Vertically Aligned Carbon Nanotubes as the Sputter Resist in Space Propulsive Systems
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Vertically Aligned Carbon Nanotubes as the Sputter Resist in Space Propulsive Systems Yoke Khin Yap, 1,* Vijaya Kayastha, 1 Jitendra Menda, 1 Lakshman Kumar Vanga, 1 Jiesheng Wang, 1 Alex Kieckhafer, 2 Dean Massey, 2 Lyon B. King, 2,** 1 Department of Physics, Michigan Technological University, Houghton, MI 49931, USA. 2 Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, MI 49931, USA. * Email: [email protected], **Email: [email protected]
ABSTRACT Two-types of vertically aligned multi-walled carbon nanotubes (VA-MWNTs) are evaluated as the protective coatings against ion erosion in electric propulsion systems. A series of experiments have been conducted to understand the erosion rate and erosion mechanism of these VA-MWNTs. These experiments were carried out with Xe propellant at an ion current density of 5 mA/cm2. We found that the erosion rates of both types of VA-MWNTs were changing with time. Such a nonlinear erosion process is explained according to a possible erosion mechanism.
INTRODUCTION The space exploration program faces enormous challenges as it seeks to achieve dramatic improvements in safety, cost, and speed of missions to the frontiers of space. Plasma propulsion systems have been recognized as far more efficient than chemical thrusters. This recognition has led to the development of highly efficient electric propulsion (EP) thrusters that are currently the only feasible technology for many deep space missions. However, these EP devices have in common electrode sputter erosion as a life-limiting process. To facilitate long thruster life, critical surfaces in EP thrusters are fabricated from sputter-resistant materials such as molybdenum (Mo). Carbon-based materials have shown nearly an order-of-magnitude improvement in sputter erosion resistance over Mo [1]. Among the tested carbon-based materials, diamond films prepared by chemical vapor deposition (CVD diamond) provide improvement by a factor of 1.5 in volumetric sputter erosion rate over others [2]. For thruster surfaces that are subject to sputter damage, yet must be electrical insulators, boron nitride ceramic has traditionally been used to increase the lifetime [3]. Recently, Meezan, et. al. found that polycrystalline diamond plates had 25% better resistance to sputtering than the traditional boron nitride ceramic [4]. On the other hand, unique mechanical properties of carbon nanotubes (CNTs) have triggered tremendous curiosities on their applications. CNTs are predicated to have extremely high Young’s modulus values, similar to that of in-plane modulus of graphite (~1000GPa). This is much higher than the bulk modulus of diamond (~443 GPa). Furthermore, the chemical bonding strength of carbon atoms in CNTs is higher than that of diamond. Thus, it is interesting to find the resistance of CNTs to ion erosion.
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Previously, we have compared two types of CNTs to CVD diamond, amorphous carbon and boron nitride films as exposed to the exhaust beam of a Hall-effect thruster [5]. We fou
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