DE976924C - Process for producing a magnetic mixed ferrite material and a magnetic core produced therefrom - Google Patents

Description

(WiGBl. P. 175)

ISSUED AUGUST 6, 1964

A ⁷ 2336 VIb / 8ob

With a ferromagnetic core for high frequencies, especially for telephone and telegraphy purposes, which is generally used at low inductions, it comes with a high initial permeability and low losses at. In the past, compact iron was used as a ferromagnetic material for low frequencies. In alternating fields, eddy current losses arose in the highly conductive metal, which with the

ίο The frequency increases extremely sharply. While for low frequencies these eddy current losses are caused by lamination of the material and could reduce mutual isolation of the individual layers, this measure was sufficient for higher frequencies by no means. To use metallic iron as the core material even at high frequencies To be able to use it, one has used it in the finest subdivisions and the so-called powder cores (Mass cores) produced. For the purpose of isolating the individual powder granules, these are then surrounded by a binding agent that is non-magnetic and, similar to air gaps, the permeability significantly lowers; in the case of iron mass cores only permeability values between approx Reached 10 and 40. as

Hilpert (German patents 226347 ^and 227 787) made the proposal as early as 1909 to use cubic ferrites or mixed ferrites for high-frequency cores instead of metallic iron, i.e. oxidic materials of the formula MeO · Fe ₂ O ₃ ,

409 655/19

where Me is at least one divalent metal. He had found that ferrites have soft magnetic properties and have a significantly lower conductivity than iron; the specific resistance can e.g. B. io ³ to io ⁵ or even io ⁷ ohm · cm. It was therefore expected that the losses caused by eddy currents should no longer appear and that ferrites would be suitable even at very high frequencies.

So the suggestion was made, through the use of combined ferrites - at which in the above formula Me consists of two different metals - the specific ones To increase resistance even further.

Nevertheless, such compact ferrites have not yet been able to find use in practice because substantial losses were still occurring. In practice it is important that the total losses stay below a maximum value. The losses are given by the loss factor tg <5, which is measured on an annular solid core without an air gap. As is well known the relationship applies

^R.

where R denotes the loss resistance, L denotes the self-induction of a coil wound on the core and ω denotes the angular frequency. In addition to the eddy current losses, which are negligible in the case of a high specific resistance, these losses also include magnetic losses, in particular the hysteresis losses. The hysteresis losses can be neglected in such materials for telegraphy and telephony purposes, which are used with small induction; the eddy current losses are already extremely weak at a specific resistance of 10 to 100 ohm · cm and can therefore also be neglected. However, ferrites still show considerable losses for other reasons, in particular the so-called residual losses. So far it has not been possible to reduce these losses to a permissible level, at least not without significantly reducing the permeability, as z. B. is the case when ferrite powder is processed into mass cores according to a known method. However, a reduction in the losses at the expense of the initial permeability μ is generally disadvantageous. One can express this connection through the

Quotient

from the loss factor and the

Initial permeability. A soft magnetic material is generally the better, the smaller it is is the value of the quotient mentioned. A reduction in the loss factor, e.g. B. by switching on an air gap, which also reduces the initial permeability accordingly the value of this quotient remains practically unchanged and in this respect does not really lead to a better one Result.

Investigations which have led to the invention have shown that even with a kornpakt Ferrite material sintered together can get very favorable results when made from a ferromagnetic ferrite at room temperature and zinc ferrite, which is inherently non-magnetic a homogeneous mixed ferrite material is formed.

In connection with investigations of the phase diagram of ferrite-containing metal oxide-iron oxide combinations depending on their composition, it has been reported that mixed ferrites have properties such as Curie point and the saturation value, lie between the corresponding values of the components, thus show approximately an additive behavior.

However, this does not generally apply to mixed ferrites containing zinc ferrite; with regard to the The saturation magnetization and the permeability are extraordinarily obtained with such mixed ferrite values much higher than those shown by the ferromagnetic ferrite component.

Ferromagnetics generally show a temperature increase to below the Curie point Increase in permeability, such that it reaches a maximum just below the Curie point will. It has been known that adding zinc ferrite to nickel ferrite will decrease it of the Curie point can reach, so that the permeability maximum closer to the operating temperature could be laid out. But it was impossible to overlook the absolute value would then assume the relative maximum. Since zinc ferrite is non-magnetic, was - just below Consideration of the above-mentioned reference to the allegedly additive behavior of the Properties of mixed ferrite - a substantial deterioration in the ferromagnetic Properties, especially saturation magnetization and initial permeability, are to be expected.

The studies on which the invention is based have surprisingly shown that contrary to expectations, the addition of zinc ferrite makes it important for cores for high frequencies Properties of a ferromagnetic ferrite can be significantly improved.

It has also been found that in the manufacture of the mixed ferrite material to achieve a low loss factor with regard to compliance with a certain oxygen content Much greater care must be taken than otherwise with similar chemical-technical Processes is common.

An older, out-of-the-art proposal is a method of manufacturing a soft magnetic, made of homogeneous iao Cubic crystals with a spinel structure, sintered in an oxygen atmosphere Mixed ferrite material specified, in which a mixture of zinc and / or cadmium ferrite and a Ferromagnetic ferrite at room temperature, i »5 such as copper, magnesium or nickel ferrite or

a mixture of these ferrites forming constituents, e.g. B. the oxides, after sintering at 100 to 1400 ⁰ C while maintaining the oxygen atmosphere of a cooling at a rate of less than 100 ° C per minute, z. B. 5 to io ° C per minute is subjected. The starting point was to produce a ferrite with a high initial permeability whose loss factor is less than 0.06 in the frequency range from 10 to 100 Hz and whose specific resistance reaches at least 1000 ohm · cm. In order to achieve the oxygen content required for this, it is necessary not to use an excessively high temperature when producing such a ferrite or when heating it for other reasons; on the other hand, by avoiding a high temperature, an optimal value for the permeability is often not achieved. The measures that are necessary to achieve the large frequency range sought after in the older proposal can therefore lead to the maximum value of the initial permeability not being able to be used.

The invention is based on the knowledge that in the case of cores for telegraphic and telephony purposes, for example for filter coils, pupin coils, etc., it may be sufficient that the loss factor in an area up to less high frequencies is below 0.06 and that it is below 0.06 at higher frequencies can exceed this limit. It is then possible, through higher sintering temperatures and a different oxygen setting, to achieve considerably higher values of the initial permeability, so that the quotient of the loss factor and the initial permeability in a limited frequency range achieves significantly more favorable values and z. B. is less than io ~ ⁴ . This is of great importance for the mentioned purpose, e.g. B. for the production of filter coils for frequencies of about 10 to 100 kHz and for Pupin coils, which are used at frequencies of 300 to 2000 Hz.

This also gives an improvement and an enlargement compared to the older proposal the scope of the ferrite material when in a method of making a sintered, soft magnetic, especially suitable for telegraphic and telephony purposes, homogeneous mixed ferrite material consisting of cubic crystals with low losses and preferably high initial permeability according to the invention a powdery mixture which is a combination of zinc ferrite with one or more ferrites with a higher Curie point is formed, sintered and thereafter slowly, preferably at a speed of less than 10 ° C per minute, in particular less than 3 ° C per minute, is cooled, the oxygen content the mixed ferrite material produced in this way is adjusted during sintering and / or cooling so that that the loss factor in the frequency range from 10 to 100 kHz has a value less than 0.06 and in at least a part of the frequency range from 100 to 1000 kHz a value greater than Has 0.06.

With regard to the oxygen content, it can be noted that a ferrite is known to occur when heated at high temperatures, such as those used in production, split off oxygen can. In order to avoid a resulting lack of oxygen, the heating can be done in pure Oxygen can be carried out. But it has been found that even if a z. B. for manufacturing required heating was carried out in pure oxygen, often a little Oxygen deficiency occurs and that, surprisingly, such a low oxygen deficiency, the makes up only a few hundredths percent by weight of the total weight of the ferrite material, for the loss is of great disadvantage.

The most favorable oxygen content can be achieved in different ways, depending on the type and the composition of the ferrite.

Basically it is easiest if you go through a suitable choice of the conditions for it ensures that the heating temperature required for the preparation of the ferrite is sufficiently low is, so that a disruptive loss of oxygen does not occur. It has been found that the sintering temperature in some cases somewhat reduced by lowering the iron oxide content of the ferrite will.

Even if the change in the conditions under which a ferrite is prepared leaves room for maneuver the required heating temperature, it often does not succeed in practice, even with the Production of the ferrite from the components to achieve a sufficiently high oxygen content. According to a further development of the invention can be such a ferrite with too low an oxygen content, which is therefore unsaturated with regard to oxygen, oxygen at a lower temperature let record. The appropriate conditions, z. B. Temperature at which the oxygen uptake are carried out depend on various factors, e.g. B. on the amount of oxygen, which should be recorded in order to achieve a sufficiently low value of the loss factor, of the accessibility of all parts of the ferrite for oxygen, of the fine grain and hence the porosity of the material and also the ferrite composition.

It has been shown that the maximum amount of oxygen that can be absorbed at the same oxygen pressure is greater at lower temperatures. On the other hand, the fact that the The rate of oxygen uptake is lower at lower temperatures. This recording speed is also to a large extent dependent on the fine grain size and porosity of the material iao addicted; with regard to the time it takes for a certain amount of oxygen to be absorbed, it is therefore desirable that the ferrite be fine-grained and porous.

In the case of a ferrite, which is produced by heating an intimate mixture of powdery oxides

the sintering temperature depends on the intensity with which the mixture was ground, and the particle size achieved. A very fine mixture that has been ground for a long time a completely reacted homogeneous product at a lower level within a reasonable time Sintering temperature result than a coarser mixture ground for a shorter time. With consideration on the initial permeability of the final product

to such a full response is of great importance.

The final ferrite should approach the state of a single homogeneous phase as possible, d. H. that it should also be avoided that the once fully reacted ferrite is formed on cooling a second phase is eliminated. The latter can be the case if one of the oxides contained, the was kept at high temperature, as it were, in a supersaturated solid solution, separates out or if the ferrite breaks down into its oxide components when it cools down. Then a faster one can Cooling down may be useful.

As a rule, however, the cooling should be carried out slowly, e.g. B. at a speed of less than 10 ° C per minute in order to avoid quenching stresses that are detrimental to the permeability. The most favorable cooling rate can be determined in each individual case on the basis of a few samples easily set.

^ o If after sintering there is an oxygen absorption is carried out, the temperature used should preferably be above the value remain, in which a second phase can form.

H5 There may be a risk that with consideration Measures required for a high initial permeability contrast with measures the oxygen content required for a sufficiently high, desired for low losses are. A compromise solution must then be found. Since the invention is based knowledge that the losses above 100 kHz should not be taken into account needs, a higher initial permeability can be achieved.

It is expedient to ensure that the Curie point of the ferrite has a value between 40 and 250 ⁰ C. This can be achieved through the ratio between zinc ferrite and one or more ferrites with a higher Curie point in the mixed crystal. It is also possible to influence the Curie point by regulating the iron oxide content and, if necessary, the oxygen content.

Very good results can be achieved if the iron oxide content is less than 50 mol percent, preferably 47 to 48 mole percent.

The mixed crystal material according to the invention is expediently produced in that a mixture the ferrite forming oxides or a corresponding mixture of compounds that are used in The heating is converted into oxides, compressed and then sintered.

In order to achieve the most complete, fully reacted product possible, the starting mixture should have great delicacy and responsiveness. This can be done e.g. B. achieve by that the starting mixture is milled for a long time and with greater intensity. Preferably, an average particle size of less than 1 micron should be achieved.

It is also possible to precipitate a solution which contains all the metals required for the ferrite by means of a base and to dry the precipitate obtained, which may already have a ferrite structure in part. To improve the moldability, the precipitate is then heated preferably at 500 to 700 ⁰ C.

Repeated sintering can also be used in the manufacture of ferrite; H. that this first sintered mixture is finely ground again and sintered again. The first Sintering carried out at lower temperatures, with the mixture not yet complete reacted through. The product obtained can then easily be ground again to great fineness. A Compression of the mixture to be pre-sintered is preferably omitted, also for the sake of convenience of milling. This production has the advantage that ultimately a well-reacted product with a high initial permeability arises.

It should also be noted that the term "magnetic core" in the present case is not just one inside a coil, but generally with regard to its magnetic core Properties of applied parts of electromagnetic assemblies, e.g. B. also parts for magnetic shielding.

Example I.

An intimate mixture of pure magnesium oxide, zinc oxide and iron oxide in a molecular ratio of 26.5: 26.5: 47 is ground for 3 hours in an iron centrifugal mill. A ring 3 cm in diameter and 4 × 4 mm in cross section is pressed from the mixture under a pressure of 4 t / cm ^{2 with water as a plasticizer and binder.} This ring is heated for 1 hour at 1300 ° C in oxygen, followed by cooling at a rate of about 3 ⁰ C per minute in oxygen. The initial permeability is

350. The values of

tg '

are plotted in Fig. 1 as a function of the frequency (curve a). This curve shows that at frequencies higher than 800 kHz the value for tg d becomes greater than 0.06.

If the temperature is raised to 1400 ^° C. instead of 1300 ^° C., the values of curve b according to FIG. 1 are obtained. The initial permeability in this case is 525. At frequencies higher than 350 kHz, tg (5 becomes greater than 0.06.

Example II

A mixture of technical copper oxide, zinc oxide and iron oxide in a molecular ratio

from 20.7: 31.6: 47.7, calculated on the pure oxides, which 1 percent by weight of manganese dioxide is added, is milled for 3 hours and then pressed into a ring in the manner described in Example I.

This ring is sintered in oxygen ^{at 1050 ° C. for 1 hour;} it is then slowly cooled down to 600 ° C., which temperature is maintained for hours, all in oxygen, whereupon it is further cooled. The copper-zinc ferrite core obtained has an initial

permeability of 385. The values of —-— are in

Fig. 2 plotted as a function of the frequency. From this figure it follows that at frequencies higher than 900 kHz the value for tg (5 becomes greater than 0.06.

Claims (17)

PATENT CLAIMS: 20 i. Process for the production of a sintered, particularly suitable for telegraphic and telephony purposes, soft magnetic, homogeneous mixed ferrite material consisting of cubic crystals with low losses and preferably high initial permeability, characterized in that a powdery mixture, from which a combination of zinc ferrite with one or more ferrites with a higher Curie point is formed, sintered and then slowly ^{cooled, preferably at a rate of less than 10 0} C per minute, in particular less than 3 ⁰ C per minute, the oxygen content of the mixed ferrite material produced in this way during sintering and / or cooling it is set so that the loss factor in the frequency range from 10 to 100 kHz has a value less than 0.06 and in at least part of the frequency range from 100 to 1000 kHz has a value greater than 0.06. 2. The method according to claim 1, characterized in that that the mixture to be sintered is nickel oxide, zinc oxide and iron oxide or compounds which convert into these oxides when heated. 3. The method according to claim 1 or 2, characterized in that the mixture to be sintered Contains magnesium oxide, zinc oxide and iron oxide or compounds which when heated pass into these oxides. 4. The method according to claim 1, 2 or 3, characterized in that the mixture to be sintered Contains copper oxide, zinc oxide and iron oxide or compounds which are converted into these oxides when heated. 5. The method according to any one of the preceding claims, characterized in that of a mixture of pure raw materials is assumed. 6. The method according to any one of the preceding claims, characterized in that a mixture is assumed which contains less than 50 mol percent, preferably about 47 to 48 mol percent iron oxide (Fe ₂ O ₃ ). 7. The method according to any one of the preceding claims, characterized in that in with respect to oxygen unsaturated ferrite, preferably at a lower temperature, e.g. B. at 600 ° C, can absorb oxygen, in particular by sintering and / or cooling in a pure Oxygen atmosphere. 8. The method according to any one of the preceding claims, characterized in that by low heating ensures a sufficiently porous structure of the ferrite. 9. The method according to any one of the preceding claims, characterized in that a to sintering starting mixture is used, the particle size of which is less than 1 micron. 10. The method according to any one of the preceding claims, characterized by the use a starting mixture that consists of a solution of all the ferrite Metals is obtained by precipitation by means of a base. 11. The method according to claim 10, characterized in that the precipitated ferrite mixture is ^{heated to about 500 to 700 0} C and then pressed. 12. The method according to any one of the preceding claims, characterized in that the ferrite-forming starting mixture is sintered repeatedly. 13. The method according to claim 12, characterized in that that the first sintering or the first sintering (presintering) at a lower temperature than the last sintering is carried out. 14. The method according to claim 12 or 13, characterized characterized in that the pre-sintering (s) without prior compression of the ground Particle is (are) made. 15. The method according to any one of claims 1 to 11, characterized in that on a so '° 5 high temperature is heated so that the mixture is completely closed even during sintering a homogeneous cubic ferrite reacted through, whereupon preferably slowly, in particular slower than 10 ° C / min. cooled down te ^s becomes such that the quotient - ^ - is less than io ~ ⁴ at 10 to 100 kHz. 16. The method according to claim 15, characterized in that with copper-zinc ferrite at a temperature of 1050 ° C and with magnesium-zinc ferrite at a temperature of 1300 ⁰ C is sintered. 17. The method according to any one of the preceding claims, characterized in that it is assumed that a starting mixture contains zinc ferrite and one or more ferrites with a higher Curie point or the compounds forming these ferrites in such a ratio that an end product with a Curie point of 40 to 250 ⁰ C results. 409 655/19 i8. Magnetic core, especially for telegraphy or telephony purposes, e.g. B. for Filter coils or Pupin coils, produced according to the method according to one of the preceding Expectations. Considered publications:
German Patent Nos. 226347, 227787; U.S. Patent No. 1,946,964; Report of the German Chemical Society, 10 II, 42 (1909), pp. 2248 to 2261; Zeitschrift für Physikalische Chemie (B), 1932, pp. 291 and 298, and 1935, pp. 1 to 11; Comptes Rendues, 1939, pp. 164 to 167; Physica, III (6), 1936, pp. 463 to 483. 15 Older patents considered: German Patent No. 976 355. 1 sheet of drawings ι 409 655 / 1Ϊ7.64