Abstract
During the ballistic atmospheric re-entry, a space vehicle has to withstand huge thermo-mechanical solicitations because of its high velocity and the friction with the atmosphere. According to the kind of the re-entry mission, the heat fluxes can be very high (in the order of some MW m−2) ;thus, an adequate thermal protection system is mandatory in order to preserve the structure of the vehicle, the payload and, for manned mission, the crew. Carbon phenolic ablators have been chosen for several missions because they are able to dissipate the incident heat flux very efficiently. Phenolic resin presents satisfying performance but also environmental drawbacks. Thus, a more environmental-friendly solution was conceived: a high-performance thermoplastic material, polybenzimidazole (PBI), was employed instead of phenolic resin. In this work PBI-ablative material samples were manufactured with and without the addition of nano-ZrO2 and tested with an oxyacetylene flame. For comparison, some carbon-phenolic ablators with the same density were manufactured and tested too. Thermogravimetric analysis on PBI samples was carried out at different heating rates, and the obtained TG data were elaborated to evaluate the activation energy of PBI and nano-filled PBI. The thermokinetics results for PBI show an improvement in thermal stability due to the addition of nano-ZrO2, while the oxyacetylene flame test enlightens how PBI ablators are able to overcome the carbon phenolic ablators performance, in particular when modified by the addition of nano-ZrO2.
Highlights
Ablative thermal shields have been used since the first Apollo mission and, after 50 years, they still represent the best choice for the ballistic atmospheric re-entry [1]
Carbon phenolic ablators are composite materials consisting of a phenolic resin matrix and a carbon felt as reinforcing phase
Thermogravimetric analysis on polymer samples were carried out at different heating rates to observe the behavior of the PBI/m and PBI/m-Z and to appreciate the difference induced by addition of nanoparticles
Summary
Ablative thermal shields have been used since the first Apollo mission and, after 50 years, they still represent the best choice for the ballistic atmospheric re-entry [1]. Polymer with a reduced environmental impact compared to the phenolic resin, but able to provide the same beneficial effects in terms of thermal protection. PBI (polybenzimidazole) has been mainly used as electrolyte material for proton exchange membrane (PEM) fuel cells which operate under anhydrous conditions at high temperature (~ 200 °C). For this kind of application, it could be doped with phosphoric acid to improve its performance [12, 13] and, in other studies [14, 15], PBI was successfully modified by the addition of different compounds and with inorganic fillers in order to increase its proton conductivity
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