YTTRIUM OXIDE CERAMIC BODY

Abstract: A sinterable yttrium oxide powder is produced useful for pressureless sintering a compact thereof into an optically translucent or transparent body. The yttrium oxide powder has a specified particle size range and specific surface area. It is formed by preparing yttrium oxalate hydrate powder by means of the wet chemical oxalate method, comminuting the yttrium oxalate hydrate powder to a required degree and calcining the yttrium oxalate hydrate.

Description: This invention relates to the production of an optically transparent polycrystalline yttrium oxide body. Yttrium oxide (Y₂O₃) has been long considered to be an excellent candidate material for high temperature lamp envelopes provided an economical process could be used to prepare translucent or transparent bodies without darkening in the reducing conditions of a typical sodium vapor discharge lamp. U.S-A-3,878,280 discloses vacuum hot pressing of yttrium oxide powder without additives in a graphite die at 1300°C to 1500°C to produce transparent yttrium oxide having an ultrafine grain size of less than 1 micrometer. This patent makes no disclosure of the stability of such hot pressed product. For example, a relatively small amount of carbon introduced by the vaporization of the graphite die and locked into the pores of the hot pressed body can cause bloating when heated in air at the operating temperatures of a sodium vapor discharge lamp ranging from 1100°C to 1200°C. In addition, hot pressing is not operable for producing a thin-walled hollow body of yttrium oxide such as on envelope for a metal vapor discharge lamp. U.S-A-3,545,987 discloses the preparation of transparent Y₂O₃-based ceramics containing ThO₂, ZrO₂ or HfO₂ as sintering additives. These compositions make a cubic solid solution of high transparency, but they have a strong tendency to darken within several hours during use as lamp envelopes in a sodium vapor discharge lamp. This darkening problem cannot be tolerated with these Y₂O₃ solid solutions because these materials eventually become totally absorbing in the visible region and useless during lamp operation. U.S-A-3,873,657 discloses the use of beryllium additives to prepare transparent Y₃O₃-based ceramics, but commercialization of this composition is probably not practical because of the toxic nature of the beryllium compounds. U.S-A-4,147,744 discloses a sintering process for producing a substantially transparent Y₂O₃-based body of Y₂O₃ and lanthana (La₂O₃). These compositions can be sintered to transparency by a special high temperature (1900-2150°C) heat treatment which makes use of the cubic-to-hexagonal phase transition responsible for retarding grain growth and enhancing sintering. This patent also discloses that lanthana and yttria precursors such as co-precipitated oxalates, carbonates, hydroxides and organic carboxylates may be used. U.S-A-4,174,973 discloses the incorporation of 0.1 to 5 wt. % of MgO or MgAl₂O₄ as a sintering aid to Y₂O₃ to make transparent Y₂O₃-based ceramics containing 0.1 to 5 wt. % of MgO or MgAl₂O₄. The trivalent (Al³⁺) and/or divalent (Mg²⁺) additives still permit good chemical stability of these sintered Y₂O₃ compositions toward reduction and darkening in high temperature, low oxygen environments such as those present in sodium vapor discharge lamps. However, a disadvantage of these Y₂O₃ ceramics containing MgO or MgAl₂O₄ is that residual secondary phase(s) remain in the microstructure of the sintered product and can cause an unknown amount of light scattering which degrades the overall optical quality. DE-A-2 056 763 describes a process for making a translucent yttrium oxide body comprising pressing an yttrium oxide powder of high purity having on average particle size ranging from 0.4 to 10 µm to form a body, firing for at least 30 minutes at a temperature of 2150 to 2330°C the formed body in a dry hydrogen atmosphere or under vacuum and thereafter at 1950 to 2300°C in a hydrogen atmosphere having a dew point of 0°C. From figure 2 of DE-A-2 056 763 it is clear that when firing temperatures less than 2100°C are used drastic drops both in the final densities and the transmittance of the bodies, are experienced. An embodiment of the process of the present invention may produce optically transparent Y₂O₃ ceramics without any sintering additives. Embodiments of the present invention may eliminate the subsequent darkening problems of the Y₂0₃ ceramic that is usually associated with the incorporation of many of the aforementioned sintering additives. In addition, an embodiment of the present invention may permit the fabrication of complex-shaped parts (i.e., thin-walled tubes) on an economical production basis and overcomes the adverse economics and impracticality of the hot-pressing method of consolidation of Y₂O₃ powders. Non-limitative aspects, embodiments and examples of the present invention will be discussed below, and reference may be had to the accompanying drawings, in which: FIG. 1 is a photomicrograph (magnified 200X) showing the microstructure of a polished sintered body of yttrium oxide produced in accordance with an embodiment of the present invention. FIG. 2 shows spectral transmittance, i.e. in-line transmission, as a function of wavelength for an as-sintered body, i.e. a sintered yttrium oxide body produced in accordance with an embodiment of the present invention, and for the same body after it was fired in dry hydrogen at 1450°C for 50 hours. In accordance with an aspect of the present invention, an yttrium oxide powder is produced, formed into a compact and pressureless sintered to produce a polycrystalline body which is optically transparent. By pressureless sintering herein it is meant the densification or consolidation of the compact without the application of mechanical pressure into a ceramic body which is at least optically translucent. By an optically transparent body, it is meant herein a body having an in-line spectral transmission of at least 10% taken at a wavelength of 590 manometers on a thickness of 0.85 millimeters of said body. The yttrium oxide powder is white in color and is crystalline. Generally, it has a purity of at least 99.9% and higher, preferably at least 99.99%, and most preferably greater than 99.99%. % purity herein denotes % by weight. The yttrium oxide powder is free of particles greater than 5 micrometers in size, preferably free of particles greater than 4 micrometers, and more preferably free of particles greater than 3 micrometers. Also, the preferred yttrium oxide powder has an average particle size of less than 5 micrometers preferably less than 4 micrometers, and more preferably less than 2 micrometers. Generally, it has on average particle size ranging from 1 micrometer up to 5 micrometers, preferably from 1 micrometer up to 4 micrometers, and typically, the average particle size ranges from 1 micrometer to 1.5 micrometers. Average particle size is determinable in a standard manner by measurement of the particle size distribution. An yttrium oxide particle herein is defined as consisting essentially of a cluster of smaller size crystals weakly bonded together autogeneously, i.e., such bonding is believed to be caused by Van der Waal's forces or by self-bonding. The yttrium oxide powder has a specific surface area ranging from 4 to 25 square meters per gram, preferably from 5 to 15 square meters per gram, and typically, it ranges from 6 to 10 square meters per gram. By specific surface area or surface area of a powder herein, it is meant the specific surface area according to BET surface area measurement. A process for producing a sintered yttrium oxide body which is at least optically transparent having an in-line spectral transmission of at least 10% taken at a wavelength of 590 nanometers on a thickness of 0.85 millimeter of said sintered body and having an average grain size ranging from 5 micrometers to 50 micrometers characterized in that it comprises : producing an yttrium oxide powder having an average particle size of less than 5 micrometers and being free of particles greater than 5 micrometers and having a specific surface area ranging from 4 square meters per gram to 25 square meters per gram; said yttrium oxide powder being produced by providing an agueous yttrium nitrate solution of at least 0.1 mole of yttrium per liter of solution, providing an aqueous oxalic acid solution having a concentration of at least 10% excess of that required for complete reaction with said yttrium nitrate, admixing said solutions thereby precipitating yttrium oxalate hydrate, recovering said yttrium oxalate hydrate precipitate, washing said yttrium oxalate precipitate with water to neutralize it, drying said yttrium oxalate hydrate precipitate to remove absorbed water, comminuting the resulting yttrium oxalate hydrate powder in air at ambient temperature, said yttrium oxalate hydrate precipitate being deflocculated in aqueous solution of a basic dispersant after neutalization and before it is dried, or before it is communited, to produce a powder wherein the average agglomerate size is less than 20 micrometers and which is free of agglomerates having a size greater than 20 micrometers, and thermally decomposing said yttrium oxalate hydrate at a temperature ranging from 650°C to 1000°C in air at ambient pressure to produce said yttrium oxide powder; forming said powder into a compact having a density of at least 45% of the density for yttrium oxide; firing said compact at a temperature ranging from 1700°C to 1900°C for a time sufficient to produce said sintered body, said firing being carried out in an atmosphere of hydrogen, said hydrogen atmosphere containing at least a sufficient partial pressure of oxygen at least after said compact becomes a closed pore body to produce said optically translucent sintered body. Briefly stated, in a more specific embodiment, the yttrium oxide powder is produced by a process which comprises providing an aqueous yttrium nitrate solution of at least 0.1 mole of yttrium per liter of solution, providing an aqueous oxalic acid solution having a concentration of at least 10% excess of that required for complete reaction with the yttrium nitrate, admixing said yttrium nitrate solution with said oxalic acid solution thereby precipitating yttrium oxalate hydrate, recovering said yttrium oxalate hydrate, washing said yttrium oxalate hydrate with water to at least substantially neutralize it, drying said yttrium oxalate hydrate to remove adsorbed water, deflocculating and flocculating said dried yttrium oxalate hydrate and powder wherein the average agglomerate size is less than 20 micrometers and which is free of agglomerates having a size greater than 20 micrometers, and thermally decomposing sold yttrium oxalate hydrate at a temperature ranging from 650°C to 1000°C in air at ambient pressure to produce said yttrium oxide powder. By ambient pressure herein, it is meant atmospheric or about atmospheric pressure. By ambient temperature herein, it is meant at or about room temperature, i.e. at or about 21°C. In the above embodiment the yttrium oxalate hydrate is produced by the wet chemical oxalate method. In this method, yttrium nitrate solution is admixed with oxalic acid solution to precipitate yttrium oxalate hydrate. The yttrium nitrate solution can vary in concentration and generally ranges from 0.5 moles to 3 moles of yttrium, preferably about 1 mole of yttrium, per liter of solution. The yttrium nitrate solution can be formed by a number of conventional techniques. Preferably, it is formed by dissolving yttrium oxide powder, preferably having a purity of at least 99.9% or higher, in a mixture of distilled or deionized water and concentrated nitric acid. Alternatively, the yttrium nitrate solution can be formed by dissolving yttrium nitrate, preferably having a purity of at least about 99.9% or higher, in distilled or deionized water. The oxalic acid solution can be formed by dissolving oxalic acid preferably having a purity of at least 99.9% or higher in distilled or deionized water. The oxalic acid solution can vary in concentration and generally it ranges from 0.5 to 1.0 mole of oxalic acid, preferably about 0.8 mole, per liter of solution. Preferably, oxalic acid in excess of that stoichiometrically required to complete reaction is used to ensure complete reaction. The yttrium nitrate solution and oxalic acid solution are admixed to precipitate yttrium oxalate hydrate. Such mixing can be carried out in a conventional manner and preferably it is carried out at ambient pressure and temperature. The formation of yttrium oxalate hydrate is illustrated by the following reaction: where n can typically vary from 4 to 10. The yttrium oxalate hydrate, i.e. precursor, can be recovered by a number of conventional techniques. Preferably, it is collected into a filter cake by centrifuging or by vacuum filtration. The precursor is washed generally with distilled or deionized water or methyl alcohol to remove acid therefrom to produce a substantially neutralized material, i.e. a material generally having a pH ranging from 5 to 7. The neutralized material can be collected in a conventional manner, preferably by vacuum filtration or centrifuging. The collected neutralized material is dried preferably in air to remove excess water therefrom, i.e., water physically adsorbed thereon. Typically, it is dried in flowing air at a temperature ranging from 110°C to 125°C for a period of time determinable empirically generally ranging from 12 hours to 24 hours. Typically, the dried yttrium oxalate hydrate is a white fluffy powder and is comprised of agglomerates. X-ray diffraction analysis of this yttrium oxalate hydrate powder indicates that it has some degree of crystallinity. The yttrium oxalate hydrate powder may be comminuted to reduce its agglomerate size, especially to significantly reduce the fraction of its larger sized agglomerates. Preferably, the yttrium oxalate hydrate powder is comminuted to produce a powder which, when calcined, will produce the preferred yttrium oxide powder. Such comminution is determinable empirically. Generally, the yttrium oxalate hydrate powder is comminuted to produce a powder comprised of agglomerates having an average size which is less than 20 micrometers and which is free of agglomerates greater than 20 micrometers size. Preferably, the yttrium oxalate hydrate powder is comminuted to produce a powder having an average agglomerate size of less than 10 micrometers and free of agglomerates greater than 15 micrometers in size. Specifically, the comminuted precursor powder should be of an average size which is approximately or roughly about three times larger than the size desired of the resulting yttrium oxide powder. Generally, the comminuted yttrium oxalate hydrate powder has a specific surface area ranging from 3 to 10 square meters per gram and typically it ranges from 4 to 6 square meters per gram. Comminution of the precursor powder can be carried out by a number of conventional techniques which do not introduce contaminants into the powder which would prevent production of optically translucent sintered body. Comminution time varies widely and depends largely on the amount and particular size reduction desired and type of equipment used. Generally, the precursor powder is comminuted in air at ambient temperature. Preferably, the precursor powder is air milled, i.e. fluid energy milled. When the powder is fluid energy milled, it is preferably homogenized to attain substantially uniform size distribution in a conventional manner such as, for example, by tumbling or blending in a plastic jar. Also, preferably, such blended powder is screened through a 1.7 mm to 850 µm (10 to 20 mesh) nylon screen to reduce the size of any large agglomerates which might have formed during blending. Before drying of the collected neutralized yttrium oxalate hydrate precipitated, or before comminution of the dried yttrium oxalate hydrate powder, it is dispersed, i.e. deflocculated, in an aqueous solution of a basic dispersant preferably formed with distilled or deionized water. The dispersant should be one which effectively disperses the precursor powder into a suspension and has no significant deleterious effect on the sintered product. Preferably, the dispersant is removed by water-washing, leaving no significant amount thereof, preferably leaving no amount thereof which is detectable by standard techniques. Representative of a useful dispersant is an organic hydroxide, preferably an organic ammonium hydroxide. Most preferably, the dispersant is tetramethylammonium hydroxide (TMAH). The concentration of dispersant solution and period of time that the precursor powder is dispersed in the dispersant solution is determinable empirically. This treatment of the precursor powder with dispersant solution is preferably carried out at ambient temperature and pressure and usually significantly improves the clarity of the resulting sintered body. Generally, from a 0.1 volume % to a 10 volume %, preferably from a 2 volume % to a 6 volume %, and most preferably from a 3 volume % to a 4 volume % solution of dispersant is useful. Preferably, the precursor powder is stirred in the dispersant solution, usually for less than 1 hour, generally from ten minutes to 40 minutes, and typically for about 30 minutes. The dispersed precursor powder can be flocculated in a conventional manner by adding an acid to the suspension. The acid should be one which is an effective flocculant and which has no significant deleterious effect thereon. The amount of acid flocculant is determinable empirically and preferably should be just sufficient to flocculate the precursor powder. Preferably, the acidic flocculant is an aqueous solution of oxalic acid which is added to the suspension until flocculation takes place. The flocculated precursor powder can be recovered in a conventional manner. Preferably, it is collected by means of vacuum filtration or centrifuging. The recovered precursor powder is washed preferably with distilled or deionized water to remove the acid therefrom and produce a substantially neutralized material, i.e. a material generally having a pH ranging from 5 to 7. The neutralized material can be recovered in a standard manner and preferably it is collected into a cake by centrifuging or vacuum filtration. If desired, washing and centrifuging can be carried out by means of centrifuging. The washed precursor powder is dried preferably in air to remove adsorbed water therefrom. Typically, it is dried in flowing air at a temperature ranging from 110°C to 125°C for a period of time determinable empirically and generally ranging from 12 hours to 24 hours. The resulting dried powder is then ready to be comminuted. The comminuted precursor powder is thermally decomposed, i.e. calcined, to produce the preferred yttrium oxide powder. Calcining of the comminuted precursor powder is preferably carried out in air at ambient pressure at a temperature ranging from 650°C to 1000°C, preferably from 750°C to 850°C, and most preferably it is about 800°C. At a temperature below 650°C, the decomposition of the precursor may not be complete, and above 1000°C, large hard aggregates may form that can reduce translucency. Calcining time is determinable empirically, for example, by weight loss. Calcining is completed when there is no more weight loss on further firing. Generally, calcining time ranges from one hour to four hours. The present thermal decomposition of yttrium oxalate hydrate is illustrated by the following reaction: Y₂(C₂O₄)₃·nH₂O (s) → Y₂O 3(s) + 3CO (g) + 3CO 2(g) + nH₂O (g) where (s) represents a solid product and (g) represents a gaseous product. In the present invention a process for producing an optically transparent polycrystalline yttrium oxide body comprises forming the preferred yttrium oxide powder into a compact having a density of at least 45% of the theoretical density for yttrium oxide, and sintering said compact at a temperature ranging from 1700°C to 1900°C in an atmosphere of wet hydrogen, or in an atmosphere of dry hydrogen until the sintered body has become a closed pore body followed by sintering or firing it in wet hydrogen. Generally, the firing or sintering atmosphere is at or about ambient pressure. A number of techniques can be used to shape the yttrium oxide powder into a compact, i.e. green body. For example, it can be extruded, injection molded, die-pressed. isostatically pressed or slip cast to produce the green body of desired shape. The compact can vary in form and size and can be simple, hollow and/or complex shape. Any lubricants, binders or similar materials used in shaping the powder should have no significant deleterious effect on the resulting sintered body. Such materials are preferably of the type which evaporate on heating at relatively low temperatures, preferably below 500°C, leaving no significant residue. The green body should have a density of at least 45%, preferably greater than 45%, more preferably greater than 50% and most preferably greater than 55% of the theoretical density of 5.03 g/cc for yttrium oxide to promote densification during sintering and achieve attainment of an optically transparent sintered body. Preferably, before sintering, the green body or compact is prefired in an oxygen-containing atmosphere such as air at a temperature below 1000°C, generally ranging from 500°C to below 1000°C, to eliminate impurities including shaping aids which would have a significant deleterious effect on the optical translucency of the sintered body. The particular prefiring temperature and prefiring period is determinable empirically and depends largely on the level of impurities present and on the thickness of the body, and generally ranges from 1 to 5 hours. Such prefiring allows the sintering atmosphere to be free of impurities, and imparts sufficient strength to the compact allowing it to be more easily handled and machined. The green or prefired body is fired in an atmosphere of hydrogen to produce a sintered body. At least at some stage of the firing of the body, the hydrogen atmosphere should be provided with at least a sufficient oxygen partial pressure to produce and maintain an optically translucent sintered body. Such oxygen partial pressure is determinable empirically. In one embodiment of the present invention, the green or prefired body is fired in an atmosphere of wet hydrogen to produce the sintered body. The wet hydrogen atmosphere should contain at least a sufficient partial pressure of oxygen to produce and maintain an optically transparent body, i.e. to prevent discoloration of the sintered body in the present process, and is determinable empirically. Specifically, the wet hydrogen atmosphere should be chosen to be reducing to the furnace elements which might be tungsten or molybdenum in a refractory metal furnace and should be slightly oxidizing to the sample being sintered so that it will not be discolored or darkened by too low an oxygen partial pressure, i.e. too much of a reducing atmosphere. Such a wet hydrogen atmosphere can be provided by incoming hydrogen gas having a dewpoint temperature ranging from 0°C to 25°C, and preferably from 15°C to 22°C. Such incoming wet hydrogen gas determines how much oxygen will be in the furnace atmosphere at any particular firing temperature. In the preferred sintering temperature range, the oxygen partial pressure in the sintering atmosphere generally ranges from 10⁻³ to 10⁻⁷ Pa (10⁻⁸ to 10⁻¹³ atmospheres). In a preferred embodiment of the present invention, the green or prefired compact is fired initially in an atmosphere of dry hydrogen until the sintered body becomes at least a closed pore body followed by firing in an atmosphere of wet hydrogen. Generally, the dry hydrogen atmosphere is provided by incoming hydrogen gas having a dewpoint temperature ranging from -50°C to -60°C. Generally, the dry hydrogen atmosphere has an oxygen partial pressure at sintering temperature of less than 10⁻¹⁵ Pa (10⁻²⁰ atmosphere). When the sintered body becomes a closed pore body, at which point it typically has a density ranging from 92% to 96%, or preferably when the sintered body has a density higher than that of the closed pore body, more preferably when the sintered body has a density of at least about 99%, and most preferably when the sintered body has a density greater than about 99.9%, an atmosphere of wet hydrogen is introduced. The wet hydrogen atmosphere is used to remove or substantially remove discoloration of the sintered body caused by firing in dry hydrogen and to complete any remaining sintering of the body. Therefore, such a wet hydrogen atmosphere should contain at least a sufficient partial pressure of oxygen to remove or at least substantially remove the discoloration of the sintered body caused by dry hydrogen, i.e. to produce and maintain the body optically transparent, and is determinable empirically. Generally, such a wet hydrogen atmosphere can be provided by incoming hydrogen gas having a dewpoint temperature ranging from 0°C to 25°C, preferably from 15°C to 22°C. Generally, a sintered body with significantly increased clarity is produced by firing initially in the dry hydrogen atmosphere to the closed pore stage followed by the firing in wet hydrogen. Generally, the firing or sintering atmosphere is at or about ambient pressure. The sintering temperature, i.e. maximum firing temperature, ranges from 1700°C to 1900°C, and preferably from 1725°C to 1800°C. Sintering temperatures lower than 1700°C either would require too long a period of sintering time to be practical or are not operable to produce a satisfactory product. On the other hand, temperatures higher than 1900°C tend to produce a sintered body with no significant advantages in optical quality but with the disadvantage of grains that are too large thereby rendering it with poor strength. The rate of heating to sintering temperature can vary and should have no significant deleterious effect on the body. Generally, heating rates can range from 100°C per hour up to 700°C per hour, and usually from 200°C per hour to 400°C per hour. An intermediate soak time of 5 to 10 hours at temperatures between 1700 and 1800°C has been found useful in providing sintered bodies with high translucency/transparency. This intermediate soaking period is carried out in the sintering furnace during the sintering cycle. The particular time period at the maximum firing temperature depends largely on such temperature and is determinable empirically. Specifically, increasing sintering or firing temperature requires less sintering or firing time. Generally, however, where only a wet hydrogen atmosphere is used, a sintering temperature of 1700°C requires a sintering time period of 8 hours, and a sintering temperature of 1900°C requires a sintering time period of 2 hours to produce the transparent sintered body. The sintered body should be cooled, i.e. the firing temperature is reduced, preferably to ambient temperature, under conditions which have no significant deleterious effect thereon. Preferably it is cooled in a wet hydrogen atmosphere, preferably an atmosphere provided by incoming hydrogen gas having a dewpoint temperature ranging from 0°C to about 25°C. Generally, cooling rate should be less than about 1500°C per hour, and usually range from 500°C per hour to 1000°C per hour. Typically the sintered body consists essentially of cubic yttrium oxide phase as determined by X-ray diffraction and electron microscope measurements. The sintered body usually has a microstructure with an average grain size generally ranging from 5 micrometers to 50 micrometers, and still in another embodiment it ranges from 5 micrometers to 20 micrometers. The grains are normally cubic in symmetry, as ascertained by X-ray diffraction studies. Generally, the present sintered body has a density of at least 99.7%, usually at least 99.8%, preferably at least 99.9%, and more preferably greater than 99.9%. In the present specification, unless otherwise stated, the density of the sintered body as well as that of the green body or compact is given as a fractional density of the theoretical density of yttrium oxide (5.03 g/cc). The sintered body should be optically transparent. More specifically it should have an in-line spectral transmission of at least 10%, and preferably at least 20%, all values taken at a wavelength of 590 nm. By the term "in line spectral transmission" used herein, it is meant the ratio of the intensity of transmitted light to the intensity of incident light, obtained when parallel light of a certain intensity is incident perpendicular to the surfaces of a sample of a certain thickness. In the present embodiments, the in-line spectral transmission was determined on a polished plate of the present sintered body of a thickness of 0.85 millimeter at a wavelength of 590 nanometers. The sintered body is preferably stable, i.e. it does not exhibit bloating in air at temperatures of up to 1300°C. Embodiments of the present invention may make it possible to fabricate simple, hollow and/or complex shaped polycrystalline yttrium oxide articles directly. Specifically, embodiments of the sintered product can be produced in the form of a useful simple, hollow and/or complex shaped article without machining, or without significant or substantial machining, such as a thin walled tube, a long rod, a spherical body or a hollow shaped article. The sintered body may have a wide variety of uses. It may be useful in any system where a ceramic protective material or plate having the present light-transmitting properties is needed. Specifically, it may be useful as a light-transmitting filter or light-transmitting window for infrared domes and solar cells. It may be especially useful as a lamp envelope for a metal vapor discharge lamp such as a sodium vapor discharge lamp. Such a lamp envelope usually has a wall thickness ranging from ½ to 1 millimeter. The invention is further illustrated by the following examples wherein the procedure and materials were as follows unless otherwise stated: The yttrium oxalate hydrate powder used in all of the examples was prepared in substantially the same manner. Specifically, 160 grams of yttrium oxide powder of about 99.99% purity was added to 1120 cc of distilled water to which 320 cc of concentrated nitric acid was added. The mixture was heated with stirring until a clear yttrium nitrate solution was formed. The solution was concentrated by boiling down to about 700 cc of volume and vacuum filtered. Then 400 grams of oxalic acid with a purity of about 99.9% was dissolved in 4.8 liters of distilled water at room temperature and vacuum filtered. The yttrium nitrate solution was rapidly dripped into the stirred oxalic acid solution at ambient temperature thereby precipitating yttrium oxalate hydrate. When the precipitation was completed, the precipitate was recovered by decantation and substantially neutralized by washing it with distilled water. The pH of the final wash water was about 5.5. The precipitate was collected by vacuum filtration and oven dried for about 23 hours at about 110°C. X-ray diffraction analysis of the yttrium oxalate hydrate powder showed it to have a number of diffraction lines of a complex structured material that has some degree of crystallinity. A portion of this as-prepared powder was used to produce the yttrium oxide powder in Example 8. In Examples 1-5 and 7, the as-prepared yttrium oxalate hydrate powder was treated with an aqueous solution of tetramethylammonium hydroxide (TMAH) in substantially the same manner. Specifically, 100 grams of the yttrium oxalate hydrate powder was stirred into 300 cc of distilled water and soaked with stirring at ambient temperature for about 25 minutes. A 3 volume % aqueous solution of TMAH was stirred into the mixture causing the yttrium oxalate hydrate to deflocculate. After about 30 minutes of stirring of the mixture at ambient temperature, an aqueous oxalic solution was added to flocculate the yttrium oxalate hydrate. The hydrate was recovered by decantation and washed with distilled water at ambient temperature to substantially neutralize it. It was then vacuum filtered and oven dried at about 110°C. A portion of this powder was used to produce the yttrium oxide powder in Example 7. In Examples 1-5, the TMAH-treated yttrium oxalate hydrate powder was air milled. In Example 6, the yttrium oxalate hydrate powder was not treated with TMAH but it was air milled. Air milling. i.e. fluid energy milling, was carried out at ambient temperature in substantially the same manner in all of the examples. Specifically, the yttrium oxalate hydrate powder was milled by passing it through a standard fluid energy mill two times. The milled powder was then blended by tumbling at ambient temperature for 20 to 30 minutes in a plastic jar. It was then screened through a -850 µm (-20 mesh) nylon screen. This processed powder was used to produce the yttrium oxide powder. Thermal decomposition, i.e. calcining, of the yttrium oxalate hydrate powder was carried out in flowing air at ambient pressure at temperatures ranging from 820°C to 850°C for periods of time ranging from 1.5 to 2 hours. Size distribution of the powders was determined on a Horiba Model #CAPA-500 particle analyzer. Size analysis was carried out in the same manner. Specifically, the powder was dispersed ultrasonically for 2½ minutes in a weakly basic distilled water solution prior to particle size measurement. The yttrium oxide powder was formed into a compact, i.e. disk, in substantially the same manner at ambient temperature to produce compacts of substantially the same size having a density ranging from 50% to 60% of theoretical density. Specifically, 1.5 grams of the yttrium oxide powder was die pressed into a disk under a pressure of about 28 MPa and then isostatically pressed at 420 MPa, producing green compacts in the form of disks 2.5 cm in diameter and 0.14 cm thick. Firing and sintering of the compact was carried out in a tungsten resistance furnace. The compact was placed on a tungsten setter and placed in the furnace and sintered on a sintering schedule. Specifically, in all of the examples, except Example 5, the compact was heated at a rate of 390°C per hour to 1725°C, held for 8 hours at 1725°C, and then heated at a rate of 390°C per hour to the given sintering temperature. In Example 5, a heating rate of 390°C per hour to the sintering temperature of 1660°C was used. All of the sintered bodies were cooled at a rate of about 975°C per hour to ambient temperature. All of the examples in Table I were carried out in an atmosphere of hydrogen at or about ambient pressure. In Examples 1 and 7-9, the furnace atmosphere was wet hydrogen. In Examples 2-4 and 6, the furnace atmosphere was initially dry hydrogen and the wet hydrogen atmosphere was not introduced until after the given sintering treatment in the dry hydrogen atmosphere. The furnace was provided with a wet hydrogen atmosphere by incoming hydrogen gas having a dewpoint temperature ranging from 20°C to 22°C. The furnace was provided with a dry hydrogen atmosphere by incoming dry hydrogen gas having a dewpoint ranging from -50°C to -60°C. The flow rate of incoming hydrogen gas was about 10 cubic feet per hour. The compacts were sintered under the given sintering conditions in Table I. Density of the sintered body was determined before it was polished and was determined by water displacement using Archimedes method. Each sintered disk was polished in a standard manner using 0.3 micrometer aluminum oxide paste. Each disk was polished so that both of its large faces were substantially smooth, flat and parallel to each other. The final thickness of each polished disk was 0.85 millimeters. Spectral transmittance, i.e. in-line spectral transmission, of the polished disks was measured in a standard manner on a Perkin-Elmer Model 330 spectrometer at a wavelength of 590 nanometers in the visible wavelength. Average grain size of the sintered body was determined in a standard manner. Specifically, the polished disk was chemically etched with a 50% solution of hydrochloric acid and water for 30 seconds to reveal the grains. The average grain size was then determined by the lineal intercept method. The examples are illustrated in Table I. In Examples 1-5, the processed yttrium oxalate hydrate powder had an average agglomerate size of 3.1 micrometers with no agglomerates larger than 16 micrometers, and the resulting yttrium oxide powder had an average particle size of 1.1 micrometers with no particles larger than 4.3 micrometers. Specifically, in Example 1, the compact was sintered for two hours at 1950°C in wet hydrogen and the resulting polished sintered body was optically transparent. In Examples 2-4, where the compacts were initially sintered in dry hydrogen followed by sintering in wet hydrogen for the given times at the given temperature, all of the resulting polished sintered bodies were optically transparent, i.e. an image was visible when the polished disk was placed on printed words. A comparison of Examples 2-4 with Example 1 illustrates the improved optical transparency obtainable when initially sintering in dry hydrogen followed by firing in wet hydrogen. The microstructure of the sintered body of Example 4 is illustrated in Figure 1. The spectral transmittance of the polished sintered body of Example 4 is illustrated by the graph in Figure 2 labelled as-sintered. The polished sintered body was then annealed in an atmosphere of dry hydrogen at 1450°C (oxygen partial pressure of ≈ 10⁻¹³ Pa (10⁻²⁰ atm.)) for 50 hours and its spectral transmittance was again determined and is shown in Figure 2. Figure 2 illustrates that the severe annealing in hydrogen only slightly reduced the spectral transmittance in the visible region of the electromagnetic spectrum. Specifically, Figure 2 shows that there is only about a 3% drop in light transmittance from 48.7 to 45.4 for the transparent yttrium oxide with good optical quality. The resistance to darkening makes such a sintered yttrium oxide body useful for special optical elements of complex shape (e.g. thin-walled tubes) at low and high temperatures in severe environments. In Example 5, sintering was carried out at a relatively low temperature in an atmosphere of wet hydrogen producing an optically transparent body which was not as transparent as those produced in Examples 1-4. In Example 6, the comminuted yttrium oxalate hydrate powder had an average agglomerate size of 2.8 micrometers and was free of agglomerates greater than 13 micrometers. The resulting yttrium oxide powder had an average particle size of 1.3 micrometers no particles larger than 3.5 micrometers. The resulting polished sintered body in Example 6 was optically transparent. However, its transparency was not as high as that of Example 4 where the yttrium oxalate hydrate powder was also treated with TMAH illustrating the importance of treatment of the yttrium oxalate hydrate powder with dispersant. The processed yttrium oxalate hydrate powder in Examples 1-5 had a specific surface area of about 4.5 square meters per gram and the resulting yttrium oxide powder had a specific surface area of about 7 square meters per gram. X-ray diffraction analysis of the sintered bodies of Examples 1-6 showed all of them to be cubic polycrystalline materials having a lattice parameter of 10.6034x10⁻⁴ 0.0003x10⁻⁴ µm (10.6034 ± 0.0003Å), typical of pure Y₂O₃. Examples 3 and 4 illustrate present invention. The sintered bodies produced in Examples 3 and 4 have a wide variety of uses and are especially useful as lamp envelopes, especially as lamp envelopes for sodium vapor discharge lamps. In Example 7, where the yttrium oxalate hydrate was not air milled, the resulting yttrium oxide powder had an average particle size of 1.2 micrometers with 12% of the yttrium oxide particles being larger than 5 micrometers. The resulting polished sintered body was opaque, illustrating the importance of the present comminution of the yttrium oxalate hydrate powder to produce the present yttrium oxide powder. In Example 8, where the yttrium oxalate powder was not treated with TMAH or air milled, the powder had an average agglomerate size of 3.3 micrometers with no agglomerate larger than 13 micrometers, but the resulting yttrium oxide powder had an average particle size of 1.2 micrometers with about 12% by volume of the particles being greater than 5 micrometers. The resulting polished sintered body was opaque illustrating the criticality of the present processing of the yttrium oxalate hydrate powder. In Example 9, commercially available yttrium oxide powder was used which had an average particle size of 2.7 micrometers with about 30% by volume of the particles being larger than 5 micrometers. The resulting polished sintered body was opaque.

Claims: A process for producing a sintered yttrium oxide body which is transparent having an in-Line spectral transmission of at least 10% taken at a wavelength of 590 nanometers on a thickness of 0.85 millimeter of said sintered body and having an average grain size ranging from 5 micrometers to 50 micrometers characterized in that it comprises : producing an yttrium oxide powder having an average particle size of less than 5 micrometers and being free of particles greater than 5 micrometers and having a specific surface area ranging from 4 square meters per gram to 25 square meters per gram; said yttrium oxide powder being produced by providing an aqueous yttrium nitrate solution of at least 0.1 mole of yttrium per liter of solution, providing an aqueous oxalic acid solution having a concentration of at least 10% excess of that required for complete reaction with said yttrium nitrate, admixing said solutions thereby precipitating yttrium oxalate hydrate, recovering said yttrium oxalate hydrate precipitate and washing said yttrium oxalate hydrate precipitate with water to neutralize it, drying said yttrium oxalate hydrate precipitate to remove absorbed water, comminuting the resulting yttrium oxalate hydrate powder in air at ambient temperature, said yttrium oxalate hydrate precipitate being deflocculated in aqueous solution of a basic dispersant after neutralization and before it is dried, or before it is comminuted, to produce a powder wherein the average agglomerate size is less than 20 micrometers and which is free of agglomerates having a size greater than 20 micrometers, and thermally decomposing said yttrium oxalate hydrate at a temperature ranging from 650°C to 1000°C in air at ambient pressure to produce said yttrium oxide powder; forming said powder into a compact having a density of at least 45% of the density for yttrium oxide; firing said compact at a temperature ranging from 1700°C to 1900°C for a time sufficient to produce said sintered body, said firing being carried out in an atmosphere of hydrogen, said hydrogen atmosphere containing at least a sufficient partial pressure of oxygen at least after said compact becomes a closed pore body to produce said optically translucent sintered body. The process according to claim 1 characterized in that said firing is carried out in an atmosphere of wet hydrogen. The process according to claim 1 characterized in that said firing is initially carried out in an atmosphere of dry hydrogen until said compact becomes a closed pore body, then said firing is carried out in an atmosphere of wet hydrogen. The process according to claim 1 characterized in that said yttrium oxide powder has a specific surface area ranging from 6 to 10 m²/g. The process of claim 1 wherein said basic dispersant is an aqueous solution of tetramethyl ammonium hydroxide. A sintered polycrystalline yttrium oxide body characterized in that it has an in-line spectral transmission of at least 10% taken at a wavelength of 590 nanometers on a thickness of 0.85 millimeter of said body, an average grain size ranging from 5 to 50 micrometers, said grains being cubic in symmetry and said body being free of bloating in air at temperatures at least up to 1300°C. The body according to claim 6 in the form of a lamp envelope. The body according to claims 6 or 7 having a wall thickness ranging from 1/2 to 1 millimeter. The body according to anyone of claims 6 to 8 characterized in that said in line transmission is at least 20%.

Inventors: GRESKOVICH, CHARLES DAVID, O'CLAIR, CHESTER ROBERT
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1987
Main classification: C04B35
IPC codes: C04B35, C01F17, H01J61