Thermochemical characterization of polybenzimidazole with and without nano-ZrO2 for ablative materials application

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.

Design of New Carbon-Phenolic Ablators: Manufacturing, Plasma Wind Tunnel Tests and Finite Element Model Rebuilding

“…The Laboratory of Materials and Surface Engineering of Sapienza University of Rome has worked on carbon-phenolic ablative materials since 2009. Several low density ablators were developed and optimized by varying raw materials, density, manufacturing strategy [15,18,34] with a special effort for a complete characterization of mechanical and thermal properties [16], thermochemical and kinetics parameters [15,34]. Some ablators were manufactured enriching the polymeric matrix with ceramic nano-particles of Al 2 O 3 and ZrO 2 [15,18].…”

“…Some ablators were manufactured enriching the polymeric matrix with ceramic nano-particles of Al 2 O 3 and ZrO 2 [15,18]. An oxyacetylene flame torch system was designed and set up in order to provide a useful screening of the ablative material potentialities [16,23,34]. The data collected were used for FEM simulations [14] and one of the first prototype was also tested in a plasma wind tunnel, Scirocco, CIRA (Italian Center of Aerospace Research) [14].…”

Effect of ceramic nano‐particles on the properties of a carbon‐phenolic ablator

“…[1,6,7] According to the kind of matrix (ceramic, metal or polymer) the toughening and strengthening mechanisms of the nano-fillers can be very different: for example, for ceramic or metallic matrix, nano-fillers are able to modify the microstructure during the manufacturing process of sintering, changing the shape and the size of the crystal grains [5] ; in a polymer the nano-fillers can create an interphase zone between reinforcement and matrix with unique thermo-mechanical and thermo-physical properties able to deeply modify the composite behavior. [2,8] This interphase zone is present both in conventional and nano-composites but, for the last ones, the interfacial area per unit volume (R, m 2 /cm 3 ) becomes ultra-large because of the huge surface/volume ratio of nano-fillers and its influence on the overall properties of the materials is more relevant. [7,8] Since small amounts of nano-fillers are able to modify the properties of a bulk material, the final density of the nano-composite does not differ significantly from the bulk material's one.…”

“…[9] Vehicles going through an interplanetary atmosphere suffer intense heating because of the friction between the surfaces of the vehicle and the atmospheric gasses. [6,8] When heat fluxes exceed 1 MW/m 2 , for example for ballistic atmospheric reentry, ablative carbon-phenolic heat shields are typically selected: these materials consist of reinforcing carbon fibers fully or partially infiltrated by a phenolic resin and they are semi-active insulators, able to protect the inner part of the vehicle because of their insulating properties and because of the endothermic reactions related to the polymer matrix decomposition. The produced pyrolysis gasses, flowing through the char and into the boundary layer, are able both to absorb part of the incoming heat and to hinder the convective exchanges (this is the so called blockage effect).…”

“…The produced pyrolysis gasses, flowing through the char and into the boundary layer, are able both to absorb part of the incoming heat and to hinder the convective exchanges (this is the so called blockage effect). [8,[10][11][12] During the last 20 years, several studies were carried out with the attempt of modifying ablative composite materials with the addition of nano-fillers. Srikanth et al [13] modified a high density carbon-phenolic ablator with nanoparticles of SiO 2 and tested the blank and the modified materials under a plasma arc jet at 2.5 MW/m 2 .…”