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 coating for selectively absorbing and reflecting microwaves, comprising a polymeric matrix containing an inorganic dispersed phase, said dispersed phase containing substantially dense spherical sintered beads of a hexagonal alkaline earth ferrite composition of the general formula AFe 12 O 19 , wherein A=Ba or Sr, said beads derived from a sol-gel process followed by hydrothermal reaction and having a final diameter less than about 1 mm and a grain size greater than about 0.1 μm.  
     
     
         2 . The coating of  claim 1  wherein said sintered beads have a diameter between about 50 μm and about 1 mm.  
     
     
         3 . The coating of  claim 2  wherein said beads are substantially monosized.  
     
     
         4 . The coating of  claim 2  wherein said beads have a selected range of diameters whereby the microwave reflectivity of said coating may be controlled over a selected frequency range.  
     
     
         5 . The coating of  claim 1  wherein said sol-gel process is selected from the group consisting of internal gelation, external gelation, and water extraction processes.  
     
     
         6 . The coating of  claim 5  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.  
     
     
         7 . The coating of  claim 6  wherein said broth further contains additional metal species in partial substitution for said iron.  
     
     
         8 . The coating of  claim 7  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.  
     
     
         9 . The coating of  claim 1  wherein said beads are sintered in air at a temperature of about 1000° to 1300° C.  
     
     
         10 . The coating of  claim 1  wherein all of said beads have substantially the same composition.  
     
     
         11 . The coating of  claim 1  wherein said beads are selected from at least two batches having substantially different compositions, whereby the microwave properties of said coating are controlled over a desired range.