Compositions – Magnetic – Iron-oxygen compound containing
Patent
1994-08-08
1996-03-12
Bonner, C. Melissa
Compositions
Magnetic
Iron-oxygen compound containing
252 6263, 252 6256, C04B 3538
Patent
active
054983614
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to manganese-zinc system ferrite.
BACKGROUND ART
Among manganese-zinc system ferrites, high magnetic permeability ferrites comprising 50 to 56 mol % of Fe.sub.2 O.sub.3, 22 to 39 mol % of MnO, and 8 to 25 mol % of ZnO, for example, have been widely used as magnetic cores for wide frequency band transformers and magnetic cores for line filters and noise filters of various communication equipment or the like. These high magnetic permeability ferrites are now required to have high magnetic permeability particularly in the high-frequency range of about 100 kHz to about 500 kHz.
Manganese-zinc system polycrystalline ferrites have the general tendency that initial magnetic permeability .mu.i increases with a crystal grain size. One exemplary substance known to promote grain growth is Bi.sub.2 O.sub.3, which is practically used in Japanese Patent Publication (JP-B) No. 29439/1977 to establish a mean grain size of 50 .mu.m or greater for improving .mu.i. However, a greater grain size concurrently entails increased losses, especially at high frequencies, due to which only lower frequencies are available.
Then JP-B No. 49079/1976 attempted to improve high-frequency response by adding Bi.sub.2 O.sub.3 and CaO in admixture. This improvement, however, is effective only at frequencies of up to about 100 kHz, and this material is not practically acceptable as the high magnetic permeability material for line filters or the like which is required to have high .mu.i at higher frequencies, for example, over the entire range of from 10 to 500 kHz.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a manganese-zinc system ferrite having high magnetic permeability in the high-frequency range, that is, high magnetic permeability over the entire range of from 10 to 500 kHz.
This and other objects are achieved by the present invention which is defined below as (1) to (7). Fe.sub.2 O.sub.3 of iron oxide, 22 to 39 mol % calculated as MnO of manganese oxide, and 8 to 25 mol % calculated as ZnO of zinc oxide, up to 800 ppm calculated as Bi.sub.2 O.sub.3 of a bismuth oxide component and up to 1,200 ppm calculated as MoO.sub.3 of a molybdenum oxide component. diameter of 5 to 50 .mu.m. Fe.sub.2 O.sub.3 of iron oxide, 22 to 39 mol % calculated as MnO of manganese oxide, and 8 to 25 mol % calculated as ZnO of zinc oxide, 45 .mu.m. further includes 50 to 500 ppm calculated as CaO of calcium oxide. initial magnetic permeability of at least 8,000, at least 8,000, and at least 2,000 at 10 kHz, 100 kHz, and 500 kHz, respectively. initial magnetic permeability of at least 9,000, at least 9,000, and at least 3,000 at 10 kHz, 100 kHz, and 500 kHz, respectively. during the sintering step, an oxygen partial pressure of at least 25% is maintained at least upon heating from a temperature of at least 1,000.degree. C. to the holding temperature.
EFFECT AND ADVANTAGES OF THE INVENTION
At frequencies of up to 100 kHz, eddy-current losses can be reduced by increasing the electric resistance at grain boundaries. However, grain boundaries switch to act as a capacitance at high frequencies of from 100 to 500 kHz, which means that an increase in electric resistance at grain boundaries does not lead to an increase in the resistance of the entire core, failing to reduce the loss. This is the reason for the drop of .mu.i at high frequencies in the prior art.
Presuming that the grain size must be large and distributed uniformly before the loss at 500 kHz can be reduced and .mu.i at 100 to 500 kHz can be increased, the inventors have found that use of MoO.sub.3 in combination with Bi.sub.2 O.sub.3 is effective to this end and thereby succeeded in significantly improving .mu.i at 10 to 500 kHz.
It is believed that while MoO.sub.3 partially sublimates during firing, it is effective for suppressing the growth of crystal grains probably due to this sublimation, thus yielding uniform crystal grains. In the prior art, JP-B No. 28078/1974 proposed in conjunction with magnetic head material
REFERENCES:
patent: 3180833 (1965-04-01), Pierrot et al.
patent: 3837910 (1974-09-01), Van der Laan et al.
Inoue Shoji
Matsukawa Atsuhito
Bonner C. Melissa
TDK Corporation
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