Process for coating ceramic particles and compositions formed from the same

Abstract

Methods of coating ceramic (e.g., barium titanate-based) particles, as well as articles and compositions formed from the coated particles are provided. The methods involve forming a mixture of barium titanate-based particles and at least two dopant metal solutions. According to some methods, a second solution (e.g., a base) is added to the mixture to cause the dopant metals to sequentially precipitate onto surfaces of the particles. The resulting particles are coated with respective layers having different dopant metal compositions. According to other methods, the mixture of barium titanate-based particles and at least two dopant metal solutions are added to a second solution (e.g., a base) to precipitate the dopant metal or metals to form a coating on surfaces of the barium titanate-based particles. The resulting particles are coated with a homogeneous coating. The coated barium titanate-based particles produced according to the methods of the invention may be further processed to form, for example, green layers and/or dielectric layers in electronic devices such as MLCCs. The methods provide a uniform distribution of the dopant metals throughout the barium titanate-based particulate composition and limit dopant segregation which can improve properties of the dielectric layers amongst other advantages.

What is claimed is:  
     
         1 . A method of coating barium titanate-based particles comprising: 
 forming a mixture of barium titanate-based particles and at least two dopant metal solutions; and    sequentially precipitating the dopant metals from the mixture to form a coating on surfaces of the barium titanate-based particles.    
     
     
         2 . The method of  claim 1 , wherein the pH of the mixture is increased to sequentially precipitate each of the dopant metals.  
     
     
         3 . The method of  claim 2 , wherein the pH of the mixture is increased by adding a base to the mixture.  
     
     
         4 . The method of  claim 3 , wherein the base comprises barium hydroxide.  
     
     
         5 . The method of  claim 3 , further comprising mixing the mixture while adding a base to the mixture.  
     
     
         6 . The method of  claim 5 , wherein mixing the mixture creates a region of high shear and the base is introduced into the region of high shear.  
     
     
         7 . The method of  claim 2 , wherein a first dopant metal is precipitated at a first pH and a second dopant metal is precipitated at a second pH greater than the first pH.  
     
     
         8 . The method of  claim 2 , wherein a first dopant metal is precipitated at a first pH, a second dopant metal is precipitated at a second pH greater than the first pH, a third dopant metal is precipitated at a third pH greater than the second pH, and a fourth dopant metal is precipitated at a fourth pH greater than the third pH.  
     
     
         9 . The method of  claim 2 , wherein the pH of the mixture is increased from a value of between about 4 and about 5 to a value of between about 7 and about 12.  
     
     
         10 . The method of  claim 1 , further comprising mixing at least two dopant metal solutions and adding the barium titanate-based particles to form the mixture of barium titanate-based particles and at least two dopant metal solutions.  
     
     
         11 . The method of  claim 10 , wherein the at least two dopant metal solutions are mixed to form a solution having a pH of less than about 4.  
     
     
         12 . The method of  claim 1 , wherein the dopant metal solutions comprise a metal selected from the group consisting of Mg, Mn, W, Mo, V, Cr, Si, Y, Ho, Dy, and Ce.  
     
     
         13 . The method of  claim 12 , wherein the dopant metal solutions comprise a metal selected from the group consisting of Y, Mg, and Mn.  
     
     
         14 . The method of  claim 1 , wherein the mixture comprises at least four dopant metal solutions.  
     
     
         15 . The method of  claim 1 , wherein the coating includes a plurality of layers, each layer comprising a different dopant metal.  
     
     
         16 . The method of  claim 1 , wherein the coating comprises a dopant metal oxide or metal hydroxide.  
     
     
         17 . The method of  claim 1 , further comprising forming the barium titanate-based particles in a hydrothermal process.  
     
     
         18 . The method of  claim 17 , further comprising maintaining the barium titanate-based particles in an aqueous slurry after the hydrothermal process and adding the aqueous slurry of barium titanate-based particles to the at least two dopant metal solutions.  
     
     
         19 . The method of  claim 1 , further comprising processing the coated barium titanate-based particles to form a dielectric layer in a multi-layer ceramic capacitor.  
     
     
         20 . The method of  claim 1 , wherein the dopant metal solutions are aqueous solutions.  
     
     
         21 . The method of  claim 1 , wherein forming the mixture of barium titanate-based particles and the at least two dopant metal solutions comprises adding barium titanate-based particles to at least one metal solution to form a first mixture and adding the first mixture to a second dopant metal solution to form the mixture of barium titanate-based particles and at least two dopant metal solutions.  
     
     
         22 . A method of coating barium titanate-based particles comprising: 
 forming a mixture of barium titanate-based particles and at least two dopant metal solutions; and    adding the mixture to a second solution to precipitate the dopant metals to form a coating on surfaces of the barium titanate-based particles.    
     
     
         23 . The method of  claim 22 , further comprising mixing at least two dopant metal solutions and adding the barium titanate-based particles to form the mixture of barium titanate-based particles and at least two dopant metal solutions.  
     
     
         24 . The method of  claim 22 , wherein the at least two dopant metal solutions are mixed to form a solution having a pH of less than about 4.  
     
     
         25 . The method of  claim 22 , wherein the mixture includes at least four dopant metal solutions.  
     
     
         26 . The method of  claim 22 , wherein the dopant metal solutions comprise a metal selected from the group consisting of Mg, Mn, W, Mo, V, Cr, Si, Y, Ho, Dy, and Ce.  
     
     
         27 . The method of  claim 26 , wherein the dopant metal solutions comprise a metal selected from the group consisting of Y, Mg, and Mn.  
     
     
         28 . The method of  claim 22 , wherein the second solution is a basic solution.  
     
     
         29 . The method of  claim 28 , wherein the basic solution comprises barium hydroxide.  
     
     
         30 . The method of  claim 22 , wherein the second solution increases the pH of the mixture to a value of between about 7 and about 12.  
     
     
         31 . The method of  claim 22 , further comprising mixing the solution while adding the mixture of barium titanate-based particles and at least two dopant metal solutions to the solution.  
     
     
         32 . The method of  claim 31 , wherein mixing the solution creates a region of high shear and the mixture of barium titanate-based particles and at least two dopant metal solutions is introduced into the region of high shear.  
     
     
         33 . The method of  claim 22 , wherein each dopant metal is distributed throughout the coating.  
     
     
         34 . The method of  claim 22 , wherein the coating comprises a dopant metal oxide or metal hydroxide.  
     
     
         35 . The method of  claim 22 , further comprising forming the barium titanate-based particles in a hydrothermal process.  
     
     
         36 . The method of  claim 35 , further comprising maintaining the barium titanate-based particles in an aqueous slurry after the hydrothermal process and adding the aqueous slurry of barium titanate-based particles to the at least two dopant metal solutions.  
     
     
         37 . The method of  claim 22 , further comprising processing the coated barium titanate-based particles to form a dielectric layer in a multi-layer ceramic capacitor.  
     
     
         38 . The method of  claim 22 , wherein the dopant metal solutions are aqueous solutions.  
     
     
         39 . The method of  claim 22 , wherein forming the mixture of barium titanate-based particles and the at least two dopant metal solutions comprises adding barium titanate-based particles to at least one metal solution to form a first mixture and adding the first mixture to a second dopant metal solution to form the mixture of barium titanate-based particles and at least two dopant metal solutions.  
     
     
         40 . A method of coating barium titanate-based particles comprising: 
 forming a mixture of barium titanate-based particles and at least two dopant metal solutions, the mixture being free of a chelating agent; and    precipitating each of the dopant metals to form a coating on surfaces of the barium titanate-based particles.    
     
     
         41 . The method of  claim 40 , wherein the pH of the mixture is increased to precipitate each of the dopant metals.  
     
     
         42 . A method of coating barium titanate-based particles comprising: 
 forming a mixture of barium titanate-based particles and at least two dopant metal solutions, the mixture being at a first temperature condition; and    precipitating each of the dopant metals to form a coating on surfaces of the barium titanate-based particles while maintaining the mixture within 25° C. of the first temperature condition.    
     
     
         43 . The method of  claim 42 , wherein the pH of the mixture is increased to precipitate each of the dopant metals.  
     
     
         44 . A green layer comprising a doped barium titanate-based particulate composition and having a surface that includes at least one 100 micron by 100 micron area that includes less than about twenty five dopant segregates having a size of greater than about 2.0 micron.  
     
     
         45 . The green layer of  claim 44 , wherein the doped barium titanate-based particulate composition includes barium titanate-based particles that include a coating comprising at least two dopant metals.  
     
     
         46 . The green layer of  claim 45 , wherein the coating comprises at least two dopant metal oxides or metal hydroxides.  
     
     
         47 . The green layer of  claim 45 , wherein the coating includes a plurality of layers, each layer comprising a different dopant metal.  
     
     
         48 . The green layer of  claim 45 , wherein each dopant metal is distributed throughout the coating.  
     
     
         49 . The green layer of  claim 44 , wherein the barium titanate-based particles are formed in a hydrothermal process.  
     
     
         50 . The green layer of  claim 44 , wherein the barium titanate-based particles are substantially spherical.  
     
     
         51 . The green layer of  claim 44 , wherein the green layer includes at least one 1 cm by 1 cm micron area that is substantially free of dopant segregates having a size of greater than about 2.0 micron.  
     
     
         52 . The green layer of  claim 44 , wherein the total weight percentage of dopants in the composition is between about 0.05 weight percent and about 10 weight percent based on the total weight of composition.  
     
     
         53 . The green layer of  claim 44 , further comprising a polymeric binder.  
     
     
         54 . The green layer of  claim 44 , wherein the green layer is a green tape.  
     
     
         55 . The green layer of  claim 44 , wherein the surface includes at least one 100 micron by 100 micron area that includes less than about ten dopant segregates having a size of greater than about 2.0 micron  
     
     
         56 . The green layer of  claim 55 , wherein the surface includes at least one 100 micron by 100 micron area that includes less than about five dopant segregates having a size of greater than about 2.0 micron  
     
     
         57 . The green layer of  claim 56 , wherein the surface includes at least one 100 micron by 100 micron area that is substantially free of dopant segregates having a size of greater than about 2.0 micron.  
     
     
         58 . A multi-layer ceramic capacitor including at least one dielectric layer formed from the green layer of  claim 44 .  
     
     
         59 . A doped barium titanate-based particulate composition having a surface including at least one 100 micron by 100 micron area that includes less than about twenty five dopant segregates having a size of greater than about 2.0 micron.  
     
     
         60 . The doped barium titanate-based particulate composition of  claim 59 , wherein the surface includes at least one 100 micron by 100 micron area that includes less than about ten dopant segregates having a size of greater than about 2.0 micron.  
     
     
         61 . The doped barium titanate-based particulate composition of  claim 60 , wherein the surface includes at least one 100 micron by 100 micron area that includes less than about five dopant segregates having a size of greater than about 2.0 micron.  
     
     
         62 . The doped barium titanate-based particulate composition of  claim 61 , wherein the surface includes at least one 100 micron by 100 micron area that is substantially free of dopant segregates having a size of greater than about 2.0 micron.