Methods for making high strength ceramic elements

Abstract

One embodiment of the present invention relates to spherical ceramic elements, such as proppants, for maintaining permeability in subterranean formations to facilitate extraction of oil and gas therefrom. The strength of the ceramic element may be enhanced by combining materials having different coefficients of thermal expansion. Methods of making the ceramic elements are also disclosed.

1 - 14 . (canceled) 
     
     
         15 . A process for manufacturing a ceramic element, comprising the steps of:
 (a) forming a spherically shaped non-sintered base of sinterable ceramic material;   (b) depositing a non-sintered layer of sinterable material on the surface of the base thereby forming a spherically shaped non-sintered composite having a base coated with at least one layer; and   (c) exerting a compressive force on the ceramic element by exposing the composite to a complete thermal cycle comprising a thermal ramp up phase and a thermal cool down phase wherein during the ramp up phase said base bonds to and shrinks more than said layer and after said cool down phase said base exerts a compressive force on said layer.   
     
     
         16 . The process of  claim 15  wherein during the exposure of the composite to the complete thermal cycle the layer material's rate of linear change versus temperature accelerates and then decelerates, after said deceleration has begun and prior to said cool down phase said base material's rate of linear change versus temperature exceeds said layer material's rate of linear change versus temperature. 
     
     
         17 . The process of  claim 16  wherein said base material has a sintering profile that defines a nominal sintering temperature and said layer material has a sintering profile that defines a nominal sintering temperature at least 5° C. but no more than 250° C. less than said base material's nominal sintering temperature. 
     
     
         18 . The process of  claim 17  wherein said layer material's nominal sintering temperature at least 20° C. but no more than 150° C. less than said base material's nominal sintering temperature. 
     
     
         19 . The process of  claim 15  wherein the step of forming a non-sintered base comprises forming a spheroid. 
     
     
         20 . A process, for forming a ceramic element, comprising the steps of:
 (a) forming a spherically shaped non-sintered base of sinterable ceramic material;   (b) heating said base to achieve at least partial sintering of the base;   (c) depositing a non-sintered layer of sinterable material on the surface of the base thereby forming a spherically shaped composite having at least a partially sintered base coated with an non-sintered layer; and   (d) exposing the composite to a complete thermal cycle that exceeds the sintering temperatures of the base and layer, wherein during said thermal cycle the base and layer bond to one another and the base contracts more than the layer thereby exerting a compressive force on the layer.   
     
     
         21 . The process of  claim 20  wherein said base material has a maximum theoretical density and, during step b, said base densifies to at least 25% of the base material's maximum theoretical density. 
     
     
         22 . The process of  claim 21  wherein said base densifies to less than 75% of the base material's maximum theoretical densification. 
     
     
         23 . The process of  claim 20  wherein the sintering temperature of the base in step b is at least 25° C. less than the temperature at which the composite is heated in step d. 
     
     
         24 - 33 . (canceled) 
     
     
         34 . A process for manufacturing a ceramic element, comprising the steps of:
 (a) forming a spherically shaped non-sintered base of sinterable ceramic material;   (b) depositing a non-sintered layer of sinterable material on the surface of the base thereby forming a spherically shaped non-sintered composite having a base coated with at least one layer; and   (c) exposing the composite to a complete thermal cycle comprising at least a first thermal ramp up phase and a final thermal cool down phase wherein after the initiation of said first ramp up phase said base shrinks and said layer applies a compressive force to said base, and after the initiation of said final cool down phase at least a portion of said layer separates from said base   
     
     
         35 . The process of  claim 34  wherein said base has a known surface area and said layer covers no more than 75 percent of said surface area. 
     
     
         36 . The process of  claim 34  wherein said base has a known surface area and said layer covers no more than 50 percent of said surface area. 
     
     
         37 . The process of  claim 34  wherein said base has a known surface area and said layer covers no more than 25 percent of said surface area. 
     
     
         38 . The process of  claim 34  wherein said layer fractures prior to separating from said base. 
     
     
         39 . The process of  claim 34  wherein said base has a coefficient of thermal expansion and said layer has a coefficient of thermal expansion greater than said base's coefficient of thermal expansion. 
     
     
         40 . The process of  claim 39  wherein the difference between said base's coefficient of thermal expansion and said layer's coefficient of thermal expansion is at least 40% but not more than 80% of said base's coefficient of thermal expansion. 
     
     
         41 . The process of  claim 39  wherein after the initiation of said cool down phase said layer applies an additional compressive force to said base.