Classifications

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  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B17/00—Forming molten glass by flowing-out, pushing-out, extruding or drawing downwardly or laterally from forming slits or by overflowing over lips
  • C03B17/06—Forming glass sheets
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  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B17/00—Forming molten glass by flowing-out, pushing-out, extruding or drawing downwardly or laterally from forming slits or by overflowing over lips
  • C03B17/06—Forming glass sheets
  • C03B17/064—Forming glass sheets by the overflow downdraw fusion process; Isopipes therefor
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
  • C04B35/584—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
  • C04B35/584—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride
  • C04B35/593—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride obtained by pressure sintering
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/71—Ceramic products containing macroscopic reinforcing agents
  • C04B35/78—Ceramic products containing macroscopic reinforcing agents containing non-metallic materials
  • C04B35/80—Fibres, filaments, whiskers, platelets, or the like
  • C04B35/82—Asbestos; Glass; Fused silica
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  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
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  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
  • C04B41/50—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials
  • C04B41/5025—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials with ceramic materials
  • C04B41/5035—Silica
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  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
  • C04B2235/3217—Aluminum oxide or oxide forming salts thereof, e.g. bauxite, alpha-alumina
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  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
  • C04B2235/3224—Rare earth oxide or oxide forming salts thereof, e.g. scandium oxide
  • C04B2235/3225—Yttrium oxide or oxide-forming salts thereof
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  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/34—Non-metal oxides, non-metal mixed oxides, or salts thereof that form the non-metal oxides upon heating, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
  • C04B2235/3418—Silicon oxide, silicic acids or oxide forming salts thereof, e.g. silica sol, fused silica, silica fume, cristobalite, quartz or flint
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  • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
  • C04B2235/658—Atmosphere during thermal treatment
  • C04B2235/6583—Oxygen containing atmosphere, e.g. with changing oxygen pressures
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
  • C04B2235/96—Properties of ceramic products, e.g. mechanical properties such as strength, toughness, wear resistance
  • C04B2235/9607—Thermal properties, e.g. thermal expansion coefficient
• • 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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  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P40/00—Technologies relating to the processing of minerals
  • Y02P40/50—Glass production, e.g. reusing waste heat during processing or shaping
  • Y02P40/57—Improving the yield, e-g- reduction of reject rates

Definitions

• This disclosure relates to isopipes used in the production of sheet glass by the fusion process and, in particular, to techniques for reducing the sag which such isopipes exhibit during use.
• isopipe is used in the specification and claims to refer genetically to a body having a configuration suitable for use as a glass forming structure in a fusion downdraw process, irrespective of the particular shape and construction of the body or whether formation of the body involves isopressing or not.
• silicon nitride material is used in the specification and claims to refer to a refractory material which comprises at least 34 wt. % N and at least 51 wt.% Si. BACKGROUND
• LCDs liquid crystal displays
• manufacturers of flat panel displays use glass substrates to produce multiple displays simultaneously, e.g., six or more displays at one time.
• the width of a substrate limits the number of displays that can be produced on a single substrate, and thus wider substrates correspond to increased economies of scale.
• the fusion process produces glass sheets whose surfaces have superior flatness and smoothness.
• the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of flat panel display devices, e.g., liquid crystal displays (LCDs).
• LCDs liquid crystal displays
• the system includes a supply pipe 9 which provides molten glass to a collection trough 11 formed in a free-space spanning, refractory body 13 known as an "isopipe.”
• molten glass passes from the supply pipe to the trough and then overflows the weirs 19 (i.e., the tops of the trough on both sides; see FIGS. 2 and 3), thus forming two sheets of glass that flow downward and inward along the outer surfaces of the isopipe.
• the two sheets meet at the bottom or root 15 of the isopipe, where they fuse together into a single sheet, e.g., a sheet having a thickness of -700 microns.
• the single sheet is then fed to drawing equipment (represented schematically by arrows 17 in FIG. 1), which controls the thickness of the sheet by the rate at which the sheet is drawn away from the root.
• a vertical temperature gradient imposed on the isopipe is used to manage the viscosity of the glass.
• the glass viscosity is typically in the range of approximately 100 to 300 IcP.
• the outer surfaces of the final glass sheet do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces only see the ambient atmosphere.
• the inner surfaces of the two half sheets which form the final sheet do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.
• isopipe 13 is critical to the success of the fusion process as it makes direct contact with the glass during the forming process.
• the isopipe needs to fulfill strict chemical and mechanical requirements to have a lifetime that is not too short and to deliver a quality sheet glass product.
• the isopipe should not be rapidly attacked by or be the source of defects in the glass.
• it should be able to withstand a vertical temperature gradient of, for example, 100°C during use, and transient gradients larger than that during heat up.
• the rate of deflection due to creep at the use temperature should be low.
• the dimensional stability of the isopipe is of great importance since changes in isopipe geometry affect the overall success of the fusion process. See, for example, Overman, U.S. Patent No. 3,437,470, and Japanese Patent Publication No. 11- 246230.
• the conditions under which the isopipe is used make it susceptible to dimensional changes.
• the isopipe operates at elevated temperatures on the order of 1000°C and above.
• the isopipe operates at these elevated temperatures while supporting its own weight as well as the weight of the molten glass overflowing its sides and in trough 11, and at least some tensional force that is transferred back to the isopipe through the fused glass as it is being drawn.
• the isopipe can have an unsupported length of two meters or more.
• Current business trends are towards ever larger glass sheets requiring ever larger isopipes for their formation.
• the weight of an isopipe made from zircon is estimated to be in excess of 15,000 pounds.
• isopipes 13 have been manufactured from isostatically pressed blocks of refractory material (hence the name "iso-pipe”).
• isostatically-pressed zircon refractories such as those sold by St. Gobain-SEFPRO of Louisville, Kentucky, have been used to form isopipes for the fusion process.
• zircon dissolves into the glass at hotter regions near the weirs of the isopipe, and then precipitates in the cooler regions near the root to form secondary zircon crystals. See U.S. Patent Publication No. 2003/0121287, published July 3, 2003, the contents of which are incorporated herein by reference. These crystals can be sheared off by the glass flow, and become inclusions in the sheet. Secondary crystals incorporated into the drawn glass are visual defects. Panels with such defects are rejected.
• Creep is the permanent change in the physical shape of a refractory or other material as a result of an imparted stress usually at elevated temperature.
• the creep acts in such a way as to relieve the stress, and is usually attributed to grain boundary sliding or material diffusion.
• An isopipe undergoing creep sags in the middle and deforms the weirs over which the glass flows. When the weirs are no longer straight, the glass flow distribution across the length is disturbed and it becomes more difficult and eventually impossible to manage glass sheet formation, thus ending production.
• Intrinsic Rate of Creep it is desirable to reduce the intrinsic rate of creep for any material used as an isopipe to: 1) enable use of a wider pipe, 2) extend the fusion draw process to higher temperature glasses (e.g., higher strain point glass that is more compatible with poly-silicon display manufacturing processes), and/or 3) extend the service life of the isopipe and thus minimize process down time and replacement costs.
• Rate of isopipe sag is proportionate to its length raised to the fourth power and inversely proportionate to the square of its height. A doubling in the length of the isopipe (with the same life requirement and temperature capability) requires either a 16 fold decrease in intrinsic creep rate or a four fold increase in height.
• the current process for fabrication of zircon isopipes cannot accommodate a four fold increase in isopipe height.
• the maximum length for a zircon isopipe which still has a reasonable service life has thus in essence been reached in the art or shortly will be reached with the current isopipe manufacturing technology. Accordingly, the ability to satisfy future requirements of flat panel display manufacturers for larger substrates will be substantially compromised with current technology.
• the present disclosure provides isopipes that have significantly improved creep rates compared to isopipes made from commercially available zircon, e.g., well below the 16-fold decrease in creep rate needed to compensate for a doubling in the length of an isopipe.
• the isopipes have a silica coating that is compatible with the types of glass compositions used to make substrates for flat panel displays. [0023] Accordingly, these isopipes are well-suited for producing flat glass by the fusion process because they can address some or all of the length, processing temperature, and/or sag problems of isopipes made from existing refractory materials, specifically, commercially available zircon.
• cost reductions can be achieved in, for example, the following ways: (1) longer isopipe lifetimes requiring less rebuild; (2) an expanded process window enabling yield improvements; (3) long term stability of isopipe shape allowing for reduced complexity of operation, especially near the end of an isopipe's life; and/or (4) increased glass delivery temperatures to the isopipe ( ⁇ 1300°C and higher) thus allowing the platinum delivery system to be shortened thus reducing material costs.
• An isopipe (13) for making a glass or a glass-ceramic (e.g., a display glass or a display glass-ceramic) which comprises a body having a configuration adapted for use in a fusion process, said body comprising a silicon nitride refractory material that:
• a method of making an isopipe (13) which has a configuration adapted for use in a fusion process includes in order:
• a method for reducing the sag of an isopipe (13) used in a fusion process that produces glass or glass-ceramic sheets which includes forming said isopipe (13) from a silicon nitride refractory material that is: (a) produced in block form in an atmosphere having a p ⁇ 2 of less than 0.1 (in one embodiment, less than 0.01) using less than 10 weight percent of one or more sintering aids (in one embodiment, less than or equal to 7 weight percent),
• the present disclosure also includes a method for making a glass sheet using an isopipe described summarily above and in detail below.
• Such method can be a fusion down-draw process.
• the higher mechanical performance of the isopipe results in better process stability and may enable the formation of glass sheet using glass materials otherwise unsuitable for a conventional fusion down-draw process using an isopipe made of traditional materials such as zircon.
• the reference numbers used in the above summaries are only for the convenience of the reader and are not intended to and should not be interpreted as limiting the scope of the invention. More generally, it is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention.
• FIG. 1 is a schematic drawing illustrating a representative construction for an isopipe for use in an overflow downdraw fusion process for making flat glass sheets. This drawing is not intended to indicate scale or relative proportions of the elements shown therein.
• FIG. 2 and 3 are schematic drawings illustrating a protective silica layer 31 produced by post-formation treatment of the outer surface of a silicon nitride isopipe.
• FIG. 2 shows the isopipe prior to post-formation treatment and
• FIG. 3 shows it after the treatment.
• protective coating 31 has been enlarged for purposes of illustration.
• Silicon nitride is a widely used material as a cutting tool and in such applications as turbine blades and bearings. It is also used in the semiconductor industry. However, to applicant's knowledge, it has not been used in the glass making industry under conditions where the silicon nitride could come into contact with molten oxide glasses. In particular, it has not been used under such conditions in the manufacture of isopipes for use in the fusion downdraw process. [0033] The reason for this lack of use is basically a prejudice against bringing non- oxide materials into contact with an oxide glass. It has previously been thought that such contact will have adverse effects on the glass, including the generation of bubbles at the glass/silicon nitride interface, e.g., nitrogen bubbles.
• Silicon nitride is difficult to sinter. Thus, it is normally sintered under applied pressure and with the use of sintering aids. The applied pressure is normally free of oxygen, e.g., approximately 95% of all silicon nitride is formed in an inert atmosphere. As a consequence, the exposed surface of the silicon nitride is capable of reacting with molten oxide glasses to produce nitrogen gas which has the potential to form bubbles.
• this problem can be overcome by post-formation treatment of the silicon nitride in an oxygen-containing atmosphere. The post-formation treatment, however, has to be carefully performed or the resulting product can still significantly react with oxide glasses.
• silicon nitride when heated in an oxygen-containing atmosphere, silicon nitride can enter into active or passive oxidation mechanisms.
• the active mechanisms are characterized by a net weight loss of the original silicon nitride product, while the passive mechanisms are characterized by a net weight increase.
• the active mechanisms are also characterized by continuing reactions with oxygen over time. Accordingly, once the active mechanism has been established, the silicon nitride can continue to react with oxygen, including oxygen available from contact with molten glass.
• the passive mechanism on the other hand, is self-limiting and stops once a silica layer on the order to a few microns forms on the surface of the silicon nitride.
• the active versus passive mechanism depends on the conditions under which the silicon nitride is exposed to oxygen, as well as the composition of the silicon nitride.
• the primary variables are temperature, oxygen pressure, and to some extent, time, ha particular, the combination of higher temperatures and low oxygen partial pressures promote the active mechanism, e.g., temperatures/O 2 combinations of T > 1600°C and p ⁇ 2 ⁇ 0.1.
• the time variable is less important, but if the silicon nitride is exposed to oxygen for extended periods of time, the SiO 2 layer that forms can be subject to spalling, which can make the material susceptible to the active mechanism.
• silicon nitrides which have more sintering aids are more likely to exhibit the active mechanism, while silicon nitrides which have less aids are less susceptible.
• sintering aids tend to dirninish silicon nitride's creep properties so that keeping such aids at low levels is beneficial both for the physical and for the chemical properties of the material.
• Some sintering aids are generally required to avoid the need for extreme measures to density silicon nitride, but in general terms, the sintering aids should be held to be less than 10 wt.%.
• the silicon nitride employed in the isopipes disclosed herein is produced in a low oxygen atmosphere, e.g., an inert atmosphere, so that the amount of sintering aids can be kept below the 10 wt.% level. Thereafter, the silicon nitride is machined into a configuration suitable for use as an isopipe. This structure is then treated hi an oxygen-containing atmosphere and at a temperature selected to promote the passive mechanism and to inhibit the active mechanism (the "post-formation treatment"). For example, the post-formation treatment of the machined silicon nitride can take place in air at a temperature on the order of 1200°C for a period of time of at least 24 hours.
• the silicon nitride is found to have a silica layer on its exposed surfaces having a thickness which is usually greater than about 2 microns and less than about 50 microns.
• the thickness of this layer can be determined by taking a cross-section of the material and examining it by SEM. In some cases, a Si 2 N 2 O interlayer will also be seen between the silicon nitride material and the outer silica layer.
• the post-formation treatment can be performed in place in the fusion machine if the muffle in which the isopipe is mounted is constructed so as to be able to generate a sufficiently high temperature to perform the treatment.
• isopipes are surrounded by muffles which are made of refractory materials and equipped with heating elements for adjusting the temperature within the muffle.
• the package of one or more sintering aids used with the silicon nitride needs to be selected with the particular glass that is to be formed using the silicon nitride isopipe in mind.
• alumina and yttria are commonly used as sintering aids for silicon nitride. Between the two, yttria may be preferable for various glasses since it has a. lower diffusion rate than alumina and is less compatible with most common glasses, including LCD glasses, thus making it less likely to combine with the glass and produce undesirable devitrification.
• Silicon nitride blocks suitable for making isopipes can be made in various ways. For example, one can start with a mixture of silicon nitride powder and one or more sintering aids. As discussed above, the weight percent of the sintering aids is less than or equal to 10% and in some embodiments, less than or equal to 7 wt.%. Suitable sintering aids include silica, yttria, alumina, and combinations thereof. [0044] In a study of oxidation behavior of silicon nitride, Themelin et al.
• compatibility triangle which results in a stable silica layer when yttria is used as a sintering aid. See Themelin et al., "Oxidation Behavior of a Hot Isostatically Pressed Silicon Nitride Material," Journal De Physique FV 5 Vol. 3, December 1993, pages 881- 888. The reference identifies particular temperature ranges and oxygen concentrations which result in the final product being in the compatibility triangle. Similar compatibility triangles can be constructed for other sintering aids, or combinations of aids, using techniques of the type employed in the Themelin et al. reference.
• a source of silicon in the original batch materials e.g., silica as a sintering aid, so that the final silicon nitride material is silicon rich, e.g., 1 wt.% silicon rich.
• silicon rich e.g. 1 wt.% silicon rich.
• the powdered batch materials can be formed into a block suitable for making an isopipe using conventional ceramic processing techniques. See, for example, Reed, James S., Principles of Ceramics Processing, 2nd Edition, Wiley Interscience, New York, 1995.
• a green body can be formed using the powder by a process that comprises, for example, uniaxial pressing, isostatic pressing, extrusion, slip cast molding, gel casting, or a combination thereof, hi particular, the green body can be formed with isostatic pressing, with low pressure isostatic pressing, or without isostatic pressing, as appropriate to the application.
• the green body once formed, is fired to produce a block of silicon nitride of suitable dimensions to form an isopipe. Due to the very strong covalent bonds in the material, classic sintering in atmospheric air will not take place for most compositions. Quasi-traditional sintering, e.g., pressure-less or atmospheric sintering, can be performed by using large amounts of sintering aids and/or extremely small particle sizes. However, as discussed above, excessive amounts of sintering aids leads to degraded physical properties of the final product, especially high temperature ones. It also promotes the undesirable active mechanism of SiO 2 formation.
• the firing of the silicon nitride blocks disclosed herein is performed in a substantially inert atmosphere, i.e., an atmosphere having a p ⁇ 2 of less than 0.1 (e.g., an atmosphere having a p ⁇ 2 of less than 0.01).
• the firing can be performed at a temperature in the range of 1700-2000°C, although higher and lower temperatures can be used if desired.
• pressure-assisted firing is employed, e.g., hot pressing, srnter-HIP or straight HTP (hot isostatic pressing). Even in these cases, some sintering aids are normally used, usually in the 2-10 mol% range.
• RBSN reaction bonded silicon nitride
• HPSN hot pressed silicon nitride
• SSN- sintered silicon nitride SRBSN—sintered reaction bonded silicon nitride
• HTP-SN- hot isostatic pressed silicon nitride SiAlON—Silicon- Aluminum-Oxygen-Nitrogen (a variant of Si 3 N 4 where alumina is used in sufficient quantity to force significant solid solution).
• the silicon nitride block is machined into a configuration suitable for use as an isopipe in a fusion process.
• the isopipe can consist of a single block of silicon nitride or it can be made of multiple pieces, some or all of which are composed of silicon nitride. Whatever configuration is chosen, the isopipe will have at least one surface which comes into contact with molten glass and is composed of silicon nitride with a protective silica layer formed on the silicon nitride after it has been machined.
• the protective silica layer is formed by a post-formation treatment, i.e., by exposing the machined surface to a partial pressure of oxygen equal to or greater than 0.1 (e.g., a partial pressure equal to or greater than 0.2) at an elevated temperature, e.g., a temperature equal to or greater than I 3 OOO 0 C (e.g., a temperature on the order of 1200°C), for an extended period of time, e.g., in one embodiment, for at least 12 hours, and in another embodiment, for at least 24 hours.
• a partial pressure of oxygen equal to or greater than 0.1 (e.g., a partial pressure equal to or greater than 0.2) at an elevated temperature, e.g., a temperature equal to or greater than I 3 OOO 0 C (e.g., a temperature on the order of 1200°C)
• an extended period of time e.g., in one embodiment, for at least 12 hours, and in another embodiment, for at least 24 hours.
• the conditions used in the post-formation treatment are selected to promote the passive oxidation mechanism and to inhibit the active mechanism, where the passive mechanism is characterized by a net increase in the weight of the part and the active mechanism by a net decrease.
• the post-formation treatment can be performed off-line or after the isopipe has been installed in the fusion machine. In either case, the treatment can be performed by exposing the machined surface to the oxygen by using air as a treatment gas or by preparing a gas mixture containing oxygen at a partial pressure of at least 0.1. As a further alternative, the treatment can be performed by exposing the surface to a heated liquid, e.g., molten glass, whose p ⁇ 2 is equal to or greater than 0.1.
• Isopipes made from commercially available zircon are not able to withstand these higher temperatures, while still having practical configurations (practical heights) and use lifetimes.
• the intrinsic rate of creep for commercially available zircon has been observed to increase by a factor of more than 30 when going from 1180 to 1250°C (see Table 2). Accordingly, fusion formation of a glass substrate having a strain point that is ⁇ 70°C higher than current glasses at the same width would require a 5.3 fold increase in the height of the isopipe to maintain even the most minimal of practical lifetimes.
• the numbers and size of defects resulting from dissolution of zircon into the glass will increase with temperature.
• the data was obtained from NAS A/TM— -2000-210026, Silicon Nitride Creep Under Various Specimen-Loading Configurations, Sung R. Choi, Ohio Aerospace Institute, Brook Park, Ohio; Frederic A. Holland, Glenn Research Center, Cleveland, Ohio Available from NASA Center for Aerospace Information 7121 Standard Drive Hanover, MD 21076.
• Both the zircon data and the silicon nitride data are based on a flexural creep strain rate evaluation based on bending. As shown in the table, the silicon nitride samples were tested at higher temperatures and stresses than the zircon samples. Also, the silicon nitride test samples were shorter in length and tested in general for longer times (100's of hrs).
• the values set forth in Table 2 are the averages of the steady state creep strain rates at the start and end of test. The starting creep rates were higher than the ending rates, but substitution of those into the reported averages above does not alter the outcome.
• Table 2 clearly shows that the major improvement in creep strain rates achieved through the use of silicon nitride. Comparing the lowest stress silicon nitride material to the standard zircon material gives a three order of magnitude improvement in creep strain rate, at 4X the applied stress and 120°C hotter test condition. Advanced zircon, which was designed to minimize creep rate does better, but is still two orders of magnitude below the silicon nitride material.
• Table 3 provides a further comparison between silicon nitride and zircon.
• silicon nitride has a higher decomposition temperature, 1900°C vs. 1625°C, than zircon, thus enabling high temperature operation; silicon nitride has an approximately 30% lower density than zircon, which, all other things being equal, reduces the load on the isopipe, which is the source of creep; silicon nitride has an ⁇ 5-6X improvement in MOR at room temperature and >3-4X at elevated temperature; silicon nitride has a -2-3X improvement in fracture toughness; silicon nitride has a -20-25% lower thermal expansion coefficient; and silicon nitride has a higher thermal shock parameter, although in this case, the measurement technique used for the Si 3 N 4 and zircon materials may have been different.
• Example 1 A silicon nitride isopipe is prepared by pressurized firing in a nitrogen atmosphere of a green body composed of silicon nitride powder and less than 10 wt.% sintering aids. The fired green body has a length greater than 1.5 meters, a height greater than 0.25 meters, and a depth greater than 0.1 meters.
• the fired green body is machined into an isopipe configuration.
• the machined isopipe is mounted in a fusion machine and heated in air to a temperature of 1200°C.
• the isopipe is held at this temperature for 24 hours during which time a silica layer is formed on the machined surface which exhibits substantially only a passive oxidation mechanism.
• the isopipe is used in the fusion process to produce a ribbon of glass which is cut into sheets which, after finishing, are used as substrates for liquid crystal displays.
• the molten glass remains in contact with the isopipe for substantial periods of time at elevated temperatures.
• the surface of the isopipe is found to be compatible with the molten glass in that the finished substrates exhibit a defect level, including crystals, blisters, and other onclusions and inclusions, below 0.001 defects per pound.
• the silicon nitride material has a flexural creep strain rate at 1250°C and 1000 psi that is less than 1 x 10 ⁇ 8 /hour and the isopipe exhibits substantially less sag than a zircon isopipe of the same dimensions and configuration used under the same conditions.

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