ferrite carbon

When they talk aboutferrite carbon, many people immediately think of the classic strontium or barium ferrites, but carbon is a different story. Often in conversations in production you hear that carbon is for steels, but in ferrites it is an impurity that is removed. But in practice everything is more complicated. I myself thought so for a long time until I came across a batch of material from one supplier, where carbon-containing phases gave an unexpected effect on thermal stability - they did not improve, but worsened it in a certain range. This forced me to dig deeper.

What actually happens to the carbon in the charge

In classical technology for the production of ferrites, especially hard ones, carbon often enters the charge through raw materials - iron oxides, carbonates. They try to burn it out at the pre-calcination stage. But burning it out completely is ideal. In reality, especially with large furnace volumes, there are always local zones where carbon is stored in the form of graphite or even forms iron carbides during the sintering process. It's not always a disaster. In some brands of ferrites for low-frequency applications, a small residual carbon content, conventionally up to 0.2%, can even smooth out the hysteresis loop a little, making it a little more rectangular - but this is a very fine line, crossed - and the coercive force creeps up uncontrollably.

I remember at one of the old production facilities associated withferrite carbonsystems, there was a problem with a batch of rings for pulse transformers. Magnetic permeability varied from batch to batch. They began to look into it - it was not the main composition that was to blame, but the method of preparing iron oxide. Reduced iron powder was used, which was then oxidized. And depending on the oxidation mode, carbon chains from the original carbonyl process remained in the structure. It was they who created additional centers for fixing the domain walls. The solution was found not in ideal burning, but in the controlled introduction of calcite into the charge, which in the process bound this carbon into a stable phase at the sintering stage. But this added a stage to control.

Another point is that carbon can migrate. If the sintered ferrite is then subjected to mechanical processing - cutting, grinding - local overheating at the edge can lead to the diffusion of carbon from the surrounding atmosphere (oil, CO2) into the surface layer. This creates a thin, brittle layer, which then develops microcracks during thermal cycling. This was observed on products that operated in power supplies with large temperature differences. I had to change the coolant during grinding and introduce low-temperature annealing after machining.

Practical cases and failed experiments

We had an experiment about ten years ago - an attempt to create ferrite with improved machinability for complex shapes. The idea was to deliberately introduce fine carbon in the form of soot into the press powder before molding. They thought like this: during sintering, the carbon will burn out, leaving micropores that will reduce fragility and facilitate subsequent finishing with a diamond tool. The theory partly worked - workability actually improved. But the magnetic properties, especially the saturation induction and eddy current losses, dropped noticeably. The pores acted like breaks in a magnetic circuit. For non-responsible products it might work, but for power elements it would be a complete failure. I had to refuse.

And here is the opposite example, where carbon played a positive role, but in a different role. We are talking about ferrites for sensors. There you need the stability of the Curie point. We were faced with the fact that when using secondary raw materials (waste recycling), the material inevitably contained traces of organic bonds, that is, the same carbon. After optimizing the sintering cycle - a longer exposure at an intermediate temperature - these residues did not simply burn out, but managed to restore part of the ferric iron to divalent iron, which is insignificant, but shifted the Curie point to a higher temperature region and made the transition more abrupt. This was useful for temperature sensors. But repeating this effect stably under production conditions is another challenge. Depends on too many factors: granulometry, atmosphere in the furnace, heating rate.

Here it is worth mentioning the experience of major players who have been working with magnetic materials on an industrial scale for a long time. For example,LONGI Corporation(official website -https://www.ljmagnet.ru), which since 1993 has been developing and producing mining equipment, including, of course, magnetic separators. Their engineers must have encountered the problem of the influence of carbon on the magnetic properties of ferrites in the context of wear and stability of the separating elements. In the context of their production in Fushun, with an area of ​​140,000 m2 and the production of about 4,000 units of equipment per year, issues of reproducibility and durability of magnetic systems are acute. I think their practical approach to materials, when the reliability of an industrial unit is at stake, is very different from laboratory research. They most likely follow the path of maximum purification of the charge from carbon and the use of proven, conservative sintering technologies to ensure the stability of the magnetic field of the separators for years. This is a different scale and a different philosophy of working with material.

Impact on process and quality control

What does it look like on the shop floor? The main point of control is, of course, calcination. But even here, not everything is simple. If you overdo it with the temperature or holding time when calcining the oxides, you can get a too coarse-grained structure, which cannot be corrected later. Carbon, if it remains, behaves especially insidiously in such a structure - during subsequent sintering, it can cause local re-reduction and the formation of metallic iron. And this is already a marriage, and not always visible to the eye. It is detected only when measuring losses or after long-term operation at a high frequency - the product overheats.

Therefore, additional control was introduced - not only a chemical analysis for total carbon (this takes a long time), but a quick test for the resistivity of the press powder after calcination. An indirect method, but it works. If the resistance is lower than expected, there is a possibility of residual carbon or low-valent forms of iron. The batch is sent for additional annealing.

Another practical nuance is the mold. To form complex products, such as U-shaped cores, organic binders are used. This is, in fact, also carbon. Their quantity and type (polyvinyl alcohol, paraffin) are critical. Too little and the part will fall apart when transported for sintering. Too much - when burned out, large pores and deformation will remain. We selected it experimentally, and for each standard size of the product there was a different recipe. There is no universal solution. You remember this only after several unsuccessful batches and complaints from collectors.

Specifics for different types of ferrites

In manganese-zinc ferrites (Mn-Zn), which are used for highly permeable cores, the attitude towards carbon is the strictest. There, even traces can increase residual losses. All technology is designed to remove it. Sintering in an oxygen-containing atmosphere is often used at the final stage precisely to oxidize any residual carbon compounds.

With nickel-zinc (Ni-Zn) ferrites for RF applications the story is a little different. There is initially a higher resistivity, and the effect of carbon on electrical conductivity is not so fatal. But it can influence the kinetics of phase formation during reaction sintering. There have been cases where the deliberate addition of a small amount of carbonate to a Ni-Zn ferrite charge for microwave absorbers allowed better control of the grain size - the carbon temporarily inhibited the diffusion of cations at a certain stage. But this, again, is the know-how of a specific production, which is not advertised.

For barium and strontium ferrites (hard magnets), carbon is mainly the enemy. It reduces the coercive force. But there is an interesting point in the production of anisotropic magnets by dry pressing in a magnetic field. Carbon-containing additives (stearates) are used to improve powder flowability and particle orientation. The goal is for them to completely decompose and leave before the grains of the main phase begin to grow. If the temperature regime is disturbed, carbon from stearates penetrates into the grain boundaries and sharply worsens the magnetic properties of the finished magnet. Monitored by thermogravimetry when developing the regime.

Thoughts out loud and non-obvious conclusions

So, going back toferrite carbon. It's not just 'admixture'. This is a technological parameter that can be both a destabilizing factor and, within a very narrow framework, a tool for influencing certain properties. It all depends on what you produce, on what equipment and for what operating conditions. Absolute purity cannot be achieved on an industrial scale, and it is not always necessary. The main thing is to understand where it can appear (raw materials, binders, atmosphere), how it will behave in a specific temperature profile, and how to stabilize this process from batch to batch.

Often problems with the reproducibility of properties rest not on the main composition, but on such “little things” - the content and form of carbon that float due to a change in the oxide supplier or due to wear and tear of the furnace lining, changing the atmosphere. Therefore, in the technological process maps for critical products, we now write in a separate line not only the “Fe2O3 content”, but also the “maximum C content after calcination” and even the “recommended type of binder for pressing.” This reduces risks.

As a result, working with ferrites is always a compromise between magnetic characteristics, manufacturability, cost and stability. Carbon is one of the players in this field. It cannot be ignored, but it should not be demonized either. You just need to know its habits in your specific technological chain. Like many things in materials science, there is more empirics and accumulated experience than pure theory from textbooks. And this experience, including failed experiments, is what distinguishes a practitioner from a theorist.