Composite interconnect materials for space solar cells and preparation method thereof
Solar cells of low orbit (200km ~ 1000km) space vehicles are the source of power for space vehicles. On the one hand, they provide electric energy for the spacecraft load and charge the battery bank on the other hand. The panel is composed of a separate battery piece by the bus bar, connected to each other in parallel or series, connected to each other to collect the charge produced on a single battery piece, and the battery piece is connected together; The bus bar connects and collects the charge collected by each string of interconnecting pieces for output. Currently, interconnectors used in space vehicles are made mainly of silver, the most conductive metal.
Low orbit space environment is a relatively complex environment, mainly including space debris, high-energy particles, micrometeorites, space radiation, great temperature difference between cold and heat, atomic oxygen and nitrogen molecules, etc. The concentration of atomic oxygen at a distance of more than 200km from the sea level is 109atoms cm-3 orders of magnitude. When the speed of the aircraft is 8km s-1, the beam density of atomic oxygen hitting the surface of the aircraft is about 1015atoms cm-2s, and the energy of each atomic oxygen reaches 4-5eV. The etching and impact effect on the surface of interconnecting materials are very great.
General solar shall each is made of pure silver foil coining, working in solar panels, with strong oxidation and high kinetic energy of atomic oxygen and continuous surface reaction generates silver oxide, silver from silver oxide flake substrate above, show fresh surface, cycle, silver foil will be run out of atomic oxygen corrosion, so as to make the solar panels cannot be powered aircraft. In order to further protect the corrosion of atomic oxygen, gold film, aluminum film, polyimide, silicon and so on are sprayed on the surface of the silver foil. However, due to the large temperature difference effect, these methods produce huge thermal cycling stress and thermal fatigue, which lead to the cracking and peeling between the coating and the silver foil.
In order to overcome the corrosion of atomic oxygen, the felled alloy is used as solar cell interconnectors, but silver plating on the felled alloy surface is necessary to improve the welding performance. The felled alloy has the same thermal property with silver, which improves the poor infiltration and shedding of thin films to a certain extent.
Molybdenum and molybdenum alloy have excellent heat resistance, high temperature mechanical properties, low molybdenum thermal expansion coefficient and high electrical conductivity, thermal conductivity, with very good atomic oxygen corrosion resistance, but poor weldability. Silver has good weldability and electrical conductivity, so the molybdenum/silver layered metal matrix composite material is prepared, which has high thermal conductivity, low thermal expansion coefficient, superior electrical conductivity and good weldability, and is resistant to atomic oxygen erosion, not magnetic, and has no magnetic interference to signal transmission and other components.
Molybdenum and silver are inmiscible systems with positive enthalpy of mixing. Under equilibrium conditions, they do not combine, nor do they melt, nor do they infiltrate, resulting in very poor binding between silver layer and molybdenum foil. After thermal expansion and cold contraction cycles, interface cracking and film detachment are easy to occur. By introducing a metal interlayer between molybdenum and silver, the obtained molybdenum substrate-metal interlayer-silver layer composite structure is annealed, so that the metal elements comprising the metal interlayer are continuously diffused into the molybdenum substrate and silver layer to increase the binding strength.
By magnetron sputtering technology, molybdenum silver alloy transition layer is deposited on the surface of molybdenum foil, and then silver film is deposited. Without introducing the third metal element of non-film base, the bonding strength of film base is significantly enhanced, and a new composite interconnect material is obtained. The fabrication cost of interconnect materials for space solar cells is reduced, and the uncontrollable problems of the structure and performance of welding interface caused by the introduction of other elements, as well as the interference of spacecraft signal transmission when used, are avoided.
