Method for preparing spherical ferrite beads and use thereof

Abstract

The invention allows the fabrication of small, dense, highly polished spherical beads of hexagonal ferrites with selected compositions for use in nonreciprocal microwave and mm-wave devices as well as in microwave absorbent or reflective coatings, composites, and the like. A porous, generally spherical bead of hydrous iron oxide is made by a sol-gel process to form a substantially rigid bead having a generally fine crystallite size and correspondingly finely distributed internal porosity. The resulting gel bead is washed and hydrothermally reacted with a soluble alkaline earth salt (typically Ba or Sr) under conditions of elevated temperature and pressure to convert the bead into a mixed hydrous iron-alkaline earth oxide while retaining the generally spherical shape. This mixed oxide bead is then washed, dried, and calcined to produce the desired (BaFe 12 O 19 or SrFe 12 O 19 ) crystal structure. The calcined bead is then sintered to form a dense bead of the BaFe 12 O 19 and SrFe 12 O 19 phase suitable for polishing and incorporation into various microwave devices and components.

What is claimed is:  
     
         1 . A nonreciprocal microwave device having a resonator comprising a sintered spherical bead of a hexagonal alkaline earth ferrite composition of the general formula AFe 12   0   19 , wherein A=Ba or Sr, said bead derived from a sol-gel process and having a diameter less than about 1 mm and a grain size greater than about 0.1 μm.  
     
     
         2 . The article of  claim 1  wherein said sintered bead has a diameter between about 50 μm and about 1 mm.  
     
     
         3 . The article of  claim 1  wherein said sol-gel process is selected from the group consisting of internal gelation, external gelation, and water extraction processes.  
     
     
         4 . The article of  claim 3  wherein said internal gelation process comprises the precipitation and gelation of hydrous iron oxide from an aqueous broth containing at least one soluble iron species, urea, and hexamethylenetetramine.  
     
     
         5 . The article of  claim 4  wherein said broth further contains additional metal species in partial substitution for said iron.  
     
     
         6 . The article of  claim 5  wherein said additional metal species comprises at least one metal selected from the group consisting of Al, Ga, Ti, Ge, Mn, Zn, Cr, and Ni.  
     
     
         7 . The article of  claim 1  wherein said bead is sintered in air at a temperature of about 1000° to 1300° C. to substantially full density.