In a ground-breaking experiment, HSE MIEM researchers subjected Lavsan (polyethylene terepfthalate, polyester) and Kapton (polypiromellitimide, polyimide) polymers, commonly used in space technology, to ionising radiation for durations ranging from microseconds to several hours at temperatures of -170°C and +20°C, while comparing their electrical conductivity under extreme conditions. The study reveals that at -170°C, Kapton's conductivity is ten times lower than at +20°C. These findings can assist engineers in developing more effective protection for spacecraft against static discharges induced by ionising radiation. The study has been published in Journal of Applied Physics.
Beyond Earth's atmosphere, spacecraft are exposed to an array of factors, including vacuum conditions, ionising radiation, space debris, micrometeorite impacts, and temperature fluctuations, among others. Ionising radiation stands as one of the most perilous factors, originating from sources like the solar wind, cosmic rays, and other phenomena. According to statistics, electrostatic discharges resulting from ionising radiation account for half of all spacecraft failures.
Polymer materials are commonly used in various components of spacecraft. They are significantly lighter than metals and prove to be excellent choices for applications such as cable insulation, screen-vacuum thermal shielding, and structural components. The probability of electrostatic discharges occurring on polymer components depends on the electrical conductivity of the polymer. A higher level of conductivity reduces the likelihood of breakdown, as it allows charges to disperse throughout the material rather than becoming concentrated in a single location.
When charged particles come into contact with a polymer characterised by high resistance and low electrical conductivity, they may become trapped within it. If charges are unable to disperse across the spacecraft's surface, they accumulate in a single location, resulting in a high electric field intensity and triggering an electrostatic discharge. In other words, a spark is generated, which can result in damage to a spacecraft’s structural components and failure of its instruments.
According to scientists, during the launch into space, the surface temperature of satellites can fluctuate in the range of -150°C to +150°C. While the behaviour of materials at high temperatures has been extensively studied, there remains a limited body of research on polymer materials at low temperatures, even though it is under low-temperature conditions that the likelihood of discharges and spacecraft malfunctions is higher.
HSE researchers investigated the electrophysical properties of polymer materials at low temperatures and determined the electrical conductivity values for two commonly used polymers in the space industry: Lavsan (polyester) and Kapton (polyimide). The experiment was carried out as follows: a polymer sample was positioned within an apparatus, the air was evacuated to create a vacuum, and subsequently, the sample was exposed to electron irradiation at an energy level of 50 keV. The experiments ranged in duration from a few microseconds to several hours and were conducted at a temperature of -170°C. Sensors inside the apparatus recorded data which the scientists then used to perform calculations and conduct an analysis.
It was found that the electrical conductivity of Kapton at -170°C differs from the results obtained at +20°C. At room temperature, the electrical conductivity of Kapton under radiation gradually increases, reaches a maximum, exhibits a slight decrease, and then rapidly surges after 100 seconds of irradiation. At low temperatures, there is no increase in electrical conductivity; instead, it decreases. Consequently, at room temperature, the electrical conductivity of Kapton after one hour of irradiation was 5*10-13 ohms-1 m-1, while at low temperatures, it was ten times lower.
The Rose-Fowler-Vaisberg model is typically used to describe the behaviour of charge carriers in polymers at room temperature. In this experiment, the scientists have confirmed its suitability for describing electrical conductivity under radiation conditions at a temperature of -170°C as well.
Until now, this model had only been tested for a few microseconds at low temperatures. We modified the laboratory setup and carried out an experiment lasting over an hour, enabling us to establish the parameters which engineers can use in their calculations for spacecraft construction. From a fundamental perspective, our findings reveal that multiple trapping formalism used in the Rose-Fowler-Vaisberg model accurately describes the transport of charge carriers in these two polymers, even under low-temperature conditions.
In the future, the researchers intend to investigate other materials which are used in extreme environments—primarily, those employed in nuclear power plants.