Consecutive manufacturing process of epsilon iron oxide. (Image: Korea Institute of Materials Science)

Iron oxide material with a high-coercive epsilon crystal phase is just about the only magnetic material that absorbs ultra-high frequencies (e.g., a potential 6G frequency band). Until now, it was only formed in a nano-sized particle of 50 nanometers or less. Japan produced pure epsilon iron oxide through a batch type wet process, but it involves a lengthy multi-stage process resulting in a low yield.

Now a research team at the Korea Institute of Materials Science (KIMS) has developed the world’s first technology to consecutively manufacture epsilon iron oxide capable of absorbing a millimeter wave with a high coercive force on the level of neodymium (Nd) magnets.

The team adopted the aerosol process to thwart the low-yield issue and produced a composite powder in which epsilon iron oxide nanoparticles are embedded in silica particles by spray-drying precursor solutions in a hot chamber. When the precursor material solution is continuously injected and the droplets are instantly dried, the iron precursor is trapped in the silica xerogel particles and limited to grow during heat treatment.

The epsilon iron oxide nanoparticles could be continuously produced through a micrometer-sized powder manufacturing process — which showed the possibility of commercialization of millimeter-wave-absorbing materials.

Epsilon iron oxide has great potential as a material for future communication parts thanks to its absorption capacity in the ultra-high-frequency (30-200GHz) band. Continuous manufacturing technology of epsilon iron oxide with millimeter-wave-absorption capability can be used for mm-wave 5G/6G wireless communication, radar sensors for driverless car, as well as stealth and low-Earth orbit satellite communication components. Plus, since it’s a high-coercivity magnetic material, it can be used for electric motor parts for future mobility.

No companies commercially produce products with applied magnetic materials capable of absorbing mm waves, while only a few companies in the U.S., Japan, and Germany produce 5G-band-absorbing and -shielding materials. The KIMS technology is expected to be localized and exported to the global market in the future.

“The epsilon iron oxide can selectively absorb ultra-high frequencies in a wide band (30 to 200 GHz),” said Principal investigator Dr. Youn-kyoung Baek. “The significance of the study is that it developed the first continuous manufacturing process of epsilon iron oxides. The technology is expected to accelerate the commercialization of wireless communication devices using millimeter waves, self-driving car radars, and absorber technology for space satellite communication in the future.”

Here is Tech Briefs interview — edited for clarity and length — with Baek.

Tech Briefs: What inspired your research?

Baek: The commercialization of 6G mobile communication is anticipated to occur around 2030. It is anticipated that 6G mobile communication will be able to quickly exchange vast amounts of data without space limitations, thanks to 50-times-faster transmission speeds than 5G communication, real-time data processing, and transmission-delay resolution. Specifically, because all sectors of industry and society are connected to networks, convergence between industries and real-time interconnection is possible, intelligent non-face-to-face services and space-based communication are anticipated to expand, and new convergence industries will be created.

The foundation of this 6G technology is ultra-high-capacity, instantaneous data transmission, which requires the control of the millimeter-wave (30 GHz to 300 GHz) operational frequency spectrum. In addition, as noise issues resulting from the extreme thinning and downsizing of electronic devices and the use of ultra-high-frequency electromagnetic signals are anticipated to intensify, it is urgent to develop a material that can absorb ultra-high frequency electromagnetic waves. However, there are almost no materials that can selectively absorb millimeter waves of 80 GHz and over. Thus, we have focused on the epsilon iron oxides.

Tech Briefs: What were the biggest technical challenges you faced?

Baek: Epsilon iron oxide shows a ferromagnetic resonance phenomenon proportionate to its high magnetic anisotropy, hence demonstrating electromagnetic wave absorption ability in the 180 GHz band and tunable frequency region. However, epsilon phase can only be formed when the particle size is less than 50 nm. To obtain high-purity epsilon iron oxide, the growth of iron oxide particles must be limited to 50 nanometers or less.

Previously reported methods by using a silica matrix, surfactant, or porous template successfully prevented the excessive growth of crystallites, however, they involve multiple time-consuming steps, produce low yields, and require expensive sacrificial templates, which limit large-scale production and practical applications. Thus, we need to find the scalable approach to synthesize the precursor particles containing a mixture of Si and Fe element to limit the growth of iron oxide particles.

Tech Briefs: Can you explain in simple terms how the technology works?

Baek: We chose spray drying as a technology that can produce particles containing multiple elements in a rapid single step. Spray drying has been widely used for the industrial manufacturing of dried fine powders for pharmaceuticals, foods, and chemicals. This process involves instant and continuous evaporation of aerosol droplets in a hot chamber, enabling the production of powders in a single step without additional washing.

Further, this approach has no limit in terms of the combination of multiple compounds in particle form. Rapid evaporation of the precursor solution during spraying induced the instant formation of microparticles consisting of elemental Fe and Si. In the annealing step, the polymerization of Si precursor occurs first, creating an amorphous silica network in the microsphere. Therefore, it was speculated that the Fe salt embedded in the densified silica underwent spatial confinement during heating. The excessive growth of iron oxide in the sphere could be restricted, causing the formation of the ε-phase. These results show that spray drying can be an efficient means of producing high-purity epsilon iron oxides without the use of surfactants or sacrificial templates in a single step.

Furthermore, compared to the previously reported wet reaction conducted in a batch reactor, this aerosol-assisted method enables continuous production of dried composite particles without washing or post-drying. Thus, this strategy provides an efficient and scalable manufacturing means of synthesizing epsilon iron oxides.

Tech Briefs: When will you start/conclude the follow-up study?

Baek: Since our technology is based on spray drying, which has already been commercialized, we are investigating the possibilities of scaling up production of epsilon iron oxides with companies.

With the development of information technologies, the demand is rising for sending heavy data such as high-resolution images at very high rate. Especially, in the era of 6G wireless communications, all the things including humans should be connected to each other so tremendous data should transmit in real time. The required frequency of carrier waves for these technologies would continue to increase to sub-terahertz. Thus, we need to improve absorption ability of the epsilon iron oxide in the frequency region.