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The multipactor effect has been on the European Space Agency’s (ESA) list of concerns for more than 40 years. It is an avalanche of electrons that occurs in vacuum, inside components such as antennas or waveguides, and can ultimately cause irreversible damage to equipment.
For decades, the standard solution has been to coat these parts with Alodine, a chromium-based compound that offers good protection but is harmful to the health of those who handle it and to the environment. European authorities have long been pushing for its ban, although none of the alternatives proposed so far has managed to match its technical performance.
Lidia Martínez, a CSIC researcher at the Institute of Materials Science of Madrid (source in Spanish) (ICMM), explains that the sector needed a substitute that would emit few secondary electrons, as these are what trigger the chain reaction.
Most previous attempts had focused on modifying the surface of materials at the micrometre scale, without achieving the leap in performance demanded by the space industry.
In this case, size really does matter, and not just the material
The ICMM team decided to change scale. Instead of working with micrometric structures, it has developed a rough surface at the nanometric level, that is, with features on the order of a billionth of a metre. To do so, they produced gold and silver nanoparticles between four and eight nanometres in size using an ultra-high-vacuum technique known as a gas aggregation source, which allows the materials to be deposited without solvents or residues.
The result is porous, metallic, chemically clean films which, according to tests carried out in ESA-accredited laboratories, reduce secondary electron emission by around 30% compared with Alodine.
In addition, the so-called cut-off energy threshold, the point at which the material starts to generate more electrons than it receives, improves by up to 300% in some configurations.
The researchers also subjected the coatings to six-month ageing tests and thermal treatments at 150°C, similar to those a satellite experiences when exposed to the Sun, and found that although performance drops slightly over time, it remains higher than that of freshly applied Alodine.
From patent to space, a journey that is not yet complete
The technology is already covered by a joint European patent application by CSIC and Nanostine (source in Spanish), filed with the European Patent Office in July 2025 and currently under examination.
Nanostine, a spin-off created by CSIC itself, will be responsible for commercialising the coating, aimed mainly at the aerospace sector. The development has also been funded by an Industrial PhD programme of the Community of Madrid and supported by the ESA Business Innovation Centre, coordinated in Madrid by the Madri+d Foundation.
Despite the promising results, Martínez is cautious about the timetable: taking a new coating all the way to actual use in space usually requires around a decade of testing and validation.
The researcher says ESA has expressed satisfaction with the solution put forward, but stresses that the project still has to clear several stages before these materials can fly on board a satellite.