ferrite mixture

When they say “ferrite mixture?”, many people immediately imagine the finished powder in a bag, which just needs to be poured and compressed. This is perhaps the most common simplistic view that I often see among new technologists. In fact, this is a whole story that begins long before the bag. If the mixture has arrived at production, it means that dozens of acceptances and test batches have already been carried out with it, and it is not just dust, but a material with a given fate - be it a core for an RF transformer or a sensor element. But even with all this, each new batch is a small risk, and here’s why.

What's behind the gray powder?

The composition is not just Fe2O3, oxides of zinc, nickel, manganese in a certain proportion. This is, first of all, the history of raw materials. The same chemical composition, but from a different iron carbonate or oxide, will give the output after sintering a different granulometry and, critically, different magnetic losses. We once took two batches of the mixture, formally identical according to the passport, from two suppliers. After pressing and sintering, the scatter in the initial magnetic permeability (μi) reached 15%. And this is despite the seemingly strict GOST. It all came down to precursors and the fineness of grinding the components before mixing.

There is often a pitfall here for industries that purchase ready-madeferrite mixture. Savings at the stage of input of raw materials from the supplier can result in instability of the parameters of your final product. High-frequency ferrites are especially sensitive to this. Therefore, for many years, in our cycle we have focused on control of incoming raw materials, and not just the finished mixture. It's more expensive, but more predictable.

By the way, about stability. An ideal mixture is one whose parameters lie within a narrow corridor not only from batch to batch, but also within one batch. Heterogeneity of mixing is a scourge. Imagine that in one part of the mold the granules are larger, in the other they are smaller. The compaction density will be different, and during sintering this is guaranteed to cause deformation or a crack. I saw this on old ribbon mixers. We switched to planetary ones - the situation leveled out, but energy costs also jumped.

From theory to the workshop: where it’s thin, it breaks

The transition from a laboratory batch of 5 kg to an industrial batch of 2 tons is always a quest. In the laboratory, I mixed everything by hand in a ceramic drum and sintered it in a chamber furnace with an ideal profile. In the workshop there is a half-cube mixer, a tunnel oven, with its zones, flows of a protective atmosphere (usually nitrogen) and inevitable temperature gradients. And here the parameters of the finished mixture, which you developed under ideal conditions, begin to “float”.

One of the key points is binders and plasticizers. They are added toferrite mixtureto improve compressibility and give strength to the green (unfired) workpiece. It’s not enough to add - the powder will generate dust, the mold will wear out instantly, and the workpiece will crumble during transportation on a pallet. If you overdo it, during sintering the binder will burn out too intensely, leaving pores, or even causing swelling of the material. We selected the percentage almost by eye, based on experience, making test pressings and observing the behavior of the workpiece during heat treatment. Standard polyvinyl alcohol (PVA) is good, but hygroscopic. They changed the humidity in the workshop - and all the settings went wrong.

A real case from practice. Once they installed a new tunnel oven. Display the temperature profile according to the material data sheet. The first batches are defective at 30%. The cores were warping. They began to figure it out. It turned out that the supplierferrite mixturechanged the type of binder to a more heat-stable one, but did not warn. The old profile with a sharp rise in temperature in the burnout zone of the bond was not suitable - the new material required a flatter curve. Trifle? No, a week of downtime and a ton of spoiled material.

Equipment and scale: a view from LONGI

When it comes to large-scale production, such as in enterprises producing mining equipment, the requirements for materials shift. What is important here is not so much the maximum magnetic permeability as stability, reliability and, importantly, cost. For example, takeLONGI Corporation(Shenyang Scientific Electromagnetic Company LONGJI LLC), which has grown into a large manufacturer since 1993. On their websiteljmagnet.ruit is clear that the scale is serious: an area of 140,000 m2, more than 1,200 employees, production of about 4,000 units of equipment per year.

For such production, supplyferrite mixture- this is not buying a bag of powder, but a long-term contract for thousands of tons with strict control regulations at each stage. I assume that they, like many large players, have an approved list of suppliers whose technological processes are audited. They are interested in a mixture that is guaranteed to give the same properties in each magnetic circuit for, say, a dredge power converter or a magnetic separation system. There is no time for experimenting with new additives - we need a proven “worker”. compound.

It would be interesting to know whether, like some, they practice vertical integration - that is, the production of ferrite powders or mixtures for their own needs through subsidiaries. This gives maximum control, but also requires enormous capital investments in metallurgical processes. Judging by the scale and specialization in the final equipment, most likely they work with trusted external suppliers of mixtures, but have their own powerful laboratory for incoming and outgoing control. This is standard and reasonable practice for an enterprise of this size.

Failures as a point of growth

You can't talk about practice without remembering failures. The most offensive one was related to an attempt to save money. We decided to purchase not a ready-made calibrated mixture, but basic oxides and do the mixing ourselves. The logic was: cheaper, plus full control. On paper - yes. In practice, they did not take into account the need for ultra-fine and ultra-uniform grinding of the components before mixing. Our own ball mills did not provide the required dispersion. As a result, after sintering, magnetic losses (Pcv) were several times higher than the nominal ones. The batch had to be disposed of. We saved pennies, lost thousands and, most importantly, time. Conclusion: you shouldn’t get involved in the material creation chain without the full cycle of necessary equipment. Sometimes it’s safer to trust a specialized manufacturerferrite mixture.

Another lesson is dependence on the human factor. Even with automated loading of components into the mixer, the operator may make a mistake in the order. Oxides must be loaded in a strict order for better homogenization. I once saw how, due to an error in the loading order, local carbonate inclusions formed in the mixture, which then, during sintering, gave rise to gas pockets inside the product. Test annealing of the sample before putting the entire batch into the furnace saved us from a major defect. Now this is an ironclad rule.

And, of course, logistics. Ferrite mixture is a hygroscopic material. Storing in an unheated warehouse in winter and then bringing it into a warm workshop leads to moisture condensation on the particles. Pressing such a mixture is torture. It was necessary to introduce a rule of mandatory conditioning of an unpacked bag in a workshop environment for at least 24 hours before use. It’s a small thing, but if you miss it, problems with pressing are guaranteed.

A look into the future of the material

Nowadays there is a lot of talk about nano-sized ferrites and mixtures for additive technologies (3D printing of magnetic elements). This is certainly promising, but for the bulk of industry - from electronics to heavy engineering - classicferrite mixturebased on manganese-zinc and nickel-zinc ferrites. Their evolution follows the path of even greater stabilization of parameters, reduction of losses at high frequencies (for power electronics) and adaptation to new, more productive pressing methods, for example, isostatic.

I also see a trend towards “sharpened” ones. mixtures. Not a universal material, but a composition optimized for the specific application of a particular manufacturer. Let's say forLONGI Corporationa mixture with increased mechanical strength after sintering can be developed if their magnetic cores operate under vibration conditions of mining equipment. This requires close cooperation between the mixture manufacturer and the final product manufacturing plant engineers.

Ultimately,ferrite mixture- this is not a product, but a semi-finished product with a high degree of responsibility. Its quality is the key to the quality of the entire device into which it will fit. And there are no small details in this process: from the choice of ore from the raw material supplier to the temperature and atmosphere in the tunnel furnace in your workshop. Experience comes precisely through understanding this chain and through the analysis of those very “inexplicable” things. failures, which, as a rule, have a completely material cause somewhere at the junction of technological stages.