Abstract:
Disclosed is a method for making a material having a controlled microstructure, the method including providing particles of a ceramic mineral material, the particles having a metal oxide dopant therein. The particles of the ceramic mineral material are consolidated into larger aggregates of a size relevant to the desired application using standard industrial mixing and pelletizing technology. The aggregates are heated under reducing conditions so that at least part of the dopant is reduced to form a transient, metastable liquid phase among the particles. The liquid phase includes at least part of the reduced dopant and promotes sintering of the particles and forms islands of reduced metal within the material and on the surface of the aggregates. Following heating of the aggregates under reducing conditions, the aggregates are heated under oxidizing conditions such that the islands of reduced metal are oxidized and/or go into solid solution within the particles, thereby creating voids within and form a shell thereon the particles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of U.S. Provisional Patent Application Ser. No. 60/807,012, filed Jul. 11, 2006, entitled “Dopant Enhanced Densification of Aluminosilicate Aggregate” which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The inventions disclosed herein relate to materials and to methods for their fabrication. Certain embodiments of the invention relate to a microstructured material having a core/shell structure. Certain other embodiments relate to a material having a ceramic-based core structure with tailored porosity surrounded by a dense metal or metal-oxide based shell.  
       BACKGROUND OF THE INVENTION  
       [0003]     Ceramic type mineral materials generally combine high strength with chemical and thermal stability. Hence they have significant utility in many products and processes. For example, ceramic-based materials are often used as supports for catalysts and as casting sands and mold materials used for the fabrication of a variety of articles in high temperature fabrication processes. Particulate ceramic materials also have significant utility as proppants in hydrocarbon recovery processes. Such materials are injected, under very high pressures, into geological structures, together with carrier fluids in a process called hydrofracturing. The injected fluid opens cracks in rock structures allowing for the passage of hydrocarbon products therethrough. The proppant materials wedge into these opened cracks and serve to maintain the integrity and permeability of the cracked structure during the extraction process. Such materials need to have high strength and chemical inertness, and should also have densities comparable to that of the carrier fluid. In addition, since these materials are used in very large amounts, their cost should be low.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention relates to ceramic-based, tailored microstructure materials comprising a ceramic core having a plurality of voids therein, and further including a metal or metal-oxide shell on the outer surface thereof. According to embodiments of the present invention, the ceramic-based, tailored microstructure materials are prepared by a dopant induced transient liquid phase sintering process under controlled oxygen fugacity. The methods of such embodiments may be utilized to prepare a variety of materials having selectably controllable properties such as density, chemical reactivity, thermal reactivity, strength and the like.  
         [0005]     According to certain embodiments of the method of the present invention for making a material having a controlled microstructure, the method includes providing particles of a ceramic mineral material the particles having a metal oxide dopant therein. The particles of the ceramic mineral material are consolidated into larger aggregates of a desired size. The aggregates of the ceramic mineral particulates are heated initially under reducing conditions so that at least part of the metal oxide dopant is reduced to form a metastable, transient liquid phase among the particles. The liquid phase includes at least part of the reduced metal oxide dopant, promotes sintering among the particles and forms islands of reduced metal oxide dopant within and on the surface of the aggregates. Following heating under reducing conditions, tie aggregates are heated under oxidizing conditions such that the islands of reduced metal oxide are oxidized and/or go into solid solution within the particles, thereby creating voids within and forming a shell on the aggregates.  
         [0006]     According to certain embodiments of the present invention, the ceramic mineral material can include oxides of aluminum, oxides of silicon, aluminosilicates and combinations thereof. In some embodiments, tie ceramic mineral material can include kaolinite, bauxite, fly ash and combinations thereof. In some embodiments, the metal oxide dopant can include an oxide of a polyvalent transition metal and in some embodiments can include oxides of iron, and/or oxides of titanium, chromium, manganese, or others, and combinations thereof. In one embodiment, the dopant includes hematite. In some embodiments, the amount of metal oxide dopant can range from approximately 0.1 to 30 weight percent of the particles. In some embodiments, the aggregates can have a sieve size in the range of 12-20 to 20-40 mesh and at least one of the heating steps includes a temperature in the range of 1100-1800° C. In some embodiments, larger or smaller aggregate size ranges are used. When heated within this temperature range under a controlled oxygen fugacity, the liquid phase forms a metastable eutectic with the ceramic mineral material, thereby promoting sintering. Post sintering thermal treatment under an alternative oxygen fugacity causes the formation of voids within the aggregates.  
         [0007]     In an exemplary embodiment of the present invention, the reducing conditions include a reducing atmosphere with a partial pressure of oxygen of less than 0.005 atm. The oxidizing conditions include an oxidizing atmosphere made from air and can be carried out at atmospheric pressure. Alternative oxidizing conditions can be selected to promote the evolution of the metastable transient liquid phase, as well as to alter the subsequent phase stability of the dopant oxides to promote pore evolution. The aggregates can be stirred during at least one of the heating steps so as to prevent sticking and agglomeration thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a scanning electron image of the cross-section of an aggregate of the present invention after being heated under reducing conditions;  
         [0009]      FIG. 2  is a scanning electron image of the circle region shown in  FIG. 1 ;  
         [0010]      FIG. 3  is a scanning electron image of the cross-section of an aggregate of the present invention after being heated under oxidizing conditions; and  
         [0011]      FIG. 4  is a higher magnification of the aggregate shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0012]     In general, the methods of the present invention may be implemented utilizing a variety of mineral materials. Typically, the materials used in the various embodiments of the present invention comprise ceramics, and in specific instances, they may comprise oxide and/or silicate-based ceramics such as alumina, aluminosilicates, silicates, and the like. These minerals may include natural products such as kaolin or bentonites, or they may comprise specifically synthesized ceramic materials, or they may comprise industrial byproducts such as fly ash. A dopant material comprising a metal oxide is incorporated into the ceramic material in an amount sufficient so as to facilitate the reactions described hereinbelow. In specific instances, the ceramic material may naturally include some amount of the dopant species therein, and an additional amount of metal oxide dopant can be added so as to raise the total concentration of the dopant material in the ceramic material to between approximately 0.1-30 percent by weight of tie ceramic material. In some instances, the as-obtained mineral material will include relatively large amounts of the dopant species therein, while in other instances, the concentration will be low to negligible. Appropriate supplementation may be made by one of skill in the art.  
         [0013]     Following the incorporation of the dopant, the mineral material is prepared for further processing, typically by forming it into aggregates having a size and shape consistent with the intended end use of the resultant product. Typically, the starting materials are ground or otherwise pulverized, together with the dopant material. In some instances, the resultant particles are pelletized into substantially larger aggregates so as to provide the resultant product with a desired texture, geometry and size. The specific size of particles employed will depend upon particular applications; however, for many typical applications, including applications as hydrocarbon recovery proppants, particle sizes in the range of 12-20 mesh to 20-40 mesh will be employed. As is known to those of ordinary skill in the art, particles having a size range of 12-20 mesh will pass through a sieve having a 12 mesh size, but be retained by a sieve having a 20 mesh size. A similar relationship holds true for particles having a 20-40 mesh size, It should be understood that in other applications, larger or smaller particle sizes may be employed.  
         [0014]     Following the formation of the aggregates, the dopant-containing particles are heated under reducing conditions. Heating may, for example, be carried out in a kiln or furnace, and in specific instances, a rotary kiln is employed to assure that particle-particle adhesion during heating is minimized. The reducing conditions are typically supplied by introducing a reducing atmosphere into the vessel in which the heating is taking place. This atmosphere may comprise a forming gas (N 2 /H 2  mixture) as is known in the art, or it may comprise other reducing atmospheres such as a hydrogen atmosphere, a hydrocarbon-containing atmosphere or the like. In this heating step, the reducing atmosphere at least partially reduces the metal oxide dopant and assists in the formation of a metastable transient liquid phase. This reduction of the metal oxide dopant produces a reduced species comprising lower oxides, a free metal, or a combination of the foregoing. Heating is carried out at a temperature sufficient to liquefy the at least partially reduced dopant material, and this liquid phase serves to facilitate sintering of the mineral material. In some embodiments, the liquid phase forms a metastable eutectic with the mineral material and thereby causes sintering to occur at a lower temperature than would be the case if the liquid phase was not present. This liquid phase migrates through the mineral material, and in general, is further reduced in the course of the heating under reducing conditions. In many instances, the further reduction increases the amount of free metal in tie liquid phase, and since the free metal is less likely to wet the mineral material, it thereby aggregates to produce metallic islands both in the bulk of the material also known as the matrix material and on the surface of the particles. The net result of the heating under reducing conditions is the production of a sintered particulate material having islands of a metal and/or a metal-containing species disposed both in the matrix material and on the surface thereof. The matrix material includes polycrystalline ceramic material with glassy grain boundaries, the glassy grain boundaries having an amorphous and/or non-amorphous structure.  
         [0015]     The sintered particulate material prepared by heating under reducing conditions is then subjected to heating under oxidizing conditions. In particular embodiments, the heating tinder oxidizing conditions is implemented by heating the sintered particulate material in ambient air, at atmospheric pressure; although, in other instances, more or less vigorous oxidation conditions may be found to be beneficial. When the sintered particulate material is heated under oxidizing conditions, the islands of metal in the matrix material interact with the matrix material and the oxidizing atmosphere and are at least partially reabsorbed back into the matrix material either as oxidized metal or as a component of a complex with the matrix material. The oxidizing conditions can be judiciously selected to tailor the resulting stability of selected crystalline and/or amorphous constituents to promote pore evolution via oxidative decomposition. The net result is that a plurality of voids are formed in the matrix material. The metallic material disposed on the outer surface of the sintered particulate material is not reabsorbed into the matrix material, or is at least reabsorbed to a lesser degree and typically consolidates and disperses so as to form a relatively voidless shell-like structure surrounding the inner structure of the particle having a plurality of voids therein. It is to be understood that this shell may comprise a continuous or a discontinuous body. In some instances, the metallic material on the surface of the particle interacts to some degree with the oxidizing atmosphere so as to form an at least partially oxidized metallic material,  
         [0016]     The net result of the process is the production of an aggregate comprised of sintered primary particles, resulting in a relatively high strength ceramic body having a plurality of voids therein and further including a relatively voidless outer shell of a metal or metal-oxide based material. These aggregates have high strength and relatively low density owing to the void structure therein. It will be appreciated by those of ordinary skill in the art that by the appropriate selection of materials and reaction conditions, properties of the aggregate comprised of sintered primary particles may be readily controlled over a wide range of compositional and processing parameters (e.g. temperature, time, oxygen fugacity, etc.).  
         [0017]     In one specific group of embodiments of the process of the present invention, starting mineral ores of kaolinite and bauxite were ground to a fine powder (approximately 30 micron) and doped with up to ten weight percent of hematite (Fe 2 O 3 ) each. The powders were spheroidized in an industrial mixer and size classified by sieving to isolate aggregates in the size range of 12-20 to 20-40 mesh. The resultant mixtures were then heated in a rotary kiln at a temperature in the nominal range of 1400-1600° C. under a forming gas atmosphere (N 2 /H 2  mixture). The oxygen partial pressure was below 0.05 atm, and it was noted that during the sintering process the Fe 2 O 3  was initially reduced to FeO, which forms a metastable eutectic mixture with alumina, silica, and aluminosilicates at a temperature of approximately 1148° C. The formation of this eutectic mixture promoted the sintering of the mineral material at a relatively low temperature. Capillary forces aided in the passage of the iron-rich aluminosilicate liquid through the particulate material, thereby redistributing solids and minimizing porosity through solution precipitation reactions between the liquid and the solid.  
         [0018]     As the reduction process proceeded, the iron-rich aluminosilicate liquid was further reduced to form elemental iron. Elemental iron does not readily wet ceramic materials, and hence the liquid phase in the interior portions of the aggregate formed pockets or islands while the liquid phase at the exterior segregated to the surface to form islands thereupon. This process resulted in formation of a ceramic sphere having islands of metal dispersed on the outer surface thereof, and further containing metal islands therein. This material could be used as is for a catalyst, and appropriate catalytic activity could be selected by an appropriate choice of metal oxide dopants.  FIG. 1  shows a scanning electron micrograph of a cross section of a bauxite-derived aggregate doped with 10% hematite after being heated in the nominal range of 1400-1600° C. under the forming gas.  FIG. 2  is a higher magnification of the circled region in  FIG. 1 . As shown in these two figures, the aggregate is relatively dense and includes significant amounts of iron on the surface thereof.  
         [0019]     Next, the thus-produced particles were heated in an oxidizing atmosphere, which in this embodiment was comprised of air at atmospheric pressure. Heating was again carried out in a rotary kiln at temperatures in the nominal range of 1400-1 600° C. This heating process caused the internal islands of elemental iron to be taken back up by the solid matrix material. The iron dissolved into the matrix and/or reacted with surrounding crystal phases, as for example to produce iron titanates. The take-up of the metal produced a number of voids in the sintered aggregate via oxidative decomposition of less stable metal oxides and further served to densify and consolidate the outer metal shell/layer, as shown in  FIGS. 3 and 4 .  
         [0020]     The materials thus produced exhibited a structure which combined high strength together with a controlled density. An example of the experimental parameters used in the above-described process are listed below in Table 1. In addition, Table 2 provides properties of particles produced according to the present invention wherein the characteristic strength, specific gravity and specific strength are compared with current state of the art materials CARBOHSP® and CARBOPROP® with a 20-40 mesh size, manufactured by CARBO Ceramics of Irving, Tex. As shown by Table 2, particles of this type have improved properties in terms of a reduction of specific gravity while maintaining strength and an increase in specific strength (i.e. the ratio of strength to density).compared to current state of the art materials and have significant utility as proppant materials.  
                               TABLE 1                           Rate   Temperature   Dwell Time   Oxygen Partial       Step #   (° C./min)   (° C.)   (min)   Pressure (atm)                   Step 1   16   1450   30   1.00 × 10 −8         Step 2   16   1450   30   0.21 (air)                  
 
         [0021]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
               
               
                 Ceramic 
                 Iron Oxide 
                   
                 Alumina 
                 Characteristic 
                 Specific 
                 Specific 
               
               
                 Mineral 
                 Content 
                 Sintering 
                 Content 
                 Strength 
                 Gravity 
                 Strength 
               
               
                 Material 
                 (wt %) 
                 Stage 
                 (wt %) 
                 (MPa) 
                 (g/cc) 
                 (MPa/(g/cc)) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Bauxite 
                 5 
                 Reducing 
                 72 
                   
                   
                   
               
               
                 Bauxite 
                 5 
                 Oxidizing 
                 72 
                 193 
                 3.16 
                 61 
               
               
                 Bauxite 
                 10 
                 Reducing 
                 72 
               
               
                 Bauxite 
                 10 
                 Oxidizing 
                 72 
                 166 
                 3.43 
                 48 
               
               
                 Kaoilnite 
                 5 
                 Reducing 
                 48 
                 236 
                 2.68 
                 88 
               
               
                 Kaolinite 
                 5 
                 Oxidizing 
                 48 
                 147 
                 2.51 
                 59 
               
               
                 Kaolinite 
                 10 
                 Reducing 
                 48 
                 136 
                 2.75 
                 49 
               
               
                 Kaolinite 
                 10 
                 Oxidizing 
                 48 
                 102 
                 2.85 
                 36 
               
               
                 CARBOHSP ® 
                   
                   
                 80 
                 250 
                 4.00 
                 63 
               
               
                 CARBOPROP ® 
                   
                   
                 72 
                 210 
                 3.70 
                 57 
               
               
                   
               
             
          
         
       
     
         [0022]     It will be appreciated that by appropriate control of reaction conditions, dopant materials, dopant quantities and the like, the ultimate microstructure of the aggregate may be selectably determined.  
         [0023]     While the foregoing has described the preparation of microstructured particles utilizing an iron-based dopant, it is to be understood that other dopant species may be similarly employed. For example, other polyvalent transition metals such as titanium, tin, chromium, manganese and the like may be employed singly or in combination so as to produce composites which include those specified metals. Such materials may have particular utility as catalysts, electro- and/or magnetically-active materials or the like. It is also to be understood that the thus-prepared particles may be subjected to further reactions. For example, the metal-containing particles may be subjected to other reactive atmospheres such as carburizing atmospheres, nitriding atmospheres and the like so as to produce particles having coatings of metal carbides, nitrides and the like on their surfaces. Such particles and/or aggregates can exhibit enhanced hardness and may have utility as high-strength materials, abrasives and the like.  
         [0024]     According to certain embodiments, the dopant materials used in the present invention will be present in a range of between 0.1-30 weight percent of the ceramic material. In some embodiments, the dopant materials will be present in a range of between 0.1-10 weight percent of the ceramic material, and in other embodiments in a range of between 5-10 weight percent of the ceramic material.  
         [0025]     Typical reduction reactions will be carried out at a temperature range sufficiently high to promote the formation of the transient: metastable liquid phase, and this range will typically be 800-1800° C. In some embodiments, the temperature range will be 1100-1800° C., and in other embodiments the temperature range will be 1400-1600° C. Reaction times will depend upon the temperature and the nature of the materials employed, but it is anticipated that most commercially feasible processes will utilize reaction times for the reduction step in the range of 20-120 minutes. The oxidation reaction will typically be carried out under time and temperature conditions similar to those for the reduction step.  
         [0026]     While the foregoing has described the use of rotary kilns for the heating step, it is to be understood that other heating systems may be employed. These can include fluidized bed reactors, stirred bed reactors, tube furnaces, microwave heating, and other such heating systems. The heating may be carried out in a single reactor by varying the atmosphere therein or by using different atmospheric zones. The process may also be carried out using separate reactors.  
         [0027]     In view of the foregoing, it will be understood that numerous modifications and variations of the inventions presented herein will be apparent to those of skill in the art. The forgoing is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. It is the following claims, including all equivalents, which define the scope of these inventions.