ferrite structure

When talking about the structure of ferrite, many people immediately imagine ideal crystals from a textbook. In practice, everything is different. Often customers ask for the “highest coercive force”, not realizing that for their specific unit - say, a separator at a processing plant - the stability of magnetic properties during vibration and temperature changes is much more critical. This is where the whole point lies: the macroscopic parameters of a product are determined by what happens at the level of grain, domain, and boundaries. And this connection cannot always be predicted only by chemical composition.

From powder to product: where control is lost

It all starts with precursors. Iron oxides, strontium, sometimes additives - it would seem that everything is according to the recipe. But one batch of oxide may have different activity due to the size of the primary particles, and this is the first call. At the stage of mixing and pre-sintering, nuclei of the future phase are formed. If you do not maintain the temperature profile, instead of a homogeneous hexaferrite structure you will get a mixture of phases, and the magnet will “soap”.

I remember ten years ago at one of the old production facilities they tried to speed up the calcination cycle and raised the temperature. The mass yield was excellent, but when they began to measure the magnetic induction on ready-made magnets for drum separators, the scatter was catastrophic. It turned out that overheating resulted in the formation of large but imperfect grains with a high density of dislocations inside. Externally, the material was beautiful, dark gray, but its properties were defective.

It is worth noting here that serious manufacturers, such as the LONGI Corporation, have automated this stage long ago. On their websiteljmagnet.ruyou can see their emphasis on complete cycle control. And this is not marketing: for mining equipment, which they produce in thousands of pieces per year, the stability of the magnetic core is a matter of reputation and lack of downtime for the client.

Microstructure: under a microscope and in work

The ideal structure of ferrite is a dense, non-porous body with small, isometric grains and clearly defined boundaries. But in life there is always a compromise. Fine grain gives high coercivity, but can reduce the residual induction. Large grains are the opposite. The technologist’s task is to find the equilibrium point for a specific task.

For powerful suspended separators, where a large force of attraction is needed, they often increase the grain size, sacrificing a little resistance to demagnetization. But for motors or sensors operating in alternating fields, it is the fine-grained, “hard” grain that is important. structure. When injection molding with ferrite plastic is a completely different story - it is important that the ferrite particles are not only magnetic, but also have the correct surface for adhesion to the polymer.

One of the most insidious problems is pores. They don't just reduce density. They concentrate mechanical stress and become points of initiation of destruction during thermal shock or vibration load. In mining equipment, which is known to operate under harsh conditions, this defect is unacceptable. I saw samples where, due to an incorrect sintering mode, the pores were lined up in chains along the grain boundaries - such a magnet crumbled in the hands after several cycles of impact tests.

The influence of alloying: not only for the passport

The addition of cobalt, lanthanum, and bismuth are standard methods for correcting properties. But in the shop they look at it differently. Cobalt is expensive and volatile at high temperatures. Its introduction requires precise control of the atmosphere in the furnace, otherwise, instead of improving temperature stability, you will get heterogeneity across the cross-section of the product. About fifteen years ago there was a fashion for complex alloyed compounds, but then many, including our Chinese colleagues fromLONGI Corporation, returned to more classic recipes, but with filigree control over the purity of the source materials and the geometry of the grains.

In their case, judging by the scale of production (more than 4,000 units of equipment per year), this is economically justified. The reliability and predictability of the material for their magnetic separator systems is probably more important than record values ​​for one parameter. Alloying often leads to the appearance of side non-magnetic phases at the boundaries, which in the dynamic mode of equipment operation can cause local overheating and cracking.

Practical observations and typical mistakes

A common mistake in small-scale production is saving on grinding. It seems that the longer we grind, the finer and better the grain. But overgrinding leads to hardening of the powder, the introduction of impurities from grinding media and, paradoxically, to grain growth during sintering due to excess surface energy. The optimal time is always the result of many trials.

Another point is cooling after sintering. Rapid cooling (quenching) fixes the high-temperature structure, but can create internal stresses. Slow cooling, especially within certain temperature windows, allows for secondary recrystallization, which is sometimes useful. But here we need to know exactly what properties we want to extract. For standard magnets, separators often simply use controlled oven cooling - stable and without surprises.

Working with different factories, I noticed that where they have their own metallographic laboratory and can regularly examine thin sections under a microscope, the percentage of suitable products is consistently higher. It is not enough to measure the finished magnet with a Teslameter. You need to see what's inside. The structure of ferrite is its DNA, and you need to be able to read it.

A look into the future: what remains relevant

Despite the boom in rare earth magnets, ferrites are here to stay. Their niche is large volumes, strict conditions, and cost requirements. Improvement is taking place not so much in the area of ​​new compositions, but in the area of ​​precision control of existing processes. Digitalization of furnaces, precision dosing, atmosphere control - this is what gives the increase.

Big players like the one mentionedLONGI Corporation(Shenyang Longji Scientific Electromagnetic Co.,Ltd.), with its history since 1993 and a staff of 1,200 people, is investing in exactly this. Because for their core product - mining equipment - the key is not the absolute strength of the magnet, but its ability to work for decades in dust, vibration and with minimal degradation. And this is laid precisely at the stage of formation of that very microstructure.

So, as I think about the structure of ferrite, I increasingly come to the conclusion that this is more an engineering problem than a scientific one. It is necessary not to discover new formulas, but to hone known processes down to the millikelvin and milligram. So that each grain in the material takes its predetermined place. It is this, and not the passport data, that ultimately determines whether the magnet will pull ore from the pulp for years or fail in six months, failing the entire enrichment complex.