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Abstract: Carbonyl iron powder is a micron- to nanometer-scale elemental iron powder produced by thermal decomposition of iron pentacarbonyl. It features high purity, ultrafine particle size, and a unique "onion-head" layered structure. Owing to its excellent magnetic properties, high reactivity, and good sinterability, it has become an indispensable base material in high-end manufacturing. This paper systematically reviews the mainstream preparation processes, performance characteristics, application scenarios, and technology development trends of carbonyl iron powder.
Keywords: carbonyl iron powder, production process, powder metallurgy, soft magnetic materials
1 Introduction
Carbonyl iron is a compound formed by the reaction of iron with carbon monoxide; the common form is iron pentacarbonyl (carbonyl iron for short), with the chemical formula Fe(CO)5. Since Ludwig Mond and Carl Langer discovered nickel carbonyl in 1889 [1], industrial production of carbonyl iron powder products truly began in Germany in 1924. As the industrially produced iron-based powder with the finest particle size, highest purity, and best sphericity, its applications have expanded from traditional powder metallurgy to frontier fields such as high-frequency electronics, defense stealth, and biomedicine.
2 Preparation processes
At present, there are mainly two preparation technologies for carbonyl iron powder: high-pressure gas-phase synthesis and medium-pressure gas-phase synthesis[2].
2.1 High-pressure gas-phase synthesis The most mature industrial process for producing carbonyl iron powder is high-pressure gas-phase synthesis, with a synthesis pressure exceeding 20 MPa, a reaction temperature of 150–200°C, and a reaction time of >120 h. It is divided into two major processes: high-pressure synthesis and thermal decomposition powder production. The two reactions can be expressed as:
2.1.1 High-pressure synthesis Industrially, sponge iron (about 95% iron) lumps or iron oxide scale are used as synthesis raw materials, ball-milled into powder, then reduced under a hydrogen atmosphere and loaded into a synthesis reactor. Carbon monoxide gas is pressurized by a high-pressure compressor and introduced into the reactor, while the reactor body is heated to carry out the synthesis reaction, producing iron pentacarbonyl, which is cooled under reduced pressure into a liquid. The iron pentacarbonyl liquid produced by the reaction is yellow or orange-yellow, with a boiling point of 103°C and a density of 1.154 g/cm3 at 20°C. When exposed to sunlight or ultraviolet light, it undergoes dimerization to produce Fe2(CO)9 and carbon monoxide. It begins to decompose slightly at 60°C, and the decomposition rate increases significantly at 155°C. It should be stored in a light-protected container with carbon monoxide gas at a certain pressure. The core factors affecting the carbonyl iron synthesis reaction: (1) Carbon monoxide pressure and temperature Carbonyl iron synthesis is a volume-reducing exothermic reaction. Increasing CO pressure (usually requiring >15 MPa) and temperature (>150°C) can significantly accelerate the reaction; conversely, when pressure is <15 MPa or temperature is <150°C, the reaction rate drops sharply or even stops. (2) Gas purity control Oxidizing impurities in CO gas (especially free oxygen) must be strictly limited. Excess oxygen will cause oxidation and carbonization of the iron raw material, causing a sudden temperature rise in the reactor and forcing the reaction to terminate. (3) Upper temperature limit constraint When the synthesis temperature exceeds 220°C, the already formed Fe(CO)5 will catalyze side reactions (2CO → CO2 + C), and the precipitated carbon deposits on the iron surface, hindering contact between CO and iron and significantly reducing the carbonyl iron extraction rate. (4) Synergistic effect of catalysts Adding trace catalytic substances (such as selenium, sulfur, or ammonia) can increase the surface activity of iron, accelerate the synthesis reaction, and improve the carbonyl iron yield. (5) Dynamic renewal of reactants Timely removal of the generated Fe(CO)5 and replenishment with fresh CO gas can maintain the CO concentration at the reaction interface and increase the effective contact probability between iron and CO, thereby improving the reaction rate and product yield.
2.1.2 Thermal decomposition process The decomposition of iron pentacarbonyl is carried out in a vertical cylindrical pyrolysis furnace heated by wall-type electrical heating or gas spiral heating. After vaporization, iron pentacarbonyl is introduced from the center of the top of the cylinder into the pyrolysis furnace. Under a certain temperature, gaseous iron pentacarbonyl decomposes, nucleates, and grows. By adjusting the thermal decomposition process, carbonyl iron powder with the required performance can be obtained. The thermal decomposition reaction of iron pentacarbonyl can also be expressed as: This reaction is endothermic. After iron nuclei form, the Fe(CO)5 concentration around the iron nuclei decreases, the CO concentration increases, and the temperature drops, which slows the decomposition reaction. But at this instant, due to the catalytic action of active iron, the decomposition reaction of carbon monoxide close to the iron nuclei is promoted and releases heat; the CO concentration around the iron nuclei decreases and the temperature rises. The carbon precipitated by decomposition adsorbs on the surface of the iron nuclei, reducing the catalytic action of iron, slowing the CO decomposition reaction, and the Fe(CO)5 concentration rises to the initial level, after which the decomposition reaction starts again. This cycle repeats, and the resulting iron powder has an onion-like spherical structure. The larger the particle size, the more onion layers. Factors affecting powder performance (1) Average powder particle size: once the pyrolyzer structure (length-to-diameter ratio) is determined, the average particle size of the iron powder mainly depends on the gas flow rate and the temperature of each zone. After the iron pentacarbonyl gas enters the pyrolyzer, it proceeds from a lower-temperature zone to a higher-temperature zone, achieving nucleation and growth of thermal decomposition. Experiments show that a higher pyrolysis temperature and a larger gas flow rate are necessary conditions for obtaining carbonyl iron powder with a smaller average particle size, because these factors increase the probability of nucleation and reduce the residence time and collision probability of iron nuclei in the hot zone. Conversely, at a lower pyrolysis temperature and a smaller gas flow rate, the average particle size of the iron powder obtained by pyrolysis is larger. (2) Carbon, nitrogen, and oxygen components: the carbon, nitrogen, and oxygen components are closely related to process conditions. During production, if no gas is added, the resulting iron powder has relatively high carbon and oxygen contents, in the range of 0.8% to 1.4%, and nitrogen content not greater than 0.1%; if a small amount of nitrogen is added as carrier gas, the carbon and oxygen contents are the same as above, while the nitrogen content increases, fluctuating approximately in the range of 0.3%–0.5%. The main source of oxygen in the powder is that the powder surface is highly active and quickly adsorbs moisture and oxygen from the air when it contacts air during discharge and sieving; if 0.5%–2% dry ammonia gas relative to the Fe(CO)5 gas is added during pyrolysis, it plays a role in passivation and inhibiting CO decomposition. At lower temperatures, the powder is less sensitive to moisture and oxygen, and when it is discharged and sieved in contact with air, the hydrogen loss value is significantly reduced. Experiments show that the performance of carbonyl iron powder with an average particle size above 3 μm produced with added ammonia gas is: hydrogen loss value not greater than 0.4%, carbon content not greater than 0.8%, and nitrogen content significantly increased.
2.2 Medium-pressure gas-phase synthesis At present, medium-pressure carbonyl iron powder production technology has become a key research direction in the new materials field. Compared with the high-pressure method, the medium-pressure carbonyl iron powder production technology has a carbonyl iron pressure of 8.0 MPa, only 1/3 that of the high-pressure method; a synthesis speed of 60 h per vessel, while the high-pressure method requires 120 h per vessel; and an iron conversion rate greater than 75%, while the high-pressure method is only about 65%. In addition, in the medium-pressure carbonyl iron powder production technology, the auxiliary raw material CO can not only be recycled at nearly 100%, but also adopts an energy-saving recycling mode. The medium-pressure gas-phase synthesis method can not only reduce equipment manufacturing costs and improve capacity, facilitating scale-up, but also has low energy consumption in the production process and fully utilizes raw materials, with production costs far lower than those of the high-pressure method. At present, this technology is in the stage of industrialization implementation.
3 Application fields
3.1 Injection molding industry [3] High-density alloys in traditional powder metallurgy require highly active iron powder as the liquid phase during sintering to form a relatively continuous binder phase and suppress the formation of brittle phases, thereby obtaining high-density or fully dense high-performance products. In military projects, it is mainly used to replace the already banned depleted uranium material to produce armor-piercing projectile cores. The injection molding process has very high requirements for maximum powder loading. Therefore, the apparent density requirement in powder standards is very high. However, the smaller the powder particle size, the more likely bridging occurs, and the lower the apparent density. In this case, it is very important that particles do not stick together. Metal powder injection molding technology is an advanced molding technology that combines the principles of plastic injection molding with the advantages of powder metallurgy technology. This process is particularly suitable for manufacturing parts with complex internal structures and small dimensions, such as hinges and zippers for foldable phones.
3.2 Soft magnetic industry[4] The indicators for measuring the performance of soft magnetic materials mainly include resistivity, permeability, DC bias characteristics, and saturation magnetic induction. Iron powder cores made from carbonyl iron powder have excellent bias characteristics and good high-frequency adaptability. The DC bias characteristics of carbonyl iron powder cores are far higher than those of other common soft magnetic materials, such as iron powder cores, Fe-Si-Al powder cores, and amorphous nanocrystalline powder cores. In addition, carbonyl iron powder cores also have relatively low high-frequency eddy current losses, and their applicable frequency range is relatively wide[5]. Based on the above characteristics, carbonyl iron powder can be used to manufacture high-frequency switching chokes, inductors, high-frequency tuning magnetic core bodies, etc.
3.3 Microwave absorbing industry [6] Carbonyl iron powder is usually regularly spherical. This morphology can reduce the contact resistance between particles, allowing magnetic flux to pass smoothly through the gaps between particles. In addition, the internal structure of the particles is dense, with almost no pores or other defects, further reducing magnetic flux leakage and improving permeability. In addition, carbonyl iron powder has an onion-like internal structure composed of multiple concentric spherical shell crystals, similar to the structure of an onion. This structure can enhance the flexibility of magnetic domain wall movement and improve permeability. Because of the extremely high permeability, when electromagnetic waves (especially microwaves and radar waves) are incident on microwave absorbing materials containing carbonyl iron powder, the powder particles will produce strong magnetization oscillations under the action of the alternating electromagnetic field, ultimately consuming electromagnetic wave energy and achieving electromagnetic wave absorption. In addition, the size and shape of carbonyl iron powder particles can match electromagnetic waves of a specific frequency to form resonant units. When the frequency of the incident electromagnetic wave is close to the natural resonance frequency of the particles, a strong magnetic resonance phenomenon is triggered, greatly enhancing the microwave absorption effect. By regulating particle size, shape, and packing mode, targeted absorption of electromagnetic waves in specific frequency bands can be achieved.
3.4 Magnetorheological fluid industry[7] Carbonyl iron powder has important applications in the magnetorheological fluid industry, mainly due to its unique physical and magnetic properties. First, carbonyl iron powder is pure iron material and responds easily when an external magnetic field is applied; second, the spherical structure and particle size of carbonyl iron powder are conducive to forming ordered magnetic chain structures, improving the shear yield strength and response speed of magnetorheological fluids; in addition, coating can give carbonyl iron powder better stability and dispersibility. Carbonyl iron powder plays a core role in the magnetorheological fluid industry and is widely used in various smart material systems that require rapid and precise adjustment of fluid dynamic characteristics, such as damping, braking, vibration control, and shape memory devices in fields such as automobiles, aerospace, precision instruments, medical equipment, and robotics.
3.5 Food additive industry[8] Because of its production process of selectively extracting iron, carbonyl iron powder has extremely low metal impurity content. The contents of metals such as lead, arsenic, mercury, cadmium, aluminum, manganese, and thallium are all at the ppb level, making it the process with the lowest metal impurity content among all current iron powder preparation processes. Therefore, carbonyl iron powder can be directly added to food as an iron supplement. Data show that the absorption rate of carbonyl iron powder by the human body exceeds 80%, far exceeding the compound iron supplements currently used.
4 Conclusion As a core material of high-end manufacturing, the technological development of carbonyl iron powder shows three major trends: Process innovation: medium-pressure methods and micro-nano bubble technology promote low-cost nanoscale production, breaking the monopoly of international giants such as BASF;
Deepening of application scenarios: from military armor-piercing projectiles to biological targeted therapy, penetrating into more than 10 cutting-edge fields; Rise of Chinese technology: enterprises such as Jiangsu Tianyi and YUEAN New Materials have achieved significant increases in global market share through patent layout. In the future, continuous efforts are needed in directions such as corrosion-resistant coatings, permeability optimization, and toxicity control to meet emerging needs such as 6G communications and precision medicine, and to promote the transformation of carbonyl iron powder from an “industrial additive” to a “functional strategic material.”
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