Full length articleMultilayer protective coatings obtained by pulsed laser deposition
Introduction
All electronic devices working in space applications have to comply with the harsh conditions of this environment, from temperature variations to different incident radiations. Antennas for space applications have to withstand different conditions than antennas used for terrestrial applications. In space, apart from the high gain antennas that cover long distance communications, which usually require large dishes, moderate and low gain antennas are also often employed in order to provide communication links between different subsystems. One of the biggest problems of low gain antennas, such as a microwave antenna, is the passive intermodulation distortion (PIM), which is due to the fact that these devices are passive, and in most of the cases they can be described as linear devices. Intermodulation distortion appears in antennas when two strong signals of frequencies very close one to another (often called two microwave or radio-frequency tones) are applied to the transmitter antenna. The complexity of the sources of intermodulation distortions is not yet fully understood by the scientific community. Electro-thermal modulation of conductivity, thermionic or field emission, ferromagnetic effects (chrome and nickel are often used in antennas), tunneling through metal-oxide junctions, micro-discharges are just a few of the physical phenomena considered to be responsible for passive intermodulation distortions [1].
In certain conditions, the phenomenon of passive intermodulation (PIM) occurs when components or just some parts of the components behave in a non-linear way. Therefore, PIM is harmful to communication systems and limits their performance, especially the dynamic range. Moreover, the impact of the ionizing radiation on the microwave antenna devices has to be reduced as well. Despite investigations on intermodulation effects in dielectric materials, dielectric resonators, microwave filters etc., there is a very little knowledge on the intermodulation effects of microwave antennas, as well as the impact of ionizing radiations.
The microstrip microwave antenna is usually composed by a dielectric substrate, a metal reflector and a thin sheet of metal foil deposited on top of dielectric substrate. The thickness of metal sheet can have an order of few microns. The effect of radiation on metal foil can introduce the impurities in metal by transmutation of nuclei in other nuclei. When a proton or alpha particle becomes neutralized in material can introduce impurities by creation of helium or hydrogen. Another effect is the displacement of atoms from normal position in structure and causes the lattice vacancies or the material can be ionized by removal of electrons under effect of radiation. When the large energy is released in a small volume of material can cause thermal heating. In case of microwave antenna the quality of materials are direct connected with the response in frequency and with existence of passive intermodulation effect [2,3]. The induced any damage on thin sheet of metal foil by space radiation (neutrons and alpha particles), even for low energy radiation, will result in an increase PIM in antenna and for this reason, the electronic circuits (ex: microstrip antenna) must be protected against the effect of radiation. Different coatings can be applied to the microwave antenna to reduce the passive intermodulation due to the natural ionizing environment, but also to protect against temperature variations. Most important, the used coatings must not interfere with the performances of the antenna. Yttria stabilized zirconia (YSZ) [[4], [5], [6]] and aluminium oxide (Al2O3) are currently used for protection against wear, high temperature or corrosion environments [7,8]. Apart from these functional applications, there is a major interest in YSZ also for solid oxide fuel cells (SOFCs) devices, which are capable to generate clean energy. Moreover, efforts are being made to increase the electrical conductivity of YSZ [9,10]. Aluminium oxide exhibits low electrical conductivity, high thermal and chemical stability, as well as high hardness and mechanical resistance. Among alumina's polymorphs, α-phase is the thermodynamically stable one, while θ, κ or γ- phases are metastable. The existence of these phases makes the obtaining of pure α-phase complicated, especially in thin film form. Different deposition techniques have been used in order to obtain thin films from these materials. YSZ and Al2O3 thin films have been obtained by chemical vapour deposition [11,12] or pulsed laser deposition (PLD) [13]. Among these deposition techniques, PLD is the most suitable for the obtaining of heterostructures having different layer ordering or geometries [14]. Several studies have been made on PLD deposited YSZ and Al2O3 layers to reveal their functional properties in connection with the deposition parameters. For example, epitaxial α and γ- phase alumina has been grown on different substrates such as SrTiO3, MgO [15] or sapphire [16] for electrical, magnetic and electronic device applications.
Taking into account the above-mentioned properties of YSZ and Al2O3, YSZ/Al2O3 heterostructures could be a solution to achieve protective coatings needed to tackle issues arising from the passive intermodulation and temperature variations in devices working in space.
Both Al2O3 and Y:ZrO2 presents very good properties in term of space applications, and both materials can be used for protection against temperature variations or for accidentally interaction with space debris or nano-micro meteorites or for radiation shielding. The Al2O3 can be used for radiation shielding for satellite in low earth orbit in pure form or by doping with rare earth elements [17]. The Y:ZrO2 also present a radiation damage tolerance.
According with Wang et al. [18], the irradiation with 30 KeV energy α-particles will damage both materials with formation of He bubble in Al2O3 and ribbon or cracks in YSZ. However, when the multilayers of Al2O3/YSZ are subjected to the same irradiation conditions, the bubbles are concentrate in Al2O3 layer and the dimensions of those bubbles are much lower than those produced in monolithic Al2O3 (without YSZ). A multilayer heterostructure of Al2O3/YSZ has a higher tolerance on radiation effect than monolithic Al2O3 or YSZ. The mechanism proposed by Wang et al., called “load-unload” is based on preexistence of nanovoids in multi-layers which results in the abnormal He bubbles growth in the Al2O3 layers [18].
In this work, we present our results on the obtaining of YSZ/Al2O3 heterostructures by Pulsed Laser Deposition (PLD) for the protection of planar monopole antennas without changing their performances after the deposition process. The behavior of the planar monopole antenna coated with YSZ/Al2O3 thin layers shows no degradation in performances, despite the used deposition conditions. The physical properties of the YSZ/Al2O3 thin films obtained by the PLD technique are presented as a function of experimental parameters. The simulation on the effects of ionized radiations incident on a YSZ/Al2O3 heterostructure have been performed using SRIM-TRIM code. The SRIM studies show that at the same energy range the proton penetration depth is higher than the alpha penetration depth, giving insights about the penetration depth of proton and alpha particles in the studied targets. Our goal is to obtain a multilayer structure able to enhance the endurance of the antenna and microwave circuitry in harsh space environment without reducing the performances under nominal operation conditions.
Section snippets
Methods and experiments
The deposition of Al2O3/YSZ heterostructures by PLD technique was carried out by ablating commercial Al2O3 and Y:ZrO2 ceramic targets with an ArF excimer laser (λ = 193 nm) working at 10 Hz pulse repetition rate. The silicon (Si) substrate was heated at a rate of 50 °C/min up to 700 °C for Y:ZrO2 and respectively, 700 and 750 °C for Al2O3, and was kept at this temperature during the deposition process. The distance between substrate and target was set at 4 cm. The laser fluence was kept at
Results and discussion
The topography analysis of samples grown on Si, at substrate temperature of 700 °C and 5 × 10−2 mbar oxygen pressure, reveals smooth surfaces, without major defects such as droplets, cracks or pores.
The Al2O3 and YSZ samples deposited on silicon (Fig. 1) have smooth and compact surfaces, with RMS roughness of ~0.4 nm for Al2O3 over an area of 1 × 1 μm2 and ~3 nm for YSZ, respectively.
The Al2O3 layer deposited on YSZ also reveals a relatively smooth surface (Fig. 2). However, the RMS roughness
Conclusions
The obtaining of YSZ/Al2O3 heterostructures by Pulsed Laser Deposition (PLD) for the protection of planar monopole antennas without changing their performances after the deposition process has been presented, as well as the simulation results on this heterostructure viability if exposed to ionized radiation. The measured reflection characteristics for Al2O3/YSZ coated and uncoated antennas are almost identical, which proves that the Al2O3/YSZ coating has no detrimental effects on the
Acknowledgements
This work was supported by a grant of the Ministry of National Education and Scientific Research, RDI Programme for Space Technology and Avanced Research - STAR, project number 168/20.07.2017.
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