Space: ENEA tests resistance of space materials to UV degradation

ENEA has developed a UV irradiation system to evaluate the resistance of space materials exposed to solar ultraviolet radiation outside the Earth's atmosphere. The activities were conducted by researchers at the Frascati Research Center in collaboration with the National Institute for Nuclear Physics (INFN) and the Italian company BEAMIDE.

 “This system allows to expose samples to continuos 24 hours cycles for up to several thousand equivalent hours of exposure to solar radiation, providing real-time and remote control of the dose of radiation released and of any variations in the irradiation period”, explained Paolo Di Lazzaro, researcher at the ENEA Laboratory of Inertial Fusion, Plasmas and Interdisciplinary Experiments. “This technology enables to perform accelerated testing to evaluate very accurately the effects of UV radiation^[1] on the materials used in long-duration space missions”, he said.

So far, the system has been used for solar radiation resistance tests in the space program “ARTES”, the project “Nano Hybrid Transparent Materials”^[2] to create materials^[3] suitable for space telescopes, and the project “FIRST-FLEX”,  to develop a new type of optical solar reflectors (OSR). The latter are passive thermal components that play a crucial role in the thermal design of a spacecraft: they are used to dissipate heat into outer space and reduce the absorption of solar flux. “Our system has been used to irradiate a new type of inorganic coating placed on the flexible surface (metallic or polymeric) of the OSR with a dose of UV radiation of 300 equivalent solar hours, according to the standards of the European Space Agency”, Di Lazzaro pointed out.

In addition to these tests conducted in an inert atmosphere (free of reactive substances), ENEA researchers irradiated some samples in air to determine the synergistic effects between UV and oxygen in the degradation process of the materials.

The UV radiation source developed by ENEA, INFN and BEAMIDE consists of a medium-pressure mercury vapor lamp powered by a 500 W electric discharge. The ‘heart’ of the lamp is a cylindrical bulb placed in the focus of an aluminum parabolic reflector and in a thin quartz window protecting the bulb. The lamp is cooled by an external fan and positioned above a cylindrical vacuum chamber, 18 cm in diameter and 12 cm deep, with an external water-cooled jacket. The vacuum chamber is closed by a quartz window that allows 90% transmission of the UV radiation.

The samples to be irradiated are placed in the chamber that can be filled with inert gas to both prevent oxidation of the samples and transfer heat to the entire surface of the chamber walls to limit the maximum temperature of the samples to 40 °C, even for irradiations lasting over a month. The pressure in the chamber, the emission in the UV and visible and the temperature of the samples are constantly monitored by a control and data acquisition system managed by a dedicated data logger, based on an Arduino board, monitored and managed remotely during the entire duration of the irradiation, capable of automatically deactivating the lamp if the temperature, UV emission and gas pressure parameters go outside the permitted range.

The complex irradiation and diagnostic system was developed by the “Eccimeri” laboratory of the ENEA Frascati Research Center, which since the 1980s has been developing ultraviolet (UV) and extreme ultraviolet (EUV) radiation sources, both coherent (laser) and non-coherent and related diagnostics. These radiation sources have been used in many fields, from the creation of thin-film transistors^[4] to the controlled stripping of graffiti on metal, plastic and stone materials, from the modification of the chemical properties of the surface of electronic materials and fabrics to the latest generation EUV micro-lithography, from invisible writing on dielectric thin films for anti-counterfeiting to the sanitization of surfaces from viruses and bacteria.

 “Thanks to our long standing expertise, we are studying an improvement of the irradiation system that involves replacing the current mercury lamp with a matrix of UV LEDs in order to more faithfully reproduce the solar spectrum (in the spectral range 200 nm - 400 nm) and reduce thermal effects, thus allowing irradiation in a vacuum as well as in a controlled atmosphere”, concluded Paolo Di Lazzaro of ENEA.