JPH01158704A - Manufacture of nonmagnetic core for air core coil - Google Patents

Abstract

PURPOSE:To inexpensively manufacture air core coils having good shape retension and large Q in mass production by polishing a core made of a sintered specific substance and then oxidizing the surface at a rising temperature in an oxidative atmosphere. CONSTITUTION:A core made of Zn ferrite of diaferromagnetic material or sintered Zn ferritic oxide is manufactured, and polished in a predetermined size. It is further heat treated in an oxidative atmosphere thereby to obtain a core made of Zn ferrite or sintered Zn ferritic oxide having high surface resistance. Since an eddy current occurs on the surface, the core can provide an inductor having excellent high frequency properties. Since the core can be fired at much lower firing temperature than that of ceramics, such as alumina or the like, its manufacturing cost is reduced, and a mass production can be achieved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a non-magnetic core, and more particularly to a core for use as a winding core of an air-core coil.

(Prior Art) Air-core coils are open magnetic path type inductors and are widely used in electronic devices. Air-core coils have good DC superimposition characteristics because they are less likely to be saturated by the current flowing through the coils. However, the winding structure of air-core coils tends to change due to external forces, resulting in poor reliability.

Therefore, it is necessary to improve shape retention so that it is not affected by external forces. In order to improve shape retention, insulating materials such as alumina and steatite and polymeric materials such as phenol and epoxy are used to form the core, and the inductor is formed using wire rings and conductor-plated wiring wrapped around the core. is forming. However, these elements have advantages and disadvantages, and none are yet satisfactory.

(Problems with the Prior Art) Among the above elements, insulating materials such as alumina and steatite have high electrical insulation properties and are useful for high frequency applications. However, the manufacturing cost is high because the sintering temperature is extremely high when manufacturing the core by firing. Furthermore, due to its high hardness, machinability with a grindstone or the like is poor, and it has the disadvantage that it cannot meet the design requirements for high dimensional accuracy due to SMT.

Polymer resins such as phenol and epoxy resins have the disadvantage of poor heat resistance and easy deformation, and are also relatively expensive.

In addition, there is Zn ferrite, which is an antiferromagnetic material, as a ferrite-type oxide sintered body, and it is possible to use this as a core, but Zn ferrite has an extremely low electrical resistance of 10 to 103 Ωcm, so it cannot be rolled. The insulation against wires is low, and in high frequency bands, eddy current loss increases, and it is not uncommon for Q to decrease by as much as 50% compared to an air-core coil.

Therefore, when Zn ferrite is used as a core, there arise problems such as an increase in charging loss when configuring a tunable filter or the like.

However, since Zn ferrite can be easily manufactured by sintering at low temperatures, the inventors
We thought that it would be possible to provide a core that takes advantage of these characteristics, and came up with the present invention.

(Object of the Invention) An object of the present invention is to provide a method for manufacturing a core body suitable as a winding core of an air-core coil.

(Summary of the Invention) The method of the present invention involves manufacturing a core made of Zn ferrite, which is an antiferromagnetic material, or a sintered Zn ferrite-based oxide similar to the antiferromagnetic material, polishing it to a predetermined size, and then polishing it in an oxidizing atmosphere. This method of manufacturing a non-magnetic core is characterized by oxidizing the surface under elevated temperature. Such a core is formed from a raw material mixture into the shape of a rod-shaped core or a drum-shaped core, and then fired at a predetermined high temperature, but according to the research of the present inventor, the electrical resistance on the surface is relatively high. However, the internal electrical resistance is low. Therefore, when finished to a predetermined size by machining, the electrical resistance decreases. Therefore, by further heat-treating in an oxidizing atmosphere, it was possible to provide a core made of Zn ferrite or Zn ferrite-based oxide sintered body with high surface resistance. Since the eddy current phenomenon is a phenomenon on the surface, the core manufactured by the method of the present invention can provide an excellent inductor for high frequency applications.

(Detailed Description of the Invention) The method of the present invention will be described in detail below. More specifically, the core produced by the method of the present invention has an oxide sintered body composition (Z n I-x MX ) OF e 203
(where M is a divalent metal and X is O or a small amount of Fe, Cu, Ni selected to be non-magnetic or almost non-magnetic,
One or more types of Co. ) and ZnO mixed with Fe2O3 in an amount less than the stoichiometric amount. CuO
Some products containing Fe, Zn, and C have high electrical resistance, so the surface oxidation treatment of the present invention is not necessary.
Depending on the molar ratio of u, some have low electrical resistance, and the treatment of the present invention can significantly improve the properties.

After creating a Zn ferrite oxide sintered body with the component ratio shown in Table 1 using a conventionally used method, polyvinyl alcohol was added and granulated to obtain an outer diameter of 6 m.
A round bar with a length of 12 mm was press-formed. Thereafter, firing was performed in air at 1250°C to obtain a sintered body. The resistance value was measured on the surface of the obtained round bar core using a resistance meter at a distance between measurement terminals of 7 mm in the longitudinal direction of the core, which was defined as an electrical resistance value. Thereafter, the surface of the round bar core was removed by a thickness of 1.5 mm using a grindstone, and the electrical resistance of the internal surface was measured in the same manner as above.

Table 1 shows the electrical resistance values of the surface and the internal surface after polishing for each component ratio. This shows that the electrical resistance value of the surface and internal surface of the Zn ferrite oxide sintered body is significantly different.

These polished round bar cores were then heated again in air for 11 hours.
After heat treatment at a temperature of 50° C. for 1 hour, the electrical resistance was measured in the same manner as above. The results are shown in Table 1. It can be seen that the surface resistance has clearly recovered.

Expressed in molar ratio, Fe20342.5% and Zn○
57.5 (sample 1), and Fe203425%, Cu
A round bar core having a composition of Zn01% and Zn056.5% (Sample 2) was manufactured by the same method as in Example 1. An oxide sintered body was manufactured by the method of Example 1. Then, when the electrical resistance was measured by the method of Example 1, Sample 1 was 4×
107Ω and sample 2 was 4×lo 8Ω. Next, the surface layer was removed in the same manner as in Example 1, and the electrical resistance was measured again. Sample 1 was 6 x 102 Ω, and sample 2 was I
It was 10'Ω. Then, when the surface was oxidized under the same conditions as in Example 1, the electrical resistance value of the base was recovered.

(Effects) As described above, according to the present invention, a nonmagnetic core made of a Zn ferrite oxide sintered body with high surface resistance was obtained. Therefore, it was possible to provide an air-core coil that has good shape retention against external stress and has a large Q. Additionally, since the core of the present invention can be fired at a much lower firing temperature than ceramics such as alumina, manufacturing costs can be reduced.
Also, mass production becomes possible.

Claims (1)

[Claims] (1) Producing a core made of Zn ferrite, which is an antiferromagnetic material, or a sintered Zn ferrite-based oxide similar to this,
A method for manufacturing a non-magnetic core for an air-core coil, which comprises polishing to a predetermined size and then oxidizing the surface at an elevated temperature in an oxidizing atmosphere.