96%, in certain embodiments at least 97%, in certain embodiments at least 98%, in certain embodiments as least 99%, in certain embodiments at least 99.5%) by mole of (Y 2 O 3 )m-(P 2 O 5 )n.
• the sintered YPO 4 composite material has a geometric porosity of less than 10%; in certain embodiments less than 9%; in certain other embodiments less than 8%; in certain other embodiments less than 7%, in certain other embodiments less than 5%.
• the sintered YPO 4 composite material has an average grain size of less than 300 ⁇ m, in certain embodiments less than lOO ⁇ m, in certain other embodiments less than 40 ⁇ m. In certain embodiments, the sintered YPO 4 material has an average grain size of at least 10 ⁇ m.
• the sintered YPO 4 composite material has a creep rate of less than 200 ppb/hr at 1180°C. In certain embodiments, the sintered composite material has a creep rate of less than 600 ppb/hr at 1250°C.
• the sintered YPO 4 composite material has a Weibull stress of at least 60 MPa at 1200 0 C, in certain embodiments at least 100 MPa, in certain other embodiments at least 120 Mpa.
• a second aspect of the present invention relates to an article operable at a temperature at higher than 1000°C, comprising the sintered YPO 4 composite material according to the first aspect of the present invention described summarily above and in detail below.
• the article is an isopipe for fusion draw process for making a glass article.
• a third aspect of the present invention is an apparatus for making glass comprising the article according to the second aspect of the present invention.
• a fourth aspect of the present invention is a method for making an article comprising a sintered YPO 4 composite material, comprising the following steps: (I) mixing a P 2 O 5 -SOUrCe material and a Y 2 O 3 -source material to produce an admixture comprising more Y 2 O 3 -SOUTCe material than required for forming stoichiometric YPO 4 ; (II) heating the admixture to obtain a reacted material; (III) producing particles of the reacted material; (FV) forming the particles into a preform; and (V) sintering the preform to obtain an article comprising sintered YPO 4 composite material comprising a plurality of grains, having a composition comprising (Y 2 O 3 )m-(P 2 O 5 )n, and an excessive amount of De(P) OfP 2 O 5 relative to Y 2 O 3 , where m and n are mole fractions Of Y 2 O 3 and P 2 O
• the P 2 O 5 -source material comprises P 2 O 5 ; and the Y 2 O 3 -SOUTCe material comprises Y 2 O 3 .
• the particles formed have an average particle size from 0.1 to 100 ⁇ m; in certain embodiments from 0.1 to 45 ⁇ m; in certain other embodiments from 0.1 to 30 ⁇ m; in certain other embodiments from 0.1 to 15 ⁇ m.
• step (IV) comprises isopressing the particles.
• step (V) comprises a step of heating the preform in the temperature range from 1000°C to 1475 0 C for a period of at least 3 hours.
• step (V) comprises a step of heating the preform in the temperature range from 1600°C to 1775°C for a period from 24 hours to 1,000 hours.
• step (I) in step (I), the amounts OfP 2 O 5 -SOUTCe materials and Y 2 Os-SOUTCe materials are chosen such that in step (V), in the resultant sintered YPO 4 composite material, 0.10% ⁇ De(P) ⁇ 0.50%, in certain embodiments 0.15% ⁇ De(P) ⁇ 0.50%.
• the sintered YPO 4 composite material in step (V), has a geometric porosity of less than 10%; in certain embodiments less than 9%; in certain other embodiments less than 8%; in certain other embodiments less than 7%.
• the pores in the sintered YPO 4 composite material are essentially closed (at least 90% by number, in certain embodiments at least 95%, in certain other embodiments at least 98%, in certain embodiments at least 99%).
• the resultant sintered YPO 4 composite material in step (V), has an average grain size of less than 300 ⁇ m, in certain embodiments less than lOO ⁇ m, in certain other embodiments less than 40 ⁇ m.
• the resultant sintered YPO 4 composite material in step (V), has an average grain size of at least 10 ⁇ m. In certain embodiments, in step (V), the resultant sintered YPO 4 composite material has a creep rate of less than 200 ppb/hr at 1 18O 0 C. In certain embodiments, in step (V), the resultant sintered composite material has a creep rate of less than 600 ppb/hr at 125O 0 C. [0026] Certain embodiments of one of the present invention has one or more of the following advantages: The material and the isopipe can have higher strength and lower creep rate even at a high operating temperature for a long period of time.
• FIG. 1 shows a plot of geometric porosity of a series of synthetic sintered
• FIG. 2 shows a plot of geometric porosity as a function of sintering time for a series of synthetic sintered YPO 4 material samples.
• FIG. 3 shows optical photomicrographs of a series of polished and thermally etched synthetic sintered YPO 4 material samples that were sintered at l,750°C for 96 hours.
• FIG. 4 shows optical photomicrographs of a series of polished and thermally etched synthetic sintered YPO 4 material samples that were sintered at 1,600 0 C for 1,000 hours.
• FIG. 5 shows a plot of grain size as a function of sintering time for a series of synthetic sintered YPO 4 material samples.
• FIG. 6 shows a plot of grain size as a function OfP 2 O 5 deficiency for several samples sintered at l,750°C at 4 hours and 96 hours, respectively.
• FIG. 7 shows a plot of grain size as a function of P 2 O 5 deficiency for a series of synthetic sintered YPO 4 material samples sintered at 1,600 0 C at 48, 240, and 1,000 hours, respectively.
• FIG. 8 shows creep rates at 1 , 180 °C and 1 ,250 °C as a function of P2O5 deficiency for a series of synthetic sintered YPO 4 material samples sintered at 1,750 °C for 4 and 100 hours, respectively.
• the deficient amount OfP 2 O 5 is equal to the excess amount OfY 2 O 3 in the composite material.
• De(P) is sometimes called "P 2 O 5 deficiency” or “deficiency in P 2 O 5 " in the present application.
• the sintered YPO 4 composite material has modified microstructural properties. Therefore, for the convenience of description, the sintered YPO 4 composite material of the present invention is sometimes referred to as "modified YPO 4 material" in the present application.
• modified YPO 4 material is the properties or characteristics of the composite material at the microscopic level that influence the overall performance of an article composed of the modified YPO 4 material (e.g., mechanical properties). For example, by modifying one or more microstructural properties, it is possible using the methods described herein to produce articles with increased strength.
• step (I) the amounts OfP 2 O 5 -SOUrCe material and the Y 2 O 3 -source material are selected such that an excess amount of Y 2 O 3 is provided than needed for producing stoichiometric YPO 4 .
• step (I) the amounts of P 2 O 5 -source material and the Y 2 O 3 -source material are selected such that an essentially stoichiometric YPO 4 is produced in step (II), and in step (IV), additional Y 2 O 3 -SOUTCe material is included in the preform.
• the first step involves admixing a P 2 O 5 -source and an Y 2 O 3 -source to produce a mixture.
• the amount of P 2 O 5 -source and an Y 2 O 3 -source is sufficient so that after the heating step excess yttrium oxide remains.
• the P 2 O 5 -SOUTCe and Y 2 O 3 -source can be admixed using techniques known in the art.
• the P 2 O 5 -source and Y 2 O 3 -source can be turbula mixed as dry powders.
• the P 2 O 5 -source and/or Y 2 O 3 -source can be in a solution and admixed with one another.
• the terms "phosphorous oxide source” and "P 2 O 5 - source” are used interchangeably.
• the "P 2 O 5 -source” is any material that can provide the phosphorous in the reaction to form YPO 4 .
• elemental phosphorous can be used as a P 2 O 5 -SOUrCe in the production of YPO 4 or modified YPO 4 materials.
• HPO 3 , H 3 PO 4 at various concentrations, may be used as P 2 O 5 -source for making YPO 4 and modified YPO 4 materials.
• the P 2 O 5 -source can be a salt that can be readily converted to P 2 O 5 in air at elevated temperatures.
• the P 2 O 5 -source is P 2 O 5 .
• the terms "yttrium oxide source” and "Y 2 O 3 -source” are used interchangeably in the present application.
• An "Y 2 O 3 -source” is any material that can provide yttrium in the reactions to form YPO 4 .
• the Y 2 O 3 -source can be Y 2 O 3 per se.
• the Y 2 O 3 -source can be a salt (e.g., nitrate, halide, etc.) that can be readily converted to Y 2 O 3 in air at elevated temperatures.
• the P 2 O 5 source and the Y 2 O 3 source are mixed in dry form.
• the P 2 Os-source comprises phosphoric acid and the Y 2 O 3 -source comprises a molar excess of Y 2 O 3 -source (e.g., Y 2 O 3 ). Heating the mixture to an elevated temperature results in the reactions between the source materials and the production of modified YPO 4 material as defined herein.
• Y 2 O 3 -source (such as Y 2 O 3 per se or a yttrium salt as mentioned above) can be admixed with stoichiometric YPO 4 material prepared beforehand.
• the Y 2 O 3 -source material is distributed in the stoichiometric YPO 4 to obtain a modified YPO 4 of the present invention.
• yttrium oxide can be precipitated on stoichiometric- YPO 4 in a liquid- containing dispersion (suspension, e.g.) by converting an yttrium salt (e.g., yttrium hydroxide or oxalate) to the oxide in situ.
• the mixture is heated for a sufficient time and at a sufficient temperature to produce modified YPO 4 material.
• the heating time and temperature can vary depending upon the amount of Y 2 ⁇ 3 -source relative to the amount of P 2 O 5 -source used, the particle size and distribution of the staring materials, and the desired microstructural and mechanical properties of the modified YPO 4 material.
• the microstructural and mechanical properties of the modified YPO 4 material can be monitored over different times and temperatures to optimize the desired properties (see Examples).
• the mixture is first heated at a temperature from 1,000 °C to 1,475 °C, 1,100 °C to 1,475 °C, or from 1,200 °C to 1,475 °C.
• the mixture is sintered at a temperature from 1,600 0 C to 1,775 0 C from 24 hours to 1,000 hours.
• the resulting material comprises excess Y 2 O 3 as well as YPO 4 as the reaction product.
• the modified YPO 4 material comprises an excess amount of less than 0.5% Y 2 O 3 .
• the modified YPO 4 material comprises an excess amount Of Y 2 O 3 (De(P)) of 0.01% to less than 0.5%, 0.05% to less than 0.5%, 0.1% to less than 0.5%, 0.15% to less than 0.5%, 0.15% to 0.45%, 0.15% to 0.40%, or 0.20% to 0.40%.
• the excess Y 2 ⁇ 3 present in the final modified YPO 4 material functions to modify one or more microstructural properties of the final product.
• modify includes changing the value of the microstructural property of the modified YPO 4 material relative to stoichiometric- YPO 4 comprising no excess of Y 2 O 3 .
• an increase or decrease in the value of the microstructural property is contemplated.
• mechanical properties such as, for example, creep rate
• microstructural properties that can be modified using the methods described herein include, but are not limited to, porosity, linear shrinkage, grain size, and grain shape. Each microstructural property is discussed below with respect to altering a mechanical property of the modified YPO 4 material.
• modified YPO 4 material having a porosity of less than 10%, less than 9%, or less than 8% can be achieved.
• the porosity is from 7 to 9%.
• the modified YPO 4 material has a linear shrinkage less than 10% after sintering at a temperature greater than 1 ,600 0 C after at least 24 hours.
• Another microstructural property that can be modified using the methods described herein includes grain size and grain shape. Creep rate of polycrystalline ceramic materials are generally inversely proportionate to the grain size. Although, ceramic bodies with larger grain sizes are most resistant to creep, they will be weaker at the service temperature as compared to a finer grained material. Excessively large grain sizes can also produce microcracking if the grain size exceeds a threshold value.
• the presence of excess Y 2 O 3 in the modified YPO 4 material assists in pore elimination during the sintering process by slowing grain growth.
• the Y 2 O 3 phase reduces the mobility of grain boundaries, which prevents the formation of larger grains during sintering. Slower rate of grain growth provides more time for removal of pores during sintering.
• the excess Y 2 O 3 can also control grain size so that porosity is minimized yet creep rate is not adversely affected due to the formation of small grains.
• the modified YPO 4 material comprises one or more grains, wherein the size of the grains is less than 300 ⁇ m in certain embodiments, less than 100 ⁇ m in certain other embodiments, or less than 40 ⁇ m in certain other embodiments, hi another embodiment, the modified YPO 4 material has a creep rate of less than 200 ppb/hr at 1,180 °C or less than 600 ppb/hr at 1,250 0 C. [0059] By controlling the grain size, the modified YPO 4 material can have good mechanical strength. In general, the presence of large grain sizes can reduce mechanical strength.
• the modified YPO 4 material has a Weibull stress of at least 60 MPa, in certain embodiments from 100 MPa to 140 MPa, in certain embodiments from 110 MPa to 140 MPa, in certain other embodiments from 120 MPa to 140 MPa at 1,200 °C.
• the excess Y 2 O 3 present in the modified YPO 4 material can also act as a scavenger to sequester common impurities like silica and alumina. Not wishing to be bound by theory, it is believed that the impurities are drawn away from sensitive grain boundaries by the Y 2 O 3 , which can reduce creep but not compromise the strength of the modified YPO 4 material.
• the modified YPO 4 materials produced herein can be used in an apparatus for producing a glass or a glass-ceramic material.
• the modified YPO 4 material can be used to form an entire piece of apparatus used in a glass or glass-ceramic manufacturing system or just part of the piece of apparatus.
• the apparatus e.g., an isopipe
• the apparatus can have a core and a coating where the coating contacts molten glass and covers all or part of the core, in which case the modified YPO 4 material can form all or part of the core and/or all or part of the coating.
• the core can be a second refractory material.
• suitable materials for such a core include, without limitation, alumina, magnesium oxide, a spinel, titanium oxide, yttrium oxide, or a combination thereof.
• Other refractory materials that can be used for the core include zircon, silicon carbide, and zirconia.
• the coating can be applied by standard methods for applying ceramic coatings, such as, flame spraying or plasma spraying.
• the coating can comprise a second refractory material such as a refractory metal, a spinel, zircon, alumina, or a combination thereof.
• a second refractory material such as a refractory metal, a spinel, zircon, alumina, or a combination thereof.
• suitable refractory metals include platinum, molybdenum, rhodium, rhenium, iridium, osmium, tantalum, tungsten, and alloys thereof.
• Isopipes used in the production of LCD glass sheets are very expensive to manufacture and maintain.
• a reduction in porosity can extend the lifetime of the isopipe.
• the isopipe is made of a material with a high porosity, particularly open pores, molten glass can infiltrate the open pores. This can lead to radical changes in the isopipe' s properties over time.
• the sintered YPO 4 composite material of the present invention has a geometric porosity of less than or equal to 10%, in certain embodiments less than or equal to 8%, in certain other embodiments less than or equal to 7%, in certain embodiments less than or equal to 5%. In certain embodiments, it is desired that at least 90% (such as 93%, 95%, 97%, 98%, 99%) of the pores are closed.
• the modified YPO 4 material can also be used to form all or part of the following components of glass or glass-ceramic manufacturing equipment which, in typical applications, come into contact with molten glass or molten glass-ceramic: pipes, vessels, channels, weirs, bells, stirrers, bricks, blocks, gates, walls, bowls, ladles, needles, sleeves, plugs, molds, rings, plungers, tweels, and the like.
• samples 1-7 The influence of stoichiometry on sintering behavior, grain growth, and creep was studied for seven sintered YPO 4 composite materials (samples 1-7). Batch and compositional information is listed in TABLE 1. Platinum crucibles were used for all reactions with the exception; the sample 7 was reacted in a silica crucible. As a reference, the sample 8 is compositionally equivalent to sample 3.
• Powder synthesis and processing [0072] Reacted materials were dry ground and sieved to -325 mesh. After sieving, half of each batch was air-classified to -15 ⁇ m. Particle size of the -325 mesh sample 8 material was measured by light scattering as the technique is better suited to measure broad size distributions. A Coulter particle counter was used for the sample 2 powders. Particle size distributions (PSD's) of sample 8 after sieving to 325 mesh and both -325 mesh/+15 ⁇ m and -15 ⁇ m fraction of sample 2 after air-classification was determined.
• PSD's Particle size distributions
• Billets used to produce machine creep specimens were formed by cold isostatic pressing (CIP) of -325 mesh powders to a pressure of 18,000 psi in a cylindrical rubber mold (Trexler Rubber) that measures ⁇ 50 mm in diameter and 180 mm in length. Two billets were pressed for each composition, one for firing at 1,750 0 C for 4 hours and the other at 1,600 0 C for 48 hours. Heating and cooling rates of 100°C/hour were used in each case. Dimensions of the billets were tracked from mold filling, pressing, and firing to monitor shrinkage and changes in geometric density/porosity. Standard size creep samples that measure -3x5x155 mm were cut from the billets by wire-sawing.
• Rate of flexural creep was measured in a three-point bend configuration.
• the testing rig provides a support span of 150 mm. Creep rate was measured at 1 , 180 and then 1,250 0 C for each sample under an applied stress of 1,000 psi. Creep rates were determined by analysis of curves of deflection of the sample at mid span with time. The values in TABLE 4 correspond to secondary or steady-state creep.
• Modulus of rupture was measured by the four-point technique to understand the changes in strength of the standard sintered YPO 4 composite material with temperature and to determine how changes in phosphorous oxide deficiency affect high temperature strength. Bend-bars used in the four-point test were simply modified creep samples. The modifications were to cut the samples in half to a length of —77 mm and to chamfer the tensile edges. The creep samples were cut in half to fit within the testing fixture and to provide a larger number of measurements and thereby increase statistical confidence. Cross-section of samples measured approximately 3 x 5 mm. A support span of 50.8 mm and load span of 19.05 mm were used in all tests. The strain rate was 17.8 ⁇ m/s, and the total duration of each test was typically between 20 and 40 seconds. [0082] II. CHARACTERIZATION OF MODIFIED YPO4 MATERIAL [0083] Sintering and Microstructure
• Porosity should be maintained below 10% and preferably below 8% to ensure that pores are closed.
• the -325 mesh powder fills space and packs more efficiently after cold isostatic pressing than the -15 ⁇ m powder.
• the formed green body has a higher density and has a lower capacity for shrinkage during sintering as indicated on FIG. 3.
• the -325 mesh powder has a higher tap density and more effectively fills molds so there is less shrinkage during the cold isostatic pressing.
• FIG. 1 shows the impact of excess yttria on the porosity of a series of sintered YPO 4 composite material samples.
• the reference numerals in this figure represent the following:
• Porosity is higher when excess yttria is present for otherwise identical sintering temperatures and times. For example, porosity increases gradually with yttria excess for pills prepared from -15 ⁇ m air-classified powders that were sintered at l,600°C for 240 hours as indicated by the squares in the figure. Porosity of samples made of -15 ⁇ m powder is plotted as a function of time in FIG. 2. The plot shows that excess yttria slows the rate of sintering but does not necessarily impose a lower limit on porosity that is different than a stoichiometric batch.
• the reference numerals in this figure represent the following:
• Example No. in FIGs. 3 and 4 The size of grains decreases progressively as levels of excess yttria in the batch are increased. An attempt was made to quantify the effect by measuring grain sizes.
• TABLE 3 provides a summary of average grain size measured by the linear intercept technique. Average grain size is plotted as a function of sintering time in FIG. 5 and as a function of yttria excess in FIGs. 6 and 7 for sintering temperatures of
• 506 Sample No. 6 sintered at 1600 0 C; [00119] 507: linear fit of data of 501 to show trend;
• 601 Samples made from -15 ⁇ m powders sintered for 4 hours;
• 602 Samples made from - 15 ⁇ m powders sintered for 96 hours;
• 604 Samples made from -325 mesh powders sintered for 96 hours;
• [00135] 702 Samples made from -325 mesh powders sintered for 1000 hours;
• the reference numerals represent the following: [00138] 801 : Samples sintered for 4 hours and tested at 1180°C; [00139] 802: Samples sintered for 4 hours and tested at 1250°C; [00140] 803 : samples sintered for 100 hours and tested at 1180°C; [00141] 804: samples sintered for 100 hours and tested at 125O 0 C. [00142] Initial average particle size for the -325 mesh and -15 ⁇ m powders are indicated in the plots as reference points from which to judge the extent of grain growth. No change in average grain size occurs relative to the original particle size when approximately 1.0-1.5% excess yttria is added to the batch. The experiments demonstrate that grain size of sintered YPO 4 composite materials can be controlled by selection of initial PSD and addition of the appropriate amount of excess yttria to the batch. [00143] Creep and Modulus of Rupture
• Rates of flexural creep measured at 1 , 180 and 1 ,250°C under a 1 ,000 psi load for sintered YPO 4 composite materials with a range of stoichiometrics are tabulated in TABLE 4 and plotted in FIG. 8. Also included in the table are geometric porosities of the creep specimens. Geometric porosity (P(g)) is calculated as follows: , where p m is measured density of the actual material, andp t is the theoretical density of the densified material. It is assumed that the theoretical density of all specimens regardless of the specific compositions is 4.22 g em " .
• the Weibull stress which is an indicator of the strength of the material ranges from 120 to 140 MPa across the studied range of excess yttria.
• the Weibull stress increases slightly moving from the stoichiometric composition to the slight excess of yttria in sample 2.
• the Weibull stress increases further for sample 3. It then begins to fall at higher concentrations of excess yttria.
• the strengthening effect encountered by the addition of a slight excess of yttria can be related to reduction of grain size (see TABLE 3 and FIG. 6). Further increases in excess yttria do not markedly reduce grain size but begin to interfere with densification. This can be seen by inspection of porosity listed in TABLE 4.
• grain sizes only vary by a factor of two from the stoichiometric composition (sample 1) to the material with the greatest quantity of excess yttria (sample 6).
• Weibull stresses and parameters derived from analysis of stress at rupture are given in TABLE 5. The difference in Weibull stresses for samples originating from near an edge and those from bulk was less than 5 percent. The Weibull moduli, however, differed significantly from one another and from the random sample set. The Weibull moduli from the edge, bulk and random locations were 11, 15.8, and 22.6. Weibull moduli from edge and bulk location are both lower than from the random sample set. This indicates that the selected edge and bulk samples may have been handled roughly before testing.

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• 2008
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