Abstract:
A method of making aluminum oxynitride includes introducing a mixture having aluminum oxide and carbon into a chamber, agitating the mixture within the chamber, and heating the mixture to make aluminum oxynitride.

Description:
BACKGROUND 
     This invention relates to ceramic compounds and more particularly to aluminum oxynitride. 
     As is known in the art, aluminum oxynitride having the chemical composition Al 23−1/3x O 27+x N 5−x , 0.429≦x≦2 (AlON) is a ceramic material that has a wide range of uses because of its relatively high degree of transparency over the optical wavelength range of about 0.2 microns up to about 5 microns. Accordingly, AlON can be used in applications that require transmission and imaging capabilities in the visible and infrared wavelength ranges. These applications include both commercial and military applications, such as, for example, transparent envelopes for vapor lamps, optical windows, ballistic armor, scanner windows, watch crystals, and transparent domes for airborne optical imaging systems, such as those found on infrared heat-seeking missiles. 
     In addition, because AlON can have a relatively high degree of strength, AlON can also serve as a protective barrier for systems that may be exposed to certain demanding external environments. For example, AlON can be formed as a window or a dome for an exterior portion of a missile. 
     AlON can be synthesized by a process sometimes called carbothermal nitridation. Generally, in this process, alumina (Al 2 O 3 ) is mixed with carbon (C), and this mixture is reacted under a nitrogen-containing atmosphere, e.g., dinitrogen (N 2 ), at high temperatures, e.g., about 1650-1850° C. The specific reactions that occur in the process are represented in equations 1-2.
 
23Al 2 O 3 +15C+5N 2 →18Al 2 O 3 +10AlN+15CO↑  (1)
 
(9+1/3 x )Al 2 O 3 +(5 −x )AlN→Al 23−1/3x O 27+x N 5−x   (2)
 
     As shown in Equation 1, a portion of alumina, carbon, and nitrogen react to form aluminum nitride, and carbon monoxide gas is produced. This reaction can occur at about 1650-1750° C. The formed aluminum nitride then reacts with alumina, e.g., at about 1750-1850° C., to form AlON. Synthesis of AlON by carbothermal nitridation, e.g., by conventional batch processing, can take up to about 20 to 30 hours to complete. 
     SUMMARY 
     In accordance with the present invention, a method is provided for making aluminum oxynitride. The method includes: introducing aluminum oxide particles into a chamber, dispersing the particles within the chamber, and forming the aluminum oxynitride including passing nitrogen gas over the dispersed particles. 
     With such method, large quantities of aluminum oxyniride may be practically produced. 
     In one embodiment, the method includes a batch rotary process or a semi-continuous rotary process in which a reaction mixture is dispersed or tumbled during reaction. Dispersing the reaction mixture can shorten the reaction time, produce a more uniform AlON composition, and produce a free flowing AlON powder compared to conventional carbothermal nitridation, thereby, e.g., lowering costs of production. 
     In addition, the semi-continuous process can shorten the time needed to synthesize multiple batches of AlON, for example, by reducing the time needed to ramp the furnace to a soak temperature, the time needed for the furnace to cool, and the time needed to re-load the retort and to remove the formed AlON from the retort. The semi-continuous process also provides convenient handling of reactants and products. 
     In another aspect of the invention, a method of making aluminum oxynitride includes introducing a mixture having aluminum oxide and carbon into a chamber, agitating the mixture within the chamber, and heating the mixture to make aluminum oxynitride. 
     Embodiments of the invention may include one or more of the following features. The method further includes introducing nitrogen gas into the chamber. Agitating the mixture includes rotating the chamber. The method further includes cooling the aluminum oxynitride, removing the aluminum oxynitride from the chamber, and introducing a second mixture having aluminum oxide and carbon into the chamber. The method further includes forming the aluminum oxynitride into a transparent structure. Forming the aluminum oxynitride includes forming a green body having the aluminum oxynitride, and sintering the green body. The method further includes isostatically pressing the sintered green body under heat. The aluminum oxynitride includes Al 23−1/3x O 27+x N 5−x , where 0.429≦x≦2. 
     In another aspect of the invention, a method of making aluminum oxynitride includes introducing a first reaction mixture having aluminum oxide and carbon into a chamber, agitating the first reaction mixture within the chamber, heating the chamber to a temperature to form aluminum oxynitride from the first reaction mixture, removing the aluminum oxynitride while maintaining the temperature of the chamber, and introducing a second reaction mixture having aluminum oxide and carbon into the chamber while maintaining the temperature of the chamber. The chamber can include an exit opening, and removing the aluminum oxynitride may include retracting a plunger within the chamber, thereby allowing the aluminum oxynitride to flow through the exit opening. 
     In another aspect of the invention, a continuous method of making aluminum oxynitride includes heating a chamber, continuously introducing a reaction mixture comprising aluminum oxide and carbon into the chamber, agitating the reaction mixture within the chamber, and continuously providing the aluminum oxynitride. The continuous process can offer similar advantages as the semi-continuous process. 
     These and other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a method for making an aluminum oxynitride (AlON) structure; 
         FIG. 2  is a schematic diagram of an embodiment of a system for making AlON; 
         FIGS. 3A-B  are cut-away perspective views of embodiments of retorts; 
         FIG. 4  is a schematic diagram of an embodiment of a system for making AlON having a plunger in an extended position; 
         FIG. 5  is a schematic diagram of an embodiment of a system for making AlON having a plunger in a retracted position; 
         FIG. 6  is a schematic diagram of an embodiment of a system for making AlON; and 
         FIG. 7  is a schematic diagram of an embodiment of a system for making AlON. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a method of making an AlON structure, e.g., an AlON dome. Generally, AlON powder is formed by reacting Al 2 O 3  and carbon in a nitrogen atmosphere at high temperatures. The formed AlON powder is calcined to remove unreacted carbon and milled to reduce the particle size of the AlON powder. The milled AlON is then sprayed dried to agglomerate the AlON particles and to form a flowable powder that can be conveniently poured into a mold to form a green body. The green body, having the approximate size of the structure, is formed by cold isostatic pressing. After pressing, the green body is heated to remove organic material, sintered to densify the structure, and further densified by hot isostatic pressing. The structure is then annealed and finished to the structure&#39;s final tolerances by grinding and polishing. 
     Referring to  FIG. 2 , the AlON powder is here formed by a batch rotary process. A batch rotary system  10  includes a furnace  20 , a cylindrical graphite chamber, here a retort  30  inside furnace  20 , and a flow tube  35  for delivering nitrogen gas into retort  30 . Retort  30  is connected to a drive motor  40  via a drive shaft  50  so that motor  40  can rotate retort  30 . Retort  30  also defines a plurality of agitator or lifter bars  60  (shown in  FIGS. 3A-B ) on the interior surface of retort  30 . Lifter bars  60  are ridges or bumps integrally formed with retort  30  that extend or spiral alone the length of retort  30 . Furnace  20  and retort  30  are available from Centorr Vacuum Industries (Nashua, N.H.). 
     Generally, as retort  30  rotates, lifter bars  60  help to disperse a reaction mixture  70  of alumina and carbon within retort  30  by lifting and allowing reaction mixture  70  to fall inside retort  30 . It is believed that tumbling mixture  70  during reaction exposes fresh, unreacted mixture  70  to nitrogen, thereby aiding diffusion of nitrogen and shortening the reaction time needed to from AlON. 
     Reaction mixture  70  is formed by mixing alumina with carbon. Typically, reaction mixture includes between about 4.5 to about 8% by weight carbon, and more preferably between about 4.5% to about 6.5% by weight carbon. The alumina is a gamma-Al 2 O 3  available from, e.g., Condea Vista (Tucson, Ariz.), although other forms of alumina, such as, for example, alpha-Al 2 O 3  may be used. The carbon is a pure (dry) carbon black, e.g., Monarch 880, available from Cabot Corp. (Billerica, Mass.). Other grades of carbon black may also be used. Preferably, the moisture content and volatile content of the alumina and the carbon are accurately known so that the carbon content in mixture  70  can be controlled within prescribed limits. 
     A homogeneous mixture of alumina and carbon is preferred so that a uniform composition of AlON can be formed. The alumina and carbon can be dry blended by mixing in a blender, ball milled (dry or wet), and/or shaken in a mixer (e.g., in a paint shaker). Preferably, the carbon is a carbon ink, which is convenient to handle and provides good mixing with the alumina. The carbon ink is formed by mixing carbon black with water or an organic solvent, such as methanol, ethanol, and propanol. A dispersant, such as, for example, Disperbyk 190 or 191, available from, e.g., BYK-Chemie, is added to the ink to wet the carbon black and to help keep the carbon black in suspension. Carbon ink is also commercially available from, e.g., Sun Chemical (Winston-Salem, N.C.) and Borden Chemical Inc. (Cincinnati, Ohio). Preferably, the carbon ink solution is formed and/or diluted to form a solution having between about 5 and about 10% by weight carbon to provide uniform wetting of the alumina. 
     The alumina and carbon ink are mixed in a cone blender having an intensifier blade or bar for about 30 to 90 minutes, or until reaction mixture  70  is blended and uniform. A typical charge for mixing is 15 kg, although the size of charge is a function of the equipment used. The liquid (e.g., water and solvent) is removed by evaporation prior to charging retort  30 , preferably in the same cone blender used for mixing. Alternatively, the mixture of alumina and carbon ink can be spray dried to form a free-flowing powder prior to charging. A charge of reaction mixture  70 , e.g., about 500 g, is then placed in retort  30  by removing an end cap (not shown) of retort  30 , charging retort  30 , and placing the end cap back on retort  30 . 
     After placing a charge of reaction mixture  70  in retort  30 , retort  30  is evacuated of air and purged with high purity nitrogen gas. Generally, the nitrogen gas flow rate must be sufficient to restrict the carbon monoxide or carbon dioxide local concentrations from poisoning of the AlON reaction. Suitable gas flow rates depend on the size of the reaction retort and the mass of the mixture  70  being processed. For mixtures  70  of about 5 to about 20 kg, the nitrogen flow rates are about 2 to about 20 L/min. 
     Retort  30  is then rotated by activating drive motor  40 , and reaction mixture  70  is heated. The rotation speed is about 2-50 rotations per minute. The effect of the rotation is to disperse the alumina/carbon particles in the chamber and enable the nitrogen gas to pass around the dispersed particles and thereby facilitate the reaction between the alumina/carbon particles and the nitrogen. Accordingly, the rotation speed should be fast enough to disperse reaction mixture  70  inside retort  30 , but not so fast that reaction mixture  70  is centrifuged inside retort  30 , thereby preventing the reaction mixture from tumbling. Reaction mixture  70  is heated in the chamber at ramp rate of greater than 10-20° C./min to a soak temperature of about 1700-1900° C., preferably about 1825° C. The soak time is about 10-30 minutes, preferably about 15 minutes. After reaction mixture  70  has soaked at the predetermined soak temperature and for the predetermined soak time, furnace  20  is turned off, and reaction mixture  70 , now AlON powder, is allowed to cool, typically taking about 4 hours. The cooled AlON powder is removed from retort  30 , and another charge of reaction mixture  70  is added to retort  30  to form another batch of AlON. 
     Thus, rotary system  10  described above is used as part of a batch process. Each batch of AlON powder is formed by loading reaction mixture  70  in retort  30 , heating mixture  70 , cooling mixture  70 , and removing formed AlON from retort  30 . For every batch of AlON powder formed, furnace  20  is heated and cooled. 
     In another method of forming AlON, the AlON powder is formed by a semi-continuous rotary process in which the furnace is held at a constant soak temperature for multiple batches of AlON. Referring to  FIG. 4 , a semi-continuous rotary system  100  includes a furnace  110 , a feed hopper  140 , a graphite retort  120  inside furnace  110 , a drive motor  130 , and a collection hopper  160 . Furnace  110  includes a nitrogen inlet  112  and a nitrogen outlet  114 . When furnace  110  is in use, nitrogen gas flows from inlet  112 , through retort  120 , and is vented through outlet  114 . Feed hopper  140  includes a valve  145  and is used for supplying a reactant mixture  150  (similar to mixture  70 ) to retort  120 . Retort  120 , similar in construction to retort  30 , is angled downwardly from hopper  140  and is rotatable inside furnace  110  via drive motor  130 . Retort  120  includes a graphite plunger  125  moveably located, e.g., extendable to a plurality of positions, inside retort  125 . Retort  120  also includes an opening  180  for removing formed AlON powder, as described below. Collection hopper  160  is used for receiving formed AlON powder  170 . Because collection hopper  160  receives hot AlON powder  170 , the interior surface of drum  160  is made of a non-contaminating material or refractory, such as, for example, alumina and graphite. Rotary system  100  is available from Centorr Vacuum Industries and Harper International (Lancaster, N.Y.). 
     In operation, furnace  110  is heated to the soak temperature (e.g., about 1700-1900° C.) and retort  120  (with agitator bars, not shown) is rotating. Here again the retort, or chamber action causes the alumina/carbon particles to disperse. Nitrogen gas flows in inlet  112  and out outlet  114 . Plunger  125  is extended past opening  180  so that when reactant mixture  150  is charged into retort  120 , mixture  150  is retained in retort  120 . Reactant mixture  150 , generally the same as reactant mixture  70 , is charged into retort  120  from hopper  140  by opening valve  145  until a predetermined amount of mixture  150  is in retort  120 . Reaction mixture  150  is dispersed in retort  120  and allowed to react, e.g., for about 10-30 minutes. Thus, here again the nitrogen is able to pass around the dispersed alumina particles 
     After mixture  70  has reacted to form AlON powder, plunger  125  is retracted ( FIG. 5 ) so that the AlON powder can flow through opening  180  and into collection drum  160 . The temperature of retort  120  is still at the soak temperature. Plunger  125  is then repositioned, e.g., extended past opening  180  ( FIG. 4 ), and another batch of AlON powder can be formed by charging reactant mixture  150  into retort  120  from hopper  140 . Thus, in this semi-continuous process, multiple batches of AlON can be formed without having to turn furnace  110  on and off for each batch of AlON. This process can shorten the time needed to synthesize multiple batches of AlON by reducing the time needed to ramp the furnace to the soak temperature, the time needed for the furnace to cool, and the time needed to charge the retort and to remove the formed AlON from the retort. 
     In another method of forming AlON, the AlON powder is formed by a continuous rotary process. Referring to  FIG. 6 , a continuous rotary system  300  includes a furnace  110 , a graphite retort  120  inside furnace  110 , a drive motor  130 , and a collection hopper  160  for collecting the product mixture  170  (formed AlON). System  300  further includes a feeder  310 , a feed hopper  320 , a load-lock hopper  330 , and a gate valve  340  between feed hopper  320  and load-lock hopper  330 . Retort  120  is similar in construction to the retorts described above, e.g., angling downwardly from feeder  310  and rotatable inside furnace  110  via drive motor  130 . Furthermore, retort  120  of system  300  is constructed with sufficient length such that as reaction mixture  150  travels from input end  350  of retort  120  to output end  360  of retort  120 , the residence time of mixture  150  in heated retort  120  is sufficient for mixture  150  to form AlON. In other words, retort  120  is made sufficiently long such that reaction mixture  150  that is introduced into retort  120  from feeder  310  is converted into AlON  170  by the time the reaction mixture reaches opening  180 . 
     In operation, furnace  110  is heated to the soak temperature and retort  120  is rotating to disperse the alumina/carbon particles that are introduced into retort  120 . Nitrogen gas flows in inlet  112  and out outlet  114 . With gate valve  340  closed, reactant mixture  150  is loaded into load-lock hopper  330 , which is evacuated of air and purged with nitrogen. Purging system  300  with nitrogen helps to prolong the life of the graphite retort. Reactant mixture  150  is then introduced into feed hopper  320  by opening gate valve  340 . Feed hopper  320  introduces mixture  150  into feeder  310 , which introduces mixture  150  into retort  120  at a predetermined rate, e.g., volumetrically or gravimetrically. Feeder  310  can be, for example, a screw feeder and shaker feeder. As the charge in feed hopper  320  decreases, more reactant mixture  150  can be provided through load-lock hopper  330 , as described above. Feed hopper  320  can be made sufficiently large to minimize the frequency at which hopper  320  needs to be re-loaded. 
     Thus, by selecting the proper dimensions of retort  120  and adjusting the feed rate of reactant mixture  150 , the temperature of the furnace  110 , and rotation speed of retort  120 , system  300  can produce a continuous output of AlON. Other experimental parameters, e.g., nitrogen flow rate and particle size, can be adjusted to optimize output and composition uniformity. 
       FIG. 7  shows another embodiment of a continuous rotary process for making AlON. System  400  is generally similar to system  300  but includes two feeding assemblies  410  and  420  placed in parallel with retort  120 , and two valves  430  and  440 . Each assembly  410  and  420  includes a feeder  310  and a feed hopper  320  generally as described above. 
     An exemplary operation of system  400  will now be described. With valves  430  and  440  closed, reactant mixture is introduced into feed hopper  320  of assembly  410 , which is then closed, evacuated and purged. Valve  430  is then opened for assembly  410  to introduce reactant mixture into retort  120 . Meanwhile, reactant mixture is introduced into feed hopper  320  of assembly  420 , which is then closed, evacuated and purged. When assembly  410  is almost empty of reactant mixture, valve  440  is opened for assembly  420  to introduce reactant mixture into retort  120 , and valve  430  is closed to load assembly  410  with more reactant mixture, as described above. When assembly  420  is near empty, valve  430  is opened, assembly  410  is again used to introduce reactant mixture into retort  120 , and valve  440  is closed. The above-described procedure is repeated as needed. 
     Referring back to  FIG. 1 , the formed AlON powder is removed from retort  30  or collection hopper  160  and calcined to remove unreacted carbon. Calcining is performed in air or oxygen between about 700 and about 800° C. In an alternative embodiment, collection hopper  160  can be connected to a calcining furnace having flowing air or oxygen. Accordingly, hopper  160  can charge product mixture  170  (the formed AlON) directly into the calcining furnace. In another embodiment, hopper  160  can be equipped with flowing air so that unreacted carbon can be removed as the formed AlON powder cools to between 700 and 800° C. 
     The calcined AlON powder is ball milled to reduce the particle size of the powder. Ball milling is performed in polyurethane-lined, rubber-lined, or AlON-lined mills using methanol as a milling fluid, and an alumina or AlON grinding media, e.g., pellets. Milling time is about 20-30 hours, typically about 24 hours. This produces an AlON powder having a particle size less than about 8 microns, typically having an average particle size less than 3 microns. The milled powder is placed in a rotary evaporator to remove the milling fluid (methanol). Calcined AlON powder can also be ball milled using water in the place of methanol. The milled slurry can then be spray dried to form a dried powder or slip cast into shapes. 
     The AlON powder, milled in methanol and dried in a rotary evaporator, is then reconstituted with water, a dispersant agent (such as Duramax D3005 (Rohm and Haas, Philadelphia, Pa.)), and preferably a binder (such as Duramax B1020) to form a slurry. Adding the binder typically provides strong AlON structures with high and uniform green body density that can be made in different shapes. The slurry is spray dried to agglomerate the AlON powder into particles about 100 microns in size. This produces a free-flowing powder having a uniform fill density that can be easily poured into a mold. 
     The AlON powder is then formed into a green body. The AlON powder is poured into a rubber mold having a predetermined shape, agitated to uniform fill density, and cold isostatically pressed at pressures greater than 15,000 psi to produce a green body to be used in sintering (described below). Formation of the green body can include other techniques such as, for example, slip casting, and injection molding. The green body typically has a density of about 60% of the theoretical density and is formed about 20% oversized of the finished structure to compensate for shrinking from sintering and hot pressing operations (e.g., hot isostatic pressing, uniaxial hot pressing, and multiaxial hot pressing), as described below. After cold pressing, the green body is slowly heated to about 500-750° C. in air to remove residual organic material, e.g., the binder. If no binder was added to the reconstituted, milled powder, the slow heat treatment described above (500-750° C. in air) can be omitted. The formed green body is then placed in a graphite, alumina, or AlON container, which is then placed in a sintering furnace. 
     The AlON green body is sintered to remove and to close its porosity and to increase its density. Sintering is performed in a flowing or static atmosphere of nitrogen for about 6-10 hours, typically about 8 hours. Final sintering temperatures are higher than 1900° C., but lower than the melting temperature of AlON, which is approximately 2140° C. Sintering produces a polycrystalline body having an average grain size of less than about 20 microns. The sintered body typically has a density of about 95-99% of the theoretical density, or about 3.5 g/cc to about 3.7 g/cc. 
     The sintered body is then hot isostatically pressed (HIP) to further increase its density. Hot isostatic pressing is performed at about 1900° C. and about 20,000-30,000 psi, preferably about 28,000-30,000 psi, in a convective argon or nitrogen atmosphere for about 6-20 hours. This produces a body having an average grain size of about 100-350 microns, and a density of about 100% of the theoretical density. After hot isostatic pressing, the body has the appearance of light gray to dark, tinted glass. 
     To obtain a “water clear” structure, the hot isostatically pressed AlON body is heat treated in a flowing nitrogen atmosphere enclosed in an alumina or AlON crucible (to provide a low partial pressure of oxygen and to avoid reducing the formed AlON) at about 1900° C. for about 6-10 hours, typically 8 hours. 
     The heat treated body is finished to the structure by optical grinding and polishing to the final tolerances of the structure. 
     Other embodiments of the above-described methods are possible. For example, in addition to or as an alternative to hot isostatic pressing, sintering aids, in the form of small amounts (&lt;0.5 wt percent) of doping additives, can be added to the AlON powder. The aids can include an element such as yttrium and lanthanum, or compounds thereof. Other lanthanides and their compounds, preferably the oxides, may also be used. A combination of the doping additives may also be used. A preferred combination includes 0.08 wt % Y 2 O 3  and 0.02 wt % La 2 O 3 . The doping additives may be added, e.g., during the ball milling step. 
     Other methods of forming AlON and AlON structures, e.g. by carbothermal nitridation, are described in U.S. Pat. Nos. 4,520,116; 4,686,070; 4,720,362; 4,481,300; and 5,135,814, all hereby incorporated by reference in their entirety. 
     Other embodiments are within the claims.