Fine-Grain Nickel Ferrite for Microwave Applications at High Peak-Power Levels

Abstract

Fine-grain ferrites were fabricated by hot-pressing ball-milled and flame-sprayed powders of Ni1−xCoxFe2O4 (where 0≤x≤0.04) and Ni0.973Mn0.027Fe2O4. Threshold fields for subsidiary absorption at Ku-band were significantly increased by decreasing the grain size to 5 μ or less. Between 5 and 10 μ, the critical field hcrit decreased gradually and above 10 μ was independent of grain size. The highest value observed was 192 Oe on a fine-grained nickel-cobalt ferrite vs ∼24 Oe for conventionally fired nickel ferrites. Dielectric losses were reduced to values below 5×10−4 at X-band, and low-field magnetic losses were approximately two to four times those of conventionally fired ferrites of the same composition. No systematic variation in threshold field or magnetic loss was observed with cobalt substitution in fine-grain samples, although threshold fields in general were found to be higher when cobalt was present. The threshold field increased linearly with cobalt substitution on large-grain samples. A Ku-band 90° ferrite phase shifter using a fine-grained, hot-pressed ferrite was capable of operating at power levels in excess of 750 kW/750 W, whereas the same device with a conventional ferrite exhibited nonlinearity below 20-kW peak power.