JPS61190045A - Soft magnetic sintered alloy having reduced specific resistance and its production - Google Patents

Abstract

PURPOSE:To obtain a soft magnetic sintered alloy having reduced specific resistance and high magnetic flux density by mixing iron powder with a proper amount of copper powder, compacting the mixture, and sintering the green compact at the temp. below the melting point of copper. CONSTITUTION:The copper powder (8-20% by weight) is blended with the atomized iron powder, a lubricant is added, and the mixture is compacted into required shape. The resulting green compact is sintered at 800-1,070 deg.C, below the melting point of copper. The magnetic material made of this sintered alloy is capable of holding high magnetic flux density required of a magnetic core, and yet it has reduced specific resistance. Accordingly, when this material is used for yokes of voice coils, the necessity of shorting out an eddy current by putting a copper crown on a center pole is obviated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a sintered soft magnetic alloy with low resistivity suitable for the yoke of a voice coil of a speaker, for example.

Generally, sintered metal magnetic cores are used in DC equipment and relatively low frequency AC equipment, and some are made of pure iron, but when it comes to AC core materials, iron and Si are used.

An alloy to which elements such as AI, Ni, and Go are added is used. This is because when such elements are added, the resistance value increases, and as a result, iron loss can be lowered.

However, depending on the use of the magnetic core, such as when a high frequency is applied, it may be preferable to have a lower specific resistance. One example is the yoke of the voice coil that drives the speaker, and the vibration mechanism of the voice coil is also applied. A so-called voice coil motor has recently been used to drive the head of a magnetic disk device, and its yoke also falls under this category.

The reason why low resistance is desired in these cases will be briefly explained below with reference to FIG.

Figure 1 is a cross-sectional view of the voice coil section, where 1 is the yoke;
5 is the center ball at the center, and 3 is a permanent magnet. When a current (signal) is input to the voice coil 2, the coil generates a magnetic field, and due to the interaction of this magnetic field and the magnetic field from the permanent magnet 3 and the yoke 1, the voice coil 2
This means that it moves up and down while repeating suction and repulsion.

By the way, when the voice coil 2 is energized, an eddy current is excited near the skin of the center ball 5 due to the action of the magnetic field, producing a magnetic field with a phase different from that of the voice coil.As a result, the magnetic field of the coil is distorted and the output is reduced. A phenomenon occurs in which the voice coil does not operate according to the input signal. This phenomenon becomes more pronounced as the frequency of the current (signal) flowing through the coil increases.

Therefore, as a countermeasure for this, we use pure iron sintered material or low carbon steel for magnetic disks and high-end speakers.
Furthermore, a copper crown 4 with low resistance is placed on the outer peripheral surface of the center ball 5.
is installed to prevent the effects of eddy currents by shorting them.

However, this countermeasure has disadvantages in terms of man-hours and cost for installing the copper layer 4, and furthermore, the distance between the center ball and the voice coil becomes large due to the presence of the copper layer 4, which increases magnetic loss. There was a problem.

This invention is based on the above-mentioned background, and is the result of research aimed at creating a magnetic core material that is less affected by eddy currents even when the copper layer 4 is attached and does not cause short circuits, and a soft magnetic alloy that has low resistivity and high magnetic flux density in terms of physical properties. The obtained product, that is, the present invention is a sintered soft magnetic alloy with low resistivity, in which 8 to 20% by weight of copper is dispersed in an iron base, and a structure in which iron and copper are not diffused; As a method for obtaining such a structure, the gist is to sinter a mixed green compact of iron powder and copper powder at a temperature of 800 to 1070° C. below the melting point of copper.

An embodiment of the present invention will be described below.

First, electrolytic copper powder with a weight ratio of 8% is mixed with atomized iron powder,
A yoke with a center ball having a diameter of 25 mm and a height of 20 m5 was formed by adding 0.5% zinc stearate as a lubricant, and sintered at a temperature of 1060 DEG C. for 20 minutes. The density of the sintered body is 7.1Q/cd. In addition, a sample with a density of 7.30/cj was prepared by setting the content of copper to 20%, which is the upper limit of this invention, and keeping the other conditions the same.

Next, as a comparison sample, a yoke with the same dimensions was made from general steel 8834 and subjected to a characteristic test.

In the test, a 6Ω coil wound 120 times was used as a voice coil, and when an input of 40W was applied to it, the attenuation of the output and the noise level generated on the output side were measured, and the relationship with the frequency on the input side was determined. I asked for it.

The graph in Figure 5 shows the results.
In the case of No. 34 (no copper layer installed), as the input frequency increases, the output is significantly attenuated and the noise level also increases. Note that the data for the case where a copper layer is attached to 8834 is not shown because the graph would be complicated, but it shows a value slightly closer to 8% Cu, which is between the two curves according to the present invention.

In contrast, the sample according to the present invention has an upper limit of copper content.
Both the lower limit and the output attenuation and noise level in the high frequency range are lower than those of conventional materials without a copper layer, and even when compared to conventional materials with a copper layer, the performance is the same or better, with no reduction in quality. This shows that the intended purpose of omitting the copper layer without removing the copper layer can be achieved.

This concludes the description of the embodiment, and next, each requirement of the present invention will be described.

First, in order to leave the copper without diffusing when sintering the mixed green compact of iron powder and copper powder, the sintering temperature conditions are particularly important. When copper diffuses into the iron base, it loses its properties as a soft magnetic material and falls into the category of semi-hard magnetic materials.

FIG. 4 shows the relationship between the sintering temperature and the magnetic flux density of the sintered body obtained by sintering a green compact with a copper content of 12% at different temperatures. The large difference in magnetic flux density between 700° C. and 800° C. in this graph indicates that a large magnetic flux density cannot be obtained unless mutual sintering of the iron powder progresses. Furthermore, when the sintering temperature exceeds the melting point of copper, 1083°C, the magnetic flux density decreases rapidly. This is thought to be due to a decrease in density. From these points, industrially, the sintering conditions that can guarantee high magnetic flux density without copper diffusion are 800 to 1070°C. Note that the higher the sintering density, the higher the magnetic flux density, so in practice it is 7g/cr1.
The above is desirable.

Next, the composition range of copper will be described. Figure 2 is 1060
The relationship between the copper content and magnetic flux density of a sintered body sintered at °C is
FIG. 3 similarly shows the relationship with resistivity, and both decrease almost linearly as the number of copper pots increases.

In the magnetic core material, the higher the magnetic flux density, the better.
Usually a value of 10KG or more is required. For this reason, the upper limit of the copper content is set at 20% in view of cost and margin for mass production.

On the other hand, for the purposes of this invention, the lower the resistivity, the better. According to the results of comparing each sample with different copper content at the same time as the above example, it was found that the copper content was 8.
% or more, it has been found that even without installing a copper layer on the center ball, it is comparable to conventional materials. Therefore, the lower limit of the copper content is set to 8%.

Note that A1 is an example of a good conductive material other than copper, but copper is preferable because it is easily oxidized and alloys with iron to increase the specific resistance.

As explained above, the magnetic material according to the present invention has a low specific resistance while maintaining the high magnetic flux density required for the magnetic core. Therefore, when this material is used for the yoke of a voice coil, it is necessary to attach a copper layer to the center ball as in the past. There is no need to short-circuit eddy currents. Therefore, the present invention has the effect of eliminating the power loss and cost disadvantage due to the attachment of the copper layer.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view illustrating the coil operating portion of a voice coil in an embodiment of the present invention, Fig. 2 shows the relationship between the copper content of the magnetic material and the magnetic flux density, and Fig. 3 shows the relationship between the copper content and the magnetic flux density. Figure 4 is a graph showing the relationship between sintering temperature of magnetic material and magnetic flux density. Figure 5 is a graph showing the relationship between input current frequency, output level, and output noise level. This is a graph showing. 1...Yoke 2...Voice coil 3...
・Permanent magnet 4...Copper layer 5...Center ball

Claims (1)

[Scope of Claims] 1. A sintered soft magnetic alloy with low specific resistance, characterized by having a composition of 8 to 20% copper and the balance iron by weight, and exhibiting a structure in which iron and copper are not diffused. 2. A sintered soft magnetic alloy with low specific resistance, characterized by mixing 8-20% by weight of copper powder with iron powder, compacting it into a desired shape, and sintering it at a temperature of 800-1070°C. manufacturing method.